C57.12.20-2005

IEEE Std C57.12.20™-2005 (Revision of IEEE Std C57.12.20-1997) IEEE Standard for Overhead-Type Distribution Transformers, 500 kVA and Smaller: High Voltage, 34 500 V and Below; Low Voltage, 7970/13 800Y V and Below I E E E 3 Park Avenue New York, NY 10016-5997, USA 9 December 2005 IEEE Power Engineering Society Sponsored by the Transformers Committee The Institute of Electrical and Electronics Engineers, Inc. 3 Park Avenue, New York, NY 10016-5997, USA Copyright © 2005 by the Institute of Electrical and Electronics Engineers, Inc. All rights reserved. Published 9 December 2005. 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. Print: ISBN 0-7381-4752-4 SH95354 PDF: ISBN 0-7381-4753-2 SS95343 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 Std C57.12.20™-2005 (Revision of IEEE Std C57.12.20-1997) IEEE Standard for Overhead-Type Distribution Transformers, 500 kVA and Smaller: High Voltage, 34 500 V and Below; Low Voltage, 7970/13 800Y V and Below Sponsor Transformer Committee of the IEEE Power Engineering Society Approved 9 June 2005 IEEE-SA Standards Board Abstract: This standard covers certain electrical, dimensional, and mechanical characteristics and safety features of single- and three-phase, 60 Hz, mineral-oil-immersed, self-cooled, overhead-type distribution transformers 500 kVA and smaller, high voltages 34 500 V and below and low voltages 7970/13 800Y V and below. Keywords: distribution transformer, overhead, mineral oil, transformer IEEE Standards documents are developed within the IEEE Societies and the Standards Coordinating Committees of the IEEE Standards Association (IEEE-SA) Standards Board. The IEEE develops its standards through a consensus development process, approved by the American National Standards Institute, which brings together volunteers representing varied viewpoints and interests to achieve the final product. Volunteers are not necessarily members of the Institute and serve without compensation. While the IEEE administers the process and establishes rules to promote fairness in the consensus development process, the IEEE does not independently evaluate, test, or verify the accuracy of any of the information contained in its standards. Use of an IEEE Standard is wholly voluntary. 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Introduction In 1938, the EEI-NEMA (Edison Electric Institute-National Electrical Manufacturers Association) Joint Committee on Standards for Distribution Transformers was organized to develop standards for the various types of distribution transformers. For 23 years, the continuous work of this Joint Committee was evidenced by a series of reports published jointly by the Edison Electric Institute and the National Electrical Manufacturers Association. The last report of overhead-type distribution transformers was the Seventh Report published in April 1961. The Joint Committee's last two reports on overhead-type distribution transformers were approved as American National Standards, the Seventh Report being reissued as American National Standard C57.12.20-1962. To avoid further duplication of effort and to simplify future revisions, the EEI-NEMA Joint Committee on Standards for Distribution Transformers was dissolved and replaced by new subcommittees of the Accredited Standards Committee on Transformers, Regulators, and Reactors, C57. The work on standards for overhead-type distribution transformers was taken over by the Subcommittee on Distribution Transformers, Overhead and Pad-mounted, C57.12.2. This standard was prepared by IEEE Subcommittee C57.12.2 on behalf of C57 and is a revision of ANSI C57.12.20-1997, Standard for Overhead Type Distribution Transformers, 500 kVA and Smaller: High Volt- age, 34 500 Volts and Below; Low Voltage 7970/13 800Y Volts and Below. The 1981 edition included detailed requirements on relief of excessive pressure. The 1988 revision recognized the step-down and series-multiple designs in distribution transformers, and omitted the platform-type or station-type trans- former. In recognition of the dramatic reduction in catastrophic failures of overhead transformers since the formulation of NEMA TR-P7-1975, the 1988 revision included in Section 8 this updated NEMA proposal that was previously designated 2.02 of NEMA Standards Publication for Transformers, Regulators, and Reactors, NEMA TR 1-1980 [B1]. The general industry consensus is that transformers that are built in con- formance with this standard and that have adequate overcurrent protection, and recognizing the limitations included therein, will provide for enclosures with reasonable withstand capability. This standard was processed and approved for submittal to ANSI by the Accredited Standards Committee on Transformers, Regulators, and Reactors, C57. Committee approval of the standard does not necessarily imply that all committee members voted for its approval. Notice to users Errata Errata, if any, for this and all other standards can be accessed at the following URL: http:// standards.ieee.org/reading/ieee/updates/errata/index.html. Users are encouraged to check this URL for errata periodically. Interpretations Current interpretations can be accessed at the following URL: http://standards.ieee.org/reading/ieee/interp/ index.html. This introduction is not part of IEEE Std C57.12.20™-2005, IEEE Standard for Overhead-Type Distribution Transformers, 500 kVA and Smaller: High Voltage, 34 500 V and Below; Low Voltage, 7970/13 800Y V and Below. iii Copyright © 2005 IEEE. All rights reserved. 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 with respect to the existence or validity of any patent rights in connection therewith. The IEEE shall not be responsible for identifying patents or patent applications for which a license may be required to implement an IEEE standard or for conducting inquiries into the legal validity or scope of those patents that are brought to its attention. Participants At the time this standard was completed, the Overhead-Type Distribution Transformers, 500 kVA and Smaller: High Voltage 34 500 Working Group had the following membership: Alan Wilks, Chair Tommy Cooper, Vice-Chair The following members of the individual balloting committee voted on this guide. Balloters may have voted for approval, disapproval, or abstention. Ignacio Ares Israel Barrientos Bikash Basu Thomas Callsen Tommy Cooper John Crotty, III Mike Culhane Kevin dela Houssaye Charlie Drexler Don Duckett Carlos Gaytan Ali Ghafourian Myron Gruber Ken Hanus Richard Hollingsworth Gael Kennedy Brian Klaponski Tim Olson Gerry Paiva Tom Pekarek Juan Jose Saldivar Jeff Schneider Stephen Shull Ed Smith Ron Stahara Al Traut Donnie Trivitt Alan Wilks Mike Williams William Wimmer David Aho Ignacio Ares Wallace Binder John Borst Carl Bush Thomas Callsen Tommy Cooper Jerry Corkran Michael Culhane R. Daubert Byron Davenport Kevin Delahoussaye Randall Dotson Charles Drexler Stephen Early Jorge Fernandez-Daher Rabiz Foda Marcel Fortin Fredric Friend Dudley Galloway George Gela Ali Ghafourian Ron Greenthaler Randall Groves Myron Gruber Bal Gupta Ajit Gwal N. Kent Haggerty Kenneth Hanus Michael Hardin William Hopf Edward Horgan, Jr. James D. Huddleston, III Saumen Kundu Gael R. Kennedy Vladimir Khalin Boyd Leuenberger Gregory Luri Al Maguire Keith Malmedal John Matthews Lee Matthews Frank Mayle Samuel Michael G. Michel Dan Mulkey Krste Najdenkoski Robert Olen Gerald Paiva Dhiru Patel Jesse Patton Michael Pehosh Thomas Pekarek Donald Platts Dinesh P. Sankarakurup Johannes Rickmann Michael Roberts John Rossetti Devki Sharma Stephen Shull H. Jin Sim Chuck Simmons Tarkeshwar Singh David Singleton James E. Smith Jerry Smith Steve Snyder Ronald J. Stahara Giuseppe Termini Alan Traut Joseph Tumidajski Joseph Vaschak Tom Wandeloski Alan Wilks James Wilson Bill Wimmer Copy iv right © 2005 IEEE. All rights reserved. When the IEEE-SA Standards Board approved this guide on 9 June 2005, it had the following membership: Steve M. Mills, Chair Richard H. Hulett, Vice Chair Judith Gorman, Secretary *Member Emeritus Also included are the following nonvoting IEEE-SA Standards Board liaisons: Satish K. Aggarwal, NRC Representative Richard DeBlasio, DOE Representative Alan H. Cookson, NIST Representative Michael D. Fisher IEEE Standards Project Editor Mark D. Bowman Dennis B. Brophy Joseph Bruder Richard Cox Bob Davis Julian Forster* Joanna N. Guenin Mark S. Halpin Raymond Hapeman William B. Hopf Lowell G. Johnson Herman Koch Joseph L. Koepfinger* David J. Law Daleep C. Mohla Paul Nikolich T. W. Olsen Glenn Parsons Ronald C. Petersen Gary S. Robinson Frank Stone Malcolm V. Thaden Richard L. Townsend Joe D. Watson Howard L. Wolfman Cop v yright © 2005 IEEE. All rights reserved. Contents Contents......................................................................................................................................................... vi 1. Overview .................................................................................................................................................... 1 1.1 Scope ................................................................................................................................................... 1 1.2 Purpose ................................................................................................................................................ 1 2. Normative references.................................................................................................................................. 1 3. Definitions .................................................................................................................................................. 2 4. Ratings........................................................................................................................................................ 2 4.1 Kilovolt-ampere ratings ....................................................................................................................... 2 4.2 Voltage ratings and tap ratings ............................................................................................................ 2 5. Basic lightning impulse insulation levels and dielectric test levels ............................................................ 3 5.1 Basic lightning impulse ....................................................................................................................... 3 5.2 Dielectric test....................................................................................................................................... 3 6. Tests ........................................................................................................................................................... 3 6.1 General ................................................................................................................................................ 3 6.2 Dielectric tests ..................................................................................................................................... 3 7. Construction ............................................................................................................................................... 3 7.1 Bushings and Terminals ...................................................................................................................... 3 7.2 Accessory equipment........................................................................................................................... 4 7.3 Polarity, terminal markings, and angular displacement ....................................................................... 7 7.4 Oil preservation ................................................................................................................................... 8 7.5 Tanks ................................................................................................................................................... 8 8. Storage and installation ............................................................................................................................ 10 8.1 Storage............................................................................................................................................... 10 8.2 Installation ......................................................................................................................................... 10 9. Design tests for fault current capability of overhead distribution transformer enclosures ....................... 10 9.1 Objective............................................................................................................................................ 10 9.2 General requirements......................................................................................................................... 10 9.3 Tests................................................................................................................................................... 11 9.4 Test number 1—an arcing fault in an enclosure ................................................................................ 11 9.5 Test number 2—for enclosures with internal fusible elements.......................................................... 11 9.6 Test results......................................................................................................................................... 12 Annex A (informative) Auxiliary Mounting Devices................................................................................... 34 Annex B (informative) Bibliography............................................................................................................ 40 vi Copyright © 2005 IEEE. All rights reserved. IEEE Std C57.12.20-2005 IEEE Standard for Overhead-Type Distribution Transformers, 500 kVA and Smaller IEEE Standard for Overhead-Type Distribution Transformers, 500 kVA and Smaller: High Voltage, 34 500 V and Below; Low Voltage, 7970/13 800Y V and Below 1. 1.1 1.2 2. Overview Scope This standard covers certain electrical, dimensional, and mechanical characteristics and safety features of single- and three-phase, 60 Hz, mineral-oil-immersed, self-cooled, overhead-type distribution transformers 500 kVA and smaller, high voltages 34 500 V and below and low voltages 7970/13 800Y V and below. Purpose This standard is intended for use as a basis for determining the performance, interchangeability, and safety of overhead-type distribution transformers and to assist in the proper selection of this equipment. NOTE―For polychlorinated-biphenyl (PCB) contamination in transformers, refer to the latest federal regulations. Normative references The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any ame