IEEE Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers

IEEE Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers

IEEE Power and Energy Society

Sponsored by the Transformers Committee

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

IEEE Std C57.12.90™-2015

(Revision of IEEE Std C57.12.90-2010)

Authorized licensed use limited to: Universidad del Valle. Downloaded on March 09,2017 at 21:27:35 UTC from IEEE Xplore. Restrictions apply.


IEEE Std C57.12.90™-2015

(Revision of IEEE Std C57.12.90-2010)

IEEE Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers Sponsor

Transformers Committee of the

IEEE Power and Energy Society Approved 5 December 2015

IEEE-SA Standards Board


IEEE Std C57.12.90-2015 IEEE Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers

Abstract: Methods for performing tests specified in IEEE Std C57.12.00™ and other standards applicable to liquid-immersed distribution, power, and regulating transformers are described. Instrument transformers, step-voltage and induction voltage regulators, arc furnace transformers, rectifier transformers, specialty transformers, grounding transformers, and mine transformers are excluded. Resistance measurements, polarity and phase-relation tests, ratio tests, no-load loss and excitation current measurements, impedance and load loss measurements, dielectric tests, temperature tests, short-circuit tests, audible sound level measurements, and calculated data are covered in this standard. Keywords: IEEE C57.12.90™, tests, transformer tests, transformers •

The Institute of Electrical and Electronics Engineers, Inc. 3 Park Avenue, New York, NY 10016-5997, USA Copyright © 2016 by The Institute of Electrical and Electronics Engineers, Inc. All rights reserved. Published 11 March 2016. 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 978-1-5044-0769-4 ISBN 978-1-5044-0770-0

STD20842 STDPD20842

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Participants At the time this draft standard was completed, the Continuous Revision of C57.12.90 Working Group had the following membership: Stephen Antosz, Chair Peter Balma, Vice Chair William Bartley Marcel Fortin Ramsis Girgis

Paulette Payne Powell Mark Perkins Bertrand Poulin Pierre Riffon

Steven Snyder Ed teNyenhuis Loren Wagenaar

The following members of the individual balloting committee voted on this standard. Balloters may have voted for approval, disapproval, or abstention. Samuel Aguirre Gregory Anderson Tauhid Haque Ansari Stephen Antosz I. Antweiler Javier Arteaga Roberto Asano Donald Ayers Roy Ayers Robert Ballard Peter Balma Thomas Barnes Paul Barnhart William Bartley Christopher Baumgartner Barry Beaster Jeffrey Benach W.J. (Bill) Bergman Mats Bernesjo Enrique Betancourt Steven Bezner Wallace Binder Thomas Bishop Thomas Blackburn Daniel Blaydon William Bloethe W. Boettger Alain Bolliger Paul Boman Jeffrey Britton Adam Bromley Chris Brooks Darren Brown Steven Brown David Buckmaster Carl Bush Thomas Callsen Paul Cardinal Juan Castellanos Donald Cherry Bill Chiu

Craig Colopy Stephen Conrad Jerry Corkran John Crouse Willaim Darovny Alan Darwin Frank Decesaro Dieter Dohnal Gary Donner Charles Drexler Fred Elliott James Fairris Jorge Fernandez Daher Joseph Foldi Bruce Forsyth Marcel Fortin Michael Franchek Fredric Friend George Frimpong Jose Gamboa Robert Ganser Carlos Gaytan Frank Gerleve Ali Ghafourian David Gilmer Ramsis Girgis Jalal Gohari Edwin Goodwin James Graham William Griesacker Randall Groves Bal Gupta Ajit Gwal Said Hachichi John Harley J. Harlow David Harris Roger Hayes Jeffrey Helzer William Henning Martin Hinow

Gary Hoffman Timothy Holdway Thomas Holifield Jill Holmes Philip Hopkinson John Houdek Mohammad Iman Richard Jackson Erwin Jauch Brad Jensen Song Jin John John Charles Johnson Wayne Johnson Laszlo Kadar John Kay Gael Kennedy Sheldon Kennedy Vladimir Khalin Gary King Joseph L. Koepfinger Boris Kogan Axel Kraemer Neil Kranich Jim Kulchisky Saumen Kundu John Lackey Chung-Yiu Lam Thomas La Rose William Larzelere Aleksandr Levin Hua Liu Mario Locarno John Luksich Thomas Lundquist Richard Marek J. Dennis Marlow Lee Matthews Omar Mazzoni James McBride Mark McNally

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Copyright © 2016 IEEE. All rights reserved.



Susan Mcnelly Charles Mc Shane Joseph Melanson Tom Melle Michael Miller Michael R. Miller Daleep Mohla Charles Morgan Daniel Mulkey Jerry Murphy Paul Mushill Ali Naderian Jahromi K. R. M. Nair Kris K. Neild Dennis Neitzel Arthur Neubauer Michael Newman Raymond Nicholas Ryan Niemerg Joe Nims Robert Olen Tim Olson David Ostrander Lorraine Padden Dwight Parkinson Luke Parthemore Bansi Patel Wesley Patterson Paulette Payne Powell Verena Pellon Brian Penny Mark Perkins Branimir Petosic Christopher Petrola Donald Platts

Klaus Pointner Alvaro Portillo Bertrand Poulin Lewis Powell Tom Prevost Jarrod Prince Ulf Radbrandt Martin Rave Jeffrey Ray Timothy Charles Raymond Leslie Recksiedler Jean-Christophe Riboud Johannes Rickmann Pierre Riffon Diego Robalino Michael Roberts Charles Rogers Oleg Roizman John Rossetti Thomas Rozek Dinesh Sankarakurup Daniel Sauer Roderick Sauls Bartien Sayogo Ewald Schweiger Jeremy Sewell Nikunj Shah Devki Sharma Hemchandra Shertukde Stephen Shull Hyeong Sim Charles Simmons Kenneth Skinger Jerry Smith

Steve Snyder Sanjib Som Thomas Spitzer Ronald Stahara David Stankes John Sullivan Troy Alan Tanaka Ed teNyenhuis Malcolm Thaden Juan Thierry James Thompson Robert Thompson Alan Traut Ajith Varghese Roger Verdolin John Vergis Jane Verner Richard Vongemmingen Loren Wagenaar David Wallace David Wallach Joe Watson Lee Welch Yingli Wen Kenneth White Roger Wicks Alan Wilks William Wimmer Baitun Yang Jennifer Yu Jian Yu Kipp Yule Shibao Zhang James Ziebarth Waldemar Ziomek

When the IEEE-SA Standards Board approved this standard on 5 December 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. Koepfinger* David J. 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

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Introduction This introduction is not part of IEEE Std C57.12.90-2015, IEEE Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers.

This document is a voluntary consensus standard. Its use may become mandatory only when required by a duly constituted legal authority or when specified in a contractual relationship. To meet specialized needs and to allow innovation, specific changes are permissible when mutually determined by the purchaser and manufacturer, provided that such changes do not violate existing laws and are considered technically adequate for the function intended. When this standard is used on a mandatory basis, the word “shall” indicates mandatory requirements, and the words “should” and “may” refer to matters that are recommended or permissive, but not mandatory. The word “must” has been removed from this revision and replaced with “shall” to conform with the IEEESA Standards Style Manual. This standard is on a continuous revision cycle and is constantly being reviewed and updated. One can go to the website www.transformerscommittee.org to seek out information on select activities and participate in upcoming changes. The following is a brief summary of the non-editorial changes in this revision: 

Subclause 6.2, Polarity Tests for single-phase transformers, has been revised to remove the comparison method and the remaining methods have been re-arranged in order of preference.



Subclause 6.3, Polarity and phase-relation tests for polyphase transformers, has been slightly revised.



Subclause 7.1.4, Ratio test of three-phase transformers with inaccessible neutrals, has been rewritten.



Subclause 9.5, Zero-squence impedance testing, has additional notes added.



Subclause 10.1.5.1, Dielectric Test sequence, has been revised.



New subclause 10.2.5, Connection of neutral terminal during switching impulse tests.



Revisions to 10.2.1, 10.3, 10.3.2.5, and 10.3.3. The number of full wave impulses applied during the test sequence has increased from one to two or three.



Subclause 10.3.2.4, Tap connections (during lightning impulse test), has been completely rewritten.



Subclause 11.1, Test Methods, the order of the two simulated loading methods has been reversed.



Subclause 11.1.2.1, Loading back method, the text, Figure 29, and Figure 30 have been revised.



Subclause 11.2.2, Hot resistance measurements, list items a) through f) have been revised.



Clause 12, Short-circuit tests, has been revised; and a new Annex C added on connection diagrams for short-circuit testing of a three-phase transformer using single-phase source. Multiple references are added to Annex D.



Clause 13, Audible sound testing, has been completely revised, adding: load sound measurements; sound-intensity measurement methods; new corrections for wall reflection, near-field effects, and environmental; a method to add no-load & load sound; a change in microphone locations.

Technical revisions were prepared by various groups within the IEEE Transformers Committee and have been surveyed and approved by these groups up through the subcommittee level. viii




Contents 1. Overview .................................................................................................................................................... 1 1.1 Scope ................................................................................................................................................... 1 1.2 Purpose ................................................................................................................................................ 2 1.3 Word usage .......................................................................................................................................... 2 2. Normative references.................................................................................................................................. 2 3. Definitions .................................................................................................................................................. 3 4. General ....................................................................................................................................................... 4 4.1 Types of tests ....................................................................................................................................... 4 4.2 Test requirements ................................................................................................................................ 4 4.3 Test sequence ....................................................................................................................................... 4 4.4 Instrumentation .................................................................................................................................... 5 5. Resistance measurements ........................................................................................................................... 5 5.1 Determination of cold temperature ...................................................................................................... 5 5.2 Conversion of resistance measurements .............................................................................................. 6 5.3 Resistance measurement methods ....................................................................................................... 6 5.4 Resistance measurement connections and reporting ............................................................................ 7 6. Polarity and phase-relation tests ................................................................................................................. 8 6.1 Subtractive and additive polarity ......................................................................................................... 8 6.2 Polarity tests: single-phase transformers ............................................................................................. 9 6.3 Polarity and phase-relation tests: polyphase transformers ..................................................................10 7. Ratio tests ..................................................................................................................................................14 7.1 General ...............................................................................................................................................14 7.2 Tolerances for ratio .............................................................................................................................15 7.3 Ratio test methods ..............................................................................................................................15 8. No-load losses and excitation current ........................................................................................................17 8.1 General ...............................................................................................................................................17 8.2 No-load loss test .................................................................................................................................17 8.3 Waveform correction of no-load losses ..............................................................................................20 8.4 Temperature correction of no-load losses ...........................................................................................20 8.5 Determination of excitation (no-load) current ....................................................................................21 8.6 Frequency conversion of no-load losses and excitation current .........................................................21 9. Load losses and impedance voltage ...........................................................................................................21 9.1 General ...............................................................................................................................................21 9.2 Factors affecting the values of load losses and impedance voltage ....................................................22 9.3 Tests for measuring load losses and impedance voltage .....................................................................23 9.4 Calculation of load losses and impedance voltage from test data .......................................................30 9.5 Zero-phase-sequence impedance ........................................................................................................33 10. Dielectric tests .........................................................................................................................................37 10.1 General .............................................................................................................................................37 10.2 Switching impulse test procedures ...................................................................................................40 10.3 Lightning impulse test procedures ....................................................................................................41 10.4 Routine impulse test for distribution transformers ...........................................................................49 ix




10.5 Low-frequency tests..........................................................................................................................51 10.6 Applied-voltage tests ........................................................................................................................52 10.7 Induced-voltage tests for distribution and Class I power transformers .............................................52 10.8 Induced-voltage test for Class II power transformers .......................................................................53 10.9 Partial discharge measurement .........................................................................................................54 10.10 Insulation power-factor tests ...........................................................................................................55 10.11 Insulation resistance tests ...............................................................................................................56 11. Temperature-rise tests .............................................................................................................................58 11.1 Test methods .....................................................................................................................................58 11.2 Resistance measurements .................................................................................................................61 11.3 Temperature measurements ..............................................................................................................63 11.4 Correction of temperature-rise test results ........................................................................................65 12. Short-circuit tests .....................................................................................................................................67 12.1 General .............................................................................................................................................67 12.2 Test connections ...............................................................................................................................68 12.3 Test requirements .............................................................................................................................71 12.4 Test procedure ..................................................................................................................................72 12.5 Proof of satisfactory performance.....................................................................................................73 13. Audible sound emissions .........................................................................................................................75 13.1 General .............................................................................................................................................75 13.2 Instrumentation .................................................................................................................................76 13.3 Test conditions ..................................................................................................................................77 13.4 Microphone positions .......................................................................................................................79 13.5 Sound pressure level measurements .................................................................................................80 13.6 Determination of audible sound level of a transformer ....................................................................84 13.7 Presentation of results .......................................................................................................................87 14. Calculated data ........................................................................................................................................90 14.1 Reference temperature ......................................................................................................................90 14.2 Losses and excitation current ............................................................................................................90 14.3 Efficiency .........................................................................................................................................90 14.4 Voltage regulation of a constant-voltage transformer.......................................................................90 Annex A (informative) Partial discharge measurement using radio-influence voltage instrumentation and its failure detection .................................................................................................................................93 A.1 Partial discharge measurement ..........................................................................................................93 A.2 Failure detection ................................................................................................................................94 Annex B (normative) 50/60-Hz frequency conversion of measured performance parameters .....................95 B.1 No-load loss and excitation current ....................................................................................................95 B.2 Load loss ............................................................................................................................................96 B.3 Temperature-rise test..........................................................................................................................97 B.4 Short-circuit test .................................................................................................................................99 B.5 Audible sound ....................................................................................................................................99 Annex C (informative) Connections diagrams for short-circuit testing of a three-phase transformer using single-phase source as an alternative (see 12.2.1.2)...........................................................................102 Annex D (informative) Bibliography ..........................................................................................................105

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IEEE Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers 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 standard describes methods for performing tests specified in IEEE Std C57.12.00™ 1 and other standards applicable to liquid-immersed distribution, power, and regulating transformers. It is intended for use as a basis for performance and proper testing of such transformers. This standard applies to all liquid-immersed transformers, except instrument transformers, step-voltage and induction voltage regulators, arc furnace transformers, rectifier transformers, specialty transformers, grounding transformers, and mine transformers. Transformer requirements and specific test criteria are not a part of this standard, but they are contained in appropriate standards, such as IEEE Std C57.12.00, IEEE Std C57.12.10™ [B17], 2 IEEE Std C57.12.20™, and IEEE Std C57.12.40™ [B18], or in user specifications.

1 2

Information on references can be found in Clause 2. The numbers in brackets correspond to those of the bibliography in Annex D

1




1.2 Purpose The purpose of this standard is to provide test procedure information for tests specified in IEEE Std C57.12.00 and other standards applicable to liquid-immersed distribution, power, and regulating transformers. It is intended for use as a basis for performance and proper testing of such transformers.

1.3 Word usage When this standard is used on a mandatory basis, the word “shall” indicates mandatory requirements. The words “should” or “may” refer to matters that are recommended or permitted but not mandatory.

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. ANSI C84.1, American National Standard for Electric Power Systems and Equipment—Voltage Ratings (60 Hertz). 3 ANSI S1.4, American National Standard for Sound Level Meters. ANSI S1.11, American National Standard for Octave Band and Fractional-Octave-Band Analog and Digital Filters. IEEE Std 4™, IEEE Standard Techniques for High-Voltage Testing. 4,5 IEEE Std C57.12.00™, IEEE Standard General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers. IEEE Std C57.12.20™, 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. IEEE Std C57.12.23™, IEEE Standard for Submersible Single-Phase Transformers167 kVA and smaller; High Voltage 25 000 V and Below; Low Voltage 600 V and below. IEEE Std C57.12.24™, IEEE Standard for Submersible, Three-Phase Transformers, 3750 kVA and Smaller: High Voltage, 34 500 GrdY/19 920 Volts and Below; Low Voltage, 600 Volts and Below. IEEE Std C57.12.34™, IEEE Standard for Requirements for Pad-Mounted, Compartmental-Type, SelfCooled, Three-Phase Distribution Transformers, 5 MVA and Smaller; High Voltage, 34.5 kV Nominal System Voltage and Below; Low Voltage, 15 kV Nominal System Voltage and Below. IEEE Std C57.12.36™, IEEE Standard Requirements for Liquid-Immersed Distribution Substation Transformers. 3 ANSI publications are available from the Sales Department, American National Standards Institute, 25 West 43rd Street, 4th Floor, New York, NY 10036, USA (http://www.ansi.org/). 4 IEEE publications are available from the Institute of Electrical and Electronics Engineers Inc., 445 Hoes Lane, Piscataway, NJ 08854, USA (http://standards.ieee.org/). 5 The IEEE standards or products referred to in this clause are trademarks of the Institute of Electrical and Electronics Engineers, Inc.

2




IEEE Std C57.12.38™, IEEE Standard for Pad-Mounted-Type, Self-Cooled, Single-Phase Distribution Transformers; High Voltage, 34 500 GrdY/19 920 V and Below, Low Voltage, 240/120 V; 167 kVA and Smaller. IEEE Std C57.12.80™, IEEE Standard Terminology for Power and Distribution Transformers. IEEE Std C57.19.00™, IEEE Standard General Requirements and Test Procedures for Power Apparatus Bushings. IEEE Std C57.19.01™, IEEE Standard Performance Characteristics and Dimensions for Outdoor Apparatus Bushings. IEEE Std C57.91™, IEEE Guide for Loading Mineral-Oil-Immersed Transformers. IEEE Std C57.98™, IEEE Guide for Transformer Impulse Tests. IEEE Std C57.113™, IEEE Recommended Practice for Partial Discharge Measurement in Liquid-Filled Power Transformers and Shunt Reactors. IEEE Std C57.123™, IEEE Guide for Transformer Loss Measurement.

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. 6 NOTE—Although the term “liquid” is used in some places in this document for simplicity, it is synonymous with the term “insulating liquid” and includes, as appropriate, insulating liquids such as mineral oil, natural esters, synthetic esters, silicone, or high molecular weight hydrocarbons. 7

actual time to crest: The time interval from the start of the transient to the time when the maximum amplitude is reached. ambient sound pressure level: The sound pressure level measured at the test facility or at the substation without the transformer energized. A-weighted sound level: Loudness that is measured with a sound level meter using the A-weighted response filter that is built into the meter circuitry. The A-weighting filter is commonly used to measure community noise, and it simulates the frequency response of the human ear. C-weighted sound level: Loudness that is measured with a sound level meter using the C-weighted filter that is built into the sound level meter. The C-weighting has only little dependence on frequency over the greater part of the audible frequency range. efficiency (of a transformer): The ratio of the useful power output of a transformer to the total power input. guard: One or more conducting elements arranged and connected on an electrical instrument or measuring circuit to divert unwanted currents from the measuring means.

6

IEEE Standards Dictionary Online subscription is available at: http://ieeexplore.ieee.org/xpls/dictionary.jsp. Notes in text, tables, and figures are given for information only and do not contain requirements needed to implement the standard.

7

3




semi-reverberant facility: A room with a solid floor and an undetermined amount of sound-absorbing materials on the walls and ceiling. sound pressure level Lp in decibels: Twenty times the logarithm to the base ten of the ratio of the measured sound pressure ( P ) to a reference pressure ( Po ) of 20 µPa or

P L p = 20 log10    Po  sound power level Lw in decibels: Ten times the logarithm to the base ten of the emitted sound power (w) to a reference power ( wo ) of 10–12 W or

 w Lw = 10 log10    wo  time to first voltage zero on the tail of the wave: The time interval from the start of the transient to the time when the first voltage zero occurs on the tail of the wave. total losses: The sum of the no-load losses and the load losses.

4. General 4.1 Types of tests Various types of tests (routine, design, conformance, and other) are defined in IEEE Std C57.12.80.

4.2 Test requirements A general summary of test requirements is included in Table 17 of IEEE Std C57.12.00-2015 and indicates which tests are normally considered routine, design, or other, by size and class.

4.3 Test sequence See 10.1.5.1 for the sequence of dielectric tests when lightning impulse or switching impulse tests are specified. NOTE—To minimize potential problems for the transformer during testing, the resistance, polarity, phase relation, ratio, no-load loss and excitation current, impedance, and load loss tests (and temperature-rise tests, when applicable) should precede dielectric tests. Using this sequence, the beginning tests involve voltages and currents, which are usually reduced as compared with rated values, thus tending to minimize problematic effects to the transformer.

4




4.4 Instrumentation Although the figures in this standard show conventional meters, adequate digital readout measuring devices and digital sampling techniques with computer calculations are considered to be satisfactory alternatives.

5. Resistance measurements Resistance measurements are of fundamental importance for the following purposes: a)

Calculation of the I 2 R component of conductor losses

b)

Calculation of winding temperatures at the end of a temperature-rise test

c)

As a quality control test of the manufacturing process

d)

As a base for assessing possible damage in the field

5.1 Determination of cold temperature The cold temperature of the winding shall be determined as accurately as possible when measuring the cold resistance. The precautions in 5.1.1, 5.1.2, and 5.1.3 shall be observed. 5.1.1 General Cold-resistance measurements shall be made on a transformer only when the liquid or winding temperature is stable. The temperature is considered stable if the top liquid temperature does not vary more than 2 ºC in a 1-h period. 5.1.2 Transformer windings immersed in insulating liquid The temperature of the windings shall be assumed to be the same as the average temperature of the insulating liquid, provided: e)

The windings have been under insulating liquid with no excitation and with no current in the windings for a minimum of 3 h for a transformer without pumps and for 1 h for transformer with pumps running before the cold resistance is measured.

f)

The temperature of the insulating liquid has stabilized, and the difference between top and bottom temperature does not exceed 5 ºC.

5.1.3 Transformer windings out of insulating liquid The temperature of the windings shall be recorded as the average of several thermometers or thermocouples inserted between the coils, with care used to see that their measuring points are as nearly as possible in actual contact with the winding conductors. It should not be assumed that the windings are at the same temperature as the surrounding air.

5




5.2 Conversion of resistance measurements Cold winding resistance measurements are normally converted to a standard reference temperature ( Ts ) equal to the rated average winding temperature rise plus 20 ºC. In addition, it may be necessary to convert the resistance measurements to the temperature at which the impedance loss measurements were made. The conversions are accomplished by Equation (1):

Rs = Rm

Ts + Tk Tm + Tk

(1)

where

Rs Rm Ts Tm Tk

is the resistance at desired temperature Ts (Ω) is the measured resistance (Ω) is the desired reference temperature (ºC) is the temperature at which resistance was measured (ºC) is 234.5 ºC (copper) or 225 ºC (aluminum)

NOTE—The value of Tk may be as high as 230 ºC for alloyed aluminum.

5.3 Resistance measurement methods 5.3.1 Voltmeter–ammeter method The voltmeter-ammeter method is the most common method used for transformer winding resistance measurement. Resistance-measuring systems employing computer-controlled digital voltmeters, currentmeasuring shunts, and/or digital ammeters of appropriate accuracy are commonly used for cold-resistance measurements and in connection with temperature-rise determinations. To use this method, the following steps should be taken: a)

Measurement is made with direct current, and simultaneous readings of current and voltage are taken using the connections in Figure 1. The required resistance is calculated from the readings in accordance with Ohm’s Law. Electronic switching power supplies may be used as voltage sources; however, batteries or filtered rectifiers may also be used, especially in instances where less ripple is desired in the measurement. Automatic recording of the resistance data is recommended so that the time to saturation and the variability of the resistance readings after stabilization can be documented.

DC SOURCE

Figure 1 —Connections for voltmeter-ammeter method of resistance measurement

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b)

The voltmeter leads shall be independent of the current leads and shall be connected as closely as possible to the terminals of the winding to be measured. This step is to avoid including in the reading the resistances of current-carrying leads and their contacts and of extra lengths of leads.

c)

When making manual resistance measurements: 1)

The voltmeter may be disconnected from the circuit before switching the current on or off to protect the voltmeter from injury by off-scale deflections. To protect test personnel from inductive kick, the current may be switched off by a suitably insulated switch with a protective circuit to discharge the energy.

2)

Due to inaccuracy of deflecting ammeters and voltmeters, current shunts and digital voltmeters, high-accuracy digital ammeters, or other high accuracy instrumentation should be used that meets the requirement of IEEE Std C57.12.00.

d)

Resistance is recommended to be measured at intervals of 5 s to 10 s, and the readings used shall be after the current and voltage have reached steady-state values.

e)

When measuring the cold resistance, preparatory to making a temperature-rise test, note the time required for the readings to become constant. That period of time should be allowed to elapse before taking the first reading when final winding hot-resistance measurements are being made. The residual flux in the core should be made the same for both the cold-resistance and hotresistance measurements by saturating the core with direct current prior to the measurement. In general, the winding will exhibit a long time constant. To reduce the time required for the current to reach its steady-state value, a noninductive external resistor may be added in series with the dc source. It may then be necessary to increase the source voltage to compensate for the voltage drop in the series resistor. The time will also be reduced by passing a direct current through other windings in either the same polarity as the winding being tested for windings on the same phase or opposite polarity for other phases during these tests. For delta-connected windings, the time can also be reduced by opening the delta connection.

f)

It is recommended that ten or more readings, but a minimum of four readings, should be used for each cold-resistance measurement, and the average of the resistances calculated from these measurements shall be considered to be the resistance of the circuit. The current used shall not exceed 15% of the rated current of the winding whose resistance is to be measured. Larger values may cause inaccuracy by heating the winding and thereby changing its temperature and resistance.

5.3.2 Bridge method Bridge methods may be used. NOTE—For resistance values of 1 Ω or more, a Wheatstone Bridge (or equivalent) is commonly used; for values less than 1 Ω, a Kelvin Bridge (or equivalent) is commonly used.

5.4 Resistance measurement connections and reporting The individual phase- or terminal-to-terminal resistance readings shall be reported along with the sum total winding resistance.

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5.4.1 Wye windings For wye windings, the reported resistance measurement may be from terminal to terminal or from terminal to neutral. For the reported total winding resistance, the resistance of the lead from the neutral connection to the neutral bushing may be excluded. For terminal-to-terminal measurements, the total resistance reported is the sum of the three measurements divided by two. 5.4.2 Delta windings For delta windings, the reported resistance measurement may be from terminal to terminal with the delta closed or from terminal to terminal with the delta open to obtain the individual phase readings. The reported total winding resistance is the sum of the three-phase readings if the delta is open. If the delta is closed, the reported total winding resistance is the sum of the three phase-to-phase readings times 1.5. 5.4.3 Autotransformer windings For autotransformer winding resistance measurements, the following method or an equivalent method shall be used. For the series winding resistance, the current shall be circulated between the high-voltage and neutral terminals, and the voltage shall be measured between the high-voltage terminal and the low-voltage terminal. For the common winding resistance, the current shall be circulated between the high-voltage and neutral terminals, and the voltage shall be measured between the low-voltage and neutral terminals. If needed, for the resistance of the lead and in-line windings (if any) between the low-voltage terminal and the neutral connection, the current shall be applied between the high-voltage terminal and the low-voltage terminal, and the voltage shall be measured between the low-voltage terminal and the neutral terminal.

6. Polarity and phase-relation tests Polarity and phase-relation tests are of interest primarily because of their bearing on paralleling or banking two or more transformers. Phase-relation tests are made to determine angular displacement and relative phase sequence.

6.1 Subtractive and additive polarity Windings arranged for subtractive polarity and additive polarity are shown in Figure 2 and Figure 3.

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Figure 2 —Windings: subtractive polarity

Figure 3 —Windings: additive polarity

Leads and polarity marks for subtractive polarity and additive polarity are shown in Figure 4 and Figure 5.

Figure 4 —Leads and polarity marks: subtractive polarity

Figure 5 —Leads and polarity marks: additive polarity

6.2 Polarity tests: single-phase transformers Polarity tests on single-phase transformers shall be made in accordance with one of the following methods: a)

Ratio meter

b)

Inductive kick

c)

Alternating voltage

6.2.1 Polarity by ratio meter The ratio meter described in 7.3.3 can also be used to test polarity.

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6.2.2 Polarity by inductive kick The polarity of transformers with leads arranged as shown in Figure 2 through Figure 5 may be determined with a low voltage dc supply and analog high-voltage voltmeter as follows: a)

With direct current passing through the high-voltage winding, connect a high-voltage direct-current voltmeter across the high-voltage winding terminals to obtain a small deflection of the pointer.

b)

Transfer the two voltmeter leads directly across the transformer to the adjacent low-voltage leads, respectively. For example, in Figure 5, the voltmeter lead connected to H1 will be transferred to

X 2 as the adjacent lead, and the lead connected to H 2 transferred to X 1 . c)

Break direct-current excitation, thereby inducing a voltage in the low-voltage winding (inductive kick), which will cause deflection in the voltmeter. The deflection is interpreted in items d) and e).

d)

When the pointer swings in the opposite direction (negative), the polarity is subtractive.

e)

When the pointer swings in the same direction as before (positive), the polarity is additive.

6.2.3 Polarity by alternating-voltage test The H1 lead shall be connected to the adjacent low-voltage lead (X1 in Figure 6).

Figure 6 —Polarity by alternating-voltage test Any convenient value of alternating voltage shall be applied to the full high-voltage winding. Readings are taken of the applied voltage and the voltage between the right-hand adjacent high-voltage and low-voltage leads. When the latter reading is greater than the former, the polarity is additive. When the latter voltage reading is less than the former (indicating the approximate difference in voltage between the high-voltage and low-voltage windings), the polarity is subtractive.

6.3 Polarity and phase-relation tests: polyphase transformers 6.3.1 Polarity of polyphase transformers Each phase of a polyphase transformer shall have the same relative polarity when tested in accordance with one of the methods described for single-phase transformers.

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6.3.2 Phase-relation tests 6.3.2.1 Test of phase relation with ratio meter The ratio meter described in 7.3.3 can also be used to test phase relationships. 6.3.2.2 Test for phasor diagram for transformers The phasor diagram of any three-phase transformer that defines the angular displacement and phase sequence can be verified as follows: 

Connect the H1 and X 1 leads together to excite the unit at a suitable, low, three-phase voltage.



Take voltage measurements between the various pairs of leads.



Either plot these values or compare them for their relative order of magnitude with the help of the corresponding diagram in Figure 7 or Figure 8.

Typical check measurements are to be taken and their relative magnitudes are also to be indicated.

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Figure 7 —Transformer lead markings and voltage-phasor diagrams for three-phase transformer connections 6.3.2.3 Six-phase windings Six-phase windings with no neutral connection shall be temporarily connected in delta (Δ) or wye (Y) for the test for phasor diagram.

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Figure 8 —Transformer lead markings and voltage-phasor diagrams for six-phase transformer connections 6.3.2.4 Zigzag windings Equal zig and zag windings are usually necessary for zigzag transformers, although unequal windings may be used for special applications. No required test is proposed to determine the phase relationships between the line-end and neutral-end sections of a zigzag winding. However, it is recommended that a test connection be made to the junction of the two winding sections and that tests be made during manufacture to prove the desired phase relationships. For the purpose of designation in Figure 7, zigzag windings are arbitrarily defined as windings whose lineend section is rotated 60º counterclockwise with respect to the neutral-end section.

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7. Ratio tests 7.1 General The turns ratio of a transformer is the ratio of the number of turns in the high-voltage winding to that in the low-voltage winding. 7.1.1 Taps When a transformer has taps for changing its voltage ratio, the turns ratio is based on the number of turns corresponding to the normal rated voltage of the respective windings to which operating and performance characteristics are referred. When the transformer has taps, the turns ratio shall be determined for all taps and for the full winding. 7.1.2 Voltage and frequency The ratio test shall be made at rated or lower voltage and rated or higher frequency. 7.1.3 Three-phase transformers In the case of three-phase transformers, when each phase is independent and accessible, single-phase power should be used, although, when convenient, three-phase power may be used. 7.1.4 Three-phase transformers with inaccessible neutrals Transformers that have wye connections but do not have the neutral of the wye brought out may be tested for ratio and polarity with three-phase or single-phase power. For single-phase power the circuit is configured such that there is no shift in the neutral (typically this requires shorting out one phase) as shown in Figure 9. When using three-phase power, and inequality of the phase voltage is found, the connection should be changed. Short out one phase of the winding to create a virtual neutral for testing purposes. Figure 9 shows the connections for such a transformer with a delta HV and a wye LV with inaccessible neutral.

For B phase and C Phase, short H1 to H3 (B Phase) – Apply H1-H2 Measure X1 – X2 (C Phase) – Apply H2-H3 Measure X1 – X3

For A Phase, short H1 to H2 (A Phase) – Apply H3-H1 Measure X2 – X1

Figure 9 —Connections for ratio test on a transformer with inaccessible neutral 14




7.2 Tolerances for ratio See 9.1 of IEEE Std C57.12.00- 2015.

7.3 Ratio test methods 7.3.1 Voltmeter method Two voltmeters shall be used (with voltage transformers when necessary): one to read the voltage of the high-voltage winding and the other, the low-voltage winding. The two voltmeters shall be read simultaneously. A second set of readings shall be taken with the instruments interchanged, and the average of the two sets of readings shall be taken to compensate for instrument errors. Voltage transformer ratios should yield approximately the same readings on the two voltmeters. Compensation for instrument errors by an interchange of instruments will otherwise not be satisfactory, and it will be necessary to apply appropriate corrections to the voltmeter readings. Tests shall be made with no less than four voltages in approximately 10% steps, and the average result shall be taken as the true value. These several values should check within 1%. Otherwise, the tests shall be repeated with other voltmeters. When appropriate corrections are applied to the voltmeter readings, tests may be made at only one voltage. When several transformers of duplicate rating are to be tested, work may be expedited by applying the foregoing tests to only one unit and then comparing the other units with this one as a standard, in accordance with the comparison transformer method discussed in 7.3.2. 7.3.2 Comparison method A convenient method of measuring the ratio of a transformer is by comparison with a transformer of known ratio. The transformer to be tested is excited in parallel with a transformer of the same nominal ratio, and the two secondaries are connected in parallel but with a voltmeter or detector in the connection between two terminals of similar polarity (see Figure 10). The voltmeter or detector indicates the difference in voltage. This method is more accurate than the following alternative method. For an alternative method, the transformer to be tested is excited in parallel with a transformer of known ratio, and the voltmeters are arranged to measure the two secondary voltages (see Figure 11). The voltmeters shall be interchanged, and the test shall be repeated. The averages of the results are the correct voltages. NOTE—Readings are repeated after interchanging voltmeters.

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Figure 10 —Voltmeter arranged to read the difference between the two secondary voltages

Figure 11 —Voltmeters arranged to read the two secondary voltages 7.3.3 Ratio meter A meter using the basic circuit of Figure 12 may be used to measure ratio. When detector DET is in balance, the transformer ratio is equal to R R1 . NOTE 1—Measurement of ratio using circuits of this type has also been described as ratio by resistance potentiometer. NOTE 2—More accurate results can be obtained using a ratio meter that provides phase-angle correction. NOTE 3—The ratio meter can also be used to test polarity, phase relation, and phase sequence.

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Figure 12 —Basic circuit of ratio meter

8. No-load losses and excitation current 8.1 General No-load (excitation) losses are losses that are incident to the excitation of the transformer. No-load losses include core loss, dielectric loss, conductor loss in the winding due to excitation current, and conductor loss due to circulating current in parallel windings. These losses change with the excitation voltage. Excitation current (no-load current) is the current that flows in any winding used to excite the transformer when all other windings are open-circuited. It is generally expressed in percent of the rated current of the winding in which it is measured. The no-load losses consist primarily of the core loss in the transformer core, which is a function of the magnitude, frequency, and waveform of the impressed voltage. No-load losses also vary with temperature and are particularly sensitive to differences in waveform; therefore, no-load loss measurements will vary markedly with the waveform of the test voltage. In addition, several other factors affect the no-load losses and excitation current of a transformer. The design-related factors include the type and thickness of core steel, the core configuration, the geometry of core joints, and the core flux density. Factors that cause differences in the no-load losses of transformers of the same design include variability in characteristics of the core steel, mechanical stresses induced in manufacturing, variation in gap structure, and core joints.

8.2 No-load loss test The purpose of the no-load loss test is to measure no-load losses at a specified excitation voltage and a specified frequency. The no-load loss determination shall be based on a sine-wave voltage, unless a different waveform is inherent in the operation of the transformer. The average-voltage voltmeter method is the most accurate method for correcting the measured no-load losses to a sine-wave basis and is recommended. This method employs two parallel-connected voltmeters: one is an average-responding [but root mean square (rms) calibrated] voltmeter ( Va ), and the other is a true rms-responding voltmeter ( Vr ). The test voltage is adjusted to the specified value as read by the average-responding voltmeter. The readings of both voltmeters are employed to correct the no-load losses to a sine-wave basis, using Equation (2) in accordance with 8.3. 17




8.2.1 Connection diagrams Tests for the no-load loss determination of a single-phase transformer are carried out using the schemes depicted in Figure 13 and Figure 14. Figure 13 shows the necessary equipment and connections when instrument transformers are not required. When instrument transformers are required, as is generally the case, the equipment and connections shown in Figure 14 apply. If necessary, correction for losses in connected measurement instruments may be made by disconnecting the transformer under test and noting the wattmeter reading at the specified test circuit voltage. These losses represent the losses of the connected instruments (and voltage transformer, if used). They may be subtracted from the earlier wattmeter reading to obtain the no-load loss of the transformer under test.

Figure 13 —Connections for no-load loss test of a single-phase transformer without instrument transformers

Figure 14 —Connections for a no-load loss test of a single-phase transformer with instrument transformers

Tests for the no-load loss determination of a three-phase transformer shall be carried out by using the three wattmeter method. Figure 15 is a schematic representation of the equipment and connections necessary for conducting no-load loss measurements of a three-phase transformer when instrument transformers are necessary.

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Figure 15 —Three-phase transformer connections for no-load loss and excitation current tests using three-wattmeter method 8.2.2 Voltmeter connections When correcting to a sine-wave basis using the average-voltage voltmeter method, attention shall be paid to the voltmeter connections because the line-to-line voltage waveform may differ from line-to-neutral voltage waveform. Therefore, depending on whether the transformer windings energized during the test are connected delta or wye, the voltmeter connections shall be such that the waveform applied to the voltmeters is the same as the waveform across the energized windings. 8.2.3 Energized windings Either the high- or low-voltage winding of the transformer under test may be energized, but it is generally more convenient to make this test using the low-voltage winding. In any case, the full winding (not merely a portion of the winding) should be used whenever possible. If, for some unusual reason, only a portion of a winding is excited, this portion shall not be less than 25% of the winding. 8.2.4 Voltage and frequency The operating and performance characteristics of a transformer are based on rated voltage and rated frequency, unless otherwise specified. Therefore, the no-load loss test is conducted with rated voltage impressed across the transformer terminals, using a voltage source at a frequency equal to the rated frequency of the transformer under test, unless otherwise specified. To determine no-load losses of a single-phase transformer or a three-phase transformer, the frequency of the test source should be within ± 0.5% of the rated frequency of the transformer under test. The voltage shall be adjusted to the specified value as indicated by the average-voltage voltmeter. Simultaneous values of rms voltage, rms current, electrical power, and the average-voltage voltmeter readings shall be recorded. For a three-phase transformer, the average of the three voltmeter readings shall be the desired nominal value. 19




8.3 Waveform correction of no-load losses The eddy-current component of the no-load loss varies with the square of the rms value of excitation voltage and is substantially independent of the voltage waveform. When the test voltage is held at the specified value as read on the average-voltage voltmeter, the actual rms value of the test voltage may not be equal to the specified value. The no-load losses of the transformer corrected to a sine-wave basis shall be determined from the measured value using Equation (2):

Pc (Tm ) =

Pm P1 + kP2

(2)

where is the average liquid temperature at the time of test (ºC) Tm Pc (Tm ) is the no-load losses, corrected for waveform, at temperature Tm

Tm

Pm

is measured no-load losses at temperature

P1 P2

is per-unit hysteresis loss is per-unit eddy-current loss

k

is

Er Ea

is the test voltage measured by rms voltmeter

 Er     Ea 

is the test voltage measured by average-voltage voltmeter

The actual per-unit values of hysteresis and eddy-current losses should be used if available. If actual values are not available, it is suggested that the two loss components be assumed equal in value and that they each be assigned a value of 0.5 per unit. For more information, see IEEE Std C57.123. Equation (2) is valid only for test voltages with moderate waveform distortion. If waveform distortion in the test voltage causes the magnitude of the correction to be greater than 5%, the test voltage waveform shall be improved for an adequate determination of the no-load losses and currents.

8.4 Temperature correction of no-load losses A reference temperature is required when stating no-load losses because the no-load losses vary with core temperature. The standard reference temperature Tr for transformer no-load losses is specified in 5.9 of IEEE Std C57.12.00-2015. The observed decrease in no-load losses for an increase in temperature results from several mechanisms acting together. Changes in the core steel resistivity, changes in mechanical stress in the core structure, and variations in the temperature gradients in the core cause the no-load loss to change with temperature. Because these factors vary from design to design and between transformers of the same design, it is not practical to specify an exact formula to account for temperature variation throughout the operating temperature range of transformers. However, ordinary variations of temperature encountered when performing the no-load loss test will not affect no-load losses materially, and no correction for temperature needs to be made, so long as the following conditions are met: 20




a)

The average liquid temperature is within ± 10 ºC of the reference temperature Tr .

b)

The difference between the top and bottom liquid temperatures does not exceed 5 ºC.

If conducting the test with temperatures outside the specified ranges is necessary, the empirical formula in Equation (3) may be used to correct the measured no-load losses to the reference temperature:

Pc (= Tr ) Pc (Tm ) {1 + (Tm − Tr ) KT }

(3)

where

Pc (Tr ) is the no-load losses, corrected to the standard reference temperature Tr Pc (Tm ) is the no-load losses, corrected for waveform, at temperature Tr KT

Tm

is the standard reference temperature (ºC) is an empirically derived per-unit change in no-load loss per degree Celsius

If the actual value of KT is not available, a value of 0.000 65 per unit change per degrees Celsius should be used. This value is typical for cores constructed of grain-oriented silicon steel and is satisfactory as a correction for no-load losses when the transformer cannot be tested within the specified temperature range.

8.5 Determination of excitation (no-load) current The excitation (no-load) current of a transformer is the current that maintains the rated magnetic flux excitation in the core of the transformer. The excitation current is usually expressed per unit or in percent of the rated line current of the winding in which it is measured. (Where the cooling class of the transformer involves more than one kilovoltampere rating, the lowest kilovoltampere rating is used to determine the base current.) Measurement of excitation current is usually carried out in conjunction with the tests for noload losses. Rms current is recorded simultaneously during the test for no-load losses using the averagevoltage voltmeter method. This value is used in calculating the per-unit or percent excitation current. For a three-phase transformer, the excitation current is calculated by taking the average of the magnitudes of the three line currents.

8.6 Frequency conversion of no-load losses and excitation current In the event that the no-load loss and excitation current test cannot be done at the rated power frequency, see Annex B for 50/60 Hz frequency conversions.

9. Load losses and impedance voltage 9.1 General The load losses of a transformer are losses incident to a specified load carried by the transformer. Load losses include I2R loss in the windings due to load current and stray losses due to eddy currents induced by leakage flux in the windings, core clamps, magnetic shields, tank walls, and other conducting parts. Stray losses may also be caused by circulating currents in parallel windings or strands. Load losses are measured by applying a short circuit across either the high-voltage winding or the low-voltage winding and applying

21




sufficient voltage across the other winding to cause a specified current to flow in the windings. The power loss within the transformer under these conditions equals the load losses of the transformer at the temperature of test for the specified load current. The impedance voltage of a transformer is the voltage required to circulate rated current through one of two specified windings when the other winding is short-circuited, with the windings connected for rated voltage operation. Impedance voltage is usually expressed in per unit or in percent of the rated voltage of the winding across which the voltage is applied and measured. The impedance voltage comprises a resistive component and a reactive component. The resistive component of the impedance voltage, called the resistance drop, is in phase with the current and corresponds to the load losses. The reactive component of the impedance voltage, called the reactance drop, is in quadrature with the current and corresponds to the leakage-flux linkages of the windings. The impedance voltage is the phasor sum of the two components. The impedance voltage is measured during the load loss test by measuring the voltage required to circulate rated current in the windings. The measured voltage is the impedance voltage at the temperature of test, and the power loss dissipated within the transformer is equal to the load losses at the temperature of test and at rated load. The impedance voltage and the load losses are corrected to a reference temperature using the formulas specified in this standard (see 9.4.2). The impedance kilovoltampere is the product of the impedance voltage across the energized winding in kilovolts times the winding current in amperes. The ratio of the load losses in kilowatts at the temperature of test to the impedance kilovoltampere at the temperature of test is the load loss power factor of the transformer during the test and is used for correction of phase-angle error as specified in this standard.

9.2 Factors affecting the values of load losses and impedance voltage The magnitudes of the load losses and the impedance voltage will vary depending on the positions of tap changers, if any, in various windings. These changes are due to the changes in the magnitudes of load currents and associated leakage-flux linkages as well as changes in stray flux and accompanying stray losses. In addition, several other factors affect the values of load losses and impedance voltage of a transformer. Considerations of these factors, partly, explain variations in load loss values and impedance voltage for the same transformer under different test conditions as well as variations between load loss values and impedance voltage of different transformers of the same design. These factors are discussed in 9.2.1 through 9.2.4. 9.2.1 Design The design-related factors include conductor material, conductor dimensions, winding design, winding arrangement, shielding design, and selection of structural materials. 9.2.2 Process The process-related factors that affect load loss values and impedance voltage are the dimensional tolerances of conductor materials, the final dimensions of completed windings, phase assemblies, metallic parts exposed to stray flux, and variations in properties of conductor material and other metallic parts. 9.2.3 Temperature Load loss values are also a function of temperature. The I2R component of the load losses increases with temperature, whereas the stray loss component decreases with temperature. Procedures for correcting the load losses and impedance voltage to the standard reference temperature are described in 9.4.2.

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9.2.4 Measurements At low power factors, such as those encountered while measuring load losses and impedance voltage of power transformers, judicious selection of measurement method and test system components is essential for accurate and repeatable test results. The phase-angle errors in the instrument transformers, measuring instruments, bridge networks, and accessories affect load loss test results. Procedures for correcting the load losses for metering phase-angle errors are described in 9.4.1.

9.3 Tests for measuring load losses and impedance voltage Regardless of the test method selected, the following preparatory requirements shall be satisfied for accurate test results: a)

To determine the temperature of the windings with sufficient accuracy, the following conditions shall be met, except as stated in the NOTE in 2): 1) The temperature of the insulating liquid has stabilized, and the difference between top and bottom liquid temperatures does not exceed 5 ºC. 2) The temperature of the windings shall be taken immediately before and after the load losses and impedance voltage test in a manner similar to that described in 5.1. The average shall be taken as the true temperature. 3) The difference in winding temperature before and after the test shall not exceed 5 ºC. NOTE— For distribution and pad-mounted transformers up to 2500 kVA, where it may not be practical to wait for thermal equilibrium, the method used to determine the winding temperature should take into consideration the lack of thermal equilibrium and the effect of ohmic heating of the winding conductors by load current during the test. The method used can be verified by staging a repeated measurement of the load losses and impedance voltage at a later time when condition 1, condition 2, and condition 3 are met.

b) The conductors used to short-circuit the low-voltage high-current winding of a transformer shall have a cross-sectional area equal to or greater than the corresponding transformer leads. c) The frequency of the test source used for measuring load losses and impedance voltage shall be within ± 0.5% of the nominal value. d) The maximum value of correction to the measured load losses due to the test system phase-angle error is limited to ± 5% of measured losses. If more than 5% correction is required, test methods and/or test apparatus should be improved for an adequate determination of load losses. 9.3.1 Wattmeter-voltmeter-ammeter method The connections and apparatus needed to determine load losses and impedance voltage of a single-phase transformer are shown in Figure 16 and Figure 17. Figure 16 applies when instrument transformers are not required. If instrument transformers are required (as is generally the case), then Figure 17 applies. NOTE—Instrument transformers to be added when necessary.

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Figure 16 —Single-phase transformer connections for load loss and impedance voltage tests without instrument transformers

Figure 17 —Single-phase transformer connections for load loss and impedance voltage tests with instrument transformers For three-phase transformers, a three-phase power measurement using two wattmeters is possible, but it can result in very large errors at low power factors in load loss tests of transformers. The two-wattmeter method should not be used for loss tests on three-phase transformers. For three-phase transformers, Figure 18 shows the apparatus and connections using the three-wattmeter method.

Figure 18 —Three-phase transformer connections for load loss and impedance voltage tests using three-wattmeter method

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The selection of test method and test system components should comply with accuracy requirements as specified in 9.4 of IEEE Std C57.12.00-2015. 9.3.2 Impedance bridge methods Impedance bridge methods may be used as an alternative to the wattmeter-voltmeter-ammeter method for measurement of load losses and impedance voltage. Although many configurations of impedance bridge networks are possible, the choice of a particular network is determined by considerations of the measurement environment and available test facility. The general form of the impedance bridge as shown in Figure 19 is an electrical network arranged so that a voltage proportional to the current through the transformer under test is compared with a reference voltage that is a function of the applied voltage ET . The voltage comparison is made by adjusting one or more of the bridge arms ( Z1 , Z 2 , and Z 3 ) until the voltages across Z 2 and Z 3 are exactly equal in magnitude and phase. Voltage balance is indicated by a null reading of the detector DET. The impedance characteristics of the transformer under test can then be calculated from the values of Z1 , Z 2 , and Z 3 .

Figure 19 —General impedance bridge network Two of the most commonly used bridge networks for transformer testing are shown in Figure 20 and Figure 21. In Figure 20, a bridge technique employs a precision, low-loss, high-voltage capacitor and precision current transformer. It has some similarities to classic Schering and Maxwell bridges. In Figure 21, another bridge technique employs a high-voltage capacitor, precision current transformer, and transformer ratio arm bridge. In general, the bridge network adjustments for voltage balance are frequency-dependent; therefore, excitation of the bridge shall be made with a power source that has low harmonic distortion and excellent frequency stability. The factors that affect overall accuracy of test results by the wattmeter-voltmeter-ammeter method also impact the accuracy of test results by impedance bridge methods.

25




Figure 20 —RC-type impedance bridge

Figure 21 —Transformer-ratio-arm bridge Loss measurements on three-phase transformers using a three-phase source are made by connecting the bridge network to each phase in turn and calculating the total losses from the three single-phase measurements. This approach is analogous to the three-wattmeter method of measuring losses by switching a single wattmeter from phase to phase. To verify that switching the bridge from phase to phase does not affect the result on the remaining phases and to demonstrate that the time involved in switching the bridge does not result in undue heating of the transformer windings during the test, the losses can be monitored for stable readings by wattmeters in all phases. 9.3.3 Transformer test procedures 9.3.3.1 Two-winding transformers and autotransformers Load loss and impedance voltage tests are carried out using the connections and apparatus shown in Figure 16 and Figure 17 for single-phase transformers and Figure 18 for three-phase transformers. With one winding short-circuited, a voltage of sufficient magnitude at rated frequency is applied to the other winding and adjusted to circulate rated current in the excited winding. Simultaneous readings of wattmeter, voltmeter, and ammeter are taken. If necessary, the corrections for losses in external connections and connected measuring instruments should be made. The procedure for testing three-phase transformers is similar, except that all connections and measurements are three-phase instead of single-phase and a balanced three-phase source of power is used for the tests. If the three line currents cannot be balanced, their average rms value should correspond to the desired value. Simultaneous readings of wattmeters, voltmeters, and ammeters should be recorded.

26




Single-phase and three-phase autotransformers may be tested with internal connections unchanged. The test is made using the autotransformer connection. The input (or output) terminals are shorted, and voltage (at rated frequency) is applied to the other terminals. The voltage is adjusted to cause rated line current to flow in the test circuit as shown in Figure 22. Simultaneous readings of wattmeter, voltmeter, and ammeter are recorded to determine load losses and impedance voltage.

Figure 22 —Connections for impedance loss and impedance voltage tests of an autotransformer For the purpose of measuring load losses and impedance voltage, the series and common windings of autotransformers may be treated as separate windings, one short-circuited and the other excited. When the transformer is connected in the two-winding connection for the test, the current held shall be the rated current of the excited winding, which may or may not be the same as rated line current. The load loss watts and applied voltamperes will be the same, regardless of whether series and common windings are treated as separate windings in the two-winding connection or are connected in the autotransformer connection, so long as rated winding current is held in the first case and rated line current in the second case. 9.3.3.2 Three-winding transformer For a three-winding transformer, which may be either single-phase or three-phase, three sets of impedance measurements are made between pairs of windings, following the same procedure as for two-winding transformers. Measurements of the impedances Z12, Z23, and Z31 are obtained between windings 1, 2, and 3. If the kilovoltampere capacities of the different windings are not alike, the current held for the impedance test should correspond to the capacity of the lower rated winding of the pair of windings under test. However, all data, when converted to percentage form, should be based on the same output kilovoltampere, preferably of the primary winding. An equivalent three-winding impedance network, as shown in Figure 23, can be derived from Equation (4), Equation (5), and Equation (6):

Z1 =

Z12 − Z 23 + Z 31 2

(4)

Z= 2

Z 23 − Z 31 + Z12 = Z12 − Z1 2

(5)

Z= 3

Z 31 − Z12 + Z 23 = Z 31 − Z1 2

(6)

where

Z12 , Z 23 , and Z 31

are the measured impedance values between pairs of windings, as indicated, all expressed on the same kilovoltampere base. 27




These equations involve complex numbers, but they may be used for the resistance (in-phase) component, the reactance (quadrature) component of the impedance voltage, or of the impedance voltamperes. The treatment of the individual load losses and impedance voltages for temperature corrections, and so on, is the same as for two-winding single-phase transformers. Total load losses of a three-winding transformer is the sum of losses in the branches of the equivalent circuit of Figure 23 for any specific terminal load conditions.

Figure 23 —Equivalent three-winding impedance network

9.3.3.3 Interlacing impedance voltage of a Scott-connected transformer The interlacing impedance voltage of Scott-connected transformers is the single-phase voltage applied from the midtap of the main transformer winding to both ends, connected together. The voltage is sufficient to circulate a current equal to the rated three-phase line current in the supply lines. The current in each half of the winding is 50% of this value. The percent interlacing impedance is the measured voltage expressed as a percent of the teaser voltage. The percent resistance is the measured losses expressed as a percentage of the rated kilovoltampere of the teaser winding. 9.3.3.4 Test of three-phase transformer with single-phase voltage To determine the load losses and impedance voltage of a three-phase transformer with single-phase voltage, the setup schematic shown in Figure 24 is recommended.

28




Figure 24 —Test of three-phase transformer with single-phase voltage The three line leads of one winding are short-circuited, and single-phase voltage at rated frequency is applied to two terminals of the other winding. The applied voltage is adjusted to circulate rated line current. Three successive readings are taken on the three pairs of leads, for example, H1 and H2, H2 and H3, H3 and H1. Then

 P + P23 + P31  Measured load losses (W ) = 1.5  12  3  

Measured impedance voltage =

(7)

3  E12 + E23 + E31   2  3 

(8)

where

P E

is the individual reading of measured load losses as indicated by subscripts is the individual reading of measured impedance voltage as indicated by subscripts

The stray loss component shall be obtained by subtracting the I 2 R losses from the measured load losses of the transformer. Let R1 be the resistance measured between two high-voltage terminals and R2 be the resistance between two low-voltage terminals; let I1 and I2 be the respective rated line currents. Then the total I2R loss of all three phases will be as in Equation (9):

Total I 2 R= ( watts ) 1.5 ( I12 R1 + I 22 R2 )

(9)

This formula applies equally well to wye- or delta-connected windings. Temperature correction shall be made as in 9.4.2.

29




9.4 Calculation of load losses and impedance voltage from test data Load losses and impedance voltage measurements vary with temperature and, in general, shall be corrected to a reference temperature. In addition, load loss measurement values shall be corrected for metering phaseangle error. 9.4.1 Correction of load loss measurement due to metering phase-angle errors Load loss error can be magnitude related, such as instrument transformer ratio errors and meter calibration. Correction of load loss measurement due to phase-angle errors in the wattmeters, voltage-measuring circuit, and current-measuring circuit shall be applied in accordance with Table 1 using the correction formula in Equation (10):

(

PC =Pm − Vm Am −ϕWd − ϕVd + ϕ Cd

)

(10)

where

PC Pm Vm Am

is the wattmeter reading, corrected for phase-angle error (W)

ϕWd

is the phase-angle error of wattmeter where applicable (rad)

ϕVd

ϕ Cd

is the actual wattmeter reading (W) is the voltmeter reading across wattmeter voltage element (V) is the ammeter reading in wattmeter current element (A) is the phase-angle error of voltage transformer (rad) is the phase-angle error of current transformer (rad) Table 1 —Requirements for phase-angle error correction

Apparent load loss power factor (PF = Pm/VA)

Comments

PF ≤ 0.03

Apply phase-angle error correction

0.03 < PF ≤ 0.10

Apply phase-angle error correction if −ϕW − ϕV + ϕ C > 290 µrad (1 min) d d d

PF > 0.10

Apply phase-angle error correction if −ϕW − ϕV + ϕ C > 870 µrad (3 min) d d d

In general, instrument transformer phase-angle errors are a function of burden and excitation. Likewise, wattmeter phase-angle errors are a function of the scale being used and the circuit power factor. Thus, the instrumentation phase-angle errors used in the correction formula shall be specific for the test conditions involved. Only instrument transformers meeting 0.3 metering accuracy class, or better, are acceptable for measurements. Use of Equation (10) is limited to conditions of apparent power factor less than 0.20 and the total system phase-angle error less than 20 min. If corrections are required with apparent power factor or system phase error outside this range, the following exact formulas in Equation (11) apply:

30




 Pm    Vm Am 

ϕa = cos −1 

(11)

= PC Vm Am cos (ϕ a − ϕWd − ϕVd + ϕ Cd ) For three-phase measurements, the corrections are applied to the reading of each wattmeter employed. The transformer load loss at temperature Tm is then calculated using Equation (12) as follows: 3

P(Tm) = ∑ Rv Ra Pci

(12)

i =1

where

P(Tm ) Pci Rv Ra

is the transformer load losses, corrected for phase-angle error at temperature Tm (W) is the corrected wattmeter reading of the ith wattmeter (W) is the true voltage ratio of voltage-measuring circuit is the true current ratio of current-measuring circuit

9.4.2 Temperature correction of load losses Both I 2 R losses and stray losses of a transformer vary with temperature. The I 2 R losses Pr (Tm ) of a transformer are calculated from the ohmic resistance measurements (corrected to the temperature Tm at which the measurement of load losses and impedance voltage was done) and the current that was used in the impedance measurement. These I 2 R losses subtracted from the measured load loss watts P(Tm ) give the stray losses Ps (Tm ) of the transformer at the temperature at which the load loss test was made, as shown in Equation (13):

Ps= (Tm ) P (Tm ) − Pr (Tm )

(13)

where

Ps (Tm ) P(Tm )

is the calculated stray losses at temperature Tm (W ) is the transformer load losses corrected in accordance with 9.4.1, for phase-angle error at temperature Tm (W )

Pr (Tm )

is the calculated I 2 R loss at temperature Tm (W )

The I 2 R component of load losses increases with temperature. The stray loss component diminishes with temperature. Therefore, when it is desirable to convert the load losses from the temperature at which it is measured Tm to another temperature T , the two components of the load losses are corrected separately, as shown in Equation (14) and Equation (15):

 T +T  Pr (T ) = Pr (Tm )  k   Tk + Tm 

(14)

31




 T + Tm  Ps (T ) = Ps (Tm )  k   Tk + T 

(15)

and, then, as shown in Equation (16):

P= (T ) Pr (T ) + Ps (T )

(16)

where

Pr (T )

is I 2 R loss (W) at temperature T (ºC)

Ps (T )

is stray losses (W) at temperature T (ºC)

P(T ) Tk

is transformer load losses (W) corrected to temperature T (ºC) is 234.5 ºC (copper) or 225 ºC (aluminum; see NOTE below)

NOTE—The temperature 225 ºC applies for pure electrical conductor (EC) aluminum. Tk may be as high as 230 ºC for alloyed aluminum. Where copper and aluminum windings are employed in the same transformer, a value for Tk of 229 ºC should be applied for the correction of stray losses.

9.4.3 Impedance voltage Impedance voltage and its resistive and reactive components are determined by the use of Equation (17) through Equation (20):

Er (Tm ) =

= Ex

P(Tm ) I

(17)

Ez (Tm ) 2 − Er (Tm ) 2

Er (T ) =

(18)

P(T ) I

= Ez (T )

(19)

Er (T ) 2 + Ex2

(20)

where

Er (Tm ) Er (T ) Ex Ez (Tm ) Ez (T ) P(T ) P(Tm ) I

is the resistance voltage drop of in-phase component at temperature Tm (V ) is the resistance voltage drop of in-phase component corrected to temperature T (V ) is the reactance voltage drop of quadrature component (V ) is the impedance voltage at temperature Tm (V ) is the impedance voltage at temperature T (V ) is the transformer load losses corrected to temperature T (W ) is transformer load losses measured at temperature Tm (W ) is the current in excited winding ( A)

32




Per-unit values of the resistance, reactance, and impedance voltage are obtained by dividing Er (T ) , Ex , and Ez (T ) by the rated voltage. Percentage values are obtained by multiplying per-unit values by 100. 9.4.4 Frequency conversion of load losses and impedance In the event that the load loss and impedance test cannot be done at the rated power frequency, see Annex B for 50/60 Hz frequency conversions.

9.5 Zero-phase-sequence impedance 9.5.1 Zero-phase-sequence impedance tests of three-phase transformers The zero-phase-sequence impedance characteristics of three-phase transformers depend on the winding connections and, in some cases, on the core construction. Zero-phase-sequence impedance tests described in this standard apply only to transformers having one or more windings with a physical neutral brought out for external connection. In all tests, one such winding shall be excited at rated frequency between the neutral and the three line terminals connected together. External connection of other windings shall be as described in zero-phase-sequence impedance for various transformer connections. Transformers with connections other than as described in zero-phase-sequence impedance shall be tested as determined by the individuals responsible for design and application. The excitation voltage and current shall be established as follows: a)

If no delta connection is present on the transformer and the transformer is a three leg core design, there is a risk of excessive tank wall heating due to the return flux from the core going into the tank wall. To avoid this, the applied voltage should be such that the current is no more than 20% of the base rating of the winding being excited. This applies to both open-circuit tests and short-circuit tests as described in 9.5.3. If the transformer is a five leg core or a shell form design, the zero-sequence impedance is equal to the positive sequence impedance and the zero-sequence test is generally not needed. However, should the test be done, the applied voltage should not exceed 30% of the rated line-to-neutral voltage of the winding being energized for the open-circuit test, and the phase current should not exceed the base rated value of the winding being excited for the short-circuit test.

b)

If a delta connection is present, the applied voltage should be such that the base rated phase current of any delta winding or the maximum allowable neutral current is not exceeded.

The percent excitation voltage at which the tests are made shall be shown on the test report. The time duration of the test shall be such that the thermal limits of any of the transformer parts are not exceeded. Single-phase measurements of excitation voltage, total current, and power shall be similar to those described in 9.3. The zero-phase-sequence impedance, in percent, on kilovoltampere base of excited winding for the test connection is as in Equation (21):

 E I  = Z 0 (%) 300  × r   Er I 

(21)

33




where

Z 0 is the zero sequence impedance, in percent, on kilovoltampere base of excited winding for the test connection E is measured excitation voltage (V) Er is rated phase-to-neutral voltage of excited winding (V) I is measured total input current flowing in the three parallel-connected phases (A) I r is rated current per phase of the excited windings (A) 9.5.2 Transformers with one neutral externally available, excluding transformers with interconnected windings The zero-phase sequence network, given the external characteristics for transformers of this type, is shown in Figure 25. Winding 1 has the available neutral, whereas windings 2, 3, and so forth do not.

Figure 25 —Equivalent zero-phase-sequence network for transformers with one externally available neutral A zero-sequence test shall be made on the winding with the available neutral. A single-phase voltage shall be applied between the three shorted line terminals and the neutral. The external terminals of all other windings may be open-circuited or shorted and grounded. The term interconnected windings shall be interpreted to mean windings in which one or more electrical phases are linked by more than one magnetic phase. 9.5.3 Transformers with two neutrals externally available, excluding transformers with interconnected windings The zero-phase sequence network, given the external characteristics for transformers of this type, is shown in Figure 26. Windings 1 and 2 have the externally available neutrals, whereas Windings 3, 4, and so forth do not. The diagram is drawn for the case of 0º phase shift between Windings 1 and 2. NOTE—This also applies also to autotransformers. For wye-wye transformers or autotransformers without a delta tertiary that are 5 leg core form or shell form, Z 3 is very large and the zero sequence impedance is equal to the positive sequence impedance. For three leg core form wye-wye transformers, the Z 3 value is typically 5 to 10 times the Z12 measured value due to the “phantom delta” effect of the tank and/or tank wall shielding. The Z 3 value should be taken into account in short circuit calculations involving the zero sequence impedance. Measurements may be made at different current levels to establish the non-linear curve for all four different measurements.

34




Figure 26 —Equivalent zero-phase-sequence network for a transformer with two externally available neutrals and 0° phase shift between Windings 1 and 2 Four tests may be made to determine the zero-phase-sequence equivalent network, one of which is redundant. a)

Test 1. Apply a single-phase voltage to winding 1 between the shorted line terminals of Winding 1 and its neutral. All other windings are open-circuited. The measured zero-phase-sequence impedance is represented by Z1 N 0 .

b)

Test 2. Apply a single-phase voltage to winding 1 between the shorted line terminals of winding 1 and its neutral. Short the line terminals and neutral of winding 2. All other windings may be opencircuited or shorted. The measured zero-sequence impedance is represented by Z1 N s .

c)

Test 3. Apply a single-phase voltage to winding 2 between the shorted line terminals of winding 2 and its neutral. All other windings are open-circuited. The measured zero-phase-sequence impedance is represented by Z 2 N 0 .

d)

Test 4. Apply a single-phase voltage to winding 2 between the shorted line terminals of winding 2 and its neutral. Short the line terminals and neutral of winding 1. All other windings may be opencircuited or shorted. The measured zero-phase-sequence impedance is represented by Z 2 N s .

Test 4 or Test 2 are redundant, and Test 4 need not be performed. If performed, however, it may be used as a check. All measured zero-phase-sequence impedances should be expressed in percent and placed on a common kilovoltampere base. The constants in the equivalent circuit are as shown in Equation (22) through Equation (24):

(

)

(

Z3 = + Z 2 No Z1N o − Z1N s = + Z1No Z 2 No − Z 2 N s

)

(22)

= Z 2 Z 2 No − Z 3

(23)

= Z1 Z1No − Z 3

(24)

NOTE—These equations involve complex numbers. The plus sign before the radical in Equation (22) is appropriate for most common cases in which windings 1 and 2 are physically adjacent in the design and no delta winding (3, 4, etc.) is interleaved with them. A minus sign may be appropriate when a delta winding (3 or 4) is physically located within or between windings 1 and 2. The correctness of the sign can be checked by comparison with design calculations of zerosequence impedance.

35




If Z1N o and Z 2 N o approach infinity, then Z 3 approaches infinity, and the equivalent circuit is that shown in Figure 27.

Figure 27 —Equivalent zero-phase-sequence network for transformers with two externally available neutrals and 0° phase shift if Z1No and Z2No approach infinity In the case of wye–wye connected transformers, the zero-sequence impedance, in general, is a nonlinear function of the applied voltage, which in turn may require more than one set of measurements to characterize the nonlinear behavior. 9.5.4 Autotransformers The tests and equivalent circuits of 9.5.2 and 9.5.3 apply equally well for autotransformer connections, except that the externally available neutral of a common winding shall be considered as two externally available neutrals, one for the common winding and one for the series-common combination. 9.5.5 General test method for zero-phase-sequence impedance measurement on transformers with interconnected windings (see Rosselli [B27]) The zero-phase-sequence impedance test described in this subclause applies to transformers with interconnected windings, generally of zig-zag connections with and without delta winding and with and without external neutral(s) available. Typical phasor and zero-sequence impedance diagrams are shown in Figure 28(a) and Figure 28(b).

Figure 28 —Typical phasor and zero-sequence impedance diagrams 36




Exciting voltage, current and voltage measurements, and terminal connections shall be established as follows: a)

Primary winding shall be excited with a three-phase voltage at rated frequency and with one phase of secondary interconnected winding shorted to its grounded neutral. If external neutral is not available, the phase should be shorted to ground. The other two secondary terminals shall be opencircuited. When no delta connection is present on the transformer, the applied voltage should not exceed 30% of the rated line-to-neutral voltage of the winding being excited, nor should the phase current exceed its rated value. When a delta winding is present, the applied voltage should be such that the rated phase current of any delta winding is not exceeded. The source shall be ungrounded to eliminate any system zero-sequence impedance that could influence the final calculation.

b)

Measure the primary voltage on each phase and the secondary voltages to ground of open-circuited phases.

c)

Measure the applied current to each phase of the primary windings and current in the shorted phase of the secondary winding.

d)

Repeat step a) through step c) for other two phases of the secondary winding, one at a time.

e)

Calculate the zero-phase-sequence impedance for each phase using the equation in Equation (25). The final zero-phase-sequence impedance shall be the average of the calculated zero-sequence impedances for all phases, or if more conservative values are required, then the lowest Z 0 can be used. All values shall be included in the certified test report.

f)

For simplifying calculations, resistance component impedance is considered negligible.

Ir  E Z 0= (%) 300  AV ×  − [ 2 Z + + 3Z N ]  Er I 

(25)

where

E AV Er I Ir Z+ ZN

is the average of applied three-phase measured voltages divided by transformer ratio is the rated phase-to-neutral voltage of the shorted winding is the measured total current flowing in the shorted phase is the rated current per phase of the shorted winding at the megavoltampere rating of the transformer is the percent positive-phase-sequence impedance based on the megavoltampere rating is the percent neutral impedance based on the megavoltampere rating (This value will be zero if no neutral impedance is used.)

10. Dielectric tests 10.1 General 10.1.1 Factory dielectric tests The purpose of dielectric tests in the factory is to demonstrate that the transformer has been designed and constructed to withstand the specified insulation levels.

37




10.1.2 Test requirements Test levels and other test parameters shall be as outlined in IEEE Std C57.12.00 or as otherwise specified. 10.1.3 Measurement of test voltages Unless otherwise specified, the dielectric test voltages shall be measured or applied, or both, in accordance with IEEE Std 4, with the following exceptions: a)

A protective resistance may be used in series with sphere gaps, on either the live or the grounded sphere. When unnecessary to protect the spheres from arc damage, it may be omitted.

b)

The bushing-type potential divider method shall be considered a standard method for transformer tests.

c)

The rectified capacitor-current method shall be considered a standard method for transformer tests.

d)

In conducting low-frequency tests for transformers of 100 kVA and less to be tested at 50 kV or less, it is permissible to depend on the ratio of the testing transformer to indicate the proper test voltage.

10.1.4 Type of power transformer The terms Class I and Class II power transformers, as used in this standard, are defined in 5.10 of IEEE Std C57.12.00-2015. 10.1.5 Factory dielectric tests and conditions 10.1.5.1 Test sequence Impulse tests, when required, shall precede the low-frequency tests. The dielectric tests should be performed in the preferred sequence as follows: a)

Lightning impulse tests (on all terminals)

b) Switching impulse tests c)

Applied voltage test

d) Induced voltage test By agreement between the manufacturer and purchaser, switching impulse tests (when required) may be performed before lightning impulse tests. The induced voltage test shall be the last dielectric test performed. 10.1.5.2 Temperature Dielectric tests may be made at temperatures assumed under normal operation or at the temperatures attained under conditions of routine test.

38




10.1.5.3 Assembly Transformers, including bushings and terminal compartments when necessary to verify air clearances, shall be assembled prior to making dielectric tests. However, assembly of items that do not affect dielectric tests, such as radiators and cabinets, is not necessary. Bushings shall, unless otherwise authorized by the purchaser, be those to be supplied with the transformer. 10.1.5.4 Transformers for connection to gas-insulated equipment During dielectric testing of transformers for direct connection to gas-insulated substations, testing with the in-service bushings is preferred, but substitute air-oil bushings may be used unless otherwise specified by the purchaser. Live part clearances and locations of the substitute bushings inside the transformer shall be identical, within normal manufacturing tolerances, to those of the in-service bushings. When required internal clearances and/or external air clearances cannot be achieved, suitable arrangements should be agreed upon between the manufacturer and purchaser prior to design of the transformer. 10.1.6 Tests on bushings When tests are required on outdoor apparatus (air-to-oil) bushings separately from the transformers, the tests shall be made in accordance with IEEE Std C57.19.00 and IEEE Std C57.19.01. Details of separate testing of bushings for use on transformers connected to gas-insulated equipment shall be agreed upon by the manufacturer and purchaser prior to the design of the transformer. 10.1.7 Dielectric tests in the field Field dielectric tests may be warranted based on detection of combustible gas or other circumstances. However, periodic dielectric tests are not recommended because of the severe stress imposed on the insulation. Where field dielectric tests are required, low-frequency applied-voltage and induced-voltage tests shall be used. For distribution transformers and Class I power transformers, the line-to-ground or line-to-line voltage stress imposed shall not exceed 150% of normal operating stress or 85% of full test voltage, whichever is lower. The duration of the tests shall be the same as that specified in 10.6 and 10.7 for applied and induced voltage, respectively. For Class II power transformers, the line-to-ground or line-to-line voltage stress imposed shall not exceed 150% of maximum system operating voltage. The duration of the test shall not exceed the limits given in Table 2. Table 2 —Maximum test duration Test voltage as a percentage of maximum system operating voltages

Allowable duration (min)

150

5

140

12

130

36

120

120

When inducing a transformer in excess of its rated voltage, the test frequency should be increased as necessary to avoid core saturation. Guidance in this area is provided in 10.7.2.

39




10.2 Switching impulse test procedures The switching impulse test, when specified, shall consist of applying or inducing a switching impulse wave between each high-voltage line terminal and ground with a crest value equal to the specified test level. 10.2.1 Number of tests The test series shall consist of one reduced voltage wave at 50% to 70% of the specified test level followed by three full voltage waves at the specified test level. 10.2.2 Switching impulse waves 10.2.2.1 Polarity Negative polarity waves shall be used. NOTE 1—The transformer core may be pre-biased using either a dc supply or reduced switching impulse waves of positive polarity. Reduced switching impulse waves of positive polarity not exceeding 70% of the full test level may be applied prior to the first full wave and in between full-wave tests. The purpose is to bias the core in the opposite flux polarity to delay saturation during negative polarity full-wave test. NOTE 2—Negative polarity waves are specified for the actual test to minimize the risk of erratic external flashover (in the gap forming the air clearance to ground or in between phases). Negative polarity waves are deemed to stress the internal insulation in a similar manner as positive polarity waves.

10.2.2.2 Wave shape The switching impulse voltage wave shall have a crest value in accordance with the assigned insulation level, subject to a tolerance of ± 3%, and shall exceed 90% of the crest value for at least 200 µs. The actual time to crest shall be greater than 100 µs, and the time to the first voltage zero on the tail of the wave shall be at least 1000 µs. Occasionally, core saturation will cause the time to the first voltage zero to be less than 1000 µs. Successive transients of the same polarity may cause the time to the first voltage zero to become even shorter. To increase the time to the first voltage zero, it may be necessary to magnetically bias the core in the direction opposite to that caused by the switching impulse transient. This goal can be accomplished by passing a small direct current through the winding between impulses by reversing the switching impulse polarity on successive applications or by applying reduced impulses of opposite polarity before each full switching impulse transient. If biasing cannot be accomplished to obtain 1000 µs to the first voltage zero, the shorter tail may be used because the duration of a switching impulse in actual service will similarly be reduced because of core saturation. 10.2.2.3 Time to crest The actual time to crest shall be defined as the time interval from the start of the transient to the time when the maximum amplitude is reached. See IEEE Std 4 for more information on high voltage testing techniques. The time to crest should be defined and determined as described in 8.1 of IEEE Std 4-2013. 10.2.2.4 Time to first voltage zero The time to the first voltage zero on the tail of the wave shall be defined as the time interval from the start of the transient to the time when the first voltage zero occurs on the tail of the wave.

40




10.2.2.5 Ninety-percent time A smooth wave sketched through any oscillations on the switching impulse voltage oscillogram may be used to determine the time that the applied wave is in excess of 90% of the specified crest value. 10.2.3 Failure detection A voltage oscillogram shall be taken of each applied or induced transient. The test is successful if there is no sudden collapse of voltage indicated on the oscillograms. Successive oscillograms may differ, however, because of the influence of magnetic saturation on impulse duration. 10.2.4 Tap connections The choice of tap connections for all windings shall be made by the manufacturer. 10.2.5 Connection of neutral terminal(s) during switching impulse tests Neutral terminals, which are known to be effectively grounded in service, shall be solidly grounded or grounded through a low-ohmic shunt during switching impulse tests, unless otherwise agreed upon between manufacturer and purchaser prior to design of the transformer. If the neutral is not effectively grounded during service, the purchaser should advise the manufacturer at the time of the inquiry of the specific service grounding condition since it may affect the transformer design and/or the testing connections. Other testing configurations may be agreed upon if the dielectric stresses across the winding, to ground and between phases are representative of service conditions. If an alternative testing circuit is proposed, it should be agreed upon between manufacturer and purchaser prior to design of the transformer. NOTE 1—A low-ohmic shunt is generally a coaxial tubular resistor used for the purpose of measuring impulse current. Such shunts have a low resistance value typically ranging from 0.1 Ω to 10 Ω (see 9.6 of IEEE Std 4-2013 for more information). NOTE 2—Effectively grounded neutral systems are systems grounded through sufficiently low impedances such that for all system conditions the ratio of the zero-sequence reactance to the positive-sequence reactance (X0/X1) is positive and less than 3, and the ratio of the zero-sequence resistance to the positive-sequence reactance (R0/X1) is positive and less than 1. Normally such systems are solidly grounded (neutral) systems or low impedance grounded (neutral) systems. For the correct assessment of the grounding conditions not only the physical grounding conditions around the relevant location but the total system should be considered.

10.3 Lightning impulse test procedures Lightning impulse tests, when required as a routine test or when otherwise specified, shall consist of and be applied in the following order: one reduced full wave, one full wave, two chopped waves, and two full waves. The time interval between applications of the last chopped wave and the following full wave should be minimized, without intentional delays, to avoid recovery of dielectric strength. When front-of-wave tests are also specified, the front-of-wave impulses should be inserted within the impulse test sequence as follows: one reduced full wave, one full wave, two front-of-waves, two chopped waves, and two full waves. The order of the chopped wave and front-of-wave tests is not mandatory. However, a reduced full wave shall be applied first, and a full wave shall be the last wave to be applied to the terminal under test. Other reduced front-of-waves, reduced chopped waves, or reduced full waves may be applied at any time during the intervening sequence.

41




This sequence applies to line terminals. See 10.3.3 for sequence for neutral terminals. For information on impulse testing techniques, interpretation of oscillograms, and failure detection criteria, see IEEE Std C57.98. See IEEE Std 4 for more information on high voltage testing techniques. 10.3.1 General Impulse tests shall be made without excitation. 10.3.1.1 Full-wave test The test wave rises to crest in 1.2 µs and decays to half of crest value in 50 µs from the virtual time zero. The crest value shall be in accordance with the assigned basic impulse insulation level (BIL), subject to a tolerance of ±3%, and no flashover of the bushing or test gap shall occur. The tolerance on virtual front time should be ±30%, and the tolerance on time to half of crest should be ±20%. However, as a practical matter, once the manufacturer has proven that they have test equipment limitations, the following shall be considered: a)

The virtual front time shall not exceed 2.5 µs except for windings of large impulse capacitance (low-voltage, high-kilovoltampere and some high-voltage, high-kilovoltampere windings). To demonstrate that the large capacitance of the winding causes the long front, the impulse generator series resistance may be reduced. The reduction should cause superimposed oscillations. Only the inherent generator and lead inductances should be in the circuit.

b)

The impedance of some windings may be so low that the desired time to the 50% voltage point on the tail of the wave cannot be obtained with available test equipment. For such cases, shorter waves are considered acceptable provided that the optimum impulse generator connection is used (i.e., using parallel stages, if applicable, and the largest available capacitance). However, if by using the optimum impulse generator connection, the minimum tail time specified (40 µs) cannot still be achieved, the following alternative methods shall be used in the following preferred order: 1)

Use of resistor(s) on the nonimpulsed winding(s). The resistor value shall be the minimum necessary to achieve the required minimum tail time of 40 µs and shall not exceed the values given in Table 3. If the transformer is to be connected to a cable or to a gas-insulated switchgear or line (GIS or GIL), the resistor value shall not exceed 30 Ω for cable-connected terminals or 75 Ω for GIS- or GIL-connected terminals.

2)

Use of resistor on the grounded terminal of the impulsed winding. Here also, the resistor value shall be the minimum necessary to achieve the required minimum tail time of 40 µs and shall not exceed the values given in Table 3. If the transformer is foreseen to be connected to a cable or to a GIS or GIL, the resistor value shall not exceed 30 Ω for cable-connected terminals or 75 Ω for GIS- or GIL-connected terminals.

NOTE 1—The impulse voltage applied to the resistor should not exceed 80% of the rated lightning impulse level of the terminal(s) on which the resistor(s) is(are) connected, unless the manufacturer has consented. For delta-connected windings, the impulse voltage applied to the resistor, if used on nonimpulsed terminals of the delta, should not exceed 50% of the rated lightning impulse level of the terminal(s) on which the resistor(s) is(are) connected. NOTE 2—IEEE Std C57.98 and IEC 60076-4 [B15] give background information regarding the effect of added resistors to the dielectric stresses applied to the transformer.

42




NOTE 3—In general, the voltage peak appearing across the resistor is considerably delayed compared to the instant of the voltage peak of the applied lightning impulse. Thus, the resulting difference between the applied impulse and the voltage across the resistor (e.g., voltage across the winding) is similar to the one that would appear across the winding if the resistor were not used and the terminal were directly grounded. If the resistor applied voltage peak coincides within 10 µs of the voltage impulse peak, then the voltage drop across the winding is significantly reduced, and a special procedure should be agreed upon between the manufacturer and the purchaser. NOTE 4—The use of the Glaninger circuit (see IEEE Std C57.98) is also an effective method to increase the tail time. If such circuit is used, care should be exercised on the overswing in the opposite polarity. Overswing in opposite polarity up to 75% is common. NOTE 5—If the specific application (line, cable, GIS, or GIL) is not indicated at the time of order, the transformer is deemed to be overhead line-connected.

If the calculated tail time for a particular connection and transformer design is such that the minimum time to 50% (e.g., 40 µs) cannot be achieved, the manufacturer shall notify the purchaser of this possibility. The manufacturer shall also state the strategy to be taken to obtain the best achievable wave shape. Notification should be given during the bidding stage for cases where the minimum tail time cannot be obtained for a particular transformer design and/or because of test laboratory limitations. In such cases, shorter wave shapes may be agreed upon between manufacturer and purchaser. NOTE 6—The minimum impulse generator energy required to meet the minimum tail time (40 µs) during an impulse test on a particular transformer design and connection can be estimated by using Equation (26):

2π f ( t2 )  U BIL    VA zU 2  η  2

Emin . =

2

(26)

where

Emin . f t2 z U U BIL

η

VA

is the minimum energy required from the impulse generator (J) is the power frequency, 60 Hz is the tail time (s) ( t2 equals 40 µs) is the impedance (in per unit) seen from the impulsed terminal is the winding rated voltage ( V , phase-to-phase) is the rated BIL of the tested winding ( V ) is the impulse generator efficiency (in per unit) ( η = 1.0) is the three-phase power rating ( VA ) for which the impedance z is defined

NOTE 7—This equation has been derived from the equations given in IEC 60076-4 [B15]. More information about wave shape control can be found in IEEE Std C57.98 and IEC 60076-4. For single-phase transformers, the three-phase bank power rating and associated phase-to-phase voltage should be used in the above equation.

10.3.1.2 Reduced full-wave test A reduced full wave is the same as a full wave, except that the crest value shall be between 50% and 70% of the full-wave value. 10.3.1.3 Chopped-wave test A chopped wave is inherently a full lightning impulse wave, except that the crest value shall be at the required level and the voltage wave shall be chopped at or after the required time to flashover (time to chopping) but not later than 6 µs after virtual origin. The virtual front time of the chopped wave may be different than the virtual front during a full-wave test because of the presence of the chopping gap. Nevertheless, the tolerance on the virtual front time for the chopped-wave test should remain as defined for full-wave test. 43




The gap or other equivalent chopping device shall be located as close as possible to the terminals of the transformer without disrupting its electrical field distribution. The distance between the chopping device and the test object shall not exceed a lead length greater than the total height of the transformer (tank + bushing). The impedance between the tested terminal and the grounded end of the chopping device shall be limited to that of the necessary leads. The voltage zero following the instant of chopping shall occur within 1 µs. However, for some winding designs (particularly low-voltage windings of high stray capacitance and some layer windings), the circuit response after chopping may not be oscillatory; it may be overdamped. For such cases, the time interval to the first voltage zero after the instant of chopping may be significantly greater than 1 µs. Only for cases where the overswing to the opposite polarity is greater than 30% is it permissible to add a series-connected resistor in the chopping circuit to limit the amount of overswing. When a resistor is added in the chopping circuit, the resistor shall not decrease the overswing below 30% of the amplitude of the chopped wave. The use of a resistor in the chopping circuit may increase the time interval to the first voltage zero after the instant of chopping. If the use of a resistor in the chopping circuit does conflict with the above requirement of the maximum time interval to the first voltage zero after the instant of chopping, the priority shall be given to the maximum limit of the time interval. For such cases, it may not be possible to reduce the overswing to the opposite polarity to 30%. NOTE 1—This method will increase the likelihood that the steepness of the voltage collapse (dv/dt) is as high as possible. NOTE 2—The use of a chopping gap made of sphere gap(s) (single or multiple sphere gaps) is the preferred chopping method since it usually gives faster voltage collapse. The use of a rod-rod chopping gap is also permissible since this method more accurately replicates in-service flashover of an air insulator. Notably, the rod-rod gap requires a greater distance between its electrode for a given operating voltage than does a sphere gap. The extended arc length of the rodrod gap provides more natural circuit damping than the shorter arc length of a sphere gap. NOTE 3—If the above prescribed maximum lead length to the chopping gap cannot be achieved because of the presence of transformer accessories such as coolers or any other transformer accessories, then the shortest possible lead length should be used during tests.

10.3.1.4 Front-of-wave test The wave to be used in a front-of-wave test is similar to a full wave, except that it is chopped on the front of the wave at the assigned crest level and time to sparkover. The time to sparkover for front-of-wave impulse tests shall be the time from virtual zero to the time of sparkover. As with the chopped-wave test, the manufacturer shall be permitted to add resistance in the circuit to limit the amount of overswing to the opposite polarity to 30% of the amplitude of the front-of-wave. 10.3.1.5 Wave polarity For liquid-immersed transformers, the test waves are normally of negative polarity to reduce the risk of erratic external flashover in the test circuit. 10.3.1.6 Impulse oscillograms All impulses applied to a transformer shall be recorded by an oscilloscope or by a suitable digital transient recorder, unless their crest voltage is less than 40% of the full-wave level. These oscillograms shall include voltage oscillograms for all impulses and ground-current oscillograms for all full-wave and reduced fullwave impulses. Sweep times should be on the order of 2 µs to 5 µs for front-of-wave tests, 5 µs to 10 µs for chopped-wave tests, 50 µs to 100 µs for full-wave tests, and 100 µs to 600 µs for ground-current measurements.

44




When reports require oscillograms, those of the first reduced full-wave voltage and current, the last two chopped waves, and the last full wave of voltage and current shall represent a record of the successful application of the impulse test to the transformer. When transformers receiving front-of-wave impulse tests require reports that include oscillograms, those of the first reduced full-wave voltage and current, the last two front-of-waves, the last two chopped waves, and the last full wave of voltage and current shall represent a record of the successful application of the front-of-wave impulse test to the transformer. 10.3.2 Connections for impulse tests of line terminals In general, the tests shall be applied to each terminal, one at a time. 10.3.2.1 Terminals not being tested Neutral terminals shall be solidly grounded. Line terminals, including those of autotransformers and regulating transformers, shall be either solidly grounded or grounded through a resistor with an ohmic value not in excess of the values given in Table 3. Table 3 —Grounding resistor values Nominal system voltage (kV)

Resistance (Ω)

345 and below

450

500

350

765

300

NOTE—These values are representative of typical transmission-line surge impedances.

The following factors shall be considered in the actual choice of grounding for each terminal: a)

The voltage to ground on any terminal that is not being tested should not exceed 80% of the fullwave impulse voltage level for that terminal.

b)

When a terminal has been specified to be directly grounded in service, then that terminal shall be solidly grounded.

c)

When a terminal is to be connected to a low-impedance cable connection in service, then that terminal shall be either directly grounded or grounded through a resistor with an ohmic value not in excess of the surge impedance of the cable.

d)

Grounding through a low-impedance shunt for current measurements may be considered the equivalent of a solid ground.

10.3.2.2 Windings for series or multiple connections When either connection is 25 kV nominal system voltage or above, the windings shall be tested on both series and multiple connections. The test voltage for the two conditions shall correspond to the BIL of the winding for that connection. For nominal system voltages of 15 kV and below, only the series connections shall be tested, unless tests on both connections are specified. 10.3.2.3 Windings for delta or wye connections When either connection is 25 kV nominal system voltage or above, the three-phase transformer shall be tested on both delta and wye connections. The test voltage for each connection shall correspond to the BIL

45




of the winding for that connection. For nominal system voltages of 15 kV and below, only the wye connection shall be tested, unless tests on both connections are specified. 10.3.2.4 Tap connections Dielectric stresses during lightning impulse testing are distributed differently depending upon the tapping and the general design of the transformer. The two extreme tap positions and the middle tap position shall be used, one tap position for testing each of the three individual phases of a three-phase transformer or three single-phase transformers of a three-phase bank. When fewer than three single-phase transformers are tested (e.g., for a single-phase application or for a replacement unit or as a spare, etc.), the tap position for the first unit shall be set to achieve the minimum effective turns in the winding under test. In case of class II transformers, for additional single-phase units (greater than one or three units), the tap position shall be changed from one unit to the other, taking into account the requirements given above. NOTE 1—As an example, if two single-phase units are to be tested, the first unit is to be tested with a tap position giving the minimum effective turns. The second unit may be tested either at the middle tap position or the maximum tap position. NOTE 2—The above required tap connections may not necessarily produce the maximum stress conditions. If different tap positions are proposed to be used during test, it should be agreed upon between manufacturer and purchaser prior to design of the transformer.

As an exception to the previous rule for selecting the two extremes and middle tap changer positions, for transformers using a reactor-type load tap changer, one of the tests shall be performed in a bridging position. This test shall be performed on a tap position as close as possible to the middle position, e.g., one tap higher or lower. If only one single-phase transformer is tested, the tests shall be performed in a bridging position closest to the minimum effective turns position. NOTE 3—Tests in bridging position are required only if this position can be used in service for continuous operation.

The tap position of the windings of the same phase not being directly impulse tested shall follow the same basic rule described above and shall be set in order to obtain the maximum, minimum, and middle turn ratios, with one of these prescribed ratios for each of the individual phases. If fewer than three single-phase transformers are tested (e.g., for single-phase application or for replacement unit or as a spare unit, etc.), the tap position of windings of the same phase not being directly impulse tested shall be set to the maximum turns to ensure maximum voltage transfer for the first unit. In case of class II transformers, for additional single-phase units (greater than one or three units), the tap position shall be changed from one unit to the other taking into account the requirements given above. NOTE 4—As an example, a transformer having a load tap changer on the (primary) HV winding and a de-energized tap changer on the (secondary) LV winding, possible tap changer position combinations during an impulse test are as follows. Other combinations may be used. 

Impulse on HV winding phase A: HV: max. tap; LV min. tap (maximum ratio);



Impulse on HV winding phase B: HV: nom. tap; LV nom. tap (nominal ratio);



Impulse on HV winding phase C: HV: min. tap; LV max. tap (minimum ratio);



Impulse on LV winding phase A: LV: max. tap; HV min. tap (minimum ratio);



Impulse on LV winding phase B: LV: nom. tap; HV nom. tap (nominal ratio);



Impulse on LV winding phase C: LV: min. tap; HV max. tap (maximum ratio).

The impulse generator adjustments to achieve the required wave-shape shall be performed on the first phase under test on a tap changer position(s) yielding to the lowest ohmic impedance. Because of the impedance variation due to a different tap position, the waveshape time parameters may change beyond the 46




required tolerances for the two remaining phases. The resultant waveshape time parameters for these two phases shall be considered valid and no waveshape time adjustments are required. 10.3.2.5 Protective devices that are an integral part of the transformer Transformers and regulators may have, as an integral part of their design, nonlinear protective devices connected across whole or portions of windings. Operation of these protective devices during impulse testing may cause differences between the reduced full-wave and the full-wave and/or chopped-wave oscillograms. In order to demonstrate that these differences are solely caused by the operation of the protective devices and not by a transformer failure, additional reduced full-wave impulse tests at different voltage levels shall be applied to show the effect of the operation of the nonlinear devices on voltage and current oscillograms and its reproducibility. The purpose of the nonlinear protective devices is to limit transient overvoltages, which may be impressed or induced across the windings during lightning impulse surges (high-frequency voltage surges). However, during the standardized switching impulse tests, because of the potentially high energy level contained in switching surges, provisions shall be made so that these devices will not operate within any of the winding sections. If, during switching impulse tests, a flashover is observed across line bushings, the nonlinear devices may limit the resulting overvoltages produced within the winding sections after flashover. Typical oscillograms depicting the operation of protective devices during impulse testing are shown in IEEE Std C57.98. The following test sequence shall be performed:  One reduced full wave between 50% and 70% of the required full-wave impulse level  One or more intermediate reduced full waves between 75% and 100% of the required full-wave impulse level (see NOTE 1)  One full wave at 100% of the required full-wave impulse level  Two chopped waves at 100% of the required chopped-wave impulse level  Two full waves at 100% of the required full-wave impulse level  One or more intermediate reduced full waves at the same voltage levels as used before the first full-wave test  One reduced full wave between 50% and 70% of the required full-wave impulse level With the exception of the special cases given below in NOTE 2 and NOTE 3, the intermediate reduced fullwave impulse level shall show the operation of the nonlinear devices and its effect on the current and voltage oscillograms. NOTE 1—The voltage level to be applied for the intermediate reduced full wave is not specifically given. Only a range is proposed because the threshold operating level of the nonlinear devices is transformer design dependent. Generally, a lightning impulse within that specified voltage range will cause the operation of the nonlinear devices. The specific number of intermediate full-wave tests and their voltage levels cannot be given here. The number of intermediate fullwave tests and their respective voltage level for a given transformer should be chosen by the manufacturer and agreed to by the purchaser. NOTE 2—In some cases, tests at the required full-wave impulse level with the standardized lightning impulse wave shape will not show the operation of the nonlinear devices. In such cases, additional intermediate reduced full-wave tests are not necessary and may be waived.

47




NOTE 3—In some other special cases, the operation of the nonlinear devices can be observed only during the choppedwave impulse tests. In such cases, the intermediate reduced full-wave tests are also not necessary and may be waived. As explained in 10.3.4.2a), comparison of the recorded oscillograms may be done by comparing the two chopped-wave tests together up to the time of chopping. Chopped-wave tests cannot be compared during and after chopping. For such cases, reduced chopped-wave impulses at a test level of approximately 75% of the required chopped-wave test level may be a useful tool to assess that the differences on the recorded oscillograms are solely caused by the operation of the nonlinear devices. If reduced chopped-wave tests are performed, they should, by agreement between the manufacturer and the purchaser, be performed before and after the required chopped-wave tests.

Because of the operation of the nonlinear devices, the comparison of the voltage and current oscillograms shall be made only between two tests performed at the same voltage level, e.g., comparing, for example, the two 80% reduced full-wave tests. Each reduced full-wave test performed after a full-wave test shall be compared with the corresponding reduced full-wave test performed prior to the full-wave tests. All three full wave tests at 100% shall be compared with each other. 10.3.3 Impulse tests on transformer neutrals Impulse tests on the neutral of a transformer or a separate regulator connected in the neutral of a transformer require one reduced full wave and three full waves to be applied directly to the neutral or regulator winding with an amplitude equal to the insulation level of the neutral. The winding being tested shall be either on the minimum voltage connection or on the maximum voltage connection. A wave having a front of not more than 10 µs and a tail of 50 µs (± 20%) to half-crest shall be used, except that, when the inductance of the winding is so low that the desired voltage magnitude and duration to the 50% point on the tail of the wave cannot be obtained, a shorter wave tail may be used, taking into account the considerations given in 10.3.1.1. 10.3.4 Detection of failure during impulse test Given the nature of impulse test failures, one of the most important matters to consider is the detection of such failures. Several indications of insulation failure exist. 10.3.4.1 Ground current oscillograms In the ground current method of failure detection, the impulse current in the grounded end of the winding tested is measured by an oscilloscope or by a suitable digital transient recorder connected across a suitable shunt inserted between the normally grounded end of the winding and the ground. Any differences in the wave shape between the reduced full wave and the final full wave detected by comparison of the two current oscillograms may be indications of failure or deviations due to noninjurious causes. They should be fully investigated and explained by a new reduced full-wave test and full-wave test. Examples of probable causes of different wave shapes are operation of protective devices, core saturation, or conditions in the test circuit external to the transformer. The ground current method of detection is not suitable for use with chopped-wave tests. 10.3.4.2 Other methods of failure detection Other methods of detecting failure include the following: a) Voltage oscillograms. Any unexplained differences between the reduced full wave and the final full wave detected by comparison of the two voltage oscillograms, or observed by comparing the chopped waves to each other and to the full wave up to the time of chopping, are indications of failure. b)

Failure of gap to sparkover. In making the chopped-wave test, failure of the chopping gap or any external part to sparkover, although the voltage oscillogram shows a chopped wave, is a definite indication of a failure either within the transformer or in the test circuit.

48




c)

Noise. Unusual noise within the transformer at the instant of applying the impulse is an indication of trouble. Such noise should be investigated.

d)

Measurement. Measurement of voltage and current induced in another winding may also be used for failure detection.

10.4 Routine impulse test for distribution transformers For distribution transformers, the impulse tests specified in 10.3 are design tests. This subclause defines a routine quality control test that is suitable for high-volume production-line testing. The routine impulse test for distribution transformers applies to overhead, pad-mounted, and underground liquid-immersed distribution transformers with requirements specified in IEEE Std C57.12.20, IEEE Std C57.12.23, IEEE Std C57.12.24, IEEE Std C57.12.34, IEEE Std C57.12.36, and IEEE Std C57.12.38. 10.4.1 Terminals to be tested In routine testing, impulse tests are applied to all high-voltage line terminals. Impulse tests of the lowvoltage terminals or the neutral terminal are not required. Line terminals rated more than 600 V are considered high voltage. 10.4.2 Procedure The windings under test are connected to ground through a low-impedance shunt. The tank, the core, and either one of the low-voltage terminals or the neutral terminal are also connected to the shunt or are directly grounded. This shunt shall consist of either of the following: a)

Ground current method. A suitable resistance shunt or wide-band pulse current transformer is employed to examine the waveform of the ground current.

b)

Neutral impedance method. A low-impedance shunt, consisting of a parallel combination of resistance and capacitance (RC), is employed. The voltage across this neutral impedance shunt is examined.

An impulse voltage with 1.2 × 50 µs wave shape and with specified crest magnitude shall be applied in each test. The tolerances, polarity, and method of determining the wave shape shall be as specified in 10.3.1.1 and 10.3.1.5. During each test, the waveform of the ground current or the voltage wave across the neutral impedance shall be examined. The required impulse tests shall be applied using either of the test series in Method 1(10.4.2.1) or Method 2 (10.4.2.2). 10.4.2.1 Method 1 One reduced full-wave test is performed, followed by one 100% magnitude full-wave test. The appliedvoltage wave in the first test shall have a crest value of between 50% and 70% of the assigned BIL. The applied-voltage wave in the second test shall have a crest value of 100% of the assigned BIL. Failure detection is accomplished by comparing the reduced full-wave test with the 100% magnitude full-wave test, using either the ground current waveform or the neutral impedance voltage waveform. A dielectric breakdown will cause a difference in compared waveforms. Observed differences in the waveforms may be indications of failure, or they may be due to non-injurious causes. The criteria used to judge the magnitude of observed differences shall be based on the ability to detect a staged single-turn fault made by placing a loop of wire around the core leg and over the coil.

49




10.4.2.2 Method 2 Two full-wave tests, with crest magnitude equal to the assigned BIL, are applied to the transformer under test. A neutral impedance shunt, using suitable values of resistance and capacitance, is employed to record waveforms for comparison. The waveforms in both tests are compared with pre-established levels. A dielectric breakdown will cause a significant upturn and increase in magnitude of the voltage wave examined across the neutral impedance. The pre-established levels are based on a staged single-turn fault test, made by placing a loop of wire around the core leg and over the coil. 10.4.2.3 Failure detection The failure detection methods described in 10.4.2.1 and 10.4.2.2 for the routine impulse test are based on the following two conditions: a)

The transformer connections during the test are such that no low-voltage windings are shorted.

b)

Chopped-wave tests are not applied.

In addition to these methods of failure detection, other methods of failure detection, as described in 10.3.4.2, are also indications of failure and should be investigated. When the test is complete and the process of failure detection is complete, the waveform records may be discarded. As an exception to the test sequence given in 10.1.5.1, the routine impulse test may be conducted either before or after the low-frequency dielectric tests; however, the preferred sequence is for the impulse test to precede the low-frequency dielectric tests. 10.4.3 Terminals not being tested All high-voltage terminals not being tested shall be solidly grounded for impulse tests of the high-voltage windings. However, if two high-voltage terminals are grounded, causing a short circuit across one or more of the high-voltage windings, the failure detection sensitivity of the test may be impaired, and a single-turn fault may not be detectable. In such cases, only one high-voltage terminal should be grounded. Grounding through a low-impedance shunt for current measurements may be considered the equivalent of a solid ground. The low-voltage windings shall be solidly grounded for impulse tests of the high-voltage windings by applying the ground to only one low-voltage terminal in order to minimize the risk of a deliberate short circuit across any low-voltage winding. Selection of the low-voltage terminal to be grounded should be as follows: a)

For a single-phase three-wire connection, where X2 would be grounded in service, terminal X2 shall be solidly grounded and terminals X1 and X3 shall be open, except as provided in the paragraph that follows this list.

b)

For a single-phase two-wire connection, where either X1 or X2 may be grounded in service, either terminal X1 or X2 shall be solidly grounded and the remaining terminal shall be open, except as provided in the paragraph that follows this list.

c)

For a three-phase four-wire connection, where X0 would be grounded in service, terminal X0 shall be grounded and terminals X1, X2, and X3 shall be open, except as provided in the paragraph that follows this list.

d)

For a three-phase three-wire delta connection, only one of the low-voltage terminals X1, X2, or X3 shall be solidly grounded and the two remaining terminals shall be open, except as provided in the paragraph that follows this list.

For series multiple or other low-voltage connections not covered specifically above, the low-voltage windings shall be grounded in accordance with the principle of grounding the winding without causing a 50




direct short circuit across any low-voltage winding and preferably selecting the terminal that will be grounded in service. It is permissible to limit the voltage to ground of any low-voltage terminal by connecting a resistor across the low-voltage windings. This resistor shall be sized to limit the induced voltage to less than 80% of the BIL of the terminal. Current flowing in the limiting resistor shall not interfere with the ability to detect a staged single-turn fault. 10.4.4 Windings for series or multiple connections For high-voltage windings with series or multiple connections, the routine impulse test shall be conducted on each connection at its assigned BIL. 10.4.5 Windings for delta or wye connections For high-voltage windings with delta or wye connections, the routine impulse test shall be conducted on each connection at its assigned BIL. 10.4.6 Tap connections For windings with taps, the routine impulse test shall be performed in the tap connection for shipment in accordance with Clause 10 of IEEE Std C57.12.00-2015.

10.5 Low-frequency tests Low-frequency tests shall be performed in accordance with the requirements of 5.10, Table 3, and Table 4 of IEEE Std C57.12.00-2015. For distribution transformers and Class I power transformers, the low-frequency test levels are developed by the applied-voltage and induced-voltage tests described in 10.6 and 10.7, or combinations thereof. The induced-voltage test may involve either single-phase or three-phase excitation. For single-phase transformers with a BIL of 150 kV or less that have only one high-voltage bushing, the high-voltage neutral terminal permanently connected to ground, and no secondary windings permanently grounded, no applied-voltage test is required. These transformers shall receive an induced-voltage test between the high-voltage terminal and ground with duration of 7200 cycles but not less than 15 s. This voltage shall be 1000 V plus 3.46 times the rated transformer winding voltage, but in no case shall the lineto-ground voltage developed exceed 40 000 V for 125 kV BIL or 50 000 V for 150 kV BIL. An appliedpotential test shall be applied to all windings that are not permanently grounded. For Class II power transformers, the low-frequency tests involve a special induced test as described in 10.8 and applied-voltage tests as described in 10.6. 10.5.1 Induced-voltage test for transformers with series or multiple connections Transformers with windings that have multiple connections (series-parallel or delta-wye) and whose connections each have a nominal system voltage of 25 kV or above shall receive two induced tests, one in each connection. If more than one winding has such multiple connections, then the connections in each of the windings shall change between the tests, and the manufacturer shall determine the relative connections for each test. The test voltage and duration (Class I—7200 cycle or Class II—1 h test) shall be contingent on the system voltage level of the high-voltage winding for the connection being tested. In all cases, the last induced test shall be for the connection with the highest test voltage. 51




10.6 Applied-voltage tests 10.6.1 Duration, frequency, and connections The test shall be performed at low frequency (< 500 Hz), and the duration of the test shall be 1 min. The winding being tested shall have all its parts joined together and connected to the line terminal of the testing transformer. All other terminals and parts (including core and tank) shall be connected to ground. 10.6.2 Relief gap A relief gap set at a voltage 10% or more in excess of the specified test voltage may be connected during the applied-voltage test. 10.6.3 Application of test voltage The voltage should be started at one quarter or less of the full value and be brought up gradually to full value. After being held for the time specified in 10.6.1, it should be reduced gradually before the circuit is opened. 10.6.4 Failure detection Careful attention should be maintained for evidence of possible failure, such as an indication of smoke and bubbles rising in the insulating liquid, an audible sound such as a thump, or a sudden increase in test circuit current. Any such indication should be carefully investigated by observation, by repeating the test, or by other tests to determine whether a failure has occurred.

10.7 Induced-voltage tests for distribution and Class I power transformers 10.7.1 Test duration The induced-voltage test shall be applied for 7200 cycles, or 60 s, whichever is shorter. 10.7.2 Test frequency As an induced-voltage test applies greater-than-rated volts per turn to the transformer, the frequency of the impressed voltage shall be high enough to limit the flux density in the core to that permitted by 4.1.6.1 of IEEE Std C57.12.00-2015. The minimum test frequency to meet this condition is given in Equation (27):

Minimum test frequency =

Et × rated frequency 1.1 × Er

(27)

where

Et Er

is the induced voltage across winding (V) is the rated voltage across winding (V) 52




10.7.3 Application of voltage The voltage should be started at one quarter or less of full value and be brought up gradually to full value. After being held for the time specified in 10.7.1, it should be reduced gradually before the circuit is opened. 10.7.4 Grounding of windings When a transformer has one end of the high-voltage winding grounded, the other windings should be grounded during the induced-voltage test. This ground on each winding may be made at a selected point of the winding itself or of the winding of a step-up transformer that is used to supply the voltage or that is connected for the purpose of furnishing the ground. 10.7.5 Need for additional induced tests When the induced test on a winding results in a voltage between terminals of other windings in excess of the low-frequency test voltage specified in Table 3 or Table 4, as applicable, of IEEE Std C57.12.00-2015, the other winding may be sectionalized and grounded. Additional induced tests shall then be made to give the required test voltage between terminals of windings that were sectionalized. 10.7.6 Failure detection Careful attention should be maintained for evidence of possible failure, such as an indication of smoke and bubbles rising in the insulating liquid, an audible sound such as a thump, a sudden increase in test circuit current, or an appreciable increase in partial discharge level. Any such indication should be carefully investigated by observation, by repeating the test, or by other tests to determine whether a failure has occurred.

10.8 Induced-voltage test for Class II power transformers 10.8.1 General Each Class II power transformer shall receive an induced-voltage test with the required test levels induced in the high-voltage winding. The tap connections shall be chosen, when possible, so that test levels developed in the other windings during the 1 h test are x times their maximum operating voltages, as specified in ANSI C84.1, where x is the ratio of the test voltage on the high-voltage winding to the maximum operating voltage. 10.8.2 Test procedure The voltage shall first be raised to the 1 h level and held for a minimum of 1 min or until a stable partial discharge level is obtained to verify that there are no partial discharge problems. The level of partial discharges shall be recorded just before raising the voltage to the enhancement level. The voltage shall then be raised to the enhancement level and held for 7200 cycles. The voltage shall then be reduced directly to the 1 h level and held for 1 h. During this 1 h period, partial discharge measurements shall be made at 5 min intervals. Partial discharge acceptance criteria shall be based on each line terminal rated 115 kV and above. These measurements shall be made in accordance with 10.9. 10.8.3 Connections The transformer shall be excited exactly as it will be in service. The voltage may be induced from any winding or from special windings or taps provided for test purposes. Single-phase transformers shall be excited from single-phase sources. Three-phase transformers shall be excited from three-phase sources. The 53




neutral terminals and other terminals that are normally grounded in service shall be solidly grounded. This will stress all of the insulation at the same per unit of overstress. 10.8.4 Frequency The test frequency shall be increased, relative to operating frequency, as required to avoid core saturation. The requirements in 10.7.2 are also applicable in the case of this induced test. 10.8.5 Failure detection Failure may be indicated by the presence of smoke and bubbles rising in the insulating liquid, an audible sound such as a thump, or a sudden increase in the test current. Any such indication shall be carefully investigated by observation, by repeating the test, and by other diagnostic tests to determine whether a failure has occurred. In terms of interpretation of partial discharge measurements, the results shall be considered acceptable and no further partial discharge tests required under the following conditions: a)

The magnitude of the partial discharge level does not exceed 500 pC during the 1-h test period.

b)

The increase in partial discharge levels during the 1-h period does not exceed 150 pC.

c)

The partial discharge levels during the 1-h period do not exhibit any steadily rising trend, and no sudden sustained increase in the levels occurs during the last 20 min of the test.

Judgment should be used on the 5-min readings so that momentary excursions of the partial discharge readings caused by cranes or other ambient sources are not recorded. Also, the test may be extended or repeated until acceptable results are obtained. A failure to meet the partial discharge acceptance criterion shall not warrant immediate rejection, but it shall lead to consultation between purchaser and manufacturer about further investigations.

10.9 Partial discharge measurement 10.9.1 Internal partial discharges Apparent internal partial discharges (apparent charge) shall be measured at the terminals of the transformer windings under test and reported in units of picocoulombs (pC). Where agreed to by both the purchaser and the manufacturer, radio influence voltage (RIV) measurements may be used in lieu of, or in conjunction with, apparent charge measurements. The procedure for RIV measurements is included in Annex A. 10.9.2 Instrumentation A partial discharge meter shall be used to measure the apparent charge generated by any internal partial discharges. The partial discharge detector, based on IEEE Std C57.113, is used to measure the partial discharge levels at the terminals. The partial discharge meter shall be coupled to the line terminal(s) of the winding(s) under test through the voltage tap of the bushing(s) or through a suitable coupling capacitor connected in parallel with the bushing. General principles and circuits are described in IEEE Std C57.113. External shielding may be used to avoid air corona, such as may occur at the bushing terminals or grounded projections. Radio-frequency chokes or tuned filters may be used to isolate the transformer under test and the partial-discharge-measuring circuit from the remainder of the test circuit, including its energy source.

54




10.9.3 Calibration The test circuit shall be calibrated according to IEEE Std C57.113.

10.10 Insulation power-factor tests The insulation power factor is the ratio of the power dissipated in the insulation in watts to the product of the effective voltage and current in voltamperes when tested under a sinusoidal voltage and prescribed conditions. The methods described in this standard are applicable to distribution and power transformers of present-day design that are immersed in an insulating liquid.

10.10.1 Preparation for tests The test specimen shall have the following: a)

All windings immersed in insulating liquid.

b)

All windings short-circuited.

c)

All bushings in place.

d)

The average temperature of the windings and insulating liquid should be between 10 °C and 40 °C, but preferably as close to 20 °C as practical. The top liquid temperature shall be measured and recorded.

10.10.2 Instrumentation The insulation power factor may be measured by special bridge circuits or by the voltampere-watt method. The accuracy of measurement should be within ± 0.25% insulation power factor, and the measurement should be made at or near a frequency of 60 Hz. 10.10.3 Voltage to be applied The voltage to be applied for measuring insulation power factor shall not exceed half of the low-frequency test voltage given in Table 3 or Table 4 of IEEE Std C57.12.00-2015 for any part of the winding or 10 000 V, whichever is lower. 10.10.4 Procedure Insulation power-factor tests shall be made from windings to ground and between windings as shown in Table 4.

55




Table 4 —Measurements to be made in insulation power-factor tests Method I Test without guard circuita

Method II Test with guard circuita

Two-winding transformersb

Two-winding transformersb

High to low and ground

High to low and ground

Low to high and ground

High to ground, guard on low

High and low to ground

Low to high and ground

— Three-winding transformers

Low to ground, guard on high

b

Three-winding transformersb

High to low, tertiary, and ground

High to low and ground, guard on tertiary

Low to high, tertiary, and ground

High to ground, guard on low and tertiary

Tertiary to high, low, and ground

Low to tertiary and ground, guard on high

High and low to tertiary and ground

Low to ground, guard on high and tertiary

High and tertiary to low and ground

Tertiary to high and ground, guard on low

Low and tertiary to high and ground

Tertiary to ground, guard on high and low High and low to tertiary and ground

High, low, and tertiary to ground

High and tertiary to low and ground

NOTE 1—Although the real significance that can be attached to the power factor of liquid-immersed transformers is still a matter of opinion, experience has shown that the power factor is helpful in assessing the probable condition of the insulation when good judgment is used. NOTE 2—In interpreting the results of power-factor test values, the comparative values of tests taken at periodic intervals are useful in identifying potential problems rather than an absolute value of power factor. NOTE 3—A factory power-factor test will be of value for comparison with field power-factor measurements to assess the probable condition of the insulation. It has not been feasible to establish standard power-factor values for liquid-immersed transformers for the following reasons: a) Experience has indicated that little or no relation exists between power factor and the ability of the transformer to withstand the prescribed dielectric tests. b) Experience has shown that the variation in power factor with temperature is substantial and erratic so that no single correction curve will fit all cases. The power factor should be reported along with the top liquid temperature measured and the bottom liquid temperature if available. No temperature correction should be applied. Temperature correction of the power factor results for trending basis may be applied by the purchaser. c) The various liquids and insulating materials used in transformers result in large variations in insulation power-factor values. a In this table, the term guard signifies one or more conducting elements arranged and connected on an electrical instrument or measuring circuit to divert unwanted currents from the measuring means. b Permanently connected windings, such as in autotransformers and regulators, shall be considered as one winding.

10.11 Insulation resistance tests Insulation resistance tests shall be made when specified. Insulation resistance tests are made to determine the insulation resistance from individual windings to ground or between individual windings. Insulation resistance tests are commonly measured in megohms or may be calculated from measurements of applied voltage and leakage current. NOTE 1—The insulation resistance of electrical apparatus is of doubtful significance compared to the dielectric strength. It is subject to wide variation in design, temperature, dryness, and cleanliness of the parts. When the insulation resistance falls below prescribed values, it can, in most cases of good design and where no defect exists, be brought up to the required standard by cleaning and drying the apparatus. Therefore, the insulation resistance may be useful to indicate whether the apparatus is in suitable condition for application of dielectric test.

56




NOTE 2—The significance values of insulation-resistance tests generally require some interpretation, depending on the design and the dryness and cleanliness of the insulation involved. When a purchaser decides to make insulation resistance tests, it is recommended that insulation resistance values be measured periodically (during maintenance shutdown) and plotted. Substantial variations in the plotted insulation resistance values should be investigated for cause. NOTE 3—Insulation resistances may vary with applied voltage, and any comparison should be made with measurements at the same voltage. NOTE 4—Under no circumstances should tests be made while the transformer is under vacuum.

10.11.1 Preparation for tests The test specimen shall have the following: a)

All windings immersed in insulating liquid

b)

All windings short-circuited

c)

All bushings in place

d)

Temperature of windings and insulating liquid near (the reference temperature of) 20 ºC

10.11.2 Instrumentation Insulation resistance may be measured using the following equipment: a)

A variable-voltage dc power supply with means to measure voltage and current (generally in microamperes or milliamperes)

b)

A megohmeter

NOTE—Megohmeters are commonly available with nominal voltages of 500 V, 1000 V, and 2500 V; dc applied test equipment is available at higher voltages.

10.11.3 Voltage to be applied The dc voltage applied for measuring insulation resistance to ground shall not exceed a value equal to the rms low-frequency applied voltage allowed in 10.6. NOTE 1—Partial discharges should not be present during insulation resistance tests because they may result in erroneous values of insulation resistance. NOTE 2—When measurements are to be made using dc voltages exceeding the rms operating voltage of the winding involved (or 1000 V for a solidly grounded wye winding), a relief gap may be employed to protect the insulation.

10.11.4 Procedure Insulation-resistance tests shall be made with all circuits of equal voltage above ground connected together. Circuits or groups of circuits of different voltage above ground shall be tested separately. Examples of procedures include the following: a)

High voltage to low voltage and ground, low voltage to high voltage and ground.

b)

Voltage should be increased in increments, typically 1 kV to 5 kV, and held for 1 min while the current is read.

c)

The test should be discontinued immediately if the current begins to increase without stabilizing.

d)

After the test has been completed, all terminals should be grounded for enough time to allow any trapped charges to decay to a negligible value. 57




11. Temperature-rise tests A temperature-rise test is defined as a test to determine the temperature rise above ambient of one or more of the transformer’s windings, as measured at the terminals. The result for a given terminal pair or winding is the average value of the temperature of the entire circuit; it is not the temperature at any given point in a specific winding. The term average temperature rise refers to the value determined by measurements on a given terminal pair of the winding. It does not refer to the arithmetic average of results determined from different terminal pairs of the transformer. See 4.1.2 and 5.11.2 of IEEE Std C57.12.00-2015 for conditions under which temperature limits apply. All temperature-rise tests shall be made under normal (or equivalent to normal) conditions for the means of cooling, as follows: a)

Temperature-rise tests shall be conducted on transformers that are completely assembled and filled to the proper liquid level.

b)

Temperature-rise tests shall be made in a room that is as free from drafts as practicable; defined as a wind speed of 0.5 m/s, or less.

c)

When it is not possible or practical to test the transformer as a completed assembly, the transformer shall be tested with the components required to replicate normal means of cooling the transformer during temperature-rise tests. When the transformers are equipped with thermal indicators, bushing current transformers, or the like, such devices shall be assembled with the transformer.

11.1 Test methods Tests shall be made by one of the following methods: a)

Actual loading

b)

Simulated loading 1)

The loading back (opposition) method, in which rated voltage and current are induced in the transformer under test

2)

The short-circuit method, in which appropriate total losses are produced by the effect of shortcircuit current

11.1.1 Actual loading Actual loading is the most accurate method, but energy requirements are excessive for large transformers. Transformers of small output may be tested under actual load conditions by loading them on a rheostat, bank of lamps, water box, and so on. 11.1.2 Simulated loading 11.1.2.1 Loading back method Some smaller transformers (typically distribution class and small power) may be tested by connecting their respective high-voltage and low-voltage windings in parallel (see Figure 29 and Figure 30). Transformers shall be tested with the combination of connections and taps that give the highest average winding temperature rise. This process will generally involve those connections and taps resulting in the highest losses. In the case of internally grounded transformers, caution should be taken for possible elevated tank voltages.

58




a)

Apply rated voltage at rated frequency to one set of windings. Circulate load current by opening the connections of either pair of windings at one point and impress a voltage across the break just sufficient to circulate rated current at rated frequency for the connection and loading used. The total loss applied during this test shall be the same as the sum of the no-load loss and load loss measured according to Clause 8 and Clause 9.

b)

Measure liquid temperatures, and determine liquid temperature rises as described in liquid temperature rise determination.

c)

Immediately shut down, and measure the hot resistance in accordance with hot-resistance measurements.

d)

When needed to meet the time limit criteria of hot-resistance measurements, resume the temperature-rise test for 1 h, holding rated voltage at rated frequency for the connection and loading used. Measure the liquid temperatures, immediately shut down, and measure the hot resistance of additional terminal pairs in accordance with hot-resistance measurements.

e)

Determine average winding temperature rises in accordance with average winding temperature-rise determination.

Figure 29 —Example of loading back method: single-phase It is preferred to use duplicate transformers, but it is not mandatory as long as the transformer under test is energized at the required voltage and current levels. This method provides the closest simulation to actual loading as both no-load and load losses are present in the transformer. The excitation and loading power sources do not necessarily need to be in phase; the power sources may be in phase or 120 degrees out of phase.

59




NOTE—In the connection scheme shown in Figure 29, even if transformers T1 and T2 are duplicate, their respective loading conditions are not identical. T1 will see an apparent inductive load at nearly zero power factor whereas T2 will see an apparent capacitive load at nearly zero power factor. As a consequence, the voltage appearing on the secondary side of the transformers will be different. Voltage on T1 will drop due to its impedance while voltage on T2 will increase. Therefore, parts of the transformer cores will be either over-excited (T2) or under-excited (T1) and the noload losses and noise will vary accordingly. Which parts of the core would experience this depends on the construction of the transformers. This test method is not an exact representation of an actual loading of a transformer at a power factor near unity, but is nevertheless a good approximation.

Figure 30 —Example of loading back method: three-phase For transformers with three windings, the same principles can be applied to load either simultaneously all three windings if the test facility allows it, or separately by pairs of windings. All loads being at nearly zero power factor. 11.1.2.2 Short-circuit method Conduct the short-circuit method as follows: a)

Prior to making the total loss run (see step b), measure load loss at rated current and frequency for the combination of connections and taps that give the highest average winding temperature rise. This process will generally involve those connections and taps resulting in the highest losses. This 60




thermal connection load loss shall be measured in accordance with Clause 9 and referenced to a temperature equal to rated average winding rise plus 20 °C. The required total losses for the total loss run shall be the sum of thermal connection load loss plus no-load loss measured in accordance with Clause 8. b)

For the total loss run, short-circuit one or more windings and circulate sufficient current at rated frequency to produce the required total losses as determined in step a and in 11.4.2.

c)

Determine liquid temperature rises as described in 11.3.2.

d)

For the rated current run, reduce the current in the windings to the rated current (or reduced current according to 11.4.1) value for the connection and the loading used. Hold the current constant for 1 h. Measure the liquid temperatures, immediately shut down, and measure the hot resistances in accordance with 11.2.2.

e)

Repeat the rated current run (step d) for hot-resistance measurements on additional terminal pairs if needed to meet the time limit criteria of 11.2.2.

f)

Determine average winding rises in accordance with 11.3.3.

11.2 Resistance measurements 11.2.1 Cold-resistance measurements Cold-resistance measurements shall be taken on all terminal pairs in accordance with Clause 5. The same test equipment shall be used for both cold-resistance and hot-resistance measurements. Normally, coldresistance measurements are taken prior to loading the transformer for temperature-rise test. However, it is permissible to allow the transformer to cool to ambient temperature and perform cold-resistance measurements after the loading test. Whenever it is necessary to make cold-resistance measurements following the temperature-rise test, the cool-down time shall be sufficient to allow the criteria in 5.1 to be met. 11.2.2 Hot-resistance measurements When the transformer is shut down for hot-resistance measurements, fans and cooling water shall be shut off. Insulating liquid pumps may be shut off or left running during shutdown. Hot-resistance measurements shall be taken as soon as possible after shutdown, allowing sufficient time for the inductive effects to disappear as indicated from the cold-resistance measurement. To minimize inductive effects when transferring measuring instrument leads from one terminal pair to another, the same relative polarity should be maintained between measuring leads and transformer terminals. a)

The time from instant of shutdown, i.e., time zero, shall be recorded for each resistance measurement.

b) For distribution transformers, a hot resistance measurement shall be made on each terminal pair (corresponding to a phase of a winding), and a series of hot resistance measurements shall be made on at least one terminal pair under test. For power transformers, a series of hot resistance measurements shall be made on each terminal pair (corresponding to a phase of a winding) under test, and all terminal pairs shall be tested at maximum nameplate MVA rating. A series of hot resistance measurements may be made on only one terminal pair under test at lower nameplate ratings. The terminal pair used for those tests should be the one found by prior testing to have the highest winding temperature rise.

61




c) The first hot resistance measurement shall be taken as quickly as possible after shutdown and after the inductive effects disappear and in any case, no later than 4 min after shutdown. In order to achieve core saturation in a short time period, the dc power supply voltage should be large enough to saturate the cores in a short time. The dc source should be selected such that it can also provide sufficient current to keep the core saturated without causing excessive heating of windings. A series resistor added to the circuit will decrease time to stabilization. When a transformer has a winding connected in delta, all involved core legs shall be saturated, unless the delta is open during shutdown, for stable and accurate readings. When performing winding resistance measurements on a transformer that utilizes series transformers for load tap changing, care is required to increase the likelihood that the whole circuit, consisting of the main and series transformer windings, is saturated before stable readings of the combined winding resistances can be obtained. d) The hot resistance versus time data shall be used as the basis for correction to time zero. The cooling curve format shall consist of a series of hot resistance measurements in accordance with c). For distribution transformers, at least ten resistance measurements shall be made on one terminal pair corresponding to a phase of a winding. All resistance measurements shall be recorded at no longer than 30-s intervals and no less than 10-s intervals. For power transformers, the last hot resistance measurement shall be taken no sooner than 10 min from shutdown. All hot resistance measurements taken within the 10-min period after shutdown shall be recorded at no longer than 15-s intervals. e)

The hot resistance-time data collected in d) shall be extrapolated to the instant of shutdown using a computerized curve fitting program to determine the hot resistance-time cooling curve. The hot resistance-time data collected shall be fitted to an exponential decay curve using the method of least squares. NOTE 1— Comparative to large power transformers, the time constant of small distribution transformers may be short due to transformer thermal characteristics and measuring equipment, which may affect the appropriateness of an exponential decay curve. NOTE 2— The mean liquid temperature surrounding the winding and the actual location of the winding may result in top and bottom liquid temperatures not giving the intended average winding liquid temperature, which could affect the appropriateness of the exponential decay curve. NOTE 3— The Working Group is initiating an investigation to define appropriate curves concerning NOTE 1 and NOTE 2 for a future modification to this clause

f) When the series of hot resistance measurements on a distribution transformer is made on only one terminal pair under test, the resistance-time data obtained on one phase of a winding shall be used to determine the correction to shutdown for the other phases of the same winding, provided the first measurement of each of the other phases has been taken within 4 mins after shutdown.

62




11.3 Temperature measurements 11.3.1 Ambient temperature measurements 11.3.1.1 Air-cooled transformers For air-cooled transformers, the ambient temperature shall be taken as that of the surrounding air, which should not be less than 10 °C nor more than 40 °C. For temperatures within this range, no correction factor shall be applied. Tests may be made at temperatures outside this range when suitable correction factors are available. The temperature of the surrounding air shall be determined by at least three thermocouples or thermometers in containers spaced uniformly around the transformer under test. They shall be located about mid-height of the transformer and 1 m to 2 m (3 ft to 6 ft) from the transformer. They shall be protected from drafts and radiant heat from the transformer under test or other sources. When the liquid time constant of the transformer (calculated at rated kVA according to IEEE Std C57.91) is 2 h or less, the time constant of the containers shall be between 50% and 150% of that of the transformer under test. When the liquid time constant of the transformer under test is more than 2 h, the time constant of the containers shall be within 1 h of the liquid time constant of the transformer under test. The time constant of a container shall be taken as the time necessary for its temperature to change 6.3 °C when the ambient temperature is abruptly changed 10 °C. 11.3.1.2 Water-cooled transformers For water-cooled transformers, the flow rate in liters per minute and the temperature of the incoming and outgoing water shall be measured. The ambient temperature shall be taken as that of the incoming water and should not be less than 20 °C nor more than 30 °C. For temperatures within this range, no correction factor shall be applied. Tests may be made at temperatures outside this range when suitable correction factors are available.

11.3.2 Liquid temperature rise determination Liquid temperature rise is the difference between liquid temperature and ambient temperature. The ultimate liquid temperature rise above ambient shall be considered to be reached when the top liquid temperature rise does not vary more than 2.5% or 1 °C, whichever is greater, during a consecutive 3-h period. It is permissible to shorten the time required for the test by the use of initial overloads, restricted cooling, and so on. The top liquid temperature shall be measured by a thermocouple or suitable thermometer immersed approximately 50 mm (2 in) below the top liquid surface. The bottom liquid temperature shall be measured by one of the following methods: a)

Thermocouples may be attached to an insulated rod and located inside the tank so that the thermocouples are in the liquid flow path from the external cooling means to the bottom of the windings.

63




CAUTION Exercise caution when employing this method. This method may be hazardous for transformers with very high voltage windings. b)

If heat exchangers or radiators are mounted on a common manifold with a single entrance to the tank, the thermocouples may be located in the piping of the single entrance.

c)

If heat exchangers or radiators have multiple entrances into the tank, thermocouples may be installed in the bottom of one radiator or heat exchanger. For accuracy, a radiator or heat exchanger located in the middle of the bank is preferred.

d)

If it is not possible to measure the temperature of the liquid inside the tank, radiators, or heat exchangers, surface temperature measurements may be used with the results corrected to account for the temperature difference between the surface and the liquid inside the tank. If surface temperature measurements are made on radiator headers, choose headers one third or one half the way in from either end of a bank of radiators. For transformers without radiators, locate the thermocouples on the tank wall at the elevation of the bottom of the winding. Thermocouples located on external cooling surfaces, for the purpose of determining internal liquid temperatures, shall be shielded and insulated so that their readings are not significantly affected by the air movement from fans or thermally induced air currents.

The average liquid temperature shall be determined as equal to top liquid temperature minus half the difference in temperature of the moving liquid at the top and bottom of the cooling means. When bottom liquid temperature cannot be measured directly, the temperature difference may be taken as the difference between the surface temperature of the liquid inlet and the outlet. A thermocouple is the preferred method of measuring surface temperature (see 11.3.4 for method of measurement). Infrared measurement devices may also be used to measure surface temperatures. 11.3.3 Average winding temperature-rise determination The average winding temperature of a terminal pair corresponding to a winding phase shall be determined from the terminal pair’s hot resistance at shutdown. When it is not possible to determine the hot resistance (e.g., with extremely low resistance windings), other methods may be used. The average winding temperature of a terminal pair shall be determined by Equation (28): θ w=

Rh θ + θ rc − θ k Rc k

(

)

(28)

The average temperature rise of a terminal pair corresponding to a winding phase shall be determined by Equation (29): (29)

∆θ w = ∆θ l + θ w − θ l

where

∆θ w ∆θl= θl ,TL −θ a

θw

θl ,TL θl

is the average winding temperature rise of a terminal pair (°C) is the average liquid temperature rise as determined from the total loss run (°C) is the average winding temperature of a terminal pair corresponding to hot resistance Rh (°C) is the average liquid temperature at end of total loss run (°C) is the average liquid temperature at shutdown (°C) 64




θa θ rc

is the ambient temperature (°C) is the temperature at which cold resistance Rc was measured (°C)

Rc Rh

is the cold resistance, measured according to Clause 5 (Ω)

θk

is 234.5 °C for copper and 225.0 °C for aluminum

is the hot resistance of a terminal pair (Ω)

NOTE—The value for θ k may be as high as 230 °C for alloyed aluminum.

Average winding rise shall be calculated by using either top liquid rise or average liquid rise. When otherthan-rated winding current is used, the average liquid rise method shall be used to determine winding rises. a)

In the top liquid rise method, the average winding temperature rise is equal to the top liquid rise, measured during the total loss run, plus the quantity (average winding temperature at shutdown minus top liquid temperature at shutdown).

b)

In the average liquid rise method, the average winding temperature rise is the average liquid rise, measured during the total loss run, plus the quantity (average winding temperature at shutdown minus average liquid temperature at shutdown).

The average winding temperature rise for each terminal pair corresponding to a winding phase shall be corrected for actual test currents, test losses, and altitude as prescribed in 11.4. The corrected average winding temperature rise shall be reported for each terminal pair of the transformer. 11.3.4 Other temperature measurements When measured, the temperature rise of metal parts other than windings shall be determined by use of a thermocouple, suitable thermometer, fiber optic temperature sensor, or other appropriate temperature measurement techniques. A thermocouple is the preferred method of measuring surface temperature. When used for this purpose, the thermocouple should be soldered to the surface. When this option is not practical, the thermocouple should be soldered to a thin metal plate or foil approximately 645 mm2 (1 in2) in size. The plate should be held firmly and snugly against the surface. The thermocouple should be thoroughly insulated thermally from the surrounding medium. The surface temperature of metal parts surrounding, or adjacent to, outlet leads or terminals carrying heavy current may be measured at intervals or immediately after shutdown.

11.4 Correction of temperature-rise test results For any of the loading methods adopted, temperature-rise test results shall be corrected for the predictable effects caused by: a)

Difference in winding rated current and the winding test current

b)

Difference in required loss and test loss

c)

Difference in altitude of operation

65




11.4.1 Correction for differences between winding rated current and test current When test equipment limitations dictate, it is permissible to hold winding current at a value lower than rated current for the winding, but not less than 85% of rated winding current. When the current held in any of the windings under test differs from the rated current, the observed differences between the average winding temperature at shutdown and the average liquid temperature at shutdown shall be corrected to give the average temperature rise of the windings at the rated current by using Equation (30):

∆θ w, c = ∆θ w, o

 rated current     test current 

2m

(30)

where

∆θ w, c is the corrected difference between average winding temperature corrected to shutdown, and the average liquid temperature at shutdown (°C) ∆θ w, o is the observed difference between average winding temperature corrected to shut own, and the average liquid temperature at shutdown (°C) m

is 0.8 for Classes ONAN, ONAF, OFAF, and OFWF is 1.0 for Classes ODAF and ODWF

The corrected average winding rise is the average liquid rise plus ∆θ w, c . 11.4.2 Correction of liquid temperature rise for differences in required total loss and actual loss This method may be used when actual loss is within 20% of the required total loss, as in Equation (31):

 P  n  ∆θl , c = ∆θl , o  r  − 1  PT  

(31)

where

∆θl , c is the liquid temperature rise correction (°C) ∆θl , o is the observed liquid temperature rise (°C)

Pr PT n

is the required total loss (W) is the actual test total loss (W) is 0.8 for Class ONAN is 0.9 for Classes ONAF, OFAF, and OFWF is 1.0 for Classes ODAF and ODWF

Corrected liquid temperature rise = observed liquid temperature rise + ∆θl , c . Corrected average winding temperature rise = observed winding temperature rise + ∆θl , c .

66




11.4.3 Correction of liquid temperature rises for differences in altitude When tests are made at an altitude of 1000 m (3280 ft) or less, no altitude correction shall be applied to the temperature rises. When a transformer tested at an altitude less than 1000 m (3280 ft) is to be operated at an altitude above 1000 m (3280 ft), it shall be assumed that the liquid temperature rise will increase in accordance with Equation (32):

 A  ∆θ A = ∆θ o  − 1 F A  o 

(32)

where

∆θ A ∆θ o A A0 F

is the increase in liquid temperature rise (°C) at altitude A meters (ft) is the observed liquid temperature rise (°C) is altitude meters (ft) is 1000 m (3300 ft) is 0.04 for self-cooled mode or 0.06 for forced-air-cooled mode

NOTE—Winding temperature rise above liquid temperature is not affected by altitude.

11.4.4 Frequency conversion of temperature-rise test results In the event that the temperature-rise test cannot be done at the rated power frequency, see Annex B for 50/60 Hz frequency conversions.

12. Short-circuit tests 12.1 General This test code applies to liquid-immersed distribution and power transformers 5 kVA and above. Within this range, four categories shall be recognized as listed in Table 5. Table 5 —Transformer categories covered by this test code Category

Single-phase (kVA)

Three-phase (kVA)

a

5 to 500

15 to 500

II

501 to 1667

501 to 5000

III

1668 to 10 000

5001 to 30 000

IV

Above 10 000

Above 30 000

I

NOTE—All kilovoltampere ratings are minimum nameplate kilovoltampere for the principal windings. a Category I shall include distribution transformers manufactured in accordance with IEEE Std C57.12.20 up through 500 kVA, single-phase or three-phase. In addition, autotransformers of 500 kVA-equivalent two-winding kilovoltampere or less that are manufactured as distribution transformers in accordance with IEEE Std C57.12.20 shall be included in Category I even though their nameplate kilovoltampere may exceed 500 kVA.

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The code defines a procedure to demonstrate the mechanical capability of a transformer to withstand shortcircuit stresses. The prescribed tests are not designed to verify thermal performance. Conformance to short-circuit thermal requirements shall be by calculation in accordance with Clause 7 of IEEE Std C57.12.00-2015. The short-circuit test procedure described in this standard is intended principally for application to new transformers to verify design. Tests may be conducted at the manufacturer’s facilities, at test laboratories or in the field, but it shall be recognized that complete equipment is not usually available in the field for conducting tests and verifying results. NOTE—Some transformers may not be fully tested because of limitations of testing facility (test laboratory, manufacturer’s facility, or field test capability). Thermal ability to withstand short-circuit can be demonstrated by calculations per IEEE Std C57.12.00 (or IEC 60076-5 [B16]), upon agreement between manufacturer and purchaser. The ability to withstand the dynamic effects of short-circuit should preferably be demonstrated by test; if not possible it may be demonstrated by calculation and design considerations per IEC 60076-5, upon agreement between manufacturer and purchaser.

12.2 Test connections 12.2.1 Two-winding transformers and autotransformers without tertiary windings 12.2.1.1 Fault location The short circuit may be applied on the transformer primary or secondary terminals as dictated by the available voltage source, but the secondary fault is preferred because it is the fault condition producing the highest fault current in each winding. The short circuit shall be applied by means of suitable low-resistance connectors. In order of preference, the secondary fault tests may be conducted by either of the following:

Source

Secondary

Closing a breaker at the source terminal to apply energy to the previously short-circuited transformer, pre-set method, Figure 31

Primary

a)

` Figure 31 —One line diagram for the “pre-set” method b)

Closing a breaker at the faulted terminal to apply short circuit to the previously energized transformer, post-set method, Figure 32

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Source

Secondary

Primary


Figure 32 —One line diagram for the “post-set” method NOTE 1—When using pre-set method, magnetizing inrushes may occur, particularly on autotransformers or axial split coils transformers. NOTE 2—When using the pre-set method for a transformer with single concentric windings, the supply should preferably be connected to the winding farther from the core, the winding closer to the core being short-circuited. This will help to avoid saturation of the core, which could lead to an excessive magnetizing current superimposed on the short-circuit current during the first few cycles. NOTE 3—For type tests on distribution transformers or when there is no identified purchaser involved, the preferred location (secondary fault) and method (pre-set) should be used. NOTE 4—When using the post-set method, excessive excitation voltages can occur prior to fault initiation unless a large enough power source with low enough source impedance is employed.

See Bergonzi, et al. [B8], Bertagnolli [B9], Foldi, et al. [B12], Jansen and te Paske [B24], and Leher [B26]. 12.2.1.2 Fault type For single-phase transformers a single-phase supply shall be used. For three-phase transformers a threephase supply is preferable, as long as the fault current requirements defined in IEEE Std C57.12.00 can be met. If this is not possible, an alternate single-phase source can be used: a)

For delta-connected windings, the single-phase supply is provided between two corners of the delta and the voltage during the test shall be the same as the voltage between phases during a three-phase test.

b)

For wye-connected windings, the single-phase voltage is supplied between one line terminal and the other two line terminals connected together. The single-phase voltage during the test shall be equal to √3 /2 times the voltage between phases during the three-phase test.

c)

Single-phase source with short-circuit on one phase at a time.

Annex C gives typical connection diagrams for testing a three-phase transformer using a single-phase source. 12.2.1.3 Tap connection for test When the transformer is provided with taps in any winding, at least one test satisfying the asymmetrical current requirement shall be made on the tap connection that calculations predict will produce the most severe mechanical stresses. Extremes of the tap range (all taps out or all taps in) normally produce the most severe stresses, and they are recommended. The tap connection that produces the most severe stress is determined by calculation and should be agreed upon between the manufacturer and the purchaser. Often the maximum forces occur with the lowest voltage tap connection. Tests on other taps, or connections in the case of dual voltage windings, may be required to verify design adequacy.

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12.2.2 Multiwinding transformers, including autotransformers 12.2.2.1 Fault location and type Fault types and terminals to which they are to be applied shall be determined individually for each particular transformer. The maximum fault current for each winding shall be determined from calculations of fault types specified in Clause 7 of IEEE Std C57.12.00-2015 using various fault types, fault locations, and applicable system data. During testing, each winding shall be subjected to its maximum calculated fault current on at least one test. In general, a given fault type and location will not produce the maximum fault current in more than one winding; therefore, it will be necessary to make tests with several different connections to fully evaluate the capability of all windings. In order of preference, the tests may be conducted by either of the following methods: a)

Closing a breaker at the source terminal to apply energy to the previously short-circuited transformer, pre-set method.

b)

Closing a breaker at the faulted terminal to apply short circuit to the previously energized transformer, post-set method.

NOTE 1—When using pre-set method, magnetizing inrushes may occur, particularly on autotransformers or axial split coils transformers. NOTE 2—When the primary winding is connected to the supply, either one or both of the secondary windings, or either one or both of the common or tertiary windings for auto-transformers, may be short circuited for the test. NOTE 3—For auto-transformers with tertiary windings, it may also be necessary to consider other fault conditions, such as single-phase or double-phase-to-ground faults with either common or series, or both, as the source(s) of supply. NOTE 4—It is recommended that the tolerance on the agreed test current values and the duration of the tests are in line with those prescribed for two-winding transformers and that the test sequence is selected according to the expected increase of electrodynamics forces.

See Bergonzi, et al. [B8], Bertagnolli [B9], Foldi, et al. [B12], Jansen and te Paske [B24], and Leher [B26]. 12.2.3 Axial split transformers Axial-split transformers comprise transformers that have three or more windings with two or more windings physically assembled axially on the same magnetic leg. Each of the axial-split windings can be connected to different sources and/or loads by different sets of bushings. Some transformers use axial-split winding arrangement for which the windings are only connected internally to other windings. Such transformers are not considered as axial-split transformers and shall be short-circuit tested with the usual short-circuit test method. 12.2.3.1 Fault location and type For axial-split transformers, the fault shall be applied to only one of the axial-split windings. The other axial-split winding(s) shall remain open circuited with possibly one of the terminals grounded for voltage reference. The short-circuit test sequence (six tests, see 12.3) shall be repeated for each axial split winding. Axial-split transformers shall preferably be tested by closing a breaker at the faulted terminal to apply short-circuit to the previously energized transformer, post-set method. In case of power limitations from the Laboratory, by agreement between the manufacturer and the purchaser, the test may be done by closing a breaker at the source terminal to apply energy to the previously short-circuited transformer, pre-set method. See Figure 31. Using the pre-set method, closing a breaker at the source terminal to apply energy to the previously shortcircuited transformer, necessitate special precautions in order to control magnetizing inrushes and not cause

70




undue high instantaneous current (high inrush superposed to the peak ac current) and related undue electromagnetic forces on the windings. The post-set method does not demonstrate that the transformer would be able to withstand energizing the transformer on a short-circuit, i.e., forgotten grounds. If such a performance is required per the client requirements, the transformer should be tested using the pre-set method with the pre-set short circuit applied on one of the axial split windings. See Figure 32. See Bergonzi, et al. [B8], Bertagnolli [B9], CIGRE WG A2.26 report [B11], Foldi, et al. [B12], Jansen and te Paske [B24], and Leher [B26].

12.3 Test requirements 12.3.1 Symmetrical current requirement, two-winding transformers For two-winding transformers, the required value of symmetrical current for any test shall be determined from the equations in Clause 7 of IEEE Std C57.12.00-2015. For Categories I and II, calculate ISC using transformer impedance only; except for Category I, the symmetrical current magnitude shall not exceed the values listed in 7.1.4.1 and Table 12 of IEEE Std C57.12.00-2015. For Categories III and IV, calculate ISC using transformer plus system impedance. See Clause 7 of IEEE Std C57.12.00-2015 for additional clarifying information on determining ZS. 12.3.2 Symmetrical current requirement, multiwinding transformers, and autotransformers For multiwinding transformers and autotransformers, the required peak value of symmetrical current in each winding shall be determined by calculation based on applicable system conditions and fault types. NOTE—The transformer category is determined based on the primary (common) winding ratings.

12.3.3 Asymmetrical current requirement The required first cycle peak for asymmetrical current tests shall be calculated in accordance with the equations in Clause 7 of IEEE Std C57.12.00-2015. 12.3.4 Number of tests Each phase of the transformer shall be subjected to a total of six tests satisfying the symmetrical current requirement specified in 12.3.1 or 12.3.2, as applicable. Two of these tests on each phase shall also satisfy the asymmetrical current requirements specified in 12.3.3. 12.3.5 Duration of tests The duration of short-circuit tests shall be in accordance with Clause 7 of IEEE Std C57.12.00-2015.

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12.4 Test procedure 12.4.1 Fault application To produce the fully asymmetrical current wave specified in 12.3.3 in the phase winding under test, the moment of switching on shall be adjusted by means of a synchronous switch or another controlled switching device. 12.4.2 Calibration tests Calibration tests shall be carried out at less than 70% of specified current to check the proper functioning of the test set-up with regard to the moment of switching on, the current setting, the damping and the duration. Tests that result in current of 95% or more of the specified current may be counted toward fulfillment of the required number of tests. 12.4.3 Terminal voltage limits When tests are to be made by applying a short circuit to energized transformers, the no-load source voltage shall not exceed 110% of the rated tap voltage, unless otherwise approved by the manufacturer. Throughout the course of any test, the voltage at the transformer source terminals shall be maintained within a range of 95% to 105% of that necessary to produce the required symmetrical short-circuit current as determined in 12.3.1 or 12.3.2, as applicable. When choosing the test voltage, the possibility of over excitation and high inrush current shall be considered. All reasonable precautions shall be taken in order to reduce these. Autotransformers and split-coil transformers require special precautions and it may be required to use the post-set method instead of the preferred pre-set method. 12.4.4 Temperature limits For liquid-immersed transformers, top liquid temperature at the start of the test shall be between 0 ºC and 40 ºC. NOTE—For some small transformers, particularly those having a low X/R, special attention should be given to not exceed the winding temperature limits. Additional delay between tests may be required in order to let the winding cool.

12.4.5 Current measurements A recording shall be made of the currents on the transformer terminals connected to the energy source. Oscillograms of currents shall be representative of stresses in the winding. When the transformer winding connected to the energy source is delta-connected, the first cycle peak asymmetrical current cannot be determined directly from terminal measurements at the source terminals. The following alternatives exist: a)

When the faulted winding is wye-connected, record oscillograms of the currents on the faulted terminals. Convert to source winding current by inverse turns ratio.

b)

When all windings are delta-connected, connect current measuring devices inside the delta of the source winding, and measure first cycle peak asymmetrical current from oscillograms obtained from these current. measuring devices. 72




c)

When all windings are delta-connected, determine peak and symmetrical currents from oscillogram obtained by proper connection of current measuring devices or by digital calculations.

Whenever possible record the tank current by connecting the transformer tank to ground through a current measuring device. Any appropriate measuring devices may be used, as long that they give correct measurements with appropriate sensitivity, precision, and uncertainty. Those measuring devices may be, for example, current transformers, shunts, or Rogowski coils. The oscillograms shall show a scaled image of the current passing through the primary of the measuring device or a scaled image of the current passing in the corresponding winding. For example, the Rogowski coil voltage output shall be properly treated by digital or analog means in order to show an image of the real primary current instead of the primary current derivative. 12.4.6 Tolerances on required current After the measured impedance is taken into account, the measured current (symmetrical or asymmetrical) in the tested phase(s) shall not be less than 95% of the required current. NOTE—For transformers having a low X/R and for those having allowable impedance variation over 5% special attention should be made to readjust as required the source settings (voltage and/or source impedance) in order to maintain the test current above 95% of the required current.

12.4.7 Frequency conversion of short-circuit test In the event that the short-circuit test cannot be done at the rated power frequency, see Annex B for 50/60 Hz frequency conversions.

12.5 Proof of satisfactory performance The transformer under test shall be judged to have performed satisfactorily when the visual inspection (12.5.1) and dielectric test (12.5.2) criteria have been satisfactorily met. In 12.5.3 through 12.5.6, recommended measurements listed can be made during the course of the tests, but they are not required unless specified. When the measurements are made and the requirements of 12.5.3 through 12.5.6 have been met following all tests, it is probable that the transformer has sustained no mechanical damage during the test. A composite evaluation of the degree to which all criteria of 12.5.3 through 12.5.6 have been met may indicate the need for a greater or lesser degree of visual inspection to confirm satisfactory performance. A decision to waive all or part(s) on the extent of the visual inspection shall be based on discussion and negotiation of all parties involved in specification and performance of short-circuit tests. NOTE 1—Note that the proof of satisfactory performances is determined by visual inspection and the standard dielectric tests defined in IEEE Std C57.12.00. It may be advisable to proceed to a visual inspection before undertaking standard dielectric test. NOTE 2—If acetylene is detected by dissolved gas analysis, extensive visual inspection is required in order to identify and correct the defect before proceeding to other tests. NOTE 3—Other recommended measurements listed are mainly to identify if something may go wrong, or something may evolve and cause failure of the transformer under test.

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12.5.1 Visual inspection Visual inspection of the core and coils shall give no indication that any change in mechanical condition has occurred that will impair the function of the transformer. The extent of the visual inspection shall be established on the basis of combined evidence obtained from the measurements described in 12.5.3 through 12.5.6. The extent of necessary visual inspection may range from an inspection through openings on the tank (hand hole, manhole) to complete dismantling of the core and coils. The appropriate level of visual inspection shall be based upon discussion and negotiation of all parties involved. The measurements and observations made during the tests and the result of the standard dielectric test should then be considered during the negotiation. 12.5.2 Dielectric tests The transformer shall withstand the standard dielectric tests of IEEE Std C57.12.00 at the full specification level following the short-circuit test series. 12.5.3 Wave shape of terminal voltage and current No abrupt changes shall occur in the terminal voltage or short-circuit current wave shapes during any test. NOTE—When substantial changes occur in the terminal voltage or current, it is then advisable to proceed to a dissolved gas analysis (DGA) before performing other tests. If dissolved gases generally associated with power arc are present it is advisable to investigate the situation in order not to have a catastrophic failure.

12.5.4 Leakage impedance Leakage impedance measured on a per-phase basis after the test series shall not differ from that measured before the test series by more than the following values:  Category I and Distribution Transformers: The allowable variation shall be a function of the transformer impedance ZT as in Table 6. Table 6 —Leakage impedance variation for Category I and distribution transformers ZT (per unit) 0.0299 or less

Percentage variation (ZT) 22.5 – (500) × (ZT)

0.0300 or more

7.5

 Categories II and III: 7.5% allowable for noncircular concentric coils; 2% allowable variation for circular coils.  Category IV: 1% allowable variation. The measuring equipment shall have the demonstrated capability of giving reproducible readings within an accuracy of ± 0.2%. NOTE 1—The 4-wire measuring method is generally required to attain these reproducible readings. NOTE 2—It may be worthwhile to measure impedance after each short-circuit. These measurements will allow identification of progressive variations, or early substantial impedance change.

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12.5.5 Dissolved gas analysis (DGA) Dissolved gas analysis made after tests shall not show signs that a power arc has occurred in the transformer. See ASTM D923 [B4], ASTM D3612 [B6], ASTM D3613 [B7], and IEEE C57.104 [B20]. 12.5.6 Excitation current Excitation current measured after the test series shall not increase above that measured before the test series by more than 5% for stacked-type cores. For transformers with wound core construction, the increase shall not exceed 25%. 12.5.7 Other diagnostic measurements Other diagnostic measurements may be made during the course of the tests to evaluate whether any sudden or progressive changes have occurred in the mechanical condition of the transformer. Such results may be useful to understand the response to short-circuit forces, but they shall not form part of the proof criteria. Frequency response analysis (FRA) and/or frequency response of stray losses (FRSL) are measurements that may serve to qualify the mechanical condition of winding or part of it (i.e., tap winding). See Boulduc, et al. [B10] and IEEE C57.149 [B22]. Transient oil pressure measurement (TOP) may also help to detect some movements or the loss of clamping pressure of the coils or the coil assembly. See Kraetge, et al. [B25].

13. Audible sound emissions 13.1 General 13.1.1 Introduction Audible sound from transformers is composed of the following three components: a)

Core audible sound: This sound component originates in the transformer core and transmits through the dielectric fluid and structural supports to the tank, where it radiates as airborne sound. The frequency spectrum of core sound consists primarily of the even harmonics of the power frequency; thus, for a 60 Hz power system, the main frequency components are 120, 240, 360, and 480 Hz.

b)

Load audible sound: This sound component is primarily produced by vibrations of the windings and tank walls when the transformer is loaded. The frequency of this sound component is primarily twice the power frequency i.e., 120 Hz for a 60 Hz transformer. When a transformer is highly loaded, load sound can be a significant contributor to the total sound of the transformer, especially for low no-load noise medium and large power transformers. The magnitude of load sound is highly dependent on the transformer load. For example, load sound level at 60% of full load is about 9 dB lower than that at full load.

c)

Cooling system audible sound: The frequency spectrum of this sound component typically consists of broadband fan noise, plus discrete tones (of low levels) at the fan blade passage frequency and its harmonics.

The sum of core and cooling system sound components is typically referred to as the no-load noise of a transformer. The total audible sound of the transformer, however, is the sum of all three components of the transformer audible sound. 75




Described in this clause are methods for: a)

Measuring transformer sound pressure levels in terms of A-weighted, one-third octave, or discrete frequency bands, using both the sound pressure and sound-intensity measuring methods. Included are methods of correcting the measurements for environmental factors, namely ambient noise and wall sound reflections.

b)

Determining the audible sound level of the transformer using the measured sound pressure data. Included are methods for:

c)

1)

Calculating the average sound pressure level

2)

Adding no-load and load audible sound level

3)

Calculating the total sound power level

Reporting the results in a standard manner.

13.1.2 Applicability The procedures specified for measuring transformer sound pressure levels or for calculating transformer sound power levels are intended to be applicable to transformers being tested in an indoor or outdoor laboratory, to transformers being tested in a factory, or to transformers that have been installed in substations (either single or in combination, with other equipment).

13.2 Instrumentation 13.2.1 Sound level meter Sound pressure level measurements shall be made with instrumentation meeting the requirements of ANSI S1.4 for Type 1 meters. 13.2.2 One-third octave filter One-third octave band frequency measurements shall be made when specified with instrumentation meeting the requirements of ANSI S1.4 for Type 1 meters together with ANSI S1.11 for Type E, Class II performance, or their equal. 13.2.3 Narrowband filter Discrete frequency measurements may be made when specified or when ambient noise test conditions required by the specification cannot be attained. Suitable analyzer bandwidth characteristics are one-tenth octave; a bandwidth of 10% of the selected frequency; or 3 Hz, 10 Hz, or 50 Hz bandwidths. 13.2.4 Wind screen A suitable wind screen shall be used when measuring sound.

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13.2.5 Calibration Sound-measuring instrumentation shall be calibrated as recommended by the sound level meter manufacturer before and after each set of measurements. When calibration change is greater than 1 dB, sound measurements shall be declared invalid, and the test shall be repeated.

13.3 Test conditions 13.3.1 Test environment The test environment, represented by “ambient noise” and “wall sound reflections,” can significantly impact the accuracy of the measurement of transformer audible sound. Therefore, environmental corrections explained in 13.5.5.2 and 13.5.5.3 shall be made when applicable. 13.3.2 Transformer location The transformer shall be located so that no acoustically reflecting surface is within 3 m (10 ft) of the measuring microphone, other than the floor or ground. When a transformer is to be tested within a semireverberant facility, it shall be located in an asymmetrical manner with respect to the room geometry. If the specified conditions cannot be met, the transformer shall be no closer than 3 m (10 ft) from a sound reflecting surface. When transformer sound emissions are measured in an enclosed space, sound reflections from walls or other large objects can influence the results because the sound contains discrete tones that are affected by room acoustics, room geometry, or reflecting objects. Thus, larger differences may exist between the sound measured in an indoor transformer installation and the sound measured in an acoustical laboratory or an outdoor installation, if the proper correction for “wall sound reflections,” as described in 13.5.5.3, is not made. 13.3.3 Determination of total sound level of a transformer 13.3.3.1 No-load audible sound level When measuring no-load audible sound level, the transformer shall be connected and energized at rated voltage and rated frequency and shall be at no load with the tap changer, if any, on the rated tap position, unless otherwise specified. NOTE—If 60 Hz power frequency is not available at the test facility, and if agreed to by manufacturer and purchaser, then the test can be performed at 50 Hz and the results converted according to B.5.

When a transformer is equipped with a tap changer, the transformer may, on certain tap changer positions, produce sound levels that are higher than the audible sound level at the rated tap position. For these transformers, the measurements may, upon purchaser request, and as agreed upon, be made with the transformer on the highest sound producing tap position. Also, other excitation conditions may occur in service leading to lower, or higher, no-load audible sound levels. For example, transformers designed to operate with variable flux; the core audible sound level is strongly impacted by the tapping position. Again, for these transformers, the measurements may, upon purchaser request, and as agreed upon, be made with the transformer operating at the highest soundproducing condition. In all cases, the specified test condition(s) shall be clearly indicated in the test report. 77




The rated voltage shall be measured line to line for delta-connected windings and line to neutral for wyeconnected windings. The voltage shall be measured with a voltmeter responsive to the average value of the voltage but scaled to read the rms value of a sinusoidal wave having the same average value. The voltmeter shall be connected between the terminals of the energized windings. Pumps and fans shall be operated as appropriate for the rating being tested. When cooling equipment is not connected to the transformer, it shall be so noted on the data sheet. When suitable cooling equipment sound emission data are available, the cooling equipment sound data may be appropriately added to the measured transformer sound if agreed to by the manufacturer and the purchaser. The addition of cooling equipment sound data to measured transformer sound data shall be clearly noted on the data sheet. Sound measurements shall begin after the transformer being tested is energized and steady-state sound level conditions are established. Measurements may be made immediately on transformers in continuous operation. 13.3.3.2 Load audible sound level Similar to load loss measurements (Clause 9), the load audible sound level at rated current is measured by applying a short circuit across either the high-voltage winding or the low-voltage winding and applying sufficient voltage across the other winding resulting in the rated current to flow in the windings. The load audible sound level shall be measured at rated frequency and at the rated tap position unless otherwise specified by the purchaser. Since load audible sound becomes a contributor to the total audible sound of the transformer at higher loads, the load audible sound level shall be measured at the ONAF measuring contour. NOTE—For simplicity, the term “ONAF” is used in this document to refer to all operating conditions where fans are running, namely ONAF, ODAF, and OFAF.

When test equipment limitations dictate, or when the ambient noise is not sufficiently lower than load audible sound of the transformer at full load, it is permissible to measure load audible sound of the transformer at a different current than rated current. When the test current differs from the rated current, but is within the range of equation validity (60%–130% of rated current), Equation (33) can be used to convert the value of the measured load audible sound level to the corresponding level at rated current.

I  L= LP ,T + 40 log10  R  P,R  IT 

(33)

where

LP , R

is the load audible sound level of the transformer at rated current

LP ,T is the measured load audible sound level of the transformer at test current

IR IT

is the rated current is the test current

13.3.4 Frequency conversion of measured audible sound levels In the event that the audible sound test cannot be done at the rated power frequency, see Annex B for 50/60 Hz frequency conversions.

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13.4 Microphone positions 13.4.1 Reference sound-producing surface The reference sound-producing surface of a transformer is a vertical surface that follows the contour of a taut string stretched around the periphery of the transformer or integral enclosure (see Figure 33). The contour shall include radiators, coolers, tubes, switch compartments, and terminal chambers; but it shall exclude bushings and minor extensions, such as valves, liquid gauges, thermometers, conduit terminal boxes, and projections at or above cover height.

Figure 33 —Microphone location for measuring audible sound from transformers 13.4.2 Safety considerations In consideration of safety and consistency of measurement, the reference sound-producing surface near unenclosed live parts of field-assembled items (such as switches, switchgear, terminal compartments, wallmounted bushings, and SF6 air-to-oil adapter bushings) shall be moved outward from the taut string contour to be consistent with safe working clearances as determined by the manufacturer for the voltage class of the live parts involved.

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13.4.3 First measurement position The first microphone locations shall coincide with the main drain valve. Additional microphone locations shall be at 1 m (3 ft) intervals in a horizontal direction, proceeding clockwise as viewed from above along the measurement surface defined in 13.4.1. 13.4.4 Number of microphone locations No fewer than four microphone locations shall be used. For small transformers, intervals of less than 1 m (3 ft) may result. The microphone shall be located on the measurement surface. When fans are not in operation, the microphone shall be spaced 0.3 m (1 ft) from the reference sound-producing surface (ONAN contour). When fans are in operation, the microphone shall be located 2 m (6 ft) from the reference soundproducing surface (ONAF contour), refer to Figure 33. 13.4.5 Height of microphone locations For transformers having an overall tank or enclosure height of less than 2.4 m (7.9 ft), measurements shall be made at half height. For transformers having an overall tank or enclosure height of 2.4 m (7.9 ft) or more, measurements shall be made at one-third and at two-thirds height.

13.5 Sound pressure level measurements The sound pressure level of a transformer can be measured using either the sound pressure measuring method or the sound-intensity measuring method. When using the sound pressure measuring method, corrections for ambient noise, wall sound reflections, and near-field effect are to be made as explained in 13.5.5. Since the sound-intensity measuring method allows measurement of only the sound radiating from the transformer, these corrections need not be made. Instead, a correction, specified in 13.5.6.1, shall be made when ambient noise and wall sound reflections are such that they affect the accuracy of the measurements. Transformer sound pressure levels shall be measured in conformance with 13.2, 13.3, and 13.4 using Aweighted sound level measurements. Furthermore, one-third octave band sound level measurements or narrowband sound level measurements may be made when specified. When specified, the sound pressure level can be measured using the sound level meter set to the non-weighted linear network or the Cweighted network. Discrete frequency sound emissions may be measured when agreed to by the purchaser and the manufacturer or when ambient sound pressure levels required by this specification (see 13.5.5.1) for either the A-weighted or the one-third octave band sound level measurements cannot be attained. 13.5.1 A-weighted sound pressure level measurements The A-weighted sound pressure level shall be measured with a sound level meter set to the A-weighted network. 13.5.2 One-third octave sound pressure level measurements The one-third octave band sound pressure levels shall be measured at the mid-band frequencies of 63 Hz through 4000 Hz, inclusive.

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13.5.3 Narrowband sound pressure level measurements Discrete frequency sound pressure levels shall be measured at the fundamental power frequency (60 Hz, for example) and at least at each of the next six even harmonics (120 Hz, 240 Hz, 360 Hz, 480 Hz, 600 Hz, and 720 Hz). 13.5.4 C-weighted sound pressure level measurements The C-weighted sound pressure level shall be measured with a sound level meter set to the C-weighted network. 13.5.5 Sound pressure level measurements using the sound-pressure method Measured sound pressure levels shall be corrected for ambient noise, wall sound reflections, and near-field effects. The correction for ambient noise shall be made using the procedure described in 13.5.5.2. The correction for wall sound reflections shall be performed by subtracting the sound reflection correction “K” as calculated by Equation (34) in 13.5.5.3. The correction for the near-field effect shall be made per 13.5.5.4. The reported final test data shall include the measured sound levels along with the corrections used in determining the final value(s). 13.5.5.1 Measuring ambient sound pressure level The ambient sound pressure level shall be established by averaging the ambient sound pressure levels measured immediately preceding and immediately following the sound measurements with the transformer energized. The ambient sound shall be measured at a minimum of four locations, and the instruments shall be in conformance with 13.2. However, additional measurements may be made if agreed to by the manufacturer and purchaser or if the ambient measurements vary by more than 3 dB around the transformer. At least one of the locations for measuring ambient sound pressure levels shall be on the center of each face of the measurement surface. Ambient sound pressure level corrections may be made at each microphone location before the average transformer sound pressure level is computed. Ambient sound pressure levels shall be measured using the same bandwidth (A-weighted, one-third octave, or discrete frequencies) specified for measuring the transformer sound. The ambient sound corrections shall be made with the identical frequency bandwidths that are used to measure the combined transformer and ambient sound pressure level. 13.5.5.2 Correction for ambient sound Measurements shall be made in an environment having an ambient sound pressure level at least 5 dB below the combined transformer and ambient sound pressure level. When the ambient sound pressure level is 5 dB or more, below the combined transformer and ambient sound pressure level, the corrections shown in Table 7 shall be applied to the combined transformer and ambient sound pressure level to obtain the transformer sound pressure level. When the difference between the ambient sound pressure level and the combined transformer and ambient sound pressure level is less than 5 dB, and it is desired only to know the sound pressure level that the transformer does not exceed, a correction of –1.6 dB may be used. Alternatively, the manufacturer may attempt to reduce the ambient noise during the sound test. For onethird octave or narrowband measurements, the 5 dB difference shall apply to the main frequency components of the transformer noise; refer to NOTES 1, 2, and 3 in 13.5.6.1. When ambient sound conditions do not comply, suitable corrections may be feasible when the ambient sound conditions are steady and discrete frequency sound levels are measured.

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Table 7 —Ambient sound corrections Difference between the ambient sound pressure level and the combined transformer and ambient sound pressure level (dB) 5 6 7 8 9 10 Over 10

Correction to be added to the combined transformer and ambient sound pressure level to obtain the ambientcorrected sound pressure level of the transformer (dB) –1.6 –1.3 –1.0 –0.8 –0.6 –0.4 0.0

13.5.5.3 Wall sound reflection correction K This correction accounts for the influence of undesired sound reflections from test room boundaries (walls, ceiling, and floor) and reflecting objects near the test transformer. The magnitude of K depends principally on the ratio of the transformer sound measuring surface S to the sound absorption area of the test room A. The sound reflection correction K shall be calculated using Equation (34), or obtained from Figure 34 using the calculated value of A/S.

 4  = K 10 log10 1 +   (A/ S)

(34)

The value of “A,” in square meters, is given by Equation (35):

A = α SV

(35)

where

α SV

is average acoustic absorption coefficient (from Table 8) is total area of surface of test room (walls, ceilings, and floors), in square meters.

The sound measuring surface area S is the vertical area (in square meters) enveloping the transformer (measurement surface) on which the sound measurement points are located plus the horizontal plane bounded by the vertical measurement surface. The measurement surface area is approximately equal to 125 % of the vertical area enveloping the transformer. Therefore, the sound measuring surface area S, is calculated using the following Equation (36):

S= 1.25 × h × lm

(36)

where

h lm

is the height of the transformer tank, in meters is the length of the sound measuring contour, in meters

The 1.25 empirical factor accounts for the sound radiated by the transformer tank cover The maximum value allowed for this sound reflection correction is 4 dB. This is equivalent to allowing sound reflection equivalent to 1.5 times the sound radiated from the transformer. In cases of high wall sound reflections, the transformer manufacturer should, if applicable/feasible, move the transformer to a larger test area or outdoors. NOTE—The sound reflection correction described above is based on the IEC 60076-10 (Determination of sound levels) for transformer sound measurements.

82




Table 8 —Approximate values of the average acoustic absorption coefficient Description of room Nearly, or partly empty, room with smooth hard walls Industrial room with machinery and no sound absorbing material on walls or ceiling Industrial room with a small amount of sound absorbing material on ceiling or walls Room with substantial amount of sound absorbing materials on both ceilings and walls Sound room or room with an open side

Average acoustic absorption coefficient, α 0.1 0.2 0.3 0.4 0.5

4.0 3.5 3.0

K, dB

2.5 2.0 1.5 1.0 0.5 0.0

2

3

4

5

6

7

8

9

10

11

12

13

14

15

A/S

Figure 34 —Sound reflection correction factor K calculated per Equation (34) 13.5.5.4 Near-field correction This correction compensates for the measuring error caused by the near-field reactive sound power around the transformer at the ONAN sound measuring contour. The value to be used for this correction is –1.0 dB for the ONAN measurements. No correction is to be made to the ONAF measurements. 13.5.6 Sound pressure level measurements using the sound-intensity method As stated earlier, in 13.5, measurements using the sound-intensity method do not need to be corrected for ambient sound or wall sound reflections. However, ambient sound and/or wall sound reflections, when high, affect the accuracy of the measurements using this method. Under those conditions, the correction, described in 13.5.6.1 shall be made. 83




13.5.6.1 Environmental correction The sound-intensity measuring equipment provides two measured values for the sound pressure level of the transformer; one using the sound pressure method LP , and one using the sound-intensity method LI . Under favorable environmental conditions (reasonable levels of ambient sound and wall sound reflections), the difference between the two measured values of the sound pressure level of the transformer is typically ≤ 4 dB. Under these conditions, no correction, to the measurements using the sound-intensity method, is needed. However, when the environmental conditions are such that this difference ( LP − LI ) is > 4 and ≤ 6 dB, the following correction shall be made:

4 < ( LP − LI ) ≤ 5 dB , corrected LI = Measured LI + 1

5 < ( LP − LI ) ≤ 6 dB , corrected LI = Measured LI + 2 where LP and LI are average sound pressure levels measured using the sound pressure and sound-intensity measuring methods, respectively. When the environmental conditions are such that ( LP − LI ) > 6 dB, using the sound-intensity method would be considered invalid. Alternatively, the transformer sound could be measured using the sound pressure method while following the recommended corrections described in 13.5.5. Alternatively, as above environmental conditions are typically caused by high ambient noise and/or high wall sound reflections, the transformer manufacturer could attempt to reduce the ambient sound and/or move the transformer into a larger test area/room, if feasible. When making one-third octave or narrowband-octave measurements, conditions stated above, for sound measurements using the sound-intensity method, only apply to the main frequency components of the transformer noise. NOTE 1—For core noise, the range of the main frequency components depends on core size, material, and operating flux density. For example, these main frequency components for large core sizes made of highly grain oriented silicon steel operating at low flux densities, are 100/120 Hz, 200/240 Hz, 300/360 Hz for 50/60 Hz transformers, respectively. Correspondingly, small sizes made of regular grain oriented steel operating at high flux densities would typically have main frequency components of to 600/720 Hz. NOTE 2—For no–load noise with fans/pumps operating, the main frequency components can extend to the 4000 Hz and 8000 Hz range. NOTE 3—For load noise, the main frequency component is the 100/120 Hz.

13.6 Determination of audible sound level of a transformer 13.6.1 Average sound pressure level (Lp) The energy average transformer sound pressure level (measured either by the sound pressure or the soundintensity method) shall be computed by logarithmically averaging the ambient-corrected sound pressure levels measured at each microphone location and for each frequency band (A-weighted, one-third octave band, or discrete frequency) using Equation (37):

1 LP= 10 × log10  N

N

∑10 i =1

( Li /10)

  

(37)

84




where

Li N

is the sound pressure level measured at the ith location for the A-weighted sound level, for a onethird octave frequency band, or for a discrete frequency (dB) is the total number of sound measurements

13.6.2 Determination of total sound pressure level of a transformer 13.6.2.1 Addition of no-load and load audible sound levels The total sound pressure level of a transformer at a certain operating condition of voltage, current, and cooling stage can be calculated by logarithmically adding the measured sound levels of no-load and load audible sound level corresponding to this operating condition. It can be calculated using Equation (38) below:

(

10 ⋅ log10 100.1LPNL + 100.1LPL LPT =

)

(38)

where

LPT is the total sound pressure level of the transformer at the specified operating condition LPNL is the no-load sound pressure level of the transformer at the specified operating condition LPL is the load sound pressure level of the transformer at the specified operating condition 13.6.2.2 Determination of total sound pressure level of a transformer at different loading conditions The determination of the total sound pressure level of a transformer at different loading conditions requires the following measurements: a)

No-load audible sound with no fans/pumps running; at ONAN contour

b)

No-load audible sound with half of the fans/pumps running; at ONAF contour

c)

No-load audible sound with all fans/pumps running; at ONAF contour

d)

Load audible sound with no fans/pumps running; at ONAF contour

For the determination of the total sound level of the transformer at a specific loading condition, the following procedure shall be used: At top power rating of transformer Total audible sound level = Logarithmic sum of the measured no-load audible sound level with all fans/pumps operating and load sound at the top power rating At a load up to the ONAN rating of transformer Total audible sound level = Logarithmic sum of the measured ONAN no-load audible sound level and load sound level at the specified loading condition corrected to the ONAN contour using the adder (10 × log10 (ONAFcontour / ONANcontour )) where (ONAF contour/ONAN contour) is the ratio of the lengths of the two contours.

85




At an intermediate load between the ONAN rating and top rating of the transformer Total audible sound level = logarithmic sum of the measured no-load audible sound level at the corresponding cooling stage and load sound level at the specified loading condition NOTE 1—When determining load audible sound level at a specific load using the load sound level measured at a different loading condition, refer to 13.3.3.2, Equation (33). NOTE 2—Use Equation (38) for the determination of the logarithmic sum of no-load and load sound level. NOTE 3—The above calculations would not be needed when purchasers require adhering to specified no-load noise level guarantees, e.g., NEMA TR1.

This is demonstrated in the example below: Example a) Transformer power ratings are 48 / 64 / 80 MVA. b)

Measured sound pressure levels are as follows: 1)

Measured no-load audible sound level: i)

With no fans running: 59 dB (A) @ ONAN contour

ii)

With half fans running: 62 dB (A) @ ONAF contour

iii)

With all fans running: 63 dB (A) @ ONAF contour

2)

Measured Load audible sound level at 80 MVA: 61 dB (A) @ ONAF contour

3)

Length of ONAN & ONAF measuring contours are 20 & 24 meters, respectively

Total audible sound level of transformer at top rating (80 MVA)

(

)

Total sound pressure level = 10 ⋅ log10 10( 0.1×63.0) + 10( 0.1×61.0) = 65.1 dB (A) Total audible sound level of transformer at 40 MVA At this MVA loading condition, no-load audible sound = 60 dB (A) Load audible sound level at 40 MVA corrected to ONAN contour =

  40    24  61 + 40 ⋅ log10    + 10 ⋅ log10   = 61 − 12.0 + 0.8 = 49.8 dB (A)  80    20  

(

)

Total sound pressure level = 10 ⋅ log10 10( 0.1×59.0) + 10( 0.1×49.8) = 59.5 dB (A)

Total audible sound level of transformer at 60 MVA At this MVA loading condition, no-load audible sound = 62 dB (A)

  60   Load audible sound level at 60 MVA = 61 + 40 ⋅ log10    = 61 − 5.0 = 56.0 dB (A)  80   

86




(

)

Total sound pressure level = 10 ⋅ log10 10( 0.1×62.0) + 10( 0.1×56.0) = 63.0 dB (A) Total audible sound level of transformer at 70 MVA At this MVA loading condition, no-load audible sound level = 63 dB (A)

  70   Load audible sound level at 70 MVA = 61 + 40 ⋅ log10    = 61 − 2.3 = 58.7 dB (A)  80   

)

(

Total sound pressure level = 10 ⋅ log10 10( 0.1×63.0) + 10( 0.1×58.7 ) = 64.4 dB (A)

13.6.3 Sound power level calculation (Lw) The A-weighted, one-third octave band, or discrete frequency sound power levels shall be computed using corresponding sound pressure level measurements using Equation (39):

Lw = L p + 10 ⋅ log10 ( S )

(39)

where

LW is the sound power level of the transformer, in dB (A) LP is the sound pressure level of the transformer, in dB (A) Refer to Equation (36) for the calculation of the sound measuring surface area “S.”

13.7 Presentation of results Reports describing transformer sound ratings shall include, as a minimum, the following data and Figure 35 (for sound pressure measurements) and Figure 36 (for sound-intensity measurements), as applicable: a)

A statement that the measurements were made and reported as described in this standard.

b) A detailed description of all deviations from this test code. c)

A description of the transformer being tested, including rated power, voltage, voltage ratio, tap position, and connections.

d) Measured voltage at the start of the no-load sound tests and measured current at the start of the load sound test. e)

The name of the transformer manufacturer, the location of manufacturer, the transformer type and serial number, the date and time of the test, and the name of the engineer approving the test.

f)

A description of the sound-measuring instruments, the microphone, and the calibration method. Include the following information for the sound-measuring instruments: manufacturer, model, serial number, and calibration source.

87




g) A description of the test environment, including a dimensioned sketch. Show the position of the transformer with respect to other objects, and show the location of the measuring surface, the microphone positions, and sound reflecting or absorbing surfaces. h) The descriptor specified by the purchaser and manufacturer for measuring one of the following sound pressure levels: 1) A-weighted sound pressure level 2) One-third octave band sound pressure level 3) Narrowband sound level i)

Clear descriptions of the sound levels measured for any of the following operating conditions: 1) Transformer fully equipped with its auxiliaries in service 2) Transformer fully equipped with its auxiliaries not in service 3) Cooling equipment in service with transformer not energized

j)

The measurement surface area and the distance between the measured microphone locations and the transformer.

k) When using the sound pressure method: 1) Ambient sound pressure levels measured at each location. 2) Combined transformer and ambient sound pressure level measured at each location and the logarithmical average sound pressure level. 3) Average A-weighted transformer sound pressure levels, and when specified, the one-third octave sound pressure levels, or the narrowband sound pressure levels, corrected for background sound conditions, wall sound reflections, and near-field effects. l)

When using the sound-intensity method: 1) Transformer sound pressure level measured at each location and the logarithmical average sound pressure level. 2) Average A-weighted transformer sound pressure levels, and when specified, the one-third octave sound pressure levels, or the narrowband sound pressure levels, corrected for environmental conditions if needed.

m) The A-weighted sound power level of the transformer and, when specified, the one-third octave band sound power level or the narrowband sound power levels. Sound level data shall be rounded to the nearest whole decibel; values of 0.5 and above shall be rounded to the next higher integer. Figure 35 and Figure 36 present an example format for presenting the transformer sound level measurements. For one-third octave and narrowband octave measurements, the same tables can be used to report measurements at the individual frequencies (1 table/frequency).

88




Measuring position

(Transformer + ambient) noise

Ambient noise @ > 4 points

1/3-height

1/3-height

2/3-height

2/3-height

1 2 3 4 5 . . . n Logarithmic average sound level of (transformer + ambient) noise Logarithmic average sound level of ambient noise Sound level of transformer (corrected for ambient noise) Correction for wall sound reflection Near-field correction Measured sound level corrected for ambient noise, wall sound reflection, and nearfield effect

dB (A) dB (A) dB (A) dB dB dB (A)

Figure 35 —Measurements using the sound-pressure method

Measuring position

Sound pressure measurements 1/3-height

2/3-height

Sound-intensity measurements 1/3-height Level

2/3-height +/-

Level

+/-

1 2 3 4 . . . n Logarithmic average of sound pressure measurements Logarithmic average of sound-intensity measurements (Pressure – Intensity) index Environmental correction Measured sound level corrected for environmental correction

dB (A) dB (A) dB dB dB (A)

Figure 36 —Measurements using the sound-intensity method

89




14. Calculated data 14.1 Reference temperature The reference temperature for determining load losses, voltage regulation, and efficiency shall be equal to the sum of the rated average winding temperature rise plus 20 ºC.

14.2 Losses and excitation current 14.2.1 Determination of no-load losses and excitation current No-load losses and excitation current shall be determined for the rated voltage and frequency on a sinewave basis, unless a different form is inherent in the operation of the transformer. 14.2.2 Load losses Load losses shall be determined for rated voltage, current, and frequency and shall be corrected to the reference temperature (see 14.1). 14.2.3 Total losses Total losses are the sum of the no-load losses and the load losses.

14.3 Efficiency The efficiency of a transformer is the ratio of its useful power output to its total power input as shown in Equation (40):

P Pi

P − PL P PL 1 L =− 1 =− Pi Pi Po + PL

η =o =i

(40)

where

η

is efficiency

Po Pi PL

is output (W) is input (W) is losses (W)

When specified, efficiency shall be calculated on the basis of the reference temperature for the average winding temperature rise of the transformer.

14.4 Voltage regulation of a constant-voltage transformer 14.4.1 General The voltage regulation of a constant-voltage transformer is defined in IEEE Std C57.12.80. The regulation may be expressed in percentage (or per unit) on the basis of the rated secondary voltage at full load.

90




14.4.2 Reference temperature When specified, voltage regulation calculations shall be based on the reference temperature described in 14.1. 14.4.3 Load loss watts and impedance volts The load loss watts and impedance volts used to compute voltage regulation are derived from the measurement of the factors described in 9.2 corrected to reference temperature described in 14.1. 14.4.4 Voltage regulation computation, two-winding transformers When specified, the voltage regulation shall be computed according to exact formulae for the calculation of regulation through three-phase to two-phase transformers. 14.4.4.1 Exact formula for the calculation of regulation The exact formulae for calculating regulation are shown in Equation (41) and Equation (42): a) When the load is lagging:

reg =

( R + Fp ) 2 + ( X + q ) 2 − 1

(41)

b) When the load is leading:

reg =

( R + Fp ) 2 + ( X − q ) 2 − 1

(42)

where

Fp

is power factor of load

q

is

R

is resistance factor of transformer =

X

is reactance factor of transformer = + Z 2 − R2 impedance kilovoltamperes is impedance factor of transformer = rated kilovoltamperes

Z

+ 1 − Fp2 load loss in kilowatts rated kilovoltamperes

The quantities Fp , q , R , X , and Z are on a per-unit basis. Results shall be multiplied by 100 to obtain the percentage of regulation. 14.4.4.2 General expression for calculation of transformer regulation A general expression for the calculation of transformer regulation permitting any degree of precision justified by the supporting data is Equation (43):

1 2 1 3 5 4 7 5 21 6 33 7 reg =− a a + a − a + a − a + a 2 2 8 8 16 16

(43)

91




where

reg a

is the regulation on a per-unit basis is a quantity depending on the angle and magnitude of the transformer impedance, the power factor of the load, and the number of windings in the transformer.

The quantity a for the calculation of the per-unit regulation of a two-winding transformer is determined by Equation (44):

a Z cos (φ + θ ) + =

Z2 2

(44)

where

load loss in kilowatts rated kilovoltamperes

R

is the resistance factor of transformer =

Z

is the impedance factor of transformer =

X φ cos φ Fp

is reactance factor of transformer = + Z 2 − R2 is the impedance angle of transformer impedance is R/Z

θ

is the phase angle of load current; positive for leading current, negative for lagging current

impedance kilovoltamperes rated kilovoltamperes

is the power factor of load cos φ

14.4.4.3 Three-phase to two-phase transformers For the calculation of the regulation for three-phase to two-phase transformation, proceed as follows: a)

For the per-unit regulation of main phase, use the impedance of the main transformer for substitution in the formula selected for use.

b)

For the per-unit regulation of the teaser phase, use the sum of the impedance of the teaser transformer plus the interlacing impedance of the main transformer for substitution in the formula selected for use.

c)

To determine the interlacing impedance, connect the two ends of the three-phase winding of the main transformer together and impress between this common connection and the 50% tap a voltage sufficient to pass three-phase line current in the supply lines.

d)

The voltage thus determined is the interlacing impedance voltage and is to be put on a per-unit basis by reference to the rated voltage of the teaser transformer on the 86.6% tap.

92




Annex A (informative) Partial discharge measurement using radio-influence voltage instrumentation and its failure detection A.1 Partial discharge measurement A.1.1 Internal partial discharges Apparent internal partial discharges shall be determined in terms of the RIV generated and measured at the line terminals of the winding under test. A.1.2 Instrumentation A radio noise and field strength meter conforming to ANSI C63.2 [B1] shall be used to measure the RIV generated by any internal partial discharges. The measurement shall be on a quasi-peak basis at a nominal frequency of 1 MHz; a frequency from 0.85 MHz to 1.15 MHz may be used to discriminate against local radio-station signal interference. The radio-noise meter shall be coupled to the line terminal(s) of the winding under test through the capacitance tap of the bushing(s). A suitable device shall be used to compensate for the capacitance dividing effect produced by the bushing tap-to-ground capacitance plus that of all elements between the bushing tap and the meter (e.g., coaxial cables, adapters). This device shall be tuned to minimize the dividing effect of the capacitances and to convey the RIV signal to the radio-noise meter with minimum attenuation. External shielding may be used to minimize the risk of air corona, which may occur at the bushing terminals or grounded projections. Radio-frequency chokes or tuned filters may be used to isolate the transformer under test and the RIV-measuring circuit from the remainder of the test circuit, including its energy source. A.1.3 Calibration The test circuit components connected to the winding under test may attenuate the generated RIV level and add to the measured RIV background level. Therefore, it is necessary to determine the relationship between the RIV at the terminal of the winding under test and the RIV reading of the radio-noise meter connected in typical location in the test circuit. The steps for this calibration ratio are as follows: a)

Apply a signal to the terminal under test of approximately 100 µV at the measuring frequency.

b)

Measure the voltage at the terminal with the radio-noise meter connected directly to the terminal.

c)

With the same radio-noise meter, measure the voltage provided by the test circuit at the location where the radio-noise meter will be connected during the partial discharge test on the transformer. A second radio-noise meter may be used for this measurement, provided its relationship to the first has been established at the measuring frequency.

d)

Use the ratio of the calibration signal voltage measured at the transformer terminal to that measured at the normal meter location in the test circuit as a multiplier on the RIV at the terminal of the winding under test.

e)

Establish that this calibration ratio remains valid over the RIV range of interest.

93




A.2 Failure detection Failure may be indicated by the presence of smoke and bubbles rising in the insulating liquid, an audible sound such as a thump, or a sudden increase in test current. Any such indication shall be carefully investigated by observation, by repeated test, or by other tests to determine whether a failure has occurred. In terms of interpretation of partial discharge measurements, the results shall be considered acceptable and no further partial discharge tests required under the following conditions: a)

The magnitude of the partial discharge level does not exceed 100 µV.

b)

The increase in partial discharge levels during the 1 h does not exceed 30 µV.

Judgment should be used on the 5 min readings so that momentary excursions of the RIV meter caused by cranes or other ambient sources are not recorded. Also, the test may be extended or repeated until acceptable results are obtained. A failure to meet the partial discharge acceptance criterion shall not warrant immediate rejection, but it shall lead to consultation between purchaser and manufacturer about further investigations.

94




Annex B (normative) 50/60-Hz frequency conversion of measured performance parameters It is preferred to perform tests at the rated frequency of the transformer to be tested. However, tests cannot always be done at the rated frequency. In such cases, upon agreement between the purchaser and the manufacturer at the tender stage, the conversion factors given in this annex shall be used to convert the tested values from the frequency used for test to the required rated frequency. The purpose of the frequency conversion factors is to have uniformity among manufacturers when such cases arise. These conversion factors are intended for 50-Hz-to-60-Hz conversion; however, the 60-Hz-to-50-Hz conversion factors are essentially the reciprocal of the 50-Hz-to-60-Hz factors as shown in this annex. See Girgis, et al. [B13] for additional information.

B.1 No-load loss and excitation current The following conversion factors shall be used to convert the values of measured no-load loss and excitation current at the design flux density from the frequency used for measurement to the required rated frequency. Voltage measured according to the average voltmeter method should be applied so that the resulting core flux density is equal to the flux density at the rated frequency of the transformer. For example, in the case of the test performed at 50 Hz on a 60 Hz transformer, the applied voltage should be 5/6 of the voltage corresponding to the 60 Hz operation. a)

No-load loss conversion factors (50 Hz to 60 Hz): 1)

2)

d)

For single-phase transformers:

B ≤ 1.4T = 1.32

(B.1)

B > 1.4T = 1.32 − 0.05 × ( B − 1.4 )

(B.2)

For three-phase transformers:

B ≤ 1.4T = 1.33

(B.3)

B > 1.4T = 1.33 − 0.05 × ( B − 1.4 )

(B.4)

No-load loss conversion factors (60 Hz to 50 Hz): 1)

For single-phase transformers:

95




2)

1 B ≤ 1.4T = 1.32

(B.5)

1 B > 1.4T = 1.32 − 0.05 × ( B − 1.4 )

(B.6)

For three-phase transformers:

1 B ≤ 1.4T = 1.33

(B.7)

1 B > 1.4T = 1.33 − 0.05 × ( B − 1.4 )

(B.8)

where

B = flux density [ T ] The value of each of these conversion factors is an average value for all core materials. This averaging translates into a maximum of 1% uncertainty of the corrected value. The excitation current conversion factor is 1.00; thus the measured value of the excitation current does not need to be converted from one frequency to the other. Numerical Example: 50-Hz-to-60-Hz conversion for a three-phase transformer: 

Design flux density

= 1.7 T



Rated voltage at 60 Hz

= 13.8 kV



Voltage applied at 50 Hz = 13.8 × 5/6

= 11.5 kV



Conversion factor = 1.33 – 0.05 × (1.7 – 1.4) = 1.315



Measured no-load loss at 50 Hz

= 22.8 kW



Converted no-load loss at 60 Hz

= 22.8 × 1.315 = 30 kW



Measured excitation current at 50 Hz

= 2.3 A



Excitation current at 60 Hz

= 2.3 A

B.2 Load loss The following conversion factors shall be used to convert the values of load loss measured at 50 Hz to their corrected values at 60 Hz.

96




Conversion values are given below to convert the measured winding eddy and stray loss separately if a magnetic field program was available to calculate at least the winding eddy losses. Otherwise, a conversion factor is given below for the sum of winding eddy + stray losses. 



Load loss conversion factors (50 Hz to 60 Hz): 

Winding eddy loss:

1.44



Stray loss:

1.23



Winding eddy + stray loss:

1.34

Load loss conversion factors (60 Hz to 50 Hz): 

Winding eddy loss:

1/1.44



Stray loss:

1/1.23



Winding eddy + stray loss:

1/1.34

The voltage to be applied on the transformer shall be such that the resulting current in the transformer is equal to the rated current. For transformers larger than approximately 10 MVA, the impedance voltage is nearly proportional to frequency. In the case of the test performed at 50 Hz on a 60 Hz transformer, the applied voltage would be nearly 5/6 of the impedance voltage of the transformer corresponding to the 60 Hz operation. Since the value of each of the conversion factors is an average value for the different types of tank wall shielding, this averaging would represent an additional 1% uncertainty to the accuracy of the converted loss values. Numerical Example; 50-Hz-to-60-Hz conversion: 

Measured losses at 50 Hz: 



Total Load Loss

= 142.4 kW

2



I R

= 124.6 kW



Winding Eddy + Stray

= 17.8 kW

Converted Losses for 60 Hz: 

Winding Eddy + Stray

= 17.8 × 1.34

= 23.9 kW



Total Load Loss

= 124.6 + 23.9

= 148.5 kW

B.3 Temperature-rise test The following current multipliers shall be applied to achieve the correct rated frequency loss. The injected test currents for the initial estimate of the total losses shall be adjusted so that the ohmic I2R loss is increased to offset the decreased winding eddy and stray loss at 50 Hz operation. a)

Multipliers for heat run currents (50 Hz to 60 Hz): 1)

Multiplier for total losses heat run current:

97




P0 + ( PI 2 R + Pe + s ⋅1.34 )

(B.9)

P0 + ( PI 2 R + Pe + s )

2)

Multiplier for winding rise run current:

PI 2 R + 1.44 ⋅ Pe PI 2 R + Pe

(B.10)

where P0

is the converted no-load loss at 60 Hz (measured at 50 Hz and converted to 60 Hz)

PI 2 R Pe + s Pe

is the calculated I 2 R losses at rated current from the dc resistance measurement is the measured (winding eddy + stray) loss at 50 Hz

b)

is the measured winding eddy loss at 50 Hz

Multipliers for heat run currents (60 Hz to 50 Hz): 1)

Multiplier for total losses heat run current:

P  P0 +  PI 2 R + e + s 1.34   P0 + ( PI 2 R + Pe + s ) 2)

(B.11)

Multiplier for winding rise run current

PI 2 R +

Pe

1.44 PI 2 R + Pe

(B.12)

where P0

is the converted no-load loss at 50 Hz (measured at 60 Hz and converted to 50 Hz)

PI 2 R Pe + s Pe

is the calculated I 2 R losses at rated current from the dc resistance measurement is the measured (winding eddy + stray) loss at 60 Hz is the measured winding eddy loss at 60 Hz

Numerical Example: Temperature-rise test at 50 Hz for a 60 Hz transformer: Total Loss Heat Run Current for 60 Hz Rated Current for 60 Hz P0 Measured at 50 Hz and Converted to 60Hz

= 272 A = 246.6 A = 14.4 kW 98




PI 2 R Pe + s Measured at 50 Hz Pe Measured at 50 Hz

= 17.8 kW

Total Loss Run Current at 50 Hz

14.4 + (124.6 + 17.8 × 1.34 ) = 272.0 × = 277.2 A 14.4 + (124.6 + 17.8 )

Winding Rise Current at 50 Hz

124.6 + 1.44 × 4.3 = = 246.6 × 248.4 A 124.6 + 4.3

= 124.6 kW = 4.3 kW

The manufacturer shall provide an adequate supply to operate the cooling equipment at the rated frequency. The measured average liquid and average winding rises will be considered accurate for rated frequency condition since the correct rated frequency losses are applied. It should be noted that direct hot spot temperature measurements (e.g., fiberoptic probes) would need to be corrected for winding eddy losses at the rated frequency. The measured tank temperature rises could be in error by a few degrees Celsius since the stray losses will not be correct and should thus be noted on the certified test reports.

B.4 Short-circuit test Voltage shall be applied so that the transformer is subjected to the calculated symmetrical and asymmetrical currents (calculated for the rated frequency). Thus, in the case of the test performed at 50 Hz on a 60 Hz transformer, the applied voltage would be nearly 5/6 of the voltage required for a 60 Hz operation test. For large power transformers, it should be noted that mechanical winding resonance in the 100 Hz to 120 Hz range may impact the short circuit response of the windings at one frequency versus the other.

B.5 Audible sound B.5.1 A-weighted sound level conversion of no-load noise The following conversion factors for both ONAN and ONAF stages shall be used to convert the value of A-weighted sound level measured at 50 Hz to the corrected values at 60 Hz and conversely from 60 Hz to 50 Hz. The manufacturer shall provide an adequate supply to operate the cooling equipment at the rated frequency. Using these frequency conversion factors requires that the manufacturer verify that neither the core nor the windings or the tank plates/stiffeners will experience mechanical resonance at the rated frequency. In such a case, the purchaser and the manufacturer should agree on any other adjustments that should be applied to the tested values. If measurements, however, indicate core, tank, or winding resonance at the frequency at which the measurement is being performed, the purchaser and the manufacturer should agree on whether any other adjustment to the tested values would be allowed as these resonances may not be excited when the transformer is operating at the rated frequency. The uncertainty is greater, and the possibility of resonance(s) exists when the transformer is operating at the rated frequency. Therefore, if the converted measured sound level is within 2 dB below the guaranteed value, then upon agreement with the purchaser, verification of the sound level at site may be required. 99




B.5.1.1 50/60 Hz conversion of ONAN (core) sound level Voltage should be applied so that the resulting flux density in the core is equal to the design flux density at the rated frequency of the transformer. For example, in the case of the test performed at 50 Hz on a 60 Hz transformer, the applied voltage should be 5/6 of the voltage corresponding to the 60 Hz operation. The corrected sound level for 60 Hz operation is obtained by adding 3.6 dB to the measured ONAN sound level at 50 Hz (LPONAN-50). Conversely, the corrected sound level for 50 Hz operation is obtained by subtracting 3.6 dB from the measured sound level at 60 Hz (LPONAN-60). The same approach can be used for transformers with OD and OF ratings. B.5.1.2 50/60 Hz conversion of ONAF sound level B.5.1.2.1 Conversion from 50-Hz tested sound levels to corresponding 60-Hz levels First, the ONAF sound pressure level of the transformer ( LPONAN −50/ 60 ) operating at 50 Hz with the cooling equipment operating at 60 Hz should be measured. Then the following conversion is to be made:

LPONAN −60 : ONAN Sound Pressure Level at 60 Hz = LPONAN −50 + 3.6 The corresponding ONAF sound pressure level at 60 Hz is

 0.1⋅ LPONAF −50/60  − 100.1⋅ LPONAN −50 + 100.1⋅ LPONAN −60  LPONAF − 60 = 10 × log 10  

B.5.1.2.2 Conversion from 60-Hz tested sound levels to the corresponding 50-Hz levels First, the ONAF sound pressure level of the transformer ( LPONAN − 60/50 ) operating at 60 Hz with the cooling equipment operating at 50 Hz should be measured. Then the following conversion is to be made:

LPONAN −50 : ONAN Sound Pressure Level at 50 Hz = LPONAN − 60 − 3.6 The corresponding ONAF sound pressure level at 50 Hz is

10 × log 100.1⋅ LPONAF −60/50 − 100.1⋅ LPONAN −60 + 100.1⋅ LPONAN −50  LPONAF −50 = B.5.1.2.3 Numerical example Measured sound levels: LPONAN −50 0

LPONAN −50/ 60 Calculated sound levels: LPONAN −60

LPONAN −60

= 59.5 dBA = 66.1 dBA

= 59.5 + 3.6 = 63.1 dBA

= 10 × log 100.1⋅( 66.1) − 100.1⋅(59.5) + 100.1⋅( 63.1)  = 67.2dBA 100




B.5.1.3 Load sound level conversion Per 13.3.3.2 Load audible sound; for this test, impedance voltage should be applied so that the resulting current in the windings is equal to the desired current of the transformer. For example, in the case of the test performed at 50 Hz on a 60 Hz transformer, the applied voltage should be 5/6 of the impedance voltage for the 60 Hz operation. The corrected sound level in dBA for 60 Hz operation (in the absence of current harmonics) is obtained by adding 4.6 dB to the load sound level measured at 50 Hz. Conversely, the corrected sound level for 50 Hz operation is obtained by subtracting 4.6 dB from the measured sound level at 60 Hz. The 4.6 dB conversion is the sum of the 1.6 dB difference in sound power produced at 60 Hz versus 50 Hz [20 Log (60/50)] and the 3.0 dB difference in the magnitude of the A-weighting attenuation at 100 Hz versus at 120 Hz. B.5.2 Frequency spectrum conversion The corrected sound levels at the main frequency components of the transformer sound in dBA, for 60 Hz operation, are obtained by adding the following values to the measured levels at 50 Hz: Center frequency (Hz)

< 100/120

100/120

200/240

300/360

400/500

> 400/500

Adder (dB)

5.5

4.6

3.9

3.4

3.2

2.5

Conversely, the corrected sound levels at the main frequency components for 50 Hz operation is obtained by subtracting the values in the table above from the measured sound levels at 60 Hz. For linear sound level values of the frequency spectrum, a constant value of 1.6 dB is to be added to all frequency components of the measured frequency spectrum at 50 Hz in order to obtain the corresponding frequency spectrum for the 60 Hz operation. Conversely, the frequency spectrum for 50 Hz operation is obtained by subtracting 1.6 dB from the measured spectrum at 60 Hz.

101




Annex C (informative) Connections diagrams for short-circuit testing of a three-phase transformer using single-phase source as an alternative (see 12.2.1.2) For three-phase transformers three-phase supply is preferable. When the test capabilities do not allow threephase tests at the fault current requirement defined in Clause 7 of IEEE Std C57.12.00-2015, an alternate single-phase source can be used The following Figure C.1 to Figure C.8 show connection diagrams of the alternate single-phase source. Figure C.1 to Figure C.5 show connections for phase to phase tests and Figure C.6 and Figure C.8 show connections for phase-to-ground test.

S2

S1

Figure C.1—Single-phase test, phase-to-phase wye-wye connected windings

S2

S1

Figure C.2—Single-phase test, phase-to-phase wye-delta connected windings

S2

S1

Figure C.3—Single-phase test, phase-to-phase delta-wye connected windings

102




S2

S1

Figure C.4—Single-phase test, phase-to-phase delta-delta connected windings

S2

S1

Figure C.5—Single-phase test, phase-to-phase wye-wye autotransformer

Single-phase source with short-circuit on one phase at a time [12.2.1.2 c)] applies for wye-wye connected transformers or auto-transformers and wye – delta transformers. Source is then applied between a phase terminal and the ground terminal. The ground terminal shall be capable of carrying the fault current for the specified test duration. In order to control the voltage on the two other phases, it is recommended to connect their terminals together. See Figure C.6 to Figure C.8. S2

S1

Figure C.6—Single-phase test, phase-to-ground wye-wye connected windings

S1

S2

Figure C.7—Single-phase test, phase-to-ground wye-wye autotransformer

103




S2

S1

Figure C.8—Single-phase test, phase-to-ground wye-delta transformer

In the above figures: 

S 1:

for pre-set method: synchronous switch for post-set method: source circuit breaker



S 2:

for pre-set method: rigid connection bar for post-set method: synchronous switch.

104




Annex D (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] ANSI C63.2, American National Standard for Electromagnetic Noise and Field Strength Instrumentation, 10 kHz to 40 GHz—Specifications. 8 [B2] ASTM D117, Standard Guide for Sampling, Test Methods, and Specifications for Electrical Insulating Oils of Petroleum Origin. 9 [B3] ASTM D877, Standard Test Method for Dielectric Breakdown Voltage of Insulating Liquids Using Disk Electrodes. [B4] ASTM D923, Standard Practices for Sampling Electrical Insulating Liquids. [B5] ASTM D1816, Standard Test Method for Dielectric Breakdown Voltage of Insulating Liquids Using VDE Electrodes. [B6] ASTM D3612, Standard Test Method for Analysis of Gases Dissolved in Electrical Insulating Oil by Gas Chromatography. [B7] ASTM D3613 Standard Practice for Sampling Insulating Liquids for Gas Analysis and Determination of Water Content, (Withdrawn 2007). [B8] Bergonzi L., Bertagnolli G., Cannavale G., Caprio G., Iliceto F., Dilli B. and Gülyesil O., Power transmission reliability. technical and economic issues relating to the short-circuit performance of power transformers, CIGRE 12-107, session 2000. [B9] Bertagnolli, Giorgio, Short-Circuit Duty of Power Transformers, The ABB Approach, Second Revised Edition, May 1998. Golinelli Editor. [B10] Bolduc L., P. Picher, G. Paré, R.J. Demers, J. Bélanger, Detection of Transformer Winding Displacement by Frequency Response of Stray Losses (FRSL), ©CIGRÉ Session 2000 paper no 12/33-02. [B11] CIGRÉ WG A2.26 report, Mechanical Condition assessment of Transformer Windings using Frequency Response Analysis (FRA), Electra No 228, October 2006. [B12] Foldi J., Bérubé D., Bertagnolli G., Maggi R. and Riffon P., Recent Achievement in Performing Short-Circuit Withstand Tests on Large Power Transformers in Canada, CIGRE 12-2001, Session 2000. [B13] Girgis, Ramsis S., Beaster, Barry, and teNyenhuis, Ed G., “Proposed Standards for Frequency Conversion Factors of Transformer Performance Parameters”, IEEE Transactions on Power Delivery, Pages 1262 – 1267, Vol. 18, No. 4, October 2003. [B14] Hemmers, R. T., and Graham, D. C., “Measurement of self-cooled transformer sound levels in relatively high ambients,” IEEE Transactions on Power Apparatus and Systems, vol. PAS-89, no. 7, pp 1657–1662, Sept./Oct. 1970.

8 ANSI publications are available from the Sales Department, American National Standards Institute, 25 West 43rd Street, 4th Floor, New York, NY 10036, USA (http://www.ansi.org/). 9 ASTM publications are available from the American Society for Testing and Materials, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, USA (http://www.astm.org/).

105




[B15] IEC 60076-4, Power Transformers—Part 4 Guide to the Lightning Impulse & Switching Impulse Testing – Power Transformers and Reactors. 10 [B16] IEC 60076-5, Power Transformers—Part 5 Ability to Withstand Short-Circuit. [B17] IEEE Std C57.12.10™ 11, Transformers.

12

, IEEE Standard Requirements for Liquid-Immersed Power

[B18] IEEE Std C57.12.40™, IEEE Standard for Network, Three-Phase Transformers, 2500 kVA and Smaller; High Voltage, 34 500 GrdY/19 920 and Below; Low Voltage, 600 V and Below, Subway and Vault Types (Liquid Immersed). [B19] IEEE Std C57.12.70™, IEEE Standard for Standard Terminal Markings and Connections for Distribution and Power Transformers. [B20] IEEE Std C57.104™ IEEE Guide for the Interpretation of Gases Generated in Oil – Immersed Transformers. [B21] IEEE Std C57.106™, IEEE Guide for Acceptance and Maintenance of Insulating Oil in Equipment. [B22] IEEE Std C57.149™ IEEE Guide for the Application and Interpretation of Frequency Response Analysis for Oil-Immersed Transformers. [B23] IEEE Std 454™-1973 (withdrawn), IEEE Recommended Practice for the Detection and Measurement of Partial Discharges (Corona) During Dielectric Tests. [B24] Janssen A.L.J., te Paske L.H., Short-Circuit Testing Experience with large Power Transformers, CIGRÉ paper 12-105, Session 2000. [B25] Kraetge, A., Kalkner, W., Plath, R., Plath, K.-D., Jaekelmann, R., Eisemann, D., Hoffmann, U., The transient oil pressure measurement as a diagnostic method to assess the short-circuit duty of power transformer, CIGRÉ A2 Transformer Colloquim 2007, Bruges, Belgium. [B26] Leher G., Investigation of Inrush Currents during a Short Circuit Test on a 440 MVA, 400 kV GSUTransformer, CIGRÉ paper 12-104, Session 2000. [B27] Rosselli, Girolamo, “Transformer Test to Calculate Z0 for Interconnected Windings Transformers Using Symmetrical Sequence Components,” IEEE proceedings paper O5GM0335 presented in 2005 at IEEE meeting in San Francisco.

10

IEC standards are available from the International Electrotechnical Commission (http://www.iec.ch/). IEEE publications are available from the Institute of Electrical and Electronics Engineers Inc., 445 Hoes Lane, Piscataway, NJ 08854, USA (http://standards.ieee.org/). 12 The IEEE standards or products referred to in this clause are trademarks of the Institute of Electrical and Electronics Engineers, Inc. 11

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IEEE Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers

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