Std C57.12.10 2010 Standard Requirements for Liquid-Inmersed Power Transformers

IEEE Standard Requirements for Liquid-Immersed Power Transformers IEEE Power & Energy Society Sponsored by the Transformers Committee IEEE 3 Park Avenue New York, NY 10016-5997 USA IEEE Std C57.12.10™-2010 (Revision of ANSI C57.12.10-1997) 6 January 2011 Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10™-2010 (Revision of ANSI C57.12.10-1997) IEEE Standard Requirements for Liquid-Immersed Power Transformers Sponsor Transformers Committee of the IEEE Power & Energy Society Approved 30 September 2010 IEEE-SA Standards Board Approved 8 August 2011 American National Standards Institute Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. Abstract: This standard sets forth the requirements for power transformer application. This standard is intended to be used as a basis for performance, interchangeability, and safety of the equipment covered and to assist in the proper selection of such equipment. This document is a product standard that covers certain electrical, dimensional, and mechanical characteristics of 50 Hz and 60 Hz, liquid-immersed power transformers and autotransformers. Such power transformers may be remotely or integrally associated with either primary switchgear or substations, or both, for step-down or step-up purposes and base rated as follows: 833 kVA and above single-phase, 750 kVA and above three-phase. This standard applies to all liquidimmersed power transformers and autotransformers that do not belong to the following types of apparatus: instrument transformers, step voltage and induction voltage regulators, arc-furnace transformers, rectifier transformers, specialty transformers, grounding transformers, mobile transformers, and mine transformers Keywords: autotransformer, dimensional characteristics, electrical characteristics, load tap changer, mechanical characteristics, power transformer, single-phase, three-phase • The Institute of Electrical and Electronics Engineers, Inc. 3 Park Avenue, New York, NY 10016-5997, USA Copyright © 2011 by the Institute of Electrical and Electronics Engineers, Inc. All rights reserved. Published 6 January 2011. Printed in the United States of America. IEEE is a registered trademark in the U.S. Patent & Trademark Office, owned by the Institute of Electrical and Electronics Engineers, Incorporated. PDF: ISBN 978-0-7381-6444-1 STD97010 Print: ISBN 978-0-7381-6445-8 STDPD97010 IEEE prohibits discrimination, harassment and bullying. For more information, visit http://www.ieee.org/web/aboutus/whatis/policies/p9-26.html. No part of this publication may be reproduced in any form, in an electronic retrieval system or otherwise, without the prior written permission of the publisher. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Standards documents are developed within the IEEE Societies and the Standards Coordinating Committees of the IEEE Standards Association (IEEE-SA) Standards Board. The IEEE develops its standards through a consensus development process, approved by the American National Standards Institute, which brings together volunteers representing varied viewpoints and interests to achieve the final product. 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Comments and recommendations on standards, and requests for interpretations should be addressed to: Secretary, IEEE-SA Standards Board 445 Hoes Lane Piscataway, NJ 08854 USA Authorization to photocopy portions of any individual standard for internal or personal use is granted by The Institute of Electrical and Electronics Engineers, Inc., provided that the appropriate fee is paid to Copyright Clearance Center. To arrange for payment of licensing fee, please contact Copyright Clearance Center, Customer Service, 222 Rosewood Drive, Danvers, MA 01923 USA; +1 978 750 8400. Permission to photocopy portions of any individual standard for educational classroom use can also be obtained through the Copyright Clearance Center. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. Introduction This introduction is not part of IEEE Std C57.12.10-2010, IEEE Standard Requirements for Liquid-Immersed Power Transformers. This standard was prepared by the Revision of C57.12.10 Working Group of the Power Transformers Subcommittee of the Transformers Committee of the IEEE Power and Energy Society. The purpose of this standard is to cover the dimensional, electrical, and mechanical characteristics for liquid-immersed power transformers and autotransformers. This standard is a revision of ANSI C57.12.10-1997, American National Standard for Transformers— 230 kV and Below 833/958 through 8333/10 417 kVA, Single-Phase, and 750/862 through 60 000/80 000/100 000 kVA, Three-Phase Without Load Tap Changing; and 3750/4687 through 60 000/80 000/100 000 kVA with Load Tap Changing—Safety Requirements. The focus of this revision was to expand the scope of the standard and to include the requirements for power transformers and autotransformers with high voltage up to 765 kV and with no limit on the megavoltampere rating. This revised standard includes the following significant changes: ⎯ The title was changed. ⎯ The scope was expanded to include autotransformers, increase the upper voltage limit to 765 kV, and remove the maximum megavoltampere limit. ⎯ Distribution substation transformers, as defined in IEEE Std C57.12.36™ [B1],a were excluded from this standard. ⎯ Most of the clauses were revised, rewritten, or rearranged. ⎯ Significant changes were made in the load tap changer (LTC) section. Additional requirements for transformer paralleling operation were added. ⎯ An informative annex on LTC considerations was added. This standard is a voluntary consensus standard. Its use may become mandatory only when required by a duly constituted legal authority or when specified in a contractual relationship. To meet specialized needs and to allow innovation, specific changes are permissible when mutually determined by the user and the producer, provided that such changes do not violate existing laws and are considered technically adequate for the function intended. When this standard is used on a mandatory basis, the words shall and must indicate mandatory requirements; the words should or may refer to matters that are recommended or permissive, but not mandatory. a The numbers in brackets correspond to the numbers in the bibliography in Annex B. iv Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. Notice to users Laws and regulations Users of these documents should consult all applicable laws and regulations. Compliance with the provisions of this standard does not imply compliance to any applicable regulatory requirements. 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Participants At the time this standard was submitted to the IEEE-SA Standards Board for approval, the Revision of C57.12.10 Working Group had the following membership: Gary Hoffman, Chair Saurabh Ghosh, Vice Chair James Graham, Secretary Don Anderegg Javier Arteaga Donald Ayers Peter Balma Stephen Beckman Thomas Beckwith Enrique Bentacourt Wallace Binder Carlos Bittner Donald Cherry Craig Colopy Frank Damico Ronald Daubert Beth Dumas Eduardo Garcia Charles Garner Everett Hager Jr. James Harlow David Harris Roger Hayes Martin Heathcoate Rowland James Jr. Marion Jaroszewski Erwin Jauch Sheldon Kennedy Stanley Kostyal Michael Lau Gilbert Lemos Thomas Lundquist Dennis Marlow John Mathiews Vinay Mehrotra Van Nhi Nguyen Ray Nicholas Gylfi Olafsson Tony Pink Donald Platts Paulette Powell Thomas Prevost Scott Reed John Rossetti Steven Schapell Stephen Schroeder Devki Sharma Thomas Spitzer Craig Stiegemeier Raman Surbramanian Robert Tillman Jane Ann Verner Richard von Gemmingen Peter Zhao The following members of the individual balloting committee voted on this standard. Balloters may have voted for approval, disapproval, or abstention. William J. Ackerman Michael Adams S. Aggarwal Samuel Aguirre Steven Alexanderson Stephen Antosz I. Antweiler Stan Arnot Donald Ayers Peter Balma Paul Barnhart William Bartley Barry Beaster Thomas Beckwith W. J. Bill Bergman Steven Bezner Wallace Binder Thomas Bishop Thomas Blackburn William Bloethe W. Boettger Paul Boman Harvey Bowles Steven Brockschink Kent Brown Steven Brown Carl Bush Donald Cash Yunxiang Chen Bill Chiu Tommy Cooper Jerry Corkran William Darovny Dieter Dohnal Gary Donner Donald Dunn Fred Elliott Gary Engmann Joseph Foldi George Forrest Bruce Forsyth Marcel Fortin Eduardo Garcia James Gardner Saurabh Ghosh Jalal Gohari Eduardo Gomez-Hennig James Graham William Griesacker Randall Groves Bal Gupta Ajit Gwal J. Harlow David Harris vi Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. Lee Matthews Phillip McClure Susan McNelly Joseph Melanson Gary Michel Daleep Mohla Kimberly Mosley Jerry Murphy Raymond Nicholas Joe Nims T. Olsen Bansi Patel Shawn Patterson J. Patton Brian Penny Howard Penrose Paul Pillitteri Donald Platts Alvaro Portillo Gustav Preininger Iulian Profir Jeffrey Ray Jean-Christophe Riboud Michael Roberts Charles Rogers John Rossetti Robert Hartgrove Roger Hayes William Henning Steven Hensley Gary Heuston Gary Hoffman R. Jackson Erwin Jauch James Jones Stephen Jordan Lars Juhlin C. Kalra Gael Kennedy Sheldon Kennedy Tanuj Khandelwal Ethan Kim J. Koepfinger Neil Kranich Jim Kulchisky Saumen Kundu John Lackey Chung-Yiu Lam Thomas la Rose Thomas Lundquist Richard Marek J. Dennis Marlow John W. Matthews Marnie Roussell Thomas Rozek Dinesh Sankarakurup Bartien Sayogo Lubomir Sevov Devki Sharma Gil Shultz Hyeong Sim James Smith Jerry Smith Steve Snyder Sanjib Som Brian Sparling Allan St. Peter David Tepen S. Thamilarasan T. Traub Joseph Tumidajski Joe Uchiyama John Vergis Jane Verner Loren Wagenaar David Wallach Barry Ward Kenneth White James Wilson Murty V. V. Yalla When the IEEE-SA Standards Board approved this standard on 30 September 2010, it had the following membership: Robert M. Grow, Chair Richard H. Hulett, Vice Chair Steve M. Mills, Past Chair Judith Gorman, Secretary Karen Bartleson Victor Berman Ted Burse Clint Chaplin Andy Drozd Alexander Gelman Jim Hughes Young Kyun Kim Joseph L. Koepfinger* John Kulick David J. Law Hung Ling Oleg Logvinov Ted Olsen Ronald C. Petersen Thomas Prevost Jon Walter Rosdahl Sam Sciacca Mike Seavey Curtis Siller Don Wright *Member Emeritus Also included are the following nonvoting IEEE-SA Standards Board liaisons: Satish Aggarwal, NRC Representative Richard DeBlasio, DOE Representative Michael Janezic, NIST Representative Lisa Perry IEEE Standards Program Manager, Document Development Matthew J. Ceglia IEEE Standards Program Manager, Technical Program Development vii Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. Contents 1. Overview .................................................................................................................................................... 1 1.1 Scope ................................................................................................................................................... 1 1.2 Mandatory requirements ...................................................................................................................... 2 2. Normative references.................................................................................................................................. 2 3. Definitions .................................................................................................................................................. 3 4. Rating data.................................................................................................................................................. 3 4.1 Usual service conditions ...................................................................................................................... 3 4.2 Kilovoltampere ratings ........................................................................................................................ 3 4.3 Voltage ratings..................................................................................................................................... 5 4.4 Insulation levels ................................................................................................................................... 5 4.5 Taps ..................................................................................................................................................... 5 4.6 Impedance voltage ............................................................................................................................... 6 4.7 Top-liquid temperature-range limits .................................................................................................... 7 4.8 Routine tests ........................................................................................................................................ 7 5. Construction ............................................................................................................................................... 7 5.1 Accessories .......................................................................................................................................... 7 5.2 Bushings ............................................................................................................................................ 13 5.3 Lifting, moving, and jacking facilities ............................................................................................... 15 5.4 Nameplate .......................................................................................................................................... 17 5.5 Ground pads....................................................................................................................................... 18 5.6 Polarity, angular displacement, and terminal markings ..................................................................... 18 5.7 Liquid preservation system ................................................................................................................ 19 5.8 Tanks ................................................................................................................................................. 21 5.9 Auxiliary cooling equipment ............................................................................................................. 22 5.10 Power supply for transformer auxiliary equipment and controls ..................................................... 23 5.11 Terminal board ................................................................................................................................ 24 5.12 Junction boxes ................................................................................................................................. 24 5.13 Disconnecting switches with interlocks and terminal chambers...................................................... 24 5.14 Throat connection ............................................................................................................................ 25 5.15 Current transformers ........................................................................................................................ 25 5.16 Surge arresters ................................................................................................................................. 26 5.17 Other insulating liquid ..................................................................................................................... 26 5.18 Loading ............................................................................................................................................ 26 5.19 “Other” tests .................................................................................................................................... 27 6. LTC equipment – basic construction features .......................................................................................... 27 6.1 Load tap changer (LTC) .................................................................................................................... 27 6.2 Tap selector switch ............................................................................................................................ 27 6.3 Motor and drive mechanism .............................................................................................................. 28 6.4 Position indicator ............................................................................................................................... 28 6.5 Control equipment and accessories.................................................................................................... 29 Annex A (informative) LTC considerations ................................................................................................. 36 A.1 Constant and variable flux LTC applications ................................................................................... 36 A.2 Transformer paralleling .................................................................................................................... 38 A.3 Control of the high-voltage voltage or the low-voltage voltage ....................................................... 41 Annex B (informative) Bibliography............................................................................................................ 48 viii Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Standard Requirements for Liquid-Immersed Power Transformers IMPORTANT NOTICE: This standard is not intended to ensure safety, security, health, or environmental protection. Implementers of the standard are responsible for determining appropriate safety, security, environmental, and health practices or regulatory requirements. This IEEE document is made available for use subject to important notices and legal disclaimers. These notices and disclaimers appear in all publications containing this document and may be found under the heading “Important Notice” or “Important Notices and Disclaimers Concerning IEEE Documents.” They can also be obtained on request from IEEE or viewed at http://standards.ieee.org/IPR/disclaimers.html. 1. Overview 1.1 Scope This voluntary consensus standard sets forth the requirements for power transformer application. This standard is intended to be used as a basis for performance, interchangeability, and safety of the equipment covered and to assist in the proper selection of such equipment. This document is a product standard that covers certain electrical, dimensional, and mechanical characteristics of 50 Hz and 60 Hz, liquid-immersed power transformers and autotransformers. Such power transformers may be remotely or integrally associated with either primary switchgear or substations, or both, for step-down or step-up purposes and base rated as follows: 833 kVA and above single-phase, 750 kVA and above three-phase. This standard applies to all liquid-immersed power transformers and autotransformers that do not belong to the following types of apparatus: a) Instrument transformers b) Step voltage and induction voltage regulators c) Arc-furnace transformers d) Rectifier transformers e) Specialty transformers 1 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers f) Grounding transformers g) Mobile transformers h) Mine transformers 1.2 Mandatory requirements When this standard is used on a mandatory basis, the words shall and must indicate mandatory requirements, and the words should and may refer to matters that are recommended and permitted, respectively, but not mandatory. NOTE—The introduction of this standard describes the circumstances under which the document may be used on a mandatory basis.1 2. Normative references The following referenced documents are indispensable for the application of this document (i.e., they must be understood and used; therefore, each referenced document is cited in text and its relationship to this document is explained). For dated references, only the edition cited applies. For undated referenced, the latest edition of the referenced document (including any amendments or corrigenda) applies. ANSI C84.1, American National Standard for Electric Power Systems and Equipment—Voltage Ratings (60 Hertz).2 ASME B1.1, American National Standard for Unified Inch Screw Threads (UN and UNR Thread Form).3 ASME B1.20.1, American National Standard for Pipe Threads, General Purpose, Inch. IEC 60038:2009, IEC standard voltages, ed7.0.4 IEEE Std C37.90.1™, IEEE Standard for Surge Withstand Capability (SWC) Tests for Relays and Relay Systems Associated with Electric Power Apparatus.5, 6 IEEE Std C57.12.00™, IEEE Standard General Requirements for Liquid-Immersed Distribution, Power and Regulating Transformers. IEEE Std C57.12.70™, IEEE Standard Terminal Markings and Connections for Distribution and Power Transformers. IEEE Std C57.12.80™, IEEE Standard Terminology for Power and Distribution Transformers. IEEE Std C57.13™, IEEE Standard Requirements for Instrument Transformers. 1 Notes in text, tables, and figures of a standard are given for information only and do not contain requirements needed to implement the standard. 2 ANSI publications are available from the Sales Department, American National Standards Institute, 25 West 43rd Street, 4th Floor, New York, NY 10036, USA (http://www.ansi.org/). 3 ASME publications are available from the American Society of Mechanical Engineers, 3 Park Avenue, New York, NY 10016-5990, USA (http://www.asme.org/). 4 IEC publications are available from the Sales Department of the International Electrotechnical Commission, Case Postale 131, 3, rue de Varembé, CH-1211, Genève 20, Switzerland/Suisse (http://www.iec.ch/). IEC publications are also available in the United States from the Sales Department, American National Standards Institute, 25 West 43nd Street, 4th Floor, New York, NY 10036, USA. 5 IEEE publications are available from the Institute of Electrical and Electronics Engineers, 445 Hoes Lane, Piscataway, NJ 08854, USA (http://standards.ieee.org/). 6 The IEEE standards or products referred to in this clause are trademarks of the Institute of Electrical and Electronics Engineers, Inc. 2 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers IEEE Std C57.19.00™, IEEE Standard General Requirements and Test Procedure for Power Apparatus Bushings. IEEE Std C57.19.01™, IEEE Standard Performance Characteristics and Dimensions for Outdoor Apparatus Bushings. IEEE Std C57.91™, IEEE Guide for Loading Mineral-Oil-Immersed Transformers. IEEE Std C57.131™, IEEE Standard Requirements for Load Tap Changers. 3. Definitions For the purpose of this document, the following terms and definitions shall apply. For other terms, the standard transformer terminology in IEEE Std C57.12.807 shall apply. Other electrical terms are defined in The IEEE Standards Dictionary: Glossary of Terms & Definitions.8 product standard: An industry product manufacturing or performance specification. 4. Rating data 4.1 Usual service conditions Service conditions shall be in accordance with IEEE Std C57.12.00. 4.2 Kilovoltampere ratings 4.2.1 General Kilovoltampere ratings are continuous and based on not exceeding 65 °C average winding temperature rise by resistance and 80 °C hottest spot temperature rise. The temperature rise of the insulating fluid shall not exceed 65 °C when measured near the top of the tank. These kilovoltampere ratings are based on the usual temperature and altitude service conditions specified in IEEE Std C57.12.00. 4.2.2 Kilovoltampere rating base The kilovoltampere rating of the transformer shall be based on its capacity at ONAN cooling stage. When fans and/or pumps are added to the transformer (forced cooling), its rating shall be increased by the percentage indicated in Table 1. 7 8 Information on references can be found in Clause 2. The IEEE Standards Dictionary: Glossary of Terms & Definitions is available at http://shop.ieee.org/. 3 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers Table 1 — Transformer kilovoltampere rating ONAN < 2500 kVA three-phase ONAN< 833 kVA single-phase ONAN 100% Forced cooling 1st stage 2nd stage 115% N/A 2500 ≤ ONAN ≤ 10 000 kVA three-phase 833 ≤ ONAN ≤ 3333 kVA single-phase ONAN 100% Forced cooling 1st stage 2nd stage 125% N/A ONAN > 10 000 kVA three-phase ONAN > 3333 kVA single-phase ONAN 100% Forced cooling 1st stage 2nd stage 133% 167% For a transformer without a self-cooled rating, the applicable multiplying factor given in Table 20 of IEEE Std C57.12.00-2006 shall be applied to the maximum nameplate kilovoltampere rating to obtain the equivalent base kilovoltampere rating. Typical transformers ratings are given in Table 2. Actual ratings shall be mutually agreed between the user and manufacturer. In transformers with concentric winding arrangement, two or more separate windings may be situated one above the other. In this case, the average winding temperature rise limit shall apply to the average of the individual readings for the stacked windings if they are of equal size and kilovoltampere rating and similar design. If they are not, the evaluation should be subject to agreement between the user and the manufacturer. For all rated loading conditions that are evaluated, a hot spot temperature rise limit of 80 °C shall apply to all windings. Table 2 — Typical transformer kilovoltampere rating Single-phase transformers ONAN Forced cooling 1st stage Three-phase transformers ONAN Forced cooling 1st stage 2nd stage 833 1041 750 862 — 1250 1562 1000 1150 — 1667 2500 2084 3125 1500 2000 1725 2300 — — 3333 5000 4167 6250 2500 3750 3125 4688 — — 6667 8333 — 8333 10 417 — 5000 7500 10 000 6250 9375 12 500 — — — — — 12 000 16 000 20 000 — — — — — — 15 000 20 000 25 000 20 000 26 667 33 333 25 000 33 333 41 667 — — 30 000 40 000 50 000 — — — — 37 500 50 000 50 000 66 667 62 500 83 333 — — 60 000 80 000 100 000 An autotransformer with a tertiary winding for external loading has no standard basis for megavoltampere rating. All simultaneous loading conditions including megavoltampere rating and power factor shall be specified by the user. 4 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers An equivalent two-winding kilovoltampere rating of an autotransformer is the rated power of the autoconnected winding multiplied by the auto-factor. Auto-factor is also known as “reduction factor” or “co-ratio.” Co-ratio = (N – 1)/N = (HV – LV)/HV As an example, a 138/69 kV, 100 MVA autotransformer has a co-ratio of (138 – 69)/138 = 0.5 and an equivalent two-winding rating equal to 100 × 0.5 = 50 MVA. If the transformer in addition is provided with a nonautoconnected tertiary winding of 35 MVA rated power, then its equivalent two-winding rating will be (50 + 50 + 35)/2 = 67.5 MVA. 4.3 Voltage ratings Voltage ratings for power transformers shall conform to the nominal and maximum system voltages defined in Table 4 and Table 5 of IEEE Std C57.12.00-2006. 4.4 Insulation levels Basic impulse insulation levels (BILs) for transformers shall conform to the BIL levels in Table 4 of IEEE Std C57.12.00-2006. 4.5 Taps 4.5.1 High-voltage winding taps for de-energized operation If specified, the de-energized tap changer (DETC), the following four high-voltage rated kilovoltampere taps shall be provided: 2.5% and 5.0% above rated voltage, and 2.5% and 5% below rated voltage. Voltages and currents should be listed in accordance with 5.4. When a load tap changer (LTC) is furnished per 4.5.2, the high-voltage DETC may not be required. 4.5.2 Taps for LTC transformers When an LTC transformer is specified, LTC equipment shall be furnished in the low-voltage winding to provide approximately ± 10% automatic regulation of the low-voltage winding voltage in approximately 0.625% steps, with 16 steps above and 16 steps below rated low voltage. The transformer shall be capable of delivering rated kilovoltamperes at the rated low-voltage position and on all positions above rated low voltage. The transformer shall be capable of delivering low-voltage current corresponding to rated low voltage at all positions below rated low voltage. When agreed on by the user, the LTC may be located in an alternate winding to regulate the high- or lowvoltage winding. This application may make the transformer operate with variable flux voltage operation when the tap positions are changed. Annex A indicates the effect in the transformer operation during this condition and other variations. 5 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers When required by the user, the transformer may be designed to deliver rated kilovoltampere output on all tap positions. 4.6 Impedance voltage 4.6.1 Percent impedance voltage The percent impedance voltage at the self-cooled rating as measured on the rated voltage connection shall be as listed in Table 3 if the user does not specify another value. For cases not covered in Table 3, the percent impedance voltage value shall be agreed between user and manufacturer, and the user should perform a system study to determine the proper value of impedance. For autotransformers, the percent impedance voltage shall be as specified by the user, or it should be the lower of the value from Table 3 and the value obtained according to the following equation: Autotransformer impedance voltage = (Value from Table 3) × (Autotransformer co-ratio) × 1.5 where Autotransformer co-ratio = (High-Voltage – Low-Voltage)/(High-Voltage) This impedance voltage is the autotransformer impedance and not the equivalent autotransformer impedance. Table 3 — Percent impedance at self-cooled (ONAN) rating High-voltage BIL (kV) Without LTC With LTC ≤ 110 5.5 — 150 6.5 7.0 200 7.0 7.5 250 7.5 8.0 350 8.0 8.5 450 8.5 9.0 550 9.0 9.5 650 9.5 10.0 750 10.0 10.5 4.6.2 Tolerance on impedance voltage The tolerance shall be as specified in IEEE Std C57.12.00. 6 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers 4.6.3 Percent departure of impedance voltage on taps for de-energized operation The percent variation of tested impedance voltage on any tap from the tested impedance voltage at rated voltage shall not be greater than the value of the total tap voltage range when expressed as a percentage of the rated voltage. NOTE— This requirement does not apply to LTC taps. 4.7 Top-liquid temperature-range limits The transformer shall be suitable for operation over a range of top-liquid temperatures from –20 °C to 105 °C, provided the liquid level was established by following the manufacturer’s filling procedure. NOTE—Operation at these temperatures may cause the mechanical pressure-vacuum bleeder device (5.1.6), if provided, to function to relieve excessive positive or negative pressures. 4.8 Routine tests 4.8.1 General Routine tests shall be made in accordance with IEEE Std C57.12.00. 4.8.2 LTC transformers Additional routine tests for LTC transformers listed in IEEE Std C57.12.00 shall be made. 5. Construction 5.1 Accessories Accessories as required and identified in Table 4 shall be located as shown in Figure 1. 7 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers CL Accessories HIGH-VOLTAGE COVER BUSHINGS CL SEGMENT 4 SEGMENT 2 SEGMENT 3 SEGMENT 1 LOW-VOLTAGE COVER BUSHINGS Clause ref. Table 4 Locations DETC operating handle S1, S4, see Clause ref. Liquid level indicator S1 5.1.2 Liquid temperature indicator S1 5.1.3 Winding temperature indicator S1 5.1.4 Pressure-vacuum gauge S1 or S4 5.1.5 Pressure-vacuum bleeder valve S1 5.1.6 Pressure relief device Cover 5.1.7 Drain and filter valves S1 5.1.8 Jacking facilities See ref. 5.3.4 Nameplate S1 5.4 Ground pad(s) See ref. 5.5 †Auxiliary cooling control S1 or S2 5.9 †LTC equipment S1 or S2 6 †When furnished. NOTE—Some designs include accessories and wiring connections as part of the LTC equipment assembly. In such cases, accessories may be located in the same segment as the LTC and may be viewed parallel to the segment centerline. Figure 1 — Accessories See Table 4 for information on accessories and construction features to be provided on transformers. Table 4 — “Basic standard” construction features Clause Items Without LTC With LTC DETC A A 5.1.2 Liquid Level Indicator S S 5.1.3 Liquid Temperature Indicator S S 5.1.4 Winding Temperature Indictor S S 5.1.5 Pressure-Vacuum Gauge A A 5.1.6 Pressure-Vacuum Bleeder Valve A A 5.1.7 Pressure Relief Device S S 5.1.8 Drain and Filter Valves S S 5.1.9 Sudden Pressure Relay A A 5.1.10 Alarm Contacts S S 5.1.11 Contact Wiring and Wire Color Coding S S 5.2 Bushings S S 5.2.1 Neutral Terminations S S 5.2.1.1 Y-Connected High-Voltage Windings A A 5.2.1.2 Y-Connected Low-Voltage Windings A A 5.1 Accessories Table 4 8 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers Table 4 — “Basic standard” construction features (continued) Clause Items Without LTC With LTC 5.2.1.3 Constructions for Neutral Terminations A A 5.3 Lifting, Moving, and Jacking Facilities S S 5.3.3.2 Other Moving Facilities (Wheels) A A 5.3.4 Jacking Facilities S S 5.4 Nameplate S S 5.5 Ground Pads S S 5.6 Polarity, Angular Displacement, and Terminal Markings S S 5.7 Liquid Preservation System S S 5.8 5.8.3.2 Tanks Bolted Cover S A S A 5.9 5.9.1 Auxiliary Cooling Equipment Controls for Auxiliary Cooling Equipment A A A A 5.9.2 5.9.2.2 Fans Future Forced-Air Cooling A A A A 5.9.3 5.10 Pumps Auxiliary Equipment Power Supply A A A A 5.11 5.12 Terminal Board Junction Box A A A A 5.12.1 5.12.2 High Voltage Low Voltage A A A A 5.13 Disconnecting Switches A A 5.13.1 High-Voltage Terminal Chamber A A 5.13.2 5.14 Low-Voltage Terminal Chamber Throat Connection A A A A 5.14.1 5.14.2 High-Voltage Throat Low-Voltage Throat A A A A A A 5.15 Current Transformers 5.15.1 Bushing Type Current Transformer 0 Terminal Blocks A A 5.16 Surge Arresters A A 5.17 Other Insulating Liquid A A 6 6.1 LTC Equipment LTC − − S 6.2 6.3 6.4 Tap Selection Switch Motor and Drive Mechanism Position Indicator − − − S S S 6.5 A.2 Control Equipment and Accessories Transformer Paralleling − − S A NOTE: “S” indicates “standard”, “A” indicates “available when specified.” 9 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers 5.1.1 De-energized tap changer (DETC) When a DETC is provided, its operating handle shall be brought out through the side of the tank in Segment 1 or 4 at a height convenient for operators to safely change the taps. If the user requires operation from ground level, then the height should not exceed 2 m (79 in). If for design reasons it cannot be located in Segment 1 or 4, it may be located in the sidewall of one of the other segments. The tap changer handle shall have provision for padlocking and shall provide visible indication of the tap position without unlocking. A hole with a minimum diameter of 9.5 mm (0.375 in) shall be provided for the padlock. The plate indicating tap changer position shall be marked with letters or Arabic numerals in sequence. The letter “A” or the Arabic numeral “1” shall be assigned to the voltage rating providing the maximum ratio of transformation. 5.1.2 Liquid level indicator A magnetic level gauge with vertical face shall be mounted on the side of the tank in Segment 1 and shall be readable to a person standing at the level of the base. The gauge shall have a dark-face dial with light markings and a light-colored indicating hand. The diameter of the dial (inside bezel) shall be as follows: a) 82.6 mm (3.25 in) ± 6.4 mm (0.25 in) minimum when the 25 °C liquid level is 2.44 m (96 in) or less above the bottom of the base b) 140 mm (50.5 in) ± 12.7 mm (0.5 in) minimum when the 25 °C liquid level is more than 2.44 m (96 in) above the bottom of the base Dial markings shall show the 25 °C level and the maximum and minimum levels with the letters HI-LO or MAX-MIN. The words “Liquid Level” shall be on the dial or on a suitable nameplate adjacent to the gauge. The 25 °C liquid level shall also be shown by suitable permanent markings on the tank or by an indication on the nameplate of the distance from the liquid level to the highest point of the handhole or manhole flange surface. The change in liquid level per 10 °C change in temperature shall be indicated on the nameplate. Nonadjustable alarm contacts shall be provided and shall be set to close at the minimum safe operating level of the liquid. The contacts shall be in accordance to 5.1.10 and 5.1.11. 5.1.3 Liquid temperature indicator A thermometer that measures top-liquid temperature shall be mounted on the side of the tank and shall be readable to a person standing at the level of the base. Gauges, when required to have operating controls on their cases, shall be mounted between 1.22 m (4 ft) and 1.83 m (6 ft) above the base. The minimum scale range shall be 0 to 120 °C. The thermal sensing element shall be mounted in a closed well at a suitable level to indicate the top-liquid temperature. The well shall be positioned so that it is covered by at least 2.5 cm (1 in) of fluid at the lowest permissible fluid level. For dimensions of the well, see IEEE Std C57.12.00. 10 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers The gauge’s dial shall have an analog (dial and pointer) or alphanumeric display that is readable in low and high ambient light conditions. The measurement title “Liquid Temperature” shall be marked or displayed on the dial or on a suitable nameplate mounted adjacent to the indicator. Indicators with analog displays shall have highly contrasting light markings on a dark dial or dark markings on a white dial. The minimum dial diameter (inside the bezel) shall be 114 mm (4.5 in). Two indicating pointers, one for present temperature and one for peak (maximum recorded historical) temperature, shall be provided. The present temperature pointer may be light or dark, in contrast to the dial. The peak temperature pointer shall be orange-red and shall have a provision for resetting without opening any covers or windows. For digital indicators, the measurement title may be shown on an alphanumeric display separately from the measured value or marked on the dial or a plate mounted adjacent to the gauge. A method of displaying the peak temperature, using externally operated controls, shall be provided. Display colors may be black, red, green, or amber. The contacts on the gauge when required shall be in accordance with 5.1.10 and 5.1.11 and have independent field-adjustable set-point values. Each of the three relay contacts shall provide the ability to turn on a cooling stage, alarm, or actuate another relay or contactor. See 5.9.1.1 and 5.9.1.2 for switches and relays or contactors that allow for redundant manual control of cooling equipment. The alarm contacts shall be adjustable over a minimum range of 40 °C to 120 °C. 5.1.4 Winding temperature indicator A thermometer that indicates winding temperature shall be mounted on the side of the tank and shall be readable to a person standing at the level of the base. Gauges, when required to have operating controls on their cases, shall be mounted between 1.22 m (4 ft) and 1.83 m (6 ft) above the base. The minimum scale range shall be 0 to 180 °C. The winding temperature indicator shall use direct-measurement, simulated or calculated methods to determine winding hottest spot temperature. Depending on the type of technology, the gauge may require that the transformer be equipped with a heated thermowell, a load current signal from a bushing current transformer, or ports through the tank wall for sensor passage. When a top fluid temperature input is required, the thermal sensing element shall be mounted in a closed well at a suitable level to indicate the top-liquid temperature. When a heated thermowell is required, the tank wall shall be ported to accept the specified heater. The well shall be positioned so that it is covered by at least 2.5 cm (1 in) of fluid at the lowest permissible fluid level. For dimensions of the well, see IEEE Std C57.12.00. The thermometer’s dial shall have an analog (dial and pointer) or alphanumeric display that is readable in low and high ambient light conditions. The measurement title “Winding Temperature” shall be marked or displayed on the dial or on a suitable nameplate mounted adjacent to the indicator. Indicators with analog displays shall have highly contrasting light markings on a dark dial or dark markings on a white dial. The dial diameter (inside the bezel) shall be minimum 114 mm (4.5 in). Two indicating pointers, one for present temperature and one for maximum historical (peak) temperature, shall be provided. The present temperature pointer may be light or dark, in contrast to the dial. The peak temperature pointer shall be orange-red and shall have a provision for resetting without opening any covers or windows. For digital indicators, the measurement title may be shown on an alphanumeric display separately from the measured value or marked on the dial or a nameplate mounted adjacent to the gauge. A method of 11 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers displaying the historical highest recorded temperature, through externally operated controls, shall be provided. Display colors may be black, red, green, or amber. The gauge shall have a minimum of three sets of contacts in accordance with 5.1.10 and 5.1.11 and have independent field-adjustable set-point values. Each of the three relay contacts shall provide the ability to turn on a cooling stage, alarm, or actuate another relay or contactor. The alarm contacts shall be adjustable over a minimum range of 40 °C to 140 °C. 5.1.5 Pressure-vacuum gauge A pressure-vacuum gauge shall be provided for power transformers without a conservator. The diameter of the dial (inside bezel) shall be 89 mm (3.5 in) ± 6.4 mm (0.25 in). The gauge shall have a dark-face dial with light-colored markings and a light-colored pointer, and it shall be located either in Segment 1 or in the half of Segment 4 that is adjacent to Segment 1. The scale range for the pressure-vacuum gauge shall be between 69 kPa (10 lb/in²), positive and negative. 5.1.6 Pressure-vacuum bleeder valve A pressure-vacuum bleeder device set to operate at the maximum operating pressures (positive and negative) indicted on the nameplate shall be furnished for power transformers without a conservator. 5.1.7 Pressure relief device A pressure relief device shall be provided on the cover of the transformer, with a minimum pressure relief rating of 142 m3/min (5000 CFM) at 69 kPa (10 lb/in²). This relief rating (rate of release) applies for all pressure relief devices regardless of pressure setting. The pressure relief device shall be supplied with an alarm contact in accordance with 5.1.10 and 5.1.11. 5.1.8 Drain and filter valves A combination drain and lower filter valve shall be located on the side of the tank in Segment 1. This valve shall provide for drainage of the liquid to within 25 mm (1 in) of the bottom of the tank. The drain valve shall have a built-in 0.375 in sampling device, which shall be located in the side of the valve between the main valve seat and the pipe plug. The sampling device shall be supplied with a 5/16-in×32-threads-per-inch (5/16-32 in) male thread for the user’s connection and shall be equipped with a cap. The size of the drain valve shall be 2 in National Pipe Thread (NPT) and shall have tapered pipe threads (in accordance with ASME B1.20.1) with a pipe plug in the open end. The upper filter valve shall be provided and located below the 25 °C liquid level in Segment 1. The size of the upper filter valve shall be 2 in NPT, and the upper filter valve shall have 51 mm (2 in) NPT (in accordance with ASME B1.20.1) with a pipe plug in the open end. 12 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers 5.1.9 Sudden pressure relay When specified, a sudden pressure relay shall be provided for the indication of transformer faults and to minimize damage to equipment. The relay shall not actuate under normal transformer operating pressures. The sudden pressure relay may be either a gas space mounted relay or a under fluid relay. Under fluid relays shall actuate under rapidly changing pressures of 10 kPa/s to 38 kPa/s (1.5 lb/in²/s to 5.5 lb/in²/s). The gas space mounted relays shall actuate with a pressure change of 3.5 kPa/s to 21 kPa/s (0.5 lb/in²/s to 3.0 lb/in²/s). The relay shall actuate within 3 cycles of the rated power frequency. The sudden pressure relay shall be able to withstand full vacuum or positive pressure of 103 kPa (15 lb/in²) without damage. The relay shall as a minimum be supplied with an alarm contact and a trip contact in accordance with 5.1.10 and 5.1.11. 5.1.10 Alarm contacts Nongrounded alarm contacts shall be suitable for interrupting the following: ⎯ 0.02 A dc inductive load ⎯ 0.20 A dc noninductive load ⎯ 2.5 A ac noninductive or inductive load ⎯ 250 V maximum in all classes 5.1.11 Contact wiring and wire color coding Contacts shall be wired with cable having the color coding shown in Figure 2 or with cable having permanent labeling. 5.2 Bushings The insulation level of line bushings shall be equal to or greater than the insulation level of the windings to which they are connected. The insulation level of the low-voltage neutral bushing having a grounded Y-connected low-voltage winding shall be the same as that of the low-voltage line bushings for windings 25 kV and below. For windings above 25 kV, a 25 kV neutral bushing with 150 kV BIL shall be provided. Unless otherwise specified, bushings shall be mounted on the cover and located as shown in Figure 3. Electrical characteristics and dimension of outdoor transformer bushings shall be as listed in IEEE Std C57.19.00 and IEEE Std C57.19.01 where applicable. 13 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers Figure 2 — Contact wiring and wire color coding H 0 OR X0 WHEN REQUIRED H1 H2 H3 X1 X2 X3 NOTE—For single-phase transformers, omit H3, X3, and neutral bushings. Figure 3 — Bushing arrangement 14 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers 5.2.1 Neutral terminations Four cover bushings shall be provided for each permanently connected Y-winding on three-phase transformers. When specified, other neutral terminations shall be provided as listed in 5.2.1.1, 5.2.1.2, and 5.2.1.3. 5.2.1.1 Neutral termination of Y-connected high-voltage windings When specified, designated neutral terminations of Y-connected high-voltage windings shall be one of the following: a) The neutral shall be ungrounded and not accessible. b) The neutral shall be brought through the cover in Segment 2. c) Provision for a future high-voltage neutral bushing shall be made on the cover in Segment 2. A fully insulated neutral shall be brought to a terminal board for isolated neutral operation of the transformer. d) High-voltage windings of transformers with a Y-Δ terminal board supplied in accordance with 5.11 item a) shall be available in one of the following constructions: 1) Neutral ungrounded and not accessible 2) Neutral brought through the cover in Segment 2 5.2.1.2 Neutral termination of Y-connected low-voltage windings When specified, one of the following neutral terminations of Y-connected low-voltage windings shall be provided: a) Permanently Y-connected low-voltage windings shall have the low-voltage neutral bushing furnished as provided for in 5.2. b) Low-voltage windings of transformers with a Y-Δ terminal board supplied in accordance with 5.11 item b) shall be provided in one of the following constructions: 1) Without neutral bushing 2) With a neutral bushing of the same voltage class as that of the winding to which it is connected 5.2.1.3 Constructions for neutral terminations Neutral terminations, when furnished, shall be provided on the cover or in the junction box, terminal chamber, or throat as necessary. 5.3 Lifting, moving, and jacking facilities 5.3.1 Safety factor Lifting, moving, and jacking facilities shall be designed to provide a safety factor of 5. This safety factor is the ratio of the ultimate stress of the material used to the working stress. The working stress is the 15 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers maximum combined stress developed in the lifting facilities by the static load of the component being lifted. This factor does not apply to pulling facilities since the unit is not suspended. For pulling, a safety factor of 2 is acceptable. 5.3.2 Lifting facilities Lifting facilities shall be provided for lifting the cover separately and also for lifting the core and coil assembly from the tank using four lifting cables. Facilities for lifting the complete transformer (with the cover securely fastened in place) shall be provided. Lifting facilities shall be designed for lifting with four vertical slings. (For large transformers, the use of spreaders or a lifting beam may be involved.) The bearing surfaces of the lifting facilities shall be free from sharp edges and shall be provided with a hole having a minimum diameter of 21 mm (0.8125 in) for guying purposes. 5.3.3 Moving facilities 5.3.3.1 General The base of the transformer shall be of heavy plate or have members forming a rectangle that shall permit rolling or skidding in the directions of the centerlines of the segments. The points of support shall be located so that the center of gravity of the transformer as prepared for shipment does not fall outside these points of support when the base is tilted 15° or less from the horizontal, with or without oil in the transformer. Provision shall be made on or adjacent to the base for pulling the transformer parallel to the centerline of Segments 1 and 3 and to the centerline of Segments 2 and 4. The base shall be constructed so that the external edges on all sides are rounded or slope upward at an angle of approximately 45°. A flat bottom base with material thickness of less than 12.7 mm (0.5 in) does not require rounded or upward sloping edges. 5.3.3.2 Other moving facilities When specified, flanged wheels for a 1.435 m (56.5 in) rail gauge for motion parallel to the centerline of Segments 1 and 3 shall be available. 5.3.4 Jacking facilities Jacking facilities shall be located near the extreme ends of the junctions of the segments. Dimensions and clearances for jacking provisions shall be as shown in Figure 4. 16 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers CL OF JACK CLEARANC TANK SIDEWALL F E JACKING POINT F TUBES, ETC TANK BASEPLATE A FLOOR LINE B Weight 15,900 kg (35,000 lb) or less Dim. (mm) (in) A 88.9 3.5 B 63.5 2.5 E 686.0 27.0 F 127.0 5.0 G 76.2 3.0 H 127.0 5.0 G H G H Weight 15,900–29,500 kg (35,000–65,000 lb) Dim. (mm) (in) A 127.0 5.0 B 63.5 2.5 E 686.0 27.0 F 127.0 5.0 G 76.2 3.0 H 127.0 5.0 Weight over 29,500 kg (65,000 lb) Dim. (mm) (in) A 457.0 18 B 102.0 4 E 508.0 20 F 127.0 5 G 76.2 3 H 127.0 5 NOTE 1—Dimensions E, F, G, and H are minimum free clearances. NOTE 2—Where required in manufacturer’s standard designs, any dimensions may be in excess of those shown. NOTE 3—Dimension E applies to nonremovable coolers only. NOTE 4—Weight includes completely assembled transformer and fluid. NOTE 5—Dimension A clarifies minimum jacking clearance. Figure 4 — Provision for jacking 5.4 Nameplate The nameplate shall conform to the requirements of nameplate C as described in IEEE Std C57.12.00. It shall be located in Segment 1 near the centerline and near eye level. It may be located in Segment 2 when LTC equipment is located in Segment 2. For LTC transformers, the phrases “LTC transformer” or “LTC autotransformer” shall be used instead of the word “transformer.” Voltage and current ratings shall be given as follows: 0 100 1 000 10 000 100 000 to to to to and 99.9 999 9 999 99 999 greater to nearest 0.1 to nearest 1 to nearest 5 to nearest 10 to nearest 25 17 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers 5.5 Ground pads A tank-grounding pad shall consist of a copper-faced steel pad or a stainless-steel pad without copper facing, 50.8 mm × 88.9 mm (2 in × 3.5 in) with two holes horizontally spaced on 44.5 mm ± 0.8 mm (1.75 in ± 0.03 in) centers and drilled and tapped for 0.5 in - 13 Unified National Coarse (UNC) thread (as defined in ASME B1.1). Minimum thickness of the copper facing shall be 0.4 mm (0.015 in). Minimum threaded depth of the holes shall be 13 mm (0.5 in). Thread protection for the ground pad shall be provided. The ground pad shall be welded on the base or on the tank wall near the base. If the base is detachable, the ground pad shall be located on the tank wall. Ground pads shall be located toward the extreme left of Segment 1 and diagonally opposite in Segment 3 and located so that they do not interfere with the jacking facilities. 5.6 Polarity, angular displacement, and terminal markings 5.6.1 Polarity All single-phase transformers shall have subtractive polarity. 5.6.2 Angular displacement The angular displacement between high-voltage and low-voltage terminal voltages of three-phase transformers with Δ-Δ connections shall be 0°. The angular displacement between high-voltage and lowvoltage terminal voltages of three-phase transformers with Y-Δ or Δ-Y connections shall be 30°, with the low voltage lagging the high voltage as shown in Figure 5. Phasor relations shall be as shown in Figure 5. 5.6.3 Terminal markings External terminals shall be marked in accordance with IEEE Std C57.12.70. The high-voltage and lowvoltage bushing arrangements shall be as shown in Figure 3. 18 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers H2 X2 H1 H3 X3 X1 H2 X2 X1 H1 X0 H3 X3 H2 X2 H0 H1 X1 H3 X3 Figure 5 — Angular displacement 5.7 Liquid preservation system One of the preservation systems in 5.7.1 through 5.7.4 shall be provided. In these systems, the interior of the transformer shall be sealed from the atmosphere at a top-liquid temperature of 105 °C. 5.7.1 Sealed-tank system A sealed-tank system is one in which the gas plus liquid volume remains constant. It shall be designed so that the internal gas pressure does not exceed 69 kPa (10 lb/in²) gauge positive or 55 kPa (8 lb/in²) gauge negative. 19 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers The system shall include a pressure-vacuum gage indicated in 5.1.5 and a pressure-vacuum bleeder valve indicated in 5.1.6. The use of this system may result in the introduction of oxygen and moisture into the transformer due to the operation of the pressure-vacuum bleeder valve. 5.7.2 Inert-gas pressure system An inert-gas pressure system is a system in which a positive pressure of inert gas is maintained from a separate inert-gas source and reducing valve system so that the interior of the transformer shall be sealed from the atmosphere. The internal gas pressure shall not exceed 55 kPa (8 lb/in²) gauge. 5.7.3 Conservator-tank system without diaphragm A conservator-tank system without diaphragm is a system that, by means of an auxiliary tank partly filled with liquid and connected to the completely filled main tank, seals the oil in the main tank from the atmosphere. The internal top oil pressure in the main tank shall not exceed 34 kPa (5 lb/in²) gauge. The system shall include the devices described in 5.7.3.1 through 5.7.3.5. 5.7.3.1 Shut-off valve A combination of valves shall be provided in the conservator tank and the main tank to close the flow of liquid between both tanks. The size of the valves shall be at the manufacturer’s option. 5.7.3.2 Drain valve A drain valve shall be located on the conservator tank side as near the bottom as possible. The size of the drain valve shall be 2 in and shall have tapered pipe threads (NPT, in accordance with ASME B1.20.1), with a pipe plug in the open end. 5.7.3.3 Liquid level indicator The liquid level indicator indicated on 5.1.2 shall be installed in the conservator tank. 5.7.3.4 Dehydrating breather A dehydrating breather to prevent the normal moisture in the air from coming in contact with the liquid in the conservator tank shall be provided. The dehydrating breather shall be filled with silica gel that absorbs 20% of its own weight in moisture and is provided with an oil trap to prevent the continuous contact between the moist air and the silica gel. 5.7.3.5 Gas accumulation relay The gas accumulation relay shall be located in the liquid connection between the main tank and the conservator tank in order to monitor the gas and liquid movements. During normal operation, the relay is completely filled with liquid to keep its internal float in their top limit or rest position. The accumulation of gases in the float gas chamber causes the float to actuate an electrical contact system. The inside mechanism shall comprise upper and lower contact systems for alarms and tripping positions. 20 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers 5.7.4 Conservator-tank system with diaphragm The conservator tank shall have the same characteristics of operation and accessories as in 5.7.3, but with the additional following accessories: 5.7.4.1 Diaphragm The interior of the conservator tank shall have a rubber air cell to isolate the transformer liquid and the air and to prevent contamination, oxygen, and/or moisture from coming in contact with the transformer liquid. The size of the rubber air cell shall be selected to assure the internal operating pressures indicated in 5.7.3 are not exceeded and to compensate the liquid volume displacement due to the temperature variations specified. 5.7.4.2 Vent valve A vent valve shall be provided at the top of the conservator tank in order to release any air trapped in the liquid side. The size of the valve shall be determined by the manufacturer. 5.7.4.3 Vacuum equalizing valve When the conservator tank is designed for full vacuum filling, a valve between the liquid side and the air side of the conservator tank shall be provided so liquid filling of the conservator tank can occur under vacuum while the pressures between both sides are equalized. The size of the valve shall be determined by the manufacturer. 5.8 Tanks 5.8.1 Pressure design Maximum operating pressures (positive and negative) for which the transformer is designed shall be indicated on the nameplate. The completely assembled transformer shall be designed to withstand, without permanent deformation, a pressure 25% greater than the maximum operating pressure. 5.8.2 Vacuum filling Tanks shall be designed for vacuum filling (external pressure of one atmosphere, essentially full vacuum) in the field. 5.8.3 Cover 5.8.3.1 General A welded main cover shall be provided. Handholes or manholes shall be provided in the cover. Handholes, if circular, shall be a minimum of 229 mm (9 in) in diameter. If rectangular, they shall be at least 114 mm (4.5 in) wide and shall have an area of at least 419 cm² (65 in²). Manholes, if circular, shall be a minimum 21 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers of 381 mm (15 in) in diameter. If rectangular or oval, they shall have minimum dimensions of 254 mm × 406 mm (10 in × 16 in). 5.8.3.2 Bolted cover When specified by the user, a bolted main cover shall be provided. 5.8.4 LTC compartment In LTC transformers, if the arcing tap switch has components that involve direct arcing in liquid, these components shall be located in a compartment sealed so it prevents transfer of liquid to any other compartment or to the main tank. 5.9 Auxiliary cooling equipment 5.9.1 Control of auxiliary cooling equipment When auxiliary cooling equipment is provided or future provisions are provided, a suitably sized relay shall be provided for control from the winding temperature indicator if supplied, the liquid temperature indicator if the winding temperature is not supplied, or both if specified. The relay shall be mounted inside the cabinet. 5.9.1.1 Control by the liquid temperature indicator The equipment for automatic control of auxiliary cooling equipment controlled from the liquid temperature indicator shall consist of the following: a) A liquid temperature indicator defined in 5.1.3. b) A manually operable switch connected in parallel with the automatic control contacts and enclosed in a weatherproof cabinet located on the side of the tank of Segment 1 at a height not greater than 1.52 m (60 in) above the base. 5.9.1.2 Control by the winding temperature indicator When specified, or for transformers with forced-cooled ratings of 133% or greater of the self-cooled ONAN rating, the equipment for automatic control of auxiliary cooling equipment for transformers controlled from the winding temperature indicator shall consist of the following: a) A winding temperature indicator defined in 5.1.4, with alarm contacts as follows: Contact b) Function 1 Supply power to first-bank cooling 2 Supply power to second-bank cooling 3 Initiate alarm or actuate relay A manually operable switch connected in parallel with the automatic control contacts and enclosed in a weather-resistant cabinet located on the side of the tank in Segment 1 at a height not greater than 1.52 m (60 in) above the base. 22 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers 5.9.2 Fans 5.9.2.1 General When specified, fan motors shall be 240 V, 60 Hz or 400 V, 50 Hz, single phase, without centrifugal switch, and shall be individually fused or otherwise thermally protected. If the power supply for 240 V, 60 Hz or 400 V, 50 Hz single-phase motors is not available, provision shall be made to accommodate another single-phase motor supply voltage in accordance with ANSI C84.1 for 60 Hz operation or IEC 60038 for 50 Hz operation, not in excess of 600 V. 5.9.2.2 Provisions for future forced-air cooling When cooling class ONAN transformers are to have provision for future forced-air cooling and the control of the forced-air equipment is to be by the liquid temperature indicator, the following equipment shall be provided: ⎯ The necessary mechanical arrangement ⎯ A thermally operated liquid temperature indicator per 5.1.3 ⎯ Provision for mounting the control cabinet ⎯ Provision for mounting the fans When cooling class ONAN transformers are to have provision for future forced-air cooling and the control of the forced-air equipment is to be by the winding temperature indicator, the following equipment shall be provided: ⎯ The necessary mechanical arrangement ⎯ A thermally operated winding temperature indicator per 5.1.4 ⎯ Provision for mounting the control cabinet ⎯ Provision for mounting the fans 5.9.3 Pumps When specified, pump motors shall be 240 V, 60 Hz or 400 V, 50 Hz, single-phase, without centrifugal switch, and shall be individually fused or otherwise thermally protected. Pump facilities shall include valves to allow removal of the pump with minimum loss of insulating oil. 5.10 Power supply for transformer auxiliary equipment and controls The power supply voltage for the transformer auxiliary equipment and controls should be specified and provided by the user. It should be in accordance with ANSI C84.1. The voltage rating for auxiliary equipment and controls supplied with the transformer should also be in accordance with ANSI C84.1. 23 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers 5.11 Terminal board When specified, only one of the following types of terminal boards may be selected for a transformer: a) A terminal board that provides for a series-multiple connection for transformers listed in the appropriate rating table. b) A Y-Δ terminal board that provides angular displacements as shown in Figure 5 for transformers with three-phase windings of 110 kV BIL (15 kV nominal system voltage) or less. The other winding of the transformer shall be permanently Δ-connected. 5.12 Junction boxes When specified, junction boxes shall be provided for the cable entrance for windings of 110 kV BIL (15 kV nominal system voltage) or less. (See 5.2.1.3 when neutral termination is required.) NOTE—Certain kilovoltampere and voltage ratings may impose design limitations on the availability or location of these items. 5.12.1 High-voltage junction box The high-voltage junction box shall be mounted either a) On the side of the tank in Segment 2, or b) On the cover in Segment 3. 5.12.2 Low-voltage junction box The low-voltage junction box shall be mounted either a) On the side of the tank in Segment 4, or b) On the cover in Segment 1, provided no high-voltage junction box is on the cover. 5.13 Disconnecting switches with interlocks and terminal chambers When specified, disconnecting switches with interlocks and terminal chambers shall be provided for the cable connection to the windings. (See 5.2.1.3 when neutral termination is required.) NOTE—Certain kilovoltampere and voltage ratings may impose design limitations on the availability or location of these items. 5.13.1 High-voltage terminal chamber The high-voltage terminal chamber shall be mounted on the side of the tank in Segment 2. 5.13.2 Low-voltage terminal chamber The low-voltage terminal chamber shall be mounted on the side of the tank in Segment 4. 24 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers 5.14 Throat connection When specified, a throat connection or connections shall be provided. (See 5.2.1.3 when neutral termination is required.) NOTE—Certain kilovoltampere and voltage ratings may impose design limitations on the availability or location of these items. 5.14.1 High-voltage throat The high-voltage throat shall be located either a) On the side of the tank in Segment 2, or b) On the cover in Segment 3, provided a low-voltage throat is not on the cover. 5.14.2 Low-voltage throat The low-voltage throat shall be located either a) On the side of the tank in Segment 1 or 4, or b) On the cover in Segment 1, provided a high-voltage throat is not on the cover. See 5.2.1.3 when neutral termination is required. 5.15 Current transformers 5.15.1 Bushing-type current transformers (or provision for their addition in the future) Bushing current transformers shall be provided as specified by the user in accordance with IEEE Std C57.13 and with accuracy classifications (full winding) as listed in Table 5 of this standard. Provisions shall be made for a maximum of two current transformers per bushing, not including current transformers for winding temperature indicators or line drop compensation. All secondary leads shall be brought to an outlet box. Provision shall be made for short-circuiting the current transformer secondary windings. Provisions shall be made for removing bushing-type current transformers without removing the tank cover. When revenue metering current transformers are provided, a certified test report shall be provided. In addition to this information, the manufacture of these current transformers shall specify the accuracy at specified burdens at all available taps as specified by IEEE Std C57.13. 25 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers Table 5 — Recommended accuracy classification of bushing current transformers Bushing current transformer ratio Revenue metering accuracy class at full winding ratio Relay accuracy class at full winding ratio 46 and below 600:5 1200:5, 2000:5, 3000:5 4000:5 and higher .3B-0.9 .3B-1.8 .3B-1.8 C200 C400 C800 69 600:5 1200:5 2000:5 and higher .3B-0.9 .3B-1.8 .3B-1.8 C200 C400 C800 Above 69 600:5 1200:5 and higher .3B-1.8 .3B-1.8 C400 C800 Bushing insulation class (kV) 5.15.2 Terminal blocks A nonsplit terminal block shall be provided in a weatherproof enclosure, located near the transformer base in Segment 1, for termination of all current transformer secondary leads. 5.16 Surge arresters When specified, one or more of the following types of construction for surge protection shall be provided: a) Provision only for the mounting of surge arresters. b) Mounting complete with surge arresters. c) A surge arrester ground pad consisting of a tank grounding pad (in accordance with 5.5) that is mounted near the top of the tank and that may be specified for each set of arresters—except that individual ground pads may be supplied where the separation of the arrester stacks is such that individual pads for grounding each phase arrester represent better design. NOTE—Material for connecting surge arresters to live parts and to ground pads is not included in item a) through item c). 5.17 Other insulating liquid When specified, another suitable insulating liquid shall be furnished instead of mineral oil. NOTE—When alternate insulating liquids are specified instead of conventional mineral oil, it is the responsibility of the manufacturer to factor the specified fluid properties in meeting this standard. 5.18 Loading IEEE Std C57.91 provides guidance and information concerning loading under various conditions, some of which may be limited by the capability of the ancillary components of the transformer. When specified, ancillary components and other construction features (e.g., cables, bushings, tap changers, liquid expansion space) shall be supplied in such a way that they in themselves do not limit the loading to less than the capability of the windings. 26 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers NOTE—IEEE Std C57.91 provides the best known general information for loading transformers under various conditions based on typical winding insulation systems and is based upon the best engineering information available at the time of preparation. It discusses “limitations” of ancillary components other than windings that may limit the capability of transformers to meet its guidelines. 5.19 “Other” tests When specified, “other” tests as described in IEEE Std C57.12.00 shall be performed. 6. LTC equipment—Basic construction features 6.1 Load tap changer (LTC) The LTC equipment, when supplied, shall consist of a liquid-immersed tap selector switch, a diverter switch with an arcing tap switch or a diverter switch with vacuum interrupter, motor drive mechanism, tap position display apparatus, and control devices. The equipment shall be located in Segment 1 or 2 of the transformer. The equipment shall meet the requirements of IEEE Std C57.131. NOTE—The LTC equipment is considered for use in the low-voltage winding of a voltage step-down application. Other applications are considered in Annex A. 6.2 Tap selector switch 6.2.1 General The tap selector switch equipment shall be liquid immersed and described by one of the following technologies: a) Arcing tap switch b) Tap selector with arcing switch c) Tap selector with vacuum interrupter 6.2.2 Tap selection switch features Tap selection switch technologies shall incorporate the following features: a) Components located in one or more liquid-filled compartments with removable bolted cover(s) for access to such components. Access shall be accomplished without exposing or draining any liquid in the transformer main tank. All covers shall have handles and be removable; covers weighing more than 20 kg (44 lb) shall additionally be hinged. b) A drain valve located in each liquid-filled compartment to provide maximal drainage. The valve shall be 1 in NPT in accordance with ASME B1.20.1, with a pipe plug in the open end. The drain valve shall have a built-in 0.375 in sampling device located on the side of the valve between the main valve seat and the pipe plug. The device shall be supplied with a 5/16-32 in male thread for the user’s connection and shall be equipped with a cap. 27 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers c) A filling plug located in the top of each liquid-filled compartment. The plug shall be 1 in NPT. d) A magnetic liquid-level gauge with a vertical face mounted on the side of each oil-filled compartment. For details, see 5.1.2. e) For arcing tap switch and tap selector with arcing switch technologies only: provision for venting to the atmosphere of gases produced by the arcing. f) Provisions for liquid temperature, pressure relief device, oil filtration for arcing-in-oil LTCs, online dissolved-gas-in-oil monitoring, and dehydrating breathers for nonsealed LTCs. 6.3 Motor and drive mechanism The motor and drive mechanism assembly shall have the following features: a) A single-phase motor without centrifugal switch suitable for operation from a 240/120 V, 60 Hz or 230/400 V, 50 Hz, three-wire source. When specified, a lightning surge arrester shall be provided for surge protection of the motor and power supply. The power source for the motor shall be 240/120 V, 60 Hz or 230/400 V, 50 Hz, three-wire, singlephase, 60 Hz, with maximum voltage to ground at 60 Hz not to exceed 150 V. This power source shall be provided by the user and shall be separate from the transformer. In some cases, the user may additionally use this source for powering forced-air-cooling fans. b) A hand crank or similar apparatus for manual operation of the driving mechanism. An electrical interlock shall be provided to prevent LTC operation by the motor drive while the manual means is engaged. A place for storing of the manual drive means, if detachable, shall be provided. WARNING Hand crank operation of the LTC may not be designed for operation under load. Consult the transformer supplier’s instructions. c) Mechanically actuated electric limit switches and mechanical stops on the LTC drive mechanism to prevent travel beyond the maximum raise and lower positions. 6.4 Position indicator 6.4.1 General An indicator of the operating position of the LTC shall be supplied. The indicator shall include the means for displaying the past maximum and minimum operating tap position of the LTC. An operator with access to the control shall have the means to reset the past maximum and minimum display function to the then present operating position. The indicator shall be located so that it can be read while the LTC is operated by hand. 6.4.2 Position indicator markings The position indicator shall be marked in accordance with the following (see Figure 6). a) The nominal (rated low-voltage) tap position shall be located on the centerline at the top of a circular dial and shall be indicated by the letter “N.” 28 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers b) The raise range (output voltage is greater than the rated low-voltage) shall be located clockwise from the “N” position. A single letter “R” (Raise) shall be located in the right half with an arrow indicating the direction of raise. The 16 tap positions in the raise range shall be marked, and a number shall appear opposite at least every fourth position. Number 16 shall be the highest voltage position. c) The lower range (output voltage is less than the rated low-voltage) shall be located counterclockwise from the “N” position. A single letter “L” (Lower) shall be located in the left half with an arrow indicating the direction of lower. The 16 tap positions in the lower range shall be marked, and a number shall appear opposite at least every fourth position. Number 16 shall be the lowest voltage position. N 4 4 L 8 R 12 8 12 16 16 NOTE—This figure is intended to present a schematic rather than a pictorial illustration of the dial face. See 6.4.2. Figure 6 — Position indicator for LTC 6.5 Control equipment and accessories 6.5.1 General Control devices to accommodate manual and automatic control of the LTC equipment shall be provided unless the user specifies that the LTC transformer be supplied with no control for automatic LTC. 6.5.2 Control equipment enclosure 6.5.2.1 General A weather-resistant cabinet shall be provided for housing the automatic control and related devices. The cabinet shall be equipped with breather, hinged doors, and provision for entrance of up to three 1.5 in conduits in the bottom. The doors shall provide access to the control and accessory devices and shall have provision for padlocking consisting of matching holes having a minimum diameter of 9.5 mm (0.375 in). 29 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers Space shall be provided for mounting the control equipment required for parallel operation by the method specified by the user (see 1.1). 6.5.2.2 Terminal blocks Terminal blocks shall be provided in the control enclosure for terminating contacts specified in 5.1.10 for liquid level and temperature indicators and for current transformer secondaries (two leads per current transformer) specified in 5.15.1. 6.5.3 Control equipment The LTC control system is composed of the following: a) Sensing apparatus to provide signals proportioned to the transformer low-voltage and load current. b) A control device to interpret the voltage and current signals of the sensing apparatus, relate this information to conditions desired by the operator, and automatically command the LTC to hold the output thereby required. 6.5.3.1 Sensing apparatus The usual sensing apparatus consists of current transformer(s) and voltage transformer(s). 6.5.3.1.1 Current transformers The manufacturer shall furnish current transformer(s) to deliver not less than 0.15 A and not more than 0.2 A to the control circuits when the transformer is operating at the maximum continuous current for which it is designed, including increases that may be obtained by normal cooling modifications. a) For a Y-connected winding, the current transformer(s) shall deliver to the line drop compensator of the control a current that is nominally in phase with the current at the X1 load terminal of the transformer. b) For a Δ-connected winding, the current transformers shall deliver to the line drop compensator of the control a current that is nominally in phase with a phasor derived from the relationship of the current at the X1 load terminal minus the current at the X2 load terminal. 6.5.3.1.2 Voltage transformers It is the responsibility of the user to install appropriate voltage transformer(s) that match the phasing of the current transformers provided with the transformer. 30 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers 6.5.4 Control device 6.5.4.1 General 6.5.4.1.1 Environmental The control device shall withstand –40 °C to 80 °C control enclosure temperature, relative humidity from zero to 100%, and altitude of up to 3000 m (9840 ft) without loss of control. 6.5.4.1.2 Response time A step change in applied voltage of 0.75 V from outside the band to within the band shall cancel any raise or lower signal within 0.3 s. 6.5.4.2 Set-point adjustment ranges The control device shall permit parameter adjustment as follows: a) Voltage level setting adjustable from at least 108 V to 132 V (related to line-voltage-by-voltagesupply ratio). b) Bandwidth setting adjustable from at least 1.5 V to 3.0 V (total range). c) Actuation time delay setting adjustable from at least 15 s to 90 s. (The time delay applies only to the first required change if subsequent changes are required to bring the system voltage within the bandwidth setting.) d) Line drop compensation adjustment including independently adjustable resistance and reactance. The resistance shall be adjustable in the range of at least 0 V to +24 V. The reactance shall be adjustable in the range of at least –24 V to +24 V. The voltage refers to line drop compensation at the nominal control base voltage of 120 V and rated base current of 0.2 A. 6.5.4.3 Components and accessories 6.5.4.3.1 General The following components shall be provided as part of the control device or as accessories to the control system: a) Test terminals for measuring voltage proportional to transformer output voltage. The test terminal voltage shall not be changed more than ± 1% by connecting a burden of 25 VA at 0.7 power factor across the test terminals, unless otherwise specified. This voltage change is not included in the specification of accuracy of the control relays. b) Manual-automatic control switch. c) Manual raise/lower switch(es). d) Operation counter to indicate accumulated number of tap changer operations. e) Band limit indication means. 31 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers f) A screw-base lamp socket with a switch and a ground-fault-protected convenience outlet for a 120 V, single-phase 60 Hz supply. g) A heater with a manual switch. 6.5.4.3.2 Control of LTC transformers in parallel Additional components and accessories shall be required when the LTC transformer is planned for operation in parallel with another LTC transformer. Several alternative means are available for such applications. The user shall specify the procedure to be used. The more commonly used procedures are indicated in 1.1. 6.5.5 Control system accuracy requirements The LTC control shall have an overall system error not exceeding ± 1%. The accuracy requirement is based on the combined performance of the control device and sensing apparatus including instrument current and voltage transformers, utility windings, transducers, etc., with the voltage and current input signals of a sinusoidal wave shape. Since it is not practical to test the overall control system accuracy, it is permissible to individually test the control system components. The accuracy of individual components is then combined to arrive at the overall control system accuracy. Accuracy tests are design tests, not made on every unit. For the test, voltage and current signals should have a sinusoidal wave shape. No analytical correction is permitted to remove effects of harmonics in the accuracy test results. 6.5.5.1 Sensing apparatus 6.5.5.1.1 Voltage source The voltage transformer shall be presumed to be of accuracy class 0.3; refer to IEEE Std C57.13. 6.5.5.1.2 Current source The current source accuracy shall be determined on a nominal 0.2 A secondary current and a burden of 3.5 VA; refer to IEEE Std C57.13. 6.5.5.2 Control device The accuracy of the control device shall be determined based on testing at an ambient temperature of 25 °C, rated frequency, a nominal input voltage of 120 V and a base current of 0.2 A at 1.0 power factor. NOTE—The user should be aware that harmonic distortion of the control device input voltage and/or current can result in differences in the sensed average or root-mean-square (RMS) magnitude that affects the overall accuracy of the control device and control system. Such differences may be inherent in the product design and do not constitute an additional error in the context of control accuracy. 6.5.5.2.1 Control device errors Each individual error-producing parameter is stated in terms of its effect on the response of the control device and is determined separately with the other parameters held constant. Errors causing the control 32 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers device to hold a higher voltage level than the reference value are plus errors and those causing a lower voltage level are minus errors. The overall error of the control device is the sum of the individual errors as separately determined causing a divergence from the voltage level setting, presuming a bandwidth of zero volts. 6.5.5.2.2 Factors for accuracy determination of control device The greater magnitude of the sum of the positive or negative errors of the following three areas shall constitute the accuracy of the control device: a) Variations in ambient temperature of the control environment between –30 °C and 65 °C. b) Frequency variation of ± 0.25% in rated frequency (0.15 Hz for 60 Hz application). c) Line drop compensation: 1) Resistance compensation of 12 V and an in-phase base current of 0.2 A with reactance compensation of zero. 2) Resistance compensation of 12 V and a 90° lagging base current of 0.2 A with reactance compensation of zero. 3) Reactance compensation of 12 V and an in-phase base current of 0.2 A with resistance compensation of zero. 4) Reactance compensation of 12 V and a 90° lagging base current of 0.2 A with resistance compensation of zero. 6.5.6 Tests 6.5.6.1 Design tests 6.5.6.1.1 Determination of accuracy of control device Subclause 6.5.6.1 outlines procedures for determining values of errors contributed by the factors described in 6.5.5.2.2. The voltage and current sources applied shall be as free of harmonics or other distortions as the test facility permits. 6.5.6.1.1.1 Tests for errors in voltage level With the control device set at a voltage level of 120 V and at an ambient temperature of 25 °C, energize the control device for one hour using a 120 V source of rated frequency. The control is calibrated at this point. Errors in voltage level in the three tests below determine the control device accuracy: a) Tests for error in voltage level due to temperature: The control device shall be tested over a temperature range of –30 °C to 65 °C in not more than 20 °C temperature increments. The air temperature surrounding the control device shall be held constant and uniform within ± 1 °C of each increment for a period of not less than one hour before taking a test reading. Tests are made at rated frequency with zero current in the line drop compensation circuit. b) Tests for error in voltage level due to frequency: The control device shall be tested over a sufficient range of frequencies to accurately determine the error over the specified range of rated frequency, ± 0.25%. Tests are made at a constant temperature of 25 °C with zero current in the line drop compensation circuit. 33 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers c) Tests for errors in voltage level due to line drop compensation: Four tests shall be made at rated frequency and a constant temperature of 25 °C and a voltage level setting of 120 V. Determine the voltage level required to balance the control with 0.2 A in the compensator circuit of the control under the following conditions: Test Set LDC-R (V) Set LDC-X (V) Current phasing Determine voltage error relative to expected (V) 1 12 0 in-phase V = 132.0 2 0 12 in-phase V = 119.4 3 12 0 90° lagging V = 119.4 4 0 12 90° lagging V = 132.0 Use the individual test error (plus or minus) that produces the largest overall error magnitude when summed in accordance with 6.5.5.2.1. 6.5.6.1.2 Set point marks Deviation of set point marks for voltage level, bandwidth, line drop compensation, and time delay settings are not considered as a portion of the errors in determining the accuracy classification. 6.5.6.1.2.1 Bandwidth center marking deviation The difference between the actual bandwidth center voltage and the marked value at any setting over the range of 120 V ± 10% shall not exceed ± 1%. 6.5.6.1.2.2 Bandwidth marking deviation The difference between the actual bandwidth voltage and the marked value shall not exceed ± 10% of the marked value set. 6.5.6.1.2.3 Compensator marking deviation The arithmetic difference between the actual compensation voltage, expressed as a percent of 120 V, and the marked value of any setting of either the resistance or reactance element of the compensator, expressed as a percent of 120 V, with 0.2 A in the compensator circuit shall not exceed ± 1%. 6.5.6.1.2.4 Time delay set marking deviation The difference between the actual time delay and the marked value of any setting shall not exceed ± 10%. This statement is true in an integrating type circuit when the delay is initiated with no stored delay. 6.5.6.1.3 Surge withstand capability (SWC) test The SWC test is a design test for the control device in its operating environment. In order to pass this test, the control device shall continue to operate properly and not have any unintentional tap change during and after the test. Refer to IEEE Std C37.90.1. 34 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers 6.5.6.2 Routine tests 6.5.6.2.1 Applied voltage The control device shall withstand a dielectric test voltage of 1000 V, 60 Hz from all terminals to case for 1 min. The test shall be performed with the control totally disconnected from equipment. After the test, it shall be determined that no change in calibration or performance has occurred. NOTE—To minimize excessive damage or failure, use of a resistor to limit the current is recommended. 6.5.6.2.2 Operation All features of the control device and its peripherals shall be operated and checked for verification of proper functioning. The control is also calibrated at this point. 35 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers Annex A (informative) LTC considerations The low-voltage winding is the standard location for an LTC. There are applications when an LTC could better be located in the high-voltage winding for step-up or step-down applications. At present, it is beyond the scope of this standard to establish the standards for the LTC to be located in the high-voltage winding. This annex provides information on issues for the transformer user and transformer manufacturer to discuss to ensure that the transformer design and application are properly coordinated for high-voltage LTC applications. Fundamental questions to answer are the following: a) Will the LTC function by constant flux voltage variation (CFVV) or variable flux voltage variation (VFVV)? b) Will the LTC be used to control the high-voltage voltage or the low-voltage voltage? c) Where will the user install the voltage transformer for monitoring the voltage and controlling the LTC tap position? A.1 Constant and variable flux LTC applications A.1.1 CFVV LTC regulation CFVV LTC operation regulates the transformer secondary by increasing or decreasing the turns in the regulated winding (typically the low-voltage winding) while the unregulated winding (typically the highvoltage winding) turns are constant. The high-voltage system voltage is relatively constant; therefore, flux density of the transformer is also relatively constant, impedance and sound levels are constant, and step voltage is also constant with step voltage tolerances according to IEEE Std C57.12.00. For a CFVV tap changer to increase the low voltage, turns are added to the low-voltage winding by operating the LTC in the raise direction. A.1.2 VFVV LTC regulation VFVV LTC operation regulates the transformer winding (typically the low-voltage winding) by increasing or decreasing the turns in the unregulated winding (typically the high-voltage winding) while the regulated winding turns are constant. The high-voltage system voltage is relatively constant; therefore, flux density of the transformer varies as the high-voltage turns are varied, impedance and sound levels also vary, and step voltage is also variable. For a VFVV tap changer to increase the low voltage, turns are subtracted from the high-voltage winding. As the volts per turn are increased—and, therefore, the low-voltage winding voltage is increased, the flux density is also increased, the transformer sound level increases until reaching a maximum level at highest tap position, and the transformer impedance is decreased. CFVV results in constant no-load losses and sound level, uniform tap step voltages, and relatively small variations of the transformer impedance. VFVV results in varying no-load losses and sound level, nonuniform tap step voltages, and relatively large variations of the transformer impedance as shown in Table A.1. 36 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers In some cases, to regulate low voltage in two- and three-winding transformers, the LTC is installed in the high-voltage winding to reduce the cost. For low-voltage constant flux designs, the regulating winding is placed inside the low-voltage winding and increases the length of mean turn of the low-voltage and highvoltage windings. As a result, cost and losses are increased. With VFVV operation, the regulating winding is typically located over the high-voltage winding, and the mean turns of the low-voltage and high-voltage windings are smaller and should result in lower losses. A second benefit is that the winding currents are significantly lower in the high-voltage winding, and this level allows smaller conductors for the regulating winding and eliminates the necessity of a booster/series transformer if the coil currents are less than the rating of the LTC. Table A.1 — VFVV cases Winding connection Design High voltage Low voltage High-voltage DETC Low-voltage LTC requirements Variable flux/ variable voltage solution Benefits Two- and three-winding transformers 1 Δ Grounded Y ± 2 at 2.5% ± 10% LTC ± 15% in highvoltage Δ Reduced losses Reduced cost Eliminate high cost of DETC 2 Grounded Y Grounded Y ± 2 at 2.5% ± 10% LTC ± 10% in highvoltage neutral Reduced losses Reduced cost 3 Grounded Y Grounded Y ± 2 at 2.5% ± 10% LTC ± 15% in highvoltage neutral Reduced losses Reduced cost Eliminate high cost of DETC 1 Grounded Y Grounded Y Autotransformers ± 2 at 2.5% ± 10% LTC ± 10% in neutral Reduced losses Reduced cost Eliminate high cost of DETC 2 Grounded Y Grounded Y ± 2 at 2.5% ± 10% in neutral Reduced losses Reduced cost Eliminate high cost of DETC ± 10% LTC Generally, the results of a variable flux design are as follows: a) Nonuniform voltage steps and paralleling concerns. b) No-load losses change with LTC tap position and are highest at the highest tap position (highest voltage level of low voltage). c) Total losses change with LTC position. d) Impedance varies with LTC position due to the flux variation; impedance varies inversely proportionally to the square of the volts per turn. e) Sound level varies with LTC tap position and is highest at the highest tap position. f) LTC affects the voltage of the tertiary winding for three-winding transformers. 37 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers g) Operating the high-voltage LTC in the raise direction reduces the low voltage; operating the highvoltage LTC in the lower direction increases the low voltage. h) Placing the regulating winding outside the high-voltage winding can increase the induced transient voltages and may require nonlinear resistors to mitigate the induced transients. For design 1 (in Table A.1), high-voltage Δ connections and low-voltage grounded Y connections, the DETC and LTC are combined into one, and this design incorporates the DETC range into the high-voltage LTC. Since the DETC needs to be installed in the center of the high-voltage Δ-connected winding and the LTC should also be installed in the center of the high-voltage winding, a combination is usually the best solution for cost and technical considerations. For design 2, the same benefits generally occur, but it is possible to use another regulating winding to supply the DETC separately from the LTC. For design 3, the DETC tap range is combined into the range of the LTC, and this design eliminates the cost of the DETC and its installation on the transformer core and coils. Autotransformers are unique. Instead of a separate high-voltage and low-voltage winding, the high voltage and low voltage share the turns of the common winding, and the high-voltage winding is the only winding using the turns of the series winding. In an autotransformer connection, the flux density variation depends on the location of the LTC. In a neutral end LTC application, both the common and series windings share the turns being varied in the common winding. Therefore, it also affects the high-voltage winding turns, the highest flux density and sound level occur at the lowest tap position (16L), and the impedance increases as the LTC position increases toward the highest tap position (16R). Up to 138 kV low voltages, on-tank tap changers are available to allow CFVV designs. Above 138 kV secondary voltages, single-phase in-tank LTCs may be required, and the cost will be significantly higher than variable flux designs. If the tertiary windings, common in autotransformers, are buried and for harmonic suppression only, variable flux regulation of the autotransformer will not cause concern in the variations of the tertiary winding voltage. If the tertiary is brought out for station service, capacitive, or reactance loading, special LTC designs are required to stabilize the tertiary winding voltage when VFVV LTC connections are chosen for voltage regulation. A.2 Transformer paralleling A.2.1 Fundamental control premises and basic methods There are three basic requirements for the appropriate control of tap position of multiple LTC transformers operating in parallel: a) The transformers must continue their basic function of controlling the load bus voltage as prescribed by the basic settings on the control: voltage set point (band center), tolerance bandwidth, and line drop compensation. b) The tap changers must operate to maintain tap position to minimize the current that circulates between them. Depending on the control method and transformer design, the appropriate tap positions on the paralleled transformers are not necessarily all the same to achieve this requirement. c) Actions a) and b) above must operate correctly in applications with multiple paralleled transformers regardless of planned system configuration changes or breaker operations that would alter the parallel connection or operation of the transformers. 38 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers Three basic methods are employed to control tap changer operation of paralleled transformers: 1) Direct operation of multiple tap changers from one control source (master/follower). 2) Biasing or restraining of multiple control set points with interconnections or communications between parallel transformers (circulating current, volt-ampere reactive (VAR) control, and power factor). 3) Biasing or restraining multiple controls without interconnections or communications between parallel transformers (reverse reactance). None of these paralleling methods are capable of correcting the imbalance between the active (watts) components of load currents in parallel transformers with different impedances. The methods above are typically used to reduce the imbalance between reactive (VARs) components of transformers’ currents. Some methods in items 2) and 3) above can be used for paralleled transformers connected to separate primary sources with limited differences between the applied primary voltages (that is, not beyond their tapping ranges). All methods, albeit sometimes involving special auxiliary apparatus, can be used for paralleling transformers with different megavoltampere capacities or impedances. However, in all cases, the more nearly identical are the transformers, the better will be the overall system performance. A.2.2 Master/follower The master/follower paralleling method assumes that, under all system operating configurations, the desired operation objectives are met by maintaining the same turns ratio on all paralleled transformers. This operation is usually accomplished by maintaining the same physical tap position. The operation consists of one active master control commanding the tap changes of additional transformers to follow. A tap operation and position feedback scheme is mandatory to confirm to the master unit that the following unit has operated properly. If that feedback is not received, the controls usually are set to lock out all further operations. The use of this method is usually confined to transformers with equivalent design parameters. This method is not applicable for paralleled transformers connected to separate primary sources. A.2.3 Circulating current As it is commonly defined, the circulating current paralleling method assumes that, under all system operating configurations, any circulating current between transformers is representative of the disparity of tap positions between the paralleled transformers. Variations in load currents do not affect the correct operation of the circulating current method in minimizing circulating current. This method derives the magnitude of the circulating current between transformers using external balancing modules and communications between controls. The circulating current method causes any circulating current between the transformers to bias the set points in opposite directions and thus cause subsequent tap operations to be in the direction to reduce the circulating current. This operation is achieved while maintaining the voltage set point accuracy. This method should not be used when the transformers are, or may be, with primary circuit switching connected to separate primary sources. A.2.4 Circulating VARs Several methods are based on controlling the VARs in the parallel transformers. Each transformer’s VAR loading is made up of the following: a) Its share of the total load VARs as determined by its relative impedance and megavoltampere capacity compared to the other transformers. b) The circulating VARs due to differences in secondary induced voltages. 39 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers In some methods, the objective is to reduce the circulating VARs to a minimum. In other methods, the objective is for the transformers’ VARs to be shared in proportion to their megavoltampere capacities. The VAR control methods are suitable for cases where the primaries are connected to different sources A.2.5 Power factor The power factor paralleling method is an implementation of the circulating current method where the basis of operation is to recognize the disparity of power factor, rather than circulating amperes, as recognized by each transformer. It assumes that, under all system operating configurations, any difference in power factor between transformer loads indicates the relative tap positions of the paralleled transformers. With the same voltage applied, the relative angle of the transformer currents will indicate the relative power factors. The transformer control with the more lagging relative load current angle is allowed only to lower tap position, if necessary to maintain bus voltage. The transformer control with the less lagging (or perhaps leading) relative load current angle is allowed only to raise tap position, if necessary to maintain bus voltage. To the extent that, from the perspective of each transformer, equalizing the power factor is equivalent to equalizing the VARs, some of the attributes of the VAR control method are realized with the power factor method. A.2.6 Reverse reactance The reverse reactance paralleling method uses a biasing voltage with X and R components in a somewhat similar way to line drop compensation, except that the X component is reversed in direction (polarity). Thus, whereas line drop compensation bias results in control of the voltage down the line, reverse reactance bias results in control based on the transformer’s induced voltage upstream from the transformer impedance. If all parallel transformers have the same induced (no-load) voltage, there will be no circulating current. The R setting compensates for the transformer impedance drop so that in fact the load bus voltage is controlled. Thus the voltage will be correct at no load and at full load, with a very small variation of set point voltage (typically around 0.1% of set point) at all loads in between. More variation will be experienced, but still typically less than 0.5% of set point, with reasonably anticipated variation of the load power factor. The control is unaffected by switching other transformers in and out of parallel since each transformer’s control takes care of itself without interconnections or communications between them. Reverse reactance control thus meets the three basic requirements for LTC control of parallel transformers. There are two caveats. First, if line drop compensation is required, each controller needs an input for the summated load current, and this input is the only interconnection required between transformers in this case. Thus, the use of line drop compensation obviates a principal advantage, that of no required control interconnections. Second, the X and R settings are optimized for the average power factor at typical load. If there are large variations in load power factor, such as might occur with crude power factor correction systems where large banks of capacitors are switched, then reverse reactance control is not recommended. However, experience has shown that the method works well in most practical situations feeding domestic, industrial, or mixed loads. A.2.7 Conclusion The paralleling method chosen must be compatible with all the circuit conditions expected to occur during the life of the paralleling. This compatibility must include all configurations that can occur due to protective relaying operations, maintenance conditions, and system loading conditions. Each of these methods has additional considerations for settings, field commissioning, and troubleshooting, depending on specific equipment or system characteristics. 40 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers A.3 Control of the high-voltage voltage or the low-voltage voltage An LTC in the high-voltage winding could be used to control either the high voltage or the low voltage. Under this IEEE standard, the user is expected to install the voltage transformer that monitors the transformer voltage for controlling the LTC tap position. It is important to address the issues listed in A.3.1 and A.3.2. A.3.1 Location of the voltage transformer Under this IEEE standard, the user is expected to install the voltage transformer that monitors the transformer voltage for controlling the LTC tap position. With a high-voltage LTC, it is possible to get a situation where the manufacturer believes that the user will have the voltage transformer on the high voltage to control the LTC position. But if the user locates the voltage transformer on the low voltage in a step-down application, the LTC lowers the voltage when the opposite is required. A.3.2 Issues to consider for high-voltage LTC applications A.3.2.1 Impedance variation Variable-volts-per-turn transformers have large variations in impedance with changes in the LTC tap position. Impedance varies inversely with the square of the volts per turn. A typical high-voltage LTC with variable volts per turn (VFVV) causes the impedance to span over a 40% range about the neutral tap position impedance. The user needs to consider the impedance range in the system design and impedance value to specify. The manufacturer must consider the lowest impedance value for the transformer shortcircuit design. A.3.2.2 Sound VFVV (variable-volts-per-turn) transformers have increasing sound levels from the neutral tap to the highest tap (16R). The manufacturer and the user must understand what the sound level limits should be for all taps. Standard test procedures test sound levels only in the neutral tap position. A.3.2.3 Ratios A high-voltage LTC to control the low voltage provides other than the standard 0.625% steps. A 10% change in the number of high-voltage turns produces something other than a 10% change in voltage of the low voltage (e.g., 1/1.1 does not equal 0.9). An increase of the number of high-voltage winding turns by 10% causes a reduction in the low-voltage winding voltage of 9.09%. A decrease in the number of highvoltage winding turns by 10% causes a low-voltage winding voltage increase of 11.11%. In such an application, the manufacturer and the user need to come to an agreement on what the actual output voltages shall be and what shall be shown on the nameplate. A.3.2.4 Nameplate Should the nameplate show the low-voltage voltage changing with tap position, or should the high-voltage voltage be shown changing with tap position? What voltages should be shown? 41 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers A.3.2.5 De-energized tap changer (DETC) Should the high-voltage winding have a DETC? Typically, there is no need for a DETC in the high-voltage winding when the LTC is located in the high voltage. A.3.2.6 Over-excitation capability The standard application requires that the transformer be able to continuously operate at no-load and 110% of rated voltage without exceeding temperature limits. A variable-volts-per-turn (VFVV) application needs to maintain the same 110% rule at the worst-case tap position. Test procedures should also be considered to verify over-excitation capability. A.3.2.7 Location of LTC switch and accessories When the LTC is located in the high voltage, frequently the best location for the LTC switch is on the same side of the transformer tank as the high-voltage bushings. Generally, that location is Segment 3. Generally, the control cabinet should also be located near the LTC switch and preferably where the control cabinet operator has a line of sight to the LTC switch. Also then, the gauges, the nameplate, and other accessories would usually be located near the control cabinet. The manufacturer and the user need to agree on the location of these items while considering manufacturing limitations and substation design. A.3.2.8 Line drop compensation Normally, a line-drop-compensation current transformer is located on the same bushing(s) as the voltage transformer. For a low-voltage Y-connection with LTC, that location would require that the line-dropcompensating current transformer be located on the X1 bushing. When the LTC is in the high voltage, that standard location may not be appropriate. The key issues to determine the proper current transformer location is where the voltage transformer is located and whether the high-voltage or low-voltage voltage is being controlled. A.3.2.9 Kilovoltampere rating in all taps Should a full kilovoltampere rating be expected in all taps? For a high-voltage LTC controlling the highvoltage voltage, a full kilovoltampere rating for all taps is generally appropriate. When a high-voltage LTC is used to control the low-voltage voltage, then full capacity is generally unnecessary. The transformer user should consider the question of kilovoltampere rating in all taps. A.3.2.10 Paralleling Paralleling of a high-voltage LTC transformer with other LTC transformers needs to consider the following issues: a) Similarity of impedances of the transformers over the range of the LTC taps b) Similarity of the high-voltage/low-voltage ratios of the transformers over the range of the LTC taps c) Compatibility of controls to maintain the correct tap positions on all the transformers while minimizing circulating current 42 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers A.3.2.11 Load sharing of transformers connected in parallel Transformers operated in parallel share the load based on their impedances when connected as shown in Figure A.1. Figure A.1 — Load sharing of transformers connected in parallel The load of each transformer can be calculated using the following equations based on transformers where ZA, ZB IA, IB IL is per-unit impedance of transformers A and B @ stated megavoltampere base is per-unit load current of transformers A and B is per-unit load current of transformers A and B in parallel Assuming the voltage drop through both transformers is equal, then IA × ZA = IB × ZB and IL = IA + IB Solving these equations gives the following load distribution between the two transformers based on the ratio of their separate impedances calculated at the same base megavoltamperes: Per-unit loads: IA = ZB ZA + ZB and IB = ZA ZA + Z B 43 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers A.3.2.12 Some general paralleling operational basics a) Transformers of the same megavoltampere rating and equal impedance share the load equally (Case I). b) Transformers of different ratings share loads based on their ratings as long as the impedances at their base megavoltamperes are equivalent (Case II). c) If transformers of different impedances are paralleled, the total capacity of the transformers connected in parallel is limited to less than the sum of their capacities (Case III). d) If transformers of different impedances are paralleled, the total capacity of the transformers connected in parallel is limited to less than the sum of their capacities (Case IV). A.3.2.12.1 Case I, identically rated transformers Bank A = Bank B: 24/32/40 MVA Impedance = 8% @ 24 MVA First, state the per-unit impedances on the same megavoltampere base: Bank A = Bank B: ZA = ZB = 0.08 @ 24 MVA base The transformers share load inversely to the ratio of the impedance of the bank to the sum of the impedances of the two banks in parallel. Bank A share = Bank B share = 0.08 0.08 = = 0.50 per-unit load 0.16 (0.08 + 0.08) NOTE—When connected in parallel, the total bank rating is 80 MVA; Bank A and Bank B both carry 0.5 per unit (40 MVA); neither unit is loaded in excess of its nameplate rating; and the bank capacity equals the sum of the transformer nameplate ratings, 80 MVA. A.3.2.12.2 Case II, transformers of different ratings Transformer: Bank A 12/16/20 MVA Impedance 8% @ 12 MVA Bank B 24/32/40 MVA Impedance 8% @ 24 MVA First, state the per-unit impedances on the same megavoltampere base: Bank A: ZA = 0.16 @ 24 MVA base Bank B: ZB = 0.08 @ 24 MVA base The transformers share load inversely to the ratio of the impedance of the bank to the sum of the impedances of the two banks in parallel. Bank A share = 0.16 0.08 = 0.33 per-unit load = (0.08 + 0.16) 0.24 44 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers Bank B share = 0.16 0.16 = = 0.67 per-unit load 0.24 (0.08 + 0.16) NOTE—When connected in parallel, the total bank rating is 60 MVA; Bank A carries 0.33 per unit (20 MVA); Bank B carries 0.67 per unit (40 MVA); neither unit is loaded in excess of its nameplate rating; and the bank capacity equals the sum of the transformer nameplate ratings, 60 MVA. A.3.2.12.3 Case III, transformers of different cooling rating NOTE—When the transformers are both rated with different cooling ratings (Bank A is ONAN/ONAF and Bank B is ONAN/ONAF/ONAF) and both have identical impedances on their self-cooled bases, each share load according to its rating but parallel operation loading is limited. Bank A: 10/12.5 MVA Impedance 8% @ 10 MVA Bank B: 12/16/20 MVA Impedance 8% @ 12 MVA First, state the per-unit impedances on the same megavoltampere base: Bank A: ZA = 0.16 @ 20 MVA base Bank B: ZB = 0.13 @ 20 MVA base The transformers share load inversely to the ratio of the impedance of the bank to the sum of the impedances of the two banks in parallel. Bank A share = 0.13 0.13 = 0.45 per-unit load = 0.29 (0.16 + 0.13) Bank B share = 0.16 0.16 = 0.55 per-unit load = 0.29 (0.16 + 0.13) Maximum total load without exceeding the nameplate rating of Transformer A = 12.5 = 27.8 MVA. 0.45 NOTE—When connected in parallel, the total bank rating is 32.5 MVA. Limiting the loading of Bank A to its nameplate rating of 12.5 MVA limits the total capacity of the paralleled transformers to a capacity of 27.8 MVA; Bank A carries 0.45 per unit (12.5 MVA); Bank B carries 0.55 per unit (15.3 MVA); neither unit is loaded in excess of its nameplate rating; and the bank capacity equals the sum of the transformer nameplate ratings, 60 MVA. A.3.2.12.4 Case IV, transformers of different cooling rating NOTE—When the transformers are both rated with different cooling ratings (Bank A is ONAN/ONAF and Bank B is ONAN/ONAF/ONAF), it is possible to specify an impedance that permits each transformer to share load according to its rating while allowing loading to the sum of the individual ratings. For this study, it has been assumed that the 12/16/20 MVA transformer is being added to increase the substation capacity and the goal is to optimize the parallel operation. Bank A: 10/12.5 MVA Impedance 8% @ 10 MVA Bank B: 12/16/20 MVA Impedance X% @ 12 MVA 45 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers First, state the per-unit impedances on the same megavoltampere base: Bank A: ZA = 0.16 @ 20 MVA base Bank B: ZB = X @ 20 MVA base The goal is to have Bank A carry 12.5 MVA while the total bank is carrying a total of 32.5 MVA. Bank A share = 12.5MVA = 0.385 per-unit load 32.5MVA The transformers share load inversely to the ratio of the impedance of the bank to the sum of the impedances of the two banks in parallel. Bank A share = X = 0.385 per-unit load (0.16 + X ) Bank B share = 0.16 = 0.615 per-unit load (0.16 + X ) Solving the Bank A equation for X: X = 0.385 (0.16+X) 0.615 X = 0.0616 X = 0.10 per unit ZB = 0.10 @ 20 MVA base, ZB = 6% @ 12 MVA base NOTE—When connected in parallel, the total bank rating is 32.5 MVA. Specifying the impedance of the 12/16/20 MVA at 6.0% @ 12 MVA base permits parallel operation to the full capacity of both transformers without exceeding the ratings of either transformer. A.3.2.13 Autotransformer LTC application considerations See Figure A.2. a) The ± LTC is located in the neutral end of the common winding resulting in a VVFV LTC application used when the tertiary voltage (TV) (tertiary winding) is buried for harmonic suppression or there is no tertiary winding to be affected by the variation in flux density. Unlike two- and three-winding VVFV LTCs, the LTC affects both the common and series windings; the minimum flux density is at tap position 16R; the maximum flux density is at 16L; and impedance variation is not as large as for the two-winding VVFV application. b) A linear LTC is located between the series and common windings, and the low voltage is regulated by moving the LTC in the raise and lower direction. This application is CFVV as the turns from the high-voltage line to the H0X0 at the neutral end of the common winding remain constant regardless of the LTC position. c) The ± LTC is located in the end of the series winding connection to the common winding and lowvoltage line bushing resulting in a VFVV LTC application used when the TV (tertiary winding) is buried for harmonic suppression or there is no tertiary winding to be affected by the variation in 46 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers flux density. Like two- and three-winding VFVV LTCs, the LTC affects only the series winding; turns in the low voltage and TV are unchanged; the minimum flux density occurs at 16L; the maximum flux density is at 16R; maximum sound level is at position 16R; and impedance variation is as large as for the two-winding VFVV application. d) The ± LTC is located in the bushing connection from the common point of the series and common windings resulting in a CFVV LTC application which does not affect TV (tertiary winding). However, if the low voltage is greater than 69 kV, an in-tank LTC is required. Such an LTC would dramatically increase the costs and tank size. e) The ± LTC is connected to a separate regulating winding for CFVV application while a series transformer polarity and output voltage is varied based on the LTC position. Figure A.2 — Autotransformer LTC application 47 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply. IEEE Std C57.12.10-2010 IEEE Standard Requirements for Liquid-Immersed Power Transformers Annex B (informative) Bibliography [B1] IEEE Std C57.12.36™, IEEE Standard Requirements for Liquid-Immersed Distribution Substation Transformers. 9, 10 [B2] IEEE Std C57.12.90™, IEEE Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers. 9 IEEE publications are available from the Institute of Electrical and Electronics Engineers, 445 Hoes Lane, Piscataway, NJ 08854, USA (http://standards.ieee.org/). 10 The IEEE standards or products referred to in this clause are trademarks of the Institute of Electrical and Electronics Engineers, Inc. 48 Copyright © 2011 IEEE. All rights reserved. Authorized licensed use limited to: Juan Navarro. Downloaded on May 22,2014 at 16:58:48 UTC from IEEE Xplore. Restrictions apply.

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