Publication No. 317 Manual on Transformers Editors M. Vijayakumaran V.K. Lakhiani V.K. Kanjlia P.P. Wahi Central Board of Irrigation & Power Malcha Marg, Chanakyapuri, New Delhi 110021 April 2013 2013 ISBN 81-7336-324-2 “Reproduction of any part of this publication in any form is permissible subject to proper acknowledgement and intimation to the publisher. The publisher/author/editors have taken utmost care to avoid erros in the publication. However, the publisher/author/editors are in no way responsible for the authenticity of data or information given in the book.” (ii) Expert Group CHAIRMAN Shri M. Vijayakumaran Sr. Technical Expert ALSTOM T&D India Ltd Naini, Allahabad 211008 SUB-GROUP CONVENORS Shri P. Ramachandran Sr. Advisor - Design & Development Power Transformers Division ABB Ltd., Maneja Vadodara – 390013 Shri M.M. Goswami General Manager Power Grid Corporation of India Ltd. Saudamini, Plot No. 2, Sector 29 Gurgaon – 122001 Shri Dinkar Devate General Manager NTPC Ltd. A-8A, Sector 24 Noida – 201301 Shri Sanjay Kar Chowdhury Sr. Dy. Manager CESC Ltd., Poddar Court 18, Rabindra Sarani Kolkata – 700001 Shri A. Kulshreshtha Addl. General Manager Bharat Heavy Electricals Ltd. BHEL, Jhansi (U.P.) – 284129 Shri S.K. Mahajan Dy. General Manager (TRE) Bharat Heavy Electricals Ltd. Piplani, Bhopal (M.P.) – 462022 Ms. Elizabeth Johnson Sr. Manager - Technical Solution Group Alstom Grid Plot No.46, Zuzuwadi Village SIPCOT Industrial Complex Hosur 635 126 Tamil Nadu (iii) Members Shri Mata Prasad Founder President CIGRE India 5/100, Vinay Khand Gomti Nagar Lucknow 226010 Shri V.K. Lakhiani Technical Director Transformers and Rectifiers (India) Limited Survey No. 427 P/3-4, & 431 P/1-2 Sarkhej-Bavla Highway, Moraiya, Taluka: Sanand, Dist. Ahmedabad–382213 Shri Ratish Kumar Executive Director – Design E&M NHPC Ltd. NHPC Office Complex, Sector 33 Faridabad – 121003 Shri R.K. Tiwari General Manager (TCB) Bharat Heavy Electricals Ltd. Piplani, Bhopal – 462022 Shri S.K. Ray Mohapatra Director Central Electricity Authority Sewa Bhavan, R.K. Puram New Delhi 110066 Shri Y.V. Joshi Executive Engineer – Equip. Engg. Gujarat Energy Transmission Corp. Ltd. Sadar Patel Vidyut Bhavan Race Course, Vadodara – 390007 Shri B.V. Raghavaiah Unit head Central Power Research Institute Switchgear Testing & Development Station Govindpura, Bhopal – 462023 Shri Ranjan Banerjee General Manager – Technical Services and R&D – Engg. & Technology Larsen & Toubro Limited Vadodara - 390 019, Gujarat Shri M.L. Jain Sr. Vice President (Technology & Quality) Transformer Business Unit EMCO Ltd. Plot No. F-5, Road No. 28 Wagle Industrial Estate, Thane (W) 400604 Shri V.M. Varkey Head – Transformer Design Siemens Ltd. Transformer Works, Thane Belapur Road Airoli, Navi Mumbai – 400708 Shri Amit Mittal Addl. General Manager NTPC Ltd. A-8A, Sector 24 Noida – 201301 Shri Anilkumar Bhatia Deputy General Manager Design & Technology Crompton Greaves Limited Kanjur Marg (East), Mumbai 400 042 (iv) Shri S. Victor P. Selvakumar Addl. General Manager (OS) Power Grid Corporation of India Ltd. Plot No. 2, Sector 29, Gurgaon, Haryana Shri J.S. Kuntia AGM – Design BHEL Piplani, Bhopal (M.P.) Shri R.K. Tyagi Dy. General Manager Power Grid Corporation of India Ltd. Saudamini, Plot No. 2, Sector 29 Gurgaon – 122001 Shri Aseem Dhamija AGM (Bushing,Capacitor & Instrument Trfr Engg.) BHEL, Bhopal (M.P.) Shri Sudhir Agarwal Dy. General Manager Power Grid Corporation of India Ltd. Saudamini, Plot No. 2, Sector 29 Gurgaon – 122001 Shri S.K. Gupta Dy. General Manager (TRE) Bharat Heavy Electricals Ltd. Piplani Bhopal – 462022 Shri T.K. Ganguly Sr. General Manager – Engg. EHV Power Transformers Division Vijai Electricals Ltd. Rudraram – 502329, A.P. Shri N.K. Dahiwale DGM – Manufacturing Transformers BHEL, Piplani, Bhopal (M.P.) Shri Oommen P. Joshua General Manager (Technical) TELK (A joint Venture of Govt. of Kerala & NTPC Ltd), Angamally, Kerala – 683573 Shri R.K. Mohapatra DGM – TRE Bharat Heavy Electricals Ltd. Piplani, Bhopal – 462022 Dr. K. Rajamani Chief Consultant Reliance Infrastructure Ltd DAKC, I Block, North Wing, Thane Belapur Road, Koparkhairane, Navi Mumbai - 400 709 Shri Pradeep Singh Manager Bharat Heavy Electricals Ltd. Piplani, Bhopal – 462022 Shri B.M. Mehra Joint Director Central Power Research Institute Noida Shri Manish Kumar Bharat Heavy Electricals Ltd. Piplani, Bhopal – 462022 (v) Shri. N.G. Patel Divisional Engineer – Testing Gujarat Energy Transmission Corp. Ltd. Sadar Patel Vidyut Bhavan Race Course, Vadodara – 390007 Shri R.S. Thakkar Junior Engineer Gujarat Energy Transmission Corp. Ltd. Sadar Patel Vidyut Bhavan Race Course, Vadodara – 390007 Shri Vibhu Tripathi Assistant Director – I, DP&D Central Electricity Authority Sewa Bhavan, R.K. Puram New Delhi 110066 Shri A.K. Panwar Assistant Director – I DP&D Central Electricity Authority Sewa Bhavan, R.K. Puram New Delhi 110066 Shri Gunjan Agrawal Manager Power Grid Corporation of India Ltd., Plot No. 2, Sector 29, Gurgaon, Haryana Shri Minal Kataria NTPC Ltd. A-8A, Sector 24 Noida – 201301 Shri K. Jayakrishnan NHPC Ltd. NHPC Office Complex Sector 33 Faridabad – 121003 Haryana Shri Govind Srivastava Sr. Manager Siemens Ltd. Transformer Works, Thane Belapur Road, Airoli, Navi Mumbai– 400708 Miss Tanvi Srivastava Dy. General Manager Alstom, Allahabadad Ms. Mary Mody General Manager EMCO Ltd, Thane Shri Pramod Srivastava Dy. General Manager (Design) Alstom T&D India Limited Ms. Anagha Dixit General Manager - Engineering (E) EMCO Limited Shri Abhilash Mishra Alstom T&D India Ltd. Vadodara Shri Meet Patel L&T Power Ltd. Vadodara Shri Pravin Jain Chief Manager (Corp Engineering) The Tata Electric Company Mumbai Shri T. Murlikrishna Chief Manager (Testing) The Tata Electric Company Mumbai (vi) Shri Tarun Garg Design Head, Power Transformers ABB Ltd Shri Vikrant Joshi Crompton Greaves Ltd. Mumbai Shri Abhay Agrawal Design Head, Small Power Transformers ABB Ltd. Shri Manish Yadav DGM – Design Testing Crompton Greaves Ltd. Shri Maneesh Jain Business Unit manager - Bushings ABB Limited Maneja, Vadodara - 390013, Gujarat Ms. Shubhangi Kulkarni Sr. Manager Design Crompton Greaves Ltd. Mumbai Shri Surojit Roy Director Schneider Electric Infrastructure Ltd. Naini, Allahabad Shri Prasenjit Paul Head – Technology Schneider Electric Infrastructure Ltd. Naini, Allahabad Shri Subrata Dutta Schneider Electric Infrastructure Ltd. Naini, Allahabad Shri Sameer Gaikwad Manager, Regional Sales – South Asia Doble Engineering Company 2nd Floor “Suvidhi Pride”, Gorwa Refinery Road, Vadodara 390 003 Shri R.V. Talegaonkar President Group I CTR Mfg Ind Ltd Nagar Road, Pune 411014 Shri Rakesh Sardana President & CEO Skipper Electricals (India) Ltd. F-667-668, RIICO Indl. Area Ph-II Bhiwadi 301019 Distt. Alwar Shri S.A. Rajan Sr. Design Manager CTR Mfg. Ind Ltd. Nagar Road, Pune Shri R. Prakash Head - Mktg & Service Easun - MR Tap Changers Pvt. Ltd. 612, (232) M.T.H Road Thiruninravur - 602 024, Chennai Shri B.D. Raut Sr. Development Manager CTR Mfg. Ind Ltd. Pune Shri D.M. Jadhav Sr. Applications Manager CTR Mfg. Ind Ltd. Pune (vii) Shri V.K. Kanjlia Secretary Central Board of Irrigation & Power Malcha Marg, Chanakyapuri New Delhi 110021 Shri P.P. Wahi Director Central Board of Irrigation & Power Malcha Marg, Chanakyapuri New Delhi 110021 Shri Manish Kataria Jt. General Secretary Hydraulic Trailers Owner’s Association (HTOA) 429, Vyapar Bhavan, P. D’Mello Road, Mumbai 400 009 Shri S.K. Batra Sr. Manager (Technical) Central Board of Irrigation & Power Malcha Marg Chanakyapuri New Delhi 110021 (viii) Message Central Board of Irrigation and Power (CBIP) has been playing a key role to disseminate the latest technological advancement information covering almost all aspects of power and renewable sector and it was in 1976, CBIP brought out detailed first edition of the Transformer Manual. I understand that the manual issued by CBIP is being widely used by power engineers as a reference book in the country & elsewhere. I am happy to note that this manual is being revised/updated now under the chairmanship of Shri M. Vijayakumaran, Sr. Technical Expert, ALSTOM T&D India Ltd. and with the help of Expert Group members from all eminent organizations in the country. This contains the latest updated technological information on the subject of Transformer. I congratulate CBIP and all experts of the Expert Group for bringing out this manual covering latest state-of-art technology and I am sure that this document will be of great benefit to engineering fraternity as a reference book. A.S. Bakshi (ix) PREFACE “Manual on Transformers” is the most popular publication of CBIP. It has been widely appreciated by practising transformer engineers associated with all facets of transformers: Design, Materials, Manufacturing, Testing, Erection and Commissioning. Utility Engineers and End users have also found this manual, a good guide and reference book. Although the transformer is a matured product of Electrical Engineering with matured science and technology, yet, to keep pace with the needs of changing times, several technological advancements take place on continuous basis. To keep pace with the fast changing technology, it is desired that this Manual is updated from time to time, at least once in 5 years. Third Revision was undertaken in 2005 with a reprint in 2007 with minor modifications. The working group reconstituted in 2012 with experts drawn from Indian Utilities, Institutes, Transformer Manufacturers etc. reviewed the Manual and finalized modifications for Fourth revision to be published now in 2013. The manual is reformatted and divided in to two Volumes: Vol. I to cover specifications of standard transformers and Vol. II to include various guidelines and specifications of fittings and accessories used in the transformers. Chapters have been reorganized in these two volumes being published together. In this revision, existing sections have been updated as recommended by the Expert group. New sections viz. 420 kV & 800 kV Shunt reactors, Furnace transformers, Rectifier Transformers, Traction Transformers etc. have been added. A chapter on new technologies describing Smart Transformers, UHV Transformers, Phase shifting transformers etc. has been included to indicate emerging trends in Technology. A chapter on Reference standards and Books (xi) also has been added to facilitate readers to locate the standards, as a ready reckoner. Technology evolves dynamically. Full-fledged revision is a voluminous task. It is decided that the revision of Manual shall be normally undertaken in 5 years’ time span. However, a core group has been formed to address urgent amendments to make the Volumes updated all the time. Core group shall welcome suggestions / improvements / amendment proposals from the distinguished users of this Manual. I am sure that the users will find the Fourth Edition of the Manual more meaningful. M. Vijayakumaran Chairman of the Expert Group (xii) Foreword With the Indian economy growing year by year, the target of providing reliable power supply to consumers is becoming increasingly important. To meet this challenge new power stations are being added and T&D networks are continuously being strengthened. As you know, the transformer is one of the most important and vital asset in a power system. Reliability and availability of such an asset plays an important role in the operation of a power system. Emphasis needs to be laid on improved design, quality control during manufacturing, use of right components / accessories, maintenance and safety during operation of such vital equipment. The adoption of state-of-art technology for important components like bushings & OLTC which have been the major items causing failure of transformer is the urgent need for all concerned professionals. The review of the protection philosophy would further improve the performance of the system. The residual life assessment and condition monitoring of the transformers will also add reliability to the power system. CBIP has brought out the first Manual on Transformers in 1976. This was updated in 1987, 1999, 2005 and again in 2007. To incorporate the latest developments and innovations, this manual has again been updated and new sections viz. 420 kV & 800 kV Shunt reactors, Furnace transformers, Rectifier Transformers, Traction Transformers etc. have been added. A chapter on new technologies describing Smart Transformers, UHV Transformers, Phase shifting transformers etc. have also been included to indicate emerging trends in Technology. For updating this manual, CBIP had constituted the Expert Group, comprising of highly experienced engineers from large power utilities, designs organization, manufacturers etc. This group was headed by Shri M. Vijayakumaran, National Representative in CIGRE Study Committee A2 on Transformers and Sr. Technical Expert, ALSTOM T&D India Ltd., who is recipient of many National and International awards. The expert group after working ceaselessly brainstorming/ working for more than one year have helped CBIP in updation of this document covering all aspects of Transformers for various voltages as mentioned above. The Central Board of Irrigation & Power wishes to acknowledge the valuable contributions made by Shri M. Vijayakumaran, Chairman of the Expert Group (xiii) for revision of this manual. Contribution made by Shri Virendra K. Lakhiani, Technical Director, Transformers and Rectifiers (India) Limited is deserves special mention, who has put in the best efforts for updation and synthesizing this manual. Our thanks are also due to Conveners Shri P. Ramachandran, Sr. Advisor - Design & Development, Power Transformers Division, ABB Ltd., Shri M.M. Goswami, General Manager, Power Grid Corporation of India Ltd., Shri Dinkar Devate, General Manager, NTPC Ltd., Shri R.K. Tiwari, General Manager (TCB), Bharat Heavy Electricals Ltd., Shri A. Kulshreshtha, AGM (TRE-SPTR, FES, RPD & T), Bharat Heavy Electricals Ltd., Shri S.K. Mahajan, DGM (TRE), Bharat Heavy Electricals Ltd., Ms. Elizabeth Johnson, Sr. Manager - Technical Solution Group, Alstom Grid, Shri Sanjay Kar Chowdhury, Sr. Dy. Manager, CESC Ltd. & Members of all the Sub-Group for revision of this manual. Contibution made by Shri R.K. Tyagi, Dy. General Manager, Power Grid Corporation of India Ltd., Shri Ranjan Banerjee, General Manager – Technical Services and R&D – Engg. & Technology, Larsen & Toubro Limited, Shri S.K. Ray Mohapatra, Director, Central Electricity Authority, Shri B.V. Raghavaiah, Unit Head, CPRI, Bhopal, Shri M.L. Jain, Sr. Vice President (Technology & Quality), Transformer Business Unit, EMCO Ltd., Shri Y.V. Joshi, Executive Engineer – Equip. Engg., Gujarat Energy Transmission Corp. Ltd., Shri Anilkumar Bhatia, Deputy General Manager - Design & Technology, Crompton Greaves Limited needs special mention. I also appreciate the dedication & the contribution made by Shri S.K. Batra, Sr. Manager, CBIP for getting this document revised. I trust that this manual would cover the existing knowledge gap on this subject and help the practising engineers in the power sector as well as students in the technical institutions in enhancing their technical skills. V.K. Kanjlia Secretary Central Board of Irrigation and Power (xiv) CONTENTS Page Preface (v) Foreword (vii) Vol I: Standard Specifications of Transformers SECTION A General 3 SECTION B Specifications for Outdoor type, Completely Self protected, 3 Phase Distribution Transformers (up to and including 100 kVA) 41 SECTION C Specifications for Outdoor type, Completely Self Protected, Single Phase Distribution Transformers (Single Phase 11 kV/250 V & 11/√3 kV/250 V, 5, 10, 16 & 25 kVA ratings) 57 SECTION D Specifications for Three Phase Distribution Transformers (above 100 kVA and up to 33 kV class) 71 SECTION E Specifications for Power Transformers of Voltage Class below 145 kV 83 SECTION F SECTION G Specifications for 145 kV Class Power Transformers Specifications for 245 kV Class Power Transformers 91 99 SECTION H Specifications for 420 kV Class Power Transformers 107 SECTION I Specifications for 800 kV Class Power Transformers 119 SECTION J Specification for 420 kV class Shunt Reactors and associated 145 kV Neutral Grounding Reactors 129 SECTION K Specifications for 800 kV class Shunt Reactors and associated 145 kV Neutral Grounding Reactor 145 SECTION L Specifications for Earthing Transformers 155 SECTION M Specifications for Furnace Transformers 165 SECTION N Specifications for Rectifier Transformers 175 SECTION O Specifications for Electrostatic Precipitator Transformers 185 SECTION P Specifications for Traction Transformers 191 SECTION Q Specifications for Dry type Transformers 197 Vol II: Application, Standard Fittings and Accessories SECTION AA SECTION BB Capitalization Formula for Transformer Losses Test Requirements for Transformers (xv) 211 215 SECTION CC Guidelines for Erection, Commissioning and Maintenance 277 SECTION DD Guidelines for Condition Monitoring and Diagnostic Techniques for Power Transformers and Reactors 347 SECTION EE Guidelines for Fire Protection of Power Transformers 371 SECTION FF Guidelines for Repair of Power Transformers at Site 386 SECTION GG Guidelines for Voltage Control of Power Transformers 409 SECTION HH Guidelines for Protective Schemes for Power and Distribution Transformers 427 SECTION II Specifications for Transformer Bushings up to 1200 kV Voltage Class 439 SECTION JJ Specifications for Valves for Transformers 455 SECTION KK Specifications for Cable Boxes for SF6 Gas Insulated Transformer Terminations for Rated Voltages of 72.5 kV and above 463 APPENDICES APPENDIX-I New Technologies, UHV AC/DC Transformers, Phase Shifting Transformers, Controlled Shunt Reactors 479 APPENDIX-II Reference Standards and Books 503 APPENDIX-III Typical Quality Assurance Plan 523 APPENDIX-IV Guaranteed Technical and Additional Technical particulars 527 APPENDIX-V List of Transformer Accessories and test certificates required 537 APPENDIX-VI Design Review Parameters 543 APPENDIX-VII Salient Technical Specifications and Particulars of Power Transformers for Power Plant Application 549 APPENDIX-VIII Test Windings for Bushing Current Transformers 559 APPENDIX-IX Pictures of Transformer Installations 563 (xvi) Vol. I Standard Specifications of Transformers SECTION A General SECTION A General 1.0 GENERAL DESIGN OF APPARATUS 1.1 Compliance with Specifications 1.1.1 Except where otherwise specified or implied herein, the transformers shall comply with the latest edition of Indian Standard 2026 (hereinafter referred to as “IS”). 1.2 Design and Standardization 1.2.1 The transformer and accessories shall be designed to facilitate operation, inspection, maintenance and repairs. All apparatus shall also be designed to ensure satisfactory operation under such sudden variations of load and voltage as may be met with under working conditions on the system, including those due to short circuits. 1.2.2 The design shall incorporate every reasonable precaution and provision for the safety of all those concerned in the operation and maintenance of the equipment keeping in view the requirements of Indian Electricity Rules. 1.2.3 All material used shall be of the best quality and of the class most suitable for working under the conditions specified and shall withstand the variations of temperatures and atmospheric conditions arising under working conditions without undue distortion or deterioration or the setting up of undue stresses in any part, and also without affecting the strength and suitability of the various parts for the work which they have to perform. 1.2.4 Corresponding parts liable to be replaced shall be interchangeable. 1.2.5 Cast iron shall not be used for chambers of oil filled apparatus or for any part of the equipment which is in tension or subject to impact stresses. This clause is not intended to prohibit the use of suitable grades of cast iron for parts where service experience has shown it to be satisfactory, e.g., large valve bodies. 1.2.6 All outdoor apparatus, including bushing insulators with their mountings, shall be designed so as to avoid pocket in which water can collect. 1.2.7 Means shall be provided for the easy lubrication of all bearings and where necessary of any mechanism or moving part, that is not oil immersed. 1.2.8 All mechanism shall, where necessary, be constructed of stainless steel, brass or gunmetal to prevent sticking due to rust or corrosion. 1.2.9 All taper pins used in any mechanism shall be of the split type complying with IS: 2393 for these items. 1.2.10 All connections and contacts shall be of ample section and surface for carrying continuously the specified currents without undue heating and fixed connections shall be secured by bolts or set screws of ample size, adequately locked. Lock nuts shall be used on stud 5 6 Manual on Transformers connections carrying current All leads from the winding to the terminal board and bushings shall be adequately supported to prevent injury from vibration including a systematical pull under short circuit conditions. Guide pulls shall be used where practicable. 1.2.11 All apparatus shall be designed to minimise the risk or accidental short-circuit caused by animals, birds or vermin. 1.2.12 Provision shall be made to fix safety fence around top cover of transformers of rating 100 MVA and above, for safe working during installation and servicing for large capacity transformers. 1.2.13 In tank on load tap changers shall be located such that the space above the diverter switch chamber will be free of inter connecting pipes etc. for lifting the diverter switch unit for inspection and maintenance purposes. 1.2.14 Dryness of the insulation may be ensured by measuring the water extraction during vacuum drying. The water extraction per tonne of insulation per hour may be limited to 5030 grams maximum. Alternatively dryness can be judged by dew point measurement. 1.3 Galvanising 1.3.1 Galvanising where specified shall be applied by the hot-dipped process or by clectrogalvanising process and for all parts other than steel wires shall consist of a thickness of’zinc coating equivalent to not less than 610 gm of zinc per square meter of surface. The zinc coating shall be smooth, clean and of uniform thickness and free from defects. The preparation of galvanising and the galvanising itself shall not adversely affect the mechanical properties of the coated material. The quality will be established by tests as per IS: 2633. Alternative to galvanising, zinc spraying or aluminising can also be considered. 1.3.2 All drilling, punching, cutting, bending and welding of parts shall be completed, and all burrs shall be removed before the galvanising process is applied. 1.3.3 Galvanising of wires shall be applied by the hot-dipped process and shall meet the requirements of the relevant Indian Standard. The zinc coating shall be smooth, clean and of uniform thickness and free from defects. The preparation for galvanising itself shall not adversely affect the mechanical properties of the wire. 1.3.4 Surfaces which are in contact with oil shall not be eleclrogalvanised/cadmium plated. 1.4 Labels 1.4.1 Labels shall be provided for all apparatus such as relays, switches, fuses, contained in any cubicle or marshalling kiosks. 1.4.2 Descriptive labels for mounting indoors or inside cubicles and kiosks shall be of material that will ensure permanence of the lettering. A matt or satin finish shall be provided to avoid dazzle from reflected light. Labels mounted on dark surfaces shall have white lettering on a black background. Danger notices shall have red lettering on a while background. 1.4.3 All plates shall be of material which is corrosion resistant. General 7 1.4.4 Labels shall be attached to panels with brass screws or with stainless steel screws or these can be stuck with suitable adhesive also. 1.5 Bolts and Nuts 1.5.1 Steel bolts and nuts exposed to atmosphere shall be of following material: • Size 12 mm or below - stainless steel • Above 12 mm - steel with suitable finish like electrogalvanised with passivation /hot dip galvanised. 1.5.2 All nuts, bolts and pins shall be locked in position with the exception of those external to the transformer, under gasket pressure. 1.5.3 All bolts, nuts and washers exposed to atmosphere and in contact with non-ferrous parts which carry current shall be of phosphor bronze. 1.5.4 If bolts and nuts are placed so that they are inaccessible by means of ordinary spanners, suitable special spanners shall be provided by the supplier. 1.5.5 Bolts and nuts shall not be less than 8 mm in diameter except when used for small wiring terminals. 1.6 Cleaning and Painting 1.6.1 Before painting or filling with oil or compound, all ungalvaniscd parts shall be completely clean and free from rust, scale and grease, and all external surface cavities on castings shall be filled by metal deposition. 1.6.2 All blast cleaned surfaces (except machined faces that have to be protected) must be cleaned in accordance with ISO specification no. ISO 8501 Part l (This standard specification is based on and now supersedes Swedish Standard SIS 05 59 00) to a minimum standard of ‘ASa21/2’ or ‘BSa21/2’ prior to paint application. 1.6.3 External and internal surfaces of all transformer tanks and chambers and other fabricated steel items shall be cleaned of scale, rust and surface dirt by blast cleaning or other suitable approved method. After cleaning, these surfaces should be immediately covered with paint. Hot oil resistant varnish on white synthetic enamel/epoxy paint is to be used for painting the inside of all oil filled chambers, including transformer tanks. Only one thin layer (~ 25 microns) of this is to be applied. 1.6.4 Except for hardware, which may have to be removed at site, all external surfaces shall receive at least four coats of paint. The type and thickness of paint shall be chosen to suit pollution level at site. 1.6.5 Selection of paint system for different environmental conditions shall be in line with ISO: 12944. 1.6.6 For rural or mild atmosphere, alkyd enamel primer and finish system may be used in four coats to give a total dry film thickness of at least 80 microns. 1.6.7 For urban or industrial situation two coats of epoxy zinc phosphate or zinc chromate primer topped with two coats of aliphatic polyurethane glossy finish paint is recommended. 8 Manual on Transformers The total dry film thickness should preferably be between 100 and 130 microns. 1.6.8 In case of highly polluted area, chemical atmosphere or at a place very near the sea coast, paint as above with one intermediate coat of high build MIO (Micaceous iron oxide) as an intermediate coat may be used to give a total dry film thickness of 150 to 180 microns. 1.6.9 All interior surfaces of chambers or kiosks that are in contact with air shall receive at least three coats of paint, of which the topcoat shall be of a light shade. If heaters are not provided in the chamber, then the top coat should be of anti condensation type. 1.6.10 Any scratch, bruise or paint damage incurred during transportation and unloading at site should be made good by the purchaser as soon as the damage is detected. This is to be done by thoroughly cleaning the damaged area and applying the full number of coats as was applied originally. Manufacturer should supply the necessary paint for this touch up painting at site. 1.6.11 One coat of additional paint shall be given at site over all external surfaces, including hardware, after erection by the purchaser. Supplier shall furnish necessary information on the make and grade of the lop-coat paint. In general, it is possible to apply enamel paint over epoxy polyurethane coating and the vice versa is not recommended. As far as possible the make and grade of the recoat shall be same as the original coat. 1.7 Oil 1.7.1 The transformers and all associated oil-filled equipment shall normally be supplied alongwith the first filling of oil and 5/10 percent (as per user requirement) excess quantity of oil shall also be supplied in non-returnable drums. The oil shall conform to IS: 335 or IEC 60296 (as per user requirement). Alternatively, if the purchaser so desires, oil may be supplied in tankers directly from the refinery for transformers which are dispatched from factory to site in gas filled condition. 1.8 Prevention of Acidity 1.8.1 The design and all materials and processes used in the manufacture of the transformer, shall be such as to reduce to a minimum the risk of the development of acidity in the oil. Special measures, such as nitrogen sealing or the use of inhibited oil shall not be resorted to, unless otherwise specified by the purchaser. 2.0 ELECTRICAL CHARACTERISTICS AND PERFORMANCE 2.1 Type of Transformers and Operating Conditions 2.1.1 All transformers, unless otherwise specified shall be oil immersed and may be either core or shell type and shall be suitable for outdoor installation. Normally oil immersed transformer shall be provided with conservator vessels. Where sealed transformers are specified, there shall be no conservator but adequate space shall be provided for expansion of oil without developing undue pressure. The types of cooling shall be as stated in the relevant specifications. General 9 2.1.2 Transformers designed for mixed cooling shall be capable of operating under the natural cooled condition upto the specified load. The forced cooling equipment shall come into operation by pre-set contacts in WTI and the transformer will operate as a forced cooled unit. 2.1.3 Transformer shall be capable of remaining in operation at full load for 10 minutes after failure of the oil and/or water circulating pumps or blowers without the calculated winding hot-spot temperature exceeding 140° C. Transformer fitted with two coolers each capable of dissipating 50 percent of the losses at Continuous Maximum Rating (CMR) shall be capable of remaining in operation for 20 minutes in the event of failure of the oil and/or water circulating pumps or blowers associated with one cooler without the estimated winding hot-spot temperature exceeding 140° C. 2.2 Continuous Maximum Rating and Overloads 2.2.1 Transformers provided with mixed cooling shall comply, as regards its rating, temperature rise and overloads, with the appropriate requirements of IS: 2026 when operating with natural cooling and with mixed cooling. 2.2.2 All transformers, except where stated shall be capable of operation continuously, in accordance with IS loading guide at their CMR and at any ratio. In case bi-directional flow of power is required, that shall be specifically stated by the purchaser. 2.2.3 Temperature rise test shall be performed at the tapping as desired by the purchaser. If nothing has been stated by the purchaser, the test shall be carried out at the tapping with the highest load losses. 2.2.4 The transformer may be operated without danger on any particular tapping at the rated kVA provided that the voltage does not vary by more than ±10 percent of the voltage corresponding to the tapping. 2.2.5 The transformer shall be suitable for continuous operation with a frequency variation of ±3% from normal 50 Hz. Combined voltage and frequency variation should not exceed the rated V/f ratio by 10%. Note: Operation of a transformer at rated kVA at reduced voltage may give rise to excessive tosses and temperature rise. 2.3 Voltage Ratio 2.3.1 The voltage between phases on the higher and lower voltage windings of each transformer measured at no-load and corresponding to the normal ratio of transformation shall be those stated in the ordering schedule. 2.3.2 Means shall be provided in accordance with clauses 8 and 9 for varying the normal ratio of transformation. 10 2.4 Manual on Transformers Electrical Connections 2.4.1 Transformers shall be connected in accordance with the IS vector symbol specified in ordering schedule of the requirements. 2.4.2 Auto connected and star/star connected transformers shall have delta connected stabilising windings if specified in the order. Two leads from one open comer of the delta connection shall be brought out to separate bushings. Links shall be provided for joining together the two terminals so as to complete the delta connection and earthing it external to the tank. 2.5 Duty under Fault Conditions 2.5.1 Except where modified below, it is to be assumed that the capacity of generating plants simultaneously connected is such that normal voltage will be maintained on one side of any transformer when there is a short-circuit between phases or to earth on the other side. Any transformer may be directly connected to an underground or overhead transmission line and switched into and out of service together with its associated transmission line. 2.5.2 All transformers shall be capable of withstanding any external short-circuit according to IS: 2026 without damage. 2.5.3 Transformers with tertiary windings shall be capable of withstanding the mechanical and thermal effects of any external short-circuit to earth with the short-circuit MVA available at the terminals not exceeding the values given in the ordering schedule with the neutral points on both HV and LV windings directly connected to earth as per the requirements of IS: 2026. 2.5.4 Transformers directly connected to generator (generator step-up transformers) shall be designed for exceptional circumstances arising due to sudden disconnection of the load and shall be capable of operating at approximately 25 percent above normal rated voltage for a period not exceeding one minute and 40 percent above normal rated voltage for a period of 5 seconds. However, the purchaser will install the over fluxing protection device in case of generator stepup transformers. Note : All inter-connected Transformers of 50MVA and above shall also be provided with over fluxing protection by the purchaser. 2.6 Stabilising Windings 2.6.1 If specified in the order, the stabilising winding shall be capable of carrying continuously the load specified therein. 2.6.2 The design of stabilising winding shall be such as to take care of the effect of transferred surges and the tenderer shall offer suitable surge protection wherever necessary. General 2.7 11 Losses 2.7.1 The accepted losses of each transformer shall be stated in the order. The tolerance on the losses of each transformer shall be in accordance with IS: 2026, except where maximum losses are specified. 2.8 Regulation and Impedance 2.8.1 The impedance voltage at principal tap and rated kVA shall be stated in the order and tolerance shall be in accordance with IS: 2026. 2.8.2 For all transformers, the value of impedance on any other tapping shall be generally subject to the approval of the purchaser at the time of order. Any specific requirement may be mentioned at the time of enquiry as a prequalification instead of at the time of order. 2.9 Flux Density 2.9.1 The maximum flux density in any part of the core and yokes, of each transformer at normal voltage and frequency shall be such that the flux density in over-voltage condition as per clause 2.2.5 shall not exceed 1.9 Tesla (19,000 lines per cm2). However, in case of transformers with variable flux the voltage variation which would affect flux density at every tap shall be kept in view while designing transformers. 2.10 Vibration and Noise 2.10.1 Every care shall be taken to ensure that the design and manufacture of all transformers and auxiliary plant shall be such as to have minimum noise and vibration levels following good modem manufacturing practices. 2.10.2 The manufacturers will ensure that the noise level shall not exceed the figures as per NEMA Pub. No. TR - 1. 2.11 Suppression of Harmonics 2.11.1 All the transformers shall be designed with particular attention to the suppression of harmonic voltage, especially the third and fifth, so as to eliminate wave-form distortion and from any possibility of high frequency disturbances, inductive effects or of circulating currents between the neutral points at different transforming stations reaching such a magnitude as to cause interference with communication circuits. 3.0 CORES The cores shall be constructed from high grade cold rolled non-ageing grain oriented silicon steel laminations or Amorphous Metal. For medium rating transformers, the core may be constructed of similar to M4 or better grade cold rolled non-aging grain oriented silicon steel laminations. For larger rating transformers, core may be constructed from high-grade non-aging, cold rolled, super grain oriented, silicon steel laminations similar to MOH / Hi-B steel or better grade. The selection of Grade much depends upon loss capitalization formula(Section AA ) for an optimized design and also upon stipulation in customer specification. 3.1 Magnetic Circuit 3.1.1 The design of the magnetic circuit shall be such as to avoid static discharges, development of short-circuit paths within itself or to the earthed clamping structure and the production of flux components at right angles to the plane of the laminations which may cause local heating. 12 Manual on Transformers 3.1.2 Every care shall be exercised in the selection, treatment and handling of core steel to ensure that as far as is practicable, the laminations are flat and the finally assembled core is free from distortion. 3.1.3 Adequate oxide/silicate coating is to be given on the core steel. However, laminations can be insulated by the manufactures if considered necessary. 3.1.4 Oil ducts shall be provided where necessary to ensure adequate cooling. The winding structure and major insulation shall not obstruct the free flow of oil through such ducts. Where the magnetic circuit is divided into pockets by cooling ducts parallel to the planes of the laminations or by insulating material above 0.25 mm thick, tinned copper strip bridging pieces shall be inserted to maintain electrical continuity between pockets. 3.1.5 The framework and clamping arrangements shall be earthed in accordance with clause 5.2. 3.1.6 When insulation is provided for the core to core bolts and core to clamp plates, the same shall withstand a voltage of 2000 2500 V AC for one minute. 3.1.7 Transformers shall withstand, without injurious heating, combined voltage & frequency fluctuations, which produce the following over fluxing condition: • • • 110 %- continuous 125%- for one minute 140%- for five seconds 3.2 Mechanical Construction of Cores 3.2.1 All parts of the cores shall be of robust design capable of withstanding any shocks to which they may be subjected during lifting, transport, installation and service. 3.2.2 All steel sections used for supporting the core shall be thoroughly sand blasted or shot blasted after cutting, drilling and welding. Any non-magnetic or high resistance alloy shall be of established quality. 3.2.3 Adequate lifting lugs shall be provided to enable the core and windings to be lifted. 3.2.4 Adequate provision shall be made to prevent movement of the core and winding relative to the tank during transport and installation or while in service. 3.2.5 The supporting framework of the cores shall be so designed as to avoid the presence of pockets which would prevent complete emptying of the tank through the drain valve, or cause trapping of air during filling. 4.0 WINDINGS 4.1 General 4.1.1 All star connected windings for system of 66 kV and above shall have graded insulation as defined in IS: 2026. All windings for system voltages lower than 66 kV shall be fully insulated. All neutral points shall be insulated for the voltages specified in IS: 2026. General 13 4.1.2 Power transformers shall be designed to withstand the impulse and power frequency test voltages as specified in IS: 2026. 4.1.3 The windings shall be designed to reduce to a minimum the out-of-balance forces in the transformer at all voltage ratios. 4.1.4 The insulation of transformer windings and connection shall be free from insulating composition liable to soften, ooze out, shrink or collapse and be non-catalytic and chemically inactive in transformer oil during service. 4.1.5 The slacks of windings shall receive adequate shrinkage treatment before final assembly. Adjustable devices shall be provided for taking up any possible shrinkage of coils in service. 4.1.6 The coil clamping arrangement and the finished dimensions of any oil ducts shall be such as will not impede the free circulation of oil through the ducts. 4.1.7 No strip conductor wound on edge shall have a width exceeding generally six times its thickness. 4.1.8 The conductors shall be transposed at sufficient intervals in order to minimize eddy currents and equalize the distribution of currents and temperatures along the windings. 4.2 Bracing of Windings 4.2.1 The windings and connections of all transformers shall be braced to withstand shocks which may occur during transport, or due to switching short-circuit and other transient conditions during service. 4.2.2 Coil clamping rings, if provided, shall be of steel or of suitable insulating material. 5.0 INTERNAL EARTHING ARRANGEMENTS 5.1 General 5.1.1 All metal parts of the transformer with the exception of the individual core laminations, core bolts and associated individual clamping plates shall be maintained at same fixed potential. 5.2 Earthing of Core Clamping Structure 5.2.1 The top main core clamping structure shall be connected to the tank body by a copper strap. The bottom clamping structure shall be earthed by one or more of the following methods: (a) By connection through vertical tie-rods to the top structure (b) By a connection to the loop structure on the same side of the core as the main earth connection to the lank 14 5.3 Manual on Transformers Earthing of Magnetic Circuit 5.3.1 The magnetic circuit shall be earthed to the clamping structure at one point only through a link placed in an accessible position beneath an inspection opening in the tank cover. The connection to the link shall be on the same side of the core as the main earth connection. The link should be brought out using bushing/terminal board on all transformers above 31.5 MVA. 5.3.2 When magnetic circuits are subdivided into separate isolated sections by duels perpendicular to the plane of laminations all such sections should be earthed. 5.4 Earthing of Coil Clamping Rings 5.4.1 Where coil clamping rings are of metal at earth potential, each ring shall be connected to the adjacent core clamping structure on the same side of transformer as the main earth connections. 5.5 Size of Earthing Connections 5.5.1 All earthing connections with the exception of those from the individual coil clamping rings shall have a cross-sectional area of not less than 0.8 cm, Connections inserted between laminations of different sections of core as per clause 5.3.2 shall have a cross-sectional area of not less than 0.2 cm2. 6.0 TANKS 6.1 Tank Construction All transformer reactor tanks should generally be of conventional type i.e., tank body with top cover, Bell shaped construction can be specified for 100 MVA and higher rating transformer unless otherwise mutually agreed between Purchaser and Manufacturer Top cover of conventional type transformer and Bell type construction may be bolted or welded to the tank body rim. Transformers with bell type tank may have joint either at tank bottom or close to bottom of the tank. Inspection covers shall always be bolted type. 6.1.1 The transformer tank and cover shall be fabricated from low carbon steel suitable for welding and of adequate thickness. The tanks of all transformers shall be complete with all accessories and shall be designed so as to allow the complete transformer in the tank and filled with oil, to be lifted by crane or jacks, transportation by road, rail or ship/boat without over straining any joints and without causing leakage of oil. 6.1.2 The transformer conservator tank, if equipped with an air cell, need not be designed for full vacuum but a vacuum-tight valve should be provided in the Buchholz relay pipe connection. Alternatively an equalizing connection may be provided between the inside of air cell and conservator for evacuating the conservator along with air cell, which may be removed after evacuation and oil filling. 15 General 6.1.3 The main tank body excluding tap-changing compartments, radiators and coolers shall be capable of withstanding vacuum given in the following tabic: Highest system voltage kV Up to 72 kV Above 72 kV MVA rating Vacuum gauge pressure kN/m2 (mm of Hg) Up to 1.6 34.7 250 above 1.6 and up to 20 68.0 500 above 20 100.64 760 100.64 760 for all MVA ratings 6.1.4 The base of each tank shall be so designed that it shall be possible to move the complete transformer unit by skidding in any direction without any damage when using plates or rails. 6.1.5 Normally a detachable base will be used, but in case transport facilities permit, a fixed base can be used. 6.1.6 Where the base is of a channel construction, it shall be designed to prevent retention of water. 6.1.7 Tank stiffeners shall be designed to prevent retention of water. 6.1.8 Wherever possible the transformer tank and its accessories shall be designed without pockets where gas many collect. Where pockets cannot be avoided, pipes shall be provided to vent the gas into the main expansion pipe. The vent pipes shall have a minimum inside diameter of 15 mm except for short branch pipes which may be 6 mm minimum inside diameter. 6.1.9 All joints other than those which may have to be broken shall be welded when required they shall be double welded. All bolted joints to the tank shall be fitted with suitable oil-tight gaskets which shall give a satisfactory service under the operating conditions and guaranteed temperature rise conditions. Special attention shall be given to the methods of making hot oil tight joints between the tank and the cover as also between the cover and the bushing and all other outlets to ensure that the joints can be remade satisfactorily at site and with ease with the help of semi-skilled labour. 6.2 Lifting and Haulage Facilities 6.2.1 Each tank shall be provided with: (a) Lifting lugs suitable for lifting the transformer complete with oil. (b) A minimum of four jacking lugs, in accessible positions to enable the transformer complete with oil, to be raised or lowered using hydraulic or screw jacks. The minimum height of the lugs above the base shall be: • Transformers upto and including 40 tonnes weight - 300 mm (approx.) so as to accommodate suitable jacks beneath the jacking parts 16 Manual on Transformers Transformers above 40 tonnes weight - 500 mm (approx.) so as to accommodate suitable jacks beneath the jacking lugs • When bell joint is at tank bottom and thick bottom plate is used, 300 and 500 mm jacking pad height may not be applicable. (c) • Suitable haulage holes shall be provided 6.2.2 To facilitate safe handling at site, the longitudinal and transverse axes and the center of gravity of main transformer tank should be marked permanently on all four sides. 6.3 Tank Cover 6.3.1 Each tank cover shall be of adequate strength, and shall not distort when lifted. Inspection openings shall be provided as necessary to give easy access to bushings or changing ratio or testing the earth connection. Each inspection opening shall be of ample size for the purpose for which it is provided and at least two openings one at each end of the tank, shall be provided. 6.3.2 A ladder (with anti-climbing lock arrangement) shall be provided for tank above 3 m height. 6.3.3 The tank cover and inspection covers shall be provided with suitable lifting arrangements. Unless otherwise approved inspection covers shall not weigh more than 25 kg each. 6.3.4 Tank shall be designed so as to avoid collection of rain water at the tank top. 6.3.5 The tank cover shall be fitted with pockets for a thermometer and for the bulbs of oil and winding temperature indicators. Protection shall be provided, where necessary, for each capillary tube. 6.3.6 The thermometer pocket shall be fitted with a captive screwed top to prevent the ingress of water. 6.3.7 The pockets shall be located in the position of maximum oil temperature at CMR and it shall be possible to remove the instrument bulbs without lowering the oil in the tank. 6.4 AXLES AND WHEELS 6.4.1 Requirement of the roller will be specified for plinth mounted transformers. If required only one set of roller of each size to be asked for. 6.4.2 If specified, transformers are to be provided with wheels and axles. They shall be of such dimensions and so supported that under any service conditions they shall not deflect sufficiently to interfere with the movement of the transformer. Suitable locking arrangements will be provided to prevent the accidental movement of the transformer. 6.4.3 Roller/skid shall be provided with suitable rail gauge as per user requirement. 6.4.4 All rollers should be detachable and shall be made of cast iron or steel as required. The direction of withdrawal shall be specified. General 17 6.4.5 Wherever specified, flanged wheels shall be provided suitable for use on gauge track as specified in the detailed specification and shall be so placed that pinch bar can be used to move the transformer. 6.4.6 If wheels are required to swivel, they shall be arranged so that they can be turned through an angle of 90° when the tank is jacked up clear of the rails or floor. Means shall be provided for locking the swivel movements in positions parallel to and at right angles to the longitudinal axis of the tank. 6.5 Conservator Vessels, Oil Gauges and Breathers 6.5.1 A conservator complete with sump and drain valve shall be provided in such a position as not to obstruct the electrical connections to the transformer having a capacity between highest and lowest visible levels of 7.5% of the total cold oil volume in the transformer and cooling equipment. The minimum indicated oil level shall be with the feed pipe from the main tank covered with not less than 15 mm depth of oil and the indicated range of oil level shall be from minimum to maximum. For 10.0 MVA & above rating, the conservator shall be equipped with aircell separator, unless otherwise specified. 6.5.2 If the sump is formed by extending the feed pipe inside the conservator vessel, this extension shall be for at least 25 mm. The conservator shall be designed so that it can be completely drained by means of the drain valve provided, when mounted as in service. 6.5.3 One end of the conservator shall be bolted into position so that it can be removed for cleaning purposes. 6.5.4 Normally one oil gauge, magnetic/prismatic/plain type as specified shall be provided. 6.5.5 The oil level at 30°C shall be marked on the gauge. 6.5.6 Taps or valves shall not be fitted to oil gauge. 6.5.7 The oil connection from the transformer tank to the conservator vessel shall be arranged at a rising angle of 3 to 7 degrees to the horizontal up to the Buchholz Relay and shall consist of: (a) For transformers up to and including 1000 kVA 25 mm inside diameter pipes as per IS: 3639 (b) For transformers from 1001 to 10,000 kVA 50 mm inside diameter pipes as per IS: 3639 (c) For transformers of over 10,000 kVA 80 mm inside diameter pipes as per IS: 3639 6.5.8 A valve shall be provided at the conservator to cut-off the oil supply to the transformer, after providing a straight run of pipe for at least a length of five times the internal diameter of the pipe on the tank side of the gas and oil actuated relay and at least three times the internal diameter of the pipe on the conservator side of the gas and oil actuated relay. 6.5.9 Each conservator vessel shall be fitted with a breather in which silica gel is the dehydrating agent and deigned so that: 18 Manual on Transformers (a) The passage of air is through the silica gel. (b) The external atmosphere is not continuously in contact with the silica gel. (c) The moisture absorption indicated by a change in colour of the tinted crystals, can be easily observed from distance. (d) All breathers shall be mounted at approximately 1,400 mm above ground level. (e) Self indicating (blue) silica gel contains the dye cobalt chloride which has potential health hazards. An alternative to the blue self indicating silica gel is SILICA GEL ORANGE with an organic indicator. The color changes from orange to light yellow as it absorbs moisture. Automatic regenerative breather with internal heater & humidity sensor are also available. 6.5.10 One non-return valve, which may automatically cut off the flow of oil from conservator towards the main tank may be provided in the pipe connection between the Buchholz relay and conservator for transformers 10 MVA and above. 6.6 Filter and Drain Valves Sampling Devices and Air Release Plugs 6.6.1 Each transformer shall be fitted with the following: (a) The filter and drain valves as specified. (b) A drain valve as specified below shall be fitted to each conservator. For diameter up to 650 mm: Size of the valve 15 mm. For diameter above 650 mm: Size of the valve 25 mm. (c) A robust oil sampling device shall be provided at the top and bottom of the main tank. The sampling device shall not be fitted on the filler valves specified under (a) above. (d) One 15 mm air release plug. (e) For transformers above 100 MVA rating, one 100 mm bore valve shall be provided for attaching vacuum connection and with provisions for attaching a vacuum gauge, a pressure gauge or an oil level indicator. 6.6.2 All other valves opening to atmosphere shall be fitted with blank flanges. 6.7 COOLER AND RADIATOR CONNECTIONS Valves and valve mountings shall be provided as specified under “Cooling Plant” Clause 7. 6.7.1 All valves up to and including 50 mm shall be of gunmetal or of cast steel. Larger valves may be of gunmetal or may have cast iron bodies with gunmetal fittings. They shall be of full General 19 way type with internal screw and shall be opened by turning counter clock-wise when facing the handwheel. 6.7.2 Means shall be provided for padlocking the bottom valves in the open and closed positions. This is required for the valves where opening device like hand-wheel, keys, etc., are the integral part. 6.7.3 Every valve shall be provided with an indicator to show clearly the position of the valve. 6.7.4 All valves shall be provided with flanges having machined faces. 6.7.5 The drilling of valve flanges shall comply with the requirements of IS: 3639. 6.8 PRESSURE RELIEF DEVICE 6.8.1 The pressure relief device shall be provided for 16 MVA & above rating transformer. PRD shall be of sufficient sizes for rapid release of any pressure that may be generated within the tank, and which might result in damage to the equipment. The device shall operate at a static pressure of less than the hydraulic test pressure for transformer tank. Means shall be provided to prevent the ingress of rain water. 6.8.2 Unless otherwise approved the relief device shall be mounted on the main tank, and, if on the cover, shall be fitted with skirt projecting 25 mm inside the tank and of such a design to prevent gas accumulation. 6.8.3 If a diaphragm is used it shall be of suitable design and material and situated above maximum oil level. 6.8.4 If a diaphragm is put at the base of pipe, an oil gauge is required on the stand pipe for indicating fracture of diaphragm. 6.8.5 One of the following methods shall be used for relieving or equalising the pressure in the pressure relief device: (a) An equaliser pipe connecting the pressure relief device to the conservator, or (b) The fitting of a silica gel breather to the pressure relief device. The breather being mounted in a suitable position for access at ground level. 6.8.6 If specified, the pressure relief valve (spring operated type) capable of releasing the pressure in the lank when it rises above a predetermined safe limit, shall be provided. It shall be provided with a micro switch for actuating trip contact when it operates. It shall also give a visual indication of valve operation by raising a flag. The flag and the switch shall remain operated until they are reset manually. The operating pressure of the pressure relief valve shall always be less than the tank test pressure. The micro switch shall have IP 55 protection and the fasteners shall be of rust proof material. 20 Manual on Transformers 6.8.7 PRD shall be provided with an outlet pipe which shall be taken right up to the bottom of the transformer up to the oil catchment pit. This is to avoid injury to personnel in event of PRD operation and subsequent splashing of oil. 6.9 Accommodation for Auxiliary Apparatus 6.9.1 If specified, facilities shall be provided for the mounting of internal/external neutral current transformer(s) adjacent to the neutral terminal(s) and tank. 6.10 EARTHING TERMINAL 6.10.1 Two earthing terminals capable of carrying for 4 seconds the full lower voltage. Short circuit current of the transformer. Provision shall be made at positions close to each of the bottom two corners of the tank for bolting the earthing terminals to the tank structure to suit local conditions. The design of earthing terminals shall be as per IS 3639 - Part 3 (Fittings and accessories for Power Transformers Part 3: Earth Terminals. 6.11 RATING, DIAGRAM AND PROPERTY PLATES 6.11.1 The following plates shall be fixed to the transformer tank at an average height of about 1750 mm above ground level as shown in Fig. 1. (a) A rating plate bearing the data specified in the appropriate clauses of IS: 2026 (b) A diagram plate showing the internal connections and also the voltage vector relationship of the several windings in accordance with IS: 2026 and in addition a plan view of the transformer, giving the correct physical relationship of the terminals. When links are provided in accordance with clause 2.3 for changing the transformer ratio, then approved means shall be provided for clearly indicating ratio for which the transformer is connected. No load voltage shall be indicated for each tap. R&D plate shall also consist of copper weight, core weight, commissioning date & factory tested capacitance value. A warning plate shall also be provided for OCTC operation during de-energized condition only. (c) Where specified a plate showing the location and function of all valves and air release cocks or plugs is to be provided. This plate shall also warn operators to refer to the maintenance instructions before applying the vacuum treatment for drying (Fig. 2). 6.11.2 The above plates shall be of material capable to withstanding continuous outdoor service. 6.12 Joints and Gaskets 6.12.1 All gaskets used for making oil tight joints shall be of proven material such as granulated cork bonded with synthetic rubber or synthetic rubber gaskets conforming to IS: 4253, unless otherwise specified. General 21 22 Manual on Transformers General 23 Fig. 3 Typical value schedule for power transformer 7.0 COOLING PLANT 7.1 General 7.1.1 Radiators and coolers shall be so designed as to avoid pockets in which moisture may collect and shall withstand the pressure tests. 7.1.2 Unless the pipe work is shielded by adequate earthed metal the clearance between all pipe work and live parts shall be more than the clearance for live parts to earth. 24 Manual on Transformers 7.2 Radiators Mounted Directly to the Tank/Banked 7.2.1 Detachable radiators as per Section JJ of this manual. 7.2.2 Valves shall be provided on the tank at each point of connection to the tank. 7.2.3 Where separate radiator banks are provided, the conservator vessels specified in clause 6.5 can be mounted thereon. 7.2.4 All coolers shall be suitable for mounting on a flat concrete base. 7.2.5 The oil circuit of all coolers shall be provided with the following: (a) A valve at each point of connection to the transformer tank (b) Removable blanking plates to permit the blanking off the main oil connection of each cooler. (c) A drain valve of 25 mm at the lowest point of each bank of cooler (d) A thermometer pocket fitted with a captive screwed cap on the inlet and outlet oil branches of each separately mounted cooler bank. (e) A filter valve as specified in clause 6.6 at the top and bottom of each cooler bank of cooler. (f) Air release plugs of 15 mm. 7.2.6 In addition the following are to be provided only with water cooled oil coolers which shall be as per IS: 6088. (a) A suitable differential pressure gauge or equivalent suitable device fitted with electrical contacts to give an alarm when differential pressure between cooler oil outlet and water inlet pressure drops below a preset value. (b) Oil and water flow switches, fitted with electrical contacts, in the pipe work adjacent to the coolers. 7.2.7 The disposition of flow indicators is to be as shown in Fig. 3. 7.2.8 Water cooled oil coolers shall be double tube type in which water shall circulate through the inner tube and oil in between the outer tube and shell. The design of shell and tube assembly shall be such as to facilitate cleaning without any risk of water mixing with the oil. The material of the tube plates and tube shall be such that corrosion shall not take place due to galvanic action. A water analysis report shall be furnished, in time, to enable supplier to ensure a suitable material for tube and tube plates. 7.2.9 Any leakage which may take place in the oil cooler shall be of the oil into the water and not the reverse, and means shall be provided to ensure that the pressure of the oil in the cooler General 25 is always greater than the pressure of the water. The water pressure in the cooler will be kept as low as possible. Further, the cooling water discharge should be free to the atmosphere to reduce the pressure in the cooler. Provision for leakage detector system shall be provided along with alarm contacts for water coolers. 7.3 Oil Piping and Flanges 7.3.1 The necessary oil piping shall be provided for connecting each transformer to the coolers and oil pumps. The oil piping shall be with flanged gasket joints. Cast iron shall not be used. 7.3.2 The drilling of all water and oil pipe flanges shall comply with IS: 3639 and IS: 1536 (Section JJ -specification for valves for transformers.) Fig. 4 Flow indicators and alarms 26 Manual on Transformers 7.3.3 A suitable expansion piece shall be provided in each oil pipe connection between the transformer and the separately mounted oil coolers. 7.3.4 Drain valves/plugs shall be provided in order that each section of pipework can be drained independently. 7.4 Oil Pumps 7.4.1 Each forced oil cooler shall be provided with a motor driven oil pump of the submerged motor type and of adequate capacity. It shall be possible to remove the pump and motor from the oil circuit without having to lower the level of oil in the transformer or coolers and without having to disturb the pump foundation fixing. Oil pump shall be capable of dealing with the maximum output of transformer and total head which may occur in service and with the varying head due to changes in the viscosity of the oil. 7.4.2 Each pump assembly shall be furnished with oil flow indicator with alarm contacts to indicate normal pump operation and oil flow. 7.4.3 For mixed type cooling, the pump should be of axial flow type to permit oil circulation when pump is idle. 7.4.4 For forced oil cooling, the coolers shall be of 2 X 50% and oil pumps 2 X 100%, one pump running and one standby in each group. 7.4.5 Under no circumstances, the degree of forced circulation creates a static electrification hazard in any part of a transformer under any operating condition. 7.5 Air Blowers and Ducts 7.5.1 Air blowers for use with oil coolers or for air blast cooling shall be motor driven. They shall be suitable for continuous operation outdoors and capable of dealing with the maximum output and total head required in service. The bearings shall be of sealed type, which does not require frequent lubrication. 7.5.2 Air blowers shall be capable of withstanding the stresses imposed when brought up to full speed by the direct application of full line voltage to the motor. 7.5.3 Air blowers shall be complete with all necessary air ducting and coolers shall be designed so that they operate with a minimum of noise or drumming. In order to reduce the transmission of noise and vibration the blowers shall be either mounted independently from the coolers or, alternatively, an approved form of antivibration mounting shall be adopted. It shall be possible to remove the blower complete with motor without disturbing or dismantling the cooler structure framework. 7.5.4 Blades or runners fabricated to form hollow sections shall not be used. 7.5.5 Blades shall be suitably painted for outdoor use. 7.5.6 If fans are mounted at a height less than 2.5 m suitably painted wire-mesh guards with a mesh not greater than 25 mm shall be provided to prevent accidental contact with the blades. Fans mounted at more than 2.5 m height shall be provided with outside guards against birdage. Guards shall be provided over all moving shaft and couplings. General 7.6 27 Motors 7.6.1 Motors shall be of the squirrel cage totally enclosed weather-proof type and shall comply with Indian Standards as applicable for continuously rated machine. The motors shall be capable of operating at all loads without undue vibration and with a minimum of noise. They shall be suitable for direct starting and for continuous running from 415-240 volts three-phase, 4 wire 50 Hz supply. 7.6.2 All motors shall be capable of continuous operation at any frequency between 48 and 51 Hz, together with any voltage within 5 percent of the nominal value. Motors upon which the primary equipment depends for its continued operation at full load shall also be capable of continuous operation at 85 percent of the nominal voltage at normal frequency without injurious over-heating. 7.6.3 All motors shall have ball or roller bearings and grease lubricators shall be fitted with hexagon nipples to relevant Indian Standard. 7.6.4 Vertical spindle motors shall have bearings capable of withstanding the thrust due to the weight of the moving parts and the action of impeller. 7.6.5 The stator windings shall be adequately braced and suitably impregnated to render them non-hygroscopic and oil resistant. Weather-proof motors shall be provided with suitable means of breathing and drainage to prevent accumulation of water. 7.6.6 Each terminal box shall be fitted with means of terminating the external wiring for outdoor use. 7.6.7 Varnished cambric or glass insulation shall be used for connections from the winding to the terminals. All motor terminals shall be of the stud type and totally enclosed. 7.6.8 Each pump, or blower and its motor shall be mounted on a common base plate and the drive shall be direct. 7.7 Cooler Control 7.7.1 Each motor or group of motors shall be provided with a three pole electrically operated contactor and with control gear of suitable design both for starting and stopping the motor manually and also automatically from the contacts on the winding temperature indicating device specified in clause 13. Additional terminals for remote manual electrical control of motors shall be provided. Overload and single phasing protection shall be provided but no-volt release shall not be fitted. MCB/MCCB shall be provided for the main supply. This equipment shall be accommodated in the marshalling box specified in clause 15. 7.7.2 Where small motors are connected in groups, the group protection shall be arranged so that it operates satisfactorily in the event of a fault occurring on a single motor. 28 Manual on Transformers 7.7.3 Where blowers and oil pumps are provided, the connections shall be arranged as to allow the motors or groups of motors to be started up and shutdown either collectively or individually. 7.7.4 All motor contactors and their associated apparatus shall be capable of holding in and operating satisfactorily and without over heating for a period of ten minutes if the supply voltage falls for the period, to 75 per cent of normal at normal frequency. The motor contactors and associated apparatus shall be capable of normal operation with a supply voltage of 85 per cent of the normal value and at normal frequency. 7.7.5 All contacts and other parts which may require renewal, adjustment or inspection shall be readily accessible. 7.7.6 The control arrangements are to be so designed as to prevent the simultaneous starting of motors of a total rating of more than 20 HP. 7.7.7 Alarm indication for failure of group of fans and oil pump shall be provided. Also for forced oil cooled transformers, alarm indication for “low oil flow” shall be provided. 7.7.8 Alarm indication shall be provided to indicate failure of power supply. 7.7.9 The start up or shut down of any pump or combination of pumps must not cause mal-operation of any gas and oil actuated relay. 7.7.10 For transformers with OFWF cooling required to meet peak load requirements and are thus switched on or off during the day, the oil pump shall be kept running when the transformer is off for a short period but water circuit is switched off. In case the transformer is switched off for a longer time, the oil pump can also be switched off but it shall be ran at least one hour earlier before the transformer is energised again. 7.7.11 Transformers with only OFAF cooling with unit coolers shall have provision of alarm for “more than one cooler fail”. Cooler failure alarm shall be interfaced with DCS/ECMS/SCADA system as applicable. 7.7.12 An alternate to the conventional cooler control “Intelligent Cooler Control & Monitoring System” which is a PLC based system can be recommended for critical power transformers, where cooler control and monitoring are interfaced with DCS/ECMS/SCADA system as applicable. 8.0 VOLTAGE CONTROL (OFF-CIRCUIT TYPE) Voltage Control (off-circuit type) should conform to Section GG of the specification. 9.0 VOLTAGE CONTROL (ONLOAD TYPE) Voltage control (on-load type) should conform to Section GG of the specification. 29 General 10.0 PARALLEL OPERATION TAPCHANGER OF TRANSFORMERS WITH ONLOAD 10.1 Besides the local and remote electrical control specified in clause 9, on-load tap changers, when specified, should be suitable for remote electrical parallel control as in clause 10.2. 10.2 Remote Electrical Parallel Control 10.2.1 In addition to the methods of control as in clause 9, the following additional provision shall be made. 10.2.2 Suitable selector switch be provided, so that any one transformer of the group can at a time be selected as “Master”, “Follower” or “Independent”. 10.2.3 Necessary interlock blocking independent control when the units are in parallel, shall be provided. 10.2.4 The scheme will be such that only one transformer of a group can be selected as “Master”. 10.2.5 An out-of-step device shall be provided for each transformer which shall be arranged to prevent further tap changing when transformers in a group operating in “Parallel control” are one tap out-of-step. 11.0 BUSHING INSULATORS AND TERMINALS The bushing should comply with IS 2099, IS 12676 and Section II of this specification. The over voltage power frequency test level or the BIL of bushings should be one step higher than that of the windings. 11.1 Transformers shall be fitted either with bushing insulators or with cable boxes, as stated in order. Where accommodation for current transformers is required on 72.5 kV bushings and above, the requisite details will be notified to the supplier at the time of tendering. 11.2 Special precautions shall be taken to avoid ingress of moisture into paper insulation during manufacture, assembly, transport and erection. 11.3 Each porcelain bushing or insulator, and paper bushing shall have marked upon it the manufacturer’s identification mark, and such other mark as .may be required to assist in the representative selection of batches for the purposes of the sample tests. Clamps and fittings made of steel or malleable iron shall be hot dip galvanised. All fasteners of size 12 mm and above shall be hot dip galvanised and fasteners of size less than 12 mm shall be of stainless steel. LV Bushing (rated current≥ 1000 Amps) palm shall be silver/tin plated. 11.4 The bushing flanges shall not be of re-entrant shape which may trap air. 30 Manual on Transformers 11.5 Bushing turrets shall be provided with vent pipes which shall be connected to route any gas collection through the Buchholz relay. The take off point of the vent pipes shall be the top most point on the bushing turret so that there will not be any air trapped in the -bushing turret. 11.6 The minimum clearances in air between live conductive parts and conductive parts to earthed structure shall be as follows: Highest Voltage for Equipment (kV) Basic Insulation level kV peak 12 24 36 52 72.5 145 145 245 245 420 800 75 125 170 250 325 550 650 950 1050 1425 1950 Minimum clearances Phase to phase (mm) 280 330 350 530 700 1220 1430 2000 2350 4000 6700*/5800* Phase to earth (mm) 140 230 320 480 660 1050 1270 1800 2150 3500 5800*/5000* * depending upon lightening & switching impulse level. Note: (1) These clearances are applicable for transformers to be installed up to an altitude of 1000 m above mean sea level. (2) For altitude exceeding 1000 in the clearance should be increased by 3 percent for every additional 300 m. (3) Air clearance of 3500 mm between phase to earth for 420 kV system can be relaxed by maximum 200 mm as far as air release pipe emanating from bushing turret is concerned. 11.7 Vertical Bushings for 52 kV & above shall be of the oil filled condenser type (OIP) & shall be of draw lead/ rod type to facilitate removal. Bushings of rating below 52 kV may be solid porcelain or oil communicating type. Condenser type bushings shall be provided with : (1) (2) Oil level gauge Oil filling plug & drain valve (if not hermetically sealed) (3) Tap for capacitance & tan delta test. An alternate to OIP Bushing is Resin Impregnated Paper (RIP) Bushing. However it has to be specified by the user. 12.0 CABLE BOXES AND DISCONNECTING CHAMBERS 12.1 Cable boxes shall be suitable for terminating the cables directly or alternatively shall be in the form of sealing end-chambers for accommodation sealing ends into which the cable will be terminated, as specified in the order. 12.2 Cable boxes shall be designed to accommodate all the cable joint fittings or sealing ends General 31 required by the manufacturers of the cables, including stress/cones or other approved means for grading the voltage stress on the terminal insulation of cables operating at voltages of 22 kV and above, between phases. They shall also be provided with expansion chambers for the filling medium and means of preventing the formation of air spaces when filling. Drain plugs of ample size shall be provided to enable the filling medium to be removed quickly. 12.3 The cable boxes shall be fitted with suitable non-ferrous wiping glands with combined armour and earthing clamps. The ends of all wiping glands shall be tinned before dispatch to site. Wiping glands for single core cables shall be insulated from the box. Wiping glands insulation cables shall be capable of withstanding a dry high voltage test of 2,000 volts AC for one minute. Air insulated cable boxes for PVC cables may be provided with compression glands. Sufficient wiping glands shall be provided for the termination of required number of cables. 12.4 Where cable boxes are provided for three core cables, the seating sockets on the two outer phases shall preferably be inclined towards the centre to minimize bending of the cable cores. Where there is more than one core per phase, the socket block shall be so designed as to minimize bending of the cable cores. 12.5 Where cables for 1 kV and above are terminated in the cable box, oil filled disconnecting chamber with removable links shall be provided for testing purposes. A barrier shall be provided on both sides of the disconnecting chamber to prevent ingress of the oil used for filling the chamber into the cable box or the transformer. It shall only be necessary to remove part of the oil in the chamber itself when making the necessary testing connections. 12.6 Where sealing end chambers are provided, the disconnecting chamber may be omitted and the facilities for testing shall be provided in the sealing end chamber itself. A barrier shall then be provided between the sealing end chamber and the main tank subject to the provision of the next paragraph. 12.7 The barrier between the main tank and the disconnecting or cable sealing end chamber may be omitted, where the design is such that the cover of the disconnecting or cable sealing end chamber can be removed without lowering any oil level other than in the chamber itself, in order to make the necessary testing connections. 12.8 The disconnecting or sealing end chamber shall have a removable cover and the design of the chamber shall be such that ample clearances are provided to enable either the transformer or each cable to be subjected separately to high voltage tests when filled with transformer oil. Insulating Fire resistant barriers should be provided between each phase of HV cable box. 12.9 An earthing terminal shall be provided in each disconnecting or sealing end chamber to which the connections from the transformer winding can be earthed during cable testing. 12.10 The cable boxes and disconnecting or sealing end chambers shall also be capable of withstanding for 15 minutes, both at the time of the first tests on the cables and at any subsequent time as may be required, between phases and to earth a DC test equal to 2E kV or an AC test equal to 4E/3 kV. 32 Manual on Transformers 12.11 During these tests the links in the disconnecting or sealing end chamber or cable box will be withdrawn and the transformer winding with connections thereto will be earthed. 12.12 Unless otherwise approved the creepage distances and clearance to earth and between phases shall not be less than those specified in Table 1. In case of compound filled cable box with shrinkable tape, the allowable minimum clearances shall be subject to the agreement between manufacturer and the user. 12.13 Cable boxes suitable for semi-fluid compound filling shall be tested with transformer oil at room temperature and at a pressure of 0.7 kg/cm for 12 hours during which no leakage shall occur. 12.14 Terminals shall be marked in a clear and permanent manner. 12.15 Unless otherwise specified main cabling jointing and filling of cable boxes will be carried out by the customer. However, filling medium will be supplied as a part of the cable box by the manufacturer. Table 1 Highest System Voltage kV Insulating medium Clearance between phases (mm) Clearance to earth direct (mm) 1.1 3.6 7.2 12 AilAir AilAir Compound Air Semi-fluid compound or oil Air Semi-fluid compound or oil 25 50 90 130 50 241 100 351 125 24 36 13.0 Creepage over cable surface (mm) 20 50 70 80 50 140 75 Creepage over porcelain to similar material (mm) 25 90 192 75 384 125 222 100 576 150 576 250 192 125 384 190 TEMPERATURE INDICATING DEVICES AND ALARM 13.1 Oil temperature indicator shall be provided as required in detail specification, i.e., Section E to K. 13.2 All transformers above 10 MVA shall be provided with a device for indicating winding temperature. The device shall have a dial type indicator and in addition a pointer to register the highest temperature reached. The number of contacts as specified will be provided. 13.3 Except where outdoor types of indicators are supplied, the temperature indicators shall be housed in the marshalling box. If specified, for transformers above 10 MVA a remote repeater indicator electrically operated from winding temperature indicator is to be provided for mounting General 33 on the control panels. Unless otherwise specified the remote repeater indicator shall be of flush mounting type. 13.4 The tripping contacts of winding temperature indicators shall be adjustable to close between 60°C and 120°C and alarm contacts to close between 50°C and 100°C and both shall re-open when the temperature has fallen by about 10°C. 13.5 The contacts used to control the cooling plant motors on the above devices shall be adjustable to close between 50°C and 100°C and re-open when the temperature has fallen by any desired amount between 15°C and 30°C. 13.6 All contacts shall be adjustable on a scale and shall be accessible on removal of the cover. Micro switches shall be preferred to mercury switches. 13.7 The temperature indicators shall be so designed that it shall be possible to check the operation of the contacts and associated equipment. 13.8 box. Connections shall be brought from the device to terminals placed inside the marshalling 13.9 Cooler failure or oil and water flow alarm shall be provided as specified in clause 7.2.6. 13.10 Accuracy class of temperature indicators shall be ± 1.5% or better. 13.11 An alternate to conventional dial type WTI/OTI is Digital WTI/OTI. 14.0 GAS AND OIL ACTUATED RELAYS 14.1 Each transformer shall be fitted with gas and oil actuated relay equipment to IS; 3637 having contacts which close following oil surge or low oil level conditions. Micro switches shall be preferred to mercury switches. 14.2 Each gas and oil actuated relay shall be provided with a test cock to take a flexible pipe connection for checking the operation of the relay. 14.3 Where specified to allow gas to be collected at ground level, a pipe approximately 5 mm inside diameter shall be connected to the gas release cock of the gas and oil-actuated relay and brought down to a point approximately 1.25 m above ground level, where it shall be terminated by a cock. 14.4 A machined surface shall be provided on the top of each relay to facilitate the setting of the relays and to check the mounting angle in the pipe and the cross level of the relay. 14.5 The design of the relay mounting arrangements, the associated pipework and the cooling plant shall be such that mal-operation of the relays shall not take place under normal service conditions. 34 Manual on Transformers 14.6 The pipework shall be so arranged that all gas arising from the transformer shall pass into the gas and oil-actuated relay. The oil circuit through the relay shall not form a delivery path in parallel with any circulating oil pipe, nor shall it be tied into or connected through the pressure relief vent. Sharp bends in the pipework shall be avoided. 14.7 When a transformer is provided with two conservators, the gas and oil actuated relays shall be arranged as follows: • If the two conservators are connected to the transformer by a common oil pipe, one relay shall be installed in the common pipe. • If the two conservators are piped separately to the transformer, two relays shall be installed, one in each pipe connection. • Adequate clearance between oil pipework and live metal shall be provided. 15.0 MARSHALLING BOX 15.1 A steel weather and vermin proof enclosure having degree of protection IP 55 shall be provided for the transformer ancillary apparatus. The box shall have domed or sloping roofs and the interior and exterior painting shall be in accordance with clause 1.6. 15.2 The marshalling box, wherever provided shall accommodate the following equipments, alternatively weather proof instruments can be mounted outdoor. (a) Temperature indicators (b) Control and protection equipment for the local electrical control of tap changer, if the same cannot be accommodated in the motor driving gear housing. (c) Control and protection equipment for the cooling plant; and (d) Terminal boards and gland plates for incoming and outgoing cables (e) Capillary entrance should preferably be from bottom of the box. 15.3 All the above equipments except (d) shall be mounted on panels and back of panel wiring shall be used for interconnection. 15.4 The temperature indicators shall be so mounted that the dials are not more than 1600 mm from ground level and the door(s) are of adequate size. 15.5 To prevent internal condensation, an approved type of metal clad space heater shall be provided, controlled by a suitable thermostat. For illumination, a suitable lamp or CFL shall be provided whose switching shall be controlled by the door switch. General 35 15.6 All incoming cables shall enter the kiosks from the bottom and the gland plate shall be not less than 450 mm from the base of box. The gland plate and associated compartment shall be sealed in suitable manner to prevent the ingress of moisture. 15.7 Undrilled gland plate shall be provided for accommodating glands for incoming and outgoing cables. 15.8 Separate kiosk may be provided for control & power circuit of OFAF cooled Power Transformers. 16.0 CONTROL CONNECTIONS AND INSTRUMENT WIRING, TERMINAL BOARD AND FUSES 16.1 All wiring connections, terminal boards, fuses and links shall be suitable for tropical atmosphere. Any wiring liable to be in contact with oil shall have oil resisting insulation and the bared ends of stranded wire shall be sweated together to prevent creepage of oil along with wire. 16.2 There shall be no possibility of oil entering connection boxes used for cables or wiring. 16.3 Panel connections shall be neatly and squarely fixed to the panel. All instruments and panel wiring shall be run in PVC or non-rusting metal cleats of the limited compression type. All wiring to a panel shall be taken from suitable terminal boards. 16.4 Where conduits are used, the runs shall be laid with suitable falls, and the lowest parts on the-run shall be external to the boxes. All conduit runs shall be adequately drained and ventilated. Conduits shall not be run at or below ground level. 16.5 When 415 volt connections are taken through junctions boxes or marshalling boxes they shall be adequately screened and 415 “VOLTS DANGER” notices must be affixed to the outside of the junction boxes or marshalling boxes. 16.6 All box wiring shall be in accordance with relevant IS. All wiring shall be of stranded copper of 660 V grade and size not less than 4.00 sq mm for CT leads and not less than 2.5 sq mm for other connections. 16.7 All wires on panels and all multicore cables shall have ferrules which bear the same number at both ends. 16.8 At those points of interconnections between the wiring carried out by separate contractors, where a change of number cannot be avoided, double-ferrules shall be provided on each wire. The change of numbering shall be shown on the appropriate diagram of the equipment. 16.9 The same ferrule number shall not be used on wires in different circuits on the same panels. 36 Manual on Transformers 16.10 Ferrules shall be provided with glossy finish to prevent the adhesion of dirt. They shall be clearly and durably marked and shall not be affected by damp or oil. 16.11 Stranded wires shall be terminated with tinned Ross Courtney terminals, claw washers or crimped tubular lugs. Separate washers shall be used for each wire. The size of the washer shall be suited to the size of the wire terminated. Wiring shall in general be accommodated on the sides of the box and the wires for each circuit shall be separately grouped. Back of panel wiring shall be arranged so that access to the connecting stems of relays and other apparatus is not impeded. 16.12 Wires shall not be jointed or tied between terminal points. 16.13 Wherever practicable, all circuits in which the voltage exceeds 125 volts, shall be kept physically separated from the remaining wiring. The function of each circuit shall be marked on the associated terminals boards. 16.14 Where apparatus is mounted on panels, all metal cases shall be separately earthed by means of copper wire or strip having a cross-section of not less than 2 sq mm where strip is used, the joints shall be sweated. 16.15 All wiring diagram for control and relay panel shall preferably be drawn as viewed from the back and shall show the terminal boards arranged as in service. All diagrams shall show which view is employed. 16.16 Multicore cable tails shall be so bound that each wire may be traced without difficulty to its cable. 16.17 The screens or screen pairs of multicore cables shall be earthed at one end of the cable only. The position of the earthing connections shall be shown clearly on the diagrams. 16.18 All terminal boards shall be mounted obliquely towards the rear doors to give easy access to terminations and to enable ferrule numbers to be read without difficulty. 16.19 Terminal board rows should be spaced adequately not less than 100 mm apart to permit convenient access to wires and terminations. 16.20 Terminal boards shall be so placed with respect to the cable gland (at a minimum distance of 200 mm) as to permit satisfactory arrangement of multicore cable tails. Terminal boards shall have pairs of terminals for incoming and outgoing wires. Insulating barriers shall be provided between adjacent connections. The height of the barriers and the spacing between terminals shall be such as to give adequate protection while allowing easy access to terminals. The terminals shall be adequately protected with insulating dust-proof covers. One dummy terminal block in between each trip wire terminal shall be provided. No Loop in Loop out shall be adopted for power circuit. 16.21 No live metal shall be exposed at the back of the terminal boards. 16.22 All fuses shall be of the cartridge type. 16.23 Fuses and links shall be labeled. 16.24 The terminal blocks preferably shall be fully enclosed with removable covers & made of molded, non-inflammable plastic material with blocks & barriers molded integrally. The terminal blocks shall be 650V grade & have 10 A continuous rating. At least 20 % spare terminals shall be 37 General provided on each panel. Terminal blocks for CT/PT secondary leads shall be provided with test links & isolating facilities. Also current transformer secondary leads shall be provided with short circuiting & earthing facilities. All CT/PT terminals shall be provided as fixed type terminals in M. Box to avoid any hazard due to loose connection leading to CT opening or any other loose connection. In no circumstances Plug In type connectors shall be used for CT/PT connections in M. Box. Terminal block shall preferably be stud type suitable for ring (O) type lugs. 17.0 TESTS 17.1 Tests shall be carried out to evaluate the performance of the material and appliance generally as per the provision of IS: 2026 and as detailed out in Section BB of this specification. A test method is referred in Appendix-VIII for Bushing CT Characteristic testing without passing current to primary winding. 17.1.1 Where customers’ inspection is specified, not less than 15 days notice shall be given to the customer in order that he may be represented. Four copies of test certificates will be supplied. 17.2 Tests are not required to be performed on bought out equipments like oil coolers, oil actuated relays, etc., at the works of the transformer manufacturer. Furnishing test certificates from the original equipment manufacturer works shall be deemed to be satisfactory evidence. Inspection of tests at the sub-contractors works will be arranged by the supplier wherever required. 17.3 Tanks 17.3.1 Routine Tests (a) Fabrication stage: (al) The tank shall be tested for leakage by being completely filled with air at a pressure corresponding to twice the normal head of oil or to normal pressure plus 35 kN/m2 whichever is lower. The pressure shall be maintained for a period of minimum one hour during which time no leakage shall occur. The equivalent air pressure corresponding to oil pressure calculated at the base of the tank to be considered for air pressure test. Permanent deflection of flat plates shall be measured on one tank of each design, if specified by customer, after the excess pressure has been released and shall not exceed the figures specified below: Horizontal length of flat plate (total length of tank wall) in mm Up to and including 750 751 to 1250 1251 to 1750 1751 to 2000 2001 to 2250 2251 to 2500 2501 to 3000 Above 3000 (a2) Permanent deflection (in mm) 5 6.5 8 9 11 12.5 16 19 The conservator shall be tested for leakage by being completely filled with air at 35 kN/m2. The pressure shall be maintained for a period of one hour during which time no leakage shall occur. 38 Manual on Transformers (a3) The radiators shall be tested for leakage by placing them horizontally in a tank filled with clean water and applying air pressure 2 kg/cm2 for atleast 15 minutes during which time no leakage shall occur. (a4) The pipes shall be tested for leakage by applying air pressure of 4 kg/cm2 for 15 minutes during which time no leakage shall occur. (b) Transformer assembly stage Oil pressure test to be conducted on tank with turret and all other accessories as assembled for routine test by filling completely with oil at a pressure corresponding to twice the normal head of oil or to normal pressure plus 35 kN/m2 whichever is lower. The pressure to be maintained for eight hours during which time no leakage shall occur. 17.3.2 Type Tests (a) Vacuum test (at fabrication stage) When required by customer, one transformer tank of each design shall be subjected to the specified vacuum as in clause 6.1.3. The tanks designed for full vacuum (760 mm of mercury at sea level or the barometric reading at the location of test) shall be tested at a maximum internal pressure of 3.33 kN/m2 (25 mm of Hg) for one hour i.e., 760-25=735 mm of Hg at sea level and (Barometric reading -25) mm Hg at other location. The permanent deflection of flat plates after vacuum has been released shall not exceed the values specified in clause 17.3.1 (al) without affecting the performance of the transformer. (b) Pressure test When specified, one transformer tank of each design with its active part as assembled for type test (i.e., including pipe work and cooling equipment and excluding PRV and conservator when air cell is provided) shall be subjected to a pressure corresponding to twice the normal head of oil or to normal pressure plus 35 kN/m2 whichever is lower measured at the base of the tank and will be maintained for 8 hours during which time no leakage shall occur. Before conducting the pressure test, the following are to be taken care of: (i) Pressure relief valve/relief vent are to be removed and the opening blanked. (ii) Transformer and tap changer conservators are to be disconnected. (iii) Diverter switch compartment of tap changer to be connected with transformer tank for equalizing the pressure on both sides. (iv) Oil should be completely filled and all trapped air released. 17.3.3 Note: User may also specify Tank Routine/Type Tests in line with latest revision of IEC 60076-1 When IS eventually gets revised in line with latest revision of IEC, CBIP Manual shall be amended to fall in line with IS. General 18.0 39 QUALITY ASSURANCE The supplier should include a quality assurance programme (QAP) that will be used to ensure that the transformer design, materials, workmanship, tests, service capability, maintenance and documentation, will fulfill the requirements stated in the contract documents, standards, specifications and regulations. The QAP should be based on and include relevant parts to fulfill the requirements of ISO-9001. A quality plan describes: • Lists of activities involved in design, procurement of raw materials and components, manufacture, stage inspection and final testing, preparation for dispatch, delivery, installation and commissioning. • The identification reference of all documentation, standards, procedures, works instructions, drawings, test methods, acceptance criteria etc. Typical QAP format is provided for illustration as Appendix III. 19.0 A list of guaranteed technical particulars and additional technical particulars are given in Appendix IV. A list of standards for transformers is given in Appendix II. A list of transformeraccessories and routine test certificate required for them is given .at Appendix V. 20.0 NEUTRAL EARTHING ARRANGEMENT /FORMATION Neutral terminals of winding of three single phase Transformer shall be formed by connection to an overhead common copper grounding bars, supported from tank and firewalls by using insulators. Puncturing/support from firewall may not be permitted. 21.0 DESIGN REVIEW The design of critical Power Transformer/ Reactor may be reviewed by user/their consultant. The design review shall be finalized before commencement of manufacturing activity. This design review may be carried out inline with CIGRE “Guideline for Conducting Design Review”, 204, SC WG12.22. A format for required Design Review parameters is given in Appendix-VI for reference. 22.0 REFERENCE TECHNICAL PARTICULARS OF POWER TRANSFORMERS & SITE PHOTOGRAPHS Salient Technical Particulars of Power Transformers for Coal Fired or Gas Based Thermal Power Plants are given in Appendix-VII for reference. Photographs of transformers & accessories already installed in various power projects are shown in Appendix-IX. SECTION B Specifications for Outdoor type, Completely Self Protected, 3 Phase Distribution Transformers (upto and including 100 kVA) SECTION B Specifications for Outdoor type, Completely Self Protected, 3 Phase Distribution Transformers (upto and including 100 kVA) 1.0 SCOPE 1.1 This section covers, technical requirements/parameters of Completely Self Protected distribution transformers of rating upto and including 100 kVA, 11 kV 3 phase and does not purport to include all the necessary provisions of a contract. 1.2 Standard Ratings The standard ratings shall be 16, 25, 63 and 100 kVA. For general requirement reference shall be made to Sections A & BB of this manual. 2.0 STANDARDS 2.1 The materials shall conform in all respects to the relevant Indian / International Standard Specification, latest, amendments thereof, some of them are listed below: Indian Standard Title International & Internationally recognisedstandard IS-2026 Specification for Power Transformer IEC 60076 IS-1180 Outdoor Distribution Transformer upto and including 100 kVA IS 12444 Specification for Copper Wire Rod ASTM B-49 IS-3347 Specification for Porcelain Transformer Bushing DIN 42531,23,3 IS-335 Specification for Transformer Oil BS 148, D-1473, D-15331934 IEC Pub 296 IS 5 Specification for Colours for Ready Mixed Paints IS-2099 Specification for High Voltage Porcelain Bushings IS-7421 Specification for Low Voltage Bushings IS-3347 Specification for Outdoor Bushings DIN 42531 to 33 IS-5484 Specification for Al Wire Rods ASTM B-233 IS - 9335 Specification for Insulating Kraft Paper IEC 60554 IS- 1576 Specification for Insulating Press Board IEC 60641 IS/6600 2026-7 Guide for Loading of Oil Immersed Transformers IEC 60076 Refer Appendix II of the Manual for List of Standards. 3.0 SERVICE CONDITIONS The Distribution Transformers to be supplied against this Specification shall be suitable for satisfactory continuous operation under the climatic conditions prevailing at site and to be specified by the purchaser as follows, as per IS 2026 (Part -1) Latest Revision/International 43 44 Manual on Transformers Standards tabulated above : (i) Location (ii) Max ambient air temperature (deg.C) (iii) Min. ambient air temperature (deg.C) (iv) Max average daily ambient air temperature (deg.C) (v) Max. yearly weighted average ambient temperature(deg.C) (vi) Max. altitude above mean sea level (meters) 4.0 : : : : : STANDARD RATINGS Transformers shall be suitable for outdoor installation with three phase, 50 Hz, 11 kV system in which the neutral is effectively earthed and these should be suitable for service under fluctuations in supply voltage upto plus 10% to minus 15%. The transformer shall conform to the following specific parameters: SL. No Item 1. Specification 25 kVA 63kVA 100 kVA 2. Continuous rated 16kVA capacity System highest Voltage 12 kV 12 kV 12 kV 12 kV 3. 4. 5. 6. 7. 8. 9. Rated Voltage HV Rated Voltage LV BIL Frequency No. of phases Connection HV Connection LV 11 kV 433 V 95 kV Peak 50Hz +/- 5% Three Delta Star (neutral brought out and directly earthed Dyn-11 ONAN 4.5 11 kV 433 V 95 kV Peak 50Hz +/- 5% Three Delta Star (neutral brought out and directly earthed Dyn-11 ONAN 4.5 11 kV 433 V 95 kV Peak 50Hz +/- 5% Three Delta Star (neutral brought out and directly earthed Dyn-11 ONAN 4.5 35 35 35 40 40 40 As per IS-1180 latest 255/ 140 As per IS-1180 latest 255/ 140 As per IS-1180 latest 255/ 140 As per IS-1180 latest 255/ 140 75/40 75/40 75/40 75/40 Not applicable Not applicable Not applicable Not applicable 10. 11. 12. 13. 11 kV 433 V 95 kV Peak 50Hz +/- 5% Three Delta Star (neutral brought out and directly earthed Vector group Dyn-11 Type of cooling ONAN Percentage impedance at 4.5 75°C Permissible temperature rise over ambient (i) Of top oil measured 35 by thermometer (ii) Of winding measured by resistance 40 14. Minimum clearances in 15. air (a) HV phase to phase / phase to earth (mm) (b) LV phase to phase / phase to earth (mm) Tap changer Specifications for Outdoor type, Completely Self Protected, 3 Phase Distribution Transformers (upto and including 100 kVA) 5.0 TECHNICAL REQUIREMENTS 5.1 Core 5.1.1 (a) Material - Cold Rolled Grain Oriented (CRGO) /Amorphous Metal Material- CRGO 45 5.1.2 (a) The core shall be stacked / wound type, of high grade cold rolled grain oriented steel laminations having low loss and good grain properties, coated with hot oil proof insulation, bolted / banded together and to the frames firmly to prevent vibration or noise. The complete design of core must ensure permanency of the core losses with continuous working of the transformers. The value of the maximum flux density allowed in the design and grade of lamination used shall be clearly stated in the offer. The manufacturer should offer the core for inspection and approval by the purchaser during manufacturing stage. Manufacturer's shall give notice for inspection with the following documents as applicable as a proof towards use of prime core material • • • • • Invoice of the Supplier Mills Test Certificate Packing List Bill of Lading Bill of Entry Certificate to Customs 5.1.3 (a) Core Clamping for CRGO Stacked Core • • • • MS Channel shall be used on top and bottom Core Channel on LV side to be reinforced at equidistance, if holes / cutting is done for LT lead in order to avoid bending of channel. MS Channels shall be painted with varnish or oil-resistant paint. Clamping & Tie-rods shall be made of HT steel and shall be parkarised 5.1.4 (a) Core Clamping for CRGO Wound Core • Core clamping shall be with top and bottom U-shaped core clamps made of sheet steel clamped with HT steel tie rods for efficient clamping. MS core clamps shall be painted with varnish or oil-resistant paint. MS rods shall be used as tie rods. Suitable provision shall be made in the bottom core clamp / bottom plate of the transformer to arrest movement of the active part. • • • 5.1.5 (a) The transformers core shall be suitable for over fluxing (due to combined effect of voltage and frequency) upto 12.5% without injurious heating at full load conditions and 46 Manual on Transformers shall not get saturated. The Manufacturer shall furnish necessary design data in support of this situation. 5.1.6 (a) No load current shall not exceed 1% of full load current and will be measured by energising the transformer at 433 volts, 50 c/s on the secondary. Increase of voltage of 433 volts by 12.5% shall not increase the no load current beyond 6% of full load current. (b) Material - Amorphous Metal 5.1.8 (b) The core shall be high quality Amorphous ribbons having very low loss formed into wound cores of rectangular shape, bolted together to the frames firmly to prevent vibration or noise. The complete design of core must ensure permanency of the core loss with continuous working of the transformers. The value of the flux density allowed in the design shall be clearly stated in the offer. Curve showing the properties of the metal shall be attached with the offer. 5.1.7 • (b) Core Clamping for Amorphous Metal Transformers Core clamping shall be with top and bottom U-shaped core clamps made of sheet steel clamped HT steel tie rods for efficient clamping. • MS core clamps shall be painted with varnish or oil-resistant paint. • MS rods shall be used as tie rods. • Suitable provision shall be made in the bottom core clamp / bottom plate of the transformer to arrest movement of the active part. 5.1.9 (b) The transformers core shall be suitable for over fluxing (due to combined effect of voltage and frequency) upto 12.5% without injurious heating at full load conditions and shall not get saturated. The Manufacturer shall furnish necessary design data in support of this situation. 5.1.10 (b) No load current shall not exceed 1% of full load current and will be measured by energising the transformer at 433 volts, 50c/s on the secondary. Increase of voltage of 433 volts by 12.5% shall not increase the no load current beyond 5% of full load current. Note : "Equal weightage shall be given to the transformers with Amorphous metal core and CRGO". 5.2 Windings 5.2.1 Material: Super enamel covered copper conductor / double paper covered copper conductor. Aluminium conductors can also be used with same loss levels as of copper wound transformers. 5.2.2 5.2.3 5.2.4 5.2.5 LV winding shall be in even layers so that neutral formation is at top. The winding construction of single HV coil preferred to crossover coil over LV coil Inter layer insulation shall be electrical grade Epoxy dotted kraft Paper for rectangular coils. Proper bonding of inter layer insulation with the conductor shall be ensured. Specifications for Outdoor type, Completely Self Protected, 3 Phase Distribution Transformers (upto and including 100 kVA) 47 5.2.6 Dimensions of winding coils are very critical. Dimensional tolerances for winding coils shall be with in limits as specified in GTP. 5.3 Oil The insulating oil shall comply with the requirements of relevant standards IS 335 / IEC 60296 5.4 Insulation Material 5.4.1 Material: Electrical Grade Epoxy Dotted Kraft Paper shall he used for Rectangular Coil 5.4.2 All spacers, axial wedges / runners used in windings shall be made of pre-compressed solid press board, conforming to type B 3.1 of IEC 641-3-2. In case of cross-over coil winding of HV all spacers shall be properly sheared and dovetail punched to ensure proper locking. All axial wedges / runners shall be properly milled to dovetail shape so that they pass through the designed spacers freely. Insulation shearing, cutting, milling and punching operations shall be carried out in such a way, that there is no burr or dimensional variations. 5.5 Losses : The maximum losses at rated voltage and rated frequency permitted at 75 Deg. C. are indicated below: Note: Voltage Ratio kVA Rating No load losses in watts (Max) Full load losses in watts (Max) at 75 Deg C 11000/433-250 V 16 65 425 11000/433-250 V 25 80 615 11000/433-250 V 63 150 1100 11000/433-250 V 100 220 1575 The losses mentioned above are under discussion and finalisation by BIS to revise IS:1180. The above losses eventually may get revised if any, after said revision. The above losses are maximum allowable and there would not be any positive tolerance. Transformers with higher losses than the above specified values would be treated as Nonresponsive. Transformers with losses less than those specified above will be capitalized during transformer evaluation as indicated below. (Refer to Loss Capitalisation formula, Section AA, Loss Capitalization factors: A. Iron Losses - Rs. 89.67 X Ec / Watt B. Copper Losses - Rs. 26.9 X Ec / Watt Where Ec is the cost of energy ( in Rupees per unit at 11 kV feeder level). In case of non availability of Ec (Energy cost per unit) at 11 kV feeder level, utility should consider the Bulk Rate Tariff plus 5% as the cost of energy at 11 kV feeder level 48 • Manual on Transformers PRICE SCHEDULE The following format shall be filled by the manufacturer for evaluating TOC (TOC) price. SL. No Price Component 1. Unit ex-works price 2. Freight & Insurance 3. (i) Excise duty Rating 1 Rating 2 Rating 3 Rating 4 (ii) Educational cess on Ex cise duty 5.6 4. Sales Tax 5. Total Cost per Unit without Capitalization (1+2+3+4) 6. NLL 7. Cost of NLL/Watt 8. Cost of Total NLL ( 6 x 7 ) 9. LL 10. CostofLL/Watt 11. Costoftotal LL(9x 10) 12. TOC price per Unit with Capitalization (5+8+11) 13. No. of Transformers 14. Type test charges 15. TOC price((12xl3)+14) Percentage Impedance The value of impedance of transformers at 75 Deg. C shall be 4.5% subject to the tolerance specified in the standard IS:2026. 5.7 Temperature Rise : The temperature rise over ambient of 50 deg. C shall not exceed the limits given below: Top oil temperature rise measured by thermometer Winding temperature rise measured by resistance : 35 Deg.C : 40 Deg.C Transformers not meeting the above limits of temperature rise shall not be accepted. 5.8 Penalty for Non Performance 5.8.1 During testing, if it is found that the actual measured losses are more than the values quoted by the manufacturer penalty shall be recovered from the manufacturer at double the loss capitalization rate arrived at clause 5.5. For fraction of a kW, proportionate penalty will be recovered. Specifications for Outdoor type, Completely Self Protected, 3 Phase Distribution Transformers (upto and including 100 kVA) 5.8.2 49 Transformers with temperature rise and impedance beyond guaranteed values: 5.8.2.1 Purchaser reserves the right to reject any transformer during the test at supplier's works, if the temperature rise exceeds the guaranteed values. 5.8.2.2 Purchaser reserves the right to reject any transformer during the test at supplier's works, if the impedance values differ from the guaranteed values including tolerance. 5.8.2.3 Purchaser also reserves the right to retain the rejected transformer and take it into service until the manufacturer replaces it with a new transformer at no extra cost. The delivery as per contract will be counted when the new transformer as per specification is provided by the manufacture. 5.9 Tank The transformer tank can be with radiator fins/ rounded or elliptical cooling tubes or made of corrugated panels. 5.9.1 For Rectangular / Octogonal Plain Tank The transformer tank shall be of robust construction rectangular /octogonal in shape and shall be built up of tested MS sheets. The internal clearance of tank shall be such that, it shall facilitate easy lifting of core with coils from the tank without dismantling LV bushings. All joints of tank and fittings shall be oil tight and no bulging should occur during service. The tank design shall be such that the core and windings can be lifted freely. The tank plate shall be of such strength that the complete transformers when filled with oil may be lifted bodily by means of lifting lugs. Inside of tank shall be painted with varnish/hot oil resistant paint. The four walls of the tank shall be made of Two "L" shaped sheets (without joints) fully welded at the corners from inside and outside of the tank for withstanding a pressure of 0.8 kg/cm2 for 10 minutes. The tank shall be reinforced by angle welded on all the outside walls on the edge of the tank to form two equal compartments. Permanent deflection when the tank without oil is subject to a vacuum of 525 mm of mercury for octogonal tank and 760 mm of mercury for round tank, shall not be more than 5 mm upto 750 mm length and 6.5 mm upto 1250 mm length. The tank shall further be capable of withstanding a pressure of 0.8 kg/sq cm (g) and a vacuum of 0.3 kg/sq cm (g) without any deformation. The radiators can be tube type or fin type or pressed steel type to achieve the desired cooling and the same shall be capable of giving continuous rated output without exceeding the specified temperature rise. 4 Nos. welded heavy duty lifting lugs of MS plate 8 mm thick (min) suitably 50 Manual on Transformers reinforced by vertical supporting flat welded edgewise below the lug shall be provided on the side wall. 4 Nos. of welded heavy duty pulling lugs of MS plate 8 mm thick (min) shall be provided to pull the transformer horizontally. Top cover fixing bolts of Galvanised Iron adequately spaced and 6 mm Neoprene bonded cork gaskets conforming to IS 4253 part-II shall be placed between tank and cover. The bolts outside tank shall have 2 flat washers and one spring washer. 5.9.2 Corrugated Tank Corrugated tanks may be offered for 63 kVA and 100 kVA. The transformer tank shall be of robust construction corrugation in shape and shall be built up of CRCA sheets of 1.2 mm thick. The internal clearance of tank shall be such that, it shall facilitate easy lifting of core with coils from the tank, with CCA (core-coil assembly), HV & LV bushings mounted on Top cover. All joints of tank and fittings shall be oil tight and no bulging should occur during service. The tank design shall be such that the core and windings can be lifted freely. The tank plate shall be of such strength that the complete transformers when filled with oil may be lifted bodily by means of lifting lugs. Inside of tank shall be painted with varnish / hot oil resistant paint. Corrugation panel shall be used for cooling. The transformer shall be capable of giving continuous rated output without exceeding the specified temperature rise. 2 Nos. welded heavy duty lifting lugs of MS plate 8 mm thick (min) suitably reinforced are to be provided. Top cover fixing bolts of galvanized iron and 6 mm Neoprene bonded cork gaskets conforming to IS 4253 part-II / nitrile rubber shall be placed between tank and cover. The bolts outside tank shall have 2 flat washers and one spring washer. Tanks with corrugations and without conservator shall be tested for leakage test at a pressure of 0.15kg/sq cm measured at the top of the tank 5.9.3 Sealed Transformer with Radiators In this type of construction tank is designed to have cover welded to the curb of tank. Space is provided above the core coil assembly where inert gas cushion system accommodates the oil expansion under variable pressure. The tank should be of stiff construction able to withstand pressure of 2 atmospheres. Specifications for Outdoor type, Completely Self Protected, 3 Phase Distribution Transformers (upto and including 100 kVA) 5.10 51 Conservator On Transformers of 100 kVA rating with rectangular plain tank the provision of conservators is obligatory. For other ratings manufacturer may adopt their standard practice. Conservator is not required in transformers with corrugated tank. When a conservator is provided, oil gauge and the plain or dehydrating breathing devise shall be fixed to the conservator which shall also be provided with a drain plug and a filling hole (M30 normal size thread) with cover. The capacity of a conservator tank shall be designed keeping in view the total quantity of oil and its contraction and expansion due to temperature variations. In addition the cover of main tank shall be provided with an air release plug to enable air trapped within to be released, unless the conservator is so located as to eliminate the possibility of air being trapped within the main tank. The inside diameter of the pipe connecting the conservator to the main tank should be within20 to 50 mm and it should be projected into the conservator so that its end is approximately 20 mm above the bottom of the conservator so as to create a sump for collection of impurities. The minimum oil level (corresponding to -5 deg C) should be above the sump level. 5.11 Surface Preparation and Painting 5.11.1 For Surface Preparation refer to section A of this Manual 5.12 Bushings The bushings shall conform to the relevant standards specified and shall be outdoor type. The bushing rods and nuts shall be made of brass material 12 mm diameter for both HT & LT. The bushings shall be fixed to the transformers on side with straight pockets and in the same plane or on the top cover. Arcing horns or lightning arrestors shall be provided on HV bushings. For 11 kV, 17.5 kV class bushings and for 0.433 kV, 1.1 kV class bushings shall be used. Bushings with plain sheds as per IS-3347 shall be mounted on the side of the Tank and not on top cover. A minimum phase to phase clearance of 75 mm for LV (upto 1.1 kV bushings) and 255 mm for HV bushings shall be obtained with the bushing mounted on the transformer. The terminal arrangement shall not require a separate oil chamber not connected to oil in the main tank. 5.13 Terminal Connectors The LV bushing and HV bushing stems shall be provided with suitable terminal connectors so as to connect the jumper without disturbing the bushing stem. Connectors shall be with eye bolts so as to receive 55 sq mm conductor for HV. 52 5.14 Manual on Transformers Terminal Markings High voltage phase windings shall be marked both in the terminal boards inside the tank and on the outside with capital letter 1U, IV, 1W and low voltage winding for the same phase marked by corresponding small letter 2U, 2V, 2W. The neutral point terminal shall be indicated by the letter 2N. Neutral terminal to be brought out and connected to local grounding terminal by an Earthing strip. 5.15 Current Transformers (for 63 and 100 kVA ratings only) • • CT's shall be provided if required on secondary side. Current transformer shall be mounted inside the tank on LV side of the transformer. Ring Type CTs in air, mounted in LV cable boxes are also accepted The current transformers shall be comply with IS: 2705. All secondary leads of bushing mounted CT's shall be brought to a terminal box near each bushing. The CT terminals shall have shorting facility. • • CT should not get saturated upto 200% of rated current. 5.16 Transformer Rating 63 kVA 100 kVA Current Ratio 100/5 A 150/5 A Class 0.5 0.5 Burden 20 VA 20 VA Application Metering Metering ISF 5 5 Lightning Arresters 9 kV, 5 kA Metal Oxide Lightning Arresters as per relevant standard, one number per phase shall be fitted under the HV bushings with GI earth strip 25x4 mm connected to the body of the transformer with necessary clamping arrangements. The metal oxide lightning arresters shall be of reputed make. 5.17 Protection The transformers shall have the following Completely Self Protection (CSP) features: (i) Internal HV fuse on the HT side of transformer Specification for the HV fuses: Expulsion/any other suitable type of fuse placed in series with the primary winding. This fuse is mounted normally inside of the primary bushing for the three phases and is connected to the high voltage winding through a terminal block. This has to protect that part of the electrical distribution system which is ahead of the distribution transformers from faults which occur inside the distribution transformer i.e., either in the windings or some other Specifications for Outdoor type, Completely Self Protected, 3 Phase Distribution Transformers (upto and including 100 kVA) 53 part of the transformer. It shall be ensured that this fuse does not blow for faults on the secondary side (LT side) of the transformer i.e., the blowing characteristics of the fuse and LT breaker shall be so coordinated such that the fuse shall not blow for any faults on the secondary side of the transformer beyond LT breakers and those faults shall be cleared by the LT breaker only. (ii) 3 Pole LT circuit breaker (a) Internally mounted oil immersed LT breaker on the LV side of the transformer: All LT faults after the breaker shall be cleared by this breaker. As such it shall be designed for perfect coordination with the HT fuse link. The Manufacturer shall furnish the time / current characteristics of LT circuit breaker and 11 kV fuses for various current multiples. The two characteristics shall be drawn on the same sheet to indicate coordination between the circuit breaker and fuse. The Manufacturer shall carry out coordination test as indicated above and this forms one of the tests for acceptance test. The breaker shall be coordinated thermally with the transformer design to follow closely the variations of coil temperature due to fluctuating loads and ambient temperatures. This is to be accomplished by connecting the breaker in series between the secondary winding and the secondary bushings. The breaker shall be located in the same oil as the core and coil assembly so that the bimetal are sensitive to the temperature of oil as well as the load current The circuit breaker shall also be closed and opened manually standing on ground. The current carrying parts of breakers shall be copper plus a set of copper tungsten current interrupting contacts. The cross-section of the current carrying parts of the breaker shall withstand the full load current at a current density not more than 2.5A/sq. mm (for additional mechanical strength the area should be more). (b) MCCB 3 pole MCCB (confirming to IS 13947) from reputed manufacturers of appropriate rating with inverse time characteristics for overload & instantaneous magnetic trip (trip time less than 10 mS at 0.4 lagging p.f.) for short circuits shall be provided after the LT bushing in the distribution box. All plastic material shall comply to glow wire test as per relevant IS. Type test report from NABL accredited laboratory shall be submitted. A distribution box (made of MS or SMC or FRP) shall be provided as an integral part of the transformer to be mounted on the tank before installation on the pole. LT bushing shall be inside the distribution box and a facility for sufficient number of outgoing feeders through cable glands shall be provided. Distribution box shall also have provision for installation of energy meter. Distribution box shall be designed for out-door duty with minimum IP - 55 protection. It shall have pad-locking arrangement. 54 (iii) Manual on Transformers Load Management Signal Light A signal light shall be provided to give information about the loading condition of the transformer. It shall forewarn any overloading problem at the installation such that a change out of the existing transformer with a higher capacity transformer can be planned. The signal light mechanism shall not reset itself when the load drops from the overloaded condition. The signal light shall remain lighted once the signal light contacts close due to overload and can be turned off by manual operation. (The signal light shall not give indication for momentary overloading). 5.18 Fittings The following standard fittings shall be provided: (a) Rating and terminal marking plates non-detachable (b) Earthing terminals with lugs - 2 Nos. (c) Lifting lugs for main tank & top cover (d) Pulling lugs - 4 Nos (e) HV bushings - 3 Nos. (f) LV bushings - 3 Nos. (g) Neutral bushings-1 No. (h) Terminal connectors on the HV/LV bushings (i) 9 kV 5 kA lightning arrestors on HT side - 3 no. (j) Thermometer pocket with cap -1 No. (k) Air release device (1) Stiffener angle 40 x 40 x 5 mm and vertical strip of 50 x 5 mm flat (m) Radiators (n) Prismatic oil level guage (o) (p) (q) (r) (s) (t) (u) (v) 5.19 Drain cum sampling valve Oil filling hole having M30 thread with plug and drain valve on the conservator Silicagel breather Pressure relief device or explosion vent. Base channel 75 x 40 mm MCCB or Oil immersed LT circuit breaker along with operating rod HV fuse links Signal light Fasteners All bolts, studs, screw threads, pipe threads, bolt heads and nuts shall comply with the appropriate Indian Standards for metric threads, or the technical equivalent. Specifications for Outdoor type, Completely Self Protected, 3 Phase Distribution Transformers (upto and including 100 kVA) 55 Bolts or studs shall not be less than 6 mm in diameter except when used for small wiring terminals. All nuts and pins shall be adequately locked. Wherever possible bolts shall be fitted in such a manner that in the event of failure of locking resulting in the nuts working loose and falling off, the bolt will remain in position. All ferrous bolts, nuts and washers placed in outdoor positions shall be treated to prevent corrosion, by hot dip galvanising, except high tensile steel bolts and spring washers which shall be electro galvanised/ plated. Appropriate precautions shall be taken to prevent electrolytic action between dissimilar metals. Each bolt or stud shall project at least one thread but not more than three threads through the nut, except when otherwise approved for terminal board studs or relay stems. If bolts nuts are placed so that they are inaccessible by means of ordinary spanners, special spanners shall be provided. Taper washers shall be provided wherever necessary. 5.20 Mounting Arrangement The under base of all transformers shall be provided with two 75 x 40 mm channels 460 mm long with holes to make them suitable for fixing on a platform or plinth 5.21 Overload Capacity The transformers shall be suitable for loading as per IS: 6600 / 2026-7. 6.0 TESTS 6.1 Routine Tests • • • • Ratio, polarity and phase sequence. Insulation Resistance (IR) & Polarisation Index (PI) Magnetic Balance Test No load current and losses at rated frequency, rated voltage and at 90% & 110% voltage. Load loss at rated current and normal frequency Impedance voltage test Resistance of windings Induced over voltage withstand test. Separate source voltage withstand test. LT Circuit Breaker coordination test. • • • • • • 56 Manual on Transformers • • Efficiency & regulation tests as per cl. 19 of IS 1180. Air pressure test on assembled transformer at 0.35 kg/cm2 for 10 min. 6.2 Special Tests Lightning impulse with chopped on the tail. Impulse voltage test: As per clause no. 13 (with chopped wave) of IS-2026 part-III as per latest version. BIL for 11 kV shall be 95 kV peak. Short circuit test. Short Circuit Withstand Test: Thermal and dynamic ability including test as per IS 2026 part-V. 6.3 Additional Tests • • • Neutral current measurement Air pressure test: As per Cl.22.5 of IS -1180 / part-I Transformer tank shall be subjected to specified vacuum. The tank designed for vacuum shall be tested at an internal pressure of 0.35 kg/cm2 absolute (250 mm of Hg) for one hour. The permanent deflection of flat plates after the vacuum has been released shall not exceed the values specified below: • Horizontal length of flat plate (in mm) Permanent deflection (in mm) Upto & including 750 5.0 751 to 1250 6.5 • • Transformer tank together with its radiator and other fittings shall be subjected to pressure corresponding to twice the normal pressure or 0.35 kg/cm2 whichever is lower, measured at the base of the tank and maintained for an hour. The permanent deflection of tie flat plates after the excess pressure has been released, shall not exceed the figures for vacuum test. The pressure relief device shall be subject to increasing fluid/air pressure. It shall operate before reaching the test pressure as specified in the above clause. The operating pressure shall be recorded. The device shall seal-off after the excess pressure has been released. Oil samples (one sample per lot) to comply with IS 1866. Single phase LV excitation current at all three phases (for reference) 6.4 Type Tests to be Conducted on one Unit • • • In addition to the tests mentioned above the following tests shall be conducted. Temperature rise test. Lightning impulse withstand voltage test: Oil samples (before and after short-circuit and temperatures rise test) for each tested transformer. SECTION C Specifications for Outdoor Type, Completely Self Protected, Single Phase Distribution Transformers (Single Phase 11 kV/250 V & 11/√3 kV/250V, 5, 10, 16 & 25 KVA Ratings) SECTION C Specifications for Outdoor Type, Completely Self Protected, Single Phase Distribution Transformers (Single Phase 11 kV/250 V & 11/√3 kV/250V, 5, 10, 16 & 25 KVA Ratings) 1.0 SCOPE 1.1 This section covers oil immersed naturally cooled 11 kV / 250V* and 11 kV/√3 / 250V* single phase Completely Self Protected distribution transformers, but does not purport to include all the necessary provisions of a contract. For general requirements and tests, reference shall be made to sections A to BB of the Manual. 1.2 Standard Ratings The Standard Ratings of 1- Phase Transformers shall be 5, 10, 16 & 25 kVA. 2.0 STANDARDS 2.1 The materials shall conform in all respects to the relevant Indian Standard Specifications with latest amendments/edition thereof: Note : * REC Specification mention 230 V Indian Standard Title International and Internationally recognized standard IS-2026 (Part-I to IV) Specification for Power Transformer IEC-60076 IS-1180 (Part l& 2) Outdoor Three Phase Distribution Transformer IS-3347 Specification for Porcelain Transformer Bushings DIN 42531,2,3 IS-7421 Specification for Low Voltage Bushings IS - 12444 Specification for Copper Wire Rods ASTM B - 49 IS-335 Specification for Transformer Oil BS 148/ASTM DI275, DI533, ILC Pub 296 1S-3070 Specification for Lightning Arresters 1HC99-1 1S-6600/ Guide for Loading of Oil Immersed Transformers 1EC 60076-7 (ILC 354) 2026-7 1S-2099 High Voltage Porcelain Bushings IEC 60137 IS 9335 Specification for Insulating Kraft Paper IEC 60554 IS 1576 Specification for Insulating Press Board IEC 60641 IS 5 Specification for Colours for Ready Mixed Paints Refer Appendix II of the Manual for List of Standards. 59 60 3.0 Manual on Transformers SERVICE CONDITIONS The Distribution Transformers to be supplied against this Specification shall be suitable for satisfactory continuous operation under the climatic conditions prevailing at site and to be specified by the purchaser as follows as per IS 2026 (Part-I) latest revision. International Standards tabulated above. (i) (ii) (iii) (iv) (v) (vi) Location Max ambient air temperature (Deg.C) Min. ambient air temperature (Deg.C) Max. average daily ambient air temperature (Deg.C) Max. yearly weighed average ambient temperature (Deg.C) Max. altitude above mean sea level (m) 4.0 STANDARD RATINGS : : : : : : The transformers shall be suitable for outdoor installation with Single phase, 50 c/s 11 kV systems in which the neutral is effectively earthed and they should be suitable for service under fluctuations in supply voltage upto plus 10% to minus 15%. The transformer shall conform to the following specific parameters. Rated HV side value (11/√3 or 11 kV) shall be specified in the detailed Guaranteed Technical Particulars by Purchaser Table 1 SI. No ITEM SPECIFICATION 1. Continuous rated capacity 5kVA 10 kVA 16kVA 25 kVA 2. System Voltage (Max) 12 kV 12 kV 12 kV 12 kV 3. Rated Voltage HV 11 kV/√3 or 11 kV 11 kV/√3 or 11 kV 4. Rated Voltage I.V 230 V (250 V Max) 230 V (250 V Max) 230 V(250 V Max) 230 V(250 V Max) 5. BIL 95 kV Peak 95 kV Peak 95 kV Peak 95 kV Peak 6. Frequency 50 Hz ± 3% 50 Hz ± 3% 50 Hz ± 3% 50 Hz ± 3% 7. No of phases Single Single Single Single 8. Type of cooling ONAN ONAN ONAN ONAN 9. Tap changing arrangement Not provided Not provided Not provided Not provided 10. Percentage 4 % (Tolerance as 4 % (Tolerance 4% (Tolerance 11. Rated frequency and 75" C Permissible temperature rise over ambient per IS: 2026) as per IS: 2026) as per IS: 2026) impedance at 2.5% (Tolerance as per IS: 2026) 11 kV/√3 or 11 kV 11 kV/√3 or 11 kV Specifications for Outdoor Type, Completely Self Protected, Single Phase Distribution Transformers (Single Phase 11 kV/250 V & 11/√3 kV/250V, 5, 10, 16 & 25 KVA Ratings) 12. (i) of top oil measured by thermometer 35 "C 35 "C 35 "C 35 ' C (ii) of winding measured by resistance 40 "C 40 "C 40 "C 40 "C (a) HV phase to phase / phase to earth (mm) 255/ 140 255/140 255/ 140 255/ 140 (b) LV phase to phase / phase to earth (mm) 75/40 75/40 75/40 75/40 61 Minimum clearances in air 5.0 TECHNICAL REQUIREMENTS 5.1 Winding Connection and Terminal Arrangements For 11 kV transformers both ends of primary winding shall be brought out through HV bushings. For 11 kV/ √3 transformers, neutral end of the primary HV winding shall be bought out for connecting to ‘Neutral’ supply wire through 1.1 kV bushing. There shall be provision for connecting ‘Neutral’ terminal, to local ‘Earth’ by way of a tinned Copper strip, of adequate size and dimension. The secondary winding shall be connected to two LV bushings. 5.2 Core 5.2.1 Core Material Transformer core shall have wound core construction using new and high quality CRGO steel with heat resistant insulating coating or Amorphous Metal. The laminations shall be free from burrs & edge bends. Air gap in the core assembly shall be avoided. The core shall be properly stress relieved by annealing in inert atmosphere as per core material manufacturer’s recommendations. Amorphous metal core shall be annealed under magnetic field. The transformer shall be suitable for over fluxing (due to combined effect of voltage and frequency) upto 12.5% without injurious heating. The operating flux density shall be such that there is a clear safe margin over the overfluxing limit of 12.5%. Following documents as applicable shall be submitted during inspection as a proof towards use of prime core material • • • • • 5.3 Invoice of the supplier Mills Test Certificate Packing List Bill of Lading Bill of entry certificate to customs Winding HV and LV windings shall be wound from copper conductors.Aluminium conductors can also be used with same loss levels as of copper wound transformers. The HV winding conductor 62 Manual on Transformers shall be covered with Super enamel & LV winding with DPC / Super Enamel. The inter layer insulation shall consist of epoxy resin coated paper or epoxy resin dotted paper. The windings shall be progressively wound in LV - HV coil design for better voltage regulation and mechanical strength. Compression bonding of the windings shall be ensured by deploying suitable press in the winding process or by curing of epoxy paper insulation in a suitable press. The core coil assembly shall be dried in an oven. The type of winding shall be indicated, whether LV windings are of conventional to type or foil wound. Joints in the winding shall be avoided. However if joining is necessary, the joints shall be properly brazed & finished. Oil 5.4 5.4.1 The insulating oil shall be new and shall comply with the requirements of IS 335. Use of recycled oil is not acceptable. 5.4.2 Oil shall be filtered & tested for BDV & moisture content before filling. Oil filling shall be carried under vacuum of 5 torrMin. Testing of oil after filling 5.4.3 Testing of oil sample for BDV & moisture content from assembled transformers shall be carried out as acceptance test in line with IS 1866. 5.4.4 Insulation Material The inter layer insulation shall be of epoxy resin bond paper. The core/coil assembly shall be securely held in position to avoid any movement under short circuit conditions. 5.5 Losses : The maximum losses at rated voltage and rated frequency permitted at 75 Deg.C. are indicated below: KVA Rating No load losses (Max) - CRGO watts 5 25 8 135 10 30 12 180 16 45 17 230 25 55 25 310 Note: No load losses (Max) Amorphous watts Full load losses in watts (Max) at 75 Deg C watts The losses mentioned above are under discussion and finalisation by BIS to revise IS: 1180. The above losses eventually may get revised if any, after said revision. The above losses are maximum allowable and there would not be any positive tolerance. Transformers with higher losses than the above specified values would be rejected. Transformers with losses less than those specified above will be capitalized during bid evaluation as per loss capitalization formula as given below. Specifications for Outdoor Type, Completely Self Protected, Single Phase Distribution Transformers (Single Phase 11 kV/250 V & 11/√3 kV/250V, 5, 10, 16 & 25 KVA Ratings) 63 Total Ownership Cost (TOC) after loss capitalization = Quoted Price + NLL X A + LLxB Transformers with losses less than those specified above will be capitalised during evaluation as indicated below. (Refer to Loss Capitalisation formula, SectionAA). Loss Capitalization factors: A. B. Iron Losses - Rs. 89.67 x Ec / Watt Copper Losses - Rs. 26.9 x Ec / Watt Where Ec is the cost of energy ( in Rupees per unit at 11 kV feeder level). In case of non availability of Ec (Energy cost per unit) at 11 kV feeder level, utility should consider the Bulk Rate Tariff plus 5% as the cost of energy at 11 kV feeder level. Price schedule: The following format shall be filled by the manufacturer for evaluating TOC. SI. No Price Component 1 Unit ex-works price (Rupees) 2 Freight & Insurance (Rupees) 3 Excise duty (Rupees) (2) Educational cess on Excise duty (Rupees) 4 Sales Tax (Rupees) 5 Total Cost per Unit without Capitalization (1+2+3+4) (Rupees) 6 NLL (Watts) 7 Cost of NLUWatt (Rupees) (A) 8 Cost of Total NLL (6 X 7) (Rupees) 9. LL (Watts) 10. Cost of LL/Watt (Rupees) (B) 11. Cost of total LL(9 x 10) (Rupees) 12. TOC price per Unit with Capitalization (5+8+11) (Rupees) 5.6 Penalty for Non-Performance 5.6.1 Loss values beyond guaranteed values, but less than Max. losses specified at cl. 5.5 -During testing, if it is found that the actual measured losses are more than the values quoted, penalty shall be recovered from the manufacturer at double the loss capitalization rate arrived at clause 5.5. For fraction of a Watt, proportionate penalty will be recovered. 5.6.2 Transformers with temperature rise and impedance beyond guaranteed values: 5.6.2.1 Purchaser reserves the right to reject any transformer during the test at supplier’s works, if the temperature rise exceeds the guaranteed values. 64 Manual on Transformers 5.6.2.2 Purchaser reserves the right to reject any transformer during the test at supplier’s works, if the impedance values differ from the guaranteed values including tolerance. 5.6.2.3 Purchaser also reserves the right to retain the rejected transformer and take it into service until the Manufacturer replaces it with a new transformer at no extra cost. The delivery as per contract will be counted when the manufacturer provides the new transformer as per specification. 5.6.2.4 Transformers having losses above the values specified in the clause 5.5 shall be rejected. 5.7 Tank The oil volume inside the tank shall be such that even under the extreme operating conditions, the pressure generated inside the tank does not exceed 0.4 kg/cm2 positive or negative. There must be sufficient space from the core to the top cover to take care of oil expansion. The tank cover shall have plasticised surface at the top to guard against bird faults. Alternately, suitable insulating shrouds shall be provided on the bushing terminals. The Transformer tank shall be of robust construction round in shape and shall be built up of tested CRCA / MS sheet. The tank shall be capable of withstanding a pressure of 1 kg/cm2 (g) and a vacuum of 760 mm of Hg for 30 minutes without any permanent deflection (Air pressure test shall be conducted as per IS -1180) The L - seam joint, C - seam joint and all fittings and accessories shall be oil tight and no deflection / bulging should occur during service. The circular base plate edges of the tank should be folded upward, for at least 25 mm, to have sufficient overlap with vertical sidewall of the transformer. Tank shall have permanent lugs for lifting the transformer bodily and there shall be facilities for lifting the core coil assembly separately. The transformer tank and the top cover shall be designed in such a manner as to leave no external pockets in which water can lodge. The transformer shall be provided with two mounting lugs suitable for fixing the transformer to a single pole by means of 2 bolts of 20 mm diameter as per ANSI C 57.12.20. • Both mounting lugs are made with steel of min. 5 mm thickness. • Minimum Oil level mark shall be embossed inside the tank. • Jump proof lips shall be provided for upper mounting lug. • Mounting lugs faces shall be in one plane. Specifications for Outdoor Type, Completely Self Protected, Single Phase Distribution Transformers (Single Phase 11 kV/250 V & 11/√3 kV/250V, 5, 10, 16 & 25 KVA Ratings) 65 • • The top cover shall be fixed to the tank through clamping only. HV bushing pocket shall be embossed to top side of the top cover so as to eliminate ingressing of moisture and water. • The edges of the top cover shall be formed, so as to cover the top end of the tank and gasket. Sealing gaskets - Continuous (without joint) oil resistant high quality Nitrile / neoprene / Polyurethane rubber gaskets conforming to Type-III as per latest IS-11149 shall be provided between tank & top cover and for fixing the bushings. Tank sealing - The transformer shall be of sealed tank construction with welded or bolted cover which seals the interior of the tank from atmosphere and in which gas volume plus oil volume remains constant. The space on the top of the oil shall be filled with dry air or nitrogen. The nitrogen plus oil volume inside the tank shall be such that even under extreme operating conditions, the pressure generated inside the tank does not exceed 0.4 kg/cm2 positive or negative. The nitrogen shall conform to commercial grade of the relevant Standard. 5.8 Surface Preparation and Painting For surface preparation refer to section ‘A’ of this Manual. 5.9 Winding Connection & Bushing Terminal Arrangements For 11/√3 kV Transformers, Neutral end of the primary HV winding shall be brought out for connection to ‘Neutral’ supply wife through 1.1 kV bushings. ‘Neutral’ terminal shall be connected to transformer tank by way of a tinned Copper strip of adequate size and dimension. The secondary winding shall be connected to two LV bushings. HV terminal shall be designed to directly receive ACSR conductor upto 7/2 59 mm (without requiring the use of lug) and the LV terminals shall be suitable for directly receiving LT cables (aluminium) ranging from 10 sq mm to 25 sq mm both in vertical and horizontal position and the arrangements should be such as to avoid bimetallic corrosion. Terminal connectors must comply as per IS : 5561. 5.10 Bushings The bushings shall conform to the relevant standards specified. For HV, 12 kV class bushings shall be used and for LV, 1.1 kV class bushings shall be used. The HV bushings shall be fixed to the top cover of the transformer and the LV bushings shall be fixed to transformer on sides and in the same plane. The bushing rods and nuts shall be of brass. The terminal arrangement shall not require a separate oil chamber not connected to oil in the main tank. The HV bushings shall not have arcing horns. 66 Manual on Transformers 5.11 Rating and Terminal Plates There shall be a rating plate on the transformer containing the information given in the relevant Indian Standard. The HV winding terminals shall be marked 1.1 and 1.2 for 11 kV/√3 HV winding. 1.2 terminal will be connected to neutral. In case of 11 kV HV winding the terminal shall be marked as 1.1 - 1.2. The corresponding secondary terminal shall be marked as 2.1 and 2.2. 5.12 Protection The transformer shall have the following CSP features: 5.12.1 Internal HV Fuse on the HT Side of Transformer Suitable replaceable fuse shall be placed inside the transformer, in series with the primary winding. The fuse on primary side of the distribution transformer shall take care of any fault occurring inside the transformer itself. The characteristic of the fuse and LT breaker shall be well coordinated so that the fuse shall not blow for any faults on the secondary side of the transformer beyond LT Breaker and those faults shall be cleared by LT breaker only. The fuse shall confirm to IS 9385 and shall be tested for short circuit current. 5.12.2 LT Circuit Breaker This breaker shall clear all LT faults after the breaker. As such it shall be designed for perfect coordination with the HT fuse for various current multiples. The two characteristics shall be drawn on the same sheet to indicate coordination between the circuit breaker and HV fuse. The manufacturer shall carry out coordination test as indicated above, and this forms one of the tests for acceptance. The LT breaker shall also be coordinated with overload characteristics of the transformer. The reference temperature for calibration shall be 40 Deg C. Arrangement shall be made so that the circuit breaker can be closed and opened manually standing on ground. The cross section of the current carrying parts of the breaker shall withstand the full load current at a current density not more than 2.5 A/sq.mm (for additional mechanical strength the area should be more). Rated short circuit breaking capacity of the breaker shall not be less than 2.5 kA. The circuit breaker shall confirm to IS-13947 with power factor 0.4 lagging. Time current characteristics & other requirements shall be in line with IS 13947 (latest version). Time current characteristics of LT CB SL. No. Test current Initial test conditions Time limits of tripping/ no Remarks tripping 1. 1.05 In Cold l ≥ 2.5h No tripping 2. 1.20 In Immediately after Sl.no. 1 I0 min ≤ t ≥ 2h Tripping 3. 1.30 In Cold t ≤ 30 min Tripping Specifications for Outdoor Type, Completely Self Protected, Single Phase Distribution Transformers (Single Phase 11 kV/250 V & 11/√3 kV/250V, 5, 10, 16 & 25 KVA Ratings) 4. 1.40 In Cold t≤10 min Tripping 5. 2.50 In Cold t ≤ 1 min Tripping 6. 4.00 In Cold t ≥ 5S Tripping 7. 6.00 In Cold t≤5S Tripping 8. 12.00 In Cold t ≥ 40mS Tripping 67 Where In is full load LV current of the transformer. Manufacturer shall submit the coordination of time-current characteristics of LT & HT side plotted on the same sheet of paper for approval of the owner. Manufacturer shall offer either internally mounted oil immersed circuit breaker or MCCB as specified below. (Utilities may specify any one option). (a) Internally mounted, Oil immersed ‘LT’ Breaker The breaker shall be located in the same oil as the core and coil assembly so that the bimetal is sensitive to the temperature of oil as well as the load. (b) MCCB Double pole MCCB (confirming to IS 13947) from reputed manufacturers of appropriate rating with inverse time characteristics for overload & instantaneous magnetic trip (trip time less than 10 mS at 0.4 lagging p.f.) for short circuits shall be provided after the LT bushing in the distribution box. All plastic material shall comply to glow wire test as per relevant IS. Type test report from NABL accredited laboratory shall be submitted. A distribution box (made of MS or SMC or FRP) shall be provided as an integral part of the transformer to be mounted on the tank before installation on the pole. LT bushing shall be inside the distribution box and a facility for sufficient number of outgoing feeders through cable glands shall be provided. Distribution box shall also have provision for installation of energy meter. Distribution box shall be designed for out-door duty with minimum IP - 55 protection. It shall have pad-locking arrangement. 5.12.3 Signal Light for Tripping of LT Breaker The distribution box shall be equipped with an LED to indicate tripping of LTCB. On resetting of LTCB the LED shall be automatically switched off. 5.12.4 11 kV Lightening Arresters High surge capacity 9 kV, 5 kA metal oxide lightening arrester conforming to 1S-3070 (Pt-III) shall be mounted on the transformer & clamped securely to the lank, to protect the transformer and associated line equipment from the occasional high voltage surges resulting from lightning or switching operations. The Earthing terminal of the lightning arresters shall be connected 68 Manual on Transformers solidly to the Earthing terminal on the tank body. Lightning arrestors with polymer insulators in line with relevant IEC shall also be acceptable. 5.12.5 Pressure Release Device The transformer shall be equipped with a self-sealing pressure release device, designed to operate at a minimum pressure of 8 psi (0.564 kg/ cm2). The PRD shall be provided in the lowvoltage terminating portion of the tank above top oil level. Inlet port shall be V4 inch or longer NPT. Resealing pressure shall be 0.3 kg/cm2 5.13 Fittings The following standard fittings shall be provided: (i) Rating and Terminal marking plates. (ii) Earthing terminals - 2 Nos. (iii) Lifting lugs - 2 Nos. (iv) HV bushings. (v) LV bushings. (vi) Bird guard. (vii) HV & LV Terminal connectors. (viii) HV side Neutral ‘Earthing’ strip; (ix) LV earthing arrangement. (x) Metal oxide lightning arrestors (9kV, 5kA) (xi) MCCB or Oil immersed LT circuit breaker along with operating rod a. (make, type and technical details to be provided.) (xii) HV fuse links (xiii) Signal light (xiv) Oil level indicator (xv) Top cover fixing clamps. (xvi) Pressure relief device. (xvii) Mounting lugs - 2 Nos. (xviii) 5 year guarantee plate (xix) Any other fitting necessary for satisfactory performance of the manufacture. 5.14 Fasteners All bolts, studs, screw threads, pipe threads, bolt heads and nuts shall comply with the appropriate Indian Standards for metric threads, or the technical equivalent. Bolts or studs shall not be less than 6 mm in diameter except when used for small wiring terminals. Specifications for Outdoor Type, Completely Self Protected, Single Phase Distribution Transformers (Single Phase 11 kV/250 V & 11/√3 kV/250V, 5, 10, 16 & 25 KVA Ratings) 69 All nuts and pins shall be adequately locked. Wherever possible bolts shall be fitted in such a manner that in the event of failure of locking resulting in the nuts working loose and falling off, the bolt will remain in position. All ferrous bolts, nuts and washers placed in outdoor positions shall be treated to prevent corrosion, by hot dip galvanising, except high tensile steel bolts and spring washers which shall be electro-galvanised/ plated. Appropriate precautions shall be taken to prevent electrolytic action between dissimilar metals. Each bolt or stud shall project at least one thread but not more than three threads through the nut, except when otherwise approved for terminal board studs or relay stems. If bolts nuts are placed so that they are inaccessible by means of ordinary spanners, special spanners shall be provided. Taper washers shall be provided where necessary. 6.0 TESTS 6.1 Routine Tests • Ratio, polarity tests. • No load current and loss at rated voltage and frequency. • Load loss at rated current and normal frequency. • Impedance voltage test. • Resistance of windings. • Insulation Resistance (IR) & Polarisation Index (PI). • Induced over voltage withstand test. • Separate source voltage withstand test. • LT Circuit Breaker coordination test. • Efficiency & regulation tests as per cl. 19 of IS 1180. • Air pressure test on assembled transformer at 0.35 kg/cm2 for 10 min. 6.2 Special Tests Lightning impulse with chopped on the tail. Impulse voltage test: As per clause no. 13 (with chopped wave) of IS-2026 part-III as per latest version. BIL for 11 kV shall be 95 kV peak Short circuit test Short Circuit Withstand Test: Thermal and dynamic ability including test as per IS 2026 part-V. 70 Manual on Transformers 6.3 Additional Tests • • Oil samples test (one sample / lot) to comply with IS 1866 Air pressure Test: As per clause 24.5.1 of IS-1180/ Part-II. 6.4 Type Tests to be Conducted on one Unit In addition to the tests mentioned above the following tests shall be conducted. • • • Temperature rise test. Lightning/Impulse withstand voltage test. Oil samples (before and after short circuit and temperature rise test). SECTION D Specifications for Three Phase Distribution Transformers (above 100 kVA and upto 33 kV class) SECTION D Specifications for Three Phase Distribution Transformers (above 100 kVA and upto 33 kV class) 1.0 SCOPE 1.1 This section covers, Three phase distribution transformers of above 100 kVA to 3150 kVA, 11 & 33 kV (outdoor & indoor use) but does not purport to include all the necessary provisions of a contract. For general requirements and tests, reference shall be made to sections A and BB respectively of the Manual. 1.2 Standard Ratings kVA 160 Voltage Ratio 200 250 315 400 500 630 800 11000/433V 1000 1250 1600 2000 2500 3000 3150 315 400 500 630 800 1000 33000/433V 1250 1600 2000 2500 3000 3150 The above ratings are also applicable for 22 /0.433 kV transformers. 2.0 STANDARDS 2.1 The materials shall conform in all respects to the relevant Indian / International Standard 73 74 Manual on Transformers Specification, with latest amendments thereof, some of them are listed below: However, prior agreement with customer is necessary for standard to be followed. Indian Standard Title International & Internationally recognised standard IS -2026 IS- 1180 IS 12444 Specification for Power Transformer Outdoor Distribution Transformer upto and including 100 kVA Specification for Copper Wire Rod IEC 60076 IS-3347 IS-335 Specification for Porcelain Transformer Bushing Specification for Transformer Oil IS -5 IS-2099 IS – 7421 IS– 3347 IS - 5484 IS -9335/IS- 1576 IS – 6600 / IS 2026-7 Specification for Colours for Ready Mixed Paints Specification for High Voltage Porcelain Bushings Specification for Low Voltage Bushings Specification for Outdoor Bushings Specification for Al Wire Rods Specification for Insulating Kraft Paper Specification for Insulating Press Board Guide forLoading of Oil Immersed Transformers ASTM B-49 DIN 42531,23,3 BS 148, D-1473, D-1533- 1934 IEC Pub 296 DIN 42531 to 33 ASTM B - 233 IEC 60137 Refer Appendix II of the Manual for List of Standards. 3.0 SERVICE CONDITIONS 3.1 The Distribution Transformers to be supplied against this Specification shall be suitable for satisfactory continuous operation under the climatic conditions prevailing at site and to be specified by the purchaser as follows as per IS 2026 (Part-I) Latest Revision/International Standards tabulated above : (i) (ii) (iii) (iv) (v) (vi) 4.0 Location Max ambient air temperature (Deg.C) Min. ambient air temperature (Deg.C.) Max average daily ambient air temperature (Deg.C) Max. yearly weighted average ambient temperature (Deg.C) Max. altitude above mean sea level (m) TAPPINGS AND TAP CHANGING : : : ---------- : ---- : : ------- 4.1 Tappings shall be provided on the higher voltage winding for variation of HV Voltage from plus 5% to minus 10% in steps of 2.5%. However no. of taps & steps to be decided as per requirement of the customer. Preference shall be given for off-circuit tap link arrangement. Tap change arrangement is not preferred in an ideal case. 4.2 Tap changing, if at all provided shall be carried out by means of an off circuit externally operated self positioning tap switch when the transformer is in de-energised condition. Switch position No. 1 shall correspond to the maximum voltage tapping. Each tap change shall result in variation of 2.5% in voltage. Provision shall be made for locking the tap changing switch handle in position. Specifications for Three Phase Distribution Transformers (above 100 kVA and upto 33 kV class) 75 4.3 For ratings greater than 500 kVA On-load tapchanger may be provided for variation of HV voltage from plus 5% to minus 15% in steps of 1.25%. 5.0 TYPE OF COOLING The transformers shall be oil immersed with natural oil circulation type-ONAN. 6.0 7.0 INSULATION LEVELS Voltage Impulse voltage (kV Peak) Power frequency (kV) 433 — 3 11000 75 28 33000 170 70 WINDING CONNECTIONS HV------Delta. LV------Star Vector Symbol.......Dyn11 8.0 LOSSES AND IMPEDANCE Losses and Impedance shall be guided as per recommendations of BEE star rating plan. Note: 1. Note: The losses mentioned above are under discussion and finalisation by BIS to revise IS:1180. The above losses eventually may get revised if any, after said revision. 2. For 22/0.433 kV transformers losses of33/0.433 kV shall be applicable. 76 Manual on Transformers 9.0 TECHNICAL REQUIREMENTS 9.1 Core 9.1.1 Material - CRGO The core shall be stacked type generally of high grade cold rolled grain annealed steel lamination having low loss and good grain properties, coated with hot oil proof insulation, bolted together and to the frames firmly to prevent vibration or noise. The complete design of core must ensure permanency of the core losses with continuous working of the transformers. 9.1.2 Core Clamping for CRGO Stacked Core MS channel or plate shall be used on top and bottom. Channel frames on LV side to be reinforced at equidistance, if holes / cutting is done for LT lead in order to avoid bending of channel. MS channels/plate frames shall be painted with hot oil-resistant varnish or paint. 9.1.3 The transformers core shall be suitable for over fluxing (due to combined effect of voltage and frequency) upto 10% without injurious heating at full load conditions and shall not get saturated. 9.1.4 No load current shall not exceed 1% of full load current for all ratings covered under this section Increase in secondary voltage of 433 volts by 10% shall not increase the no load current beyond 6% of full load current for ratings below 315 kVA and 4% of full load current for ratings above 315 kVA. 9.2 Windings 9.2.1 Material: Super enamel covered/Double paper covered (DPC) Copper / Aluminium round/strip conductor. 9.2.2 LV winding shall be of strip type copper / Aluminium conductor or copper / aluminium foil type. 9.2.3 HV coil is wound over LV coil as crossover coils or continuous disc coils. The choice of copper / aluminium as winding material should be left to customer at the time of tendering. 9.2.4 Inter layer insulation shall be Kraft paper/Epoxy dotted paper. Proper bonding of inner layer insulation with the conductor shall be ensured. Specifications for Three Phase Distribution Transformers (above 100 kVA and upto 33 kV class) 9.3 77 Oil The insulating oil shall comply with the requirements of relevant standards IS 335 / IEC:60296 9.4 Temperature Rise The temperature rise over ambient of 50 deg C shall not exceed the limits described below: Top oil temperature rise measured by thermometer Winding temperature rise measured by resistance 9.5 : : 35 Deg.C 40 Deg.C Insulation Material Material: Electrical grade insulation Kraft paper. All spacers, axial wedges / runners used in windings shall be made of pre-compressed Pressboard— solid, conforming to type B 3.1 of IEC 641-3-2. In case of cross-over coil/continuous disc winding of HV all spacers shall be properly sheared and dovetail punched to ensure proper locking. All axial wedges / runners shall be properly milled to dovetail shape so that they pass through the designed spacers freely. Insulation shearing, cutting, milling and punching operations shall be carried out in such a way, that there is no burr or dimensional variations. 10.0 TANK 10.1 Rectangular Plain Tank The transformer tank shall be of robust construction rectangular in shape and shall be built up of tested MS sheet. The internal clearance of tank shall be such that it shall facilitate easy lifting of core with coils from the tank without dismantling LV bushings. All joints of tank and fittings shall be oil tight and no bulging should occur during service. The tank design shall be such that the core and windings can be lifted freely. The tank plate shall be of such strength that the complete transformers when filled with oil may be lifted bodily by means of lifting lugs. Inside of tank shall be painted with varnish / hot oil resistant paint. The four walls of the tank shall be made of two “L” shaped sheets (without joints) fully welded at the corners from inside and outside of the tank for withstanding a pressure of 0.8 kg/cm2 for 10 minutes. The tank shall be reinforced by welded angle on all the outside walls on the edge of the tank to form two equal compartments. Permanent deflection when the tank without oil is subject to a vacuum of 525 mm of mercury for rectangular tank and 760 mm of mercury for round tank shall not be more than 5 mm upto 750 mm length and 6 mm upto 1250 mm length. The tank shall further be capable of withstanding a pressure of 0.8 kg/sq cm (g) and a vacuum of 0.3 kg/sq cm (g) without any deformation. 78 Manual on Transformers Only Pressed steel radiators shall be used for cooling. The transformer shall be capable of giving continuous rated output without exceeding the specified temperature rise. 4 Nos. welded heavy duty lifting lugs of MS plate 8 mm thick (min) suitably reinforced by vertical supporting flat welded edgewise below the lug shall be provided on the side wall. Top cover fixing shall be with galvanised iron bolts and 6 mm Synthetic resin bonded cork sheet gasket (type RC 70C) conforming to IS 4253 part-II shall be placed between tank and cover. The bolts outside tank shall have 2 flat washers & one spring washer. 10.2 Corrugated Tank The transformer tank shall be of robust construction corrugated in shape and shall be built up of CRCA sheets of thickness. Thickness of corrugated CRCA sheet shall be decided by manufacturer & customer jointly. The internal clearance of tank shall be such that, it shall facilitate easy lifting of core with coils from the tank with CCA (core-coil assembly), HV & LV bushings mounted on Top cover. All joints of tank and fittings shall be oil tight and no bulging should occur during service. The tank design shall be such that the core and windings can be lifted freely. The tank plate shall be of such strength that the complete transformers when filled with oil may be lifted bodily by means of lifting lugs. Inside of tank shall be painted with varnish / hot oil resistant paint. Corrugated panel shall be used for cooling. The transformer shall be capable of giving continuous rated output without exceeding the specified temperature rise. 2 Nos. welded heavy duty lifting lugs of MS plate 8 mm thick (min) suitably reinforced shall be provided. Top cover fixing shall be with GI (Galvanised Iron) bolts and 6 mm Synthetic resin bonded cork sheet gasket (type RC 70C) bonded cork gaskets conforming to IS 4253 part-II shall be placed between tank and cover. The bolts outside tank shall have 2 flat washers & one spring washer. Tanks with corrugations and without conservator shall be tested for leakage test at a pressure of 0.15kg/cm2 measured at the top of the tank. 10.3 Sealed Transformer with Radiators In this type of construction, tank is designed to have cover welded to the curb of tank. Space is provided above the core coil assembly where inert gas cushion system accommodates the oil expansion under variable pressure. The tank should be of stiff construction, able to withstand pressure of 2 atmospheres. 10.4 Conservator The provision of conservator is obligatory for plain Tanks mentioned in clause 10.1 above Specifications for Three Phase Distribution Transformers (above 100 kVA and upto 33 kV class) 79 i.e., where pressed steel radiators are used for oil circulation. Conservator is not required for corrugated tanks. When a conservator is provided, oil gauge and the plain or dehy drating breathing devise shall be fixed to the conservator, which shall also be provided with a drain plug and a filling hole (M30 normal size thread) with cover. The capacity of a conservator tank shall be designed keeping in view the total quantity of oil and its contraction and expansion due to temperature variations. In addition, the cover of main tank shall be provided with an air release plug to enable air trapped within to be released, unless the conservator is so located as to eliminate the possibility of air being trapped within the main tank. The inside diameter of the pipe, connecting the conservator to the main tank, should be within 20 to 50 mm and it should be projected into the conservator so that its end is approximately 20 mm above the bottom of the conservator so as to create a sump forcollection of impurities. The minimum oil level (corresponding to -5 deg.C.) should be above the sump level. 10.5 Surface Preparation and Painting For surface preparation refer to section A of this Manual. 11.0 TERMINALS The terminals arrangement alternatives are given below: kVA Voltage Details of Terminals All ratings 11000 17.5 kV porcelain Bushings as per IS 3347 for normally polluted atmosphere, or 3p 1G air or compound filled cable box suitable for 3core XLPE /PILC aluminium cables. All ratings 33000 36 kV porcelain Bushings as per IS3347 for normally polluted atmosphere, or 3p 1G air or compound filled cable box suitable for 3core XLPE /PILC aluminium cables. 160/200 433 4plG air filled cable box suitable for3 l/2 core 135mm2 PVC aluminium cable 250 315, 400, 500 433 4plG air filled cable box suitable for3 l/2core 400mm2 PVC aluminium cable 433 4p2G air filled cable box suitable for3 l/2core 400mm2 PVC aluminium cable 630.800 433 4p4G air filled cable box suitable for3 l/2core 400mm2 PVC aluminium cable 1000,1250 433 4p6G air filled cable box suitable for3 l/2core 400mm2 PVC aluminium cable 1600 433 4p21G air filled cable box suitable for 1 core 1000mm2 PVC aluminium cable 2000, 2500 3000 433 4p28G air filled cable box suitable for 1 core 1000mm2 PVC aluminium cable Note: (a) Alternatively 433V terminal could be provided with 1.1 kV bushings as per 1S.3347 for normally polluted atmosphere. 80 Manual on Transformers (b) Alternatively 433 V terminal could be provided with 1.1 kV bushing suitable for bus duct connections. (c) Alternatively 433 V terminal could be provided with1.1 kV epoxy bushings in cable box or bus duct. (d) P & G denote ‘Pole’ and ‘Gland’ respectively. (e) Epoxy may be used as the filling medium instead of bitumen compound. (i) The bushings shall conform to the relevant standards specified and shall be outdoor type. The bushing rods and nuts shall be made of brass material 12 mm diameter for both HT & LT. The bushings shall be fixed to the transformers on side with straight pockets and in the same plane or on the top cover. The tests as per IS 2099 / IS 7421 shall be conducted on the transformer bushings. (ii) For 0.433/11 kV/33 kV service voltage, 1.1/17.5/36 kV class bushings shall be used. Bushings of plain sheds as per IS-3347 shall be mounted on the tank/cover. For 1.1 kV class indoor transformers, 1.1 kV class epoxy busbar bushings /porcelain bushings can be used. Bushings in HV cable box as per BS2562 may be used for compound filled cable box or termination in air with boots covering the live terminals. (iii) Dimensions of the bushings shall conform to the Standards specified. (iv) A minimum phase to phase clearance of 75 mm for LV (upto 1.1 kV bushings) and 255 mm for HV bushings shall be obtained with the bushings mounted on the transformer. (v) The bushings shall be fixed on the sides with pockets in the same plane or on the top cover. (vi) Brazing of all inter connections, jumpers from winding to bushing shall have cross-section larger than the winding conductor. (vii) The design of the cable box internal bushing for LV shall be such as to provide adequate earth clearance and creepage distance as stipulated in the standards specified. All other tests as per relevant standards shall be applicable. (viii) The terminal arrangement shall not require a separate oil chamber not connected to oil in the main tank. (ix) HV and LV bushings shall be mounted on top cover in case of corrugated tank. 11.1 Terminal Clearances The minimum clearance shall be as under: Voltage Medium Clearance phase to phase (mm) terminal chamber Open Closed Clearance phase to earth (mm) terminal chamber Open Closed 433 Air 40 25 40 20 11000 Air *Compound 280 165 140 102 — 75 — 60 351 351 320 222 — 125 — 100 33000 Air *Compound * Clearances given against compound filled cable box are applicable for same cable box in air if terminals insulated with boots are used and cable is ofXLPE type. Specifications for Three Phase Distribution Transformers (above 100 kVA and upto 33 kV class) 81 For outdoor bare bushings the LV and HV bushing stems shall be provided with suitable terminal connectors so as to connect the jumper without disturbing the bushing stem. 11.2 Terminal Markings High voltage and low voltage phase windings shall be marked both in the terminal boards inside the tank and on the outside with capital letter 1U, IV, 1W and low voltage winding for the same phase marked by corresponding letter 2U, 2V, 2W. The neutral point terminal shall be indicated by the letter 2N. 11.3 Fittings to be provided The following fittings shall be provided for transformers with conservator: (a) (b) Rating and terminal marking plates. Two earthing terminals (studs and bolts should be properly galvanized and conform to IS: 1363 and IS: 1367. (c) Two lifting lugs to lift core assembly. (d) Two lifting lugs to lift complete transformer (e) Lifting lugs for tank cover. (f) Thermometer pocket in accordance with IS: 3580. (g) Air release plug on the transformer tank to release air trapped inside the tank when filling oil through conservator. (h) Conservator tank shall have inter connection pipe projection, 20 mm above bottom of the conservator so as to create a sump for collection of impurities. It shall have 30 mm dia drain valve, oil filling hole with cap on the top of the conservator. (i) Oil level gauge with toughened glass with “minimum” marking. (j) De-hydrating breather. (k) One drain cum sampling valve. (1) One filter valve on the upper side of the tank (m) Unidirectional flat rollers. (n) Inspection hole For sealed transformers with radiators and nitrogen cushion, the following accessories are recommended: (a) (b) (c) (d) (c) 11.4 Oil level guage Pressure guage Oil temperature indicator and winding temperature indicator (optional). One drain cum sampling valve One filter valve on upper side of tank. Fasteners Bolts or studs shall not be less than 6 mm in diameter except when used for small wiring terminals. 82 Manual on Transformers Wherever possible, bolts shall be fitted in such a manner that in the event of failure of locking, resulting in the nuts working loose and falling off, the bolt will remain in position. All ferrous bolts, nuts and washers placed in outdoor positions shall be treated to prevent corrosion, by hot dip galvanizing, except high tensile steel bolts and spring washers which shall be electro-galvanized/ plated. Appropriate precautions shall be taken to prevent electrolytic action between dissimilar metals. Each bolt or stud shall project at least one thread but not more than three threads through the nut, except when otherwise approved for terminal board studs or relay stems. If bolts, nuts are placed so that they are inaccessible by means of ordinary spanners, special spanners shall be provided. Taper washers shall be provided where necessary. SECTION E Specifications for Power Transformers of Voltage Class Below 145 kV SECTION E Specifications for Power Transformers of Voltage Class Below 145 kV 1.0 SCOPE 1.1 This section covers technical requirements/parameters for power transformers for voltage below 132 kV. This part specification does not purport to include all the necessary provisions of a contract. For general requirements, loss capitalization and tests, reference shall be made to Section ‘A’, ‘AA’ and ‘BB’ respectively of this Manual. For 110 kV / 100 kV class transformers reference can be drawn from section F for appropriate rating of transformer. 1.2 Standard Rating 1.2.1 66 kV Class Transformers Three-phase power rating MVA Voltage ratio kV Cooling 6.3 66/11 ONAN 8.0 66/11 ONAN 10.0 66/11 ONAN 12.5 66/11 ONAN/ONAF 20.0 66/11 ONAN/ONAF ONAN rating-60 percent of ONAF. 1.2.2 33 kV Class Transformers Three-phase power rating MVA Voltage ratio kV Cooling 1.0 33/11 ONAN 1.6 33/11 ONAN 3.15 33/11 ONAN 4.0 33/11 ONAN 5.0 33/11 ONAN 6.3 33/11 ONAN 8.0 33/11 ONAN 10.00 33/11 ONAN 85 86 Manual on Transformers 1.2.3 11 kV Class Transformers Rating MVA (A) (B) Voltage ratio kV Impedance voltage percent Cooling 3.15 11/6.6 6.25 ONAN 4 11/6.6 7.15 ONAN 3.15 11/3.3 6.25 ONAN 4 11/3.3 7.15 ONAN 5 11/3.3 7.15 ONAN 6.3 11/3.3 7.15 ONAN 2.0 WINDING CONNECTIONS AND VECTOR GROUP 2.1 Transformers of 11 kV and 33 kV Class 2.2 HV — Delta LV — Star Vector Group — Dy n11 Transformers of 66 kV Class HV — Star or Delta LV — Star Vector Group — YNyn0 or Dyn 11 Note: No Tertiary Winding is required for 66 kV Class Transformers. 3.0 TAPPINGS 3.1 OLTC is not recommended for 11 kV and below 5 MVA. For other kV class transformers, this may be provided for higher ratings, if required. In case of off-circuit tap changer, the tappings shall be such as to provide for a voltage adjustment on the high voltage of + 3 percent to - 9 percent in steps of 3 percent, the tappings being located on the high voltage winding. In case of on-load tap changer, the tappings shall also be on the high voltage winding. A voltage adjustment of high voltage of + 5 to - 15 percent in 16 equal steps is recommended. With transformers having OLTC, these tappings may be used to get 10 percent over-voltage on low voltage ‑side at no-load. When under this condition the high voltage side experiences Specifications for Power Transformers of Voltage Class below 145 kV 87 an over-voltage, the tappings shall be changed so that the over-excitation is limited to 10 percent only. 4.0 INSULATION LEVELS Highest Voltage for equipment kV rms Rated lighting impulse with-stand voltage kV peak Rated Short duration power frequency withstand voltage kVrms 3.6 7.2 12 36 72.5 40 60 75 170 325 10 20 28 70 140 Note: (i) 66 kV Windings should be with graded insulation. (ii) Some utilities are specifying higher lightning impulse level of 350 kVp for 72.5 kV non-uniform winding. 4.1 Clearances of Line Terminals in Air The minimum clearances in air between live conductive parts and conductive parts to earthed structure shall be as follows: 5.0 Minimum clearances Highest System Voltage Basic Insulation level kV kV peak Phase to Phase (mm) Phase to Earth (mm) 12 75 280 140 24 125 330 230 36 170 350 320 52 250 530 480 72.5 325 700 660 145 550 1220 1050 TEMPERATURE RISE For the purpose of standardization of maximum temperature rises of oil and windings, the following ambient temperatures are assumed: Cooling medium Maximum ambient temperature Maximum daily average ambient temperature Maximum yearly weighted average temperature : Air : 50°C : 40°C : 32°C 88 Manual on Transformers With the above ambient temperature condition the temperature rises are as given below: 6.0 • • • • • 7.0 (a) (b) (c) (d) (e) (f) (g) (h) (i) (j) (k) (1) (m) (n) (o) Oil oC Winding oC 50 55 TERMINALS 3.3 kV-3.6 kV porcelain bushings with plain sheds as per IS: 3347. 6.6 and 11 kV-17.5 kV porcelain bushings with plain sheds as per IS: 3347. 33 kV-36 kV porcelain bushings with plain sheds as per IS: 3347. 66 kV-72.5 kV condenser bushings as per section II. Transformers shall be fitted either with bushing insulators or cable boxes as required by the purchaser. FITTINGS AND ACCESSORIES Rating and diagram plate. 2 Nos. earthing terminals. Cover lifting lugs. Lifting lugs. Skids and pulling eyes on both directions. Oil-filling hole and cap. Jacking pads. Pocket on tank cover for thermometer. Air release devices. Conservator with oil filling hole, cap and drain plug-size 19 mm nominal pipe (3/4 in. BSJ/M 20). (i) Prismatic oil level gauge for all transformers up to and including 1.6 MVA. (ii) Magnetic type oil gauge for transformers above 1.6 MVA, with low oil level alarm contact. Silica gel breather with oil seal. Pressure relief device (16 MVA & above) Valves: (i) Drain valve with plug or blanking flanges. The same can be used for filtering purpose. (ii) A sampling valve. (iii) 1 No. Top & 1 No. Bottom Filter Valve. Buchholz relay with alarm and trip contacts with one shut-off valve on conservator side (i) Size of Buchholz relay up to 10 MVA-50 mm (ii) 10 MVA and above-80 mm Specifications for Power Transformers of Voltage Class below 145 kV 89 (p) Oil temperature indicator with one electrical contact shall be provided with anti-vibration mounting. (q) Winding temperature indicator with two electrical contacts for alarm and trip purposes. Switching of fans shall be done by winding temperature indicator for all transformers having ONAF rating. The winding temperature indicator shall be provided with antivibration mounting. Tank mounted weather-proof marshalling box for housing control equipment and terminal connectors. Wiring up to marshalling box with PVC SWA PVC copper cables 660/1100 volts grade. (r) (s) Air cell (7.5 MVA & above) (t) Rollers-4 Nos. Sl. No. (v) Rating Type Gauge Shorter axis Longer axis 1 Up to 5 MVA Flat, uni-directional As per manufacturer’s practice, however, not to exceed 1000 mm 2 6.3 MVA Flanged, bi-directional 1435 mm 1435 mm 3 10 MVA and above Flanged, bi-directional 1676 mm 1676 mm Cooling accessories ONAN/ONAF cooling (i) Radiators with shut-off valves and air release plugs. (ii) Fans. (iii) Filter valves. (iv) Drain and sampling device. (v) Air release device. SECTION F Specifications for 145 kV Class Power Transformers SECTION F Specifications for 145 kV Class Power Transformers 1.0 SCOPE 1.1 This section covers technical requirements/parameters for power transformers of 145 kV class. This part specification does not purport to include all the necessary provisions of a contract. For general requirements, loss capitalization and tests, reference shall be made to Sections ‘A’, ‘AA’ and ‘BB’ respectively of this Manual. For 100/110 kV class transformers, reference can be drawn from this section for appropriate rating of the transformers. 1.2 Standard Ratings 1.2.1 Two Winding Transformers (i) (ii) (iii) Three-phase power rating MVA Voltage ratio kV Impedance voltage per cent Cooling (a) 16 132/33 10 ONAN/ONAF 16 132/33 10 ONAN/ONAF 16 132/33 12.5 ONAN/ONAF (b) 16 132/11 10 ONAN/ONAF 25 132/11 10 ONAN/ONAF 31.5 132/11 12.5 ONAN/ONAF Connections : HV-Star with neutral directly earthed LV-Star with neutral directly earthed Vector Group YNyn0 : Alternatively HV-Star with neutral directly earthed LV-Delta Vector Group : YNd 11 Tappings On-load tappings at the neutral end of HV winding for : HV variation. Tapping Range + 5 to -15 per cent in 16 equal steps. : ONAN rating for 16 MVA, 25 MVA and 31.5 MVA transformers shall be 10, 16 and 20 MVA respectively. 93 94 Manual on Transformers 1.2.2 Interconnecting Auto-Transformers Three-phase power rating MVA 50 63 (i) Connections Vector Group Note : : : Voltage ratio kV 132/66 132/66 Impedance voltage per cent 10 10 Cooling ONAN/ONAF ONAN/ONAF HV and LV Star auto with neutral directly earthed. YNao (i) No stabilizing winding up to 100 MVA for 3-Phase, 3 limbed core type construction. ii) Tappings: On-load for the variation of 66 kV voltages from - 5 to + 15 per cent in 16 steps. iii) ONAN rating shall be 60 per cent of ONAF rating. 1.2.3 Generator Transformers Three-Phase Power Rating MVA Voltage ratio Impedance voltage per cent Cooling 140 11/138 12.5 ONAN/ONAF or ONAN/OFAF or ODAF or OFWF/ODWF 140 13.8/138 12.5 -do- 250 15.75/138 14.5 ONAN/ONAF or ONAN/OFAF or ODAF or OFWF/ODWF (i) Connection : HV-Star with neutral directly earthed LV-Delta Vector Group : YNd11/ YNd1/ YNd7 /YNd5 (As per User’s requirement) (ii) Tappings : Off-circuit taps on HV for HV variation from +2½ to -7½ percent in 2½ per cent steps or On-load tap changer on HV for HV variation from + 5 per cent to -10 per cent in 1.25 per cent steps. (iii) ONAN rating in case of ONAN/ONAF, ONAN/OFAF, cooling shall be 60 per cent of OFAF rating. OFAF/ODAF = 100 per cent. (iv) The standardized ratings are for three-phase units only. If single phase units are required due to transport limitations, then these ratings will be one-third of the three-phase unit. 95 Specifications for 145 kV Class Power Transformers 2.0 * INSULATION LEVELS Highest voltage for equipment kVrms Rated lightning impulse with- stand voltage kV peak Rated short duration power frequency withstand voltage kVrms 12 75 28 17.5 95 38 36 170 70 72.5 325* 140 145 550** 230** Some utilities specify lightning impulse level of 350 kVp ** Some utilities specify 650 kVp lightning impulse and 275 kVrms power frequency voltage level. 2.1 Clearances of Line Terminals in Air The minimum clearances in air between live conductive parts and conductive parts to earthed structure shall be as follows: 3.0 Highest System Voltage Basic Insulation level kV kV peak Phase to phase (mm) Phase to earth (mm) 12 75 280 140 24 125 330 230 36 170 350 320 52 250 530 480 72.5 325 700 660 145 550 1220 1050 145 650 1430 1270 COOLING EQUIPMENT (a) ONAN/ONAF (b) Minimum clearances 1-100 per cent tank or separately mounted cooling system consisting of radiators and fans and one standby fan 2-50 per cent group and 2 standby fans, one in each 50 per cent group ONAN/OFAF or ODAF 2-50 per cent groups 2-100 per cent pumps per group, one of which will be standby for each 50 per cent bank 2-standby fans one in each 50 per cent group 96 Manual on Transformers 3-50 per cent group with independent pump and fans out of which one group to act as standby, (c) 2-100 per cent heat exchangers out of which one is standby. 4.0 OFWF or ODWF TEMPERATURE RISE For the purpose of standardization of maximum temperature rises of oil and winding the following ambient temperatures considering the transformer to be operating at extreme tap position incurring extra copper losses Cooling medium Air Maximum ambient temperature Maximum daily average ambient temp. Maximum yearly weighted average temp. 50°C 40°C 32°C Water 30°C 25°C – With the above ambient temperature conditions temperature rises considering the transformer to be operating at extreme tap position incurring extra copper losses are as given below : External Cooling Medium Part 5.0 Air Water Winding (measured by resistance) °C 55. when the oil circulation is natural or forced nondirected. 60, when the oil circulation is forced directed. 60, when the oil circulation is natural or forced non-directed. 65, when the oil circulation is forced directed. Top oil (measured by thermometer) °C 50 55 TERMINAL BUSHINGS (a) Two-winding and auto-transformers The terminal bushings shall be as per Section II of the manual. (b) Generator transformer LV side : LV bushings shall be mounted on turrets suitable for connection to busbars in isolated phase bus ducts. HV side : As per Section II. 6.0 FITTINGS AND ACCESSORIES (a) (b) (c) (d) (e) (f) (g) Rating and diagram plate. 2 Nos. earthing terminals. Lifting bollards. Jacking pads. Haulage lugs. Pocket on tank cover for thermometer. Air release devices. 97 Specifications for 145 kV Class Power Transformers (h) (i) (j) (k) Conservator with oil filling hole, cap and drain valve, aircell (above 7.5 MVA). Magnetic type oil level gauge with low oil level alarm contacts of 0.5 A, 220 V DC rating. Silica gel breather with oil seal-2 Nos. of 100 per cent for 140 and 250 MVA ratings. Pressure relief device. (1) Valves (i) Oil valve between each cooler and main tank. (ii) Drain valve. (iii) 2 Nos. filter valves, one on top and another at bottom on diagonally opposite corners. (iv) 2 Nos. sampling valves at top and bottom of main tank. The sampling valve shall be provided with provision for fixing PVC pipe. (m) Valve schedule plate for transformers above 31.5 MVA. (n) Buchholz relay with alarm and trip contacts of 0.5A, 220 V DC rating and one shut-off valve size 80 mm. (o) (i) Oil temperature indicator with maximum-pointer and one electrical contact. (ii) Oil temperature indicator with maximum pointer and two sets of contacts for above 31.5 MVA. (p) Winding temperature indicator with ‘maximum pointer and 3 sets of contacts for ONAN/ ONAF and 4 sets of contacts for ONAF/OFAF or ODAF and 2 sets of contacts for OFWF/ ODWF. (q) Repeater dial of winding temperature indicator for remote indication for transformers above 16 MVA. For transformer above 50 MVA, the remote indication shall be a separate measuring system. (r) Rollers. Gauge Rating Type Shorter axis Longer axis 1. Two winding and auto transformers Flanged bi-directional with locking and bolting device. 1676 mm 1676 mm 2. Generator transformers Flanged, bi-directional with locking and bolting device. 2 rails with 1676mm gauge 2 rails with 1676mm gauge Alternatively 3 rails with 1676 mm gauge between adjacent rails. Alternatively 4 rails in two pairs with 1676 mm gauge for each pair and centre distance between pair 3486 mm. (s) Inspection cover. (t) Wiring up to marshalling box with PVC copper cables, 660/1100 volts grade. (u) Tank mounted/floor mounted weather-proof marshalling box for housing control equipment and terminal connections. (v) On-load tap changing gear with remote control panel as required. 98 Manual on Transformers (w) Cooling accessories: (I) ONAN/ONAF Cooling (i) Requisite number of radiators with top and bottom shut-off-valves, air release plug and drain plug. (ii) Fans. (iii) For header mounted radiator 2 Nos. valves, one at top header and other at bottom header to be used for filtration and oil filling. Bottom valve to be used as drain valve also. (iv) Drain and sampling device. (v) Air release device. (II) ONAN/ONAF-OFAF/ODAF Cooling (i) Requisite number of radiators with shut-off-valves. (ii) Fans. (iii) Oil pumps. (iv) Oil flow indicator with one alarm contact. (v) For header mounted radiators 2 Nos. valves, one at top header and other at bottom header to be used for filtration and oil filling. Bottom valve to be used as drain valve also. (vi) Drain-cum-filter valve for cooling system. (vii) Air release plug. (III) OFAF/ODAF Cooling (i) OFAF coolers with integral fans. (ii) Oil pumps. (iii) Oil flow indicator with one alarm contact. (iv) Brass encased thermometers. (v) Drain plug and air release devices. (TV) OFWF/ODWF Cooling. (i) Oil/Water heat exchangers. (ii) Oil pumps. (iii) Oil flow indicator with one alarm contact. (iv) Water flow indicator with one alarm contact. (v) Pressure gauges. (vi) Brass encased thermometers. (vii) Differential pressure gauge with one alarm contact. (viii) Reflux valves (Non-return). (ix) Drain-cum-filter valve for cooling system. SECTION G Specifications for 245 kV Class Power Transformers SECTION G Specifications for 245 kV Class Power Transformers 1.0 SCOPE 1.1 This section covers technical requirements/parameters for power transformers of 245 kV voltage class but does not purport to include all the necessary provisions of contract. For general requirements, loss capitalization and tests, reference shall be made to Sections ‘A’, ‘AA’ and ‘BB’ respectively of this Manual. 1.2 Standard Ratings 1.2.1 Two Winding Transformers (A) Three-phase power rating MVA Voltage ratio kV Impedance voltage percent Cooling 50 220/66 12.5 ONAN/OFAF or ONAN/ODAF 100 220/66 12.5 ONAN/OFAF or ONAN/ODAF (i) Connections : Stabilizing winding : HV—Star with neutral effectively earthed. LV—Star with neutral effectively earthed. (ii) Vector Group : (iii) Tapping : (iv) Cooling : YNyn0. On-load tappings at the neutral end of HV for HV ±10 per cent in 16 equal steps. ONAN : 60 per cent OFAF : 100 per cent ODAF : 100 per cent The rating under ONAF condition although not guaranteed should be about 80 percent. No stabilizing winding up to 100 MVA for 3-phase, 3 limbed core type construction. (B) Three-phase power rating MVA Voltage ratio kV Impedance voltage percent Cooling 50 220/33 12.5 ONAN/OFAF or ONAN/ODAF 100 220/33 15.0 ONAN/OFAF or ONAN/ODAF 101 102 Manual on Transformers (i) Connections : Vector Group : Stabilizing : (ii) Tappings : (iii) Cooling : HV—Star with neutral effectively earthed. LV—Delta. Alternatively star with neutral effectively earthed. Yd11. Alternatively YNyno. No stabilizing winding for YNyn0 up to 100 MVA for 3-phase, 3 limbed core type construction. On load tappings at the neutral end of HV for HV variation from +10 to -10 per cent in 16 equal steps. ONAN : 60 per cent OFAF : 100 per cent ODAF : 100 per cent Note : The rating under ONAF condition although not guaranteed shall be about 80 per cent. 1.2.2 Auto-Transformers Three-phase power rating MVA Voltage ratio Percentage impedance voltage 100 220/132 12.5 160 220/132/11 12.5 200 220/132/11 12.5 Cooling ONAN/OFAF or ONAN/ODAF ONAN/OFAF or ONAN/ODAF ONAN/OFAF or ONAN/ODAF ONAN/ONAF cooling can also be specified for above transformers. (i) Connections : HV and LV — Star auto with neutral effectively earthed. (ii) Stabilizing winding : Delta. (No stabilizing winding for 100 MVA 3 phase 3 limbed Core type construction) (iii) Vector Group : YNa0dl (iv) Tappings : On-load for the variation of 132 kV voltage from-5 to +15 per cent in 16 equal steps. (v) Cooling ONAN : 60 per cent OFAF/ODAF : 100 per cent. Note : (1) In case auto-transformers are provided with L. V. winding (Tertiary Winding) for loading purpose then the MVA rating, voltage rating, percentage impedance between HV winding to L. V. winding and IV winding to L.V. winding shall be specified by the customer. The minimum rated lightning impulse withstand voltage level shall be 170 kV peak. Rated short duration power frequency voltage shall be 70 kV. (2) Rating under ONAF condition although not guaranteed shall be about 80 per cent. 1.2.3 Generator Transformers for Thermal Stations Three-phase power rating MVA Voltage Ratio Percentage impedance voltage 140 11/235 12.5 140 13.8/235 12.5 250 15.75/235 14.0 315 15.75/235 14.0 Cooling ONAN/OFAF or ODAF or OFWF/ ODWF ONAN/OFAF or ODAF or OFWF ONAN/OFAF or ODAF or OFWF/ODWF -do- 103 Specifications for 245 kV Class Power Transformers (i) Connection : HV — Star with neutral effectively earthed. LV — Delta. (ii) Vector Group : YNd11/ YNd1/ YNd5/ YNd7 (As per User’s requirement) (iii) Tappings : Off-circuit taps on HV for HV variation from + 2½ to -7½ percent in 2½ per cent steps or On-load tap changer on HV for HV variation from + 5 percent to -10 per cent in 1.25 per cent steps. (iv) The standardized ratings are for three phase units only. If single phase units are required due to transport limitations then these ratings will be one-third of the three-phase unit. (v) Cooling : ONAN : 60 per cent OFAF/ODAF : 100 per cent. Two 50% cooling radiator banks shall be provided. Each bank shall have one stand by fan and one stand by pump. 2.0 INSULATION LEVELS Highest voltage for equipment kV rms Rated lightning impulse withstand voltage kV peak Power frequency rated short duration withstand voltage kV rms 12.0 75 28 17.5 95 38 36 170 70 72.5 325* 140 145 550** 230** 245 950*** 395*** * Some utilities specify lightning impulse level of 350 kVp ** Some utilities specify 650 kVp lightning impulse and 275 kVrms power frequency voltage level. *** Some utilities specify 1050 kVp lightning impulse and 460 kVrms power frequency voltage level. 2.1 Clearances of Line Terminals in Air The minimum clearances in air between live conductive parts and conductive parts to earthed structure shall be as follows: Highest System Voltage Basic Insulation level kV kV peak Phase to phase (mm) Phase to earth (mm) 12 24 36 52 72.5 145 145 245 245 75 125 170 250 325 550 650 950 1050 280 330 350 530 700 1220 1430 2000 2350 140 230 320 480 660 1050 1270 1800 2150 Minimum clearances 104 Manual on Transformers 3.0 COOLING EQUIPMENT (a) ONAN/OFAF - 2-50 per cent groups or ODAF 2-100 per cent pumps for each 50 per cent bank, One of which will be standby. 2-Standby fans one in each 50 per cent group, or 3-50 per cent groups with independent pumps and fans out of which one group to act as standby (b) 2-100 per cent heat exchangers out of which one is standby. OFWF or ODWF 4.0 TEMPERATURE RISE For the purpose of standardization of maximum temperature rises of oil and winding, the following ambient temperatures are assumed: Cooling medium Air Water Maximum ambient temperature 50°C 30°C Maximum daily average ambient temperature 40°C 25°C Maximum yearly weighted average temperature 32°C - With the above ambient temperature conditions, temperature rises are as given below: External cooling-medium Part Windings (measured by resistance) Top oil (measured) by thermometer) Air °C Water °C 55, when the oil circulation is 60, when the oil circulation is natural or forced non-directed. natural or forced non directed. 60, when the oil circulation is forced directed. 65, when the circulation is forced directed. 50 55 5.0 TERMINAL BUSHINGS (a) Two windings and Auto-Transformers. The terminal bushings shall be as per Section II. (b) Generator Transformer LV Side : LV bushings shall be mounted on turrets suitable for connection to bus bars in isolated phase bus ducts. HV Side : As per Section II. 105 Specifications for 245 kV Class Power Transformers 6.0 FITTINGS AND ACCESSORIES (a) (b) (c) (d) (e) (f) (g) (h) (i) (j) Rating and diagram plate. 2 Nos. earthing terminals. Lifting bollards. Jacking pads. Haulage lugs. Pocket on tank cover for thermometer. Air release devices. Conservator with oil filling hole, cap drain valve. Magnetic type oil gauge with low oil level alarm contacts of 0.5 A, 220 V DC. Silicagel breather with oil seal-2 Nos. Note: In addition to the silicagel breather more advanced oil preservation system like air dryers, molecular sieve flexible membrane can also be considered. (k) Required Nos. of pressure relief vents or spring operated pressure relief devices. For transformer above 50 MVA, the remote indication shall be a separate measuring system. (1) Valves (i) Oil valve between each cooler and main tank. (ii) Drain valve (iii) 2 Nos. filter valves on diagonally opposite corners. (iv) 2 Nos. sampling valves at top and bottom of main tank. The sampling valve shall be provided with provision for fixing PVC pipe. Rating Type Gauge Shorter axis 1 (m) (n) 1. 2 Two winding and auto-transformers 3 Flanged bidirectional with locking and bolting device. 2. Generator Transformers Flanged bidirectional With locking and Bolting device. Longer axis 4 5 1676 mm 1676 mm 2 rails with 1676 mm gauge 2 rails with 1676 mm gauge Alternatively 3 rails with 1676 mm gauge between adjacent rails. Alternatively 4 rails in two pairs with 1676 mm gauge for each pair and centre distance between pair 3486 mm. Valve schedule plate. Buchholz relay with alarm and trip contacts of 0.5A, 220 V DC and one shut-off valve on conservator side, size 80 mm. 106 (o) (p) (q) (r) (s) (t) (u) (v) (w) Manual on Transformers Oil temperature indicator with maximum-pointer and two sets of contacts. Winding temperature indicator with maximum pointer and 3 sets of contacts of ONAN/ ONAF and 4 sets of contacts for ONAF/OFAN or ODAF and 2 sets of contacts for OFWF/ ODWF. Repeater dial of winding temperature indicator for remote indication. Rollers. Inspection cover. Wiring up to marshalling box with PVC copper cables, 660/1100 volts grade. Tank mounted/floor mounted weather-proof marshalling box for housing control equipment and terminal connections. On-load tap changing gear with remote control panels as required. Cooling accessories. (I) ONAN/OFAF or ODAF Cooling (i) Requisite number of radiators with shut-off-valves. (ii) Fans. (iii) Oil pumps. (iv) Oil flow indicator with one alarm contact. (v) For header mounted radiators 2 Nos. valves, one at top header and other at bo tom header to be used for filtration and oil filling. Bottom valve to be used as drain valve also. (vi) Drain-cum-filter valve for cooling system size. (vii) Air release plug of size 19 mm nominal pipe (3/4in. BSP). (II) OFAF/ODAF Cooling (i) OFAF/Coolers with integral fans. (ii) Oil pumps. (iii) Oil flow indicator with one alarm contact. (iv) Brass encased thermometers. (v) Drain plug and air release devices. (III) OFWF/ODWF Cooling (i) Oil / Water heat exchangers (ii) Oil pumps. (iii) Oil flow indicator with one alarm contact. (iv) Water flow indicator with one alarm contact. (v) Pressure gauges. (vi) Brass encased thermometers. (vii) Differential pressure gauge with one alarm contact. (viii) Reflux valves (Non-return) (ix) Drain-cum-filter valve for cooling system. SECTION H Specifications for 420 kV Class Power Transformers SECTION H Specifications for 420 kV Class Power Transformers 1.0 SCOPE 1.1 This section covers technical requirements/parameters for power transformers of 420 kV class but does not purport to include all the necessary provisions of a contract. For general requirements, loss capitalization and tests reference shall be made to Sections ‘A’, ‘AA’ & ‘BB’ respectively of this Manual. 1.2 Standard Ratings 1.2.1 Generator Transformers Three phase rating MVA 250 or 315 600 or 780 or 945MVA Bank Voltage ratio Tapping range per cent 15.75 A-Off circuit taps or 16.5 + 2.5% to- 7.5% /420 B-On-load taps + 5% to -10% 20 to 27 kV /420 A-Off circuit taps B - On load taps Percent impedance voltage Cooling 14.5 ONAN/OFAF or OFAF or OFWF or ONAN/ ODAF or ODAF or ODWF 13.5 to 16.0 ONAN/ OFAF or OFAF or OFWF or ONAN/ ODAF or ODAF or ODWF Note : The ratings of generator transformers for hydro generating sets have not been standardized as the sizes of these sets depend upon site characteristics. The purchaser shall specify the type of cooling required. Other Parameters (i) Connections HV Star neutral effectively earthed, LV delta (ii) Connections symbol YNd11/YNd1/ YNd5/ YNd7 (As per User’s requirement) (iii) Tappings Full power tappings on HV winding for HV voltage variation. Tap changing shall be by: (a) Off-circuit tap changer, tapping range + 2.5% to - 7.5% in steps of 2.5 per cent alternatively (b) On-load tap changer, tapping range + 5% to - 10% in steps of 1.25 per cent 109 110 Manual on Transformers (iv) Three-phase rating should be understood as three phase bank rating and not necessarily three-phase unit rating. Wherever transport restrictions impose, single phase ratings equal to one-third of the three phase bank rating may be specified. (v) ONAN rating shall be guaranteed at 60 per cent of the OFAF, or ODAF rating. Rating under ONAF condition shall be about 80 per cent. Two 50% cooling radiator banks shall be provided. Each bank shall have one stand by fan and one stand by pump. (vi) Air core reactance of HV winding shall not be less than 20 per cent and knee point voltage shall not be less than 1.1 pu. (vii) Short circuit level Transformer shall be suitable for connection to for 420 kV system the system having the following short circuit and duration levels and duration: 40,50 and 63 kA for one second. (viii) Terminal Bushings (a) LV Terminals: Oil - sealed / Oil communicating Condenser type bushings mounted on turrets suitable for connections to busbars in isolated phase busducts which shall have spacing of 1250 mm for 250 MVA three-phase unit and 1500 mm for each 200 MVA single-phase unit of a 600 MVA three-phase bank. (b) HV Terminals-Line End: 420 kV oil filled condenser bushing. No arcing horns shall be provided. For details refer Section II. Neutral End: 17.5 kV porcelain bushing. No arcing horns shall be provided. 1.2.2 Auto - Transformers The purchaser may specify auto-transformer with constant ohmic value of impedance or constant percentage impedance as given below: Standard Ratings (a) Auto - Transformers (Constant Percentage Impedance) Three-phase Voltage ratio Tapping range Percent impedance voltage 100/100/33.3 400/132/33 + 10% to-10% 16 steps of 1.25% HV-1V HV-LV IV-LV 12.5 27 12 ONAN/OFAF 200/200/66.7 400/132/33 + 10% to -10% 16 steps of 1.25% 12.5 36 22 ONAN/OFAF or ONAN/ODAF 250/250/83.3 400/220/33 + 10% to-10% 16 steps of 1.25% 12.5 45 30 ONAN/OFAF or ONAN/ODAF 315/315/105 400/220/33 + 10% to-10% 16 steps of 1.25% 12.5 45 30 ONAN/OFAF or ONAN/ODAF 500/500/166.7 400/220/33 + 10% to-10% 16 steps of 1.25% 12.5 45 30 ONAN/OFAF or ONAN/ODAF 630/630/210 400/220/33 + 10% to-10% 16 steps of 1.25% 12.5 45 30 ONAN/OFAF or ONAN/ODAF HV/IV/LV Cooling per cent MVA 111 Specifications for 420 kV Class Power Transformers (b) Auto-Transformers (Constant Ohmic Impedance) Three-Phase HV/ IV/LV Voltage Ratio Tapping Range per cent MVA Per cent impedance voltage Cooling HV-IV HV-LV IV-LV (min)* 100/100/33.3 400/132/33 + 10 to-10% 16 steps of 1.25% 12.5 45 30 ONAN/OFAF ONAN/OFAF 200/200/66.7 400/132/33 + 10 to-10% 16 steps of 1.25% 12.5 45 30 ONAN/ODAF ONAN/OFAF 250/250/83.3 400/220/33 + 10 to-10% 16 steps of 1.25% 12.5 60 45 ONAN/ODAF ONAN/OFAF 315/315/105 400/220/33 + 10 to-10% 16 steps of 1.35% 12.5 60 45 ONAN/ODAF ONAN/OFAF 500/500/166.7 400/220/33 + 10 to - 10% 16 steps of 1.25% 12.5 60 45 ONAN/ODAF ONAN/OFAF 630/630/210 400/220/33 + 10 to-10% 16 steps of 1.25% 12.5 60 45 ONAN/ODAF * No limit is specified on higher side. ONAN/ONAF cooling can also be specified for 100,200,260 1nd 315 MVA ratings. Other Parameters (i) Connections HV Star auto with neutral IV effectively earthed LV Delta (ii) Connection symbol YNa0d11 (iii) Full power tappings shall be provided on series winding for the variation of voltage on HV side. The tap changers shall be suitable for bi-directional flow of rated power. The tap changers shall be in accordance with Section GG. (iv) Three-phase rating should be understood as three phase bank rating and not necessarily three phase unit rating. Wherever transport restrictions impose, single phase ratings equal to one third of the three phase bank ratings may be specified. (v) ONAN rating shall be guaranteed at 60 per cent of the OFAF or ODAF rating. Rating under ONAF condition although not guaranteed shall be about 80 per cent. (vi) For these transformers the temperature rise of the top oil refers to the specified loading combination for which the total losses are highest. Individual winding temperature rises shall be considered relative to that specified loading combination which is the most severe for the particular winding under consideration. (vii) Air core reactance of HV winding shall not be less than 20 per cent and knee point voltage shall not be less than 1.1 pu. (viii) The specified percentage impedance voltage is at principal tapping and on the MVA base 112 Manual on Transformers corresponding to HV/TV rating. Tolerance on percentage impedance voltage shall be as under: Pairs of windings HV-IV (normal tap) HV-IV (max & min tap) Tolerance ±10 per cent ±15 per cent For constant percentage impedance auto-transformer HV-LV IV-LV ±15 percent ± 15 per cent 1.2.3 Short Circuit Level Transformer shall be suitable for connection to the system having the following short circuit level: • 420 kV 40,50 and 63 kA (rms), 1 second • 245 kV 40 kA (rms), 1 second • 145 kV 31.5/40 kA (rms), 1 second 1.2.4 Terminals (a) LV Terminals: 52 kV oil-filled condenser bushings. The bushing shall be arranged in a line with 1000 mm spacing to allow mounting of phase to phase barriers. No arcing horns shall be provided. (b) IV Terminals: 145/245 kV oil-filled condenser bushings with test taps. No arcing horns shall be provided. (c) HV Terminals - Line End: 420 kV oil-filled condenser bushing. No arcing horns shall be provided. For details refer Section II. Neutral End: 17.5 kV porcelain bushing. No arcing horns shall be provided. 1.2.5 Insulation Levels 1.2.5.1 Lightning Impulse And Power Frequency Voltage Test Level Highest voltage for equipment kV (rms) Rated lightning impulse withstand voltage kV (peak) 12 75 17.5 95 24 125 145 550* 245 950** 420 1300*** * Some utilities specify lightning impulse level of 650 kVp ** Some utilities specify lightning impulse level of 1050 kVp. *** Some utilities specify lightning impulse level of 1425 kVp. Rated power frequency, short duration withstand voltage kV (rms) 28 38 50 - 113 Specifications for 420 kV Class Power Transformers 1.2.5.2Rated Switching Surge kV (Peak) 1050 Or 1175 Withstand Voltage For 400 Kv Terminal. 1.2.5.3 Partial Discharge At 1.5 Um/√3 Shall Be Limited To 500 Pc. Notes : (i) Insulation of tertiary winding of auto-transformer should be adequate to withstand the transferred surge voltage appearing across them due to an impulse striking on HV or IV terminals. Therefore, 33 kV LV winding shall be designed for a minimum lightning impulse withstand voltage of 250 kV (peak) and short duration power frequency withstand voltage of 95 kV (rms). (ii) The shunt reactor or capacitors connected to the LV side would be required to be frequently switched on and off. The LV winding should be capable of withstanding the stresses as may be caused by frequent switching. 1.2.5.4 TEMPERATURE RISES (a) Temperature rise of top oil (b) Measured by thermometer Air - cooled transformers Water-cooled transformers 50°C 55°C 55°C 60°C 55°C 65°C Temperature rise of winding Measured by resistance: - When oil circulation Natural or forced non-directed - When oil circulation is Forced directed Notes : (i) For the purpose of standardization of maximum temperature rises of oil and winding as measured by resistance, the following ambient temperatures are assumed : Air Water Cooling medium ambient temperature 50°C 30°C Maximum daily average ambient temperature 40°C 25 °C Maximum yearly weighted average temperature 32°C - (ii) Maximum yearly weighted temperature is based on ambient temperature cycle and its duration. (iii) Wherever ambient temperatures are higher than those specified above, the temperature rises, reduced by corresponding amount, shall be specified. (iv) Guaranteed temperature rise limits are valid for all the tappings. (v) The above temperature rises are applicable to transformers required for operation at an altitude notexceeding 1000 metres above sea level. 1.3 Cooling 1.3.1 ONAN/OFAF or ONAN/ODAF Two 50 per cent banks. One number of pump and one standby pump in each bank. Adequate number of fans and one standby fan in each 50 per cent bank. 114 Manual on Transformers 1.3.2 OFAF or OD AF 1.3.3 OFWF or ODWF Adequate number of coolers with one cooler as standby (6x 20% OFAF coolers or 4x33% OFAF coolers can be adopted). Two 100 per cent coolers. Notes : (i) The transformer shall be filled up with mineral oil, conforming to IS: 335 or IEC 60296. (ii) For auto transformers 100 per cent cooling equipment should be capable of dissipating losses occurring in all the three windings, at any tap. It is required only in case specifically called fort. 1.4 Bushings 1.4.1 The voltage and current ratings, basic insulation level and creepage distances of the bushings shall be in accordance with the following table: SI. No. Voltage rating kV (rms) Current rating (Amps) Creepage distance (mm) Basic insulation level (kVP) 1. 420 10500 1425 2. 3. 4. 245 145 52 1250 1250 2000 800 1250 3150 6125 3625 1300 1050 650 250 1.4.2 The Dimensions of Bushings are as per Section II. 1.5 Clearances of Line Terminals in Air Clearances in air between live parts and to earthed structures for LV terminals of generator transformers and auto-transformers shall be determined as per spacing given in clause 1.1.1 (viii) and 1.2.1.4 respectively. The clearances for HV and IV terminals shall be as tabulated below: Highest voltage for equipment kV (rms) 12 24 36 52 72.5 145 145 245 245 420 Clearances Phase to phase earth (mm) 280 330 350 530 700 1220 1430 2000 2350 4000 Phase to (mm) 140 230 320 480 660 1050 1270 1800 2150 3500 Specifications for 420 kV Class Power Transformers 115 Air clearances of 3500 mm between phase to earth can be relaxed to the extent of maximum of 200 mm so far as air release pipe emanating from bushing turret is concerned. 1.6 (a) (b) (c) (d) (e) (f) (g) (h) (i) (j) (k) (l) (m) (n) (o) (p) (q) (r) (s) Fittings and Accessories Rating and diagram plate. Two earthing terminals. Lifting bollards. Jacking pads. Haulage lugs. Pocket on tank cover for thermometer. Air release devices, Sudden/Rapid Pressure Rise Relay (Optional) Conservator with oil- filling hole, cap and drain valve (size: 25 mm.) Magnetic type oil gauge with low oil level alarm contacts (ratings: 0.5 Amp, 220 Volts DC.) Dial size 250 mm. Silica gel breather with oil seal. Air cell type oil preservation system. Pressure relief device (atleast 2 Nos) capable of resealing after release of pressure. Valves • Oil valves between cooler and main tank • Drain valve preferably with padlocking arrangement (size 100 mm). • Two filter valves (size: 50 mm) on diagonally opposite ends - one at top and other at bottom preferably with padlocking arrangement on bottom valve. • Two sampling valves (size: 15 mm) at top and bottom of main tank. Oil flow indicator with alarm contacts (ratings, 0.5 Amps, 220 Volts DC) with each pump. Valve schedule plate. Buchholz relay with alarm and trip contacts (Ratings: 1.0 Amp 220 Volts DC) shall have: • One number shut-off valve (Size: 80 mm) on conservator side • Test cock • Gas collection box and gas check valve at ground level. Copper tube interconnection between gas collection box and relay shall also be provided. In transformers, for installation in areas subject to high seismic forces, i.e., horizontal acceleration of 0.3 g or more at and above a frequency of 8 Hz pitot type or reed type of gas and oil relay shall be used. Dial type oil temperature indicator with ‘maximum-reading’ pointer and two sets of contacts (ratings, 5 Amps, 220 Volts DC). 1 No. dial type winding temperature indicator for a two winding transformer and one dial type windings temperature indicator for each winding of a multi winding transformer with ‘maximum-reading’ pointer and two sets of contacts rating : 5 Amps, 116 (t) (u) (v) Manual on Transformers 220 Volts DC (for OFAF/ODAF and OFWF/ODWF) and four sets of contacts (for ONAN/OD AF/OFAF). Remote indication for each winding temperature shall be through a separate measuring system. Cover lifting lugs. Provision for mounting bi-directional flanged rollers with locking and bolting device for rail gauge specified below: Type of construction Shorter axis Longer axis Single-phase Two rails with 1676 mm gauge Two rails with 1676 mm gauge Three-phase 2/3/4 rail combination according to layout and size of the transformer Two rails with 1676 mm gauge (w) Weather proof marshalling box for housing control equipment and terminal connections. (x) Wiring up to marshalling box with PVC SWA copper cables of 650/1100 volt grade. (y) Cooling accessories. I ONAN/OFAF or ONAN/ODAF cooling (i) Requisite number of radiators provided with: - One shut off valve on top (size: 80 mm) - One shut off valve at bottom (size: 80 mm) - Air release device on top - Drain and sampling device at bottom - Lifting lugs (ii) Fans (iii) Oil pumps with shut off valve on both sides (if required for ONAN cooling pumps can be by-passed using by-pass pipes and valves). (iv) Expansion joints, one each on top and bottom cooler pipe connections. (v) Air release device and oil drain plug on oil pipe connections. II OFAF or ODAF cooling (i) OFAF coolers with integral fans (ii) Oil pumps with shut-off valves on both sides. (iii) Brass encased thermometers. (iv) Air release devices and oil pipe connections. (v) Lifting lugs. III OFWF or ODWF Cooling (i) Oil/water heat exchangers with segregated oil and water headers Specifications for 420 kV Class Power Transformers (ii) (iii) (iv) (v) (vi) 117 Oil pumps with shut-off valves on both sides. Water flow indicator with alarm contacts (ratings: 0.5 Amp, 220 Volts DC). Brass encased thermometer. Pressure gauges. Differential pressure gauge with alarm contacts, operating when difference between oil outlet pressure and water inlet pressure is less than 0.2 kg./cm2 (vii) Reflux valve if required as per scheme. (viii) Drain and sampling device on cooler pipe connection. (z) Online DGA (aa) Online Moisture Removal (for GT). SECTION I Specifications for 800 kV Class Power Transformers SECTION I Specifications for 800 kV Class Power Transformers 1.0 SCOPE 1.1 This section covers power transformers of 800 kV class but does not purport to include all the necessary provisions of a contract. For general requirements, capitalization and tests reference shall be made to Sections ‘A’, ‘AA’&‘BB’respectively of this Manual. 1.2 Standard Ratings 1.2.1 Generator Transformers Single phase rating MVA Voltage ratio kV Tapping range Percent impedance Voltage Cooling 200 21/765√3 15-16% (with ±5% tolerance) at principal tap OFAF/OFWF ODAF/ODWF 260 315 24/765√3 27/765√3 ± 5% in 8 steps with off circuit taps/ links Three single-phase units will form a bank of 3-phase. Note : The purchaser shall specify the type of cooling required before purchase. Above mentioned Transformer rating (in Table) shall be decided by the Utility based on generator rating, power factor & system requirement. Other Parameters (i) Connections - HV star neutral effectively earthed, LV delta (ii) Connections symbol – YNd11 / YNd1/ YNd5/ YNd7(As per User’s requirement)in 3-phase bank. (iii) Tappings - Full power tappings on HV winding for HV voltage variation. (iv) Air core reactance of HV winding shall not be less than 20 per cent and knee point voltage shall not be less than 1.1 pu. (v) Short circuit level Transformer shall be suitable for connection to the system having the following short circuit level: 800 kV - 40 kA or 50 kA (rms) for 1 second as required. (vi) Terminal bushings (a) LV Terminals: 36 kV, 12500 Amps. Oil filled condenser type bushings mounted on turrets, suitable for connections to bus bars in isolated phase bus ducts which shall have spacing of 1500 mm for each 200 MVA single-phase unit of a 600 MVA three-phase bank. 121 122 Manual on Transformers (b) For 260 MVA single-phase unit of a 780 MVA three-phase bank, 1 no. 36 kV, 16000 Amps, rating. HV Terminals-Line End: 800 kV, 1250/2000/2500 Amps,oil filled condenser Bushing with test tap. No arcing horns shall be provided. For details refer Section II. (c) Neutral End: 36 kV porcelain bushing. No arcing horns shall be provided. (vii) Temperature rises (a) Top oil measured by Thermometer - 40°C (b) Winding rise measured by Resistance method - 45°C (c) Maximum design ambient temperature (also refer para 1.4 note i) - 50°C Total capacity of coolers for each transformer shall be minimum 120% of actual requirements. 1.2.2 Auto - Transformers Single-phase rating HV/TV/LV Voltage ratio MVA kV 333.33/333.33/111.1 765/400/33 √3 √3 Tapping range Per cent impedance voltage at principal tap HV-IV ± 5.5% in 22 steps 14.0 HV-LV 60 Cooling IV-LV 45 ONAN/ONAF/ OFAF or ODAF Alternatively ONAN/ONAF1/ ONAF2 500/500/167.67 Note : 765/400/33 √3 V3 ± 5.5% in 22 steps 14.0 195 180 -do- tolerance tolerance tolerance ±10% ±15% ±15% Three single-phase units will form a bank of 3-phase. Rating of stabilizing LV winding may be of 1/3rd reactive rating. However, continuous thermal rating shall be at least 5 MVA Active loading. Other Parameters (i) Connections - HV/IV Star auto with neutral effectively earthed LV Delta (ii) Connection symbol - YNa0d11(3-phase) (iii) ONAN rating shall be guaranteed at 60 per cent of the OFAF or ODAF rating. Rating under ONAF condition although not guaranteed shall be about 80 per cent. Alternatively ONAN/ONAF 1/ONAF2 (60%/ 80%/ 100%) cooling with 2 x 50% or with radiator banks or 4x33.3% unit coolers can be used. (iv) For these transformers the temperature rise of the top oil refers to the specified loading combination for which the total losses are highest. Individual winding temperature rises shall be considered relative to that specified loading combination which is the most severe for the particular winding under consideration. 123 Specifications for 800 kV Class Power Transformers (v) Air core reactance of HV winding shall not be less than 20 per cent and knee point voltage shall not be less than 1.1 pu. (vi) Short circuit Level Transformer shall be suitable for connection to the system having the following short circuit level: 800 kV – 40, 50 kA (rms) for 1 second 420 kV – 40,50 and 63 kA (rms) for 1 second (vii) Terminals (a) LV Terminals: 52 kV oil-filled condenser bushings. The bushing shall be arranged in a line with 1000 mm spacing. No arcing horns shall be provided. (b) IV Terminals: 420 kV oil-filled condenser bushings with test taps. No arcing horns shall be provided. (c) HV Terminals: 800 kV oil-filled condenser bushing with test tap. No arcing horns shall be provided. Neutral End: 36 kV porcelain bushing. No arcing horns shall be provided. (viii) Temperature Rises: 1.3 (a) Top oil measured by thermometer - 40°C (b) Winding rise measured by resistance method - 45°C (c) Maximum design ambient temperature (Also refer to Notes under clause 1-4) - 50°C Insulation Levels 1.3.1 Impulse and Power Frequency Voltage Test Level for Transformer Windings. Highest voltage for equipment Urn kV (rms) Rated lightning impulse withstand voltage kV (peak) Rated switching impulse withstand voltage kV (peak) Rated power frequency short duration withstand voltage kV (rms) 17.5 (Neutral) 24 52 (LV of Auto Transformer) 420 800 95 125 250 — — — 38 50 95 1300 1950 1050 1550 — — 124 1.3.2 Manual on Transformers Partial discharge at 1.5 Um/√3 shall be limited to 500 pC Notes (i) Insulation of tertiary winding of Auto-transformer should be adequate to withstand the transferred surge voltage appearing across it due to an impulse striking on HV or IV terminals. Therefore, 33 kV LVwinding shall be designed for a minimum lightning impulse withstand voltage of 250 kV (peak) andshort duration power frequency withstand voltage of 95 kV (rms).Surge Absorbers may be adopted between Tertiary winding Terminals to limit transferred surge value to 250 kVp and is a discretion between purchaser and manufacturer. (ii) The shunt reactor or capacitors connected to the LV side would required to be frequently switched onand off. The LV winding should be capable of withstanding the stresses as may be caused by frequentswitching. 1.4 Temperature Rises Notes (i) For the purpose of standardization of maximum temperature rises of oil and winding as measured by resistance, the following ambient temperatures are assumed. Air Water Cooling medium ambient temperature 50°C 30”C Maximum daily average ambient temperature 40ºC 25°C Maximum yearly weighted average temperature 32°C - (ii) Maximum yearly weighted temperature is based on ambient temperature cycle and its duration. (iii) Wherever ambient temperature is higher than those specified above, the temperature rises, reduced by corresponding amount, shall be specified. (iv) Guaranteed temperature rise limits are valid for all the tapping. (v) The above temperature rises are applicable to transformers required for operation at an altitude not exceeding 1000 meters above sea level. 1.5 Cooling 1.5.1 ONAN/OFAF or ONAN/ODAF Two 50 percent banks. One number of pump and one standby pump in each bank. Adequate number of fans and one standby fan in each 50 per cent bank. 1.5.2 ONAN/ONAF1/ONAF2 Two 50 percent banks or four 33.3% unit coolers. Adequate number of fans and one standby fan in each 50 per cent bank or in each 33.3% bank. 1.5.3 OFAF or ODAF Adequate number of coolers with one cooler as standby. 1.5.4 OFWF or ODWF Two 100 per cent coolers. Note:(i) The transformer shall be filled up with mineral oil, conforming toIS: 335 or IEC 60296. (ii) Tor auto transformers 100 per cent cooling equipment should be capable of dissipating losses occurring in all the three windings, at any tap. 125 Specifications for 800 kV Class Power Transformers 1.6 Bushings 1.6.1 The voltage and current ratings, basic insulation level and creepage distances of the bushings shall be in accordance with the following table: Voltage rating kV (rms) Current rating (Amps) Creepage distance (mm) Basic impulse level (kVp) Switching impulse level (kVp) 800 420 52 2500 1250 / 2000 3150 / 5000 16,000 10500 1300 2100 1425 250 1550 1050 -- 1.6.2 Partial Disc/large Level Pico-Coulombs :as per IEC 60137.500 at 1.5 p.u. 1.7 Clearances of Line Terminals in Air Clearances in air between live parts and to earthed structures for LV terminals of generator transformers and auto - transformers shall be determined as per spacing given in clause 1.1.1 (ix) and 1.2.1 (x) respectively. The clearances for HV and IV terminals shall be as tabulated below: Highest voltage for equipment kV (rms) 12 24 36 52 72.5 145 145 245 245 420 800 Clearances Phase to phase (mm) 280 330 350 530 700 1220 1430 2000 2350 4000 5800* / 6700* Phase to earth (mm) 140 230 320 480 660 1050 1270 1800 2150 3500 5000* / 5800* * depending upon lightening & switching impulse level. Airclearances between phase to earth can be relaxed to the extent of maximum of 200 mm so far as air release pipe emanating from bushing turret is concerned. 1.8 Fittings and Accessories (a) Rating and diagram plate. (b) Two earthing terminals, (c) Lifting bollards. (d) Jacking pads. (e) Haulage lugs. (f) Pocket on tank cover for thermometer. 126 Manual on Transformers (g) Air release devices. (h) Conservator with oil- filling hole, cap and drain valve (size: 25 mm ) (i) Magnetic type oil gauge with low oil level alarm contacts (ratings: 0.5 Amp, 220 Volts DC.) Dial size 250 mm. (j) Silicagel breather with oil seal. (k) Air cell type oil preservation system. Aircell rupture detector may be provided. • In addition to provision of air cell in conservators for sealing of the oil system against the atmosphere, an on line insulating oil drying system shall be provided. This on line insulating oil drying system shall be designed for very slow removal of moisture that may enter the oil system or generated during cellulose decomposition. (1) Required number of pressure relief device capable of resealing after release of pressure. (m) Valves • Oil valves between cooler and main tank • Drain valve preferably with padlocking arrangement (minimum size 80 mm). • Two filter valves (size: 50 mm) on diagonally opposite ends - one at top and other at bottom preferably with padlocking arrangement on bottom valve. • Two sampling valves (size: 15 mm) at top and bottom of main tank. (n) Oil flow indicator with alarm contacts (ratings, 0.5 Amps, 220 Volts D.C.) with each pump. (o) Valve schedule plate. (p) Buchholz relay with alarm and trip contacts (Ratings: 1.0 Amp. 220 Volts D.C.) shall have • One number shut-off valve (Size: 80 mm) on conservator side • Test cock • Gas collection box and gas check valve at ground level. Copper tube interconnection between gas collection box and relay shall also be provided. In transformers, for installation in areas subject to high seismic forces, i.e., horizontal acceleration of 0.3 g or more at and above a frequency of 8 Hz pitot type or reed type of gas and oil relay shall be used. (q) Online dissolved gas monitoring device. (r) Dial type oil temperature indicator with maximum reading pointer and two sets of contacts (ratings, 0.5 Amps, 220 Volts D.C). Specifications for 800 kV Class Power Transformers 127 (s) 1 No. dial type winding temperature indicator for a two winding transformer and one dial type windings temperature indicator for each winding of a multi winding transformer with ‘maximum reading’ pointer and two sets of contact ratings: 5 Amps, 220 Volts D.C. (for OFAF/ODAF and OFWF/ODWF) and four sets of contacts (for ONAN/ODAF/OFAF). (t) Remote indication for each winding temperature shall be through a separate measuring system. (u) Cover lifting lugs. (v) Provision for mounting bi-directional flanged rollers with locking and bolting device for rail gauge specified below: Type of construction Shorter axis Longer axis Single-phase 2/3 rail combination with 1676 mm gauge according to layout and size of Transformer Two rails with 1676 mm gauge (w) Weather proof marshalling box for housing control equipment and terminal connections. (x) Wiring up to marshalling box with PVC SWA copper cables of 650/1100 Volt grade. (y) Cooling accessories. (I) ONAN/OFAF or ONAN/ODAF cooling (i) Requisite number of radiators provided with: (ii) - One shut off valve on top (minimum size: 80 mm) - One shut off valve at bottom (minimum size: 80 mm) - Air release device on top - Drain and sampling device at bottom - Lifting lugs Fans (iii) Oil pumps with shut off valve on both sides (if required for ONAN cooling pumps can be by-passed using by-pass pipes and valves). (iv) Expansion joints, one each on top and bottom cooler pipe connections. (v) Air release device and oil drain plug on oil pipe connections. (II) OFAF or ODAF cooling 128 Manual on Transformers (i) OFAF coolers with integral fans (ii) Oil pumps with shut-off valves on both sides. (iii) Brass encased thermometers. (iv) Air release devices and oil pipe connections. (v) (III) Lifting lugs. OFWF or ODWF cooling (i) Oil/water heat exchangers with segregated oil and water headers (ii) Oil pumps with shut-off valves on both sides. (iii) Water flow indicator with alarm contacts (ratings: 0.5 Amp, 220 Volts DC). (iv) Brass encased thermometer. (v) Pressure gauges. (vi) Differential pressure gauge with alarm contacts, operating when difference between oil outlet pressure and water inlet pressure is less than 0.2 kg/cm2 (vii) Reflux valve if required as per scheme. (viii) Drain and sampling device on cooler pipe connection. SECTION J Specification for 420 kV Class Shunt Reactors & Associated 145 kV Class Neutral Grounding Reactors SECTION J Specification for 420 kV Class Shunt Reactors & Associated 145 kV Class Neutral Grounding Reactors A. Specifications for 420 kV Class Shunt Reactors 1.0 Scope 1.1 This section covers technical requirements/parameters for Shunt Reactors of 420 kV class but does not purport to include all the necessary provisions of a contract. For general requirements, loss capitalization and tests, reference shall be made to Sections ‘A’, ‘AA’ & ‘BB’ respectively of this Manual. 1.2 Introduction The AC power system networks are operated as voltage source networks. One essential requirement, therefore is that under all operating conditions of load the voltage, profile of entire network should be maintained nearly flat, regulating the voltage of each bus within narrow band of permitted tolerance. No special compensation facilities are needed when the Power transfer is over small distances. With increase in power network and generating station connected to load centers employing long transmission lines, this requirement becomes difficult to meet. The need, thus arises to compensate these long transmission lines (generally above 200 km). 1.3Problem Associated with- Reactive Power Power transmission lines are characterised by their line inductance and shunt capacitance. For shorter lines their inductive reactance dominates. As a result, when they carry load current, which normally is inductive in nature, the receiving end voltage reduces in magnitude and hence shunt reactors are not required for such lines. Long transmission lines present a problem of a different kind; once energised the line charging becomes a source of reactive power. Under light load conditions the VAR generation exceeds the VAR consumption which causes excessive voltage at mid-point. The consumed reactive power is equal to generated power for a certain transmitted load. This transmitted load is called surge impedance or natural load of the line. Voltage profile under these conditions becomes flat. Under Heavy load condition, generation of reactive power in lines reduces while its consumption increases substantially. 1.4 Function of Shunt Reactor In practice, on account of the transient stability considerations, the permissible loading of long -lines is kept below surge impedance loading and therefore one faces the challenge to restrict over voltage along the length of the line. This is accomplished by the connection of shunt reactor at intermediate buses. This solution is satisfactory but, when used, the total transmission capacity of the line is reduced. Mid-point shunt compensation not only improves the voltage profile but also enhances the power transfer capacity of a long line. Shunt Reactors are, thus, important components-for better utilization of existing and new lines since they compensate for large capacitive currents generated by HV transmission lines over long distances, restricting optimum system operation under low load conditions. 131 132 Manual on Transformers Capacitive energy is thus balanced with reactive energy and the shunt reactors: • Maintain grid voltage within limits compatible with the systems insulation level under normal service conditions. (Lightly loaded condition.) • Control dynamic over voltage under abnormal conditions (loss of system, interconnections. resulting from load shedding operations. or from a line-ground fault) • Take care of switching transients. 1.5 Reactors may be permanently connected, or switched in and off type, depending upon voltage variations. 1.6 Connection In The System The reactor is connected either directly on the line and or HV bus or connected to a low-voltage tertiary winding of a large transformer. 1.7 Applicable Standard Except otherwise specified or implied herein, the Reactors shall comply with latest edition of International Standard IEC 60076-6. 1.8 Standard Ratings Based on length of transmission lines & reactive power compensation in India following three phase ratings of Shunt Reactor have been standardised for 400 kV lines – 1. 50 MVAR 2. 63 MVAR 3. 80 MVAR 4. 125 MVAR Note: Single Phase Rating option also possible for specific requirement. 1.9 Major Technical Parameters 1.9.1 Type: Gapped Core or Magnetically Shielded Air Core type Construction 1.9.2 Application & Operation: Shunt Reactors will be connected to the 400kV transmission system for Reactive Power Compensation and shall be capable of controlling the dynamic over voltages occurring in the system due to load rejection. 1.9.3 Shunt Reactors shall be capable of operating continuously at a voltage 5% higher than their rated voltage without exceeding hot spot temperature of 140°C at any part of the reactor. 1.9.4 The Neutral Grounding Reactors(NGR) are required for grounding of the neutral point Specification for 420kV Class Shunt Reactors & Associated 145 kV Class Neutral Grounding Reactors 133 of Shunt Reactors to limit the secondary arc current & recovery voltage to a minimum value in case Shunt Reactor is connected in the line termed as “Line Reactor”. The Reactor directly connected to the bus termed as “Bus Reactor” and shall have its neutral directly earthed without NGR. In such cases utility may decide to have BIL for Neutral of Shunt Reactor as 95 kVp & 38kVrms with 36 kV Neutral Bushing. 1.9.5 The Reactor shall be capable of withstanding Switching Surge Overvoltage of 2.5 p.u and temporary overvoltage of the order of 2.3 p.u for few cycles followed by power frequency overvoltage upto 1.5 p.u. 1.9.6 Rating (MVAR) 50/63/80/125 (a) (b) (c) (d) (e) Rated voltage System Fault Level Connection Tolerance on Impedance Ratio of Zero sequence Reactance to positive Reactance (X0/X1) (f ) Range of Constant Impedance (Linearity) (g) Harmonic Content in Phase Current 420kV (1.0 pu) 40/50/63 kA as per System Requirement Star with Neutral brought Out -0 to +5% 0.9 to 1.0 (h) (i ) (j) (k) (l) Up to 1.5 pu The crest value of the third harmonic component in phase current not to exceed 3% of the crest value at rated voltage ±2% 200 microns p-p, 60 microns average 2 kg/mm2 maximum 80 dB ONAN with separate/tank mounted radiator bank* Permissible unbalance current among different phase Vibration Level at rated voltage & frequency Stress on tank wall Noise Level *OFWF/OFAF due to space constraint/ customer specific requirement ONAN - One 100% bank Cooling OFAF - Adequate number of coolers with one cooler as standby OFWF - Two 100 per cent coolers (m) Maximum Partial Discharge level (n) Temperature Rises* Temperature rise of top oil measured by thermometer Temperature rise of winding measured by resistance Radiator bank/ Cooler can be tank or separately mounted 500 pico coulomb at 1.5 pu (as per IEC 60076-3) 40oC 45oC Notes: (i) For the purpose of standardization of maximum temperature rises of oil and winding as measured by resistance, the following ambient temperatures are assumed : Cooling medium ambient temperature Air at 50°C, Water at 30°C Maximum daily average ambient temperature Air at 40°C, Water at 25°C Maximum yearly weighted average temperature Air at 32°C (ii) Maximum yearly weighted temperature is based on ambient temperature cycle and its duration. (iii) Wherever ambient temperatures are higher than those specified above, the temperature rises, reduced by corresponding amount, shall be specified. (iv) The above temperature rises are applicable to transformer reactors required for operation at an altitude not exceeding 1000 metres above sea level. 134 1.10 Manual on Transformers Terminals Line Terminals: 420 kV oil-filled condenser bushings with test taps. No arcing horns shall be provided. Neutral Terminal: 145 kV oil-filled condenser bushings with test taps. No arcing horns shall be provided. 1.10.1 Technical Parameters Bushings Parameter Bushing Line Terminal Bushing Neutral Terminal (a) Rated Voltage (kV) 420 145 (b) Rated Current (A) 1250 1250 (c) Creepage distance (mm) 10,500 3625 Tank Cover Tank Cover (e) Rated Lightning Impulse withstand voltage (kVp) 1425 650 (f ) Rated Switching Impulse withstand voltage (kVp) 1050 - (g) One minute Power frequency withstand voltage (kVrms) 695 305 (d) Mounting Notes(i) The 1250 amps bushings shall be suitable for draw lead type assembly (ii) The Dimensions of Bushings are as per Section II. (iii) In case of GIS termination, bushing to meet the requirements as per IEC 61639. 1.11 INSULATION LEVEL WINDINGS Line End Neutral End (a) Lightning Impulse withstand voltage (kVp) Parameter 1300 550 (b) Switching Impulse withstand voltage (kVp) 1050 - (c) Power frequency withstand voltage (kVrms) - 230 1.12 (a) Clearances of Line Terminals in Air Clearances Highest voltage for equipment kV (rms) 420 145 Phase to Phase 4000 mm (Min) NA Phase to Earth 3500 mm (Min) 1050 mm (Min) Note - Air clearances of 3500 mm between phases to earth can be relaxed to the extent of maximum of 200 mm so far as air release pipe emanating from bushing turret is concerned. 1.13 TESTS 1.13.1 Routine Tests (a) Measurement of winding resistance. (b) Measurement of insulation resistance between winding & earth by 5 kV megger. Specification for 420kV Class Shunt Reactors & Associated 145 kV Class Neutral Grounding Reactors (c) (d) (e) (f) (g) (h) (i) (j) 135 Measurement of impedance by bridge method. Measurement of loss and current by bridge method at rated voltage and ambient temperature. Correction factor per degree centigrade shall be established for each rating of reactor by measuring losses at ambient temperature and elevated temperature. Correction factor thus established shall be applied for computation of losses at 75oC. Loss value thus obtained shall be corrected to rated current. For purpose of similarity for applying this coefficient the shunt reactor under test shall have identical rating & guaranteed losses within ±5%. Measurement of Capacitance and Tan delta Jacking test on reactor tank without fitting & accessories. Frequency response analysis test. Measurement of vibration & stress. Isolation test between Core-End Frame, End-frame-tank and Core-Tank by 2.5kV AC for one minute. Dielectric Test (i) Separate source voltage withstand test at 230 kV AC for one minute. (ii) Induced over voltage withstand test with P.D. indication at 412 kV AC (1.7pu/√3) for 5 minutes and at 364 kV AC (1.5pu//√3) for one hour. Note: Large value of reactive compensation is required during induced over voltage test on reactors. Considering the test plant limitations level of 1.7pu can be omitted for larger ratings of Shunt Reactors as per the agreement between the manufacturer and customer. This is permitted by IEC. (iii) Full wave lightning impulse voltage withstand test at 1300 kVp on line terminals. (iv) Switching impulse voltage withstand test at 1050 kVp on line terminals. 1.13.2 Type Test (On One Unit Only) (a) (b) Temperature rise test as per IEC 60076-2 along with DGA before and after temperature rise test. Measurement of acoustic noise level 1.13.3 Special Test (On One Unit Only) (a) (b) (c) (d) (e) Measurement of zero sequence reactance.(for three phase reactors only) Full wave lightning impulse voltage withstand test at 550 kVp on neutral terminal. Magnetization curve test/ knee point voltage measurement. Measurement of Harmonics Measurement of Mutual Reactance (for three phase reactors only) 1.13.4 Test on Reactor Tank (a) As per requirements of GENERAL section 136 Manual on Transformers 1.13.5 Reactor Oil Testing The following tests are to be conducted on oil samples for the reactor tank assembled for testing. Acceptance norms for insulating oil after filling into reactor shall be as given below – Characteristic Permissible limit ( IEC 60422) Electrical strength 60 kV (min.) Water content 10 ppm (max.) Ten Delta at 90oC 0.01 (max.) Resistivity at 90 C 6 x 1012 ohm-cm (min.) Interfacial tension 0.035 N/m (min.) o 1.14 GUARANTEED TECHNICAL PARTICULARS 1. Manufacture name & country 2. Type of reactor (Gapped/ Air core) 3. Standards applicable 4. Rated MVAr capacity 5. Rated voltage (kV) 6. Type of cooling 7. Thermal data a) Temperature rise in oil above ambient temperature (Deg C) b) Temperature rise of winding by resistance above ambient temperature (Deg C) c) Rated frequency (Hz) 8. Number of phases 9. Guaranteed max. losses at rated voltage and frequency at rated output at 75 Deg C (kW) 10. Noise level & reference (dB) 11. Insulation level (winding & bushing) a) Lightning impulse (1.2/50 microsecs.) withstand voltage (kVp) b) Power frequency withstand voltage (kVrms) c) Switching surge withstand voltage (kVp) 12. a) Range of voltage upto which impedance will be constant (p.u.) b) Impedance value at 1.0 pu (ohms) c) Impedance value at 1.5 pu (ohms) 13. X0/X1 14. a) Vibration & maximum stress on the tank b) Vacuum withstand capacity of tank Winding Bushing Line/ Neutral Line/ Neutral Specification for 420kV Class Shunt Reactors & Associated 145 kV Class Neutral Grounding Reactors 15. Harmonic content in phase current 16. Shipping weights & Dimensions (a) Size of largest package (mm)x(mm)x(mm) (b) Weight of largest package (kg) (c) Gross weight to be handled (kg) (d) Gross volume to be handled (cu.m) (e) Approx. overall dimension (lxbxh) (mm) (f) Approx. quantity of oil required for first filling (g) Un tanking height 17. Proposed method of transportation 18. Compliance to technical specification w.r.t. parameter specified for (a) Oil (b) Bushing CT (c) Bushings (d) Terminal connectors 19. Whether similar equipment are type tested & are in successful operation for at least two years (If yes, furnish type test reports) 20. Overall general arrangement drawing of Shunt Reactor with all accessories to be enclosed 21. Additional Data 1. System Fault Level 2. Connection 3. Amount of unbalanced current in each phase when connected to symmetrical voltages 4. Tolerance on Current 5. Capacitance Value (Phase to ground) 6. Clearances from 400 kV terminal (a) Phase to phase (b) Phase to ground (c) Neutral to ground 7. Tank (a) Type (b) Material (c) Thickness 8. Gasket Details (a) Material (b) Temperature withstand capability 9. Conservator (a) Total Volume 137 138 Manual on Transformers (b) Volume between highest & lowest visible oil level (c) Type of Conservator 10. No. of Pressure Relief device provided (a) Operating Pressure 11. Temperature Indicators (a) OTI/WTI (i) Make (ii) Range (oC) (iii) Accuracy (%) (b) RWTI (i) Make (ii) Range (oC) (iii) Accuracy (%) (iv) Auxiliary Supply required 12 Winding (a) Material (b) Cross-sectional area of conductor (c) Current density 13. Core (a) Type of Core (b) Justification for type of Core adopted (c) Technical details of the core (No. of limbs) (d) Material of Core, its grade & thickness 14. Bushings (a) Type (b) Maker’s Name (c) Standard Applicable (d) Visible discharge voltage for falling power frequency voltage (e) One minute power frequency withstand voltage (i) Dry (ii) Wet (f) Full wave Impulse withstand voltage (g) Switching Impulse withstand voltage (h) Total Creepage distance in air (Approx.) (i) Weight of assembled bushing (Approx.) (j) Cantilever Strength (Approx.) (k) Rated Current 15. Radiator Line (400 kV) Neutral(145 kV) Specification for 420kV Class Shunt Reactors & Associated 145 kV Class Neutral Grounding Reactors (a) Make (b) Material (c) Thickness (d) Pressure Withstand Capability (e) Vacuum Withstand Capability 16. Marshalling box (a) Type of Mounting (b) Degree of Protection 17. Bushing Current Transformer (a) Line Side (i) Type or voltage class (ii) Ratio (iii) Accuracy class (iv) Burden (VA) (vi) Knee point voltage (Volts) (Minimum) (vii) Maximum resistance of secondary winding (ohms) 139 (viii) Maximum exciting (mA) current (b) Neutral Side (Before/ After Neutral formation) (i) Type or voltage class (ii) Ratio (iii) Accuracy class (iv) Burden (VA) (vi) Knee point voltage (Volts) (Minimum) (vii) Maximum resistance of secondary winding (ohms) (viii) Maximum exciting (mA) current B. SPECIFICATION FOR NEUTRAL GROUNDING REACTORS 1.0 Scope 1.1 This section covering specification for Neutral Grounding Reactors, does not purport to include all the necessary provisions of a contract. For general requirements and tests, reference shall be made to other Sections ‘A’ and ‘BB’ of the Transformer Manual. 2.0 General 2.1 Unless otherwise modified in this section the earthling transformers shall comply with latest versions of IEC 60076 2.2 Single phase Neutral Grounding Reactors are connected between neutral point of 400 kV Shunt Reactors and earth, where neutral of Shunt Reactor is designed for 145 kV class insulation. 140 Manual on Transformers 2.3 In the high voltage transmission system installed with shunt reactor and when single pole reclosing is envisaged, a small inductor called neutral grounding reactor is required to be put between neutral of shunt reactor and earth to compensate the capacitive current during single line to earth fault. 2.4 Thus NGR is required to carry high current for a very short time. 2.5 In a shunt reactor maximum permissible unbalance current among different phases is 2%, which implies that for 125 MVAR shunt reactor current of 3.5Amp may flow through continuously. 2.6 Based on above consideration NGR is designed for continuous current of 10 Amp & 60 Amp for 10 seconds (time during which the fault would be definitely cleared). 2.7 NGR consists of air core coil of suitable impedance (as specified in the contract based on system requirement) immersed in oil filled tank. The Line terminal is brought out through 145kV OIP condenser Bushing which is connected to neutral terminal of shunt reactor. Neutral terminal is brought out through 36 kV porcelain bushing. 2.8 As continuous losses of NGR are negligible, reactor tank surface is adequate for dissipation of these losses & therefore no cooling equipment is required. 3.0 TECHNICAL PARAMETERS (a) Rated Voltage from insulation strength consideration 145 kV (b) Rated frequency 50 Hz (c) No. of Phases 1 (d) Type Outdoor (e) Insulation Graded (f ) Max. Continuous Current 10A(rms) (g) Rated Short time current for 10 seconds 60A(rms) (h) Rated Impedance at rated Short time current 400 to 2500 ohm (actual value to be defined by Customer) (i) Natural Oil Cooling(ONAN) Cooling System (j ) Max Temperature rises over 50°C ambient Winding measured by resistance : 50°C Top Oil measured by Thermometer: 45°C (k ) Connection Between neutral of Shunt Reactor and Ground (l) 145 kV oil-filled condenser bushings with test taps. No arcing horns shall be provided Line Terminal (m) Neutral Terminal 36 kV Porcelain bushing. No arcing horns shall be provided. 141 Specification for 420kV Class Shunt Reactors & Associated 145 kV Class Neutral Grounding Reactors 3.1 Technical Parameters of Bushings Line Terminal Neutral Terminal (a) Rated Voltage (kV) 145 36 (b) Rated Current (A) 1250 630 (c) Creepage distance (mm) (d) Mounting 3625 600 Tank Cover Tank Cover (e) Rated Lightning Impulse withstand voltage (kVp) 650 170 (f ) One minute Power frequency withstand voltage (kVrms) 270 305 75 77 Notes: (i) The 1250 amps bushings shall be suitable for draw lead type assembly. (ii) The Dimensions of condenser Bushings are as per Section II. (iii) In case of GIS termination, bushing to meet the requirements as per IEC 61639 3.2 Insulation Level Windings Line End Neutral End Lightning Impulse withstand voltage (kVp) 550 95 Power frequency withstand voltage (kVrms) 230 38 3.3 Clearances of Line Terminals in Air Voltage Level Phase to Earth 145 1050 24 230 3.4 Method of Grounding 3.5 Tests : Solidly (a) Measurement of winding resistance. (b) Measurement of insulation resistance between winding & earth by 5 kV megger. (c) Measurement of impedance by V/I method at 10A & 60A. (d) Separate source voltage withstand test at 38 kV AC for one minute. (e) Full wave lightning impulse voltage withstand test at 550 kVp on line terminals. (f) Jacking test on reactor tank without fitting & accessories. 142 Manual on Transformers 3.6 Tests on Reactor Tank 3.6.1 As per requirements of GENERAL section 3.7 Guaranteed Technical Data 1. Manufacture name & country of manufacture 2. Type of reactor (gapped/ air core) 3. Standards applicable 4. Type of cooling 5. Rated voltage (kV) 6. Max. continuous current (Amps) 7. Rated short time current at 10 sec. (Amps) 8. Ohmic value (ohms) Rated reactive capacity for continuous operation at rated voltage (MVAR) 9. Thermal data: (a) Temperature rise in oil at rated current above ambient temperature of 500C (Deg C) (b) Temperature rise of winding over ambient temperature of 500C (Deg C) 10. Rated frequency (Hz) 11. Guaranteed max. losses at rated voltage and frequency at rated output (kW) 12. Insulation level (winding & bushing) (a) Lightning impulse (kVp) (b) Power frequency (kVrms) (c) Neutral brought out at (kV) 13. Type of insulation (Graded or Uniform) 14. Overall dimensions & weights (a) Length with coolers (mm) (b) Breadth with coolers (mm) (c) Height with bushing (mm) (d) Quantity of oil (kl) (e) Un tanking heights (mm) 15. Shipping data (a) Size of largest package (mm)x(mm)x(mm) (b) Weight of largest package (kg) (c) Gross weight to be handled (kg) (d) Gross volume to be handled (cu.m) 16. Compliance to technical specification w.r.t. parameter specified for (a) Oil (b) Bushing CT (c) Bushings Specification for 420kV Class Shunt Reactors & Associated 145 kV Class Neutral Grounding Reactors 143 (d) Terminal connectors 17. Whether similar equipment are type tested & are in successful operation for at least two years (If yes, furnish type test reports) 18. Overall general arrangement drawing of Neutral Grounding Reactor with all accessories to be enclosed 19. Bushing Current Transformer (a) Line Side (i) Type or voltage class (ii) Ratio (iii) Accuracy class (iv) Burden (VA) (v) Accuracy limit factor (vi) Knee point voltage (Volts) (Minimum) (vii) Maximum resistance of secondary winding (ohms) (viii) Maximum exciting (mA) current (b) Neutral Side (i) Type or voltage class (ii) Ratio (iii) Accuracy class (iv) Burden (VA) (v) Accuracy limit factor (vi) Knee point voltage (Volts) (Minimum) (vii) Maximum resistance of secondary winding (ohms) (viii) Maximum exciting (mA) current SECTION K Specifications for 800 kV Class Shunt Reactors & Associated 145 kV Neutral Grounding Reactor SECTION K Specifications for 800 kV Class Shunt Reactors & Associated 145 kV Neutral Grounding Reactor A. Specifications for 800 kV Class Shunt Reactors 1.0 Scope 1.1 This section covers technical requirements/parameters for Shunt Reactors of 800 kV class but does not purport to include all the necessary provisions of a contract. For general requirements, loss capitalisation and tests, reference shall be made to Sections ‘A’, ‘AA’ & ‘BB’ respectively of this Manual. 1.2 Refer clause no. 1.2 ,1.3, 1.4, 1.5, 1.6, 1.7 of Section J “Specifications for 420 kV Class Shunt Reactors” for general information , functions & application. 1.3 Standard Ratings Based on length of transmission lines & reactive power compensation in India ,following single phase ratings of Shunt Reactor have been standardised for 800kV lines – 1. 2. 1.4 1.4.1 80 MVAR - 765kV 110 MVAR - 765kV Major Technical Parameters Type: Gapped Core or Magnetically Shielded Air Core type Construction 1.4.2 Application & Operation: Shunt Reactors will be connected to the 800 kV transmission system for Reactive Power Compensation and shall be capable of controlling the dynamic over voltage occurring in the system due to load rejection. 1.4.3 Shunt Reactors shall be capable of operating continuously at a voltage 5% higher than their rated voltage and thermal and cooling system shall be designed accordingly. 1.4.4 The Neutral Grounding Reactors are required for grounding of the neutral point of Shunt Reactors to limit the secondary arc current & recovery voltage to a minimum value in case Reactor is connected in the line termed as “Line Reactor”. The Reactor directly connected to the bus termed as “Bus Reactor” and its neutral is directly earthed. 1.4.5 The Reactor shall be capable of withstanding Switching Surge Overvoltage of 1.9 p.u and temporary overvoltage of the order of 1.4 p.u for about 10 cycles followed by power frequency overvoltage upto 1.8 p.u. 147 148 Manual on Transformers 1.4.6 (a) Rating (MVAr) 80/110 single phase at 765/√3 kV (b) (i) Rated voltage 765/√3 kV (ii) Max. continuous operating voltage(Um) 800/√3 kV (c) System Fault Level 40/50/63 kA as per System requirement (d) Connection Star after 3 phase bank formation (e) Tolerance on Impedance -0 +5 % (f) Ratio of Zero sequence Reactance to positive Reactance (X0/X1) 0.9 to 1.0 (g) Range of Constant Impedance (Linearity) Up to 1.25 pu (h) Harmonic Content in Phase Current The crest value of the third harmonic component in phase current not to exceed 3% of the crest value of the crest value of fundamental when reactor is energized at rated voltage with sinusoidal wave form (i) Permissible unbalance current among different phases of a bank +/- 1% (j) Vibration Level at 800√3 kV voltage & 50Hz 200 microns p-p, 60 microns average (k) Stress on tank wall at 800√3kV 2 kg/mm2 max. (l) Noise Level at 800/√3kV 80 dBA (m) Cooling ONAN with separate radiator bank (n) Maximum Partial Discharge level 500 pico coulomb at 1.5 pu (as per IEC 60076-3) (o) Temperature Rises over an ambient temp. of 50 Deg. Cent. And at 800/√3kV Temperature rise of top oil measured by thermometer 400C Temperature rise of winding measured by resistance 450C The above temperature rises are applicable to reactors required for operation at an altitude not exceeding 1000 metres above sea level. 1.5 Terminals Line Terminals : 800 kV oil-filled condenser bushings with test taps. Neutral Terminal: 145 kV oil-filled condenser bushings with test taps. No arcing horns shall be provided. 1.5.1 Technical Parameters Bushings Parameter Line Terminal Neutral Terminal (a) Rated Voltage (kV) 800 145 (b) Rated Current (A) 1600/2500 1250 (c) Creepage distance (mm) 20000 3625 (d) Mounting Tank Cover Tank Cover 149 Specifications for 800kV Class Shunt Reactors & Associated 145 kV Neutral Grounding Reactor (e) Rated Lightning voltage (kVp) Impulse withstand 2100 650 (f) Rated Switching Impulse withstand voltage (kVp) 1550 - (g) One minute Power frequency withstand voltage (kVrms) 970 275 305 Notes: (a) The bushings shall be suitable for draw lead type or draw rod type assembly (b) The Dimensions of Bushings are as per Section II. (c) In case of GIS termination, bushing shall meet the requirements as per IEC 61639. 1.6 Insulation Level Windings Parameter 1.7 Lightning Impulse withstand voltage (kVp) 1950 550 (b) Switching Impulse withstand voltage (kVp) 1550 - (c) Power frequency withstand voltage (kVrms) - 230 Clearances of Line Terminals in Air Clearances 1.8 TESTS 1.8.1 Routine Tests (e) (f) (g) (h) Neutral End (a) (a) (a) (b) (c) (d) Line End Highest voltage for equipment kV (rms) Phase to earth Clearance (mm) 800 5800 mm (Min) 145 1050 mm (Min) Measurement of winding resistance. Measurement of insulation resistance between winding & earth by 5 kV megger. Measurement of reactance by bridge method. Measurement of loss and current by bridge method at rated voltage and ambient temperature. Correction factor per degree centigrade shall be established for each rating of reactor by measuring losses at ambient temperature and elevated temperature. Correction factor thus established shall be applied for computation of losses at 75ºC. Loss value thus obtained shall be corrected to rated current. Measurement of Capacitance and Tan delta Jacking test on reactor tank without fitting & accessories. Frequency response analysis test. Measurement of vibration & stress. 150 (i) (j) Manual on Transformers Isolation test between Core-End Frame, End-frame-tank and Core-Tank by 2.5kV AC for one minute. Dielectric Test (i) eparate source voltage withstand test at 230 kV AC for one minute. (ii) Induced over voltage withstand test with P.D. indication at 785 kV AC (1.7pu/√3) for 30 seconds and at 693kV AC (1.5pu/√3) for one hour. Note: Large value of reactive compensation is required during induced over voltage test on reactors. Considering the test plant limitations level of 1.7pu can be omitted for larger ratings of Shunt Reactors as per the agreement between the manufacturer and customer. This is permitted by IEC. (iii) Full wave lightning impulse voltage withstand test at 1950 kVp on line terminals. (iv) Switching impulse voltage withstand test at 1550 kVp on line terminals. 1.8.2 (a) (b) 1.8.3 (a) (b) (c) 1.8.4 (a) 1.8.5 Type Test (On One Unit Only) Temperature rise test as per IEC 60076-2 & DGA before and after temperature rise test. Measurement of acoustic noise level at 800/√3kV Special Test (On One Unit Only) Full wave lightning impulse voltage withstand test at 550 kVp on neutral terminal. Magnetization curve test/ knee point voltage measurement. Measurement of Harmonic content of current Test on Reactor Tank As per requirements of GENERAL section Reactor Oil Testing The following tests are to be conducted on oil samples for the reactor tank assembled for testing. Acceptance norms for insulating oil after filling into reactor shall be as given below – 1.9 Characteristic Permissible limit ( IEC 60422) Electrical strength 70 kV (min.) Water content 10 ppm (max.) Ten Delta at 900C 0.01 (max.) Resistivity at 900C 6 x 1012 ohm-cm (min.) Interfacial tension 0.035 N/m (min.) Guaranteed Technical Particulars Refer Clause no 1.14 of Section J “Specification of 420 kV Class Shunt Reactors” Specifications for 800kV Class Shunt Reactors & Associated 145 kV Neutral Grounding Reactor B. SPECIFICATION FOR NEUTRAL GROUNDING REACTORS 1.0 Scope 151 This section covering specification for Neutral Grounding Reactors, does not purport to include all the necessary provisions of a contract. For general requirements and tests, reference shall be made to other Sections ‘A’ and ‘BB’ of the Transformer Manual. 2.0 General 2.1 Unless otherwise modified in this section the Neutral Grounding Reactors shall comply with latest versions of IEC 60076 2.2 Single phase Neutral Grounding Reactors are connected between neutral point of 800 kV Shunt Reactors and earth, where neutral of Shunt Reactor is designed for 145 kV class insulation. 2.3 In the high voltage transmission system installed with shunt reactor and when single pole reclosing is envisaged, a small inductor called neutral grounding reactor is required to be put between neutral of shunt reactor and earth to compensate the capacitive current during single line to earth fault. 2.4 Thus NGR is required to carry high current for a very short time. 3.0 Technical Parameters (a) Rated Voltage from insulation strength consideration 145 kV (b) Rated frequency 50 Hz (c) No. of Phases 1 (d) Type Dry type air core for outdoor application (e) Insulation Graded (f ) Max. Continuous Current 20A (rms) (g) Rated Short time current for 60 seconds 240A (rms) (However the NGR shall be designed for a current rating of 600A(rms) short time current to ensure mechanical robustness) (h) Rated Impedance at rated Short time current (Ohms) Actual value to be defined by Customer (i ) Cooling System Natural Air Cooled (k) Connection Between neutral of Shunt Reactor and Ground (l) Line Terminal 145 kV oil-filled condenser bushings with test taps. No arcing horns shall be provided (m) Neutral Terminal 36 kV Porcelain bushing. No arcing horns shall be provided. 152 Manual on Transformers 3.1 Technical Parameters of Bushings Line Terminal Neutral Terminal (a) Rated Voltage (kV) 145 36 (b) Rated Current (A) 1250 630 (c) Creepage distance (mm) (d) Mounting (e) (f) 3625 600 Tank Cover Tank Cover Rated Lightning Impulse withstand voltage (kVp) 650 170 One minute Power frequency withstand voltage (kVrms) 305 77 Notes (i) The 1250 amps bushings shall be suitable for draw lead type assembly. (ii) The Dimensions of condenser Bushings are as per Section P. (iii) In case of GIS termination, bushing to meet the requirements as per IEC 61639 3.2 3.3 Insulation Level Windings (a) Lightning Impulse withstand voltage (kVp) (b) Power frequency withstand voltage (kVrms) Line End Neutral End 550 95 230 38 Clearances of Line Terminals in Air Clearances Phase to Earth (a) 145 1050 (b) 36 230 3.4 Method of Grounding: Solidly 3.5 Mounting of NGR : On Pedestal Insulator 3.5.1 Requirement of Pedestal Insulator 3.6 (a) (b) Type Porcelain/ Silicon rubber (a) Minimum Creepage 438 mm (b) One minute Power frequency withstand voltage 55 kVrms (c) Lightning Impulse Withstand voltage 125 kVp (d) Mounting Structure Non magnetic material Tests Measurement of winding resistance. Measurement of insulation resistance between winding & earth by 5 kV megger. Specifications for 800kV Class Shunt Reactors & Associated 145 kV Neutral Grounding Reactor (c) (d) (e) Measurement of impedance by V/I method Separate source voltage withstand test at 38 kV AC for one minute. Full wave lightning impulse voltage withstand test at 550 kVp on line terminal. 3.7 Test on Reactor Tank 3.7.1 As per requirements of Section A 3.8 Guaranteed Technical Data 1. Manufacture’s name & country of manufacture 2. Type of reactor 3. Standards applicable 4. Type of cooling 5. Rated voltage (kV) 6. Max. continuous current (Amps) 7. Rated short time current for 10 sec. (Amps) 8. Ohmic value (ohms) Rated reactive capacity for continuous operation at rated voltage (MVAR) 9. Rated frequency (Hz) 10. Guaranteed max. losses at rated voltage and frequency at rated output (kW) 11. Insulation level (winding & bushing) (a) Lightning impulse (kVp) (b) Power frequency (kVrms) (c) Neutral brought out at (kV) 12. Type of insulation (Graded or Uniform) 13. Overall dimensions & weights (a) Length (mm) (b) Breadth (mm) (c) Height (mm) (d) Un tanking heights (mm) 14. Shipping data (a) Size of largest package (mm)x(mm)x(mm) (b) Weight of largest package (kg) (c) Gross weight to be handled (kg) (d) Gross volume to be handled (cu.m) 15. Compliance to technical specification w.r.t. parameter specified for (a) Bushing CT (b) Bushings (c) Terminal connectors 153 154 Manual on Transformers 16. Whether similar equipment are type tested & are in successful operation for at least two years (If yes, furnish type test reports) 17. Overall general arrangement drawing of Neutral Grounding Reactor with all accessories to be enclosed 18. Bushing Current Transformer (a) Line Side (i) Type or voltage class (ii) Ratio (iii) Accuracy class (iv) Burden (VA) (v) Accuracy limit factor (vi) Knee point voltage (Volts) (Minimum) (vii) Maximum resistance of secondary winding (ohms) (viii) Maximum exciting (mA) current (b) Neutral Side (i) Type or voltage class (ii) Ratio (iii) Accuracy class (iv) Burden (VA) (v) Accuracy limit factor (vi) Knee point voltage (Volts) (Minimum) (vii) Maximum resistance of secondary winding (ohms) (viii) Maximum exciting (mA) current SECTION L Specifications for Earthing Transformers SECTION L Specifications for Earthing Transformers 1.0 SCOPE 1.1 This section covering specification for earthing transformers, does not purport to include all the necessary provisions of a contract. For general requirements,loss capitalization and tests, reference shall be made to Sections ‘A’, ‘AA’ and ‘BB’ of the Transformer Manual. 2.0 GENERAL 2.1 Unless otherwise modified in this section the earthling transformers shall comply with latest versions of IS 5553 (Part 6) and IS 2026. 2.2 Three phase earthing transformers provide an artificial neutral and are used for the following purposes : (a) (b) (c) (d) to earth an otherwise unearthed system. to connect single phase loads between lines and neutral. to connect an arc suppression coil. to limit fault current during a line to earth fault determined by the zero sequence impedance of earthing transformers and also by the possible addition of resistors and thereby permitting selective protection. *Note : The provision of the earthing transformer does not necessarily make the system effectively earthed. (e) Earthing transformers with zigzag (inter-star) connected winding can have a star connected secondary winding to provide an auxiliary supply. 2.3 Construction of earthing transformer is similar to conventional oil filled transformer. Usually cooling specified is ONAN type. 3.0 WINDING CONNECTIONS Earthing transformers are usually connected either in zigzag (inter-star) or star-delta. For stardelta transformer the secondary delta winding shall always be connected in closed delta. The neutral of star connected main winding is earthed. Earthing transformer which consists of a single winding connected in inter-star may also be provided with, an auxiliary (secondary) winding. This secondary winding when provided shall be connected in star. (a) Primary - Zigzag (inter-star), Secondary - Star (b) Primary - Star Secondary - Delta (c) Primary - Zigzag (inter-star) 3.1 For the purpose of fault current limitation resistors/reactors can be inserted either between primary neutral point and earth or in series with primary terminals of inter-star or star connected primary windings to adjust the zero sequence impedance (Figs. 1 and 2). 157 158 Manual on Transformers For star-delta connected earthing transformer the delta connected winding may be of the open type in order to permit the insertion of a resistor or reactor to adjust the zero sequence impedance. 3.2 Also connecting the resistor/reactors at the neutral end would be preferable. 4.0 TAPPINGS AND TAP CHANGING 4.1 For zigzag connected earthing transformer having auxiliary winding if tappings are required for voltage variation, it shall be provided on zigzag connected main winding. Equal and uniform number of tappings shall be provided on both zig and zag windings of main windings. Fig. 1 Interconnected star (zigzag) neutral earthing transformer Fig. 2 3 phase star-delta neutral earthing transformer Rangeof variation: +5 to -5% in steps of 2.5% Specifications for Earthing Transformers 159 4.2 Tap changing shall be carried out by means of an off circuit externally operated selfpositioning switch (when the transformer is in de-energised condition. Position No. 1 shall correspond to maximum plus tappings. Provisions shall be made for locking the tap changing switch handle in position. 4.3 However, tappings are not preferred for earthing transformer. 5.0 INSULATION LEVEL The insulation level for the line terminals of an earthing transformer shall correspond to those specified for transformers as per IS: 2026 (Part 3). 6.0 LOSSES AND IMPEDANCE 6.1 Losses 6.1.1 Only no-load losses should be specified for earthing transformer not provided with additional auxiliary windings. The tolerance on specified no. load losses will be subject to limits specified in IS: 2026. 6.1.2 Both no-load and load losses will be specified for earthing transformers provided with windings’ suitable for supplying auxiliary loads. The load losses specified should be based on the rating of the auxiliary winding. These losses are also subject to tolerance in accordance with IS : 2026. 6.2 Impedance 6.2.1 Zero sequence impedance of each earthing transformer shall be specified in ohms per phase and this impedance will be subject to a tolerance of +20%. -0%. 6.2.2 When earthing transformers are provided with auxiliary winding impedance between the auxiliary winding and the main inter-star (zigzag) winding must be specified and this impedance shall be subject to tolerance as per IS : 2026. However, if any difficulty arises to achieve both the specified zero sequence impedance of main winding and the percentage impedance between the main winding and auxiliary winding, in such cases either external resistors/reactors may be provided on main windings to adjust the zero sequence impedance or current limiting resistors/reactors may be provided on auxiliary side to limit the fault current on auxiliary side to the specified value. 7.0 CONTINUOUS AND SHORT TIME CURRENT RATING 7.1 Continuous Current 7.1.1 Rated Neutral Continuous Current Continuous neutral current is specified either in the case where phase unbalance of the system exists or when the earthing transformer is to be designed for connection of single phase loads between line and the neutral. 160 7.1.2 Manual on Transformers Rated Continuous Current The current flowing through the line terminals continuously when a rated power of a secondary winding is specified. Note : 7.2 The earthing transformer shall carry the specified neutral or rated continuous current and comply as regards the temperature rise with appropriate requirements of IS : 2026 when it is energised at rated voltage and frequency Rated Short Time Current in the Neutral The earthing transformer shall carry the specified neutral fault current for the specified duration without exceeding the winding temperature of 250°C for copper and a temperature of 200°C for aluminium. When an earthing transformer is designed for the neutral point to be connected to a current limiting impedance in the connection to earth, it should also be capable of withstanding, for a period of 5 seconds, the maximum earth fault current that can flow without the additional impedance in circuit. This safe guard is necessary should, for instance, the bushing of an earth resistor flash over. When earthing transformer are operated without external resistor, the rated short time current and zero sequence impedance shall have the following relationship : Ish = 3. Vph Zo Vph is the maximum permissible operating phase voltage Zo is the zero sequence impedance per limb of earthing transformer Ish is the short time neutral current of the transformer 7.3 Ability to Withstand Rated Short Time Current 7.3.1 The earthing transformers shall be capable of withstanding the mechanical and thermal stresses caused by the rated short time current flowing for the specified duration. The thermal ability can be demonstrated by calculation using the following formula as per clause 9.1 of IS 2026 (Part 5) θ1 = θ0 + a J2t x 103 °C where Q1 is the highest average temperature attained by the winding due to short time current maintained over the specified duration and shall not exceed 250°C for copper winding and 200°C for aluminium winding. Specifications for Earthing Transformers 161 θo is the initial temperature in degree Celsius J is the short time current density in ampere per square millimetre t is the duration in seconds a is a function of 1/2 (θ2 + θ0), in accordance with Table 1. θ2 is the maximum permissible average winding temperature, 250°C for copper and 200°C for aluminium. 7.3.1.1 Where earthing transformers are used with external resistor/reactors to limit the earth fault current, the earthing transformer should also be able to withstand dynamically and thermally the maximum earth fault current without external resister/reactors for a period of 5 seconds. 7.3.1.2 For earthing transformers without secondary winding θ0 shall be taken as the sum of the maximum ambient temperature and manufacturers guaranteed average oil temperature rise of the earthing transformer under normal operating conditions. 7.3.2 For earthing transformer with loaded secondary windings θ0 shall be the sum of the appropriate maximum ambient temperature and the relevant temperature is specified inIS : 2026 measures by change in resistance. Table 1 1/2(Ө0 + Ө2) a - function of 1/2(Ө1 + Ө2) °C Copper windings Aluminium windings 140 7.41 16.5 160 7.80 17.4 180 8.20 18.3 200 8.59 19.1 220 8.99 - 240 9.38 - 7.3.3 Ability of earthing transformer to withstand mechanical stresses due to the rated short time current flowing in the windings under fault conditions shall be determined by tests described as per clause 8.6 of IS 5553 (part 6). 8.0 TESTS 8.1 Type Test • Impulse Voltage Withstand Test (IS : 2026 (Part 3)). • Heat run test (IS : 2026 (Part 2)). Applicable only in the case of earthing transformers having auxiliary winding. 162 8.2 Manual on Transformers Special test • Short circuit withstand test. Clause 8.6 of IS : 5553 (Part 6). 8.3 • • • Routine Test Measurement of winding resistance IS : 2026 (Part 1). Measurement of insulation resistance IS : 2026 (Part I). Measurement of zero sequence impedance IS : 2026 (Part I). Note : Zero sequence impedance may be measured at any current between 25 per cent to 100 per cent rated short time neutral current and is expressed in ohms per phase. It shall be ensured that the applied current shall not exceed the current carrying capability of the winding or metallic constructional parts. • • • • • • • • Measurement of no load loss and no load current IS : 2026 (Part I). Measurement of impedance voltage and loss (in case of auxiliary winding) Dielectric tests (IS: 2026 (Part 3). Separate source voltage withstand Test IS : 2026 (Part 3). Induced over voltage test IS : 2026 (Part 3).Applicable only in the case of earthing transformer with a secondary winding. Check of voltage vector relationship and polarity IS : 2026 (Part 1). Measurement of voltage ratio IS : 2026 (Part 1). Applicable only in the case of earthing transformer with a secondary (auxiliary) winding. Ratio measurement of zigzag connected earthing transformer with star connected auxiliary winding. For a zigzag connected earthing transformer the zig and zag windings constituting one phase arc physically wound on two different limbs of the core. Hence if a single phase supply is applied between line and neutral of inter-star (zigzag) connected winding, the voltage induced in zig and zag winding, will be different. Due to this, voltage induced on secondary winding of same phase will not be the same as that defined by the per phase voltage ratio of the transformer. Thus voltage ratio measurement with single phase application will give misleading results if application and measurement is made on per phase basis, (ie between line and neutral). A vector diagram of a 33/0.435 kV ZNynl connected earthing transformer is given as Fig. 3. The zig and zag windings per limb are designed for 11 kV and LV is designed for 0.435/3kV. Here 3 phase voltage ratio is defined as the ratio of the HV line to neutral voltage to LV line to line voltage (IR-IN/2R-2Y) ie 33/3/0.435 = 43.799 Fig. 3 Vector and voltage relationship or a 33/0.435 kV ZNynl connected earthing transformer For single phase application, the ratio IR-IN/2R-2Y will be : Specifications for Earthing Transformers 11 kV x 2/0435 √3 x2 163 = 33 /3 x 2 /0.435 / 3 x 2 = 33/√3/ 0.435 kV = 43.799 i.e., for a zigzag connected earthing transformer to get actual design ratio with single phase application, the ratio measurement shall be made by applying line to neutral voltage (per phase) on inter-star connected main winding and measuring the induced line to line voltage on corresponding star connected secondary windings or vice versa. Note : For ratio measurement with 3 phase application equal and balanced supply (w.r. to voltage and phase difference) shall be applied, otherwise ratio error will be high. SECTION M Specifications for Furnace Transformers SECTION M Specifications for Furnace Transformers 1.0 SCOPE This section covers specifications for transformers having application to be used along with the Furnaces. However, this does not purport to include all the necessary provisions of a contract. For general requirements and tests, reference shall be made to other Sections ‘A’ and ‘BB’ of the transformer manual. 2.0 GENERAL Standards IS 2026 and IS 12977 to be followed for furnace transformers. Following are the major application for the furnace transformers. 2.1 Melting Ferrous and Non Ferrous scraps. 2.2 Refining of steels and many other materials etc. Furnace Transformers are described as below. 1. Arc Furnace Transformers which includes submergedarc furnace, Electric arc furnace and Ladle refining arc furnace transformers. 2. Induction furnace Transformers. Arc Furnace transformers are withlow secondary voltageswith very high secondary currentsand areused for various process applications.Whereas Induction furnace transformers are having medium & lowsecondary voltages. Constructions of both furnace transformers aresimilar to power transformers to some extent excluding arc furnace transformers which are dealing with very low secondary voltage with higher currents. Here the secondary voltages are intentionally varied with current to melt and boil the product mix for attaining defined chemical composition and characteristics. The variance in voltage and current does depend on the quality of charge which is not predefined with fixed chemical composition. Hence, to make the same Ferro alloys or produce the same steel you have to selectively vary the KVA rating of the transformer alongwith the secondary voltage and current which will ultimately vary from mother ore to mother ore as the ore made available is various underground mines from various parts of the globe. 3.0 WINDING CONSTRUCTION Arc furnace transformers based on the process optimization requirements have possibilities to connect in Star and Delta configuration onprimary winding in order to get additional range of secondary voltages. Star and Delta change overs is done using either external bushings or with the help of star delta change over switch. 167 168 Specifications for Furnace Transformers Secondary windings of Arc furnace transformers are having open delta construction. Closing to Delta configuration is done within the furnace for such transformers. In order to provide stability, higher rating Arc Furnace Transformers is often provided with series reactor. Such reactors may be provided as in built or can be of separate unit.One of the major purpose of providing the series reactor is also because of fast changing of impedance of the molten bath in the crucible where the metal is both in solid-liquid state. Fig. 1 Typical HVwinding connection of Arc Furnace Transformer having Star Delta Change over mechanism Fig.2 Typical winding connections of Arc Furnace Transformer with separate Auto Transformer for tappings. Converter & inverter circuits are connected at the secondaryterminals of Induction furnace transformers as the furnace panel needs to change the frequency of the molten metal bath depending upon flux penetration and magnetic coupling required. Induction furnace Transformers are mainly three winding transformers with secondary windings connected in Star and Delta or in Star configuration. The number of secondary outputs will also depend on the number of phases or pulse required in the melting system for more optimal power utilization and reduction in system harmonics to a large extent. Phase shifting is also provided for induction furnace transformers for larger furnacesto get pulses of required numbers. 4.0 TAPPINGS AND TAP CHANGING Tappings in the arc furnace transformers are provided on Primary windings in order to get optimized process. Depending upon the process and designrequirement either of the following tapping scheme is adopted. 4.1 Direct Regulation. Tappings are provided on the main winding. This is a variable flux voltage variation (VFVV) arrangement. This arrangement which consists of taps at end of primary winding is used for low rating furnace transformers. The cost of OLTC is minimum due to lower voltage and current values (the primary winding, may be of the order Specifications for Furnace Transformers 169 of 33 or 66 kV). In this arrangement the step voltage is variable throughout the range of voltage regulation. Fig. 3 Direct Regulation with Tappings provided on the Main winding. 4.2 Regulation with Auto Transformer This arrangement is used mainly for larger furnace applications. In this arrangement separate auto transformer is used for voltage regulation.This arrangement gives linear or equal steps secondary voltage variation with tapping positions. The auto transformer and main transformer may be housed either in common tank or in separate tanks. For single phase arc furnace transformers, auto transformers are mounted within the same tank of main transformer. Practice of providing separate 3 phase auto transformer for a bank of 3 single phase arc furnace transformers is also seen. This configuration is preferred beyond 12MVA requirement mostly to reduce the copper content of the external secondary bus bars of the furnace system beyond the transformer till the electrodes. These transformers calls for very high material content and hence it is heavy. Logistics always remains a question and moreover these transformers are mounted at a height of 14 to 18 meters platform from the ground zero level. To make the transformers lighter to handle such transformers are manufactured in single phase units rather than 3 phase unit. Fig. 4. Typical Schematic for Regulation with Auto Transformer. 4.3 Regulation with Auxiliary Booster Transformer Such arrangement is used for medium and large rating furnace transformers to avoid constraints related to OLTC current Rating. In this arrangement it is possible to get voltage variation in 170 Specifications for Furnace Transformers equal steps throughout the full range of regulation. Main and Booster transformers are located in the same tank, to minimize the length of connections between the secondary windings for both the transformers in such arrangement. Since the Booster transformer is only for regulation, its rating is much smaller than the main transformer rating. Centre to centre distances and window heights of Booster and Main transformer are generally kept same to facilitate connections between their secondary windings. In this arrangement Secondary current of Main and Booster transformers are equal; the two winding sets are often connected by their leads in a figure of eight which is known as the “eight-eight arrangement”, thus avoiding extra connections between them. Fig. 5. Typical Schematic for Regulation with Auxiliary Booster Transformer. Fig. 6. Eight – Eight Arrangement for LV connection of Auxiliary Booster Transformer. Since frequent tap changing is required for the process applications, Arc Furnace Transformers are mainly provided with On Load Tap Changer.Since the number of tap changing per day of continuous operation can go upto 250 operations a day, it is preferred to use Vacuum type OLTC for this purpose. It is also a practice to use oil online filtration device to maintain oil quality in the diverter chamber. Induction Furnace Transformers are mainly provided with Off Circuit Tap Changers in case the system voltage is stable. In case of unstable system voltage oil type OLTC is required in the primary where there is no phase shifting done in the HV winding. In case phase shifting is required to have higher pulse system then either LV phase shifting can be done with OCTC or OLTC in primary winding. In case phase shifting is done in HV then it is advised not to use OCTC or OLTC in the HV winding as the phase angle will get distorted with change in tapping position with respect to nominal tap at which the phase difference angle has been defined. 5.0 INSULATION LEVEL Insulation level for the line terminals of shall correspond to as specified in IS 2026 (Part 3). 171 Specifications for Furnace Transformers 6.0 LOSSES AND IMPEDANCE In case of specific requirement of auto transformer the impedance of the auto transformer needs to be kept little higher with sufficient hoop resistance to withstand short circuit current flowing through it. Losses of the auto transformer need to be little higher. On addition of both these attributes the auto transformer will have higher withstand-ability of high inrush current. This will save the transformer from failing during switching operationof furnace system. There need not be too much reduction in losses of the transformer that will unnecessarily give rise to cost of transformer as the furnace system has higher losses compared to the transformer only in the secondary bus bar path that carries power from the transformer secondary to the electrode system. Standardization of losses might not work out feasible as KVA rating, secondary voltage and current is always varying from rating to rating and application to application. 7.0 RATINGS The MVA, LV voltage range and current are major factors in case the furnace is operating in fixed pitch mode or variable pitch mode of the electrodes. The thermal delta (the physical triangle formed in the molten bath) will decide the power density required to melt the charge. However, manufacturer of induction furnace based on adopted converter& invertor circuits and majorly the coil voltage required at the furnace crucible decides secondary voltage rating for Induction furnace transformers. 8.0 TEMPERATURE RISE For the purpose of standardization of maximum taemperature of oil and winding, the following ambient temperatures are assumed. Cooling Medium Air Water Maximum Ambient Temeprature 50 30 Maximum Daily average ambient temperature 40 25 Maximum yearly weighted average ambient temperature 32 -- With the above ambient temepratures, temeprature rises are as given below. 9.0 Part Air Water Windings (Measured by Hot Resistance) 55 60 Top Oil (Measured by Thermometer) 50 50 TESTS IS 2026 is referred to for testing of Furnace Transformers. 9.1 Routine Tests: 9.1.1 Ratio Measurement Test 172 Specifications for Furnace Transformers 9.1.2 Check of voltage vector relationship and polarity. 9.1.3 Measurement of winding resistance. 9.1.4 Measurment of Insulation resistance. 9.1.5 Separate Source high voltage withstand test. 9.1.6 Induced over voltage withstand test. 9.1.7 Measurement of No Load Loss. 9.1.8 Measurement of Load Loss. 9.2 Type Tests 9.2.1 Temperature Rise Test 9.2.2 Impulse Test Dynamic short circuit test is not conducted for furnace transformers. The same is demonstrated using IEC / IEEMA guidelines and sophisticated software. Higher time for disconnecting the secondary terminal shorting is expected during hot resistance measurement in temperature rise test of Arc Furnace transformers, efforts should be made to minimize this duration. 10.0 FITTINGS AND ACCESSORIES Unless otherwise specified within the contract, following minimum fittings and accessories are to be provided with transformer. 10.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8 10.9 10.10 10.11 Rating and Diagram Plate Two Earthing terminals Lifting Bollard Jacking pads Haulage Lugs Pocket on tank cover for thermometer Air Release device Conservator with oil- filling hole, cap and drain valve Magnetic type oil gauge with low oil level alarm contacts Silicagel breather with oil seal. Required number of pressure relief device capable of resealing after release of pressure. 10.12 Valves 10.12.1 Drain valve. 10.12.2 Two filter valves on diagonally opposite ends - one at top and other at bottom. 10.12.3 Oil Sampling valve at bottom of main tank. 10.13 Valve schedule plate. Specifications for Furnace Transformers 10.14 10.15 10.16 10.17 10.18 10.19 173 Buchholz relay with alarm and trip contacts One number shut-off valve (Size: 80 mm) on conservator side Dial type oil temperature indicator. Dial type winding temperature indicator for a two winding transformer. Cover lifting lugs. Weather proof Marshaling box for housing control equipment and terminal connections. 10.20 Cooling accessories. 10.20.1 ONAN/OFAF cooling Requisite number of radiators provided with: Air release device on top Drain and sampling device at bottom Lifting lugs 10.20.2 OFWF or ODWF cooling Oil/water heat exchangers with segregated oil and water headers Oil pumps with shut-off valves on both sides. Oil and Water flow indicator with alarm and trip contacts Dial type thermometer. Pressure gauge. Differential pressure gauge with alarm contacts, operating when difference between oil outlet pressure and water inlet pressure is less than 0.2 kg/cm2 Drain and sampling device on cooler pipe connection. 10.21 It is alwaysrecommended to provide Surge suppressors and RC Elements at the line terminals of Furnace transformer at users end. SECTION N Specifications for Rectifier Transformers SECTION N Specifications for Rectifier Transformers 1.0 SCOPE This section covers specifications for transformers having application to be used along with the low voltage rectifier circuits. However, this does not purport to include all the necessary provisions of a contract. For general requirements and tests, reference shall be made to Sections ‘A’ and ‘BB’of the transformer manual. 2.0 2.1 GENERAL Standards IS 2026, IEC 61378 and IEC 146 to be followed for rectifier transformers. Following are the major application of the rectifier transformers. Electrolytic process for Aluminum, Zinc, Copper etc. Chemical process of Caustic Soda, Zinc, Copper, Chlorine etc. Rectifier Transformers are used with either of the following DC circuits. Rectifier Bridge connection 6 Pulse or 12 pulse for medium to high DC voltages Rectifier single-way inter-phase connection for low DC voltage levels & high DC 2.1.1 2.1.2 2.2 2.2.1. 2.2.2. currents 2.2.3. Thyristor or diode rectifiers Construction of rectifier transformer is decided based on the configuration of the DC circuit scheme adopted. The rectifier transformers have typically low voltages in the range of few hundreds only. The Rectifier Transformers require wide range of voltage regulation on secondary side and automatic constant current control. For voltage regulation, it is extremely difficult to have taps on the secondary winding because of few turns in LV and high current value. The taps are either provided on the primary winding, or a separate regulating transformer (autotransformer) is used (feeding the primary of the main transformer) which can be accommodated in the same tank. The required double-wound or auto-connected regulating transformer can, depending on transport or site limitations, be built into the same tank as the rectifier transformer, or into a separate tank. For large rating rectifier transformers, the field due to high currents causes excessive stray losses in structural parts made from magnetic steel. Hence, these parts are usually made of nonmagnetic steel. Due to a large variety of applications, there are several influencing factors to consider: • Voltage range and step voltage. 177 178 • Manual on Transformers Double-tier: HV and LV windings in two levels, and Star and Delta connection to achieve a 12-pulse reaction. Pulse numbers higher than 12: requires additional phase shifting windings. LV bushing arrangement: adapted to suit rectifier design and to limit structural heating. Bushings /risersare typically mounted on the tank side wall. • • 3.0 WINDING CONSTRUCTION Rectifier transformers are provided with two or multi windings based on DC circuit adopted. In case of Bridge configuration, two windings connected either in Star or Delta is adopted. In case of Configuration with inter phase transformer multi windings with secondary connected in Star configuration is provided. In both of the cases, Configuration of primary winding can either be star or delta and decided by the rectifier manufacturer. Fig. 1 Typical vector diagram of 12 pulse Scheme with Inter phase Transformer (a) Fig.2 Typical vector diagram of 6 pulse Bridge configuration. Windings HV winding: The HV windings of the rectifier transformer are usually disc type and are connected in either star or delta or zigzag. These type of connection are used to get the phase shift between the input and output voltages to increase the no of pulses of the rectified DC output. More number of pulses of the DC output system reduce the ripples there by improving the quality of DC output. LV winding: The secondary voltage of the rectifier transformer is very less and of the order of a few volts. Thereby, the turns in the secondary winding are also very less. On the other hand, current carried by these windings is very high. For this reason a special winding called half & Sections is used. This resembles the disc winding to the extent that each coil is wound in two disc accommodating the total number of turns. A number Specifications for Rectifier Transformers (b) 179 of coils of this type are connected in parallel by bus-bars for sharing total current. The advantage of this coil is that odd number of turns can be accommodated in two discs of the coil without any loss of space, which cannot be done with the normal disc-winding technique. This improves the space factor of the winding and insures compactness. LV winding arrangements: adapted to minimize the winding hotspots and influence of harmonics. Interleaving of LV windings The two groups of coils of both the secondary are axially interleaved to ensure that the impedance between the two secondary is minimum and the impedance of the secondary with respect to the primary windings is same so that all the coils of both the secondary share the current equally. The ends of the bus-bars are then connected in star or delta as the case may be. The bus bars are placed in go-no go arrangement in such a manner that the fluxes are getting cancelled as a result there would be minimum reactance between bus-bars and stray losses.. Coil disposition Since the secondary coils carry heavy currents and the coils are connected in parallel by means of bus-bars, it is essential that the secondary coils are placed outermost for ease of connection. Consequently, HV coil is placed concentrically over the core, over which the LV coils are placed. Sometimes an electrostatic shield between coils is provided to limit transferred surges. Voltage Regulation By OLTC/OCTC In case of OLTC, the voltage variations are being achieved on line by changing OLTC taps. The OLTC is generally used with Diode Type Rectifier Unit. The OCTC tap-changers are generally used for the transformer coupled with Thyristor rectifier. In this case the voltage variations are being achieved by change of firing angle of the thyristor, which is not economical alway By Auto transformer The diode rectifiers have a longer range and a higher number of smaller voltage steps in the transformer. A multi-coarse-fine on-load tap changer (OLTC) or an OLTC/NLTC combination is preferred, together with LV-side saturable reactors for the voltage finetuning. Electrolysis processes require wide regulation ranges, fast and tune voltage regulation. The Wide regulation is carried out by a regulating transformer. The fast & tune regulation is carried out either by a set of saturable reactors. Aluminium electrolysis Rectiformer requires wide range of secondary voltage variation from 0% to 100% to start pot line from beginning. The whole range of regulation is 180 Manual on Transformers split up into smaller ranges with a combination of OLTC (On load tap changer) + Saturable reactor. (c) Saturable Reactor core (Transductors) When connected to a diode technology rectifier, Self-Saturable Reactors offer solutions for fast & tune voltage regulation. These Saturable Reactors are placed in rectifier transformer tank. Magnetic cores mounted on LV copper busbars carrying the main current produces a voltage drop. The saturation level is controlled by a DC current circuit permitting a controlled variation of voltage drop. In normal practice, there will be two DC windings of Self-Saturable Reactors namely bias winding & control winding. The control and bias windings are in the form of copper rod passing through the ring core. This fine regulation is equivalent to 2 or 3 steps of the OLTC and discrete voltage achieved. Ring type cores are being used. (d) Interphase Transformers Two or three rectifier systems may need to be paralleled, when the current rating increases. The paralleling is done with the help of inter phase transformer which absorbs at any instant the difference between the direct voltages of individual systems so that there are no circulating current. Since the flux in the magnetic circuit of the interphase transformer is alternating with 3 times the supply frequency i.e 150Hz, when two systems are paralleled, the core losses in IPT are high. Hence, the operating flux density in the interphase transformer is designed to be around 50 to 67% of the value used for the conventional transformer. The ‘C’ type cores are used in IPT. 4.0 INSULATION LEVEL Insulation level for the line terminals of shall correspond to as specified in IS 2026 (Part 3). 5.0 LOSSES AND IMPEDANCE Unless otherwise specified, Losses and the Impedance of transformer are specified at the principal tap. Principal tap is having maximum LV voltage with highest MVA. In case of transformer having application of self Saturable reactor, values and measurement scheme of losses and percentage impedance shall be agreed between purchaser and supplier. Losses in inter phase transformer and Saturable reactors, if not measured shall be demonstrated by calculations. 6.0 STANDARD RATINGS There are no standard ratings of Rectifier Transformers. The rating is decided based on the defineda pplications. 181 Specifications for Rectifier Transformers 7.0 TEMPERATURE RISE Rectifier Transformers are subjected to non linear loads having significant harmonic contents during service conditions. Pattern of such harmonics shall be considered during limiting top oil and winding rises. OFWF cooling is standardized for rectifier transformers. For the purpose of standardization of maximum temperature of oil and winding, the following ambient temperatures are assumed. Cooling Medium Water Maximum Ambient Temeprature 30 Maximum Daily average ambient temperature 25 Maximum yearly weighted average ambient temperature -- With the above ambient temepratures, temeprature rises are as given below. 8.0 Part Water Windings (Measured by Hot Resistance) 60 Top Oil (Measured by Thermometer) 50 TESTS IS 2026 is referred to for testing of Rectifier Transformers. 8.1 Routine Tests 8.1.1 Ratio Measurement Test 8.1.2 Check of voltage vector relationship and polarity. 8.1.3 Measurement of winding resistance 8.1.4 Measurment of Insulation resistance 8.1.5 Separate Source high voltage withstand test 8.1.6 Induced over voltage withstand test 8.1.7 Measurement of No Load Loss 8.1.8 Measurement of Load Loss, Calculation of Load Loss shall be as oer IEC 146. 8.2 Type Tests 8.2.1 Temperature Rise Test considering increased value of losses due to harmonics 8.2.2 Impulse Test Dynamic short circuit test is not conducted for Rectifier transformers. The same is demonstrated using IEC / IEEMA guidelines and sophisticated software. Higher time for disconnecting the secondary terminal shorting is expected during hot resistance measurement in temperature rise test of Rectifier transformers, efforts should be made to minimize this duration. 182 9.0 Manual on Transformers FITTINGS AND ACCESSORIES Unless otherwise specified within the contract, following minimum fittings and accessories are to be provided with transformer. 9.1 Rating and Diagram Plate 9.2 Two Earthing terminals 9.3 Lifting Bollard 9.4 Jacking pads 9.5 Haulage Lugs 9.6 Pocket on tank cover for thermometer 9.7 Air Release device 9.8 Conservator with oil- filling hole, cap and drain valve 9.9 Magnetic type oil gauge with low oil level alarm contacts 9.10 Silicagel breather with oil seal. 9.11 Required number of pressure relief device capable of resealing after release of pressure. 9.12 9.12.1 9.12.2 9.12.3 Valves Drain valve. Two filter valves on diagonally opposite ends - one at top and other at bottom. Oil Sampling valve at bottom of main tank. 9.13 Valve schedule plate. 9.14 Buchholz relay with alarm and trip contacts 9.15 One number shut-off valve (Size: 80 mm) on conservator side 9.16 Dial type oil temperature indicator. 9.17 Dial type winding temperature indicator for a two winding transformer. 9.18 Cover lifting lugs. 9.19 Weather proof Marshaling box for housing control equipment and terminal connections. 9.20 Cooling accessories. OFWF or ODWF cooling Oil/water heat exchangers with segregated oil and water headers Oil pumps with shut-off valves on both sides. Oil and Water flow indicator with alarm and trip contacts Dial type thermometer. Pressure gauge. Specifications for Rectifier Transformers 9.21 183 Differential pressure gauge with alarm contacts, operating when difference between oil outlet pressure and water inlet pressure is less than 0.2 kg/cm2 Drain and sampling device on cooler pipe connection. It is recommended to provide Surge suppressors and RC Elements at the line terminals of Rectifier Transformer. Location of the same may be decided between purchaser and manufacturer. SECTION O Specifications for Electrostatic Precipitator Transformers SECTION O Specifications for Electrostatic Precipitator Transformers 1.0 Scope This section covers specifications High voltage rectifier transformers (HVR) having application for supplying power to Electrostatic Precipitator (ESP) used for cleaning the flue gases. However, this does not purport to include all the necessary provisions of a contract. For general requirements and tests, reference shall be made to Sections ‘A’ and ‘BB’ of the transformer manual. 2.0 General Since several other components are part of the transformer housed in the same tank, generally Standard IS 2026, where ever applicable, is followed for HVR transformers. HVR Transformer is used for thyristor controlled HV DC power supply to the high voltage fields of Electrostatic Precipitator (ESP). An Electrostatic Precipitator (ESP) used in industries and especially in power plants, is equipment which utilizes an intense electric field for extraction and removal of suspended particles / dust from flue gases to clean the flue gases before discharging the same into the atmosphere to avoid pollution. 3.0 Construction The HVR Transformer is a single phase step up transformer fitted with various components as HV Rectifier, HF Choke, HV Resistor column, and a linear reactor inside the same tank. The single phase AC input supply of 415 V ± 10 % is fed to the transformer through a microprocessor based electronic controller (EC). EC-HVR combined unit provides controlled HVDC supply to the ESP electrodes. EC controls the input voltage of the HVR in-order to optimize the ESP operation based on the HVR output current & voltage feedbacks. The high voltage side output of the transformer is connected to Diode Bridge Rectifier for full wave rectification to DC output up to 95 kVp and up to 1600 mA. The positive polarity of the DC output is earthed during operation. The negative polarity of the transformer is taken out through a bushing. This negative polarity output is connected to the ESP field coils. The flue gases passing between the electrodes of ESP are subjected to an Intense Electric Field. Continuous sparks take place across the electrodes during dust extraction from the Flue gases, which shorts the HV output of the transformer to the Earth. An HVR transformer has to withstand these momentary short circuits, in the form of sparks throughout its life. A linear reactor is connected in the input of the transformer to increase the impedance and limit the short circuit current in the event of short circuit.The supply to transformer is continuously switched on and off by electronic controller for controlling spark and maximum dust collection. The high frequency choke is connected to the output to provide high impedance to high frequency currents. HV Resistor column is provided as potential divider for providing feedback of output DC voltage to the electronic controller. 187 188 Manual on Transformers The transformer may be manufactured with a breather or may be made in hermetically sealed construction without a breather. The transformer tank is filled with Mineral transformer oil (IS-335) or Silicone transformer fluid (IEC-60836) having high flash point based on the requirement. 4.0 Tappings Changing The input voltage supplied to the HVR transformer is controlled through the electronic controller for the required output voltage and current. Tappings are not required for the HVR transformers. 5.0 Insulation Level HV winding of the HVR transformer is tested for separate source withstand voltage at 10 kV since the positive polarity of DC output is earthed at site during operation. However, separate source withstand voltage for LV winding, induced over voltage withstand and Impulse voltage withstand levels are as specified in IS 2026 (Part 3). 6.0Losses and Impedance Since the transformer experiences frequent short circuits during operation, the impedance offered to the short circuit is kept high of the order of 25 to 35 % by providing a linear reactor in the LV side of the transformer. 7.0 Standard Ratings The voltage rating of the transformer is standardized with DC peak output voltage of 70 kVP and 95 kVp. The DC output current rating of the transformer varies from 400 mA to 1600 mA as per the requirement. 8.0 Temperature Rise Temperature rises of winding and oil above ambient temperature of 50 °C are as given below: Part 9.0 Temperature Rise (°C) Windings (Measured by Hot Resistance) 55 Top Oil (Measured by Thermometer) 50 Tests IS 2026 in general is followed for testing of HVR Transformers, where ever applicable. Specifications for Electrostatic Precipitator Transformers 189 9.1Routine Tests 9.1.1 Ratio Measurement Test1 9.1.2 Measurement of winding resistance1 9.1.3 Measurment of Insulation resistance 9.1.4 Separate Source high voltage withstand test2 9.1.5 Induced over voltage withstand test 9.1.6 Measurement of No Load Loss and current 9.1.7 Measurement of Load Loss and impedance 9.2 Type Tests: 9.2.1 Temperature Rise Test3 9.2.2 Impulse Test4 9.2.3 Spark Test (Short Circuit Test) 5 9.2.4 Functional Tests – Measurement of parameters pertaining to voltage and current feedback signals to electronic controller, and check of different functional operational requirement with the electronic controller. Ratio and resistance of the transformer is measured during assembly of the transformer with other components, since these tests are not possible after assembly and tanking due to connection of various other components, 1 HV winding of the HVR transformer is tested for separate source withstand voltage at 10 KV since the positive polarity of DC output is earthed at site during operation 2 The temperature rise will be computed from the hot winding resistance of the LV winding since various components are connected in series with the HV winding of the transformer. 3 Impulse test is be conducted on the transformer without linear reactor, Rectifiers, HF choke and Resistor column 4 Continuous sparks take place across the electrodes of ESP during dust extraction which shorts the HV output of the transformer to the Earth. An HVR transformer has to withstand these momentary short circuits, throughout its life. The short circuit test is conducted on the HVR transformer along with the electronic controller, by simulating this site condition with a spark gap connected across the output. The gap is adjusted to get the required no. of sparks per minute at required peak voltage which is the short circuit condition while the electronic controller controls the voltage to quench the spark. 5 10.0 Fittings and Accessories Unless otherwise specified within the contract, following minimum fittings and accessories are to be provided with transformer. 10.1 10.2 10.3 Rating and Diagram Plate Two Earthing terminals Jacking pads 190 Manual on Transformers 10.4 10.5 10.6 10.7 10.8 Air Release plug Conservator with oil- filling hole Magnetic oil level indicator with low oil level alarm contacts Silicagel breather Pressure relief valve / vent 10.9 10.10 10.11 10.12 10.13 10.14 Drain plug Valves – Two valves on diagonally opposite ends - one at top and other at bottom. Buchholz relay with alarm and trip contacts One number shut-off valve on conservator side Dial type oil temperature indicator. Weather proof Marshaling box for housing control equipment and terminal connections (housing relays, feedback resistor, OTI etc. and the associated wiring). SECTION P Specifications for Traction Transformers SECTION P Specifications for Traction Transformers 1.0 Scope This section covers specifications for transformers used on board rolling stock. However, this does not purport to include all the necessary provisions of a contract. For general requirements, loss capitalization and tests, reference shall be made to Sections ‘A’, ‘AA’ and ‘BB’ of the transformer manual. 2.0 General IEC-60310 is followed for traction transformers. Traction transformers are deployed to power the main traction circuit as well as various auxiliaries circuits such as train air conditioning, train pantry car, locomotive/EMU auxiliaries etc. Traction transformers are used in: 2.1 Electric locomotives 2.2 Electric multiple units (EMU) Traction Transformers can be subdivided as: l Fixed ratio transformers l Transformers with HV taps l Transformers with LV taps Traction transformers are basically step down transformers – used to step down the overhead catenary (OHE) voltage levels (typically 25 kV) to traction system levels (around 1000 V). Traction transformers are generally forced cooled to save the precious equipment space. Inhibited Mineral oil is generally used for the cooling of the transformers. However other types of synthetic oils are also gaining popularity owing to high flash points – directly related to fire safety. Traction transformers are under slung mounted and should be suitable to withstand the shock/ vibrations experienced during traction service. The transformer base should be rigid enough to avoid the damages due to the hitting by extraneous objects during service. 3.0 Winding Construction Traction transformers are single phase transformers. Both shell type and core type configurations are used. Windings are generally sandwich coils or concentric coils. The complete core-coil assembly is finally fitted in horizontal position inside the tank. The core coil assembly needs adequate strengthening and support - to sustain the shocks and vibrations experienced during traction service. 193 194 4.0 Manual on Transformers Tappings For traction power control, one or more of the windings may be equipped with tappings. Tappings can be on the HV side or the LV side as per the system configuration. 5.0 Insulation Level Insulation level for the primary winding and secondary windings shall be as per IEC-60310 or as specified by the purchaser. 6.0Losses and Impedance Losses are normally not specified for traction transformers by the purchaser. For traction transformer, generally the efficiency is specified at rated load. Losses are generally higher as compared to the equivalent rating power transformers – in order to pack more power in the limited space. Impedance is specified by the system designer based on the traction controls. 7.0Ratings Traction transformers usually have several secondary windings e.g. traction, auxiliary, hotel load etc. The rating of the transformer is specified in kVA which is individually specified for each winding. 8.0 Temperature Rise Limits of temperature rises shall be as per IEC-60310 or as specified by the purchaser. 9.0 Tests IEC 60310 is followed for testing of traction transformers. List of applicable tests is as under: 9.1Routine Tests 1. Measurement of winding resistance 2. Measurement of voltage Ratio 3. Measurement of No Load Loss & No-load current 4. Measurement of Impedance voltage 5. Measurement of load loss 6. Induced voltage withstand test 7. Separate Source voltage withstand test Specifications for Traction Transformers 9.2 9.3 Type Tests 1. Determination of total losses 2. Temperature Rise Test 3. Full wave impulse voltage withstand Test 4. Shock & vibration withstand test Investigation tests: 1. 10.0 195 Behaviour under short circuit conditions (optional) Fittings and Accessories Fittings and accessories shall as agreed mutually between the manufacturer and purchaser. However guidance may be taken from other sections of the manual. SECTION Q Specifications for Dry Type Transformers SECTION Q Specifications for Dry Type Transformers 1.0 SCOPE 1.1 This section of the specification covers the different types of dry type transformers. This section does not purport to include all the necessary provisions of a contract. For general requirements, tests, erection, maintenance and commissioning, reference shall be made to Sections ‘A’, ‘BB’&‘CC’ of the Manual. 1.2 It is not the intent to specify completely all details of design and construction of the equipment. However, the equipment shall conform in all respect to high standard of design, engineering and workmanship and be capable of performing in continuous commercial operations. 2.0 STANDARDS 2.1 Except where specified otherwise herein, all material, equipment and construction shall conform to Indian Electricity Act and rules and latest versions of Indian standards specified below: 2.2 List of Standards (a) IS-11171 : Dry type transformers (b) IS - 2026 (Part-I) : Power transformers - General (c) IS - 2026(Part-II) : Power transformers Temperature rise (d) IS - 2026 (Part-Ill) : Insulation Levels, Dielectric Tests and External Clearances in Air (e) IS - 2026 (Part-IV) : Terminal markings, tappings and connections. (f) : Ability to Withstand Short Circuit (g) IS-12063 : Degree of protection provided by enclosures (h) IEC-60076-11 : Power transformers Dry type transformers 3.0 IS - 2026 (Part-V) SERVICE CONDITIONS The transformer to be supplied against this specification shall be suitable for satisfactory continuous operation under the climatic condition prevailing at site and to be specified by the purchaser as per IS2026/ International Standard as under, 199 200 (i) (ii) (iii) (iv) (v) (vi) Manual on Transformers Location : .... Max ambient air temperature (Deg.C) : .... Min. ambient air temperature (Deg.C) : .... Max. average daily ambient air temperature (Deg.C): .... Max. yearly weighed average ambient temperature (Deg.C): .... Max. altitude above mean sea level (m) : .... 4.0 TERMINOLOGY 4.1 Dry Type Transformer A transformer in which mineral oil or any liquid is not employed either as a cooling or insulating medium. Cooling will be by natural circulation of air or by forced air cooling. (AN or AN/AF as per IS11171). 4.2 Dry Type Transformers are classified into following categories depending on the insulation potting process: (a) Vacuum Pressure Impregnated (VPI) Transformers have coils impregnated with polyester varnish/resin under vacuum & pressure. These transformers are generally available upto 1800C (Insulation Class ‘H’). Some manufacturers also use silicon resin for impregnation. (b) Cast Resin Transformers have coils encapsulated in epoxy resin by molding process. Depending on the temperature class of resin used these transformers are available upto 1550C (Insulation class ‘F’) or 180°C (Insulation Class ‘H’). 5.0 STANDARD RATINGS FOR 3 PHASE TRANSFORMERS Standard ratings of dry transformers with losses & impedances are recommended in Table 1 Table 1 : Standard ratings and losses of Dry type transformers. Rating kVA 100 160 200 250 315 400 500 630 800 1000 1250 1600 2000 2500 Total Loss Total Loss at 50% load at rated load %Z kV * kV * HV upto 22kV class/ LV upto 600V 0.94 2.4 4 1.29 3.3 4 1.5 3.8 4 1.7 4.32 4 2 5.04 4 2.38 6.04 4 2.8 7.25 5 3.34 8.82 5 3.88 10.24 5 4.5 12 5 5.19 13.87 6 6.32 16.8 6 7.5 20 6 9.25 24.75 6 Total Loss Total Loss %Z at 50% load at rated load kV kV * kV * HV 33kv class/LV upto 600V 1.12 2.4 5 1.42 3.3 5 1.75 4 5 1.97 4.6 5 2.4 5.4 5 2.9 6.8 5 3.3 7.8 5 3.95 9.2 5 4.65 11.4 5 5.3 12.8 5 6.25 14.5 6 7.5 18 6 8.88 21.4 6 10.75 26.5 6 Specifications for Dry Type Transformers 201 Total Loss values given in above table are applicable for thermal classes,B,E & F and have component of load loss at reference temperature according to clause 17 of IEC 60076-11. An increase of 7% in above losses for thermal class H is allowed. * Reference temperature for load losses = average winding temperature rise as given in column 2 of table 2 plus 30 deg C Other higher ratings of dry type transformers upto 15 MVA having HV upto 33 kV and LV upto 11 kV are possible depending on the application for which they are required. 6.0 RATED FREQUENCY The standard frequency shall be 50 Hz with a tolerance of ± 3 percent. 6.1 Operation other than the Rated Voltage and Frequency 6.1.1 Transformer built in accordance with this specification may be operated at its rated kVA at any voltage within ± 10 percent of the rated voltage at that particular tap. 6.1.2 The transformer shall be capable of delivering rated current at a voltage equal to 105 percent of the rated voltage. Note : The slight temperature rise increase which would correspond to the 5 percent over voltage due to high no load loss is disregarded. 6.1.3 A transformer for two or more limits of voltage or frequency or both shall give its rated kVA under all the rated conditions of voltage or frequency or both; provided an increase in voltage is not accompanied by decrease in frequency. 7.0 (a) (b) 7.1 ELECTRICAL CHARACTERISTICS AND PERFORMANCE Thermal classification of insulation and permissible temperature rises should confirm to class ‘F or class ‘H’ as per relevant clause of IS 11171. Impedance voltage and short circuit performance against, thermal and dynamic requirements arc applicable as per relevant clauses of IS 2026 & IS 11171. Core The core shall be stacked type generally of high grade cold rolled grain oriented silicone steel lamination having low loss and good grain properties coated with carlite. The stacked lamination will be bolted or tied with belts firmly to prevent vibration or noise. The complete design of core must ensure permanency of the core losses with continuous working of transformers. Cruciform core with Mitred/ step lap joints are used to reduce the core losses and magnetizing current. 7.2 Windings The low voltage and high voltage windings can be made of Copper, Foil/ insulated rectangular 202 Manual on Transformers strips. Aluminium foils/conductors can also be used. However, losses recommended in Table 1 Note: The Low Voltage (<600V) windings of cast resin transformer can be with no casting in mold if insulation , pre impregnated with heat activated epoxy resin, is used. 7.3 Insulating Material The insulating material shall conform to the thermal class of Insulation specified. Table-2: Winding Temperature Rise Limits 7.4 Insulation system temperature (°C) Average winding temperature rise limits at rated current K 105 (A) 50 120 (E) 65 130 (B) 70 155(F) 90 180(H) 115 200 125 220 140 Protective Housing for Dry Type Transformer Dry type transformer shall be provided with suitable protective sheet steel housing, if required by the site conditions with minimum IP 43 degree of protection for the enclosure for outdoor and IP20 for indoor transformers. The housing shall have ventilation louvers/ opening provided with wire mesh screens and shall be supplied with suitable lifting lugs. Safety limit switches shall be provided and wired in such a way that the incoming supply may be disconnected whenever any one of the sides of the enclosures are opened with the transformer in energized condition. The provision for suitable padlocking shall be provided on the doors of enclosure for safety. 7.5 Off-Circuit Links The off-circuit tapping links shall be provided on the HV side with appropriate register plate to show the link location. The links shall be provided with inspection window for viewing purpose. 7.6 Insulators & Bushings HV & LV insulators & Bushings shall be of epoxy mould type. 7.7 Termination LV & HV termination shall be suitable for cable/ busduct termination as per the purchaser’s specifications. 7.8 Transformer Fittings Each transformer shall be fitted with following accessories : (i) Inspection covers. 203 Specifications for Dry Type Transformers (ii) (iii) (iv) (v) (vi) (vii) Off circuit links in the primary for voltage variations Rating & diagram plates Terminal marking plate Two nos. earthing terminals Lifting lugs and haulage lugs / holes Winding temperature detectors with solid state type temperature signaliser with digital read out and requisite sets of remote signaling contacts for alarm and trip operation. (viii) Under carriage with bi-directional rollers with locking and bolting devices. Suitable arrangement for core and winding assembly to draw out the same. (ix) Marshalling box complete with all instruments, accessories and fittings as required for the transformer. (x) Danger plate indicating “entry prohibited under energized condition” of the transformer. 8.0 INSULATION LEVELS Highest voltage for equipment Um kV (rms) Rated short duration power frequency withstand voltage kV (rms) Rated lightning impulse withstand voltage kV (peak) List 1 List 2 < 1.1 3 3.6 10 20 40 7.2 20 40 60 12.0 28 60 75 17.5 38 75 95 24.0 50 95 125 36.0 70 145 170 Choice between List 1 and List 2 as per relevant clause of IS 11171 & IEC 60076 Part 11 9.0 EXTERNAL CLEARANCES The following clearances are to be maintained in air between line to earth for the respective voltage: Highest voltage for equipment Um kV (rms) Minimum external clearance between line to earth mm < 1.1 3.6 7.2 12.0 17.5 24.0 36.0 25 60 90 110 170 210 280 204 10.0 Manual on Transformers TESTS AND TEST CERTIFICATES The transformer shall be subjected to all the routine, type and special tests as per IS—11171 and 2026 / IEC 60076 as agreed upon between the purchaser and the manufacturer. 10.1 Routine Tests The following shall constitute the routine tests: (a) (b) (c) (d) (e) (f) (g) 10.2 Measurement of winding resistance. Measurement of voltage ratio and check of voltage vector relationship. Measurement of Insulation Resistance. Measurement of impedance voltage (principal tapping), short circuit impedance and load losses. Measurement of no-load losses and current. Separate source voltage withstand lest. Induced over-voltage withstand test. Type Tests The following shall constitute the typetests: (a) (b) 10.3 Lightning impulse test Temperature rise test Special Tests The following tests may be carried out by mutual agreement between the purchaser and the supplier: (a) (b) (c) (d) 11.0 Short circuit withstand test Measurement of acoustic sound level Partial discharge measurement Mechanical tests: IP test on enclosure SURFACE TREATMENT AND PAINTING Surface treatment and painting shall be done as per Section A of this Manual. 12.0 PACKING, TRANSPORT, STORAGE AND INSTALLATION 12.1 Packing • Dry type transformers with Enclosure need not be packed and can be dispatched only wrapped with polythene sheet. Specifications for Dry Type Transformers • • 12.2 205 Dry Type transformers without enclosures should be wrapped with polythene sheet and packed in wooden crates. The Dry Type transformers should be dispatched fully assembled, unless until transport restrictions do not permit. Transport To lift the transformer, all four lifting lugs must be loaded uniformly with equal length straps of suitable length in order to avoid distortion to the top core clamping structure and system of core and coil supports. 12.3 Storage Dry transformers must be stored in a dry, well-ventilated room and covered with a plastic sheet. After a long storage period at extreme low temperatures or after a lengthy period of being deenergised in very humid surrounding, the transformers must be dried before putting back into service. Drying can be achieved by warm air heaters or by industrial dehumidifiers. 12.4 • • • • 13.0 Installation After packing materials and any other blocking means used during transport are removed, the transformer should be cleaned and dusted-off, taking special care of the air cooling ducts between the windings and between the low voltage coils and the magnetic core. Once in final position ensure that the transformer is secured by blocking the rollers. The installation location should be well ventilated. Suitable proper air circulation should be ensured around the transformer during operation at all times. If the Dry Type transformer is required for outdoor installation, then the same shall be specified by customer in initial specifications. Before the putting in service of the transformer, all electrical connections must be checked (incl. the tapping link connections). A poor electrical connection willcause unnecessary heating, resulting in possible damage to the transformer insulation. TEMPERATURE RISE TEST Temperature rise of dry type transformers can be conducted as per following methods which is based on loading type: (i) Direct Loading Method This method is generally followed for small transformersdue to loading consideration. In this method, one winding preferably the inner winding of the transformer is excited at rated voltage with the other connected to a suitable load such that rated currents flow in both windings. 206 (ii) Manual on Transformers Back-To-Back Method This method is appropriate when there are two similar transformers. Two transformers, one of which is the transformer under test, are connected in parallel, and the inner winding is excited at the rated voltage of the transformer under test. By means of different voltage ratios or an injected voltage, therated current is made to flow in the transformer under test until stabilization of the core and winding temperatures (refer Fig-1 below). The hot resistance of the winding is measured and compared with cold resistance to calculate average winding temperature rise. Fig-1 (Back to Back Method) A Voltage source at rated frequency for no-load losses B Source for rated current at rated frequency for load losses C Booster transformer (iii) Simulated Load Method: In this method, Temperature rise is established by combining the short-circuited test (load loss) and the open circuit test (no-load loss) in two stage. In each stage, hot resistance of the winding is measured and compared with cold resistance to calculate average winding temperature rise. The final temperature rise is calculated by adding them together as per IEC 60076-11. Out of all the above three methods, the widely followed method is simulated loading method Following procedure may be followed for temperature rise test: 1. 2. 3. Test procedure shall be followed as per IEC 60076-11. Test should be carried out with/without enclosure as per user requirement. Test shall be conducted at Minimum Tap. Specifications for Dry Type Transformers 4. 5. 6. 7. 8. 207 For measurement of ambient temperature, atleast 4 nos. sensors shall be placed at each side around the transformer at a level approximate half way up to cooling surface, at a distance of 2 meters from the cooling surface. For direct measurement of winding temperature through WTI, atleast 4 nos. RTD sensors (Two in center limb & one in each outer phase) shall be placed as close as possible to the innermost LV winding conductors at the top of the winding and for measurement of core temperature, 1 no. RTD sensor shall be placed in the centre of top yoke. Hot resistance of all winding (centre phase) shall be measured and their final winding temperature rise shall be calculated inline with IEC 60076-11. Accuracy of all the instruments used in testing should be equal to or better than 1 oC. In critical Transformers temperature of core, winding, LV terminals/Busbar/Flange & enclosure shall also be recorded using Laser Gun/Thermo vision camera immediately after test. Vol. II Application, Standard Fittings and Accessories SECTION AA Capitalization Formula for Transformer Losses SECTION AA Capitalization Formula for Transformer Losses A. Capitalization Formula for Power Transformers 1.0 The rate of capitalisation of transformer losses depends upon the rate of interest, rate of electrical energy per kWh, life of transformer and average annual loss factor. The annual loss factor takes into account the loading of the transformer during the year. In computing the rate of capitalisation of ‘Iron losses’, ‘Load losses’ and ‘Auxiliary losses’, following methodology is recommended : (i) Rate of interest (r) (ii) Rate of electrical energy (EC): It is the cost of energy per kWh at the ‘Bus’ to which the transformer is to be connected. This has been taken as Rs. per kWh at 11 kV. (iii) Life of the transformer (n): It is taken as 25 years. (iv) The transformer may be considered in service (LF) for (365x24) 8760 hours in a year. With modern techniques in design of Condition-monitoring etc, for calculation of losses no downtime is considered. (v) The cooling auxiliaries system has been considered in service for 40 percent of the time, the transformer is in service. (vi) Annual losses factor: LS = 0.2LF+0.8(LF)2 where: LS is the annual loss factor LF is the annual load factor. This may be decided by the purchaser depending on the application of transformer. However assuming annual load factor (LF) as 60 percent, annual loss factor (LS) works out to 0.408. (vii) Capitalisation Formula Suggested, Capitalised Cost of Transformer = Initial Cost (IC) + Capitalised Cost of annual iron losses (Wi) + Capitalised cost of annual Load losses (Wc) + Capitalised cost of annual auxiliary losses (Wp). Note: (i) Actual value can be worked out by the purchaser by considering appropriate values of r, EC, LF and LS. (ii) For auto transformer, the load losses capitalisation shall consider the losses due to both HV and IV loaded to their rating with tertiary unloaded unless otherwise required by the purchaser. For other three winding transformers the loading combinations for capitalisation of losses shall be indicated by purchaser. 213 214 B. Manual on Transformers Capitalization Formula for Shunt Reactor As the losses of the shunt reactor cannot be separated in to Iron Loss and Copper Loss, It is customary to guarantee the Total Losses of the shunt reactor. In such a case it is recommended that the capitalization formula as applicable for Iron losses of the power transformers shall be applied to the total losses of the Shunt Reactor. C. Capitalization Formula for Distribution Transformers For distribution transformers, the following illustrates the method for calculating the loading factors for evaluation of loss capitalization to be specified by the purchaser. The values indicated are typical values and the utility may adopt values different from those indicated in case the rates of interest, cost of energy and the number of hours of operation are different from those indicated in the example below. A life expectancy of less than 25 years is not recommended. LOADING FACTOR FOR NO LOAD LOSS A= H x Ec x [{(l+r)n -1} / r(l+r)n ] Rate of Interest in per unit r Expected life n Number of hours of operation in a year H Cost of energy to the utility at UkV level Ec 1+r (1+r) n (l+r) n-l [{(l+r) n–l} /r(l+r) n] A=loading factor per kW of iron loss 0.08 25 8400 Ec 1.08 6.848475 5.84848 10.67477619 Rs. 89668iEc LOADING FACTOR FOR COPPER LOSS LLF = 0.2*LF + 0.8 *LF*LF 1 = 0.3 1 1 B = A x LLF Where A B H = Loading factor in Rs. per kW of no load loss. = Loading factor in Rs. Per kW for load loss. = No. of hours, the transformer will remain charged in a year, i.e., no. of hours of operation (taken as 8400 hrs.). Ec = Cost of energy to the utility at 11 kV level. r = Rate of interest per unit (Taken as 8%) n = Expected life of the transformer (Taken as 25 years) LLF = Loss load factor (where LLF=0.2 x Load factor + 0.8 x L.F.2) LF = Load factor (taken as 0.5) Ec = Cost of energy (in Rupees per unit at 11 kV feeder level). Note: In case of non-availability of Ec (Energy cost per unit) at 11 kV feeder level, utility should consider the Bulk Rate Tariff plus 5% as the cost of energy at 11 kV feeder level. Load factor considered for Bfactor is 0.5, higher load factor may be considered for urban areas. SECTION BB Test Requirements for Transformers SECTION BB Test Requirements for Transformers 1.0 General Test requirements, procedures and criteria for successful testing of transformers are defined in national and international standards, i.e. IS 2026 and IEC Publication 60076 This Section describes specific requirements for performing tests specified in IEC Publication 60076, IS 2026 and other standards applicable to distribution, power and regulating transformers. It is intended for use as a guide and reference for testing of transformers. The chapter covers purpose, interpretation and explanation of specific conditions pertaining to the testing of transformer and procedure for correction when ideal test conditions can not be achieved. The main objectives of this manual are following: • • • • 2.0 To ensure system needs are met To obtain technical uniformity To provide inputs for proper interpretation of test results To eliminate unsuccessful practices Necessity of tests on transformer When all manufacturing processes have been completed, tests are performed on transformer at the manufacturer’s works to ensure the following purposes: (1) To prove that the design meets the specified job requirements and to obtain transformer characteristics. (2) To check that the quality requirements have been met and that performance is within the tolerance guaranteed. Tests performed for the former purpose are referred to as Type Tests and that for the latter purpose are referred to as Routine Tests (carried out on every unit manufactured). In addition to the aforesaid two categories of tests, Special Tests may also be performed to obtain information useful to the user during operation or maintenance of the transformer. Transformer is important and vital equipment, it is therefore necessary to ensure its proper performance throughout its service life. Also during transportation, installation and service operation, the transformer may be exposed to conditions, which adversely affect its reliability and useful life. It is therefore necessary to do the field testing to ensure good operating health of transformers. 3.0 Tests The general requirements and details of the various categories of tests (Routine Tests, Type Tests and Special Tests) are in accordance with IEC Publication 60076 (latest edition). The Indian 217 218 Manual on Transformers standard IS: 2026 is under revision and is expected to be revised in accordance with IEC. The customer specific requirements are referred hereto as Additional Special tests and Mechanical Tests. The following tests are generally performed on the transformer which may also form part of the customer acceptance: (A) Factory Tests • 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. • 11. 12. • 13. 14. 15. 16. 17. 18. 19. Routine Tests Measurement of winding resistance Measurement of voltage ratio, polarity and check of voltage vector relationship Measurement of no-load loss and excitation current Measurement of short-circuit impedance and load loss Measurement of Insulation resistance Tests on on-load tap-changers, where appropriate Switching impulse withstand voltage test, transformer winding Um > 170 kV Lightning impulse withstand voltage test, transformer winding Um > 72.5 kV Separate-source withstand voltage test Induced AC over voltage withstand test with partial discharge measurement (The tests at sl. no.7, 8, 9 and 10 above are referred to as Dielectric Tests) Type Tests Lightning impulse voltage withstand test, for Um ≤ 72.5 kV Temperature rise test Special Tests Lightning impulse test on neutral terminal Long-duration induced AC voltage test (ACLD) transformer winding Um < 170 kV Short-circuit withstand test Measurement of zero-sequence impedances on three phase transformers Measurement of acoustic sound level Measurement of the harmonics of the no-load current Measurement of the power taken by the fan and oil pump motors •Additional Special Tests 20. 21. 22. Test with lightning impulse chopped on the tail Magnetic circuit (Isolation) test Determination of capacitances and dissipation factor between winding-to-earth and Test Requirements for Transformers 219 26. 27. 28. 29. between windings. Magnetic balance test on three-phase transformers Determination of transient voltage transfer characteristics Dissolved gas analysis (DGA) of oil filled in the transformer before and after temperature rise test Recurrent surge oscillographic (RSO) test Determination of core hot spot temperature Frequency response analysis (FRA) test Measurement of magnetization current at low voltage 30. 31. 32. Functional tests on auxiliary equipments Tests on oil filled in transformer Dew point measurement before dispatch 23. 24. 25. Mechanical Tests 33. 34. 35. Oil pressure test on completely assembled transformer Jacking test and Dye-penetration test Pressure relief device test (B) Recommended Field tests 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. Dew point measurement for large transformer filled with dry air or nitrogen filled Winding resistance measurement Vector group and polarity Voltage ratio test Measurement of magnetizing current Magnetic balance test on three phase transformer Magnetic circuit (Isolation) test Measurement of short circuit impedance at low voltage Insulation resistance measurement Measurement of capacitance and dissipation factor on windings and Bushings of 72.5 kV class and above. 11. Dissolved gas analysis (DGA) on transformers 12. Tests on oil filled in transformer as per IS 1866 / IEC 60422 13. Frequency response analysis (FRA) test The dielectric tests (Test Nos. A.7 to A.12) may be routine, type or special tests depending upon the voltage rating, specific customer requirements and referred standards. The purpose, interpretation and explanation for specific conditions of the tests are briefly described as below. 220 Manual on Transformers The tests and their sequence shall be mutually agreed between the manufacturer and the user. 3.1 Measurement of Winding Resistance 3.1.1 General Resistance measurement helps to determine the following: (a) Calculation of the I2R losses. (b) Calculation of winding temperature at the end of a temperature rise test. (c) As a base for assessing possible damage in the field. 3.1.2 Determination of Cold Temperature The resistance is measured at ambient (cold) temperature and then converted to resistance at 75 0C, for all practical purpose of comparison with specified design values, previous results and diagnostics. The cold temperature of the winding shall be determined as accurately as possible when measuring the cold resistance. The following should be observed. 3.1.2.1 Transformer Windings Immersed in Insulating liquid The temperature of the winding shall be assumed to be the same as the temperature of the insulating liquid, provided: (a) (b) The windings have been under insulating liquid with no excitation and with no current in the winding from three hours to eight hours (depending upon the size of the transformer) before the cold resistance is measured. The temperature of the insulating liquid has stabilized, and the difference between top and bottom temperature does not exceed 5 0C. 3.1.2.2 Transformer windings without insulating liquid The temperature of the winding shall be recorded as the average of several thermometers or thermocouples inserted between the coils, with care taken to see that their measuring points are as nearly as possible in actual contact with the winding conductors. It should not be assumed that the windings are at the same temperature as the surrounding air. 3.1.3 Resistance Measurement Methods The resistance of each winding shall be measured by any one of the following methods. If winding has tapping, then resistance shall be measured on at least principal, maximum and minimum taps. 3.1.3.1 Voltmeter-Ammeter method This method should be employed if the rated current of the transformer winding is one ampere or more. The following steps are performed to conduct this test. (a) Measurement is made with direct current, and simultaneous readings of current and Test Requirements for Transformers (b) (c) (d) (e) (f) 221 voltage are taken. To minimize errors of observation: (1) The measuring instruments shall have such ranges as will give reasonably large deflection. (2) The polarity of the core magnetization shall be kept constant during all resistance readings. The voltmeter leads shall be independent of the current leads and shall be connected as closely as possible to the terminals of the winding to be measured. This is to be avoid including in the reading the resistance of current-carrying leads, their contacts and extra length of leads. Readings shall not be taken until after the current and voltage have reached steady-state values. Readings shall be taken with not less than four values of current when deflecting instruments are used. The current used shall not exceed 15% of the rated current of the winding whose resistance is to be measured. Larger values may cause inaccuracy by heating the winding and thereby changing its temperature and resistance. 3.1.3.2 Bridge Method Bridge methods or high-accuracy digital instrumentation are generally preferred because of their accuracy and convenience. The current rating of the measuring instrument should not be very low for large inductive objects. In case of delta connected windings of a large rating transformer, the resistance meter should have adequate current rating. For star connected windings with neutral brought out, the resistance shall be measured by two methods (1) (2) Between line and neutral For small transformer with star connected windings, the resistance shall be measured between phases (line to line), and then resistance of the individual windings shall be determined by dividing the value by 2. This will rule out the effect of the resistance of the neutral lead and bus bars which is significant in comparison to phase resistance of small transformers. However, for the delta connected windings, measurements shall be made between pairs of line terminals. In this case the resistance per winding will be 1.5 X measured resistance between the pair of line terminals. In case of open delta connected winding, the resistance can be measured across all the three windings are in series and also individual winding resistance can be measured. Few precautions are to be carried out to minimize errors while performing the test as follows: (a) (b) Charged battery of sufficient capacity or at least 10 A shall be used with the bridge to avoid errors due to drop in battery voltage during measurements. To reduce the high inductive effect, it is advisable to use a sufficiently high current to 222 (c) (d) (e) (f) (g) Manual on Transformers saturate the core. Therefore the measuring instruments shall have high ranges as well as large deflection. The polarity of the core magnetization shall be kept same during all resistance readings. A reversal in magnetization of the core can change the time constant and result in erroneous readings. The voltmeter leads shall be independent of the current leads and shall be connected as closely as possible to the terminals of the winding to be measured. This is to avoid including in the reading the resistances of current-carrying leads and their contacts and of extra lengths of leads. To protect the voltmeter from injury by off-scale deflections, the voltmeter should be disconnected from the circuit before switching the current on or off. To protect the personnel from inductive kick, the current should be switched off by a suitably insulated switch. Readings shall not be taken until after the current and voltage have reached steady-state values. The current used shall not exceed 15% of the rated current of the winding whose resistance is to be measured. Larger values may cause inaccuracy due to heating of the winding and thereby changing its temperature and resistance. 3.2 Measurement of Voltage Ratio, Polarity and Check of Voltage Vector Relationship 3.2.1 Ratio Test 3.2.1.1General The turn ratio of a transformer is the ratio of the number of turns in the high-voltage winding to that in the low-voltage winding. When the transformer has taps, the turn ratio shall be determined for all taps and for the full winding. The ratio tests shall be made at rated or lower voltage and the voltage shall be applied to the winding with higher voltage rating. In the case of three-phase transformers, when each phase is independent and accessible, singlephase supply should be used; although, when convenient, three-phase supply may be used. 3.2.1.2 Tolerances for ratio The tolerances for ratio shall be as specified in IS 2026 Part 1 and IEC 60076-1. Ratio test methods Various types of ratio test methods are possible. Out of those, Ratio Bridge method is most commonly adopted. In this method, the turn ratio on each tapping between pairs of winding shall be measured by a direct reading ratio meter. This method gives more accurate results as Test Requirements for Transformers 223 compared to other methods described in aforesaid standards. The modern ratio bridge can also be used to test polarity, phase relation and phase sequence. More accurate results can be obtained using a ratio bridge that provides phase-angle correction. 3.2.2 Polarity and Vector Group Verification Polarity and phase-relation tests are of interest primarily because of their bearing on paralleling or banking two or more transformers. Phase-relation tests are made to determine angular displacement and relative phase sequence. Phase-relation or vector group verification test is performed on a three phase transformer or on a bank of three singlephase transformers. The details of Additive and Subtractive polarity are given in IS: 2026-Part 1 and IEC 60076-1. 3.2.2.1 Polarity By Alternating-Voltage Test For a single-phase transformer having a ratio of transformation of 30 to 1 or less, the polarity test shall be done as follows. The line terminal of high voltage winding (1.1) shall be connected to the adjacent line terminal low-voltage winding (2.1) as shown in Fig. 1 Fig.1 Polarity by alternating voltage test Any convenient value of alternating voltage shall be applied to the full high-voltage winding and readings shall be taken of the applied voltage and the voltage between the right-hand adjacent high-voltage and low-voltage leads. When the later reading is greater than the former, the polarity is additive. When the later reading is less than the former (indicating the approximate difference in voltage between that of the high-voltage and low-voltage windings), the polarity is subtractive. 3.2.2.2 Verification of Vector Group The phasor diagram of any three-phase transformer that defines the angular displacement and phase sequence can be verified by connecting the HV and LV leads together to excite the 224 Manual on Transformers unit at a suitably low three-phase voltage, taking voltage measurements between the various pairs of leads and then either plotting these values or comparing them for their relative order of magnitude with the help of the corresponding phasor diagrams, e.g. as shown in Fig. 2 and 3. Typical check measurements are to be taken and their relative magnitudes are then compared. Example 1 CONNECT 1U TO 2U MEASURE 1W-2V, 1W-2W, 1U-2W, 1V-2V, 1V-2W VOLTAGE RELATION 1W-2V= 1W-2W 1W-2V< 1W-1U 1V-2V<1V-2W 1V-2V <1U-1W Fig 2 : For HV-Delta / LV-Star Transformer Example 2 Connect 1U to 2U MEASURE 1W-2V, 1W-2W, 1U-2W, 1V-2V, 1V-2W VOLTAGE RELATION 1W-2W = 1V-2W 1W-2V > 1V-2V 1U-N= (1U-2W)+(2W-N) Yd11 Fig 3 : For HV-Star / LV-Delta Transformer 3.3 Measurement of No-load Loss and Excitation Current 3.3.1 General No-load (excitation) losses are those losses that are incident to the excitation of the transformer. No-load (excitation) losses include core loss, dielectric loss, conductor loss in the winding due to excitation current, and conductor loss due to circulating current in parallel windings. These losses change with the excitation voltage. Excitation current (no-load current) is the current that flows in any winding used to excite the transformer when all other windings are open-circuited. It is generally expressed in percent of the rated current of the winding in which it is measured. Test Requirements for Transformers 225 3.3.2 No-load Loss Test The purpose of the no-load loss test is to measure no-load losses at a specified excitation voltage and a specified frequency. The no-load loss determination shall be based on a sine-wave voltage. The average-voltage voltmeter method is the most accurate method for correcting the measured no-load losses to a sine-wave basis and is recommended. This method employs two-parallelconnected voltmeters; one is an average-responding (possibly rms calibrated) voltmeter; the other is a true rms-responding voltmeter. The readings of both voltmeters are employed to correct the no-load losses to a sine-wave basis, using equation given in paragraph for waveform correction of no-load losses. Test voltage will be 90%, 100%, 110%, guaranteed at 100% and for reference purpose at 90 and 110% 3.3.2.1 Connection Diagrams Tests for the no-load loss determination of a single-phase transformer are carried out using the schemes depicted in Figs. 4 & 5. Fig. 4 shows the necessary equipment and connections for the case where instrument transformers are not required. When instrument transformers are required, which is the general case, the equipment and connections shown in Fig. 5 apply. If necessary, correction for losses in connected measuring instruments may be made by disconnecting the transformer under test and noting the wattmeter reading at the specified test circuit voltage. These losses represent the losses of the connected instruments (and voltage transformer, if used). They may be subtracted from the earlier wattmeter reading to obtain the no-load loss of the transformer under test. Tests for the no-load loss determination of a three-phase transformer shall be carried out by using the three wattmeter method. Figure 6 is schematic representation of the equipment and connections necessary for conducting no-load loss measurements of a three-phase transformer when instrument transformers are necessary. Nowadays, digital power analysers or power meters are available for determination of losses (both no-load and load). Selection of these power analysers shall be based on the desired accuracy at low power factors Fig 4 : Connections for no-load loss test of a single-phase transformer without instrument transformers 226 Manual on Transformers without instrument transformers Fig : 5 - Connections for no-load loss test of a single-phase transformer with instrument transformers Note : Source neutral should be available for Y-Y connected transformers when no delta winding is present to provide return path from transformer neutral Note : Voltmeters should be connected Line-to-Neutral for Y-connected winding or Line-to-Line for delta-connected winding Fig : 6 - Three phase transformer connections for no-load loss and excitation current tests using three-wattmeter method 3.3.2.2 Voltage and frequency for no-load loss test The operating and performance characteristics of a transformer are based upon rated voltage and rated frequency, unless otherwise specified. Therefore, the no-load loss test is conducted with rated voltage impressed across the transformer terminals, using a voltage source at a frequency equal to the rated frequency of the transformer under test, unless otherwise specified. For the determination of the no-load losses of a single-phase transformer or a three-phase transformer, the frequency of the test source should be within ± 0.5% of the rated frequency of the transformer under test. If the excitation frequency is beyond the specified tolerance, then the test voltage shall be adjusted to maintain the V/f ratio corresponding to the ratio of rated voltage and rated frequency. The voltage shall be adjusted to the specified value as indicated by the average-voltage voltmeter. Simultaneous values of rms voltage, rms current, electrical power and the average voltmeter readings shall be recorded. For a three-phase transformer the average of the three voltmeter readings shall be the desired nominal value of the voltage. The most difficult cases, both with regard to voltage wave shape distortion and power measurements usually arise when testing large single-phase transformers. Test Requirements for Transformers 227 3.3.2.3 Instrument error at low power factor At low power factors, judicious selection of measurement method and test system components is essential for accurate and repeatable test results. The phase-angle errors in the instrument transformers, measuring instruments, bridge networks and accessories affect the loss test results. Procedures for correcting the losses for meeting phase-angle errors are described in IEC Publication 60076-8 3.3.2.4 Correction of no-load losses The eddy current component of the no-load loss varies with the square of the rms value of excitation voltage and is substantially independent of the voltage waveform. When the test voltage is held at the specified value as read on the average-voltage voltmeter, the actual rms value of the test voltage may not be equal to the specified value. The no-load losses of the transformer corrected to a sine-wave basis shall be determined from the measured value by means of the following equation: The above equation is valid only for voltage with moderate waveform distortion. If waveform distortion in the test voltage causes the magnitude of the correction to be greater than 5%, then the test voltage waveform must be improved for an adequate determination of the no-load losses and currents. For large single phase transformers, it is expected that the difference between rms voltages and average voltage will be greater than 5%, which should be accepted in view of test voltage source limitation. The new generation of power analysers are equipped with software for automatic calculation of corrected losses based on the input data of voltages and power. 3.3.3 Measurement of Excitation (no-load) Current The excitation (no-load) current of a transformer is the current that maintains the rated magnetic flux excitation in the core of the transformer. The excitation current is usually expressed in per unit or in percent of the rated line current of the winding in which it is measured. Measurement of excitation current is usually carried out in conjunction with the tests for no-load losses. RMS current is recorded simultaneously during the test for no-load losses using the average-voltage voltmeter method. This value is used in calculating the per unit or percent excitation current. 228 Manual on Transformers For a three-phase transformer, the excitation current is calculated by taking the average of the magnitude of the three line currents. The tolerance for no-load current should be as per IS 2026 Part -1 3.4 Measurement of Short-circuit Impedance andLoad Loss 3.4.1 General The load losses of a transformer are those losses incident to a specified load carried by the transformer. Load losses include I2R loss in the windings due to load current and stray losses due to eddy currents induced by leakage flux in the windings, core clamps, magnetic shield, tank walls and other conducting parts. Stray losses may also be caused by circulating currents in parallel windings or strands. Load losses are measured by applying a short circuit across either the high voltage winding or the low voltage winding and applying sufficient voltage across the other winding to cause a specified current to flow in the windings. The power loss within the transformer under these conditions equals the load losses of the transformer at the temperature of test for the specified load current. The impedance voltage of a transformer between a pair of windings is the voltage required to circulate rated current through one of two specified windings when the other winding is short circuited, with the windings connected as for rated voltage operation. Impedance voltage is usually expressed in per unit or percent of the rated voltage of the winding across which the voltage is applied and measured. The impedance voltage is measured during the load loss test by measuring the voltage required to circulate test current in the windings. The measured voltage is the impedance voltage at the test frequency and the power loss dissipated within the transformer is equal to the load losses at the temperature of test and at rated load. The impedance voltage is corrected to the rated frequency and the load losses are corrected to a reference temperature using the formulas specified in this standard. 3.4.2 Factors Affecting the Values of Load Losses and ImpedanceVoltage The magnitude of the load losses and the impedance voltage will vary depending on the positions of tap changers, if any in various windings. These changes are due to the changes in the magnitudes of load currents and associated leakage-flux linkages as well as being due to changes in stray flux and accompanying stray losses. 3.4.2.1 Temperature Load losses are also a function of temperature. The I2R component of the load losses increases with temperature, while the stray loss component decreases with temperature. Procedures for correcting the load losses to the standard reference temperature are described in 3.5.5. Test Requirements for Transformers 229 3.4.2.2 Instrument error at low power factor At low power factors, judicious selection of measurement method and test system components is essential for accurate and repeatable test results. The phase-angle errors in the instrument transformers, measuring instruments, bridge networks and accessories affect the load loss test results. Procedures for correcting the load losses for meeting phase-angle errors are described in IEC Publication 60076-8 3.4.3 Methods for Measuring Load Losses and Impedance Voltage Test Conditions To determine the load losses and impedance voltage with sufficient accuracy, the following conditions shall be met. 1. 2. 3. 4. 5. The temperature of the insulating liquid has stabilized and the difference between top and bottom oil temperatures does not exceed 5 0C. The temperature of the windings shall be taken immediately either before or after the load losses and impedance voltage test in a manner similar to that described in 3.1.1. The average shall be taken as the winding temperature for computation of losses. The conductors used to short-circuit the low voltage, high current winding of a transformer shall have a cross-sectional area greater than the corresponding transformer winding leads. The test current shall be at least 50 % of the rated current of the winding across which the voltage is applied. The measurement of losses shall be done at the earliest after excitation of the transformer to the test current to avoid heating of the winding resulting in increase in resistance. 3.4.3.1. Wattmeter-voltmeter-ammeter method for load loss and impedance voltage test The connection and apparatus needed for the determination of the load losses and impedance voltage of a single-phase transformer are shown in Figures 7 and 8. Figure 8 applies when the instrument transformers are required, which is the general case. For three phase transformers, three-phase power measurement utilizing two wattmeter is possible but can result in very large errors at low power factors encountered in load loss tests of transformers. It is recommended that the two-wattmeter method should not be used for loss tests on three-phase transformers of ratings preferably above 20 MVA, 66 kV class. For three phase transformers, Figure 9 shows the apparatus and connections using the threewattmeter method. Fig 7 : Basic circuit for load loss and impedance measurement 230 Manual on Transformers Fig 8 : Single-Phase transformer connection for load loss and impedance tests with instrument tranformers Fig 9 : Three phase transformer connection for load loss and impedance voltage tests using three- wattmeter method 3.4.3.2Measurement with Power Analyser Nowadays, digital power analysers or power meters are available for determination of load losses. Selection of these power analysers shall be based on the desired accuracy at low power factors. The new generation of power analysers are equipped with software for automatic calculation of corrected losses based on the input data of voltage, current, power, frequency and temperature. 3.4.4 Test Procedure 3.4.4.1Two-winding transformers and auto transformers Load loss and impedance voltage tests are carried out using the connections and apparatus shown in figure 8 for single-phase transformers and figure 9 for three-phase transformers. With one winding short-circuited, a voltage of sufficient magnitude is applied to the other winding and adjusted to circulate test current in the excited winding. Simultaneous readings of wattmeter, voltmeter and ammeter are taken. If necessary, the corrections for the losses in external connections and connected measuring instruments should be made. Test Requirements for Transformers 231 The procedure for testing three-phase transformers is very similar, except that all connections and measurements are three-phase instead of single-phase and a balanced three-phase source of power is used for the test. If the three line currents cannot be balanced, their average rms value should correspond to the desired value, at which time simultaneous reading of wattmeters, voltmeters and ammeters should be recorded. Single phase and three-phase auto transformers may be tested with internal connections unchanged. The test is made using the auto transformer connection. The input (or output) terminals are shorted and voltage is applied to the other terminals. The voltage is adjusted to cause test current to flow in the test circuit as shown in Figure10. Simultaneous readings of wattmeters, voltmeters and ammeters are recorded for determination of load losses and impedance voltage. Fig 10 : Connection for impedance loss and impedance-voltage tests of an auto-transformer For the purpose of measuring load losses and impedance voltage, the series and common windings of auto transformers may be treated as separate windings, one short circuited, the other excited. In this situation, where the transformer is connected in the two-winding connection for the test, the current held must be the test current of the excited winding, which may or may not be the same as rated line current. The load loss watts and applied volt-amperes will be same, whether series and common windings are treated as separate windings in the two-winding connection or are connected in the auto-transformer connection, so long as rated winding current atleast 50 percent is held in the first case and rated line current atleast 50 percent in the second case. 3.4.4.2 Three winding transformers For a three winding transformer, which may be either single phase or three phase, three sets of impedance measurements are made between pairs of windings, following the same procedure as for two winding transformers. Measurement of the impedances Z12, Z23 and Z31 are obtained between windings 1, 2 & 3. If the kVA capacities of the different windings are not alike, the current held for the impedance test should correspond to the capacity of the lower rated winding of the pair of the windings under test. However, all of these data when converted into percentage form should be based on the same output kVA, preferably that of the primary winding. An equivalent three-winding impedance network as shown in Figure 11 can be derived from the following equations: 232 Manual on Transformers Fig 11 : Equivalent three-winding impedance network Where Z12, Z23 and Z31 are the measured impedance values between pairs of windings, as indicated all expressed on the same kVA base. These equations involve complex numbers, but they may be used for the resistance (in-phase) component or the reactance (quadrature) component of the impedance voltage or of the impedance volt-amperes. The treatment of the individual load losses for temperature corrections, etc., is the same as for two-winding, single phase transformers. The total load losses of the three winding transformer is the sum of the losses in the branches of the equivalent circuit of Figure 11 for any specific terminal load conditions. 3.4.5 Calculation of Load Losses and Impedance Voltage Load loss measurements vary with temperature and in general must be corrected to a reference temperature. In addition, load loss measurement values must be corrected for metering phase angle error. Impedance voltage measurement to vary with frequency and the values must be corrected for rated frequency. • Temperature correction of load losses Both I2R losses and stray losses of transformer vary with temperature. The I2R losses, Pr(Tm), of a transformer are calculated from the ohmic resistance measurements (connected to the temperature, Tm, at which the measurement of the load losses and impedance voltage was done) and the current that were used in the impedance measurement. These I2r losses subtracted Test Requirements for Transformers 233 from the measured load loss watts P(Tm), give the stray losses, Ps(Tm), of the transformer at the temperature at which the load loss test was made. Ps (Tm) = P(Tm) – Pr(Tm) Where Ps(Tm) is the calculated stray losses (watts) at temperature Tm. P(Tm) is the transformer load losses (watts), corrected in accordance with phase angle errors in wattmeter at temperature Tm. Pr (Tm) is the calculated I2R loss (watts) at temperature Tm The I2R component of load losses increases with temperature. The stray loss component diminishes with temperature. Therefore, when it is desirable to convert the load losses from the temperature at which it is measured, Tm, to another temperature, T, the two components of the load losses are corrected separately. Thus, Then P (T) = Pr (T) + Ps (T) Where Pr (T) = I2R loss (watts) at temperature T, 0C Ps (T) = stray losses (watts) at temperature T, 0C P (T) = Transformer load losses (watts) corrected to temperature T, 0C Tk = 234.5 0C (copper) » 235 0C Tk = 225 0C (aluminium) • Calculation for impedance The impedance shall be measured at rated frequency by applying an approximately sinusoidal supply to one winding, with the terminal of other winding short circuited, and with possible other winding open circuited. The supplied current should be equal to the relevant rated current. However, in case of limitation in the rating of supply source the current should not be less than the 50% of the rated current. Due to fluctuation in load the supply frequency may not be always 234 Manual on Transformers be the rated frequency. Then frequency correction should be applied to calculate the actual impedance at rated frequency as following. The formula for calculating the percentage impedance with current and frequency correction is Z (%)= Vtest I rated fr X X X 100 Vrated I test ft Where Vtest = Test voltage Vrated = Rated voltage Itest = Test current Irated = Rated current ft = Test frequency fr = Rated frequency 3.5Measurement of Insulation Resistance Insulation resistance tests are made to determine the insulation resistance from individual winding to ground or between individual windings. The insulation resistance in such tests is commonly measured in mega-ohms, or may be calculated from measurements of applied voltage and leakage current. Note (1) The insulation resistance of electrical apparatus is subjected to wide variation in design, temperature, dryness, and cleanliness of the parts. When the insulation resistance falls below prescribed values, it can, in most cases of good design and where no defect exists, be brought up to that required standard by cleaning and drying the apparatus. The insulation resistance, therefore, may offer a useful indication as to whether the apparatus is in suitable condition for application of dielectric tests. (2) Under no conditions, test should be made while the transformer is under vacuum. • Instrumentation Insulation resistance may be measured using the following equipment: (a) A variable-voltage dc power supply with means to measure voltage and current (generally in micro-amperes or milli-amperes) (b) A mega-ohmmeter Mega-ohmmeters are commonly available with nominal voltages of 500 V, 1000 V, 2500 V, and 5000 V; dc or in multiples of 1000 V upto 10,000 V. • Voltage to be applied The dc voltage applied for measuring insulation resistance to ground shall not exceed a value Test Requirements for Transformers 235 equal to the half of the rated voltage of the winding or 5 kV whichever is lower. • Procedure Insulation resistance tests shall be made with all circuits of equal voltage above ground connected together. Circuits or groups of circuits of different voltages above ground shall be tested separately. All external insulating parts of the transformer shall be cleaned thoroughly to remove dust, moisture etc. before the test. • Examples: (a) High voltage to low voltage and ground, low voltage to high voltage and ground. (b) Voltage should be increased in increments of usually one kilovolt and held for one minute while the current is read. (c) The test should be disconnected immediately in the event the current begin to increase without stabilizing. (d) After the test has been completed, all terminals should be grounded for a period of time sufficient to allow any trapped charges to decay to a negligible value. • Polarisation Index (PI) The purpose of polarisation index test is to determine if equipment is suitable for operation or even for an overvoltage test. The polarisation index is a ratio of insulation resistance value at the end of 10 min test to that at the end of 1 min test at a constant voltage. The total current that is developed when applying a steady state dc voltage is composed of three components: (1) Charging current due to the capacitance of the insulation being measured. This current falls off from maximum to zero very rapidly. (2) Absorption current due to molecular charge shifting in the insulation. The transient current decays to zero more slowly. (3) Leakage current which is the true conduction current of the insulation. It has a component due to the surface leakage because of the surface contamination. The advantage of PI is that all of the variables that can affect a single IR reading, such as temperature and humidity, are essentially the same for both the 1 min and 10 min readings. Since leakage current increases at a faster rate with moisture present than does absorption current, the IR readings will not increase as fast with insulation in poor condition as with insulation in good condition. After 10 min the leakage current becomes constant and effects of charging current and absorption current die down. Acceptable PI value for power transformer shall be better than 1.5. For distribution transformer it should be at least 1.3. 236 • Manual on Transformers Interpretation of Results Insulation resistance may vary with applied voltage and temperature any comparison must be made with measurements at the same voltage. The significance of values of insulation resistance tests generally requires some interpretation, depending on the design and the dryness and cleanliness of the insulation involved. When a user decides to make insulation resistance test, it is recommended that insulation resistance values be measured periodically (during maintenance shutdown) and that these periodic values be plotted. Substantial variations in the plotted values of insulation resistance should be investigated for cause. 3.6 Tests on On-load Tap-Changers 3.6.1 Operation Test With the tap-changer fully assembled on the transformer the following sequence of operations shall be performed without failure: (a) (b) (c) (d) With the transformer un-energised, eight complete cycles of operations (a cycle of operation goes from one end of the tapping range to the other, and back again). With the transformer un-energised, and with the auxiliary voltage reduced to 85% of its rated value, one complete cycle of operation. With the transformer energized at rated voltage and frequency at no load, one complete cycle of operation With one winding short circuited and, as far as practicable, two rated current according to IEC 60076-1 in the winding, 10 tap-change operations across the range of two steps on each side from where a coarse or reversing changeover selector operates, or otherwise from the middle tapping. 3.6.2 Auxiliary Circuits Insulation Test After the tap changer is assembled on the transformer, a power frequency tests according to IEC 60076-1 shall be applied to the auxiliary circuits as specified in IEC 60076-3. 3.7 Dielectric Tests The purpose of dielectric tests is to demonstrate that the transformer has been designed and constructed to withstand the specified insulation levels. The insulation requirements for the transformers and the corresponding dielectric tests are given in IS 2026 Part-3 and IEC Publication 60076-3 with reference to specific windings and their terminals. For oil immersed transformers, the requirements apply to the internal insulation only. The dielectric tests shall generally be made at the manufacturer premises with the transformers approximately at ambient temperature. Transformers, including bushings and terminal compartments when necessary to verify air clearances, shall be assembled prior to making dielectric tests, but assembly of items, Test Requirements for Transformers 237 such as radiators and cabinets, which do not affect dielectric tests is not necessary. Bushing shall, unless otherwise authorised by the purchaser, be those to be supplied with the transformer. If a transformer fails to meet its test requirements and the fault is in a bushing, it is permissible to replace this bushing temporarily with another bushing and continue the tests on the transformer to completion without delay. A particular case arises for tests with partial discharge measurements, where certain types of commonly used high-voltage bushings create difficulty because of their relatively high level of partial discharge in the dielectric. When such bushings are specified for the transformer, it is permitted to exchange them for bushings of a partial discharge free type during the testing of transformer. Test levels and other test parameters shall be as per IEC Publication 60076-3 and the corresponding IS 2026 Part-3. It is recommended to measure voltage at the high voltage terminal of its transformer. The measuring system shall be in accordance with IEC Publication 60071-2. In conducting low frequency tests for transformers of 100 kVA and less to be tested at 50 kV or less, it is permissible to depend on the ratio of testing transformer to indicate the proper test voltage. 3.7.1 Rules for Some ParticularTransformers In transformers where uniformly insulated windings having different Um values are connected together within the transformer, the separate source AC withstand test voltages shall be determined by the insulation of the common neutral and its assigned Um. In transformers which have one or more non uniformly insulated windings, the test voltages for the induced withstand voltage test, and for the switching impulse test, are determined by the winding with highest Um value, and the windings with lower Um values may not receive their appropriate test voltages. During switching impulse tests, the voltages developed across different windings are approximately proportional to the ratio of turns. Rated switching impulse withstand voltages shall only be assigned to the winding with the highest Um. Test stresses in other windings are also proportional to the ratio of numbers of turns and are adjusted by selecting appropriate tappings to come as close as possible to the assigned value. 3.7.2 Insulation Requirements and DielectricTests The basic rules for insulation requirements and dielectric tests for different categories of windings are described in Table 1(Refer IEC Publication 60076-3) 238 Manual on Transformers Category of winding Highest voltage for equipment Um kV Uniform Uniform and non-uniform insulation Tests Lightning impulse (LI) Switching impulse (SI) Long duration AC(ACLD) Short duration AC(ACSD) Separate source AC Um ≤ 72.5 Type Not applicable Not applicable Routine Routine 72.5 < Um ≤ 170 Routine Not applicable Special Routine Routine 170 < Um < 300 Routine Routine* Routine Special* Routine Um ≥ 300 Routine Routine Routine Special Routine * If ACSD test is specified, the SI test is not required. The standard dielectric requirements are verified by dielectric tests. They shall, where applicable and not otherwise agreed upon, be performed in the sequence as given below. (1) (2) (3) (4) (5) (6) Switching impulse test (SI) for the line terminal Lightning impulse test (LI) for the line terminals Lightning impulse test (LI) for neutral terminal Separate source AC withstand voltage test (applied potential test) Short-duration induced AC withstand voltage test (ACSD) Long-duration induced AC voltage test (ACLD) 3.7.3 Switching Impulse withstand Voltage Test, Transformer Winding Um > 300 kV This test is intended to verify the switching impulse withstand strength of the line terminals and its connected windings to earth and other windings, the withstand strength between phases and along the winding under test. The impulses are applied either directly from the impulse voltage source to a line terminal of the winding under test, or to a lower voltage winding so that the test voltage is inductively transferred to the winding under test. The detailed test procedures and specific test requirements are addressed in IEC Publication 60076-3. • Switching impulse waves Polarity The polarity of test voltage shall be negative because this reduces the risk of erratic external flashovers in the test circuit. Wave shape The voltage impulse shall have a virtual front time of at least 100 μs, a time above 90% of the specified amplitude of at least 200 μs, and a total duration from the virtual origin to the first zero passage of at least 500 μs but preferably 1000 μs. Test Requirements for Transformers • 239 Test sequence and records The test sequence shall consists of one impulse of a voltage between 50% and 75 % of the full test voltage and three subsequent impulses of full voltage. If the oscillographic or digital recording should fail, that application shall be disregarded and a further application made. Oscillographic or digital records shall be obtained of at least the impulse wave-shape on the line terminal under test and preferably the neutral current. • Test connections During the test the transformer shall be in a no-load condition. Windings not used for the test shall be solidly earthed at one point but not short-circuited. For a single phase transformer, the neutral terminal of the tested winding shall be solidly earthed. A three-phase winding shall be tested phase by phase with the neutral terminal earthed and with the transformer so connected that a voltage of opposite polarity and about half amplitude appears on the two remaining line terminals which may be connected together. To limit the voltage of opposite polarity to approximately 50% of the applied level, it is recommended to connect high ohmic damping resistors (10 kΩ to 20 kΩ) to earth at the non tested phase terminals. • Failure detection The test is successful if there is no sudden collapse of voltage or discontinuity of the neutral current if recorded on the oscillographic or digital records. Additional observation during the test (abnormal sound effect etc.) may be used to confirm the oscillographic records, but they do not constitute evidence in themselves. 3.7.4 Lightning Impulse withstand Voltage Test This test is intended to verify the impulse withstand strength of the transformer under test. This test shall only be made on windings that have terminals brought out through the transformer tank or cover. When non-linear elements or surge diverters are installed for the limitation of transferred over voltage transients, the evaluation of test records may be different compared to the normal impulse test. These non-linear protective devices connected across the windings may cause difference between the reduced full wave and the full-wave impulse oscillograms. To prove that these differences are indeed caused by operation of these devices, this should be demonstrated by making two or more reduced full-wave tests at different voltage levels to show the trend in their operation. The detailed test procedure and specific test requirements are addressed in IEC 60076-3. • Impulse wave The test impulse shall be a full standard lightning impulse: 1.2 µs ± 30% / 50 µs ± 20 %. 240 Manual on Transformers But in some cases this standard impulse shape cannot reasonably be obtained, because of low winding inductance or high capacitance to earth. In such cases wider tolerance may be accepted by the agreement between purchaser and customer. It is recommended to use IEC Publication 60722 as a guide for non-standard wave shapes. • Test sequence The test sequence shall consists of one impulse of a voltage between 50% to 75% of full test voltage, and three subsequent impulses at full voltage. If, during any of these applications, an external flashover in the circuit or across a bushing spark gap should occur, or if the oscillographic recording should fail on any of the specified measuring channels, that application shall be disregarded and a further application made. • Test Connections • During test on line terminals The impulse test sequence is applied to each of the line terminals of the tested winding in succession. In the case of a three phase transformer, the other line terminals of the winding shall be earthed directly or through a low impedance, not exceeding the surge impedance of the connected line. If the winding has neutral terminal, it shall be earthed directly or through a low impedance such as a current measuring shunt. In the case of separate-winding transformer, terminals of windings not under test are earthed directly or through impedances, so that in all circumstances, the voltage appearing at the terminals is limited to not more than 75% of their rated lightning impulse withstand voltage for star connected windings, and 50% for delta- connected windings. In case of auto transformer, when testing the line terminal of the high voltage winding the nontested line terminal shall be earthed through resistors not exceeding 400 Ω to get the impulse waveform as needed. • Impulse test on a neutral terminal Impulse withstand capability of neutral may be verified by : (a) (b) Indirect application: Test impulses are applied to any one of line terminals or to all three line terminals connected together. The neutral is connected to earth through an impedance or is left open. Then standard lightning impulse is applied to the line terminal which shall not exceed 75% of the rated LI withstand voltage of the line terminal. Direct application: Test impulse corresponding to the rated withstand voltage of the neutral is applied directly to the neutral with all line terminals earthed. In this case, however a longer duration of front time is allowed, upto 13µs. Test Requirements for Transformers • 241 Records of test The oscillographic or digital records obtained during calibrations and tests shall clearly show the applied voltage impulse shape (front time, time to half value and amplitude). The oscillograms of the current flowing to earth from the tested winding shall also be recorded. • Test sequence The test sequence shall consist of one impulse of a voltage between 50% to 75% of full test voltage, and three subsequent impulses at full voltage. If, during any of these applications, an external flashover in the circuit or across a bushing spark gap should occur, or if the oscillographic recording should fail on any of the specified measuring channels, that application shall be disregarded and a further application made. Failure detection • Grounded current oscillograms In this method of failure detection, the impulse current in the grounded end of the winding tested is measured by means of an oscilloscope or by a suitable digital transient recorder connected across a suitable shunt inserted between the normally grounded end of the winding and ground. Any differences in the wave shape between the reduced full-wave and final full-wave detected by comparison of the two current oscillograms, may be indication of failure or deviations due to no injurious causes. They should be fully investigated and explained by a new reduced wave and full-wave test. Examples of probable causes of different wave shapes are operation of protective devices, core saturation, conditions in the test circuit external to the transformer. The ground current method of detection is not suitable for use with chopped-wave tests. • Other methods of failure detection Voltage Oscillograms: Any unexplained difference between the reduced full-wave and final full-wave detected by comparison of the two voltage oscillograms, or any such differences observed by comparing the chopped-waves to each other and to the full-wave up to the time of flashover, are indications of failure. Noise: Unusual noise within the transformer at the instant of applying impulse is an indication of trouble. Such noise should be investigated. Measurement: Measurement of voltage and current induced in another winding may also be used for failure detection. 3.7.5 • Separate Source Voltage withstand Test Duration, frequency and connections A normal power frequency, such as 50 Hz, shall be used and the duration of the test shall be one minute. 242 Manual on Transformers The winding being tested shall have all its parts joined together and connected to the terminal of the testing transformer. All other terminals and parts (including core and tank) shall be connected to ground and to the other terminal of the testing transformer. • Application of voltage for Separate Source Withstand test The test shall be commenced at a voltage not greater than one-third of the full value and be brought up gradually to full value in not more than 15 s. After being held for the specified time of 60 seconds, it should be reduced (in not more than 5s) to one third or less of the maximum value and the circuit opened. • Failure detection Careful attention should be started given for evidence of possible failure that could include items, such as an indication of smoke and bubbles rising in the oil, an audible sound such as a thump, or a sudden increase in test circuit current. Any such indication should be carefully investigated by observation, by repeating the test, or by other test to determine if a failure has occurred. 3.7.6 Induced AC voltage withstand tests with partial discharge measurement ACSD test is intended to verify the AC withstand strength of each line terminal and its connected winding(s) to earth and other windings, the withstand strength between phases and along the winding(s) under test As per IS 2026 Part 3 and IEC Pub. 60076-3, the test is normally performed with partial discharge measurement (Method 2) for transformers with highest voltage winding of ≥ 300 kV. For transformer with highest voltage winding of < 300 kV, the test is performed without partial discharge measurement (Method 1). However, with the latest revision of IEC 60076-3, the methods for induced over-voltage withstand test are reformed as AC short duration test (ACSD) and AC long duration test (ACLD). ACSD test is intended to verify the AC withstand strength of each line terminal and its connected winding(s) to earth and other windings, the withstand strength between phases and along the winding(s) under test. For Um >72.5 kV, the test is normally performed with partial discharge measurements to verify partial discharge free operation of the transformer under operating condition. However, the requirements for partial discharge measurement during the ACSD test may be omitted. This shall be clearly stated at the enquiry and order stages. ACLD test is always performed with the measurement of partial discharge during the whole application of test. This is test is not a design proving test, but a quality control test and is intended to cover temporary over voltages and continuous service stress. It verifies PD free operation of the transformers under operating conditions. 243 Test Requirements for Transformers An alternating voltage shall be applied to the terminals of one winding of the transformer. The voltage shall be as nearly as possible sinusoidal and its frequency is sufficiently above the rated frequency to avoid excessive magnetizing current during the test. The test voltage is the peak value of voltage divided by √2 .The test time at full test voltage shall be 60 sec for test frequency up to and including twice the rated frequency. For frequency above twice the rated frequency the time duration of test shall be: 120 X Rated frequency ,but not less than 15 sec Test frequency Table below shows the different conditions of induced AC voltage test as defined in IEC publication 60076-3. The time duration for the application of test voltage with respect to earth is shown in Figure 12 Type of test Type of winding Uniformly insulated Table 2 :Induced AC Voltage test Highest Test voltage level voltage of equipment Um Test Duration (Refer Fig 12) Remarks ≤ 72.5 kV As per Table 2 of IEC 60076-3 60 sec No PD measurement > 72.5 kV U1= from Table D.1 of IEC 60076-3 U2= 1.3 Um/√3 C= 120 x Rated Freq. Test freq. D=5 min PD level should be ≤ 300 pC at level U2 Phase to earth test C= 120 x Rated Freq. Test freq. PD level should be ≤ 500 pC at level U2 AC Short duration (ACSD) Non-uniformly insulated >72.5 kV U1=from Table D.2 of IEC 60076-3 U2=1.5 Um/√3 D=5 min Phase to U1=from phase test Table D.2 of IEC 60076-3 U2= 1.3 Um/√3 AC Long Duration (ACLD) Uniformly and nonuniformly insulated Delta connected HV 72.5<Um ≤ 245 kV U1= 1.7 Um/√3 U2= 1.5 Um/√3 Star connected HV 72.5<Um <300 kV U1= 1.7 Um/√3 U2= 1.5 Um/√3 ≥ 300 kV U1= 1.7 Um/√3 U2= 1.5 Um/√3 PD level should be ≤ 300 pC at level U2 D=30 min C= 120 x Rated Freq. Test freq. D=30 min C= 120 x Rated Freq. Test freq. D=60 min C= 120 x Rated Freq. Test freq. PD level should be ≤ 500 pC at level U2 PD level should be ≤ 500 pC at level U2 244 Manual on Transformers Fig.12: Time sequence for the application of test voltage with respect to earth Where Um = Highest voltage for equipment U1 = Test voltage U2 = Partial discharge evaluation level The detailed procedure and specific test requirements are addressed in IEC-60076-3 3.7.7 Switching ImpulseVoltage withstand Test, Transformer Winding Um < 300 kV Refer to the clause no 3.7.3 of this Section 3.7.8 Lightning Impulse Voltage withstand Test, Transformer Winding Um < 300 kV Refer to the clause no 3.7.4 of this Section 3.7.9 Long-duration Induced AC Voltage test (ACLD), TransformerWinding Um < 170 kV The test procedure is same as clause 3.7.6 of this Section 3.8 Temperature Rise Test Temperature rise test is performed to prove that temperature rise comply to limits specified in standards and to derive thermal characteristics for the transformer. The test is carried out supplying full load losses for sufficient time to ensure that the temperature rise of the winding and oil reach steady state values. The transformer shall be assembled completely with its cooling equipment. It is desirable to put the specified conservator with the transformers, if available. Alternatively, temporary conservator of approximately same capacity can be used for the purpose of the test. Test Requirements for Transformers 245 The top oil temperature is measured by a thermometer in a pocket at the top of the transformer tank, and this is used to verify that steady conditions have been reached. Final winding temperatures cannot be measured directly. The transformers shall be tested in the combination of connections and taps that give the highest winding temperature rises as determined by the manufacturer and reviewed by the purchaser’s representative when available. This will generally involve those connections and taps resulting in the highest losses. All temperature rise test shall be made under normal (or equivalent to normal) conditions of the means of cooling. The temperature–rise test shall be made in a room that is free from drafts as practicable and equipped with its protective device. • Cooling air temperature Precautions should be taken to minimise variations of cooling air temperature specially when the steady state is approached. Rapid variation of reading should be prevented by providing at least three sensors, and average of their readings shall be used for evaluation. The sensors shall be distributed around the tank 1m to 2m away from the tank or cooling surface and protected from direct radiation. • Cooling water temperature The temperature is measured at the intake of the cooler. Readings of temperature and rate of water flow should be taken at regular interval. Test Method • Short Circuit Method During this test the transformer is subjected to the calculated total losses, previously obtained by two separate determination of losses, namely load loss at reference temperature and no load loss. The purpose of this test is - to establish the top oil temperature rise in steady-state condition with dissipation of total losses - to establish the average winding temperature rise at rated current and with the top oil temperature rise as determined above. This is achieved in two steps: (a) Total loss injection First the top oil and average oil temperature rises are established when the transformer is subjected to a test voltage such that the measured power is equal to the total losses of the 246 Manual on Transformers transformer. The test current will be above rated current to the extent necessary for producing an additional amount of loss equal to the no-load losses, and winding temperature rise will be correspondingly elevated. The oil temperature and cooling medium temperature are monitored, and the test is continued until a steady- state oil temperature rise is established. The test may be terminated when the rate of change of top oil temperature rise has fallen below 1oC per hour and has remained there for a period of 3 hour. (b) Rated current injection When the top oil temperature rise has been established, the test shall immediately continued with the test current reduced to the rated current for the winding combination connected. This condition is maintained for 1 h, with continuous observation of oil and cooling medium temperatures. At the end of one hour, the resistance of windings are measured with suitable method. During the hour with rated current the oil temperature falls. The measured values of winding temperature shall therefore be raised by the same amount as the average oil temperature rise has fallen from the correct value. The corrected winding temperature value minus the cooling medium temperature at the end of the total losses injection period is the average temperature rise. By the agreement, the two steps of the test may be combined in one single application of the power at a level between load loss and the total loss. The temperature- rise figures for the top oil and for the windings shall then be determined using the correction rules. The power injected during the test shall however be at least 80% of the total loss figure. (c) Determination of average winding temperature The average winding temperature is determined via measurement of winding resistance. A reference measurement (R1,θ1) of all winding resistances is made with the transformer at ambient temperature, in a steady condition. When the resistance R2 at different temperature (θ2) is measured this yields the temperature value R2 Copper: θ2 = R1(235+θ1) - 235 R2 Aluminum : 02= Rl ( 225+ θ1) - 235 The external cooling medium temperature at the time of shutdown is θa The winding temp. rise is then, finally : ∆θw =θ2-θa Test Requirements for Transformers (d) 247 Determination of winding temperature before shutdown Immediately after disconnection of test power supply and removal of short circuit connection the resistance of winding is measured with a suitable measuring circuit. The winding has large electrical time constant therefore accurate reading obtained only after a certain time delay. The resistance of the winding varies with time as the winding cools down. It shall be measured for a sufficient time to permit the extrapolation back to instant of shutdown. The detailed procedure to determine the resistance at the instance of shutdown is accordance with IEC-60076-2. (e) Corrections If the specified values of power or current have not been obtained during the test, the result shall be corrected according to the following relation. They are valid within a range of +20% from target value of power and +10% from target value of current. The oil temperature rise above ambient during the test is multiplied by : X Total losses Test losses X= 0.8 for distribution transformers X= 0.9 for larger transformers with ON cooling X= 1.0 for transformers with OF or OD cooling The average winding temperature rise above average oil temperature during the test is multiplied by: Y Rated Current Test losses Y=1.6 for ON and OF cooled transformers Y= 2.0 for OD cooled transformers 3.9 Short Circuit WithstandTest This test identifies the requirement for power transformer to sustain without damage the effects of over current originated by external short-circuit. The test demonstrates the thermal ability and dynamic effects of power transformer to withstand the rated short-circuit forces. The detailed procedures describing the magnitude of current, test duration, no. of tests and evaluation criteria shall be as per IEC 60076-5. 3.10Measurement of zero–phase–sequence impedances on 3-phase transformers • Zero–phase–sequence impedance tests of three–phase transformers 248 Manual on Transformers The zero–phase–sequence impedance characteristics of three–phase transformers depend upon the winding connections, and in some cases, upon the core construction. Zero–phase–sequence impedance tests apply only to transformers having one or more windings with a physical neutral brought out for external connection. In all tests, one such winding shall be excited at rated frequency between the neutral and the three line terminals connected together. External connection of other windings shall be as described in succeeding paragraphs for various transformer connections. Transformers with connections other than as described in succeeding paragraphs shall be tested as determined by those responsible for design and application. The excitation voltage and current shall be established as follows: If no delta connection is present on the transformer, the applied voltage should not exceed 30% of the rated line–to–neutral voltage of the winding being energized, nor should the phase current exceed its rated value. If a delta connection is present, the applied voltage should be such that the rated phase current of any delta winding is not exceeded. The percent excitation voltage at which the tests are made shall be shown on the test report. The time duration of the test shall be such that the thermal limits of any of the transformer parts are not exceeded. Single–phase measurements of excitation voltage, total current, and power shall be similar to those described in for load loss measurements. The zero–sequence impedance in percent on kVA base of excited winding for the test connection is: Ir E Z (%)=300 E I r Where E = measured excitation voltage Er = rated phase–to–neutral voltage of excited winding I = measured total input current flowing in the three parallel–connected phases Ir = rated current per phase of the excited winding A zero-sequence test shall be made on the winding with the available neutral. A single–phase voltage shall be applied between the three shorted line terminal and neutral. The external terminals of all other windings may be open–circuited or shorted and grounded. The zero-sequence impedance is dependent upon the physical disposition of the windings and the magnetic parts and measurement of different windings may not therefore agree. 3.11Measurement of Acoustic Sound Level This test shall be done in accordance with the clauses given in NEMA TR1 and IEC-60076-10. The guidelines of testing method is given in IEC 60076-10-1 Audible sound from transformer originates principally in the transformer core and is transmitted, Test Requirements for Transformers 249 either through the dielectric fluid or the structural support, to other solid surfaces from which it is radiated as airborne sound. The audible sound also contains the noise emitted by any dielectric fluid mechanical cooling system. Measurement should be made in an environment having an ambient sound pressure level at least five decibels below the combined sound pressure level of the transformer and the ambient sound pressure level. The transformer shall be located so that no acoustically reflecting surface is within 3 m of the measuring microphone, other than the floor or ground. The transformer shall be connected and energised at rated voltage and rated frequency, and shall be at no load with the tap changer on principal tap. Pumps and fans shall be operated as appropriate for the rating being tested. Sound measurements shall begin after the transformer being tested is energised and steady- state sound level conditions are established. Measurements may be made immediately on the transformers that have been in continuous operation. The rated voltage shall be measured line-line for ∆ connected windings and line-neutral for Y connected windings. The voltage shall be measured with a voltmeter responsive to the average value of the voltage but scaled to read the rms value of a sinusoidal wave having the same average value. The voltmeter should be connected between the terminals of the energized windings. The reference sound-producing surface is a vertical surface that follows the contour of a taut string stretched around periphery of the transformer or integral enclosure (Fig 12). The contour shall include radiators, coolers, tubes, switch compartments, and terminal chambers, but exclude bushing and minor extensions. The measurement shall be done with the microphone, which shall be calibrated as recommended by the sound level meter manufacturer before and after measurement. The first microphone locations shall coincide with the main drain valve. The number of microphone position is not less than 4. The microphone shall be located on the measurement surface spaced 0.3 m from the reference sound- producing surface. When fans are in operation, the microphone shall be located 2 m from any portion of radiators and coolers. For transformers having an overall tank or enclosure height of les than 2.4 m, measurements shall be made at half height. For transformers having an overall tank height of 2.4 m or more, measurements shall be made at one-third and at two-thirds height. The sound power rating of the transformer is determined using the following five steps: (a) Measure ambient sound pressure level (b) Measure combined ambient and transformer sound pressure levels (c) Compute ambient corrected sound pressure levels (d) Compute average sound pressure levels (e) Calculate sound power levels 250 Manual on Transformers The detailed calculation is done in accordance with the ANSI/IEEEC57.12.90. The average sound level of transformers should not exceed the values given in table 0-2 through 0-4 of NEMA TR1 when measured at the factory in accordance with the conditions outlined in ANSI/IEEEC57.12.90. Fig. 13 Microphone location for measuring audible sound from transformers 3.12 Measurement of the Harmonics of the No-load Current The harmonics of the no–load current in all the phases are measured by means of harmonic analyzer and the magnitude of the harmonics is expressed as a percentage of the fundamental component. 3.13 Measurement of Power taken by the Fans and Oil Pump Motors The measurement shall be done by suitable instruments at rated voltage. 3.14 Test with Lightning Impulse Chopped onTail This test is a special test and should be used for special applications on the line terminals of a winding. When this test is performed it shall be combined with the full lightning impulse test. The peak value of the chopped impulse shall be 1.1 times the amplitude of the full impulse. Usually the same settings of the impulse generator and measuring equipment are used, and only the chopping gap instrument is added. The standard chopped lightning impulse shall have a time to chopping between 2 µs and 6 µs. The triggered type chopping gap should be used with adjustable timing, although a plain rod-rod gap is allowed. The chopping circuit should be so arranged that the amount of over swing to opposite polarity of the recorded impulse will be limited to not more than 30% of the amplitude of the chopped impulse. For this purpose, it is permitted to put a resistance in service with the chopping gap. The test is combined with the full impulse test in a single sequence. The order of application is : Test Requirements for Transformers 251 one reduced level impulse one full level impulse one or more reduced level chopped impulse(s) two full level chopped impulses two full level impulses The same type of measuring channels and oscillographic or digital records are specified as for the full-wave impulse test. The detection of faults during chopped impulse test depends essentially on a comparison of the oscillographic or digital records of full level and reduced level chopped impulses. The neutral current record presents a superposition of transient phenomena due to the front of the original impulse and from the chopping. Account should therefore be taken of the possible variations, of the chopping time delay. The recordings of successive full impulse tests at full level constitute a supplementary criterion of a fault, but they do not constitute in themselves a quality criterion for the chopped impulse test. 3.15Magnetic Circuit (Isolation ) Test This test is done to detect the presence of inadvertent ground if exists. This test is done with help of megger or by AC supply. During this test other terminals should be in open circuit position. This test is done by applying the voltage alternate between the core clamp to end frame, core clamp to tank and between end frame to tank. The value of test voltage is varying according the customer requirement and electrical specification. The duration of test voltage application is 60 seconds. Alternatively the test is performed with the help of megger. In which the value of insulation resistance is measured between two terminals. This test shall be conducted in accordance with IS-2026 Part 1. The tests will be successful if the terminals withstand the required AC voltage for test duration. The values of the insulation resistance in mega-ohm (MΩ) should as follows; Test voltage : 2.5 kV New equipment : > 10 MΩ Service aged equipment : > 1 MΩ Insulation deterioration : < 1 MΩ Destructive circulating current : < 100 KΩ 252 3.16 Manual on Transformers Determination of Capacitance and Dissipation Factor Between Winding to Earth and between Windings Capacitance and tan delta are usually determined for winding to earth and between windings by bridge measuring technique, such as Schering Bridge. The test specimen shall have the following requirements: All windings immersed in insulating liquid in case of liquid filled transformer. All winding short-circuited. All bushings are in place. The applied voltage for measuring capacitance and tan delta shall not exceed half of the low frequency test voltage, for any part of the winding or 10 kV whichever is lower. This test may be performed with or without guard for the circuit combination as shown below. • Method 1 Method II Test without guard Test with guard Two winding HV to LV and ground LV to HV and ground HV and LV to ground Two winding HV to LV and ground LV to HV and ground HV and LV to ground LV to ground, Guard on HV Three winding HV to IV, LV and ground IV to HV, LV and ground LV to IV, HV and ground HV and IV to LV and ground IIV and LV to IV and ground IV and LV to HV and ground IIV, IV and LV to ground Three winding HV to LV and ground, guard on IV HV to ground, guard on LV & IV LV to IV & ground, guard on HV LV to ground, guard on HV & IV IV to HV & ground, guard on LV IV to ground, guard on HV & LV HV &LV to IV & ground HV & IV to LV & ground Temperature correction factors The temperature correction factors for the insulation power factor depend upon the insulating material, their structure, moisture, etc. Values of correction factor ‘K’ listed in the below table are typical and satisfactory for practical purpose for use as given in equation Test temperature T, 0C 10 15 20 25 30 35 40 45 50 55 60 65 70 Correction Factor ‘K’ 0.80 0.90 1.00 1.12 1.25 1.40 1.55 1.75 1.95 2.18 2.42 2.70 3.00 Test Requirements for Transformers 253 FP20 = Fpt /K Where, FP20 is the power factor corrected to 20 0C Fpt is the power factor measured at T T is the test temperature 0C K is the correction factor as per table above Insulation temperature may be considered to be that of the average liquid temperature. When insulation power factor is measured at a relatively high temperature and the corrected values are unusually high, the transformer should be allowed to cool and the measurements should be repeated at or near 20 0C 3.17 Magnetic Balance Test on 3-phase Transformers This test is conducted only in three phase transformers to check the imbalance in the magnetic circuit. In this test, no winding terminal should be grounded; otherwise results would be erratic and confusing. • Evaluation criteria The voltage induced in the center phase shall be 50% to 90% of the applied voltage on the outer phases. However, when the center phase is excited then the voltage induced in the outer phases shall be 30 to 70% of the applied voltage. Zero voltage or very negligible voltage induced in the other two windings should be investigated. The purpose of this test basically is to ensure that there is no inter turn fault in the winding which is generally reflected in high excitation current in faulty winding. 3.18 Determination of Transient Voltage Transfer Characteristics When the low-voltage winding cannot be subjected to lightning over voltage from the low voltage system, this winding may, by agreement between supplier and purchaser, be impulse tested with surges transferred from high voltage winding. This method is also used when the design is such that an impulse directly applied to the low voltage winding could result in unrealistic stressing of higher voltage windings, particularly when there is a large tapping winding physically adjacent to the low voltage winding. With the transferred surge method, the tests on the low voltage winding are carried out by applying the impulse to the adjacent high voltage winding. The line terminals of the low voltage winding is connected to earth through resistance of such value that the amplitude of the 254 Manual on Transformers transferred impulse voltage between line terminals and earth, or between different line terminal or across a phase winding, will be as high as possible but not exceeding the rated impulse withstand voltage. The magnitude of the applied impulses shall not exceed the impulse level of the winding to which the impulses are applied. The details of the procedure shall be same as the lightning impulse test on line terminal of HV winding. 3.19 • Dissolved Gas Analysis (DGA) of Oil Filled in the Transformer Introduction For many years the method of analyzing gasses dissolved in the oil has been used as a tool in transformer diagnostics in order to detect incipient faults, to supervise suspect transformers, to test a hypothesis or explanation for the probable reasons of failures or disturbances which have already occurred and to ensure that new transformers are healthy. The evaluation criteria with dissolve gas analysis is based on the fact that during its lifetime the transformer generates decomposition gasses–essential from the organic insulation – under the influence of various stresses– both normal and abnormal. The gasses that are of interest for the DGA analysis are the following; - H2 Hydrogen CH4 Methane C2 H4 ethylene C2H6 Ethane C2H2 acetylene (C3H6 propene )– not always measure (C3H8 propane ) – not always measure CO carbon monoxide CO2 carbon dioxide O2 oxygen N2 Nitrogen TCG total combustible gas content (= H2 + CH4 + C2H4 + C2H6 + C2H2 + CO) All these gases except oxygen and nitrogen may be formed during the degradation of the insulation. The amount and the relative distribution of these depend on the type and severity of the degradation and stress. Around the world and during the years several different schemes have been proposed as evaluation scheme for the DGA. The most commonly known schemes are the one proposed by Rogers and the scheme laid down in IEC publication 60599 Test Requirements for Transformers • 255 Procedure The DGA procedure consists of essential four steps: - Sampling of oil from the transformer Analysis of these gases from oil - Analysis of the extracted gas mixture in a gas chromatography, GC. Interpretation of the analysis according to an evaluation scheme. Sampling, extraction and analysis procedures are given in IEC publication 60599 • Interpretation There are several different approaches how to explain and interpret the analyzed gas composition and to diagnose the condition of the transformer. The well known DGA analysis techniques are - Identification of the key gas, The key gas identify a particular problem, e.g., H2 indicates a PD Determination of rations between gasses, normally between gas levels. Determination of rates of increase (“production rates”), in ppm / day or ml gas/day The most common known schemes are the one proposed by Rogers forming the basis for the ANSI method and the scheme laid down in IEC Publication 60599. Both these methods are using ratios gas concentrations. This scheme can also be used to understand the evaluation – scheme based on ratios. For instance, the IEC method uses 3 ratios, C2H2/ C2H4, CH4/ H2, C2H4/ C2H6 CH4/H2 is used to discriminate between a thermal fault and an electric fault. C2H2/C2H4 indicates the presence of a strong discharge of very severe electric problem and C2H4/ C2H6 is an indication of the oil temperature. 3.20 Recurrent Surge Oscillographic (RSO) Test This test is generally performed at pre-stage of transformer manufacturing after completion of terminal gear. The insulation of a transformer must be proportioned to the surge voltages, which will appear at the various points throughout the windings. The surge voltage distribution in the winding is independent of the magnitude of the applied voltage and that the same results may be obtained by applying a reduced surge voltage, of the order of a few hundred volts. This test is conducted with a recurrent surge generator, which consists of a capacitor charged to a suitable voltage and discharged by means of a thyratron into a circuit which is designed to generate the required low-voltage surge of the standard wave shape. The charge and discharge sequence is repeated fifty times per second. The output voltage from the recurrent surge 256 Manual on Transformers generator is applied to the terminal of the transformer being tested, in a similar manner to that in which a high voltage impulse test would be conducted. The surge voltage appearing at any point of the winding can be measured and displayed on the screen of the cathode-ray oscilloscope. The time base is arranged so that it is synchronized with the recurrent discharge of the capacitor. By this means it is possible to obtain a standing picture on the screen of the applied voltage and of the voltage appearing at the points along the winding, together with a time calibration wave. The test provides information on impulse distribution along the winding and transferred surge voltages on the other windings. 3.21 Determination of Core Hot Spot Temperature This test is done to check uniform distribution of flux on every point on the core & to determine the core hot spot temperature. This test is done by exciting the core with suitable voltage, which is the voltage per turn is multiplied with the wound turn around the core. The required voltage is applied to transformer and note down the reading of temperature at the different point on the core using thermo vision camera or laser temperature scanner. The reading of the temperature should not vary too much from point to point. If the temperature is varying from one point to other then the flux is not distributed uniformly around the core. Scan the point around the core where the highest temperature occurs. This is the hot spot temperature of the core. 3.22 Frequency Response Analysis (FRA) Frequency response analysis (FRA) test is conducted on transformers & reactors to determine the frequency response of windings. The reference frequency responses obtained during laboratory testing serve as ‘fingerprints’ to monitor the condition of the transformer or reactor during service. The frequency response of an electrical winding is obtained by application of sweep frequency (sinusoidal). The winding will have a characteristic frequency response for the applied signal at different frequencies. The response is uniquely determined by the winding arrangement involved and any winding movement or other fault will modify the frequency response due to changes in inductances and capacitances. The sweep frequency voltage is applied through network analyzers. The frequency response of the winding is determined between the frequency ranges of 10 Hz to 2 MHz. The FRA test is performed on one winding of the electrical equipment at a time. The transformer / reactor shall be electrically isolated from any other electrical connections or systems, including earth connections during FRA test. The two end terminals of each winding shall be made available for measuring the frequency response across the winding. Test Requirements for Transformers 257 # For star connected winding, the response shall be measured across the terminal & neutral. # For delta connected winding, the response shall be measured across two line terminals & in case of open-delta, across individual winding. # For auto connected winding, the response of series & common windings shall be measured separately. For a transformer, it is normal practice to earth one end of every winding that is not being tested, leave the other open end. Alternatively, all other windings may be left unconnected from each other and from earth. In every case, the termination of each winding for each test should be recorded. The frequency response of the winding is determined by plotting the ratio of the output from the winding to the input at atleast following frequency ranges. * 10 Hz to 2 kHz * 100 Hz to 20 kHz * 1 kHz to 200 kHz * 10 kHz to 2 MHz Alternatively frequency ranges specified by the customer can be selected. The test is normally conducted at maximum, mean and minimum taps, in case of windings having tapping. While making measurements at mean tap, care should taken to move the tap from higher voltage taps, for proper comparison of FRA results of different phases of same transformer or different transformers. The FRA results is analyzed for Changes in response of the winding Difference between the FRA records of different phases of the same transformer. FRA test is primarily a condition assessment test and can be used in conjunction with other diagnostic tests for detailed analysis and interpretation of the transformer. 3.23Measurement of Magnetization Current at Low Voltage This test is performed at 415 V, 3-phase (neutral un-grounded) for three phase transformer and 230 V 1-phase for single phase transformer. This test is performed to locate defect in magnetic core structure, failures in turn insulation or problem in tap changers. The acceptance criteria for the results of exciting current measurement should be based on the comparison with the previous site test results or factory test results. The general pattern is two similar high readings on the outer phases and one lower reading on the center phase connected in star, in case of three phase transformers. An agreement to within 25% of the measured exciting current with the previous test is usually considered satisfactory. If the measured exciting current value is 50% higher than the value measured during pre-commissioning checks, then the winding needs further analysis. 258 Manual on Transformers 3.24Functional Tests on Auxiliary Equipments Acceptance test for Oil (OTI) and winding (WTI) temperature indicator A. Routine test 1. OTI (Range 20º-140º C) (i) 2. Each completely assembled instrument shall be tested for accuracy over the complete range i.e. at 40º, 60º, 80º, 100º & 120 º C by keeping the bulb in the hot oil bath continuously stirred. The accuracy of indication shall be ±1.5 % full scale deflection (FSD). WTI ( Range 30º - 150º C) (i) Each completely assembled instrument shall be tested by injecting the current to its heater coil. Oil bath shall be maintained at 60 0C, Total temperature and temperature rise shall be recorded for 0, 2, 3 and 4 amperes current. The accuracy of the indication i.e. oil bath temperature measured by standard. Thermometer plus rise in temperature due to injection of current in heating coil i.e. total temperature indicated by WTI shall be within ± 1.5 % FSD. Error allowed shall be 1.5 / 100 x 150 = ± 2.25 0C Max. (ii) In case of repeater, both WTI and repeater shall be tested together by injecting the current as mentioned in 2(I). Accuracy of the repeater readings shall be within ± 1.5 FSD i.e. ± 1.5 0C considering WTI readings as the reference temperature. • High Voltage Test on Insulation test of auxiliary wiring Unless otherwise specified the wiring for auxiliary power and control circuitry shall be subjected to a one minute power frequency withstand test of 2 kV r.m.s. to earth. Motors and other apparatus for auxiliary equipment shall fulfill insulation requirements according to the relevant IEC standard (which are generally lower than the value specified for the wiring alone, and which may sometimes make it necessary to disconnect them in order to test the circuits) B. Type tests (one instrument of each lot / batch) Switch setting and operations: Switches shall be able to set between 50–140 0C and their operation shall be within ± 2.5% of pointer indication unless otherwise specified on purchase order. Switch setting will be done as below: OTI: Alarm (S1) Trip (S2) WTI: Alarm (S1) Trip (S2) Fan start (S3) Pump start (S4) – – – – – – 95 0C 100 0C 115 0C 125 0C 85 0C 95 0C 259 Test Requirements for Transformers Switch differential Each switch shall have adjustable differential (difference between make and break temperature) of 6 0C to 90 0C and will be set for 6 ± 1 0C differential. Switch Rating 5 Ampere, continuous 250 V, AC or DC for make or break. 3.25 Tests on Oil Filled in Transformer Following test on the oil filled in the transformer shall be necessary performed before conducting electrical test to ensure proper oil impregnation of the insulation system. 3.25.1 Dielectric Strength The voltage at which the oil breaks down when subjected to an AC electric field with continuously increasing voltage contained in the specified apparatus is called dielectric strength. The voltage is expressed in kV. The dielectric strength of oil is determined by the two methods. First method utilizes spherical capped electrode in the test cell, which is recommended primarily for filtered, degassed and dehydrated oil prior to and during filling of electrical power equipment rated above 230 kV and above. The second method utilizes flat electrodes and recommended for all other apparatus. The detailed test procedure is in accordance with IS 6792. The acceptance value of oil for the different test voltage of transformer in general is recommended as per the table given below. 3.25.2 Water Content The recommended value of water content are given in table below System voltage of transformer kV Above 72.5 245 420 Upto and including 72.5 245 420 800 BDV kV Water content ppm, max. Tan delta at 90° C 60 65 70 75 20 15 10 10 <0.05 <0.02 < 0.01 < 0.01 High water content accelerates the chemical deterioration of the insulating paper and is indicative of the undesirable operating conditions or maintenance requiring correction. 260 Manual on Transformers 3.25.3 Dielectric Dissipation Factor (Tan delta at 900C) This test covers the determination of the power factor of new and service aged oil. This test is used to indicate the dielectric losses in the oil when used in an alternating electric field and of the energy dissipated as heat. A low power factor indicates low dielectric losses. It is useful as a means to ensure that sample integrity is maintained, and as an indication of changes in quality resulting from contamination and deterioration in service or as a result of handling. This test is satisfactorily performed in the field, as well as in a laboratory environment. The detailed test procedure and test equipment shall be in accordance with IS 6262. Acceptable limit for the dielectric dissipation factor largely upon the type of apparatus and application. The power factor limits given for oil are based upon the understanding that this is an indicator test for contamination by excessive water or polar or ionic materials in the oil. High level of dissipation factor (0.5 % at 25º C) is because of contaminants may collect in the areas of high electrical stress and concentrate in the winding. Very high dissipation factor ( > 1.0% ) in oil may be caused by the presence of free water which could be hazardous to the operation of a transformer. 3.25.4 Resistivity The resistivity (specific resistance) in ohm-centimeters of a liquid is the ratio of the dc potential gradient in volts per centimeter paralleling the current flow within the specimen, to the current density in amperes per square centimeter at a given instant of time and under prescribed conditions. This is numerically equal to the resistance between opposite faces of a centimetre cube of liquid. Resistivity measurements are made at many different temperatures. But for acceptance test, it is generally done at a temperature of 90º C, while for routine testing, it is usually made at room temperature or 90º C. The average electrical stress to which specimen is subjected to shall not be less than 200 V/mm nor more than 1200 V/mm. the upper limit is set with the purpose of avoiding possible ionization if higher stresses are permitted. The detailed test procedure is as accordance with IS 6103. Useful information can be obtained by measuring resistivity at both ambient and at higher temperature such as 90º C. A satisfactory result at 90º C coupled with an unsatisfactory value at lower temperature is an indication of the presence of water or degradation products precipitated. 3.26Measurement of vibration on transformer tank The vibration measurement on the completely assembled transformer is done to ascertain the safety of the transformer against any impending mechanical damage due to looseness of the Test Requirements for Transformers 261 component leading to high vibrations. The vibration in the transformer caused by the attraction between the poles at the level of each air gap. Mechanical resonance may appear in the system formed by the core packets, the yokes and the insulating spacers. Moreover, other components not located in the magnetic circuit can also enter into resonance, particularly some parts of the tank, which receive impulses transmitted through oil. For conducting vibration measurements, the transformer is energized at rated voltage and the rated frequency. A magnetic accelerometer is fixed at the location where measurement is intended. The vibration signal from accelerometer is transmitted to a vibration meter which indicates the vibration level in microns. For thorough checking of vibration pattern, the transformer tank is marked into several small sections for measurement of the highest amplitude vibrations. The measurement shall commence after allowing minimum 30 minutes stabilization time after energizing the transformer at rated voltage. There is no acceptable criterion for vibrated level on a transformer. However, one should look for only appreciable increase in the load during the test period and subsequent readings taken at periodic intervals at site. 3.27 Vacuum Test on Transformer Tank Vacuum test is carried out to check out any permanent deflection of flat plate after the vacuum has been released. This test is to be carried out at test figures specified on the tank drawing. This test is carried out on only one tank of same work order until otherwise specified by customer. Tank and its temporary structure are connected as given in the drawing and connect the tank to the vacuum pumping instrument. The tank shall be capable of withstanding an absolute pressure as stated on the drawing for at least 60 minute. During and after vacuum test we should examine all welds for cracks. In the event of repair work, vacuum test should be repeated. Maximum temporary deflection during test and permanent deflection after test should be measured. Permanent deflection should not exceed the value specified in the specification. 3.28 Oil Pressure Test on Completely Assembled Transformer This test is done after completion of all electrical and temperature rise test. Transformer with cooling bank, bushing and other accessories shall be tested for any oil leakage at high pressure (normal pressure plus 35 kN per sq.m measured at the base of tank) and at room temperature as specified by customer. The procedure for conducting this test is as follows: 1. 2. 3. Conservator along with the protective relay shall be disconnected. Calibrated pressure gauge shall be mounted at the bottom of the tank. Bushings will remain mounted however conservator along with buchholz relay shall be 262 4. 5. 6. 7. • Manual on Transformers isolated. In welded cover type construction cooler bank, bushings shall be removed but all turrets and cover pipe work shall remain. Fill the oil completely and release all trapped air. The specified pressure shall be maintained for the specified test duration as specified in the test schedule or quality plan. The test duration should be at least one hour unless otherwise specified. Criteria for oil pressure test During the pressure test, there shall not be any leakage. If there is pressure drop during the test either because of some trapped air inside the transformer or due to ambient temperature variation, the pressure shall be raised to the specified level. The unit will be considered to pass the test only if there is no visual oil leakage. Pressure drop shall be considered as failure of the unit in the test. 3.29 Jacking Test and Dye-penetration Test This test is done to check out the mechanical capability of jacking pads on bottom tank of transformer. The procedure for conducting the above test is as follows: 1. 2. 3. 4. 5. 6. 7. 8. Bring the transformer on to rail track The transformer should be filled with oil. Place jack under jacking pads such that the C.L. of the jack ram coincides with the jack points on jacking pads at appropriate height. connect the jacks to hydraulic pump unit Raise pressure of oil slowly so that the transformer is lifted gradually. Continue lifting till, the flanges of the rollers are above the R/L and its possible to turn the wheel. The transformer must be held in the raised condition for 15 minutes. Lower the transformer gradually after the expiry of time period by releasing oil pressure in stages till the wheel again rest on the rails. The transformer jacking pads should withstand this test without any deformation or any cracks. The dye-penetration test is done simultaneously with jacking test to check out any welding cracks in jacking pads. This is done by applying the dye paint to the welded joints of jacking pads. If there is no leakage of dye at welding joints, then the welded joint is perfect. 3.30 Pressure Relief Device Test The pressure relief device shall be subjected to rising pressure (pneumatically). The pressure at which the device operates shall be noted. The operating pressure should be less than normal pressure plus 35 kN/m2. Test Requirements for Transformers (B) • 263 RECOMMEND FIELD TESTS INTRODUCTION Transformer is important and vital equipment between generation station and the utility and therefore necessary to ensure its proper performance through out its service life. During transportation, installation and service operation, the transformer may be exposed to conditions, which adversely affect its reliability and useful life. Field-testing and condition monitoring are the techniques to ensure good operating health of power transformers. Interpretations are also included to provide additional information on the particular test and to provide guidance on acceptable criteria. There is not necessary any direct relationship between field tests and factory tests. Interpretation of measured results is usually based on a comparison with data obtained previously on the same unit under similar condition. It should be noted that some times the results of several types of tests should be interpreted together to diagnose a problem. Manufactures acceptance criteria shall also be consulted. B.1 Dew Point Measurement for Large Transformers Filled with dry air or nitrogen filled Large rating transformers are transported to site from manufacturing works, without oil and filled with dry air or nitrogen due to weight limitations. Positive gas pressure is generally maintained at 0.175 kg/m2 during transportation and storage. As the insulation of transformer is hygroscopic, it absorbs moisture from atmosphere if positive pressure of gas is not maintained. After arrival of transformer at site it is necessary to check the gas pressure and if it is not positive there is every possibility that moisture must have gone inside the transformer during transportation. To ascertain this factor and to check the dryness of the insulation, dew point measurement is carried out at site. Dew point is the temperature at which the water vapours present in the gas filled in the transformer begin to condense. It will not be possible to define a limit of dew point of nitrogen gas as dew point depends on the ambient temperature, pressure of the gas, moisture level of cellulose insulation etc. The procedure and acceptance limits are given in Section CC of this manual. • ELECTRICAL TESTS B.2 Winding Resistance Measurement Transformer winding resistances are measured at site in order to check for abnormalities due to loose connections, broken strands of conductor, high contact resistance in tap changers, high voltage leads and bushings. The resistance is measured by two methods (a) Voltmeter Ammeter method 264 (b) Manual on Transformers Bridge method The detailed test procedure of above methods is same as factory testing and is covered in section A.3.1.2 of this manual. Precautions shall be taken during field testing as given below. The test shall be conducted at all taps of the transformer winding and the measured value shall be converted to 75 0C. The acceptance criterion is usually agreement to within 5% of resistance measurements made separately on different phases, under field condition. But, for large transformers it is recommended to compare the resistance values with original data measured in the factory and in case of large variation, connection tightness to be checked. The current used for these measurements should not exceed 15% of the rated current in order to avoid heating the winding thereby changing its resistance. However, the current should not be too small, which may not be sufficient to avoid inductive effect, due to core magnetization The winding resistance shall be preferably done when the difference in the top and bottom temperature of the winding (temperature of oil in steady-state condition) is equal to or less than 5oC. Winding resistance measurement shall be done only after measurement of magnetization current (excitation current), magnetic balance & SFRA. The polarity of the core magnetization shall be kept constant during all resistance measurement. A reversal in magnetization of the core can change the time constant and result in erroneous readings. B.3 Vector Group and Polarity To determine the phase relationship and polarity of transformers The procedure to find out vector group shall be general be same as defined in section A3.2.2.2. (a) Connect neutral point and LV phase with earth (b) Join 1R1 of HV and 3B1 of tertiary (c) Apply 415V, 3 phase supply to HV terminals (d) Measure voltage across following terminals 1R1- 1Y1, 1Y1 – 1B1, 1B1 – 1R1, 3Y1 – 1B1, 3Y1 -1Y1, 3R1-N, 3Y1 – N, 1R1- N, 2R1 – N, 2Y1-N, 2B1-N Example 1 Connect 1U to 2U MEASURE 1W-2V, 1W-2W, 1U-2W, 1V-2V, 1V-2W VOLTAGE RELATION 1W-2V= 1W-2W 1W-2V< 1U-1W 1V-2V<1V-2W 1V-2V <1U-1W Fig 1 : For HV-Delta / LV-Star Transformer Test Requirements for Transformers 265 Example 2 Connect 1U to 2U MEASURE 1W-2V, 1W-2W, 1U-2W, 1V-2V, 1V-2W VOLTAGE RELATION 1W-2W = 1V-2W 1W-2U > 1V-2U 1U-N=1U-2W+2W-N Yd11 Fig 2: For HV-Star / LV-Delta Transformer The test shall be conducted with three phase supply and voltmeters. By the measured voltage data it should ensure that the desired conditions of vector group and polarity are fulfilled. Ensure the isolation of transformer from high voltage and low voltage side with physical inspection of open condition of the concerned isolators. In case tertiary is also connected, ensure the isolation of the same prior to commencement of testing. B.4 Voltage Ratio Test To determine the turns ratio of transformers during commissioning and periodic interval decided by the customer for preventive maintenance The procedure shall be following (a) Keep the terminals of IV and LV open. (b) Apply 3 phase or single phase supply according to the transformer type on HV terminals. (c) Measure the voltage ratio of HV and IV. (d) Repeat the steps for each tap position separately. The test shall be conducted with three phase supply and voltmeters. Results of the transformation turns/voltage ratio are absolute, and may be compared with the specified values measured during factory testing. One should also consider the trend of voltage ratio values with reference to the ratio values measured during the commissioning tests. The voltage should be applied only in the high voltage winding in order to avoid unsafe voltage. 266 Manual on Transformers B.5 Measurement of Magnetizing Current This test is performed to locate defect in the magnetic core structure, shifting of windings, failure in turn to turn insulation or problems in tap changers. These conditions change the effective reluctance of the magnetic circuit thus affecting the current required to establish flux in the core. The procedure is as follows: (a) (b) First of all keep the tap position in the lowest position and IV and LV terminals open. Apply 3 phase 415V supply on the line terminal for 3-phase transformers and 1-phase, 230 V supply on single phase transformer. (c) Measure the voltage applied on each phase (Phase-Phase) on line terminals and current in each phase of the line terminal. (d) After completion of the above steps keep the tap positioning Normal position and repeat the steps a to c. (e) After completion of the above steps keep the tap positioning at Highest position and repeat the steps a to c. (f) Repeat the test with tap position in normal position. The test shall be conducted with single phase or three phase supply according to test requirement, voltmeter and multimeter. The acceptance criteria for the results of exciting current measurement should be based on the comparison with the previous site test results or factory test results. The general pattern is two similar high readings on the outer phases and one lower reading on the center phase, in case of three phase transformers connected in star. An agreement to within 30% of the measured exciting current with the previous test is usually considered satisfactory. If the measured exciting current value is 50 times higher than the value measured during pre-commissioning checks, then there is likelihood of a fault in the winding which needs further analysis. Care should be taken during exciting current measurement to avoid the effect of residual magnetism in the transformer core. The residual magnetism results in the measurement of higher than normal exciting current. B.6Magnetic balance test on 3-phase transformer This test is conducted only on three phase transformer to check the imbalance in the magnetic circuit. The procedure for conducting test is as follows (a) (b) (c) (d) Keep the tap in nominal tap position Disconnect the transformer neutral from ground Apply single phase 230 V across one of the HV winding terminal and neutral then measure voltage in other two HV terminals across neutral. Repeat the test for each of the three phases. Repeat the above test for IV winding also Test Requirements for Transformers 267 The test shall be conducted with 230 V single phase supply and voltmeter The voltage induced in the center phase shall be 50 to 90% of the applied voltage. However, when the center phase is excited then the voltage induced in the outer phases shall be 30 to 70% of the applied voltage. Zero voltage or very negligible voltage induced in the other two windings should be investigated. Disconnect transformer neutral from ground and no winding terminal should be grounded, otherwise results would be erratic and confusing. The purpose of this test basically to ensure that there is no inter turn fault in the winding which is generally reflected in high excitation current in faulty winding. B.7 Magnetic circuit (Isolation) test This test should be performed prior to a unit being placed in-service or following modifications to the transformer which could affect the integrity of its core insulation. Refer clause 3.15 part A of this section for test procedures and acceptance criterion. B.8Measurement of short circuit impedance at low voltage To find out the short circuit impedance of transformer the measurement is performed in single phase mode. This test is performed for the combination of two winding. The one of the winding is short circuited and voltage is applied to other winding. The voltage and current reading are noted. The test shall be conducted with variac of 0-280 V, 10 A, precision RMS voltmeter and ammeter. The acceptable criteria should be the measured impedance voltage having agreement to within 3 percent of impedance specified in rating and diagram nameplate of the transformer. Variation in impedance voltage of more than 3% should be considered significant and further investigated. The conductors used for short-circuiting one of the transformer windings should have low impedance (less than 1m-ohm) and short length. The contacts should be clean and tight. B.9 Insulation resistance measurement Insulation resistance test are made to determine the insulation resistance from individual winding to ground or between individual windings. The insulation resistance in such tests is commonly measured in mega-ohms, or may be calculated from measurements of applied voltage and leakage current. The test is conducted with the help of Mega-ohmmeter. IR is proportional to the leakage current through/over the insulation after capacitive charging and absorption currents become negligible 268 Manual on Transformers on application of DC voltage. Insulation resistance shall be measured after the intervals of 15 sec, 60 sec and 600 sec. The polarization index (PI) is defined as the ratio of IR values measured at the intervals of 600 and 60 seconds respectively. Whereas, the dielectric absorption is the ratio of IR values measured after 60 sec and 15 sec. IR is normally measured at 5 kV DC or lower test voltage, but the test voltage should not exceed the rated power-frequency test voltage of transformer windings. Polarization Index (PI) is useful parameters for logistic interpretation of IR test results. This ratio is independent of temperature and gives more reliable data for large power transformers. A PI of more than 1.25 and dielectric absorption factor of more than 1.3 are considered satisfactory for a transformer when the results of other low voltage tests are found in order. PI of less than 1 calls for immediate corrective action. For bushings, an IR value of above 10000 M-ohms is considered satisfactory. The IR value of transformer is dependent on various factors such as configuration of winding insulation structures, transformer oil, atmosphere condition etc. therefore, present trend is to monitor oil characteristics for judging the condition of dryness of the transformer and not to rely solely on absolute values of IR. It may be note that no national/international standards specify minimum insulation resistance values of transformers. The value of IR could be even zero under heavy fog or humid conditions. During IR measurement, we must ensure following conditions. - Transformer is disconnected from other associated equipment Bushings are cleaned and free of moisture Transformer tank and core are properly grounded Both ends of winding under test are short-circuited. B.10 Measurement of capacitance and dissipation factor The insulation dissipation factor (tan delta) is the ratio of the resistive current to the capacitive current flowing through the insulation on application of sinusoidal voltage under prescribed condition. The capacitance values are relatively independent of temperature and prevailing atmospheric conditions. Normally low dissipation factor is indicative of problem in insulation structure and predictive aging of insulation. The voltage to be applied shall not exceed half of the power frequency test voltage or 10 kV whichever is lower. Detailed test procedure is concerned under this manual earlier. The test is conducted with high voltage supply and Schering bridge High dissipation factor is indicative of problem in insulation structure and predictive aging of insulation. But, the comparative values of tests taken at periodic intervals are useful in identifying potential problems rather than an absolute value of tan delta. The initial reference can be drawn from the tan delta values measured during factory testing. Test Requirements for Transformers 269 The acceptance criterion to assess the probable condition of the insulation of the transformer is no substantial variation in the measured values of tan delta (dissipation factor) at periodic interval when compared with previous references. For bushings, the tan delta value shall not exceed 0.7% during operation. Environmental factors like variation in temperature, relative humidity, surrounding charged objects etc. have great influence on measurement of dissipation factor. Care shall be taken to control the above factors during measurements. B.11 Dissolved gas analysis (DGA) Dissolved gas analysis is a well-established condition-monitoring tool for power transformers. The method is based on analyzing the types of gases dissolved in the transformer oil and their production rates. The relative concentration of these gases depends upon the type and severity (energy density) of fault. The test procedure to conduct DGA test is accordance with the IS 9434. The main gases formed as result of thermal and electrical faults in a transformer are Hydrogen (H2), Methane (CH4), Ethane (C2H6), Ethylene (C2H4), Acetylene (C2H2), Carbon Monoxide (CO) and Carbon dioxide (CO2). Whereas Acetylene is mainly associated with arcing at very high temperature ranges, Ethylene is related with hot spots and medium temperature ranges. Hydrogen is mainly result of ‘cold’ gas plasma of corona discharges. Carbon monoxide and carbon dioxide are the result of thermal decomposition of cellulose material. For proper interpretation of DGA results, it is essential to collect data at periodic intervals during the service life of the transformer and the additional information regarding age of the transformer, past history of failures, loading pattern, history of filtration etc. B12. Tests on oil filled in transformer as per IS 1866 Refer clause 3.25 part A of this Section 4.0METHOD OF DECLAIRING EFFICIENCY 4.1 The efficiency to be declared is the ratio of the output in kW to the sum of the output in kW and the following losses : (a) No load loss, which is considered to be constant at all loads; and (b) Load loss, which varies with load. The total loss, on load is the sum of (a) & (b). 5.0 CALCULATION OF INHERENT VOLTAGE REGULATION 5.1 Two Winding Transformers 5.1.1 The inherent voltage regulation from no-load to a load of any assumed value and power factor may be computed from the impedance voltage and corresponding load loss measured 270 Manual on Transformers with rated current in the winding as follows: Let I = rated current in winding excited; E = rated voltage of winding excited; ISC = current measured in winding excited; EZsc = voltage measured across winding excited (impedance voltage); PSC= watts measured across winding excited; EXSC = reactance voltage= Ö[ E2zsc-(Psc/Isc)2] P=PSc corrected to 75 dcg. C, and from current ISC to I; Ex=EXSC X (I/ISC) Er = P ; I EX%=100EX/E; Er%=100Er/E; = Ia/I ; and Ia = current in winding in excited during the short circuit test corresponding to that obtained when loading at the assumed load on the output side and with rated voltage on the input side. 5.1.2 For rated load at unity power factor, the percentage regulation is approximately equal to: 2 Er + (Ex%) 200 5.1.3 For rated load at any power factor cosФ, the percentage regulation is approximately equal to: Er% cos Ф + Ex% sin Ф + (Ex% cos Ф - Er% sin Ф)2 200 5.1.4 For any assumed load other than rated load and unity power factor, the percentage regulation is approximately equal to: n. Er + (n. Ex%) 200 2 Test Requirements for Transformers 271 5.1.5 For any assumed load other than rated load and at any power factor cos Ф, the percentage regulation is approximately equal to: (n .Er% cosφ + n. Ex% sin φ) + (n .Ex%cos φ - n .Er% sin φ)2 200 5.1.6 The above formula are sufficiently accurate for transformers covered by this specification. 5.2 Three Winding Transformers 5.2.1 The formula given in 2.1 for two-winding transformers can be applied to three-winding transformers and their regulation calculated with an accuracy comparable to that of the data available by assuming that the currents in the windings remain constant both in magnitude and phase angle even though the output terminal voltage change, due to the regulation, from their no-load values. On a three-winding transformer the no-load voltage of a winding will change with current in the other windings (even though it remains itself unloaded). Therefore, the voltages regulation of a winding on a three-winding transformer is expressed with reference to its no-load open-circuit terminal voltage when only one of the other windings is supplied and the third winding is on no-load, that is the basic voltage for each winding and any combination of loading is the line no-load voltage obtained from its turns ratio. For the frequent case of two output winding (W2 and W3) and one input winding (W1) the voltage regulation is usually required for three loading conditions, namely: Only W2 loaded, Only W3 loaded, and Both W2 and W3 loaded. For each condition two separate figures should be quoted, that is the regulation of each output winding W2 and W3 (whether carrying current or not) for constant voltage supplied to the winding W1. Note:The regulation between W2 and W3 relative to each other for this simple and frequent case is implicit in the values (W1 to W2) and (W1 to W3) and nothing is gained by expressing it separately. 5.2.2 The data required are the impedance voltage and load losses derived by testing the three windings in pairs and expressing the results on a basic KVA, which can conveniently be the rated kVA of the smallest winding. It should be determined from the transformer as built. 272 Manual on Transformers From the data, an equivalent circuit 3 winding transformer is derived as shown below: 5.2.3 The equivalent circuit is derived as follows: Let a12 and b12 be respectively the percentage resistance and reactance voltages referred to the base kVA and obtained on a test, short-circuiting either winding W1 or W2 and supplying the other, with the third winding W3 on open-circuit: a23 and b23 similarly apply to a test on the windings pair W2 and W3 (with W1 on open-circuit); a31 and b31similarly apply to a test on the windings pair W3 and Wj(with W2on open-circuit); d = the sum (a12 + a23 + a31); and f = the sum (b12 + b23 + b3I) Then the mathematical values to be inserted in the equivalent circuit are: Arm W1a1 = d/2 - a23, and b1 = f/2 - b23 Ami W2a2 = d/2 - a31, and b2 = f/2 - b31 Arm W3a3 = d/2 - a12, and b3 = f/2 - b12 It is to be noted that some of these mathematical values may be negative or may even be zero (depending on the actual physical relative arrangement of the windings on the core). Test Requirements for Transformers 273 For the desired loading conditions the kVA operative in each arm of the equivalent circuit is determined and the regulation of each arm is calculated separately. The regulation with respect to the terminals of any pair of windings is the algebraic sum of the regulations of the corresponding two arms of the equivalent circuit. 5.2.4 The detailed procedure to be followed in the case of two output windings and one supply winding is as follows (a) (b) (c) Determine the kVA in each winding corresponding to the loading being considered. For the output windings W2 and W3, this is the specified loading under consideration; deduce n2 and n3, where n is the ratio of the actual loading to the base kVA used in the equivalent circuit. For the input winding W1 kVA should be taken as the vectorial sum of the outputs from W2 and W3 windings, and the corresponding power factor and quadrature factor (sin φ) deduced from the in-phase and quadrature components. 5.2.5 When greater accuracy is required, an addition should made to the above vectorial sum of the outputs as follows: Add to the quadrature component, to obtain the effective input kVA to windings W1, b2 (output kVA from winding W2) x 100 x n2 + (output kVA from winding W3) x W x b3 100 xn n for each arm being the ratio of the magnitude of the actual kVA loading of the winding to the base kVA employed in determining the network. A more accurate solution is obtained by adding the corresponding quantities (a x n x output kVA) to the in-phase component of the vectorial sums of the outputs, but the difference is rarely appreciable. 5.2.6 Apply the final formula of 2.1.5 separately to each arm of the network, taking separate values of n for each arm as defined in 2.2.5. 5.2.7 To obtain the regulation between the input winding and either of the loaded windings, add the separate regulations determined under 2.2.6 for the corresponding two arms, noting that one of these may be negative. (Note that the summation is algebraic, but not vectorial). Note: A positive value for the sum determined indicates a voltage drop from no-load to the loading considered while a negative value for the sum indicates a voltage rise. 5.2.8 Repeat the operation described in 2.2.7 for the other loaded winding. 274 Manual on Transformers 5.2.8.1 The above procedure is applicable to auto-transformers if the equivalent circuit is based on the effective impedances measured at the terminal of the auto-transformer. 5.2.8.2 In the case of input to two windings and load from one winding, the above procedure can be applied if the division of loading between the two supplies is known. 5.2.8.3 Example of Application to a Three-Winding Transformer. Assume that: W1 is a 66 kV primary winding, W2 is a 33 kV output winding loaded at 2000 kVA having a power factor cos φ of 0.8 lagging, and W3 is a 11 kV output winding loaded at 1000 kVA having a power factor cos φ of 0.6 lagging. The following information is available, having been calculated from test data and is related to a base kVA of 1000: a12 = 0.26 b12 = 3.12 a23 = 0.33 b23=1.59 a31=0.32 b31=5.08 It can be deduced that d = 0.91 and f = 9.79 If follows, therefore, that for W1, a1 = 0.125 and b1 = +3.305 W2 ,a2= 0.135 and b2 = -0.185 W3 , a3 = 0.195 and b3 =+1.775 The effective full load input kVA to winding W1 is: (a) with only output = 2000 kVA winding W2loaded at power factor 0.8 lagging. (b) With only output = 1000 kVA winding W3 loaded at power factor 0.6 lagging. (c) With only output = 2980 kVA Test Requirements for Transformers windingW2and W3 loaded 275 at power factor 0.74 lagging. Applying the formula of 2.1.5 separately to each arm of the network we have for the regulation of that arms alone: W1 under condition (a) where n1 = 2.0 is + 4.30 per cent W1 under condition (b) where n1 = 1.0 is + 2.74 per cent W1 under condition (c) where n1 = 2.98 is + 7.15 per cent W2 under condition where n2 = 2.0 is - 0.005 per cent W3 under condition where n3 = 1.0 is +1.54 per cent Therefore, the total transformer regulation is: For condition (a) - with output winding W2 fully loaded and W3 unloaded: At terminals W2 - 4.30 - 0.005 = 4.295 per cent At terminals W3 = 4.30 + 0 = 4.30 per cent For condition (b) - with output winding W2 unloaded and W3 fully loaded: At terminals W2= 2.74 + 0 = 2.74 per cent At terminals W3= 2.74 + 1.54 = 4.28 per cent For condition (c) - with both output windings W2 and W3 loaded: At terminals W2= 7.15 - 0.005 = 7.145 per cent At terminals W3= 7.15 + 1.54 = 8.69 per cent SECTION CC Guidelines for Erection, Commissioning and Maintenance SECTION CC Guidelines for Erection, Commissioning and Maintenance As the continuity of supply is of paramount importance in modern power systems, it is necessary to take all possible precautions during erection and commissioning of the transformers followed by regular preventive maintenance. This section covers erection, commissioning and maintenance of Power and Distribution Transformers. 1.0 PACKING AND DISPATCH 1.1 After testing each transformer it shall be dispatched from the works ready for reassembly of external components which are dismantled for facilitating transportation. According to the transport facilities available and weight/ height restriction for the route, transformers are transported either by rail, road or sea depending on the size of transformer. Power transformers should be dispatched with external fittings dismantled and packed separately. However distribution transformers can be transported in fully assembled condition. 1.2 Transformer tank is filled with oil or pure dry nitrogen/ air depending upon the transport weight limitations. Dry air should be preferred due to safety of personnel entering inside the Transformer. If nitrogen is used, the information shall be stenciled on the tank prominently. In case the tank is filled with oil, sufficient space is left above the oil to take care of the expansion of oil. This space is filled with pure dry air/nitrogen gas under atmospheric pressure. In case the tank is filled with inert gas/nitrogen a positive pressure according to the manufacturer’s standard practice and dew point of -50 deg C or better shall be maintained. The temperature, pressure and dew point at the time of gas filling (reading taken after stabilization) shall be painted on the transformer tank. External gas cylinders should be provided to make up any gas leakage during transit for transformers having rating 50 MVA and above and voltage rating 132 kV and above. 1.3 Transformer shall be accompanied by escort decided by the manufacturers or the customers as per the agreement. For large rating transformers of above 100 MVA the digital impact recorders may be provided during shipment from transformer manufacturer’s works to site. Provision of SMS alert needs to be provided during transportation and in case of any major impact, SMS alert shall be sent to representatives of manufacturers and respective utilities. If required by utility, GPS may also be installed on Transformers to locate the exact position during transportation. Manufacturer shall specify the limiting values to the purchaser in advance. The impact recorder is required to be sent back to the manufacturer for their analysis. Joint analysis of impact recorder may also be carried out at trailer itself after receipt at site to check any movement outside specified limit during transportation. Manufacturer shall send the analysis report of impact recorder back to user / utility for their record. Interpretation of impact recorder may be carried out as per standard DIN EN 13011. In case the impact recorder indicates some serious shock during shipment, the core and coil will have to be subjected to thorough inspection before erection. Higher impacts may also be verified with SFRA results comparison of factory and site. 279 280 Manual on Transformers Note: (i) Impact recorders are attached to the main body of the transformer during transportation to monitor the shock and impact, which the transformer may be subjected to during transportation. The impact recorder is a electronic or mechanical recording accelerometer whose main feature is its ability to record shock and impact from three directions axial, lateral and longitudinal. (ii) Check at factory that impact recorders have sufficient paper, battery life and memory capacity for the expected duration of transport. (iii) Mount the impact recorder as low as possible near the transformer centerline. Always keep the impact recorder axis aligned with the transformer major axis. This provides the best possible recorder data (Refer DIN EN 13011). (iv) For transformers with a N2 gas/ dry air pressure of 0.3 PSI, the acceptable limits of dew point shall be as under: (Source: Courtesy BHEL, Bhopal) Temperature of insulation in °F Maximum permissible dew point in °F Temperature of insulation in °C Maximum permissible dew point in °C 0 -78 -17.77 -61.11 5 -74 -15.0 -58.88 10 -70 -12.22 -56.66 15 -66 -9.44 -54.44 20 -62 -6.66 -52.22 25 -58 -3.33 -49.99 30 -53 -1.11 -47.22 35 -48 +1.66 -44.44 40 -44 +4.44 -42.22 45 -40 +7.44 -39.39 50 -35 +9.99 -37.22 55 -31 12.77 -34.99 60 -27 15.55 -32.77 65 -22 18.33 -29.99 70 -18 23.11 -27.77 75 -14 23.88 -25.55 80 -10 26.66 -23.33 85 -6 29.44 -21.11 90 -1 32.22 -18.33 95 +3 34.99 -16.11 100 +7 37.75 -13.88 110 +16 43.33 -8.88 120 +25 48.88 -3.88 130 +33 54.44 +0.55 140 +44 59.99 +5.55 Guidelines for Erection, Commissioning and Maintenance 281 1.4 All external fitting such as Conservator, Buchholz relay, Dehydrating Breather, Turrets, Bushings, Explosion Vent/ Pressure Relief Devices (PRDs), OLTC - Driving Mechanism and Motor Operated Mechanism (MOM) Boxes, Marshalling Kiosks, Radiators, Rollers, Cooling Fans, Pumps, Gasket etc., which are liable to damage in transit, shall be removed and packed separately. All openings created on the tank by the removal of components arc blanked with identifiable blanking plates & suitable gaskets. All openings on the fittings removed are also closed with blanking plates & suitable gaskets. 1.5 When any internal parts like tap changers, etc., are removed for transportation, they are dispatched in tanks filled with oil/inert gas or suitable measures taken so that they do not absorb moisture. • All fragile parts such as temperature indicators, oil level gauges, etc., shall be carefully packed to avoid breakage in transit. • All blanking plates, tank valve guards, etc., used exclusively for transport is to be preserved in safe custody by the purchaser for future use. 1.6 Rail Transport 1.6.1 Transformers may be transported by rail / road trailers depending on size of transformer, destination, delivery time and the route limitations. In case of transformers where the weight and dimensions of the main body exceed limits, special well wagons are employed. Detached parts are packed/crated and normally dispatched along with the main body of the unit, so that all the parts are received at the destination station with the unit. 1.6.2 Loading 1.6.2.1 Most of the transformer manufacturers have their own railway siding and transformers are loaded at their works using their cranes. In cases where separate siding facilities are not available, road tractor-trailers are used for the transport from works to loading railway stations. Mobile cranes and railway cranes are also used for loading the units into the wagons. In the absence of any crane facility, the transformers are unloaded from the trailers near the railway track into a platform of adequate height built up of wooden sleepers. Jacks and pull lifts chain pulley block of adequate capacity are used to slide the transformer over a pair of rails placed on the sleeper platform bridging the trailer width in full. To prevent the trailer from toppling when the transformer is moved to the platform the stage end of the trailer is supported by a sleeper pack. From the platform the transformer is slid onto the wagon taking care to have the rails for the full width of wagon. The rails are greased for easy movement. The rollers of the transformer are removed before leaving the works. While providing a sleeper stage it should slightly be at a higher level to allow for the increase in height of the trailer while the load is released due to the springs getting relaxed. 1.6.3 Lifting and Jacking 16.3.1 Transformers should be lifted by jacking at the jacking pads provided for the purpose and simultaneous use should be made of all such lugs or lifting bollards in order to avoid any unbalance in lifting. Before lifting the complete transformer it should be ensured that all cover 282 Manual on Transformers bolts arc tightened. Apart from the main lifting points designed to take the total weight of the unit, the transformer has subsidiary lifting points suitable for particular components only. Care must be taken to distinguish between them. It is advisable to use a spreader between slings so that the lift on the hooks is in the vertical direction. The slinging angle is not to exceed 60°. Safe loads of wire ropes and the multiplying factors to be used corresponding to the lifting angles are furnished in Fig. 1. Gunnies are used on the slings to avoid metal contact and consequent damage to the slings. Sale load of wire ropes Dia. of wire rope (mm) Multiplying factor for different lifting angles Safe load (Kg) Lifting angles Multiplying factor 8 600 0 1 12 1,300 20 1.015 16 2,300 40 1.065 20 3,500 60 1.155 24 5,000 28 7,000 32 9,000 36 11,000 40 14,000 44 17,000 56 24,500 64 33,500 70 40,000 1.6.3.2 Where it is necessary to use jacks for lifting, only the Jacking pads provided for the Lifting angle Slinging angle Fig. 1 Correct method of slinging Guidelines for Erection, Commissioning and Maintenance 283 purpose of jacking should be used. Jacks should never be placed under valves or cooling tubes or stiffeners. Not more than two jacks should be operated at the same time. When two jacks are being operated the opposite side of the transformer should be firmly supported by sleepers. Jacks are also not to be left in position with load for a long time. The transformer should always be handled in the normal upright position. During the handling operation care must be taken to prevent overturning or even tilting. 1.6.3.3 Loading on the railway wagons is done as centrally as possible to distribute the weight equally on all axles and wheels. Manufacturer’s drawing in this regard is to be referred to. It is desired that the transformer be loaded longitudinally on the wagon. To prevent damage to the transformer base, the unit is loaded on a row of sleepers or wooden planks placed on the wagon floor. 1.6.4 Lashing The transformer is lashed on all four sides by wire ropes or chain of adequate size and lightened using turn buckles with locking facility. Wooden props are also used. After the movement of the wagon for a short distance the tightness of the lashing is to be checked. When enroute transhipment is involved, lashing is to be checked again. 1.7 Road Transport 1.7.1 Transformers may be shipped by road, where well developed roads exist & the route conditions permit. Multi-axle tractor driven low-platform trailers are used for road transport. Hydraulic trailers to be used for transportation of heavy Transformers, i.e., weighing more than 50 Tonnes including weight of trailer. The tractors are to have adequate hauling capacity and the trailers loading capacity. 1.7.2 Route Survey 1.7.2.1 The road system is examined in detail on the following points: (i) Width of the road Normally not less than 5 m (ii) Bridges and culverts To have sufficient strength to take the moving load; consultation with the Highways Department is necessary (Refer Fig. 2 for assessing the axle load.) (iii) Encumbrances enroute Like telephone, telegraph, traction and electric utility wires, avenue trees, cross beams of bridges, subways, aqueducts, etc., across the roadway. (iv) Sharp bends Steep gradients up or down with respect to the maneuverability of the tractor-trailer. (v) Road worthiness Of the route like sandy stretches, waterlogged areas, crowded localities like market places, schools and other public places. (vi) Operational Constraints of the tractor-trailer to be used. To clear up any 284 Manual on Transformers doubts as to the feasibility of the route a rehearsal drive of the tractor-trailer unit is performed. All turnings clearance from local authorities, PWD, Motor Highways Department should be obtained before movement. All transport diversions should be thoroughly checked. WTa = Self weight of tractor Fig. 2 Self weight of trailer Pay load 50 tonnes Load in the front axle of tractor Load in all the 3 axles of trailer (Divide by 3 for axle load) Load in all the 2 axles of this trailer (Divide by 2 for axle load) Take the momentum at the points at Ra, Rb and Rc and solve the equations, to get Ra, Rb and Rc. In this case if we assume tractor and trailer of equal weight (i.e.) WTa = WTi; = 10 tonnes and WP = 50 tonnes. Ra = 5 tonnes axle load in front axle Rb = 35 tonnes axle load in 2nd axle Rc = 30 tonnes axle load in 3rd axle Load per rear axle of the tractor = 35/3 = 11.66 tonnes Load per rear axle of semi-trailer = 30/2 WTi = WP = Ra = Rb = Rc = Guidelines for Erection, Commissioning and Maintenance 285 = 15 tonnes To limit the load per axle to 10 tonnes at the rear axle of trailer, the payload should not exceed 30 tonnes Otherwise the number of axles for the trailer should be increased. 1.7.2.2 Movement of the transformer shall be through the route surveyed for the purpose and no deviation of the route shall be followed unless it is surveyed again. 1.7.3 Strengthening the Route 1.7.3.1 Such of those bridges and culverts which require strengthening are strengthened by adopting suitable measures like propping up using timber and steel in consultation with the Highways Department. For uniform loading, chequer plates, M.S. plates, etc., are used. For details refer Figs 3, 4 and 5. In some cases an alternative route by-passing such bridges and culverts may be cheaper than propping up. 1.7.4 Loading, Lifting, Jacking and Lashing Refer clauses 1.2.2, 1.2.3 and 1.2.4 (Rounded of thicknesses) Fig. 3 Spreading gravel cushion and placing steel plates above Fig. 4 Dry stone packed with lose earth in-between 286 Manual on Transformers Fig. 5 Wooden sleeper stage platform built on river bed with the plates underneath 1.7.5 Movement 1.7.5.1 A pilot vehicle with all tools and tackles, jacks, sleepers, chequer plates, crowbars, etc., and sufficient trained staff should run in front of the vehicle. Red flags and danger lamps should be exhibited at prominent places to warn traffic on the route. 1.7.5.2 The branches of avenue trees that are likely to foul the equipment should be cleared while the load is moved. Electric utility power lines likely to foul should be switched off and lifted temporarily / dismantled while the load is moved. 1.7.5.3 After moving the load for a short distance tightness of the lashing should be checked. 1.7.5.4 In the case of night halt or stoppage of the loaded trailer for a fairly long duration the trailer should be supported either by sleepers or providing supporting jacks on all sides thus releasing the load from the tyres. Danger lights should be displayed in the front and rear of the vehicle. 1.7.5.5 For the normal running, it is desirable not to run the vehicle over 15-20 km per hour with no load and 10-15 km with loads on good surfaced roads. For bad roads it is desirable to run the vehicle at much lower speeds. 1.7.5.6 The brake system on the tractor-trailer has to be carefully operated whenever the vehicle is running with load. While running over any bridge or culvert the vehicle should be run only at a very slow speed. Long before the approach to the bridge the speed should be brought down and the vehicle allowed to proceed over the bridge without creating any impact, which is sometimes caused by applying brakes when running at high speeds. Till all the wheels of the tractor-trailer are clear of the bridge, the speed should not be increased. Transportation should be avoided during heavy rains. 1.7.5.7 In case of the vehicle getting locked up in a slipping soil, the safest procedure would be to detach the tractor after the trailer had been anchored suitably by sleepers. The tractor may be moved forward and anchored suitably with sufficient sleepers. The trailer can be pulled up by winches in the rear of the tractor. Guidelines for Erection, Commissioning and Maintenance 1.8 287 Water Transport Water transport is the cheapest mode of transport. While ocean going ships are used for the high seas, barges are used for inland navigation routes. Special care is to be taken for prevention of rusting of parts and ingress of moisture like use of anti-corrosive paints, silica gel packing, sealing using polythene covers etc. Packages should not be left on wharves for more than 2 weeks. Storing of heavy packing is to be done only in consultation with port authorities so that the safety of the wharf is not endangered. 1.8.1 Loading Usually the wharf cranes may not have sufficient capacity for handling very heavy packages. Special floating cranes or cranes of ships are used for loading the packages from the wharf to ships or barges. In the case of barges, special care is required to prevent overturning of the barge at the time of loading. Packages are to be placed inside the hold. Decks of ships are not to be used for keeping the packing during transport. 1.9 Unloading at Site In cases where the substations are having adequate crane facility, the transformer is unloaded by crane. Alternatively, mobile cranes are used. Where no crane facility is available a trench is dug to a depth equal to height of the trailer platform and the transformer is slid to position. If this also is not possible the transformer is unloaded into a sleeper platform and gradually lowered to plinth level (Fig. 6 for guidance). The sleeper platform level is to be at a slightly higher level to allow for the increase in height of the trailer while the load is released due to the springs getting relaxed. Winches are to be used for putting the transformer into position. Fig. 6 288 1.10 Manual on Transformers Inspection and Storage A thorough external examination shall be made immediately on arrival of the transformer at site. If any damage is suspected, open delivery is to be taken and a claim made against the carriers in accordance with the terms of contract. The manufacturers and under-writers are also to be informed about the details of inspection done jointly in carriers. 1.10.1 Unpacking and Inspection 1.10.1.1 Packages arc to be opened carefully so that the tools used for opening do not cause damage to the contents. 1.10.1.2 When the transformers are dispatched N2/ dry air filled, pressure shall be checked and it should be ascertained whether there was any leakage of atmospheric air into the tanks. In case of any leakage of N2/ dry air, dryness of insulation should be checked by dewpoint measurement (clause 1.1.2) after fillingwith dry air/nitrogen and after a stabilization period of 12-24 hours. If insulation is wet, the transformer has to be dried. If the pressure is positive and varies with temperature of the surrounding air the seal can be taken to be effective. 1.10.1.3 When the transformer is dispatched filled with oil, oil level in main tank at the time of receipt is to be verified by comparing the oil level before dispatch. Any shortage is to be duly recorded in the documentation and intimated promptly to the supplier. A sample of oil shall be taken from the bottom of the tank and tested to IS 1866. If the sample of oil does not meet the requirements, the matter should be reported to the supplier along with insulation resistance values of the various windings to earth. 1.10.1.4 Core Insulation Test (at 2.5 kV AC one minute withstand alternatively with 2.5 kV Megger) shall be carried out to check insulation between Core (CC&CL) and Ground. (Not applicable for Air Core Reactors). Core insulation value should be comparable with factory value and generally more than 500 M Ohm. 1.10.1.5 Drums containing transformer oil which have been dispatched separately shall beexamined carefully for leaks. All drums are dispatched filled up to their capacity and any shortage should be reported, 1.10.1.6 In case of bushings, oil level shall be checked. The porcelain portion is to be checked for any crack or chipping. The terminals should be checked for any bends. 1.10.1.7 Fragile instruments like oil level gauge, temperature indicator, etc. are to be inspected for breakages or other damages. 1.10.1.8 Any damaged or missing components should be reported to the supplier within the insurance liability period, so that the same can be investigated and shortage made up as per the terms of contract. Guidelines for Erection, Commissioning and Maintenance 289 1.10.1.9 Any paint damages are to be touched up after proper cleaning with wire brush, emery and applying a coat of primer. Generally, manufacturers supply adequate quantity of finish paint for such purposes. 1.10.2 Storage 1.10.2.1 After arrival at site, it is desirable to erect and commission the transformer with minimum delay. In case this is not possible the transformer should be erected at its permanent location with conservator and breather fitted, the transformer should be vacuum dried and processed oil should be filled in the tank and oil condition to be monitored during long storage. If this is also not possible, then, it is preferable that the tank containing core and coil be placed under shed. The tank shall preferably be filled with processed oil up to core & winding level with dry air/nitrogen filling above the oil. Gas pressure is to be monitored periodically. 1.10.2.2 Transformer/ Reactor should be kept in Dry air/ N2 filled condition for the maximum period of 6 months including storage at works, transportation and storage at site. If the transformer/ Reactor is to be stored at site with Dry air/ N2 filled condition, N2 pressure to be monitored on daily basis so that chances of exposure of active part to atmosphere are avoided. In case of drop in N2 pressure, dew point of N2 has to be measured to check the dryness of the Transformer/ Reactor. If there is drop in dew point, fresh nitrogen need to be filled. Leaks are to be identified and rectified and Nitrogen to be filled to the required pressure. 1.10.2.3 In case the transformer/ reactor is to be stored for more than 6 months, it needs to be stored in oil filled condition. Processed oil to be filled which complies to the required specification and ppm ≤ 10 ppm and BDV ≥ 60 kV. In case of storage of transformer in oil-filled condition, the oil filled in the unit should be tested for BDV and moisture contents once in every three months. The oil sample should be taken from bottom valve. If BDV is less and moisture content is more than as given for service condition then oil should be filtered 1.10.2.4 The transformer shall never be stored in polluted areas likely to cause corrosion. 1.10.1.5 Equipment meant for indoor use, such as control panels should be stored indoor. Fragile components are to be stored carefully. 1.10.2.6 Bushings, if not mounted on the transformer, should be stored in the cases under the shed. 1.10.2.7 All packing shall be kept above ground by the use of supports so as to allow free airflow underneath. It is preferable to store all loose parts under cover. 1.10.2.8 Transformer oil when received in drums shall be stored under cover. Drums should not stand on end but are to be placed on their sides with the bung at 45° downward. (3-9 O’ clock position) 1.10.2.9 If the accessories are to be stored for a long period they can be repacked. It is advisable 290 Manual on Transformers to store the accessories; especially the electrically operated ones’ in a rain protected area and packing list should be retained so that the contents of cases or crates can be checked at the end of the storage period. Heaters for marshalling kiosks, etc., shall be kept energized. 1.10.2.10 While in storage, the gas pressure, oil samples, breather condition are to be checked frequently. 2.0 INSTALLATION 2.1 Erection Schedule When the erection work is to be carried out by the supplier under the contract agreement, the supplier and the user shall prepare a schedule of the works to be carried out with specific period for each item of work involved. All the assembly and erection drawings should be available at site. 2.2 Precautions 2.2.1 As far as possible no work shall be done during rainy season to avoid moisture absorption. 2.2.2 Extreme care must be taken to prevent any foreign material from being dropped into the transformer. Workmen having access to the interior of a transformer should empty their pockets of all loose materials. Any spanners or other tools used shall be securely tied so that they can be recovered easily if accidentally dropped. 2.2.3 Fibrous cleaning materials shall not be used. The presence of loose fibres in suspension in transformer oil can reduce its insulating properties. 2.2.4 All components dispatched separately shall be cleaned inside and outside before being fitted. 2.2.5 If any internal temporary transportation braces are provided they are to be removed without disturbing any permanent internal arrangements. Such parts should be clearly marked on the transportation drawings. 2.2.6 The transformer shall be erected on a level foundation. 2.3 Site Preparation 2.3.1 Since all electrical installation shall comply with the requirements of the Indian Electricity Act and Rules made thereunder, it is essential that they are complied with. The provisions of the Factories Act and Rules are also to be complied with to the extent applicable. Guidelines for Erection, Commissioning and Maintenance 291 2.3.1.1 All tools, tackles and other equipments required for the erection work may be arranged at the site before the work is started. Generally the following items will be required. (i) Lifting Equipment: Depending upon the transformer, crane of sufficient capacity will be required. In the absence of crane facility, a derrick is erected and work carried out using a chain and pulley block. Wire rope slings, D-shackles, etc. are also required. (ii) Oil Purifier: A vacuum oil purifier of sufficient capacity provided with thermostatically controlled heating and filtering facility is required. The following table may be used as a guide for selecting the oil purifier capacity. Oil purifier capacity litres/ hr Quantity of oil to be purified litres 2000 Up to 20,000 4000 More than 20,000 but Less than 50,000 6000 More than 50,000 but Less than 80,000 10000 and above More than 80000 Note: (a) For ODAF cooled Transformers, hot oil flow through the coil to be ensured during the oil circulation. (b) In 765 kV AC & 500 kV HVDC Transformers and above rating, particle counting to be checked during oil circulation. The procedure and interpretation shall be in accordance with the recommendation of CIGRE report WG-12.17 - “Effect of particles on transformer dielectric strength”. (c) All the hoses, filter machine to be thoroughly flushed before using them for oil circulation. (d) All the oil tanks should be properly painted inside and sealed with breather provided to prevent entry of moisture. (e) A higher vacuum filter with a vacuum of 0.1 torr or lower is to be used for a voltage class of 245 kV and above. (iii) Vacuum Pump: A vacuum pump with a vacuum hose and other fittings capable of producing a vacuum up to 759 mm mercury. (iv) Oil Storage Tank: One or more oil tanks of sufficient capacity to store the entire quantity of the transformer oil will be useful for filling from drums and also when oil is received at site in tankers for transformers dispatched gas-filled. This will reduce the time of filling. (v) Pressure Vacuum Gauge: To read - 0.5 to + 0.5 kg /cm2 for checking inert gas pressure. (vi) Oil Testing Apparatus: Conforming to IS: 335 (vii) 5000/ 2500/ 1000 V motorized megger (viii) Voltmeters, 0 to 500 V, 0 to 100 V range and 0 to 5 V range, milli-ammeter, low power factor watt meter. (ix) Set of spanners suitable for metric sizes and B.S. sizes. (x) Set of drum opener, crowbar, pipes, hammer etc. 292 Manual on Transformers (xi) Set of screwdrivers, cutting pliers, screw spanners and pipe wrench. (xii) Clean cotton cloth and cotton waste. (xiii) Electric hand lamp. (xiv) 12 mm vinyl hose of approximate 10 m length for being used as an oil level indicator during erection. (xv) Brushes & spray gun with compressor for painting. (xvi) PVC wires for connecting meters during testing. (xvii) Set of tarpaulins of suitable size. (xviii) Earth discharge rods. The above is only a general list of tools, and measuring instruments etc., used for erection of transformers. The capacity, sizes, etc., of each tool would depend on the type and size of transformer. 2.3.2 The installation site shall have easy accessibility for inspection and routine maintenance etc. Foundation for the transformer should have level floor strong enough to support the weight, vibrations and prevent accumulation of water. The transformer foundation should be provided with adequate oil soak pits and drains. Typical designs of such pits are shown in Fig. 7. Fig. 7 Guidelines for Erection, Commissioning and Maintenance 293 2.3.3 In case of large and costly transformers, fire protection walls may be necessary on either side of the transformer for isolating each transformer from the rest. 2.3.4 The transformer installation position shall be such that the breather, thermometers, oil level indicators, diagram plates, etc., can be safely examined with the transformer energized. It should also be possible to have access to the operating mechanism of on load / off circuit tap changing equipment, marshalling box, etc. Sampling valve, drain valve, etc., shall be at accessible and convenient locations, if need be through construction of raised platforms. 2.3.5 For outdoor transformers where rollers are not fitted, level concrete plinth with bearing plates of sufficient size and strength can be adopted. The plinth shall be above the maximum flood or storm water level of the site and of the correct size to accommodate the transformer in such a way that no unauthorized person may step on the plinth. 2.3.6 Where rollers are fitted, suitable rail tracks shall be provided and when the transformer is in the final position, the wheels shall be locked to prevent accidental movement of the transformer. 2.4 Unit Erection 2.4.1 Positioning of the Unit 2.4.1.1 The transformer tank containing the core and coil-assembly should be first placed in the position selected for assembly. 2.4.1.2 In case the rollers are to be fitted, it should be ensured that the wheel shafts are well greased and the wheels rotate freely. While fixing the rollers, the flanges should come on the inner side of the rails. The transformer shall then be jacked up and the roller fitted to the bottom frame. In some of the transformers, skid is provided in place of rollers. 2.4.1.3 All parts, gaskets, bushings, radiators and coolers etc., should be readily available in good condition. The transformer oil required for filling must also be readily available. 2.4.1.4 In case special foundation bolts are supplied, these are to be used for arresting the movement of the transformer, including anti-earthquake devices. 2.5 Oil When oil is dispatched to site separately, it may be done in sealed steel / HDPE drums or epicoated road tankers. At the time of filling the drums, it is ensured that the oil is filtered, clean and dry. 2.5.1 Oil Specification 2.5.1.1 The oil as received at site for filling and topping up in the transformer must comply with IS 335 / IEC 60296/Customer specification for acceptance criteria. Many utilities have different 294 Manual on Transformers specifications of oil (stringent than IS/IEC). Some Utilities prefer napthenic oil and others specify inhibited oil. IEEMA has taken an initiative to standardize a common specification for oil to be used by all Indian Utilities. Once IEEMA Specification on oil is published, CBIP Manual shall adopt the same. For the present, extracts from IS 335 Oil Specifications are reproduced below for reference. 2.5.1.2 The Oil Sample from the Transformer tank, after filling (new/unused) in tank before commissioning should meet the following specifications as per IS 1866. Guidelines for Erection, Commissioning and Maintenance 295 Note: 1. These properties are very sensitive to storage and processing, i.e., the temperature and vacuum of filtration, the cleanliness of the processing system including filter machine, pipes, valves, cleanliness of transformer and its cooling system, etc., Extreme care should hence he taken in these areas to achieve the values indicated above. 2. When unused oil has been filled in the lank then tests given against SI. nos. 7, 9, 10 & 11 in the above table are important. Other parameters only form base data for future comparison. 3. Used oil shall meet the BDV and moisture content requirement given in the Table whereas other parametersmay lie in between the above values and the limiting values given under cl. 2.5.1.3. 2.5.1.3 The recommended limits for mineral insulating oil filled in power transformers in service (as per IS: 1866) are as follows: 296 Manual on Transformers SL. No. Characteristics / property IS 1866 1. Appearance Clear and transparent and without visible sediments 2. 3. Interfacial tension (IFT) min. Hash point 4. Neutralization value (total acidity) max. 0.015 N/m, min Max. decrease of 15° C from initial value. However the absolute value should not be less than 125 °C 0.3 mg kOH / g Increase frequency of testing if more than 0.2 mg kOH/g 5. Di-electric strength (breakdown voltage) BDV min 6. 7. Above 170 kV - 50 kV 72.5 KV- 170 kV -40 kV Below 72.5 kV - 30 kV Dielectric dissipation factor (Tan 5) DDF at 90 Above 170 kV - 0.2 Max. Below 170 kV- 1.0 max. ° C and 40-60Hz, max. 8. Specific resistance (resistivity) (i) At 90 °C, min. (ii) At 20 °C, min. Water content, max 9. Sediment and sludge 0.1*1012Q-cm 1.0* 1012I2-cm Above 170 kV- 20 ppm max 72.5 kV - 170 kV - 40 ppm max. Below 72.5 kV — No free moisture at room temperature No sediment or precipitable sludge (below 0.02 % by mass) Note : Recondition/replace the oil if one or more of the above parameters are beyond the specified limits. 2.5.1.4 POWERGRID Specification of oil for 400 / 800 kV Transformers is different and is given below for reference. Guidelines for Erection, Commissioning and Maintenance 2.5.2 297 Precautions 2.5.2.1 Oil is easily contaminated. When sampling the oil and filling the oil in the tank, it is very important to keep the oil free from contamination. 2.5.2.2 All equipment used in handling the oil must be clean and should be washed with clean transformer oil before use. (The oil used for washing must be discarded). Particular attention shall be paid to the cleanliness of bungs, valves and other points where the dirt or moisture tends to collect. 2.5.2.3 Sampling and type of container for DGA, wherever specified, shall be as per IS: 9434 or IEC: 600567. 298 Manual on Transformers 2.5.2.4 Flexible steel hose is recommended for handling insulation oil. Some kinds of synthetic rubber hose are also suitable but only those known to be satisfactory should be used. Ordinary rubber hose should not be used for this purpose as oil dissolves the sulphur from the rubber and thereby gets contaminated. Proper flushing of hoses to be carried out before using them for oil filtration and oil filling in the Transformer. Hose used for handling oil should be clean and free from loose rust or scale. 2.5.2.5 Transformers must always be disconnected from the electricity supply system before the oil level in the tank is lowered. 2.5.2.6 Oil must not be emptied near naked lights heater/fire, as vapour released is inflammable. 2.5.2.7 Minute quantities of moisture (particularly in the presence of fibres or dust) lower the dielectric strength of the oil. Therefore, to reduce the risk of condensation of the moisture entering the oil, containers taken into a warm room shall not be opened until the entire body has attained the same temperature as the room temperature. It is preferable not to mix oils from different suppliers. However, if the oil-required to be mixed meet the requirements of IS: 335 and if these are made from the same feed stock, these can be mixed. 2.5.3 Oil Sampling Oil takes up moisture readily and its condition should always be checked before use. Oil of a muddy colour is certain to be wet. Water and water-saturated oil are both heavier than dry oil and sink to the bottom of any container. Samples shall, therefore, be taken from the bottom. Samples should not be taken unless the oil has been allowed to settle for 24 hours, if from a drum or two / three days if from a large transformer. 2.5.4 Samples from Tank Dirt from the draw-off valve or plug should be removed. To ensure that the valve is clean, some quantity of oil should be allowed to flow into a separate container before collecting samples for testing. Samples shall be collected either in glass bottle (refer IEC 60567) or in stainless steel bottle. Detailed sampling procedure for steel bottle is given in Annexure-III to this section. It is recommended that: Oil sample for any of the major tests (BDV, PPM, Resistivity, Tan Delta, DGA) must be taken from both top and bottom sampling valves and while drawing the sample the corresponding top oil temperature must be furnished. The lesser of the values obtained from the 2 samples shall be considered for decisions regarding BDV and resistivity, while the higher values shall be reckoned for PPM, Tan Delta. 2.5.5 Sample from Oil Drum The drum should first be allowed to stand with the bung vertically upwards for at least 24 hours. The area around the bung should be cleaned. A clean glass or brass tube long enough to reach to within 10 mm of the lowermost part of the drum should be inserted, keeping the uppermost end Guidelines for Erection, Commissioning and Maintenance 299 of the tube sealed with the thumb whilst doing so. Remove the thumb, thereby allowing oil to enter the bottom of the tube. Reseal the tube and withdraw an oil sample. The first two samples should be discarded. Thereafter, the samples should be released into a suitable receptacle. 2.6 Oil Filling 2.6.1 Before filling oil in the tank, it should be tested to meet the requirements as IEC 60296/ IS 335 (with its latest amendments). In case the oil does not meet the requirement, it should be processed and shall only be used when it meets the requirements. 2.6.2 For transformers dispatched gas filled, the filling of oil inside the tank is done under vacuum. Transformers of high voltage rating (132 kV and above) have their tanks designed to withstand full vacuum. Below 132 kV manufacturer’s instructions should be followed regarding the creation of full or partial vacuum during filling the oil in the tank. 2.6.3 When filling a transformer with oil it is preferable that the oil be pumped into the bottom of the tank through a good quality filter machine. 2.6.4 Some radiators may be suitable only for partial vacuum. No higher vacuum than as could be withstood by radiators should be applied to such radiators type tanks even if the radiator valves are closed. It should also be ensured that the bushings, tap changer board, relief vent diaphragm, Buccholz relay, conservator, etc. are not subjected to full vacuum as these may not designed for the same. (Specific guidelines from manufacturer may be followed) Note: Now a days pressed steel radiators, bushings - condenser as well as porcelain, buchholz relay, conservator etc. are designed for full vacuum and are readily available. 2.6.5 In case the transformer is provided with an on load tap changer of in-tank type, while evacuating the main transformer tank, the diverter switch compartment must also be evacuated simultaneously so that no undue pressure is allowed on the tap changer chamber. While releasing vacuum, the tap chamber vacuum should also be released simultaneously. For this one pressure equalizer pipe should be connected between main tank and tap changer. 2.7 Drying of Transformers Before the drying out is started all fittings except coolers and associated accessories shall be fitted. The coolers, etc., can be conveniently fitted after the successful dry-out of windings and insulation. The process of drying out a transformer is one requiring care and good judgment. If the drying out process is carelessly or improperly performed, great damage may result to the transformer insulation through overheating etc. In no case shall a transformer be left unattended during any part of the dry-out period. Transformer should be carefully watched throughout the dry out process and also observations shall be carefully recorded. 300 Manual on Transformers When the transformer is dried out, it is necessary to ensure that the firefighting equipment is available near the transformer. The dry out of transformer is necessary in the following cases: (a) (b) (c) (d) On first commissioning After prolonged storage at site without nitrogen After detection of free moisture/ high moisture content in oil Due to exposure of core and coil assembly for 48 hours or more in case of inspection at site Various methods can be adopted for drying out a transformer depending upon the facilities available at site. Some of them are described below: 2.7.1 Drying out a Transformer using Filter Machine 2.7.1.1 The most practical method of drying out is by circulation of hot oil through a high vacuum filter machine incorporating oil heater and vacuum chamber (or other oil cleaning and moisture removing device). The vacuum pump of the filter machine should have the capacity of creating vacuum as high as possible but not less than 710 mm of mercury. Where possible, a vacuum pump can be connected to the tank top cover to keep the oil in tank under vacuum consistent with tank suitability. This may speed up the drying out process. It is preferable to lag or blanket the transformer tank to prevent loss of heat. Oil is drawn from the bottom and let into Guidelines for Erection, Commissioning and Maintenance 301 the transformer at the top. This will remove any settled moisture/ impurities. After about 8-12 hours circulation in this manner, the cycle is reversed and oil is drawn from the top and fed at the bottom. 2.7.1.2 The oil temperature as measured by the oil temperature indicator should be of the order of 55°C. It should be seen that the oil temperature at the filter machine in no case exceeds 65°C. The circulation is continued till the insulation resistance and oil samples tests are satisfactory. 2.7.1.3 Plot IR values taken at regular intervals against temperature readings. It will be observed that in the beginning IR values drop down as the temperature goes up. The IR values will be low till moisture is coming out of the insulation and start rising before steadying. A typical dry out curve is shown in Fig. 8. 2.7.1.4 Measuring winding Tan delta also a criteria for knowing the dryness of the Transformer insulation. During dry-out, winding Tan delta may be measured between different cycles to check the effectiveness of Dry-out. 2.7.1.5 The heat can also be provided by short circuit heating method as mentioned as clause 2.7.2 Hot Air Circulation Time in hours Fig. 8 Typical dry-out curve 2.7.2.1 Hot air from external heaters is blown into the transformer after draining the oil, through an opening in the bottom of the tank and is allowed to escape through an opening in the tank top cover (Fig. 9). The speed of the air must not exceed 600 m per minute. The temperature of the air is raised gradually to 55°C in the first 8 hours, 65°C in the second 8 hours, and 75°C-80°C in the third 8 hours. In this method also, hourly readings of temperature and insulation resistances are taken till steady values are obtained. 302 2.7.3 Manual on Transformers Short Circuit Heating Fig. 9 2.7.3.1 The transformer can also be heated up by short-circuiting the low voltage winding and supplying at the high voltage terminals a reduced voltage. The supply voltage should be maintained in such a way that the current in the windings does not exceed 70 per cent of normal full load current and the oil temperature about 75°C. In this case, temperature of oil should be measured at the bottom of the tank also. Constant watch is to be kept to ensure that the temperature limits are not exceeded. The temperature of the windings which can be measured by the following formula should in no case exceed 90°C : T2 =R2/R1(235+T1])-235 where, T2 = Final average temperature of copper T1 = Initial average temperature of the copper R2 = Final resistance of the windings. R1 = Initial resistance of the windings. This method should be used in conjunction with streamline filter as described in clause 2.7.1. • Caution: Short circuit heating to be carried out with due precaution 2.7.4 Drying by Vacuum Pulling, N2 Filling and Heating 2.7.4.1 This is the most effective method and the quickest too but cannot be applied in case Guidelines for Erection, Commissioning and Maintenance 303 of transformers not designed for withstanding the vacuum pressure. The vacuum should be drawn from the top of the tank connecting suitable pump to any valve fitted at the top of the tank. Vacuum is applied after draining the oil. The transformer core and coil are heated externally by the use of heater. Temperature at the tank wall to be of the order of 60-70 °C. Another method of heating active part is to do hot oil circulation for 2-3 volumes and then drain oil immediately. 2.7.4.2 The leakage rate has to be less than 40 m bar-lit/sec- so as to get better vacuum during the drying out process. After ascertaining that there is no leakage in the transformer from gaskets/ valves etc., pull out vacuum and keep the transformer under near absolute vacuum (1 torr or less) for about 96 hours running the vacuum pump continuously. The duration of vacuum can vary between 48 to 96 hrs. depending upon the dew point being achieved. Keep Vacuum machine ON and collect condensate for measurement. 2.7.4.3 Then the vacuum is broken with dry nitrogen. The dew point of the inlet of nitrogen is to be measured and will be of the order of - 50 °C or below. When the nitrogen comes to the positive pressure of 0.15 kg/cm2, it is stopped and kept for 48 hours. Then the nitrogen pressure is released and the outlet nitrogen dew point is measured. If the dew point is about -30 °C or below then the dryness of transformer is achieved. If not again the transformer is taken for vacuum treatment and then nitrogen is admitted as mentioned above and tested. The cycle to be continued till dew point of -30°C or below is achieved. 2.7.4.4 Duration of vacuum cycle may vary between 48-96 hrs. Initially one or two nitrogen cycles may be kept for 24 hrs. After that it may be kept for 48 hrs depending upon dew point being achieved. 2.7.4.5 When condensate collection rate is less than 40 ml/hr for 24 hrs and Dew point of Nitrogen is about -30 °C at outlet. 2.7.4.6 After completion of the drying of the transformer the following parameters are to be checked. (a) PI (Ratio of R600 and R60) may be 1.5 or more (b) BDV and moisture content of the oil as per IS 1866 or as recommended by transformer manufacturer (c) Power factor of winding - Less than 0.5%.at 20 deg C 2.8 Circulation of Oil in Coolers and Tap Changers 2.8.1 Coolers and tap changers are filled with clean dry oil. Oil samples are taken out from them and tested. Further circulation of oil is carried out till the oil results are satisfactory and meet the requirements as per IS 1866. It is advisable to carry out the circulation in the main tank and selector switch/ diverter switch tank simultaneously to remove moisture from the tap changer terminal board/diverter switch cylinder provided on the tank. 304 Manual on Transformers 2.9 Important Fittings and Accessories 2.9.1 Gaskets 2.9.1.1 Whenever blanking plates are removed to fix detached parts such as bushing turrets, etc., a new gasket shall be used while fixing the same. A set of new unused gaskets of correct size and thickness is supplied with every transformer for this purpose. 2.9.1.2 Gaskets shall be stored in hermetically sealed containers in a cool place. They must be protected from damp, oil and grease. 2.9.1.3 To make a gasketed joint, first clean the metal surfaces ensuring that they are free from oil, rust scale, etc. Then a film of the special gasket adhesive if supplied by the manufacturer may be applied to one of the surfaces. The gasket may be then stuck to the surface after the lapse of a few minutes. The other metal surface may also be given a film of adhesive and placed over the gasket. Both may then be tightened according to the special instructions of the manufacturers. Some type of gasketed joints is shown in Fig. 10. (a) Metal Fit Type: In this case the flanges are tightened uniformly till the two metals touch each other. (b) Ordinary Type: In this case the gasket should be uniformly compressed such that its thickness comes down to above 60 per cent of its original thickness. (c) Distance Piece Type: In this system the flanges are tightened uniformly till the upper flange touches the metal piece welded to the lower flange. Joints in gaskets should be scarfed or dove tailed as shown in Fig. 11. Bushings 2.9.2 2.9.2.1 Normally three types of bushings are used: (i) Plain porcelain type (ii) Plain oil filled (iii) Condenser type • Plain porcelain type bushings are used up to rated voltage 36 kV. Plain oil-filled type bushings can either have its oil in communication with the main tank or separately sealed. These bushings will be bulkier in appearance and used up to 110 kV. Guidelines for Erection, Commissioning and Maintenance 305 Fig. 10 Fig. 11 2.9.2.2 Condenser type bushings are also of three categories, one is the SRBP type, resin impregnated paper type (RIP) and the other is the oil impregnated paper condenser (OIP) type. The last one is distinguishable by the presence of porcelain shell below the flange level. 2.9.2.3 The bushings shall be checked for any damage at the oil end as well as the porcelain before fixing and shall be cleaned thoroughly. The bushings shall be lifted by using the lifting eyes and soft manila ropes. Steel Wire ropes or slings shall not be used (Fig. 12). The line lead of H.V. winding if coiled inside the transformer is drawn through the bushing using a string when the bushing is lowered into position. The cable ferrule is fixed in position at the top of the bushing brass tube. The lower end of the bushing shall be inspected through the inspection cover for proper sealing. The line connection should be tight and should not strain the terminals. Sufficient flexibility in the connection leads should also be provided to avoid mechanical stress 306 Manual on Transformers Fig. 12 Guidelines for Erection, Commissioning and Maintenance 307 on the bushing. The shield barriers, if any shall be inspected through the inspection cover for proper seating. The line connection should be tight and should not strain the terminals. The arcing horns, if any, shall be in proper position as shown by the supplier in general arrangement drawing. 2.9.3 Tap Changers 2.9.3.1 Change over switch or link arrangement provided in a multi-ratio transformer has to be checked for proper ratio. 2.9.3.2 Off-Circuit Tap Changer: The off-circuit tap changer forms an integral part of the transformer. Since the operation is to be carried out from outside, the operating handle may at times be dispatched separately. This has to be fitted as per manufacturer’s instructions. Care has to be taken to have correct alignment of the handle. The actual position of tap changer is confirmed when the ratio tests are done. Before changing taps, isolate the transformer from supply on all windings. In no case should the tap switch handle be left half way and unlocked to prevent damage due to inadvertent operation. 2.9.3.3 On-Load Tap Changers: If the tap changer is dispatched separately from works, it is to be fitted on the tank. Before mounting on the tank, the insulation resistance value of each tap changer lead to earth should be measured and in case of low value, the cause should be investigated. The leads from the tap changer are then connected to their respective position on the terminal board provided on the tank. The tightness of all connections on the selector switch and terminal board is to be ensured. The tap changer is then to be filled with clean oil and drying out is to be carried out. Oil filling and drying out is carried out simultaneously along with the transformer as explained earlier. 2.9.4 Cooling Equipments 2.9.4.1 The cooling equipments and associated pipe work and fittings are to be thoroughly cleaned and flushed with clean dry transformer oil before assembly. The pressure gauge, differential pressure gauges, etc., if any are fitted in position. The cooler and associated pipe work is then filled with clean dry oil keeping all the cooler circuit open, except the transformer inlet and outlet valves. Air is released from all the pipe work during filling. The oil is circulated through a filter press using the filter valves provided in the cooler inlet and outlet branches. The cooler control circuit is to be checked for correct operation in all positions of the selector switch. Test push buttons are provided for checking of the working of motors individually. The cooler system is then connected to the main tank by opening the tank inlet and outlet valves. 2.9.4.2 Cooling Fans: Cooling fans are mounted as per manufacturer’s instructions. The fans are tested for insulation value and normal running before they are mounted. 308 Manual on Transformers 2.9 4.3 Separate Coolers (i) Forced Oil Cooled Transformers In the case of forced oil cooled transformer, oil pumps are provided for circulating the oil. The pumps are dispatched separately after blanking both suction and delivery sides. The pump is connected at the proper position as per the general arrangement drawing. New gaskets should be used at the joints and the bolts tightened. The pipe work at the pump connection is done as per the matching marks on the flanges to avoid undue stress on the flanges of the pump when the bolts are tightened. In some pumps an air release plug is provided on the body. This plug should be checked for tightness. Oil flow meters are provided on the pipeline connecting the pump. The flow meter being a delicate instrument is packed separately and sent. The flow meter should be taken out carefully and mounted on the flange provided on the pipe connection. The mounting position should be as per the outline general arrangement drawing. In large transformers the radiators are sometimes separately mounted. In such cases there will be a header each at top and bottom, which arc supported on frames. Flanges are provided on these headers for fixing the radiators. Radiator valves are fitted to the headers and dispatched. The end frames are to be erected first. The frames should be positioned correctly with respect to the transformer. The distances between center lines of transformer and cooler should be strictly as per the general arrangement drawings as otherwise the connecting pipe work will not match. After erecting the end frames the top and bottom headers are mounted. The headers will have to be properly leveled so that the connecting pipe work can be easily fixed. Then radiators are fixed. If the conservator is to be provided on the cooler the same may be mounted on it and all fittings for the same attached. The interconnecting pipe work may be done taking care to connect correct pieces at the correct location. Usually expansion joints are provided in the pipeline connecting the transformer tank to the cooler. Special care should be taken to see that these are installed correctly. (ii) Forced Water Cooled Transformers In the case of forced water-cooled transformers the oil to water shell tube heat exchangers are dispatched separately and properly blanked. On receipt at site, it shall be checked whether blanking is all right. If the blanking is found to be defective, the matter has to be referred to the manufacturer. In such a case, moisture/rain water might have entered the heat exchanger oil circuit and there might be rusting. It may be necessary to take out the different parts of heat exchangers separately and clean them thoroughly and put them back. The brackets for mounting the heat exchanger may be attached to the transformer first taking care of the matching marks. The heat exchanger may then be mounted on the support in the correct position after referring to the general arrangement drawing. The oil pump, oil flow meter and the connecting pipes may be fixed after this, in the correct position. In the water circuit the necessary water pipes may be connected. It is also to be made Guidelines for Erection, Commissioning and Maintenance 309 sure that on the outlet side water is allowed to discharge freely without any obstructions. Usually a water flow meter is placed on the outlet pipe to indicate that there is a positive water flow. It is to be made sure that there is no restriction in the water outlet pipe as any obstruction in this pipe will increase the pressure in the water circuit and may result in the water pressure exceeding the oil pressure and creating leakage of water into oil circuit, which is detrimental to the transformer. The heat exchanger oil circuit is sealed from the water circuit with special seals and the circuit is pressure tested at the supplier’s works to make it absolutely sure that there is no leakage. The sealing should not be tempered in any manner, as it is detrimental to the transformer. If there is any doubt about this sealing, the matter should be intimated to the manufacturer. 2.9.5 Conservator 2.9.5.1 Conservator, where fitted, should be assembled with its pipe work, etc., making sure that all gasketed joints are oil-tight and the pipe work is clean and free from moisture. The mechanism of the float type oil gauge inside the conservator might be locked to prevent damage during transit. After placing the conservator in position, it should be released by turning the locking belt in the direction indicated on the plate. 2.9.5.2 While topping up oil in the transformer, it should be ensured that oil is filled to a level indicated by the oil gauge on the conservator in commensurate with the filling oil temperature. 2.9.5.3 Procedure for mounting air cell and oil filling inside the conservator. Fig. 13 Oil conservator with air cell 310 Manual on Transformers Set up the air cell inside the conservator. Care should be taken to see that the hooks on air cell are properly engaged in the brackets provided inside the conservator. Inflate the air cell at a pressure as shown in the instruction plate (DO NOT APPLY EXCESS PRESSURE AS IT MAY DAMAGE THE AIR CELL) through the breather connection pipe. Follow the instructions given in the Instruction Plate fixed on the transformer. Ensure that there is no leakage. The conservator with Air Cell is pressure tested and dispatched from the factory at a slightly positive pressure. Confirm that there is no oil leakage. Fix three numbers air release valves on the conservator. Keep air release valves open. Fix air filling adapter on breather pipe and inflate the air cell at an air pressure indicated on the INSTRUCTION PLATE affixed on the transformer and hold air pressure. Open the air release valves and start oil filling from the bottom filter valve of the transformer. • Observe the air release valves and as soon as oil starts overflowing, close the air release valves one by one. Stop oil filling when all air release valves are closed. • Remove the air-filling adapter. • Continue oil filling and observe the Magnetic Oil Level Gauge (MOG) • Stop the filling when the needle of MOG shows the level corresponding to the ambient temperature at the time of filling. • Fix silica gel breather. Caution: 2.9.6 • Do not open any of the air release valves after completion of oil filling. If air release valve is opened, air will enter and oil level will drop. • The plain oil level gauge on the end cover of the conservator should indicate full oil level always. If air enters the conservator, it can be seen by a fall in the oil level in plain oil level gauge. • The plain oil level gauge should be monitored on regular basis. Buchholz Relay 2.9.6.1 The Buccholz is checked for correct functioning of the mercury switches by injecting air through the test petcock when full of oil. When mounting on the pipe work, the correct direction is maintained with the help of arrow provided. The angle of inclination is also to be checked and should be between 3 to 7°. The gas release pipe, if provided is to be connected to the top petcock. In service the top petcock should be open and gas release pipe should be full of oil. When the gas is to be collected through the gas release pipe, initially the oil will flow out and then the gas can be collected. Guidelines for Erection, Commissioning and Maintenance 2.9.7 311 Dehydrating Breather 2.9.7.1 The breather pipe work shall be properly cleaned. The oil level in the oil seal at the bottom should be filled to the correct level with transformer oil. Any oil that might have over flown should be wiped off. It is to be ensured that the breathing hole at the bottom of the seal is not blocked by dirt, etc. and silica gel to be filled into the breather is dry. 2.9.8 Pressure Relief Device (PRD)/ Explosion Vent 2.9.8.1 PRD: Mount PRD as per manufacturer’s leaflet and also the G.A. drawing of transformer. Check operation of alarm/ trip contacts. 2.9.8.2 Explosion Vent: The temporary cover, which is provided over the explosion vent flange on the tank cover, should be removed and the explosion vent fitted with suitable gaskets. Care being taken to ensure that the top diaphragm with its gaskets makes an airtight joint. As the top diaphragm is sent blanked from works, the blanking plate shall not be removed till the oil level inside the transformer comes above the tank cover. The fixing can be done after vacuum application. 2.9.9 Temperature Indicators 2.9.9.1 Before installing, the accuracy of the instrument shall be checked by hot oil or water bath. The switches are adjusted to make contact at the desired temperature depending upon the site conditions, i.e., ambient temperature, loading conditions, etc. The check is done through hot oil bath. 2.9.9.2 The capillary tube is protected adequately to withstand all normal handling. It should not, however, be bent sharply or repeatedly and should be supported by clips to prevent sagging. On no account it must be cut. 2.9.9.3 The thermometer pocket should be filled with transformer oil 2.9.9.4 The connection of the winding temperature indicator C.T. is made to the thermometer pocket as per instructions given on the WTI Terminal Board. 2.9.10 Bushing Current Transformers 2.9.10.1 It is not advisable to remove the bushing CTs unless situation warrants for the same. In such cases great care shall be taken in handling current transformers. Current transformer should be kept flat at all time. If it is not handled properly, it will deform in shape resulting in an increase in excitation current. All C.T. Secondary terminals should be short circuited or loaded before energizing the transformer. This will prevent excessive voltage developing across. C.T. secondary, which can damage the C.T and be a hazard if touched. 312 2.11 Manual on Transformers Completion of Erection Work Final topping up is now done up to a level in conservator commensurate with filled oil temperature. Any other work such as wiring of various alarm/trip contacts, fan motors, pump motors and other apparatus, earthing of neutral and tank is also to be completed. The interposing valves between the radiators and the tank are opened. Tank surface is retouched with paint wherever required and transformer is made ready for the commissioning tests. 3.0 TESTING AND COMMISSIONING If the foregoing instructions have been carefully followed, the transformer can now be safely put into service after pre-commissioning tests. 3.1 Tests The following pre-commissioning tests shall be carried out. 3.1.1 Checking of Ratio, Polarity and Phase Relationship 3.1.1.1 The ratio shall be checked on all taps and between all the windings and the results should tally with the manufacturer’s factory test results. Preferably Transformer Automatic Turns Ratio Meter (TTR) should be used. In case of OLTC, continuity during tap change is to be ensured. This is best done by applying single phase voltage on each HV winding in turn and observing variations of LV voltage at the instance of diverter operation by using analogue voltmeter. Break will be indicated by any major deflection of the analogue meter towards zero. 3.1.1.2 Polarity and interphase connections shall also be checked 3.1.2 Resistance Measurement of Windings 3.1.2.1 Kelvin Bridge meter or automatic winding resistance measurement kit (ohm meter, preferably 25 A kit) should be used for the measurement of resistance. Tapped winding resistance shall be measured at all tap positions. Pre-commissioning values are to be compared with factory values after applying temperature correction factors. 3.1.3 Insulation Resistance 3.1.3.1 IR values between windings and between windings to earth are checked; while checking these values no external lines, lightning arrestors, etc., should be in circuit. Bushings are thoroughly cleaned before taking IR values. A 5000 / 2500 /1000 volts megger preferably motor operated should be used for measuring IR values. One minute and 10 minute IR values can be taken to find out the polarization index also. (PI = R600/R60) Care should be taken that the lead wires of the megger do not have joints. Note Now a-days digital meters are also available in the market which can read Polarisation Index (PI) directly and compensate for magnetic interference. Guidelines for Erection, Commissioning and Maintenance 3.1.4 313 Magnetizing Current 3.1.4.1 Magnetizing current may be measured using single-phase 230 volts supply for each phase individually and compare the results with manufacturer’s factory test results. If the test results at the factory are available with 3 phase supply then magnetizing current at site may also be measured using 3 phase 415 volts supply. It is recommended to measure magnetizing current at 10 kV for Transformers having voltage rating in the range of 400 kV and 800 kV Note: Measurement of magnetizing current is a standard feature available in most of the available Automatic C &Tan Delta Test kits. 3.1.5 C and Tan Delta I Power Factor Measurement of Transformer Windings and Bushings (a) For Transformer windings, measurements shall be done after opening the jumpers and isolating the transformer from other equipment and the ground. (b) The test kit should be suitable to work in charged switchyard environment, i.e., induction suppression unit should be provided. (c) Test modes shall be selected as below : Bushings UST mode between HV and test tap. Windings (i) Between two windings - UST mode (ii) Between winding to Earth- GSTg mode with other winding (s) guarded. (iii) Between HV to earth in GST mode for comparison of test results taken in (i) & (ii) above Notes While carrying out the test, all 3 phases of the same winding are to be shorted to compensate/nullify the effect of winding reactance. The bushing porcelain and test tap are to be properly cleaned before the commencement of test. Pre-commissioning values are to be compared with factory values after applying temperature correction factors. Tan Delta/ Power Factor values should be more frequently monitored if faster deterioration trend is observed. 3.1.5.1 Frequency Response Analysis (FRA)- Measures mechanical movement of windings and core during transit or in operation.FRA to be carried out at site in same tap position where it was measured at factory to have better interpretation. Normally, it is preferred to have FRA signatures in extreme and nominal tap position. These signatures also act as base signatures for any further measurement to be carried out during operation. 3.1.6 Tap Changer 3.1.6.1 The sequence of operation of the tap changers shall be checked. Check should be made for: (a) Manual Operation. (b) Local Electrical Operation. 314 Manual on Transformers (c) Remote Electrical Operation. (d) Group Operation, if applicable. 3.1.7 Buccholz Relay Test 3.1.7.1 Check whether the Buccholz relay is mounted at an angle by placing a spirit level on the top of the relay. Confirm that the relay does not operate when pumps are switched on in forced oil cooled transformers. Buccholz relay operation for alarm and trip are checked by injecting air through the test petcock (For transformers with ATMOSEAL conservators relay may be tested either by putting in test position or by draining oil from relay after closing valves from either sides). 3.1.8 Magnetic Oil Level Gauge 3.1.8.1 The float level of the oil level indicator is moved up and down between the end positions to check that the mechanism does not stick at any point. The low oil level alarm of the oil gauge shall be checked. This can only be checked before installation. 3.1.9 Temperature Indicators 3.1.9.1 The contacts of WTI and OTI for alarm and trip are checked and set at required temperatures depending upon ambient temperatures and loading conditions. 3.1.10 Fans and Pumps 3.1.10.1 It shall be checked that the specified number of fans are mounted on radiators as per general arrangement drawing. IR values and settings for operation of fan motors and oil pumps are checked. Check also that the direction of rotation of fans and pumps is correct. 3.1.11 Marshalling Box 3.1.11.1 The wiring from various accessories to marshalling box shall be checked. 3.1.12 Oil 3.1.12.1 Oil samples from top and bottom of main tank are tested as per IS: 1866 (table given in clause 2.5.1). DGA tests are to be done to obtain benchmark before charging, one month after charging, three months after charging, and thereafter every year. 3.1.12.2 Oil of diverter switch should be checked for BDV at the time of commissioning and subsequently yearly or 5000 operations, whichever is earlier. 3.1.1.12.3 General Checks (a) All oil valves are in correct positions, closed or opened as required. (b) All air pockets are cleared. (c) Thermometer pockets are filled with oil. Guidelines for Erection, Commissioning and Maintenance 315 (d) Oil is at correct level in the bushings, conservator, diverter switch and tank etc. (e) Earthing connections are done. (f) The colour of silica gel and oil in the breather cup. (g) Arcing horn gaps on bushings (where provided) are properly adjusted. (h) Heaters in cubicles, conservator, etc., where provided should be checked. (i) To check alarm/trip contacts of all accessories, instruments flow meters, differential pressure gauges etc. (j) In the case of water cooled transformers, the pressure gauge readings on both water and oil sides to confirm that the water pressure is less than the oil pressure. The oil and water flow should not be less than that specified. If all the above tests/ checks are found satisfactory, allow a settling time of at least 24 hours for oil and release air from all venting points. Now the transformer can be energized after setting the protective relays to the minimum extent possible. Wherever possible, the voltage should be built up in steps. Any abnormality during commissioning such as vibration of radiator parts, hum etc., should be observed. After a few hours of energisation at no load, the transformer shall be switched off. The Buccholz relay should be checked for collection of air/ gas. Abnormalities noticed should be corrected. All protective relays should be reset to normal values. Transformer can now be re-energized and loaded gradually. After commissioning, the following details should be furnished to the manufacturer: (i) (ii) (iii) (iv) 4.0 4.1 Details of transformer including its serial number. Date of commissioning, with test results. Substation/generating station where commissioned. Protection given to the transformer such as lightning arrestor, differential protection, circuit breaker on H.V/L.V etc. (v) Loading details with complete temperature log. MAINTENANCE General 4.1.1 If a transformer is to give long and trouble-free service it should receive a reasonable amount of attention and maintenance. Following are the causes of breakdown of transformers: (i) Faulty design and construction. (ii) Incorrect erection, operation and maintenance. (iii) Wear and tear, ageing and other deterioration. (iv) Accidents. 4.1.2 A rigid system of inspection and preventive maintenance ensures long life, trouble free service and low maintenance cost. Maintenance consists of regular inspection, testing and reconditioning where necessary. 316 Manual on Transformers 4.1.3 Records must be kept giving details of any unusual occurrence and also if any test results taken. 4.1.4 The principal objective of maintenance is to maintain the insulation in good condition. Moisture, dirt and excessive heat are the main causes of insulation deterioration and avoidance of these will in general keep the insulation in good condition. 4.1.5 No work should be done on any transformer unless it is disconnected and isolated from all external / energized circuits, and all windings have been solidly earthed. 4.2 Factors Affecting the Life of a Transformer 4.2.1 Transformer oil readily absorbs moisture from the air. This reduces the dielectric strength of the oil. It is also reduced by solid impurities present in the oil. Care should be taken that moisture does not penetrate inside the transformer. 4.2.2 Much of the mechanical strength of paper and-pressboard comes from the long chain cellulose polymer. Although temperature is a major factor, Oxygen and water clearly have a significant effect on the degradation of cellulosic material (Kraft paper). It is seen that moisture is formed in service-aged transformers due to thermal ageing, which results in lower degree of polymerization (DP) indicating weakening of mechanical strength of paper. 4.3 Maintenance Procedure CIGRE WG A2.18 – Life Management Techniques for Power Transformers shall be referred. The maintenance procedure listed in subsequent clauses is to be attended to at the intervals of time noted against each item in Annexure I. 4.3.1 Oil 4.3.1.1 For maintenance of oil reference may be made to “IS 1866: Code of Practice for Electrical Maintenance and Supervision of Mineral Insulating Oil in Equipment” which gives recommendations in detail for the maintenance of insulation oil. A few short notes on the subject are given below: The oil level should be checked at frequent intervals and any excessive leakage of oil investigated. There may be slight loss of oil by evaporation; this need cause no concern if the tank is topped up at regular intervals. All leaks should be repaired as quickly as possible so as to avoid possible trouble caused by low oil level. Oil for topping up should comply with IS 335 / IEC 60296: New insulating oils and should preferably be from the same source as the original oil because the oil refined from different crudes may not be completely miscible and may separate into layers. Furthermore, there may Guidelines for Erection, Commissioning and Maintenance 317 be a greater tendency to form acidity or sludge in a mixture than in oil from a single source of supply. Used oil shall not be mixed. New oil may be added as make up only, not exceeding about 10 per cent. Samples of the oil should be taken at regular intervals preferably half yearly and tested for DGA and oil parameters. It may be mentioned that the dielectric strength does not give a true indication of the deteriorated condition of the oil. Even oil, which is highly deteriorated, may give a high dielectric strength, if dry. Normal methods of oil purification only maintain the dielectric strength, but do not improve the deteriorated condition of the oil. It is, therefore, advisable not to rely solely on the dielectric strength of the oil by periodic tests. In addition to chemical tests other tests as given in Clause 2.5.1 should also be carried out. If the dielectric strength is below, the recommended limit, the oil should be reconditioned by passing it through either a centrifugal separator or a filter. After reconditioning, the breakdown voltage should be more than 50 kV r.m.s. across a standard gap (2.5+0.05 mm apart) and other parameters to be tested as per clause 2.5.1 (IS 1866). Other Oil parameters play important role for healthy operation of the transformer. In case any of the parameters like resistivity, IFT reaches the limiting value given in clause 2.5.1 (IS 1866), oil should be monitored more frequently and in case the values continue to deteriorate, a decision regarding change of oil is required to be taken. It may be noted that reconditioning by vacuum filtration only improves BDV, moisture content and removes dust, dirt suspended material etc. from oil. This process does not improve any other parameters of oil and will tend to retard the process of deterioration of oil. Other methods may have to be followed to improve resistivity, acidity etc. In such a case it is better to change the oil and the old oil may be sent to an oil refinery for reclaiming it. 4.3.2 Rollers 4.3.2.1 Alter a transformer has been in service for a long period, rollers should be examined carefully. They should be greased and rotated to see that they turn freely. Rollers should also be inspected for overheating when moved on tracks, during initial erection. 4.3.3 Transformer Body 4.3.3.1 The transformer tank and other parts should be inspected periodically for any rust or and oil leak. Rusted portions, if any, should be cleaned thoroughly and repainted with proper paints. Transformer should be completely painted at proper intervals. If any leak is found, it should be investigated. If it is due to defective welding, the same should be rectified after consulting the manufacturer. Leaking joints can be rectified by tightening the bolts to the correct pressure or by replacing the gaskets. 4.3.4 Core and Winding 4.3.4.1 It is recommended that the core and winding be removed from the tank for visual 318 Manual on Transformers inspection as per time schedule given in inspection table. The windings should be examined to ensure that no sludge has been deposited blocking the oil ducts. Any loose nuts and bolts should be tightened. On transformers with bell type tank, the inspection of core and coil can be done by lifting the tank. Inspection of core and coil assembly of transformers with conventional type tank has to be done with extreme care so as not to damage the insulation structure which might have become brittle due to long service life of transformer 4.3.4.2 Before lifting the core and winding from the tank, it is usually necessary to disconnect the windings from the bushings or cable boxes inside the tank to disconnect the off-circuit tap switch handle or leads of the on-load tap changer and to remove any earthing strips between the core clamps and the tank. 4.3.4.3 The core and winding must be removed with great care. It should be placed under cover and in a dry place. 4.3.5 Bushings 4.3.5.1 The bushings should be inspected for any cracks or drippings of the porcelain at regular intervals and kept free from dust and dirt. In location where special and abnormal conditions prevail, such as sand storm, salt deposits, cement dust, oil fumes etc., bushings should be cleaned at more frequent intervals. 4.3.5.2 Oil level in oil filled bushings should be checked periodically. 4.3.6 Cable Boxes 4.3.6.1 The sealing arrangements for filling holes should be checked each year. When screwed plugs are sealed with a bituminous compound, the compound should be examined for cracks. If the compound has cracked it should be replaced as the cracks may lead to an accumulation of water around the plug. Gasketed joints should be examined and tightened whenever required. 4.3.7 External Connections 4.3.7.1 All external connections should be tight. If they appear to be blackened or corroded, the same can be cleaned or should be replaced, if required. 4.3.8 Conservator and Magnetic Oil Gauge 4.3.8.1 Conservators are so arranged that the lower part acts as a sump in which any impurities entering the conservator will collect. A valve/plug is fitted at the lowest point of the conservator for draining and sampling. The inside of the conservator should be cleaned every two to three years. A removable end is generally provided for this purpose. 4.3.8.2 The oil level indicator should be kept clean. Generally the oil level is visible through a transparent material. In case of breakage immediate replacement is essential. When conservator is stripped for cleaning, the mechanism of the oil gauge should be cleaned. Guidelines for Erection, Commissioning and Maintenance 4.3.9 319 Dehydrating Breather 4.3.9.1 Breathers should be examined to ascertain if the silicagel requires changing. The frequency of inspection depends upon local climatic and operating conditions. More frequent inspections are needed when the climate is humid and when the transformer is subjected to fluctuating loads. So long as the silicagel is in active stage its colour is blue but as it becomes saturated with moisture its colour gradually changes to pale pink. The gel should then be replaced or reactivated. The saturated gel can be regenerated by heating up to 110-130 °C for 8 to 10 hours or 150-200 °C for two to three hours and can be used again. 4.3.9.2 The level in the oil seal must be maintained at the level marked in the cup. 4.3.10 Buccholz Relay 4.3.10.1 Routine operation and mechanical inspection tests should be carried out at one and two yearly intervals respectively. 4.3.10.2 During operation if gas is found to be collecting and giving alarm, the gas should be tested and analysed to find out the nature of fault. Sometimes, it may be noticed that the gas collected is only air. The reasons for this may be that trapped air if any is getting released or due to leakage of the suction side of the pumps. The trapped air is released in initial stages only when vacuum is applied during filling of oil. The internal faults can be identified to a great extent by chemical analysis of collected gas. 4.3.10.3 Buccholz relay may also give alarm/trip signal due to the oil level falling below the required level. 4.3.11 Explosion Vent 4.3.11.1 The diaphragm, which is fitted at the open end of the vent should be inspected at frequent intervals and replaced, if damaged. Failure to replace the diaphragm quickly may allow the ingress of moisture which will contaminate the oil. If the diaphragm has broken because of a fault in the transformer an inspection must be carried out to determine the nature and cause of the fault. 4.3.12 Gaskets 4.3.12.1 Gaskets sometimes shrink during service. It is, therefore, necessary to check the lightness of all bolts fastening gasketed joints. The bolts should be tightened evenly round the joints to avoid uneven pressure. Leaking gaskets should be replaced at the earliest opportunity. 4.3.13 The pipe work should be inspected at least once a year. Leaks may be due to slack unions, which should be tightened, or to badly seated joints requiring the pipes to be aligned and joints remade. 320 Manual on Transformers 4.3.14 Temperature Indicators 4.3.14.1 At each yearly maintenance inspection, the level of oil in the pockets holding thermometer bulbs should be checked and the oil replenished, if required. The capillary tubing should be fastened down again if it has become loose. Dial glasses should be kept clear and if broken, replaced as soon as possible to prevent damage to the instrument. Temperature indicators should be calibrated with standard thermometer immersed in hot oil bath if found to be reading incorrectly. 4.3.15 Coolers and Cooling Fans 4.3.15.1 There are variety of coolers. For radiator type coolers, maintenance primarily consists of replacing damaged elements, cleaning the outer surface to remove settled dust, repainting etc. 4.3.15.2 Fan blades are cleaned to remove dust; bearings of the fan motors should be lubricated occasionally. Greases should not be added while the motor is running. For other coolers, manufacturer’s instructions should be followed. 4.3.16 On-load Tap Changer 4.3.16.1 Since all on-load tap changers are not of the same design and construction, special instructions of manufacturer’s should be followed. However, a few points are enumerated. (a) Diverter Switch: The maintenance primarily consists of servicing of diverter switch contacts, checking the oil level in the diverter switch chamber, and replacement of diverter switch oil when the same becomes unsuitable for further service. (b) Motor Driving Mechanism (i) Do not allow dirt to accumulate between contact rings of notching controller. (ii) Do not use oil/grease on contacts rings on notching controller. (iii) Check the operation of anti-condensation heater. (iv) If the contacts of contactors are silver faced, no touching up be ever done, if any is worn out, it should be replaced. Copper contacts may be lightly touched up with a file when they become rough. The pole faces of electromagnet must be kept clean. (v) Do not oil/grease the contact surface of radial multi-contact switches, unless a special contact lubricant is used. The space between the rings should be cleaned occasionally. If necessary, a few drops of Benzene be used. (c) Selector Switch: The contacts do not make/break current. As such, the wear is only due to mechanical movement of moving contacts. These may be inspected once in 2/3 years. Guidelines for Erection, Commissioning and Maintenance 321 4.3.17 Spares 4.3.17.1 It is a healthy practice to have essential spares like one number of each type of bushings, one thermometer, one cooling fan, pump, buchholz etc., for each group of similar transformer. Suppliers’ recommendations may also be considered in this connection. 4.3.18 Inspection and Maintenance Schedule 4.3.18.1 The frequency of inspections should be determined by the size of the transformer. Local climatic and atmospheric conditions will also influence the inspection schedule. Use Annexure I as a guide for determining the inspection schedule. 4.3.19 Transformer Preservation in De-energized Condition for Long Storage 4.3.19.1 Transformers fitted with conservator oil preservation system (COPS)/ diaphragms are recommended for preservation of Transformers in de-energised condition. Suitable cautions to be put in placing advising to open the valves before energizing the transformer. Heaters in MB should be kept ON. Oil level in the conservator should be monitored. (Low oil level could be indicative of rupture in air cell of COPS (Conservator Oil Preservation System). 4.3.19.2 Following checks to be followed while the transformer is kept for long storage in deenergised condition : (A) Quarterly • Open the valves between tank and radiator, and run the oil pumps for two hours. • Carry out oil tests for BDV and Moisture content. (B) Half yearly • BDV of OLTC oil. • DGA (C) Yearly • Measurement of Tan delta of Bushings. • Operate Tap changer 2-3 times over full range. (D) Once in three years • Measurement of Winding Tan Delta 322 Manual on Transformers Annexure I Table 1 Recommended Maintenance Schedule for Transformers of Capacities Up to 10 MVa & 33 kV SL. No Inspection frequency Items to be inspected Inspection notes Action required if inspection shows unsatisfactory conditions 1. Hourly (i) Load (amperes) (ii) Temperature (iii) Voltage Check against rated figures oil temperature (OTI) and winding temperature(WTI) and ambient temperature 2 Daily Position of tap switch air cell Oil level glass to indicate If level drops, check and conservator full. re-commission air cell. 3 Fortnightly Dehydrating breather Check that air passages If silicagel is pink, change are clear. Check colour of by spare charge. The old active agent charge may be reactivated for use again. 4 Monthly (i) Oil level in transformer. (ii) Connections (iii) Explosion vent (iv) Diaphragm (Pressure relief device) Check transformer oil level. Check tightness. Check for cracks/ damages. Check for any oil spillage. 5 Quarterly Bushings Examine for cracks and Clean or replace. Tighten, dirt deposits Check for if required external connections 6 Half yearly (i) Non-conservator transformer. Check for moisture under Improve ventilation, (ii) Cable boxes, gasket joints, cover. Inspect. maintenance of breathers to gauges and general paint work be ensured. Take remedial measures. 7 Yearly (i) Oil in transformer (ii) Earth resistance (iii) Relays, alarms, their circuits, etc. Check for dielectric Strength and water content. Check for acidity and sludge. < 1 ohm Examine relay and alarm contacts, their operation, fuses, etc. Check relay accuracy, etc. 8 5 Yearly Non-conservator transformers Internal inspection above Filter oil regardless of core. condition. 9 10 Yearly If low, top up with dry oil. Examine transformer for leaks. If loose, tighten, Replace if cracked. Take suitable action to restore quality of oil. Take suitable action if earth resistance is high. Clean the components and replace contacts and fuses if necessary, Change the setting, if necessary, Overall inspection Wash by hosing down with including lifting of core clean dry oil. and coils. Note : Inspection of core & coil to be done in consultation with manufacturer, 323 Guidelines for Erection, Commissioning and Maintenance Table 2 Maintenance Schedule Recommended Maintenance Schedule for Capacities of 10 MVa and Above SL. No Inspection frequency Items to be inspected Inspection notes Action required if inspection shows unsatisfactory conditions 1 Hourly (i) Ambient temperature (ii) Winding temperature (iii) Oil temperature (iv) Load (amperes) (v) Voltage (vi) Tap position of tap changed. Check that temperature rise is reasonable.Check against rated figures. Shut down the transformer and investigate if either is persistently higher than normal. 2 Daily (i) Check that air passages are free. Check colour of active agent. Positive pressure to be ensured If low, top up with dry oil, examine transformer for leaks. Contact manufacturer in case of major changes If half of silica gel is pink, change by spare charge. The old charge may be reactivated for use again. 3 Quarterly (i) Bushings Examine for cracks Clean or replace. Tighten terminals and (ii) Indoor transformers Checking and dirt deposits. top Check for clearance terminal connectors of oil leaks of arcing horns, if applicable Check ventilation. 4 Half Yearly Dissolved gas Analysis of oil sample Test for all fault DGA (IS 10593/ IEC 60599/ ANSI/ Gases IEEE C57.104) Action to be taken as per DGA test results like increase in frequency of sampling to have trend and any specialized test if required. 5 Yearly (i) Physical checking of oil level Oil in transformer (ii) Oil parameters as per cl no. 2.5.1 (IS 1866/IEC 60422) (hi) Cooler fan bearings, motors and operating mechanism. (iv) OLTC Top up, if required Take suitable action to restore quality of oil. Filter or replace based on test results Replace burnt or worn contacts or other parts Oil level in Main and OLTC conservator (ii) Oil colour and level in condenser bushings (iii) Leakage of water into cooler. (v) Dehydrating breather (vi) Cooler oil Pumps (vii) N2 pressure, if applicable Check with dip stick method Check for dielectric strength (BDV) and water content (moisture) Check for oil parameters Lubricate bearings Check gear box. Examine contacts. Check manual control and interlocks. Check oil in OLTC driving mechanisms. 324 Manual on Transformers C& tan delta of bushings Yearly (or earlier if), the transformer can conveniently be taken out for checking). 5. 6. SOS (In case of any protection tripping due to internal fault or after any major maintenance work) 10 Yearly value should be less than 0.007, rate of rise of C & tan δ to be less than 0.001/ year Replace based on test result Checking of WTI, OTI pockets Check presence of oil Calibration of WTI.OTI Checking of operation of buchholz Relay Top up if required (i) OLTC Oil Check for dielectric strength (BDV) and water content (moisture) Filter or replace based on lest results. (ii) Disconnecting Chamber Check BDV of oil Replace oil in disconnecting chamber if BDV < 50 kV (iii) Gasket Joints Tighten the bolts Replace evenly to avoid uneven leaking. pressure (iv) Cable boxes. Check for scaling arrangements for filling holes. Examine compound for cracks. Cracked compound around screwed holes to be replaced. (v) Surge diverter and gaps. Clean or replace. (vi) Relays, alarms, their circuits etc. Examine for cracks and dirt deposits. Examine relay and alarm contacts, their operation, fuses, etc. Check relay accuracy etc. (vii) Earth resistance — LV tests like ratio, winding resistance at all taps position, c & tan delta, magnetizing current and magnetic balance, Test results to be compared with precommissioning test results Take suitable action; if earth resistance is high. Life assessment test Furan measurement and calculated Degree of Polymerization gasket, if Clean the components and replace contacts and fuses, if necessary. Change the setting, if necessary. If major deviations found then specialized tests like FRA, FDS, PD tests to be carried out in consultation with manufacturer. Will help decision making for taking up any major refurbishment action on transformer. Notes SI. No. 1& 2 are in purview of Operation staff whereas the rest to be taken care of by maintenance staff. With respect to on-load tap changers, the manufacturer’s recommendation should be followed. The silica gel may be reactivated by heating it to 150 to 200 degree C. Every time the drying medium of breather is changed, oil seal should also be changed. No work should be done on any transformer unless it is disconnected from all external circuits and the tank and all windings have been solidly earthed. In case of anything abnormal occurring during service, maker’s advice should be obtained, giving him complete particulars as to the nature and the extent of occurrence, together with the name plate particulars in order to assist identification. In case of any through fault seen by the transformer or any major failure, SFRA to be measured and results to be compared with previous base signatures. Guidelines for Erection, Commissioning and Maintenance 325 Table 3 : Trouble Shooting Chart for All Transformers Trouble Rise in temperature High temperature Cause Remedy Over voltage Over current Change the circuit voltage or transformer connections. to avoid over-excitation If possible, reduce load. Heating can often be reduced by improving power factor of load. Check parallel circuits for circulating currents which may be caused by improper ratios or impedances. See Electrical Troubles, below. High ambient temperature Either improve ventilation or relocate transformer in lower ambient temperature. Insufficient cooling If unit is artificially cooled, make sure cooling is adequate. Lower liquid level Fill to proper level. Sludged oil Use filter press to wash off core and coils. Filter oil to remove sludge. Short circuited core Test for exciting current and no load loss. If high inspect core and repair. See Electrical Troubles, below. Electrical troubles Winding failure Lightning, short circuit, Usually, when a transformer winding fails, the transformer Overload, Oil of low is automatically disconnected from the power source by the dielectric strength, Foreign opening of the supply breaker or fuse. material Core insulation breakdown Smoke or cooling liquid may be expelled from the case, (core bolt, clamps, or accompanied by noise. When there is any such evidence or between laminations) a winding failure, the transformer should not be reenergized at full rated voltage, because this might result in additional internal damage. Also it would introduce a fire hazard in transformers. After disconnection from both source and load, the following observations and tests are recommended : (a) External mechanical or electrical damage to bushings, leads, potheads, (b) Level of insulating liquid in all compartments. (c) Temperature of insulating liquid wherever it can be measured. (d) Evidence of leakage of insulating liquid or sealing compound. Core failure high excitation current Incorrect voltage Short-circuited core Open Test core loss. If high, it is probably due to a short-circuited core joints core. Test core insulation. Repair if damaged. If laminations are welded together, refer to manufacturer. Core loss test will show no appreciable increase. Pound joints together and retighten clamping structure. Improper ratio voltage abnormal Supply Change terminal board connection or ratio adjuster position to give correct voltage. Change tap connections or readjust supply voltage. 326 Audible internal arcing and radio interference Manual on Transformers Isolated metallic part The source should be immediately determined. Make certain that all normally grounded parts are grounded, such as the clamps and core. Loose connections Same as above. Tighten all connections. Low liquid level, exposing live parts Maintain proper liquid level. Lightning Provide adequate lightning protection. Dirty bushings Clean bushing porcelains, frequency depending on dirt accumulation. Leakage through screw joints Foreign material in threads. Oval nipples Poor threads Improper filler Improper assembly Make tight screw joints and or gasket joints. Leakage at gasket Poor scarfed joints Make tight screw joints or gasket joints. Insufficient or uneven compression Improper preparation of gaskets and gasket surfaces. Leakage in welds Shipping strains, imperfect weld. Repair leaks in welds. Pressure relief diaphragm cracked. Improper assembly. Mechanical damage. Replace diaphragm. Inspect inside of pipe for evidence of rust or moisture. Be sure to dry out transformer if there is a chance that drops of water may have settled directly on windings or other vulnerable locations, as oil test may not always reveal presence of free water. Pressure-relief diaphragm ruptured Internal fault In conservator type transformers-obstructed oil flow or breathing. In gasseal transformer-obstructed pressure relief valve. In sealed transformers -liquid level too high. Check to see that valve between conservator and tank is open and that ventilator on conservator is not blocked. Make certain that relief valve functions and that valve discharge lines are open. Liquid level should be adjusted to that corresponding to liquid temperature to allow ample space for expansion of liquid. Moisture condensation in open type transformers and air filled compartments Improper or insufficient Ventilators Make sure that all ventilator openings are free. Moisture condensation in scaled transformers. Cracked diaphragm Moisture in oil See remedies above for cracked and ruptured diaphragms. Filter oil Audio noise Leaky gaskets and joints. Accessories andexternal transformer parts are set into resonant vibration giving off loud noise. Make certain all joints are tight. Tighten loose parts. In some cases parts may be stressed into resonant state. Releasing pressure and shimming will remedy this condition. Bushing flashover Mechanical troubles Guidelines for Erection, Commissioning and Maintenance 327 Rusting and deterioration of paint finish Abraded surfaces weathering and Bare metal of mechanical parts should be covered with grease. Fractured metal or porcelain parts of bushings Unusual strains placed on terminal connections Cables and bus bars attached to transformer terminals should be adequately supported. In the case of heavy leads, flexible connections should be provided to remove strain on the terminal and bushing porcelain. Oil Troubles (see also IS: 1866) Low dielectric strength Condensation in open type Make certain that ventilating openings are unobstructed. transformers from improper Replace diaphragm. Replace gasket, if necessary. Test ventilation Broken pressure cooling coil and repair. relief diaphragm Leaks around cover accessories Leaky cooling coil High moisture content Ingress of moisture in oil/ Filter and monitor ppm for three months. In case moisture winding content increases again, check points of moisture entry and take appropriate action. Badly dis-coloured oil Contaminated by varnishes. Retain oil if dielectric strength, resistivity and tan delta Carbonized oil due to values of oil are satisfactory as per IS 1866. CI 2.5.1 of this switching Winding or core section failure Oxidation (sludge or acidity) Exposure to air High temperatures operating ‘Wash down’ core and coils and tank. Filter and reclaim or replace oil. Same as above. Either reduce load or improve cooling. *Code of practice for maintenance and supervision of insulating oil in service (first revision). In any event, filter oil or dry transformer by heating, or both, to restore dielectric strength. Notes In addition to the above instructions given in this section reference may also be made to IS: 10028 Part 1,2&3 -Code of Practice for Selection, Installation and Maintenance of Transformers Part 1-Selection, Part 2-Installation & Part 3-Maintenance In case of anything abnormal occurring during service, manufacturer’s advice should be obtained, giving them complete particulars as to the nature and the extent of occurrence, together with the nameplate particulars in order to assist identification. Special list of testing equipment required at site are included in Annexure -II. 328 Manual on Transformers Annexure II Equipment’s Required for Pre-Commissioning and Maintenance Tests SL. No. Test Ref. 1. IR value winding. 2. DC resistance of transformer Automatic winding resistance test kit IS : 2026 (25 Amp.) winding. 3. Ratio of transformer windings. 4. Electric strength of transformer Automatic BDV test kit oil. IS : 6792 5. Moisture content of transformer Karl Fisher Apparatus oil. IS : 2362 6. Capacitance and Tan delta Automatic Capacitance and Tan delta IEC 60137 IEEE C 57 of transformer bushings and measurement kit windings. 7. Dissolved Gas Analysis of transformer oil. Portable DGA test set. IS:10569 8. Vibration measurement. Vibration cum noise level meter. IEEE C 57 9. Moisture content insulation. 10. Note: of Equipment Required transformer Battery and mains operated 5 kV IS : 2026 motorized Insulation Tester of Automatic ratio meter IS : 2026 solid On line Moisture measurement kit Frequency Response Analysis. Frequency (FDS) Domain spectroscopy Sweep Frequency Response Analyzer CIGRE Brochure 254, CIGRE Brochure 414, IEC 60076-18 HV lead of test kits like Insulation tester and tan delta kits should be with double screen and other leads should be at least single shielded. 329 Guidelines for Erection, Commissioning and Maintenance Annexure IiI Title Oil Sampling Procedures Scope Sampling of oil from oil filled electrical equipment. Purpose: This procedure describes the techniques for sampling oil from oil filled equipment such as power transformer and reactors using stainless steel sampling bottles fitted with valves on both sides. Gases may be formed in oil filled electrical equipment due to normal ageing and also as a result of faults. Operation of the equipment with fault may seriously damage the equipment. It is valuable to detect the fault at an early stage of development. During the early stages of fault the gases formed will normally dissolve in the oil. By extracting dissolved gas from a sample of oil and determining the quantity of composition of gases the type and severity of fault can be inferred. Responsibility : Maintenance engineer. I.E.C. 567 Reference: IS 9434 Apparatus: (i) Stainless steel sampling bottle of volume one litre as per IS 9434. (ii) Oil proof transparent plastic or transparent PVC tubing. (iii) A drilled flange in case sampling valve is not suitable for fixing a tube. Sampling (Refer Fig. 14) Procedure: (i) Remove the blank flange or cover (2) of the sampling valve and clean the outlet with a lint free cloth to remove all visible dirt. (ii) If the sampling valve is not suitable for fitting a tube, it may be necessary to use a separate flange with a nozzle in the centre suitable to connect the transparent plastic / PVC tube. (iii) Connect a short oil proof plastic tube (around one meter long) at both end of the stainless steel sampling bottle (5) as shown in (Fig. 14) (iv) Open the valves (4) & (6) on the stainless steel bottle (5), allow 250 ml (approx.) of oil to flow into the bottle by opening valve (1). Close (4), (6) and (1). Disconnect tube from the flange and rinse by gently tilting the bottle upside down such that no air bubble is formed inside during rinsing. Expel this oil into the waste bucket (7) by opening valves (4) and (6). (v) Connect the tube (3) to the flange (2). Hold the bottle in vertical position as shown in Fig. (14). Slowly open the equipment sampling valve so that oil flows through the sampling bottle. 330 Manual on Transformers Guidelines for Erection, Commissioning and Maintenance 331 (vi) After stainless steel sampling bottle (5) has been completely filled with oil, allow about one litre to two litres of oil to flow to waste bucket (7), till no air bubbles are seen from top outlet. (vii) Stop the oil flow by closing of first the valve (6) and then valve (4) and finally the sampling valve (1). (viii) Disconnect the sample bottle (5) and then disconnect the tubing from the main equipment and the sampling bottle. (ix) Label the sample. (Refer Annexure III-A) (x) Send the information as per Annexure-III-B along with the samples (xi) In case of critical samples furnish information as per Annexure-III-C also Precautions: (i) When sampling oil, precaution should be taken to deal with any sudden release of oil. (ii) Sample should normally be drawn from the bottom sampling valve. (iii) Proper closing of both the valves (4) & (6) of the bottle should be ensured immediately after the collection of sample. (iv) Due care should be taken to avoid exposure of oil to air while sampling. (v) Sampling should be done preferably in a dry weather condition. (vi) Sample should be taken when the equipment is in its normal operating condition. (vii) Care should be taken to hold the bottle in place inside the container when transporting. (viii) Testing should be carried out as early as possible. 332 Manual on Transformers Oil Sampling in Syringes Now a days, syringes are available of various capacity for oil sampling purpose. These oil sampling in syringes have advantage than sampling in bottles as bottles are sometimes not completely tight thus resulting in error in DGA results. Some of the utilities have started oil sampling in syringes. The specification of syringes and oil sampling procedure for syringes is given below for reference. Specification of Syringes The glass syringe and three way stop cock valve shall meet the following specification Dimensions General Volume 50 ml ± 1.5% Volume Piston Outside Diameter 27.45 mm ± 0.20 mm Barrel Diameter (OD) 32.35 mm ± 0.55 mm Barrel Collar Diameter 44.00 mm ± 0.75 mm Piston Collar Diameter 34.05 mm ± 0.65 mm Length (L) 178.00 mm ± 0.50 mm Increment 2.0 ml The Syringe shall be made from heat Resistant borosilicate Glass The material and construction is resistant to breakage from shock and sudden temperature changes Reinforced at luer lock tip at centre and barrel base. The Cylinder –Plunger fit is leak proof and shall meet the requirement of IEC-60567 Plunger shall be individually ground and fitted to barrel for smooth movement with no back flow. Barrel rim is flat on both sides to prevent rolling and is wide enough for convenient finger tip grip. The syringe shall be custom fit and uniquely numbered for matching. The syringe shall be clearly marked with graduations of 2.0 ml and 10.0 ml and shall be permanently fused for life time legibility. Three way Valve Shall be made of 100 % Nylon. Two female ports and one male port. Two female ports shall be designed to accept luer lock fitting. Guidelines for Erection, Commissioning and Maintenance 333 334 Manual on Transformers Guidelines for Erection, Commissioning and Maintenance 335 Annexure Iii (a) Labelling of the Oil Sample Bottle/ Syringe (a) Bottle / SYRINGE Number : (b) Manufacturer’s Name : (c) Name of the site : (d) Equipment Name or Serial No. : (e) Sampling Date : 336 Manual on Transformers Annexure Iii (B) Details to be furnished along with the Samples 1.0 Sample Sent By Name / Designation Organisation Name Address Tel No & Fax E-mail 2.0 and Oil Sample Details Date & Time of Sampling Sampling Point Oil Temp DegC Load at the time of sample (MW/MV A/AMPS) Sample Remarks 3.0 Equipment Details Substation / Plant Name Make Capacity of Equipment (MVA)(l Ph/3 Ph) Type of Cooling Type of Oil Date of Installation Any Other Information 4.0 Bottle Numbers Weather Condition Winding Temp, DegC Voltage at the Time of Sampling (kV) Trans Name / Feeder Name / Location ID Manufacturer SI. No. Voltage Rating (In kV) ONAN/OFAN / Breather Arrangement Diaphragm / Air O N A F / / O FA F Cell/Conventional / OFWF Dry col Paraffinic / Quantity of oil in the Nephthenic Equipment, kL Inhibited / Uninhibited Date of Last Filtration Tests to be done: (please tick desired standard and required tests) Guidelines for Erection, Commissioning and Maintenance 337 Reference Standard: Oil Parameters: IS-1866 (Before Charging) / IS-1866 (In-Service) / IS-335/ IS-12463/ Dissolved Gas Analysis (IS- Water Content (IS-13567) 9434) Specific Resistance at 90degC Dielectric Dissipation Factor at 90degC (IS-6103) (IS-6262) Inter Facial Tension at 27°C Flash Point (IS-1448-P-21) (IS-6104) Kinematic Viscosity at 27°C Pour Point (IS-1448P-10) (IS-1448-P-25) Carbon Type Composition Oxidation Stability (AnnexCofIS-335) (IS-13155) Oxidative Aging (IS-12177 Corrosive Sulphur (Annex(Method-A)) BofIS-335) Furan Analysis (IEC-61198) Dielectric Strength (IS-6792) Total Acidity (IEC-62021 Vol-1 / IS-1448 P-l) Sludge Content (IS-1866) Density at 29.5°C (IS-1448 P-16) SK Value (Annex-D of IS335) Inhibitor Content (IS-13631) In case of new transformer following additional information to be furnished : Date of commissioning : MVA rating : kV rating : Oil type (Parafinic / Naphthanic) : Cooling (ONAN/ONAF/OFAF/OFWF) : Type of oil preservation: (Air cell/diaphragm type/Direct breathing) Make : 338 Manual on Transformers Annexure III (C) ADDITIONAL DATA INPUT FORMAT FOR CRITICAL EQUIPMENTS 1. Voltage profile for last six months indicating maximum and minimum values and % of time voltage more than rated voltage. 2. Loading Pattern (Monthwise) of the transformer for last six months Max. load Min. load Normal load Current (A).... Current (A).... Current (A) .... mw mw mw 3. Date of last filtration carried out 4. Type of oil preservation system: 5. air cell in conservator/diaphragm in conservator/direct breathing 6. Any Buchholz alarm/trip operation in past: YES/NO 7. Any oil topping up done in the past: YES/NO 8. Whether complete oil was changed any time: YES/NO 9. Present BDV / Moisture content value: 10. Color of silicagel 11. Date of commissioning: 12. Manufacturer’s serial number: mvar mvar mvar 339 Guidelines for Erection, Commissioning and Maintenance Annexure IV SPECIFICATIONS FOR TEST EQUIPMENTS I 5 kV BATTERY OPERATED INSULATION RESISTANCE TESTER The equipment offered shall be for the measurement of insulation resistance of electrical equipment. Technical Requirements Rated Voltage selection 0.5/1.0/2.5/5 kV (DC volts) Rated insulation resistance 0-100,000 Mega ohms Type Portable, compact and direct reading type of multi voltage and multi rated insulation resistance ranges. It shall be suitable for DC battery operation. Batteries shall be rechargeable with 230V, 50 Hz AC supply. The necessary accessories for this purpose shall be supplied by the supplier. The operating temperature shall be up to 50 deg C and humidity 85%. There should be provision for infinity adjustment. The instrument shall be supplied in a carrying case with 2 m long mains lead and 25 m long test leads with carrying case. It shall generally comply with the requirements of IS: 2992 and IS: 11994 or relevant internationally acceptable standards. As per requirement of ISO - 9001, calibration certificate for each testing instrument covering entire range shall be supplied with the test kit at the time of supply. The testing equipment is generally meant for carrying out testing at site and transportationmovement from one place to another is unavoidable. Therefore equipments shall be robust in design so that it gives desired performance even in adverse site conditions. Though the instrument is capable of operating on battery and are provided with battery condition indicators, it would be advisable to conduct the tests on mains supply input power to the extent possible. Usage of battery must be resorted to sparingly. The supplier should have adequate “After sales service” in India. II Transformer DC Winding Resistance Measurement Test Kit The instrument is used for measuring DC winding resistance of the transformer/reactor where large inductance is present. The test kit shall be able to withstand inductive kicks from transformer winding. 340 Manual on Transformers Variation in test current shall not result in loss of accuracy. The display of resistance should be through LED/LCD without requiring any balancing of decades to obtain stable readings. It should employ four wire method and no lead compensation shall be required for measurement. Built-in discharge circuit should be provided to discharge the specimen when test is completed when current lead accidentally disconnects or when instrument power supply is lost. Technical Parameters Test current 25 Amp Resolution 1 milliohm Range 0 to 100 ohms Accuracy ± 0.5% of full scale reading or better Open circuit voltage min. 30 Volts, DC General Requirements The instrument shall contain all standard accessories including test leads of 25 meters with suitable clamps/connectors and carrying case. The kit should have been proven for repeatability of test results in charged switchyard conditions and documentary evidence should be furnished along with the bid. The kit should have been tested for EMI /EMC as per standards. Input supply of the kit shall be 230 Volts AC, 50 Hz, Variations + 15% and 5% in voltage and frequency respectively. As per requirement of ISO-9001, calibration certificate for each testing instrument covering entire range shall be supplied with the test kit at the time of supply. The testing equipment is generally meant for carrying out testing at site and movement from one place to another is unavoidable. Therefore equipment shall be robust in design so that it gives desired performance even in adverse site conditions. Battery / mains operated micro ohmmeters employing currents less than 5 amps are not recommended for transformers. The supplier should have adequate “after sales service” in India. Guidelines for Erection, Commissioning and Maintenance III 341 Automatic Turns Ratio Tester The equipment offered shall be used for measurement of turns ratio of various transformers and bushing current transformers automatically displaying the ratio without requiring any manual balancing of decades. Technical Requirement Operation Voltage: 240 volts: 50Hz, single phase A.C Test Voltage: 240 volts AC Measuring range: 1 to 2000 Accuracy: ± 0.5 % of FSD Measuring Principle It should display actual turns ratio of different vector groups in three phase transformers without conversion The kit should be supplied with 25 m of test lead. The kit should be tested for successful operation in charged 765 kV switch yard and be tested for EMI and EMC. The kit shall be capable of operating at a temperature of 50°C and at a humidity upto 85%. As per requirement of ISO 9001 calibration certificate for each testing instrument covering entire range shall be supplied with the test kit at the time of supply. The testing equipment is generally meant for carrying out testing at site and movement from one place to another is unavoidable. Therefore equipment shall be robust in design so that it gives desired performance even in adverse site conditions. The supplier should have adequate “after sales service” in India. IV 100 kV Automatic Transformer Oil Breakdown Voltage Test Set The equipment offered shall be suitable for determination of electrical strength (break down voltage) of insulating oil conforming to IS-335 and IS-1866 upto 100 kV when measured in accordance with IS:6792. The test cell shall be as per IS: 6792 and IEC-156 suitable for BDV upto 100 kV without external flash over. The unit shall be automatic type having control unit and high voltage transformer in a common cabinet with necessary partition. JIV chamber interlocking and zero start interlocking shall be provided. The unit shall have motorized drive to increase voltage linearly as per the rate specified in IS: 6792. Provision shall also be available for manual increase of voltage. 342 Manual on Transformers The unit shall be complete with test cell stirrer, calibrator and necessary gauges for adjusting the gap. The equipment shall be suitable for operation at 240 volts, 50Hz, single phase AC supply. As per requirement of ISO-9001, calibration certificate for each testing instrument covering entire range shall be supplied with the test kit at the time of supply. The testing equipment is generally meant for carrying out testing at site and movement from one place to another is unavoidable. Therefore equipment shall be robust in design so that it gives desired performance even in adverse site conditions. The supplier should have adequate “after sales service” in India. V Moisture Content Measurement Set The test kit shall make use of automatic Karl Fischer titrator capable of measuring water in oil upto 1 ppm. The measured moisture shall be displayed in microgram or PPM or percentage. The set should have following features : • • • • • • • • Should be compact and portable. Should operate with Karl Fischer reagent which is available. Should have error indicator for indicating any defect in vessel charge or generator solution and electrodes. Should have facility for auto-display of ions in parts per million. Should be free from effect of humidity, parasitic reactions and inherent drift of circuitry. Should have 3 sets of syringes required for measurement and 6 sets of vessel charge bottles. Should be suitable for 240 V 50 Hz AC supply. Should have back up indication for mains on, instrument error, stirrer moving and titration over. Resolution range = 1 in ppm The speed of stirrer should preferably be controllable. The set should be accompanied with six sets of bottles of reagent. As per requirement of ISO 9001, calibration certificate for each testing instrument covering entire range shall be supplied with the test kit at the time of supply. The testing equipment is generally meant for carrying out testing at site and movement from one place to another is unavoidable, Therefore equipment shall be robust in design so that it gives Guidelines for Erection, Commissioning and Maintenance 343 desired performance even in adverse site conditions. The supplier should have adequate “after sales service” in India. VI Specifications for Automatic Capacitance and Tan Delta Kit The instrument should be suitable for automatic offline measurement of C& Tan δ of the switchyard equipments as well as excitation current of transformers/reactors, in live switchyard upto 800 kV AC and 800 kV DC level. The test results should have repeatability, consistency & immunity to electromagnetic interference in live switchyard. Output V: 0-10 kV AC Test Frequency Accuracy I: 0-100 mA (Min) continuous, 200 mA(Min) intermittently 45 Hz to 55 Hz, Independent of line voltage & frequency variations Cp : 0.5 % of reading ± 1pF Measurement Range Tan δ : 1 % of reading ± 0.02% Cp : 10 pF to 1 µ F Tan δ : 0 to 100 % It shall work on single phase 230 Volts ±10 %, 50 Hz ±5 % supply with standard socket. Operating temperature & 0 to +50 deg C humidity Humidity upto 90 % Power Supply Against short circuit, over voltage, improper ground connection over load & transient surges, the kit should have alarm/cut-off features to protect the instrument. Also the kit should have facility of stopping automatically on power failure as well as interlock for HV. One complete set of cables of sufficient length (Min 25Metre) with suitable clamps & connectors, compatible with the instruments should be provided for successfully carrying out the test. The supplier should have adequate “after sale service” in India. VII Sr.No 1) Technical Specifications frequency Domain Spectroscopy (FDS) Test Kit Description Function of the kit Specifications Instrument shall accurately carry out following, (a) Analysis of Water content in Oil-Paper, and insulation of Power and Instrument Transformer. (b) Diagnosis of all type of bushings and CB insulation. (c) Diagnosis of Cable and Motor Insulation. The kit shall be based on Frequency Domain Spectrograph (FDS) as well as Dielectric Frequency Response (DFR) technique for analysis. The measurements shall be at lower frequencies and effected through Tan delta and capacitance measurement of insulation. 344 Manual on Transformers (2) Mains Input Power 230V ±10% at 50 Hz (3) Voltage out put 0-200 V, Current Out put 0-50mA Frequency range 0.1m Hz to 5 kHz min Measurements Capacitance- 10pF to 10μF (4) Dissipation Factor -0 to 10 @ accuracy of 2% max (5) Measurement Channels Channel 1, Channel 2. (6) Measurement Modes UST,GST,GSTg. minimum (7) PC requirement A reputed laptop with latest specifications such as Core 2 Duo processor, 2GB DDR RAM 320GB HDD DVD writer,15’’TFT screen or with better specs. Preloaded with MS Windows XP,/VISTA/latest with antivirus.Original CDs etc (8) Software The test record should download via RS 232 C port to a PC in windows Excel/equivalent format. Software shall be able to assist in analyzing the measurements and its reporting in user friendly manner.Softwareupgradation shall be done as and when applies with no cost to POWERGRID. (9) Accessories All testing/measurement low noise cables 18 meter long. PC inter phase cables, Power supply cables,USB cables, Operating manual, Original CD and software, Application CD etc. Hard carrying case etc.connectors.AC power adopter. (10) Repeatability It should offer repeatability of test results in charged switchyard. (11) Operating conditions Shall operate at Temperature 0 to 50 deg C, Humidity not condensing up to 95%, (12) Safety Standards The test set shall meet international safety standard for IEC 61010-1 safety and IEC 61326-1 EMC The cost of PC shall be quoted separately. VIII Specification of F.R.A. Test Set The Instrument shall be able to detect and aid identification of dielectric and mechanical failures in large power transformers in 400 kV/765 kV charged area, preceded by mechanical changes in the winding structure due to Transportation damage, Short circuit forces, Natural effects of aging on the insulating structures etc. The testing method should be based on Sweep Frequency Response Analysis of complex R-L-C-network. The test instrument shall be designed and supplied complete with accessories cables connectors, leads etc. alongwith required licensed software for proper analysis and storage of test data. Detailed specifications of various components of FRA test set are as below: SL.No. Particulars Technical Specifications 1 FREQUENCY RANGE 10 Hz to 20 MHz minimum 2 INPUT AND OUTPUT IMPEDANCE 50Ω 3 MEASURING POINTS 1600 minimum 5 DYNAMIC RANGE >80dB Guidelines for Erection, Commissioning and Maintenance 345 SL.No. Particulars Technical Specifications 6 ACCURACY RANGE ±1dB (down to – 80 dB) 7 PLOT FREQUENCY VS. Magnitude, phase etc. 8 SCALING /SWEEP MODE Log/Linear 9 CONTROL AND DISPLAY UNIT Shall be controlled through built in industrial PC with touch screen/keyboard. Or external laptop PC of reputed brand with latest specifications such as Core 2 Duo Intel processor, 4GB DDRRAM 320GB HDDDVD writer, 15’’TFT/LCD screen or with better specs. preloaded with Operating system MS Windows-7 or latest with antivirus. Original CDs of PC diagnostics from OEM etc and application software for the test kit. 10 TEST LEADS One set of 30meters coaxial or equivalent with clamps and connectors, compatible with the test instrument. 11 POWER SUPPLY It shall work on single-phase 230±10% V, 50±5% Hz, supply with variations in voltage and frequency respectively. 12 OPERATING TEMPERATURE 0 to +50°C 13 STORAGE TEMPERATURE -20 to +70°C 14 RELATIVE HUMIDITY 10 to 90 % non-condensing 15 PROTECTION Against short circuit & overload 16 REPEATABILITY It should offer repeatability of test results in 400 kV/765 kV charged area including for 315/500MVA Transformers and 50/60/80/125 MVAR reactors. SOFTWARE Software of the kit should be Windows based, Menu driven and user friendly. It should have all the templates/features required for complete testing of transformer, reactors etc. and facility for comparison of past template of the tested unit. 18 ACCESSORIES PC cables, Licensed OS software & antivirus for PC, Licensed software of the testing kit, , combination plugs, power-supply cables, original hard & soft carrying case (which should be robust/ rugged enough for proper safety of the kit during transportation), manual (both in soft copy & hard copies) etc, required for carrying out all types of testing. 19 ENVIRONMENT The test kit shall be compatible for EMI/EMC environment required for EHV switchyard as per IEC61000. 17 20 21 CALIBRATION CERTIFICATE SERVICES AFTER SALE Unit shall be duly calibrated before supply and the date of calibration shall not be older than two month from the date of supply of Kit. Bidder will have to submit the documentary evidences of having established mechanism for prompt services in India as required as per the specifications. SECTION DD Guidelines for Condition Monitoring and Diagnostic Techniques for Power Transformers and Reactors 1.0 SECTION DD Guidelines for Condition Monitoring and Diagnostic Techniques for Power Transformers and Reactors GENERAL 1.1 Condition monitoring may be defined as a predictive method making use of the fact that most equipment will have a useful life before maintenance is required. It includes the life mechanism of individual parts of equipment or the whole equipment, the application and development of special purpose equipment, the means of acquiring the data and the analysis of that data to predict the trends. 1.2 Initial stage of a condition monitoring programme consists of establishing the base line parameters and then recording the actual base line (or finger prints) values. The next stage is the establishment of routine testing of plants and equipment observing the running condition and assessing the parameters previously determined for the baseline. These readings are then compared with the fingerprint and the state of the present plant condition can be determined from the absolute figures. The rate of degradation and an assessment of the likely to failure can be estimated from the trend. 1.3 The benefits of condition monitoring can be summarized as below: • • • Reduced maintenance costs Results provide a quality - control feature Limit the probability of destructive failures, leading to improvements in operator safety and quality of supply For assessing possibility & severity of any failure and consequential repair activities. Provides information on the plant operating life, enabling business decisions to be made either on plant refurbishment or replacement. • • 1.4 Condition Monitoring Techniques The following condition-monitoring techniques are currently being used by Power Utilities world over to assess the health of transformer/ reactors in service: 1.4.1 Dissolved Gas Analysis (DGA) provides an early warning of various incipient faults in transformer winding or core. 1.4.2 Oil Parameters Testing: Low BDV indicates moisture or particulate contaminants in the oil. High moisture content varying with temperature indicates wet winding. Acidity, resistivity and interfacial tension (IFT) indicate oil condition. (For other details refer clause 2.5 of section CC) 1.4.3 Capitance & Tan δ measurement of bushing and winding to assess the condition of insulation, bushing and winding (For details refer clause 6.9 & 8.11 of section BB) 1.4.4 Winding resistance measurement to detect problem of broken sub-conductors, winding 349 350 Manual on Transformers contact joints and OLTC connections (For details refer clause 4.1 & 8.3 of section BB) 1.4.5 Impedance measurements with precision instruments to check for dynamic movements of winding due to system short-circuit faults 1.4.6 Turn ratio test indicates problem in winding and verifies correct tap changer connections 1.4.7 Excitation/ magnetization current tests to locate faults in the magnetic core structure such as shorted laminates or core bolt insulation breakdown or shorted turns due to insulation failures, which have resulted in conducting paths between winding turns. 1.4.8 IR measurement to indicate the presence or absence of harmful contamination (dirt, moisture etc.) and stress degradation of insulation. 1.4.9 Frequency response analysis (FRA) to check for system resonance condition and dynamic movements and detection of winding mechanical distortion during transportation and through fault. 1.4.10 PD measurement through UHF, Acoustic, RFI methods for localization of faults 1.4.11 Furfuraldehyde (FFA) analysis in oil (HPLC chromatography) to detect ageing in cellulosic material without taking paper samples 1.4.12 On-line dielectric dissipation factor (DDF) monitoring of bushings: Signals from transducers connected to test taps of bushings are collected and transmitted to User Interface Module for processing the data by proprietary software and converted to dielectric loss angle/ dielectric dissipation factor (DDF) and leakage current values. 1.4.13 Dielectric Response Methods for Diagnostics for Power Transformers giving general indication of moisture in insulation and possible paper ageing and oil condition 1.4.14 On-line Gas Monitors (a) On-Line hydrogen monitors to provide earliest possible detection of gas build up and alert the user to the need for more detailed analysis (b) On-line moisture content measurement to continuously monitor water content in oil, which indicates the status of solid insulation. The data is stored in monitor’s memory and can be down loaded remotely or locally which can be subsequently analyzed using window based control software. The software automatically gives trend analyses; generate graphs and reports for the user to take action based on the same. Guidelines for Condition Monitoring and Diagnostic Techniques for Power Transformers and Reactors 351 1.4.15 On-line PD detection using probe connected to the test tap 1.4.16 On-line temperature monitoring-direct measurement through fibre optic sensors for continuous monitoring of hot spot temperature can be the basis for an on-line load management system which can be accessed by operating staff to control loading pattern and thermal ageing of the transformer. 1.4.17 Thermo-vision Scanning of Transformer 2.0 DISSOLVED GAS ANALYSIS (DGA) 2.1 The transformer undergoes electrical, chemical and thermal stresses during its service life which may result in slow evolving incipient faults inside the transformer. The gases generated under abnormal electrical or thermal stresses are hydrogen(H2), methane(CH4), ethane (C2H6), ethylene (C2H4), acetylene (C2H2), carbon monoxide(CO), carbon dioxide(CO2), nitrogen(N2) and oxygen (O) which get dissolved in oil. Collectively these gases are known as Fault Gases, which are routinely detected and quantified at extremely low level, typically in parts per million (ppm) in dissolved Gas Analysis (DGA). Most commonly used method to determine the content of these gases in oil is using a Vacuum Gas Extraction Apparatus and Gas Chromatograph. 2.2 DGA is a powerful diagnostic technique for detection of slow evolving faults inside the transformer by analyzing the gases generated during the fault which gets dissolved in the oil. For Dissolved Gas Analysis to be reliable, it is essential that sample taken for DGA should be representative of lot and no dissolved gas shall be lost during transportation and laboratory analysis. Suggested sampling procedure based on IEC 60567 is given in Annexure-III of Chapter CC. Effective fault gas interpretation should basically tell us first of all, whether there is any incipient fault present in the transformer. If there is any problem, what kind of fault it is. Whether the fault is serious and the equipment needs to be taken out of service for further investigation. DGA can identify deteriorating insulation and oil, hot spots, partial discharge, and arcing. The health of oil is reflective of the health of the transformer itself. DGA analysis helps the user to identify the reason for gas formation and materials involved and indicate urgency of corrective action to be taken. 2.3 The evolution of individual gas concentrations and total dissolved combustible gas (TDCG) generation over time and the rate of change (based on IEC 60599 and IEEE C 57-104 standards) are the key indicators of a developing problem. Some of the recognised interpretation techniques are discussed below: 2.3.1 Individual Fault Gases Acceptable Limits To ensure that a transformer (with no measured previous dissolved gas history) is behaving 352 Manual on Transformers normal, the DGA results are compared with the gassing characteristics exhibited by the majority of similar transformers or normal population. As the transformer ages and gases are generated, the normal levels for 90% of a typical transformer population can be determined. From these values and based on experience, acceptable limits or threshold levels have been determined as given in Table 1 below: Table 1 : Ranges of 90% Typical Conc Values (all Types of Transformers) as per IEC 60599 /1999 Transformer FAULT GASES (in µ 1/1) or ppm Sub Type H2 CH4 C2H6 C2H4 C2H7 CO CO2 No OLTC 60-150 40-110 50-90 60-280 3-50 540-900 5100-13000 Communicating 75-150 35-130 50-70 110-250 80-270 400-850 5300-12000 OLTC Note 1 - The values listed in this table were obtained from individual networks. Values on other networks may differ. Note 2 - “Communicating OLTC” means that some oil and /or gas communication is possible between the OLTC compartment and the main tank or between the respective conservators. These gases may contaminate the oil in the main tank and affect the normal values in these types of equipment. “NO OLTC” refers to transformers not equipped with an OLTC, or equipped with an OLTC not communicating with or leaking to the main tank. Note 3 - In some countries, typical values as low as 0.5µ l/ l for C2H2 and 10µ l/l for C2H4 have been reported. However it is improper to apply threshold level concept without considering the rate of change of the gas concentration. When an abnormal situation is indicated by above table, a testing schedule is devised with increased sampling frequency. 2.3.2 Total Dissolved Combustible Gas (TDCG) Limits The severity of an incipient fault can be further evaluated by the total dissolved combustible gas (TDCG) present. Limits for TDCG are as given in Table 2. An increasing gas generation rate indicates a problem of increasing severity and therefore we should resort to shorter sampling frequency. Table 2 : Action based on TDCG limits (IEEE standard C:57.104-1991) TDCG Limits, PPM Action < or = 720 Satisfactory operation, Unless individual gas acceptance values are exceeded 721-1920 Normal ageing/ slight decomposition, Trend to be established to see if any evolving incipient fault is present. 1921-4630 Significant decomposition, Immediate action to establish trend to see if fault is progressively becoming worse. >4630 Substantial decomposition, Gassing rate and cause of gassing should be identified and appropriate corrective action such as removal from service may be taken. Guidelines for Condition Monitoring and Diagnostic Techniques for Power Transformers and Reactors 353 Note :TDCG value includes all hydrocarbons, CO & LH, and does not include C02 which is not a combustible gas. 2.3.3 The relationship of evolved gas with temperature and the type of faults are shown in Tables 3 and 4 respectively: Table 3 : Relationship of evolved gases with temperature Relationship with temperature Methane (CH4) > 120° CEthane (C2H6 ) > 120° CEthylenc (C2H4 ) > 150" CAcetylene (C2H2 ) > 700° C Table 4 : Associated faults with different fault gases Associated faults with different gases Oil Overheating :C2H4, C2H6, CH4 Traces of acetylene with smaller quantity of Hydrogen may be evolved Overheated Cellulose: CO Large quantity of Carbon-Di-Oxide (CO2) and Carbon Monoxide (CO) are evolved from overheated cellulose. Hydrocarbon gases such as Methane and Ethylene will be formed if the fault involves an oil impregnated structure. Partial discharge in Oil (Corona): H2, CH4 Ionisation of high stressed area where gas / vapour filled voids are present or ‘wet spot’ produces Hydrogen and methane and small quantity of other hydrocarbons like ethane and ethylene. Comparable amounts of carbon monooxide and di-oxide may result due to discharges in cellulose. Arcing in Oil :C2H2, H2 Large amount of Hydrogen and acetylene are produced with minor quantities of methane and ethylene in case of arcing between the leads, lead to coil and high stressed area. Small amounts of carbon mono-oxide and di-oxide may also be formed, if fault involves cellulose. It is well known that there is no definite interpretation method in the world, which can indicate the exact location and type of the fault. The different interpretation methods only provide guidelines to take an engineering judgment about the equipment. Apart from the DGA results various other factors are taken into consideration such as past history of the transformer, grid condition, loading patterns etc. 2.4 Ratio Methods Several well- known methods/criteria (like Rogers ratio, IEC 60599, Dornenberg, Key gas etc.) are being used by utilities to interpret the DGA results, based mostly on the relative concentrations (i.e. ratios) of the constituent gases. These ratios generally give an indication of the existence and nature of a problem. Some of the interpretation methods used for DGA arediscussed here in brief: 2.4.1 IEC 60599 Method This method is applicable only when the fault gas results are ten times the sensitivity limit of 354 Manual on Transformers the Gas Chromatograph (GC). As per IEC 60567 the sensitivity limit for the GC should be maximum 1 ppm for all the hydrocarbons and 5 ppm for hydrogen. In this method three ratios viz. C2H2/C2H4, CH4/H2& C2H4/C2H6 are used for interpretation. Various combinations of the ratios are used for diagnosis of type of fault such as PD, Discharge of low energy, Discharge of high energy, Thermal fault < 300 deg C, Thermal fault 300 - 700 Deg C and Thermal fault > 700 Dcg C. The table (as per IEC 60599) showing different type of faults depending upon the three key ratios is given in Table 5 : Case Characteristic Fault C2H4 H2 PD Partial discharges NS <0.1 <0.2 Dl Discharges of low energy >1 0.1 - 0.5 >1 D2 Discharges of high energy 0.6 - 2.5 0.1 -1 Tl Thermal fault T < 300°C NS >1 but NS T2 Thermal fault 300°C < 1 < 700°C <0.1 T3 Thermal fault <0.2 1 2 >2 1 <1 >I 1-4 >1 >4 Note 1 — In some countries, the ratio C2H2 / C2H6 is used, rather than the ratio CH4 / HZ Also in some countries, slightly different ratio limits are used. Note 2 - The above ratios are significant and should be calculated only if at least one of the gases is at a concentration and a rate of gas increase above typical values (see clause 9). Note 3 - CH4 / H2<0.2 for partial discharges in instrument transformers.CH4/H2<0.07 for partial discharges in bushings. Note 4 - Gas decomposition patterns similar to partial discharges have been reported as a result of the decomposition of thin oil film between over-heated core laminates at temperatures of 140 °C Though this method is quite comprehensive, still there are cases where it does not fit into any of the cases listed in the diagnosis table. These cases should be dealt through trend analysis and other interpretation methods. Again interpretation through the above method is meaningless unless it is correlated with the earlier sample results. 2.4.2 IEEE Method-C57-104/1991 2.4.2.1 Key Gas Method Characteristic “Key Gases” have been used to identify particular type of fault. Laboratory simulations and comparison of results of DGA tests combined with observations from the tear down of failed transformers have permitted the development of a diagnostic scheme of the characteristic gases generated from thermal and electrical (Corona and arcing) deterioration of electrical insulation. Table 6 shown the lists of the key gases for the conditions of arcing, corona, overheating in oil and overheating in paper in the order of decreasing severity. Guidelines for Condition Monitoring and Diagnostic Techniques for Power Transformers and Reactors 355 Table 6 Key gases associated with typical fault Fault type Key gases Arcing Acetylene (C,H,), Hydrogen (H,) Corona Hydrogen (H7) Overheated Oil Ethylene (C,H4), Methane (CH4) Overheated Cellulose Carbon Mono-oxide (CO) and Dioxide (CO2) 2.4.2.2 Ratio Method These methods are used to determine the type of fault condition by comparing ratios of characteristic gases generated under incipient fault conditions. The advantages to the ratio methods are that they are quantitative, independent of transformer capacity and can be computer programmed. The disadvantages are that they may not always yield an analysis or may yield an incorrect one. Therefore it is always used in conjunction with other diagnostic methods such as key gas method. (a) The Doernenberg Ratio method is used when prescribed normal levels of gassing are exceeded. It provides a simple scheme for distinguishing between pyrolysis (overheating) and PD (corona and arcing). In this method four ratios viz. CH4/H2,C2 H2C2 H4 C2H6/C2H2 & C2H7/CH4 are used. Table 7 : Ratios for key gases - Doernenburg Suggested fault diagnosis Ratio I (Rl) CH2/H2 Ratio 2 (R2) C2H2/ C2H4 Ratio 3 (R3) C2H2/CHd4 Ratio 4 (R4) C2H6/C2H2 1.Thermal Decomposition >1.0 <0.75 <0.3 >0.4 2.Corona (Low Intensity PD) <0.1 Not Significant <0.3 >0.4 3.Arcing (High Intensity PD) >0.1 >0.75 >0.3 <0.4 In Doemenburg’s method for declaring the unit faulty at least one of the gas concentrations (in ppm) for H2, CH4, C2H2and C2 H4should exceed twice the values from limit LI (Table 8) and one of the other three should exceed the values for Limit LI. Having established that the unit is faulty, for determining the validity of ratio procedure at least one of the gases in each ratio Rl, R2, R3 or R4 should exceed limit LI. Otherwise the unit should be re-sampled and investigated by alternative procedures. Table 8 : Concentrations of dissolved gas Key gas Concentration LI (in ppm) Hydrogen(H2) 100 Methane (CH4) 120 Carbon Monoxide (CO) 350 Acetylene (C2H2) 35 Ethylcne(C2H4) 50 Ethane (C2H6) 65 356 (b) Manual on Transformers The Rogers Ratio method is a more comprehensive scheme using only three ratios viz. CH4/H2, C2H2/C H4& C2H4/C2H6, which details temperature ranges for overheating conditions based on Halstead’s research and some distinction of the severity of incipient electrical fault conditions (Table 9). A normal condition is also listed. Table 9 : Rogers ratio for Key Gases Case R2 (C2,/H4) Rl R5 (CH,/H2) (C2H4/C2H6) Suggested fault diagnosis 0 <0.1 >0.1 <0.1 Unit normal 1 <0.1 <0.1 <0.1 Low-energy (See Note) 2 0.1-3.0 0.1-1.0 >3.0 Arcing - High energy discharge 3 <0.1 >0.1<1.0 1.0-3.0 Low temperature thermal 4 <0.1 >1.0 1.0-3.0 Thermal <700°C 5 <0.1 >1.0 >3.0 Thermal >700°C density arcing -PD Note There will be a tendency for the ratios R2 and R4 to increase to a ratio above 3 as the discharge develops in intensity lEEIi C57.104 - 1991 standard gives an elaborate way of analysing the type of fault using Doernenberg, Rogers’s method and TDCG limits. However it is again emphasized that DGA shall give misleading results unless certain precautions are taken. These are proper sampling procedure, Type of sampling bottle, cleanliness of bottle, duration of storage, method of gas extraction, good testing equipment and skilled manpower. 2.5 Trend Analysis Transformers from the same manufacturer and of same type some time exhibit initially specific pattern of gas evolution which subsequently slows down (or reach a plateau) is called Fingerprints or normal characteristics, which are characteristic to the transformer and do not represent an incipient fault condition. When a possible incipient fault condition is identified for first time, it is advisable to determine gassing trend with subsequent analysis giving information such as which gases are currently being generated and rate of generation of these gases. The level of gases generated in subsequent analysis provides a baseline from which future judgement can be made. In the examination of trends, key gases, TDCG, CO2/CO ratio, rate of gas generation and fingerprints (of normal trends) of particular transformer should also be considered. Guidelines for Condition Monitoring and Diagnostic Techniques for Power Transformers and Reactors 357 The ratio of gas generation is a function of load supplied by the transformers and this informatioris vital in determining the severity of fault condition and decision of removal of the equipment from service for further investigation. Two methods are suggested in literature for assessing the gassing rate: • Change of concentration of gas in ppm • Determination of actual amount of gas generated General guidelines for rate of gas generation for removal of transformer from service are 100 ppm/day and 0.1 cub feet (0.003 m3) gas per day A flow chart is given at Fig. 1 for step by step action to be taken based on DGA test results giving recommendations: Examination of DGA results Compare with DGA of previous sample and with typical values At least one gas above typical values ol gas concentrations and rates of gas increase Calculate gas ratios All gases below typical values of gas concentrations and rales of gas increase Report as typical DGA/healthy equipment Fault identified by table 4 Gas concentration above alarm values of gas concentrations and rates of gas increase, or change in fault type D2 ALERT condition ALERT condition Institute more frequent sampling Consider on-line monitoring Take immediate action Consider on-line monitoring, inspection or repair Store data Fig. 1 Flow chart for DGA from IEC 60599/ 1999 3.0 FURFURALDEHYDE ANALYSIS FOR REMNANT LIFE MEASUREMENT OF PAPER INSULATION Degradation of insulating paper can be ascertained by direct or indirect methods. Direct method employs Degree of Polymerization (DP), which requires a physical paper sample from the winding. However, this method being destructive to transformer, cannot be employed as a routine condition monitoring exercise. 358 3.1 Manual on Transformers Degree of Polymerization (DP) One of the most dependable means of determining paper deterioration and remaining life is the DP test of the cellulose. The cellulose molecule is made up of a long chain of glucose rings which form the mechanical strength of the molecule and the paper. DP is the average number of these rings in the molecule. For DP measurement remove a sample of the paper insulation about 1 centimeter square from a convenient location near the top of center phase with a pair of tweezers. In general, in a three-phase transformer, the hottest most thermally aged paper will be at the top of the center phase. If it is not possible to take a sample from the center phase, take a sample from the top of one of the other phases. Table 10 DP values for estimating remaining paper life 3.2 New insulation 1,000 DP to 60% to 66% life remaining 500 DP 30% life remaining 300 DP 0 life remaining 200 DP 1,400 DP Furan Analysis It is known that in addition to CO and C02 the ageing process of the paper produces several oil soluble by products, most notably the furanoid compounds (FFA). When cellulose insulation decomposes due to overheating, chemicals, in addition to C02 and CO, are released and dissolved in the oil. These chemical compounds are known as furanic compounds or furans. The most important one, for our purposes, is 2-furfuraldehyde. When DGAs are required, always request that furans testing be completed by the laboratory to check for paper deterioration. In healthy transformers, there are no detectable furans in the oil, or they are less than 100 part per billion (ppb). In cases where significant damage to paper insulation from heat has occurred, furan levels have been found to be at least 100 ppb and up to 70,000 ppb. The monitoring of furanic compounds by annual sampling of the oil and its analysis using High Performance Liquid Chromatography (HPLC) has beenis under used for condition monitoring on a routine basis for somein recent years by some utilities. Source : EPRI’s Guidelines for the Life Extension of Substations, 2002 Update, chapter 3. Table 7: DP Values for Esti mating Remaining Paper Life Generally FFAs are extracted from the oil either by solvent extraction or solid phase extraction and measured by HPLC by UV detector. The major FFA in oil is 2-Furfural and others are present in very low or undetected levels. 2-Furfural can be measured colorimetrically using spectrophotometer. This method is rapid and accurate and measures only 2-Furfural in oil. This technique is useful for quick screening of FFA in transformer oil. The relationship between the generation of these by-products and condition of in service paper is not well established. However, it has generally been seen that there is a linear relationship between FFA and DP. FFA may be used as a complimentary technique to DGA for condition monitoring. Guidelines for Condition Monitoring and Diagnostic Techniques for Power Transformers and Reactors 359 Table 11 : Furans, DP, Percent of Life Used, of Paper Insulation Non-thermally upgraded paper 55 °C Rise Transformer 2FAL (ppb) Thermally upgraded paper 65 °C Rise Transformer Total Furans (ppb) Estimated Degree of Polymerization (DP) Estimated Percentage of Remaining Life Interpretation 58 51 800 100 Normal 130 100 700 90 Aging 292 195 600 79 Rate 654 381 500 66 1,464 745 400 50 1,720 852 380 46 2,021 974 360 42 2,374 1,113 340 38 Excessive 2,789 1,273 320 33 Aging 3,277 1,455 300 29 Danger Zone 3,851 1,664 280 24 High Risk of 4,524 1,902 260 19 Failure 5,315 2,175 240 13 End of Expected 6,245 2,487 220 7 Life of Paper 7,337 2,843 200 0 Insulationand of the Transformer Accelerated Aging Rate Testing is done for five different furans which are caused by different problems. The five furans and their most common causes are listed below: 5H2F (S-IIydroxymethyl-2-Furaldchyde) caused by oxidation (aging and heating) of the paper 2FOL (2-Furfurol) caused by high moisture in the paper 2FAL (2-Furaldchyde) caused by overheating 2ACF (2-Acetylfuran) caused by lightning (rarely found in DGA) 5M2F (5-Methyl-2-Furaldehyde) caused by local severe overheating (hotspot) (Source: Transformer Diagnostic USBR, June 2003, Page-38) 4.0 CONTINUOUS DGA MONITORING BY ON-LINE GAS SENSORS 4.1 Laboratory DGA is carried out at a predefined interval and the fault developing within that interval cannot be ascertained till the transformer has actually failed. It is important to appreciate that some faults may take less than one year to progress from onset to failure; others may remain in a stable state for a much longer period but have the potential for a rapid increase. In case of sudden rise in gas levels indicated by on-line sensor, sample should be sent for immediate laboratory analysis for confirmation and to decide further course of action. One of the most widely used on line Gas Monitoring system has a membrane, which allows preferentially lighter 360 Manual on Transformers molecules to pass through and be detected in a gas reaction cell. Some of the on-line gas monitors also provide continuous moisture measurement thereby ascertaining the wetness of the winding. 4.2 More recently various companies have developed Fourier transform infra Red (FTIR) detectors which will detect most of the gases of interest and quantify their amounts. These systems are not seen currently as an alternative to the very low cost of a laboratory based oil analysis. Their current role is to investigate particular transformer with gassing problems. However, an added benefit being gained is the greater understanding of the load temperature, time relationships for detected gases. This should eventually lead to better understanding and interpretation of the laboratory analysis. 5.0 CAPACITANCE & TAN δ MEASUREMENT OF BUSHINGS AND WINDINGS Monitoring of insulation requires knowledge of insulation characteristics and judgement based on experience. Factors like ambient temperature, humidity, cleanliness of surface of bushings and electrostatic interference are to be considered before taking any reading so that error in measurement could be avoided and readings are accurate. To overcome interference due to charged switchyard, the polarity of the input voltage is reversed w. r. t. electrostatic interference and second measurement is made; the two averaged and interference is effectively reduced. The testing kit with its own sinusoidal source and interference suppression / cancellation unit is used which generates its own signal opposite in phase and magnitude to cancel out the interference. While carrying out this test in EHV substation, certain precautions like use of Interference Suppression Unit along with double-shielded leads, disconnection of jumpers and cleaning of surface of the bushings are taken. All phases of a particular winding is shorted together and also with neutral to minimize the effect of inductive currents during measurements. (a) Bushing Dissipation factor and capacitance values are measured in UST (Ungrounded Specimen Test) mode. (b) Measurement for windings is carried out as per the following combinations: • Winding to winding in UST mode. • Winding to Ground in GSTg mode with other winding guarded. While carrying out measurement for HV winding LV winding should be guarded whereas for LV winding HV winding shall be guarded. • Winding to Ground in GST mode may be done for verification of test results taken in UST and GSTg mode. • Tan delta value for healthy insulation does not change appreciably with change in Guidelines for Condition Monitoring and Diagnostic Techniques for Power Transformers and Reactors 361 temperature. In case of variation of Tan delta with change of temperature, condition of insulation needs to be investigated. Use of temperature correction factor should not be applied. In case of any doubt, measurement should be taken as near to 20 °C as possible. Rate of change of tan delta and capacitance is very important. Normally Tan delta of bushings and windings at 20°C should be less than 0.007 during O&M. For new bushings, Tan delta value generally lower than 0.50.4% and for windings also, Tan delta value should be less than 0.5% at 20 deg C. Capacitance value can be within +10%, -5% of previous capacitance values. Rate of rise of Tan delta for bushings and windings should not be more than 0.001 per year. The rate of change of tan delta more than 0.001 per year needs further investigation 6.0 FREQUENCY RESPONSE ANALYSIS Frequency Response Analysis (FRA) is made to assess the mechanical integrity of the transformer. Transformers while experiencing severity of short circuit current looses its mechanical property by way of deformation of the winding or core. These changes cannot be detected through conventional condition monitoring techniques such as Dissolved Gas Analysis, Winding Resistance Measurement, Capacitance and Tan delta measurement etc. Sometimes even transportation without proper precaution may cause some internal mechanical damages. FRA measurement, which is signature analysis, provides vital information of the internal condition of the equipment so that early corrective action could be initiated. Short circuit forces can cause winding movement and changes in winding inductance or capacitance in Power Transformers. Recording the frequency response with these changes gives information regarding the internal condition of the equipment. Frequency Response Analysis (FRA) has proved to be an effective tool to detect such changes. Fig. 2 FRA test set up Sinusoidal signal output of approximately 2 V rms from the Frequency Response Analyzer is applied and one measuring input (R1) is connected to the end of a winding and the other 362 Manual on Transformers measuring input (T1) is connected to the other end of the winding. The voltage is applied and measured with respect to the earthed transformer tank. While the low frequency analysis reveals the winding movements, the high frequency analysis reveals the condition of joints. It is ensured that winding which is not under test is terminated in open condition in order to avoid response difference among the three phases. The same procedure is followed on subsequent tests on the same or similar transformer, to ensure that measurements are entirely repeatable. The voltage transfer function T1/R1 is measured for each winding for four standard frequency scans from 5 Hz to 2 MHZ and amplitude & phase shift results are recorded for subsequent analysis • Analysis of Measured Frequency Responses Interpretation of the test results is based on subjective comparison of FRA responses taken at different intervals. If changes are observed in the later FRA spectrum with respect to the reference FRA spectrum, it is left to the experience of the analyst for quantitative condition assessment of the transformer. However, one should check for any significant shift in the resonance frequencies and emergence of new resonant frequencies in the later FRA response, which could be the result of any mechanical deformation in the transformer winding. As FRA is signature analysis, data of signature of the equipment when in healthy condition is required for proper analysis. Signatures could also be compared with unit of same internal design or with other phases of the same unit. Normally measured responses are analysed for any of the following: • Changes in the response of the winding with earlier signature. • Variation in the responses of the three phases of the same transformer. • Variation in the responses of transformers of the same design. In all the above cases the appearance of new features or major frequency shifts are causes for concern. The phase responses are also being recorded but normally it is sufficient to consider only amplitude responses. Fig.3 Guidelines for Condition Monitoring and Diagnostic Techniques for Power Transformers and Reactors 363 As per EuroDoble Client Committee, the traces in general will change shape and be distorted in the low frequency range (below 5 KHz) if there is a core problem. The traces will be distorted and change shape in higher frequencies (above 10 KHz), if there is winding problem. Changes of less than 3 decibels (dB) compared to baseline traces are normal and within tolerances. In general, changes of +/- 3 dB (or more) in following frequency range may indicate following faults: Table 12 • Following international standards are available for FRA and its interpretation: (a) IEC 60076-18 Edition 1.0 (b) CIGRE Technical Brochure 342; Mechanical condition assessment of Transformer winding using FRA 7.0 PARTIAL DISCHARGE MEASUREMENT PD techniques for detection and location are important for diagnostics, as they not only help to identify the inception of damage caused, but also assist to monitor the evolving and deteriorating situation affected by the various stress factors existing in the service condition. There can be various reasons for PD inception. It can be the result of electrical stress caused by mechanical deformation, overheating of insulated conductor or even can be the result of inherent defect. The PD detection at site can be done by various techniques such as: • Dissolved Gas Analysis • Conventional IEC 60270 • Acoustic measurement • RFI measurement • UHF measurement All these techniques have their strength and weakness in terms of sensitivity and accuracy. The sensitivity and effectiveness of these techniques depends upon the type of defect and location of the fault. 364 • Manual on Transformers Analysis of Dissolved Gases for PD detection Normally Oil samples are taken once or twice a year from the main tank for laboratory analysis. From dissolved gas analyses it is possible to detect damage caused by both overheating and PD. However a direct measurement of PD will be much helpful to ascertain a rapidly increasing rate of PD than relying upon the trending of dissolved gases. • Conventional IEC 60270 PD measurement for factory acceptance testis based upon IEC 60270 standard. The measurement with this method is usually made using the bushing tap as a capacitance divider and measuring PD transients in the 30-500 kHz range. The PD measurement at the bushing taps is affected by the attenuation, the winding resonance, external corona, various electromagnetic discharges present at the site and the weather condition. Fig. 4 • Acoustic PD Measurement Acoustic discharge detection is based on detection of the mechanical signals emitted from the discharge. The discharge appears as a small “explosion,” which excites a mechanical wave that propagates throughout the insulation. These waves propagate through different media before reaching the container of the equipment under test. AE sensors mounted on the container of the equipment pickup these AE signals and convert into electrical signals, which could be analysed using a data acquisition and processing system. Acoustic PD detection system usually consists of AE sensor, preamplifier and a data acquisition, storing and analyzing instrument. Guidelines for Condition Monitoring and Diagnostic Techniques for Power Transformers and Reactors • 365 RFI Scanning The important advantage of this method is that it can be included in the routine monitoring schedule and will survey all substation equipment. It will indicate bushing problems and this is one area where dissolved gas analysis is limited by the difficulties in extracting a gas sample. Sensors such as telescopic, duck antenna or high frequency split core current transformers can be used. The main advantage is that this method can detect the PD without the need for outages or any special connections. For effective analysis, the RFI signals are monitored in frequency domain from 50 MHz to 1000 MHz as well as in time domain mode. A typical baseline reading is taken prior to measurement on any HV equipment. A baseline reading will show all the RF frequency signature for the area where a substation is located. The frequency spectrum from 50 MHz to 1000 MHz consists of several different continuous signals like FM radio, mobile signals, TV stations, Essential wireless services and other telecommunication sources. Example of a typical frequency scan is shown in Figure given below, where these telecommunication signals are present. Fig. 5 The broadcasted signals are narrowband in nature whereas the RF signals produced by PD are broadband in nature. It is therefore necessary to discriminate between these two signals to recognize the presence of PD source. The facility of both peak and average detectors in the instrument facilitates discrimination between the narrowband characteristics of broadcast RF emissions and the broadband impulsive RF emissions from PD. The amplitudes of the peak and average measurements for broadcasted 366 Manual on Transformers RF emissions are nearly same whereas in case of broadband RF emissions resulting from PD the average value has much lower amplitude. Figure given below shows the variation in average and peak amplitude measurements. Average and peak amplitude Suspected PD detected Fig. 6 • PD Detection- with UHF Probe In this method, UHF probe inserted in an oil valve or access port allowing the transformer tank to screen interference acting like a Faraday cage. This probe detects the electromagnetic radiation from the PD and is not affected by winding resonances. The UHF method technique can be used on site even when the transformer is energized, in a high electromagnetic interference environment. UHF method cannot be calibrated ay injecting a known charge across the winding as is done in case of conventional method as per IEC 60270. Performance/Sensitivity Check is performed by Injecting high-frequency test impun the additionally integrated electrode. Fig. 7 Guidelines for Condition Monitoring and Diagnostic Techniques for Power Transformers and Reactors 367 The PD output of the UHF probe is connected via coaxial cable to a pre-amplifier and the measuring instrument. Optimum measuring frequencies for better signal and S/N are selected for measurements and measurements are performed Figures given below shows a typical probe inserted through an oil valve and typical curve obtained during measurement. Typical UHF measurement UHF probe inserted in drain valve Fig. 8 8.0 Dielectric Response Methods for Diagnostics for Power Transformers The dryness and ageing state of the oil-paper insulation is a key factor for deciding reliability of a power transformer since moisture has deleterious effects on dielectric integrity and insulation ageing rates. Today the water content of the cellulose of a transformer in service is determined indirectly via moisture measured from oil. As moisture distributes unequally between the oil and the pressboard, the greater part resides within the solid insulation. Because the water concentration in the oil is highly temperature dependent, the measurement of moisture in oil is not a particularly reliable indicator of dryness in the cellulose, particularly not for lightly loaded transformers. Recently, methods are developed of determining moisture content and ageing of the pressboard and paper more directly by measuring the effects of moisture on electrical properties. The two foremost techniques are: (a) Dielectric spectroscopy in time domain, i.e. measurements of polarization and Depolarization currents (PDC) (b) Dielectric frequency domain spectroscopy (FDS), i.e. measurements of electric capacitance C and loss factor tan δ in dependency of frequency. • Polarization and Depolarization Currents Measurement (PDC) In the polarization and depolarization current (PDC) method, a DC voltage is applied to the insulation system under test for a specific time and the polarization current is measured. After then, the insulation system is shortened and the depolarization current is measured. From the polarization and depolarization currents the dielectric response is evaluated and the dissipation factor frequency characteristic is calculated. The PDC method is much faster than the FDS at very low frequencies. Measurement results are usually presented in a log/log scale with charging and discharging current over time. According to the common interpretation guideline the first 1-100 seconds are influenced by oil conductivity. The end value of polarization current is determined by the pressboard resistance and therefore by moisture 368 Manual on Transformers Fig. 9 Measurement of the relaxation currents using PDCAnalyser& Principle waveform of relaxation currents Interpretation of PDC Measurement data Fig. 10 • Dielectric Frequency Domain Spectroscopy (FDS) In frequency domain spectroscopy (FDS), the dissipation factor of the insulation system under test is measured by frequency sweep. Frequency versus Tangent Delta measurements method is called Frequency Domain Spectroscopy (FDS). In this method, frequency range is much enhanced especially to low frequencies. The dissipation factor plotted via frequency shows a typical s-shaped curve. With increasing moisture content, temperature or aging the curve shifts towards higher frequencies. Moisture influences the low and the high frequency parts. The middle part of the curve with the steep gradient reflects oil conductivity. Insulation geometry conditions Guidelines for Condition Monitoring and Diagnostic Techniques for Power Transformers and Reactors 369 determine the “hump” left of the steep gradient. In order to determine the moisture content of the insulation, the measurement should also provide data left of the “hump”, where the properties of the solid insulation dominate. Fig. 11 Following table may be considered for determining the level of wetness and taking necessary action for Dryout. Table 13 : Moisture content in paper Source : IEEE Std. 62-1995 Insulation condition Dry (at commissioning) Moderate to Wet (Lower no. indicate fairly dry whereas large no. indicate moderately wet insulation) Wet Extremely wet % Moisture by dry weight in paper (Wp) % Saturation of Waterin oil 0.5-1.0 % <5% <2% 6-20 % 2-4 % 21-30 % > 4.5 % > 30 % (Source : CIGRE DOC. No. 227. Life Management Technique for Power Transformer) 9.0 ON LINE WINDING TEMPERATURE MEASUREMENT 9.1 The life of a transformer is shortened considerably if it is operated consistently at elevated temperatures. A winding temperature indicator (WTI) is used to monitor the temperature of winding by indirect methods and give alarm / trip commands to protection circuits when temperature rises beyond the set limits to protect the transformer. This method uses a thermal image of the transformer and is based on the transformer load current. Calibration consists of assigning an assumed hot temperature to full load current. However it is not a direct measurement of the winding temperature and cannot reliably represent the transient conditions. 370 Manual on Transformers 9.2 The direct measurement of winding temperature of transformer by hot probes with fibreoptic links is based on fluoroptic thermometry technology where a temperature sensitive phosphor is connected to the detector. Blue violet light pulses are sent down the fibre causing the phosphor to glow. Decay of fluorescence after each pulse varies accurately with temperature. The same optical fibre transmits the excitation pulses and returns the fluorescent signal. It has a very wide temperature range of measurement (-200°C to + 450°C) and has a high accuracy of + 0.1 °C. The optical signals do not get distorted in presence of even very high electromagnetic fields. The cables/links are non-corrosive in transformer oil and they do not influence the electric field inside transformer. 9.3 The change in decay time of fluorescent light is detected by a detector, which is directly calibrated in terms of temperature. The response time of the change is very high and of the order of 0.2 sec. So, fibre-oplic point sensors for direct hot-spot measurement can measure the actual hot-spot and have a much faster response. Accurate knowledge of thermal behaviourand hotspot temperature enables the manufacturer to refine designs and calculating procedures and the customer to fully utilize the overload capabilities of plant without reducing life expectancy and degrading the dielectric integrity. 10.0 THERMO VISION SCANNING OF TRANSFORMER A thermo vision camera determines the temperature distribution on the surface of the tank as well as in the vicinity of the Jumper connection to the bushing. The information obtained is useful in predicting the temperature profile within the inner surface of tank and is likely to provide approximate details of heating mechanism The following temperature rises above ambient have been found to be practical during infrared inspections: Table 14 : Action based on temperature rise above ambient Temperature rise above ambient (°C) Recommendation (based on IEEE Std 62-1995) 0-10 Repair in regular maintenance schedule : Little probability of physical damage 11-39 Repair in near future; Inspect for physical damages 40-75 Repair in the immediate future. Disassemble and check for probable damage >76 Critical problems; Repair immediately Note : Any decision to remove any equipment from service has to he taken based on test results, special tests if any (in totality) and manufacturers’ recommendation. 11.0 CONCLUSION Condition monitoring uses sensors to provide raw data and warning signals from the equipment under surveillance. The diagnostic and condition monitoring together is a powerful tool for optimizing assets life. It also helps to reduce maintenance, failure and consequential losses and to assist in predicting residual life. Based on the results, refurbishment strategy, upgrading and replacement decisions can be taken. SECTION EE Guidelines for Fire Protection of Power Transformers SECTION EE Guidelines for Fire Protection of Power Transformers 1.0 GENERAL 1.1 Introduction The hazard of fire originating in or spreading to power transformers has always been recognised in the Power Industry. With the increasing size of generating units and associated transmission and distribution networks the number of transformers or large capacities has increased phenomenally thus necessitating more stringent protection measures to prevent fire risk to transformers and damage to equipment. This section of the transformer specification discusses the various aspects of transformer fire protection. 1.2 Strategy The strategy for safeguarding against fire is to emphasize fire prevention rather than fire fighting. Nevertheless, it is equally important to provide adequate fire fighting arrangements. This applies also to associated equipment like bushings, circuit breakers, instrument transformer, cables, etc., since a fire originating in these can easily spread to the power transformer. In effect, all the transformer installation should be provided with the prevention as well as fire detection and fighting systems. 1.3 Other Factors Factors like proximity of the transformer to buildings and other equipment such as switchgear play an important role in design of the fire prevention scheme. It is desirable that these equipments be segregated from the transformer installation or be provided with fire prevention measures to avoid spread of fire to the transformer installations. 2.0 DESIGN CONSIDERATIONS A majority of fires originating in transformers are due to inadequate design and installation, apart from faulty operation and maintenance practices, Proper installation, house keeping and maintenance can reduce fire hazards to a great extent. Hence, fire hazards must be given utmost attention while designing, selecting and installing power transformers and correct operation and maintenance procedures must be adhered to strictly. 2.1 Bushings Bushings are often the source of transformer failures and consequent fires. This is due to the fact that dielectric stresses in bushings are very high and sometimes oil tightness may not be ensured. Only bushings of proven design, which have been fully type tested and have passed all acceptance tests shall be used, Bushings shall be provided with test taps and regular (annual) checks of bushing tan delta shall be carried out. Also, the oil level, in oil filled bushings, shall be checked daily. 373 374 2.2 Manual on Transformers Tapchanger Tap changer, particularly on-load type are a potential fire risk. The selection of tap changer design, and proper maintenance of its mechanism are important. In no case should an off-load tap changer be operated when the transformer is energised from any of its windings. 2.3 Cable Sealing Ends The level of compound in the sealing ends should be checked periodically. 3.0 INSTALLATION REQUIREMENTS The general recommendations for safeguarding power transformers are given below and specific recommendations arc given in clause 4. The requirement relevant to transformers, in increasing order of importance of installation are: (a) Soak pits (b) Drain pits (c) Barrier walls (d) Fire detection system (c) Fire hydrant (f) Deluge, spray or mulsifire system (g) Nitrogen injection system In case of remote controlled or unattended substations, automatic fire detection and fighting system (e.g. nitrogen injection system) along with conventional system can also be provided. 3.1 Outdoor Transformers (a) Soak Pit and Drain Pit The transformers foundation shall be surrounded by a suitable soak pit enclosed by a 150 mm high non-combustible curb. This soak pit shall be filled with coarse crushed stones about 25 mm in diameter to a minimum depth of 300 mm. The volume of the soak pit minus the volume of the stones should be sufficient to contain the entire oil content of the transformer if the oil content is less than or equal to 5 kl. In case the oil content is more than 5 kl, the volume of soak pit minus the volume of stones should be sufficient to contain at least one third of the total oil content. The excess should be led through two or more hume/concrete pipes (min. 150 mm dia.) from bottom of pit to a central remote burnt oil tank. Control cables emanating from transformers shall be led through the soak pit through hume RCC pipes. The marshalling kiosk of the transformer shall be installed outside the pit and away from the potential fire hazard zone. Guidelines for Fire Protection of Power Transformers (b) 375 Barriers between Transformers Barrier wall of brick or reinforced cement concrete shall be provided for separation of transformers wherever adequate space is not available (Refer clause 4). The barriers shall extend at least 300 mm above the highest transformer bushing and pressure relief vent and lengthwise 600 mm beyond the transformer including any radiators and tap changer enclosures. (c) Fire Detection, Hydrant and Deluge Systems Selection of these systems should be based on the importance of an installation. The fire detection system only detects a fire and sends an alarm, whereas the other systems are active fire fighting systems. Selection guidelines are given in clause 4. (d) Nitrogen Injection Transformer Explosion and Fire Protection Systems Selection of this system is based on the importance of installation, criticality of availability of power supply and permitted restoration time of transformer after failure of transformer. The fire detection system only detects an incidence of fire and sends an alarm to other water based systems, which operates only after fire, whereas nitrogen injection systems are designed for automatic tank explosion prevention apart from fire extinguishing. Selection guidelines are given in clause 4 and details in clause 7.4 . 3.2 Indoor Transformers (a) For indoor installation with oil filled transformers rated at more than 75 kVA, each of the transformer should be located in a vault. Transformer vaults should preferably have a minimum fire resistance rating of 3 hours but where transformers are protected by water spray and Carbon dioxide systems, construction with one hour rating is adequate. Facilities for remote monitoring of vault temperature shall be provided. (b) It is recommended that a trapped floor drain be provided which discharges burning oil to a safe location. A fire door and a noncombustible curb at least 100 mm high should be provided at each doorway. Switchboards etc. should be physically separated from the vaults and the latter should never be used as offices, filing cabinets, work rooms etc. (c) Adequate ventilation should be provided in the transformer vault. Self cooled transformer should be separated by 600 to 750 mm to permit free air circulation. It is recommended that air filters be provided on all vents wherever the installation is located in dusty, high pollution areas. The vent should automatically close in the event of a fire in the vault. 3.3 General The requirements laid down in Section 5 of Tariff Advisory Committee’s “Regulations for the Electrical Equipment of Buildings” and Section 7.9 of IS : 10028 (part II) shall be followed for all transformer installations. In addition, the following measures are recommended for cables: 376 Manual on Transformers (a) The power cables entering the transformer shall be coated with fire resistant material in the immediate vicinity of the transformer cable box entry so as to prevent spreading of fire from or to the transformer cable. (b) Cable trenches shall be filled with sand to prevent spread of fire. It is recommended that trenches of more than 1000 cm2 cross-sectional area be provided with incombustible barriers at intervals not exceeding 45 meters. The barriers shall be at least 50 mm in thickness and of the same height as the cable trench. The cables shall be carried through holes in the barriers which shall be made good thereafter to prevent passage of fire beyond the barriers. 4.0 INSTALLATION REQUIREMENTS: SPECIFIC A summary of various recommendations for fire protection and fighting systems for indoor and outdoor transformers is given below: 4.1 (a) (b) Fire Protection for Outdoor Power Transformers Size (each) Under 10 MVA 10 to 100MVA Number One or more One only (See Note 2) More than one Transformer Protection Hydrant protection Hydrant (protection with water spray)Equipment as per IS -3034: OR Nitrogen Injection System 1. 2. 3. (c) Above 100 MVA One only 4. (See Note 2) More than one 1. (Sec Note 3) 2. (d) Above 200 MVA One only (See Note 2) 1. More than one (Sec Note 3) 2. Provide a 7.5 m clearance between units and 60 mm hose with two portable spray nozzles: or Masonry barriers between units with 60 mm hose and two portable spray nozzles: or Fixed automatic water spray with separate piping for each unit or Nitrogen Injection System for each unit. Hydrant (protection with water spray) Equipment as per IS -3034 or Nitrogen Injection Injection System Fixed automatic water spray(mulsifire) separate riser and piping for each unit; or Nitrogen Injection System for each unit and Provide a 7.5 m clearance or masonry barriers between units. Hydrant (protection with water spray) Equipment however, fixed water spray protection may be desirable in addition. as per IS -3034 or, Nitrogen Injection System Fixed automatic water spray with separate piping for each unit as per IS 3034 or Nitrogen Injection System for each unit Provide a 15 m clearance or masonry barriers between units. Notes 1. The enclosure should consist of a masonry barrier with wing walls of the same height extending 0.6 to 1 m beyond the transformer, including any radiators and tapchanger enclosures. The enclosure should also be provided with a roof of equal fire resistance to the walls. Where there are important or high value bus structures exposed to a transformer oil fire and/or electric service or production could be interrupted for an extended period resulting in a large loss, a fixed automatic water spray system should be provided to minimize the physical damage from fire and reduce the downtime for repairs. Guidelines for Fire Protection of Power Transformers 377 Multiple transformer of 100 MVA and above may be protected as single units if separated by a minimum of 35 m. For those separated by a distance between the minimum clearance shown and 35 m transformer protection may be either a barrier or fixed water spray. Cables, isolated bus duct, or cable tray penetrating an exposed wall should be sealed with a fire barrier or stop. Ventilation louvers should be relocated to an unexposed area. “Wherever water spray nozzles are provided, the nozzle should be separated from the transformer by over 4 in for voltages below 250 kV. For voltages beyond this and for solid hose streams, this distance varies with factors such as water pressure, wind velocity and direction, size of nozzle etc. 4.2 Fire Protection for Indoor Power Transformers Typical size of Transformer (a) (b) Upto 100 kVA Upto 30 MVA Type of Building/ Transformer encl. (See Note 1) Protection (Sec Note 2) Combustible Automatic sprinklers. Alternatively, use dry type transformers. Non-combustible Usual first aid protection. Non-combustible Combustible 1. Fire resistive vault 2. 40 mm fire hose with two water spray nozzles; and 3. Usual portable extinguishers. Or, Nitrogen Injection system. 1. Fire resistive vault, and 2. Automatic sprinklers, water spray, CO2 system Or, Nitrogen Injection System (c) Above 30 MVA Any & upto 100 MVA 1. Fire resistive vault, and 2. Automatic sprinklers, water spray, CO2 system. Or, Nitrogen Injection System (d) Above 100 MVA Any 1. Fire resistive vault 2. Automatic sprinklers water spray, CO2 system Or, Nitrogen Injection System Notes 1. Combustible and non-combustible buildings are generally classified based on type of construction materials used and type of occupancy. 2. This protection applies for all new installations and at existing locations when a large loss possibility exists without improvement. 3. If transformers are of unusual importance or large (above 10 MVA) located within fire resistive transformer houses or vaults, install fire resistive walls between units to reduce the exposure to adjacent transformers unless automatic fixed extinguishing systems of 7.5 m clearances are provided. 378 5.0 Manual on Transformers RECOMMENDED MAINTENANCE AND TESTING PRACTICES It is essential to monitor certain parameters to check the healthiness of the transformer and to minimise fire risks. The parameters to be checked and their frequencies shall be as brought out elsewhere (Section CC) in the transformer manual. Some of the parameters more relevant to transformer fires are discussed below. 5.1 Oil Leakage Oil leakage from transformer tank, bushings or radiators may become sources of major fires. It is recommended that all transformer installations be inspected daily for leakage of oil. Any leakage detected should be immediately attended to. In case of excessive leakage the transformer should be de-energised and repair work carried out. 5.2 Hot-oil Circulation During hot-oil circulation in the transformer, it must be ascertained that all combustible materials are kept at a safe distance from the transformer. The transformer shall be covered with noncombustible materials. Under such condition, it is essential that the transformer is kept under close watch. 5.3 Terminal Equipment Sparks from improper terminal connectors and neighbouring fuses etc., falling on the transformer can cause great fires. To prevent such occurrences it is recommended that the terminal connectors be regularly inspected for over-heating/sparking. Infra-red temperature scanners can be used for this purpose. It is also recommended that fuses be installed at a distance from the transformer such that sparks generated during their operation will not reach the transformer. 5.4 Transformer Oil The condition monitoring of transformer oil can give valuable insights into the healthiness of transformers. It is recommended that dielectric strength, acidity and oil tan delta (90°C) be monitored continuously and detailed investigation be carried out whenever any of these characteristics indicate signs of deterioration of oil quality. 5.5 Housekeeping The importance of good housekeeping and cleanliness in reducing fire hazard cannot be overemphasised. Many fires have been caused by oil drips and collection of rags in dirty cluttered surroundings. It is very important to remove possibilities of such fires, by ensuring that the installations arc spacious and the vacant spaces are periodically cleaned to remove obstructions to ventilation and movement of personnel. 6.0 FIRE FIGHTING EQUIPMENT In addition to the fire protection systems described above, it is essential to provide primary fire fighting equipment for every transformer installation. The equipment required for indoor and outdoor installations should be as per following guidelines : 379 Guidelines for Fire Protection of Power Transformers Typical size of transformer For the first two units 45 ltrs. foam type extinguishers 2 llrs. from type extinguishers Sand & water buckets Upto 20 MVA 2 4 Upto 50 MVA 3 6 50 MVA 4 8 2 each (Indoor) 4 each (Outdoor) 2 each (Indoor) 4 each (Outdoor) 6 each For every additional two Units or part thereof 45 ltrs. 2 ltrs. Sand & foam type from type water extinguishers extinguishers buckets 1 2 2 2 2 2 2 each (Indoor) 4 each (Outdoor) 2 each (Indoor) 4 each (Outdoor) 6 each Notes 1. The sand and water buckets shall be of 9 litre capacity each and shall have round bottoms. 2. Sandpits of adequate sizes shall be provided in central location for catering to any sand requirements. These pits should always be kept filled with dry sand. 7.0 FIRE FIGHTING SYSTEMS Some of the major types of fire fighting systems are discussed below: 7.1 Automatic Mulsifire System This system is widely used for fire fighting of outdoor transformers. Fire detectors located at various strategic points are used to sense high temperature near the transformer. If the temperature exceeds the set value the automatic mulsifire system sprays water at a high pressure on the surface of the transformer to control fire on any burning oil spilled over. Various subsystems are used to make a complete mulsifire system. (a) Main Hydrant: This is used to carry the water to various parts of the switchyard or transformer substation and forms the backbone of the system. Sturdy corrosion free pipes and valves should be used for this purpose. The materials should be able to withstand fire for a reasonable duration. (b) Fire Detectors: Fire detectors can either be thermocouples or specially designed bulbs which burst when a high temperature is applied and release any valves or checking device to start the water spray. (c) Ring Mains and Nozzles: Ring mains which surround the transformer are provided to feed the water to the nozzles at various levels. Since the water pressure is high, the ring mains should be designed to withstand this pressure. Nozzles should be located such that the water spray, in the event of a fire, envelopes the entire surface of the transformer. The whole system should be periodically checked to detect any leakages. (d) Pumps: Pumps are provided to fill the hydrants initially and to maintain its pressure. Pumps driven by electrical motors are a standard provision; however, the standby pumps should preferably be diesel engine driven. It is recommended that the main and standby pumps in a pump house be segregated. 380 7.2 Manual on Transformers Sprinkler and Hydrant System This system is similar to the mulsifire system but water in this case is sprayed on the transformer body at lower pressure, Hence the name sprinkler. This system is normally not capable of extinguishing large fire. For this reason, it is desirable to connect an alarm to the sprinkler system. Whenever the sprinkler operates an alarm is given to signal requirement of additional fire fighting arrangement. The auto-starting pump valves and alarm must be periodically checked. 7.3 Foam System This system is used to cut off the oxygen supply to the burning parts and thereby extinguish the fire. For this it is essential that the entire surface of the burning part or oil be covered with foam. The system includes a foam compound chamber, which contains the compound. In the event of a fire, this compound comes in contact with water and air to form foam. This foam is then sprayed onto the burning parts. The compound chamber should be corrosion proof and the valves should be periodically tested to ensure their healthiness. 7.4 Nitrogen Injection System for Transformers /Reactors 10 Mva and above Nitrogen injection system shall be used to prevent the transformer explosion and possible fire, in the case of internal fault and as such acts as fire preventer. In certain cases, tank explosion cannot be prevented and transformer oil catches fire. In such cases and also in the event of fire by external causes, it shall act as firefighting system. In either way it shall protect the transformer and eliminate or minimize the post fire damages. Thus, System shall be suitable for protecting the transformer tank from explosion and also transformer, OLTC and cable box from fire. The system operation shall be automatic and also be made to operate with remote control from control box and manually from extinguishing cubicle in the event of power loss. 7.4.1.1 Operation On receipt of signals, e.g Differential protection parallel with Fire detector, Buchholz (surge) parallel with PRV and transformer/reactor isolation signals, a predetermined quantity of oil drain shall commence and simultaneously Nitrogen shall be injected at a pre-determined flow rate to create stirring action and to bring down temperature of top oil surface below ignition point and shall extinguish fire within shortest possible time, say, 30 seconds. Transformer Conservator Isolation Valve (TCIV) shall block oil passage and isolate conservator tank oil and shall prevent escalation of fire. System shall have following characteristics: • • • System shall operate in automatic, remote and manual mode in the event of power failure. System shall have provision of testing on live transformers to ensure healthiness at all times. System shall have interlock to ensure operation of system only after transformer electrical isolation to avoid nitrogen injection in energized transformer. Guidelines for Fire Protection of Power Transformers • • • • • • • 381 System shall have mechanical locking arrangement for nitrogen release system as well as oil drain to avoid unnecessary operation during maintenance and /or testing of the transformer and /or system. System shall have provision to monitor nitrogen injection pressure as well as cylinder pressure. Pressure monitoring switch for back-up protection for nitrogen release as redundancy to first signal of oil draining commencement for Nitrogen release shall preferably be provided. System shall have individual mechanical release devices and provision for oil drain and nitrogen release to operate manually in case of operation DC supply failure. Nitrogen release scheme shall be designed in such a way that the nitrogen gas shall not enter the energised transformer/reactor tank even in case of passing/leakage of valve. Individual system component/equipment should preferably not have working voltage other than Station DC Voltage.AC-DC/DC-DC converter shall not be used for reliable operation. All outdoor panels / equipment shall be of IP55 protection class. Schematic diagram of typical Nitrogen Injection system, is shown in Fig.1 Fig.1 382 Manual on Transformers Pipe layout of Nitrogen Injection system as suggested by Supplier of Nitrogen injection system shall be provided by transformer manufacturer typically shown in Fig.2 Fig. 2 • Major System components Nitrogen injection system shall broadly consist of the following components. However, all other components which are necessary for fast reliable and effective working of the fire protective system are also provided. (a) Fire Extinguishing Cubicle (FEC) : shall be mounted 5-7 m away from transformer / reactor or placed next to the fire wall if fire wall exists. Cubicle shall be suitable for outdoor as well as indoor installation. It shall have hugged split doors fitted with high quality tamper proof lock. Cubicle shall contain nitrogen gas cylinder, mechanism for oil drain and nitrogen release with essential back up of pressure switch for operation alongwith control unit (b) Control box : shall be placed in the control room for monitoring system of operation, automatic control and remote operation. All individual components,instruments shall work on station DC supply. Following alarms, indications, switches, push buttons, audio signals etc. shall be provided : System on, Oil drain valve closed, Gas inlet valve closed, TCIV(see below) closed, OLTC oil surge relay trip, Oil drain valve open, Extinction in progress, Cylinder pressure low, Differential/fire detector, Buchholz relay /PRV / RPRR trip, Master relay of transformer / reactor trip, System out of service, Auto/ Manual / Guidelines for Fire Protection of Power Transformers 383 Off, Extinction release on / off, Lamp test, Visual/ Audio alarm, Visual/ Audio alarm for DC supply fail, Fault in cable connecting fault : fire detector, differential relay, Buchholz relay, PRV / RPRR, transformer /reactor trip, TCIV alarm etc. (c) Transformer Conservator Isolation Valve : (TCIV) shall be fitted in the conservator pipe line, between conservator and Buchholz relay shall operate for isolating the conservator during abnormal flow of oil due to rupture / explosion of tank or bursting of bushing including tank depressurization during system operation. The valve will not isolate conservator during normal flow of oil during filtration or filling or refilling, locking plates shall be provided with handle for pad locking to ensure no movement of valve position during service and filter position. It shall have proximity switch for remote alarm and indication glass window for visual inspection similar to Buchholz glass inspection window for physical checking of the status of valve. The TCIV shall be of the best quality as malfunctioning of TCIV could lead to serious consequence. The closing of TCIV means stoppage of breathing of transformer/reactor. Fire survival cable connecting TCIV shall be terminated in transformer marshalling box. (d) Fire detectors : shall be specially designed to generate signals after sensing higher temperature. (e) Piping : Heavy duty pipe connecting the transformer/reactor tank for oil drain, and for nitrogen injection shall be provided. Pipes connecting oil tank laid underground, shall be preferably be galvanized, medium duty. (f) Cables:Fire survival, copper cables (capable to withstand 750° C.),Fire Retardant Low Smoke (FRLS), copper cable of 12 core x 1.5 sq. mm and 4 core x 1.5 sq.mm size shall preferably be used for interconnection. 7.4.1.1 Transformer/reactor manufacturer shall provide following provisions : (a) Oil drain opening with gate valves on transformer / reactor tank at upper portion at ~120 mm from top cover. (b) Nitrogen injection openings with gate valves on transformer / reactor tank at bottom side of tank. (c) Flanges with dummy piece in conservator pipe between Buchholz relay and conservator tank for fixing TCIV. (d) Fire detector brackets on transformer / reactor tank top cover. 384 (e) Manual on Transformers Mounting support/ frame on tank side wall for signal box. 7.4.1.2 Technical particulars Typical technical parameters of a Nitrogen injection system are as follows Fire extinction period from commencement of nitrogen injection 30 s (Max.) Total time duration to bring oil temperature below flash point 30 minutes (Max.) Fire detector’s (quartz bulb) heat sensing temperature 141 deg.C Transformer Conservator Isolation valve 40 l / min setting for normal operation (valve should not close) to ensure no obstacle for transformer breathing Transformer Conservator Isolation valve setting for operation during abnormal flow of oil due to rupture / explosion of tank or bursting of bushing / oil drain during system operation 60 l / min (Minimum) Capacity of nitrogen cylinder 10 m3 gas at pressure of 150 kg/cm2 upto 60000 litres oil capacity of transformer / reactor tank and 20 m3 gas at pressure of 150 kg/cm2 above 60000 litres oil capacity of transformer / reactor tank . Power supply As per substation DC voltage. For Control Box For Fire extinguishing cubicle for lighting 230V AC 385 Guidelines for Fire Protection of Power Transformers 7.4.2. Information shall be provided to system equipment manufacturer in the following typical format Customer: Work Order Nr/Tender Nr/Project details : Quantity : Manufacturer : MVA Rating : Oil Qty in tank : Litres Oil Qty in Conservator : Litres End User : Sr. No. : Voltage ratio : Power Supply (Pl tick √ ): Substation/ Control room D.C. Supply : 220V 110V A.C. Supply available : Control room Marshaling box Conservator pipe diameter : mm Conservator pipe angle : Spare contacts in relay panel : Differential RPRR Trip 1 Nr relay trip 1 Nr NO potential NO potential free OLTC oil surge relay trip 1 Nr. No potential free 48V 24V Single phase, 240V, 50 Hz degree Buchholz Trip 1 Nr 1 Nr NO potential Free Free Master relay Trip 1 Nr NO potential fre Other Free PRV Trip 1 Nr NO potential Free Restricted Earthfault relay 1 Nr NO potential Free HV & LV circuit breaker trip, 1 Nr each NC potential free NO : Normally Open, NC : Normally Close, RPRR : Rapid Pressure Rise Relay Main Dimensions of tank : L= mm B= mm H= mm A= mm Bevelled cover Built-in On load Tapchanger : Yes No Oil Filled Cable Box : No Yes Yes No If Yes : Provide general arrangement drawings Cooling details : If OF type NR OF PUMPS : HEAD : Flow : KW/HP : BACK FLOW WHEN PUMP SWITCH OFF : Installation : New Distance from Trs to control room through cable trench : Distance from Control room to Relay Panel through cable trench Additional information : LPM Post mtrs mtrs 386 7.5 Manual on Transformers Other Systems and Selection Various other systems are also available for fire fighting of transformers notable among them being: Carbon dioxide and clean gas agent system for indoor or enclosed installations. However ,limited experience of these systems exists. 8.0 STANDARDS AND GUIDELINES RELEVANT TO TRANSFORMER FIRE PROTECTION : (a) IS: 10028 (Part II) : (b) Tariff Advisory Committee : (c) BS 5306 Part 4 (d) NFPA-15 (e) NFPA 70 : : : (f) Central Electricity Authority, The : Gazette of India, Extraordinary 2010 : : Code of practice for selection, installation and maintenance of transformer. Regulations for the electrical equipment of buildings. Carbon dioxide fire fighting systems. Water spray fixed systems. Fire protection of transformers and transformer vaults. Technical standards for constructions of sub stations and switchyards. Technical standards for construction of Thermal Generating Stations. Safety provisions for electrical installations and apparatus of voltage exceeding 650 volts SECTION FF Guidelines for Repair of Power Transformers at Site SECTION FF Guidelines for Repair of Power Transformers at Site 1.0 INTRODUCTION Transformers are amongst the most efficient equipment made by mankind and as with all manmade equipment the power transformer also fails. The reasons for the failures can be attributed to design, manufacturing, operational and maintenance issues. Design, manufacturing and process validation is ensured during the routine and type testing of the transformers at the manufacturer’s works. Over the years the manufacturers have had better understanding and reduction in the knowledge gap regarding transformer technology and this led to improved and compact designs. The inclusion of newer tests in the test schedule verify the enhanced knowledge base. With rapid growth of the network, increased fault levels and faster ageing, the equipment are subjected to increased operational stress. Increased consumer awareness, privatization, competition and profitability has forced utilities and generation companies to focus on longer life cycles and increased revenue generation from every investment made. With the increased cost of acquisition the options of refurbishment and repairs at site have gained prominence and are commercially lucrative also due to the faster turn around for product to be back into action. In this chapter effort has been made to focus on various aspects that need to be kept in mind when planning for repair at site. The focus areas are diagnosis of the fault and localization of the fault zone to determine nature and extent of repairs. Based on this assessment the logistic requirements for the repairs the quantum of work involved and various technicalities involved in the process are to be carried out. 2.0 ON-SITE REPAIRS Performing on-site repairs on a transformer has multiple advantages and the major ones are described in this section. 2.1 Advantages of Site Repairs 2.1.1 The damaged equipment can be brought into service much faster than repairs at the factory by way of saving in to & fro transportation time as well as completion of repair-tocommissioning carried out in single set-up. 2.1.2 On-site repair costs considerably less than repair at the factory particularly, for large power transformers in remote areas. 389 390 Manual on Transformers 3.0 REASONS FOR RELUCTANCE TOWARDS SITE REPAIR OF POWER TRANSFORMERS 3.1 The reluctance of the utilities/customers for encouraging repairs at site are listed below: (a) Validation techniques of the repair work carried out at site (customers generally have a mind-set that high voltage test is the only validation procedure and this testing will not be available in case of repair at site). Concern about drying out of the transformer at site, after the repairs have been carried out and the validation of the end point of drying out. Non-availability of the logistics arrangement which the owner has to necessarily make for effecting the repair at site and lack of expert manpower to assist in carrying out the work. Other reasons could be : (i) Exorbitant hourly rates of technical manpower deputed by the reputed manufacturers generally with no ceiling, for carrying out repairs at site. (ii) Unavailability of suitable storage facility for oil, equipment for drying out of power transformer at site and other logistics at site. (iii) Guarantee and warrantee requirement to match that of new transformer or in case the repairs are carried out under controlled conditions at the factory. (iv) Unavailability of suitable tooling for carrying out transformer repairs. (v) Lack of understanding of changes in maintenance methods as transformer insulation ageing would have occurred. (vi) Lack of covered high roof space for repairs, since repairs at site need dry and clean conditions. (vii) Lack of indepth knowledge about transformer manufacturing, practices and various stage checks. (viii) Play safe attitude and not to take any chance on performance after repairs. (ix) Demonstrated high voltage withstand capacity (as guarantee) of repairs carried out. From the manufacturer end the reluctance is because of the following reasons: (b) (c) (d) 3.2 (a) (b) (c) Diversion of manpower and tooling from the regular manufacturing activity. Lack of confidence on alternative drying out methods and the availability of suitable instrumentation for quantification. Other reasons are (i) Their engineers would have difficult time in delivering results and perform in conditions not under their control. (ii) Though no guarantees may have been agreed but reputation of the company remain at stake. Guidelines for Repair of Power Transformers at Site 391 (iii) Accuracy and quality of available history & available performance data. (iv) Disruption in production activity if manpower is sent to site. (v) Repair activity is not treated as a regular business activity. (vi) The quantum of work is double i.e., once for dismantling and the second for assembly which is generally not well appreciated by the end user. (vii) Difficulty of duplication of drying out process at site and reliability of instruments/ the data collected. 3.3 Repairs of power transformers at site is controversial and is still a subject of discussion amongst engineers. While examples of successful repairs of power transformers up to 400 kV class exist, still most of the utilities and the manufacturers are shy of taking up this activity at site. One of the most common reasons for the utilities not promoting this activities is the nonavailability of high-voltage test equipment at site for HV testing after repairs. The fact is, that this is a type test and a manufacturer during the manufacture of the power transformer even at his works is not using any high voltage tests in the production process or as part of in-process quality checks. However, the trend world over is for site repairs. Mobile High Voltage test facilities and mobile Low frequency heating facilities are being created to address two main issues of Insulation drying and high voltage testing at site. 4.0 SITE REPAIR CATEGORIES AND REQUIREMENTS 4.1 Repairs can be categorized in to : (a) Minor repairs. (b) Major repairs 4.1.1 Minor Repairs Generally repairs involving low volume of oil draining and attending to minor defects in the transformer like replacing a defective bushing, bushing CT, gasket, bolt or burnt-out connection at a bushing or small repairs of diverter switches. 4.1.2 Major Repairs These repairs generally involve draining of main tank oil for accessibility to the problem area and arc mostly in the core-coil assembly i.e., Winding, Core, OLTC selector switch. It involves, total oil draining, storage, oil filling under vacuum and during out activities for restoration of the transformer to service. 392 4.2 Manual on Transformers Site Repair Requirements 4.2.1 Power transformer repair at site is in no way any different from the repair process followed at the factory. The repair will be successful if all the quality check parameters adopted at factory are religiously duplicated at site and so also the tooling and the environment. 4.2.2 The information required for decision on carrying out repair at site is given below : (i) Nature of the fault. (ii) Location of the fault. (iii) Extent of damage due to the fault. (iv) Scope of work based on information collected. (v) Material required for carrying out the repairs. (vi) Sequencing of activities during the repairs. (vii) Time required for various repair activities. (viii) Facility and tooling needed at site for repairs. (ix) Suitable equipment for drying out transformer at site. (x) Validation method of the repair to be carried out. 4.2.3 The scope of work determination is the most important activity for site repair as this will govern the material, tooling and facilities required for the successful completion of the work. The whole scope of work can be divided into following main activities : • Pre repair work assessment. • In process stage checks and review of repairs work assessment. • Post repair activities including the final in-process checks. • Drying out and commissioning activities. 4.2.4 After the fault detection techno-economic assessment is done in respect of down time and cost of repairs at site, to the overall time (including transportation time) and cost of repairs at the factory and it is generally found that the down time is an important aspect than the rather pure economics. As soon as the service interruption has taken place restoration of the supply becomes the prime activity. To determine the health of the transformer and its suitability for the immediate restoration requires that the condition of the equipment be determined to the maximum possible accuracy. The manufacturer can also contribute in this assessment. It has been generally observed that after the disruption has occurred the condition determination is not done systematically as required for correct assessment. Most of the time it is attributed to lack of equipment, test instruments and at times required expertise of the available technical persons. 5.0 DIAGNOSTIC TESTING OF THE TRANSFORMER 5.1 Record of the system condition at the time of outage as well as immediately before is one of the most important activity. The load conditions at the time of disruption i.e., the load current, voltage, the status of the various equipments and protection in service, any fault history Guidelines for Repair of Power Transformers at Site 393 etc. and also weather condition needs to be recorded. Many failures in transformers have been found to be due to the abnormal system operation and/or protective device malfunction therefore the status of all the protections is extremely important and need to be correctly recorded. 5.2 Immediately after the interruption, temperature of the top oil may be recorded, and the oil samples must be collected for measuring moisture content in oil, Dissolved Gas Analysis (DGA) and for furfural content specifically for old transformers (in service for more than 10 years). As these tests are trending tests, the earlier test readings will act as reference and help in reaching some conclusion. 5.3 The fault detection comprises of the combination of the following electrical tests after the transformer has been shutdown to determine the cause of the failure and the location therefore. • Magnetising Current. • Magnetic Balance. • SFRA signature test • Voltage Ratio. • Single Phase Short Circuit Current. • Winding Resistance. • Vector Group. • OLTC Contact Resistance. • Insulation Resistance and tangent delta of windings and condenser bushings For diagnostic testing it is mandatory to repeat all the tests in all the possible tap positions as fault can exist in isolated condition even with normal operating condition. These electrical tests are indicative of the location of the fault and to further accurately establish the fault some invasive tests may need to be done. These invasive tests are generally done at the winding level after isolating the OL TC to further localize the fault and if OLTC is the culprit then also these invasive tests help in locating the fault. Condition assessment helps in determining the scope of work and the tests to be followed for determining the healthiness of the various aspects of the transformer are to be so selected that at least two diagnostic tests should indicate the same parameters/findings. 5.4 Most of the manufacturers recommend that the testing to determine the condition of the equipment after operational disruption be done in single phase and/or at reduced or elevated voltage values to exactly determine the extent of the problem. Example 1. A fault at 240 volts may be showing no abnormality but when carried out at 2 kV level can give a totally different value. Magnetising current test and magnetic balance test results have been known to show variations in values in the case of interturn and interdiscinsulation failures. Example 2. A high winding resistance value in a winding having the OLTC connected can mean improper contact for bushing termination or defective joint or improper OLTC contact in 394 Manual on Transformers the selector switch or the diverter switch. The improper diverter switch contact will reflect in multiple test results either in the odd position or the even position for in-tank OLTC construction based on the Dr. Jansen principle. For different OLTC construction the result may reflect in only one position and may be confusing and require invasive testing. Example 3. A 160 MVA power transformer in prefect working condition for 15 years is taken in for overhauling due to low IR values of the insulation. After drying out it is found that the winding resistance results of the transformer are erratic and indicated some problem. A vigorous motion given to the tap connections restores winding resistance values to near normal values. On investigation, stripping the taping of the tap lead joint revealed a burnt out joint and the best part was that the DGA history of the transformer indicated no problem of this type. The other finding here was that the transformer was being routinely tested for winding resistance only in three positions first, nominal and the last position. 5.5 For transformers in the network with history of tripping under fault conditions the FRA is an invaluable tool for the determination of the condition of the transformer. The FRA record of the transformer is to be maintained meticulously and after the transformer faces a tripping, the FRA test is to be conducted to determine any change in the geometry of the assembly or loss of clamping pressure of the winding. This helps in detecting small changes so that the repair/refurbishment/overhauling/replacement plan can be done properly to avoid unpleasant surprises. 6.0 REPAIRING OF TRANSFORMERS 6.1 Even some 400 kV class transformers have been successfully overhauled at site involving changing of old gaskets, changing of oil, drying out of the job for IR value improvement and are in successful operation. Though not in the repair category it clearly demonstrates the adequacy of the drying out method, if properly done at site. 6.2 The repair process needs either the involvement of the manufacturer’s engineers or skilled manpower experienced with similar activity at some manufacturing location. Different repair activities require different materials, tooling and facilities. The facility requirement is also governed by the manufacturing methodology adopted for the transformer manufacturing. Large power transformers are generally manufactured in three different configurations (a) Cover mounted construction (b) Cover free construction. (c) Cover free and bell cover construction. These constructions have their own advantages and disadvantages and the choice of the method of construction is either specified by customer or is decided by the manufacturer. The manufacturer is conversant with all the construction configurations and has facilities and tooling to enable manufacturing of all the configurations. Guidelines for Repair of Power Transformers at Site 6.3 395 Procedure of Major Repairs Major repairs in the core coil assembly are categorized in to three main types : (a) (b) Core components. Winding and its connections. (c) OLTC selector switch and its components. For various components/mountings fitted on the transformer, the major repairs are in : (a) High voltage bushings. (b) OLTC diverter switch and its components. These repairs require draining of oil, internal inspection for determining the extent of the problem and remedial action thereof including the components required to be procured. Generally the problem solving does not take much time compared to the time required for the pre repairing and post repairing activities leading to successful commissioning. Winding repairs are the most complicated of all the repair works carried out and generally require replacement of the defective coils or the whole composite coil of the winding. For winding replacement the maximum tooling and logistics arrangement is required including the new winding from the manufacturer or third party supplier. Winding replacement activities are preferably to be carried out under the guidance of manufacturer though such guidance is now a days available from third party sources also. During the repair activity some improvements or refurbishment can be also done to increase the reliability of the equipment and ensure trouble free operation. 6.4 Facility Planning for Site Repairs 6.4.1 Most power stations are adequately equipped for handling large and heavy weight equipment/ transformer and generally have a Service-bay equipped with crane and working area. In case of non-availability of overhead crane facility, adequate crane capacity needed for the repairs should be determined and hired out. At this point of time the need is to assess the following : (i) (ii) (iii) (iv) (v) (vi) Location where the refurbishment is to be carried out, preferably covered space. Adequate space availability for dismantling the transformer and storage of the dismantled transformer parts. Crane of adequate capacity with enough working space by height clearance. The selected area should be dry, away from any open source of water, dust free and shall allow additional covering if necessary. Power supply point of adequate capacity for lighting and supply for various equipment operations like hydraulic crimping tools, brazing machines, pneumatic spanners heating, filter machines etc. The power supply should not be very near either as it is a fire hazard in case of any problems with the supply wire or electrical equipment failure. Generally the supply should have ELCB protection facility. 396 6.4.2 Manual on Transformers Lifting Facilities for Tank Cover Transformer manufacturers use various types of tank covers and each one has its own typicality of handling. Some of the points to be taken care of when planning for the crane and lifting facilities are given below : • For Top Cover Type Transformers (i) Weight of the top cover. (ii) Dimension of the cover. (iii) Dimension of the active part. (iv) Weight of the active part. (v) Height to be lifted for minimum clearance. (vi) Generally it is preferable to have vertical overhead cranes, in case other cranes are used then adequate boom height should be available and the clearance of the boom from the active part during lifting (i.e., if 40 ft high working height is required then with the mobile cranes the same distance is to be available from the top of the active part lifted such that the active part is clear of the tank top and not from the boom top). (vii) The top cover should be inspected for unbalance load distribution and all mountings on the top cover should be removed. (viii) The mobile crane should be rated at least 75% more than the maximum load to be lifted. (ix) Four nos. slings having total load lifting capacity atleast twice the maximum load to be lifted. (x) • The OLTC mounting to be checked and dismantling procedure selected. For Bell Cover Type Transformer (i) Weight of the bell cover. (ii) Dimension of the bell cover. (iii) Height to be lifted for clearance of at least 30 cms from the top most part of the active part assembly. (iv) Lifting height is to be calculated from the top of the bottom part of the bell cover and not from the ground level. (v) Generally it is preferable to have vertical overhead cranes, in case other cranes are used then adequate boom height should be available and the clearance of the boom from the active part during lifting (i.e., if 40 ft high working height is required then with the mobile cranes the same distance is to be available from the top of the active part lifted such that the active part is clear of the tank top and not from the boom top). Guidelines for Repair of Power Transformers at Site (vi) 397 Top cover should be inspected for unbalance load distribution and all mountings on the top cover should be removed. (vii) The mobile crane should be rated at least 50 % more than the maximum load to be lifted. (viii) Four nos. slings having total load lifting capacity atleast twice the maximum load to be lifted. (ix) • The OLTC will be mounted on the top cover but resting fork will be fitted on to the end frame. For Top Covers with the Active Part Fitted to it These are nearly similar to the transformers with the top cover type and the main consideration is that the weight to be lifted is now the weight of the active part plus the weight of the top cover. Also in these type of transformers OLTC etc. are all mounted on to the top cover and weight eccentricity can be experienced. The transformer is to be first inspected from inside and then the slings of different lengths may have to be used for balancing the same during the lifting of the job. 6.4.3 OtherFacilities and Related Requirements The place selected to carry out the repair and/or refurbishment activity should preferably be covered, but in case of unavailability of such space then a temporary cover around the transformer may have to be built such that the top of the same is removable and the crane hook can be brought in after removing the cover and the required activity is carried out. Also at some places it will be preferable to build a temporary cover such that the same can be rolled out to expose the transformer for work requiring crane access and then rolled back to cover the transformer again. The area/location for repair/refurbishment activity of the transformer should have adequate space so as to store the removed fitments from the transformer at the same place. Generally when the transformer is disassembled for repair/refurbishment lot of fittings and mountings on the transformer cover will have to be dismantled. Internally the active part may be earthed to the tank and in some of the older designs of temperature sensors the WTI CT are mounted as part of the active part and the CT connections for the heater are terminated at the end of the pocket as the heater is part of the pocket and not part of the meter. Also the place should not be near to any source of open water and also no water dripping should occur near by. The other facilities for site repair of the transformers and related requirements are tabulated below: 398 SI. No. Manual on Transformers Facility Requirements 1. Lighting (i) Should be provided with cool lighting as the internal environment is going to be warm and oil saturated. (ii) All light fittings should be provided with glass cover, Hand lamps are to be explosion proof type. (iii) The routing of the supply wires to the various light fittings and facilities should be preferably new wires and without joints, and all termination's to be properly secured. 2 Fans & ventilation (i) (ii) (iii) 3 4 Access method (i) Cleanliness (i) (ii) (ii) (iii) 5 Enclosure (i) (ii) (iii) 6 Power (i) (ii) (iii) (iv) (v) 7 Storage (i) Ventilation is of paramount importance. As the transformer exposed parts are to be protected from moisture. Ingress it is desirable that the internal environment be maintained hot and dry. Humidity control will ensure faster final drying out. The enclosure environment can be kept dry by injecting dry air through a dry air generator. The routing of the supply wires to the various light and fittings should be preferably new and without joints, and all termination’s to be properly terminated. Access to the enclosure is to be highly regulated, preferably as far as possible dirt carrying objects like shoes etc. are to be kept out of the enclosure, The access opening should not be directly open type, it should be at such a place that at least two covers should overlap each other. Shoes should be left out side. The area within the shed is to be covered with tarpaulin in case it is not hard surface and should be cleaned with vacuum cleaners only. The transformer areas where people are moving while working are to be cleaned at least 2-3 times daily with vacuum cleaning and any oil wet smears are to be cleaned with oil washing. The enclosure size should be such that it should accommodate all the materials being removed from the active part assembly at the same place. The structure should be rigid and the moving part should be light and arrangements should be such that after placing it should be easily tied to the main structure tightly. Height of the enclosure should be 1½ man height above the topmost part of the transformer tank. The power supply should be made available at two places near the supply work area but not within the enclosure. Half of all the fittings are to be supplied from one of the source and the other half from the other source. The light and power fittings from one source are to be fitted all around the enclosure and the second set in the same fashion. In case any one source failure the work enclosure will not get dark and adequate light and air circulation will be maintained. No live wire insulated or otherwise is to be routed over the transformer active part during the repair process and are to be only routed along the enclosure body. All insulation materials are to be wrapped in polyethylene, bound and tagged and stored in the sequence of their removal from the active part (e.g. : The part removed first from the winding is to be kept nearest to the winding and in front while the last opened object is to be located at the farthest so that it is also the last object assembled). (ii) The windings not to be handled are to be wrapped preferably with polythene during the working hours and removed(when subjected to any hot oil bath or treatment. (iii) All common blocks, winding clamping ring, winding pressuring /packing blocks, washers are to be tied and stacked to-gather and preferably kept soaked in oil drum. Guidelines for Repair of Power Transformers at Site 399 8 Safety (i) As the transformer active part insulation is a combination of paper soaked in oil it is a potential fire hazard and should never be left unattended. (ii) All lighting fixtures are to be of cool daylight type. (iii) No electrical connections arc to be made by twisting wires, they are to be made by male female connectors. (iv) Joints of wires taped with insulation tapes should be of bright colours so that any heating could be detected easily by dis­colouration. (v) The whole enclosure is a no smoking zone. (vi) Fire extinguishers both, of Dry chemical powder and CO2 gas type arc to be located such that there are no obstacles in the path. (vii) Asbestos sheets should be available handy to cover any ignited surface. (viii) No inflammable material should be stored near the work area. (ix) There should be no fire source near the work area. 9 Safety aspects related to working inside the transformer Working within the transformer require special skills, some of the common and important safety measures are: (i) All tools being taken inside the transformer tank must be tied at one end with some object outside the transformer tank or to the wrist of the person going to work inside the transformer. (ii) For climbing in and out of the transformer it is mandatory that rope ladders be used for climbing in and out of the transformer. At no point of time metallic ladders should be used as slipping can cause damage to manpower or the active part. (iii) Person working on or within the transformer should not carry anything metallic in the pockets like loose change, pens etc. All watches etc. are to be removed before entering the transformer tank. (iv) The clothes worn during the visit inside the transformer should not contain any metallic buttons etc. Also the belts are to be left outside as generally they contain metallic buckles. (v) Movement within the transformer should be very carefully done, so that body weight is not put on any cleates or various supports used within the transformer. (vi) The active part should not be disturbed without marking the present position at two or three reference points so that the various clearance are maintained as per the. manufacturer recommendations. (vii) No body weight should be put on the various connection leads of the winding to the bushing, tap coil to the OLTC etc. (viii) Any new material being fitted within the transformer particularly insulation materials made of pressboard or permawood should be first dried in a oven at 90° C for minimum of 12 hours preferably in a vacuum oven and then soaked in filtered oil for an equal duration. Thicker items require longer soaking time. 7.0 QUALITY ASPECTS DURING THE REPAIR OF THE TRANSFORMER AT SITE 7.1 The Joints Quality The connection phase consists of joining of the winding to various cables and their further termination on to the OLTC/OCTC or to the various bushings. For the termination of the winding to cables and the subsequent connection two process have been followed: • Jointing by brazing. • Jointing by crimping. 400 Manual on Transformers As the repair unit is generally oil soaked the repair by crimping is a better method. For jointing by brazing, resistance brazing for smaller dimension cables and copper conductors can be followed but for large dimension cables and multiple parallel conductors flame brazing is followed. The flame brazing process is well established and highly skilled workers arc required with adequate provision for safety to avoid any incident of fire hazard. The process also entails surrounding the nearby oil soaked insulation with asbestos sheets. After the brazing has been carried out the joint is to be cleaned of all bum marks and the brazing material spatters the joint is filed to bring about smooth finish so as to avoid any sharp points and subsequent damage to the taping. The crimping method is a cleaner and a safer process because no heating is involved but it requires custom designed ferrules and sockets to be crimped on and the crimping machine and joint qualification be carried out before the start of the jointing process. 7.2 Shrinkage Arresting after Drying Out This is a grey area in repairs but a mandatory process is to be followed for any repair jobs for the successful completion of the repair process. Various manufacturers have their recommendations regarding the methods to be followed for this process. A standard recommendation could be that after completion of drying out process the transformer is to be oil filled and the insulation soaked in for duration of 24 hours. After that the oil be drained and the job extracted and the coils clamped to required height. The job is then retanked and the drying out process repeated for 2 more cycles before final oil filling and circulation and preparation of the job for testing and final commissioning. The above mentioned recommendations are based on site experiences. 8.0 TOOLS REQUIRED FOR SITE REPAIRS A tentative list of tools required for the activities related to transformer repairing at site is given below: 8.1 Tools SI. No. Tools 1. Special tools for dismounting of the OL TC. 2. D shackles -1 ton 3. D shackles -5 ton 4. D shackles - Max. load /3, multiple quantities 5. Slings - 1 ton (wire rope/nylon) 6. Slings - 5 tons (wire rope/nylon) 7. Slings - Max load/3, (wire rope) 4 nos. 8. Pump- 1/2 hp Guidelines for Repair of Power Transformers at Site 9. Pump - 3 hp + starter 10. Hose 25 m long minimum (multiple quantities) 11. Extension board - 3 ph & 1 ph 12. Spanners DE type (size : 8-9 to 32-36) 13. Spanners ring type (size : 8-9 to 32-36) 14 Ring &Dli spanner 55 size 15 Box spanner (size : 8-9 to 32-36) 16 Torque spanner handles 17 Sledge hammer - 5 kg 18 Hammer - 1 kg 19 Mallet 20 Screwdrivers - various sizes 21 Hacksaw frame standard size 22 Hacksaw blades HSS type 23 Tommy bars -20 mm dia 3 ft long 24 Chisels: 1 in wide, 2’ wide 25 No. punch (8×8) various sizes 26 Flat files (small & regular 27 Half round files (small & regular) 28 Round files (small & regular) 29 Welding machine 30 Drilling machine 31 Grinding machine 32 Gas cutting set with cylinders 33 Pipe 3” dia 5 ft. long 34 Knives for paper cutting etc. 401 402 Manual on Transformers 8.2 Special Tools 1 Winding lifting arrangement 2 Lead cutting tools 3 Brazing machine 4 Brazing rods 5 Brazing torch nozzles and oxy-acclylene cylinders 6 Hydraulic Crimping Tools (cutting tools & crimping dies) 7 Stools for yoke unlacing of adequate height and wt. capacity 8 Nylon slings for tying the winding to lifting frame 9 Wooden / ms pallets for lamination storage 10 Fans 11 Torches 12 Vacuum cleaners 13 Lamination stack tying arrangement 14 Lamination stack tying arrangement 15 Heat insulation during brazing. 16 Dry air generator and compressor setup. 17 Scaffolding arrangement for access to the active part during 8.3 Test Instruments 1 2.5 kV test set 2 Megger 1 kV 3 Megger 5 kV motorised 4 Ratio meter 5 Winding resistance kit 6 Multimeters 7 Dew point meter Guidelines for Repair of Power Transformers at Site 8 Tan delta kit 9 Clampmeters 10 Variac 1 ph and 3 ph 11 Continuity tester. 9.0 CONSUMABLES REQUIRED FOR REPAIR / REFURBISHMENT JOBS SI. No. Materials 1. Crepe Paper Rolls -25 mm wide 2. Crepe Paper Rolls -15 mm wide 3. 20 mill Paper Rolls 4. PCB Sheets -1.5 mm thk. (impregnated) 5. PCB Sheets -3.0 mm thk. (impregnated) 6. Cotton Tape Rolls-1/2" 7. Cotton Tape Rolls-I" 8. Newar Tape Rolls- 9. Permali studs & nuts -10 mm 10. Permali studs & nuts -12 mm 11. Permali studs & nuts -16 mm 12. Permali studs & nuts -20 mm 13. 25 thk' PCB/PW off cuts (impregnated) 14. PVA glue 15. Fibre glass tube-core bolts 16. Fibre glass washer-core bolts 17. Oil drums with 209 litres each as per IS : 335 18. Common Blocks (various sizes) 19. Terelene Tapes 20. Cotton waste 403 404 Manual on Transformers 21. Cotton cloth (markin type) 22. Sand paper 23. Blanking plates-various sizes 24. Plastic sheets/polythene 25. Carbon Tetra Chloride (cleaning agent) 26. Wooden coil bobbins 27 Empty oil drums 28 Wooden boxes with locks 29 Plastic bags (small) 30 Plastic bags (medium) 31 Undelible markers 32 Tags (fibre) 33 String roll 34 Plastic buckets and mugs 35 Tarpaulins (large) 36 Themocoal foam - 8 mm thk. Profile 10.0 DRYING OUT OF TRANSFORMERS AT SITE AFTER REPAIRS Various methods are prevalent for carrying out the final drying out activity at site after the repairs have been completed. Some of them are briefly discussed below: 10.1 Drying Out by Hot Air Blowing and Vacuum Pulling In this method the active part is heated up by blowing in hot air into the tank and the temperature of the active part is raised to a higher temperature of around 70° to 75° celcius and then vacuum pulling is done for extraction of moisture from the insulating materials. Multiple cycles of heating and vacuum pulling, are carried out till the termination criteria is reached. Oxygen present in the hot air has a tendency to oxidize the thinner insulation faster than the thicker insulation. This method is not recommended for CCA above 66 kV class. 10.2 Drying Out by Hot Air Blowing Inthis method hot air is blown into the main tank and the active part is heated up, the temperature of the hot air is themlostatically controlled at the blower end at around 70° to 75° Celsius and Guidelines for Repair of Power Transformers at Site 405 the drying process is continued in this manner. This method is adapted for transformers whose tank cannot withstand high vacuum. The hot air removes moisture from the insulation of the active part and the vapour pressure difference ensures removal of moisture from the insulating material to the hot air. The presence of hot oxygen in hot air tends to oxidize the thin insulation faster than the thick insulation. The process is continued till the termination criteria is achieved. This method is recommended only for 33 kV class CCA and below. 10.3 Heating in Inert Environment and Vacuum Pulling In this the transformer tank is first evacuated and then filled with dry N2 gas and the active part heated up by applying external heat to the tank body. The heating can be by means of Halogen or infrared lamps or by winding some turns of cable on the tank body and passing low voltage and high current through the cable. The temperature of the active part is raised between 80° to 90° Celsius and then vacuum pulled on the tank for faster extraction of moisture from the active part insulation. Due to the inert atmosphere this is the most widely adopted method and this ensures that the oxidation of the insulation paper is minimized. Multiple heating and vacuum cycles are repeated till the proper drying out criteria is achieved. 10.4 Heating by Hot Oil Circulation and Vacuum Application In this method the heating of the active part is carried out by circulation of hot oil. The maximum temperature of the oil can be 60° Celsius. On reaching the desired temperature the oil is drained very fast from the tank and vacuum applied on the tank. The fast draining is necessary to minimize the heat loss during the oil draining operation. The heating of the oil is carried out by high vacuum oil filter machine and during the process the moisture carried by the oil is removed during the passage through the filter machine. Multiple heating and vacuum cycles are repeated till suitable drying out criteria is achieved. 10.5 Heating by Oil Spray and Vacuum Pulling In this method hot oil is sprayed on to the active part through pipes mounted inside the tank with multiple nozzles in them. During this process vacuum is also applied into the tank to remove the moisture from the insulating material. The heating through oil spray is limited to again up to 60 degrees for the oil but this method is least prevalent as the installation of spray pipe is required inside the tank, but widely followed in the eastern European countries. The main advantage of this process is the simultaneous flushing of the moisture from the insulation by the oil and also the cleaning of the insulation due to the oil flow on the insulation. 10.6 Drying Out by Hot Oil Circulation Generally used for small transformers upto 10 MVA and 33 kV class of transformers where the insulation mass is not much and the tank is not designed to withstand full vacuum. Generally special transformers like furnace and rectifier transformers are also in this category of transformers and the oil does the heating and also the moisture removal from the insulation. 406 10.7 Manual on Transformers Termination Criteria for the Drying Out Process The termination criteria for the drying out process is expressed differently by different manufacturers but they are all aimed towards achieving good dry insulation which ensures trouble free operation of the equipment. The moisture content in the insulation is kept at around 0.5% of the insulation weight for new transformers at the time of manufacturing. But for older transformers the moisture content is kept between 1 to 2 % of the weight of the insulation, as beyond this value there is a chance of the thin paper insulation getting brittle during the heating and vacuum pulling process. Some of the common methods of measurement of the termination criteria are: (i) Measurement of Insulation Resistance Values Measurement of the insulation resistance values through the drying out process and this is generally followed for small transformers and those being dried out by hot air or hot oil circulation. The IR curve has a tendency to follow the bath tub curve and the subsequent calculation of the Polarization Index values form the Insulation resistance values. (ii) Dew Point Measurement The dew point of the dry gas generally N2 is taken before the same is injected into the tank and the value should be around -60° Celsius. It is then held inside the tank for 24 hours to achieve vapour phase balance and then again measure the dew point value. Dew point values at around -25° celsius at a temperature 40° celsius are considered satisfactory but lower the value better is the drying out. The process needs to be repeated with fresh dry gas everytime till the termination criteria is achieved. (iii) Tan Delta Value of the Insulation Generally followed for transformers being dried out with oil circulation and oil being used for heating of the insulation. Voltage should not be applied to the active part under vacuum. The tandelta measurement should be done after oil filling, circulation and standing time. Tan delta values of 0.5 are considered to be good values for termination of the process. (iv) Rate of Moisture Extraction Another method is the measurement of the water extraction rate during the vacuum pulling process applied after the heating phase. The rate of water extraction after certain duration is measured and the acceptable value of around 30 to 50 ml/hour/ton of insulation would be considered sufficient for termination of the process. One school of thought advocates the measurement of the water extraction rate of 100 ml/hr, reading repeated for 3 consecutive hourly readings irrespective of the quantum of the insulation. (v) Recommendation of the Manufacturer Termination criteria as recommended by the manufacturer can be considered as the final say in the matter and multiple measurements can be carried out to confirm the achievement of the Guidelines for Repair of Power Transformers at Site 407 termination criteria. The factory based drying out process using HAV or vapour phase method has the termination criteria of 30 gms /hour/ton of water extraction rate 11.0 REFURBISHMENT OF POWER TRANSFORMERS Refurbishment can be defined as an activity undertaken to improve the existing condition of the transformer or transformer insulation. The options available for the transformer refurbishment are as under: (i) (ii) 11.1 (a) (b) (c) (d) 11.2 Over loading the existing transformers with additional coolers and increased condition monitoring with insulation refurbishment. Refurbishment of the transformer. Requirement for Option (i) above For the new loading, recalculation of the temperature rises of the top oil and the windings temperature for the 145, 245 & 420 kV class transformers. Check the extent of the excess over the permitted / standardized values. If additional cooling keeps the temperature within the permitted value then the existing winding can be used with minimum risk. Parallely the diagnostic testing to determine the residual life of the transformer insulation, the present condition of the insulation and the mechanical condition of the active part should also be done. Requirement for Option (ii) above When the calculated temperature rise of the windings exceed permissible limits and extra cooling is not effective but the condition of the insulation indicate high residual life, the best effective situation could be: (a) (b) (c) (d) (c) (g) (h) Use the same core with necessary insulation change. Use of the same tank but with changing of the gaskets. Use of latest methods to reduce the stray losses. Replacement of the old winding by new winding & new insulating material. Suitable up-gradation of off circuit tap switch or OLTC. Suitable modification to use new bushings. Suitable testing process could be identified to check and prove the various parameters after refurbishment and refurbishment could be carried out even at site to minimize the time and cost implications. While planning for overhauling/refurbishment of transformers it is necessary to determine at the onset the scope of work to be carried out. The refurbishment activity can be categorized roughly into following: 408 (a) (b) (c) (d) 11.3 Manual on Transformers Improving the insulation condition of the transformer with no replacement of oil and the condition assessment and RLA indicates significantly good condition. Improving the insulation condition of the transformer with replacement of oil as the oil condition is beyond improvement by filtration and RLA indicates good life condition. Improving the insulation condition of the transformer and also minor changes to accessories for up-rated operation. Improving the insulation condition of transformer with major overhauling and repairs of the transformer for up-rated operation. The Main Activities/Benefits of Refurbishment one listed below: 11.3.1 Activities • • • • • • • • • • • Inspection, testing and repair / replacement / up-gradation of protective devices fitted on the transformer (Temp, indicators, Buccholz relay etc.) Changing of old gaskets and stoppage of leaks from the main tank and accessories. Inspection and up-gradation / repair / replacement of bushings, bushing leads. Inspection and up-gradation / repair / replacement of off -load or on -load tap changer diverter switch, selector switch and motor drive unit. Fitting of new / latest condition monitoring systems. De-sludging and tightening of the various joints with an eye not to introduce any other problem in the transformer during the over hauling. Generally most of the HV windings have wraps and removal of the same during inspection for assessing the condition of winding could be beneficial in terms of de-sludging and suspected deterioration due to heating or PD. Re-fitting of the wraps (removed for inspection) as per the original is of utmost importance particularly for transformers with DOF cooling. If the oil parameters are within limits then vacuum re-conditioning of the oil. Reconditioning or modification of the conservator or sealing system. Drying out of the transformer with heating and vacuum cycles in its own tank. 11.3.2 Benefits • • • Slowing the ageing rate of the paper insulation and oil. Humidity in the paper insulation to be kept in the range of 1 % -2%. Improving or sustaining the short circuit withstand capacity of the winding (ageing reduces the withstand capacity) generally by coil re-tightening SECTION GG Guidelines for Voltage Control of Power Transformers SECTION GG Guidelines for Voltage Control of Power Transformers This section covers voltage control of OFF-circuit as well as ON-load type. 1.0 VOLTAGE CONTROL (OFF-CIRCUIT TYPE) 1.1 When specified, each transformer shall be provided with off-circuit tap changing switch or off-circuit links for varying its effective ratio of transformation whilst the transformer is deenergised and without producing phase displacement. 1.2 The off-circuit switch handle will be provided with a locking arrangement along with tap position indicator, thus, enabling the switch to be locked in position. A warning plate indicating that switch shall be operated only when the transformer is de-energised shall be fitted. 2.0 VOLTAGE CONTROL (ONLOAD TYPE) 2.1 When specified, each transformer shall be provided with voltage control equipment of the tap changing type for varying its effective transformation ratio whilst the transformers are on-load and without producing phase displacement. 2.2 Equipment for local and remote electrical and local manual operation shall be provided and shall comply with the following conditions. Local remote switch may be housed in remote control panel or in tap changer driving unit. 2.2.1 use. It shall not be possible to operate the electric drive when the manual operating gear is in 2.2.2 It shall not be possible for any two electric controls to be in operation at the same time. 2.2.3 The equipment shall be suitable if specified for supervisory control and indication on multi-way switch, make-before-break, having one fixed contact for each tap position, shall be provided and when specified, wired to the tapchanger drive gear. This switch shall be provided in addition to any which may be required for remote tap position indication purposes. Supervisory indication shall also be provided when specified in the form of contacts to close on “Tapchange incomplete”. All other components of the supervisory gear if required will be specified separately. 2.2.4 Operation from the local of remote control switch or push button shall cause one tap movement only until the control switch or push button is returned to the off position between successive operations. 2.2.5 All electrical control switches and the local operating gear shall be clearly labelled in a suitable manner to indicate the direction of tapchanging. 2.2.6 The local control switches shall be mounted in the marshalling box, or driving gear housing. 411 412 Guidelines for Voltage Control of Power Transformers 2.3 The equipment shall be so arranged as to ensure that when a tapchange has been commenced it shall be completed independently of the operation of the control relays or switches. If a failure of the auxiliary supply during a tapchange or any other contingency such as tapchanger getting stuck would result in that movement not being completed, adequate means shall be provided to safeguard the transformer and its auxiliary equipment. 2.4 Suitable apparatus shall be provided for each transformer to give indications as follows: 2.4.1 To give indication, mechanically at the transformer and electrically at the remote control point if specified, of the number of the tapping in use on the transformer. 2.4.2 To give an indication at the remote control point that a tapchange is in progress, by means of an illuminated lamp. 2.5 For remote control, the switches, push button tap position indicator, etc., shall all be supplied as loose apparatus unless a remote control panel is specified. 2.6 All relays and operating devices shall operate correctly, at any voltage between the limits specified in the relevant Indian Standard. 2.7 The tapchanging switches and mechanism shall be mounted in oil tanks or compartments mounted in an accessible position on the transformer tank. 2.8 Any enclosed compartment not oil filled shall be adequately ventilated. Metal clad thermostatically controlled heaters shall be provided in the driving mechanism chamber and in the marshalling box. All contactors, relay coils or other parts shall be suitably protected against corrosion or deterioration due to condensation, fungi, etc. 2.9 The tap changer contacts which are not used for making or breaking current like separate selector switch contacts can be located inside main transformer tank where tapchanger construction permits such an arrangement. On load tapchangers having separate compartment for selector contacts, the oil in such compartment shall be maintained under conservator head by means of pipe connection from the highest point of the chamber to the conservator. Such connection shall be controlled by suitable valve and shall be arranged so that any gas leaving the chamber will pass into the gas and oil actuated relay. A separate buchholz relay may be provided for this compartment. 2.10 It shall not be possible for the oil in these compartments of the tapchange equipment, which contain contacts used for making or breaking current, to mix with the oil in the compartments containing contacts not used for making or breaking current. 2.11 Any ‘DROP DOWN’ tanks associated with the tapchanging apparatus shall be fitted with guide rods to control the movement during lifting or lowering operations. The guide rods shall be so designed as to take support of the associated tank when in the fully lowered position Guidelines for Voltage Control of Power Transformers 413 with oil. Lifting gear fitted to ‘DROP DOWN’ tanks shall include suitable device to prevent runaway during lifting and lowering operations. They shall be provided with adequate breathing arrangement. If specified the tapchanger shall be mounted in such a way that the cover of the transformer can be lifted without removing connections between windings and tapchanger. 2.12 Each compartment in which the oil is not maintained under conservator head shall be provided with a suitable direct reading oil gauge. 2.13 The alternating supply for electrical operation of the control and indicating gear shall be standard 415 volts, three-phase, 3 wire, 50 Hz, along with 240 volts single phase, 2 wire 50 Hz, subject to a variation of + 10 per cent so that the equipment offered can withstand variation in A.C. 2.14 Limit switches shall be provided to prevent over running of the mechanism and except where modified in clause 2.15 shall be directly connected in the circuit of the operating motor. In addition a mechanical stop or other approved device shall be provided to prevent over-running of the mechanism under any condition. 2.15 Limit switches may be connected in the control circuit of the operating motor provided that a mechanical declutching mechanism incorporated. 2.16 Thermal devices or other means like motor circuit breakers with shunt tripcoil shall be provided to protect the motor and control circuits. All relays, switches, fuses, etc., shall be mounted in the marshalling box or driving gear housing and shall be clearly marked for purposes of identification. They shall withstand the vibration associated with tapchanger gear operation. 2.17 The control circuits shall operate at 110 V single phase to be supplied from a transformer having a ratio of 415 or 240/55-0-55 V with the centre point earthed through a removable link mounted in the marshalling box or tapchanger drive. 2.18 The whole of the apparatus shall be of robust design and capable of giving satisfactory service without undue maintenance under the conditions to be met in service, including frequent operation. 2.19 A five-digit counter shall be fitted to the tapchanging mechanism to indicate the number of operations completed by the equipment. 2.20 A permanently legible lubrication chart shall be fitted within the driving mechanism chamber, where applicable. 2.21 Loose equipments shall be supplied for mounting on the purchaser’s control panel and the control panel will be additionally supplied as and when required by the purchaser. 414 3.0 Guidelines for Voltage Control of Power Transformers PARALLEL OPERATION TAPCHANGER OF TRANSFORMERS WITH ONLOAD 3.1 Besides the local and remote electrical control specified in clause 2 on-load tapchangers, when specified, should be suitable for remote electrical parallel control as in clause 3.2. 3.2 Remote Electrical Parallel Control 3.2.1 In addition to the methods of control as in clause 2, the following additional provision shall be made. 3.2.2 Suitable selector switch be provided, so that anyone transformer of the group can at a time be selected as ‘Master’, ‘Follower’ or ‘Independent’. 3.2.3 Necessary interlock blocking independent control when the units are in parallel, shall be provided. 3.2.4 The scheme will be such that only one transformer of a group can be selected as ‘Master’. 3.2.5 An out-of-step device shall be provided for each transformer which shall be arranged to prevent further tapchanging when transformers in a group operating in ‘Parallel control’ are one tap out-of-step. 4.0 ONLOAD TAP CHANGER CONTROL SCHEME 4.1 The control scheme for tap changer can be as under : (i) Non-automatic independent-as per Scheme 1. The scheme used for independent control from local or remote panel. (ii) Non-auto/automatic independent-as per Scheme 2. The scheme used for independent control with automatic voltage control relay and line drop compensation as optional. If required non-auto condition can be availed. (iii) Non-automatic simultaneous parallel operation-as per Scheme 3. The scheme used for non-automatic simultaneous parallel operation. (iv) Non-auto/automatic simultaneous parallel opcration-as per Scheme 4, Sheet 1 and 2. The scheme used for automatic simultaneous parallel operation with facility for use with nonauto condition also. Sheet I of the Scheme represents main control scheme and with sheet 2 line drop compensation control scheme can be achieved as optional. 4.2 General Local control items shall be mounted inside the on-load tapchanger driving mechanism or marshalling box. Remote control items are to be mounted on remote control cubicle installed in Guidelines for Voltage Control of Power Transformers 415 the control room. All the control items are to be mounted in easily accessible position and clearly labelled. AH the control item shall be of best quality and or class most suitable for working under the conditions specified and shall withstand the variation of temperatures and atmospheric condition arising under working conditions so also withstand vibrations. All the control items shall be wired and connected as per ‘Schematic Diagram of tapchanger control equipment given in Scheme 1. 4.3 Motor On-load tap changer driving gear Motor shall be of squirrel cage totally enclosed type and shall comply with Indian Standard IS: 325. It shall be suitable for direct starting and continuous running from 415 volts 3-phasc or 240 volts single phase 50 Hz supply. Motor shall be capable of continuous operation at any frequency between 48 and 51.5 Hz together with any voltage within 10 per cent of nominal value. Motor shall have ball or roller bearing and vertical spindle 416 Guidelines for Voltage Control of Power Transformers Guidelines for Voltage Control of Power Transformers 417 418 Guidelines for Voltage Control of Power Transformers Guidelines for Voltage Control of Power Transformers 419 420 Guidelines for Voltage Control of Power Transformers Guidelines for Voltage Control of Power Transformers 421 motor shall have bearing capable of withstanding thrust due to the weight of the moving parts. The stator windings shall be adequately braced and suitably impregnated to render them nonhygroscopic. 4.4 Overload Protection Relay The overload protection relay shall be of robust, adjustable triple-pole construction. It should provide accurate and reliable protection against overload, single phasing, over heating and short circuit. The relay should be provided with temperature compensating device to off-set the effect of ambient temperature variation. For single phase motor, over-load protection device with feature similar to those of the three-phase motor as far as these are applicable shall be provided. 4.5 Contactors/Relays Contactors/Relays shall be of robust and compact construction and shall comply with Indian Standard IS : 2959. The electromagnetically operated air break type contactor with sufficient number of contacts shall be suitable for mounting on a vertical supporting structure. The contactors shall be suitable for operation at 110 Volts A.C.-15 per cent to + 10 per cent 50 Hz. Main and auxiliary contacts of contactor shall be suitably rated. For sufficient long life these contacts shall be break type and shall make contacts practically bounce-free. 4.6 Control Supply Transformer The control supply transformer shall be single phase having ratio 240/55-0-55 or415/55-0-55. Its insulation shall be suitably impregnated to render it non-hygroscopic. 4.7 Control Selector Switches All the control selector switches shall be of robust and compact construction and shall comply with Indian Standard IS : 4064 and 4047. The control switches shall be suitable for on-load switching of resistive and inductive loads. The switches shall incorporate multi air brcak type wiping contacts housed in an assembly of packets moulded from anti tracking material. The knob of the handle of the switch shall be suitably designed so that while operating a firm grip is obtained. 4.8 Remote Tap Position Indicator Remote tap position indicator mounted on remote control panel shall show accurately same tap position as indicated by local tap position indicator on on-load tap changer. The remote indication can be by means of an analogue indicator, or digital indicator or by means of lamp indications. Transmitter switch in the driving gear shall be make before break type. This switch in the driving gear shall be mounted in accessible position so that it can be cleaned and maintained regularly. The remote indicator mounted on control panel shall not be affected by normal auxiliary voltages supply variation. 422 4.9 Guidelines for Voltage Control of Power Transformers Indicating Lamp Necessary indicating lamps provided shall be of low watt consumption and of filament type or Neon or LED type. Lamps shall be of such construction so that these can be replaced very easily. 4.10 Space Heater Space heater of adequate capacity and robust construction shall be provided inside each control cabinet to prevent moisture condensation. Space heaters shall be rated for 240 volts 1 phase, 50 Hz supply. Heater shall be complete with miniature circuit breaker or “ON-OFF” switch. HRC fuse on phase and link on the neutral. Mounting of the heater and its location shall not cause localized intensive heating of control equipment and wiring. 4.11 Wiring All the wiring shall be carried out for motor circuit with 1100 volts grade PVC insulated stranded copper conductors of size 2.5 sq mm and for control circuit with 650 volts grade PVC insulated copper conductor of size 1.5 sq mm suitable for tropical atmosphere. All wiring shall be in accordance with relevant IS. Engraved / printed core identification ferrules, marked to correspond with the wiring diagram shall be fitted at both ends of each wire. Ferrules shall fit tightly on the wires and shall not fall off when the wire is removed. All wiring shall be terminated on terminal blocks through suitable lugs. Insulated sleeves shall be provided at all the wire terminations. All wiring shall be neatly bunched and cleated without affecting access to equipment mounted within the cabinet. One piece moulded 1100 V grade terminal blocks complete with insulation barriers, terminal studs, washers, nuts and lock nuts shall be used. Terminal blocks shall be numbered for identification and grouped according to function. 10 per cent spare terminal blocks for control wire termination shall be provided on each panel. Terminal board rows should be spaced adequately apart to permit convenient access to wires and terminations. Terminal boards shall be so placed with respect to the cable gland plate (at a minimum distance of 70 mm) as to permit satisfactory arrangement of multicore cable tails without undue stress or bends. Opening of door should not disturb or stress the wire termination. 4.12 (i) Voltage Regulating Relay Introduction: Voltage Regulating Relay is used for regulating the secondary voltage of power transformers with on-load tapchangers. The required dead band settings are set by setting the nominal value, and lower and upper levels independently. The time delay setting on the front panel eliminates the relay operations for momentary fluctuations of the regulated voltage thus reducing the number of operations of the tap changer. “ Guidelines for Voltage Control of Power Transformers 423 When the regulated voltage falls below the specified under voltage limit, the control\ relays are automatically blocked, i.e., there is no voltage correction, and a pair of contacts is made available for alarm. (ii) General Description: Voltage regulating relay should be designed for maximum operational simplicity for regulating the secondary voltage of power transformer with onload tapchangers. The deadband (band width) can be set by setting the nominal value adjustment (NVA) to the required value “110V+10 per cent”. The desired time delay can be set on the front panel and the control action will take place only if the voltage continues to remain outside the deadband after the time delay has elapsed. For voltage corrections requiring more than one tap change, time delay is initiated again before further tapchange. The relay is reset automatically after the voltage is brought within the selected deadband. For repeated short duration voltage fluctuations on the same side of the deadband, the time delay is effectively reduced to provide a voltage time integral response of the regulator. Operation of the Raise Control relay is automatically inhibited when the voltage falls below the specified under-voltage limit. One pair of normally open relay contacts are provided to effect the tapchanger, Raise and Lower operation and to trigger an alarm in case of undervoltage conditions. (iii) Specifications Auxiliary Supply : -15% 50 Hz + 10% : 110 V ± 10 per cent 50 Hz Sensitivity (Dead Band) : Nominal value adjustable (NVA) and Nominal Value Range between + 0.75 to + 2.5 per cent Time Delay Setting : Fixed, i.e., (Voltage independent) time delay continuously adjustable from 10 to 110 secs. Time Delay Resetting : Instantaneous resetting with voltage deviation occurring in opposite direction. Under Voltage Blocking : Internal blocking at 80 per cent of regulated value. Restoration at 85 per cent of regulated value. Control Relays : One pair of normally open potential free contacts of suitable rating. Control Operation : Single pulsed operation of sufficient duration to initiate tapchanger. Operating Temperature : - 5° to + 50°C PT Supply (regulated Voltage) 424 Guidelines for Voltage Control of Power Transformers Option 4.13 : Line drop compensator with resistive and reactive compensation of either polarity upto 20 per cent adjustable in steps or continuously and suitable for operation with 1 Amp. current transformer. If required suitable interposing current transformer to be used to get 1 Amp. secondary current. 1 pair of NC (UV) contacts provided. Line Drop Compensator (LDC): Description I The Line Drop Compensator is an optional unit designed to match with the Automatic Voltage Regulator Relay. The unit is housed in the same enclosure or separately mounted. The voltage at the generating end and at thereceiving end are not the same due to the drop across the line. The LDC is used to compensate for this line drop, and the amount of compensation required is calculated as a per cent of the nominal voltage knowing the length of the line, its resistance/unit length, its reactance/unit length and the rated current, and set on the front panel. The line current is stepped down to 1 Amp. and fed to the LDC. The resistive and reactive drops are simulated by having 90° phase shifted voltage and their polarity selected by polarity switches. The net compensation is then fed to the stepped down PT voltage. II Specifications Resistive Compensation Reactive Compensation Input Rated Current Power Consumption Accuracy Max. Overcurrent Polarity Selection III : : : : : : : 0-20 per cent of the regulating value continuously adjustable. 0-20 per cent of the regulating value continuously adjustable. 1 Amp. 50 Hz. As required (CT burden will depend on power consumption). 10 per cent. 50 per cent of rated current (1.5 Amp.) Both positive and negative compensation. Operating and Connection Requirements Connection to the LDC unit are made through the rear panel terminals. The line current is stepped down to 1 Amp. 50 Hz and fed to the LDC. The net compensation is fed to the AVR circuit, internally or external connections. The required amount of percent R and per cent X compensation can. be set on the front panel of the LDC. The polarity selector switches provide both positive and negative compensation. The per cent R and per cent X settings can be calculated from the following formulae: √3ILRL Per cent R = ________ x 100 per cent VL Guidelines for Voltage Control of Power Transformers 425 √3I1XL Per cent X = ––––––––– X 100 per cent V, where IL = the primary rated current of the line. VL - the voltage between lines of the power transformer. XL = the line reactance in ohms/phase. RL = the line resistance in ohms/phase. Note : When LDC unit is not to be used, keep per cent R and per cent X settings to Zero, i.e., on min. position. 5.0 NOTES ON TAP CHANGER SCHEME 5.1 Note on Schematic Diagram of Tap Changer for Parallel Control The parallel scheme is prepared keeping in mind that any one of the on-load tapchanger is selected to the master position and the other to the follower position. This is being done by selector switch SSS provided on control cubicles for each transformer. With selector switches SSS in their respective position as above bus wires N 504 and N 505 become source of supply to the OLTC control circuits of all the transformers. Selecting Local/Remote switches CSS in remote position one can press Master unit push button RPB or LPB, for desired raise or lower operation, a direct impulse for Raise or Lower goes to Master unit OLTC. Also through Master unit selector switch SSS the bus wires N 506 or N 507 energize and transmit Raise or Lower impulse to follower OLTC unit through their own selector switch SSS. 5.2 Out-of-Step Circuit Out-of-step circuit is designed on the principle of odd/even position of on-load tap changers. Through the link provided on first unit between bus wire N 504 and N 513 bus wire N 518 becomes source of supply to out-of-step relays of all the units. Under normal working condition the out-of-step relays of all the units are energised. The out-of-step relays of all the follower units get supply from bus wire N 511 or N 512 through their own odd/even switch OSS. The bus wire N 511 or N 512 gets supply from bus wire N 515 through Master units selector switch SSS and odd/even switch OSS. The out-of-step relay of Master unit gets supply from bus wire N 518 through selector switches SSS and contacts of outof-step relays of follower units. When any of the on-load tapchanger from group lags behind by one step or moves ahead by one step, for what so ever the reason may be, creates condition of out-of-step of transformers running in parallel. Under the out-of-step condition the supply to the out-of-step relay is cutoff and hence it gets de-energised and one of its contactenergises a time delay relay TDR which initiate visual and audible alarm. 426 5.3 Guidelines for Voltage Control of Power Transformers Automatic Control Automatic control feature is used to keep voltage constant at desired level, 110 volts 2 wire PT supply should be arranged as input to AVC relay. OSS switch is to be set on auto mode by which AVC relay circuit comes into operation. The setting are to be done as required prior to commissioning the relay. 5.4 Line Drop Compensation The feature to be used along with AVC relay to compensate for the voltage drop across the line and to maintain desired voltage at receiving end. Input to LDC device through secondary current 1 Amp. from line CT and output from LDC is connected with AVC. Suitable adjustments for line resistive and reactive compensation to be made on LDC to suit. SECTION HH Guidelines for Protective Schemes for Power and Distribution Transformers 1.0 SECTION HH Guidelines for Protective Schemes for Power and Distribution Transformers GENERAL This section of the Transformer Manual covers recommended protection schemes for distribution and power transformers. This part specification does not purport to include all necessary provisions of a contract. For general requirements, ratings and tests reference shall be made to other sections of the Transformer Manual. 2.0 PROTECTION OF DISTRIBUTION TRANSFORMERS 2.1 Pole mounted distribution transformers of capacities ranging from 16 kVA to 200 kVA with voltage ratio of 11000/433-250 volts shall have the protection as given in Table 1. Table 1 Voltage ratio Capacity (kVA) Protection Primary side 11000/433250 Volts 16,25,63,100 and 200 Dropout/Horn fuse Secondary side Moulded case circuit breaker (MCCB) 2.2 Ground mounted distribution transformers of capacities ranging from 200 kVA to 1600 kVA with voltage ratio of 11000/433, 33000/433 and 33000/11000 volts shall have the protection as given in Table 2. However, wherever circuit breakers are provided following protections are recommended: 2.2.1 DMT type over current and earth fault relay and if required, delayed neutral earth fault protection to take care of high resistance faults in the outgoing feeders/transformer LT cable can be provided. 2.2.2 Oil temperature indicator with one electrical contact for alarm or trip shall be provided for distribution transformers of capacities 1000 kVA onwards. Winding temperature indicator with two electrical contacts for alarm and trip can be specified by purchaser as an optional item for distribution transformers of capacities 1000 kVA onwards. 2.2.3 Buchholz relay with alarm and trip contacts shall be provided for transformer of capacity 1000 kVA onwards. Table 2 Voltage ratio Capacity (kVA) 11000/433-250 33000/433 315,630, 1000 and 1600 630, 1000, 1600 33000/11000 1600 Protection Primary Secondary side side HRC/Expulsion MCCB/ACB fuse HRC/Expulsion MCCB/ACB fuse HRC/Expulsion CB fuse 429 430 Manual on Transformers 2.3 The 33 kV and 11 kV windings of the distribution transformers located outdoors and connected to overhead lines shall be protected by lightning arrestors. 3.0 PROTECTION OF 12 KV CLASS POWER TRANSFORMERS 3.1 12 kV Class Power transformers with capacities ranging above 1600 kVA as covered in Section D of Transformer Manual shall have the following protection : • Circuit breakers both on primary and secondary side. • IDMT over current and earth fault relay shall be provided on the 11kV side. • IDMT over current and earth fault relay shall be provided on the secondary side and if required, delayed neutral earth fault protection to take care of high resistance faults in the outgoing feeders/Transformer LT cable can be provided • Buchholz relay with alarm and trip contact. • Winding temperature indicator with alarm and trip contacts, pressure relief device with trip contact shall be provided. However, oil temperature indicator with alarm and trip contacts, oil level indicator with alarm contact can be specified by the purchaser as optional items. • Lightning arrestors on both primary and secondary sides when the transformer is located outdoors and connected to overhead lines. 4.0 PROTECTION OF 36 KV CLASS POWER TRANSFORMERS 4.1 36 kV class power transformers of capacities ranging from 3.15 MVA and above as included in Section ‘E’ of the Transformer Manual shall have the following protection : • Circuit breakers both on the primary and secondary sides. • High speed percentage biased differential relay with second harmonic restraint. • IDMT type overcurrent relay with high set elements on the primary side. • IDMT type over current and earth fault relay on the secondary side. • Oil temperature indicator with one electrical contact for alarm or trip contact. • Buchholz relay with alarm and trip contact. • Winding temperature indicator with three electrical contacts for (a) alarm (b) trip (c) control of fan for transformers above 10 MVA can be specified by the purchaser. • Lightning arrestors on both primary and secondary sides when the transformer is out doors and connected to overhead lines. • Oil surge protection for OLTC (if provided) diverter tank with trip contact. • Pressure relief device with trip contact. • Oil level indicator with alarm contact shall be provided Guidelines for Protective Schemes for Power and Distribution Transformers 5.0 431 PROTECTION OF 72.5 KV CLASS POWER TRANSFORMERS 5.1 72.5 kV class Power Transformers with capacities as covered in Section ‘E’ of Transformer Manual shall have the following protection : • Circuit breakers on both primary and secondary sides. • High speed percentage biased differential relay with second harmonic restraint. • Back up IDMT over current and earth fault relay on primary side. • Back up IDMT over current and earth fault relay on the secondary side. • Oil temperature indicator with alarm and trip contact. • Buchholz relay with alarm and trip contact. • Winding temperature indicator with three electrical contacts for (a) alarm (b) trip (c) control of fan for transformers above 10 MVA can be specified by the purchaser. • Magnetic oil gauge with low oil level alarm contact. • Lightning arrestors on both primary and secondary sides when the transformer is outdoors and connected to overhead lines. • Oil surge protection for OLTC diverter tank with trip contact. • Pressure relief device with trip contact. • Restricted earth fault protection • In case of non-clearance of fault on operation of transformer protection, utilities may consider tripping of all other circuit breakers feeding the fault by devising appropriate scheme logic. 6.0 PROTECTION OF 145 AND 245 KV CLASS POWER TRANSFORMERS 6.1 145 kV class power transformers with capacities as covered in Section ‘F’ of Transformer Manual and 245 kV class power transformers with capacities as covered in Section ‘G’ of Transformer Manual shall have the following protection : • Circuit breakers on both primary and secondary sides. • High speed percentage biased differential relay with second harmonic restraint • Restricted earth fault relay on star connected primary and secondary sides. • Back up IDMT over current and earth fault relay on the primary and secondary sides • Over flux relay. • Buchholz relay with alarm and trip contact. • Oil temperature indicator with alarm and trip contact. • Magnetic oil gauge with low level alarm contact. • Oil surge protection for OL TC diverter tank with trip contact. 432 Manual on Transformers • Lightning arrestors on both primary and secondary sides when the transformer is located outdoors and is connected to the overhead lines. • Pressure relief device with trip contact. • Winding temperature indicator with four electrical contacts for (a) alarm (b) trip (c) control of fan and pumps for transformers above 10 MVA can be specified by the purchaser • Transformer fire protection trip and isolation of power supply to cooling fans and pumps • Local breaker backup relays on all sides of Transformer 7.0 PROTECTION OF 145 KV AND 245 KV CLASS INTERCONNECTING AUTO TRANSFORMERS 7.1 145 kV and 245 kV class interconnecting auto transformers with capacities as covered in Section ‘F’ & Section G of Transformer Manual shall have the following protection: Circuit breakers on both primary and secondary sides. High speed percentage biased differential relay with second harmonic restraint Restricted earth fault relay. Back up directional over current and earth fault relay on the primary and secondary sides Over fluxing relay. Oil temperature indicator with alarm and trip contact. Buchholz relay with alarm and trip contact. Winding temperature indicator with three sets of contacts for alarm, trip and control of fans (ONAN/ONAF) and four sets of contacts for (ONAN/OFAF). Magnetic oil gauge with low level alarm contact. Oil surge protection for OLTC diverter tank with trip contact. Lightning arrestors on both primary and secondary sides when the transformer is located outdoors and is connected to overhead lines. Pressure relief device with trip contact. Transformer fire protection trip and isolation of power supply to cooling fans and pumps. Definite time over current relay for alarm. Local breaker backup relays on all sides of auto transformer. In addition to the protection mentioned in this clause, for large inter connecting auto transformer (inter connecting two different systems/utilities) with OLTC and tertiary delta winding along with neutral grounding transformer for tertiary winding, the following additional protection may be provided. The typical protection diagrams showing Relaying connection and trip logics are enclosed as Annexures I and II. • • • • • • • • • • • • • • • 7.2 Guidelines for Protective Schemes for Power and Distribution Transformers 433 (i) (ii) Restricted earth fault relay for neutral grounding transformer. Definite time over current relays on secondary and tertiary side of auto transformer for alarm. (iii) Overcurrent relay on primary neutral. (iv) Overcurrent and earth fault protection on tertiary side. (v) Overcurrent relays on neutral of NGT. (vi) Buchholz relay for NGT with alarm and trip contact (vii) Overload trimming relays on secondary and tertiary sides for load trimming. Teed protection is used wherever the transformer is charged on primary side and secondary side breaker/isolator are open. In that case backup directional overcurrent/earth fault relays on secondary side arc non-operative. Hence, the instantaneous overcurrent protection provided on bushing CTs of the transformer to be wired for trip through secondary side breaker/isolator ‘b’ contact, will be provided as backup protection to transformer differential protection. 8.0 8.1 • • • • • • • • • • • • • • • 9.0 PROTECTION OF 420 KV CLASS AUTO TRANSFORMERS 420 kV class Auto Transformers with capacities as covered in Section ‘H’ of Transformer Manual shall have the following protection. High speed percentage biased differential relay with harmonic restraint. Restricted earth-fault relay. Neutral displacement relay or restricted earth fault relay for protection against ground faults in the tertiary winding/associated connections depending upon the tertiary earthing arrangements. Backup directional overcurrent and earth fault or impedance relays. Over flux relay. Buchholz relay with alarm and trip contact Oil temperature indicator with alarm and trip contact. Winding temperature indicator with three sets of contacts for alarm, trip and control of fans (ONAN/ONAF) and four sets of contacts for (ONAN/OFAF). Magnetic oil gauge with low level alarm contact. Lightning arrestors on both sides of the transformer. Pressure relief device with trip contact. Transformer fire protection trip and isolation of power supply to cooling fans and pumps Definite time over current relay for alarm. Oil surge protection for OLTC diverter tank with trip contact Local breaker backup relays on all sides of auto transformer PROTECTION OF GENERATOR TRANSFORMERS 9.1 Generator transformers with capacities as covered in Sections ‘F’, ‘G’ and ‘H’ of the Transformer Manual shall have the following protection : 434 • • Manual on Transformers • • Circuit breakers on HV side. Overall differential current relay covering the generator zone also, in addition to transformer differential protection. Restricted earth fault relay on the HV side. Over fluxing relay. Neutral overcurrent relay against sustained external system earth faults. Buchholz relay with alarm and trip contact. Oil temperature indicator with alarm and trip contact. Winding temperature indicator with three sets of contacts for alarm, trip and control of fans (ONAN/ONAF) and four sets of contacts for (ONAN/OFAF). Magnetic oil gauge with low level alarm contact. Lightning arrestors on the HV side when the transformer is located outdoors and is connected to the overhead lines. Pressure relief device with trip contact. Oil flow indicator with one contact for alarm, wherever applicable. Water flow indicator with one contact for alarm wherever applicable. Transformer fire protection trip and isolation of power supply to cooling fans and pumps Oil surge protection for OLTC diverter tank with trip contact Local breaker backup relays 10.0 PROTECTION OF 800 KV AUTO TRANSFORMERS • • • • • • • • • • • • 10.1 800 kV Auto Transformers with capacities as covered in Section ‘I’ shall have the following protection: • High speed percentage biased differential relay with second harmonic restraint. • Restricted earth-fault relay. • Neutral displacement relay or restricted earth fault relay for protection against ground faults in the tertiary winding/associated connections depending upon the tertiary earthing arrangements. • Backup directional over current and earth fault relay with non-directional high-set feature or impedance relays. • Back-up neutral E/F protection with IDMT characteristics. • Over flux relay for both HV & MV/LV side. • Buchholz relay with alarm and trip contact. • Oil temperature indicator with alarm and trip contact. • Winding temperature indicator with three sets of contacts for alarm, trip and control of fans (ONAN/ONAF) and four sets of contacts for (ONAN/OFAF). • Magnetic oil gauge with low level alarm contact. • Lightning arrestors on both sides of the transformer. Guidelines for Protective Schemes for Power and Distribution Transformers • • 435 • • • • Pressure relief device with trip contact Transformer fire protection trip and isolation of power supply to cooling fans and pumps. Definite time over current relay for alarm in HV side. Oil surge protection for OLTC diverter tank with trip contact. Local breaker back up relays on all sides of Auto Transformer. Neutral over Current relay (for alarm) 11.0 PROTECTION OF 800 KV GENERATOR TRANSFORMERS • Generator transformers with capacities as covered in Section ‘I’ shall have the following protection: • • Circuit breakers on HV and LV side. Overall differential current relay covering the generator zone also, in addition to transformer differential protection. Generator transformer differential protection Restricted earth fault relay on the HV side. Overhead line connection differential protection including generator transformer HV winding Over fluxing relay. Neutral over current relay against sustained external system earth faults. Buchholz relay with alarm and trip contact. Oil temperature indicator with alarm and trip contact. Winding temperature indicator with three sets of contacts for alarm, trip and control of fans (ONAN/ONAF) and four sets of contacts for (ONAN/OFAF). Magnetic oil gauge with low level alarm contact Lightning arrestors on the H.V side when the transformer is located outdoors and is connected to the overhead lines. Pressure relief device with trip contact. Oil flow indicator with one contact for alarm wherever applicable. Water flow indicator with one contact for alarm wherever applicable. Transformer fire protection trip and isolation of power supply to cooling fans and pumps. Buchholz relay with alarm & trip contact for OLTC Oil surge protection for OLTC diverter tank with trip contact. Local breaker back up relays. • • • • • • • • • • • • • • • • • 436 Manual on Transformers Guidelines for Protective Schemes for Power and Distribution Transformers 437 SECTION II Specifications for Transformer Bushings upto 1200 kV Voltage Class SECTION II Specifications for Transformer Bushings upto 1200 kV Voltage Class 1.0 GENERAL This specification covers outdoor capacitance graded Oil Impregnated Paper (OIP) bushings with values of highest voltage for equipment (Urn) from 52 kV upto 1200 kV voltage class and with values of rated current (Ir) upto 5000A and solid porcelain and oil communicating type bushings for voltage class < 36 kV for use in oil filled transformers and reactors. The Synthetic Resin Bonded (SRBP) type and Resin Impregnated Paper (RIP) type bushings are not covered in this section. These bushings, if required by the purchaser shall be supplied as per purchaser’s specification. In case of Bushings other than those specified, dimensions will be subject to agreement between manufacturer and purchaser. The specification establishes essential details and dimension to ensure interchangeability and adequate mounting of the bushings. 1.1 The porcelain components shall be sound, free from defects, thoroughly vitrified and smoothly glazed. 1.2 Unless otherwise specified, the glaze shall be brown in colour. The glaze shall cover all exposed porcelain parts of the bushings except those areas which are required to be left unglazed. 1.3 The design of the bushing shall be such that stresses due to expansion and contraction in any part of the bushing shall not lead to deterioration. 1.4 Cement if used in the construction of the bushing shall not cause fracture by expansion or loosening by contraction. Cement thickness shall be as small and even as practicable. 1.5 All exposed ferrous metal parts shall be hot dip galvanized wherever possible. 1.6 No arcing horns shall be provided on the bushings unless otherwise specified or agreed between purchaser and supplier. 1.7 Any stress shield shall be considered as an integral part of the bushing assembly. 1.8 52 kV to 1200 kV voltage class bushings shall be Oil Impregnated Paper (OIP) type condenser bushings. The bushings below 52 kV voltage class shall be of porcelain and oil communicating type unless otherwise specified. 1.9 Each bushing shall have marked upon it the manufacturer’s identification mark. 441 442 Manual on Transformers 1.10 The duration and rated short time current of the bushing for various voltage ratings shall be as specified. 1.11 Limits of temperature rise shall be in accordance with IEC 60137 1.12 Permissible variation in the value of capacitance, and maximum value of dielectric dissipation factor (tan delta) of bushing and the test tap on transformer bushing shall be as per IEC 60137 1.13 Bushings made of solid porcelain and oil communicating type and those having insulation which docs not flow under service conditions shall be suitable for mounting at any angle of inclination. Unless otherwise specified, those having a liquid insulation shall be suitable for mounting at any angle of inclination to the vertical, not exceeding 30°. 1.14 The cantilever strength of the bushing shall be in accordance with Level – I of IEC60137, unless otherwise specified. 1.15 The profile of the porcelain and spacing of the petticoats shall suit the duty specified. The petticoats shall preferably be of aerodynamic type conforming to IEC 60815 to enable hot line washing during service. 1.16 OIP Bushings shall preferably be of hermetically sealed. 2.0 STA NDARDS 2.1 The electrical characteristics of the bushings shall be in accordance with IEC 60137 3.0 SOLID PORCELAIN, OIL COMMUNICATING AND OTHER TYPE BUSHINGS UPTO 36 KV VOLTAGE CLASS 3.1 The dimensional parameters of the bushings upto and including 36 kV voltage class have already been standardised in IS : 3347 and shall be referred to. 3.2 Rated Voltage, Current and Basic Insulation Level 3.2.1 The rated voltage, current and basic insulation levels of the bushings shall be in accordance with IS: 2099 or IEC 60137. 4.0 OIP CONDENSER TYPE BUSHINGS FROM 52 KV TO 1200 KV VOLTAGE CLASS 4.1 Interchangeability 4.1.1 All the bushings from 52 kV to 800 kV voltage rating shall have dimensions as mentioned in clause 4.3 to enable interchangeability with different makes of bushings manufactured in conformity with this specification. Dimensions of 1200kV Bushing are for information only. Specifications for Transformer Bushings upto 1200 kV Voltage Class 4.1.2 The standardized dimensions shall be kept in view by the transformer manufacturers as well, while designing the transformers, so that the transformer can accept any bushing of the parameters and dimensions specified herein. 4.1.3 Other ratings of bushings, not covered in the specification, shall be supplied, if required. 4.2 Basic Insulation Level, Voltage, Current Ratings and Creepage Distances The basic insulation level, rated voltage and current and creepage distances for various voltage class bushings shall be as under: Highest System Voltage rating (kV) Rated Switching Impulse withstand voltage (kVp) 1950 1550 Power frequency withstand voltage (kV) Current Rating (Amps) Creepage distance (mm) 1200 800 Rated Lightning Impulse withstand Voltage(kVp) 2400 2100 1200 970 2500 2500 1250 30000 20000 420 1425 1050 695 2000 10500 245 1050 850 505 145 650 NA 305 2500 1250 2000 800 1250 6125 3625 2000 800 72.5 350 NA 155 1250 1815 3150 1250 52 250 NA 105 3150 1300 5000 Note: The above insulation level refers to operation at any altitude not exceeding 1000m. For installations at an altitude higher than 1000m, the arcing distance shall be increased according to IEC 60137. 4.2.1 The minimum value of creepage distance specified is 25 mm/kV of the rated voltage of bushings. 4.2.2 For areas with very heavy or extremely heavy pollution the minimum creepage distance shall be as specified by the user. 4.3 Standardized Parameters of Bushings 4.3.1 To render the bushings interchangeable, certain parameters of the bushings have been identified and their maximum and minimum dimensions have been standardized. The identified parameters shall be as indicated in Table 1. 444 Manual on Transformers Table 1 SL. No. Parameter Identification of Parameter 1. LI Length between top of air end terminal and bottom seat of flange. 2. L2 Length between bottom seat of flange and bottom of the oil end shield/ stress relieving electrode/ oil end terminal whichever is the longest. 3. L4 Flash over length. 4. L5 Length of the air end terminal. 5. L6 Length for bushing current transformer (BCT) accommodation. 6. Dl Outside diameter of live metallic tank. 7. D2 Maximum diameter of oil immersed end. 8. D3 Outside diameter of fixing flange. 9. D4 Fixing flange holes pitch circle diameter. 10. D5 Diameter of fixing hole. 11. N Number of fixing holes. 12. D6 Maximum diameter of oil end shield/stress relieving electrode. 13. D7 Minimum inside diameter of central tube. 14. D8 Diameter of air end terminal. 4.3.1.1 The standardised dimensions for various voltage and current rating bushings shall be as indicated in Figs. 1 to 4. 4.4 Lead Arrangement The lead arrangement, inside diameter of the central tube, rod and jointing rod for different current ratings and voltage class bushings shall be as under. 4.4.1 Definitions 4.4.1.1 Draw-Lead Conductor Conductor consisting of one or more flexible leads in parallel drawn into the central tube of the major insulation body of the bushing, (preferred up to 1000 A). 4.4.1.2 Draw Rod Or Solid Stem Conductor consisting of a round rod either removable and drawn into the central tube of the major insulation body of the bushing or fixed rod or tube and not removable, (preferred from 1250 A up to 5000 A). Specifications for Transformer Bushings upto 1200 kV Voltage Class Voltage rating Current rating (kV) (Amps) 1200 800 420 245 145 72.5 52 where, Type of lead Dia. of the 2500 2500 SS SS Jointing rod/rod (mm) --- 1250 2000 2500 1250 2000 800 1250 2000 800 1250 3150 1250 3150 DL DR/SS DR/SS DR ss DL DR SS SS SS SS ss ss 45 50/NA 50/NA 45 -35 35 ------ Minimum Internal Dia. of the central tube (mm) --60 60 60 48 -38 38 ------ DL- Draw Lead DR- Draw Rod SS- solid Stem Note : Diameter of Jointing rod may be different from the standard , in case lesser diameter is chosen, it should be proved by temperature rise test at rated current. However the clamping dimensions shall be as per Fig.2 4.5 Bushing Current Transformer (BCT) 4.5.1 To accommodate the bushing current transformers, space provided on various voltage class bushings shall be as under: 800 kV : 600 mm 420 kV : 400 mm 600mm 245 kV : 300 mm 600 mm 145 kV : 100 mm 300 mm 600 mm 72.5kV : 100 mm 300 mm 600mmm 52 kV : 100 mm 300 mm 600mm 4.5.2 Bushings with space as necessary for the accommodation of bushing current transformers specified shall be also offered. 446 4.6 Manual on Transformers Lead Joint 4.6.1 Draw lead and draw rod type leads shall have joint as shown in Fig. 2a & 2b respectively. The dimensional details, material composition, finish, bolts spacing etc., have also been shown in the figure. The joint shall be provided at the bushing fixing flange level. The bottom portion of the joint shall be in flush with the bottom of the flange. The complete joint with the top portion of the lead upto the joint with nuts and bolts shall be supplied along with the bushing. The free portion of the joint shall be brazed / crimped by the purchaser with the lead to be supplied along with transformer to make the complete lead. 4.6.2 In case of draw rod type lead arrangement, the complete rod upto the joint with free end of the rod forming one portion of the joint with necessary nuts and bolts shall be supplied along with bushing. The remaining portion of the rod with one end to match the joint shall be arranged by the purchaser through the transformer supplier. 4.7 Oil End Terminal and Shield Fixing Arrangement for 245 kV Solid Stem Type Bushing The oil end terminal and shield fixing arrangement for 245 kV Solid stem type bushing shall be as shown in Fig. 3. The entire oil end terminal arrangement along with corona shields shall be supplied along with bushing. 4.8 Oil End Terminal for 72.5 kV/ 52 kV/ Solid Stem Type Bushing For 72.5 kV & 52 kV bushings the palm type terminal at the oil end of the solid stem type bushings shall be supplied in accordance with Fig. 4(a) & 4(b) as applicable. 4.9 Air End Terminal The lengths and diameters of the air end terminals for the various bushings shall be as under: Bushings upto 800 amps - 30 mm x 125 mm (D x L) Bushings of 1250 Amps - 60 mm x 125 mm (D x L) (Upto 3150 Amps) Bushings of 5000 Amps - 90 mm x 125 mm (D x L) 4.10 Test Tap OIP bushings shall be provided with test taps of proven design unless otherwise specified. Tan delta of bushing at Um/√3 should not be more than 0.005 at ambient temperature, when measured at factory. 4.11 Oil Level Indicator 4.11.1 The bushings shall be provided with oil level indicators as under: 420 kV & 800 kV Bushings: Magnetic oil level gauge / Oil sight window 245 kV and below - Oil sight window Specifications for Transformer Bushings upto 1200 kV Voltage Class The oil level indicator shall be so designed and dimensioned that oil level shall be clearly visible from ground level. 5.0 PRECAUTIONS TO BE TAKEN ON RECEIPT OF BUSHINGS AT SITE 5.1 PACKING AND STORAGE To prevent physical damage to the bushings, generally only one bushing is packed in a wooden packing case. At times, more than one bushing are also packed in the case of bushings of lower voltage rating. The bushings may be stored either in the same case or removed and placed vertically. When the packed bushings are stored outdoors, they should be kept horizontal and covered with a tarpaulin for protection from rain and other atmospheric contaminants. 5.2 CHECKS BEFORE INSTALLATION / RECIEPT OF BUSHINGS AT SITE (i) Inspect the bushing physically for any leakage or damage on the porcelains. The oil end and inner portion of the central tube should be cleaned thoroughly. Care should be taken to prevent scratching of the painted surface of the bottom stress shield / base plate. (ii) Check oil level of the bushing in vertical position. The oil level should be such that the oil level is clearly visible through the oil sight glass provided on the bushing. Topping of oil inside the bushing at site is not prohibited. (iii) Ensure the top terminal is tightened firmly with its gasket provided to avoid water/ moisture entry into the transformer. (v) Capacitance (C1) and Tan delta values of the bushing should be measured between the top terminal and test tap at 2 kV to 10 kV(Maximum). The measurement should be preferably carried out in indoors with RH not exceeding 60% and at ambient temperature. (vi) Place the bushing vertically on a suitable stand. Remove the threaded test tap cover. Insert a plug/clip into the central stud of the test tap and connect to the Schering Bridge through a screened cable. The flange body should be grounded. Connect the high voltage supply to the top terminal. Measure the capacitance and tan delta value of the bushing upto a maximum of 10kV in UST mode. CAUTION: • The voltage applied on the bushing should be limited to 10 kV, when the bottom end is not immersed in oil. Utmost care should be taken to avoid any contact with the bushing during testing as this will result in fatal injury to the personnel. • The test tap should be dry, free from any moisture condensation and dirt deposition. Ensure not to damage the soldered pin of the test tap while inserting the clip or plug during measurements. The threaded test tap cover should be fixed back to the test tap, immediately after the test. • Do not carry out measurements with bushing either placed in wooden box or in horizontal position. Factory test values of tan delta and capacitance are indicated on the name plate or in the test report of individual bushing at working voltages. However site values may vary as they do not resemble factory test condition. The site values recorded at the time of commissioning should be taken as the reference value for comparison with future measurements. 448 Manual on Transformers 5.3 PERIODICAL CHECKS AND MAINTENANCE The bushing is a self contained unit and as such there is no specific maintenance to be carried out. However a periodical check of the oil level and cleaning of the porcelains will normally suffice. In order to determine the healthiness of the bushing, measurement of capacitance and tan delta may be carried out during annual maintenance. These values are to be compared with precommissioning test results. Tan delta value of 0.007 or more and increase in capacitance by 5% or more, if observed, should be referred to bushing manufacturer. Due to the limited quantity of oil in bushings, sampling of oil from bushings is not recommended. In case of a special requirement like DGA Analysis, sampling should be done only under the supervision of manufacturer. Specifications for Transformer Bushings upto 1200 kV Voltage Class 450 Manual on Transformers Specifications for Transformer Bushings upto 1200 kV Voltage Class 452 Manual on Transformers Specifications for Transformer Bushings upto 1200 kV Voltage Class 454 Manual on Transformers SECTION JJ Specifications for Valves for Transformers SECTION JJ Specifications for Valves for Transformers 1.0 SCOPE 1.1 This section covers specification for valves for transformers. This part specification does not purport to include all necessary provision of a contract. For general requirements reference shall be made to other sections of the Transformer Manual. 2.0 GENERAL 2.1 All valves in oil line shall be suitable for continuous operation with transformer oil (IS: 335) at 100°C. 2.2 Gland packing/ gasket material shall be of teflon rope/nitrile rubber. In case of GM/CI (gun metal/cast iron) gate and globe valves, gland packing preferably of teflon rope shall be used to prevent oil seepage through gland. Asbestos or graphite (gun metal/cast IMM) packing material shall not be used as they are not compatible with hot transformer oil and asbestos is a banned item.. 2.3 Inside surface of valves shall be clean and valve ends shall be suitably blanked at the time of dispatch. Machined and flange surfaces shall be suitably protected against rusting and transit damage. Butterfly and radiator valves shall be covered in polythene bags before packing for dispatch. 2.4 After testing, inside surface of all C.I. valves coming in contact with transformer oil shall be applied with one coat of oil resisting paint/ varnish. Inside surface of all valves in water lines shall be applied with two coats of paint conforming to IS: 9862, two coats of black Japan conforming to Type B of IS: 341 or any other suitable water resistant compound. Unless specified otherwise by the purchaser, outside surface of all valves with ferrous body shall be painted with two coats of red oxide zinc chromate primer followed by two coats of full gloss finishing paint conforming to IS: 2932 and of shade matching with the transformer body or shade no. 631 of IS: 5. Outside surface of all valves with non ferrous body shall be painted with one coat of etch primer followed by two coats of red oxide zinc chromate primer followed by two coats of full gloss finishing paint conforming to IS: 2932 and of shade matching with the transformer body or shade no. 631 of IS: 5. After testing, inside surface of all C.I. valves coming in contact with transformer oil shall be applied with one coat of oil resisting paint/varnish. Inside surface of all valves in water lines shall be applied with two coasts of pant conforming to IS : 9862 or two coats of black Japan conforming to Type B of IS : 341. Outside surface of the valves shall be painted with two 457 458 Manual on Transformers coats of red oxide zinc chromate primer followed by two coats of full gloss finishing paint conforming to IS : 2932 and of distinct different shade (631 of IS - 5) to that of main tank surface. 2.5 All hardwares used shall be electro-galvanized. 3.0 SIZE OF THE VALVE AND VALVE FLANGES 3.1 The size of the valve and valve flanges shall be as per Table 1. All flanges shall be drilled at off-centres. 3.2 Recommended size of the valves for drain, filter, sampling and air release applications shall be per Table 2. Table 1 Nominal size of valve (nominal pipe bore) mm Diameter of flange mm Diameter of bolt circle mm 15 25 50 80 100 150 200 250 95 115 150 185 215 285 340 395 65 85 115 145 180 240 295 350 No. of bolts 4 4 4 4 4 8 8 12 Bolt size M12 M12 M16 M16 M16 M20 M20 M20 Diameter of bolt hole mm 14 14 18 18 18 23 23 23 Table 2 Transformer rating kVA Over — 1600 10000 50000 100000 Up to and including Size of drain valve mm 1600 10000 50000 100000 — 50 50 80 100 100 4.0 TYPE OF VALVES 4.1 Gunmetal Globe Valves Size of filter valve mm 25 25 50 50 50 Size of sampling valve mm 15 15 15 15 15 Size of vacuum valve mm --100 100 4.1.1 15 mm valves used for oil sampling or air release shall be made of gunmetal gate /globe valve (screw down stop) type generally conforming to class 1 of IS: 778 with inside screw stem. The oil sampling valve shall have provision to fix 15 mm PVC pipe. 4.1.2 The flanges shall be either drilled as per Table 1 (of this specification) or screwed as explained in Table 2 of IS: 778. 4.1.3 Preferred dimensions shall be as per Table 3. Specifications for Valves for Transformers 459 Table 3 Size of valve mm Face to face dimension mm 15 (Screwed) 57 15 (Flanged) 72 Thickness of flange mm 6.6 4.1.4 Tests (a) Pressure tests : (b) Body ..15 kg/cm2 Seat ..10 kg/cm2 Duration .. 2 minutes (c) Seepage test in open condition: Test pressure.. 2 kg/cm2 Duration .. 12 hrs. Note: Transformer oil (IS : 335) or water at ambient temperature shall be used for pressure and oil seepage tests. 4.2 Gunmetal Gate Valves 4.2.1 25 mm valves used for oil filtering, oil drainage or in Buchholz pipe shall be gunmetal gate (screw down step) type generally conforming to class 1 of IS : 778) with inside screw stem. 4.2.2 The flanges shall be drilled as per Table 1. 4.2.3 Preferred dimensions shall be as per Table 4. Table 4 Size of valve mm Face to face dimension mm Thickness of flange mm 25 (Flanged) 90 8.0 4.2.4 Tests as per clause 3.1.4 4.3 Cast Iron Taper Plug Valves 4.3.1 50 mm, 80 mm and 100 mm C.I. taper plug or gate valves may be used for oil filtering, oil draining. 4.3.2 Valves shall be self-lubricating, short pattern type, provided with valve operating wrenches. The lubricant used shall be compatible with transformer oil (IS: 335) at 100°C. 4.3.3 Preferred dimensions shall be as per Table 5. 460 Manual on Transformers Table 5 Size of valve mm Face to face dimension mm Thickness of flange mm 50 80 100 178 (7”) 203 (8”) 229 (9”) 19 19 22 4.3.4 Tests (a) Pressure tests Body ..15 kg/cm2 Seat ..10 kg/cm2 Duration ..2 minutes (b) Seepage test in open condition Test pressure..2 kg/cm2 Duration ..12hrs. Note: Transformer oil (IS : 335) at ambient temperature shall be used for pressure and seepage tests. 4.4 C.I. Gate Valves 4.4.1 50 mm, 80 mm, 100 mm, 150 mm, 200 mm and 250 mm, C.I. gate valves may be used in cooling water pipe lines. 4.4.2 C.I. gate valve shall generally conform to class PNI of IS : 780 double flanged, inside screw, non-rising spindle, Solid wedge gate and with screwed (inside screw type or bolted bonnet, Gunmetal trim material, teflon rope gland packings and nitrile rubber gaskets. The valves shall be provided with open and shut indicator. 4.4.3 Preferred dimensions shall be as per Table 6. Table 6 Size of valve mm Preferred face to face dimension mm Thickness of flange mm 215 230 255 280 318 355 16 18 20 22 25 26 50 80 100 150 200 250 4.4.4 Hydrostatic Pressure Tests Body ..15 kg/cm2 for 5 minutes Seat ..10 kg/cm2 for 2 minutes Specifications for Valves for Transformers 4.5 461 C.I. Check Valves 4.5.1 100 mm, 150 mm, 200 mm, and 250 mm C.I. check valves may be used in oil pipe lines of forced oil cooled transformers to avoid reverse flow and/or cross flow of oil where required. 4.5.2 C.I. Check valves shall be of horizontal swing check type generally conforming to IS: 5312 (Part I), without bypass arrangement. The weight of the swing (flap) shall be such that full opening is available at a minimum oil velocity of 1.0 m/s at a pressure of 1.5 kg/cm2. The pressure drop through the valve shall be less than 0.3 m of oil column (0.026 kg/cm2) with normal flow rates. The above requirement should be proved by calculations if test facilities not available. 4.5.3 Preferred dimensions shall be as per Table 7. Table 7 Size of valve mm Face to face dimension mm Thickness of flange mm 100 150 200 250 300 400 500 600 20 22 25 26 4.5.4 Testing as per CI. 7.1, 7.2 and 7.3 of IS: 5312 (Part 1). 4.6 Butterfly Valves 4.6.1 Cast iron butterfly valves shall be of the nominal sizes; 50, 80, 100, 150, 200 and 250 mm. These valves are used in oil pipe lines such as Buchholz relay pipe lines and as shut off valves between transformer main tank and cooler tank. 4.6.2 The body and cap of butterfly valves shall be made from any of the seven grades of grey iron casting conforming to IS: 210 except grade FG 150 and the disc shaft and stop pin shall be made from mild steel conforming to Fe 410-WA of IS : 2062 disc and body of the valve shall be in direct contact without any sealing ring. Valve is operated by a spanner and provision shall be given for locking in open/closed position. ‘OPEN’, ‘CLOSE’ markings on the body and an arrow mark on the tap for position indication shall be provided. To indicate the disc position in the cap removed condition, some suitable marking on the spindle head shall also be provided. 4.6.3 Preferred dimensions shall be as per Table 8. Table 8 Size of valve mm Face to face dimension mm Thickness of flange mm 50 80 100 150 200 250 40 40 40 40 40 45 16 16 16 16 16 22 462 4.6.4 Manual on Transformers Tests The valves shall be tested with transformer oil at 1.5 kg/cm2 with the assembled valve in closed/ open position, the leakage shall not be more than as specified below: (i) Leakage through body when pressure is applied for thirty minutes Nil (ii) Leakage around top spindle with cap fitted on Nil (iii) Leakage around top spindle when cap is removed: for 50, 80 and 100 mm valves-2 drops per minute (max.) for 150, 200 and 250 mm valves-8 drops per minute (max.) (iv) Leakage past diaphragm in closed position: for 50, 80 and 100 mm valves-6 drops per minute (max.) for 150, 200 and 250 mm valves-6 cc per minute (max.) 4.7 Radiator Valves 4.7.1 Radiator valves shall be of 80 or 100 mm size 4.7.2 Construction same as per clause 4.6.2 4.7.3 Preferred dimensions shall be as per Table 9. Table 9 Size of valve Size of flange square A/F mm Diameter of bolt circle mm No. of bolt Bolt hole size mm Face to face dimension mm Thickness of flange mm mm 80 150 160 4 18 40 16 100 180 180 4 18 40 16 4.7.4 Tests: Same as per clause 4.6.4 SECTION KK Specifications for Cable Boxes for SF6 Gas Insulated Transformer Terminations for Rated Voltages of 72.5 kV and above SECTION KK Specifications for Cable Boxes for SF6 Gas Insulated Transformer Terminations for Rated Voltages of 72.5 kV and above 1.0 SCOPE The purpose of this specification is to establish electrical and mechanical interchangeability between cable terminations and the gas-insulated enclosure and to determine the limits of supply. This technical specification covers the connection assembly of cables to SF6 gas insulated terminations (cable boxes) in single phase or three-phase arrangements, where the cable terminations are gas filled and there is a separating insulating barrier between the cable insulation and the gas insulation of the transformer/ switchgear. This also covers the connection assembly of extruded insulation cables to gas insulated enclosure, where the cable terminations are of dry type. In this arrangement, the cable termination design comprises an elastomeric electrical stress control component in contact with a separating insulating barrier between the cable insulation and the gas insulation of the transformer/ switchgear. The cable termination does not include any insulating fluid. 2.0 STANDARDS IS 2026 (Part 1) : Power Transformers - General. IS 2026 (Part 3) : Power Transformers - Insulation level and dielectric tests. IS 2026 (Part 4) : Power Transformer - Terminal marking, tapping and connections IS 2099 : Bushings for alternating voltages above 1000 V. 1EC 60137 : Bushings for alternating voltages above 1000 V. IEC 60141-1 : Tests on oil filled and gas pressure cables and their accessories - Part-1: Oil filled, paper or polypropylene paper laminate insulated, metal-sheathed cables and accessories for alternating voltages up to and including 500 kV. IEC 60141-2 : Tests on oil filled and gas pressure cables and their accessories - Part-2: Internal gas pressure cables and accessories for alternating voltages up to 275 kV. IEC 60517 : Gas insulated metal enclosed switchgear for rated voltages of 72.5 kV and above. IEC 60694 : Common specifications for high voltage switchgear and control gear standards. 465 466 Manual on Transformers I EC 60840 : Power cables with extruded insulation and their accessories for rated voltages above 30 kV (Um = 36 kV) up to 150 kV (Um = 170 kV) - Test methods and requirements. IEC 60859 : Cable connections for gas insulated switchgear for rated voltages of 72.5 kV and above. IEC 61639 : Direct connection between power transformers and gas insulated switchgear for rated voltages of 72.5 kV and above. 3.0 LIMITS OF SUPPLY The limits of supply for fluid filled cable terminations shall be according to Fig. 1 and for dry type cable terminations shall be according to Fig. 3. To limit the voltage under transient conditions, between parts 6 or 11 and part 13 of Fig. 1 for fluid-filled cable terminations and non-linear resistors (part 15) of Fig. 3 for dry type cable terminations may be connected across the insulated junction. The number and characteristics of the non-linear resistors shall be determined and supplied by the cable termination manufacturer, taking into consideration the requirements of the user and the switchgear manufacturer. 4.0 RATING When dimensioning the cable connection assembly, the following rated values shall apply: (a) Rated voltage (72.5 kV - 100 kV - 123 kV - 145 kV - 170 kV - 245 kV -300 kV - 362 kV - 420 kV - 550 kV) (b) Number of phases in one enclosure (One or three phases for Ur <170 kV and single phase for Ur>l 70 kV) (c) Rated insulation level (ref. IEC 50694) (d) Rated normal current and temperature rise (2000 A at a temperature 90° C is standardised for interchangeability) (c) Rated short time and peak withstand currents (ref. IEC 60517; cl. 4.5, 4.6 and 4.7) (f) Rated duration of short circuit Specifications for Cable Boxes for SF6 Gas Insulated Transformer Terminations for Rated Voltages of 72.5 kV and above 467 468 Manual on Transformers Specifications for Cable Boxes for SF6 Gas Insulated Transformer Terminations for Rated Voltages of 72.5 kV and above 5.0 DESIGN CONSTRUCTION REQUIREMENTS 5.1 Pressure Withstand Requirements 469 The maximum recommended design pressure (absolute) for the outside of the cable termination is 0.85 MPa. In addition, the bushing and cable termination shall be capable of withstanding the vacuum conditions when the transformer connection enclosure is evacuated as part of the gas filling procedure. The transformer connection enclosure shall satisfy the requirements as per IEC 60517 for the design pressure. The maximum operating gas pressure (absolute) of a direct connection assembly shall not exceed: • The design pressure of the transformer connection enclosure plus 0.1 MPa when the design pressure is lower than 0.75 MPa (gauge); • 0.85 MPa (absolute) when the design pressure equals or exceeds 0.75 MPa (gauge). 5.2 Mechanical Forces on Cable Terminations The manufacturer of the cable termination shall take into account, in the three-phase connection, the total dynamic forces generated during short circuit conditions. These forces consist of those generated within the cable termination and those coming from the main circuit of switchgear. The maximum additional force to be applied from the switchgear to the connection interface transversely and being transferred from the main circuit end terminal shall not exceed 5 kN. For single-phase connections, even taking into account lack of symmetry, it is considered that the additional force is small. However, a total mechanical force of 2 kN applied to the connection interface transversely should be assumed. It is the responsibility of the manufacturer of the switchgear to ensure that the specified forces arc not exceeded. 5.3 Mechanical Forces Applied on the Bushing Flange In addition to the maximum operating gas pressure specified 5.1, the flange of the bushing attached to the transformer connection enclosure is subjected, in service, to the following loads: • part of the weight of the switchgear not supported by the switchgear’s own supporting structures; • part of the wind load, if applicable, not supported by the switchgear’s own supporting structure’s; • expansion/ contraction stresses due to the temperature variations of the switchgear enclosures. For the evaluation of these stresses, it shall be considered that, on the transformer side the variation height of the bushing flange due to temperature variation docs not exceed ±0.0008 times the transformer tank height in the case of a steel tank. 470 Manual on Transformers These loads result in the simultaneous application, at the centre of the bushing flange, of: • a bending moment M0; • a shearing force Ft; • a tensile or compressive force Fa. The bushing and the transformer shall be capable of withstanding, in service, the values of Mo, Ft and Fa specified in Table 1, and it shall be the responsibility of the switchgear manufacturer to ensure that these values are not exceeded. Table 1 : Moment and forces applied on the bushing flange and transformer Rated voltage kV Bending moment M0 kNm Shearing force Ft kN Tensile or compressive force Fa kN 72.5 - 100 123 - 170 245 - 300 362 - 550 5 10 20 40 7 10 14 20 4 5 7 10 Except where specified otherwise by the customer, the relative positions and levels of the transformer and switchgear foundations respectively shall be considered as not varying. 5.4 Vibrations The vibrations generated inside the energised transformer are transmitted by the oil and the tank wall of the transformer to the bushing rigidly fixed on this wall and to the switchgear. The switchgear manufacturer and the transformer manufacturer shall agree to take into account these vibrations. 6.0 STANDARD DIMENSIONS AND SPECIAL REQUIREMENTS 6.1 Fluid Filled Cable Terminations Standard dimensions for fluid filled cable connection enclosures, main circuit end terminals and cable terminations applied to single phase enclosures are shown in Fig. 2. With the given four standard sizes, the voltage range (Ut) from 72.5 to 550 kV is covered. 6.2 Dry Type Cable Terminations Standard dimensions for dry type cable connection enclosures, main circuit end terminals, and cable terminations applied to single phase enclosures are shown in Fig. 4. With the given four standard sizes, the voltage range (Ut) from 72.5 to 550 kV is covered. Figure 3 shows the two types of dry type cable termination. Type A incorporates an elastomeric electrical stress control component inside the insulating barrier. For type B, the elastomeric electrical stress control component is located is located externally. 7.0 TESTS 7.1 General The testing of the cable termination and the gas insulated metal enclosed switchgear is to be performed for cable terminations in accordance with [EC 60141-1 for oil filled cables, IEC 60141-2 for gas filled cables, IEC 60840 for cables with extruded insulation and IEC 60517 for switchgear. In addition, this specification gives recommended arrangements for dielectric tests Specifications for Cable Boxes for SF6 Gas Insulated Transformer Terminations for Rated Voltages of 72.5 kV and above 471 472 Manual on Transformers Specifications for Cable Boxes for SF6 Gas Insulated Transformer Terminations for Rated Voltages of 72.5 kV and above 473 and for the tests after cable installation. 7.2 Dielectric Type Tests 7.2.1 Dielectric Type Tests of Cable Terminations The dielectric type test of the cable termination fitted with a representative cable shall be performed in an enclosure filled with insulating gas at a pressure according to the values specified in Table 2. If a shield is an integral part of the cable termination design, it shall be mounted in its service position during the test. An additional test shield may be used to screen the exposed connection interface, if required by the cable termination manufacturer, provided it does not overlap the connection interface by more than the distance l2 in Fig. 2 for fluid filled cable termination. Table 2 : Gas-pressure limits for dielectric type test of cable-terminations Range of rated voltages Ut kV 72.5 - 100 123 - 170 245 -300 362 -550 Minimum SF6 functional pressure °C (absolute) at 20°C MPa 0.10 0.30 0.35 0.40 Notes : The gas pressures are intended as a guideline for the manufacture of the cable-termination. Higher values are permissible. If a gas other than SF6 is used the minimum functional pressure should be chosen to give the same dielectric strength. 7.2.1.1 Dielectric Type Test of Single-Phase Cable-Terminations The cable-termination is surrounded by a metal cylinder connected to earth, the internal diameters of which are 300 mm, 300 mm, 480 mm and 540 mm respectively for the four standard sizes of cable connection enclosure (d5 in Fig. 2 for fluid-filled cable-terminations and Fig. 4 for dry type cable-terminations). 7.2.1.2 Dielectric Type Test of Three-Phase Cable-Terminations The non-symmetrical arrangement for Ut <170 kV with a cylinder of 650 mm internal diameter (Fig. 5) is intended to simulate the conditions for a three-phase cable connection. Cableterminations, which are intended for a single-phase cable connection, may be tested in the same cylinder of 650 mm internal diameter to cover all expected sizes of cable connection enclosures. 7.2.2 Dielectric Type Test of Cable Connection Enclosures The cable connection enclosure and main-circuit end terminal may be subjected to the dielectric type test according to IEC 60517 without the cable-termination. 474 Manual on Transformers Fig. 5 Non-symmetrical arrangement for dielectric tests of cable terminations The gas pressure for the dielectric type test may be specified by the manufacturer of the switchgear in accordance with Table 1. 7.3 Tests after Cable Installation If parts of the switchgear directly connected to the cable connection assembly cannot withstand, at rated filling density for insulation gas, the test voltage specified for the cable test (IEC 60141 and IEC 60840), or, if in the judgement of the switchgear manufacturer, it is not acceptable to apply the test voltage to the switchgear, the switchgear manufacturer should make special Specifications for Cable Boxes for SF6 Gas Insulated Transformer Terminations for Rated Voltages of 72.5 kV and above 475 provisions for the testing of the cable, for example disconnecting facilities and/or increasing gas pressure in the cable connection enclosure. If required by the user, the switchgear manufacturer shall provide the location for a suitable test bushing and provide the user with all necessary information for mounting such a bushing to the cable connection enclosure. For cases where electrical clearances are inadequate, the term bushing shall include a suitable insulated connection and test terminal. The requirement for the test bushing shall be specified by the user in the enquiry. 8.0 INFORMATION TO BE GIVEN WITH ENQUIRIES, TENDERS AND ORDERS Refer to IEC 60840, IEC 60141 and clause 9 of IEC 60517. In addition, the user and the manufacturers shall consider the installation requirements of the equipment. Manufacturers shall state the specific requirements for civil, electrical and installation clearances applicable to the switchgear, cable-termination and cable. 9.0 RULES FOR TRANSPORT, STORAGE, ERECTION, OPERATION AND MAINTENANCE Refer to IEC 60694, clause 10. The cable-termination manufacturer should ensure that during manufacture, handling, storage and installation of the cable-termination, provisions should be made to ensure that the requirements given in 5.2 of IEC 60694 can be satisfied after final assembly of the connection. The cabletermination manufacturer should supply the necessary information to enable these requirements to be satisfied, if the cable-termination is to be installed by others. Note : It should be noted that increasing the gas pressure is not a reliable method of improving the electrical strength at the surface of an insulator when tested with d. c. voltage. APPENDICES Appendix I New Technologies, UHV AC/DC Transformers, Phase Shifting Transformers, Controlled Shunt Reactors Appendix I NEW TECHNOLOGIES, UHV AC/DC TRANSFORMERS, PHASE SHIFTING TRANSFORMERS, CONTROLLED SHUNT REACTORS 1. TRANSFORMERS FOR SMART GRID Smart grid is the future of electrical systems, designed to meet the four major electricity requirements of the global society viz. capacity, reliability, efficiency and sustainability. Transformers in future will have to meet the demands of smart grid. Trends and changes required in transformer technology to meet this new development in transmission and distribution of electric power are reviewed below. Transformers will be called upon to meet more and more emergency over loadings to maintain the supply even with equipment breakdowns with minimal spare capacity. Maintenance free accessories will be demanded for transformers especially tap-changers, silica gel breather. Hot dip galvanized radiators and Distribution transformers with stainless steel tanks or superior surface treatment systems will be demanded to eliminate maintenance. Fail-safe transformers, without causing fires on failure will be demanded. More and more dry type (Resin impregnated Paper – RIP) condenser bushings will be used in critical transformers. Failure of such bushings will not cause oil spill and consequent fires. High flash point insulating fluids (e.g., Synthetic Esters) will be preferred for transformers required in thickly populated urban areas. Advanced on line monitoring, control and diagnostics will be incorporated in critical transformers. Utilities will demand early detection devices more and more. Multiple intelligent electronic devices (IED) will monitor the behaviour of the core, windings, oil, tap changer and bushings of transformer. Green Transformers are being offered by a number of manufacturers. These are transformers with high efficiency and with bio-degradable insulating fluid instead of mineral oil( natural esters made from sunflower oil). High efficiency is achieved at no-load by using amorphous cores or superior grade silicone steels, working at low magnetic flux density. Low current density with unique measures for stray loss reduction, results in low load losses. Utilities will insist for star rated distribution transformers with low levels of losses. Environment friendly manufacturing processes will be used in the production of transformers: Mercury contacts from instruments are changed to magnetic contacts. Silica gel with cobalt chloride (standard blue indication - cobalt salts are suspected to be carcinogenic) are replaced with gel with organic indicators. Asbestos paper is totally prohibited as insulation. Water based paints are replacing solvent paints to reduce carbon emission. Silent transformers will be demanded in future. Transformer noise, major pollutant, is no more tolerated in cities. Even at 100 MVA units, 45- 50 dB noise levels are possible. Increased adoption of renewable power generation will feed power directly to distribution lines, causing voltage up swings at distribution voltage. To check this, distribution transformers will have to be fitted with on-load tap changers. 481 482 Manual on Transformers Smart grid technology is not a single silver bullet but rather a collection of existing and emerging technologies working together. When properly implemented, these technologies will increase efficiency in production, transport and consumption, improve reliability and economic operation, integrate renewable power into the grid, and increase economic efficiency through electricity markets and consumer participation. 2 UHV AC & DC TRANSFORMERS 2.1 UHV AC Transformers Currently the maximum AC transmission voltage in the country is 800 kV and the next transmission voltage being planned is at 1200 kV. Such UHV lines will be required for transferring large power for short distances (say 6000 MW for 200-500 kms) for interconnecting high load centres with minimum right of way (ROW) and low transmission losses. PGCIL has put up a national 1200 kV test station at Bina, Madhya Pradesh where it will carry out field tests and evaluation of transmission and distribution equipment developed for 1200 kV power transmission. Two banks of 1000 MVA 1150/400 kV auto-transformer banks are used at Test station to step up power and then to step down to the grid. Transformers actually used for UHV transmission will be of 3000 MVA bank capacity, consisting of 1000 MVA single phase units. These will be made of 5 limbed core with three parallel wound limbs of 333 MVA capacity. Tertiary voltage will be 33-66 kV with possible use for reactive compensation. Proposed specifications of such transformer will be as below: Table 1 : Proposed specifications of 1000 MVA 1200 kV single phase Transformer Rated Power(MVA) HV/IV/LV 1000/1000/333 MVA Rated no load voltage(kV) HV/IV/LV 1150/√3/400/√3/33 Rated frequency and No of phase 50Hz , 1 Phase Percentage impedance HV-IV 18% (+/- 10% tolerance) HV-LV 40% (Min) IV-LV 20% (Min) Insulation Levels(HV/IV/LV) Lightning Impulse withstand level (kVp) 2250/1300/250 Switching Impulse withstand level(kVp) 1800/1050/- One Minute Power Frequency withstand voltage(kVrms) 1040/- Cooling OFAF/ODAF Temperature rise Top oil 400C Mean winding 450C Maximum Partial discharge level 300pC at 1.5 Um/√3 Sound pressure level at rated voltage 90dBA New Technologies, UHV AC/DC Transformers, Phase Shifting Transformers,Controlled Shunt Reactors 483 The major challenge for such units will be limiting transport weight to 325 Tonnes, probable limit of transportation.1200 kV OIP Condenser bushing, 14 m long, weighs nearly 6 tonnes. When the commercial units are to be made, on-load tap-changing will be achieved by providing tappings at the neutral end for HV or MV variation. MV variation tap changer will result in less variation in flux density in the core. Due to transport weight restrictions, tap winding and excitation winding will be accommodated in a separate tank with core for connection to neutral end of main winding. 2.2 UHV DC Transformers EHV and UHV DC transmission is used to for transferring large power for long distances with minimum right of way and line losses. It is the most environmentally friendly and economical way of transmitting large amounts of electric power. Compared to AC, DC transmission needs much narrower right-of-ways ,while higher voltage reduces both electricity losses and the cost of building large scale power lines. As generation takes place further and further away,higher and higher transmission voltgaes are required. Currently we have DC transmission lines at 500 kV and new lines at 800 kV are being executed in the country. These will be capable of transmitting up to 8000 MW for 2000-3000 km. UHV DC transmission is being developed for 1100 kV DC where two conductors will carry 10,000 MW for 5000 km. The AC to DC converters are built as ± 800 kV double circuits with eight series connected, six pulse converters. The transformers are single phase two winding units. In total 24 converter transformers are needed at each end of the line. Depending on the position of transformers within the converter ,four different designs are needed with different DC voltage ratings (800, 600, 400 and 200 kV) where the transformers connected to the uppermost and lowermost bridges had to be built for the highest DC potential. (see sketch below) 484 Manual on Transformers The basic function of HV DC converter transformer is to adjust the line voltage of the AC side to the HV DC transmission voltage. It should also meet the following requirements: • A galvanic separation between the DC and AC systems • Specified short circuit impedance • High content of current harmonics • Large range of voltage regulation by on-load tap-changer. In conventional AC/DC converters, the transformer acts as a barrier to prevent DC voltage from entering the AC network. One of the transformer windings is connected to the AC side, which is also called the line-side winding. The other winding is connected to the converter valves, called the valve-side winding. HVDC Transformer is used for stepping down the AC voltage to suit the converter or inverter valve voltages. The valve windings of transformer should be capable of withstanding DC voltages that will impinge on them during the pole reversals, necessary for reversing the power flow in lines. During DC voltage applications, resistivity of insulation structure will predominate instead of permittivity during AC application. DC voltage results in additional demands on the insulation structure in comparison to AC voltages. The design of the valve is such that the rate of current increase must be controlled when the valve starts carrying current. The rate of increase largely depends on the transformer reactance, which also has to be fulfilled within narrow limits for two individual transformer units. The high content of current harmonics requires special attention be paid to controlling additional and stray losses in the transformer, when it comes to total losses and the risks of local overheating in the windings and metallic components exposed to stray flux from windings and internal current carrying leads. In order to optimize the reactive power needed for the operation of the converter, depending on load variations the system designer generally specifies a large range of voltage ratio variation between the line and valve sides. The transformer concept normally used is a single-phase design, with two wound limbs and two outer limbs for the return flux. The windings are arranged concentrically with the valve winding on the outside. The line winding is divided into two coils – the one for the tapped part is located closest to the core, followed by the non tapped section. This arrangement is beneficial for the topology of the valve-side, which require AC as well as DC insulation. Compared with a conventional power transformer in the AC network, an HVDC converter transformer must be tested for the ability of valve-side windings to withstand. DC voltages. In operation, the valve windings are exposed to an AC voltage and a superimposed DC voltage. A DC transmission must be able to handle the fast transition of power from one direction to the other. Such transitions also mean a switch in converter polarity, from positive to negative, and vice versa . Operation with continuous DC voltage, superimposed. New Technologies, UHV AC/DC Transformers, Phase Shifting Transformers,Controlled Shunt Reactors 485 AC voltage and DC polarity reversal will be reflected in additional dielectric tests of the valveside windings; tests with DC voltage, tests with AC voltage and tests with switching surge voltage are in accordance with IEC standards. All four types of test are considered to be non transient, with a uniform voltage along the valve winding. For that reason, the two external terminals of the winding are connected together and the voltage is applied to the two terminals simultaneously. During the test with applied DC voltage, the level of partial discharge is measured. During the transient period after the application of voltage, there may be Occasional charge movements within the insulation system. These movements give rise to a noticeable partial discharge signal on the valve-side terminals. The phenomenon is well known and recognized in current standards. The industry has therefore accepted an upper limit on the number of occasions such bursts of partial discharge can take place during the tests. Furthermore, the frequency of bursts must diminish during the course of the test. 3. PHASE SHIFTING TRANSFORMER Phase shifting transformers help to control the real power flow in transmission lines. Existing transmission systems are often operated and stressed to the limit of their performance capability of their original design. In order to increase the power transfer through existing lines, phase shifting transformers is a solution to maximize asset utilization. To ensure that under these conditions the economical, reliable and secure operation of the grid is maintained, the need for various aspects of power flow management within the power systems is becoming evident. Phase shifting transformers help control the real power flow in a complex power distribution network in a very efficient way. They allow for better utilization of existing networks allowing load growths. Function of phase shifting transformer can be described through the current distribution over parallel lines. The natural distribution of current depends on impedance of lines. If X/R of two lines are extremely different then distribution is inefficient. There will be difference in phase angle and magnitude of load voltage UL1 and UL2. The two load sides are connected together, thus equivalent reactance reduces and system stability improves. For the load voltages to be equal load currents I1 and I2 are different in magnitude and phase angle. Based on the currents I1 and I2 this difference can be expressed by a circulating current IC flowing in the loop. Tie line 1 is overloaded by circulating current IC and tie line 2 is under loaded by same amount. By adding phase shifting transformer in one of the tie line the circulating current can be controlled by changing the phase angle between the two tie line voltages. The load currents can thus be equalized. 486 Manual on Transformers The Electric Power Flow Line reactance form major portion in line impedance, inserting a voltage in phase with or opposite to the line voltage i.e. changing magnitude of voltage will have impact on reactive power. The boost voltage with phase angle perpendicular to the line voltage /i.e., creating a phase shift influences active power. Following example shows calculation of active, reactive power and load angle. System specification: Base MVA: 400 MVA Phase shifting transformer: 400 MVA, 220 kV ± 10 x 1.5% & 10x1.20 %X = 12%, %R = 0.17% (based on load losses) Overhead line: Length 50 km, r =0.063Ω /km, x = 0.36Ω/km Solution: Base impedance = R=0.063 x 50 = 3.15Ω X= 0.36 x 50 = 18Ω ZTotal p.u = Ztransformer p.u + Zline p.u = (0.0017 + j0.12) + (0.026+j0.149) = 0.028 + j 0.27 ≅ j 0.27 (mainly reactive in nature) = XTotal p.u New Technologies, UHV AC/DC Transformers, Phase Shifting Transformers,Controlled Shunt Reactors Current lags 900 to due to purely inductive nature. Reactive power, Q=√S2–P2 Phase Regulation using Phase shifting Transformer: Vsource=1pu∠00 Vload=1pu∠-15.660 PST rating 220 kV±10x1.20 Δφ = ± 120 Vload=1pu∠-15.660 - Δφ)0 487 488 Manual on Transformers 1. Phase shifting transformer phase angle set at -12° i.e. - 10 x 1.2° (Δφ= - 120) 2. Phase shifting transformer phase angle set at +12° i.e. + 10 x 1.2° (Δφ =+120) New Technologies, UHV AC/DC Transformers, Phase Shifting Transformers,Controlled Shunt Reactors Operating condition Active Power (P) Reactive Power (Q) Initial Condition (∆φ=O°) 1 pu 0.14 pu ∆φ= –12° 0.2364 pu 0.01 pu 0.7636 pu 0.13 pu ∆φ=-12° 1.72 pu 0.42 pu -0.72 pu -0.28 pu 489 Change in Active Change in power (∆P) Reactive power (∆Q) Significant variation in active power is achieved by changing phase angle. Minor change is observed in reactive power. Types of Phase Shifting Transformer 1. A nonsymmetric, quadrature type and single core PST The phase shift between source and load terminals is achieved by connecting the regulating winding of phase B to the delta connection point of phase A and C, and analogous for the remaining phases. A quadrature voltage that can be regulated by means of the variable tap is added to the input voltage in order to obtain a phase shift ‘α’. The direction of phase shift can be change by using the switches. In this manner, the power flow in the line can be increased or decreased. The relation between the tap position and angle ‘α’ is non-linear and can be derived from phasor diagram: ...(1) 490 Manual on Transformers The relation between the output voltage and the injected quadrature voltage is given by: ...(2) Substitute angle ‘α’ value from equ1 to equ 2. ...(3) The power flow through the line is increased by adding an angle ‘α’ to the existing angle’δ’. ...(4) After simplification, ...(5) Relation between P, α and the quadrature voltage for a direct asymmetrical PST with δ = π 6 New Technologies, UHV AC/DC Transformers, Phase Shifting Transformers,Controlled Shunt Reactors 491 The curve of angle ‘α’ is relatively linear up to 0.6 rad i.e. 34°. 2. A symmetric, quadrature type and single core PST A symmetric design is made with an additional regulating winding and an additional tap changer. The maximum possible phase angle can be increased significantly. The relation between the quadrature voltage and the angle ‘α’ is still non-linear and can be derived from phasor diagram, ...(6) Substitute value of angle ‘α’ in active power equation. ...(7) The curve of angle ‘α’ is relatively linear up to 1.5rad i.e. 85°. Range of linearity has increased. 492 3. Manual on Transformers A symmetric, single core PST with hexagonal winding connection. An alternative implementation of a direct and symmetrical PST is shown in above fig. The resulting phasor diagram has hexagonal shape. Additional impedance, connected to the load side terminals is necessary, to protect the tap changer from short circuit currents, because no transformer impedance is present at phase angle zero. 4. A symmetric, quadrature type and dual core PST with series and exciting unit. Symmetric, quadrature type, dual core PST with series and exciting unit The series winding between source and load terminals is split into two halves, and the exciting winding of the exciting unit is connected to the connection point of these two windings. Thus, total symmetry between source and load is achieved. The regulating circuit, consisting of the tap winding in the exciting unit and an exciting winding in the series unit, can be design New Technologies, UHV AC/DC Transformers, Phase Shifting Transformers,Controlled Shunt Reactors 493 independently from main circuit, with regard to the voltage level. This provides more freedom for the selection of the tap changer, which sometimes is quite difficult and can even determine the limits for a specific design. 5. Asymmetric, quadrature type and dual core PST with series and exciting unit Series winding is not split into two halves, thus voltage does not get added symmetrically. Therefore this arrangement becomes asymmetrical. Exciting winding is connected at load terminals. This arrangement gives freedom in selection of Tap changer. Independent design of exciting winding is possible from main circuit. The selection of OLTC for PST To achieve variable phase angle shift, OLTCs are used in PSTs. Phase shifting transformer can be built to provide discrete phase angle shift, continuous variable phase angle shift, or a combination of both. Discrete phase angle shifters normally provide settings for a plus-orminus fixed degree value and zero. If a variable phase shift is desired, an On Load Tap-Changer (OLTC) is required. OLTCs are used to provide several tap positions for any desired angle range. A Phase-Shifting Transformer can be constructed as a shell or as a core form transformer. In addition to the MVA rating, the amount of phase shift also directly affects the physical size of the transformer. Two typical types of regulation are used with OLTCs. One involves direct regulation at the line end and the other involves a series and an exciting unit. 1. Direct regulation at line end The tapped windings are wound on the same core leg as the main delta winding and the OLTCs are designed as reversing types. The tapped windings of phase 1(energized by Ue) are connected between phases 2 and 3 produces the voltages UAA’ and UA’A’’. The voltages of the source (US1) 494 Manual on Transformers and load side (UL1) are out of phase by an angle α . The input and output voltage magnitudes are equal. While selecting an OLTC the maximum through-current and the maximum step voltage Ust must be taken into consideration. Because the phasor of the exciting winding is located electrically between the phasors of US and UL, account must be taken of the fact that exciting voltage changes with the phase angle. Delta hexagonal phase shifter These transformers have OLTCs with linear regulation, i.e. without changeover selector. The tapped winding is wound on the same core leg as the main delta winding. The tapped winding of phase 1 is located between phases 2 and 3 and produces the voltage UAA’. The input and output voltage magnitudes are equal. New Technologies, UHV AC/DC Transformers, Phase Shifting Transformers,Controlled Shunt Reactors • 495 Potential connection of the tap winding In case of phase-shifting transformers with regulation at the line end and reversing type OLTC, high recovery voltages can occur due to the winding arrangement. There are methods available to reduce recovery voltage viz. use of tie in resistors, use of electrostatic screens to shield the tap windings and use of Advance Retard Switch (ARS) as the change-over selector. This additional unit allows the change-over operation to be carried out in two steps without interruption. With this, the regulating winding is connected to a fixed potential for the whole operation. The limiting factors for the ARS are the processes of commutation which have to be controlled by the ARS. Minimum number of OLTCs required for regulation at the line end Regulation Number of OLTCs OLTC with reverser 3 two-phase-units Or 6 single phase units Number of ARSs None OLTC plus ARS acting as change6 single phase units over selector 3/6 units OLTC without change-over selector None 6 single phase units 2. Regulation with Series and Exciting Unit These types of transformers basically consist of a series unit and an exciting unit, located on two separate cores. These units can be enclosed in one tank, but for large capacity transformers, they are constructed in two separate tanks. 496 Manual on Transformers Minimum number of OLTCs required in case of regulation using exciting unit Regulation Number of OLTCs With reverser 3 phase-units(star point) Or 3 single phase units With one coarse winding 3 phase-units(star point) Or 3 single phase units With several coarse windings 3 phase-units(star point) Or 3 single phase units With two OLTCs in series with coarse change over selector Number of ARSs None 1/3 units 1/3 units 3 phase-units(star point) Or 3 single phase units and 3 single phase units 1/3 units Phase Shifting Transformer Ratings & Specifications - Shunt Unit Three phase rating MVA Voltage ratio Tapping range per cent Percent impedance Voltage Cooling Phase Shift Angle As per line MW rating 400/V1/V2/33 Tapping range may vary from ±10% to full tapping winding ie ±100% Between main windings it shall be as per system requirement. ONAN/ ONAF/ OFAF For PST system, shall be as per system requirement. Generally between 5ºto 15º Between other pair of windings it shall be as per system requirement. Note : V1/V2 and other voltages shall depend on the usage of PST ie the lines in which PST is required. New Technologies, UHV AC/DC Transformers, Phase Shifting Transformers,Controlled Shunt Reactors 497 - Series Unit Three phase rating MVA Voltage ratio Tapping range per cent Percent impedance Voltage Cooling Phase Shift Angle Shall depend on the amount of phase shift required. V3/V4 NA for series unit Shall be as per system requirement. ONAN/ ONAF/ OFAF For PST system, shall be as per system requirement. Generally between 5º to 15º Note : V3/V4 shall depend on the usage of PST ie the lines in which PST is required. In practice many solutions are possible to the design of PST. The user’s electric power system requirements and the manufacturer’s preference generally determine the design. The major factors determining the type of PSTs are given below: Performance factors - The power rating and phase-shift angle requirements The voltages The connected system’s short-circuit capability. Design factors: - Type of construction (core form or shell form) Shipping limitations OLTC performance specifications These factors decide whether a single-core, two-core type or a single tank with two cores has to be chosen. The most popular of all the above is two core design. Every design has its own advantages and disadvantages. Other Parameters (Shunt Unit) (i) Connections HV Star neutral effectively earthed, IV & LV Star neutral effectively earthed, SV delta (ii) Connections symbol YNyn0yn0d11, YNyn0yn0d1 etc (iii) Tappings As in table above (iv) Three-phase rating should be understood as three phase bank rating and not necessarily three-phase unit rating. Wherever transport restrictions impose, single phase ratings equal to one-third of the three phase bank rating may be specified. (v) ONAN rating shall be guaranteed at 60 per cent of the OFAF, or ODAF rating. Rating under ONAF condition shall be about 80 per cent. 498 (vi) (vii) (viii) Manual on Transformers Air core reactance of HV winding shall not be less than 20 per cent and knee point voltage shall not be less than 1.1 pu. Short circuit level - Transformer shall be suitable for connection to for 420 kV system the system having the following short circuit and duration levels and duration: 40,50 and 63 kA for one second. Terminal Bushings (a) HL/IV/LV/SV Terminals: Oil filled condenser bushing. No arcing horns shall be provided. (b) Neutral End: 36kV porcelain bushing. No arcing horns shall be provided. Other Parameters (Series Unit) (i) Connections HV Open delta, LV Delta (ii) Connections symbol Dd9, Dd3 (for quadrature boosters) (iii) Tappings As in table above (iv) Three-phase rating should be understood as three phase bank rating and not necessarily three-phase unit rating. Wherever transport restrictions impose, single phase ratings equal to one-third of the three phase bank rating may be specified. (v) ONAN rating shall be guaranteed at 60 per cent of the OFAF, or ODAF rating. Rating under ONAF condition shall be about 80 per cent. (vi) Air core reactance of HV winding shall not be less than 20 per cent and knee point voltage shall not be less than 1.1 pu. (vii) Short circuit level of transformer shall be suitable for connection to for 420 kV system the system having the following short circuit and duration levels and duration: 40,50 and 63 kA for one second. (viii) Terminal Bushings (a) HV & LV Terminals: Oil filled condenser bushing. No arcing horns shall be provided. (b) Tolerance on percentage impedance voltage shall be as under: Pairs of windings Tolerance HV-IV (for shunt unit) HV-LV (for series unit) ±10 per cent ±10 per cent Phase Shifting Transformer Testing Most of the rules and standards which exist for power transformers can be applied to PST’s as well. However, there are a few topics which are specific to PST’s and some of the more important are related to testing practice. New Technologies, UHV AC/DC Transformers, Phase Shifting Transformers,Controlled Shunt Reactors 499 One of these is heat run test, which can differ from the normal practice because of the complexity of the unit. This becomes obvious for a dual core design. Normally total losses are applied during the initial phase of the heat run, in order to determine the oil temperature rise. As this is done with a short circuit condition, no losses are actually generated in the core, and the actual core losses need to be generated by increasing the load current and thus the load losses accordingly. This works fine for any normal transformer, but it needs some additional consideration for a dual core phase shifter. Due to specific properties of dual core PST’s it is not possible to achieve the correct total losses for both active parts at the same time. The two active parts would always have the same losses in the maximum tap position. Thus either separate heat runs are necessary or a correction must be applied. For a dual core design of a quadrature type phase shifter; With the PST in service, normally both ends of the series winding are exposed to disturbances in the system. The standard lightning impulse test at all terminals of the PST helps to ensure that the series winding is able to withstand these stresses. A PST can face even more severe stresses during a lightning storm: In case of a closed by-pass breaker, a lightning impulse will appear simultaneously at the source and the load side terminal. This very likely produces oscillations with high magnitude at the center of the series winding, where the exciting winding of the exciting unit is connected. If such a condition is likely to appear during normal operation, it might be useful to specify a lightning impulse test with source and load terminals connected, to ensure that the PST can withstand the stresses of a lightning stroke in this situation. Switching impulse testing can cause different kinds of problems. As usual, the secondary side needs to be left open, in order to achieve an appropriate wave shape. It can then happen, influence by the mutual interconnection of windings in different phases, that the magnitude of the voltage oscillation at the secondary terminal exceeds the applied voltage. Therefore it is sometimes appropriate to make the switching impulse test “indirectly”, which means that the tested terminal is not the terminal, where the impulse is applied. Phase shift angle measurement shall be done by interconnecting both the units of PST. Typical Connection diagram of PST 500 Manual on Transformers REFERENCES 1 Walter Seitlinger Phase Shifting Transformers, Va Tech T&D (Va Tech Elin Transformatoren Gmbh 2. A.Kramer and J.Ruff Transformers for Phase Angle Regulation Considering the Selection of On-Load Tap Changer. IEEE Transactions on Power Delivery, Vol. 13, No. 2, April 1998. 3. Jody Verboomen, Member IEEE, Dirk Van Hertem, Member IEEE, Pieter H. Schavemaker, Wil L. Kling, Member IEEE, Ronnie Belmans, Fellow IEEE Phase Shifting Transformers: Principles and Applications. 4. Josef Rusnak Power Flow Control By Use Of Phase Shifting Transformer 5. Paola Bresesti, Member, IEEE, Marino Sforna, Vittorio Allegranza, Deniele Canever, and Riccardo Vailati Application of Phase Shifting Transformers for a secure and efficient operation of the interconnection corridors. 6. ANSI C57.135: IEEE guide for the application, specification and testing of phase shifting transformers • Controlled Shunt Reactors Application & Principle The basic principle of CSR is to control the reactive power by using a thyristor valve, which can provide the necessary speed of switching and control by means of firing angle control. The CSR generally consists of controlled shunt reactor transformer (CSRT) with apprx. 100% impedance, thyristor valves, controller, neutral grounding reactor, necessary circuit breakers and other auxiliaries. The CSRT is a power transformer with nearly 100% impedance between primary and control winding (secondary winding). In other words when the control winding is short circuited with rated voltage applied on the primary winding, the reactive power flow will be similar to the power flow in a shunt reactor of similar rating connected to the same rated voltage. The CSR is not merely a substitute of Shunt Reactor, but the advantages of CSR are much more than a shunt reactor. The CSR offers various added advantages like fully controllable reactive power, reduction in dynamic over voltages, increased power carrying capacity of lines, fast response, full compatibility to single phase auto reclosure, economy of size, minimum harmonics etc. New Technologies, UHV AC/DC Transformers, Phase Shifting Transformers,Controlled Shunt Reactors 501 The rating of CSR depends on the line parameters i.e. the line length and the compensation required. Further the control winding design is dependent upon the control winding current which is governed by the max. current allowed for the thyristor valves. Typical Ratings & Specification Three phaserating MVA Voltage ratio Generally 50, 63 400/V1/V2 and 80 MVAr for 400 kV line or as per compensation required for line. Tapping range per cent Percent impedance Voltage Cooling NA Between Primary (HV)-Secondary ONAN/ ONAF/ OFAF (Control) it shall be 100%. Between other pair of windings it shall be as per system requirement. Note : V1/V2 shall depend on the current carrying capacity of thyristor valves. In practice many solutions are possible to the design of CSR. The basic variants of CSR are : - CSR with continuous control. - CSR with On/Off thyristor control. - CSR with VCB control. Other Parameters (i) Connections HV Star neutral effectively earthed, Control Star neutral effectively earthed, SV delta (ii) Connections symbol YNynd11, YNynd1 etc. (iii) Tappings (iv) Three-phase rating should be understood as three phase bank rating and not necessarily three-phase unit rating. Wherever transport restrictions impose, single phase ratings equal to one-third of the three phase bank rating may be specified. (v) ONAN rating shall be guaranteed at 60 per cent of the OFAF, or ODAF rating. Rating under ONAF condition shall be about 80 per cent. NA 502 Manual on Transformers (vi) Air core reactance of HV winding shall not be less than 20 per cent and knee point voltage shall not be less than 1.1 pu. (vii) Short circuit level - Transformer shall be suitable for connection to for 420 kV system the system having the following short circuit and duration levels and duration: 40,50 and 63 kA for one second. (viii) Terminal Bushings (a) HL/IV/LV/SV Terminals: Oil filled condenser bushing. No arcing horns shall be provided. (b) Neutral End: 36 kV porcelain bushing. No arcing horns shall be provided. Typical Connection diagram of CSR Principle of Controlled Shunreactor Testing of CSR Testing of CSR shall be in line with IEC 60076 as is followed for 400 kV class power transformers. Appendix II Reference Standards and Books Appendix - II REFERENCE STANDARDS AND BOOKS As on April 2013 1. SPECIFICATIONS IEC ANSI / IEEE IS -Oil filled 60076-1 C57.12.00 2026-1 -Dry 60076-11 C57.12.01 11333 Reactors -Specifications -Testing -Dry type series -Smoothing reactors for HVDC 60076-6 C57.21 (Shunt) - Self protected liquid filled transformer 60076-13 Design and application of liquid-immersed power transformers using hightemperature insulation materials 60076-14 -SF-6 filled 60076-15 Transformers for Wind Turbine applications 60076-16 -Converter transformer 61378-1 C57.18.10 -HVDC transformer 61378-2 C57.129 -Traction transformer 60310 -Phase shifting transformer 62032 Specifications C57.16 1277 C57.135 Transformers < 230 kV, 1~10 MVA single phase, 0.8~100 MVA 3 Phase C57.12.10 Overhead distribution transformers < 500 kVA 34.5/13.8 kV C57.12.20 Pad Mounted Compartmental type Single Phase distribution transformers HV 34.5 kV LV 240-120 V =<167 kVA C57.12.21 Pad mounted 3 Phase DT < 2.5MVA 34.5/0.48kV C57.12.22 505 5553 Part 1-8 506 Manual on Transformers IEC ANSI / IEEE Pad mounted single Phase DT < 167 kVA 34.5/0.48kV With separable HV Connector C57.12.25 Pad mounted 3 Phase DT-with insulated HV connectors < 2.5MVA 34.5 kV with separable HV connector C57.12.26 Pad mounted 3 Phase DT < 5 MVA 34.5/0.48kV C57.12.34 Enclosure Integrity-Pad mounted C57.12.28 Enclosure IntegrityPad mounted for coastal environments C57.12.29 Enclosure Integrity-Pole mounted C57.12.31 Enclosure IntegritySubmersible equipment C57.12.32 Electronics Power Transformers 295 Liquid immersed distribution substation transformers C57.12.36 Secondary network transformers – subway and vault type C57.12.40 Secondary network protectors C57.12.44 Ventilated, Dry type power transformers 1~500 kVA single phase 15~500 kVA 3 phase C57.12.51 Sealed, dry power transformers > 0.5 MVA 3 phase 34.5 kV C57.12.52 Dry type transformers used in unit substations C57-12.55 Transformers underground -Type self-cooled, single phase distribution transformers with separable, insulated High voltage connecters highvoltage connectors: High voltage (24940 Grd Y/14400 V and below) and low voltage 240/120 V, 167 kVA and smaller C57.12.23 IS 507 Reference Standards and Books IEC ANSI / IEEE Transformers underground type three-phase distribution transformers 2500 kVA and smaller: High voltage 34500 Grd Y/19920 volts and below, Low voltage, 480 volts and below C57.12.24 Distribution transformers 1 to 500 kVA, single-phase and 15 to 500 kVA, three-phase with high-voltage 601-34500 volts, low voltage 120-600 volts. Ventilated Dry-Type C57.12.50 Ventilated dry type network transformers 2500 kVA and below. Three phase, High voltage 34500 volts and below low-voltage 216 Y/125 and 480 Y/277 volts, requirement C57.12.57 IS External Clearances in Air 2026-3 Outdoor distribution transformers up to and including 100 kVA, 11 kV: Part 1 Non-sealed type 1180-1 Outdoor distribution transformers up to and including 100 kVA, 11 kV: Part 2 Sealed type 1180-2 Earthing Transformer 3151 508 Manual on Transformers 2. TESTING IEC ANSI / IEEE IS Testing -General, Dry transformer -General, oil filled -HVDC transformer 60076-1 C57.12.91 C57.12.90 C57.129 2026-1 Dielectric tests 60076-3 C57.12.90 2026-3 Temperature rise test -Oil filled -Dry transformer 60076-2 C57.12.90,1538 2026-2 Impulse/ switching surge test 60076-4 C57.98 C57.138 (Distribution transformer) 2070 / 11349 / 15638 Short circuit withstand requirements & testing 60076-5 C57.12.90 2026-5 Determination of Sound Levels 60076-10 Measurement of frequency response 60076-18 C57.134 2026-10 Loss measurement C57.123 Temperature rise test for Overload C57.119 PD testing -Oil filled -Acoustic -Dry transformer 60270 60076-3 C57.113 C57.127 C57.124 Test for thermal evaluation of dry type transformers (Cast resin & Resin encapsulated) C57.12.60 Test for thermal evaluation of dry type transformers (Ventilated dry type) C57.12.56 Test for thermal evaluation of dry type transformers (Dry type specialty and general purpose) 259 Guide for determination of maximum winding temperature rise in liquid filled transformers 1538 Test procedure for thermal evaluation of insulation systems for liquid immersed transformers C57.100 2026-3 509 Reference Standards and Books 3. TRANSFORMER OIL IEC ANSI / IEEE IS -Mineral Oil Specifications 60296 ASTM D3487-08 335 -Silicone Oil Specifications 60836 ASTM D4652-1987 -Organic ester oil Specifications 61099 -Synthetic Aromatic oil specifications 60867 -Mineral oil Maintenance 60422 Transformer oil -Natural Ester oil maintenance C57.106 1866 C57.147 -Silicon oil maintenance 60944 -Synthetic Organic Ester Maintenance 61203 -Hydrocarbon fluid maintenance C57.111 C57.121 -Sampling 60475 60567 ASTM D923-97 ASTM D3613-98 9434 6855 -BDV 60156 ASTM D1816-97 ASTM D 877-00 6792 -Oxidation stability 61125 ASTM D2440 ASTM D2112-01 12422 / 12958 -Water content 60814 ASTM D1533-05 -DGA-Interpretation of results 60599 C57.104 -DGA- Bushings TS 61464 DGA – OLTC PC57.139 D12 -DGA -During factory test 61181 -DGA – Sampling 60567 -DGA – Test method ASTM D 3612-01 -DGA–Silicone oil filled transformers C57.146 -Reclamation of Oil Gassing of insulating liquids under electrical stress and ionization PC57.130/D17 637 60628 10593 510 Manual on Transformers 4. ACCESSORIES IEC ANSI / IEEE Fittings and accessories for power transformers Tap changers Bushing -General -HVDC Bushings -Dimensions -Terminals Bushing Application Guide Bushings Seismic Qualification 60214-1 C57.131 8468 60137 60518 C57.19.00 C57.19.03 C57.19.01 C57.19.100 TS 61463 2099 / 7421 3347 / 8603 / 13312 / 12676 15137 / 4257 Induction voltage regulator C57.15 Control Cabinet C 57.148 Surge Arresters - Part 1 : Non-Linear Resistor Type Gapped Surge Arresters for AC System - Part 3 : Artificial Pollution Testing of Surge Arresters - Part 5 Selection and Application Recommendations Voltage Transformers Part 1 General requirements Part 2 Measuring voltage transformers Part 3 Protective voltage transformers Part 4 Capacitor voltage transformers Insulating oil conditioning plants IS 3639 15086-1 15086-3 15086-5 186 C57.13.0, 13.1, 13.2 3156-1 3156-2 3156-3 3156-4 6034 Insulation resistance tester, - hand operated (magnets generator type) - electric type - Portable mains operated 2992 10656 11994 Portable earth resistance meter 9223 Pressure and vacuum gauge 3624 Danger notice plates 2551 Warning symbol for dangerous voltage 8923 511 Reference Standards and Books Galvanized strand for earthing 12776 Ready mixed paint, bushing, zinc chrome priming 104 Ready mixed paint, air drying, red-oxide-zinc chrome, priming 2074 Enamel synthetic, exterior - (a) undercoating - (b) finishing 2932 Polyurethane full gloss enamel 13213 Epoxy based zinc phosphate primer 13238 Epoxy enamel, two component, glossy 14209 Paper insulated lead-sheathed cables for electricity supply 692 PVC insulated cables for working voltages up to and including 1100 V 694 PVC insulated (heavy duty) electric cables - for working voltages up to and including 1100 V - for working voltage from 3.3 kV up to and including 11 kV Cross linked polyethylene insulated PVC sheathed cables - for working voltage up to and including 1100 V - for working voltages from 3.3 kV up to and including 33 kV - for working voltages from 66 kV up to and including 220 kV 1554-1 1554-2 7098-1 7098-2 7098-3 Compression type tubular inline connectors for Aluminum conductors of insulated cables 8308 Compression type tubular terminal ends for Aluminum conductor of insulated cables of insulated cables 8309 512 Manual on Transformers Cable sealing bores for oil immersed transformers suitable for paper insulated lead sheathed cables for highest system voltages from 12 kV up to and including 36 kV 9147 Conduits for electrical installations - General requirements - Rigid steel conduits - Rigid plain conduits of insulating materials - Pliable self-recovering conduits of insulating materials 9537-1 Brass glands for PVC cables 12943 Current transformers - General - Measuring Current transformers - Protective Current transformers - Protective Current transformers for special purpose application 9537-2 9537-3 9537-4 60044-1 60044-6 C57.13.0, 13.1, 13.2 2705-1 2705-2 2705-3 2705-4 Cork composition sheets: Part 2 Cork and rubber 4253-2 Dimensions for ‘O’ rings and grooves for vacuum flanges 6838 ‘O’ rings - Dimensions - Material selection and quality acceptance criteria - Seal housing dimensions and tolerances - Terminology and definition of terms 9975-1 9975-2 9975-3 9975-4 Rubber gaskets 11149 Hexagon head bolts, screws and nuts of product grade C - Hexagon head bolt - Hexagon head screws - Hexagon nuts 1363-1 1363-2 1363-3 513 Reference Standards and Books Hexagon head bolts, screws and nuts of product grades A &B - Hexagon head bolts - Hexagon head screws - Hexagon nuts - Hexagon thin nuts -(chamfered) - Hexagon thin nuts (un chamfered). 1364-1 1364-2 1364-3 1364-4 1364-5 Slotted counter sunk head screws 1365 Plain washers 2016 Slotted grub screws 2388 Fastners - single coil rectangular section spring washers 3063 Activated alumina 9700 Nitrogen 1747 Copper alloy gate, globe and check valves for water works purposes 778 Sluice Valve for Water Works Purposes (50 to 1200 mm Size)-Specification 14846 Swing check type reflux (nonreturn) valves Part 1 single door pattern 5312-1 Steel plug valves for petroleum Petrochemical and allied industries 11699 Steel ball valves for the Petroleum and allied industries 11792 Gas operated relays 3637 Propeller type ac ventilating fans 2312 Electric axial flow fans 3588 Shell and tube type heat exchangers 4503 Oil to water heat exchangers for transformers 6088 Silica gel 3401 Electric power connectors 5561 514 Manual on Transformers Porcelain post -insulator for systems with nominal voltage greater than 1000 V 2544 Hollow insulators for use in electrical equipment 5621 Silicone compound for application on high voltage porcelain insulators 7648 Guide for the selection of insulators in respect of pollution conditions 13134 Permissible limits of visual defects for insulating porcelain for electrical circuits 13305 Pressed Steel Radiators Tests on indoor and outdoor post insulators of ceramic material or glass for nominal voltages greater than 1000 V IEEMA 9 IEC 60168 515 Reference Standards and Books 5. INSTALLATION, OPERATION & MAINTENANCE Installation & maintenance -Oil filled -Dry type transformer Loading guide -Oil filled -Dry type transformer Failure investigation IEC ANSI / IEEE IS - C57.93, C57.12.11, C57.12.12 C57.94 10028 Part 1-3 60076-7 60076-12, 60905 C57.91,C57.92, C57.115 C57.96 2026-7, 6600 C57.125 Reporting failure data C57.117 Evaluation and Reconditioning of oil filled transformers C57.140 Diagnostic field testing 62 PC 57.152 D2 Monitoring Of transformers &accessories PC57.143 D21 Guide for safety procedures and practices in electrical work 5216–1 Guide for safety procedures and practices in electrical work (lifesaving technique) 5216-2 Code of practice for earthing. 142 3043 Steel wire ropes for general engineering purposes 2266 Dee-shackles and bow shackles 6132 Hand operated chain pulley blocks 3832 Eye bolts with collars 4190 Automotive vehicles -portable jacks for automobiles -Part I mechanical jacks 4552-1 Automotive vehicles-portable jacks for automobiles - Part 2 Hydraulic jacks 4552-2 Wire rope slings - safety criteria and inspection procedure for use 12735 Colors for ready mixed paints and enamels 5 Code of practice for painting of ferrous metals in buildings -Part I Pre-treatment 1477-1 516 Manual on Transformers Code of practice for painting of ferrous metals in Building Part 2 painting 1477-2 Code of practice for installation and maintenance of power Cables up to and including 33 kV rating 1255 Code of practice for electrical wiring installations 732 Class - 2 and Class - 3 Transformers Methods of measurement of touch current and protective conductor current. Grounding of instrument transformers secondary circuits Application of transformers connection’s in three phase distribution systems Guide for liquid-immersed transformer through fault current duration Seismic guide for power transformers and reactors Fire protection, guide for substation Recommended practice for protection and co-ordination of industrial and commercial power system Recommended current ratings for cables. Part 1 paper insulated lead sheathed cables Recommended current ratings for cable Part 2 PVC insulated and PVC sheathed heavy duty cables Recommended current ratings for cables Part 5 PVC insulated light duty cables Recommended short circuit ratings of high voltage PVC cables Recommendation for heat exchanger gasket Guide for loading power apparatus bushings 1585 60990 C57.13.3 C57.105 C57.109 C57.114 979 242 3961-1 3961-2 3961-5 5819 10864 C57.19.101 517 Reference Standards and Books 6. APPLICATION GUIDES IEC Transformers 60076-8 High temp. Insulation materials 60076-14 Converter transformers 61378-3 Tap changers 60214-2 ANSI / IEEE 1276 8478 Transformers connected to generators - C57.116 Transformers for nonsinusoidal currents (loads with Harmonics) - C57.110 Loss Evaluation Guide - C57.120 62032 C57.135 TR 60616 C57.105 (Connections) C57.12.70 (Terminal markings) Phase shifting transformer Terminal marking & connections Apparatus bushings Standard terminology 60050-421 C57.12.80 C57.12.59 C57.12.109 Insulation co-ordination 60071-1,2,3 60664-1 (Low voltage) 1313.1 (Definition & principles) 1313.2 (Application guide) Preferred voltage ratings 60038 1312 Transformers for Nuclear generating station 638 Bar coding of Distribution transformers C57.12.35 Permissible temperature rise for terminal Bushings-Seismic Qualification Direct connection details between transformers & GIS 2026-4 C57.19.100 Through fault current duration- (Equipment damage curves) Dry Oil filled Selection of insulators for polluted environments IS 2026-8 TS 60815 60943 TS 61463 61639 3716 / 15382 Part 1-4, 518 Determination of sound level Guide for sound abatement Manual on Transformers 60076-10-1 Cleaning of Insulators C57.136 C57.12.58 957 Occurrence and Mitigation of Switching transients induced by transformers C57.142 Metric conversion of transformer standards C57.144 Transient Voltage Analysis of Dry type transformer Coil C57.12.58 Determination of max winding temperature rise in liquid filled transformers Electrical Power System Device Function Numbers, Acronyms, Contact designations Recommended electrical clearances in air insulated electrical power substations Guide for protecting Transformers Guide for protection of network Transformers Guide for Protection of Shunt Reactors Application of CTs used for Protective Relays 2026-9 1538 61850-7-4 C37.2 1427 C37.91 C37.108 C37.109 C37.110 current transformers 4201 voltage transformers 4146 capacitor voltage transformers 5547 Application guide for gas operated relays 3638 Application guide for electrical relays for ac systems. Part 1 over current relays for feeders and transformers 3842-1 Reference Standards and Books 519 7. Books on Transformer Engineering 1. First Books on Transformers 1889 - Friedrich Uppenborn History of the Transformer, Pages 60 E&F.N, Spon, London (Translated from German) 1889 - John Ambrose Fleming The Alternate Current Transformer in Theory and Practice Vol 1 Induction of Electric Currents, Pages 487 Vol 2 The Utilization of Induced Currents, Pages 507 The Electrician Printing & Publishing Co. Ltd, London 2. Evergreen Classics (Theory) 1938,1951 - F Blume , G Camilli , A.Boyajian , VM Montsinger Transformer Engineering – A Treatise on the theory ,operation and application of transformers , Pages 496, John Wiley & Sons Inc , New York 1943-1965 – ed 13.0 Electrical Engineering Department, MIT, US Magnetic Circuits and Transformers- A first course for Power & Communication Engineers , Pages 742 John Wiley & Sons, New York 1942-1997 – ed 5.0 ABB Electric system Technology , Raleigh USA Electrical Transmission and Distribution Reference Book. (Earlier Westinghouse T & D Book) 3. First Transformer Books Published from India 1962 - S B Vasutinsky Principles, Operation and Design of Power Transformers PSG College of Technology , Coimbatore Incidentally the same Russian author’s title in Chinese came out in 1983- Theory and Calculation of Transformers, Pages 554 1982, 2003 - ed 2.0 BHEL Transformers 4. Current Books Accessories 2011 – Keith Ellis Bushings for Power Transformers , Pages 116 Author House , Bloomington , IN 2011 – Faiz J Siah Kolah B Electronic Tap Changers for Distribution Transformers , Pages 183 Springer-Verlong , London 2000 – Axel Kramer On- Load Tap -Changers for Power Transformers , Pages 232 MR 520 Manual on Transformers Germany 1979,1987 – HP Moser & V Dahinden Transformer Board Vol 1( 1979) & 2(1987) Weidman, Rapperswill , Switzerland Application 1925-2007 - ed 13.0 Martin Heathcote The J & P Transformer Book , Pages 973 Newness Publications , Amsterdam 14th edition expected in 2013 √2003,2007- ed2.0 James H Harlow Electric Power Transformer Engineering, Pages , CRC Press , New York 2003-2007 – ed 3.0 ABB Power Transformer Business unit Transformer Hand Book , Pages 23 Maintenance 2006-2010 – ed3.0 ABB Transformer Services Business unit Service Hand Book for Transformers √1981-2004 – ed 3.0 M Horning , J Kelly, S Meyers , R Stebbins Transformer Maintenance Guide SD Meyers Inc , Ohio Theory 2004-13 – ed2.0 SV Kulkarni , SA Kharparde Transformer Engineering Design & Practice Marcel and Dekker Inc , New York 2008 – A.V. Chiplonkar Design, Operation and Maintenance of Core type oil filled Transformers, Pages 720 2001,2010 – ed 2.0 Robert M Del Veechio, Rajendra Ahuja , Bertland Poulin , Pierre T Feghalli, DM Shah Transformer Design Principles with Applications , Pages 720 CRC Press 1987,1995 – ed 2.0 K.Karsai, D Kerenyi , L Kiss Large Power Transformers Akademiai Kiado , Budapest Theory – Specifics 1966 – ed 2.0 M Waters The Short Circuit Strength of Power Transformers Macdonald & Co. , London 1996-2008 – ed 3.0 G.Bertagnolli Short Circuit Duty of Power Transformers , Pages 225 ABB Management Services , Transformers, Zurich 1974 – Richard Stoll Eddy currents Clarendon Press , Oxford 2005 – ed 5.0 William H. Hayt , J A Buck Engineering Electromagnetics Tata Mcgraw Hill , India Reference Standards and Books 521 2006 – ed 2.0 Joseph A Edminister Electromagnetics Tata Mcgraw Hill , India 1964 – F H Kreugar Discharge Detection in High Voltage Equipment Heywood Book , London 1993 – D.Konig , Y N Rao Partial Discharges in Electric Power Apparatus VDE Verlag Gmbh , Berlin 1980 – M V K Chari and PP Silvester Finite Elements in Electrical and Magnetic Field Problems John Wiley & Sons 2008 – ed 9.0 J P Holman, Heat Transfer, Pages 676 Tata Mcgraw Hill, New Delhi 2011 – Suhas V Patankar Numerical Heat Transfer and Fluid Flow Taylor & Francis Testing 2003,2011 – ed 2.0 Ake Carlson, Jitka Fuhr Testing of Power Transformers, Pages ABB Power Transformer Business Area Transformer Oil 2012 – A K Pandey A Glance on Power Transformer Oil – 100 Questions , Pages 73 Shroff Publishers , Mumbai 2011 – ed 3.0 Nynas Naphthenics AB, Sweden Transformer Oil Hand Book, Pages 223 2000 – Marcelo M Hirschler ASTM Publication No. 1376 Electrical Insulating Materials, pages 211 1988 – H.G. Erdmann (ed) Electrical Insulating Oils , Pages 146 ASTM Publication STP 998 5. More Details www.eng-tips.com FAQ238-1287: What are good references for a Power Engineer? http://www.transformerscommittee.org/ - Bibliography on transformer books flipkart.com – On line book store abebooks.com bookfinder.com - For used books and out of print books 522 Manual on Transformers 6. List of CIGRE Brochures for Transformers S. No. Technical Brochure No. Title Year 1 TB -96 Thermal aspects 1995 2 TB-170 Static electrification 2000 3 TB-157 Effect of particles 2000 4 TB-156 Customer specifications 2000 5 TB-209 Short circuit performance 2002 6 TB-204 Design reviews 2002 7 TB-227 Life management of transformers 2003 8 TB-248 Economics of Transformer Management 2004 9 TB-298 Guide on Transformer Lifetime Data Management 2006 10 TB-349 Moisture Equilibrium and Moisture Migration within Transformer Insulation Systems 2008 11 TB-343 Recommendations for Condition Monitoring and Condition Assessment Facilities 2008 12 TB-342 Mechanical Condition Assessment of Transformer Windings 2008 13 TB-378 Cooper Sulphide in Transformer Insulation 2009 14 TB-436 Experience in service with new insulating liquids 2010 15 TB-407 HVDC converter transformers - guidelines for design review 2010 16 TB-406 HVDC converter transformers - test procedures, ageing, evaluation and reliability in service 2010 17 TB-393 Thermal Performance 2010 18 TB-445 Guide for transformer maintenance 2011 Appendix III Typical Quality Assurance Plan (Format for Illustration only) Appendix Iii TYPICAL QUALITY ASSURANCE PLAN (FORMAT FOR ILLUSTRATION ONLY) SL. No. Component / Classification operation and Type of Insp. / test Quantum of insp. REF.Doc. & ACPT. Norm description of test 1.7 1.7.1 Form of record INSP/TEST V/SV MQ P VE P P TANK AND FABRICATION Verification of welders Major Qualification / Specification Verification of Each WPS/ASME V/SV Records Welder Sec. DC IR/TC 1.7.2 Thickness verification of Steel under use Major Measurement Sampling TRQS 82403 DRG/Material Spec. No. ~ V/SV IR/ TC 1.7.3 MECH&CHEM. properties of steel Major Testing -do~ ~do~ -do- -do- VE 1.7.4 Verification of dimensions of components and assly. Major Measurement 100% for Major assly ~do~ ~do~ ~do~ -do- 1.7.5 Inspection of fit up and major weld, preparation Weld finish & size. Major Visual & Measurement ~do~ ~do~ ~do~ ~do~ -do- 1.7.6 Check for flatness of gasketing surface Major Measurement -do- —do- —do— —do— H 1.7.7 Leak proof ness of oil filled Compartments Major Testing Each unit Process spec. NO.-/ -do- -do- VE 1.7.8 Pressure test Major -do- One unit of new design -do- -do- H 1.7.9 Vacuum test Major Testing One unit of V/SV P H new design IR/ -do- VE CBIP TC 1.7.10 D.P. Testing of major weld Major Testing Jacking / Lifting pad Process Spec. No. -do- 1.7.11 Verification as per OGA Major 100% DRG. No. -do- H 1.7.12 Blast cleaning Major 100% Process Spec. No. — -do- H 1.7.13 Paint type and shade Major Sampling DRG. No. - -do~ H 1.7.14 Paint thickness Major -do- DRG. No. - -do~ H 1.7.15 Uniformity of finish Major -do- Process Spec. -do- -do- -do~ -do~ No. 1.7.16 Paint adhesion test -do- Major 525 Process Spec. No.~ 526 Legends: V Vendor SV Sub Vendor MQ Manufacturer’s Quality Assurance WPS Welding Process Specification P Perform IR Inspection Record TC Test Certificate Manual on Transformers Appendix IV Guaranteed Technical and Additional Technical Particulars Appendix IV GUARANTEED TECHNICAL AND ADDITIONAL TECHNICAL PARTICULARS I. Guaranteed Technical Particulars: 1. Name of the manufacturer and county of origin: 2. Installation indoor/outdoor: 3. Reference standard: 4. Continuous ratings under service conditions specified in IS: 2026: (a) Type of cooling (b) Rating (MVA) (c) (i) With ONAN cooling (ii) With ONAF cooling (iii) With OFAN cooling (iv) With OFAF cooling (v) With OFWF cooling (vi) With ODAN cooling (vii) With ODAF cooling (viii) With ODWF cooling: Rated Voltage : (i) HV kV : (ii) IV kV : (iii) LV kV : (d) Rated frequency Hz : (e) Number of Phases HV : Current at rated no load voltage and on principal tap: (i) 5. HVAmps : (ii) IV Amps : (iii) LV Amps: Connections : (i) HV: (ii) IV: (iii) LV: 6. Connection symbol: 7. Temperature rise: (a) Temperature rise of oil above reference peak ambient temperature (By thermometer) (°C): (i) At full ONAN rating (°C): (ii) At full OFAF/ODAF/OFWF/ODWF rating (°C): (b) Temperature rise of winding above reference peak ambient temperature (By resistance method) (°C): (i) At full ONAN rating: (ii) At full OFAF/ODAF/OFWF/ODWF rating: 529 IV LV 530 Manual on Transformers (c) 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. Limit of hot spot temperature for which the transformer is designed (°C) Over the maximum yearly weighted average ambient temperature: Type of tap changing switch: (i) Off circuit switch/links: (ii) On load: Tappings on windings HV/IV/LV for: (i) Constant flux/variable flux/combined regulation: (ii) Location (Line/Central/Neutral) end of winding: (iii) Number of steps: (iv) Variation of (HV/IV/LV): (i) No load loss at rated voltage and frequency at principal tap (kW): (ii) No load loss at the voltage corresponding to the highest tap (kW): (iii) Tolerance, if any, on the above values: (a) Load loss at rated output, rated frequency and corrected for 75°C winding temperature at: (i) Principal tap (kW): (ii) Highest tap (kW): (iii) Lowest tap (kW): (b) Tolerance, if any, on the above values: (a) Auxiliary losses at rated output, normal ratio, rated voltage, rated frequency and ambient tmperature (kW): (b) Tolerance, if any, on the above values: Total losses at normal ratio inclusive of auxiliary equipment losses (kW): Positive sequence impedance on rated MVA base at rated current and frequency at 75°C winding temperature between: HV-IV HV-LV IV-LV (i) Principal tap, per cent: (ii) Highest tap, per cent: (iii) Lowest tap, per cent: Zero sequence impedance at reference temperature of 75° C at principal tap, per cent: Reactance at rated MVA base at rated current and frequency, per cent: Regulation at full load and 75°C winding temperature expressed as a percentage of normal voltage: (i) At Unity Power Factor per cent: (ii) At 0.8 Power Factor (Lagging) per cent: Efficiency at 75°C winding: temperature as derived from guaranteed loss figures and at Unity power factor (a) At full load, per cent: (b) At 3/4 load, per cent: (c) At 112 load, per cent: (i) Maximum efficiency, per cent: (ii) Load at which maximum efficiency occur s (per cent of full load): Time in minutes for which the transformer can be run at full load without exceeding the maximum permissible temperature at reference ambient temperature when: (a) Power supply to fans is cutoff but the oil pumps are working: (b) Power supply to oil pumps is cut-off but fans are working: (c) When power supply to both fans and pumps is cut off: Short time rating for 2 seconds of: (a) HV winding: Guaranteed Technical And Additional Technical Particulars 531 (b) IV winding: (c) LV winding: 22. Permissible over loading: (a) HV winding: (b) IV winding: (c) LV winding: 23. Terminal arrangement: (i) High voltage (HV): (ii) Intermediate voltage (IV): (iii) Low voltage (LV): (iv) Neutral-HV/IV/LV: (v) Tertiary: 24. Insulating and cooling medium: 25. Test Voltage: HV / IV / LV (i) Lightning impulse withstand test voltage (kV peak): (ii) Power frequency with-stand test voltage dry as well as wet for I minute (kV rms): (iii) Switching impulse withstand test voltage (kV peak): 26. Partial discharge level at 1.5Um/ √ 3 kV rms (pC): 27. Noise level when energised at normal voltage and frequency without load (db): 28. External short circuit withstand capacity (MVA) and duration (seconds): 29. Over flux withstand capability of the transformer: II. ADDITIONAL TECHNICAL PARTICULARS (These figures are indicative only. These shall not form the basis for upward or downward revision of prices). 1. Details of Core: (a) Type of core construction: (b) Type of core joints: (c) Flux density at rated voltage and frequency and at principal tap Tesla: (d) Magnetizing current at normal ratio and frequency: (i) 90 per cent of rated voltage: (ii) 100 per cent of rated voltage: (iii) 110 per cent of rated voltage: (In case kVA ratings of windings are different, this may be specified in terms of magnetizing kVA): (c) Power factor of magnetizing current at normal voltage ratio and frequency: (f) (i) Material of core laminations: (ii) Thickness of core laminations (mm): (g) (i) Whether core construction is without core bolts: (h) (ii) (iii) (iv) (v) (vi) (i) Insulation of core bolt: Insulation of core bolt washers: Insulation between core laminations: Core bolt insulation withstand voltage for 1 minute (kV rms): Are the core bolts grounded. If so how: Material of core clamping plate: (i) (ii) Thickness of core clamping plate: (iii) Insulation of core clamping plate: Describe location/Method of core grounding: 532 Manual on Transformers (j) Details of oil ducts in core: Details of windings: HV IV LV (a) Type of winding: (b) Material of the winding conductor: (c) Maximum current density of windings (at rated current) A / Sq. mm (i) HV: (ii) IV: (iii) LV: (iv) Regulating: (d) Whether HV windings are interleaved / intershielded: (e) Whether winding are preshrunk: (f) Whether adjustable coil clamps are provided for HV and LV windings: (g) Whether steel rings used for the windings if so, whether they are split: (h) Whether electro-static shields are provided to obtain uniform voltage distribution in the HV windings: (i) Insulating material used for: (i) HV winding: (ii) IV winding: (iii) LV winding: (iv) Regulating winding: (i) Insulating material used between: (i) HV and IV, winding: (ii) IV and LV winding: (iii) LV winding and core: (iv) Regulating winding and earth: (k) Type of axial coil supports: (i) HV winding: (ii) IV winding: (iii) LV winding: (l) Type of radial coil supports: (i) HV winding: (ii) IV winding: (iii) LV winding: (m) Maximum allowable torque on coil clamping bolts: (n) DP value conductor paper: Bushings: HVIV LV Neutral (a) Make and type: (b) (i) Rated voltage class kV: (ii) Rated current (Amps): (c) Lightning impulse withstand test voltage (1.2x50microsecond) (kV peak): (d) Switching surge with-stand test voltage (kV peak): (e) Power frequency with-stand test voltage: (i) Wet for 1 minute (kV rms): (ii) Dry for 1 minute (kV rms): (f) Partial discharge level: 533 Guaranteed Technical And Additional Technical Particulars (g) Creepage distance in (mm): (h) Creepage distance (protected): (i) Whether test lap is provided: (j) Quantity of oil in bushing and specification of oil used (kg): (k) Weight of assembled bushing (kg): (1) Minimum clearance height for removal of bushing (mm): Minimum clearance (mm): In Air- (i) HV: Between Phase to (ii) IV: phases ground (iii) LV: Approximate weight (a) Core with clamping: (b) Coil with insulation: (c) Core and winding: (d) Oil required for first filling: (e) Tank and fittings with accessories: (f) Untanking weight: (g) Total weight with oil and fittings: (h) Weight of total insulation: Detail of Tank (a) Type of tank: (mm) (b) Approximate thickness of sheet (mm) (i) Sides (mm) (ii) Bottom (ton-) (iii) Cover (torr) (iv) Cooling Tubes/Radiators (torr) (c) Vacuum recommended for hot oil circulation (d) Vacuum to be maintained during oil filling in transformer tank: (e) Vacuum to which the tank can be subjected without distortion: (f) No. of bi-directional wheels provided: (g) Track gauge required for the wheels.Longitudinal Axis Transverse Axis (h) Type of pressure relief device/explosion vent and pressure at which it operates: Conservator (a) Total volume (Litres): (b) Volume between the highest and lowest visible oil levels (Litres): (c) Power required by heaters (if provided) (kW): (i) (i) Quality Governing standard: 534 Manual on Transformers Specific resistance at (ohm-cms) 27°C: 90°C: (iii) Tan delta: (iv) Water content (ppm): (v) Dielectric strength (Breakdown voltage) (kV): (vi) Characteristic of oil after ageing test: (a) Specific resistance at (ohm-cms) 27°C: 90°C: (b) Tan delta: (c) Sludge content: (d) Neutralization Number: (vii) Details of oil preserving equipment offered: Radiator (i) Overall dimensions 1 x b x h (mm): (ii) Total weight with oil (kg): (iii) Total weight without oil (kg): (iv) Thickness of Radiator tube /fins (mm): (v) Types of mounting: (vi) Vacuum withstand capability: 10. Cooling System (a) Make and type: :Fan Motor (b) No. of connected units: (c) No. of standby units: (d) Rated power input: (e) Capacity (cu m/min.) or (Litres/min.): (f) Rated voltage (Volts): (g) Locked rotor current (Amp): (h) Efficiency of motor at full load (Per cent): (i) Temp, rise of motor at full load (ºC): (j) B HP of driven equipment: (k) Temperature range over which control is adjustable (°C): (1) Whether the fan and/or pumps suitable for continuous operation at 85 per cent of their rated voltage: (m) Estimated time constant in hours for (i) Natural cooling: (ii) Forced air cooling: 11. Gas and Oil operated relay make and type: 12. Temperature Indicators Oil temperature Indicator (i) Make and type: (ii) Permissible setting ranges for alarm and trip: (iii) Number of contacts: Pump Motor Winding temperature Indicator Guaranteed Technical And Additional Technical Particulars (iv) (v) 13. 14. 15. Current rating of each contact: Whether remote indicators provided. If so, whether equipment required at purchasers control room is included: Approximate overall dimension : (a) Length : mm (b) Breadth : mm (c) Height : mm (i) Minimum clearance height for lifting core and winding from tank: (ii) Minimum clearance height for lifting tank cover: Shipping details (a) Approximate weight of heaviest package: (b) Approximate dimension of largest package: 16. Transformer will be transported with oil/gas: 17. Size of rail recommended for the track: 18. Details of bushing current transformers 19. (i) Quantity: (ii) No. of cores: (iii) Ratio: (iv) V.A. burden: (v) Accuracy class: (vi) Knee point voltage: (vii) Magnetizing current at knee point voltage: (viii) Secondary resistance: 19. (HV IV LV Neutral) Lifting jacks (i) Governing standard: (ii) No. of jacks in one set: (iii) Type and make: (iv) Capacity (tonnes): (v) Pitch (mm): (vi) Lift (mm): (vii) Height in closed position (mm): (viii) Mean diameter of thread (mm): 20. 535 Marshalling kiosk (i) Make and type: (ii) Details of apparatus proposed to be housed in the kiosk: 21. Details of anti-earth-quake device provided, if any: 22. Separate conservator and Buchholz relay provided: 23. Tap Changing Equipment (These details refer to the basic rating 536 Manual on Transformers of O.L.T.C. as guaranteed by O.L.T.C manufacturers): (a) (b) (c) (d) (e) (f) (g) (h) (i) (j) (k) (1) (m) (n) (o) (p) (q) 24. Make: Type: Power flow-unidirectional/bi-directional/restricted bi-directional: Rated voltage to earth (kV): Rated current (Amps): Step voltage (Volts): Number of steps: Control-Manual/Local Electrical/Remote Electrical: Voltage control-Automatic/Non-Automatic: Line drop compensation provided/Not provided: Parallel operation: Protective devices: Auxiliary supply details: Time for complete tap change (one step) sec.: Diverter selector switch transient time (cycles): Value of maximum short circuit current (Amps): Maximum impulse withstand test voltage with 1.2/50 micro-seconds full wave between switch assembly and ground (kV peak): (r) Maximum power frequency test voltage between switch assembly and earth (kV rms): (s) Maximum impulse withstand test voltage with 1.2/50 microseconds across the tapping range (kV peak): (t) Approximate overall dimensions of tap changer (In case of separate tank type) (mm): (u) Approximate overall weight (In case of separate tank type) (kg): (v) Approximate mass of oil (In case of separate tank type) (kg): (w) Particulars of the O.L.T.C. control panel for installation in the control room: Driving mechanism box (i) Make and type: (ii) Details of apparatus proposed to be housed in the box: Appendix V List of Transformer Accessories and Test Certificates Required Appendix V LIST OF TRANSFORMER ACCESSORIES AND TEST CERTIFICATES REQUIRED SI. No Accessory Ref. Std. Test-certificates required 1. 2. 1. Condenser Bushing IS 2099 2. IS Bushings IS 2099 3. OLTC IS 8468 3. 4. 5. 6. 7. 1. 2. 3. 1. 2. 3. 4. 5. 6. 7. 8. 1. 2. 3. 4. 5. 6. Winding 4. Temperature Indicator 5. Oil Temperature Indicator 6. Pressure Relief Valve 7. Cooling Fan 1. 2. 3. 4. 5. 1. 2. 3. IS 2312 1. 2. 3. 4. Appearance, construction and dimensional check Test for leakage of internal filling at a pressure of 1.0 kg/cm2 for 12 h Insulation resistance measurement with 2000 V megger Dry power frequency voltage withstand test Dry power frequency voltage withstand test for test tap insulation. Partial discharge measurement up to 1.5 Um / √3 kV Measurement of tangent delta and capacitance Appearance, construction and dimensional check. Insulation resistance measurement with 2000 V megger. Dry power frequency voltage withstand test. Oil tightness test for the diverter switch oil chamber at an oil pressure of 0.5 kg/ cm2 at 100°C for 1 h. Mechanical operation test. Operation sequence measurement. Insulation resistance measurement using 2000 V megger. Power frequency voltage withstand test on diverter switch to earth and between even and odd contacts. Power frequency voltage withstand test on tap selector- between stationary contacts, between max. and min. taps, between phases and supporting frames, between phases. Operation test of complete tap changer. Operation and dielectric test of driving mechanism. Calibration test. Dielectric test at 2 kV for one minute. Accuracy test for indication and switch setting scales. Test for adjustability of switch setting. Test for switch rating. Measurement of temperature rise with respect to the heater coil current. Calibration test. Dielectric test at 2 kV for one minute. Accuracy test for indication and switch setting scales. Test for adjustability of switch setting. Test for switch rating. Functional test with compressed air to check bursting pressure, indication flag operation and switch operation. Dielectric test at 2 kV for one minute. Switch contact testing at 5A 240 V AC. Insulation resistance measurement. Dielectric test at 2 kV between winding and body for 1 minute. Operation check. Appearance, construction and dimensional check. 539 540 8. Manual on Transformers Transformer Oil Pump IS 325 & IS 9137 1. 2. 3. 4. Insulation resistance measurement. Cold resistance measurement at ambient temperature. Motor efficiency at full load. No load voltage, current, power input, frequency and speed. 5. 6. Locked-rotor readings of voltage, current and power input. Water pressure test for pump casing at 5 kg/cm2 for 10 minutes at ambient temperature. Transformer oil pressure test for the pump set assembly at 2 kg/cm2 for 30 minutes at 800 C. Measurement of head, discharge, current, power input to motor and overall efficiency of the pump set at rated voltage. Appearance, construction and dimensional check. Observation of flow with respect to requirement. Switch contact test at 5A 240 V AC. Dielectric test at 2 kV for one minute. Appearance, construction and dimensional check. Leak test with transformer oil at a pressure of 3 kg/cm2 for 30 minutes at ambient temperature for relay casing. Insulation resistance measurement with 500 V megger. Dielectric test at 2 kV for 1 minute. Elements test at 1.75 kg/cm2 for 15 minutes using transformer oil at ambient temperature. Loss of oil and surge test. Gas volume test. Mechanical strength test. Velocity calibration test. Appearance construction and dimensional check. Test for oil levels. Switch operation for low level alarm. Switch contact test at 5A 240 V AC. Dielectric test at 2 kV for 1 minute. Appearance, construction and dimensional check. Air pressure test at 2 kg/cm2 under water for 15 minutes. Appearance, construction and dimension check. Appearance, construction and dimensional check. Electrical operation. Insulation resistance measurement using 500 V megger at ambient temperature. Dielectric test at 2 kV for 1 minute Appearance, construction and dimensional check. Polarity check. Measurement of insulation resistance. High voltage power frequency test. Determination of ratio error and phase angle of measuring and protection BCTs. Determination of Tunis ratio error for PS class BCT. Determination of composite error for protective class BCT. Inter turn insulation withstand test. Exciting current characteristic test. Secondary winding resistance measurement. Knee-Point Voltage, measurement for PS class BCT. 7. 8. 9. 9. 1. 2. 3. 4. 1. Oil Flow Indicator/ Water Flow Indicator 10. Buchholz Relay 11. Oil Level Indicator 12. Pressed Steel Radiators 13. OLTC Control Cubicle/Cooler Control Cubicle IS 3637 IEEMA 9 2. 3. 4. 5. 6. 7. 8. 9. 1. 2. 3. 4. 5. 1. 2. 1. 2. 3. 4. 1. 2. 3. 4. 5. 14. Bushing Current Transformers IS 2705 6. 7. 8. 9. 10. 11. List of Transformer Accessories and Test Certificates Required 15. 16. 17. Off Circuit Tap Changer Oil to Water Heat Exchanger Pressure Gauges/ Differential Pressure Gauges 541 1. Construction and dimensional check. 2. Mechanical operation check. 3. Insulation resistance measurement using 2000 V megger. 4. Millivolt drop test of contacts. 5. High voltage power frequency withstand test by applying appropriate voltages to live parts to earth, between maxi¬mum and minimum taps, between change over contact and intermediate bearing, between adjacent taps, between tap changer contact and intermediate bearing and between tap changer contact and current take off terminal. 1. Test certificates for the materials of construction. 2. Manufacturers in process inspection records for all parts, subassemblies, accessories and complete assembly. 3. Shell side pressure test at 10 kg/cm2 with transformer oil at a temperature of 70°C + 10°C for 6 h. 4. Water side pressure test at 5 kg/cm2 with water at ambient temperature for 6 h. 1. Appearance, construction and dimensional check. 2. Calibration test. 3. Alarm contact setting test. Appendix VI Design Review Parameters Appendix VI Design Review Parameters Project: Transformer Name and Rating: 1. DESIGN DETAILS Sl. No. Aspect 1 Magnetic Circuit 2 Winding Design Parameter Type of Magnetic Circuit (Nos. of Limbs etc.) Core Dia (mm) / Weight (kg)/ Flux density (T) Whether Split Core used Core Joint Details (Mitred etc.) Whether core bolts are used Core Grade Thickness of core material / Building Factor Core hot spot actual / permissible No. of cooling ducts in core No Load Loss (kW) Designed/ Guaranteed Winding Type Winding Arrangement No of turns : HV IV LV RW Winding Dimensions (in mm) (e.g. ID/OD/Height) : HV IV LV RW Asymmetry in Windings Winding Conductor details (e.g. PICC/netted CTC/paper CTC) : HV IV LV RW Winding Conductor Dimension (in mm) : HV IV LV RW Current density HV/IV/LV/RW (A / Sq. mm) Shunt details (Wall & Yoke) Load loss at Normal tap : I2R Loss (kW) Stray Loss (kW) 545 Value 546 Manual on Transformers 2 Winding 3 SC Withstand Capability 4 Dielectric Design Total Load Loss (KW) Designed / Guaranteed Load loss at Maximum tap : I2R Loss (kW) Stray Loss (kW) Total Load Loss (KW) Load loss at Minimum tap : I2R Loss (kW) Stray Loss (kW) Total Load Loss (KW) % Impedance at 75 Deg C: Normal Tap, Designed / Guaranteed Maximum Tap, Designed / Guaranteed Minimum Tap, Designed / Guaranteed Stresses During Dynamic Short Circuit : Radial (Actual/Permissible) N/mm2 HV IV LV RW Axial Bending (Actual/Permissible) N/mm2 HV IV LV RW Unbalance axial forces acting on clamping structure (kN) Compressive forces in Winding (Actual/Permissible) kN HV IV LV RW Spiraling (Tangential) Force in LV winding (Actual/ Permissible) kN SC Current density HV/IV/LV/RW (A / Sq. mm) Ring Detail (Top & Bottom) Dielectric Stresses: Lightning Impulse Distribution Transferred Surge On LV (LI) Radial Duct Arrangement Oil stress in HV (kV/mm) Oil stress in IV (kV/mm) Oil stress in LV (kV/mm) Oil stress in RW (kV/mm) Creep stress HV (kV/mm) Creep stress IV kV/mm Creep stress LV kV/mm Creep stress RW kV/mm Paper stress in HV (kV/mm) Design Review Parameters 2. 5 Cooling Design 6 Tap Changer 547 Cooling Type Maximum Oil Velocity Directed Oil flow washer details Temperature rise (above Ambient of 50 Deg C): Top Oil (Designed / Guaranteed) HV Winding (Designed / Guaranteed) IV Winding (Designed / Guaranteed) LV Winding (Designed / Guaranteed) Cooler Details: Nos. of Coolers Fans & Pumps per cooler Cooler Loss Designed / Guaranteed Cooler Inlet & Outlet Temp Difference Deg C Tap Changer Type, Range (& Steps) Tap Changer Type (Reversing / Coarse Fine / Linear) Tap Changer Rated Current Voltage Of Tap Changer To Earth Voltage Across Taps Step Voltage Maximum Switching Capacity Recovery Voltage Detail (if OLTC) Whether Tie In Resistor Required (if OLTC) ENCLOSURES Sr. No. Enclosure 1 Core hot spot calculation. 2 Calculation of inrush current & air core reactance 3 Dynamic ability to withstand short circuit (including forces details in tabular format as per IEC 60076-5.) 4 Thermal ability to withstand short circuit. 5 Cooling calculations design details. 6 Calculation to show that during total failure of power supply to cooling fans and pumps, the transformer shall be able to operate at full load for at least ten minutes without the calculated winding hot spot temperature exceeding 140 Deg c. 7 Impulse voltage distribution graph/details. 8 Transfer surge calculation graph/details. 9 Sectional view of Winding Assembly showing Top and Bottom Rings 10 Winding arrangement sketch. 11 Calculation of Ohmic resistance at different taps. (applicable for ICT) 12 Recovery voltage calculation (for OLTC). Appendix vii Salient Technical Specifications and Particulars of Power Transformers for Power Plant Application 551 Quantity Applicable Standards Service & Duty Number of phases Number of windings (per Two winding phase to be mentioned for single phase) Frequency (Hz) Winding connection 2. 3. 4. 5. 6. 7. 8. 9. Application 1. Delta (b ) LV Side YNd11 or YNd1 Star (a ) HV Side Vector Group Two winding 3-phase Outdoor & Continuous IS 2026 / IEC 60076 1 X 100% YNd11 or YNd1 Delta Star 50 Hz (Frequency variation to be 50 Hz (Frequency variation considered as per Grid frequency to be considered as per Grid variation) frequency variation) 3-phase or 3 nos. 1 phase Outdoor & Continuous IS 2026 / IEC 60076 1 X 100% Generator Transformer GENERAL A. Generator Transformer Technical Specifications/ Generator Transformer in Coal Generator Transformer in Particulars based power plants Gas based CCPP Sl. No. 1 X 100% Station Transformer YNyn0 or YNyn0yn0(3-wdg) Star Star 50 Hz (Frequency variation to be considered as per Grid frequency variation) Two/ Three winding 3-phase Outdoor & Continuous YNyn0 or YNyn0yn0(3-wdg) Star Star 50 Hz (Frequency variation to be considered as per Grid frequency variation) Two/ Three winding 3-phase 1 X 100% Unit Transformer Unit Transformer in Gas based CCPP 3-phase Star Delta 50 Hz (Frequency variation to be considered as per Grid frequency variation) Star Delta 50 Hz(Frequency variation to be considered as per Grid frequency variation) Two winding 3-phase Outdoor/ Indoor & Continuous IS 11171, IS 2026 or IEC60076 2 X 100% per system (normally) Unit Aux/Station Aux Transformer LV Transformer (Dry Type/ Oil immersed) in Coal/ Gas based power plants Dyn11 (if Gen. Dyn11 (if Gen. Trafo. Dyn11 or Dyn1 Trafo. -YNd1) or -YNd1) or Dyn1 (if Gen. Dyn1 (if Gen. Trafo Trafo - YNd11) - YNd11) Star Delta 50 Hz (Frequency variation to be considered as per Grid frequency variation) Two winding /Three Two winding winding 3-phase Outdoor & Continuous IS 2026 / IEC 60076 IS 2026 / IEC 60076 1 X 100% or 2 X 50% or 2 X 100% Unit Transformer Unit Transformer in Coal based power plants Outdoor & Continuous Outdoor & Continuous IS 2026 / IEC 60076 IS 2026 / IEC 60076 2 X 100% or 1 X 100% Station Transformer Station Transformer Station Transformer in Coal based power in Gas based CCPP plants SALIENT TECHNICAL SPECIFICATIONS AND PARTICULARS OF POWER TRANSFORMERS FOR POWER PLANT APPLICATION Annexure VII Salient Technical Specifications and Particulars of Power Transformers for Power Plant Application Generator transformer shall be sized to deliver the continuous gross output of the Generator at Steam Turbine TMCR and /or VWO conditions, minus total of excitation power (if applicable) and one (1) 50% capacity Unit Transformer loading. Generator Transformer shall be sized to evacuate MVA arrived as above at a generator terminal voltage of 0.95 PU. Station Transformer (ST) will be sized to deliver following power output (under most stringent condition) (i) tart-up loads of one Unit (ii) shutdown loads of other Units (iii) 00% station common loads (iv) % design margin (minimum) may be taken on above [For large size coal based power plant (say > 500 MW), considering the availability of secondary side equipment of ST, simultaneous starting of Unit-1 & shut-down of Unit-2 under one ST out, may be decided as per project requirements] For power plant having two or more number of units and two Station Transformers (ST), each ST will be sized to deliver following power output with other ST out of service. (under most stringent condition) (i) tart-up loads of Unit-1 (ii) hut-down loads of Unit- 2, on bypass operation (iii) 00% station common loads (iv) % design margin (minimum) may be taken on above [For large size coal based power plant (say > 500 MW), considering the availability of secondary side equipment of ST, simultaneous starting of Generator transformer shall be sized to deliver the continuous output power of the Gas Turbine at ISO condition. (i.e. at 15° C) MVA rating of this transformer shall be sized based on the gas turbine generator maximum output minus total of generator excitation power and unit auxiliary loads, depending on the plant configurations. Generator Transformer shall be sized to evacuate MVA arrived as above at a generator terminal voltage of 0.95 PU. Transformer Rating (MVA) 11. ONAN/ONAF ONAN/ONAF cooling cooling (80%/100%) (80%/100%) OR ONAN/ONAF/ OFAF cooling (60%/ 80%/ 100% or 50%/ 75%/ 100%) Type of cooling 10. Station Transformer Station Transformer in Coal based power in Gas based CCPP plants ONAN/ONAF cooling (80%/100%) OR ONAN/ ONAF/OFAF cooling (60%/ 80%/ 100%) OR ONAN/ ONAF/ODAF cooling (60%/80%/100%) RATINGS B. ONAN/ONAF cooling (80%/100%) OR ONAN/ONAF/ OFAF cooling (60%/80%/100%) OR ONAN/ONAF/ODAF cooling (60%/80%/100%) OR ODAF cooling, in case of unit coolers (100%) Technical Specifications/ Generator Transformer in Coal Generator Transformer in Particulars based power plants Gas based CCPP Sl. No. In line with IS 2026-I, MVA rating shall be sufficient to cater to the maximum running load on the respective Switchgear/Load Centre. 5% design margin (minimum) may be taken on above Unit Transformer (UT) will be sized to deliver unit loads + Balance of Plant (BoP) loads. (If there is no Station Transformer and GCB is provided) 5% design margin (minimum) may be taken on above Unit Transformer (UT) will be sized to deliver unit loads. In case of plant having Generator Circuit Breaker (GCB), Unit Transformer shall be sized to take care the loads of units as well as the common auxiliaries/ systems. 5% design margin (minimum) may be taken on above LV Transformer (Dry Type/ Oil immersed) in Coal/ Gas based power plants AN or AN/AF or ONAN Unit Transformer in Gas based CCPP ONAN/ONAF ONAN/ONAF cooling cooling (80%/100%) (80%/100%) OR ONAN/ONAF/ OFAF cooling (60%/ 80%/ 100%) Unit Transformer in Coal based power plants 552 Manual on Transformers Rated Current-HV windings (A) Rated Current- LV windings (A) Over voltage Withstand Time Sec 14. 15. 16. 17. Rated Voltage-LV (kV) 13. 60 sec 5 sec 125% of rated voltage 140% of rated voltage Continuous 60 Sec 5 sec (a) For over fluxing factor 1.10 (b) For over fluxing factor 1.25 (c) For over fluxing factor 1.40 Capability Over-fluxing Continuous 110% of rated voltage As per MVA rating & voltage rating As per MVA rating & voltage rating As per Generator terminal voltage Rated Voltage – HV (kV) 245 kV for 3-phase bank & 245/√3 for single phase bank For 220 kV Grid System. 420 kV for 3-phase bank & 420/√3 for single phase bank For 400 kV Grid System. 765 kV for 3-phase bank &765/√3 for single phase bank For 765 kV Grid System. 12. As per Plant distribution System 220 kV for 220 kV grid system. 400 kV for 400 kV grid system. 765 kV for 3-phase bank for 765 kV grid system Unit-1 & shut-down of Unit-2 under one ST out, may be decided as per project requirements] As per Plant distribution System 5 sec 60 Sec Continuous 5 sec 60 sec 5 sec 60 Sec Continuous 5 sec 60 sec Continuous 5 sec 60 Sec Continuous 5 sec 60 sec Continuous As per MVA rating & voltage As per MVA rating & As per MVA rating & rating voltage rating voltage rating Continuous Unit Transformer in Coal based power plants 5 sec 60 Sec Continuous 5 sec 60 sec Continuous As per MVA rating & voltage rating As per MVA rating & voltage rating As per Plant distribution System 220 kV for 220 kV grid As per Generator system. 400 kV for 400 terminal voltage kV grid system. 765 kV for 3-phase bank for 765 kV grid system Station Transformer Station Transformer in Coal based power in Gas based CCPP plants As per MVA rating & voltage As per MVA rating & As per MVA rating & rating voltage rating voltage rating As per Generator terminal voltage 245 kV for 3-phase bank for 220 kV grid system. 420 kV for 3-phase bank for 400 kV grid system. 765 kV for 3-phase bank for 765 kV grid system. Technical Specifications/ Generator Transformer in Coal Generator Transformer in Particulars based power plants Gas based CCPP Sl. No. LV Transformer (Dry Type/ Oil immersed) in Coal/ Gas based power plants 5 sec 60 Sec Continuous 5 sec 60 sec Continuous As per MVA rating & voltage rating As per MVA rating & voltage rating As per Plant distribution System Not required 60 Sec (for Oil filled) Continuous NA NA Continuous As per kVA rating & voltage rating As per kVA rating & voltage rating As per Plant distribution System As per Generator terminal As per Plant distribution voltage System Unit Transformer in Gas based CCPP Salient Technical Specifications and Particulars of Power Transformers for Power Plant Application 553 Maximum current density of windings 19. Temperature rise over 50°C maximum ambient of Top oil by thermometer Temperature rise over maximum ambient of winding by resistance TAPPINGS Type Range - Steps x % Variation Taps provided on HV/LV HV Neutral end Winding 21. 22. D 23. 24. 25. Indicative range: +/-5% in steps of 2.5% for OCTC and +10% to -15% for OLTC in steps of 1.25%, however exact range to be decided to meet the system requirement of evacuating the total generated power within the operating range of generator power factor. Off circuit tap changer/ On load tap changer(preferably for 3-ph bank transformer) 55°C (for OFAF) 60°C (for ODAF) Design Ambient Temperature 20. 50°C (say)/ lower ambient temperature as per plant requirement TEMPERATURE RISE 3- 3.50 A/ Sq. mm LV C 3- 3.50 A/ Sq. mm HV HV Neutral end 3- 3.50 A/ Sq. mm 3- 3.50 A/ Sq. mm As per IS - 6600 On load tap changer 55°C 50°C HV Side HV Side ± 10% in steps of 1.25% On load tap changer 55°C 50°C 50°C (say) / lower 50°C (say) / lower ambient temperature ambient temperature as per plant as per plant requirement requirement 3- 3.50 A/ Sq. mm 3- 3.50 A/ Sq. mm As per IS - 6600 Station Transformer Station Transformer in Coal based power in Gas based CCPP plants Indicative range: +/-10% in Indicative range: steps of 2.5% for OCTC and ± 10% in steps of +7.5% to -15% for OLTC 1.25% in steps of 1.25%, however exact range to be decided to meet the system requirement of evacuating the total generated power within the operating range of generator power factor. Off circuit tap changer/ On load tap changer 55°C (for OFAF) 60°C (for ODAF) 50°C 50°C (say) / lower ambient temperature as per plant requirement 3- 3.50 A/ Sq. mm 3- 3.50 A/ Sq. mm As per IS - 6600 Over loading capacity 18. As per IS - 6600 Technical Specifications/ Generator Transformer in Coal Generator Transformer in Particulars based power plants Gas based CCPP Sl. No. 3- 3.50 A/ Sq. mm 3- 3.50 A/ Sq. mm As per IS - 6600 Unit Transformer in Gas based CCPP 55°C 50°C HV Side Indicative range: ± 10% in steps of 1.25% Off circuit tap changer 55°C(for Oil filled) 90°C (for Dry type. Class-F) 50°C (for Oil filled) 50°C (say) / lower ambient temperature as per plant requirement 3- 3.50 A/ Sq. mm 3- 3.50 A/ Sq. mm As per IS – 6600, 2026& 11171 LV Transformer (Dry Type/ Oil immersed) in Coal/ Gas based power plants HV Side HV Side ± 10% or ±12.5% in steps ± 5% in steps of 2.5% of 1.25% On load tap changer On load tap changer 55°C 50°C 50°C (say) / lower 50°C (say) / lower ambient temperature ambient temperature as as per plant per plant requirement requirement 3- 3.50 A/ Sq. mm 3- 3.50 A/ Sq. mm As per IS - 6600 Unit Transformer in Coal based power plants 554 Manual on Transformers LV side SHORT CIRCUIT WITHSTAND TIME WITHOUT INJURY DUE TO TERMINAL FAULT WITH RATED VOLTAGE MAINTAINED ON THE OTHER SIDE With 3 phase dead short circuit With single phase short circuit H 31. 32. 2 sec 2 sec As per short circuit calculation at LV side 2 sec 2 sec As per short circuit calculation at LV side 2 sec 2 sec 2 sec 2 sec As per short circuit As per short circuit calculation at LV side calculation at LV side 2 sec 2 sec As per short circuit calculation at LV side 40/50/63 kA for 1/3 sec (To 40/50 kA for 1/3 sec 40/50 kA for 1/3 sec Magnitude of fault be defined as per grid system (To be defined as (To be defined as per current& Withstand fault level) per grid system fault grid system fault level) time depends level) upon system configurations Effectively earthed Non-effectively earthed a) MVA rating at AN/ ONAN b) 4-10% CFVV LV Transformer (Dry Type/ Oil immersed) in Coal/ Gas based power plants 2 sec 2 sec As per short circuit calculation at LV side 2 sec 2 sec 50/65 KA (r.m.s.) for 1 Sec Magnitude of fault 50/44/40 KA (r.m.s.) current &Withstand time for 1 Sec depends upon system configurations Non effectively earthed 30. 40/50/63 kA for 1/3 sec (To be defined as per grid system fault level) Non effectively earthed HV side Non Effectively Earthed Non effectively earthed 29. Non Effectively Earthed Non effectively earthed SYSTEM FAULT LEVEL Non Effectively Earthed Solidly earthed G Non Effectively Earthed Solidly earthed LV side Solidly earthed a) MVA rating at ONAF or ONAN b) 10-14 % 28. Solidly earthed a) MVA rating at ONAF a) MVA rating at or ONAN ONAF or ONAN b) 12-20 % b) 10-14 % For three winding transformers, impedance value to be decided based on the system requirement CFVV HV side a) MVA rating at OFAF or ONAF or ONAN b) 12-20 % For three winding transformers, impedance value to be decided based on the system requirement CFVV Unit Transformer in Gas based CCPP SYSTEM EARTHING a) MVA rating at OFAF or ONAF or ONAN or ODAF b) 12% to 16% (to be decided considering the requirement to evacuate the total generated power within the operating range of generator power factor and also to limit the short circuit contribution to grid) CFVV Unit Transformer in Coal based power plants 27. a) MVA rating at OFAF or ONAF or ONAN or ODAF b) 12% to 16% (to be decided considering the requirement to evacuate the total generated power within the operating range of generator power factor and also to limit the short circuit contribution to grid) CFVV Station Transformer Station Transformer in Coal based power in Gas based CCPP plants F a) Base MVA Rating b) Impedance at Principal Tap at Rated Frequency and 75°C winding temperature IMPEDANCES E CFVV Category of voltage variation 26. Constant Flux Voltage Variation (CFVV) Technical Specifications/ Generator Transformer in Coal Generator Transformer in Particulars based power plants Gas based CCPP Sl. No. Salient Technical Specifications and Particulars of Power Transformers for Power Plant Application 555 Class of winding insulation SI (Switching impulse)/ LI (Lightning impulse) / AC (Power Frequency AC withstand voltage) a) HV Bushing b) HV Winding 36. 37. As per projects requirement Paint shade Type of Winding Insulation Epoxy paint INSULATION Max 85 dB Noise level Finishing paint type 35. Max 85 dB One number stand-by fan per One number stand-by fan bank and one number stand-by per bank and one number pump per bank for radiator banks stand-by pump per bank for radiator banks Stand-by Fan & Pumps J One number stand-by One number stand-by fan per bank and one fan per bank number stand-by pump per bank for radiator banks 120% capacity radiator banks (minimum two (2) banks) with arrangement of isolating 20% capacity of banks at a time so that in case of leakage in one section, only that section will be isolated not the whole bank. OR 120% capacity Unit Coolers (min six (6) nos. coolers so that in case of failure of one cooler also, 100% capacity is available) As per IS 2026& IS: 2099 (Insulation level of HV winding & bushing to be verified with insulation co-ordination study) / IEC 60076 Class-A HV- Non uniformly graded LVuniformly graded As per IS 2026& IS: 2099 (Insulation level of HV winding & bushing to be verified with insulation coordination study) / IEC 60076 Class-A HV- Non uniformly graded LV- uniformly graded As per projects requirement Epoxy paint Max 85 dB As per projects requirement Epoxy paint Max 85 dB 2 X 60 % Banks As per IS 2026& IS: 2099 (Insulation level of HV winding & bushing to be verified with insulation co-ordination study) / IEC 60076 Class-A As per IS 2026& IS: 2099 (Insulation level of HV winding & bushing to be verified with insulation coordination study / IEC 60076 Class-A HV- Non uniformly HV- Non-uniformly graded LV- uniformly graded LV- uniformly graded graded As per projects requirement Epoxy paint 120% capacity radiator banks 2 X 60 % Banks (minimum two (2) banks) with arrangement of isolating 20% capacity of banks at a time so that in case of leakage in one section, only that section will be isolated not the whole bank. Unit Transformer in Coal based power plants 2 X 60 % Banks Bell type/ Conventional Unit Transformer in Gas based CCPP Class-A HV- uniformly graded LV- uniformly graded As per projects requirement Epoxy paint Max 85 dB As per IS 2026& IS: As per IS 2026& IS: 2099 / IEC 60076 2099 / IEC 60076 Class-A HV- uniformly graded LV uniformly graded As per projects requirement Epoxy paint Max 85 dB One number stand- One number stand-by by fan per bank and fan/ Bank one number standby pump per bank for radiator banks 2 X 60 % Banks Bell type/ Conventional Bell type/ Conventional Cooler type & capacity Bell type/ Conventional 34. Bell type Transformer tank cover type 33. Bell type CONSTRUCTION I Station Transformer Station Transformer in Coal based power in Gas based CCPP plants Technical Specifications/ Generator Transformer in Coal Generator Transformer in Particulars based power plants Gas based CCPP Sl. No. As per IS 2026& IS: 2099 / IEC 60076 Class-A (Oil filled) ClassF(Dry Type) HV- uniform LV- uniform As per projects requirement Epoxy paint Max 85 dB Not applicable 2 X 50 % Banks for oil immersed Conventional LV Transformer (Dry Type/ Oil immersed) in Coal/ Gas based power plants 556 Manual on Transformers LI (Lightning impulse) / AC (Power frequency withstand voltage) -LV BUSHINGS Reference standard HV Bushing (a). Type (b). Voltage Class (c). Creepage distance (d). Terminations LV Bushing (a). Type (b). Voltage Class (c). Creepage distance (d). Terminations 39. K 40. 41. 42. (a). Oil communicating condenser (b). As per IS 2026-III for rated LV voltage (c). 20 mm/kV (for IPBD terminations) (d). Bus duct terminations (a). OIP condenser with test tap (b). As per IS 2026-III for rated HV system voltage (c). 31 mm/kV (d). Suitable for terminating ACSR/AAC/AAAC overhead conductors or IPS tubes (in case outdoor cable sealing arrangement is provided) IS : 2099 / IEC 60137 As per IS 2026& IS:2099 / IEC 60076 a. Oil communicating condenser b. As per IS 2026-III for rated LV voltage c. 20 mm/kV (for IPBD terminations) d. Bus duct terminations a. OIP condenser with test tap b. As per IS 2026-III for rated HV system voltage c. 31 mm/kV d. Suitable for terminating ACSR/AAC/AAAC overhead conductors or IPS tubes IS : 2099 / IEC 60137 As per IS 2026& IS:2099 / IEC 60076 As per IS 2026& IS:2099 / IEC 60076 LI (Lightning impulse) / AC (Power frequency withstand voltage) -HV Neutral 38. As per IS 2026& IS:2099 / IEC 60076 Technical Specifications/ Generator Transformer in Coal Generator Transformer in Particulars based power plants Gas based CCPP Sl. No. a. Porcelain bushing b. As per IS 2026-III for rated LV voltage c. 25- 31 mm/kV d. SPBD a. OIP condenser with test tap/ Porcelain bushing b. As per IS 2026-III for rated HV system voltage c. 31 mm/kV d. Suitable for terminating ACSR/ AAC/AAAC overhead conductors or IPS tubes a. OIP condenser with test tap b. As per IS 2026-III for rated HV system voltage c. 31 mm/kV d. Suitable for terminating ACSR/ AAC/AAAC overhead conductors or IPS tubes (in case outdoor cable sealing arrangement is provided) a. Porcelain bushing b. As per IS 2026-III for rated LV voltage c. 25- 31 mm/kV d. SPBD IS : 2099 / IEC 60137 IS : 2099 / IEC 60137 As per IS 2026& As per IS 2026& IS:2099 / IEC 60076 IS:2099 / IEC 60076 a. Porcelain bushing b. As per IS 2026-III for rated LV system voltage c. 25- 31 mm/kV d. SPBD/ Cables a. Porcelain bushing b. As per IS 2026-III for rated HV system voltage c. 20 mm/kV (for IPBD terminations) d. Bus duct terminations a. Porcelain bushing b. As per IS 2026-III for rated HV system voltage c. 20 mm/kV (for IPBD terminations) d. Bus duct terminations a. Porcelain bushing b. As per IS 2026-III for rated LV system voltage c. 25- 31 mm/kV d. SPBD IS : 2099 / IEC 60137 IS : 2099 / IEC 60137 a. Porcelain bushing/ Epoxy Cast b. As per IS 2026 c. 25 mm / kV d. Non Segregated Phase Bus Duct (NSPBD) a. Porcelain bushing/ Cast Resin b. As per IS 2026 c. 25 mm / kV d. Cable IS : 2099 / IEC 60137 As per IS 2026& As per IS 2026& IS:2099 As per IS 2026& IS:2099 IS:2099 / IEC 60076 / IEC 60076 / IEC 60076 As per IS 2026& As per IS 2026& IS:2099 As per IS 2026& IS:2099 IS:2099 /IEC 60076 /IEC 60076 /IEC 60076 As per IS 2026& IS:2099 /IEC 60076 LV Transformer (Dry Type/ Oil immersed) in Coal/ Gas based power plants As per IS 2026& IS:2099 /IEC 60076 Unit Transformer in Gas based CCPP Unit Transformer in Coal based power plants Station Transformer Station Transformer in Coal based power in Gas based CCPP plants Salient Technical Specifications and Particulars of Power Transformers for Power Plant Application 557 Minimum clearances in air 44. TRANSFORMER OIL Type Applicable standard 45. 46. IS 335/ IEC 60296 Mineral Oil (Uninhibited) As per IS 2026-5 / IEC 60076 b) LV Side L As per IS 2026-5 / IEC 60076 a) HV Side HVN & LVN Bushing a. Porcelain bushing b. As per IS 2026-III c. 25- 31 mm/kV d. Cable IS 335/ IEC 60296 Mineral Oil (Uninhibited) IS 335/ IEC 60296 Mineral Oil (Uninhibited) As per IS 2026-5 / IEC 60076 As per IS 2026-5 / IEC 60076 HVN & LVN Bushing a. Porcelain bushing b. As per IS 2026-III c. 25- 31 mm/kV d. Cable Unit Transformer in Coal based power plants IS 335/ IEC 60296 Mineral Oil (Uninhibited) IS 335/ IEC 60296 Mineral Oil (Uninhibited) As per IS 2026-5 / IEC As per IS 2026-5 / 60076 IEC 60076 As per IS 2026-5 / IEC As per IS 2026-5 / 60076 IEC 60076 HVN & LVN Bushing a. Porcelain bushing b. As per IS 2026-III c. 25- 31 mm/kV d. Cable Station Transformer Station Transformer in Coal based power in Gas based CCPP plants As per IS 2026-5 / IEC 60076 As per IS 2026-5 / IEC 60076 HVN Bushing a. Porcelain bushing b. As per IS 2026-III c. 25- 31 mm/kV d. Flats HVN/LVN Bushing a. Type b. Voltage Class c. Creepage distance d. Terminations 43. HVN Bushing a. Porcelain bushing b. As per IS 2026-III c. 25- 31 mm/kV d. Flats Technical Specifications/ Generator Transformer in Coal Generator Transformer in Particulars based power plants Gas based CCPP Sl. No. As per IS 2026-5 / IEC 60076 As per IS 2026-5 / IEC 60076 LVN Bushing a. Porcelain bushing/ epoxy cast b. As per IS 2026-III c. 25- 31 mm/kV d. Cable LV Transformer (Dry Type/ Oil immersed) in Coal/ Gas based power plants IS 335/ IEC 60296 IS 335/ IEC 60296 Mineral Oil (Uninhibited) Mineral Oil (Uninhibited) As per IS 2026-5 / IEC 60076 As per IS 2026-5 / IEC 60076 LVN Bushing a. Porcelain bushing b. As per IS 2026-III c. 25- 31 mm/kV d. Cable Unit Transformer in Gas based CCPP 558 Manual on Transformers Appendix VIII Test Windings for Bushing Current Transformers Appendix VIII Test Windings for Bushing Current Transformers When bushing current transformers are provided in power transformers, a test winding (a one turn winding over and above secondary winding) shall be provided to test the CT characteristics without passing current through the primary (i.e., transformer bushing) which will be difficult . This one turn test winding shall be suitable to carry 10A (minimum). In certain cases, it may be more convenient to provide test tappings, one being at one end of the secondary winding and the other at the center of the secondary winding. The test terminals should be physically separated from output terminals of the secondary winding and preferably of slightly larger size. When test winding is energized, the primary winding should be open circuited. Conversely, when the primary winding is employed (i.e., when power transformer is energized), the test winding should be kept open circuited. One of the test terminals shall be kept grounded in service and shall be shown in rating & diagram plate. Test winding is not recommended for current transformers for metering and for winding temperature indication. Test winding terminals shall be marked as below: Fig. 1 Fig. 2 Current Transformer with two secondary windings having Separate magnetic coreswindings having Separate magnetic cores and Test Winding Fig. 3 Current Transformer with two secondary and Common Test Winding Transformer with an intermediate tapping on secondary winding with test taps REFERENCE (1) BS 3938: 1973 Current Transformers, Appendix H Current Transformer test windings. 561 Appendix IX Pictures of Transformer Installations Appendix IX Pictures of Transformer Installations PLATE 1: 1-PHASE GENERATOR TRANSFORMER- TOP VIEW PLATE 2:1 PHASE GENERATOR TRANSFORMER-SIDE VIEW 565 566 Manual on Transformers PLATE 3: 3-PHASE GENERATOR TRANSFORMER-TOP VIEW PLATE 4: 3-PHASE GENERATOR TRANSFORMER-FRONT VIEW Test Windings For Bushing Current Transformers PLATE 5: 3-PHASE GENERATOR TRASNFORMER- SIDE VIEW PLATE 6:3D MODEL VIEW OF A BANK OF 3 1- PHASE GENERATOR TRANSFORMERS WITH IPBD CONNECTIONS 567 568 Manual on Transformers PLATE 7: 3D MODEL VIEW OF 3 PHASE GENERATOR TRANSFORMER WITH IPBD CONNECTIONS PLATE 8:GENERATOR TRANSFORMER AT SITE Test Windings For Bushing Current Transformers PLATE 9: GENERATOR TRANSFORMER INSTALLED AT SITE WITH LIGHTNING ARRESTERS PLATE 10: STATION TRANSFORMER INSTALLED AT SITE 569 570 Manual on Transformers PLATE 11: DISTRIBUTION TRANSFORMER INSTALLED AT SITE PLATE 12: SERIES UNIT OF PHASE SHIFTING TRANSFORMER Test Windings For Bushing Current Transformers PLATE 13: SHUNT UNIT OF PHASE SHIFTING TRANSFORMER PLATE 14: CONTROLLED SHUNT REACTOR INSTALLED AT SITE 571 572 Manual on Transformers PLATE 15: RECTIFIER TRANSFORMER ALONG WITH RECTIFIER CUBICLE INSTALLED AT SITE PLATE 16: ARC FURNACE TRANSFORMER INSTALLED AT SITE Test Windings For Bushing Current Transformers PLATE 17: SILICAGEL BREATHERS ASSEMBLY PLATE 18: HV BUSHING WITH TERMINAL CONNECTION 573 574 Manual on Transformers PLATE 19: UNIT TRANSFORMER TAP-OFF OF IBPD & FIRE WALL PLATE 20: RAIL GAUGE AND FLANGED TWIN ROLLER ASSEMBLY Test Windings For Bushing Current Transformers PLATE 21: RADIATOR BANK ASSEMBLY WITH COOLING FANS PLATE 22: TRANSFORMER OIL PUMPS AND OIL FLOW INDICATION 575 576 Manual on Transformers PLATE 23: TRANSFORMER OIL FLOW INDICATOR PLATE 24: CONSERVATOR AND OLTC DRIVING SHAFT Test Windings For Bushing Current Transformers PLATE 25: MARSHALLING BOX 577 578 Manual on Transformers CORE GROUP Chairman Shri M. Vijayakumaran Sr. Technical Expert ALSTOM T&D India Ltd Naini, Allahabad 211008 Members Shri P. Ramachandran Sr. Advisor - Design & Development Power Transformers Division ABB Ltd., Maneja, Vadodara – 390013 Shri V.K. Lakhiani Technical Director Transformers and Rectifiers (India) Limited Survey No. 427 P/3-4, & 431 P/1-2 Sarkhej-Bavla Highway, Moraiya, Taluka: Sanand, Dist. Ahmedabad–382213 Shri Dinkar Devate General Manager NTPC Ltd. A-8A, Sector 24, Noida - 201301 Shri M.M. Goswami General Manager Power Grid Corporation of India Ltd. Saudamini, Plot No. 2, Sector 29 Gurgaon – 122001 Shri V.K. Kanjlia Secretary Central Board of Irrigation & Power Malcha Marg, Chanakyapuri New Delhi 110021 Shri P.P. Wahi Director Central Board of Irrigation & Power Malcha Marg, Chanakyapuri New Delhi 110021 Shri S.K. Batra Sr. Manager (Technical) Central Board of Irrigation & Power Malcha Marg, Chanakyapuri New Delhi 110021 LAN: 0532 -2699990-Extn 1408, Mob+91 9956390015 vijayakumaran.moorkath@alstom.com T:+91 265 263 7291 +91 265 260 4502 M: +91 97 243 32977 p.ramachandran@in.abb.com Cell : +91-9687659750 Landline : +91-2717-661565 virendra.lakhiani@transformerindia.com; M: 09650992237 dinkar@ntpceoc.co.in Phone 0124 2571814 M : 9910378078 mgoswami@powergridindia.com Phone 91-11-26115984 M : 9810137864 Fax 91-11-26116347 Email kanjlia@cbip.org Phone 91-11-24101592 M : 9810801555 Email wahi@cbip.org Phone 91-11-26876229 ext. 133 M: 9811943812 Fax 26116347 Email batra@cbip.org; cbip@cbip.org