16
Diagnostic testing of high-voltage machine insulation A review of ten years' experience in the field J. S. Simons, B.Sc. (Eng.), C. Eng., M.I.E.E. Indexing terms: Insulators and insulation, Nondestructive testing, Instrumentation and measuring science, Review of progress Abstract: Following a review of the basic philosophy of preventive maintenance testing and the theory of degradation mechanisms leading to failure, available test methods used for assessing the state of high-voltage machine insulation are discussed. Preferred test methods introduced ten years ago for field measurements between the winding and the grounded core are detailed, together with the reasons for their selection and their significance. Test data obtained from measurements on nearly five hundred machines are presented in terms of eight criteria and four groupings based on rated voltage, namely 3-4-9 kV, 5-7-9 kV, 8-12-9 kV, and 13 kV and above. To illustrate the practical application of the diagnostic data, a number of case histories are briefly outlined. It is concluded that a standardised programme of nondestructive measurements carried out periodically on high-voltage stator windings can identify trends in generalised degradation, reduce unplanned outages and allow refurbishment to be carried out at an early stage. Of the several criteria used, the measure- ments of integrated discharge energy and associated charge/voltage loop trace displays have been particularly helpful in indicating changes in structural integrity associated with cumulative degradation. They have also been of value in detecting anomalous discharging. The need is recognised for additional diagnostic tests to be developed to detect localised defects. 1 Introduction For more than twenty years, manufacturers of high-voltage machines have maintained an active interest in the develop- ment of improved test methods. 1 ' 2>3>4>s>6 These six references each have a substantial bibliography. Consider- able effort has been made to assess the relative merits of high voltage a.c, d.c. and low-frequency testing. Most workers have concluded that no single test is likely to provide sufficient data to allow a satisfactory assessment of insulation quality to be made. Whereas during the 1960s most activity was concerned with evaluating the test methods themselves, in the last decade 7 ' 8> 9t 10 preferred tests have been selected and applied in the field. These tests are now reviewed and proposed methods of assessing future insulation life are discussed. 2 Basic philosophy of maintenance testing A number of surveys made in different parts of the world have shown that the cost of outages arising from failure of machines in service represents a significant percentage of the intial capital cost of the machines. Whatever the incidence of insulation faults, it is clearly of advantage to the user if outages can be planned and complete shutdown of plant avoided. One way of minimising this problem is the introduction of periodic nondestructive measurements to detect significant changes in the state of the insulation. Such measurements can be supplemented by high-voltage tests at convenient times to seek out localised defects not detectable by global measurements or other monitoring techniques. Paper 676B, first received 1st November 1979 and in revised form 14th January 1980. Subject review paper Mr. Simons is with GEC Machines Limited, Mill Road, Rugby Warwickshire CV21 1BD, England IEEPROCEEDINGS, Vol. 127, Pt. B, No. 3, MA Y1980 It is evident from experience that a significant part ot the overall objective of high reliability and availability of large machines can be achieved by maintaining the integrity of the insulation of the machine. It is recognised that other». factors, such as bearing or rotor failures, need to be con-" sidered in the overall programme of preventive maintenance". However, this review is concerned solely with high-voltage stator insulation and its appraisal. When considering the science of diagnostic testing it is helpful to draw an analogy between on the one hand, the medical practitioner and his patients and, on the other, the specialist engineer applying his expertise to the appraisal of high-voltage machines. No two patients are identical in all respects, and in some cases only a relationship estab- lished over several years enables the specialist to interpret correctly the significance of the scientific data available. Further, sudden collapse or death can sometimes occur with little warning. Thus, whereas periodic medical checks do not, of themselves, ensure a definitive prediction of further life, they increase substantially the probability of detecting serious illness before it has reached a terminal condition. This enables remedial action to be taken, including, in some cases, surgery. So it is with machines; an assessment of the hazards and likely faults followed by systematic preventive maintenance testing and associated corrective action minimises unplanned outages which threaten the profitability of a user company. The advantage of planned maintenance testing is thus the prospect of the utilisation of the full potential life of the insulation of the winding. It also enables a rewind to be planned based on monitoring the ongoing rate of deterio- ration of the insulation as judged by selected successive tests. This allows the overall risk and probability of failure to be assessed. It is sometimes practicable to make localised repairs, e.g. remove individual coils, before complete replacement is carried out. 139 0143-7038/801030139 +16 $01-50/0

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Page 1: Diagnostic testing of high-voltage machine insulation. A review of ten years' experience in the field

Diagnostic testing of high-voltage machineinsulation

A review of ten years' experience in the field

J. S. Simons, B.Sc. (Eng.), C. Eng., M.I.E.E.

Indexing terms: Insulators and insulation, Nondestructive testing, Instrumentation and measuring science,Review of progress

Abstract: Following a review of the basic philosophy of preventive maintenance testing and the theory ofdegradation mechanisms leading to failure, available test methods used for assessing the state of high-voltagemachine insulation are discussed. Preferred test methods introduced ten years ago for field measurementsbetween the winding and the grounded core are detailed, together with the reasons for their selection andtheir significance.

Test data obtained from measurements on nearly five hundred machines are presented in terms of eightcriteria and four groupings based on rated voltage, namely 3-4-9 kV, 5-7-9 kV, 8-12-9 kV, and 13 kVand above. To illustrate the practical application of the diagnostic data, a number of case histories arebriefly outlined. It is concluded that a standardised programme of nondestructive measurements carried outperiodically on high-voltage stator windings can identify trends in generalised degradation, reduce unplannedoutages and allow refurbishment to be carried out at an early stage. Of the several criteria used, the measure-ments of integrated discharge energy and associated charge/voltage loop trace displays have been particularlyhelpful in indicating changes in structural integrity associated with cumulative degradation. They have alsobeen of value in detecting anomalous discharging.

The need is recognised for additional diagnostic tests to be developed to detect localised defects.

1 Introduction

For more than twenty years, manufacturers of high-voltagemachines have maintained an active interest in the develop-ment of improved test methods.1'2>3>4>s>6 These sixreferences each have a substantial bibliography. Consider-able effort has been made to assess the relative merits ofhigh voltage a.c, d.c. and low-frequency testing. Mostworkers have concluded that no single test is likely toprovide sufficient data to allow a satisfactory assessment ofinsulation quality to be made. Whereas during the 1960smost activity was concerned with evaluating the testmethods themselves, in the last decade7'8> 9t 10 preferredtests have been selected and applied in the field. Thesetests are now reviewed and proposed methods of assessingfuture insulation life are discussed.

2 Basic philosophy of maintenance testing

A number of surveys made in different parts of the worldhave shown that the cost of outages arising from failure ofmachines in service represents a significant percentage ofthe intial capital cost of the machines. Whatever theincidence of insulation faults, it is clearly of advantage tothe user if outages can be planned and complete shutdownof plant avoided. One way of minimising this problem is theintroduction of periodic nondestructive measurements todetect significant changes in the state of the insulation.Such measurements can be supplemented by high-voltagetests at convenient times to seek out localised defects notdetectable by global measurements or other monitoringtechniques.

Paper 676B, first received 1st November 1979 and in revised form14th January 1980. Subject review paperMr. Simons is with GEC Machines Limited, Mill Road, RugbyWarwickshire CV21 1BD, England

IEEPROCEEDINGS, Vol. 127, Pt. B, No. 3, MA Y1980

It is evident from experience that a significant part otthe overall objective of high reliability and availability oflarge machines can be achieved by maintaining the integrityof the insulation of the machine. It is recognised that other».factors, such as bearing or rotor failures, need to be con-"sidered in the overall programme of preventive maintenance".However, this review is concerned solely with high-voltagestator insulation and its appraisal.

When considering the science of diagnostic testing itis helpful to draw an analogy between on the one hand,the medical practitioner and his patients and, on the other,the specialist engineer applying his expertise to the appraisalof high-voltage machines. No two patients are identical inall respects, and in some cases only a relationship estab-lished over several years enables the specialist to interpretcorrectly the significance of the scientific data available.Further, sudden collapse or death can sometimes occurwith little warning. Thus, whereas periodic medical checksdo not, of themselves, ensure a definitive prediction offurther life, they increase substantially the probability ofdetecting serious illness before it has reached a terminalcondition. This enables remedial action to be taken,including, in some cases, surgery. So it is with machines; anassessment of the hazards and likely faults followed bysystematic preventive maintenance testing and associatedcorrective action minimises unplanned outages whichthreaten the profitability of a user company.

The advantage of planned maintenance testing is thusthe prospect of the utilisation of the full potential life ofthe insulation of the winding. It also enables a rewind to beplanned based on monitoring the ongoing rate of deterio-ration of the insulation as judged by selected successivetests. This allows the overall risk and probability of failureto be assessed. It is sometimes practicable to make localisedrepairs, e.g. remove individual coils, before completereplacement is carried out.

139

0143-7038/801030139 +16 $01-50/0

Page 2: Diagnostic testing of high-voltage machine insulation. A review of ten years' experience in the field

3 Theory of degradation mechanisms leading to failure

The stator winding of a high-voltage machine is subjected toa combination of thermal, electrical and mechanicalstressing. Depending on the size, rating and operating con-ditions of a particular machine, the relative magnitudes ofthese stresses can vary substantially. Typical nominalelectrical stress levels can vary between 2—2-5 kV mm"1.Normal mechanical bar forces can vary up to 10 kg cm"1.Differential movement of copper and insulation on a longstator bar can be up to 10 mm, depending on the lengthand maximum operating temperature. There is also thematter of transient stressing under impulse conditions. Peakstresses can be raised by a factor of five or greater,depending on the duty of the machine. Thus, a largewinding of long core length subjected to rapid starting andstopping, e.g. a hydrogenerator or pump storage generator,will experience thermal cyclic stresses not experienced by acontinuously loaded synchronous machine. Similarly, arelatively small machine running at full load continuouslywith some overloading will tend to have a combined stresscharacteristic dominated by thermal stress. On the otherhand, a larger direct-online-started machine, driving acompressor with associated vibrational stress, will have acombined stress characteristic dominated more bymechanical stress, i.e. bar vibration and shock loading.

High transient overstressing, e.g. a short circuit or aserious overvoltage on the system, can result in failure.Such failures are not predictable by periodic measurementsof insulation quality. Preventive maintenance testing is thusmost applicable to generalised deterioration of insulatingproperties associated with specific types of degradation.••> We can summarise the processes of degradation associated

HAvith the different stresses as follows:Thermal stress: Cumulative thermal degradation re-

sulting in delamination, cracking,embrittlement or depolymerisation.

Electrical stress: Cumulative electrochemical effectsof internal partial discharging withassociated discharge erosion insideand outside the bar.Transient pulsing from switchingsurges and system disturbance e.g.direct-online starting, tripping onoverload.

Mechanical stress: Differential expansion and contrac-tion. Bar vibration producing frettingbetween copper, insulation and core.Shock loading.

Environmental stress: Contamination from water, oil, dust,carbon, salt, rust, sand. Radiationdegradation.

As a result of the above stresses and degradationmechanisms, consequential failure is usually associated witha combination of several processes of deterioration.

In some statistical surveys of machine failures it isapparent that, because of undetected defects, environmentalcontamination prior to commissioning at hostile sites, andmalfunctioning of plant during commissioning, a significantnumber of failures occur on new windings. It is considereddesirable, therefore, that new windings should be checkedbefore commissioning and after 1—2 years service. There-after, the period between subsequent tests can be increasedas considered appropriate.

For rotating-machine insulation, it is the combinedthermal, mechanical and electrical degradation processes

which produce general or localised weakening of thestructure. Thus, if such structural weakening can beassessed, the future life of the insulation can, hopefully, beestimated from known data, relating the quantity measuredto time to failure.

In view of the markedly different dielectric character-istics of insulation systems, differences in discharge erosionresistance, in thermal endurance and in dynamic mechanicalproperties, it is necessary to establish the relationshipbetween life and the test parameter used for the particularinsulation system. This can be determined partly from dataobtained in the field and partly from combined functionaltest rigs in the laboratory.

4 Available test methods used for assessing insulationproperties

Preventive maintenance testing in the past has tended to belimited to the periodic application of an overvoltage testequivalent to 1-5 times the rated line voltage of themachine. High-voltage d.c, 50 Hz and, more recently, low-frequency have all been used and have particular advantagesand limitations. There are an increasing number of standardscurrently specified in America and Europe and manynational standards, IEEE 43, VDE 0530, BS 4999, ESI,44.5 and IEEE 433. Specific standards cover the testing ofboth single and multiturn coils. High-voltage d.c. is attractivefor overvoltage tests because of the reduced size and weightof the test equipment. It is effective for locating defects buttends to increase stress concentration in the endwindingregions. These are more vulnerable to environmentalcontamination than the slot portions. The use of 50 Hzreproduces the normal stress distribution at the end-windings, but requires a larger power supply and testequipment. In areas of higher dielectric loss it can also leadto localised dielectric heating. Low frequency offers thebest overall compromise but, as yet, has not been widelyadopted. 0-1 Hz has been the frequency most used.Recognising the limitation and potential risk of failure inapplying proof tests to windings in the field, much efforthas been directed in a search for suitable nondestructivemeasurements.

These generally take the form of tests applied to thecomplete winding to earth and/or the individual phases.When testing individual phases the rest of the windingis usually earthed. Additional information can be obtainedby testing between each phase and the others, with the core,only, earthed. Such tests can be carried out over a periodof several hours, thus allowing the machine to be recom-missioned within twenty-four hours. Usually, the machinewill be shut down overnight, some end covers removed forthe winding to be inspected and testing carried out with thewinding cool. The machine terminal connections and coversare reassembled by the following evening.

When there is a protracted outage, it is possible toremove the rotor if considered necessary, and to carry outtests on portions of the winding, scanning slots for partialdischarge. Such investigations are usually carried outfollowing a failure or when a localised problem, such asloose bars or wedges, have been diagnosed. The followingtests on complete windings and individual phases have beenselected by different workers in the field, and are docu-mented in the bibliographies of the references given:

(a) Low-voltage direct-current measurement of insulationresistance and determination of polarisation index (p.i.) i.e.ratio lOmin ij . / l min i.r.

140 IEE PROCEEDINGS, Vol. 127, Pt. B, No. 3, MA Y 1980

Page 3: Diagnostic testing of high-voltage machine insulation. A review of ten years' experience in the field

(b) High-voltage direct-current measurement, referred toas the direct-current leakage test. A step-voltage test, inwhich the voltage is increased in a specified manner, duringwhich time the measured current is observed.

(c) 50 Hz dielectric loss and capacitance measurementsover a range of voltage usually to rated line-voltage incre-ments of 0-2 Vjine.

(d) Dielectric loss analyser measurements of integrateddischarge energy (/iJ/cycle pF), recording of charge transferper cycle as a loop trace.

(e) 50 Hz resonant circuit-discharge measurements. Themagnitude of the quadratic rate of partial discharge magni-tude is measured using a narrowband amplifier at spotfrequencies in the range 10-130 kHz, and the currentexpressed as a pseudo-mean value in A/F or C/F.

( / ) Measurement of partial discharge magnitude using awideband amplifier at 130 kHz.

(g) Measurement of dielectric loss at 0-1 Hz. Measure-ment of partial discharge magnitude at 0-1 Hz.

5 Preferred test methods adopted for diagnostic testingin the field

Bearing in mind the need to limit testing time in the field,to ensure that a machine can be returned to service inapproximately 24 h, and in view of difficulties arising fromsupply harmonics and airborne interference, the testmethods detailed later in this Section were standardised.They have been employed for ten years in the field.

The several tests are carried out both on the completewinding and on individual phases with the rest of thewinding earthed. The results of the tests are analysed andan overall assessment made of the state of the winding usingselected criteria from each test. The selection and signi-ficance of these criteria are discussed in the followingSection.

Before initiating the tests, it is important to isolate thewinding from all external cables, observing appropriatesafety procedures and site regulations.

The following standardised test procedure is used:(a) A measurement of d.c. insulation resistance and

determination of polarisation index at 1000, 2500 or5000 V, as appropriate.

(ft) The measurement of 50 Hz insulation resistance,capacitance and integrated discharge energy value at Vphase>0-8Vj,ne and Vjine using a dielectric loss analyser (d.l.a.)and taking a photographic recording of the charge-transfer/voltage-loop trace, for l l kV windings in the field, it iscustomary to test at lOkV rather than l l k V (VKne),to avoid the need to change the instrument range and tomaximise the loop trace area. This facilitates evaluation ofthe shape of the loop trace. An additional measurement canbe made at rated line voltage if necessary.

(c) The measurement of 50 Hz dielectric losses at incre-ments of voltage (0-2Vhne up to line voltage, recordingcapacitance and tan 5.

(d) To establish that localised defects in the majorinsulation do not exist, each phase of the winding is givena 1 min high-voltage proof test at 0 1 Hz using a peakvoltage equivalent to l-5V/jnc 50 Hz r.m.s. and a 01 Hz:50 Hz equivalent ratio of 1-15. The rest of the winding isearthed; this test is optional. In choosing the above testsequence, the following tests were not included for thereasons stated:

(i) High-voltage direct-current leakage tests werediscarded as time consuming, difficult to interpret, sensitive

to external effects and producing an abnormal stress distri-bution in the endwindings.

(ii) Resonant-circuit partial-discharge measurementswere discarded because of difficulties in eliminatingexternal or mains borne interference and in establishingadequate resolution of individual discharge measurements.Bearing in mind the nature and number of dischargespresent and the complexity of their changing spectra withtime, it was concluded that an integrated energy measure-ment was more appropriate and more likely to yieldmeaningful data.

(iii) 0 1 Hz loss and discharge measurements are a recentdevelopment and little data has been published to date.Bearing in mind the increase in testing time associated withsuch tests and problems of collation with 50 Hz data, theadvantage of such measurements over 50 Hz tests have yetto be established.

6 Significance of preferred tests

In this Section, the significance of the preferred testsembodied in the standardised test sequence and proceduresare reviewed.

6.1 D. C. insulation resistance test

Before the application of high alternating voltage to awinding, it is necessary to establish that it is neither toodamp nor too contaminated to constitute a risk of failure.These conditions can result in excessive leakage current,leading either to surface discharging at points of discon-tinuity or to localised thermal instability and overheating.The d.c. insulation resistance test and associated compu-tation of polarisation index fulfils this function (see IEEE43). Its major limitation is that it tends to reflect surfaceconditions and a realistic assessment of the state of the bulkof the insulation is not usually possible. In the event of lowinsulation resistance and associated low polarisation indexbeing obtained, steps should be taken to dry the windingand remove severe contamination before proceeding witha.c. tests. A satisfactory p.i. value is normally > 2 but,providing the insulation resistance value is acceptably high,a somewhat lower value of p.i. need not of itself constitutea basis for rejection. A further consideration is the need totake account of the size and capacitance of the windingwhen assessing the level of insulation resistance. A smalllow-capacitance winding is likely to have substantiallyhigher initial insulation resistance than a large winding witha discontinuous-type insulation system. The value for thelatter should preferably be greater than 100 MSI at ambienttemperature and, with modern insulation systems, can besubstantially higher.

A useful criterion of insulation quality is the value ofohms x farads, the product of the 1 min insulation resis-tance and the winding capacitance, measured as M£2 andMF, respectively.

When carrying out d.c. tests, for reasons of accuracy andsafety it is essential that the winding be adequately earthedboth before and after the application of the test potential.For larger windings and a test voltage of 5000 V, a periodof thirty min is recommended, to ensure adequate dis-charging, following earthing.

The insulation resistance of individual phases with theremainder of the winding earthed should be obtainedbefore a value for the complete winding. The value of onephase to earth is usually approximately twice that of the

IEEPROCEEDINGS, Vol. 127, Pt. B, No. 3, MAY 1980 141

Page 4: Diagnostic testing of high-voltage machine insulation. A review of ten years' experience in the field

complete winding. To assist in assessing the state of thewinding, reference can be made to data plotted in Fig. 1.Here, Rw is the insulation resistance of the completewinding, and Rph ^ 2RW. Pt is a factor dependent on thecondition of the winding and on the ambient temperatureat which the test is carried out.

If the polarisation index, defined as the ratio of thelOmin/lmin values of insulation resistance, is less than1-5, and the one-min insulation resistance is below therecommended level, the winding should be given a dry-outbefore proceeding with high-voltage a.c. tests. Undercertain circumstances, the low insulation resistance mayresult solely from superficial dampness.

In carrying out a dry-out, it is important to controlcarefully the rate of temperature rise during the initialheating period and to limit the rate of temperature riseduring the initial heating period, and to limit the maximumtemperature as recommended by the manufacturer. Aconvenient method of following the progress of the dry-outis to take insulation resistance readings and to plot theseand winding temperature against time (Fig. 2). If substan-tial amounts of water are trapped in the winding, dry-outcan be a protracted business.

6.2 Dielectric loss analyser

Of the several tests which form part of the overall assess-ment programme, dielectric loss analyser (d.l.a.) testshave proved to be especially helpful9'10 in assessing thestate of winding insulation. The instrument is basically acapacitance-type bridge with loop trace display of charge

lOOOp

100

010uo

01

00110* 20* 30* 40* 50* 60#

winding temperature, *C70* 80 ' 90»

Fig. 1 Recommended minimum insulation resistance of completewinding Rw

Rw = Pf(kV + 1)MJ2, where kV is line voltage of machine inkilovoltsN.B. On a 3-phase winding when testing separate phases, then theinsulation resistance of each phase Rph is approximately twice thevalue of the complete winding i.e. Rph — 2RWa New machine — dry, imaged and cleanb Machine after service running — clean and uncontaminatedc Machine after service running for several years — with normal

industrial contamination

transfer versus voltage developed in 1959,4 a paralleldevelopment in America by Dakin3 being described also atthat time.

6.2.1 Derivation and interpretation of d.l.a. loop trace: Thecharging and discharging sequence of insulation containingmany distributed internal cavities is a complex one. Itproduces across the detector points of the bridge a poten-tial which increases in small discrete steps. The envelope ofthe charging/discharging sequence is a 50 Hz wave with alarge harmonic content. The derivation of the loop trace asa charge/voltage pattern over a cycle is shown in Fig. 3.

3-0

.oo•£ 10

L 0

120

100

80

20 40 60 80 100 120 140time, h

ten-minute (R10)insulation resistance^

160

one-minute (R,)insulation resistance

0 20 40 60 80 100 120 140 160time, h

Fig. 2 Winding dryout. Insulation resistance, polarisation indexand temperature against time of dryout

sinusoidalreference voltage

360

Fig. 3

J = }

Jc =

where

time

Derivation of d.l.a. loop trace display

Q'v<* + E vq

Q

— J/cycle /iF or /uJ/cyclepF

; is dynamic inception voltage r.m.s.M is maximum charge transfer per half cycleX is capacitance of specimen

142 IEE PROCEEDINGS, Vol. 127, Pt. B, No. 3, MAY 1980

Page 5: Diagnostic testing of high-voltage machine insulation. A review of ten years' experience in the field

This charge/voltage pattern is seen to be a summation ofincrements of charge transfer as voltage is increased abovedischarge inception together with charge retained duringnondischarging portions of each half cycle.

The charge/voltage characteristics of individual portionsof the winding vary in shape and magnitude. The resultantcharge/voltage characteristic will be a geometrical sum-mation of the individual loops (see Figs. 4, 5 and 6). Theseshow results obtained from d.l.a. measurements on a test

15-4-30-6 cm

Fig. 4 Loop trace displays - epoxy mica bar

Lengths quoted above each diagram give position and size of centraltest electrode

108-1-123-3cm

loop trace of the entire test area- 10kV0-15A-3cm

Fig. 5 Loop trace displays - epoxy mica bar

(Continuation of Fig. 4)

bar, initially subdivided into ten sections, tested separatelyand, subsequently, the whole of the bar was assessed. Thebar was selected as one with considerable variation inproperties along its length. The mean discharge energy losswas computed from the ten traces and compared with theenergy loss derived from the loop trace obtained on thewhole bar. In addition, the size and shape of the overalltrace was calculated from the ten separate traces forcomparison with the observed trace dimensions. In bothcases, agreement was obtained within 1%. Thus, the meanvalue of integrated discharge energy at high voltage reflectsthe averaged value of associated void content and associatedstructural integrity.

The loop trace shape, displaying as it does the incre-mental charge transfer in each half cycle, is responsive tochange in partial discharge inception voltages and variationsof nonlinear conduction. All these changes have diagnosticsignificance (see Figs. 7, 8 and 9). Fig. 7 shows a wide rangeof loop traces obtained on windings in the field. Fig. 8illustrates the modifying effect of moisture and oily dirton loop trace shape and Fig. 9 illustrates the detection ofspark discharging between coils and core. An importantaspect of the latter loop traces is the superposition on thenormal smooth charge-transfer/voltage-pattern of spikypulses. These represent discrete large charge transfers fromsparking between coil and core, which occur at particularinstants in the cycle. Because of their magnitude, they aredetectable on the loop trace, particularly at lower voltagewith the assistance of the d.l.a. amplifier, despite theoverall dilution effects present.

As demonstrated later in the paper, if localised degradedsites exist in the winding, which are significantly inferiorto the remainder and involve only a few per cent of totalwinding capacitance, e.g. one coil, they are detectable byan increase in integrated discharge energy level of theparticular phase (see Fig. 15 and case history).

The phenomenon of spark discharging has been widelyreported since the introduction of epoxy-mica-type coilinsulation. It arises from loosening of wedging and blockingleading to bar vibration and consequential damage to thecorona shield. This results in spark discharging between

1 2 3

guard guardtestelectrode

15 cms

Fig. 6 Analysing geometrical summation of loop traces

Calculated mean energy loss/cycle pF

10Energy loss/cycle

_ n=j10

n = 1

where Cn is incremental value of capacitance of 1S cm of test bar• observedA calculatedTotal measured energy value = 7-41 /ij cycle pFComputed value from 10 sections = 7-48 nJ cycle pF

IEE PROCEEDINGS, Vol. 127, Pt. B, No. 3, MAY 1980 143

Page 6: Diagnostic testing of high-voltage machine insulation. A review of ten years' experience in the field

coil and core. Fig. 9 loop traces a,b,c and d were obtainedin the laboratory by setting up a single bar in a simulatedslot and progressively isolating the corona shield. Looptraces / and g were obtained on two machines in the field(see case histories). Subsequent removal of wedges fromthese confirmed that localised external damage to somecorona shields had occurred. The latter were refurbishedbefore the machine was put back into service.

In considering the significance of particular charge/voltage loop traces, it is important to recognise the

normal average

high inception

large discharges

yhigh energy symmetrical

1/5

high energy slightlyasymmetrical

1/10

signif icant variationof discharge inception

nonlinearity

sharp onset

very high energymarked asymmetry

low inceptionsymmetrical

high loss

low energya " * • — - b '

complete winding phase/phaseinfluence of contamination

Fig. 7 D.L.A. loop traces of phases of windings

low-energy clean windingrbitumen moderately high-energy: bitumen

moderate-energy moisturecontamination: shellac

gross endwinding contamination:bitumen

Fig. 8 D.L.A. loop traces of phases of windings

144

influence of temperature, prolonged stressing and nonlineardielectric loss on the loop trace pattern.

6.2.2 Effect of temperature on d.l.a. patterns: An increasein temperature will result in a decrease in the time constantof charge redistribution of the insulation with associatedchanges in discharge inceptions. This will tend to reduce thetotal number of discharge sites and reduce the number ofdischarges per cycle. However, the magnitude of thedischarges may be increased. The change in temperaturewill affect the breakdown strength of any gas present invoids, and an associated change in permittivity of the soliddielectric with temperature will increase the stress andassociated energy transfer across any discharging voids. Onthe other hand, if the dielectric loss of the insulationincreases to a relatively high value > 0 1 , the charges on theinsulation will decay more rapidly. This will result in anapparent decrease in energy transfer. At elevated tempera-ture, nonlinearity effects will be increased and someincrease or decrease in energy transfer will be observed. Itis recommended that successive integrated discharge energymeasurements of windings be compared at the sametemperature i.e. near ambient. Between 20°C and 40°C,little change in discharge energy values has been observed.

6.2.3 Effect of prolonged stressing on d.l.a. pattern: Whena sample of insulation with voids is stressed for a longperiod, the currents flowing in the insulation surroundingthe voids may be sufficient to generate some heat loss inthe insulation. If significant, this localised power loss willincrease the temperature of the insulation in that area,

d,

Fig. 9 D.L.A. loop traces-spark discharging

a Single bar in simulated slotb Corona shield removed from one sidec Corona shield isolated from slot-spark discharged As c but 11-8 nF in parallele Double inception with multiple coils/ Epoxy winding: slot dischargingg Epoxy winding: slot discharging (gain X 2)

IEE PROCEEDINGS, Vol. 127, Pt. B, No. 3, MAY 1980

Page 7: Diagnostic testing of high-voltage machine insulation. A review of ten years' experience in the field

increasing the permittivity. This will result in changes in thelocalised field concentration and in the number and size ofindividual discharges per cycle.

Based on measurements made on samples in the labora-tory and in the field, it appears that the interaction of allthe above factors results in a relatively small change in totalenergy per cycle over a considerable range of temperatureand time. Thus, whereas the pattern of individual chargetransfer is affected by many factors, the integrated chargetransfer per cycle is more stable and reproducible, reflectingbasic structural configuration.

6.2.4 Detection of nonlinearity using a d.l.a.: If nonlin-earity, i.e. harmonic distortion, exists in the specimencurrent, it is not possible to balance the dJ.a. loop trace toa straight line, and characteristic nonlinear traces areobtained (see Figs. 7 and 8). Most dielectrics tend to havestress dependent properties, nonlinear to varying degrees,and amplification of the d.l.a. Y signal enables the degreeof nonlinearity to be assessed. Thus, the d.l.a. will detectsignificant ionic movement at low stress (Garton effect)and represents a more sensitive method of detecting non-linearity than a narrowband a.c. bridge.

The introduction of a 50 Hz filter in the detector circuitof the d.l.a. converts it to a 50 Hz a.c. bridge if total loss isrequired.

6.2.5 Relationship between integrated discharge energy,void content and insulation life: We have seen that the totalintegrated charge transfer is directly related to the voidcontent at high voltage when the voids are ionised. From ananalysis of the charge/voltage diagram it can be shown thatthe relationship between integrated discharge energy andvoid content at high voltage V is

Jc = (void content) 8 ( K -

(V \= (void content) 8 V] I—- 1 e

yi I

where V = applied voltageV( = the dynamic discharge inception voltage from

the loop tracee = the permittivity of the solid insulation

Jc = the integrated discharge energy in J/cycle F(usually expressed as juJ/cycle pF).

Thus, the overall void content is reflected in the globalvalue of integrated discharge energy Jc at high voltage (seeFig. 10) and Jc reflects the general structural integrity.Changes in structure associated with cumulative degradationare also detectable by corresponding changes in theintegrated discharge energy/voltage characteristic.

The empirical relationship established from practicalmeasurements for the dependence of integrated dischargeenergy on applied voltage is of the general form J = Kx~m,where J is integrated discharge energy, x = VJV and A" is aconstant. Tests on insulating materials under a.c. stress14

have indicated a relationship of life « (Vjvy1.If integrated discharge energy is related to VJV by the

general equation J' = K(yiVi)m, it is to be expected thatintegrated discharge energy will be related inversely to timeto failure.

As has already been pointed out, machines are subjectedto combined stressing with associated mechanical, thermaland electrical degradation. In so far as all three stresses con-tribute to gradual structural deterioration, reflected in an

increase in void content, a measurement of integrateddischarge energy may be expected to reflect this structuralchange. In addition, for a given insulation system andintegrated discharge energy level, there will be a corres-ponding life expressed in terms of an inverse powerrelationship. Fig. 11 shows the influence of structuraldegradation of micaeous insulation on capacitance,integrated discharge energy and increase of tan 5 withstress.

In so far as an increase in dielectric loss with increasedstress relates to the onset of internal partial discharging invoids, an approximate assessment of the integrated amountof discharge energy can be made using the relationship

power = V2 wCA tan 5

where V is the r.m.s. value of loss tangent voltage, tan 5 isthe increase of loss tangent due to discharges and C is thecapacitance of the specimen at voltage V.

In the case of windings in the field, the modifying effectof surface contamination on the endwindings influencesboth capacitance and dielectric loss. This is helpful indetecting the build up of contamination over a period inservice. However, it complicates interpretation of changesof tan 5 in terms of structural defects. By comparison,integrated discharge energy values expressed in terms of

50

u.a

30

10

very high void content

- moderate void content

low void content

A 6 8applied voltage.kV r.m.s.

10 12

Fig. 10 Integrated discharge energy against voltage: relationship tovoid content

IEE PROCEEDINGS, Vol. 127, Pt. B, No. 3, MA Y1980 145

Page 8: Diagnostic testing of high-voltage machine insulation. A review of ten years' experience in the field

per unit capacitance are less influenced by surface effectsand can be used with greater assurance for assessingstructural changes during service. Fig. 12 shows resultsobtained on a synchronous condenser before and afterthe removal of substantial endwinding contamination. Theresults show the greater influence of contamination on tan5 results. Figs. 13 and 14 show comparative integrateddischarge energy and tan 5 data obtained from three 30 MWturbine generators over a period of six years. Examinationof the comparative data displayed in Figs. 13 and 15 showsthat the calculated value of Jc from A tan 5 results in aconsiderable discrepancy, dependent on how much of thetan 5 characteristic is being influenced by changes of solidloss with stress. It is evident that the integrated dischargeenergy/voltage characteristics, significantly less affected asthey are from variations in solid loss, are easier to interpretin terms of progressive structural change. This is furtherdemonstrated by the comparative curves of capacitance,integrated discharge energy and A tan 5, plotted in Fig. 14.

Both capacitance and tan 5 values are more susceptible tochanges arising from contamination than are values ofintegrated discharge energy. This response to contaminationby the tan 6 test, however, is a valuable diagnostic aid, andis complementary to the assessment of structural change

40

35

£25>.^ 2 0a.-""15

10

5

12h

1 19783 1977

1 19763 19731 1973

2 1975-19782 19742 1972

2 4 6 8 10kV

J Jc *machine dla calculatedno. 10kV fromatanS

393140293222242223

473952454330313131

6 8voltage, kV

10 12

24h K)0h160°C 180°C

100h 100h 100h 1000h200°C 220°C 240°C 240°C

Fig. 13 Comparison of measured and calculated integrateddischarge energy values. 30 MW turbine generators

Fig. 11 Comparison of capacitance, integrated discharge energyand max A tan 8 /AV against ageing time

16

14

12

10- 3- 2

&

10

a, 6

before cleaning

after refurbishment

J Jdla calc j c a [ C

tan6 jdia

7 12 1-7

6 6 10

1974 after refurbishment

1 2 3 4 5 6voltage, kV

Fig. 12 Influence of endwinding contamination on loss-tangent/voltage characteristics and integrated discharge energy - syn-chronous condenser

32Or

310

1972 1973 1974 1975 1976 1977 1978time

Fig. 14 Capacitance, A tan b I AV and integrated discharge energyagainst years of service. 30MW turbine generators

146 IEEPROCEEDINGS, Vol. 127, Pt. B, No. 3, MA Y1980

Page 9: Diagnostic testing of high-voltage machine insulation. A review of ten years' experience in the field

obtained from the integrated discharge energy measure-ments. Fig. 15 shows data obtained on an 11 kV motor 14years old, before and after a fault on phase C. Followingremoval of the fault-line end coil, the value of integrateddischarge energy for the phase was reduced by 13-6%. Thisdemonstrates the ability of the d.l.a. to detect one seriouslydegraded coil, representing, as it does, only a few percentof the total capacitance of the winding. Here, again, thevalues of integrated discharge energy calculated fromchange in tan 5 are shown to be excessively high by avarying amount.

6.3 50 Hz dielec trie loss measuremen ts

These have been discussed in the context of the dJ.a. Thea.c. dielectric losses measured at the terminals of a windingreflect a combination of dielectric phenomena which needto be separated and analysed correctly if a useful judgmentis to be made of the relative contributions of internal andexternal effects. Thus, 50 Hz loss tangent measurements canbe strongly influenced by the type of stress grading used,particularly in the case of a machine with a short core.

This effect is illustrated by comparative tests on a shortbar length initially unguarded, guarded, and with differenttypes of stress grading (see Fig. 16). The presence of surfacecontamination can produce marked nonlinear variation ofdielectric loss with stress.

Loss tangent versus voltage measurements have beenused for many years as a shop floor quality control test forindividual coils and windings as made (BS4999, Pt.61:1977), but because of the above complications, they do noton their own provide, for complete windings, adequatediagnostic information regarding likely future life. The

influence of stress grading on winding characteristics willvary considerably from winding to winding.

6.4 0-1 Hz overvoltage proof test

The tests described in the preceding Sections are designed tobe nondestructive, while providing diagnostic informationwhich can be related to the potential risk of failure. Thereare, however, certain highly localised types of defect, e.g.pinholing from an iron particle, which cannot be detectedexcept by a go-no-go-type proof test. The relative merits of50 Hz a.c. versus d.c, versus low frequency have alreadybeen much discussed.4'6>9 0-1 Hz testing was selected asoffering the best solution in reducing size and weight of testequipment while retaining an acceptable stress distribution.This test is only applied with the specific consent of theparticular customer and taking due account of the con-sequential outage for localised repair should an electricalbreakdown occur. The voltage level used has a peak valueequivalent to l-5xline voltage. This has been foundadequate to detect local sites of significant weakness.

In only a few cases has breakdown occurred during thetest, and in every such case this has been associated withlocalised mechanical fretting or cracking. The test does notresult in significant burning or erosion of copper and/oriron and, thus, facilitates the consequential refurbishment.Such tests do not need to be carried out frequently but canassist in doubtful cases when additional assurance of limitedfuture life is required.

It is recognised that acoustic or electromagnetic probetests may, in certain circumstances, help to locate sitesof localised discharging. In addition, the possibility ofintroducing online monitoring on certain key machines isbeing explored.

20r

18

16

u

J Jd.l.a. calc Jcalc

tan6 Jdla

phase C i after removalof fault coil

CW completewinding

2 U 6 8 10kV

6 8voltage, kV

10

20

18

16

U

212X

"co>10ainmo 8

6

2

wmm25^0 m^25

mm mm

stress grading y>type 2 /

//

/ stress/ grading type 1

• 1 — - ^unguarded;no grading yS

— " ^

°/o increase incapacitance

^ ^ 29

. 50

y A

^ (datum)

guards

JcdlaS>

10 kV

182

105

11-1

5-3

6 8 10 12voltage, kV

Fig. 15 Comparison of measured and calculated integrateddischarge energy values 3730 kW motor

Fig. 16 Influence of stress grading on tan 6 and integrateddischarge energy

IEE PROCEEDINGS, Vol. 127, Pt. B, No. 3, MA Y1980 147

Page 10: Diagnostic testing of high-voltage machine insulation. A review of ten years' experience in the field

Table 1: Basis of assessment of condition of machine

Visual inspectionRecord condition of endwinding insulationCheck wedging, packing and blockingEvidence of type and amount of contamination if present

Initial criteria reviewed(a) D.C. insulation resistance 1 min value(b) Polarisation index(c) A.C. insulation resistance /?, d.l.a.id) A.C. capacitance C, d.l.a.(e) Compute tan 6 = MtjRC at 0-2 VL (see also (/))(f) Compute ohm farad value, MSI X MF(g) Observe d.l.a. loop trace at Vphase, 0-8 VXive and Vlirie

(/?) (/) Record loop trace areas and compute integrated energyvalues at l/p/, and V/irK

(/) Using 50 Hz bridge measure tan 6 and C at 0-2, 0-4, 0-6,0-8 and VUne

(k) Compute tan b VL — tan 6 0-2 VL and max A tan 6/AVCompare data with that for similar windings and any previousresults obtained (see Table 2)Assess results in relation to years of service, rate of change oftest criteria with time, and influence of environmental hazards

7.1 Criteria from d.c. test

(a) 1 min insulation resistance(b) Polarisation index(c) Ohm farad, the product of 1 min insulation re-

sistance and winding capacitance, in microfarads.

7.2 Criteria from d.l.a. test

(a) Integrated discharge energy at Vphase

(b) Loop trace shape at Vp^ase\ evidence of anomolouscharge transfer

(c) Integrated discharge energy at VUne

(d)R and C values at low stress (tan 6 = 1/coRC)

7.3 Criteria from loss tangent/voltage test

(a) Loss tangent level at 0 2 VUne

(b) Loss tangent VUne — loss tangent 0-2 VUne

(c) (Optional) max A tan 5/AK

7 Criteria selected for assessment of winding insulationquality

The general basis of assessment of winding quality is toselect preferred criteria for each of the standardised testsand, hence, compute the overall state of the insulation. Torelate the assessment to likely future life it is necessary totake account of such factors as the type and size ofmachine, the type of insulation system, the years of service,the mode of operation, any environmental hazards and thecondition of the winding as observed by visual examination(see Table 1).

The overall computation can be assisted by the intro-duction of a merit marking system2 in which marks areassigned to specific criteria. Such a system is to be thesubject of a separate presentation. The criteria which havebeen used for general assessment are:

8 Review of field test data

The very wide range of values obtained in the field over tenyears for each of the above criteria is summarised in Table2. They are presented in terms of four categories based onrated voltage, namely, 3-4-9 kV, 5-7-9 kV, 8-12-9 kV and13—16kV. In total, over 500 machines have been appraised,300 rated 11 kV, over 50 5-6-6 kV and a number of largegenerators 100-150 MW in the voltage range 13-8-18kV.The latter include both steam and water turbine generatorsand motor/generators at pumped storage sites.

It has already been noted that, in assessing the signi-ficance of the test data, it is necessary to take account ofspecific design and operational factors which dictate thetypes and magnitudes of stresses in service. It is helpful toreview such considerations in terms of the four machinevoltage categories selected.

Table 2: Range of values of selected criteria obtained in the field for different voltage levels — complete winding data

Test criteria(refer to Table 1)

Range of values obtained in the field

21 machines 347 machines

13-16kV 8-12-9 kV

63 machines

5-7-9 kV

53 machines

3-4-9 kV

1 min insulationresistance, MSI(a)Polarisation index10 min i.r./1 min i.r.(b)Loss tangenttan 6 at 0-2 Viine

(e) or (/)Capacitance, /iF(d)Ohm farad1 min i.r. X capacitance

(f)Integrated discharge energya t vphase M-J/cyclepF(h)Integrated discharge energya t vlim MJ/cyclepF

)Tan 5 i(/) (Ar)

125-1500

1-5-6-0

0-015-0-061

0-63-1-67

113-902

0-1-12-5

3-2-62-2

0003-0053

17-18 000

10-7-7

008-0-175

0025-1-45

1 -9-2753

0-05-11-3

2-5-70

0007-0-187

3-8-5000

1-1-5-7

0011-0-211

0023-0-723

2-2-1808

0-05-1-3

0-41-27-2

0003-0-116

11-17 000

1-0-10-0

0012-0-247

0-025-0-43

1-7-1440

< 001-0-30

0-01-2 4

0001-0044

148 IEE PROCEEDINGS, Vol. 127, Pt. B, No. 3, MAY 1980

Page 11: Diagnostic testing of high-voltage machine insulation. A review of ten years' experience in the field

8.1 Machines 13 kV and above

When comparing data derived from steam turbine generators,water turbine sets and individual drive motors, it is necessaryto recognise that the operating conditions, magnitude ofthe bar forces and nature of the overall duty may differsubstantially. Thus, a water-cooled steam-turbine generatorwill not be operated normally above 60°C, and thedominant stresses experienced by the insulation aremechanical/electrical.

In the case of a pumped storage mot or/generator, thedominant stresses are thermal cyclic with superposing ofmechanical/electrical stress. Fig. 17 shows the range ofintegrated discharge against voltage characteristics obtainedfrom both types of machine, and Fig. 18 shows the corres-ponding loss-tangent envelope. Selected curves are num-bered to enable the two sets of data to be compared.

8.2 Machines 8-12-9 kV

In this group are the vast majority of machines tested, someof which are generators but most being motors driving avariety of loads. The nature of the load and the environ-mental conditions can vary tremendously. Compressor drivemotors appear to suffer particularly arduous conditions ofcombined stressing, and have to be operated frequently insevere environments. An increasing number of 11 kVmotors are switched direct-online and some plant appli-cations demand frequent switching. Transient voltageswitching surges will tend to seek out degraded turninsulation of line-end coils. Fig. 19 shows the wide envelopeof integrated discharge energy characteristics obtained forthis group, and Fig. 20 shows the corresponding tan 5

60r

50-

^ 4 0 -

0.30 •o

20

10

high void content

against voltage data. Some of the curves are numbered toenable the two sets of data to be compared.

8.3 5-6-6 k V machines

The general comments made concerning 11 kV machinesapply equally to the above group. However, because of thereduced voltage and associated integrated discharge energy,the dominant stress tends to be thermal/mechanical withfailures more likely to be in the endwinding region, par-ticularly if contamination is present. In some cases, slotinsulation failures have been recorded associated withpartial discharge damage or excessive transient surgesleading to failure of line-end coils. Fig. 21 shows the rangeof integrated discharge energy characteristics obtained forthis group, and Fig. 22 shows the corresponding tan 5against voltage envelope. The curves are numbered, toenable the two sets of data to be compared.

9 Some representative case histories

The introduction of preventive maintenance testing atspecific sites tends, initially, to be directed to older machinesof relatively high risk, subsequently being extended toother machines, as the value of the maintenance programmebecomes established. In only a relatively low percentage ofthe many machines tested has a seriously degraded con-dition been detected requiring immediate replacement ofwindings. As the regular testing programme is implementedand early action taken to deal with such problems ascumulative contamination and movement and migration ofwedging and blocking, so the incidence of critical degra-dation has been diminished.

To illustrate the practical application of the preferredtest programme and its value to date, a few case histories

10r

0 6

8 10 12 14applied voltage, kV r.m.s.

16 18

. gross oilcontamination

6 8 K) 12applied voltage, kV r.m.s.

18

Fig. 17 Machines 13 kV and above; range of integrated energyagainst voltage

Fig. 18 Machines 13 kV and above; range of tan 6 against voltagecharacteristics

IEE PROCEEDINGS, Vol. 127, Pt. B, No. 3, MA Y1980 149

Page 12: Diagnostic testing of high-voltage machine insulation. A review of ten years' experience in the field

are quoted. Some test data are given in specific cases toindicate the basis of assessment.

rewound; this recommendation was accepted by the userand the work was carried out during a planned outage.

9.1 11 kVhydrogenerator (built 1961)

After 10 years' service, as a result of a flash flood themachine was submerged in water. An extended dry out wascarried out and the machine put back into service. Non-destructive tests demonstrated that, even after dry out andmany months of full load running, water was still trappedin the windings and a high risk of electrical failure existed.

The following test data were obtained after dryout:1 min insulation resistance = 64 MQPolarisation index = 1-4Ohm farad = 22-8Integrated discharge energy at 6-3 kV = 1-1 /J/cycle/pFLoop trace shape - markedly elliptical at 6-35 kV

completely unstable at 8 kVLoss tangent at 2 kV = 0 086

= 0096,load unstable0-1 Hz test lOkV peak only — voltage regulation

excessive leakage current

It was therefore recommended that the stator should be

i i

50[- very high void content

£30

20

10

-moderate voidcontent

low voidcontent

4 6 8applied voltage, kVr.m.s.

10 12

Fig. 19 Machines 8-12- 9 k V; range of integrated discharge energyagainst voltage characteristics

9.2 Large 11kVmotor (built 1952)

This machine is a vital drive at an establishment, and had,on occasions, been loaded in excess of the design rating.The user was concerned about the condition of the windingand the possibility of failure resulting in an extended shut-down for repairs. Nondestructive tests showed the windingto be sound electrically, and gave the user confidence in thereliability of his plant.

It was recommended that further tests be carried out at3-yearly intervals to assess the ongoing rate of deterior-ation. The winding was reassessed over a period of sevenyears, and the following data was obtained on the completewinding:

Criterion

Insulation resistance, MJ2Ohm faradIntegrated dischargeenergy at 6-35 kVIntegrated dischargeenergy at 10kVTan 6 at 0-2 VuneTan 6 at 10 kV

22r severe oil and carboncontamination

20

Age in years

19

550310

20

26

00520090

23

400220

2-5

24

00420091

25

325178

2-1

24

00430087

18

16

U

tan

O

y• oily dirt

cleanlow void content

5

2 4 6 8applied voltage, kV r.m.s.

10

Fig. 20 Machines 8-12-9kV; range of tan 6 against voltagecharacteristics

150 IEE PROCEEDINGS, Vol. 127, Pt. B, No. 3, MAY 1980

Page 13: Diagnostic testing of high-voltage machine insulation. A review of ten years' experience in the field

18

16

li. 12Q.a-

10

S 8

high void content

A 5 6 7applied voltage, kVr.m.s.

Fig. 21 Machines 5- 79 kV; range of integrated discharge energyagainst voltage characteristics

20-

18

16

K

severecontamination

5 6 7applied voltage, kV r.m.s.

Fig. 22 Machines 5-7-9kV; range of tan 8 against voltagecharacteristics

IEE PROCEEDINGS, Vol. 127, Pt. B, No. 3, MA Y1980

Some further satisfactory service was anticipated but, aspart of a moderisation programme, it was decided to rewindthe motor to provide an increased transient protectionlevel. The old winding was examined, and autopsy of coilsshowed the condition of the winding to be consistent withthe diagnostic conclusions. The coils tested still retained aninterturn electric strength level > 8 kV.

9.3 11 kV3730kWmotor (built 1958)

This machine drives a gas compressor. Diagnostic testsindicated that there was likely to be substantial degradationof line end coils with C phase noticeably the worst. Threemonths later, a line end coil of Cphase failed. After isolating,the rest of the winding was rechecked. A 14% reduction indJ.a. energy values was measured on C phase, with nosignificant changes on the other phases. The completewinding data showed a 10% reduction in d.l.a. energy level,with some reduction in slope of the tan 5/voltage curve (seeFig. 15). This illustrates the complementary roles of d.l.a.discharge energy and tan 5 measurements. The followingdata summarises the two sets of tests:

0-6

0-5

0-1

0 1 2 3 4 5applied voltage, kV r.m.s.

Fig. 23 Machines 3-4- 9 k V; range of integrated discharge energyagainst voltage characteristics

151

Page 14: Diagnostic testing of high-voltage machine insulation. A review of ten years' experience in the field

Criterion

Ohm faradIntegrated dischargeenergy at 6-35 kVIntegrated dischargeenergy at 10kVTan 6 at 0-2 VUne

Tan 6 at V/jnc

Complete winding

Beforefailure

1484-9

44

00420-131

Afterfailure

1093-3

40

00490-127

Phase C

Beforefailure

4-2

44

00410-156

Afterfailure

872-9

38

00520-139

9.4 Nine 11 kV generators (built 1935)

These are installed in a hydroelectric scheme. Two othermachines had already failed in service and were rewound.Following these incidents, a programme of planned rewindsfor all machines was decided upon. Diagnostic tests wereused to establish the condition of each machine, andcustomer was advised on the order in which the rewindsshould be carried out.

Two machines were found to be still in relatively goodcondition, and for these a rewind was not necessaryimmediately. The rewind programme proceeded systemati-cally over several years, and dissection of the windings fullyconfirmed the diagnostic data.

10

0,6CT

o

•25

0 1 2 3 U 5kVr.m.s.

Fig. 24 Machines 3-4-9kV; range of tan 6 against voltagecharacteristics

9.5 11 kV motor (rewound 1968)

Nondestructive tests were carried out on a newly rewoundmachine, and revealed alarmingly high discharge energyvalues > 5 0 / L J at 10 kV. The user was advised that earlyfailure was likely and that a further rewind or replacementshould be provided. This diagnosis was confirmed by afailure three months later.

9.6 Six 11 kV mo tors (built 1958-1959)

These machines drive motor generator sets. There is a largenumber of machines in the plant. Two winding failures hadoccurred. Tests carried out on six machines indicated signi-ficant variations in the general condition, winding voidcontent and failure risk. The remaining machines weresubsequently tested to obtain an overall assessment, onthe basis of which a programme of rewinds or replace-ments was recommended minimising the incidence ofunplanned outages. The diagnostic measurements identi-fied three different insulation systems having differentlife characteristics (see Figs. 25 and 26). It was sub-

50

£30

20

10

6 8v o l t a g e #kV

10

Fig. 25 Six 11 kV 1865kW drive motors; integrated energyagainst voltage characteristics

152 IEEPROCEEDINGS, Vol. 127, Pt. B, No. 3, MAY 1980

Page 15: Diagnostic testing of high-voltage machine insulation. A review of ten years' experience in the field

sequently established that the type and amount of turninsulation on systems A and B were significantly different.This illustrates the importance of correlating the diagnosticdata with the specific insulation system involved. System Crepresents an epoxy rewind.

9.7 Four 16kV hydrogenerators

Following an insulation failure during routine maintenancetesting of one machine, all four machines were tested. Thetests indicated that the winding of the machine in questionwas seriously weakened as a result of service incidents andcontamination with oil (no. 10, Fig. 18).

It was recommended that the machine should berewound at the earliest possible opportunity and thisrecommendation was accepted. The condition of the otherthree machines was reported on and the assessment of theirrelative conditions provided a basis for future action onthese also.

9.8 11 kVmotor (built 1967)

This machine drives a compressor, and had been subjectedto very heavy oil contamination. Although it had beencleaned subsequently, and varnished, the tests indicatedhigh losses in the windings, with thermal instabilityattributable to oil having dissolved in and softened thebond of the endwinding insulation.

The following data were obtained on the completewinding:

Ohm farad

287

Integrateddischargeenergy at6-35 kV

2loop traceevidenceofnonlinearsolid loss

Integrateddischargeenergy at9 k V

8-7loadregulating

Tan 6at2kV

0-167

Tan 5at 10 kV

0-190unstableload

It was subsequently established that this winding had alow-loss epoxy slot portion with bitumen mica endwindings.This explains the low discharge energy values and thesusceptibility of the endwinding to gross oil contamination.The data also indicates the complementary roles of d.l.a.and tan 5 measurements.

It was recommended that the machine should berewound, and this was carried out during a planned outage.Visual examination of the endwinding confirmed thepresence of severe oil contamination.

9.9 Large 11 kV motor (built 1949)

This is the motor of an m.g. set for a steelworks drive. Testsindicated a high void content between conductors and slotinsulation. It was concluded that severe degradation of theturn insulation of the line-end coils was present.

It was recommended that the line and neutral coils beinterchanged and that a replacement winding be provided.This recommendation was accepted. The machine remainedrunning until rewound. Severe degradation of the turninsulation was subsequently confirmed by autopsy.

9.10 Two 11kVmotors (built 1967)

These machines are installed at a petrochemical plant, andtests were undertaken as part of a routine maintenanceexercise. Visual examination showed evidence of a localisedbreakdown in the endwinding of one machine. It is believedthat this occurred when the machines were inadvertentlysoaked by a water spray. The earth leakage device operated,but, following a dry out, the machines were put backwithout this fault being located.

The tests indicated that discharge energy levels werehigh, with evidence of water contamination and delami-nation of the insulation, attributed to swelling of theinsulation during drying out of the windings on site. Themachines were returned to the works for repair of thewinding fault and for a careful drying out and revarnishing.

It was recommended that replacement windings wereprovided as part of a planned outage and this was sub-sequently carried out.

9.11 11 kV mill drive motor (built 1967)

The motor was insulated with an epoxy insulation systemwith standard wedging etc. After 9 years' service, when thewindings was checked the results were regarded as normal,except for the fact that spiky discharges were observed,superposed on the normal smooth loop trace of the d.l.a.(see Fig. 9). When the covers of the machine were fullyremoved and the winding energised, local scintillation wasnoticed along a few slots by the edge of slot wedges. Thewinding was checked by removing the slot wedges, andevidence was found of spark erosion between loose bars,wedge packing and the core. The coil corona shields wererefurbished, any loose coils repacked in the slot withconducting liner material and the winding rewedged. D.L.A.

16

U

12

X

«©C

o**. 8

loss

tangent

en

U

2

a

/ /

-

-

b

y / /^^r / / C

10voltage. kV

Fig. 26 Six 11 kV 1865kW drive motors; tan S against voltagecharacteristics

IEE PROCEEDINGS, Vol. 127, Pt. B, No. 3, MA Y1980 153

Page 16: Diagnostic testing of high-voltage machine insulation. A review of ten years' experience in the field

tests, carried out subsequently, indicated that all spikydischarges had been eliminated.

Since this case, two or three other motors insulated withepoxy-type systems have been diagnosed as having loosebars and consequential slot discharge. This phenomenon hasbeen widely reported by users, and underlines theimportance of adequate packing and wedging to ensureretention of contact between the coils and the core.

10 Conclusions

As a result of field testing, it is concluded that a standardisedprogramme of nondestructive measurements, carried outperiodically on high-voltage windings, can identify trendsin generalised degradation, reduce unplanned outages andenables refurbishment to be carried out at an early stagebefore serious deterioration of structural integrity occurs.Assessment of the diagnostic data obtained and theirsignificance in relation to the overall integrity of thewinding can be assisted by the use of selected criteria fromeach test. The measurement of integrated discharge energyand associated display of charge/voltage loop trace, usinga dielectric loss analyser, has been particularly successful indetecting anomolous discharging, including erosive sparkdischarging arising from bar vibration.

It is recommended that a standardised diagnostic testprogramme be applied to large machines comparing data asmade with similar data obtained periodically in service.From such data for a specific insulation system, it is hopedto establish a direct relationship between overall conditionand life. There is a need to supplement generalised tests byadditional diagnostic tests designed to detect localiseddefects. Further work is required on the relative merits ofdifferent probe techniques and online monitoring devices.

11 Acknowledgments

I wish to express my grateful thanks to my colleagues who

have laboured long and well in the field, and to GECMachines limited for their support and encouragement.

12 References

1 RUSHALL, R.T., and SIMONS, J.S.: 'An examination of high-voltage d.c. testing applied to large stator windings'. Proc. IEE,1955,102 A, pp. 565-576

2 STARK, K.H.: 'Assessment of the insulation serviceability ofturbo-generator stators and of high-voltage bushings', ibid.,1961,109 A, pp. 71-88

3 DAKIN, T.W., and MALINARIC, P.J.: 'A capacitance bridgemethod for measuring integrated corona. Charge transfer andpower loss per cycle, AIEE Trans., I960, pt. 3, PAS-79, pp.648-652

4 SIMONS, J.S., and RICHARDS, M.T.: 'Non-destructive electricaltest methods for evaluating high-voltage insulation', Proc. IEE,1962,109 A Suppl. 3, pp. 71-79

5 SIMONS, J.S.: 'The measurement of integrated discharge energyin high voltage insulation using a dielectric loss analyser withloop trace display'. Presented at the IEE conference on di-electric and insulating materials, April 1964

6 KOZYREV, B.I.: 'Comparative effectiveness of tests carried outon the frame insulation of the stator windings of generators withdifferent types of voltage', Elect. Stantsri, 1972, 2, pp. 32-35

7 SCHULER, R.: 'Assessing the state of the insulation of statorwindings', Bull. Assoc. Suisse Electr., 1969, 60, pp. 777-785

8 HIRABAYASHI, S., SHIBUYA, Y., HASEGAWA, T., andINUISHI Y.: 'Estimation of the size of voids in coil insulationof rotating machines', IEEE Trans., 1974, EI-9, pp. 129-136

9 SIMONS, J.S.: 'Preventative maintenance testing of largemachines - recommendations for insulation test procedures'.IEE Colloquium Digest, 1975, 3, pp. 129-136.

10 SIMONS, J.S.: 'Some aspects of the evaluation in the laboratoryand field of the serviceability of micaceous insulation forrotating machines'. Presented at the BEAMA internationalelectrical insulation conference, May 1978

11 SIMONI, L.: 'Ageing theory of engineering materials'. Report72/CRD, Nov. 1972, Institute Di Electrotechnics Industriate,University of Bologna

12 NELSON, W.: 'Graphical analysis of accelerated life test datawith inverse power law model', IEEE Trans., 1972, EI-7 pp2-11

13 WHITMAN, L.C., and DOIGAN, P.: 'Calculation of life character-istics of insulation', AIEE. Trans., 1954, 73, pp. 193-198

14 PARKMAN, N.: 'The effect of small discharges, on someinsulating materials'. ERA report, L/T 321,1954

154 IEE PROCEEDINGS, Vol. 127, Pt. B, No. 3, MAY 1980