Discussion on

Diagnostic testing of high-voltage machine insulation

R. Miller {Brighton Polytechnic): In his discussion,Mr. Simons mentioned the paper by Miller and BlackA onlow-frequency partial discharge measurements. Heconcluded from this paper that partial dischargemeasurements, undertaken at very low frequencies, had notapparent or conclusive correlation with those taken atpower frequency. It should be indicated that there is a corre-lation between tests performed at both frequencies, pro-vided the test routine outlined in Reference A is followed.

The partial discharge measurements undertaken onvarious stator-bar samples at Brighton Polytechnic havedemonstrated that tests at 1 Hz compare reasonably withthose made at power frequency. At frequencies less than1 Hz, the discharge inception voltage is slightly higherthan at power frequency, depending on the type of materialbeing tested. However, the values of integrated-dischargemagnitude and integrated energy are unaffected if thetest voltage remains at a known proportion above theappropriate inception voltage. It is therefore proposed thatthe test frequency be specified according to the type ofinsulation employed in the component under test, andthat measurements of the integrated discharge magnitude/voltage or integrated discharge energy/voltage relationshipsbe performed at a specified frequency within the range0 1 Hz to lHz.

The low-frequency work has now been extended withthe establishment of a collaborative research programmebetween Brighton Polytechnic and the University ofTrieste, Italy, the ultimate aim of which will be to assist inthe formulation of an international standard for low-frequency partial discharge measurements.

LA. Black {Brighton Polytechnic): When referring todiscrete pulse measurement, as opposed to the integrationof discharge pulses, I would point out that the new versionof the pulse discrimination system developed at BrightonPolytechnic can cope with a very high pulse-repetition rate.This could be applicable to testing machine insulation.3

W.K. Hogg {Brighton Polytechnic): I feel that theauthor could usefully have included a number of majorpublications to support his statements concerning thedegradation mechanisms described in the introduction. TheSchering Bridge and d.l.a. equipment have provided usefulinformation on the condition of the stator coil/barinsulating systems, although premature failures still occur.The loop traces shown, although informative, require adegree of expertise to interpret.

New instruments and testing techniques which arecurrently being developed and tested under serviceconditions may complement existing tests. One pointwhich should be made is that if slot discharges occur, theyare normally less than 2% of the total energy per cycle andcan be difficult to detect using the d.l.a. instrument; alsothey could be confused with harmonics in the waveforms.

Although the paper is concerned with stator coil/barand interconductor insulation, at present, prematurefailures resulting from core faults are of greater significance.

Paper 676B by SIMONS, J.S. (See IEEProc. B, Electr. Power Appl.,1980, 127, (3), pp. 139 -154 ) . Read before IEE Power DivisionProfessional Group P i , 13th May 1980

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J.W. Wood {NEI-ParsonsInsulation Engineering): Thepaper relates to machines falling into two categories:

{a) Machines having design or manufacturing faults.{b) Machines, the insulation of which is ageing in a

general manner.I agree with the other discussers that the d.l.a. and otherdischarge measuring instruments are valuable for assessingthe machines in category {a) above. Provided regular checksare made it should be possible to pick out inherent faultsor the onset of faults by discharge measurements. I wouldlike to point out that another method useful for certaintypes of faults might be to use a condition monitor whichdetects particles released into the gas stream by thedeterioration of the insulation.

Dr. Hogg commented that the d.l.a. appears to beinsensitive to slot discharge if the machine is a large one.Some time ago Dr. Hogg pointed out that, for normal sizevoids, theoretical calculations of maximum dischargemagnitude give a value of-about 1000 pC. Slot dischargeinvolves discharge of a void having two conducting surfaces,the dagged bar surface and the core. It would appear,therefore, that inherently the discharge magnitudesassociated with slot discharge will be much larger than thosedue to internal voids. I think that a wideband dischargedetector such as the ERA Mk III, which gives a pulseresolutions of about 20/us, is a more appropriate instrumentthan the d.l.a. for detecting slot discharge. I believe theon-line measurement techniques described by Mr. Kurtz ofCanada have been used to successfully detect slot dischargein waterwheel generators. I know of no instance of slotdischarge in a hydrogen cooled turbogenerator.

Mr. Simons appears to claim that there is a relationshipbetween discharge energy and life of machines in category{b) above. A tan 5 is described as differing from dischargeenergy as measured by the d.l.a. in some instances, and thisobservation is said to be useful. In what way?

There appears to me to be great difficulty in determiningthe life of insulation from a discharge energy measurement.Would Mr. Simons comment on the correct unit, is itdischarge energy per unit length or volume or some otherunit? How can the effects of geometric differences betweendifferent machine conductor dimensions be overcome?

Mr. Simons is probably more experienced than any otherperson in interpreting discharge energy measurements. Iflife is determinable from discharge energy, perhapsMr. Simons has quantitative information demonstrating this.Have breakdown tests been carried out on condemnedcoils, and can Mr. Simons therefore give a curve from thevast information available, showing a relationship betweendischarge energy and breakdown strength for a particulardesign?

J.H. Mason {South Bank Polytechnic): In Section 6,Mr. Simons refers to the possibility of absorbed moisturereducing the insulation resistance of stator windings. Ifthe i.r. is low he recommends heating to 'dry' the insulationuntil higher, stable i.r. values are attained; then the d.l.a.and tan 8 characteristics are measured at about 30°C.Would it be useful to make these measurements in the'damp' state, to assess the risk of failure occurring whenmachines are restarted after being out of service?

IEE PROC, Vol. 127, Pt. B, No. 6, NOVEMBER 1980

As the i.r. of the insulation falls by an order of magnitudewhen the temperature is raised 30°C (compare his Fig. 1),and the variation will probably be greater if the insulationhas suffered thermal degradation, I would suggest that thed.l.a. and tan 6 characteristics should also be measured atthe normal operating temperature.

How does Mr. Simons assess the insulation quality inservice from his measurements at 30°C?

If there is difficulty in resolving discharges when testsare made at 50 Hz, I wonder whether tests could be madewith d.c, at the operating temperature. With d.c, thedischarge repetition rate (fd) is highly dependent on theinsulation resistance; thus a marked increase in fd wouldoccur in insulation which had suffered thermal degradation.

In conclusion, if corona is observed using a tuned-circuitdetector, it becomes impossible to resolve the dischargesabove about 1 -5 Vi whereas with surface discharges, whichincrease in magnitude as the voltage is raised, it is possibleto distinguish occasional discharges of 50000pC in thepresence of large numbers of smaller discharges.

J.S. Simons (in reply): As always, there is a tendencyfor contributors to the discussion to take sides in arguingthe relative merits of this or that discharge test. The paperis concerned with carrying out a number of differenttests, each selected to contribute data based on specificcriteria. These are used to make an overall assessment. Toassist in the latter, a simple numerical system of meritmarking has been recently proposed.0

If measurements of both integrated discharge energyand individual partial discharges can be demonstrated inthe field to provide complementary data of relevance toinsulation life, there is a case for using both. In the contextof on-line monitoring, partial discharge measurement isalready being exploited.0

When discussing the sensitivity of resonant-circuit-typeinstruments, it is important to recognise that in the fieldthere is always the problem of achieving theoreticalresolution and maximum sensitivity in the presence ofnoise, notwithstanding some important advances made indeveloping discrimination circuits as referred to byDr. Black and Dr. Miller. Moreover, because of the largenumber of small pulses involved, resolution of partialdischarges on stator bars using a 50 Hz test is necessarilylimited to a relatively small increment of voltage aboveinception. The d.l.a. on the other hand is free of thesefundamental limitations and its full potential advantageshave yet to be realised. A recent development, at presentthe subject of a pending patent application, is a means ofsuperposing a partial discharge resonant display on theexisting loop trace. This will further facilitate themeasurement of discharging inception voltage and willenhance the display of large incremental pulses of charge.

Thus, in replying to the comments of Dr. Hogg,Mr. Wood and Dr. Mason, who suggest that resonant-circuitpartial discharge measurements are likely to be moresensitive for detecting large individual slot discharges, Iwould make the following observations.

The two types of instrument do not measure the samequantity but in terms of their ability to detect a given largepulse amplitude of incremental charge transfer, the d.l.a.circuit is limited only by the amount of amplification used.With the existing commercial instrument, the maximumgain available is 10. Our field experience, as reflected inthe paper, is that anomalous large incremental chargetransfer related to discontinuities or damage to the corona

shield are detectable on winding tests using the existinginstrument. Tests in the laboratory comparing resonantcircuit and d.l.a. loop traces indicate that for the largerdischarge pulses of diagnostic interest, we obtain similardynamic discharge inception values. As has been intimatedearlier there is considerable scope for further improvementin terms of discrimination and sensitivity if this is required.

When referring to 'slot discharge' in the paper we werespecifically dealing with external discharge between thecorona shield and the core and not internal partial dischargein the coil itself. Two types of such slot discharge have beenidentified. One is a static spark discharge derived fromdamage to the corona shield; the other, sparking associatedwith a vibrating bar loose in its slot. The former usuallyresults from the latter and can be measured using a d.l.a.The latter can be monitored on-line.D

Dr. Hogg challenges our interpretation of the loop tracesin Fig. 9 / and g of the paper. Suffice it to say that thisparticular winding was subsequently investigated byremoving the slot wedges in particular slots where anacoustic probe scan had indicated a high noise level.Inspection of the bars revealed some surface erosion of thegraphite-coated corona shields and some localised burningof the wedges and separators as a result of surface trackinginitiated by the spark discharges. When the corona shieldshad been refurbished and the coils re wedged and packed,the d.l.a. loop trace response was normal. This experiencehas since been confirmed on two other case histories.

In reply to Mr. Wood, the unit of integrated dischargeenergy normally quoted is that displayed directly by thed.l.a., i.e. integrated discharge energy per cycle per unitcapacitance. When appropriate, this can be readily translatedinto a value per coil side by multiplying by the appropriatecapacitance. The latter can be derived from the windingcapacitance divided by the number of coil sides.

The mechanisms of degradation of stator-bar insulationhave been discussed in the paper and in earlier referenceslisted. Dr. Hogg reminds us of recent work in which thedeterioration of mica under the action of electricaldischarges has been investigated in great detail by Ryder,Wood and Hogg.E'F Their studies have demonstratedthe complex and unstable nature of the individual dischargespectra and confirmed the comparative stability of integrateddischarge measurements. Their work has also confirmedthat it is not the severity of individual discharge pulsesinside the coils that limits the life of machine insulation.Rather do we find that thermal/mechanical degradationoften produces the initial loss of structural integrity. Itis at this stage that the cumulative effect of the manysmall individual discharges contributes to an increaseddegradation rate inside the coil. This is why the assessmentof progressive degradation in terms of loss of structuralintegrity is the key to assessing the remaining expectationof life of machine insulation. The dielectric loss analyserwas developed to enable this structural change to bemonitored by measuring void content as well as theintegrated discharge energy level at working voltage andabove.

Several contributors seek evidence of the correlationbetween integrated discharge and life. It is evident fromearlier comments that in establishing a correlation betweenintegrated discharge energy and expectation of life fora given insulation system, it will depend on the combinedthermal, electrical and mechanical stress conditions existing.Thus two identical windings subjected to quite different

IEEPROC, Vol. 127, Pt. B, No. 6, NOVEMBER 1980 387

operating duties will have a different overall rate of changeand life expectancy. From the many tests we have carriedout, however, clear patterns have emerged for the thermo-plastic-bonded mica systems correlating expectation of lifewith integrated discharge energy values for similar insulationsystems operating under comparable conditions. We havecarried out also a limited number of voltage endurancetests on bar samples in the laboratory, comparing stressversus time to failure and integrated discharge energyvalues measured on the bars at the different test stresses.The pattern of data obtained is consistent with thetheoretical relationship discussed in the paper.

With the introduction of epoxy insulation, if properlyengineered and executed, the problems of internal structuraldegradation have largely disappeared. The major mechanismof deterioration of epoxy windings now appears to be thatof external damage to the bar from fretting and vibration.However, the d.l.a. has demonstrated its ability to detectsuch external damage.

Difficulties in interpreting loop traces have been raised.The interpretation of such data requires a disciplinedapproach in which anomalous results are analysed interms of the particular winding, the site conditions andthe supply waveform. It is imperative to eliminate oridentify any source of loop trace distortion arising fromother than the test object. Having done this, anomalouscharge-transfer characteristics can be correlated withchanges in winding conditions in both slot and endwindingregions. The general procedures outlined in the paperenable this to be achieved.

In reply to Dr. Mason's question regarding test procedure,the sequency of tests standardised is recommended so asto minimise the risk of damage during testing. Thus theapplication of an initial d.c. test and polarisation indexdetermination provides assurance or otherwise of the

superficial condition of the insulation before high-voltagetests are applied. The initial a.c. measurements at lowvoltage indicate whether the bulk of the insulation is fitfor testing at higher voltage. It is standard practice to carryout such tests at ambient temperature. D.C. resistancevalues can be normalised to a standard temperature(IEEE 43) for purposes of comparison. For temperaturesup to about 30°C, the value of integrated discharge energyfor a given void content does not appear to change signi-ficantly.

References

A MILLER, R., and BLACK, I.A.: 'Partial discharge energymeasurements on electrical machine insulation when energisedat frequencies between 0-1 Hz and power frequency', IEEETrans., EI-14, pp. 127-136

B BLACK, I.A., and LEUNG, N.K.: 'The application of the pulsediscrimination system to the measurement of partial dischargesin insulation under noisy conditions'. IEEE InternationalSymposium on electrical insulation, Boston, Massachusetts,June 1980,p. 167

C SIMONS, J.S.: 'Diagnostic measurements on high voltage machineinsulation — recommended tests based on ten years testing inthe field and suggested merit rating system'. Proceedings CEAInternational Symposium on Generator Insulation Tests, June1980.

D KURTZ, M., and STONE, G.C.: 'In-service partial dischargetesting of generator insulation', IEEE Trans., 1979, EL-14,pp. 94-101

E RYDER, D.M., WOOD, J.W., and HOGG, W.K.: 'Thedeterioration of mica under the action of electrical discharges',IEEE Trans., 1975, PAS-94, pp. 1012-1020

F RYDER, D.M., WOOD, J.W., and HOGG, W.K.: 'A comparisonof the discharge resistance of natural and synthetic resin bondedmica insulation'. Paper presented at ISH Symposium, Zurich,1975.

DC102B

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