IEEE-295-1969-R2007.pdf

Copyright © 1998 IEEE All Rights Reserved1 IEEE Std 295-1969 (R2007) IEEE Standard for Electronics Power Transformers Reaffirmed 5 December 2007 IEEE-SA Standards Board Published by The Institute of Electrical and Electronics Engineers, Inc 345 East 47th Street, New York, NY 10017, USA ii FOREWORD The Electronics Transformer Technical Committee for several years has been working towards the generation of a Standard for electronics purpose power transformers even while under AIEE auspices. Earlier attempts seemed to parallel the distribution and power transformer Standards too closely, and hence missed the point of departure of how electronics transformers needed, at least in part, a different, treatment than power distribution transformer Standards. After the completion of the Wide-Band Standards Nos. 111 and 264 by the committee, a pattern was established that could be followed in the Power Transformer Standard to achieve the desired results. This proposed Standard can be well considered to be the product of the merger of AIEE and IRE people into the new blend of IEEE as it represents concessions oil the part of each to the other at various points. Some interesting problems had to be resolved in the process. For example, in the use of solid-state rectiÞers it is necessary to provide fast and sensitive protection for the easily destroyed diodes in the event of faulting. This, in turn, led to a new deÞnition of transformer inrush current, which we found was not previously deÞned satisfactorily. Also, the extensive use of combined alternating and direct voltages and currents in electronics transformers made it necessary to resolve certain insulation testing problems that have plagued the industry for years. Yet to say that these problems are resolved is an oversimpliÞcation of the situation. Nevertheless, a good start has been made in this regard. We have attempted to give guidance and direction in the sensitive area of corona testing without foreclosing on the adoption of new Standards and understandings on the subject. For all sizes of electronics purpose power transformers, it is expected that this Standard will supersede the use of USA C57 Standards. PURPOSE The character and applications of transformers used in electronic circuits are enough different from other categories so that none of the existing Standards does a satisfactory job. Much confusion and conßict has obtained in the past several years because of this lack, with different people having a wide variety of opinion relative to many aspects of speciÞcation and testing. This Standard is intended to control by exception in the areas where conßict and disagreement have been most noticeable. Other well-known Standards are listed that should be used in areas where general agreement exists. iii ACKNOWLEDGMENT The Institute wishes to acknowledge its indebtedness to those who have so freely given of their time and knowledge, and have conducted experimental work on which many of the IEEE publications are based. This publication was prepared by the Electronics Power Transformer Subcommittee of the Electronics Transformers Committee of the IEEE Parts, Materials, and Packaging Group. The membership of the Subcommittee was: W. W. Wahlgren , Chair G. D. Polzin , Secretary E. E. Aldrich ( deceased ) , Secretary C. E. Carter, Jr. W. J. Field S. Hannon A. D. Hasley O. Kiltie L. W. Kirkwood A. J. Kornbluh R. Lee H. W. Lord H. Mitsanas T. Pelc I. Tarr H. I. Tillinger A. B. Trussell J. P. Whistler D. Wildfeuer R. G. Wolpert © Copyright 1969 by The Institute of Electrical and Electronics Engineers, Inc. iv CLAUSEPAGE 1. SCOPE1 1.1 Related and Reference Standards. 1 2. DEFINITIONS2 2.1 Additional Definitions 2 3. SYMBOLS3 4. TRANSFORMER ELECTRICAL TESTS.3 4.1 Electrical Tests. 3 4.2 Characteristic Tests 3 5. ELECTRICAL TESTS .3 5.1 Electrical Strength Tests 3 5.2 Induced Voltage Tests 4 5.3 Repeated Electric Strength Testing 5 5.4 Corona Tests 5 5.5 Corona Test Methods. 6 5.6 Temperature Rise Tests 6 Annex I Definitions Relating to Transformer-Rectifier Systems (Informative).8 Annex II Transformer Terminal Marking Guide (Informative)9 Annex III Typical Norms and Tolerances for Transformer Specifications (Informative)13 Annex IV Electric Strength Tests for Transformers Connected to High-Voltage Lines (Informative)14 Annex V Inrush Current Considerations (Informative)16 Annex VI Service Conditions (Informative).17 Copyright © 1969 IEEE All Rights Reserved 1 IEEE Standard for Electronics Power Transformers 1. SCOPE This Standard pertains to power transformers and inductors that are used in electronic equipments and supplied by power lines or generators of essentially sine wave or polyphase voltage. Guides to application and test procedures are included. Appendices contain certain precautions, recommended practices, and guidelines for typical values. Provision is made for relating the characteristics of transformers to the associated rectiÞers and circuits. Certain pertinent deÞnitions relating to transformers and transformer applications, which have not been found elsewhere, are included with appropriate discussion. Attempts are made to alert the industry and profession to factors that are commonly overlooked. This Standard includes, but is not limited to, the following speciÞc transformers and inductors. RectiÞer supply transformers for either high- or low-voltage supplies. Filament and cathode heater transformers. Transformers for alternating current resonant charging circuits. Inductors used in rectiÞer Þlters. Autotransformers with Þxed taps. 1.1 Related and Reference Standards a)USA Standard C57: Transformers, Regulators and Reactors. USA Standard C57: 12.00Ñ1965, General. USA Standard C57: 12.20Ñ1964, Overhead-Type Distribution Transformers, 67 000 Volts and Below, 500 kVA and Smaller. USA Standard C57: 12.80Ñ1958, Terminology. USA Standard C57: 12.90Ñ1965, Test Code. USA Standard C57: 18Ñ1964, Pool-Cathode Mercury-Arc RectiÞer Transformers, Requirements, Terminology and Test Code. USA Standard C57: 31Ñ1948, Guide for Operation of Transformers at Altitudes Greater Than 1000 Meters. USA Standard C57: 32Ñ1948, Guide for Operation of Transformers, Regulators and Reactors. b)USA Standard C42. USA Standard C42.25Ñ1956, DeÞnitions of Electrical Terms (Industrial Control Equipment). USA Standard C42.65Ñ1957, DeÞnitions of Electrical Terms (Communications). 2 Copyright © 1969 IEEE All Rights Reserved IEEE Std 295-1969IEEE STANDARD FOR c)IEEE Standards Publication No. 111, Low-Power Wide-Band Transformers. d)IEEE Standards Publication No. 264, High-Power Wide-Band Transformers. e)Standards in Preparation. Corona Test Guide. 2. DEFINITIONS Electrical terms used in this Standard shall be in accordance with those given in USA Standard C42 ÒAmerican Standard DeÞnitions of Electrical TermsÓ insofar as they apply except as herein stated. The IEEE Dictionary, when issued, shall be applicable and included as a part of this Standard and shall take precedence over USA Standard C42 in case of conßict. 2.1 Additional Definitions 2.1.1 Turns Ratio: shall be preferably deÞned in terms of the primary turns as the number of turns of a given secondary divided by the number of primary turns. Thus a ratio less than one (1) is a step-down transformation, a ratio greater than one (1) is a step-up transformation, and a ratio equal to one (1) is unity ratio. 2.1.2 Loaded Voltage Ratio: shall be equal to the secondary voltage divided by the primary voltage. (See paragraph 2.1.1.) For linear loads, the ratio shall be stated for a speciÞed load current and power factor. For rectiÞer loads, the ratio should be given for the speciÞed circuit conÞguration, including the Þlters, and the rated direct-current load. Unless otherwise stated, the ratio shall be given for rated conditions, line voltage, frequency, load, and stabilized temperature. Primary voltages shall be given as line to line and secondary voltages as leg values (terminal to neutral or center tap if used) unless otherwise indicated. 2.1.3 No-Load Loss: (excitation loss) is the input power, expressed in watts, to a completely assembled transformer that is excited at rated terminal voltage and frequency, but not supplying load current. 2.1.4 Full-Load Losses: 2.1.4.1 Core Loss: is the measured power loss, expressed in watts, attributable to the material in the core and associated clamping structure, of a transformer that is excited, with no connected load, at a core ßux density and frequency equal to that in the core when rated voltage and frequency is applied and rated load current is supplied. 2.1.4.2 Winding Loss: (copper loss) is the power losses of all windings involved, expressed in watts, in an inductor or transformer with the values measured at or corrected to the rated load current, frequency, and waveshape an stabilized at the maximum ambient temperature. 2.1.4.3 Stray Losses: are those occurring in the core and case structure that result from the leakage ßux and stray ßux of a transformer when supplying rated load current. 2.1.5 Graded Insulation: is the selective arrangement of the insulation components of a composite insulation system to more nearly equalize the voltage stresses throughout the insulation system 2.1.6 Inrush Current: is the maximum root-mean-square or average current value, determined for a speciÞed interval, resulting from the excitation of the transformer with no connected load, and with essentially zero source impedance, and using the minimum primary turns tap available and its rated voltage. (See Annex V.) 2.1.7 Peak Inrush Current: is the peak instantaneous current value resulting from the excitation of the transformer with no connected load, and with essentially zero source impedance, and using the minimum turns primary tap and rated voltage. 2.1.8 Essentially Zero Source Impedance: implies that the source impedance is low enough so that the test currents under consideration would cause less than Þve (5) percent distortion (instantaneous) in the voltage amplitude or waveshape at the load terminals. (See Annex V.) Copyright © 1969 IEEE All Rights Reserved 3 ELECTRONICS POWER TRANSFORMERSIEEE Std 295-1969 3. SYMBOLS The proposed IEEE Standards Publication No. 276 ÒLetter and Graphic Symbols for Electronics TransformersÓ or revisions shall apply. 4. TRANSFORMER ELECTRICAL TESTS Transformer terminals normally grounded in service should be grounded during these tests or connected as otherwise required or noted in the following test description. 4.1 Electrical Tests (values not recorded) It is recommended that the following electrical tests be made on all transformers. Ratio, polarity, terminal marking tests. No-load excitation; exciting current (amperes), loss (watts). Corona test (when speciÞed). Induced voltage. Electric strength of insulation. 4.2 Characteristic Tests (may be performed on a limited basis unless otherwise speciÞed). Inrush current (when speciÞed). Winding loss, impedance, regulation. Leakage inductance. Impulse (when speciÞed). Temperature rise, winding resistance. 5. ELECTRICAL TESTS 5.1 Electrical Strength Tests (see paragraph 5.3 for retesting). Applied high-voltage tests to major insulation systems should be made with windings shorted. Windings and shields on one side of the insulation should be connected to frame and ground while windings or shields on the other side should be connected together. Sine wave test voltages having a frequency in the operating range of the transformer and having adequate current capacity for the application is applied between the two sets of terminals in the manner set forth herein. All voltages should be deÞned in the same terms; e.g., root mean square, peak, average. 5.1.1 Method Voltage should be increased at a convenient uniform rate of not greater than 2000 volts per second, from zero to the speciÞed value, maintained for the speciÞed period (unless breakdown occurs) then decreased to zero at the same rate. 4 Copyright © 1969 IEEE All Rights Reserved IEEE Std 295-1969IEEE STANDARD FOR 5.1.2 Primary Windings with rated voltagex over 600 volts line to line should be tested in accordance with USAS C57.12 as amended or revised (see Annex IV). 5.1.3 Primary Windings with rated voltages 600 volts or less line to line should be tested with sine wave alternating voltage equal to twice the rated voltage of the highest voltage tap, plus 1000 volts and held at that value for the duration of 3600 cycles. 5.1.4 Connections for windings not under test should be speciÞed so that unwarranted stresses will not occur during the electric strength tests. Windings with relatively low working voltage to ground should be grounded during the test of other windings to prevent the lower voltage insulation from being damaged through capacitive coupling. 5.1.5 Secondary Windings that have no special test voltage speciÞed should be tested with applied alternating voltage equal to twice the rated voltage of the highest voltage tap, plus 1000 volts and held at that value for the lesser time of 3600 cycles or one minute. 5.1.6 Secondary Windings or Inductor Windings that may have a speciÞc operating direct or alternating voltage derived elsewhere, unless otherwise speciÞed, should be tested at twice the working volts plus 1000 volts for the lesser of one minute or 3600 cycles and using the same type and frequency of voltage as the working stress. High alternating voltage should not be substituted for direct current unless speciÞcally authorized by the manufacturer. 5.1.7 When the voltage insulation strength is not the same at both ends of a winding, an induced voltage test may be substituted in lieu of the applied voltage test. 5.2 Induced Voltage Tests 5.2.1 Secondary Windings and Inductors employing graded insulation systems may be tested as described herein in lieu of other high-voltage electric strength tests. NOTE Ñ Many high-voltage rectiÞer windings have a distinctly different voltage stress to adjacent windings on one end of a winding when compared with the stress on the other end. Not only can the voltage stress be different in magnitude, but also in waveform. An alternating voltage stresses the insulation much more than a direct voltage of the same peak value. For example, a three-phase winding wye connected for a system using three-phase bridge rectiÞer will have a direct voltage to ground at the neutral equal to one-half of the bridge output, whereas the rectiÞer terminals will have an alternating voltage excursion from zero to twice the peak voltage of one leg. It is not necessary or desirable to use the same kind and amount of insulation at the neutral as at the rectiÞer terminals, yet it is desirable to test both insulations at twice the normal working voltage and with a waveform similar to that of the working voltage. In this test, apply a direct voltage between neutral and ground equal to twice the working voltage of the neutral, and at the same time excite the tran