ANSI IEEE C57.13.1-1981

ANSI/IEEE C57.13.1-1981 (reaffirmed 1986) An American National Standard IEEE Guide for Field Testing of Relaying Current Transformers Sponsor Power System Relaying Committee of the IEEE Power Engineering Society Approved 9 March 1978 Reaffirmed 18 June 1986 Reaffirmed 19 March 1992 IEEE Standards Board Secretariat Institute of Electrical and Electronics Engineers National Electrical Manufacturers Association Approved 31 December 1980 Reaffirmed 25 August 1987 Reaffirmed 2 December 1992 American National Standards Institute © Copyright 1981 by The Institute of Electrical and Electronics Engineers, Inc 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. (ANSI Reaffirmed 1987) American National Standard An American National Standard implies a consensus of those substantially concerned with its scope and provisions. An American National Standard is intended as a guide to aid the manufacturer, the consumer, and the general public. The existence of an American National Standard does not in any respect preclude anyone, whether he has approved the standard or not, from manufacturing, marketing, purchasing, or using products, processes, or procedures not conforming to the standard. American National Standards are subject to periodic review and users are cautioned to obtain the latest editions. CAUTION NOTICE: This American National Standard may be revised or withdrawn at any time. The procedures of the American National Standards Institute require that action be taken to reafÞrm, revise, or withdraw this standard no later than Þve years from the date of publication. Purchasers of American National Standards may receive current information on all standards by calling or writing the American National Standards Institute. Foreword (This Foreword is not a part of ANSI/IEEE C57.13.1-1981, IEEE Guide for Field Testing of Relaying Current Transformers.) This guide was prepared by a Working Group of the Relay Input Sources Subcommittee of the IEEE Power System Relaying Committee of the IEEE Power Engineering Society. The working group expresses its gratitude to past members, members of the Relay Input Sources Subcommittee, Power System Relaying Committee, Liaison Committees, and groups who have contributed their time and knowledge in preparing this guide. At the time it approved this standard, C57 had the following membership: I. H. Koponen, Chair C. R. Wilmore, Secretary Bonneville Power Administration ............................................................................................................ George W. Iliff Electric Light and Power Group ...................................................................................................................... R. R. Bast I. O. Berkhan I. H. Koponen J. P. Markey (Alt) B. F. Smith E. A. Villasuso Institute of Electrical and Electronics Engineers .............................................................................................S. Bennon W. P. Burt J. C. Dutton D. C. Johnson L. W. Long D. E. Massey National Electrical Manufacturers Association.............................................................................................L. C. Aicher W. R. Courtade J. D. Douglass W. C. Kendall C. W. Mayall (Alt) W. J. McNutt Norman M. Neagle (Alt) R. L. Schwab R. E. Uptegraff, Jr G. C. Wilburn Rural ElectriÞcation Administration ........................................................................................................ J. C. Arnold, Jr Tennessee Valley Authority ............................................................................................................................L. R. Smith Underwriters Laboratories .............................................................................................................. W. A. Farquhar (Alt) E. J. Huber U.S. Bureau of Reclamation .......................................................................................................................... S. J. Baxter Organization Represented Name of Representative The Relay Input Sources Subcommittee of the Power System Relaying Committee of the IEEE Power Engineering Society, at the time that it reviewed and approved this guide, had the following membership: D. R. Volzka , Chair J. W. Walton , Secretary F. G. Basso J. Berdy J. L. Blackburn C. F. Burke M. B. DeJarnette C. M. Gadsden F. B. Hunt W. C. Kotheimer W. A. Lewis M. D. Limerick J. Miller A. R. Summers A. Sweetana J. M. Vanderleck L. N. Walker E. C. Wentz At the time this guide was approved the members of the working group of the Relay Input Sources Subcommittee were: D. R. Volzka , Chair C. F. Burke M. D. Limerick F. E. Newman D. H. Colwell R. E. Linton When this guide was approved March 9, 1978, the IEEE Standards Board had the following membership: Joseph L. Koepfinger, Chair Irvin N. Howell, Jr, Vice Chair Ivan G. Easton, Secretary William E. Andrus C. N. Berglund Edward J. Cohen Warren H. Cook David B. Dobson R. O. Duncan Charles W. Flint Jay Forster Ralph I. Hauser Loering M. Johnson Irving Kolodny William R. Kruesi Thomas J. Martin John E. May Donald T. Michael Voss A. Moore William S. Morgan Robert L. Pritchard Blair A. Rowley Ralph M. Showers B. W. Whittington iv CLAUSE PAGE 1. Introduction .........................................................................................................................................................1 2. Consideration of American National Standards Institute (ANSI) Accuracy Classes .........................................1 3. Safety Considerations in Field Testing Current Transformers ...........................................................................2 4. Current Transformer Types, Construction, Effect On Test Methods..................................................................2 4.1 Bushing, Window, or Bar-Type Current Transformers with Uniformly Distributed Windings................ 2 4.2 Wound Current Transformers, or Those without Uniformly Distributed Windings ................................. 2 4.3 Consideration of Remanence ..................................................................................................................... 3 5. Insulation Resistance Tests .................................................................................................................................4 6. Ratio Tests...........................................................................................................................................................4 6.1 Voltage Method.......................................................................................................................................... 4 6.2 Current Method .......................................................................................................................................... 5 7. Polarity Check.....................................................................................................................................................6 7.1 DC Voltage Test......................................................................................................................................... 6 7.2 AC Voltage Test Ñ Oscilloscope .............................................................................................................. 6 7.3 Current Method .......................................................................................................................................... 7 8. Winding and Lead Resistance (Internal Resistance)...........................................................................................8 9. Excitation Test ....................................................................................................................................................8 10. Burden Measurements.......................................................................................................................................10 11. Specialized Situations .......................................................................................................................................10 11.1 Current Transformers in a Closed-Delta Transformer Connection.......................................................... 10 11.2 Generator Current Transformers .............................................................................................................. 10 11.3 Intercore Coupling Check ........................................................................................................................ 11 v An American National Standard IEEE Guide for Field Testing of Relaying Current Transformers 1. Introduction In the application of protective relays, the most widely used input quantity is current. A multiplicity of different protective relays either utilizes current directly, combines it with other currents as in differential schemes, or combines it with voltage to make impedance or power measurements. The source of relay input current is from current transformers which may be located on the bushings of power circuit breakers and power transformers, on the bus bars of metal clad switchgear, or installed as separate items of equipment located as required. The purpose of this guide is to describe Þeld test methods that will assure that the current transformers are connected properly, are of marked ratio and polarity, and are in condition to perform as designed both initially and after a period of service. 2. Consideration of American National Standards Institute1 (ANSI) Accuracy Classes Relaying accuracy classes have been established in ANSI/IEEE C57.13-1978, Requirements for Instrument Transformers, to specify the performance of relaying current transformers. During faults on the electric power system, relaying current transformers must operate at high overcurrent levels. ANSI classiÞcations, therefore, deÞne minimum steady-state performance at these levels. Performance is described by using a two-term identiÞcation system consisting of a letter and a number selected from: (C,T) (10, 20, 50, 100, 200, 400, 800), for example, C400. The Þrst term of this identiÞcation describes performance in terms basically relating to construction and is discussed in Section 4. The second term of this identiÞcation is the secondary terminal voltage rating. It speciÞes the secondary voltage that can be delivered by the full winding at 20 times rated secondary current without exceeding 10 percent ratio correction. As an example, a 100 V rating means that the ratio correction will not exceed 10 percent at any current from 1 to 20 times rated current with a standard 1.0 W burden. (1.0 W times 5 A times 20 times rated secondary current equals 100 V.) The ANSI voltage rating applies to the full secondary winding only. If other than the full winding is used, the voltage rating is reduced in approximate proportion to tums used. 1ANSI documents are available from the American National Standards Institute, 1430 Broadway, New York, NY 10010. Copyright © 1981 IEEE All Rights Reserved 1 ANSI/IEEE C57.13.1-1981 IEEE GUIDE FOR TESTING OF 3. Safety Considerations in Field Testing Current Transformers Many of the tests called for in this guide involve high voltage and, therefore, should be performed only by experienced personnel familiar with any peculiarities or dangers that may exist in the test setups and test procedures. While some dangers are speciÞcally pointed out herein, it is impractical to list all necessary precautions. Test procedures in 4.3, 6.1, 7.1, and 7.2 are described appropriately for the usual case where secondary tums are more numerous than primary turns. In the unusual case where primary turns are more numerous than secondary turns, primary and secondary and H1 and X1 should be interchanged in these paragraphs and related Þgures. 4. Current Transformer Types, Construction, Effect On Test Methods Current transformers for protective relay applications are divided into two general categories which affect test methods. 4.1 Bushing, Window, or Bar-Type Current Transformers with Uniformly Distributed Windings Current transformers of this type have no Òprimary windingÓ but rather utilize the primary conductor passing once through the center of a toroidal core to perform this function. Since the secondary winding is uniformly distributed about the core and only a single primary turn is used, essentially all ßux which links the primary conductor also links the secondary winding as shown in Fig 1. Figure 1ÑUniformly Distributed Secondary Winding Because there is essentially no leakage ßux in such a device, it has negligible leakage reactance. Therefore, the excitation characteristic can be used directly to determine performance. Current transformers of this type have a C classiÞcation per ANSI/IEEE C57.13-1978, indicating that ratio correction at any current can be calculated adequately if the burden, secondary winding resistance, and the excitation characteristics are known. ANSI/IEEE C57.13-1978 states that if transformers have C classiÞcation on the full winding, all tapped sections shall be so arranged that the ratio can be calculated from excitation characteristics. Previous issues of ANSI/IEEE C57.13 did not require such arrangement of tapped sections. 4.2 Wound Current Transformers, or Those without Uniformly Distributed Windings Wound-type current transformers are usually constructed with more than one primary turn and undistributed windings. Because of the physical space required for insulation and bracing of the primary winding and fringing effects of nonuniformly distributed windings, ßux is present which does not link both primary and secondary windings. Figure 2 is included to clearly illustrate the effect but does not reßect usual construction practice. 2 Copyright © 1981 IEEE All Rights Reserved RELAYING CURRENT TRANSFORMERS ANSI/IEEE C57.13.1-1981 The presence of such leakage ßux has a signiÞcant effect on current transformer performance. When this ßux is appreciable, it is not possible to calculate ratio correction knowing the burden and the excitation characteristic. Units of this type have a T classiÞcation in accordance with ANSI/IEEE C57.13-1978, indicating that ratio correction is to be determined by test. Figure 2ÑLeakage Flux Associated with Class T Current Transformers 4.3 Consideration of Remanence The performance of both C and T class current transformers is inßuenced by remanence or residual magnetism. The available core materials are all subject to hysteresis. The phenomenon is shown by plotting curves of magnetic ßux density as a function of magnetizing force as shown in Fig 3(a). When the current is interrupted, the curves show that the ßux density does not become zero when the current does. When the current contains a dc component, the magnetizing force in one direction is much greater than in the other. The curves resulting are both displaced from the origin and distorted in shape, with a large extension to right or left in the direction of the dc component as noted in Fig 3(b). If the current which is interrupted is high, or if it contains a large dc component and is interrupted when total ßux is high, remanence will be substantial, perhaps being above the ßux equivalent of the knee point shown on the excitation curve of Fig 10. When the current transformer is next energized, the ßux changes required will start from the remanent value, and if the required change is in the direction to add to the remanent ßux, a large part of the cycle may Þnd the current transformer saturated. When this occurs, much of the primary current is required for excitation, and secondary output is signiÞcantly reduced and distorted on alternate half cycles. This condition can be corrected by demagnetizing the current transformer. It is accomplished by applying a suitable variable alternating voltage to the secondary, with initial magnitude sufÞcient to force the ßux density above the saturation point, and then decreasing the applied voltage slowly and continuously to zero. If there is any reason to suspect that a current transformer has been recently subjected to heavy currents, possibly involving a large dc component, or been magnetized by any application of dc voltage, it should be demagnetized before being used for any test requiring accurate current measurement. Test connections are identical to those required for the excitation test as shown in Fig 9. Copyright © 1981 IEEE All Rights Reserved 3 ANSI/IEEE C57.13.1-1981 IEEE GUIDE FOR TESTING OF 5. Insulation Resistance Tests Insulation resistance between the current transformer secondary and ground is usually checked by the use of conventional insulation test instruments. The neutral ground must be removed and the current transformer preferably isolated from its burden for this test. Actually, the neutral can be used to test all three phases simultaneously. If relays are left connected to the current transformers during the test, the relay manufacturer should be consulted before test values above 500 V are used. Many solid-state relay designs have surge-suppression capacitors connected from input terminals to ground which may be damaged by use of a higher voltage. Figure 3ÑHysteresis Curves: (a) Ñ Normal Hysteresis Curve; (b) Ñ Hysteresis Curve with Remanence The resistance should be compared with those of similar devices or circuits. Readings lower than those known to be good should be carefully investigated. The generally accepted minimum insulation resistance is 1 MW. One of the most common reasons for low readings is the presence of moisture. Drying out the equipment and retesting should be considered before it is dismantled. 6. Ratio Tests There are two generally accepted methods of checking the tums ratio of all types of current transformers. 6.1 Voltage Method A suitable voltage, below saturation, is applied to the secondary (full winding), and the primary voltage is read with a high-impedance (20 000 W/V or greater) low-range voltmeter as shown in Fig 4. The turns ratio is approximately equal to the voltage ratio. Saturation level is usually about 1 V per turn in most low- and medium-ratio bushing current transformers. High-ratio generator current transformers and window-type current transformers used in metal-clad switchgear may have saturation levels lower than 0.5 V per turn. In the case of very high, ratio current transformers, application of a test voltage with an even lower voltage per turn may be required to avoid personnel hazard and possible damage to equipment. The ANSI relay accuracy class voltage rating should not be exceeded during this test. At the same time the overall ratio is being determined, the tap section ratios may be checked with a voltmeter by comparing tap section voltage with the impressed voltage across the full winding. An ammeter is included in the recommended test method as a means of detecting excessive excitation current. CAUTION Ñ If more convenient, voltage may be applied to a section of the secondary winding; however, voltage 4 Copyright © 1981 IEEE All Rights Reserved across the full winding will be proportionately higher because of autotransformer action. RELAYING CURRENT TRANSFORMERS ANSI/IEEE C57.13.1-1981 6.2 Current Method This method of determining the turns ratio requires a source of high current, an additional current transformer of known ratio with its own ammeter, and a second ammete