ieee std c57.12.10-2010

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IEEE Standard Requirements forLiquid-Immersed Power Transformers

Sponsored by theTransformers Committee

IEEE3 Park AvenueNew York, NY 10016-5997USA

6 January 2011

IEEE Power & Energy Society

IEEE Std C57.12.10™-2010(Revision of

ANSI C57.12.10-1997)

right The Institute of Electrical and Electronics Engineers, Inc.ded by IHS under license with IEEE

Not for Resaleproduction or networking permitted without license from I HS

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8/13/2019 IEEE Std C57.12.10-2010


IEEE Std C57.12.10 ™-2010 (Revision of

ANSI C57.12.10-1997)


Sponsor

Transformers Committeeof theIEEE Power & Energy Society

Approved 30 September 2010

IEEE-SA Standards Board



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Abstract: This standard sets forth the requirements for power transformer application. Thisstandard is intended to be used as a basis for performance, interchangeability, and safety of theequipment covered and to assist in the proper selection of such equipment. This document is aproduct standard that covers certain electrical, dimensional, and mechanical characteristics of50 Hz and 60 Hz, liquid-immersed power transformers and autotransformers. Such powertransformers may be remotely or integrally associated with either primary switchgear orsubstations, or both, for step-down or step-up purposes and base rated as follows: 833 kVA andabove 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 ofapparatus: instrument transformers, step voltage and induction voltage regulators, arc-furnacetransformers, rectifier transformers, specialty transformers, grounding transformers, mobiletransformers, and mine transformersKeywords: autotransformer, dimensional characteristics, electrical characteristics, load tapchanger, 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 13 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 ElectronicsEngineers, Incorporated.

PDF: ISBN 978-0-7381-6444-1 STD97010Print: 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 permissionof the publisher.



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IEEE Standards documents are developed within the IEEE Societies and the Standards Coordinating Committees ofthe IEEE Standards Association (IEEE-SA) Standards Board. The IEEE develops its standards through a consensusdevelopment process, approved by the American National Standards Institute, which brings together volunteersrepresenting varied viewpoints and interests to achieve the f inal product. Volunteers are not necessarily members of theInstitute and serve without compensation. While the IEEE administers the process and establishes rules to promotefairness in the consensus development process, the IEEE does not independently evaluate, test, or verify the accuracyof any of the information or the soundness of any judgments contained in its standards.

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ivCopyright © 2011 IEEE. All rights reserved.

Introduction

This introduction is not part of IEEE Std C57.12.10-2010, IEEE Standard Requirements for Liquid-Immersed PowerTransformers.

This standard was prepared by the Revision of C57.12.10 Working Group of the Power TransformersSubcommittee of the Transformers Committee of the IEEE Power and Energy Society. The purpose of thisstandard is to cover the dimensional, electrical, and mechanical characteristics for liquid-immersed powertransformers 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 through60 000/80 000/100 000 kVA, Three-Phase Without Load Tap Changing; and 3750/4687 through60 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 themegavoltampere 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 we re excluded fromthis 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 aduly constituted legal authority or when specified in a contractual relationship. To meet specialized needsand 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 adequatefor the function intended.

When this standard is used on a mandatory basis, the words shall and must indicate mandatoryrequirements; the words should or may refer to matters that are recommended or permissive, but notmandatory.

a The numbers in brackets correspond to the numbers in the bibliography in Annex B.



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vCopyright © 2011 IEEE. All rights reserved.

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.Implementers of the standard are responsible for observing or referring to the applicable regulatoryrequirements. IEEE does not, by the publication of its standards, intend to urge action that is not incompliance with applicable laws, and these documents may not be construed as doing so.

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Users of IEEE standards should be aware that these documents may be superseded at any time by theissuance of new editions or may be amended from time to time through the issuance of amendments,corrigenda, or errata. An official IEEE document at any point in time consists of the current edition of thedocument together with any amendments, corrigenda, or errata then in effect. In order to determine whethera given document is the current edition and whether it has been amended through the issuance ofamendments, corrigenda, or errata, visit the IEEE Standards Association web site athttp://ieeexplore.ieee.org/xpl/standards.jsp , or contact the IEEE at the address listed previously.

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Current interpretations can be accessed at the following URL: http://standards.ieee.org/reading/ieee/interp/index.html .

Patents

Attention is called to the possibility that implementation of this standard may require use of subject mattercovered by patent rights. By publication of this standard, no position is taken with respect to the existence



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viCopyright © 2011 IEEE. All rights reserved.

or validity of any patent rights in connection therewith. The IEEE is not responsible for identifyingEssential Patent Claims for which a license may be required, for conducting inquiries into the legal validityor scope of Patents Claims or determining whether any licensing terms or conditions provided inconnection with submission of a Letter of Assurance, if any, or in any licensing agreements are reasonableor nondiscriminatory. Users of this standard are expressly advised that determination of the validity of any

patent rights, and the risk of infringement of such rights, is entirely their own responsibility. Furtherinformation may be obtained from the IEEE Standards Association.

Participants

At the time this standard was submitted to the IEEE-SA Standards Board for approval, the Revision ofC57.12.10 Working Group had the following membership:

Gary Hoffman , ChairSaurabh Ghosh , Vice ChairJames Graham , Secretary

Don Anderegg

Javier ArteagaDonald AyersPeter BalmaStephen BeckmanThomas BeckwithEnrique BentacourtWallace BinderCarlos BittnerDonald CherryCraig ColopyFrank DamicoRonald DaubertBeth DumasEduardo GarciaCharles GarnerEverett Hager Jr.

James Harlow

David HarrisRoger HayesMartin HeathcoateRowland James Jr.Marion JaroszewskiErwin JauchSheldon KennedyStanley KostyalMichael LauGilbert LemosThomas LundquistDennis MarlowJohn MathiewsVinay MehrotraVan Nhi NguyenRay Nicholas

Gylfi Olafsson

Tony PinkDonald PlattsPaulette PowellThomas PrevostScott ReedJohn RossettiSteven SchapellStephen SchroederDevki SharmaThomas SpitzerCraig StiegemeierRaman SurbramanianRobert TillmanJane Ann VernerRichard von GemmingenPeter Zhao

The following members of the individual balloting committee voted on this standard. Balloters may havevoted for approval, disapproval, or abstention.

William J. AckermanMichael AdamsS. AggarwalSamuel AguirreSteven AlexandersonStephen AntoszI. AntweilerStan ArnotDonald AyersPeter BalmaPaul BarnhartWilliam BartleyBarry BeasterThomas BeckwithW. J. Bill BergmanSteven BeznerWallace BinderThomas Bishop

Thomas BlackburnWilliam BloetheW. BoettgerPaul BomanHarvey BowlesSteven BrockschinkKent BrownSteven BrownCarl BushDonald CashYunxiang ChenBill ChiuTommy CooperJerry CorkranWilliam DarovnyDieter DohnalGary DonnerDonald Dunn

Fred ElliottGary EngmannJoseph FoldiGeorge ForrestBruce ForsythMarcel FortinEduardo GarciaJames GardnerSaurabh GhoshJalal GohariEduardo Gomez-HennigJames GrahamWilliam GriesackerRandall GrovesBal GuptaAjit GwalJ. HarlowDavid Harris



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viiCopyright © 2011 IEEE. All rights reserved.

Robert HartgroveRoger HayesWilliam HenningSteven HensleyGary HeustonGary HoffmanR. Jackson

Erwin JauchJames JonesStephen JordanLars JuhlinC. KalraGael KennedySheldon KennedyTanuj KhandelwalEthan KimJ. Koepfinger

Neil KranichJim KulchiskySaumen KunduJohn LackeyChung-Yiu Lam

Thomas la RoseThomas LundquistRichard MarekJ. Dennis MarlowJohn W. Matthews

Lee MatthewsPhillip McClureSusan McNellyJoseph MelansonGary MichelDaleep MohlaKimberly Mosley

Jerry MurphyRaymond NicholasJoe NimsT. OlsenBansi PatelShawn PattersonJ. PattonBrian PennyHoward PenrosePaul PillitteriDonald PlattsAlvaro PortilloGustav PreiningerIulian ProfirJeffrey Ray

Jean-Christophe RiboudMichael RobertsCharles RogersJohn Rossetti

Marnie RoussellThomas RozekDinesh SankarakurupBartien SayogoLubomir SevovDevki SharmaGil Shultz

Hyeong SimJames SmithJerry SmithSteve SnyderSanjib SomBrian SparlingAllan St. PeterDavid TepenS. ThamilarasanT. TraubJoseph TumidajskiJoe UchiyamaJohn VergisJane VernerLoren Wagenaar

David WallachBarry WardKenneth WhiteJames WilsonMurty V. V. Yalla

When the IEEE-SA Standards Board approved this standard on 30 September 2010, it had the followingmembership:

Robert M. Grow, Chair Richard H. Hulett, Vice Chair

Steve M. Mills, Past Chair Judith Gorman, Secretary

Karen BartlesonVictor BermanTed BurseClint ChaplinAndy DrozdAlexander GelmanJim Hughes

Young Kyun KimJoseph L. Koepfinger*John KulickDavid J. LawHung LingOleg LogvinovTed Olsen

Ronald C. PetersenThomas PrevostJon Walter RosdahlSam SciaccaMike SeaveyCurtis SillerDon Wright

*Member Emeritus

Also included are the following nonvoting IEEE-SA Standards Board liaisons:

Satish Aggarwal, NRC RepresentativeRichard DeBlasio, DOE RepresentativeMichael Janezic, NIST Representative

Lisa Perry IEEE Standards Program Manager, Document Development

Matthew J. Ceglia IEEE Standards Program Manager, Technical Program Development



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viiiCopyright © 2011 IEEE. All rights reserved.

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



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1Copyright © 2011 IEEE. All rights reserved.


IMPORTANT NOTICE: This standard is not intended to ensure safety, security, health, orenvironmental protection. Implementers of the standard are responsible for determining appropriatesafety, 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 maybe found under the heading “Important Notice” or “Important Notices and DisclaimersConcerning IEEE Documents.” They can also be obtained on request from IEEE or viewed athttp://standards.ieee.org/IPR/disclaimers.html .

1. Overview

1.1 Scope

This voluntary consensus standard sets forth the requirements for power transformer application. Thisstandard is intended to be used as a basis for performance, interchangeability, and safety of the equipmentcovered and to assist in the proper selection of such equipment.

This document is a product standard that covers certain electrical, dimensional, and mechanicalcharacteristics of 50 Hz and 60 Hz, liquid-immersed power transformers and autotransformers. Such powertransformers 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 tothe following types of apparatus:

a) Instrument transformers

b) Step voltage and induction voltage regulators

c) Arc-furnace transformers

d) Rectifier transformers

e) Specialty transformers



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


f) Grounding transformers

g) Mobile transformers

h) Mine transformers

1.2 Mandatory requirementsWhen this standard is used on a mandatory basis, the words shall and must indicate mandatoryrequirements, 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 amandatory 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 thisdocument is explained). For dated references, only the edition cited applies. For undated referenced, thelatest 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 RelaySystems Associated with Electric Power Apparatus. 5, 6

IEEE Std C57.12.00 ™, IEEE Standard General Requirements for Liquid-Immersed Distribution, Power andRegulating Transformers.

IEEE Std C57.12.70 ™, IEEE Standard Terminal Markings and Connections for Distribution and PowerTransformers.

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 implementthe 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 avai lable from the American Socie ty 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, ruede Varembé, CH-1211, Genève 20, Switzerland/Suisse (http://www.iec.ch/). IEC publications are also available in the United Statesfrom 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.



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IEEE Std C57.19.00 ™, IEEE Standard General Requirements and Test Procedure for Power ApparatusBushings.

IEEE Std C57.19.01 ™, IEEE Standard Performance Characteristics and Dimensions for Outdoor ApparatusBushings.

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, thestandard transformer terminology in IEEE Std C57.12.80 7 shall ap ply. Other electrical terms are defined inThe 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 notexceed 65 °C when measured near the top of the tank. These kilovoltampere ratings are based on the usualtemperature 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. Whenfans and/or pumps are added to the transformer (forced cooling), its rating shall be increased by the

percentage indicated in Table 1.

7 Information on references can be found in Clause 2.8 The IEEE Standards Dictionary: Glossary of Terms & Definitions is available at http://shop.ieee.org/ .



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Table 1 — Transformer kilovoltampere rating

ONAN < 2500 kVA three-phaseONAN< 833 kVA single-phase

2500 ≤ ONAN ≤ 10 000 kVA three-phase833 ≤ ONAN ≤ 3333 kVA single-phase

ONAN > 10 000 kVA three-phaseONAN > 3333 kVA single-phase

ONANForced cooling

ONANForced cooling

ONANForced cooling

1st stage 2nd stage 1st stage 2nd stage 1st stage 2nd stage

100% 115% N/A 100% 125% N/A 100% 133% 167%

For a transformer without a self-cooled rating, the applicable multiplying factor given in Table 20 ofIEEE Std C57.12.00-2006 shall be applied to the maximum nameplate kilovoltampere rating to obtain theequivalent base kilovoltampere rating.

Typical transformers ratings are given in Table 2. Actual ratings shall be mutually agreed between the userand manufacturer.

In transformers with concentric winding arrangement, two or more separate windings may be situated oneabove the other. In this case, the average winding temperature rise limit shall apply to the average of theindividual readings for the stacked windings if they are of equal size and kilovoltampere rating and similardesign. If they are not, the evaluation should be subject to agreement between the user and themanufacturer. For all rated loading conditions that are evaluated, a hot spot temperature rise limit of 80 °Cshall apply to all windings.

Table 2 — Typical transformer kilovoltampere rating

Single-phase transformers Three-phase transformers

ONANForced cooling

ONANForced cooling

1st stage 1st stage 2nd stage

833 1041 750 862 —

1250 1562 1000 1150 —

1667 2084 1500 1725 — 2500 3125 2000 2300 —

3333 4167 2500 3125 —

5000 6250 3750 4688 —

6667 8333 5000 6250 —

8333 10 417 7500 9375 —

— — 10 000 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 62 500 — — 50 000 66 667 83 333

— — 60 000 80 000 100 000

An autotransformer with a tertiary winding for external loading has no standard basis for megavoltampererating. All simultaneous loading conditions including megavoltampere rating and power factor shall bespecified by the user.



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An equivalent two-winding kilovoltampere rating of an autotransformer is the rated power of the auto-connected 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 anequivalent 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 voltagesdefined 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 ofIEEE 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 kilovoltamperetaps 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 approximately0.625% steps, with 16 steps above and 16 steps below rated low voltage. The transformer shall be capableof delivering rated kilovoltamperes at the rated low-voltage position and on all positions above rated lowvoltage. The transformer shall be capable of delivering low-voltage current corresponding to rated lowvoltage 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 low-voltage winding. This application may make the transformer operate with variable flux voltage operationwhen the tap positions are changed. Annex A indicates the effect in the transformer operation during thiscondition and other variations.



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When required by the user, the transformer may be designed to deliver rated kilovoltampere output on alltap 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 thelower 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 autotransformerimpedance.

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.



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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 ratedvoltage shall not be greater than the value of the total tap voltage range when expressed as a percentage ofthe 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 to105 °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.



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HIGH-VOLTAGE COVER BUSHINGS

SEGMENT 3

SEGMENT 1

LOW-VOLTAGE COVER BUSHINGS

S E G M E N T 2

S E G M E N T 4

CL

CL

†When furnished.

Accessories Locations Clauseref.

DETC operating handle S1, S4, seeClause ref.

Table 4

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

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

5.1 AccessoriesTable 4 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



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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 Tanks S S

5.8.3.2 Bolted Cover A A

5.9 Auxiliary Cooling Equipment A A

5.9.1 Controls for Auxiliary Cooling Equipment A A

5.9.2 Fans A A5.9.2.2 Future Forced-Air Cooling A A

5.9.3 Pumps A A5.10 Auxiliary Equipment Power Supply A A

5.11 Terminal Board A A5.12 Junction Box A A

5.12.1 High Voltage A A5.12.2 Low Voltage A A

5.13 Disconnecting Switches A A

5.13.1 High-Voltage Terminal Chamber A A

5.13.2 Low-Volta e Terminal Chamber A A5.14 Throat Connection A A

5.14.1 Hi h-Volta e Throat A A5.14.2 Low-Voltage Throat A A5.15 Current Transformers

5.15.1 Bushing Type Current Transformer A A

0 Terminal Blocks A A

5.16 Surge Arresters A A

5.17 Other Insulating Liquid A A

6 LTC Equipment −

6.1 LTC − S

6.2 Tap Selection Switch − S

6.3 Motor and Drive Mechanism − S

6.4 Position Indicator − S

6.5 Control Equipment and Accessories − SA.2 Transformer Paralleling − A

NOTE: “S” indicates “standard”, “A” indicates “available when specified.”



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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 inSegment 1 or 4 at a height convenient for operators to safely change the taps. If the user requires operationfrom ground level, then the height should not exceed 2 m (79 in). If for design reasons it cannot be locatedin 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 insequence. The letter “A” or the Arabic numeral “1” shall be assigned to the voltage rating providing themaximum 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 diameterof 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) orless 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 orMAX-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 indicationon the nameplate of the distance from the liquid level to the highest point of the handhole or manholeflange 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 operatinglevel 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 bereadable to a person standing at the level of the base. Gauges, when required to have operating controls ontheir cases, shall be mounted between 1.22 m (4 ft) and 1.83 m (6 ft) above the base. The minimum scalerange 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-liquidtemperature. 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.



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The gauge’s dial shall have an analog (dial and pointer) or alphanumeric display that is readable in low andhigh ambient light conditions. The measurement title “Liquid Temperature” shall be marked or displayedon 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 markingson 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 peaktemperature pointer shall be orange-red and shall have a provision for resetting without opening any coversor windows.

For digital indicators, the measurement title may be shown on an alphanumeric display separately from themeasured 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 haveindependent field-adjustable set-point values. Each of the three relay contacts shall provide the ability toturn on a cooling stage, alarm, or actuate another relay or contactor. See 5.9.1.1 and 5.9.1.2 for switchesand relays or contactors that allow for redundant manual control of cooling equipment. The alarm contactsshall 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 bereadable to a person standing at the level of the base. Gauges, when required to have operating controls ontheir cases, shall be mounted between 1.22 m (4 ft) and 1.83 m (6 ft) above the base. The minimum scalerange shall be 0 to 180 °C.

The winding temperature indicator shall use direct-measurement, simulated or calculated methods todetermine winding hottest spot temperature. Depending on the type of technology, the gauge may requirethat the transformer be equipped with a heated thermowell, a load current signal from a bushing currenttransformer, 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 closedwell at a suitable level to indicate the top-liquid temperature. When a heated thermowell is required, thetank wall shall be ported to accept the specified heater. The well shall be positioned so that it is covered byat least 2.5 cm (1 in) of fluid at the lowest permissible fluid level. For dimensions of the well, seeIEEE Std C57.12.00.

The thermometer’s dial shall have an analog (dial and pointer) or alphanumeric display that is readable inlow and high ambient light conditions. The measurement title “Winding Temperature” shall be marked ordisplayed 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 markingson 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 peaktemperature pointer shall be orange-red and shall have a provision for resetting without opening any coversor windows.

For digital indicators, the measurement title may be shown on an alphanumeric display separately from themeasured value or marked on the dial or a nameplate mounted adjacent to the gauge. A method of



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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 haveindependent field-adjustable set-point values. Each of the three relay contacts shall provide the ability toturn 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 adark-face dial with light-colored markings and a light-colored pointer, and it shall be located either inSegment 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 andnegative) 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 reliefrating of 142 m 3/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 valveshall 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 thevalve 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 theuser’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 ofthe 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.



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5.1.9 Sudden pressure relay

When specified, a sudden pressure relay shall be provided for the indication of transformer faults and tominimize 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 fluidrelays 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 to3.0 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 with5.1.10 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 towhich they are connected.

The insulation level of the low-voltage neutral bushing having a grounded Y-connected low-voltagewinding shall be the same as that of the low-voltage line bushings for windings 25 kV and below. Forwindings 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 inIEEE Std C57.19.00 and IEEE Std C57.19.01 where applicable.



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Figure 2 — Contact wiring and wire color coding

H 1 H 2 H 3

21 XX 3X

H OR X 0 WHEN REQUIRED0

NOTE—For single-phase transformers, omit H3, X3, and neutral bushings.

Figure 3 — Bushing arrangement



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5.2.1 Neutral terminations

Four cover bushings shall be provided for each permanently connected Y-winding on three-phasetransformers.

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 thefollowing:

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. Afully insulated neutral shall be brought to a terminal board for isolated neutral operation of thetransformer.

d) High-voltage windings of transformers with a Y- Δ terminal board supplied in accordance with 5.11item 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- ∆ 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 isconnected

5.2.1.3 Constructions for neutral terminations

Neutral terminations, when furnished, shall be provided on the cover or in the junction box, terminalchamber, 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 isthe ratio of the ultimate stress of the material used to the working stress. The working stress is the



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maximum combined stress developed in the lifting facilities by the static load of the component beinglifted. This factor does not apply to pulling facilities since the unit is not suspended. For pulling, a safetyfactor 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 coilassembly 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 ofspreaders or a lifting beam may be involved.) The bearing surfaces of the lifting facilities shall be free fromsharp 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 permitrolling 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 forshipment 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 ofSegments 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 angleof approximately 45°. A flat bottom base with material thickness of less than 12.7 mm (0.5 in) does notrequire 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 ofSegments 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.



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JACKINGPOINT

TANKBASEPLATE

GGB

A

E

TANKSIDE-WALL

LOF JACK

CLEARANC

TUBES,ETC

FLOORLINE

FF

Weight15,900 kg (35,000 lb) or less

Weight 15,900–29,500 kg(35,000–65,000 lb)

Weight over29,500 kg (65,000 lb)

Dim. (mm) (in) Dim. (mm) (in) Dim. (mm) (in)

A 88.9 3.5 A 127.0 5.0 A 457.0 18B 63.5 2.5 B 63.5 2.5 B 102.0 4E 686.0 27.0 E 686.0 27.0 E 508.0 20F 127.0 5.0 F 127.0 5.0 F 127.0 5G 76.2 3.0 G 76.2 3.0 G 76.2 3H 127.0 5.0 H 127.0 5.0 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. Itshall be located in Segment 1 near the centerline and near eye level. It may be located in Segment 2 whenLTC equipment is located in Segment 2.

For LTC transformers, the phrases “LTC transformer” or “LTC autotransformer” shall be used instead ofthe word “transformer.”

Voltage and current ratings shall be given as follows:

0 to 99.9 to nearest 0.1

100 to 999 to nearest 11 000 to 9 999 to nearest 5

10 000 to 99 999 to nearest 10100 000 and greater to nearest 25



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5.5 Ground pads

A tank-grounding pad shall consist of a copper-faced steel pad or a stainless-steel pad without copperfacing, 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 (asdefined 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, theground 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 3and 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-phasetransformers with Δ -Δ connections shall be 0°. The angular displacement between high-voltage and low-voltage terminal voltages of three-phase transformers with Y- Δ or Δ -Y connections shall be 30°, with thelow 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 low-voltage bushing arrangements shall be as shown in Figure 3.



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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 ofthe 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 sothat the internal gas pressure does not exceed 69 kPa (10 lb/in²) gauge positive or 55 kPa (8 lb/in²) gaugenegative.

H3

H2

H1

H3

H2

H1

X1

X2

X3

X0

X2

X1X3

H0

H3

H2

H1

X3

X2

X1



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The system shall include a pressure-vacuum gage indicated in 5.1.5 and a pressure-vacuum bleeder valveindicated in 5.1.6. The use of this system may result in the introduction of oxygen and moisture into thetransformer 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 aseparate inert-gas source and reducing valve system so that the interior of the transformer shall be sealedfrom 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 filledwith liquid and connected to the completely filled main tank, seals the oil in the main tank from theatmosphere. The internal top oil pressure in the main tank shall not exceed 34 kPa (5 lb/in²) gauge. Thesystem 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 ofliquid 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 thedrain 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 inthe conservator tank shall be provided. The dehydrating breather shall be filled with silica gel that absorbs20% 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 theconservator tank in order to monitor the gas and liquid movements. During normal operation, the relay iscompletely filled with liquid to keep its internal float in their top limit or rest position. The accumulation ofgases in the float gas chamber causes the float to actuate an electrical contact system. The insidemechanism shall comprise upper and lower contact systems for alarms and tripping positions.



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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 withthe additional following accessories:

5.7.4.1 DiaphragmThe interior of the conservator tank shall have a rubber air cell to isolate the transformer liquid and the airand 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 5.7.3are not exceeded and to compensate the liquid volume displacement due to the temperature variationsspecified.

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 theliquid 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 airside of the conservator tank shall be provided so liquid filling of the conservator tank can occur undervacuum while the pressures between both sides are equalized. The size of the valve shall be determined bythe manufacturer.

5.8 Tanks

5.8.1 Pressure design

Maximum operating pressures (positive and negative) for which the transformer is designed shall beindicated 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



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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, thesecomponents shall be located in a compartment sealed so it prevents transfer of liquid to any othercompartment 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 indicatorif the winding temperature is not supplied, or both if specified. The relay shall be mounted inside thecabinet.

5.9.1.1 Control by the liquid temperature indicator

The equipment for automatic control of auxiliary cooling equipment controlled from the liquid temperatureindicator 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 than1.52 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-cooledONAN rating, the equipment for automatic control of auxiliary cooling equipment for transformerscontrolled 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 Function

1 Supply power to first-bank cooling

2 Supply power to second-bank cooling

3 Initiate alarm or actuate relay

b) A manually operable switch connected in parallel with the automatic control contacts and enclosedin a weather-resistant cabinet located on the side of the tank in Segment 1 at a height not greaterthan 1.52 m (60 in) above the base.



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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 for60 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 controlof 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 controlof 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 centrifugalswitch, 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 controlsThe 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 inaccordance with ANSI C84.1.



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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 theappropriate rating table.

b) A Y- Δ terminal board that provides angular displacements as shown in Figure 5 for transformerswith three-phase windings of 110 kV BIL (15 kV nominal system voltage) or less. The otherwinding 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 kVnominal 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 ofthese 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 thecable connection to the windings. (See 5.2.1.3 when neutral termination is required.)


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.



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5.14 Throat connection

When specified, a throat connection or connections shall be provided. (See 5.2.1.3 when neutraltermination is required.)


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 withIEEE 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 currenttransformers 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 thecurrent 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. Inaddition to this information, the manufacture of these current transformers shall specify the accuracy atspecified burdens at all available taps as specified by IEEE Std C57.13.



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Table 5 — Recommended accuracy classification of bushing current transformers

Bushinginsulation class

(kV)

Bushing currenttransformer ratio

Revenue meteringaccuracy class at full

winding ratio

Relay accuracyclass at full

winding ratio

46 and below 600:51200:5, 2000:5, 3000:5

4000:5 and higher

.3B-0.9

.3B-1.8

.3B-1.8

C200C400

C80069 600:5

1200:52000:5 and higher

.3B-0.9

.3B-1.8

.3B-1.8

C200C400C800

Above 69 600:51200:5 and higher

.3B-1.8

.3B-1.8C400C800

5.15.2 Terminal blocks

A nonsplit terminal block shall be provided in a weatherproof enclosure, located near the transformer basein 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 ismounted near the top of the tank and that may be specified for each set of arresters—except thatindividual 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) throughitem 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 ofthe 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 ofwhich 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 expansionspace) shall be supplied in such a way that they in themselves do not limit the loading to less than thecapability of the windings.



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NOTE—IEEE Std C57.91 provides the best known general information for loading transformers under variousconditions based on typical winding insulation systems and is based upon the best engineering information available atthe time of preparation. It discusses “limitations” of ancillary components other than windings that may limit thecapability of transformers to meet its guidelines.

5.19 “Other” testsWhen 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 diverterswitch 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 thetransformer. 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 followingtechnologies:

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 liquidin the transformer main tank. All covers shall have handles and be removable; covers weighingmore than 20 kg (44 lb) shall additionally be hinged.

b) A drain valve located in each liquid-filled compartment to provide maximal drainage. The valveshall be 1 in NPT in accordance with ASME B1.20.1, with a pipe plug in the open end. The drainvalve shall have a built-in 0.375 in sampling device located on the side of the valve between themain valve seat and the pipe plug. The device shall be supplied with a 5/16-32 in male thread forthe user’s connection and shall be equipped with a cap.



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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-filledcompartment. For details, see 5.1.2.

e) For arcing tap switch and tap selector with arcing switch technologies only: provision for venting tothe atmosphere of gases produced by the arcing.

f) Provisions for liquid temperature, pressure relief device, oil filtration for arcing-in-oil LTCs, onlinedissolved-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 or230/400 V, 50 Hz, three-wire source. When specified, a lightning surge arrester shall be providedfor 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, single-

phase, 60 Hz, with maximum voltage to ground at 60 Hz not to exceed 150 V. This power sourceshall be provided by the user and shall be separate from the transformer. In some cases, the usermay 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 electricalinterlock shall be provided to prevent LTC operation by the motor drive while the manual means isengaged. 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 meansfor displaying the past maximum and minimum operating tap position of the LTC. An operator with accessto 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 byhand.

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 acircular dial and shall be indicated by the letter “N.”



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b) The raise range (output voltage is greater than the rated low-voltage) shall be located clockwisefrom the “N” position. A single letter “R” (Raise) shall be located in the right half with an arrowindicating the direction of raise. The 16 tap positions in the raise range shall be marked, and anumber 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 halfwith an arrow indicating the direction of lower. The 16 tap positions in the lower range shall bemarked, and a number shall appear opposite at least every fourth position. Number 16 shall be thelowest voltage position.

N

4

8

12

16

4

8

12

16

L R

NOTE—This figure is intended to present a schematic rather thana 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 providedunless 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. Thecabinet shall be equipped with breather, hinged doors, and provision for entrance of up to three 1.5 inconduits 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).



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Space shall be provided for mounting the control equipment required for parallel operation by the methodspecified 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 forliquid level and temperature indicators and for current transformer secondaries (two leads per currenttransformer) 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 thisinformation to conditions desired by the operator, and automatically command the LTC to hold theoutput 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 than0.2 A to the control circuits when the transformer is operating at the maximum continuous current forwhich 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 ofthe control a current that is nominally in phase with the current at the X1 load terminal of thetransformer.

b) For a Δ -connected winding, the current transformers shall deliver to the line drop compensator ofthe control a current that is nominally in phase with a phasor derived from the relationship of thecurrent 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 thecurrent transformers provided with the transformer.



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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 fromzero 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 raiseor 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-voltage-supply 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 tothe 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 beadjustable in the range of at least –24 V to +24 V. The voltage refers to line drop compensation atthe 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 controlsystem:

a) Test terminals for measuring voltage proportional to transformer output voltage. The test terminalvoltage 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 thespecification 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.



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f) A screw-base lamp socket with a switch and a ground-fault-protected convenience outlet for a120 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 foroperation in parallel with another LTC transformer. Several alternative means are available for suchapplications. The user shall specify the procedure to be used. The more commonly used procedures areindicated 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 basedon the combined performance of the control device and sensing apparatus including instrument current andvoltage transformers, utility windings, transducers, etc., with the voltage and current input signals of asinusoidal wave shape.

Since it is not practical to test the overall control system accuracy, it is permissible to individually test thecontrol system components. The accuracy of individual components is then combined to arrive at theoverall 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 toremove 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 of3.5 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 resultin differences in the sensed average or root-mean-square (RMS) magnitude that affects the overall accuracy of thecontrol device and control system. Such differences may be inherent in the product design and do not constitute anadditional 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 controldevice and is determined separately with the other parameters held constant. Errors causing the control



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device to hold a higher voltage level than the reference value are plus errors and those causing a lowervoltage level are minus errors. The overall error of the control device is the sum of the individual errors asseparately determined causing a divergence from the voltage level setting, presuming a bandwidth of zerovolts.

6.5.5.2.2 Factors for accuracy determination of control deviceThe greater magnitude of the sum of the positive or negative errors of the following three areas shallconstitute 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 reactancecompensation 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 resistancecompensation 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 describedin 6.5.5.2.2. The voltage and current sources applied shall be as free of harmonics or other distortions as thetest 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 thecontrol 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 atemperature range of –30 °C to 65 °C in not more than 20 °C temperature increments. The airtemperature surrounding the control device shall be held constant and uniform within ± 1 °C ofeach increment for a period of not less than one hour before taking a test reading. Tests are made atrated 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 sufficientrange 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 dropcompensation circuit.



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c) Tests for errors in voltage level due to line drop compensation: Four tests shall be made at ratedfrequency and a constant temperature of 25 °C and a voltage level setting of 120 V. Determine thevoltage level required to balance the control with 0.2 A in the compensator circuit of the controlunder 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 whensummed 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 settingsare 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 therange 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 themarked 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, andthe marked value of any setting of either the resistance or reactance element of the compensator, expressedas 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 andafter the test. Refer to IEEE Std C37.90.1.



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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, itshall 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.



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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 beyondthe 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 discussto ensure that the transformer design and application are properly coordinated for high-voltage LTCapplications.

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 theLTC 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 theregulated winding (typically the low-voltage winding) while the unregulated winding (typically the high-voltage winding) turns are constant. The high-voltage system voltage is relatively constant; therefore, fluxdensity of the transformer is also relatively constant, impedance and sound levels are constant, and stepvoltage is also constant with step voltage tolerances according to IEEE Std C57.12.00. For a CFVV tapchanger to increase the low voltage, turns are added to the low-voltage winding by operating the LTC in theraise direction.

A.1.2 VFVV LTC regulation

VFVV LTC operation regulates the transformer winding (typically the low-voltage winding) by increasingor decreasing the turns in the unregulated winding (typically the high-voltage winding) while the regulatedwinding turns are constant. The high-voltage system voltage is relatively constant; therefore, flux density ofthe 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 thehigh-voltage winding. As the volts per turn are increased—and, therefore, the low-voltage winding voltageis increased, the flux density is also increased, the transformer sound level increases until reaching amaximum 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 smallvariations 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 inTable A.1.



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In some cases, to regulate low voltage in two- and three-winding transformers, the LTC is installed in thehigh-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 high-voltage windings. As a result, cost and losses are increased. With VFVV operation, the regulating windingis typically located over the high-voltage winding, and the mean turns of the low-voltage and high-voltagewindings are smaller and should result in lower losses. A second benefit is that the winding currents aresignificantly lower in the high-voltage winding, and this level allows smaller conductors for the regulatingwinding and eliminates the necessity of a booster/series transformer if the coil currents are less than therating of the LTC.

Table A.1 — VFVV cases

DesignWinding connection

High-voltageDETC

Low-voltageLTC

requirements

Variable flux/variable voltage

solutionBenefits

Highvoltage

Lowvoltage

Two- and three-winding transformers

1 Δ GroundedY

± 2 at 2.5% ± 10% LTC ± 15% in high-voltage Δ

Reduced lossesReduced costEliminate high cost

of DETC

2 GroundedY

GroundedY

± 2 at 2.5% ± 10% LTC ± 10% in high-voltage neutral

Reduced lossesReduced cost

3 GroundedY

GroundedY

± 2 at 2.5% ± 10% LTC ± 15% in high-voltage neutral

Reduced lossesReduced costEliminate high cost

of DETC

Autotransformers

1 GroundedY

GroundedY

± 2 at 2.5% ± 10% LTC ± 10% in neutral Reduced lossesReduced costEliminate high cost

of DETC

2 GroundedY

GroundedY

± 2 at 2.5% ± 10% LTC ± 10% in neutral Reduced lossesReduced costEliminate high cost

of DETC

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 (highestvoltage 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.



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g) Operating the high-voltage LTC in the raise direction reduces the low voltage; operating the high-voltage LTC in the lower direction increases the low voltage.

h) Placing the regulating winding outside the high-voltage winding can increase the induced transientvoltages 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, theDETC and LTC are combined into one, and this design incorporates the DETC range into the high-voltageLTC. Since the DETC needs to be installed in the center of the high-voltage Δ -connected winding and theLTC should also be installed in the center of the high-voltage winding, a combination is usually the bestsolution for cost and technical considerations.

For design 2, the same benefits generally occur, but it is possible to use another regulating winding tosupply the DETC separately from the LTC. For design 3, the DETC tap range is combined into the range ofthe LTC, and this design eliminates the cost of the DETC and its installation on the transformer core andcoils.

Autotransformers are unique. Instead of a separate high-voltage and low-voltage winding, the high voltageand low voltage share the turns of the common winding, and the high-voltage winding is the only windingusing 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 sharethe turns being varied in the common winding. Therefore, it also affects the high-voltage winding turns, thehighest flux density and sound level occur at the lowest tap position (16L), and the impedance increases asthe 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 kVsecondary voltages, single-phase in-tank LTCs may be required, and the cost will be significantly higherthan 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 tertiarywinding voltage. If the tertiary is brought out for station service, capacitive, or reactance loading, specialLTC 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 transformersoperating 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 transformersregardless of planned system configuration changes or breaker operations that would alter the

parallel connection or operation of the transformers.



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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 powerfactor).

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 aretypically 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 theirtapping ranges).

All methods, albeit sometimes involving special auxiliary apparatus, can be used for parallelingtransformers 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, thedesired operation objectives are met by maintaining the same turns ratio on all paralleled transformers. Thisoperation is usually accomplished by maintaining the same physical tap position. The operation consists ofone active master control commanding the tap changes of additional transformers to follow. A tapoperation and position feedback scheme is mandatory to confirm to the master unit that the following unithas operated properly. If that feedback is not received, the controls usually are set to lock out all furtheroperations. 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 systemoperating configurations, any circulating current between transformers is representative of the disparity oftap positions between the paralleled transformers. Variations in load currents do not affect the correctoperation of the circulating current method in minimizing circulating current. This method derives themagnitude of the circulating current between transformers using external balancing modules andcommunications between controls. The circulating current method causes any circulating current betweenthe transformers to bias the set points in opposite directions and thus cause subsequent tap operations to bein the direction to reduce the circulating current. This operation is achieved while maintaining the voltageset point accuracy. This method should not be used when the transformers are, or may be, with primarycircuit 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 VARloading is made up of the following:

a) Its share of the total load VARs as determined by its relative impedance and megavoltamperecapacity compared to the other transformers.

b) The circulating VARs due to differences in secondary induced voltages.



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In some methods, the objective is to reduce the circulating VARs to a minimum. In other methods, theobjective is for the transformers’ VARs to be shared in proportion to their megavoltampere capacities. TheVAR 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 basisof operation is to recognize the disparity of power factor, rather than circulating amperes, as recognized byeach 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 samevoltage applied, the relative angle of the transformer currents will indicate the relative power factors. Thetransformer 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 theextent that, from the perspective of each transformer, equalizing the power factor is equivalent toequalizing the VARs, some of the attributes of the VAR control method are realized with the power factormethod.

A.2.6 Reverse reactance

The reverse reactance paralleling method uses a biasing voltage with X and R components in a somewhatsimilar 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 transformerimpedance. If all parallel transformers have the same induced (no-load) voltage, there will be no circulatingcurrent. The R setting compensates for the transformer impedance drop so that in fact the load bus voltageis 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 beexperienced, 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 eachtransformer’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 thesummated load current, and this input is the only interconnection required between transformers in thiscase. Thus, the use of line drop compensation obviates a principal advantage, that of no required controlinterconnections. Second, the X and R settings are optimized for the average power factor at typical load. Ifthere are large variations in load power factor, such as might occur with crude power factor correctionsystems 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 duringthe 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 thesemethods has additional considerations for settings, field commissioning, and troubleshooting, depending onspecific equipment or system characteristics.



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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 thetransformer voltage for controlling the LTC tap position. It is important to address the issues listed in A.3.1and 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 thetransformer voltage for controlling the LTC tap position. With a high-voltage LTC, it is possible to get asituation where the manufacturer believes that the user will have the voltage transformer on the highvoltage to control the LTC position. But if the user locates the voltage transformer on the low voltage in astep-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 withvariable 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 impedancevalue to specify. The manufacturer must consider the lowest impedance value for the transformer short-circuit design.

A.3.2.2 Sound

VFVV (variable-volts-per-turn) transformers have increasing sound levels from the neutral tap to thehighest tap (16R). The manufacturer and the user must understand what the sound level limits should be forall 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 thelow voltage (e.g., 1/1.1 does not equal 0.9). An increase of the number of high-voltage winding turns by10% causes a reduction in the low-voltage winding voltage of 9.09%. A decrease in the number of high-voltage winding turns by 10% causes a low-voltage winding voltage increase of 11.11%. In such anapplication, the manufacturer and the user need to come to an agreement on what the actual output voltagesshall 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-voltagevoltage be shown changing with tap position? What voltages should be shown?



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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-voltagewinding when the LTC is located in the high voltage.

A.3.2.6 Over-excitation capabilityThe 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 needsto maintain the same 110% rule at the worst-case tap position. Test procedures should also be considered toverify 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 sameside 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 cabinetoperator has a line of sight to the LTC switch. Also then, the gauges, the nameplate, and other accessorieswould usually be located near the control cabinet. The manufacturer and the user need to agree on thelocation 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 voltagetransformer. For a low-voltage Y-connection with LTC, that location would require that the line-drop-compensating current transformer be located on the X1 bushing. When the LTC is in the high voltage, thatstandard location may not be appropriate. The key issues to determine the proper current transformerlocation 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 high-voltage voltage, a full kilovoltampere rating for all taps is generally appropriate. When a high-voltage LTCis used to control the low-voltage voltage, then full capacity is generally unnecessary. The transformer usershould 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 followingissues:

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 whileminimizing circulating current



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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 inFigure 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 is per-unit impedance of transformers A and B @ stated megavoltampere baseIA, IB is per-unit load current of transformers A and BIL 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 theratio of their separate impedances calculated at the same base megavoltamperes:

Per-unit loads: B A

B A

Z Z Z

I +

= and B A

A B

Z Z Z

I +

=



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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 attheir base megavoltamperes are equivalent (Case II).

c) If transformers of different impedances are paralleled, the total capacity of the transformersconnected 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 transformersconnected 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 theimpedances of the two banks in parallel.

Bank A share = Bank B share =)08.008.0(

08.0+

=16.008.0

= 0.50 per-unit load

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 thetransformer 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


Bank A: ZA = 0.16 @ 24 MVA base

Bank B: ZB = 0.08 @ 24 MVA base


Bank A share =)16.008.0(

08.0+

=24.016.0




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Bank B share =)16.008.0(

16.0+

=24.016.0


NOTE—When connected in parallel, the total bank rating is 60 MVA; Bank A carries 0.33 per unit (20 MVA); Bank Bcarries 0.67 per unit (40 MVA); neither unit is loaded in excess of its nameplate rating; and the bank capacity equalsthe 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 isONAN/ONAF/ONAF) and both have identical impedances on their self-cooled bases, each share load according to itsrating 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



Bank B: ZB = 0.13 @ 20 MVA base


Bank A share =)13.016.0(

13.0+

=29.013.0


Bank B share =)13.016.0(

16.0+

=29.016.0


Maximum total load without exceeding the nameplate rating of Transformer A =45.05.12

= 27.8 MVA.

NOTE—When connected in parallel, the total bank rating is 32.5 MVA. Limiting the loading of Bank A to itsnameplate 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 ofits 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 isONAN/ONAF/ONAF), it is possible to specify an impedance that permits each transformer to share load according toits 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 thesubstation 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



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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 = MVA MVA5.325.12



Bank A share =)16.0( X

X +


Bank B share =)16.0(

16.0

X + = 0.615 per-unit load

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 the12/16/20 MVA at 6.0% @ 12 MVA base permits parallel operation to the full capacity of both transformers withoutexceeding 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 LTCapplication used when the tertiary voltage (TV) (tertiary winding) is buried for harmonicsuppression or there is no tertiary winding to be affected by the variation in flux density. Unliketwo- and three-winding VVFV LTCs, the LTC affects both the common and series windings; theminimum flux density is at tap position 16R; the maximum flux density is at 16L; and impedancevariation 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 thehigh-voltage line to the H0X0 at the neutral end of the common winding remain constant regardlessof the LTC position.

c) The ± LTC is located in the end of the series winding connection to the common winding and low-voltage 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



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Annex B

(informative)

Bibliography

[B1] IEEE Std C57.12.36 ™, IEEE Standard Requirements for Liquid-Immersed Distribution SubstationTransformers. 9, 10

[B2] IEEE Std C57.12.90 ™, IEEE Standard Test Code for Liquid-Immersed Distribution, Power, andRegulating Transformers.

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