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Partial Discharge Detection and Localization in High - IJAERD

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Description

IEEE Std C57

127-2000

IEEE Trial-Use Guide for the Detection of Acoustic Emissions from Partial Discharges in Oil-Immersed Power Transformers

Sponsor

Transformer Committee of the IEEE Power Engineering Society Approved 21 September 2000

IEEE-SA Standards Board

Abstract: This trial-use guide applies to the detection of partial discharges in power transformers

It takes advantage of the acoustic emissions produced by partial discharges

Although primarily intended for field use,

it can also be used in the factory environment,

Keywords: acoustic emission (AE),

The Institute of Electrical and Electronics Engineers,

New York,

NY 10016-5997,

USA Copyright © 2000 by the Institute of Electrical and Electronics Engineers,

All rights reserved

Published 13 December 2000

Printed in the United States of America

Print: PDF:

ISBN 0-7381-2679-9 ISBN 0-7381-2680-2

SH94893 SS94893

No part of this publication may be reproduced in any form,

in an electronic retrieval system or otherwise,

without the prior written permission of the publisher

IEEE Standards documents are developed within the IEEE Societies and the Standards Coordinating Committees of the IEEE Standards Association (IEEE-SA) Standards Board

Members of the committees serve voluntarily and without compensation

They are not necessarily members of the Institute

The standards developed within IEEE represent a consensus of the broad expertise on the subject within the Institute as well as those activities outside of IEEE that have expressed an interest in participating in the development of the standard

Use of an IEEE Standard is wholly voluntary

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or provide other goods and services related to the scope of the IEEE Standard

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the viewpoint expressed at the time a standard is approved and issued is subject to change brought about through developments in the state of the art and comments received from users of the standard

Every IEEE Standard is subjected to review at least every five years for revision or reaffirmation

When a document is more than five years old and has not been reaffirmed,

it is reasonable to conclude that its contents,

do not wholly reflect the present state of the art

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regardless of membership affiliation with IEEE

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Permission to photocopy portions of any individual standard for educational classroom use can also be obtained through the Copyright Clearance Center

Introduction (This introduction is not part of IEEE Std C57

127-2000,

IEEE Trial-Use Guide for the Detection of Acoustic Emissions from Partial Discharges in Oil-Immersed Power Transformers

Publication of this trial-use guide for comment and criticism has been approved by the Institute of Electrical and Electronics Engineers

Trial-use guides are effective for 24 months from the date of publication

Comments for revision will be accepted for 18 months after publication

Suggestions for revision should be directed to the Secretary,

IEEE-SA Standards Board,

Box 1331,

Piscataway,

NJ 08855-1331,

and should be received no later than 13 June 2002

It is expected that following the 24-month period,

shall be submitted to the IEEE-SA Standards Board for approval as a full-use guide

Participants At the time this trial-use guide was completed,

the Working Group on Partial Discharge Tests in Transformers had the following membership:

Harley,

Chair Donald Ayers Ron L

Barker Tord Bengtsson William Carter Roy Colquitt Ed Cromer John Crouse Ron Dallbert Gordon Denny

Copyright © 2000 IEEE

All rights reserved

Elliott Don Fallon Norman Field Steve Jordan Emil Kowal Mike Lau Raymond Lortie Richard Lowe Andre Lux

Jack McGill Mark D

Perkins Dirk Russwurm Ewald Schweiger Hemchandra Shertukde James Smith Subhash C

Tuli Barry Ward Eduardo Garcia Wild

The following members of the balloting committee voted on this trial-use guide: Paul Alex Dennis J

Allan George Allen Jacques Aubin Donald E

Ballard Ron L

Barker Mike Barnes A

Bartek William H

Bartley Martin Baur Edward A

Bertolini Enrique Betancourt Wallace B

Binder Alain Bolliger William Carter Donald J

Cash James F

Christensen Don Chu Robert C

Degeneff Alfonso M

Delgado Dieter Dohnal Randall L

Dotson B

Dunlap,

IV John A

Ebert Fred E

Elliott Gary R

Engmann Reto H

Fausch Pierre T

Feghali Joe Foldi Michael A

Franchek Charles G

Garner Harry D

Gianakouros Donald A

Gillies Richard D

Graham Robert L

Grunert Ernst Hanique N

Wayne Hansen

Harley Tommy W

Hayes R

Hayes Keith R

Highton Peter J

Hoefler Philip J

Hopkinson Richard A

Huber Virendra Jhonsa Charles W

Johnson Anthony J

Jonnatti Lars-Erik Juhlin C

Kalra Egon Koenig Barin Kumar John G

Lackey Thomas Lundquist Joe D

MacDonald William A

Maguire Charles Mandeville John W

Matthews Terence McComb Nigel P

McQuin Sam Michael R

Minkwitz,

Jack Moffat Daleep C

Mohla Harold R

Moore Daniel H

Mulkey R

Musil Vladimir G

Neumann Russell C

Nordman E

Norton B

Patel Wesley F

Patterson Jesse M

Patton David Payne

Paulette A

Payne Mark D

Perkins Linden W

Pierce R

Leon Plaster Donald W

Platts G

Preininger Tom A

Prevost Jeewan L

Riboud Peter G

Risse John R

Rossetti Hazairin Samaulah Vallamkonda Sankar Subhas Sarkar Leo J

Savio Rick Sawyer William E

Saxon Pat Scully Dilipkumar Shah Devki Sharma Mark Siehling Hyeong Jin Sim Tarkeshwar Singh Kenneth R

Skinger Stephen D

Smith Steven L

Snyder Peter G

Stewart Ron W

Stoner Malcolm V

Thaden James A

Thompson Thomas P

Traub Subhash C

Tuli Georges H

Vaillancourt John Vandermaar Robert A

Veitch William G

Wimmer F

Copyright © 2000 IEEE

All rights reserved

When the IEEE-SA Standards Board approved this trial-use guide on 21 September 2000,

it had the following membership: Donald N

Heirman,

Chair James T

Carlo,Vice Chair Judith Gorman,

Secretary Satish K

Aggarwal Mark D

Bowman Gary R

Engmann Harold E

Epstein H

Landis Floyd Jay Forster* Howard M

Frazier Ruben D

James H

Gurney Richard J

Holleman Lowell G

Johnson Robert J

Kennelly Joseph L

Koepfinger* Peter H

Bruce McClung Daleep C

James W

Moore Robert F

Munzner Ronald C

Petersen Gerald H

Peterson John B

Posey Gary S

Robinson Akio Tojo Donald W

*Member Emeritus

Also included is the following nonvoting IEEE-SA Standards Board liaison: Alan Cookson,

NIST Representative Donald R

Volzka,

TAB Representative Don Messina IEEE Standards Project Editor

Copyright © 2000 IEEE

All rights reserved

Contents 1

Overview

1 Scope

2 Purpose

Definitions

Instrumentation

3 Filter

5 Counter

6 Display

Test procedure

Interpretation of results

Annex A (informative) Bibliography

Copyright © 2000 IEEE

All rights reserved

IEEE Trial-Use Guide for the Detection of Acoustic Emissions from Partial Discharges in Oil-Immersed Power Transformers

Overview 1

It utilizes the acoustic emissions (AEs) produced by PDs

Although primarily intended for field use,

this guide can also be used in the factory environment,

IEEE Std C57

although it can sometimes provide a rough approximation of it

Refer to factory test codes for safety warnings for these situations

PD location should only be attempted by those technicians and engineers trained in working high-voltage transformers

Copyright © 2000 IEEE

All rights reserved

IEEE Std C57

127-2000

IEEE TRIAL-USE GUIDE FOR THE DETECTION OF ACOUSTIC

WARNINGS

The transformer tank must be connected to a low resistance ground to limit the extremely high voltages being induced into the ground circuit and the tank if a high voltage to ground failure occurs

The personnel risk is very high if the transformer fails to ground

Even when grounded properly,

the voltage on the tank to a different ground source may be LETHAL at the instant the failure occurs

If the transformer is being energized or de-energized,

or there is another type of power system voltage,

all personnel should maintain a reasonable distance from the transformer and equipment electrically connected to the tank due to the possibility of a failure

It is recommended that acoustic measurement equipment connected to the tank be electrically isolated from the transformer tank,

by optical means or by high-voltage electrical insulation,

when measuring during transient events to eliminate the danger to the equipment or operators

It is preferable to make all connections to the tank with the transformer deenergized,

but in no case should the transformer voltage be above normal voltage while the sonic measuring devices are installed

Personnel must not access areas where high voltages are within striking distance,

such as on top of energized transformers or in bushing compartments

The transformer ground circuit must never be changed (connected or disconnected) while the transformer is energized

Even with the transformer deenergized,

it is possible to have circulating currents in substation ground circuits

appropriate care should be exercised when connecting or disconnecting ground circuits

Definitions For the purposes of this trial-use guide,

the following terms and definitions apply

The Authoritative Dictionary of IEEE Standards Terms [B21]1 should be referenced for terms not defined in this clause

All liquids and many gels meet this criterion

Couplants produced for ultrasonic,

nondestructive testing purposes are generally suitable

gelled glycerin or silicone grease are particularly efficient and are recommended

depending on the instrument being used

For example,

there are nine bursts shown in the time interval,

The groups of acoustic emission oscillations are also called pulses

piezo-electric crystal when perturbed by a shock wave,

which could be caused by a partial discharge

numbers in brackets correspond to those of the bibliography in Annex A

Copyright © 2000 IEEE

All rights reserved

EMISSIONS FROM PARTIAL DISCHARGES

IEEE Std C57

127-2000

Figure 1—Typical AE oscillations 2

depending on the instrument being used

For example,

there are 13 oscillations in the detail of Figure 1

The time interval in the detail is not defined

piezo-electric transducer that detects the mechanical stress waves that propagate from the partial discharge source through the internal construction materials and oil to the transformer tank wall

Note that the sensor is sensitive to stress waves in its frequency range that may not be from a partial discharge source

Instrumentation

Instrumentation Many different types of instrumentation are available for detecting and displaying AEs

A typical system,

which has been shown effective in certain transformer arrangements,

is described in this trial-use guide

However,

other detection systems may be equally or more effective,

depending on the transformer physical parameters and the location of the PD

The main elements of the system are — Sensing transducer — Preamplifier — Filter — Power amplifier — Counter — Display — Power supply

Copyright © 2000 IEEE

All rights reserved

IEEE Std C57

127-2000

IEEE TRIAL-USE GUIDE FOR THE DETECTION OF ACOUSTIC

Because the sensor is a piezo-electric device,

it will also respond to varying electromagnetic fields,

such as those found in substations

To minimize this effect,

the transducer can be either a “differential” type utilizing two crystals (mounted out of phase) or a shielded single crystal transducer with an integral preamplifier circuit

The latter is the preferred and most common configuration because its comparatively high-amplitude,

low-impedance output is less susceptible to degradation due to noise pickup in the connecting cables

The acoustic impedance of a sensing crystal differs from that of the steel transformer wall

for efficient transfer of the signal from the steel to the crystal,

some users interpose a “matching piece

” Although several materials may be used for this purpose,

a hard-epoxy resin is convenient because it also provides some thermal and electrical isolation

However,

care should be taken to select a resin that exhibits low acoustic attenuation (usually a function of the fillers used) so that it does not adversely affect the amplitude of the transmitted signal

Furthermore,

as the acoustic impedance of epoxy resin does not numerically fall between that of steel and crystal,

the thickness of the matching piece shall be equivalent to half the wavelength of the signal propagating in it—in this case,

The acoustic couplant gel or grease,

is applied to the face of the transducer or matching piece just prior to the test

The preferred configuration is the integral amplifier discussed in 3

In either case,

the preamplifier circuit should accept high-impedance (approximately 10 000 Ω),

low-amplitude (less than 100 µV) signals,

provide a gain of about 40 dB,

and be capable of working into a 50 Ω load

To preserve the integrity of the signal,

the inherent noise produced by the preamplifier itself shall not exceed 3 µV referred to its input

These are frequencies at which the response to a constant sinusoidal input voltage has fallen by 3 dB from the maximum value

In this case,

FL should be about 100 kHz,

and FH should be about 300 kHz

The roll-off characteristics of the filter shall be a minimum of 48 dB/octave (240 dB/decade) for the high-pass section

This means that,

relative to the signal of interest (150 kHz),

a 50 kHz signal would be attenuated by 48 dB

The low-pass filter should roll off at not less than 24 dB/octave (120 dB/decade) so that a 600 kHz signal would be attenuated by 24 dB

The purpose of the filter is to negate as many of the effects as possible of signals that are not associated with PDs

These include vibrations caused by the magnetostrictive action of the core (Barkhausen noise),

Most of these fall below 30 kHz

the Barkhausen noise emanating from the core has been found to be in the 50 kHz range

a 100 kHz high-pass section with a rapid,

roll-off response characteristic is needed

The reasonably generous band-pass (200 kHz) allows for variations between different transducers,

in so far as their resonant frequencies are concerned

Depending on location and source of the PD,

some users find that a lower frequency (e

particularly when higher-frequency signals are attenuated

This type of sensor is more susceptible to external or other mechanical signals

Copyright © 2000 IEEE

All rights reserved

IEEE Std C57

127-2000

EMISSIONS FROM PARTIAL DISCHARGES

Its gain must be adjustable in at least 2 dB increments over the range of 20–70 dB

The maximum inherent noise level produced by the amplifier should not exceed 100 µV referred to its input

The amplitude threshold,

in which this is to be carried out,

should require a minimum signal-to-noise ratio of 3:1 for activation

Oscillation rates from 0 to 106 counts/s should be capable of being displayed

To accomplish this,

a logarithmic or digital display is recommended

The physical display should be convenient and easy to read—bear in mind that it sometimes will be read in awkward locations in the field and often in bright sunlight

it should be powered by an isolated supply

This is necessary because the power-supply ground is unlikely to be the same physical ground as the transformer tank

Ground loops are to be avoided for both safety and noise reduction considerations

When size and transportation limitations do not preclude an adequate supply of power for the desired time period of monitoring,

Test procedure 4

Consequently,

the test can only be carried out on energized equipment,

and adequate safeguards must be involved

WARNING

The following test procedure requires that the operator make contact with the apparatus being evaluated

It is mandatory that such contacts involve only adequately grounded surfaces

Bushings and other electrical components are not necessarily adequately grounded

Therefore,

the evaluation of such components (bushings,

) by means of this test procedure is not recommended,

The technique and test procedure described are intended for exclusive use on oil-filled equipment with adequately grounded metal walls

Use in any other environment could be DANGEROUS

Copyright © 2000 IEEE

All rights reserved

IEEE Std C57

127-2000

IEEE TRIAL-USE GUIDE FOR THE DETECTION OF ACOUSTIC

which propagates to the tank wall where it can be detected by an appropriate sensor

The output of the sensor will be proportional to the energy content of the forcing function (pulse)

Because the sensor contains a resonant crystal,

it will oscillate at its natural frequency

The amplitude of these oscillations will then decay exponentially due to the mechanical damping inherent in the crystal

Consequently,

each pulse arriving at the transformer tank wall will result in a “burst” type signal from the transducer

One burst is produced for each PD detected

The number of oscillations contained within each burst is determined by the amplitude of the forcing function (pulse from the PD) that excited the crystal

An accounting of the number of these oscillations,

which occurs within a 1 s'interval,

contains information relative to both the number of discharges that occurred within that time interval as well as their amplitude

The amplitude of the mechanical pulse is attenuated as it propagates through the insulation and oil during its journey to the tank wall

Consequently,

the oscillation count rate will be at its maximum when the sensor is at its closest proximity to the source

This effect enables the operator not only to detect the presence of PDs,

but also to estimate the approximate location of their source

If the preamplifier is included in the transducer package (the preferred arrangement),

the cable should be a 50 Ω characteristic-impedance shielded coaxial cable not more than 30 m long

If the preamplifier is a separate unit,

it should first be connected to the sensor with a high-impedance shielded coaxial cable not exceeding 1 m in length

Subsequent connection between the preamplifier and power amplifier should then be made with a 50 Ω characteristic-impedance coaxial cable no longer than 30 m long

With the transducer connected and suspended in such a manner that it does not come in contact with any object,

the power amplifier gain should be increased to a point where its own noise causes random pulses to be detected when the counting threshold is set to 1 V

With the amplifier set at this level,

the counter threshold level should then be raised to 3 V,

thus requiring a signal-to-noise ratio of 3:1 for actuation of the counter circuit during testing operations

Its use for unattended longterm monitoring is not recommended due to the confusing signals produced,

AE signals produced by PDs propagate through the oil to reach the transformer tank wall

Therefore,

all attempts to detect such AEs should be carried out below the top oil level

As previously stated in 4

this technique is designed for use on metal transformer tanks that are at ground potential

The following procedures,

apply only to that mode of operation

The instrument should be set up as previously described in 4

with care being taken to ensure that the power source selected is adequate for the type and length of tests planned

Sufficient couplant is to be applied to the face of the transducer to ensure efficient transmission of the AE signal from the tank wall to the sensing crystal

Ideally,

the face of the transducer should be covered with a film of couplant approximately 0

5 mm thick

The use of more couplant will not be

Copyright © 2000 IEEE

All rights reserved

EMISSIONS FROM PARTIAL DISCHARGES

IEEE Std C57

127-2000

whereas too little couplant can seriously inhibit the transducer’s sensing capabilities

The face of the transducer with its film of couplant should be brought into contact with the transformer tank wall with only sufficient pressure applied in order to hold it in position

It is only necessary to hold the sensor steady so that no signals are generated due to relative movement between the sensor and tank wall

This can be achieved either by means of a magnetic clamp or adhesive tape

However,

with care and a little practice,

the sensor can be quite successfully hand held

If PD activity is detected at any location,

the foregoing procedure should be repeated at other positions on the transformer in order to locate the position where a maximum oscillation count rate is obtained

This is the position on the tank wall where the sensor is closest to the PD source

The oscillation count rate obtained at that location also provides the best estimate of the activity level of the emitting PD source

There will be more than one area where maximums are observed if there are multiple PD sources

The functioning of the transducer and instruments can be tested by tapping on the face of the transducer or by placing the face of the transducer against the tank wall and breaking the lead of a #2 pencil on the tank wall next to the transducer

Interpretation of results This clause is included as a guide to the types of signals that may be encountered

It is not intended as a definitive description of all types of faults and their signatures

However,

it gives little indication of the amplitude of the individual discharges that have occurred

the same oscillation count rate could be obtained from many discharges of low amplitude as could be obtained from a few discharges of high amplitude

) This situation can be resolved by also taking into account the burst count rate

A high oscillation count rate,

coupled with a high burst count rate is indicative of many low-amplitude discharges

On the other hand,

a high oscillation count rate coupled with a low burst count rate is indicative of fewer,

) all attenuate the mechanical stress wave as it propagates to the tank wall

The situation is further complicated by the fact that each of these materials exhibits different attenuation characteristics

For this reason,

it is not possible to quantify the discharge level through the oscillation or burst count rates

To achieve this,

it would be necessary to know the exact location of the PD source and details of the materials in the propagation path of the pulses

It may then be possible to calculate the amount of attenuation present and apply the necessary compensation (increase) to the observed oscillation count rate

As this is rarely a practical approach in the field,

the observed pulse rate should be taken as indicative of the minimum level of activity that is present,

since compensation for attenuation would always result in a higher estimate

Copyright © 2000 IEEE

All rights reserved

IEEE Std C57

127-2000

IEEE TRIAL-USE GUIDE FOR THE DETECTION OF ACOUSTIC

two types of result (other than zero) may be encountered

One in which “continuous” readings are obtained,

while the other produces “sporadic” readings

By continuous it is meant that activity is present all the time,

though not necessarily producing a nonvarying oscillation count rate—count rates may vary but never decline to zero

This type of signal is typical of that produced by an energetic PD source

Usually,

this sensor is associated with reasonably high count rates and a well-defined location

“Sporadic” activity can be further subdivided into two types

One in which the activity is present most of the time,

but short quiescent periods are also encountered

This type of signal is usually produced by exposed sources on conductors,

The second type of “sporadic” signal is typified by lengthy quiescent periods (perhaps minutes) followed by short periods of very high activity

This type of signal usually has been found to be associated with floating static shields

Often short-lived arcs are associated with this type of fault,

and these produce very energetic AE signals during active periods

As previously described,

it is often possible to determine the position on the tank wall where the transducer is closest to the PD source

gives no information as to the distance into the tank (from that location) to the source

However,

observing the signal on an oscilloscope (a digital transient recorder is recommended),

it is possible to form an opinion regarding this

For example,

the pulse shown in Figure 1 has suffered very little attenuation

This is evidenced by the high rise rate of the leading edge of the burst envelope,

resulting in the characteristic “arrowhead” shape

To achieve this,

the propagation path is almost entirely in oil with little solid insulation involved

If the same signal had propagated through layers of insulating materials,

the resulting attenuation would not only have affected the overall amplitude,

but also modified the burst envelope by “rounding off” the leading edge

In the extreme,

the burst envelope becomes “egg shaped” as shown in Figure 2

By utilizing this phenomenon,

it is possible to estimate whether the source lies close to the surface or is buried well within the insulation system

The foregoing involves the use of ancillary instrumentation not normally available in the field

most often applied in the field if circumstances warrant the added complication

Figure 2—Typical AE burst

Copyright © 2000 IEEE

All rights reserved

EMISSIONS FROM PARTIAL DISCHARGES

IEEE Std C57

127-2000

this correlation should not be used in the field due to the effects of attenuation in both the acoustic and electrical signals

This means that no absolute value of PD activity can be determined from AE measurements made in the field

It is important to verify whether the acoustic signal is due to internal PDs or if it is due to mechanical noise

To make this determination requires the expertise of the investigator,

and other evidences such as the presence of indicating combustible gasses or electrical PD

In general it is true that a more intense PD source will produce a higher count rate than a weak source

This is because,

at the site of an intense discharge,

there are multiple locations or perturbations that are each producing PDs and AEs

However,

it is necessary to recognize the differences in making acoustic measurements on large power transformers vs

Large power transformers: The locations of the PDs that are likely to lead to failure in large power transformers may be in areas where the attenuation of the PD signal is great

These would include areas within the windings and in the high-low spaces

In this instance,

a source that is likely to lead to failure will be attenuated to the point that the AE count rate is low

With large power transformers,

there are generally more attenuation sources due to the thicker tank walls,

the presence of more insulation barriers,

Because the value of the large power transformer is so high and the cost of a catastrophic failure is so great,

the detection of any internal PDs in large power transformers should be a cause for further investigation

This might include close monitoring of the behavior of the discharge with time,

more frequent samples of oil for combustible gas measurements,

and other advanced diagnostic measurements

Smaller core form transformers that do not have tank wall shielding and/or FOA barriers whose tank wall thickness is 1/4 in or less: A given PD will probably produce more acoustic energy at the transducer location than a larger transformer with 3/8 in tank wall thickness and tank wall shielding

Because of this,

there may be more justification for taking a less conservative approach such as characterizing the PD count rate—a detected oscillation count rate of 10 000 counts/s should be cause for further investigation

The level of activity necessary to produce oscillation count rates in the range of 100 000 counts/s should be cause for considerable concern

Typically,

this type of result requires the existence of either high levels of continuous PD activity or occasional arcing

In any case,

it should justify prompt actions aimed at determining the cause

The foregoing is included only to give guidance as to the level of AE activity detected in the field that should 1)

Give cause for little concern

Give sufficient concern to warrant further repeated testing

Be of sufficient concern that it prompts serious evaluation using all means available

Obviously,

the data obtained from this type of test is not sufficiently definitive to warrant its use as either acceptance or go/no-go criteria,

and should not be used as such

It can be seen that by taking into account the type of signal obtained,

the approximate location of the emitting source,

and an estimate of the level of the activity involved,

it is reasonable to use the acoustic measurement as a means of identifying potential PD problems

While the acoustic measurement alone may not provide an estimate of the severity of the problem or the assessment of its cause,

it can indicate the need for other diagnostic measures,

which when combined with the acoustic data,

will often provide the means for identifying the cause and severity of the problem

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IEEE TRIAL-USE GUIDE FOR THE DETECTION OF ACOUSTIC

it becomes even more useful when used in conjunction with other information,

such as dissolved gas-in-oil data

Assuming that the problem has been present for some time,

as is typical of situations that develop in the field,

good correlation should be expected between gas analysis and AE data

If a gas analysis shows the existence of constituents associated with the degradation of cellulose due to PD activity and the AE data indicates a continuously emitting source in the area of one of the coils,

there is a good possibility that a PD is present in the area indicated

) The two sources of data can often supplement each other in yet other ways

For instance,

sometimes the breakdown of constituents in the gas analysis is so complex that,

although it is obvious that a significant problem is involved,

it is not possible to determine whether the cause is due to PDs or is thermal in origin

The AE system responds only to signals produced by PDs or arcs

Purely thermal phenomena do not produce such signals

Therefore,

the existence of any AE signal together with the complex gas analysis may confirm the existence of PDs

Conversely,

the absence of AE activity in this case may indicate that the problem is basically thermal in origin

As the combination of information produced by these two techniques is so advantageous,

it is particularly recommended that gas analysis results be taken into account when interpreting AE data

The foregoing comments are particularly aimed at the evaluation of units in the field

When evaluating transformers on the shop floor,

the same good correlation between dissolved gas and AE data is not usually obtained

AE technique provides essentially real-time data relative to activity occurring at that instant

Oil analysis,

is to some extent historical in nature

It is necessary for a PD to be active for some time before sufficient gas is generated so that it is detectable in the large volume of oil present

When the unit is new,

or the insulating fluid is new or reprocessed,

it is unlikely that PD-related faults would be present for a long enough period of time to be reliably detected by gas analysis

Normally,

radio-influence voltage (RIV) or “apparent charge” detection is carried out in the factory,

and this provides an alternate database for correlation with AE data in the signal interpretation process

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Annex A (informative)

Bibliography Since the early 1950s,

there has been much activity in the area of ultrasonic/acoustic emission detection of PDs

Consequently,

the amount of literature now available is too large to allow for complete documentation

The following bibliography is intended to give a broad overview of the subject and provide references for further study

“Ultrasonic Detection and Location in Insulating Structures,” AIEE Transactions,

“Table of Ultrasonic Properties

Richland,

“Engineering Dielectrics,” Corona Measurement and Interpretation,

Harrold,

ASTM Publication,

STP669,

“Transformer PD Diagnosis using Acoustic Emission Technique,” ISH-97,

Leijon,

“Acoustic Frequencies Emitted by Partial Discharges in Oil,” ISH-93,

Acoustic Monitoring and Gas-In-Oil Analysis for Transformers,

Doble Engineering Company,

Report #62PAIC95,

831–836

“Utilization of Acoustic Emission for Detection,

Measurement,

and Location of Partial Discharges,” AEWG-Second International Conference on Acoustic Emission,

Lake Tahoe,

Kresge,

“Ultrasonic Corona Detection,” IEEE Transactions,

PAS-86,

Peter D

“Methods of Experimental Physics,” Ultrasonics,

18–19,

New York: Academic Press,

“Partial Discharge XXI: Acoustic Emission-Based PD Source Location in Transformers,” Proceedings IEEE Electrical Insulation Magazine,

Acoustic Emission Detection of Partial Discharges in Power Transformers

Ultrasonic Engineering

New York: John Wiley & Sons,

13–21,

“Acoustical Properties of Insulating Liquids and Gases,” IEEE International Symposium of Electrical Insulation,

Philadelphia,

“Acoustic Waveguides for Sensing and Locating Electrical Discharges in High Voltage Power Transformers and Other Apparatus,” IEEE Transactions On Power Apparatus and Systems,

449–457,

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IEEE TRIAL-USE GUIDE FOR THE DETECTION OF ACOUSTIC

[B15] Harrold,

“The Relationship Between Ultrasonic and Electrical Measurements of Under Oil Corona Sources,” IEEE Transactions on Electric Insulations,

8–11,

“A Statistical Study of Electrical and Acoustical Characteristics of Pulsative Corona,” TGIAO,

IEEE paper AT6 122–2,

Winter Power Meeting,

Absorption and Dispersion of Ultrasonic Waves

New York: Academic Press,

“Acoustic Emission from Stressed Dielectric Liquids,” ISA paper no

Albuquerque,

“Detection of Partial Discharges in Transformers Using Acoustic Emission Techniques,” IEEE Transactions on Power Apparatus and Systems,

PAS-97,

“Parameters Affecting the Velocity of Sound in Transformer Oil,” IEEE Transactions on Power Apparatus and Systems,

PAS-103,

May 1984

The Authoritative Dictionary of IEEE Standards Terms

104-1991,

IEEE Guide for the Interpretation of Gasses Generated in Oil-Immersed Transformers

113-1991,

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

“Partial Discharge Automatic Monitor for OilFilled Power Transformers,” IEEE paper no

PES Summer Meeting,

Ultrasonic Testing of Materials

New York: Springer-Verlag Inc

20–23

“Detection and Location of Partial Discharges by Ultrasonics,” ERA Transactions 2947,

General Electric,

225–228,

Nakamura,

Katsukawa H

“Acoustic-Based Real-Time Fault Location in Power Substation,” Third International Symposium on Electricity Distribution and Energy Management,

Proceedings,

106–111

Nakamura,

Watanabe T

“Acoustic-Based Real-Time Partial Discharge Location in Model Transformer,” Proceedings ICSPAT ’94,

[B29] Partial Discharge Detection Using Acoustic Emission

Princeton: Physical Acoustics Corporation

“Online Transformer Monitoring,” Electrical World,

Special Report,

19–26,

Alnajjar,

“Fault Detection Device For Electrical Power Transformers Using Novel DSP Scheme,” Proceedings ICPAT'96,

Boston,

7–10,

“Target Parameter Estimation in the Near Field with Two Sensors,” IEEE Transactions on Acoustic Speech and Signal Processing,

Box 1331,

Piscataway,

NJ 08855-1331,

USA (http://standards

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[B33] Shertukde,

“Manufacture of Fault Diagnostic Device for Electrical Power Transformers (FD2EPT),” Proceedings ICSPAT'96,

Boston,

7–10,

“Non-Destructive Magnetic Measurements of Steel Grain Size,” Non-Destructive Testing Journal,

England,

Mercier,

“The Detection of Partial Discharges in High Voltage Potential Transformers in Service,” IEEE Transactions on Power Apparatus and Systems,

PAS-93,

Gervais,

“Model Analysis of Pulses Generated by Partial Discharges,” IEEE paper no

A76-4168,

PES Summer Meeting,

Hickling,

Hindmarch,

“Electrical and Ultrasonic Characteristics of Partial Discharges in Oil Immersed Insulation,” IEEE Conference on Dielectric Materials,

Measurements and Applications,

Cambridge,

England,

Wavelet Theory and Its Applications

Boston: Kluwer Academic Publishers,

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IEEE TRIAL-USE GUIDE FOR THE DETECTION OF ACOUSTIC

Annex B (informative)

Instrumentation calibration B

Provide 60 dB of gain to the incoming signal Severely attenuate signals outside the frequency range of 100–300 kHz Produce a 3:1 signal-to-noise ratio for actuation of the pulse counting process

The procedure in B

Signal generator (with low-output voltage capability)

Frequency counter

Precision voltmeter with high-frequency (500 kHz) response capability and preferably peak voltage indication

Connect the voltmeter across this input so that it monitors the voltage level of the applied signal

Connect the counter to the output of the signal generator to monitor the frequency of its output

Adjust the signal generator to supply a 150 kHz sinusoidal signal with peak amplitude of 1 mV

Set the power amplifier gain to 60 dB ± 1 dB

Set the pulse counter circuitry trigger level to 1 V

The instrument display should indicate an oscillation count rate of 150 000 pulses/s

Raise the instrument counter trigger level to 3

The counter should now indicate 0

Slowly increase the amplitude of the applied signal (still at 150 kHz),

noting the voltage necessary to activate the counter circuit and result in a 150 000 pulses/s display

This amplitude should be no less than 3

With the system setup as previously described (3

reduce the frequency of the input signal to 100 kHz

The counter should now indicate 0

With the same setup,

increase the frequency of the input signal to 300 kHz

The counter should again indicate 0

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Without changing the instrumentation configuration,

slowly vary the input signal (at constant 3 mV peak amplitude) over the range of 100–300 kHz

Correct indication should only be obtained when the frequency of the input signal is between 120 kHz and 280 kHz

Completion of these checks ensures satisfactory performance of the power amplifier,

and signal-tonoise ratio discriminator and counter circuits

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Annex C (informative)

Calibration of transducer and preamplifier C

The output from the crystal is a very low-amplitude,

high-impedance signal that requires processing in a preamplifier before it is useful

The transducer element and preamplifier is to be considered as a complete system,

whether they are contained in the same package (as in the preferred shielded single crystal sensor) or as separate units

Heavily damped,

non-destructive testing immersion transducer

Transducer excitation pulse circuit—This is required to provide a positive going pulse,

achieving a peak amplitude of 300 V in 500 ns,

and decaying to 0 amplitude in 3 µs

The circuit is required to work into a high-impedance load and have a pulse repetition rate of approximately 1 kHz

Transient recorder—A digital oscilloscope with a 500 ns sampling rate is recommended

Spectrum analyzer—This should be capable of analyzing transients and processing signals with a frequency content up to at least 300 kHz

Ultrasonic immersion test tank

Appropriate preamplifier power supply

To achieve this,

the pulsing circuit excites the ultrasonic (driving) transducer so that it outputs a well-defined mechanical pulse

Having been submerged in water,

this pulse propagates exclusively in the longitudinal mode and subsequently excites the sensor being evaluated

The output of the sensor/preamplifier combination is then supplied to a transient recorder where the time-domain record is obtained

At the same time,

the signal is supplied to a spectrum analyzer for frequency analysis

Connect pulser circuit to ultrasonic (transmitting) transducer

Connect appropriate power supply to sensor/preamplifier combination

Connect sensor/preamplifier output to transient recorder and in parallel to the spectrum analyzer

Set transient recorder sampling period to approximately 500 ns and transient capture trigger level to approximately 2 V

Set spectrum analyzer in the transient analysis mode and select a frequency range that embraces at least 0–250 kHz

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Immerse both driving transducer and discharge detector sensor in the water-filled immersion tank

Ensure that they face each other squarely and are 16–17 cm apart

The transducers should be located at least 8 cm away from any reflecting objects such as the tank walls

It is also important to ensure that no bubbles adhere to the face of either the transmitting transducer or sensor

Energize transducer excitation pulse circuit and preamplifier power supply

The time-domain signal displayed by the transient recorder should be that of a “burst” made up of many oscillations

The leading oscillations should be high in amplitude with the remainder decaying to zero,

similar to that shown in Figure 1

The requirements are that the maximum peak-to-peak voltage be no less than 5

The duration of the burst should be no less than 80 µs and no longer than 150 µs

To avoid the confusing effects of random noise,

it is recommended that the spectrum be enhanced by averaging at least eight separate spectra

The resulting spectrum should show a dominant peak between 120 kHz and 160 kHz

The resonant characteristic of the crystal should be evident by the amplitude of this peak being at least 40 dB and no more than 43 dB above the spectrum reference level

In meeting these criteria,

the sensor is shown to have a lightly damped crystal of the correct resonant frequency and the preamplifier is producing the required 40 dB of gain

Note—The foregoing procedure requires that the PD detection sensor be completely immersed in water

If a sensor that is not suitable for total immersion is used,

the same result can be obtained by utilizing a vertical water column

In this case,

the driving transducer is located about 8 cm from the bottom of the column,

while only the face of the sensing transducer is required to enter the water surface

If this approach is used,

the same precautions relative to reflections from the tank sides,

and separation distance between the sensors are still appropriate

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