C57.12.59-2015 - Ieee Guide For Dry-type Transformer Through-fault Current Duration

IEEE Guide for Dry-Type Transformer Through-Fault Current Duration

IEEE Power and Energy Society

Sponsored by the Transformers Committee

IEEE 3 Park Avenue New York, NY 10016-5997 USA

IEEE Std C57.12.59™-2015

(Revision of IEEE Std C57.12.59-2001)

Authorized licensed use limited to: Chinese University of Hong Kong. Downloaded on December 07,2015 at 21:41:04 UTC from IEEE Xplore. Restrictions apply.

IEEE Std C57.12.59™-2015

(Revision of IEEE Std C57.12.59-2001)

IEEE Guide for Dry-Type Transformer Through-Fault Current Duration Sponsor

Transformers Committee IEEE Power and Energy Society Approved 3 September 2015

IEEE-SA Standards Board

Authorized licensed use limited to: Chinese University of Hong Kong. Downloaded on December 07,2015 at 21:41:04 UTC from IEEE Xplore. Restrictions apply.

Abstract: Recommendations believed essential for the application of overcurrent protective devices that limit the exposure time of dry-type transformers to short-circuit currents are set forth in this guide. This guide is not intended to imply overload capability. Keywords: dry-type transformers, fault current, IEEE C57.12.59™, mechanical damage, mechanical duty, normal base current, short-circuit current, short-circuit impedance, thermal damage

The Institute of Electrical and Electronics Engineers, Inc. 3 Park Avenue, New York, NY 10016-5997, USA Copyright © 2015 by The Institute of Electrical and Electronics Engineers, Inc. All rights reserved. Published 6 November 2015. Printed in the United States of America. IEEE is a registered trademark in the U.S. Patent & Trademark Office, owned by The Institute of Electrical and Electronics Engineers, Incorporated. PDF: Print:

ISBN 978-0-7381-9867-5 ISBN 978-0-7381-9868-2

STD20338 STDPD20338

IEEE prohibits discrimination, harassment, and bullying. For more information, visit http://www.ieee.org/web/aboutus/whatis/policies/p9-26.html. No part of this publication may be reproduced in any form, in an electronic retrieval system or otherwise, without the prior written permission of the publisher.

Authorized licensed use limited to: Chinese University of Hong Kong. Downloaded on December 07,2015 at 21:41:04 UTC from IEEE Xplore. Restrictions apply.

Important Notices and Disclaimers Concerning IEEE Standards Documents IEEE documents are made available for use subject to important notices and legal disclaimers. These notices and disclaimers, or a reference to this page, appear in all standards and may be found under the heading “Important Notice” or “Important Notices and Disclaimers Concerning IEEE Standards Documents.”

Notice and Disclaimer of Liability Concerning the Use of IEEE Standards Documents IEEE Standards documents (standards, recommended practices, and guides), both full-use and trial-use, are developed within IEEE Societies and the Standards Coordinating Committees of the IEEE Standards Association (“IEEE-SA”) Standards Board. IEEE (“the Institute”) develops its standards through a consensus development process, approved by the American National Standards Institute (“ANSI”), which brings together volunteers representing varied viewpoints and interests to achieve the final product. Volunteers are not necessarily members of the Institute and participate without compensation from IEEE. While IEEE administers the process and establishes rules to promote fairness in the consensus development process, IEEE does not independently evaluate, test, or verify the accuracy of any of the information or the soundness of any judgments contained in its standards. IEEE does not warrant or represent the accuracy or content of the material contained in its standards, and expressly disclaims all warranties (express, implied and statutory) not included in this or any other document relating to the standard, including, but not limited to, the warranties of: merchantability; fitness for a particular purpose; non-infringement; and quality, accuracy, effectiveness, currency, or completeness of material. In addition, IEEE disclaims any and all conditions relating to: results; and workmanlike effort. IEEE standards documents are supplied “AS IS” and “WITH ALL FAULTS.” Use of an IEEE standard is wholly voluntary. The existence of an IEEE standard does not imply that there are no other ways to produce, test, measure, purchase, market, or provide other goods and services related to the scope of the IEEE standard. Furthermore, the viewpoint expressed at the time a standard is approved and issued is subject to change brought about through developments in the state of the art and comments received from users of the standard. In publishing and making its standards available, IEEE is not suggesting or rendering professional or other services for, or on behalf of, any person or entity nor is IEEE undertaking to perform any duty owed by any other person or entity to another. Any person utilizing any IEEE Standards document, should rely upon his or her own independent judgment in the exercise of reasonable care in any given circumstances or, as appropriate, seek the advice of a competent professional in determining the appropriateness of a given IEEE standard. IN NO EVENT SHALL IEEE BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO: PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE PUBLICATION, USE OF, OR RELIANCE UPON ANY STANDARD, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE AND REGARDLESS OF WHETHER SUCH DAMAGE WAS FORESEEABLE.

Translations The IEEE consensus development process involves the review of documents in English only. In the event that an IEEE standard is translated, only the English version published by IEEE should be considered the approved IEEE standard.

Authorized licensed use limited to: Chinese University of Hong Kong. Downloaded on December 07,2015 at 21:41:04 UTC from IEEE Xplore. Restrictions apply.

Official statements A statement, written or oral, that is not processed in accordance with the IEEE-SA Standards Board Operations Manual shall not be considered or inferred to be the official position of IEEE or any of its committees and shall not be considered to be, or be relied upon as, a formal position of IEEE. At lectures, symposia, seminars, or educational courses, an individual presenting information on IEEE standards shall make it clear that his or her views should be considered the personal views of that individual rather than the formal position of IEEE.

Comments on standards Comments for revision of IEEE Standards documents are welcome from any interested party, regardless of membership affiliation with IEEE. However, IEEE does not provide consulting information or advice pertaining to IEEE Standards documents. Suggestions for changes in documents should be in the form of a proposed change of text, together with appropriate supporting comments. Since IEEE standards represent a consensus of concerned interests, it is important that any responses to comments and questions also receive the concurrence of a balance of interests. For this reason, IEEE and the members of its societies and Standards Coordinating Committees are not able to provide an instant response to comments or questions except in those cases where the matter has previously been addressed. For the same reason, IEEE does not respond to interpretation requests. Any person who would like to participate in revisions to an IEEE standard is welcome to join the relevant IEEE working group. Comments on standards should be submitted to the following address: Secretary, IEEE-SA Standards Board 445 Hoes Lane Piscataway, NJ 08854 USA

Laws and regulations Users of IEEE Standards documents should consult all applicable laws and regulations. Compliance with the provisions of any IEEE Standards document does not imply compliance to any applicable regulatory requirements. Implementers of the standard are responsible for observing or referring to the applicable regulatory requirements. IEEE does not, by the publication of its standards, intend to urge action that is not in compliance with applicable laws, and these documents may not be construed as doing so.

Copyrights IEEE draft and approved standards are copyrighted by IEEE under U.S. and international copyright laws. They are made available by IEEE and are adopted for a wide variety of both public and private uses. These include both use, by reference, in laws and regulations, and use in private self-regulation, standardization, and the promotion of engineering practices and methods. By making these documents available for use and adoption by public authorities and private users, IEEE does not waive any rights in copyright to the documents.

Photocopies Subject to payment of the appropriate fee, IEEE will grant users a limited, non-exclusive license to photocopy portions of any individual standard for company or organizational internal use or individual, non-commercial use only. To arrange for payment of licensing fees, please contact Copyright Clearance Center, Customer Service, 222 Rosewood Drive, Danvers, MA 01923 USA; +1 978 750 8400. Permission to photocopy portions of any individual standard for educational classroom use can also be obtained through the Copyright Clearance Center.

Authorized licensed use limited to: Chinese University of Hong Kong. Downloaded on December 07,2015 at 21:41:04 UTC from IEEE Xplore. Restrictions apply.

Updating of IEEE Standards documents Users of IEEE Standards documents should be aware that these documents may be superseded at any time by the issuance 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 the document together with any amendments, corrigenda, or errata then in effect. Every IEEE standard is subjected to review at least every ten years. When a document is more than ten years old and has not undergone a revision process, it is reasonable to conclude that its contents, although still of some value, do not wholly reflect the present state of the art. Users are cautioned to check to determine that they have the latest edition of any IEEE standard. In order to determine whether a given document is the current edition and whether it has been amended through the issuance of amendments, corrigenda, or errata, visit the IEEE-SA Website at http://ieeexplore.ieee.org/xpl/ standards.jsp or contact IEEE at the address listed previously. For more information about the IEEE-SA or IEEE’s standards development process, visit the IEEE-SA Website at http://standards.ieee.org.

Errata Errata, if any, for all IEEE standards can be accessed on the IEEE-SA Website at the following URL: http:// standards.ieee.org/findstds/errata/index.html. Users are encouraged to check this URL for errata periodically.

Patents Attention is called to the possibility that implementation of this standard may require use of subject matter covered by patent rights. By publication of this standard, no position is taken by the IEEE with respect to the existence or validity of any patent rights in connection therewith. If a patent holder or patent applicant has filed a statement of assurance via an Accepted Letter of Assurance, then the statement is listed on the IEEESA Website at http://standards.ieee.org/about/sasb/patcom/patents.html. Letters of Assurance may indicate whether the Submitter is willing or unwilling to grant licenses under patent rights without compensation or under reasonable rates, with reasonable terms and conditions that are demonstrably free of any unfair discrimination to applicants desiring to obtain such licenses. Essential Patent Claims may exist for which a Letter of Assurance has not been received. The IEEE is not responsible for identifying Essential Patent Claims for which a license may be required, for conducting inquiries into the legal validity or scope of Patents Claims, or determining whether any licensing terms or conditions provided in connection with submission of a Letter of Assurance, if any, or in any licensing agreements are reasonable or non-discriminatory. 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. Further information may be obtained from the IEEE Standards Association.

Authorized licensed use limited to: Chinese University of Hong Kong. Downloaded on December 07,2015 at 21:41:04 UTC from IEEE Xplore. Restrictions apply.

Participants At the time this IEEE guide was completed, the DC Circuit Breaker Working Group had the following membership: Paulette Payne Powell, Chair Bipin Patel, Vice Chair Timothy Holdway

Charles Johnson

Richard Marek

The following members of the individual balloting committee voted on this guide. Balloters may have voted for approval, disapproval, or abstention. Samuel Aguirre I. Antweiler Donald Ayers Robert Ballard Barry Beaster Steven Bezner Wallace Binder Thomas Bishop Thomas Blackburn William Bloethe Carl Bush Thomas Callsen Paul Cardinal Suresh Channarasappa John Crouse Ray Davis Dieter Dohnal Gary Donner Donald Dunn Jorge Fernandez Daher Joseph Foldi Marcel Fortin Derek Foster Robert Ganser Carlos Gaytan Frank Gerleve

David Gilmer Jalal Gohari Edwin Goodwin Randall Groves Ajit Gwal Michael Haas Timothy Holdway Jill Holmes Philip Hopkinson Mohammad Iman Richard Jackson Charles Johnson Laszlo Kadar Sheldon Kennedy Yuri Khersonsky Jim Kulchisky Marc Lacroix Chung-Yiu Lam Aleksandr Levin Richard Marek John Miller Daniel Mulkey Jerry Murphy K. R. M. Nair Martin Navarro Michael Newman

Raymond Nicholas Mirko Palazzo Klaus Papp Bansi Patel Wesley Patterson Paulette Payne Powell Branimir Petosic Christopher Petrola Alvaro Portillo Lewis Powell Iulian Profir Michael Roberts Charles Rogers Joseph Rostron Thomas Rozek Bartien Sayogo Jerry Smith David Stankes Kerwin Stretch David Tepen James Thompson John Vergis Kenneth White Jennifer Yu Jian Yu

6

Copyright © 2015 IEEE. All rights reserved. Authorized licensed use limited to: Chinese University of Hong Kong. Downloaded on December 07,2015 at 21:41:04 UTC from IEEE Xplore. Restrictions apply.

When the IEEE-SA Standards Board approved this guide on 3 September 2015, it had the following membership: John D. Kulick, Chair Jon Walter Rosdahl, Vice Chair Richard H. Hulett, Past Chair Konstantinos Karachalios, Secretary Masayuki Ariyoshi Ted Burse Stephen Dukes Jean-Philippe Faure J. Travis Griffith Gary Hoffman Michael Janezic

Joseph L. Keopfinger* David Law Hung Ling Andrew Myles T. W. Olsen Glenn Parsons Ronald C. Petersen Annette D. Reilly

Stephen J. Shellhammer Adrian P. Stephens Yatin Trivedi Philip Winston Don Wright Yu Yuan Daidi Zhong

*Member Emeritus

7 Copyright © 2015 IEEE. All rights reserved. Authorized licensed use limited to: Chinese University of Hong Kong. Downloaded on December 07,2015 at 21:41:04 UTC from IEEE Xplore. Restrictions apply.

Introduction This introduction is not part of IEEE Std C57.12.59™-2015, IEEE Guide for Dry-Type Transformer Through-Fault Current Duration.

This guide provides recommendations for the application of overcurrent protection devices to limit the exposure time of dry-type transformers to short circuits. It shall not be confused with IEEE Std C57.109™, IEEE Guide for Liquid-Immersed Transformer Through-Fault Current Duration, which applies only to liquid-immersed transformers. Dry-type transformers differ significantly from liquid-immersed types in several respects: a)

There are significant variations in dry-type winding constructions, including conventional varnish impregnated layered windings, vertically stacked varnish-impregnated disk windings, solid cast windings, and combinations thereof, all of which have different transient heating characteristics during time intervals greater than about 100 s.

b)

The transient heating of liquid-immersed transformer windings is considerably buffered by the insulating medium in which the windings are immersed, providing a relatively long thermal time constant as compared to dry-type transformers.

Because of the foregoing, the through-fault protection curves for dry-type transformers are limited to overload time intervals of 100 s or less. No one curve for longer time intervals would characterize the thermal performance of all the different dry-type transformer constructions and temperature ratings. Moreover, such curves are not known or, at least, not available. Consequently, the curves in this guide pertain to the temperature rise of the windings during time intervals less than 100 s, wherein nearly all the heat generated is stored in the conductors. For longer time intervals, it is recommended that reference be made to IEEE Std C57.96™ [B1], IEEE Guide for Loading Dry-Type Distribution and Power Transformers. As short-circuit time intervals become progressively less than 100 s, mechanical considerations become more important than thermal characteristics. Short-circuit performance characteristics are contained in IEEE Std C57.12.01™. This guide supplements that information, but in no way supersedes it. The substantive revisions of this guide include expansion of the scope to include Category III transformers. The basis for the equations and the thermal and mechanical damage curves of Figure 1, Figure 2, and Figure 3 were developed and are detailed in Annex A. Figure 2a was corrected for consistency with the details of Annex A.

8

Copyright © 2015 IEEE. All rights reserved. Authorized licensed use limited to: Chinese University of Hong Kong. Downloaded on December 07,2015 at 21:41:04 UTC from IEEE Xplore. Restrictions apply.

Contents 1.  Overview��������������������������������������������������������������������������������������������������������������������������������������������������� 10 1.1  Scope�������������������������������������������������������������������������������������������������������������������������������������������������� 10 1.2  Purpose����������������������������������������������������������������������������������������������������������������������������������������������� 10 1.3  General����������������������������������������������������������������������������������������������������������������������������������������������� 10 2.  Normative references�������������������������������������������������������������������������������������������������������������������������������� 11 3.  Definitions������������������������������������������������������������������������������������������������������������������������������������������������� 11 4.  Transformer coordination�������������������������������������������������������������������������������������������������������������������������� 11 4.1  General transformer coordination������������������������������������������������������������������������������������������������������� 11 4.2  Category I transformers���������������������������������������������������������������������������������������������������������������������� 13 4.3  Category III transformers������������������������������������������������������������������������������������������������������������������� 14 Annex A (informative) Basis for through-fault protection������������������������������������������������������������������������������ 16 Annex B (informative) Bibliography������������������������������������������������������������������������������������������������������������� 19

9

Copyright © 2015 IEEE. All rights reserved. Authorized licensed use limited to: Chinese University of Hong Kong. Downloaded on December 07,2015 at 21:41:04 UTC from IEEE Xplore. Restrictions apply.

IEEE Guide for Dry-Type Transformer Through-Fault Current Duration IMPORTANT NOTICE: IEEE Standards documents are not intended to ensure safety, security, health, or environmental protection, or ensure against interference with or from other devices or networks. Implementers of IEEE Standards documents are responsible for determining and complying with all appropriate safety, security, environmental, health, and interference protection practices and all applicable laws and regulations. This IEEE document is made available for use subject to important notices and legal disclaimers. These notices and disclaimers appear in all publications containing this document and may be found under the heading “Important Notice” or “Important Notices and Disclaimers Concerning IEEE Documents.” They can also be obtained on request from IEEE or viewed at http://standards.ieee.org/IPR/disclaimers.html.

1. Overview 1.1 Scope This guide for dry-type transformer through-fault current duration applies to dry-type distribution and power transformers built in accordance with IEEE Std C57.12.01™.1

1.2 Purpose This guide sets forth recommendations believed essential for the application of overcurrent protective devices that limit the exposure time of dry-type transformers to short-circuit currents. This guide is not intended to imply overload capability.

1.3 General Protective devices, such as relays and fuses, have well-defined operating characteristics that relate fault magnitude to clearing time. It is desirable that these characteristic curves be coordinated with comparable curves applicable to dry-type transformers that relate duration and fault magnitude to withstand capability. The magnitude and duration of fault currents are of utmost importance in establishing a coordinated protection practice for transformers, as both mechanical and thermal effects of fault currents shall be considered. For fault-current magnitudes near the maximum short-circuit current rating of the transformer, mechanical effects are more significant than thermal effects. The maximum symmetrical short-circuit current should not exceed 1

Information on references can be found in Clause 2.

10 Copyright © 2015 IEEE. All rights reserved. Authorized licensed use limited to: Chinese University of Hong Kong. Downloaded on December 07,2015 at 21:41:04 UTC from IEEE Xplore. Restrictions apply.

IEEE Std C57.12.59-2015 IEEE Guide for Dry-Type Transformer Through-Fault Current Duration

25 times normal base current in accordance with IEEE Std C57.12.01. At lower fault-current magnitudes approaching the overload range, mechanical effects are less important unless the frequency and duration of fault occurrence is high enough to promote mechanical degradation. The point of transition between mechanical concern and thermal concern cannot be precisely defined; mechanical effects tend to have a more prominent role in larger kVA ratings because the mechanical forces are greater.

2.  Normative references The following referenced documents are indispensable for the application of this document (i.e., they must be understood and used, so each referenced document is cited in text and its relationship to this document is explained). For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments or corrigenda) applies. IEEE Std C37.91™, IEEE Guide for Protective Relay Applications to Power Transformers.2,3 IEEE Std C57.12.01™, IEEE Standard General Requirements for Dry-Type Distribution and Power Transformers.

3.  Definitions For the purposes of this document, the following terms and definitions apply. The IEEE Standards Dictionary Online should be consulted for terms not defined in this clause. 4 normal base current: Rated current of a transformer corresponding to its rated voltage and rated base kVA. short-circuit impedance: the sum of transformer impedance and system short-circuit impedance at the transformer location, expressed in percent of rated voltage and rated base kVA of the transformer. transformer short-circuit impedance: the transformer impedance expressed in percent of rated voltage and rated base kVA of the transformer.

4.  Transformer coordination 4.1  General transformer coordination For the purposes of coordination of overcurrent protective devices, with transformer short-circuit withstand capability, Figure 1, Figure 2, and Figure 3 are presented as protection curves for transformer Categories I through III as defined in IEEE Std C57.12.01 and adopted in Table 1.

The IEEE standards or products referred to in this clause are trademarks of The Institute of Electrical and Electronics Engineers, Inc. IEEE publications are available from The Institute of Electrical and Electronics Engineers. (http://standards.ieee.org/). 4 IEEE Standards Dictionary Online subscription is available at:http://www.ieee.org/portal/innovate/products/standard/standards_ dictionary.html. 2 3

11

Copyright © 2015 IEEE. All rights reserved. Authorized licensed use limited to: Chinese University of Hong Kong. Downloaded on December 07,2015 at 21:41:04 UTC from IEEE Xplore. Restrictions apply.

IEEE Std C57.12.59-2015 IEEE Guide for Dry-Type Transformer Through-Fault Current Duration

Table 1—Transformer categories Category (see NOTE 1)

Single-phase (kVA) (see NOTE 2)

Three-phase (kVA) (see NOTE 2)

Reference protection curves

I

1–500

15–500

Figure 1

II

501–1667

501–5000

Figure 2

III

1668 – 10 000

5001 – 30 000

Figure 3

NOTE 1— Category I include autotransformers of 500 kVA or less (equivalent two winding) even through nameplate kVA may exceed 500 kVA. NOTE 2— All kVA ratings are minimum nameplate kVA for the principal winding.

For Category I, a single curve applies that reÀects both thermal and mechanical damage considerations. For Categories II and III, two curves apply, one of which reÀects both thermal and mechanical damage considerations, while the other reÀects primarily thermal damage considerations only. On curves that have both a solid and dotted portion, the solid portion represents a total fault duration beyond which thermal damage to insulation adjacent to current-carrying conductors and anneal-softening of aluminum may occur, while the dotted portion represents a total fault duration beyond which cumulative mechanical damage may occur. The increasing signi¿cance of mechanical effects for higher kVA transformers is reÀected in these curves. The frequency of faults varies with different transformer applications. Applications characterizing frequent and infrequent faults are presented in the annex of IEEE Std C37.91. The curves in Figure 1, Figure 2, and Figure 3 are based upon maximum symmetrical short-circuit current magnitude of dry-type transformers not to exceed 25 times base current limited to a short-circuit current duration of 2 s. For Category I transformers, the symmetrical short-circuit magnitude will normally be limited only by the transformer impedance. For Category II and Category III transformers, the symmetrical short-circuit current is calculated based on the sum of the transformer impedance plus a value of system impedance speci¿ed by the user. See IEEE Std C57.12.01 for additional details. NOTE— Refer to IEEE Std C57.96 [B1]5 for loading capabilities at durations longer than 100 s for Categories II and III transformers.6

The validity of these damage-limit curves cannot be demonstrated by test, since the effects are progressive over the transformer lifetime. The curves are based principally on informed engineering judgment and favorable historical ¿eld experience; they provide a uniform minimum standard for all dry-type transformers. For an explanation of how the curves are derived, refer to Annex A. The per-unit short-circuit currents shown in Figure 1, Figure 2, and Figure 3 are the balanced transformer winding currents. The line currents that relate to these winding currents depend upon the transformer connection and the type of fault present. Application engineers shall relate the winding currents to the currents seen by the protective devices in order to protect the transformer within its capability.

The numbers in brackets correspond with those of the bibliography in Annex B. Notes in text, tables, and ¿gures of a standard are given for information only and do not contain requirements needed to implement this guide.

5 6

12 Copyright © 2015 IEEE. All rights reserved. Authorized licensed use limited to: Chinese University of Hong Kong. Downloaded on December 07,2015 at 21:41:04 UTC from IEEE Xplore. Restrictions apply.

IEEE Std C57.12.59-2015 IEEE Guide for Dry-Type Transformer Through-Fault Current Duration

Figure 1—Category I transformers

4.2  Category I transformers The recommended duration limit is based on the curve in Figure 1. The curve reflects both thermal and mechanical damage considerations and should be applied as a protection curve for faults that will occur frequently or infrequently. 4.2.1  Faults that occur frequently The curve in Figure 2a, reflecting both thermal and mechanical damage considerations, should be applied as a protection curve for faults that will occur frequently (typically more than 10 in a transformer lifetime). It is dependent upon the short-circuit impedance of the transformer for fault currents above the 70% of maximum possible and is keyed to the I2t of the worst-case mechanical duty (maximum fault current for 2 s) as shown by the dashed curve for a few impedances. The remaining portion matches the thermal protection curve for faults below the 70% level. 4.2.2  Faults that occur infrequently The curve in Figure 2b reflects primarily thermal damage considerations. It is not dependent upon short-circuit impedance of the transformer and may be applied as a protection curve for faults that will occur only infrequently (typically not more than 10 in a transformer lifetime). This thermal damage curve may also be used for backup protection where the transformer is exposed to frequent faults normally cleared by high-speed relaying.

13

Copyright © 2015 IEEE. All rights reserved. Authorized licensed use limited to: Chinese University of Hong Kong. Downloaded on December 07,2015 at 21:41:04 UTC from IEEE Xplore. Restrictions apply.

IEEE Std C57.12.59-2015 IEEE Guide for Dry-Type Transformer Through-Fault Current Duration

Figure 2—Category II transformers

4.3  Category III transformers The recommended duration limits depend upon fault frequency and are based upon the curves in Figure 3. Fault frequency refers to the number of faults with magnitudes greater than 50% of the maximum as limited by short-circuit impedance. 4.3.1  Faults that occur frequently The curve in Figure 3a, reflecting both thermal and mechanical damage considerations, should be applied as a protection curve for faults that will occur frequently (typically more than five in a transformer lifetime.) It is dependent upon the short-circuit impedance for fault currents above 50% of the maximum possible and is keyed to the I2t of the worst-case mechanical duty (maximum fault current for 2s) as shown by the dashed curve for a few impedances. The remaining portion matches thermal protection curves for faults below the 50% level. 4.3.2  Faults that occur infrequently The curve in Figure 3b reflects primarily thermal damage considerations. It is not dependent upon short-circuit impedance of the transformer and may be applied as a protection curve for faults that will occur only infrequently (typically not more than five in a transformer lifetime). This thermal damage curve may also be used for backup protection where the transformer is exposed to frequent faults normally cleared by high-speed relaying.

14

Copyright © 2015 IEEE. All rights reserved. Authorized licensed use limited to: Chinese University of Hong Kong. Downloaded on December 07,2015 at 21:41:04 UTC from IEEE Xplore. Restrictions apply.

IEEE Std C57.12.59-2015 IEEE Guide for Dry-Type Transformer Through-Fault Current Duration

Figure 3—Category III transformers

15

Copyright © 2015 IEEE. All rights reserved. Authorized licensed use limited to: Chinese University of Hong Kong. Downloaded on December 07,2015 at 21:41:04 UTC from IEEE Xplore. Restrictions apply.

IEEE Std C57.12.59-2015 IEEE Guide for Dry-Type Transformer Through-Fault Current Duration

Annex A (informative)

Basis for through-fault protection A.1  Through-fault protection curves (see Figure 1, Figure 2, and Figure 3) Symmetrical short-circuit current for the curves in Figure 1, Figure 2, and Figure 3 of Clause 4 is based upon Equation (A.1):

I 2t = k

(A.1)

where I t k

is the symmetrical short-circuit current in times normal base is time in seconds is the constant determined at maximum current with t = 2 seconds

Maximum symmetrical short-circuit current magnitude for dry-type transformers per Clause 4 is 25 times base current and short-circuit current duration is limited to 2 s. At maximum short-circuit current magnitude and duration k is determined using Equation (A.1).

I 2t = (25) 2 × 2 (A.2) I 2t = 1250

(A.3)

Equation (A.3) is the basis of the through-fault protection curves for Category 1 transformers as well as Category 2 and 3 transformers for faults that occur infrequently. Table A.1 provides symmetrical short-circuit current at various overload time intervals as plotted in the curves. Table A.1—Symmetrical short-circuit current with duration for Figure 1, Figure 2b, and Figure 3b t (seconds)

2

5

10

20

50

100

Isym (times normal base current)

25

15.81

11.18

7.91

5

3.54

A.2  Through-fault protection curve (see Figure 2a) For Figure 2a, the maximum symmetrical through-fault current is dependent upon short-circuit impedance. The equation for this dependency is principally based upon informed engineering judgment and favorable historical field experience. For fault currents 70% to 100% of maximum, Equation (A.3) is rewritten to find the time that corresponds to 70% of maximum short-circuit current for Category II transformers.

t=

1250 (A.4) I2

16

Copyright © 2015 IEEE. All rights reserved. Authorized licensed use limited to: Chinese University of Hong Kong. Downloaded on December 07,2015 at 21:41:04 UTC from IEEE Xplore. Restrictions apply.

IEEE Std C57.12.59-2015 IEEE Guide for Dry-Type Transformer Through-Fault Current Duration

t=

1250

( 0.7 × 25)

= 4.08 s (A.5)

2

Per Clause 4, t equals 2 s at maximum through-fault current; and at 70% through-fault current, t equals 4.08 s per Equation (A.5). Symmetrical short-circuit current for fault currents 70% to 100% maximum is determined by the following equation:

(

I 2t = 2 100

) for 2 s ≤ t ≤ 4.08 s (A.6) 2

Z

Equation A.6 is rewritten to find the impedance (Z) at short circuit for a given duration.

Z=

100 2 × I t

(A.7)

Table A.2 shows symmetrical short-circuit current and impedance for fault currents 100% and 70% of maximum determined by Equation (A.3) and Equation (A.7) respectively, also indicated by the dotted, slanted portion of the curve in Figure 2a. Table A.2—Symmetrical short-circuit current 70% to 100% maximum for Figure 2a % Transformer impedance, Z

Symmetrical short-circuit current (times normal base current) 100% Isym, t =2 s

70% Isym, t =4.08 s

4

25

17.5

6

16.67

11.67

8

12.5

8.75

10

10

7

12

8.33

5.83

For fault currents below 70%, the symmetrical fault current is determined from Equation (A.3); the duration is determined from Equation (A.8).

t = 0.255Z 2

(A.8)

Table A.3 shows symmetrical short-circuit current and impedance for fault currents below 70% based upon Equation (A.3) and Equation (A.8), also indicated by the dotted, vertical portion of the curves in Figure 2a. Table A.3—Symmetrical short-circuit current below 70% maximum for Figure 2 % Transformer impedance, Z

t (seconds)

Isym (times normal base current)

4

4.08

17.5

6

9.18

11.67

8

16.32

8.75

10

25.5

7

12

36.72

5.83

17

Copyright © 2015 IEEE. All rights reserved. Authorized licensed use limited to: Chinese University of Hong Kong. Downloaded on December 07,2015 at 21:41:04 UTC from IEEE Xplore. Restrictions apply.

IEEE Std C57.12.59-2015 IEEE Guide for Dry-Type Transformer Through-Fault Current Duration

A.3  Through-fault protection curve (see Figure 3a) For Figure 3a, the maximum symmetrical through-fault current is dependent upon short-circuit impedance. The equation for this dependency is principally based upon informed engineering judgment and favorable historical field experience. For fault currents 50% to 100% of maximum, Equation (A.3) is rewritten as Equation (A.9) to find the time that corresponds to 50% of maximum short-circuit current for Category III transformers.

t= t=

1250 I2

(A.9)

1250

( 0.5 × 25)

2

= 8 s

(A.10)

Per Clause 4, t equals 2 s at maximum through-fault current, and at 50% through-fault current t equals 8 s per Equation (A.10). Symmetrical short-circuit current for fault currents 50% to 100% maximum is determined by the following equation:

(

I 2t = 2 100

for 2 s ≤ t ≤ 8 s Z) 2

(A.11)

Equation (A.12) is rewritten to find short-circuit impedance for a given duration.

Z=

100 2 × I t

(A.12)

Table A.4 shows symmetrical short-circuit current and impedance for fault currents 100% and 50% of maximum determined by Equations (A.3) and Equation (A.12) respectively, also indicated by the dotted, slanted portion of the curve in Figure 3a. Table A.4—Symmetrical short-circuit current 50% to 100% maximum for Figure 3a % Transformer impedance, Z

Symmetrical short-circuit current (times normal base current) 100% Isym, t =2 s

50% Isym, t =8 s

4

25

12.5

6

16.67

8.33

8

12.5

6.25

10

10

5

12

8.33

4.17

For fault currents below 50%, the symmetrical fault current is determined from Equation (A.3); the duration is determined from Equation A.13.

t = 0.5Z 2

(A.13)

Table A.4 shows symmetrical short-circuit current and impedance for fault currents below 50% based upon Equation (A.3) and (A.13), also indicated by the dotted, vertical portion of the curves in Figure 3a.

18

Copyright © 2015 IEEE. All rights reserved. Authorized licensed use limited to: Chinese University of Hong Kong. Downloaded on December 07,2015 at 21:41:04 UTC from IEEE Xplore. Restrictions apply.

IEEE Std C57.12.59-2015 IEEE Guide for Dry-Type Transformer Through-Fault Current Duration

Annex B (informative)

Bibliography Bibliographical references are resources that provide additional or helpful material but do not need to be understood or used to implement this standard. Reference to these resources is made for informational use only. [B1] IEEE Std C57.96™, IEEE Guide for Loading Dry-Type Distribution and Power Transformers.

19

Copyright © 2015 IEEE. All rights reserved. Authorized licensed use limited to: Chinese University of Hong Kong. Downloaded on December 07,2015 at 21:41:04 UTC from IEEE Xplore. Restrictions apply.

IEEE standards.ieee.org Phone: +1 732 981 0060 © IEEE

Fax: +1 732 562 1571

Authorized licensed use limited to: Chinese University of Hong Kong. Downloaded on December 07,2015 at 21:41:04 UTC from IEEE Xplore. Restrictions apply.