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IEEE Transactions on Energy Conversion, Vol. 3, N o. 4, December 1988

THE ABILITY OF DIAGNOSTIC TESTS TO ESTIMATE THEREMAINING LIFE OF STATOR INSULATIONG.C. Stone, H.G. Sedding*, B.A. Lloyd and B.K. G upta

Ontario Hydro Research DivisionToronto, Canada

833

Abstract - Stator windings fro m three generators and on e motor weresubjected to a wide variety of diagnostic and destructive tests prior tothe rewind of the machines. Diagnostic tests included insulationresistance, polarization index, capacitance, dissipation facto r tip-u p,partial discharge magnitude and discharge inception voltage. Thedestructive tests included breaking down individual coils with either ac,dc, or impulse voltages. The remaining life of the stator insulation is as-sumed (correctly or incorrectly) to be a function of the breakdown volt-age. Since the results of the diagnostic tests did not correlate with thebreakdown voltages, it seems that remaining life can no t be predicted o nthe basis of diagnostic tests alone. Equations and other relationshipsrecently developed in Japan to predict the remaining life of the statorgroundwall insulation system could not be confirmed. Diagnostic testsare most useful to indicate the trend in insulation aging in a particularmachine.Keywords: Stator insulation, diagnostic tests, remaining life

INTRODUCTIONForced outages of generators and motors in critical applica-tions can cost users millions of dollars in repair and outage costs. Reli-able operation of a rotating machine depends critically on the integrity ofits stator winding insulation, which is aged by exposure to a combination

of thermal, electrical, mechanical and environmental stresses. There-fore, utilities have a keen interest in diagnostic measurements to assessthe condition of the stator insulation in these machines. Such measure-ments help in planning maintenance and repair schedules. Minor repairscould be made in time to prevent forced outages with associated ex-pensive major repairs, enabling the utility to use its fiscal and humanresources most effectively.Many generating stations in North America are 25 years ormore old, and are nearing the end of their planned life. Financial condi-tions and regulatory constraints have made approval for construction of

new stations increasingly difficult, and extending the life of an existingolder plant may be a more palatable altemative. To determine whcthergenerator and large motor rewinds are necessary to achieve an extendedlire, methods are required to estimate the probable remaining life of ex-isting windings. The results of insulation diagnostic methods are crucialin planning the cost effective life extension of older stations.A direct measure of the integrity of an insulation system maybe the breakdown strength under dc, ac, and/or impulse stresses, asdetermined through destructive testing. Another more subjective meansof assessing the insulation's condition requires the dissection and exam-ination of some samples of the insulation. Unfortunately both of theseprocedures damage the winding, making the machine unserviceable.

* SERC Research Fellow, Brighton Polytechnic, U.K.

8 8 WM 026-7 A paper recommended and approvedtby the IEEE Rotating Machinery Committee of theIEEE Power Engineering Society or presentation atthe IEEE/PES 1988 Winter Meeting, New York, NewYork, January 3 1 - February 5 , 1988 . Manuscriptsubmitted August 31 , 1987; made available forprinting November 13 , 1987 .

Nondestructive diagnostic tests of the insulation system arerequired by maintenance engineers since, by definition. the insulationremains serviceable once such tests are completed. The credibility inestimating the insulation's condition using these diagnostic tests shouldbe well correlated to results of breakdown tests and dissections. How-ever, by their nature, experiments to establish a correlation between non-destructive and destructive tests are very costly and require availabilityof a machine for an extensive length of time. Because of the cost andlimited availability of machines for destructive testing, most authorshave evaluated the capabilities of different nondestructive diagnosticmethods with little experimental verification of any correlation with themeasured strengths of the machines [1.21.

The only extensive work on comparing diagnostic tests toremaining life, as determined by breakdown strength, has come fromseveral Japanese investigators during the last fcw years [3-6,9-121.These authors reported a correlation between the breakdown strength(expressed as a percentage of the value for new coils, or residualstrength) and various combinations of the results from diagnosticmeasurements eg, partial discharges, capacitance, dissipation factor,resistance, etc.Recently Ontario Hydro researchers [7 ] also performed manydiagnostic tests and measured strengths of coils in a 542 MW gcneratorwinding. Contrary to work reported in [3-61, the results did not show anycorrelation between the nondestructive and destructive measurements.Subsequently, tests partly funded by EPRI were performed on threemore machines of different designs to verify the existence or nonexis-tence of a similar correlation. The combined results from these ex-tensive tests on four machines are given in this paper.

WINDING S INVESTIGATEDFour different stator windings were tested to destruction in

this study. The details of the windings are outlined in Table 1. Thesmallest machine tested w as an 11 ,000 HP motor with a VP I epoxywinding; the largest machine was a 542 MW turbine generator with amicafolium insulation system. Of the four machines, three had mode minsulation systems. Only stator 1 did not have stress rclief coatings.Destructive testing of these machines was possible since theywere to be rewound. However, only one machine (stator 3) wasrewound because of deficiencies associated with the stator groundwallinsulation. Stator 3 was manufactured with imperfect semiconductiveand electric field grading coatings. When placed in service, this windingquickly deteriorated, with the slot and grading coatings "disappearing".By keeping the winding tight within the slot, rapid "slot discharge"deterioration was avoided, and no urlnding failure occurred. However,

for the 7 or so years before rewinding, stator 3 produced over 1 ppm ofozone within the generator enclosure and had the highest partial dis-charge activity of any generator in Ontario Hydro. Using the OntarioHydro on-line PDA test [8], discharges in excess of 10.000 pC weremeasured in this winding, which were correctly recognized as symptomsof deteriorated coatings.

As indicated above, the other three windings were rewoundfor reasons unconnected to the condition of the stator groundwall (Table1). In fact, based on in-service partial discharge testing as well as regu-lar inspections, the stator insulation of these three windings was ex-pected to be in good condition, in s pite of their long service lives.0885-8969/88/1200-0833$01.0001988 IEEE

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834Table 1Measurements on M achines Tested

STATOR 1 STATOR 2 STATOR 3 STATOR 4Rating 4 pole, 6.6 kV ,11,ooohT

MotorEpoxy resin bondedmica paper

4 pole, 24 kV,542 M WTurbine G eneratorShellac bondedmicafolium

48 pole, 13.8 kV80 M WHydro GeneratorEpoxy resin bondedmica paper

72 pole, 13.8 kV ,60 MWHydro G eneratorPolyester resinbonded mica split.tings20 years3000 pC

InsulationSystemOperating TimeIn-service PDMagnitude "1Reason forRewindVisual InspectionInsulationResistancePolarizationIndexdc RampCapacitanceDissipationFactorPartial Dischargedc Breakdownac BreakdownImpulseBreakdownDissection

56,000 hours700 pc

12 years2OOo Pc

13 years11,Ooopc

Coolant leaksurn-to-tuminsulation failuresYeS

Loss of semiconductivecoating on stator barsYes

Severe core prob-lemsYeses

No No Yes YesNoTo 20 kVAt 1,23,4,5,6 kV

NoTo 49 kVAt 3,6,9,12,15 kV

YesTo 30 kVAt 1,2,3,4,5,6kV

YesNoAt 1 and 8 kV

As aboveAt 24,5,6 kVWithstand onlyWithstand only

As aboveAt 6,10,14 kVWithstand onlyWithstand only

As aboveAt 4 and 6 kV

As aboveAt 6,8,10 kV

Yes YesNoYes

Withstand onlyNo

YesYes

YesNo

The ac diagnostic tests included the measurement ofcapacitance, dissipation factor and partial discharge intensity for a rangeof voltage steps up to rated line-to-groun d voltage. For the ac testing,the winding (or portion of winding) w as energized by a 30 kV , 40 VAHipotronics series resonant test set. In most cases, because the statorframe was solidly grounded, the capacitance bridge (a Guildline trans-former ratio bridge) had to "float" above ground potential. As with thedc testing, the stress relief coatings were not guarded out, because this isvirtually impossible in an installed winding.

TEST PROCEDURESDiagnosticTests

Prior to the destructive breakdown tests, the four windingswere subjected to a variety of nondestructive diagnostic tests which havebeen used in the past to help assess insulation condition. In most casesthese tests w ere done o n the com plete winding, individual phases, phasegroups and individu al coils or bars. As with all the breakdow n tests, allthe diagnostic tests were done with the coils or bars installed within thestator, that is, the coils were not removed from the stator core prior totesting.

Direct current diagnostic tests included insulation resistance(IR) and polarization index (PI), both done at 1000 V dc according toIEEE 3. These tests (as all the other tests) w ere done with the windinga t m m e mp er at ur e. No special precautions were taken to guard thestress relief coatings. A dc "ramp" test was also performed on three ofthe stators. In this test the dc leakage current was recorded as the volt-age was increased at a rate of 2 kV/min.

The partial discharge measurements were done w ith severaltypes of detectors. The Hipotronics detector operating in the"wideband, 70 kHz and 30 kHz modes was used. In addition, theOntario Hydro 10 MHz detector [l ] was employed, as was the PDA [81.In all cases, the discharge detectors were calibrated in picocoulombsusing the appropriate technique for the detector. Since the results withthe various detectors mo re or less agreed, only those from the OntarioHydro detector are given below.

Once the diagnostic tests on complete phases were finished,the windings were sectioned into coils. Diagno stic tests were then doneon individual coils. Due to time constraints, not all coils in a windingcould be tested. In general, about 60 coils were isolated in each windingfor testing. Further details of the testing are given in Table 1.

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835

W

a50 1 -aU

Destructive TestsAfter completing the diagnostic tests on individual coils, thegroundwall insulation of each coil was subjected to gradually increasingdc, ac, or lightning impulse voltages, until the insulation punctured. Thedc or 60 Hz ac voltage was manually increased at about 1kV/second. Apower transformer was required for the ac breakdown tests. since theresonant test supply could not sustain resonance at high voltages due to

discharging in the coil. The impulse voltage was a negative 1/40 pssurge. Three impulses were applied at each voltage level, with the volt-age raised in 5 kV ncrements until breakdow n occurred. Obviously , anyparticular coil or bar could be tested to breakdown with only one type ofvoltage.In some cases, flashovers occurred between the exposed coilcopper conductors and the stator frame. By insulating the exposed con-ductors and spraying SF6 in the vicinity, such flashovers were often pre-vented. How ever, in several cases surface flashover still occurred inspite of the above measures, and the true breakdown voltage could notbe determined. Details of the destructive tests are given in Tables 1and

5.

1 2 3 4

0.05 0.22 0.07 0.05-- _- -0.46 -0.06__ _ - -- 0.19

RESULTS

1 2 3 4

-- -- 0.35-0.140.24- -- --

Resultsof Diagnostic Tests

ACICO

Atan6QmaxDI VI R

Prior to destructive testing, various diagnostic tests were per-formed on the phases, parallels, coil groups, and coils in the winding asoutlined above (Table 1). Although a complete battery of nondestructivetests was performed, some of these tests essentially measure the same in-sulation parameters and are therefore well correlated. Fo r example, thechange in coil capacitance (ACICJ versus the change in dissipation fac-tor (Atans) from 1kV to operating voltage for machine 4 are highly cor-related (Figure 1 and Table 2). Physically both parameters are a measureof the void content in the insulation and we would therefore expect sucha high correlation. Other parameters such as peak partial dischargelevels (Q") at operating voltage and discharge inception voltage(DIV) show little correlation with CO , ACIC, or Atan6 (Figure 2 andTable 2). The discharge levels are a measure of the largest void dis-charging at a given applied voltage and may not be correlated withACIC, or AtanS, which are quantities averaged over the whole coil.

0.97 0.92 0.25 0.950.37 0.18 0.06 0.56

-0.31 -0.36 - 0 . 2 3 -0.22__ __ -0.6 - 0 .07

ACICo

:ator 1 2 3 4

0.4

o'5.310 1

0 0.2 0.4A T a n b ( X )

Figure I : Comparison of diagnostic quantities ACICo VS Atan6measured at 1 kV and 8 kV (stator I )3

Ev z l

0 ~ , 1 , , ( , , , , ( , , , , ,

Maximum Discharge Magni tude (1000 pC )

Figure 2 : Comparison of diagnostic quantities capacitance vs dis-charge magnitude measured at 8 kV (stator 4 )

Table 2Correlation ofDiagnostic Tests for Four Machines.Atan 6

1 2 3 4

0.45 0.32 0.53 0.53-0.31 -0.23 -0.15 -0.23

_ _ __ -0.69 -0.160.00._ __ __

I R

+ The numbers in the table are the correlation coefficients between different diagnostic tests for each stator.number close to either fl.O indicates that the tw o diagnostic tests are well correlated.about -0.5 and t0.5, they are essentially uncorrelated.

RIf the number is between

_ _ No measurements taken.

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83 6

100hz o o - ual2 80 -1t:3 70 -g 8 0 - o$ 50 -W 40 -U 30 -P

20 -LO -0

Due to the higher electrical stress on line end coils, degrada-tion of the semiconductive paint and possible void growth due to dis-charge erosion would be expected to be more severe in line end coils. InFigure 3, the capacitance at 1 kV is plotted as a function of coil positionin the winding f or stator 3. As expected if electrical aging were occur-ring, the capacitance shows a slight decrease towards the line end. Fig-ure 4 shows the change in capacitance with applied voltage which tendsto increase slightly as we move toward the line end thereby indicatinghigher void content in the line end coils. Howev er in both cases, thescatter in the data is larger than any measurable trend and only a weakcorrelation of capacitance with coil position exists for any of the ma-chines tested (Table 3). No correlation between coil position and maxi-mum discharge magnitude or discharge inception voltage was found(Table 3).

00 0

0 00 0

O D

, , , ( , , , , , , , , , , , , , , , (

8 I

1 3 5 7 9 11 13 15 17 ID 21Neutral LineEndnd Coil Loca t ion

Figure3: Capacitance as a function of coil position in the windingmeasured at I kV (stator 3)

2o5 1 _ jZ j10

-1

Nleutral 5 7 D 11 13 IS 17 I D 21LineEndnd Coil Loca t ionFigure 4: Change in capacitance as a function of coil position in thewindingfor I kV and 8 kV (stator3)

Table 3Correlation of Non-D estructive Tes ts Results with Coil Position

Sta tor 1 Sta tor 2 Sta tor 3 Sta tor 4ACICo -0.2 0.44 0.27 -0.11Atan6 -0.12 0.42 0.64 -0.12

-0.09 0.27 0.46 0.22DIV -0.38 0.10 0.11 0.66IR -0.36 -0.15PI 0.15

Results of Destructive TestsThe primary purpose of the destructive tests was to estimatethe remaining life of the insulation system, which is assumed to be afunction of the insulation strength. At present breakdown voltage is theonly indicator which has been proposed to provide a relatively quantita-

tive measure of remaining life. A full consideration of the advantagesand disadvantages of such an indicator are beyond the scope of thispaper.

Figures 5.6 and 7 show the breakdown voltages as a functionof the position of the co il in the winding of stator 3 using dc, ac, and im-pulse voltages respectively. Table 4 shows the correlations betweenbreakdown voltage and coil position for all four machines. From thedata in Table 4 it is evident that the breakdown voltage is not correlatedwith the position of the coil within the phase. If any significant electri-cal aging of the line end coils had taken place, the breakdown voltagesfor these coils would have been reduced.

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837EO

60

0

E o

00

Figure 6: AC breakdown voltage as a function of coil position in thewinding (stator3)

100

t:00

0 0 0O D0

0

o0

1 3 5 7 Q 11 13 15 17 18 21Neutral LineEndnd Coil Location

Figure 7: ImpuLre breakdown voltage as a function of coil position inthe winding (stator 3 )

Table 4Correlation of Destructive Tests Results with Coil Position

Sta tor 1 Stator 2 Stator 3 Stator 4

DC Breakdown -0.09 -0.27 0.59AC Breakdown 0.15 0.21 0.005

Table 5Breakdown Voltage (kV)

Sta tor 1 Stator 2 Stator 3 Stator 4Voltage Rating 6.6 24.0 13.8 13.8

DC BreakdownAverage 66 >I16 75 98Minimum 45 105 56 56No . of Coils Tested 9 14 22 21

AC BreakdownAverage 38 >49 38 51Minimum 29 24" 13 35No. of Coils Tested 10 5 23 21

Impulse BreakdownAverage 111 >I25 71 13 3Minimum 90 >125 60 90No. of Coils Tested 13 18 23 21

* breakdown at locations of previous insulation repairs

Comparison of D iagnostic and DestructiveTest ResultsTo examine the effectiveness of the various nondestructivetests in predicting insulation condition. we can compare the breakdownstrength of various coils in a winding with nondestmctive test results.Although exhaustive correlations were attempted for the various non-destructive and destructive measurements, only some of the parameterscommonly used to indicate insulation condition are presented here. Fig-ures 8-10 are plots of ac breakdown voltage versus some of thesemeasured or derived diagnostic parameters for stator 3. These includemaximum discharge magnitude, discharge inception voltage, and change

in dissipation factor (Atan6). Table 6 shows correlation coefficients forthese and other parameters for ac, dc. and impulse breakdown levelsfrom all four machines. The data with all four machines clearly show alack of correlation of any of the diagnostic parameters with the break-down levels.On two of the machines tested (3 and 4). insulation resistanceand polarization index were also measured. For stator 3, the insulationresistance of the coils was of the order 200-1000 GQ after 1minute andwas in excess of 1.5 TL2 after 10minutes yielding polarization indices of

5 or greater. Using the IEEE 3 criteria for polarization index levels, thewinding could be deemed to be in excellent condition. The highpolarization index values are also consistent with high dc and impulsebreakdown voltages but are suspect when considering the ac breakdownlevels (Tablet5). Th e polarization index is sensitive to the dampness andcontamination of a winding. It is essentially insensitive to the presenceof voids or delamination which are likely to result in pd under high acstress. Thus there are doubts about the usefulness of insulationresistance and polarization index (both dc tests) in assessing the condi-tion of materials operating under ac stress, at least for epoxy or polyesterwindings, when gross cracks, etc. are not present.

Impulse 0.17 _- 0.19 0.05Breakdown

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COMPARISON WITH OTHER STUDIES

- BO -5W 50 -M=:3 40 -d-U

30 -20 -

I%Ud 10 -

As described in the above section, no correlation was foundbetween the diagnostic tests and breakdown voltage (or presumablyremaining life), Figures 8.9, and 10. This finding is opposite to the find-ings of several manuf acturers [3-6, 9-12]. In a ll cases and in varyingdegrees, they found correlation betwecn breakdown voltage and parame-ters such as Atan6, Qmax , etc. As indicated in Figures 8-10 and Table 6,we could not confirm their findings. Additionally, predictive equationsfor specific applications were am ved at [5,9,12], and derived parameters[12] were postulated as predictors of insulation remaining life. Inseveral cases good correlations were indicated. When similar derivedquantities were calculated based on the Ontario Hydro data (Figures 11,12, and 13), no obvious correlation was found.

The contrast between this work and that of other studies inthis area is of considerable concem. Especially so when the other effortscame from a num ber of m ajor manufacturers and not just a single orga-nization. The other studies also covered a wide range of insulation sys-tems and applications, ie polyester and epoxy r esidm ica systems in ma-chines ranging from 3 kV motors (91 to 18 kV turbine generators [l l ] aswell as model coils [10,12]. It is therefore appropriate to consider pos-sible reasons for the discrepancies.70

00

50

40

30

20

10

0 I2W 4W BW BW

RC ohm farad

Figure 11:AC breakdown voltage vs the diagnostic quality RCmeasured at 1 kV (stator 4 )

70

0 0 0

0

8 10 12 14A = A C / C o + A t a n 8 ( X )

Figure 12:AC breakdown voltage vs the diagnostic quantity A measuredat I kV and 8 kV (stator 4 )

70

eo!!o ! , , , , , , , I ,

0 20 40 BO BO

83 9

0

QmK = Z ( A - 0.8) + 67 lo gm9Figure 13 :AC breakdown voltage vs he diagnostic parameter (stator 4 )

One possible difference is that the prior studies are primarilylaboratory-based with the testing done on individual coils which wereusually removed from the slot. In ou r studies, all coils were testedwithin the stator core, as would be the situation for diagnostic tests onnormal machine s. In the work presented in this paper, the electrode sys-tem was composed of the copper conductor - composite insulation -stator core iron. The contact between the stator bar surface and the coreiron is facilitated by a semiconductive coating. The integrity of the con-tact is dependent upon the condition of this coating, and how well thestator bar conforms to the slot shape. It is difficult to sec how so me ofthe electrode strategies in the other studies which employ silver paintground electrodes [ lo ] relate to the real situation. Obviously the use of aconductive paint provides an intimate contact. It is of relevance to notethat stator 3 had undergone severe deterioration of the semiconductivecoating. In the studies which used a simulated slot [12], a coating o fsemicond uctive paint was not applied. Anothe r significant difference insome studies [lo] was the omission or elimination of the end windinggrading paint. The distortions due to the influence of the nonlinearstress control system are well known and was acknowledged [lOj. How-ever, although acknowledged, the influence on the results does not ap-pear to have been explicitly considered.

Apart from testing differences, a major factor may also behow severely degraded the coil samples were prior to test. In the litera-ture, the insulation systems were polyester/mica or epoxy/mica. Thcsesystems were derived from machines which had service lifetimes com-parable to our study. In addition, the referenced studies made and agedsome bars specifically for experimental purposes. It is possible that theused coils employed in the other studies were very severely dcgradcdand very near to the end of their life. On the other hand most of thesamples tested by us had given no indication that their dielectric strengthhad been significantly impaired in service. Hence, diagnostic tests maybest as indicators of remaining life when the insulation is close to fail-ure. Diagnos tic measurem ents might then be relatively insensitive lifeindicators for bars which still have significant life remaining.

Since the referenced work is usually based on laboratory tcst-ing circumstances, and the results could not be verified by ourselvesusing diagnostic test procedures which can be practically applicd tostators which are expected to remain in service, equations which purportto predict insulation condition should be used with caution.

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84 0VALIDITY OF DIAGNOSTIC TESTS

Diagnostic ac and dc testing has a long history in helpingmaintcnance pcrsonnel dcterminc the condition of the insulation systemsin rotating machine stator windings . At the present time, mostmaintenance cngincers use diagnostic tests for trending, rathcr thanproviding objective estimates of remaining life [1,2]. For example, if in-sulation resistance, tip-up, partial discharge activity, etc is measured on aregular basis, when consistent dccreases in IR or consistent increases inlip-up or partial discharge magnitude arc noted over time, then a morein-dcpth invcstigation of the winding m ay be warranted. For three of thestalors tcstcd, such trending was done, and the condition of the insulationwas correctly prcdicted [1,8]. Unfortunately, cstimating the remaininglife of a winding is best done by a person who can draw on past experi-encc with similar insulation systems, manufacturers, operating practices,etc. The outcome is a very subjcctive estimate.

Since individuals with such experience are not always avail-able, it would be highly desirable if the remaining life could be objec-tivcly estimatcd from test results alone. Based on the testing of theabovc four windings, it seems that diagnostic tests alone cannot objec-tivcly estimate remaining life, especially if the testing was only doneonce. However it docs seem that at least on complete windings, somediagnostic tests, such as partial dischargc magnitude, do indecd give anindication of relative condition [1,8]. On thrce of the windings, diag-nostic tcsting done on the entire machine indicated that the insulationwas rclatively healthy, in spite of the many years of service aging (Table1). This condition was gcnerally verificd by the high average breakdownvoltages and winding dissection. The fourth machine, stator 3, had ex-trcmcly high partial discharge activity and high tip-up, and was verifiedto be in rclatively poor condition by visual inspection and some very lowac breakdown voltagcs. How evcr, given the cost of testing, too few ma-chines wcrc tested to derive any quantitative relationship between thediagnostic tests on complcte windings and lowest winding breakdownvoltagc, or somc other numerical indicator of remaining life. It is un-likely that sufficient data will ever be collccted for a quantitative rela-tionship to bc developed, especially when a rclationship would be neces-sary for every different insulation system and operating environment.

Considerable statistical data was collected however, on therclalionship bctween diagnostic tests on individual coils and the break-down voltages of those coils. As described above, no correlation wasfound. That is, coils with the worst results from the diagnostic testsoften did not have the lowest brcakdow n voltage. Thus, at least for indi-vidual coils, a sct of diagnostic tests taken at one time do not seem to beable to predict the insulation condition. If the condition of individualcoils could not be predicted, one wonders if diagnostic tests on entirewindings could reliably predict the condition of the winding with asingle measurement or group of measurements at one point in time.We therefore conclude that until a more searching diagnostictcst is dcvclopcd, the estimation of remaining life must be based on thetrcnd of the diagnostic tests over time, as wcll as the availability of ahowledgeable expert to visually examine the winding and assess otherpcrtincnt information. To help rotating machine users understand thecomplcxity of cstimating the remaining life, a Handbook is bcing devel-oped under EPRI project RF'2577-1.

CONCLUSIONS1. AC and dc diagnostic tcsting'of stator windings gives useful in-formation on the relative condition of the winding insulation.The information from such diagnostic tests is most useful whenrcsults are taken over time, so that a historical trend can bedctcrmincd.2. Based on tests on individual coils, existing diagnostic tests, whendone at one time, arc not able to predict breakdown voltages. Fur-thermore, no group of tests on individual coils scems to be able topredict breakdown voltages. Thus, other widely publicized workwhich has established such a relationship could not be duplicated.

It is assumed that coil breakdown voltages relate to rcmaining lifein some fashion.

3. To estimate remaining life of rotating machine windings not Onlyrequires that diagnostic tests be done (preferably over time), butthat an individual with expericnce in judging aging processes andaging rates, as well as other relevant data, be available to examinethe winding.The most desirable test strategy is to perform all types of diag-nostic tests, since each test can give a slightly different view ofthe winding condition. However, as a minimum, at least one ac(preferably a partial discharge test) and one dc test (insulationresistance) should be done.There is some evidence that ac hipot testing may be more sensi-tive to insulation weaknesses in real windings than dc hipot test-ing.

4.

5.

ACKNOWLEDGEMENTSThe tests described above were very time consuming and expensive, re-quiring the cooperation, funding and technical knowledge of several or-ganizations and individuals. We are indebted to the production branchof Ontario Hydro w hich allowed us access to machines, sometimes caus-ing the extension of machine outages. In particular we thank B.J.Quartemaine and J.F. Lyles as well as the staff of Pickering Nucleargencrating station, Beck GS , and Harmon G S . We also thank Messrs.DiPaul, Jack, Campbell and Anderso n who did most of the testing. Thiswork was partly funded by EPRI Projects RP 2307-1 and RP 2577-1, Dr.D.K. Sharma and Mr. B.S. Bemstein, project managers. Dr. H.G. Sed-ding would also like to thank the British SER C for funding his visit toOntario Hydro.

REFERENCES1. Kurtz, M. and J.F. Lyles, "Generator Insulation Diagnostic Test-ing," IEEE Transactions on Power Systems and Applications, Vo lPAS-98, Sept 1979, p 1596.Simons, J.S., "Diagnostic Testing of High-V oltage Machine Insula-tion," IEEProceedings, Vol 127, Pt.B, No.3, May 1980. p 139-154.Yoshida, H. and Y. Inoue, "Test Methods of Rotating Machines,"IEEE Transactions on Electrical Insulation, EI-21, No. 6. Dcccm-ber 1986, p 1069-1071.Yoshida, H. and K. U memoto, "Insulation Diagnosis for RotatingMachine Insulation," IEEE Transactions on Electrical Insulation,EI-21, N0.6, December 1986, p 1021-1025.Kadotani, K., T. Myashi ta , F. Ako, and K. Matsunobu, "An Ap -

proach for Insulation Diagnosis of Mica-Resin Coils," IEEE Trans-actions on Power Systems and Applications, Vol PAS-100, No.,September 1981, p4136- 4141.Kadotani, K., T.Hakamada and S. Yamatake, "A Proposal for In-sulation Diagnosis of 3 kV M otor Stator Windings," IEEE Transac-tions on Electrical Insulation, EI-18, No.1, February 1983, p 59-63.Gupta, B.K., M. Kurtz, G.C. Stone and D .K. Sharma, "DestructiveTests on a 542 MW Generator Winding," Conference Record of1986 IEEE International Symposium on Electrical Insulation,Washington, DC, June 9-1 1, 1986.Kurtz, M ., J.F. Lyles and G.C. Stone, "Application of Partial Dis-charge Testing to Hydrogenerator Maintenance," IEEE Transac-tions on Power Systems and Applications, August 1984, p 2148.Tsukui, T., M. Takamura and Y. Kako, "Correlations between Non-destructive and Destructive Tests on High-Voltage Coil Insulationsfor Rotating Machines," IEEE Transactions on Electrical Insula-tion, Vol. EI-15, No. 2, April 1980, p 118.10. Kadotani, K. and Y. Kako. "Capability of Insulation Diagnosis forMica-Resin Insulated Coils," IEEE Transactions on Electrical In-sulation, Vol. EI-15, o. 6, Dccember 1980, p 481.11. Kaka Y., K. Kadotani, S. Kenjo, S. Hirabayashi, T. Tani and F.Natsume, "Multi-Stress Degradation of Insulation Systems for HighVoltage Rotating M achines," CIGRE Paper No. 15-02, 1982.12. Itoh, S.. T. Tani, H. Koshiba and T. Kawakami, "An Estimation ofBreakdown Voltage of High-Voltage Motor Insulation by RCValue," Electrical Engineering in Japa n, Vol.l04A , No.1, 1984, p7.

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841Dis cus s ion

W. McDermid (Mani toba Hydro, Winnipe g, Cana da):The aut ho rs have repo r ted da ta us in g a number of commondia gno s t i c and des t ru c t ive t es t methods and have mades ome in te r es t ing compar i s ons .ance and po l a r i z a t io n index may no t be us ef u l f o rpo ly es te r wind ings . Th i s i s c o n t r a r y t o t h e e x p e ri e n ceof Manitob a Hydro.

We have twelve 12 0 MVA 13.8 kV h y d ro g e n e r a t o r s w i t hp o l y e s t e r- m i c a i n s u l a t e d m u l t i - t u r n c o i l s t a t o rwind ings. On two occas io ns , i nvo lv ing d i f f e r en t un i t s ,we have exper ien ced low i n s u l a t i o n r e s i s t a n c e a nd lo wp o l a r i z a t i o n i n d ex r e a d i n g s o n so me , b u t n o t a l l , s t a t o rc i r c u i t s f o l l ow i n g th e o p e r a ti o n of t h e f i r e p r o t e c t i o nwater de luge s ys t em.

In the cas e o f Uni t 5, o ne s p l i t p ha s e ha d a 1 minutei n s u l a t i o n r e s i s t a n c e o f 7 1 4 megohms and a p o l a r i z a t i o nindex of 1. 5 a t a t empera tu re o f 22"C, whi le mos t o th ers p l i t p h a s e s h a d a n I R of12COmegohms and PI of a tl e a s t 4 . 4 . Some clea nin g and dryin g was done but d idno t improve the cond i t ion o f t he one s p l i t phase andf i n a l l y tw o s t a t o r c o i l s h ad t o b e r e p l a c e d . B ot hdefe c t s were found t o be a t t h e p o i n t a t wh ic h t h e c o i le x i t s t h e s l o t a nd a p pe a r ed t o i n v o l v e a c r a ck o r t a p es epara t ion i n the g roundwal l . B oth co i l s had beeni n vo l ve d i n l i f t s d ur i ng t h e o r i g i n a l i n s t a l l a t i o n .a n l m i n u t e i n s u l a t i o n r e s i s t a n c e of b e tw ee n 1 an d 2megohms and po l a r i z a t io n index va l ues o f 1 . 0 a t approx-imately 60"C, w h i l e t h e t h i r d p h a s e h a d a n I R of 7 0 0megohms and PI of 6 . 4 , The above da ta were o b t a i n e da f t e r a p r e l im i n a r y d ry o u t . F o l lo w i ng e x t e n s i v ec l e a n in g of o r i g i n a l l i f t c o i l s i n t h e en d tu r n a r e ai m m ed i a te l y a d j a c e n t t o t h e s l o t , a nd f u r t h e r d r y i n g ,i t w as p o s s i b l e t o r e t u r n t h e u n i t t o s e r v i c e w i t h o u th a v in g t o r e p l a c e a n y c o i l s .

I n a d d i t i o n t o m o i st u r e , b r a k e d u s t a nd s o o t a r eb e l i e v e d t o h a v e been f ac to r s i n making the g roundwal lc r a c k s o r t a p e s e p a r a t i o n s f u l l y c o n du c t iv e [l].

I t i s r e p o r t e d t h a t m ea s ur em e nt s of i n s u l a t i o n r e s i s t -

I n t h e c a s e o f U n i t 6 , two phases were found t o have

Reference[ 11 W. McDermid, " Inves t igat ion of Groundwall Defects

in Polyes ter-Mica" , Minutes of t he 50 th AnnualIn te rna t ion a l C onference o f Doble C l i e n t s , 1983,Sec. 7-301.

Manuscript received February 17 , 1988

A . W . W. Cameron (Retired, London, ON, Canada): The reported lack ofcorrelation between short-time AC breakdown voltage and other experienceor diagnosis of remaining insulation service life may be explained by know nVoltage Endurance characteristics. The high short time AC strength of aninsulation sample is determined mainly by the effective thickness of solicinsulation in it. The droop of any AC voltage endurance curve towardsreduced breakdown voltages of identical samples, after long times ofstressing, is caused by progressive ionic erosion of voids, by localizedheating by void discharges, and by dielectric heating approaching runawayin remaining solid insulation.

This supports the Authors' suggestion that low AC strength of a samplefrom an aged machine would indicate that it would soon have failed inservice. I would suggest that long-time AC voltage endurance tests of suchsamples might be more informative for less advanced deterioration andlonger prospects.It should be emphasized that Direct Voltage tests have the sole purpose ofdetecting cracks and fissures through the groundwall. Such damage isusually caused by mechanical vibration or impact, aggravated by localizedheating from discharges, strand faults, or core faults. Direct overvoltagetests by the Ramp Method give reliable non-destructive indications whendone on a winding which has cooled to ambient temperature while open toair of normal humidity, to p rovide a little moisture to enhance conduction inany fissure. These facts are w ell known to the Authors, but still not widelyenough appreciated.These two Papers present very valuable means of assessing the futuretrend of a stator winding's reliability, in terms of probable ability towithstand "events" incidental to operation, such as overvoltages andovercurrents from external causes. There are now two requirements for aUtility to realize the economic benefits of such information:1. Assessment of the probabilities of such events and their probableseverities in future periods of time.2. Realistic estimates of the dollar values of reliability levels of givenmachines at future times, w hereby the indicated repairs and rewinds canbe times for optimum costibenefit ratio.

Manuscript received February 23, 1988.

G.C. Stone, B.A. Lloyd a n d B.K. Gupta would like tothank Mr . McDermid and Mr. Cameron for their interest in ou r paper. M r.McDerm id's examples of how the insulation resistance (IR) andpolarization index (PI) tests can be useful to determine the condition ofepoxy or polyester insulated windings are very pertinent. As stated in thepaper, such tests are useful for any type of insulation to determine ifexcessive moisture (such as occurs after a deluge) or major flaws such ascracks are present. Thes e dc tests are also uscful if direct comparisons canbe made, such as the discusser's con trasting a "good" and a "bad" phasc.Our point is that these tests taken in isolation are rclatively incffectivc fo rthe more usual types of aging which occur in thermosetting windings. Forexample, PI and IR are essentially useless in evaluating the degree ofaging caused by slot discharge.

Another variant of dc testing is mentioned by M r. Cameron.DC ramp tests to twice the phase-to-phase operating voltage were done onmany phases and coils in our test program. Thc dc ramp test resultsessentially duplicated the IR and PI data. Since none of the tested bars o rcoils failed below the test voltage, we did not have the opportunity toutilize the dc ramp test's principle advantage: the ability to abort thc tcstprior to puncture.

As ou r tests have shown, assessing insulation conditionshould not be done on the basis of any single mcasurcm ent. All tes ts ma yyield useful information. when put into perspective with past tcst data,insulation type, prior problems, etc. We believe much more cfforl isrequired bcfore objective, reliable estimates of insulation remaining lifecan be determined.Manuscript received April 15, 1988.