SPECIFICATION FOR OIL FILLED TRANSFORMER CONTENTS 1. SCOPE 2. CODES AND STANDARDS 3. GENERAL REQUIREMENTS 4 AMBIENT CONDITIONS 5. CONSTRUCTION 6. TERMINALS AND MARSHALLING BOX 7. COOLING 8. OIL 9. ACCESSORIES 10. CORE AND WINDING 11. TAPPINGS AND CONTROL 12. NEUTRAL GROUNDING RESISTOR (NGR) 13. NOISE 14. INSPECTION, TESTING AND ACCEPTANCE 15. PACKING AND DESPATCH Abbreviations: ONAN : Oil Natural Air Natural ONAF : Oil Natural Air Forced OLTC: On Load Tap Changer OTI : Oil Temperature Indicator WTI : Winding Temperature indicator RTCC: Remote Tap Changer Control 1.0 SCOPE The intent of this standard specification is to define the requirements of design, engineering, manufacture and testing of oil filled power/distribution transformers. 2.0 CODES AND STANDARDS 2.1 The transformer shall comply with the latest edition of the following and other relevant Indian Standards/Manual: IS:335 - Insulating oil IS:1271 - Thermal evaluation and Classification of electrical insulation. IS:2026 - Power transformers IS:2099 - Bushing for Alternative voltages above 1000 V. IS:2705 - Current transformers. IS:3347 - Dimensions for porcelain Transformer Bushings IS:3637 - Gas operated relays. IS:3639 - Fitting & accessories for power transformers IS:4201 - Application guide for CTs. IS:8478 - Application guide for ON-load tap changers IS:8468 - On-load tap changers (Amendment 1) IS:13947- LV switchgear and control gear - Part 1 General rules. IEEE:32 – Neutral Grounding Devices- Standard Requirements, Terminology and Test Procedure ASTM standard A240-304 CBIP Manual on Transformers 2.2 In case of any contradiction between various referred standards/ specifications/data sheets and statutory regulations, the following order of decreasing priority shall govern: - Statutory regulations - Data sheets - Job specifications - This specification - Codes and standards. 2.3 Transformer shall also conform to the provisions of the latest revisions of the Indian Electricity rules and any other statutory regulations currently in force. 3.0 3.1 GENERAL REQUIREMENTS The offered equipment shall be brand new with a proven field track record. No prototype equipment shall be offered. The vendor shall be responsible for design, engineering and manufacturing of the equipment to fully meet the intent and requirements of this specification and attached data sheets. 3.2 3.3 All equipment and accessories required for completeness of the equipment, whether specifically mentioned or not, but considered essential for satisfactory performance shall be included as a part of the offered equipment 4.0 AMBIENT CONDITIONS Transformer and all its accessories and allied systems shall be suitable for operating satisfactorily in humid and corrosive atmospheres found in fertilizer plants, refineries and petrochemical plants. Service conditions shall be as specified in the data sheets. If not specifically mentioned therein, design temperature of 40°C and an altitude not exceeding 1000m above mean sea level shall be considered. 5.0 5.1 CONSTRUCTION Transformer tank shall be of welded sheet steel construction and provided with gasketed steel cover plates. Base shall be suitably reinforced to prevent any distortion during lifting. Base channels shall be provided with skids and pulling eyes to facilitate handling. Transformer tank shall be tested as per clause 13.0 of this specification. 5.2 All covers and seals shall be oil and airtight and shall not be affected by mineral or synthetic oil action. Detachable radiators equipped with air vent, drain plug and lifting lugs shall be provided with shut-off valves to permit removal of any radiator unit without emptying the tank. Radiators shall be securely braced to prevent undue vibration. For transformers of ratings less than 500 KVA, fixed radiators welded to the tank may be provided. 5.3 All fasteners and bolts etc. shall be cadmium plated. All surfaces to be painted shall be thoroughly cleaned, de-scaled, made free from rust and transformer shall be epoxy painted. Paint shall be suitable to withstand specified atmospheric conditions. 5.4 Transformer shall be suitable for using emulsifiers as part of fire protection system. 5.5 Different parts of transformers shall be connected by copper flexibles for earth continuity purpose. 6.0 TERMINALS AND MARSHALLING BOX 6.1 Windings shall be brought out and terminated on outdoor bushings, cable boxes or bus-duct chamber, which will be located as specified on the data sheet. 6.2 When outdoor bushings are specified they shall be supplied complete with terminal connectors suitable for the specified conductor size. 6.3 Cable boxes shall be supplied with cable lugs and glands. 6.3.1 For MV PVC Power/control cables double compression cable glands and crimped type tinned copper cable lugs shall be supplied. Gland plate shall be removable type. For single core cables, gland plate shall be of non-magnetic material. HV cable box shall be suitable for termination of specified size of XLPE insulated cable .The head-room available between cable gland plate and terminals shall not be less than 600mm for cable upto 11 kV, and 900 mm for 22 kV and 33 kV cables. Cable box and disconnecting chamber shall be air insulated. 6.3.2 Cable box shall be weatherproof to IP-55. For fixed portion of cable box, an inspection cover with a lifting handle shall be provided. 6.3.3 Primary cable box (where applicable) shall be able to withstand specified primary system fault level for 0.20 secs. 6.4 Terminal chamber for bus-duct termination shall have a gasketed cover plate bolted to it and shall be weatherproof to IP-55. A separate inspection cover with lifting handle shall be provided to facilitate connection and inspection. Phase sequence of the bus bars shall be as specified in the data sheets. 6.5 Marshalling box shall be weatherproof to IP-55. All protective devices and neutral CTs shall be wired by means of PVC insulated copper conductor armoured cables upto the marshalling box. Terminals shall be clamp type. Removable gland plate with double compression type glands shall be provided. Lamp with switch & socket shall be provided in the marshalling box. Preferably, marshalling box shall be located on the front side of transformer. 6.6 For transformers having provisions for terminating TPN bus duct on the 433V side, neutral of the star connected secondary winding shall be brought out to a secondary terminal chamber. A CT shall be mounted (if specified) on the neutral terminal with CT secondary wired up to the marshalling box. A separate neutral bushing shall be provided for neutral earthing of transformers. The neutral CT shall be mounted before bifurcation of the neutral. Supporting arrangement for GI strip/cable as applicable shall be provided for connection of neutral bushing to earth/NGR. 7.0 7.1 COOLING Type of cooling shall be in accordance with data sheet. 8.0 OIL Transformer shall be supplied with first filling of oil and 10% extra oil in non-returnable drums. 9.0 9.1 9.1.1 9.1.2 9.1.3 9.1.4 9.1.5 ACCESSORIES The following accessories shall be provided as a minimum: Rating Plate Terminal marking Plate Two earthing terminals De-hydrating breather Conservator (For transformer rated 50 kVA and above, for rated voltage 11 kV and below, and all ratings above 11 kV) Air release Device (for transformers with conservator) Oil filling hole with cover Oil Level indicator with minimum marking Thermometer pocket Dial type thermo meter with contacts for OTI & WTI Oil temperature indicator Winding temperature indicator (for transformers of rating 500 KVA and above) Pressure relief valve Sampling valve Conservator drain valve Top oil filter valve 9.1.6 9.1.7 9.1.8 9.1.9 9.1.10 9.1.11 9.1.12 9.1.13 9.1.14 9.1.15 9.1.16 9.1.17 Drain cum bottom filter valve 9.1.18 Double float Buchholz relay 9.1.19 Separate neutral bushing outside terminal box with connector assembly 9.1.20 9.1.21 9.1.22 9.1.23 9.1.24 9.1.25 9.2 Inspection cover Terminal box Marshalling box (IP-55) Lifting lugs Jacking lugs Cross channels with towing lugs Conservator shall be complete with oil filling plug and cap, oil drain valve, oil level gauge in addition to magnetic oil level gauge. A flexible oil resistance air bag shall be provided for conservator as specified in data sheet. Air bag shall be designed to withstand repeated expansion and contraction due to changes in oil level. 9.3 Pressure relief valve (PRV) shall be provided with alarm contact. 9.4 Thermometer pocket for thermometer shall be provided. 9.5 Separate buchholz relay shall be provided for main tank and OLTC chamber. This shall be double float type for the main tank with separate normally open trip and alarm contacts. For the OLTC chamber the relay shall be of single float type or oil surge relay with one trip contact. Isolating valve shall be provided on either side of the relay and distance piece shall be supplied. 9.6 Oil temperature indicator shall be 150 mm dial type with two normally open contacts for tripping and alarm. It shall be provided with maximum reading pointer. 9.7 Winding temperature indicator shall have normally open contacts one each for tripping, alarm and Starting /stopping of cooling fan motors. It shall be provided with maximum reading pointer. 9.8 Contact rating of all auxiliary devices shall be 0.5A, 110V/220V DC. 9.9 For Transformer rating upto 1600 kVA the Size of valves for drain and filter shall be 25mm and that for sampling shall be 15mm. 10.0 CORE AND WINDINGS 10.1 Transformer shall be double wound, core type with high grade cold rolled non-aging grain oriented low loss, high permeability silicon steel laminations (M-4 or better grade) perfectly insulated and clamped to minimise vibration and noise. Care shall be taken to insulate core fastening bolts to reduce losses and avoid hot spots. All parts of magnetic circuit shall be bonded to earth system. 10.2 Windings shall be of copper and shall be designed to withstand the applicable thermal and dynamic short circuit stresses. 11.0 TAPPINGS & CONTROL 11.1 These shall be provided on high voltage side and connected to off circuit or on-load tap changing gear as specified on data sheet. Under conditions of external short circuit, the tap changing equipment shall be capable of carrying the same current as the windings. 11.2 Off circuit tap changing gear: Off circuit tap changing gear shall have an external operating handle mounted on the transformer side and shall meet the following requirements: - Positive snap-action contact changing - The mechanism shall be such that it is impossible for the contacts to be set in a position whereby the windings remain open-circuited or partly short-circuited. - Mechanical stops at the ends shall be provided to prevent overrun. - The driving rod through cover or tank wall shall be properly sealed against oil leakage under all service conditions. The handle shall be metallic and adequately sized in order to allow operation without the need of tools and be located in a directly accessible position. The handle shall be provided with padlock facilities to lock the tap changer in the desired position. Tap positions shall be clearly marked in line with the data given on the rating plate. 11.3 On Load Tap Changer 11.3.1 High speed on load tap changing gear with number of steps as specified on the data sheet shall be provided and mounted on the transformer. The OLTC gear shall have diverter resistance and the current diverting contacts shall be housed in a separate oil chamber segregated from the main tank of the transformer. The contacts shall be accessible for inspection and their tips shall be replaceable. OLTC oil chamber shall have oil filling, drain and sampling facility. It shall be provided with oil-level indicator, connection orifices, valves and silica gel breather shall be provided. 11.3.2 OLTC shall be provided with local and remote controls. Local Control 11.3.3 Manual-Mechanical Control The cranking device for operation of the OLTC gear shall be removable and located at a height not exceeding 1500mm above ground level for easy operation. The mechanism shall be complete with normal accessories including at least the following:- A mechanical tap position indicator (Rated tap voltages shall be marked on the diagram plate). - A mechanical operation counter. - Mechanical stops to prevent over cranking of the mechanism beyond extreme tap positions. 11.3.4 Electrical Control Control circuit shall incorporate the following: a) Local/remote manual electrical operation. b) Device to ensure a positive and full completion of tap change once it is initiated even if there is loss of power. c) An interlock to cut-off electrical control automatically upon recourse being taken to manual mechanical control in emergency. d) Electrical interlock to cut-off a counter impulse for a reverse tap change, being initiated during a progressive tap change and until the mechanism comes to rest and resets circuits for a fresh operation. e) All auxiliaries and devices for electrical control of OLTC gear should be housed in a weather-proof cabinet mounted on the transformer and shall include: - Local tap position indicator - 5 digit operation counter - Cubicle lighting - Thermostatically controlled space heater. - Miniature circuit breaker with magnetic and thermal overload devices for controlling the incoming supply to the OLTC motor. - Padlocking arrangement for the hinged cabinet door. - Removable plate with cable glands. - Inside tag with control scheme indelibly marked. 11.3.5 Remote Tap Changer Control Panel Remote Tap Changer control panel shall comprise of the following: - Individual/parallel control on Master follower sequence selector switch. - Raise/lower control switch. - Potentiometer type tap position indicator. - Out of step relay. - Time delay relay. - Indicating lamp for out of step. - Out of step buzzer. - Indicating lamp for tap changer supply available. - Indicating lamp for tap change in progress. RTCC panel shall be dust & vermin proof, floor mounting, and freestanding type. The enclosure shall be cold rolled sheet of 2.5 mm for front and back and 2.5mm thick for the rest. All doors and opening shall be provided with neoprene gaskets. 11.3.6 Automatic tap changing control shall be provided complete with voltage sensing relay if specified on data sheet. 11.3.7 OLTC wherever called for shall be suitable for bi-directional power flow. 11.3.8 OLTC shall also be rated for basic insulation level value as specified for the transformer in the data sheet. 12.0 Neutral Grounding Resistor (NGR) 12.1 Resistors: 12.1.1 The resistor elements shall be made of unbreakable, corrosion-proof, joint-less stainless steel grids conforming to ASTM standard A240-304. The grids shall be in punched form. 12.1.2 The resistor shall be sized for carrying rated current specified in the data sheet for 10 seconds with maximum temperature limited to 790 deg C for stainless steel resistor elements while limiting the temperature of Aluminium conductor / bus bar to 350 deg C. 12.1.3 If specified in the data sheets the resistors shall also be sized to carry 10% of rated current continuously with the maximum operating temperature of resistor elements limited to 415 deg C. Further, the temperature rise of resistor elements when they carry 10% of rated current followed by the specified rated current for 10 seconds shall not exceed the temperature limit specified in the above clause. Under steady-state conditions when the NGR carries 10% of rated current the surface temperature of the enclosure, excluding the roof but including the separate canopy shall be limited to a maximum of 70 deg C. 12.1.4 Grids shall be mounted on steel rods insulated by special heat-resistant insulating materials, suitable for the above temperatures. Ceramic/ Porcelain insulators shall be used to insulate the resistor elements from the enclosure. The insulators and terminal bushings shall have adequate minimum creepage values (total and protected) for the required voltage grade. 12.1.5 Resistors shall be provided in parallel and series combinations to achieve the overall resistance value. Minimum two banks in parallel shall be provided in the system unless specified otherwise. 12.2 Enclosure: 12.2.1 The resistor elements shall be housed in naturally ventilated sheet steel enclosure with minimum IP-31 degree of ingress protection and suitable for outdoor installation. 12.2.2 The enclosure thickness shall not be less than 3mm. The NGR shall have suitable arrangements for fixing it on concrete foundations / grouting in floor. 12.2.3 The terminals for neutral and earthing connections shall be housed in a separate vermin-proof, weather-proof terminal box with minimum IP-55 degree of ingress protection. The terminal box shall be provided with a bolted removable undrilled gland plate of non-magnetic material. This shall be drilled at site to suit the cable entry. A nickel plated brass double compression gland of suitable size shall be provided along with the terminal box. 12.2.4 A separate canopy shall be provided above the enclosure roof with an air gap between them. It shall also cover the terminal compartment. Hooks or handle shall be provided to lift the canopy. 12.2.5 It shall be possible to periodically clean the resistor elements and enclosure after removing the bolted side covers. 12.2.6 Facility shall be provided to earth the enclosure at two points. For this purpose, suitably sized studs shall be provided on the sides of the enclosure to accommodate the connection of GI strip. 12.2.7 The bottom of the enclosure shall be suitably sloped and provided with a drain plug to remove water that may get collected in the enclosure. 12.3 Terminals 12.3.1 The two ends of the resistor shall be brought to suitable epoxy / porcelain bushing type terminals of adequate rating for the neutral and earth connections. The terminals shall be suitable for terminating the specified size of cables / earthing strip and shall be supplied complete with tinned-copper lugs. Bolted type link shall be provided for isolating the resistor at the neutral end. 12.3.2 Suitable washers shall be provided between the grid element ends and the terminals to maintain good electrical contact. 12.3.3 The connection between transformer neutral and NGR shall be carried out by means of single core cable of size and type as specified in the data sheet. 12.3.4 The connection between NGR and earth pits is carried out using GI earthing strips / cable as specified in the data sheet. 12.4 Space Heater A suitable anti-condensation space heater shall be provided inside the NGR enclosure to prevent condensation of moisture. This shall be rated to operate from a 240V, 50 Hz single phase supply. The terminals of the space heater shall be brought out to a separate vermin-proof, weather –proof terminal box. The terminal box shall have minimum IP-55 degree of ingress protection and shall be complete with tinned-copper lugs and nickel plated brass double compression glands. The space heater shall be controlled using a double pole switch, HRC fuse, neutral link and thermostat provided in the terminal box. The switch shall be operable from the front of the enclosure without opening the terminal box cover. 12.5 The current transformer, if specified, shall be cast in resin type suitable for the applicable voltage grade and temperature conditions. The secondary connections of the current transformer shall be brought out to suitable terminals provided with shorting link. 12.6 Cooling The NGR shall be natural air cooled. Forced cooling shall not be used. If required, louvers can be provided in the enclosure provided they are completely guarded on the inside with a fine wire mesh. 12.7 Nameplate Marking The equipment shall be provided with stainless steel nameplate located predominantly indicating particulars such as make, tag number of the equipment, rating and other relevant details. 12.8 Painting After manufacture, all surfaces to be painted shall be cleaned, descaled made free from rust and given one coat of primer and two coats of epoxy paint of shade 632 or as specified in the data sheet. 13.0 NOISE Audible Noise level shall not exceed the limits indicated in CBIP manual. 14.0 INSPECTION, TESTING AND ACCEPTANCE 14.1 Owner's representative shall be given free access in the works from time to time for stage-wise inspection and progress reporting. Four weeks advance notice shall be given to witness the final routine test as per IS: 2026 and other tests as agreed upon. These tests shall be performed on the complete assembly at manufacturer's works. Test certificates duly signed by owner's representative shall be issued as part of final document. 14.2 Routine Tests: Routine tests as per IS: 2026 shall be carried out. 14.3 Additional Tests: 14.3.1 Oil leakage Test - All tanks and oil filled compartments shall be tested for oil tightness by being completely filled with air/oil of a viscosity not greater than that of insulating oil to IS: 335 at an ambient temperature and subjected to a pressure equal to the normal pressure plus 35 kN/m2 measured at the base of the tank. This pressure shall be maintained for a period of not less than 12 hours for oil and 1 hour for air, during which time no leakage shall occur. 14.3.2 Vacuum Test – The transformer tank shall be subjected to a vaccum of 34.7kN/m2 gauge pressure (250 mm Hg) for one hour. The permanent deflection of flat plates after the vacuum has been released shall not exceed the value specified below without affecting the performance of the transformer. Horizontal length of flat plate (in mm) Upto & including 750mm 750 to 1250 1251 to 1750 1751 to 2000 2001 to 2250 2251 to 2500 2501 to 3000 Above 3000 Permanent deflection (in mm) 5 6.5 8.0 9.5 11.0 12.5 16.0 19.0 14.3.3 Pressure Test - One transformer tank of each size shall be subjected to a pressure corresponding to twice the normal head of oil or to the normal pressure plus 35kN/m2 whichever is lower as measured at the base of the tank and will be maintained for one hour. The permanent deflection of the flat plates after the excess pressure is released shall not exceed the figures specified above. 14.3.4 The transformer shall be subjected to heat run test, if specified in data sheet. 14.3.5 Impulse test, if specified in the data sheet, shall be carried out on all three limbs of the transformer. 14.3.6 Transformer shall be subjected to short circuit test, if specified in the data sheet. 14.3.7 The following tests shall be conducted for the NGR. a. Measurement of resistance value at the prevailing ambient temperature by Kelvin Double Bridge. b. One minute power frequency high voltage withstand test. c. Insulation resistance measurement, both before and after the power frequency high voltage test. d. Heat run test. The temperature rise / heat run test may be conducted by passing the rated current for rated time on a limited resistor section in a proportionately sized test enclosure. The temperature rise shall be measured under simulated conditions ensuring that heat produced (I squared RT) in the test enclosure is more than or equal to the calculated value of energy dissipation required for the test enclosure in proportion to the complete Neutral Grounding Resistor enclosure as per the thermal model adopted. Details of the calculation /modeling considered shall be furnished. 15.0 PACKING AND DESPATCH All the equipment shall be divided into multiple sections for protection and ease of handling during transportation. The equipment shall be properly packed for the selected mode of transportation, i.e. by ship, rail or trailer. The equipment shall be wrapped in polythene sheets before being placed in crates/ cases to prevent damage to finish. The crates / cases shall have skid bottoms for handling. Special notations such as ‘Fragile’, ‘This side up’, ‘Centre of gravity’, ‘Weight’, ‘volume’ ‘Owner’s particulars’, ‘P.O. no.’ etc. shall be clearly and indelibly marked on the packages together with other details as per purchase order. The equipment may be stored outdoors for long periods before installation. The packing shall be completely suitable for outdoor storage in areas with heavy rains and high ambient temperature unless otherwise agreed. In order to prevent movement of equipment/components within the crates, proper packing supports shall be provided.