IEEE Std C37.91-2000 (Revision of IEEE Std C37.91-1985) IEEE Guide for Protective Relay Applications to Power Transformers Sponsor Power Systems Relay Committee of the IEEE Power Engineering Society Approved 8 March 2000 IEEE-SA Standards Board Abstract: The protection of power transformers is covered; various electrical protection schemes are explored; and guidelines are given for the application of these schemes to transformers. Alter- native detection methods including mechanical, thermal, and gas analysis are discussed. Keywords: power, protection, relaying, transformer The Institute of Electrical and Electronics Engineers, Inc. 3 Park Avenue, New York, NY 10016-5997, USA Copyright © 2000 by the Institute of Electrical and Electronics Engineers, Inc. All rights reserved. Published 9 October 2000. Printed in the United States of America. Print: ISBN 0-7381-1976-8 SH94830 PDF: ISBN 0-7381-1977-6 SS94830 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. IEEE Standards documents are developed within the IEEE Societies and the Standards Coordinating Com- mittees of the IEEE Standards Association (IEEE-SA) Standards Board. Members of the committees serve voluntarily and without compensation. They are not necessarily members of the Institute. The standards developed within IEEE represent a consensus of the broad expertise on the subject within the Institute as well as those activities outside of IEEE that have expressed an interest in participating in the development of the standard. 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. Every IEEE Standard is subjected to review at least every five years for revision or reaffirmation. When a document is more than five years old and has not been reaffirmed, it is rea- sonable 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. Comments for revision of IEEE Standards are welcome from any interested party, regardless of membership affiliation with IEEE. Suggestions for changes in documents should be in the form of a proposed change of text, together with appropriate supporting comments. Interpretations: Occasionally questions may arise regarding the meaning of portions of standards as they relate to specific applications. When the need for interpretations is brought to the attention of IEEE, the Institute will initiate action to prepare appropriate responses. Since IEEE Standards represent a consensus of all concerned interests, it is important to ensure that any interpretation has also received 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 interpretation requests except in those cases where the matter has previously received formal consideration. Comments on standards and requests for interpretations should be addressed to: Secretary, IEEE-SA Standards Board 445 Hoes Lane P.O. Box 1331 Piscataway, NJ 08855-1331 USA Note: 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 with respect to the existence or validity of any patent rights in connection therewith. The IEEE shall not be responsible for identifying patents for which a license may be required by an IEEE standard or for conducting inquiries into the legal validity or scope of those patents that are brought to its attention. IEEE is the sole entity that may authorize the use of certification marks, trademarks, or other designations to indicate compliance with the materials set forth herein. Authorization to photocopy portions of any individual standard for internal or personal use is granted by the Institute of Electrical and Electronics Engineers, Inc., provided that the appropriate fee is paid to Copyright Clearance Center. To arrange for payment of licensing fee, please contact Copyright Clearance Center, Cus- tomer Service, 222 Rosewood Drive, Danvers, MA 01923 USA; (978) 750-8400. Permission to photocopy portions of any individual standard for educational classroom use can also be obtained through the Copy- right Clearance Center. Introduction (This introduction is not part of IEEE Std C37.91-2000, IEEE Guide for Protective Relay Applications to Power Transformers.) This is a revision of IEEE Std C37.91-1985, IEEE Guide for Protective Relay Applications to Power Trans- formers. This guide will aid in the effective application of relays and other devices for the protection of power transformers. In this revision several areas have been improved. Most notably: — Several figures were corrected. — Subclauses 5.5.2 and 6.2.3 on current inrush were rewritten to include a new form of inrush restraint. — Current transformer connections were updated in 5.4 to be in line with IEEE Std C37.110-1996, IEEE Guide for Application of Current Transformers Used for Protective Relaying Purposes. — Geomagnetic influence on transformers and protective relays is discussed in Clause 13. — Clause 11, Gas analysis, was revised to reflect current philosophy and practice. Participants At the time this guide was completed, the Working Group on Protective Relay Applications to Power Trans- formers had the following membership: Roger Hedding, Chair Robert Haas, Vice Chair Richard F. Angle Philip Engel Miriam P. Sanders Graham Clough Stephen E. Grier Gerry Schmitt Stephen P. Conrad Hans Heining-Triebs James E. Stephens Mark Conroy John J. Horwath Malcolm Swanson Clifford Downs Barry Jackson Jerome B. Williams Vern Dvorak Hardy J. King Murty V. Yalla Bradley D. Nelson Copyright © 2000 IEEE. All rights reserved. iii The following members of the balloting committee voted on this standard: Richard F. Angle Roy E. Hart Bradley D. Nelson John Appleyard Irwin O. Hasenwinkle George C. Parr Robert W. Beckwith Roger A. Hedding Robert D. Pettigrew Stuart Borlase Charles F. Henville Radhakrishna V. Reb- John Boyle Jerry W. Hohn bapragada John Burger Dennis K. Holstein Mohindar S. Sachdev Jeffrey A. Burnworth Stanley H. Horowitz Miriam P. Sanders Daniel Carreau John J. Horwath Tarlochan Sidhu Carlos H. Castro James D. Huddleston, III Veselin Skendzic Simon R. Chano James W. Ingleson Peter A. Solanics Stephen P. Conrad Barry Jackson Kevin A. Stephan Carey J. Cook Herbert Jacobi James E. Stephens Robert W. Dempsey Carl Kinsley Richard P. Taylor Clifford Downs Joseph L. Koepfinger James S. Thorp Paul R. Drum W. C. Kotheimer Donald R. Volzka Walter Elmore Sture O. R. Lindahl Charles L. Wagner Ahmed Elneweihi William Lowe William P. Waudby Philip Engel W. J. Marsh, Jr. Jerome B. Williams Jonathan D. Gardell Michael J. McDonald Donna Williams Tony Giuliante Jeffrey L. McElray Murty V. Yalla Stephen E. Grier Gary L. Michel John A. Zipp Robert W. Haas George R. Nail Stan Zocholl This standard is dedicated to the memory of Graham Clough, who passed away during the final stages of preparation of this document. He will be remembered for his contributions to this document and his service to the relay industry. When the IEEE-SA Standards Board approved this standard on 8 March 2000, it had the following membership: Richard J. Holleman, Chair Donald N. Heirman, Vice Chair Judith Gorman, Secretary Satish K. Aggarwal James H. Gurney Louis-François Pau Dennis Bodson Lowell G. Johnson Ronald C. Petersen Mark D. Bowman Robert J. Kennelly Gerald H. Peterson James T. Carlo E. G. “Al” Kiener John B. Posey Gary R. Engmann Joseph L. Koepfinger* Gary S. Robinson Harold E. Epstein L. Bruce McClung Akio Tojo Jay Forster* Daleep C. Mohla Hans E. Weinrich Ruben D. Garzon Robert F. Munzner Donald W. Zipse *Member Emeritus Also included is the following nonvoting IEEE-SA Standards Board liaison: Robert E. Hebner Noelle D. Humenick IEEE Standards Project Editor iv Copyright © 2000 IEEE. All rights reserved. Contents 1. Overview.............................................................................................................................................. 1 1.1 Scope............................................................................................................................................ 1 1.2 Purpose......................................................................................................................................... 1 2. References............................................................................................................................................ 1 3. Philosophy and economic considerations ............................................................................................ 2 4. Types of transformer failures............................................................................................................... 3 5. Relay current........................................................................................................................................ 4 5.1 Minimum internal faults .............................................................................................................. 4 5.2 Maximum internal faults.............................................................................................................. 4 5.3 Through-faults.............................................................................................................................. 4 5.4 Performance of CTs ..................................................................................................................... 5 5.5 Reasons for mismatch current...................................................................................................... 6 6. Electrical detection of faults ................................................................................................................ 7 6.1 Fuse protection............................................................................................................................. 8 6.2 Differential protection................................................................................................................ 10 6.3 Overcurrent relay protection ...................................................................................................... 18 6.4 Ground fault protection.............................................................................................................. 21 6.5 Fault detection for special-purpose transformers....................................................................... 23 6.6 Backup and external fault protection ......................................................................................... 29 6.7 Temperature relays..................................................................................................................... 31 6.8 Miscellaneous relays.................................................................................................................. 31 7. Mechanical detection of faults ........................................................................................................... 31 7.1 Gas accumulator relay................................................................................................................ 31 7.2 Gas detector relay ...................................................................................................................... 32 7.3 Pressure relays ........................................................................................................................... 32 8. Thermal detection of abnormalities ................................................................................................... 34 8.1 Thermal relays for winding temperature.................................................................................... 34 8.2 Other means of thermal protection ............................................................................................ 35 8.3 Testing thermal relays................................................................................................................ 38 9. Fault clearing ..................................................................................................................................... 38 9.1 Relay tripping circuits................................................................................................................ 39 9.2 Circuit breakers.......................................................................................................................... 39 9.3 Remote tripping of circuit breakers ........................................................................................... 39 9.4 Circuit switcher.......................................................................................................................... 40 9.5 Fuses .......................................................................................................................................... 40 9.6 Other practices ........................................................................................................................... 40 Copyright © 2000 IEEE. All rights reserved. v 10. Reenergizing practice......................................................................................................................... 41 11. Gas analysis ....................................................................................................................................... 42 12. Special protective schemes ................................................................................................................ 42 12.1 Overall unit generator differential ............................................................................................. 42 12.2 Unit transformer of three-legged core form type....................................................................... 44 12.3 Grounding transformer inside the main transformer differential zone ...................................... 44 12.4 Unbalanced voltage protection for Y-connected, three-legged, core-type transformers ........... 45 12.5 Differential protection of single-phase transformers connected in three-phase banks .............. 46 12.6 Differential protection of a bank of three single-phase autotransformers with ∆ tertiary ......... 47 12.7 Differential protection of single-phase transformers in a three-phase bank with a spare transformer............................................................................................................. 48 13. Other considerations .......................................................................................................................... 49 14. Device numbers ................................................................................................................................. 50 Annex A (informative) Application of the transformer through-fault current duration guide to the protection of power transformers....................................................................................... 51 Annex B (informative) Bibliography............................................................................................................. 70 vi Copyright © 2000 IEEE. All rights reserved. IEEE Guide for Protective Relay Applications to Power Transformers 1.Overview 1.1 Scope This guide covers practical applications, general philosophy, and economic considerations of power trans- former protection. 1.2 Purpose The purpose of this guide is to aid in the effective application of relays and other devices for the protection of power transformers. Emphasis is placed on practical applications. The general philosophy and economics of transformer protection are reviewed. The types of faults experienced are described, and technical problems with such protection, including current transformer (CT) behavior during fault conditions, are discussed. Various types of electrical, mechanical, and thermal protective devices are also described and associated problems such as fault clearing and reenergizing are discussed. 2.References This guide shall be used in conjunction with the following publications. When the following standards are superseded by an approved revision, the revision will apply. IEEE Std 32-1972 (Reaff 1997), IEEE Standard Requirements, Terminology, and Test Procedures for Neu- tral Grounding Devices.1 IEEE Std C37.2-1996, IEEE Standard Electrical Power System Device Function Numbers and Contact Designations. IEEE Std C37.110-1996 IEEE Guide for the Application of Current Transformers Used for Protective Relaying Purposes. 1IEEE publications are available from the Institute of Electrical and Electronics Engineers, 445 Hoes Lane, P.O. Box 1331, Piscataway, NJ 08855-1331, USA (http://standards.ieee.org/). Copyright © 2000 IEEE. All rights reserved. 1 IEEE Std C37.91-2000 IEEE GUIDE FOR PROTECTIVE RELAY IEEE Std C57.12.00-2000, IEEE Standard General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers. IEEE Std C57.91-1995, IEEE Guide for Loading Mineral-Oil-Immersed Transformers. IEEE Std C57.92-1981 (Reaff 1991), IEEE Guide for Loading Mineral-Oil-Immersed Power Transformers 2 Up to and Including 100 MVA with 55 °C or 65 °C Average Winding Rise. IEEE Std C57.109-1993, IEEE Guide for Liquid-Immersed Transformer Through-Fault-Current Duration. 3.Philosophy and economic considerations Protective relaying is applied to components of a power system for the following reasons: a)Separate the faulted equipment from the remainder of the system so that the system can continue to function b)Limit damage to the faulted equipment c)Minimize the possibility of fire d)Minimize hazards to personnel e)Minimize the risk of damage to adjacent high-voltage apparatus In protecting some components, particularly high-voltage transmission lines, the limiting of damage becomes a by-product of the system protection function of the relay. However, since the cost of repairing faulty transformers may be great and since high-speed, highly sensitive protective devices can reduce dam- age and therefore repair cost, relays should be considered for protecting transformers also, particularly in the larger sizes. Faults internal to a transformer quite often involve a magnitude of fault current that is low relative to the transformer base rating. This indicates a need for high sensitivity and high speed to ensure good protection. There is no one standard way to protect all transformers, or even identical transformers that are applied dif- ferently. Most installations require individual engineering analysis to determine the best and most cost-effec- tive scheme. Usually more than one scheme is technically feasible, and the alternatives offer varying degrees of sensitivity, speed, and selectivity. The plan selected should balance the best combination of these factors against the overall economics of the situation while holding to a minimum a)Cost of repairing damage b)Cost of lost production c)Adverse effects on the balance of the system d)The spread of damage to adjacent equipment e)The period of unavailability of the damaged equipment In protecting transformers, backup protection needs to be considered. The failure of a relay or breaker during a transformer fault may cause such extensive damage to the transformer that its repair would not be practical. When the fault is not cleared by the transformer protection, remote line relays or other protective relays may operate. Part of the evaluation of the type of protection applied to a transformer should include how the system integrity may be affected by such a failure. In this determination, since rare but costly failures are involved, a diversity of opinion on the degree of protection required by transformers might be expected among those familiar with power system relay engineering. The major economic consideration is 2 This standard has been revised and redesignated to IEEE Std C57.91-1995. 2 Copyright © 2000 IEEE. All rights reserved. IEEE APPLICATIONS TO POWER TRANSFORMERS Std C37.91-2000 not ordinarily the fault detection equipment but the isolation devices. Circuit breakers often cannot be justified on the basis of transformer protection alone. At least as much weight should be given to the service requirements, the operating philosophy, and system design philosophy as to the protection of the transformer. Evaluations of the risks involved and the cost-effectiveness of the protection are necessary to avoid going to extremes. Such considerations involve the art rather than the science of protective relaying. 3 See [B22], [B24], [B29], and [B59] . 4.Types of transformer failures The electrical windings and the magnetic core in a transformer are subject to a number of different forces during operation, for example a)Expansion and contraction due to thermal cycling b)Vibration c)Local heating due to magnetic flux d)Impact forces due to through-fault current e)Excessive heating due to overloading or inadequate cooling These forces can cause deterioration and failure of the winding electrical insulation. Table 1 summarizes failure statistics for a broad range of transformer failure causes reported by a group of U.S. utilities over a period of years. Table 1—Failure statistics for three time periods 1955–1965 1975–1982 1983–1988 Percent of Percent of Percent of Number Number Number total total total Winding failures 134 51 615 55 144 37 Tap changer failures 49 19 231 21 85 22 Bushing failures 41 15 114 10 42 11 Terminal board failures 19 7 71 6 13 3 Core failures 7 3 24 2 4 1 Miscellaneous failures 12 5 72 6 101 26 Total 262 100 1127 100 389 100 This guide deals primarily with the application of electrical relays to detect the fault current that results from an insulation failure. Clause 5 examines the current a relay can expect to see as a result of various types of winding insulation failures. The detection systems that monitor other transformer parameters can be used to indicate an incipient electri- cal fault. Prompt response to these indicators may help avoid a serious fault. For example a)Temperature monitors for winding or oil temperature are typically used to initiate an alarm requiring investigation by maintenance staff. 3 The numbers in brackets correspond to the references listed in Annex B of this guide. Copyright © 2000 IEEE. All rights reserved. 3 IEEE Std C37.91-2000 IEEE GUIDE FOR PROTECTIVE RELAY b)Gas detection relays can detect the evolution of gases within the transformer oil. Analysis of the gas composition indicates the mechanism that caused the formation of the gas; e.g., acetylene can be caused by electrical arcing, other gases are caused by corona and thermal degradation of the cellulose insulation. The gas detection relays may be used to trip or alarm depending on utility practice. Generally, gas analysis is performed on samples of the oil, which are collected periodically. Alternatively, a continuous gas analyzer is available to allow on-line detection of insulation system degradation. c)Sudden-pressure relays respond to the pressure waves in the transformer oil caused by the gas evolu- tion associated with arcing. d)Oil level detectors sense the oil level in the tank and are used to alarm for minor reductions in oil level and trip for severe reductions. These various relays are discussed in greater detail in later clauses of this guide. 5.Relay current Two characteristics of power transformers combine to complicate detection of internal faults with current- operated relays a)The change in magnitude of current at the transformer terminals may be very small when a limited number of turns are shorted within the transformer. b)When a transformer is energized, magnetizing inrush current that flows in one set of terminals may equal many times the transformer rating. These and other considerations require careful thought to obtain relay characteristics best-suited to the particular application. 5.1 Minimum internal faults The most difficult transformer winding fault for which to provide protection is the fault that initially involves one turn. A turn-to-turn fault will result in a terminal current of much less than rated full-load current. For example, as much as 10% of the winding may have to be shorted to cause full-load terminal current to flow. Therefore, a single turn-to-turn fault will result in an undetectable amount of current. 5.2 Maximum internal faults There is no limit to the maximum internal fault current that can flow, other than the system capability, when the fault is a terminal fault or a fault external to the transformer but in the relay zone. The relay system should be capable of withstanding the secondary current of the CT on a short-time basis. This may be a factor if the transformer is small relative to the system fault and if the CT ratio is chosen to match the transformer rating. 5.3 Through-faults Fault current through a transformer is limited by the transformer and source impedance. While current through a transformer thus limited by its impedance can still cause incorrect relay operations or even trans- former failure, CT saturation is less likely to occur than with unlimited currents. The above favorable aspect may disappear if the transformer protective zone includes a bus area with two or more breakers on the same side of the transformer through which external fault current can flow with no 4 Copyright © 2000 IEEE. All rights reserved.