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IEEE Std C57.113™-2010 (Revision of

IEEE Std C57.113-1991)

IEEE Recommended Practice for Partial Discharge Measurement in Liquid-Filled Power Transformers and Shunt Reactors

Sponsor

Transformers Committee of the IEEE Power & Energy Society

Approved 17 June 2010

IEEE-SA Standards Board

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Abstract: Wideband measurement of the apparent charge of partial discharges (PDs) that may occur in liquid-filled power transformers and shunt reactors excited by ac test voltages between 40 Hz and 400 Hz are discussed. The major components of the PD measuring circuit including the calibrator are specified in compliance with IEC 60270. The PD test procedure is described and recommendations for the evaluation of PD test results are presented. Keywords: apparent charge, IEEE C57.113, partial discharge (PD), power transformer, shunt reactor, wideband PD measurement

•

The Institute of Electrical and Electronics Engineers, Inc. 3 Park Avenue, New York, NY 10016-5997, USA Copyright © 2010 by the Institute of Electrical and Electronics Engineers, Inc. All rights reserved. Published 20 August 2010. 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: ISBN 978-0-7381-6383-3 STD96098 Print: ISBN 978-0-7381-6384-0 STDPD96098 IEEE prohibits discrimination, harassment and bullying. For more information, visit http://www.ieee.org/web/aboutus/whatis/policies/p9-26.html. No part of this publication may be reproduced in any form, in an electronic retrieval system or otherwise, without the prior written permission of the publisher.

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Introduction

This introduction is not part of IEEE Std C57.113-2010, IEEE Recommended Practice for Partial Discharge Measurement in Liquid-Filled Power Transformers and Shunt Reactors.

The detection of partial discharges (PDs) was introduced for quality assurance tests of high-voltage (HV) apparatus at the beginning of 1960. Originally this technique was based on the measurement of radio interference voltages (RIV) in terms of microvolts (µV) as recommended by NEMA TR1-1974 [B112], NEMA 107-1964 [B113], and CISPR 16-1-1993 [B42].a This quantity, however, is weighted according to the acoustical noise impression of the human ear, which is not a measure of the PD activity in the insulation of HV apparatus. As a consequence, Technical Committee No. 42 of IEC decided to prepare a separate standard for PD measurements associated with the apparent charge, which was first published in 1968. Since that time, this technology is considered as an indispensable tool for an enhancement of the reliability of HV apparatus. IEEE Std C57.113-2010 covers the wideband method for apparent charge measurements in compliance with the third edition of IEC 60270, published in 2000.b

Notice to users

Laws and regulations

Users of these documents should consult all applicable laws and regulations. Compliance with the provisions of this standard does not imply compliance to any applicable regulatory requirements. Implementers of the standard are responsible for observing or referring to the applicable regulatory requirements. IEEE does not, by the publication of its standards, intend to urge action that is not in compliance with applicable laws, and these documents may not be construed as doing so.

Copyrights

This document is copyrighted by the IEEE. It is made available for a wide variety of both public and private uses. These include both use, by reference, in laws and regulations, and use in private self-regulation, standardization, and the promotion of engineering practices and methods. By making this document available for use and adoption by public authorities and private users, the IEEE does not waive any rights in copyright to this document.

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

iv Copyright © 2010 IEEE. All rights reserved.

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Updating of IEEE documents

Users of IEEE standards should be aware that these documents may be superseded at any time by the issuance of new editions or may be amended from time to time through the issuance of amendments, corrigenda, or errata. An official IEEE document at any point in time consists of the current edition of the document together with any amendments, corrigenda, or errata then in effect. In order to determine whether a given document is the current edition and whether it has been amended through the issuance of amendments, corrigenda, or errata, visit the IEEE Standards Association web site at http://ieeexplore.ieee.org/xpl/standards.jsp, or contact the IEEE at the address listed previously.

For more information about the IEEE Standards Association or the IEEE standards development process, visit the IEEE-SA web site at http://standards.ieee.org.

Errata

Errata, if any, for this and all other standards can be accessed at the following URL: http://standards.ieee.org/reading/ieee/updates/errata/index.html. Users are encouraged to check this URL for errata periodically.

Interpretations

Current interpretations can be accessed at the following URL: http://standards.ieee.org/reading/ieee/interp/ index.html.

Patents

Attention is called to the possibility that implementation of this recommended practice may require use of subject matter covered by patent rights. By publication of this recommended practice, no position is taken with respect to the existence or validity of any patent rights in connection therewith. The IEEE is not responsible for identifying Essential Patent Claims for which a license may be required, for conducting inquiries into the legal validity or scope of Patents Claims or determining whether any licensing terms or conditions provided in connection with submission of a Letter of Assurance, if any, or in any licensing agreements are reasonable or non-discriminatory. Users of this recommended practice are expressly advised that determination of the validity of any patent rights, and the risk of infringement of such rights, is entirely their own responsibility. Further information may be obtained from the IEEE Standards Association.

v Copyright © 2010 IEEE. All rights reserved.

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vi Copyright © 2010 IEEE. All rights reserved.

Participants

At the time this recommended practice was submitted to the IEEE-SA Standards Board for approval, the Dielectric Tests―TF on PD Measurement Working Group had the following membership:

Bertrand Poulin, Chair Raymond Bartnikas Alain Bolliger Carl Bush Alan Darwin Reto Fausch Marcel Fortin John Harley Peter Heinzig Thang Hochanh

Stephen Jordan Vladimir Khalin Eberhard Lemke Paul Millward Arthur Molden Martin Navarro Ron Nicholas Mark Perkins Gustav Preininger

Dirk Russwurm Hemchandra Shertukde Charles Sweetser Juan Luis Thierry Subash Tuli Dharam Vir Loren Wagenaar Hanxin Zhu Waldemar Ziomek

Most of the work in writing this document was done by the TF Chair Eberhard Lemke and TF Members Marcel Fortin, John Harley, Thang Hochanh, Stephen Jordan, Vladimir Khalin, Mark Perkins, Bertrand Poulin, and Loren Wagenaar.

The following members of the individual balloting committee voted on this recommended practice. Balloters may have voted for approval, disapproval, or abstention. Samuel H. Aguirre Stan Arnot Carlo Arpino Javier Arteaga Ali Al Awazi Martin Baur Barry Beaster Stephen Beattie W. J. Bill Bergman Steven Bezner Wallace Binder Thomas Blackburn Thomas Blair David Blew William Bloethe W. Boettger Paul Boman Harvey Bowles Jeffrey Britton Chris Brooks Kent Brown Carl Bush Donald Cash Yunxiang Chen Bill Chiu Craig Colopy Tommy Cooper Jerry Corkran John Crouse Alan Darwin

John Densley Dieter Dohnal Randall Dotson Donald Dunn Fred Elliott Gary Engmann Donald Fallon Rabiz Foda Joseph Foldi Bruce Forsyth Marcel Fortin Saurabh Ghosh Jalal Gohari Eduardo Gomez-Hennig Edwin Goodwin James Graham Randall Groves Bal Gupta Kenneth Hanus David Harris Robert Hartgrove Roger Hayes Peter Heinzig Gary Heuston James Huddleston III Wayne Johnson James Jones Stephen Jordan Lars Juhlin C. Kalra

Gael Kennedy Sheldon Kennedy Vladimir Khalin Joseph L. Koepfinger Jim Kulchisky Saumen Kundu John Lackey Chung-Yiu Lam Stephen Lambert Thomas La Rose Raymond Lings Maurice Linker Thomas Lundquist G. Luri Keith Malmedal J. Dennis Marlow John W. Matthews Joseph Melanson Gary Michel Daleep Mohla Daniel Mulkey Jerry Murphy Michael S. Newman Raymond Nicholas Miklos Orosz J. Patton Brian Penny Patrick Picher Paul Pillitteri Alvaro Portillo

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Bertrand Poulin Paulette Payne Powell Gustav Preininger Iulian Profir Pierre Riffon Michael Roberts Oleg Roizman Marnie Roussell Thomas Rozek Dinesh Sankarakurup

Daniel Sauer Gregg Sauer Bartien Sayogo Hyeong Sim Tarkeshwar Singh Charles Smith James Smith Jerry Smith John Stein

John Sullivan S. Thamilarasan James Thompson Subhash Tuli Joe Uchiyama John Vergis Jane Verner Loren Wagenaar Barry Ward Thomas Wier

When the IEEE-SA Standards Board approved this recommended practice on 17 June 2010, it had the following membership:

Robert M. Grow, Chair Richard H. Hulett, Vice Chair

Steve M. Mills, Past Chair Judith Gorman, Secretary

Karen Bartleson Victor Berman Ted Burse Clint Chaplin Andy Drozd Alexander Gelman Jim Hughes

Young Kyun Kim Joseph L. Koepfinger* John Kulick David J. Law Hung Ling Oleg Logvinov Ted Olsen

Ronald C. Petersen Thomas Prevost Jon Walter Rosdahl Sam Sciacca Mike Seavey Curtis Siller Don Wright

*Member Emeritus

Also included are the following nonvoting IEEE-SA Standards Board liaisons:

Satish K. Aggarwal, NRC Representative Richard DeBlasio, DOE Representative Michael Janezic, NIST Representative

Don Messina

IEEE Standards Program Manager, Document Development

Matthew J. Ceglia IEEE Standards Program Manager, Technical Program Development

vii Copyright © 2010 IEEE. All rights reserved.

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Contents

1. Overview .................................................................................................................................................... 1 1.1 Scope ................................................................................................................................................... 1 1.2 Purpose ................................................................................................................................................ 1

2. Normative references.................................................................................................................................. 2

3. Definitions .................................................................................................................................................. 2

4. Specification of PD measuring circuits ...................................................................................................... 3 4.1 General ................................................................................................................................................ 3 4.2 Coupling capacitor............................................................................................................................... 4 4.3 Measuring impedance.......................................................................................................................... 5 4.4 PD measuring instrument .................................................................................................................... 5 4.5 PD calibrator........................................................................................................................................ 7 4.6 Maintaining the specified parameters of PD measuring circuits ......................................................... 8

5. PD test procedure ....................................................................................................................................... 8 5.1 Calibration ........................................................................................................................................... 8 5.2 PD measurement.................................................................................................................................. 9

Annex A (informative) Design of PD coupling units ................................................................................... 12

Annex B (informative) Response of PD measuring instruments.................................................................. 17

Annex C (informative) Calibration of PD measuring circuits ...................................................................... 20

Annex D (informative) Basic sensitivity check............................................................................................ 21

Annex E (informative) Bushing tap ratio measurement ............................................................................... 23

Annex F (informative) Noise identification ................................................................................................. 24

Annex G (informative) PD pattern recognition ............................................................................................ 27

Annex H (informative) Bibliography ........................................................................................................... 29

viii Copyright © 2010 IEEE. All rights reserved.

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IEEE Recommended PracticeGuide for Partial Discharge Measurement in Liquid-Filled Power Transformers and Shunt Reactors

IMPORTANT NOTICE: This standard is not intended to ensure safety, security, health, or environmental protection. Implementers of the standard are responsible for determining appropriate safety, security, environmental, and health practices or regulatory requirements.

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

1.1Scope

This recommended practice describes the test procedure for the detection and measurement by the wideband apparent charge method of partial discharges (PDs) occurring in liquid-filled power transformers and shunt reactors during dielectric tests, where applicable. 1.2 Purpose

PD Partial discharge measurements in transformers and shunt reactors should preferably be made on the basis of measurement of the apparent charge. Relevant measuring systems are classified as narrowband or wideband systems. Both systems are recognized and widely used. Without giving preference to one or the other, it is the object of this document to describe the wideband method. General principles of PD measurements, including the narrowband method, are covered in IEEE Std 454-1973 [7]1, IEC 270 (1981) [2],60270 and IEC 60076-3 (1980) [1].[B71].1,2

1 Information on references can be found in Clause2. 2 The numbers in brackets correspond to those of the bibliography in AnnexH. 1The numbers in brackets correspond to those of the references in Section 3 2IEC publications are available from IEC Sales Department, Case Postale 131, 3 rue de Varembé, CH 1211, Genève 20, Switzerland/Suisse. IEC publications are also available in the United States from the Sales Department, American National Standards Institute, 11 West 42nd Street, 13th Floor, New York, NY 10036, USA.

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).

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For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments or corrigenda) applies. When the standards referred to in this guide are superseded by an approved revision, the latest revision shall apply.[1]

IEC 60270, High-voltage test techniques―Partial discharge measurements.3

[2] IEC 270 (1981), 76-3 (1980), Power transformers; Part 3: Insulation levels and dielectric tests.2

[3] IEEE Std 4™, IEEE Standard Techniques for High Voltage Testing.4 , 5

IEEE Std C57.12.00-1987, ™, IEEE Standard General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers (ANSI).3

IEEE Std C57.12.90-1987™, IEEE Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers (ANSI). Transformers; and Guide for Short-Circuit Testing of Distribution and Power Transformers [5] 4-1978, IEEE Standard Techniques for High Voltage Testing (ANSI).

IEEE Std C57.19.00™, [6] IEEE Std 21-1976 IEEE Standard General Requirements and Test Procedures for OutdoorPower Apparatus Bushings.(ANSI).

[7] IEEE Std 454-1973 (Withdrawn), IEEE Recommended Practice for the Detection and Measurement of Partial Discharges (Corona) During Dielectric Tests.4

3.Definitions

For the purposes of this document, the following terms and definitions apply. The IEEE Standards Dictionary: Glossary of Terms & Definitions should be referenced for terms not defined in this clause.6

partial discharge: An electric discharge that only partially bridges the insulation between conductors.

apparent charge (terminal charge), q: A charge that, if it could be injected instantaneously between the terminals of the test object, would momentarily change the voltage between its terminals by the same amount as the partial discharge itself. The apparent charge should not be confused with the charge transferred across the discharging cavity in the dielectric medium. Apparent charge, within the terms of this document, is expressed in coulombs, abbreviated C. One pC is equal to 10-12 C. apparent charge level: Mean value of the apparent charge of partial discharge (PD) pulse trains measured in terms of picocoulomb (pC) by means of PD instruments.

NOTE—As specified in 4.5.7

repetition rate, n: The partial discharge pulse repetition rate, n, is the average number of partial discharge pulses per second measured over a selected period of time.

acceptable terminal partial discharge level: The specified maximum terminal partial discharge value for which measured terminal partial discharge values exceeding said value are considered unacceptable. The method of measurement and the test voltage for a given test object should be specified with the acceptable terminal partial discharge level.

voltage related to partial discharges: The phase-to-ground alternating voltage whose value is expressed by its peak divided by 2 .

partial discharge-free test voltage: A test procedure, at which the test object should not exhibit partial discharges above the acceptable energized background noise level

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calibrating charge: Charge of artificial partial discharge (PD) pulses generated by PD calibrators.

NOTE—As specified in 4.6.

frequency response: Transfer impedance of the partial discharge (PD) measuring instrument versus the frequency characterized by the lower and upper limit frequency, f1 and f2.

NOTE—As specified voltage, applied in accordance with a4.5.

PD calibrating circuit: Interconnection of the partial discharge (PD) calibrator with the test object and the PD measuring circuit intended for the determination of the scale factor, Sf.

NOTE—As specified in 4.6.

energized background noise level: Stated in pC, the residual response of the partial discharge measurement system to background noise of any nature after the test circuit has been calibrated and the test object is energized at 50% of its nominal operating voltage. acceptable energized background noise level: Energized background noise level present during test that is considered acceptable.

bushing tap: Connection to a capacitor foil in a capacitively graded bushing designed for voltage or power factor measurement that also provides a convenient connecting point for partial discharge measurement. The tap-to-phase capacitance is generally designated as C1, and the tap-to-ground capacitance is designated as C2. See bushing potential tap, bushing test tap, and capacitance (of bushing) in IEEE Std 21-1976 [6].

3 IEC publications are available from the Central Office of the International Electrotechnical Commission, 3, rue de Varembé, P.O. Box 131, CH-1211, Geneva 20, Switzerland (http://www.iec.ch/). IEC publications are also available in the United States 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, 445 Hoes Lane, Piscataway, NJ 08854, USA (http://standards.ieee.org). 5 The IEEE standards or products referred to in Clause 2 are trademarks owned by the Institute of Electrical and Electronics Engineers, Incorporate d. 6 The IEEE Standards Dictionary: Glossary of Terms & Definitions is available at http://shop.ieee.org/. 7 Notes in text, tables, and figures of a standard are given for information only and do not contain requirements needed to implement this standard.

PD measuring circuit: Interconnection of the partial discharge (PD) measuring instrument with the measuring impedance and the coupling capacitor intended for measuring the apparent charge level.

pulse train response: Reading of the partial discharge (PD) measuring instrument versus the repetition frequency of injected calibrating pulses.

NOTE—As specified in4.6. specified apparent charge level: Apparent charge level permitted for the test object if subjected to the partial discharge (PD) test procedure and conditioning stated in IEEE Std C57.12.90 and EEE Std C57.19.00.

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4. Specification of PD measuring circuits

4.1 General To measure the apparent charge, the following major circuit components are required:

Coupling unit, which captures the PD signal from the terminals of the test object

Measuring instrument, which processes the captured PD pulses and evaluates the apparent charge level

Associated high-voltage (HV) and low-voltage (LV) leads and measuring cables, which connect the individual components

Generally the coupling unit contains a coupling capacitor, Ck, which is connected in series with measuring impedance,

Zm. If the test object is equipped with capacitive graded bushings, the capacitance between HV conductor and bushing

tap, C1, may substitute the coupling capacitor, Ck, as illustrated in Figure 1.

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Figure 1—PD measuring circuit using the bushing tap coupling mode

C0 – Calibrating capacitor

C1 – Capacitance between HV conductor and bushing tap

C2– Capacitance between bushing tap and grounded bushing flange

Ca – Virtual capacitance of the test object Dc – Coupling device

Fi – Noise rejection filter

Hv – Connection to the HV test supply Mc – Measuring cable

Mi – Measuring instrument

Pc – PD calibrator To – Test object

V0 – Step pulse generator

Zm – Measuring impedance The HV connection leads between the test object and coupling capacitor should be PD-free up to the highest applied ac test voltage level. The ground connection leads should be kept as short as possible in order to reduce the inductance and thus to minimize the impact of electromagnetic interferences disturbing sensitive PD measurements. Optional HV and LV filters may also be utilized to reduce the influence of environmental disturbances.

4.2 Coupling capacitor

The coupling capacitor, Ck, is intended for the decoupling of the high-frequency PD signal from the terminals of the

test object at low attenuation, due to the high-pass filter characteristics of this unit. Additionally the ac test voltage level appears extremely reduced at the output of Ck. This response is also achieved, if instead of the coupling

capacitor, Ck, the capacitance between HV conductor and bushing tap, C1, is utilized. To minimize the impact of

stray capacitances on PD test results, the capacitance of Ck and C1 should exceed 300 pF. Moreover, these units

should be PD-free up to the maximum applied ac test voltage level. 4.3 Measuring impedance

The measuring impedance, Zm, is intended for the conversion of PD current pulses into equivalent voltage pulses.

Using the classical coupling mode by means of a separate coupling capacitor, the measuring impedance, Zm, is

generally formed by a parallel connection of a resistor, Rm, with an inductor, Lm; see Annex A.

If the bushing tap coupling mode according to Figure 1 is used, the measuring impedance, Zm, consists of the parallel

connection of a resistor, Rm, and an inductor, Lm. Both elements are additionally shunted by the capacitance

between bushing tap and grounded bushing flange, C2; see Annex A.

Moreover, passive and active elements could be utilized for PD signal filtering and overvoltage protection. All these elements are usually integrated in a terminating box, referred to in IEC 60270 as a coupling device, Dc.

Due to the high-pass filter characteristics of the series connection of either Ck or C1 with the measuring impedance,

Zm, care should be taken that the specified lower limit frequency, f1, of the complete PD measuring circuit is not

substantially affected by the parameters of the PD coupling unit; see Annex A.

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WARNING

In order to minimize any danger for the operator and the instrumentation, as well as to ensure an optimum signal transmission, the coupling device should always be located inside the HV test area. The coupling device shall be attached physically as close as possible to the bushing tap or to the coupling capacitor.

4.4 PD measuring instrument

For measuring the apparent charge, either the analog or the digital signal processing can be utilized. Independent from the measuring principle applied, the instrumentation is generally equipped with the following major units:

Attenuator, to adjust the magnitude of the input PD pulses

Band-pass filter amplifier, to amplify and integrate the captured PD pulses

Peak detector, to evaluate the apparent charge level

To ensure comparable and reproducible PD test results, both the frequency response and the pulse train response of PD measuring instruments should be specified.

4.4.1 Frequency response

To measure the apparent charge level the captured PD current pulses are integrated. For this purpose usually a band-pass filter is utilized, characterized by the lower and upper limit frequency, f1 and f2, and the bandwidth, ∆f,

given by Equation (1):

∆f = f2 – f1 (1) To keep the integration error as low as possible, the PD measurement is performed in a frequency range where the amplitude-frequency spectrum of the PD pulses is nearly constant; see Annex B.

From a practical point of view the lower limit frequency, f1, should be located around 100 kHz. Lower values may

minimize the impact of attenuation of PD pulses propagating along transformer windings. However, this may also lead to serious disturbances, such as iron core related noises as well as harmonics from the ac test facility. To reduce the impact of interferences in the low-frequency range the high-pass filter characteristics should be such that the attenuation is about 40 dB for frequencies around 25 kHz and at least 60 dB for frequencies below 15 kHz.

To minimize the integration error, the upper limit frequency, f2, should be chosen around 300 kHz. To reduce

interferences of radio broadcast stations, the attenuation should exceed 20 dB for frequencies above 500 kHz and at

least 40 dB for frequencies above one MHz

From the specified frequencies, f1 and f2, it follows that the bandwidth, ∆f, has a value of 200 kHz. A wider

bandwidth would be useful for the localization of PD sites, but this may lead to an increasing measuring error, because the PD pulses may not be integrated as desired; see Annex B.

4.4.2 Pulse train response

To evaluate the apparent charge level of random distributed PD pulse trains, the pulse magnitudes should be averaged in compliance with IEC 60270. This is accomplished if the characteristic charging time constant, τ1, and

discharging time constant, τ2, of the peak detector, as part of the PD measuring instrument, satisfy the

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condition τ1 << τ2 < 440 ms.

NOTE 1—If the pulse train response differs significantly from those recommended in IEC 60270 the actual dependence of the reading, Ri, versus the pulse repetition rate, N, should be determined in order to judge the evaluation of the apparent charge

level.

NOTE 2—To eliminate stochastically appearing noise pulses at comparatively low repetition rate, for instance one pulse per cycle of the applied ac test voltage, some PD detectors are equipped with special features for noise suppression which may reject pulses having a repetition rate below 100 Hz. Care should be taken when using this instrumentation because PD pulses of high magnitude may not be recognized if they do not ignite in each half-cycle of the applied ac test voltage. To avoid such erroneous measurements, a visualization of the phase-resolved PD pulses is strongly recommended using a suitable display unit, such as a scope or a computer.

NOTE 3—The specified pulse train response is appropriate only for ac test voltages where the frequency may range between 40 Hz and 400 Hz. For dc test voltages or test voltages composed by ac and dc voltages, it is recommended to evaluate the number versus the magnitude of PD pulses.

4.4.3 Display unit

In addition to the measurement of the apparent charge level by means of analog or digital meters it is strongly recommended to display the phase-resolved PD patterns by means of a suitable display unit, such as an oscilloscope or a computer. This may assist not only the identification and classification of harmful PD defects but also the discrimination of disturbing electromagnetic interferences, which are often not phase-correlated.

4.4.4 Basic sensitivity

The basic sensitivity should be determined by means of calibrating pulses specified in 4.6, which are injected into the input of the measuring impedance connected to the PD measuring instrument via the associated measuring cable. A calibrating charge of 50 pC should cause a minimum deflection of 50% of the full reading of the indicating instrument or of the optional display unit.

4.4.5 Linearity

The linearity should be determined by means of calibrating pulses specified in 4.6, which should be injected in the measuring impedance connected via the associated measuring cable to the PD measuring instrument. The measuring sensitivity should be adjusted such that the full reading (100%) is obtained for an injected calibrating charge of 500 pC. After that the magnitude of the calibrating charge should be reduced stepwise by 100 pC. Under this condition the values indicated by the PD measuring instrument should not deviate by more than ±10% from the true magnitudes of the injected pulse charges.

4.5 PD calibrator

The PD calibrator is intended for the simulation of the charge transfer from the PD source to the terminals of the test object; see Annex C. To generate artificial PD pulses required for this purpose, the calibrator is generally equipped with a pulse generator connected in series with a calibrating capacitor, C0; see Figure 1. The pulse generator produces

fast rising step voltages of known magnitudes, V0. Therefore the calibrating charge is given by Equation (2):

q0 = V0 C0 (2) The PD calibrator should meet the requirements of IEC 60270. To adjust the desired magnitude of the calibrating charge, q0, the magnitude of the voltage step, V0, and the capacitance of the calibrating capacitor, C0, can

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be tuned accordingly. The calibrating charge should be adjustable between 50 pC and 1000 pC. These magnitudes should not differ by more than ±10% from the rated values.

To minimize the measuring error caused by non-controlled distortions of the pulse shape, care has to be taken that the conditions C0 < 200 pF and C0 < 0.1 Ca are satisfied. A simplified approach for the evaluation of Ca is

presented in Annex D. To avoid any superposition errors and thus to minimize the impact of the pulse train response on the reading of the PD measuring instrument, the repetition frequency of the calibrating pulses should be in the range of 100 Hz to 1000 Hz, inclusive.

The output impedance of the step pulse generator should not exceed 100 Ω. The rise time of the step pulse, which refers to the 10% and 90% values of the maximum pulse magnitude, should be less than 100 ns. After the peak is obtained the voltage magnitude should not differ more than ±5% from the mean value for a time span not shorter than 50 µs. The decay time, which refers to the 90% and 10% values of the pulse magnitude, should either be the same as the rise time, if bipolar calibrating pulses are created, or it should exceed 200 µs if pulses of only positive or negative polarity are created.

To display the calibrating pulses when the actual PD test under high voltage is running, the calibrating capacitor, C0,

which is usually designed only for low voltages, should be substituted by an HV calibrating capacitor, this should be PD-free up the maximum ac test voltage level. The measuring cable between the step pulse generator and the terminating box connected to the input of the HV calibrating capacitor should be matched with the characteristic cable impedance in order to avoid disturbing pulse reflections. Generally the HV calibrating capacitor should be located as close as possible to the HV terminal of the test object.

The objective of the calibration procedure is to determine the scale factor, Sf, which represents the ratio between the

calibrating charge, q0, injected between the terminals of the test object, and the reading, R0, of the PD measuring

instrument [see Equation (3)]:

Sf = q0 / R0 (3) To evaluate the apparent charge level, qa, under HV test conditions the reading, Ri, of the PD instrument is multiplied

by the scale factor, Sf [see Equation (4)]:

qa = Ri Sf = q0 R i / R0 (4)

This means the apparent charge level can be determined if the calibrating charge, q0, is multiplied by the reading,

Ri, due to PD events, and divided by the reading, R0, caused by the calibrating charge. 4.6 Maintaining the specified parameters of PD measuring circuits

To verify the specified technical parameters of the PD measuring circuits including the PD calibrator, performance checks should be performed at least once a year and after repair. The scale factor, Sf, of the PD measuring circuit

and the values of the pulse charges, q0, created by the PD calibrator shall be kept in a record of performance

established and maintained by the user. Additionally, type tests, routine tests, and performance tests should be performed in compliance with the recommendations of IEC 60270.

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5. PD test procedure

5.1 Calibration

5.1.1 PD measuring circuit

Before starting the first HV test, the complete PD measuring circuit according to Figure 1 should be calibrated to establish the scale factor, Sf; see 4.6. For calibration, all equipment should be set up exactly as used during the PD

test. If the test object is a three-phase transformer, the calibration should be performed at each terminal in turn, while making sure that the PD measuring instrument is always connected to each phase using either the bushing tap coupling mode or a separate coupling capacitor.

The calibrating pulses should be injected between the top of the HV bushing and the transformer tank, as evident from Figure 1. Generally, a portable battery powered PD calibrator should be used. If desired, a suitable pulse generator along with a terminating box connected to a suitable HV calibrating capacitor may also be used in order to display the calibrating pulses during the running HV test, and to adjust the magnitudes of the calibrating charge from the control room.

The connecting leads between the calibrator and test object should be kept as short as possible in order to avoid pulse distortions that may cause calibration errors. Therefore, the portable calibrator or the terminating box in connection with the HV calibrating capacitor should be placed as close as possible to the HV terminals of the test object. At least four separate calibrating charge levels should be injected to check the linearity of the PD measuring instrument. For a specified apparent charge level of qa = 500 pC, the calibration should be performed with the values

q01 = 100 pC, q02 = 200 pC, q03 = 500 pC, and q04 =1000 pC.

5.1.2 AC test voltage measuringcircuit

The ac test voltage measuring circuit should be calibrated in accordance with the requirements of IEEE Std 4 as well as in compliance with IEC 60060-1 [B69] and IEC 60060-2 [B70]. If an ac voltage measuring instrument is connected to the voltage output of the coupling device or to the bushing tap, either the capacitive divider ratio should be determined as reported in Annex E, or the complete ac measuring circuit should be calibrated using a reference measuring system.

5.2 PD measurement

5.2.1 PD test circuits

Figure 2 shows a circuit recommended for PD tests under induced voltage, i.e., the HV winding of the single-phase power transformer is excited through the LV winding. The LV test voltage source should be designed as specified in IEEE Std C57.12.00 and IEEE Std C57.12.90, which requires simulations of the actual operating configuration.

An optional LV filter may be required to reduce interferences coming from the ac power supply. The HV and LV connection leads should be kept as short as possible in order to minimize the inductance and thus to reduce the impact of electromagnetic noises.

The coupling device, which is generally equipped with the measuring impedance and additional elements for signal filtering and over-voltage protection as well as with the LV arm of the voltage divider, should be placed as close as possible to the bushing tap. The signal outputs of the coupling device for the PD pulses and the ac test voltage are

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connected via measuring cables to the PD measuring instrument and to an ac voltmeter.

Figure 2—PD measuring circuit for power transformers using induced test voltage

In addition to the reading instrument of the PD measuring instrument a display unit, such as a scope or a computerized PD measuring system should be utilized, which may be useful not only for the identification and classification of harmful PD sources (CIGRE TF 15.11/33.03.02 [B34], CIGRE WG 21-03 [B40], CIGRE WG D1.33 [B41], Fuhr [B52], Fuhr et al. [B53], Fryxell et al. [B54]) but also for the discrimination of disturbing noises in the surroundings.

Figure 3 shows a test circuit recommended for PD tests of power transformers and shunt reactors excited by a step-up transformer. If an appropriately rated step-up transformer is not available, a series resonant test circuit can also be used where the inductance of the test object is compensated by a capacitor bank. An optional HV filter may be required on the HV side in order to minimize the impact of disturbing interferences coming from the step-up transformer. Additionally a filter may be helpful for noise rejection if positioned on the LV side of the step-up transformer.

5.2.2 Test procedure

The PD test procedure and the ac test voltage level applied should be as specified in IEEE Std C57.12.00, IEEE Std C57.12.90, and IEEE Std C57.19.00. For the actual PD test, the ac test voltage level should first be raised up to 50% of the rated voltage of the test object. The energized background noise should then be evaluated in terms of pC and recorded for each phase for a time interval of 60 s, where the average energized background noise level should not exceed a relative value of 50% of the specified apparent charge level. The ac test voltage is then raised up to the one-hour test value and held constant long enough to verify if there are any PD problems. The ac test voltage is then raised to the enhancement level and held constant for 7200 cycles. The voltage is next reduced directly back to the one-hour test level and held constant for 60 min or even more if desired. During the 60 min period for each bushing terminal, the apparent charge level should be evaluated for a recording time of 60 s, which should be repeated at subsequent intervals of 5 min.

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Figure 3—PD measuring circuit for shunt reactors energized by a step-up ransformer

5.2.3 Interpretation of PD test results The PD test results should be considered acceptable and no further PD tests are required under the following conditions:

a) The apparent charge level, qa, measured during the one-hour test does not exceed the specified apparent

charge level, qs, stated in IEEE Std C57.12.90 and IEEE Std C57.19.00.

b) The increase in the apparent charge level, qa, during the one-hour test does not exceed a value of

Δqs, as stated in IEEE Std C57.12.00, IEEE Std C57.12.90, and IEEE Std C57.19.00.

c) The apparent charge level, qa, during the one-our test period does not exhibit any steadily rising trend, and

no sudden sustained increase in the level occurs during the last 20 min of the test.

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

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9. Partial Discharge Measurement During Induced Voltage Tests

Partial discharge measurements are normally performed during the induced voltage tests. The duration of the test, the time sequence for the application of test voltage, connection and grounding of windings, and the test voltage values should be as specified in IEEE C57.12.90-1987 [4] and IEEE C57.12.00-1987 [3]. Further information on partial discharge measurement may be found in IEEE Std 454-1973 [7].

9.1 Test Procedure

The voltage should first be raised to 50% of the rated voltage value of the test object, and the energized background noise should then be measured and recorded on each measured terminal. The acceptable energized background noise level should not exceed 50% of the acceptable terminal partial discharge level and, in any case, should be below 100 pC. The voltage is then raised to the one-hour test level and held there long enough to verify that there are no partial discharge problems. The voltage is then raised to the enhancement level and held for 7200 cycles. The voltage is next reduced directly back to the one-hour test level and held for 60 min or more.

During the 60 min period, partial discharge measurements should be made at 5 min intervals on each terminal of 115 kV class and above In terms of interpretation of partial discharge measurements, the results should be considered acceptable and no further partial discharge tests required if

1) The magnitude of the PD level does not exceed the acceptable terminal partial discharge level. 2) The increase in PD levels during the 60 min period does not exceed 39% of the acceptable terminal partial

discharge level. 3) The PD levels during the 60 min period do not exhibit any steadily rising trend and there is no sudden,

sustained increase in levels during the last 20 min of the tests.

If the PD level rises above the specified limit for a significant time and then returns below this level again, the test may continue without interruption until acceptable readings have been obtained for 60 min. Occasional high readings should be disregarded. As long as no breakdown occurs, and unless very high levels of partial discharge are sustained for a long period of time, the test is regarded as nondestructive. A failure to meet the partial discharge acceptance criteria should not therefore warrant immediate rejection, but should lead to consultation between purchaser and manufacturer about further investigations.

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Annex A (informative) Design of PD coupling units A.1 Conventional coupling mode

PD measuring circuits in compliance with IEC 60270 are generally equipped with a separate coupling capacitor, Ck,

which is connected in series with measuring impedance, Zm; see Figure A.1. For wideband measurement of the

apparent charge the lower limit frequency, f1, should be chosen significantly above 50 kHz in order to minimize

the impact of low-frequency noises, such as harmonics excited by the ac test supply as well as iron core related noises. Because the PD coupling circuit unit illustrated schematically in Figure A.1 represents a high-pass filter, it seems obvious to tune the desired lower limit frequency, f1, by the relevant circuit parameters Ck, Rm, and Lm.

Figure A.1—Conventional coupling unit using a separate coupling capacitor

Ca – Virtual test object capacitor Ck – Coupling capacitor Lm – Shunt inductor Rm – Measuring resistor To – Test object Zm – Measuring impedance

For better understanding a practical example will be considered based on the following parameters:

Lower limit frequency: f1 = 70 kHz

Capacitance of the HV coupling capacitor: Ck = 1 nF The lower limit frequency of the circuit according to Figure A.1 is given by Equation (A.1):

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f1 = 1 / (2π × Ck × Rm) (A.1)

Therefore the value in Equation (A.2) is required for the measuring resistor:

Rm = 1 / (2π × Ck × f1) = 2.27 kΩ (A.2) Under practical conditions, however, the ac test voltage applied may cause an HV magnitude across Rm, which

may become harmful not only for the PD measuring equipment but also for the operator. If, for instance, an induced test is performed using a frequency of fac = 400 Hz, the capacitive impedance of the coupling capacitor, Ck,

is given by Equation (A.3):

Zc = 1 / (2π × Ck × fac) = 1 / (2π × 1 nF × 400 Hz) = 398 kΩ (A.3) Based on Equation (A.2) and Equation (A.3) the divider ratio is Dr = 2.27 kΩ / 398 kΩ = 1 / 174. That means, an

assumed ac test voltage magnitude of 800 kV would cause a peak voltage across Rm of almost 7000 V. This value

can effectively be reduced if the resistor Rm is shunted by an inductor Lm; see Figure A.1. In this context care has been taken that the lower cut-off frequency, f1, of the PD measuring circuit is not decreased substantially. This

requirement is accomplished by the condition shown in Equation (A.4):

Lm > 10 × Rm / (2π × 70 kHz) = 53 mH (A.4)

From the previous example, where a test frequency of fac= 400 Hz was assumed, the inductive impedance becomes,

as shown in Equation (A.5):

Zl = 2π × 400 Hz × Lm = 130 Ω (A.5) Therefore, the divider ratio is given by Dr =130 Ω / 398 kΩ = 1 / 3060. That means that an ac test voltage level of

800 kV is attenuated from a value of originally 7000 V down to about 400 V, which can well be accepted. To record the PD pulses in a phase-resolved manner using a display unit, such as a scope or a computer- based PD measuring system, the PD coupling unit can be configured accordingly, as illustrated in Figure A.2. Here the LV arm of the capacitive divider is represented by a measuring capacitor designated as Cm. Due to the very

different frequency spectra of the PD pulses and the ac test voltage, both signals appear completely separated at the both outputs: PD and ac. The resulting impedance of the parallel connection of the circuit elements Rm and Lm is

mainly governed by the inductive impedance Zl = 130 Ω; see Equation (A.5). Consequently the impact of this circuit

on the phase shifting of the ac test voltage can be neglected due to the much higher impedance of Ck which is

Zc = 398 kΩ; see Equation (A.3). Therefore, the desired divider ratio can simply be approximated by Dr = Ck /

Cm. For an assumed voltage divider ratio of Dr = 1 / 10 000 follows, for instance, a value for the measuring capacitor Of:

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Figure A.2—PD coupling device equipped with a measuring capacitor for displaying the ac test

voltage

AC – Output for ac test voltage Ca – Virtual test object capacitor Ck – Coupling capacitor Cm – Measuring capacitor Dc – Coupling device GD – Ground termination Lm — Shunt inductor PD – Output for PD pulses

A.2 Bushing tap coupling mode

If instead of a separate coupling capacitor, Ck, the capacitance between the HV conductor and bushing tap, C1, is

utilized for capturing the PD transients, it has to be taken into consideration that the capacitance between bushing tap and grounded bushing flange, C2, may affect the frequency response, as will be discussed more in detail in the

following. The main circuit elements of the bushing tap coupling mode are llustrated in Figure A.3. For the following considerations, first the impact of the shunt inductor, Lm, shall be neglected. Therefore the lower limit frequency of

this network can be expressed by Equation (A.7):

f1 = 1 / (2π Rm (C1 + C2)) (A.7) The value of C1 is given by the bushing utilized for the decoupling of the PD signal. This parameter can therefore

not be varied. Consequently it can only be adjusted by varying the measuring resistor, Rm, as well as by an

additional capacitor connected parallel to C2. For better understanding consider the following practical example:

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Desired lower limit frequency: f1 = 70 kHz

Capacitance between HV conductor and bushing tap: C1= 500 pF

Capacitance between busing tap and grounded bushing flange: C2= 2500 pF

Figure A.3—Equivalent circuit of the bushing tap coupling mode

Bu – Bushing

Bt – Bushing tap

C1 – Capacitance between HV conductor and bushing tap C2 – Capacitance between bushing tap and grounded bushing flange

Ca – Virtual test object capacitance

Lm – Shunt inductor Rm – Measuring resistor

To – Test object

Zm – Measuring impedance

Inserting these values in Equation (A.7) we get Rm ≈ 760 Ω. To avoid a reduction of f1 by the frequency dependent

impedance of, Lm, the condition Zl > 10 Rm at f1 = 70 kHz should be satisfied, which is fulfilled for Lm > 17 mH.

To estimate the portion of the ac test voltage magnitude appearing across the shunt inductor, Lm, and thus across the

measuring impedance, Zm, a maximum test frequency of fac= 400 Hz should be assumed. Under this condition the

frequency dependent impedances of Lm and C1 are given by Zl ≈ 43 Ω and Zc ≈ 795 kΩ, respectively.

Consequently the divider ratio is given by Dr = 43 Ω / 795 kΩ ≈ 1 / 18 000. That means a maximum ac test

voltage level of, for instance, 800 kV is attenuated down to about 45 V, which is harmless for the instrumentation and the operator as well.

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The bushing tap coupling mode also provides a convenient tool for displaying the ac test voltage, as illustrated in Figure A.4. For C1 = 500 pF the measuring capacitance should be Cm = 5 µF, if a voltage divider ratio of Dr = 1 / 10

000 is desired.

Figure A.4—Bushing tap coupling mode capable for displaying the ac test Voltage

Bu – Bushing

Bt – Bushing tap C1 – Capacitance between HV conductor and bushing tap

C2 – Capacitance between bushing tap and grounded bushing flange

Ca – Virtual test object capacitance Cm – Measuring capacitor

GD – Ground connection Lm – Shunt inductor

Rm – Measuring resistor

To – Test object Zm – Measuring impedance

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Annex A (Informative)

(This appendix is not part of IEEE C57.113-1991, IEEE Guide for Partial Discharge Measurement in Liquid-Filled Power Transformers and Shunt Reactors, but is included for information only.)

A1 Partial Discharge Recognition

One of the greatest advantages of the wide-band method is the ease with which the results can be displayed on a cathode ray oscilloscope, which means that the PD signal can be observed in terms of the phase of the applied test voltage. This is of great help in determining whether or not the discharges originate inside the test object. The pulse polarity can also be identified, and pulses may be counted and sorted according to their amplitude or polarity, or both. Digital processing of PD signals by computer is also possible.

Examples of the most common oscillographic patterns encountered during partial discharge tests on large transformers appear in Fig A1 (see also [B95] 6 and [B62] ). Fig A1(a) represents the case of air corona on the high-voltage electrode. Fig A1(b) is for air corona on a point on the ground side. Such corona can usually be eliminated by selecting a high-voltage electrode of larger diameter for case (a) and by covering protrusions on and around the transformer with rounded metallic shields or semiconductive material, such as rubber, for case (b). These corona discharges are usually very large, but it should be pointed out that they appear only during one half-cycle of the applied voltage. Small discharges are present on the other half-cycle but are so low in amplitude that they usually cannot be observed.

The case shown in Fig A1(c) occurs when ungrounded metallic objects are present on or near the transformer under test. The obvious solution in this case is to remove as many of the loose objects from the test area as possible and ground the rest, especially metallic fences.

The case shown in Fig A1(d) is the result of a bad ohmic contact, usually inside the transformer, although it could also be from the connections outside. Note that, in this case, the discharges occur on both sides of and at the zero-crossings of the test voltage.

Figs A1(e) and A1(f) represent PDs occurring within the insulation structure of a transformer. They are usually present on the increasing voltage slope of both half-cycles and do not normally cross the voltage peaks; although they may extend down to zero-crossings. There is usually a fair amount of hysteresis present, but excessive hysteresis and rapidly decreasing inception voltage are indicative of PDs in gas bubbles. Fig A1(e) represents PDs in oil-paper insulation or in gas bubbles. Fig A1(f) represents creeping discharges, which are usually higher in amplitude but less numerous than those in case (e).

Figs A1(g) and A1(h) represent two cases of external interference. The first is typical of thyristor interference, the pulses being equally spaced and of roughly the same amplitude. Since the test voltage frequency for transformers is usually different from the power frequency, the pulses are not synchronized. The number of pulses appearing during one cycle of the test voltage depends on the ratio of its frequency to that of the power system and on the particular design of the equipment producing the interference. Usually a range from two to six pulses is seen, even though fewer than two pulses may be present at every cycle. This is due to the fact that the eye tends to see many superposed cycles at the same time.

Fig A1(h) is typical of a periodic signal with a frequency falling inside the bandwidth of the PD detector. One such source of interference in North America is the navigational system, LORAN C, operating at 100 kHz. Other than the fact that they are not usually synchronized to the test voltage, interference signals are not usually dependent on the test- voltage level and do not normally disappear when the test voltage is lowered as PD signals do. In normal situations, these characteristics suffice to identify the signals as interference.

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Annex B (informative) Response of PD measuring instruments

B.1 Frequency response

Due to the high-pass filter characteristics of the PD coupling unit, see Annex A, the voltage jump appearing across the terminals of the test object as a result of a PD event is differentiated. Therefore, this signal should be integrated again in order to evaluate the apparent charge of the captured PD pulses. The integration can be performed at sufficient accuracy if the measuring frequency is chosen below 500 kHz, where the amplitude frequency spectrum of the PD pulses is nearly constant (Schon [B132], [B133], [B134]); see Figure B.1. Under this condition the output signal of the PD measuring instrument is a pulse whose magnitude is a measure of the charge of the input pulse. It should be noted that the duration of the output pulse to be evaluated is much longer than those of the input PD pulse.

Figure B.1—Selection of the band-pass filter characteristics for PD instruments A – Amplitude-frequency spectrum of PD pulses

B – Band-pass filter characteristics of the PD measuring instrument B.2 Pulse trainresponse PD events occurring under ac test voltages are characterized by pulse sequences whose magnitudes may randomly be distributed over an extremely wide range. An example is illustrated in Figure B.2, which reveals that the magnitudes of the apparent charge pulses scatter between about 100 pC and 3000 pC.

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Figure B.2—PD signatures of surface discharges in a power transformer displayed for one cycle (a and b) and for 3000 s (c and d) of the applied ac test voltage To ensure comparable and well-reproducible PD test results, it seems therefore feasible to average the randomly distributed PD pulse magnitudes. As a consequence, in IEC 60270 the evaluation of the “largest repeatedly occurring PD magnitude” is recommended. This PD quantity is equivalent to the “apparent charge level” as defined in this recommended practice; see specified apparent charge level in Clause 3.

As can be seen from Figure B.3, after the maximum PD magnitudes are averaged, the apparent charge level can well be quantified by a value of approximately 2800 pC. This approach is based on a specified pulse train response of the PD measuring instrument shown in Figure B.4, where the tolerance band is well fitted for the condition τ1 << τ2 <

440 ms. Here is τ1 the charging time constant and τ2 the discharging time constant of the peak detector as part of the

PD measuring instrument. It should be mentioned that CISPR 16-1-1993 [B42] also recommends an averaging of random distributed noise pulses where the characteristic time constants are specified by τ1 < 1 ms and τ2 < 160 ms.

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a) Maximum magnitudes of subsequent PD pulse sequences

Figure B.4—Maximum and minimum reading, Rmax and Rmin, of PD instruments versus the pulse

repetition rate, N, recommended in IEC 60270

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Annex C (informative) Calibration of PD measuring circuits

The objective of the calibration is to determine the scale factor, Sf, required for evaluating the apparent charge

level, qa, from the reading of the PD measuring instrument, Ri, as shown in Equation (C.1):

qa = Ri Sf (C.1) This

procedure is in principle based on a simulation of the charge transfer from the PD source to the terminals of the test object. For this purpose artificial PD pulses are injected between the terminals of the test object by means of a calibrator (Lemke [B92], Lemke et al. [B94]); see Figure 1. The scale factor, Sf, is

determined from the ratio between the calibrating charge, q0, and the reading of the PD measuring

instrument, R0. Therefore, Sf is expressed either in terms of pC/scale of the reading instrument or in terms of pC/div

of the display unit.

For better understanding, consider the following practical example:

Calibrating charge injected between the terminals of the test object: q0 = 200 pC Reading of the PD instrument: R0 = 50

scales Resulting scale factor: Sf = q0 / R0 = 4 pC /scale Reading of the PD instrument during the actual PD test: Ri = 20 scales

Based on these values, the apparent charge can be calculated as shown in Equation (C.2):

qa = Ri Sf = (20 scales) (4 pC /scale) = 80 pC (C.2) It should

be noted that advanced computerized PD measuring instruments are equipped with a feature to scale the PD data in terms of pC automatically after the calibration procedure has been performed.

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Annex D (informative) Basic sensitivity check

To check the basic sensitivity of the PD measuring instrument, the circuit shown in Figure D.1 could be utilized. Here the virtual test object capacitance, Ca, is simulated by an LV capacitor having a value of

10 000 pF. Furthermore, the capacitance between the HV conductor and the bushing tap, C1, and the capacitance between the bushing tap and the grounded bushing flange, C2, are simulated by an LV capacitors

having values of, for instance, C1 = 500 pF and C2 = 2500 pF, respectively. The measuring impedance, Zm, of the

PD measuring circuit is connected to the junction of C1 and C2, which represents the bushing tap coupling mode.

The sensitivity should be such that when a calibrating charge of 25 pC is injected into Ca the reading of the PD

measuring instrument exceeds at least twice the internal amplifier noise level. This basic sensitivity check needs only to be performed when installing a new PD measuring system and at specified time intervals, i.e., after each year and after repair or modification of the components of the PD measuring circuit.

The circuit illustrated in Figure D.1 could also be utilized for an estimation of the virtual test object capacitance, Ca, if

the internal capacitance of the calibrator, C0, is known. This can be done by the following three steps:

1. With all the equipment configured exactly as it shall be during the PD test, a calibrating charge of

approximately 500 pC should be injected into the terminals of the test object, which is represented in Figure D.1 by Ca. The reading, R1, of the PD measuring instrument should be noted.

2. After connecting an additional capacitor, Cp, of 1000 pF in parallel to the output of the calibrator, the

appearing reading, R2, should also be noted.

3. A reading ratio of R2 / R1 > 0.8 indicates that Ca is fairly high, and the procedure should be repeated

with the parallel capacitance, Cp, increased 10 times, i.e., from originally 1000 pF up to

10 000 pF.

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The value of the virtual test object capacitance, Ca, may then be assessed applying the approximation shown in

Equation (D.1): Ca = ((Cp R2) / (R1 – R2)) – C0 (D.1)

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Annex E (informative) Bushing tap ratio measurement

To display phase-resolved PD patterns, a low ac voltage component should be available having the same phase angle as the applied ac test voltage in order to synchronize the display unit. The most convenient way is to capture the ac signal from the bushing tap. To evaluate the appearing output voltage magnitude, the ratio of the capacitive divider formed by the capacitance between the HV conductor of the bushing and the bushing tap, C1, and the capacitance

between the bushing tap and grounded bushing flange, C2, should be known. From this it can be determined

whether the maximum ac voltage magnitude appearing at the bushing tap can be accepted for the measuring purpose or if it should be reduced by means of an additional measuring capacitor, C3, connected in parallel to C2.

The capacitive divider ratio of the bushing may also be measured directly for the applied ac test voltage by means of a suitable ratio bridge. The value usually ranges between 1:10 000 and 1:50 000. As an option, an appropriate device specified in IEEE Std 4 for alternating voltage measurement can also be connected directly to the HV bushing terminal. For this case the bushing tap output voltage should be measured by means of an ac voltmeter having an input impedance of more than 1 MΩ. After energizing the transformer up to the desired ac test voltage level, the output voltage is measured. The bushing tap ratio can then be calculated by dividing the magnitude of the low voltage through the magnitude of the applied ac test voltage.

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Annex F (informative) Noise identification

During PD tests of power transformers and shunt reactors, excessive electromagnetic noises may be encountered, such as pulse-shaped noises due to external PDs and surges of switchgears and power electronics as well as continuous radio frequency noises radiated from broadcast stations. In this context it should be noted that one of the greatest advantages of the wideband method is the ease with which the electromagnetic disturbances can be displayed and thus be discriminated from the PD signal. Characteristic noise signatures are displayed in Figure F.1.

Figure F.1(a) refers to corona discharges ignited from protrusions on the surface of HV electrodes in ambient air. Such discharges can easily be identified because after exceeding the inception voltage they appear only in one half-cycle of the applied ac test voltage. If such discharges ignite on HV electrodes they can either be eliminated by cleaning of the electrode surface or by increasing the electrode curvature radius. To avoid disturbing PD events from grounded electrodes, the curvature radius should also be enlarged, which could be done by additional metallic shields or by covering the critical protrusions with semiconductive material, such as rubber.

The noise signatures shown in the left record of Figure F.1(b) may occur when ungrounded (floating) metallic objects are present and thus charged and discharged via the stray capacitance coupled to the HV electrode of the test object. Typically, regular pulse sequences may appear close to the zero-crossing of the est voltage where the pulse magnitudes in both half-cycles are often well comparable. The obvious solution in this case is to remove as many loose objects as possible from the surroundings of the test object and grounding the rest, especially metallic fences. The right record shown in Figure F.1(b) refers to a faulty metallic contact of a connection lead, which may happen outside or inside the test object due to corrosion. In most cases, the appearance of such discharges is comparable to those caused by floating electrodes; but the pulse repetition rate may become significantly higher.

Signatures of external interference due to electromagnetic transients in the power network are shown in the left record of Figure F.1(c). The regular pulses evident from the left figure are caused by power electronics, due to a 6-pulse thyristor switch of an ac/dc converter. The disturbing pulses can easily be identified because they are equally spaced and roughly of the same amplitude. If the test voltage frequency is different from the power frequency, the pulses appear running because they are not synchronized to the ac test voltage. In practice, two to six pulses can be observed during one ac test voltage cycle, even though fewer than two pulses may occur. This is due to the fact that the eye tends to see many superimposed cycles at the same time. An example of external interferences due to permanent occurring radio noises are exemplarily displayed in the right record of Figure F.1(c). To avoid such disturbances electromagnetically shielded test areas are widely used. In addition, well-grounded metallic grid structures could be helpful. The grounding impedance in the high-frequency range could be reduced effectively by means of copper foil.

In some cases, however, the noise level cannot be rejected completely, as illustrated in Figure F.1(d). One kind of noise remained even if after the grounding condition of a power transformer under test was substantially improved. The records reveal that the noisy pulses are phase-correlated to the applied ac test voltage, which is also typical for real PD events. Finally, a xenon lamp installed nearby the test area was identified as the noise source.

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For the identification and localization of electromagnetic noises, the following general rules may be helpful:

1. External noises appear often independent from the applied ac test voltage level and thus do not disappear

if the test voltage level is lowered, as the PD events do.

2. Pulse-shaped noises may appear unsynchronized with the applied ac test voltage, whereas PD pulses occur always phase-correlated.

In this context it should be noted that advanced computer-based PD measuring systems are equipped with powerful features for the recognition and rejection of disturbing noises.

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Annex G (informative) PD pattern recognition

Characteristic signatures of PDs in liquid-filled power transformers and shunt reactors have been published in many papers, for instance in (CIGRE TF 15.11/33/03/02 [B34], CIGRE WG 21-03 [B39], CIGRE 21-03 [B40], Fuhr [B52], Fuhr et al. [B53]). To emphasize the very complex nature of PDs two characteristic measuring examples are presented in the following.

Figure G.1(a) refers to creeping discharges in a power transformer, which reveals that in the positive half- cycle the characteristic PD pulse sequences appear at rising ac test voltage and disappear shortly after the positive peak value is achieved. In the negative half-cycle the PD pulses ignite at falling ac test voltage and disappear after the negative peak value is reached. Moreover, the PD events ignite simultaneously in both half-cycles after the PD inception voltage is achieved. This phenomenon is different from corona discharges in air because the positive discharges appear at a test voltage level significantly above the inception voltage of the negative Trichel discharges; see Figure F.1(a). Furthermore, the pulse magnitudes scatter over an extremely wide range and differ substantially for both half-cycles.

Another measuring example is illustrated in Figure G.1(b) which refers to discharges in gas bubbles in transformer oil. The PD signatures are very different from those of surface discharges. So the PD inception and extinction voltages are characterized by an excessive hysteresis, and the PD extinction voltage may substantially decrease after longer stressing time. Moreover, the PD events may suddenly disappear even if the ac test voltage remains constant. Finally the PD events may ignite thereafter again. Such very complex PD phenomena have to be taken into account for assessment of the insulation condition of liquid-filled power transformers and shunt reactors.

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IEEE Std C57.113-2010 IEEE Recommended Practice for Partial Discharge Measurement in

Liquid-Filled Power Transformers and Shunt Reactors

Annex H

(informative)

Bibliography

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[B2] ANSI C63.2-1987, American National Standard Specifications for Electromagnetic Noise and Field-Strength Meters, 10 Hz to 40 GHz. 8

[B3] ANSI C68.3-1976, American National Standard Recommended Practice for the Detection and Measurement of Partial Discharges (Corona) During Dielectric Tests.

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8 ANSI publications are available from the Customer Service Department, American National Standards Institute, 25 W. 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/).

29 Copyright © 2010 IEEE. All rights reserved.

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[B24] Boyles, C. R., and Hinton, R. A., “Seven Years of Corona Testing,” Paper no. 70 CP120-PWR, IEEE Winter Power Meeting, New York, NY, Jan. 25–30, 1970.

[B25] Brand, U., and Muhr, M., “New Investigations on the Measurement of Partial Discharge (PD) and Radio Interference Voltage (RIV) on High-Voltage Engineering,” Paper no. 63.13, Fourth International Symposium on High Voltage Engineering, Athens, Greece, Sept. 5–9, 1983.

[B26] Brown, R. D., “Corona Measurement on High-Voltage Apparatus Using the Bushing Capacitance Tap,” IEEE Transactions on Power Apparatus and Systems, vol. PAS-84, pp. 667–671, Aug. 1965.

[B27] Carter, W. J., “Practical Aspects of Apparent Charge Partial Discharge Measurements,” IEEE Transactions on Power Apparatus and Systems, vol. PAS-101, no. 7, pp. 1985–1989, July 1982.

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[B29] Channakeshava, Gururaj, B. I., and Jayaram, B. N., “Studies on Partial Discharge Measurement in Transformer Windings,” CIGRE paper 12-09, Sept. 1–9, 1982.

[B30] Cesari, S., and Yakov, S., “Partial Discharge Inception Tests on Oil Immersed Insulation Structures,” Paper 22.09, Fourth International Symposium on High-Voltage Engineering, Athens, Greece, Sept. 5–9, 1983.

[B31] CIGRE JWG 15.01.04, “Characterization of Partial Discharges in Transformer Insulation,” Paper 15.01.04, Session Paris, Aug. 2000.

[B32] CIGRE JWG 15/21/33-20, “Progress on High-Voltage Monitoring Systems for In-service Power Apparatus,” Session Paris, Aug. 1996.

[B33] CIGRE SC 12, “Measurement of Partial Discharges in Transformers,” Electra, no. 19, pp. 13–65, Nov. 1971.

[B34] CIGRE TF 15.11/33.03.02, “Knowledge Rules for Partial Discharge Diagnosis in Service,” Electra Brochure 226, 2003.

[B35] CIGRE TF 33.03.05, “Calibration Procedures for Analog and Digital Partial Discharge Measuring Instruments,” Electra, no. 180, pp. 123-124, Oct. 1998.

[B36] CIGRE WG 03, “Elimination of Interference in Discharge Detection,” Electra, no. 21, pp. 55–72.


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[B37] CIGRE WG 12-01, “General Report of Group 12-01, Electra, no. 37, pp. 64–74, Dec. 1974.

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[B41] CIGRE WG D1.33, “Guide for Electrical Partial Discharge Measurements in compliance to IEC 60270,” Technical Brochure 366, Electra, vol. 60, no. 241, Dec. 2008.

[B42] CISPR 16-1-1993, Comité International Spécial des Perturbation Radioélectrique.10

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[B53] Fuhr, J., et al., “Detection and Localization of Internal Defects in the Insulation of Power Transformers,” IEEE Transaction on Electrical Insulation, vol. 28, no. 6, 1993.

[B54] Fryxell, J., et al., “Performance of Partial Discharge Tests on Power Transformers,” CIGRE paper 12-04, June 10–20, 1968.

[B55] Gailhofer, G., Kury, H., and Rabus, W., “Partial Discharge Measurements on Power Transformer Insulation, Principles and Practice,” CIGRE paper 12-15 June, 10–20, 1968.

[B56] Gänger, B., and Vorwerk, H. J. “Ionization Measurements on Transformers,” The Brown Boveri Review, vol. 54, no. 7, pp. 355–367, July 1967.

10 CISPR documents are available from the Central Office of the International Electrotechnical Commission, 3, rue de Varembé, P.O. Box 131, CH-1211, Geneva 20, Switzerland (http://www.iec.ch/). They are also available in the United States from the Sales Department, American National Standards Institute, 25 West 43rd Street, 4th Floor, New York, NY 10036, USA (http://www.ansi.org/).


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[B69] IEC 60060-1, High-voltage test techniques―Part 1: General definitions and test requirements.11

[B70] IEC 60060-2, High voltage test techniques―Part 2: Measuring systems.

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11 IEC publications are available from the Central Office of the International Electrotechnical Commission, 3, rue de Varembé, P.O. Box 131, CH-1211, Geneva 20, Switzerland (http://www.iec.ch/). IEC publications are also available in the United States from the Sales Department, American National Standards Institute, 25 West 43rd Street, 4th Floor, New York, NY 10036, USA (http://www.ansi.org/). 12 IEEE publications are available from the Institute of Electrical and Electronics Engineers, 445 Hoes Lane, Piscataway, NJ 08854, USA (http://standards.ieee.org).


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[B112] NEMA TR1-1993 (R2000), Transformers, Regulators, and Reactors.13

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13 NEMA publications are available from Global Engineering Documents, 15 Inverness Way East, Englewood, Colorado 80112, USA (http://global.ihs.com/).


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