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IEEE Std 43.1974(±edi99i)aLevision of -Std d9.106i)

IEEE Recommended Practice forTesting Insulation Resistance of

Rotating Machinery

f,rl\``.

.\.I

Approved Sepiinber 7, 1977

E:RIffi#F±E±¥i';:§8:American National Standards hstitute

9 Copyright 1974 by

The Institute of Electrical and Electronics Engineers, Inc345 East 47th Street, New York, NY 10017, USA

No port. Of this p'ulil.ieatwn menu be Tepr(id:used in tLny jirrn,in. trm electrci,rke Tetrieiiut syscerrL or otlurmrise,

without prior uwitten permission Of the publisher.'

IEEE Standards documents are developed within the Technical Com-mittees of the IEEE Societies and the Standards Coordinating Commit-tees of the IEEE Standards Board. Members of the committees servevoluntarily and without co.mpensation. They are not necessarily mem-bers of the Institute. The standards developed within IEEE representa consensus of the broad expertise on the subject within the Instituteas well as those activities outside of IEEE which have expressed an in-terest in participating in the development of the standard.

Use of an IEEE Standard is wholly voluntary. 'rhe existence of anIEEE Standard does not imply that there are no other ways to pro-duce, test, measure, purchase, market, or provide other goods and ser-vices related to the scope of the IEEE Standard. Furthermore, the view-point expressed at the time a standard i§ approved and issued is subjectto change brought about through developments in the state of the artand comments received from users of the standard. Every IEEE Stan-dard is subjected to review at least once every five years for revisionor reaffirmation. When a document is more than five years old, and has

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hse-r+e`g'ardingthemean-3ific. applic ations. When'attention of IEEE, the

priate responses. SinceL concerned interests, it ishas also received the con-

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Institute wil) initiateIEEE Standardsimportant to ensurecurrence of a balance of intg`rests-:-For'-this reason IEEE and the lnem-bers of its technical committees, are not able to provide an instant re-sponse to interpretation requests 6±cept in those cases where the matterhas previously received

Comments on standadressed to:

EgrnggBoard345 East 47th StreetNew York, NY 10017USA

should be ad-

.`-...

'-.I. `I.1` '- :

\

Joseph L. Koepfinger, C#a!.rman

Jacques J. ArchambaultSaul Aronow .Robert D. BriskmanDale R. CochranWarren H. CookLouis CostrellCharles W. FlintJay Forster

+`{\.

Approved May 30, 1974

IEEE Standards Board

Sava I. SheIT, Secretary

Irvin N. Howell, JrIrving KolodnyWilliam R. KruesiBenjamin J. LeanAnthony C. LordiDonaldiT. MichaelVoss A. Moore

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:.S:arrfeg.ng€tj-=9:+.6~3,.,+inl`..i

Warren H. Cook, VI.ce-Cha!I.rman

William S. MorganHarvey C. NathansonJames D. M. PhelpsSaul W. RosenthalGustave ShapiroRalph M. ShowersRobert A. SodermanWilliam T. Wintringham

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Foreword

(This foreword i8 not a part of IEEE Std 43-1974. IEEE Recommended Practice for Testing Insulation Resistance ofRotating Machinery.)

\Insulation resistance measurement has been recommended and used for more than half a cen-

tury to evaluate the condition of electric insulation. Whereas individual insulation resistancemeasurements may be of questionable value, the carefully maintained record of periodic meas'iire-ments accumulated over months and years of service, i§ of unquestioned value as a measure of thecondition of the electrical insulation. Originally, in 1950, this Recommended Practice was pub-lished by the AIEE as a Guide to present the various facets associated with the measurement andunderstanding of electrical insulation resistance. The Guide was revised in 1961. With this publiLcation as a Recoinmended Practice, the IEEE is presenting and recommending electrical insula-tion resistance measurement as an important factor in monitoring the condition of electrical in-sulation in rotating machinery.

This Recommended Practice was prepared by a working group of the Insulation Subcommitteeof the Rotating Machinery Committee of the IEEE Power Engineering Society. Working grouppersonnel were:

R.F.Shrarif;-;>,`^€faa!.rman

C'n

3J`_'

J. M. BrownA. W. W. CameronE. a. Curdts

R. J. HillenW. J. SheetsG. Wolff

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SECTION1. Scobe

2. Purpose

Contents

3. Insulation Resistance-General Theory, Use, Limitations

4. Factors Affecting Insulation Resistance

5. Uniform conditions for Measuring Insulation Resistance

6. Winding connections for Insulation Resistahce Tests

7. Methods of Measuring Insulation Resistance; Precautions

8. Interpretation of Insulation ResistanceTest Results

10. Standards References

APPENI,IX `f -``. '`*`

:::Ere.efh:.:d::|o:nforfeT::t.:::i::?h:sn?:#i:!i[i¥;ft¥FIGURES

Figl

Fig2

' Fig 3

`g,Insulation

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Class 8

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PAGE

IEEE Recommended Practice forTesting Insulation Resistance of

Rotating Machinery

1' Scope .

1.1 This document describes the recommend-ed procedure for the measurement of insula-tion resistance of windings of rotating ma-chines rated 1 hp, 1 kw or gI.eater.

It applies to synchronous machines, in-7 '`duction machines, direct-current machines',

gether with precautions for avoiding er-roneous results

(4) Provide a basis for interpreting insula-tion resistance test results to estimate thesuitability of the winding for service or foroverpotential test.

(5) Present equations, based on machineratings, for the calculation of recommended

[t applies to armature wfndfng§ and.±{.e{agr;,¥¥.%£{E`:;sut¥pefsn:¥:::£aotFngre=;as:£Fnc:s.`va]ues forand synchronous converters.

::::c:;:1:ji,s:u;:i::::i¥e::ji:`¢Rse~;;9fu:'qabf::.i_i`-` 3. |ri9:#ul;F:ion Resistance-deneral

dy,-r2 Theory, Use, Limitations`aJ' ~

3.1.' Insula.tion resistance is the term gener-ally used to describe the quotient of the ap-plied direct potential divided by the current atsome given time measured from the start of

3.1.1 The current that results from the ap-plied direct potential consists of two parts:that in leakage paths over the surface of theinsulation and that within the volume of theinsulation. The current within the volume ofthe insulation may be further subdivided asfollows (see IEEE Std 62-1958, Guide for Mak-ing Dielectric Measurements in the Field).

(1) The capacitance charging current, ofcomparatively high magnitude and short du-ration, usually has effectively disappeared bythe time the first data are taken, and it doesnot affect the measurements.

(2) The absorption current decays at a de-creasing rate from a comparatively high ini-tial value to nearly zero. The resistance-timerelationship is a power function which may be

windings.It does not apply to

machines..

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;;:tp.enr|eecsoisi=necned:faEi:i=tui=g-vcau'rureenotfi:sdu;::rgct,-qurrent rotating-machine windings. The

i APP.endix gives inaintenance informatiofi'-i;ire|`|+±.i

§{g;ont:exr::eiEm:`unbt,e£::t::;:n:°hr,:;¥:x;ji:a]`:::ii*`;Iii§;¥;t:E:::u:i:±[:°o¥i;.,i;.±i`;e::cetf:c:ee_to:|i]a]=b::atiy.€`to dry-o'ut procedure.

inatian on insulation resistance measure-!.tfr€-listed in Section 10.#Lo.

2. Purpose±i{

pdrpose of this publication is to:)escribe and define insulation resist-s applied to the winding of a rotating

•Beview the factors which affect orifi§ulation resistance characteristics

9@inend uniform test conditionsfitline and reconinend uniform meth-Pr4ifee.asuring insulation resistance to-

IEEE43-1974

plotted on log-log graph paper as a straightline. Usually the resistance measured in thefirst few minutes of a test is largely deter-mined by the absorption current.

(3) The conduction current, which, withthe surface leakage current. i§ nearly con-stant. These currents predominate after theabsorption current has become insignificant.

3.1.2 After removal of the impressed directpotential and the provision of a suitable dis-charge circuit, there will be evident a di§-.charge which is of two parts:

(1) The capacitance discharge currentwhich will decay nearly instantaneously, de-pending u|)on the discharge resistance

(2) The absorption discharge current whichwill decay from a high initial value to nearlyzero, as does the absorption current in Section3.1.1.

3.2 The insulation resistance of a rotating-machine winding is a function of the type andassembly of insulating material. In general, itvaries directly with the thickness of the insu-lation and inversely with conductor surfacearea. To obtain meaningful insulation resist-ance measurements on water-cooled rna-chimes, water should be removed and the in-ternal circuit thoroughly dried.

3.3 Insulation resistance measurements areaffected by several factors discussed in Section4:

(1) Surface condition(2) Moisture(3) Temperature(4) Magnitude of test direct potential(5) Duration of application of test direct po-

tential(6) Residual charge in the winding

3.4 Readings of insulation resistance are usu-ally taken after test direc.t potential appli-cation of 1 min and, if facilities are available.after 10 min to provide data for obtaining thepolarization index.

3.5 The I)olarization index (ratio of 10 min to1 min insulation resistance) is described inSection 4.5.2.

3.6 The interpretation of insulation resist-ance measurements of machine windings andthe calculated polarization index is describedin Section 8.

RECOMMENI)ED PRACTICE FOR TESTING

4. Factors AffectingInsulation Resistance

4.1 Effect of Surface Condition4.1.1 Foreign matter. such as carbon dust

deposited on creepage surfaces, may lower theinsulation resistance. This factor is particu-larly important in the case of direct-currentmachines which have relatively large exposedcreepage surfaces.

4.1.2 Dust on insulation surfaces which isordinarily nonconducting u'hen dry may.when exposed to moisture, become partiallyconducting and lower the insulation resist-ance.

4.1.3 If the insulation resistance is reducedbecause of contamination or excessive surfacemoisture, it can usually be brought up to itsproper value by cleaning and by drying to re-move the moisture (see Appendix).

4.2 Effect of Moisture4.2.1 Regardless of the cleanliness of the

winding surface, if the winding temperature isat or below the dew point of the ambient air, amoisture film will form on the insulation sur-face and may lower the insulation resistance.The effect is more pl.onounced if the surface iscontaminated. It is important to make resist-ance measurements when the winding ten-perature is above the dew point.

4.2.2 Many types of winding insulation arehygroscopic, and.moisture may be drawn intothe body of the insulation from the humid am-blent air. Absorbed moisture will have a largeeffect on the insulation resistance. Machinesin service are usually at a temperature highenough to keep the insulation comparativelydry. Machines out of service may be heated tokeep the winding temperature above the dewpoint.

4.2.3 When tests are to be made on a rna-chime that has been in service. the tests shouldbe made before the machine winding temper-ature drops to room temperature. The oppor-tunity may be taken to test at several temper-atures to establish the ap|)licable temperaturecoefricient (see Section 4.3.4).

4.3 Effect of Temperature4.3.I Insulation resistance of most materi-

als varies inversely with tempel.ature.4.3.2 To minimize the effect of temperature

when comparing insulation resistance tests or

INSULATION RESISTANCE OF ROTATING MACHINERY

when applying the recommended minimumvalue of insulation resistance as given by Eq2. it is important that the test be corrected toa 40°C.base. The correction may be made byuse of Eq 1:

R® -Kt x Rt

whereRc

fit

(Eq 1)

insulation resistance (in megohms)corrected to 4o oCmeasured insulation resistance (inmegohms) at temperature t

Kt = insulation resistance temperature•coefficient at temperature t

4.3.3 The correction of insulation resist-

IEEE43-1974

temperature, Kt can be determined from sucha plot by inversion of Eq 1.

4.3.4 An approximate value for the temper-ature coefficient K t may be obtained by usingFig 1 which is based on doubling of insulationresistance for each 10 °C reduction in tempel.-ature (above dew point) and has been foundtypical of some new windings.

4.3.5 When the polarization index is usedto determine the insulation condition, it is notnecessary to make a temperature correction to40OC.

4.3.6 The effect of temperature on the po-larization index is usually small if the rna-chime temperature does not change ap|)reci-ably between the 1 and 10 min readings; but,when temperature is high, the temperature

I-'~..*characteristics of the insulation system mayindicate a reduced polarization index in which

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•\0 a 10 2L. 10 .a SO .a To .a .a loowhndJo[ T.mp.nlu*. befm.I C¢l.ltp

FiglApproxifrote Insulation Resistance Variation with Temperature

for Rotating Machines

9

rEEE43-1974 RECOMMENDED PRACTICE FOR TESTING

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Fig2Typical Variation of Insulation Resistance with Time for Class 8

Insulated Alternating-Current Armature Windings

age rating of the winding and the basic insula-tion condition. This is particularly importantin the case of small, low-voltage machines, orwet units. If the test potential is too high. theapplied test potential may overstre§s the insu-lation.

4.4.2 Insulation resistance tests are usuallymade at direct potentials of 500 to 5000V.The value of insulation resistance may de-crease somewhat with an increase in appliedpotential: however. for insulation in good con-dition and thoroughly dry, substantially thesame insulation resistance will be obtained forany test potential up to the peak value of the

4.4.3 If the insulation resistance decreasessignificantly with an increase in applied po-tential, it may be an indication of im-perfections or fractures of the insulation ag-gravated by the presence of dirt or moisture,or may be due to the effects of dirt or moisturealone, or may result from other deterioratingphenomena. The change in resistance is morepronounced at potentials considerably above

potential.

4.5 Effect Of Duration ofPotential: Polarization Index

4.5.1 The measured insulation resistanceof a winding will normally increase with the

10

duration of application of the direct test po-tential (see Fig 2). The increase will usually berapid when the potential i§ first applied, andthe readings gradually approach a fairly con-stant value as time elapses. The measured in-sulation resistance of a dry winding in go.odcondition may continue to increase for hourswith constant test potential continuously ap-plied; however, a fairly steady value is usuallyreached in 10 to 15 min. If the winding is wetor dirty, the steady value will usually bereached in one or two minutes after the testpotential is applied. The slope of the curve isan indication of insulation condition.

4.5.1.1 The change in insulation resist-ance with the duration of the test potentialapplication may be useful in appraising thecleanliness and the dryness of a winding. If fa-cilities are available, the test potential may beapplied for 10 min or more to develop the di-electric absorption characteristic. This char-acteristic may be used to detect moisture ordirt in the winding§.

4.5.2 The polarization index is the ratio ofthe 10 min resistance value to the 1 min resist-ance value. The polarization index is in-dicative of the slope of the characteristic curve(see Section 4.5.1.I and Figs 2 and 3). The po-larization index may be useful in the appraisalof the winding for dryness and for fitness for

INsuljATloN RrslsTANCE oF ROTATING MACHINERY

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IEEE43-1974

5. Conditions for MeasuringInsulation Resistance

5.1 The insulation surface must be clean anddry if the measurement i§ to provide the infer-nation on the condition within the insulationas distinguished from the Surface condition.Surface cleanliness is of great importancewhen tests are made in humid weather.

5.2 The winding temperature should be a few• degrees above the dew point to avoid co.n-

den§ation of moisture on the winding insula-tion. It is also important that for comparinginsulation resistances of machine windings, a

• 40°C basis be used. (For converting insulationresistance values to this temperature see See-

--.- '.h`?.ion 4.3 and Fig 1.)

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often desirable to make insula-measurement§ when the rotat-

is, subject to centrifugal forces

__I 1_J.__ I_I.A -:-:1^.1`, ^L+-:T`aArecords of a given machine

:fa3i3ji:nTd:::t: any special test conditions.

11

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apd later data, similarly obtained.4:'6 Effect of Existing Charge on i

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I pr,4.6.1 Insulation resistanceI be in error if residual charges exist in the5ji:i::.I:sh[:::i°cree'otefp°:.earTzeaat:::£n{:dtehx:

fidings must be completely discharged to3,grounded machine frame. If any doubt ex-faas to the sufficiency of discharge, the dis-arge current should be measured. This will

as a reverse deflection of the insulationance measuring meter after connectionsa^ade but before the voltage is applied.i,deflection should be negligible compared

}esfxpected test current.Q.2 After application of high direct po-'igiv grounding of windings is important

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6. Winding Connections forInsulation Resistance Tests

6.1 It is recommended that each phase be iso-lated and tested separately, when feasible.

6.2 The neutral end of each phase windingshould be disconnected when practicable.Testing each phase individually gives a com-pari§on between phases which is useful inevaluating the condition of the winding nowand in the future.

6.3 Tests may be made on the entire windingat one time, under certain conditions, such aswhen time is limited; however, this procedureis not the preferred method. One objection to

IEEE43-1974

testing all phases at a time is that only groundinsulation is tested and no test is made of thephase-t.o-phase insulation. The phase-to-phase i'nsulation is tested when one phase istested at a time with other phases grounded.

6.4 The connection leads, brush rigging, ca-bles, switches, capacitors, lightning arresters,and other external equipment may influencethe insulation resistance test reading on a rna-chime winding to a marked degree. Thus it isdesirable to measure the insulation resistanceof a winding exclusive of the external equip-ment of the machine.

7. Methods of MeasuringInsulation Resistance; Precautions

7.1 Direct measurement of insulation resist-ance may be made with the following instru-ments:

(1) Direct-indicating ohmmeter with self-contained hand or power-driven generator

(2) Direct-indicating ohmmeter with self-contained battery

(3) Direct-indicating ohmmeter with self-contained rectifier using .an external alter-mating-current supply

(4) Resistance bridge with self-containedgalvanometer and batteries7.2 Insulation resistance may be calculatedfrom readings of a voltmeter and micro-ammeter using an external direct-current Sup-ply.

7.2.1 The voltmeter-ammeter method is asimple method for the determination of the in-sulation resistance by measurement of the po-tential impressed across the insulation andthe current through it. A source of direct po-tential is required, and the voltmeter must beselected to fit the maximum and minimum po-tential§ which may be used. The ammeter isusually a multirange microammeter selectedto measure the full range of leakage currentswhich may be encountered at the potentialsused.

7.2.2 The microammeter must be on thehighest range or short circuited during the

RECOMMENDED PRACTICE FOR TESTING

first few seconds of charge so that it will not bedamaged by the capacitance charging currentand the initial absorption current.

7.2.3 If the microammeter is at test poten-tial, precautions should be taken to ensuresafety to the operator. To avoid measurementerrors, the instrument should be guarded (seeIEEE Std 62-1958).

7.2.4 For test potentials above 5000 V, thelead between the test set and the windingmust be well insulated, shielded, of largediameter and spaced from ground; otherwise,leakage currents and corona loss may in-troduce errors in the test data (see IEEE Std62-1958).

7.2.4.1 Both ends of the winding shouldbe connected together to minimize surges ifthe insulation should fail during test.

7.2.5 Resistance is calculated from theequation j} = E/I, where j3 is insulation re-sistance in megohms, E is the voltmeter read-ing in volts, and I is the ammeter reading inmicroamperes at a stated time after appli-cation of test potential.

7.3 In general, a finite amount of time is re-quired to bring the potential impressed on theinsulation to the desired test value. Full po-tential Should be applied as rapidly as pos-sible.

7.4 Instruments in which the test potential issupplied by motor-operated generators, bat-teries or rectifiers are usually used for makingtests of over 1 min duration, that is, for testsfor dielectric absorption or polarization.index(see Sections 8 and 9).

7.5 It is essential that the potential of anytest source be constant to prevent fluctuationin the charging current. Stabilization of thesupplied voltage may be required.

7.6 Where protective resistors are used in testinstruments, their effect on the magnitude ofthe potential applied to the insulation undertest should be taken into account. The poten-tial drop in the resistors may be an appreci-able percentage of the instrument potentialwhen measuring a low insulation resistance.

7.7 To compare with previous and futuretests, the same potential should be applied bythe same method to permit a proper com-parison of results.

12

INSULATION RESISTANCE OF ROTATING MACHINERY

8. Interpretation of InsulationResistance Test Results

8.1 In:ulation resistance history of a givenmachine, made and kept under uniform con-ditions so far as the controllable variables areconcerned, is recognized as a useful way ofmonitoring the insulation condition. Estima-tion of the suitability of a machine for the ap-plication of appropriate overpotential tests orfor operation may be based on a comparison ofpresent and previous values of the polari-zation index or insulation resistance valuescorrected to 40 °C (see Section 4.3.4).

8.2 When the insulation resistance history is

ipsu!et.ion resistapce. (a.or:ecteq.,.t§:i.4`'a :c`)

IEEE43-1974

conditions. The curves illustrate the signifi-cance of polarization index.

8.5.2 Depending upon the winding coridi-tion, insulation class, and machine type, val-ues Of 1 to 7 have been obtained for the polari-zation index. Class 8 insulation usually has ahigher polarization index than Class A insula-tion. Moisture or conducting dust on a wind-ing lowers the polarization index. When high-potential alternating-current machines haveend windings which are treated with semi-conducting material for corona eliminationpurposes, the polarization index may be some-what lower than that of a similar machinewhich is untreated.

8.5.3 If the polarization index is reducedbecause of dirt or excessive moisture, it can be

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meobs,h`ould be at I:as? that .of the,.5e63.fug-ap:P_d±:=dmininum insulation resistal!oe IedfromEq2(seeSection9.3).``

i-::i::n::ie:ae;!:a:c:iy:efe::w:1:i::.,:je;.ilit:.:::tit;:;fs:e!Si,:8i:

IiJ?+ liinitations : }'i a.3.1 Insulation resistance of a winding is .`ut'f

iesistance at which a winding

t directly related to its dielectric;!*ipFossible to specify the value

Windings having an extremely large

Ssr.eivaj,tLa::emo::,toaT:::e::ymhaacvhe£:::i::atio-n resistance that are less than theended minimum value.

lsinBle insulation resistance measu-re-st-`6ne particular potential does not in-whether foreign matter is concentrated[ibuted throughout the winding.!`''.-

i#:;`iz;tionindex(seesection4.5.2).t``ITypical resistarice versus time char-tiQS.are shown by Figs 2 and 3, illustra-ekevior of insulation under different

13

.Jf' ,,-=•` 9: Recommended Minimum Value

of Polarization Index and

giREirgare:osmulmaet:::d:::::t::cpeo,arlzationindex or the recommended minimum value ofinsulation resistance j3.in at 40°C of an alter-nating-current or direct-current rotating ma-chine winding as used herein is the least valuerecommended which a winding should havejust prior to application of an overpotential'test or operation (see Sections 9.4 and 9,5).

9.1.1 It is recognized that it may be possibleto operate machines with values lower thanthe recommended minimum value; however,it is not normally considered good practice.

9.1.2 In some cases special insulation ma-terial or designs. not injurious to the dielectricstrength, will provide lower values.

9.1.2.1 When the end winding of a rna-chime is treated with a semiconducting mate-rial, for corona elimination purposes, the ob-

LI\| <. \,J\,\, L~t,+ ,,,- ^ ,-.-.-,.

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RECOMMENDED PRACTICE FOR TESTING

9.3.3.1 If each phase is te§ted separatelyand guard circuits are used on the other twophases not under test, the observed resistanceof each phase should be divided by three to ob-tain a value, which. after correction for tem-perature, may be compared with the recom-mended minimum value of insulation resist-ance.

9.3.4 For insulation in good condition. in-sulation resistance readings of 10 to 100 timesthe value of the recommended minimum val-ue of insulation re;istance j3 in obtained fromEq 2 are not uncommon.

9.3.5 In applications where the machine isvital. it has been considered good practice toinitiate reconditioning should the insulationresistance, having been well above the min-

a-ftEe]in;ful..ation-resistance(at4o°g)atas+the minimum recommeri.dad

:a.:ig;::,s,9.5 rated above 10 000 kvA should

IEEE43-1974

served insulation resistance may be somewhatlower than that of a similar machine which isuntre`ated.

9.2 The recommended minimum value of I)o-larization index for alternating-current anddirect-current rotating machines is:

ForclassA 1.5 .ForclassB 2.0ForclassF 2.0

9.3 The recommended minimum insulationresistance ji in for alternating-current and di-rect-current machine armature windings andfor field windings of alternating-current anddirect-current machines can be determined byEq 2 (see Section 8.3.2):

fa v - :aottee:t=aa|,i:nremt:rfi:o,y`ao`i#:Qt

9.3.1 The actual winding insinati%n resist-

::Ci::dbeedu:::ilo::°vma?ua::£°:¥:¥:i+:be;:;:i¥i*fa:;:,±e:%;be°:i::1::I:a:nsp:°e]aar£::te]:::n:::,amnud:hree}cnos::

9.3.2 Temperature corrections should al- `'.I ```stffijj7£ffigg§;,.5d6e:;:a:n8[d#;ue:a:€];::fi;r±;:¥Ce:n:a:::i::

14

ed insulation resistance. corrected to 40,obtained by ap|)lying direct potential toentire winding for one minute.

¥earyast:lee:fa4doeo£€::eee¥.enc:i:ngs`.::.°3t,2.#Fig 1).

9.3.3 The insulation resistance'`6f"6-fi'gphase of a three-phase armature winding withthe other two phases grounded is a|)proxi-mately twice that of the entire winding.Therefore, when the three phases are testedseparately, the observed resistance of eachphase should be divided by two to obtain a val-ue which, after correction for temperature,may be compared with the recommended min-inum value of insulation resistance.

Large AC Rotating Machinery. 1

IEEE Std 62-1958, Guide for Making Dielec-tric Measurements in the Field.I

IEEE Std 95-1962, Guide for Insulation Test-ing of Large AC Rotating Machinery withHigh Direct Voltage.1

I Presently under revision.

INsuLATION RrslsTANCE OF ROTATING MACHINERy

Appendix

A1. Prevention of MoistureAbsorption in Winding

Insulation of Rotating Machines

The following information describes sug-gested maintenance procedures to preventmoisture absorption in the windings of map-

IREE43-1974

A1.3 An approximation of the heat requiredto raise the winding temperature of an en-closed horizontal generator or motor 5°Cabove ambient temperature, where the ma-chine is closed, is given by

.+ H=¥ (zr=#) (EqAi)chimes which are out of Service. More detailed I-~" `information may be found in IEEE Std 56-

:ogr5:i::|eaati::9#1:iLieNn:In:e5?;:5i::::,A-8uido:.,:i`;.rtating Machinery.

..j;-i, jl.

go.|]on¥ea::£enr::d?¥::aabrseor°bu:u°£.~csfee:;`€i±.,;££°g:pej:;`,a

below the recommended limits sug+geste4 in ;E#:

fry. <}

wri:ie -`A = i-heat, in kilowatts

fiacnine end-bell diameter, in feet (inmeters) -.±FEEli

*-*h-lF#`= i;'=i;i-ri6' stator length between end-rfell cJ6nters, in feet (in meters).

lure to reduce insulation resistanc_e_i_6_a.Y:1¥_: :ter= `ttS'r..

` Section9.

:..' 3f.A1.1.1 This can be prevented if the wiind`!.,„ .^-a'di+.,^'•`:-:~in886#g:rsautr::eufnsda;]nwgaayiF;:i:ttae;:epde:i;tguhrte[.y

:±t§i¥]:^:*;:tr%egaci::]{;:::?::af:hie:a:t:::s!:iuifefry,:1`.3 In rooms where wide and rapid tem-Ea`t!}r% changes are experienced, some high-winding temperature rise may be necessary.Alfl.4 In rooms with limited air exchange,humidifiers will reduce dampness in stor-ep8firj ` t

It.`inay not be necessary to maintain

SiFrttr¥se¥onr:£riEec[;n:inbue°::`syo.rgee£::alt`hy:Lg and Summer months in most parts ofJt#it€d States, while the low atmospheric

ffi:I;e.h:;;e:fe;;it:ii,:;:¥eg:e::f:,::gal:itr:.:La`'e¥ably above the recommended lnin-

given in Section 9.

A2. Removal of Moisturefrom Winding Insulation of

..:r,i.,I: g+:* =:±.. Rotating Machines

-iif`-.a:2;E:i:jE¥:ectric machinery should, when nece§-dried by circulating current in the

windings or by heaters maintaining a reason-ably constant temperature in the windings.

A2.1.1 Sufricient heat should be providedto produce a temperature on the end windingsof not higher than 80°C by thermometer or90 °C by resistance temperature detector.

A2.1.2 For machines which have beenflooded, a prolonged drying time is expected.Cycling of temperatures often accelerates dry-out in severe cases of flooding. A temperaturein excess of 80 °C by thermometer or 90 °C byresistance temperature detector may be re-quired for satisfactory drying at atmosphericpressure in a reasonable time. The use of high-er temperatures should be made with cautionand after consultation with the manufacturerof the equipment.

15

tion.

A2.1.3 Drying under a moderate vacuum isrecommended, when possible, since this meth-od reduces the maximum temperature re-quired, and drying can usually be accom-plished without exceeding the design-temper-ature values.

A2.I.4 The rate of tem|)eraturebe limited to 5 °C per hour to avoid damagethe insulation by excessive gas or vaDor fi

A2.2 The progress of the dry-out may be ob-,served without temperature col.rection bymeans of the polarization index values (see Fig3). These will increase in value as moisture isexpelled.

A2.3 I;he progress of the dry-out may also be

:::eervr::db£:a,ei:::i::figcin:F'taeti°pne::;isrt:,f't`&eg:i::eatts:atnhde-S:i!::i:g:u:i::igsE°cuo[:p::

(Eq A2)chime is heated.

proaching a constant value.

A3.1.I The heaters should preferably be lo-cated in the air chambers under the machineand distributed so as to allow for even distri-bution of heat along the length of the ma-chine.

A3.1.2 If the rotor is not in place, the endsof the machine may be closed with the end-bells or with large tarpaulins to reduce theheat loss.

A3.2 Field Winding Heating .The main field winding of a generator may

be used to introduce heat into the machinewhen some source of direct current is avail-able which can be separately controlled andallocated to the machine that requires heat. Ifthe rotor of an alternating-current machine

aching a constant value.

.-'.. {..'.` 25°C, measured at the slip ringsce has started to increase aftel. reachingit.§ -I. a I. `T_____.,__ _i____i ,I ________1 _.. -A3.2.2 Normally, about 15 I)ercent of rated" full-load field current will be required and the

=-±.fieldrvy±nding temperature rise should beabove ambient temperature.

X~'3.-9-"ELature Winding Heating. The arma-ture of the machine may be used to supplyheat by passing current through the armatureconductors.

A3.3.1 Alternating current at low potentialmay be induced in the armature, if the ma-chine can be rotated at reduced speed (as on ahydro generator), with the generator termi-nals short circuited or connected to a load.Careful control of speed and field current is re-quired.

A3.3.2 Direct current may be passedthrough the armature conductors.

A3.4 Steam HeatingThe use of steam heaters is not recommend-

ed because of the possibility of leakage con-densing, damaging the insulation system.

nimum value and preferably until it ap-aches constancy (see Fig 3).

A3. Methods of HeatingMachine Windings

IIeat may be applied by any convenientmethod as 16ng as it is safe and I)roper pre-cautions are taken to prevent rire. The follow-ing methods may be considered, the choice be-tween them being principally a matter of con-venience, flexibility, cost, and availability. Ifthe machine is enclosed to conserve heat, pro-vision should be made to vent the moisturebeing removed.

A3.1 Electric Space HeatersElectric space heaters will frequently be

found to be the most suitable for heating sincethey are usually available in various sizes andcan be placed in service quickly at low.cost.

16

ENSI/IEEE Std 95.1977(Revision o£ IEEE Std 95.1962)

An American National Standc.rd

IEEE Recommended Practice forInsulation , Testing of

LargeAfighotfi#:.gt¥3ftgeeryw±th

-.i .,#g-sStei -`

i•_...'.£;t` % fro*

i::i ...-I-.I`.i±£-f f i...? i.`'f ;df i

:i;i ih. r...i;t* -;r.

SpOps9r `-

Rotating Machinery Co'mmittee of theIEEE Power Engineering Society

I.

ReAifev§#S.:':~¥ef±`!:rFJa€j=::179491

IEEE Standards Board

Approved November 28, 1977Reaffirmed April 20, 1992

AmericanNationalStandardshstitute®Copyright 1977 by

titute of Electrical and Electronics Engineers, Inc

NO Part of this public:atiorl may be reproduced in any form.(-,` i_ __ _,_-. .in an eleetroriic retrieval system or otheru)ise,: `givithou[ the briar u)ritten perinissiori of the publisher.

Joseph L. Koepfinger, Chai.rman

Approved S?ptember 5, 1974

IEEE Standards Board

Warren H. Cook, VI.ce Cha!.rman

Jean I)acques AI'chambaultSaul AronowRobert D. Briskmai`Dale R. CochranLouis CostrellChal.les W. FlintJay Forster

Sava I. Shen, Secretary

lrvin N, Howell, JrITving KolodnyWilliam FL. KruesiBenjamin J. LeonAntholiy C. LordiDonald T. MichaelVoss A. MooreWilliam T. Wintringham t

t I)eceased

William S. MorganHarvey C. NathansonJames D. M. PhelpsSaul W. RosenthalGustave ShapiroRalph M. ShowersRobert A. Soderman

Foreword-iw:r;:h£8H|:i%?I:Tcttfyfo!t¥eF)Std95.1977.Recomiendedpracticeforln8ulationTe8tingofLargeACRotating

itionally. the insulation of rotating machines has been tested for dielectric Strength withgiri.a voltage. In 1952 attention was directed to testing with direct voltage. Since then, high

g!,!oa::::tshb:::I:sfsd£:'nysuo§fe;.{d¥[ayng{::epr::tg§o°:}Pnr{:One.dureandresultsarefoundinthefEEE7ihe Insulation Subcommittee of the IEEE Rotating Machinery Committee appointed a

:go:feu:Pigt§s!ei:;:nydvttoi:aateg:e:Si:n::ug|::t:leo:n:tt:;::::i;o:ps:n;;o:n:I:eftiha=:£r;i;tr,€:eeihce:ri:::fc:aank:n;;ttehr;i.thelnsulationSubcommitteeoftheIEEERotatingMa'chineryCommitteeappointeda

i¥::Pt£:rreers}S::tdhee::isgte!no:#;iedt:reac:e;:I?a::nf:re:npsruai:tt{icoe;testing,buttherearesti|1|aisagreement regarding the utility of such tests. In this recommended practice every ef-

#.i:h:::mpa:n::ee:tv:aels:tn:;aieow££a:C:ti]r:eEd:i:I:eec:t:vda:!C::::;:i:nsh::t|a!ts,odn°:ebsttfi:I;oTf:lisa:g°ec::::tergro¥::vteh§:\Of those who have used the methods described in this recommended practice have found

g:£da:t;eicfe;:vt:°;¥acg::::i:I::atiru:i:t:e::a.dmd::1:a:nt.°fo°rtEerre::§itt§P:°ncde|:]r,e:;sr:['ts]E°ap:adctt::i:::::.es::=!

g:5]:::;C7:7:Sf:°:n:d:efier:t¥::a::C:t:I:i:xia::o:rti:iiim::oaf:o;S:L:anrbfg:e:¥e%et:£:[tt:e:r£::teknv:a:a£:;::t';I:et::c:t:5:t:I./ 'J.` ,I

c`ument was originally developed by a working group-of the Insulation Subcommittee ofRotating Machinery Committee. The members of this working group were:

•,hit.I, C. L. SLdway, Chair.mcln

C. E. AsburyJ. S. Askeya. M. CainA. W. W. CameronE. a. CurdtsJ. L. KuehlthauH. M. Mar§den

a. R. Loxley, Secre!ary

G. L. MosesE. R. ScattergoodW. SchneiderH. R. TomlinsonH. P. WalkerW. A. WeddendorfE. S. Yates

was pre|)ared by a working group of the Insulation Subcommittee. The members

R. F. Sharrow, Cftal.rman

J. M. BrounA. W. W. CameronE. a. Curdt8

R. J. HillenW. J. SheetsG. Wolff

WANINGDue to. High Volt'i¥gte Used, Diel.ectricTests Should Be €onducted ,Only byExperienced Personnel, and AdequateSafety Precautions.Should Be Taken toAvoid Injury to Personnel and Damageto Property.

Contents

i€Breparations for Test: Test Connectionsar..I PToofTe8t preparations4.2 Air-Cooled Machines?*`.3 Hydrogen cooled Machines

Liquid Cooled Arlnature WindingsI8olation of the Winding from Cables and Auxiliary Equipment

6 Sectionalizing the winding:7 Discharge of the winding:!`,i.TT±F9_iiLET_e:i9" High-Voltage Test Connection to the Winding

?4.10 Test Connection to GroundProcedure: Proof Tests

Test Voltage for Acceptance Proof TestihgTest Voltage for Maintenance Proof Testing

3 Voltage Application.4 Grounding5 Test Results.6i Failure7 Suggested Test Record

:Test Procedure: Controlled Overvoltage Test; Controversy in InterpretationTest Method

3' Initial Voltage Step#i Current Measurement5 Plotting the Data

Interpretationit. Other Methods of Testing and lnterpretatio.n8 Fault Locationj9J Safety Precautions During High Direct Voltage Tests

;?ndards Referencesgral)hy

P..ENDIXi: General Discussion of Test Procedures`}ffiA1.1 Routine Maintenance Testing.. +

A1.2 0vervoltage Testing and Electric Strengthk`'A1.3 Proof Tests

Relationship Between Direct Current and AlternatingCurrent in Overvoltage Tests

A1.5 Controlled Direct Overvoltage Tests1.6 Acceptance Testing Using High Direct Voltage

\Alternate Test Procedure: Controlled Overvoltage Test anded-Time Method:

PAGE

7

7

7

8889999

10101212

SECTION

FIGU`RES

Fig 1 Typical circuit for High Direct voltage Test of Three phasesof Machine Winding Tested Simultaneously

Fig 2 Winding F`irst in Good condition shows warning of BreakdownFig 3 Winding slope Decrease May Indicate Imminent FailureFig 4 Plot of overvoltage Test on Three pha§e§ Tested separatelyFig 5 Plot of Insulation Resistance as calculated from current

Values Recorded During Overvoltage TestFig AI Measured Current Recol.ded at IntervalsFig A2 Ship's Curve Template for Drawing Dielectric Absorption Curves .Fig A3 Test Results of Leakage Voltage Curves

TABLETable AI Elapsed Time at the Conclusion of Each Voltage Step

An American NationcLI StcmdardIEEE Recommended Practice for

Insulation Testing ofLarge AC Rotating Machinery with

High Direct Voltage

ahoorption current (or component). A rever-sible component of the measured current,which changes with time of voltage appli-cation. resulting from the phenomenon of "di-electric absorption" within the insulationwhen §tres§ed by direct voltage. See IEEE Std62-1958, Guide. for Making Dielectric Mea-surements in the Field, Section 6.

aeeeptance proof teat. A test applied to new in-sulated winding before commercial use. Itmay, be performed at the factory or after in-

ff 1.Scopei8 recommended practice presents uni-triethods for testing insulation with di-

bltages higher than 5000 V. I It applies to

¥Scgr::tt::i::d¥aatcehd}::%oroaotevdo:th£:a:,0.02'ers acceptance testing of new equipment,'

{t,:±a:t:£rnyt::;nn::etefi::i:gafotfe:e::£:i°ens'tahnat,`sthJ'aati {ri .fierv€r.a.;bfeen in -service.S£,:?--`::-?`'T. a n d.. Z;+±t*`` a-t:;-d-£`=i-'G:.ne~r;i.fie:L~ir;;ue-i.t.s-:;.I i.;.;i.:::¥: stallatian, or both. See Ameri;an National~T¥3¥ 2. Purpose a:. i '` u.-''ua'u v-I..|a. ii.cuul..,I.c ,,.., u.8§*. + :.f atl.*€.+-;; _n~ou€_¥a~c.E.iris_sL.4N5_I_ C50.10-1977.

Spurpose of this recommended pr;-ctice,. :h:+rite,bre£;kdownT Vo`|t:ge. . The voltage at which a:i-,.rv -------.-----.-.- I .-..--- r .--..-- `T.i:.:*rdisruptive discharge takes place through Or`~..

1E°.*hpjrg°hv{gfereuc:i;°or,Tagper°accecde:::Sn::I t::i: `t `. :::,:i::t:vseurcf:ee:: t:oer£::u::tot:en:t). A revel.irtine maintenance tests on the maifu. .§ib|e component of the measured current onEinsulation ofwindings of large ac rna- charge or discharge of the winding which isi.£±'` due to the geometrical capacitance. that i§,

;3-i.proviee.pniform procedures for an- the capacitance a§ measured with alternating3the variations in measured current` so

#aft;o£:e::e:;ea:I::E;,:oesn:s&:::te£:cfeo[;§s:tt:A:d§3a;£*t_I;:§[:,_;fie::cuo:n:s¥:::satfs::::[ufo::::so:daa§:f2:.C]t:t:£8:|iLh` I-

r`define terms which have a specific;as used in this document

. 3. Definitions

!!Qwing definitions are based on thoselon use which have been modified toSpecific for the purpose of this recoin-p+ractice.

#.:toj;nv:e8Se¥t]aEnfeEms:as4u;:LE;:.t8Raetc::e:::::tjTesting Insulation Resistance of Rotating

;-.i I

ethoda have been fo.und applicable to equip-all6r Size and lower vo)tage.

-`.. -rcurrent of power or higher frequencies. With

Section 6.

controlled overvoltage test (dc leakage, mea-Bured current, or 8tep voltage test). A test inwhich the increase of applied direct voltage iscontrolled and measured currents are contin-uously observed for abnormalities with the in-tention of stopping the test before breakdownoccurs. See Section 6.2.

electric 8trebgth (dielectric Strength). Themaximum potential gradient that the materi-al can withstand without rupture.

high direct voltage. A direct voltage above5000 V supplied by portable test equipment oflimited capacity.

IEEEStd 95.1977 lEEE RECOMMENDED PRACTICE FOR INSULATlo`' TESTING 0F

leakage (conduction) current. The non-reversible constant current component of themeasured current which remains after the ca-pacitive current and absorption current havedisal)peared. Leakage curl.ent passes throughthe insulation volume, through any defects inthe insulation, and across the insulation sur-face.

maintenance proof test. A test applied to anarmature winding after being in Service to de-termine that it is suitable for continued ser-vice. It is usually made at a lower voltage thanthe acceptance proof test.

measured current. The total direct current re-§ulting from the application of direct voltageto insulation and including the leakage cur-rent, the absorption current, and, theoretical-ly, the capacitive current. Measured current isthe value read on the microammeter during adirect high voltage test of insulation.

overvoltage (overpotential). A voltage abovethe normal rated voltage or the maximum op-Crating voltage of a device or circuit. A directtest overvolt;!ge is a voltage above the peak ofthe alternating line voltage.

polarization index. The ratio of the insulationresistance of a machine winding measured at1 min after voltage has been applied dividedinto the measurement at 10 min. See IEEEStd 43-1974.

proof test (withstand test), A "fail" or "nofail'' test of the insulation system of a rotatingmachine made to demonstrate whether theelectrical strength of the insulation is above apredetermined minimum value.

4. Preparations for Test:Test Cormections

4.1 Proof Test Preparations. The prepara-tions required for proof tests are simpler thanwhen current measurements are made in con-ducting controlled overvoltage tests. If I)rooftests are to be made without current measure-ment. only those sections marked by an aster-isk need to be consulted.

4.2 Air-Cooled Machines4:2.1 Temperc.ture of the Winding

4.2.1.1 A direct /oltage test should bemade at winding temperatul.es not in excess of

40°C unless otherwise agreed upon betweenthe user and the manufacturer.

4.2.1.2 Temperature is important whenmaking leakage measurement§ at high directvoltages because it influences humidity andmoisture condensation on the surface of theinsulation. Insulation resistance and dielec-tric absorption vary with temperature, so thatconstant temperature is required for accurateand comparable measurements. Hence. ten.peratures near ambient are preferred: other.wise, resistance values must be corrected to acommon base temperature. See IEEE Std 43-1974, Section 4.

4.2.2 Humidity. The insulation resistanceand the polarization index should be at orabove the minimum value recommended inIEEE Std 43-1974 before making a high directvoltage test. Surface leakages on end wind-ings, etc, are increased if moisture is allowedto settle on them, particularly if any dirt i§present. It is difficult to attain a standard de-gree of humidification for close col.relation ofmeasurements on successive tests. For thesereasons. some users prefer to keep windingsdry until tested by maintaining their temper-ature sli.gh!!y a6oue ambient. When machinesare idle for long periods, they may be heated toprevent absorption of moisture. See IEEE Std43-1974. Appendix. Overvoltage tests will bemost significant when made under humidityconditions approaching those under whichthe machine win operate. The usual proce-dure when proof tests are made is to keep thewinding dry both prior tci the test and when inservice.

4.2.a Dirt or\ the Winding4.2.3.1 When dirt is present, the effect of

moisture on the surface is to increase theamount of leakage current. It is usually desir-able to reduce such currents by avoiding mois-ture condensation. Accumulation of dirt onthe winding may increase test voltage stressesin end windings, especially if moisture ispresent.

4.2.3.2 If dirt that is oil soaked or other-wise flammable appears to present a fire haz-ard, in view of the possibility of a flashoverduring the test, the winding should be cleanedbefore testing.

4.2.3.3 The winding may be cleaned be-fore testing if iL is considered that the cleancondition is a standard upon which measure-

s

IEEBStd 95.1977

tors and other creepage surfaces must be dryand carefully cleaned, and may be guarded asindicated in IEEE Std 62-1958. Porcelain §ur-faces may be coated with Silicone compoundto reduce leakage currents due to condensedmoisture on the surface. If auxiliary equip-ment is included in the test and weakness isdetected, the weakness should be located by8ectionalizing. If isolation is difricult and theleakages of the connected item and of thewindingr are of the same order (one not morethan twice the other). they may be tested indi-vidually to provide reference data and testedtogether thereafter until deviation from nor-mal is detected.

4.5.2 Oil-filled a|)paratus should not be in-cluded in the test when current measurementsare made because current readings may be er-ratic and may not pr6vide significant results.

sEouid not exceed the transformer test voltagespec-ir}`a`ation given in IEEE Std 262-1973

i#o:g`:`4gi;:;s2,.:-,:.9f:i,Ta,;:.:cc:.:in%r:I:=rs:t:I:I:

rEf from successive tests can conridently bepared.i.4..2.3.4 The winding Should be cleanedk is found responsible for insulation re-in.ce. below the minimum value recom-

|£§q.`i;e`nEF.E,ess:?n:3;::E4ie%,n.§ii:=tiog:isigs.t detection of incipient faults and in-ion of repairs required during the timef service, and testing again after cleaninglnting and any other work for rinal assur-of-fitness for service.

.. Varnish and Other Couting8. "ei on the winding§ of certain varnishes,be uncured state, causes high leakage;.[Such coatings should be allowed tocure thoroughly before tests are made.

i;e5:t:s¥gdo8:i:::f£:t,£:.utTthhee:oatco:£Thpm`

:Vveerr'h{afut]h:orr°to°trh:§rt:e::a:es?-!S¢i?`fty;;i~i±e4j5t:a::fi';tLfeg;;:Ci{::i:°u£°t[:sttevao:t:g:tb°e::Str:hmeosvteadt:rTwh£:di?£afgt+:i

¥anble.

has,J?e„better inspected and o~bs€'ryera, ;nd

Ed.-for. tests to mainta-in effectiv=-n-;-s-;' 2)i ti'Vity of fault detgction becomes-greater as the

gig:hiis::Tjcset:.dT{:esper:tc£::u::;:Ohroaj±a-`m-'-:Tndt£:g::¥efobnecuop::s::i:]iecrTrrentmeasure-

iTfqefiu:-.::ios.,:!£rs-.:tt,::e4i¥nsffg:¥Fa.!j¥jl¥|-!;ia:;::eiu:,:,:aT:::d::d:et.:ad:::,tio!:tafs:?n::[ggffTqe:;a.cooled

efyed when the machine is tested. For8`apled windings the normal procedure

ig¥e#et:e::;liar:]*:tr:o:uS:i::e::;;ii;::a:]i:g the test, equipment rated for ap-r 100 rnA i8 required.

ion of the Winding from C&b]eg andEquipmenti8 preferable to exclude from the

e`ms that can readily be disconnect-ime available and to apply separategpriate to them. The sensitivity of

:,a,SIer::deunctesdt3ya:hyeT::lps::;i:ftehx:ments. Pothead and busbar insula-

should be disconnected whenever practical.Testing one phase at a time gives a com-parison between phases which is useful inevaluating the condition of the winding andfor historical records.

4.6.2 Tests may be made on the entirewinding at one time under certain conditions,although this procedure is not the preferredmethod. One objection to testing all phases ata time is that only ground insulation is testedand no test is made of the phase-to-|]hase in-su)ation a§ is made when one phase is testedat a time with other I)bases grounded. Testingall phases at a time is applicable for small ma-chines and for machines that have in-accessible.neutral connections. In this casethe three line leads of the winding should be

lEEEStd 95.1977 IEEE REcoMMENDED pRACTicE FOR INSULATlor`' TESTING oF

connected together to avoid surges at an openend in the event of failure or flashover.

4.6.2.I If separation of phases is unusu-ally difficult, it may be done once to establisha reference and all phases te§ted togetherthereafter until 8ome deviation from the nor-mal is found.

4.6.2.2 A comparison of the leakage cur-rent in the three phases may be obtainedwhen all phases are tested at the same time ifspecial connections al.e made as shown in Fig1 and if the direct current is measured on eachphase. This method reduces test time. It re-quires special instrumentation technique anddoes not provide a phase-to-phase test.

4.7 Di8chal'ge of the Winding4.7.1 Following a high direct voltage test,

the winding should be grounded for a min-imum time period equal to or greater than 4times the accumulated test period, but in nocase less than 1 h to ensure that no significantenergy remains stored in the winding.

4.7.2 Unless the winding is grounded, theenergy stored in the winding which has beentested will be dangerous for periods of severalhours. Opening the test source will not removevoltage from the winding because of the storedenergy in it. The winding will not be safe topersonnel without grou.nding until completelydischarged.

4.7.3 If the ground is removed too soon, avoltage will build up in the winding and mayreach a high value. Such voltage would bedangerous to personnel who might touch thewinding and could damage the winding if un-der such condition it were placed in service orgiven other tests.

4.8 Test Equipment. General information onhigh direct voltage insulation test equipmentis given in IEEE Std 62-1958, Section 6.

4.8.1 Power Source to DC Test Equiprierit4.8.1.1 The ac circuit should be free

from intermittent loads and transients.4.8.1.2 The ac supply to the direct volt-

age test equipment must have constant non-fluctuating voltage if accurate dc measure-ments are to be made. Reg.ulating trams-forlners. electronic regulators, motor gener-ators, or combinations of these may be used.Commercially available regulators performbest when their appal.ent power capacitymatches the capacity of the load.

10

L

4.8.1.3 Normal system frequency is pref-erable to a nonsynchronized source, such as asepal.ately driven house generator, because ofthe effect of variable frequency upon voltageregulators and electronic test equipment.

4.8.1.4 A low-ampere 115 V 60 Hz powersource will supply the usual equipment forhigh direct voltage tests on rotating machineinsulation.

4.8.2 High Direct Test Vo.tage4.8.2.1 A source of adjustable direct

voltage is required. Usually a variable auto-transformer is provided in the ac supply cir-cuit.

4.8.2.2 If it is desired to raise voltage con-tinuously rather than in steps, some means ofsmooth variation is necessary. A geared driveis advantageous. A motor-driven variable au-totransformer will raise voltage in a uniformseries of Small steps. Stepless voltage increas-es, in linear or other functions of time, can bearranged with certain direct voltage supplies,electrostatic generators in particular.

4.8.2.3 The voltage control should be ar-ranged for a small voltage change betweensteps, for example, a variable autotl.ansformerof many turns.. Such equipment of the re-quired current rating and designed for 240 Vusually has more turns and will give smallersteps than when using autotransformers de-signed for 120 V.

4.8.2.4 The high-voltage polarity of thetest Set may be either positive or negative. Iftest results are to be com|)al.ed, the test shouldbe made with the same polarity. The test re-port should indicate the polarity used.

4.8.3 Direct Voztoge Afecrsz.remerit4.8.3.1 The dc test set Should be pro-

vided with a high voltage measurement unitcalibrated in kilovolts on the voltmeter. Volt-age instrumentation should be capable ofmeasuring kilovolts with several ranges avail-able.

4.8.3.2 It is occasionally desirable tocheck kilovoltmeter calibration. This may bedone by means of a sphere-gap. See IEEE Std4-1969 (ANSI C68.1-1968), Techniques for Di-electric Tests. The calibration sphere-gapmay serve as an overvoltage limiter during thetest if the gap is increased 20 percent abovethe calculated gap for the maximum voltage.At this setting there will be no needless flash-overs. Resistance of approximately 10.0 000 r}

I.ARGE AC ROTATING MACHINERY WITH HIGH DIRECT VOLTAGE

```,I-Sfty:

•_ 'Lkypica| Circuit for Higb Dire6t.Voltage Te-st'..`'i`

-` of Three phases of Machine windingTested a;

`¥(Threemicro:¥mule:::scOc:nlyberep|:ce:Lbx_,."tkl".

IEEEStd 95.1977

Shielding, and guarding. One meter or record-er may be switched between several outputleads, and meter ranges changed by insulatedcontrols. See Fig 1.

4.8.4.3 Current mea8urements are great-ly facilitated if a recorder can be used with atime or voltage base, In particular, a recorderpermits the determination of an accuratelnean when current i8 fluctuating because ofsupI)ly voltage un8teadiness.

4.8.4.4 Current measurements may alsobe made by means of a calibrated cathode-rayoscilloscope across a resistor in the groundconnection of the test set. The presence of co-rona may be detected by characteristic wave-form. It Should be noted. however, that pulsessimilar to corona pulses have been observed onapplication of direct voltage to certain insula-

•.`tions. These may be Significant at even a£'mall fraction of the direct voltage test level.•`'`.in `to 4.8.4.5 If the test set includes an over-

trip, it should be calibrated and setiTghhie.::.6tj

i¥:t::e:;a:;a•#stress

gh to avoid an inadvertent trip dur-Interruption would cause a breaknt~curve and could possibly over-nding irisulation undesil.ably.

4.8.5LLSGi=.ound Provisions of the Test Set

sion'*s4i:L5iL]btiuabdsetaanvt:#abTe°¥onrdins::tr°t¥:cL6nclu§ion of the test, and a grounding device

2 a one with switching. Arrows indicate dis- ..a` te`..;r ;i-o-:iai,-e-i:;t-a-ill-;-:;=ii=ble-for use i-n emer--,± I _L___^ -.-. _^_+ +t`-A..~t` ,-,. I+ \ .charge current through fault.) . gency.

4.8.5.2 A grounding stick with a dis-_ ` charge resistor is usually provided. The stick

:.".i..I.i..;.i:......`..:ii.:i,:.I:,.::i;.:

11

1 MQ should be connected in seriese-gap to limit surges.: When current measuTements are made, thegap should be omitted from the test circuit.

iE' 4.8.3.3 If the test equipment includesOvervoltage relay trip, it should be set andibrated.Le8.4 DC Mea€zLrement•,]4.8.4.1 The total current is measured in2roamperes. The ammeter should have sev-I current ranges when controlled over-t.age tests are to be made.¥.4.8.4.2 For special controlled overvoltageting, when three phases are metered andted simultaneously, the microammeter is§ed in the high voltage side of the dc supply;h appropriate insulation, electrostatic

insulated and safe for the maximum

€£TRE:¥g`a,tfan8;:;a]Tyheth:o{:gdfntgheshr°e::£to:eu:£ts{:voltage is reduced to zero and then connecteddirectly to ground. A resistor of 1000 to 6000a/kv of maximum test voltage should be usedwith the grounding stick. The ground wireconnected to the discharge resistor should beextra flexible and have generous current-car-lying capacity and physical strength such asNo 12 or No 14 B&S gauge.

4.8.5.3 0n 8ome test sets, opening themain breaker automatically. closes a resist-ance grounding device. When an autoinatic ormanual grounding switch with a resistor is in-corporated in the test set, a grounding stick(without a resistor) should be available foremergency and for visible protection.

IEEEStd 95.1977 IERE RECOMMENDED PRACTICE FOR INSULATION TESTING 0.F

4.9 High-Voltage Test Connection to theWinding

4.9.1 The phase under test should be ener-gizied at both line and neutral ends wheneverpractical. Other phases should be grounded atboth endg. Connecting both ends of the wind-ing together and testing only a portion of alarge machine winding is desirable to limit thedischarge and thereby to minimize possibledamaging surges in the event of failure orflashover during the test. Where a connectionbetween line and neutral is very difficult. testsmay be made by connecting to only one end ofthe winding. Extra precautions should be tak-en to avoid external flashover of the test cir-cult when only one end of the winding is ener-gized.

4.9.2 High-voltage tes`t connections shouldhave minimum leakage current and coronaloss,

4.9.2.1 High-voltage leads should bespaced a minimum of 4 in plus 1 in per 10 kvof test voltage from grounded surfaces wherepractical.

4.9.2.2 Test connections should be sup-ported in the clear, without solid insulationwherever possible. Where solid insulation i§used, it must be dry and of generous surfacelength.

4.9.2.3 The use of large diameter wire fortest leads will reduce corona.

4.9.2.4 Corona on test leads can be re-duced by the use of conductors insulated withmaterials such as polyethylene. This is partic-ularly important with reduced clearance fromgrounded Surfaces. When test lead insulationnot designed for high voltage is used, the testleads should be treated as if they were bare.The insulation reduces corona because of itsdiameter. However, the insulation may bedamaged and unsuitable for normal use afterthe test.

4.9.2.5 Corona can be reduced by round-ing off sharp I)rojections and terminals withmasses of conducting material.. Semi-conducting plastic, such as moist asbestosputty, shaped to spherical contour, may beused. Lead foil or rounded metallic caps ortubes may be used to cover sharp ends. (Alu-minum foil crinkles and creates undesirablepoints.) Connections exposed when sectiona-

:!i:[±ignaat:i:hd::pg::£.tuo`:I::Carefullytleatedto

4.9.2.6 The effect of corona on measure-ments can be reduced by enclosing terminals,etc, in conducting shields connected to aguard circuit and insulated from the measur-ing circuit.

4.9.2.7 At high elevations, corona ismore severe and all precautions to minimize itmay be necessary.

4.9.3 Leakage current resulting from testconnections should be checked at severalpoints up to the highest voltage to be used, af-ter leads are in place before connecting to thewinding for test. Record the result of this test.

4.10 Test Connection to Ground. Ground con-nections must be strong and secure for thesafety Of personnel. In addition, inadequategrounding could be responsible for incorrecttest data and resulting conclusions.

4.10.1 Bolh ends of any portion of the ar-mature winding not under test Should begI'ounded whenever practical.

4.10.2 Ground the following auxiliaryequipment to the machine frame:

(1) Armature temperature detector coils orthermocouple§ .

(2) Other devices associated with the wind-ing

(3) Current transformer secondarie§(4) Rotor winding and shaft(5) Test set frame (see Section 4.8.4)4.10.3 Certain objects close enough to be-

come charged should be gI'ounded.WARNING: During a high direct voltage test,

it is possible for nearby `ingrounded coils, met-allic objects, or 8emiconducting vanished sur-faces to develop voltages which could give dan-ge]ous shocks. It is therefore recommendedthat in the area within 10 ft of the test leads orthe machine winding under test that all Spareparts, I)ieoe8 Of equipment, tools, eke, whichcannot be removed, be grounded while the testi8 in progress.

4.10.4 The test-set frame should be con-nected to the station ground. In addition, thetest set frame should be connected directly tothe frame of the machine under test. Thisground is for the protection of the operator ofthe test equipment and must be secure andcontinuous.

4.10.5 All ground leads and connectorsmust be mechanically strong and so arrangedthat they cannot be broken or removed by ac-

12

.AC;]ROTATING MACHINERY WITH HIGH DIRECT Vol,TAGE

L€'.or error. The ground lead is usually No!Sqgauge flexible stranded conductor oriH The continuity of ground connections|d be observed. For this reason, tape anderg?clip insulators should not be used on

F8:8;eedvf;°s:bTe°:tn,du:::::C:i:phs.aTnhde::rtacno::iiji&any way. Additional ground leads withija`or connectors of ample size to preclude

B§tgEtd¥dbfroer¥£as:eh:rrg:£osfc:?:::i!g°.ns::%ue[ce34,j5..1-..Such grounds are often left in placeEthe tests have been completed when the

gQ may be unattended. All personnelfight come into contact with these leadsdi,`be advised of their purpose and im-nce. .i

r,1 5. Test Procedure:Si.',\t r` , Proof Testsgylla .

IREStd 95-1977

Acceptance or maintenance proof tests maybe applied as such or a.s a continuation of con-trolled overvoltage tests as covered in See-tion 6.

5.3 Voltage Application. Application of testvoltage should be gradual, should avoid ex-ceeding the maximum test set current, andshould avoid unnecessarily tripping the over-current device in the test set which could in-troduce undesirable surges.

Duration of either acceptance or main-tenance proof tests should be 1 min. Tiring i8to start when test voltage is reached.

Reduction of test voltage at end of testshould not be abrupt. Voltage should be al-lowed to decay to at least half value before thewinding is grounded.

5.4 Grounding. Initial grounding should bemade through the resistor provided. After ini-tial grounding, the winding should be solidly

„ gTounded to the frame of the machine as de-

2d`ui|)ment and connections. prelimi-

§T:j::etc.aretobemad.easd+escribedinVoltage for Acceptance Proof. Te8t-equipment either in the factory or in

add.is governed by the appropriate equip-t'code. See ANSI C50.10-1977. Theween the test direct voltage and the2quency (rms) test value for accept-s is I.7. The direct test voltage i§ 1.7e power frequency (rms) code. test•,. ` ,:-{`;g!,::g:eef:o:I.t¥.ai:ii::b:e:e:ngpEo.:;:'v:,:h::

dition of insulation. equipment his-ted service reliability, eta. In general.qfrequency test voltage ranging be-|i-ipercent and 150 percent of ratedminal voltage has proven adequate.ct'!test voltage I:br maintenance testsated by multiplying the power fre-ms) test voltage by 1.7.8t^`vgltage for maintenance |]roof test-e* -special conditions of insulation.Lags, or other considerations may re-riation from the range indicated. It isa.that the origi*al equiprpent manu-tie consulted on these occasions.

13

scribed in Section 4.5.1. See Section 4.8 for in-formation about grounding equipment andconnections.

5.5 Test Results.- Acceptance and main-tenance proof testing is conducted on a purelywithstand basis. If no evidence of distress orfailure is observed by the end of the total timeof voltage application, the test is taken as sat-isfactory.

5.6 Failure. Complete failure is usually in-dicated by a Sharp capacitive discharge at thejoint. of failure. There are times, however,when failure or partial failure may be in-dicated by a large abnormal change in leakagecurrent or by erratic leakage current observedon the meter.

When failure location is not readily observ-able by capacitive discharge or other signs ofdistress, such as smoke or glowing creepagepath, systematic segregation of the windingmay be required to locate the specific portionor coil involved.

Application of a low value alternating po-tential may assist in locating the point of fail-ure. The circuit should have devices to limitcurrents to 6 to 10 A to prevent burning of thecore iron or coil insulation.

Probing with a length of glounded metal foilfastened to an insulating rod, such as a

IEEEStd 95-1977 lEEE RECOMMENDED PRACTICE FOR INSULATI0t\' TESTING 0F

ground stick. can be helpful in locating thepoint of failure. The probe can be at groundI)otential with winding energized or vice versa.If th..e winding will not support any voltage,then the probe should be energized.

5.7 Suggested Test Record. A Suggested testrecord includes:

(1) Serial number of ec|uipment(2) Equipment rating. type of insulation(3) Manufacturer's name(4) Date of test(5) Time of test(6) Test voltage and duration(7) Leakage current at end of test(8) Test connection and connected ap-

paratus (if any)(9) Temperature of winding(10) Time at this temperature(11) Temperature and humidity of en-

viornment(12) Time out of service(13) Test equipment descriptionComments regarding the following are also

of value:(1) Reason for test(2) Visual inspection(3) Physical condition of winding and insu-

lation(4) Resistance and polarization index prior

to test(5) Pertinent history of equipment(6) Date winding was installed(7) Observations of distress, corona, etc,

during test(8) Result of test and action taken(9) Recommendations for maintenance. op-

eration, or future test activity

6. Test Procedure:Controlled Overvoltage Test

6.1 Controversy in Interpretation. It .shouldbe pointed out that there is some controversyin the interpretation of the controlled over-voltage test results. Many operators havefound that this test i§ a very useful main-tenance tool, while others question its value.The following procedure describes one of sev-eral methods of controlled overvoltage testingused. Another method acceptable to otherusers is described in the appendix.

14

6.2 Test Method. The controlled overvoltagetest. sometimes referred to as a dc leakage testor a step voltage test, is a high direct voltagetest in which the voltage is increased in a spec-ified manner, dul.ing which time the mea-sured current i§ observed. This type of test,done under suitable conditions. provides arecord of the condition of the winding forpresent and future use and may permit pre-diction of breakdown voltage if it is within orSlightly above the test voltage. Conclusionsare reached by recognition of abnormalities ordeviations in the curve determined by current,measured in microamperes, versus appliedvoltage, measured in volts, plotted as the testprogresses. When winding has uninsulatedconductol`§, deviations in the curve can be ex-pected.

It is recommended that an initial voltage beapplied and held constant until the polari-zation index is determined and that the volt-age be then increased at a rate not exceeding 3|]ercent of the final test level in each min-ute. If this cannot be done at a constantrate, equal 1 min steps should be used. Thesmaller the steps, the greater the I)robabilityof waming of approach to breakdown. Voltageshould be raised to the recommended max-imum level or until abnormalities are observ-ed. The recommended maximum voltage is1.25 to 1.50 times the rated alternating voltagetimes 1.7. See Sect;on 5.2, first paragraph.

It is often, but not always, possible to termi-nate the test before breakdown of defective in-sulation.

It is important that. since unexpected fail-ure can occur. there be adequate provision forrepair at the time the test is made.

The total test time required is from 30 to 45min per phase.

6.3 Initial Voltage Step. Apply the initialvoltage step of approximately one-third of therecommended maximum voltage. Maintainthis voltage constant for 10 min to obtain thepolarization index. The polarization indexmay also be determined prior to the test bymeans of insulation resistance measurements.See IEEE Std 43-1974.

Read the current at 1 min and at 10 min toprovide data for calculating the polarizationindex from which an evaluation of insulationcondition at relatively low voltage may be ob-tained.

i AC ROTATING MACHINERY WITH HIGH DIRECT VOLTAGE

a polarization index is the ratio of thein current reading to the 10 min cur-reading. A value of at least 2.0 for Cla§§ 8-Clqss F or 1.5 for Class A insulation will

ate reasonably clean and dry insulation.¥|EEE Std 43-1974. If lower values of po-

`i`on index are obtained. the test should

bp.?a and the reason determined. See

!f.i,dl::-ui::i.nanka::t::af::.5f6-L1:?g7:ing Machinery. Heating the wind-y above `ambient quay prevent mois-}nsation. which i§ often a cause for a

D]aiization index. If the polarization in-=:still low, it indicates the possibility ofsfve.dirt or moisture in the winding. The!±EE::tp8::d|E°£Ebesttedst;g.rgn7ti],£i:::nbgir+

frinmdiii`g s`hould be cooled to ambient pri-- \£ea!ipg ..... : . . _ .... f t#w*,3h``3

Lta§..initial 10 min step, if the pojifekfaj 9fx is. satisfactory, commenc

equal-step voltage increase`,``'3nts to the voltage setting i-or

|EEBStd 95.1977

liEST"ATEt)IBF3EAKDOWNI

I:

AMAFiK ED UPWARD 11

§CH ANGE IN SLOPELE IMAY PREDICT POSS

BFi IEAKDOVIN I a8

1!I:llE =

5a

BREAKpewi

KILOVOLTS D-CFIGURE 2

Fig2

i:':a:;E#:::ti-;si:::-;::::ir=B::g.:; First in Good ConditionWaning of Breakdowfl

i-:-:E:eta'a:;t,:i:::!o;tsfii;;io::ira:£|iieyj£:gee:T#;!d::::;ig;i::eiea:;¥:oc!e,.cv:#e:iecTaf:::.g.Ej.';i?:f;i`ij::;:_;e:;;;gi,:jii:_;';::,:t-:_:bf::;:giii::i:i:-est set. Voltage §tep§, once set,

6 adjusted or complex charging- <ition§ will be introduced which

!fi6'ffaticcurrentreadings.Readth\:a''~``"rL:I:,Fsd±t£::;,:::::;tta::#i,e,:seuaa§,I;e:I::::een:

35he end of each step. smooth curve with rising characteristics: Seent Measurement. Current readings Fig 2. The apparent rise is dependent upon

i;ktQhnea::thae.e::e°fd:::ho§bttea:£ineaia::`T¥#£aegi83-:E:S:e:S::;:S:e:bfee:|i:£b:e°±ti':¥;#¥V::eu:Sh::a:¥::ae}itr-

ach step should be plotted I.mmedi.-ghthe test. Many operators use lin-cition paper with current expressed

Eie£::vaos,ttsh:sO;£:n:i:c:sns:.v£]ts:ge:EE.C`9g=rdinate paper is availablelides determination of insulationritl)out calculation. It standard-

i]Orf.theCoordinates.Seetheap.

is#:idse¥.°u±:cbo:dg{!::n::ythbeeu:loafFTut.a voltage base has some ad-

:§:ib::t;i[]at;:u:I:ssl;n;c{::;n!::eo::a:f7csumr:::i

15

no abrupt deviaticin from the smooth curve.breakdown i§ probably not imminent and thetest may be continued until the recommendedmaximum voltage is reached: see Section 6.2,second I)aTagTaph.•(2) Any deviation from a smooth curve

should be viewed as a warning of possible ap-proach to the breakdown voltage of the insula-tion: see Figs 2 and 3 and Section 6.2, firstparagraph. This deviation should be con-firlned by further measurements at one ormore voltage increments. It should be relnem-bered that warnings are Sometimes obtainedwithin as little as 5 percent below the break-down voltage. When the deviation is con-firmed, the test should be stopped if possiblebreakdown is to be avoided.

IEEEStd 95.1977 lEEE RECOMMENDED PRACTICE FOR INSULATION TESTING 0F

(3) The most usual indication of approachto a breakdown voltage is an accelerating rateof increase of current with voltage. See Fig 2.This\type of behavior is associated with wind-ings at ambient temperature in air of normalto high humidity. To obtain an indication ofthe breakdown voltage, the plotted currentcurve may be extrapolated to the vertical,with somewhat accelerating curvature for thesake of conservatism. See Fig 2. If the pre-dicted breakdown voltage i§ as low as the rec-ommended maximum test voltage, the trendshould I)e verified by one more voltage step. Ifthe extrapolation still shows a low value. thetest should be stopped if possible breakdown i§to be avoided.

(4) C-urrent should be watched for any ten-dency to rise with time during constant volt-age application because this would indicateimminent breakdown.

(5) A very abrupt drop in leakage current israrely found: but when it occurs above thepeak operating voltage for the winding, it mayindicate approaching breakdown of the insu-lation. See Fig 3. No method is known for esti-mating the breakdown voltage in this case andit can only be assumed that failure is im-minent. One more voltage steb should bemade. On conflrmation of the occurrence ofthis phenomenon, the test should be stopped ifpossible breakdown is to be avoided.

(6) Cases of abrupt breakdo`rm before thecurl.ent curve approaches the vertical may oc-cur. In some cases this occurs where there ismechanical abrasion, cracking, or acute micamigration. Hence, if breakdown is to be avoid-ed, the test should be terminated con-servatively when preliminary inspectionshows that such conditions possibly exist.

(7) In case of any indication of approach toI)ossible breakdown, it should be confirmedthat the cause does not lie in corona from testconnections, insulation of test leads, etc.Proper placing and insulation of test leads, asdescribed in Section 4. and I)reliminary testsof leads alone at the maximum voltage willavoid this type of error. See Section 4.9.2.

(8) When there is no indication of im-pending breakdown, the test may be contin-ued to the recommended maximum proo/ !es!value. See Sections 5.1 and 5.2.

(9) The test is usually made individually oneach phase of the winding. Differences in

II

I

i

I

DE CRE ASE lNlsLO PE MAYI

lNlM D'CAMIN TEENT POSFA'I Sl8LLURE

1 I

II EII

I

I I

I

KILOVOLTS D-CFig3

Winding Slope Decrease May hdicate|m m€Ti ent Failure

curve characteristics between the phases notattributed to corona, temperature, or hu-midity are usually attributed to the conditionof the insulation. See Fig 4.

6.7 Other Methods of Testing and Interpreta-tion. Other methods of interpreting the test re-sults are sometimes- used. When tests havebeen made in the past. a repetition of the pre-vious test method may be desirable so that re-sults may be compared.

The interpretation of controlled overvoltagetests may also be made by plotting the insula-tion resistance (calculated from test voltage

Fi84Plot of Overvoltage Test on

Three. Phases Tested Separately

I IPOSS I'BLE IWE IAKN IESSlNOa

C OC OMP AF]E OTOACO

I

I

II I /I

I

A aa

-i

l<lLOVOLTS D-C

16

iARGE Ac ROTATING MACHINERY wlTH HIGH DIREor VOLTAGE

I 1

AB Soft PTION CuF]RENT ELl MIN^ITED

I

I I

`. II

. \ \ I

aW ... !\ I

...•`. \\ I

E T"ATE

-,,'I

\IBf!EAKOPW.\

I

k lLOVOLTS D-C

I.I i Fig 5lot of hgulation Resistancebin Chaent Values Recordedfr voltage Test

as CalculatedduringOve,;-/i

i:::Egg:B¢f;e€Lptrrvee::i:ears:]sffap::!]et:gve:1:ae:ejrg§^::

o+ther acceptable method uses ?.pS*.+±S`ee theterval between voltage

IEREStd 95.1977

(6) The isolated coil should be groundedand the winding made electrically continuouswhere the coil had been in the circuit

(7) Avoiding corona at these connectionpoints i§ important and may be difficult to ac-complish (see Section 4.9)

6.9 Sofety Precautions During High DirectVoltage TeBtB. Personnel should be advisedbefore the test that after application of highdirect voltage there will be a`residual charge inthe winding which is dangerous and that de-energizing the test Source will not immediate-ly de-energize the machine winding undertest.

Windings which have been tested must besolidly grounded before being approached by=personnel.

I-,.There is a possibility that after a test, if a

ii¥£difeirg::g:eioe!;:t:::p:en:£id¥e?gr:;u:n:,d::¥:::

removed before minimum grounding•e will be a voltage bu!ldup to a level

be dangerous to personnel or equip-

iniiiijai!;ij:i;,gi.;,:.:.:,:ui::,S;h::i:i::;joi,fffiij!:ifs#:.g:i::::I:a:;:::,:::?oi:a;:h:e:a;;e::eii:i:i;i;;;h::

`: `j -I

;d to locate the fault:

i,i]:tebr::o.:ad;:a:;;eadg{§er:;§eenit:;r:se::e:aein::i;gb=:us=`.;a;¥:::£::toe¥t:{e:t,:o;geo:f:tit:I:i:;i:du:,I:e:bt:vtou[ia::tb identify the fault location

it:act;%nf:nen:et:h:e?:e;?::oc;ei:n::tLsose:a¥isfi:RES:g¥af:::ecg:§:h:;¥ie!i:ss,a.i:ec:h:a:I::deaE,:t,;ra:;:vf,'!u:ei

17

§'ri.stationednearthemachineto.apg_¥tii"ri

`to avoid possible damage to the re-

tryLof the winding from surges that may}hen the winding dischargesF'the failure cannot be readily located!rvation of flash or discharge when di-lt.age is applied, it is often practical tor ?ltemating voltage, with current lim-e to 10 A, to locate the point of failure:

Fgi#nnd:i:d=nJitet8hueccp£!9::e:;i:9i:reealied until the point of breakdown i8 lo-ie6 Section 5.6, third and fourth para-'.;then the failed coil has been located, itle.` electrically isolated so that the re-SE of the winding!can be tested to the!rmined maximum voltage

with the test set. See Section 4.8.5.2.The winding may be solidly grounded a§

Boon as the voltage has been reduced to zero.If a high alternating voltage test is to follow

a high direct voltage test, it is advisable todouble the minimum grounding time to en-8uTe that the absorbed charge does not con-tribute to puncture when the alternating volt-age test is a|)plied. Otherwise, the absorbedcharge, superimposed upon the peak alter-nating voltage dielectTic stress, may exceedthe electric strength of the winding.

A machine should not be placed in Serviceafter a high direct voltage test until the wind-ing has been gTounded, as described in Section4.7, rirst paragra|]h.

IEEEStd 95.1977 IEEE REcoMMENDED PRACTICE FOR INsULATlor`' TESTING oF

Dissipation of the absorbed charge cannotbe accelerated by the application of alter-nating potential or by the application of directvoltage wi`th reversed polarity. Severe insula-tion voltage gradients will be introduced inthe winding if this is attempted.

7. Standards References

IEEE Std 4-1969 (ANSI C68.1-1968). Tech-niques for Dielectric Tests

IEEE Std 43-1974. Recommended Practicefor Testing Insulation Resistance of RotatingMachinery

IEEE Std 56-1977,Guide for Insulation Main-tenance of Large AC Rotating Machinery

IEEE Std 62-1958, Guide for Making Dielec-tric Measurements in the Field

IEEE Std 100-1972 (ANSI C42.100-1972),Dictionary of Electrical and ElectronicsTerms

IEEE Std 115-1965. Test Procedure for Syn-chronous Machines

IEEE Std 262-1973 (ANSI C57.12.90-1973),Test Code for Distribution. Power, and Regu-lating Transformers

IEEE Std 270-1966, Definitions of General(Fundamental and Derived) Electrical andElectronic Terms

American National Standard General Re-quirements for Synchronous M.achines, ANSIC50.10-1977.

8. Bibliography

ALKE, R. J. DC overpotential testing experi-ence on high-voltage generators. AJEE Trans-actions (Pou)er A|.paratus and Systems), vo\71,1952, pp 567-570.BETHKE, R.W., and WESTPHAL, L. C.Maintenance testing of oil circuit recloser in-sulation with high voltage direct current.AIEE Transactic)ns (Power Apparatus andSyslems/, vol 73,1954, pp 1462-1465.CAMERON, A. W. W. Diagnoses of ac gener-ator insulation condition by nondestructive

tests. AIEE Transactions (Potuer A_p_paratusand Sys!ems/, vol 71,1952, pp 263-269.CAMERON. A.W.W. The value of over-voltage tests. Presented at the IEEE Distl.ictCcinference, District Confel.ence Paper62-531.CAMERON, A. W. W., and SINCLAIR,A. M. Experience and development in nan-destructive dc testing for maintenance ofhigh-voltage stators. AJEE Transac!i.ons/Pou;er Apparatus and Systems/, vol 75,1956,pp 201-206.COMPTON, 0. R. Generator testing on the`rEPCO eystem. Electric Light and Poujer,Apr 1954.CURDTS, E. a., and ROSS, C. W. The recog-nition of possible measurement errors in dc di-electric testing in the field. AJEE Transac.i-i6-n-i icornmirnication and Electronics). vo\74.1955, pp 630-635.Diagnosis of machine windings without kill-ing the patient. Ejec.!r!.ca! Wesl, vol 108, May1952.DUKE, C.A., ROBERTS, W.J.. SMITH,L.E., and CAMERON, A.W.W. Investiga-tion of maintenance tests for generator insula-ti\or\. AIEE Transactions (Pou)er Appal-atusand Systems/, vol 80,1961, pp 471-478.DUKE, C. A„ BOSS, C. W„ and JOHNSON,J. S. Report of dielectric tests on a large hydro-g.e-n-e-ti;or. AIEE Tra_nsactiors_ !Co.T.Tuni-_

ca!i.on and EJectronl.cs/, vol 74, 1955, pp673-678.DUNKLE. W. F. Insulation field test results.Presented at the AIEE Winter General Meet-ing,1951, Miscellaneous Paper 51-43.Edison Electric Institute. I/ndergroLinc{ Sys-terns Reference Boole.

FIELD, R. F. The basis for the non-destructive testing of insulation. AJEE Trans.c[c!!.ons, vol 60,1941, pp 890-895.

FINDLAY. D.A., BREARLEY, R.G.A.,and LOUTTIT, C. C. Evaluation of the inter-nal insulation of generator coils based on pow-er factor measurements. AJEE Transac!ions/Pocuer Appara!tis and Syslems/, vol 78,1959,pp 268-274.FOUST. C.M., and BHIMANI. B.V. Pre-dicting insulation failures with direct voltage.-iiE-i Transactions (Power Apparatus and

Sys!ems/, vol 76,1957, pp 1120-1130.GARDNER, Some anomalies in electric mea-surement of cable. Presented at the Middle

18

E AC ROTATING MACHINERY WITH HIGH DIRECT VOLTAGEt- .`.

[6:51956.rR`RIS, H.R., and RHINE, F.P. Test:bings-with high-voltage dc. EJecfri.caJ

*ds-!g|Ey i:'£?,56a.nd STAFFORD. D. E.

i:i:e:;8f:i:ai:yt;ir:::;e:!n;:c:E:i:i:i::g;|i¥;i:,;;i:p;:fN. J. S. A maintenance inspection

fEte£or[[arEe_:ota^t.£Fgm::£5n~e,s..A,EFf.,roils, voi 70, 1951, pp 749_754. `.

!S.; i. S., and CLOKEY, J. W. I.ea*z,g'Ea-'` characteristics of insulatioh €,rfe-dc dielectric strength. AJEE `Trq7rsac- .I::

#;i::if:in]t:e::7ie¥:,4::::=t:3,Oo::I:in£}::.,:

•,.`+

IREEStd 95.1977

MARCROFT, H. C. Field studies of gener-ator windings. AJEE Transac!i.ons /PowerAppara!#s and SyslemsJ, vol 71, 1952, pp822-828.MCFARLIN, V. S. Test and life expectancyof generator windings. AIEE Conference Pa-per 58-1310.MCHENRY, 8. L. Generator insulation test-ing by continuous time+function applicationo{ direct vdrtage.. IEEE Transactions on Pow-er Appara!us and Sys!ems, vol FAS-86, Nov•1967, pp 1329-1333.MCILVEEN, Nondestructiv-e field testing ofhigh voltage cables. Presented at the PowerDistribution Conference, University of Texas,Austin. TX, Oct 26, 1960.MCLEAN, H. T. A versatile high voltage dcinsulation tester. Presented at the AIEE Win-ter General Meeting, 1955, Conference Pa|]er

-'k5a]£.R|s,E.R.,andcASE,R.D.Acanddc

dielectric breakdown testing of a large turbinegenefatt)I statol.. Presented at the AIEE Win-

i:i:;:e;;:::d:r::tajt{;Sg:y;;;b§`n:;j7fy;i::a+:t;#-;E§;g§;E;8;;;i¥i;cei`€;;:[a::5t:;;::,:,:i;eg:::I:itir5;:raa=:i:1-686.

CFE Transactions (Applications cnd

73,1954, pp 452-454.a.. and DALLAS. J. P. A dis-`de high potential test volt

{ol.r¢}<t+

Sac-.8trical insulation. AJEEications and Industry),

:f`355-357.£_M.. and ROBERTSON, K. D. Mi-

}::~t:?uj:e:ee::efd°ra?i:i:VA[±igEescu[±:lMeeting,1957, Conference Paper4,

H.. Insulation Control measure--atton. Electrical Engineering,

•`H{., and HOUSER, W.D. Direct

innpotential testing of large gener-ihted at the AIEE Summer Gener-ifl955, Conference Pa|]er 55-504.

Operating experience in diesel-±o'tives results in design changes.Iations (Applications-and lndus-954. pp 461-463.

MOSES',~.G. L. Review of some problems in dctesting low voltage electric machine insula-

cti7on. Presented at the AIEE Winter GeneralM-eeting, 1953, Conference Paper.MULAVEY, J.E. Testing of main turbine-generator insulation. AJEE Transac!I.ons

r'/Z3otoGr!hAppara!zis and Systems/, 1956, vol 75,',Pp|`52±155.SINANKERVIS, 8. J. High voltage dc testingof cables and cable fault location. Presentedat the AIEE South West District Meeting,1956. District Paper 56-491.NEMETZ, A. M„ KIRWEN, M.S„ andJOHNSON, J. S. Destructive breakdowntests on a large turbine-generator stator wind-ing. AIEE Transactions (Pou)er Appardtusand Sys!ems/, vol 76,1957, pp 421-425.ODOK, A. M.. and SOELAIMAN. T. M. Im-proves dc high potential testing of insulationSystems in low and medium voltage dc equi|]-ment. Presented at the AIEE Winter GeneralMeeting, 1958, Conference Paper 58-378.OLIVER. F. S. Medium voltage ac testing ofrotating machinery insulation. Presented atthe AIEE Winter General Meeting. 1958,Conference Paper 58-203.

19

IEEEStd 95.1977 IEEE REcOMMEr`.DED pRACTicE FOR iNsuLATION TESTlh'G oF

PLETENIK. A. Experience in dc testing of acgenerator insulation. Presented at the AIEEWinter General Meeting. 1955. ConferencePaper.\.

SCHLEIF, F. R. Corrections for dielectric ab-sorption in high voltage dc insulation tests.AIEE Transactions (Pou)er Apparcitus andSys!cms/. vol 75,1956. pp 513 -517.

SCHljEIF, F. R., and ENGVALL. L. R. Ex-perience in analysis of dc insulation tests formaintenance progTamming. AJEE Transac.tions (Pou)er Apparatus cind Systems). vo\ 78`1959, pp 156-161.

SCHLEIF, F. R., and ENGVALL, L. R. DCinsulation testing. Presented at the AIEEWinter General Meeting, 1958. CoriferencePaper 58-204.

SCHNEIDER, W. DC high potential main-tenance testing of tractic>n motors and genel`-ators. Presented at the AIEE Winter GeneralMeeting., 1956, Conference Paper 56-372.

SCHNEIDER. W. Dielectric absorption stud-ies at higher voltages on large rotating rna-chines. Presented at the AIEE Winter Gener-al Meeting,1951, Miscellaneous Paper 51-129.

SCHURCH, E. C. Experience with high volt-age dc insulation testing of generator statorwindings. AJEE Transacli.ons /Power Ap-paratus and Systems), vo\ 75, 1966, pp1082-1088.

SIDWAY, C. L., and LOXLEY, a. R. Tech-niques and examples of high-voltage dc test-ing of rating machine windings. AJEE rrans-

actions (Pou)er Apparatus and Systems), vo\72,1953, pp 1121-1125.STEVENS. K. M„ and JOHNSON, J. S. De-stl.uctive ac and dc tests on two large turbinegenerators of the Southel.n California EdisonCo. AIEE Trcinsactions (Power Apparatusand Systems/, vol 73,1954, pp 1115-1122.WALKER, H. P., and FLAHERTY, R. J. Se-vere moisture conditioning uncovers weak-messes in conventional motor insulation sys-tens for naval shipboard use. AJEE Transac-tlons (Power Apparatus and Systems), vo\ 80`1961. pp 23-31.WAY, W. R. Progress report on stator wind-ing insulation of large hydro-electric gener-ators in Canada. Presented at the ConferenceInternationale des Grands Reseaux Elec-triQues, Paris. France, 1954.WEBB, R. L. Report on high voltage dc test-ing of genel.ator insulation. Presented to theEdison Electric Institute Prime Movers Com-mittee, Akron, OH, Feb 3, 1953.WEDDENFORF, W. A. The use of dc over-potential testing as a maintenance tool in theindustrial plant. AJEE Transac[I.ons /Com-muni.cafi.on and Ezectroni.cs/, vol 78.1959. pp729-736.WICHMANN, A. AC and dc methods fcir theevaluation and maintenance testing of highvoltage insulation in electric machines. JEEPTTansactions on Pouer Apparatus and Sys-!ems, vol 82,1963, pp 273-280.WIESEMAN, R. W. Maintenance over-potential tests for armature winding in ser-vise. General Electric. Reuieu], ALug 1950.

20

ROTATING MACHINERY WITH HIGH DIRECT VOLTAGEIEEE

Std 95-1977

Appendixlil ii not a part of IEEE Std 95-1977, IEBB Recontnended practice for InBulation Testing of LargeMachinery with Hi.h Di]cct Voltage.)

ffiJ. General Discussion ofSsr?`i Test proceduresr``-

bro`cedures and exl)erience leading to.got high direct voltage are discussed in]'6-wing paragra|)hs, and a comparison of|irect voltage with the established a)ter-i?$6ltage test method is given.

i'§j:ii&iin;E;:a`:n:;d:I::;:t:e::tin:t:oi]t:e:§ii:g;'escu:i:

;ati:c¥;:rr:a;:,::]e:.sata's::ei;gn::be;:s:f=::e:§:ti:: :

!u§±:;imespracticaltomakeloca]ized

Fe°s:s't:h:uwt i°oucta,°rrartehp:[ractehawne¥en6e°r:i: .

]achine tests show low measured}perating conditions can be limited•otection can be installed.lance programs can be scheduledto the relative urgency for repair.

vervoltage Testing and Electr.ic

;.;3;oe:rdo£[::gaes::srtasn::eta::etp±t±:%,:i+a:lation has a certain level of electrical

;ii;s::;::nc:eai[i::eipa:tor:hF:i:s:t¥::e:i:p:I;i:cia,:lge cause failure of ori!y that insula-hitable for service.

ngf Te8t8. For many years power fre-Lvervoltage proof tests have been the!y+ accepted method of acceptance

;i;snt:nwgT;nv:];::;t:::::;oe:tjrneeq:Can::;i--.broof test values specified in IEEE,'`and ANSI standards are based on}ars of experience. It is because oftablished alternating voltage stan-.at a relation between the power fTe-

21

quency voltage and direct voltage methods issought.

Proof tests are intended to search for the ex-istence of flaws in the material and for manu-facturing defects. and to demonstrate in apractical manner that the insulation testedhas a certain agreed upon electrical strength.A primary Tequil'ement of such a test is that itshould be discerning and effective in detectingflaws at or below a minimum specifiedstrength without damaging Sound insulation.

Proof test voltages are intended to be §uf-ficiently high to break down coil insulationthat. has an insufricient factor of safety withrespect to the operating voltages, over-volta`ges. and further deterioration to be ex-pected in service. It should be recognized thattioth power frequency voltage and direct volt-age.{-proof tests ate empirical in nature and donot necessarily check the adequacy of the de-sign Qrthe inherent breakdown voltage levelof the insulation system.',.

A1.4 Relationship Between Direct Currentand Alternating Current in Overvoltage Tests.The relationship between the w.ithstand volt-age level using high direct voltage and theequivalent withstand voltage using power fre-.queney;voltage cannot be precisely stated be-causerf the relationship is composed of manyfactol.a,

The lack of a precise equivalence should notcause concern because the purpose of prooftests is to demonstrate that the insulation canwithstand the overvoltages to be expected inservice rather than to establish the I)recisevalue of electrical` strength. The electricalstrength has been found in cases studied to beassociated with impulse strength. Therefore, adirect voltage proof test may indicate abilityof the insulation to withstand surges andshort-time overvoltages approximating thesame peak value. The test overvoltage valuealso provides for insulation deterioration in afurther period of operation.

The.proper high direct voltage proof test forinsulation need not necessarily be related to

lEEEStd 95.1977 IEEE RECOMMENDED PRACTICE FOR INSULATION TESTING OF

the corresponding power frequency voltageproof test by the ratio of the electrical strengthof sound insulation under power frequencyvoltage stress to that under direct voltagestress.

Some investigators point out that until aknown equivalence can be established, the di-rect voltage test cannot be considered com-parable in searching ability to the establishedpower frequency voltage tests.

Direct voltage acts to search out a faultyarea in the insulation by establishing a leak-age current from that area. Although smallcurrents may aggravate damage and lead tobreakdown if the voltage is raised to a highenough. level, this usually does not occur un-less the weakness is signiricant and should befound. High temperature of the insulationusually increases the conductance of any solidinsulation remaining in the fault path: dcconduction in fissures, however. may be re-duced rather than increased by an increase intemperature.

ALL.4.I Ratio of Direct Test Voltage .o Pow-ei. Frequency Test Voltage. The ratio of directtest voltage to power frequency (rms) test volt-age has been reported to vary from 1 to 3. Thiswas determined from tests comparing pre-dicted direct voltage strengths with actualpower frequency voltage (rms) strengths ofmachine insulation containing incipientfaults, and from tests comparing direct volt-age and power frequency voltage (rms)strengths of large numbers of intact samplesof new and used insulation. Further researchis required to correlate the physical character-istics of breakdown locations with the associ-ated range of ratios. In general, it appearsthat (1) the higher ratios occur in well-com-pacted insulation; (2) ratios in the region ofI.41 correspond, as would be expected, to con-ditions emulating an open air gap in a uni-foI.in field (where direct voltage equals thepeak value of the alternating voltage): and (3)ratios less than 1.41 correspond to internal orsurface creepage paths, open or closed in fis-sures. along which maintained direct voltagestress may have some peculiar property of es-tablishing a considerable (but not necessal.ilydestructive) leakage current.

Values of 2.0 to 3.0 have been used widelyfor the ratio of direct to power frequency (rms)test voltages in the cable industry.

The well-compacted slot portions of arma-ture coils ap|)ear to have an average electricalstrength ratio of direct voltage to power fre-quency voltage (rms) between 2 and 3. How-ever, in a machine winding the cro§§ con-nections and leads external to the slots cannotapproach the salne conditions of mechanicalcompaction and electrical strength in theirground insulation. For testing complete arma-ture winding§ of rotating machines, therefore,the ratio between direct voltage and power ire-quency voltage (rms) suggested in this recom-mended practice is 1.7 for acceptance andmaintenance tests.

A1.4.2 VoZ!age Gred[enl8.. AztematEng Vcr-sue D{rec!. When applying a test voltage toany insulating structure, the matter of voltagegradients becomes a factor to be considered.In the case of a direct voltage, the voltage dis-tribution may be different from that under ap-plied alternating voltage.

Notwithstanding this limitation; there aremany practical and economic advantages ofusing high direct voltage for testing ac ap-paratus. High direct voltage test equipment issmall, compact. and provides an economicaltest which may serve to evaluate the physicalcondition of the insulation.

In contrast, in power frequency voltageproof testing procedures it is the usual prac-tice to check test voltages with the windingunder test connected to the test circuit to eval-uate possible distortion or peaking of the volt-age wave. No such test is necessary with directvoltage testing.

A1.5 Controlled Direct Overvoltage Tests.The use of the controlled direct overvoltagetest appears to offer the possibility of a warn-ing of breakdown of an incipient fault by ob-servation of leakage current. especially theconduction component, during step-by-Step orcontrolled application of voltage.

Many investigators have found relation-ships permitting such prediction: however,some others have failed to find such relation-ships and have challenged their existence.

A routine maintenance program has eco-nomic advantages which are not dependenton either accuracy in prediction of breakdownvoltage or certainty in avoiding breakdown.

A1.6 Acceptance Testing Uging High DirectVoltage. There has been a great quantity of

22

Eec. ROTATING MACHli{ERY WITH HIGH DIRECT VOLTAGE

IHffl

cases this has been in proof testing.e reported experience indicates very

Factory results.-.{,¢+

rA2. Altemate Test Procedure:

3ontrolled Overvoltage Test andGraded-Time Method

f~+desirabletoobtainonlythet"eleak-i`frent bn a controlled overvoltage test.I::;iipor;:#::Sb::eqfuafrt:mae:::a:rh°:o!,St;,``|`.'d: maintaining each voltage for ?. V9.ry;,€grTfty:t. . \

;a.¥::=eennt,.i:o`,I?:h,oe§§;::T-Ei[:R:ii:ry:-i':::~J`:ii

IEREStd 95.1977

itp3r:ii).\ 1is.i`5toxp

fy,

•^6CSS

I I.,

TEaTW ^0€ ITI OKVD.CPOL^ I,'Z Tl ON lNI OEX.3.12

a.2s a6O O`7s I.a t3 2 233 4 5 .709ioTlhlE IN ulNUTES

Fig A1+ ,I) I)-

used in*`diawing smooth curves of the proper

;iss,Prteessetntceod;nNe°ctte{ot:sa.t sa:'f`:i;e[+a:: ;is;§` ShiE?e¥tg:iuntt:e:ar¥. read from the smooth

;i;erdoa[;:::::I;i:v:ot,:t:angsee;:ls¢:its:;:g§nutd]ht:seife'..+i::i::it:;a:b:iu:u:¥::e:jt;::yt:iiiii::I::I:t:°if::d¥;Ce;ni:

iniform rate-of-rise method.Litial voltage step is.eppr_a_ap.?-±:|peo .:?.:2..I £O;li93#g formula for the conduction com.

23

(I.I.a X ilo.o) -(,.3'16)2

I.o + ,'lo.o) -21'3.16

Ea£;?uea]Tyarofmk#:::v]o3,.t8ag¥y=£LHi5 maintained constant for 10 min duT-¢h time the measured current is ob-

Should be logged from the initial

:-?pplication to the winding.Dents to the voltage setting on eachld be made within the first 10 a.nerally necessary to Set the initial?proximately 5 to. 10 percent belowd value to allow for voltage increasee absorption test and to end each

]e desired test voltage.6asured current should be recorded at

he:;:'fte[:5|patnod]3..OThmefs:vaa|:eseaacr:5s read during thd test on log-logie graph paper. See Fig A1. A smooth

Subtract C from the 1 and 10 min total cur-rent readings to obtain the currents due to ab-sorption. These values will then be used to cal-culate the absorption ratio rv a8 follows:

IV I- (absorl]tion current at 1 min)Iaio.o (absorption current at l0min)

As soon as the preceding calculations arecompleted, the time schedule to be used forthe remainder of the test may be selected fromTable A|.

IEREStd 95.1977 IEEE RECOMMENDED PRACTICE FOR INSULATION TESTING 0F

Fig A2Shit)'B Curve Template for Drawing

Dielectric Ab8o]ption Curves

24

LARGE AC ROTATING MACHINERY WITH HIGl] DIRECT VOLTAGE

LEAH^CE VC)LT^CE cuRVEs wiHoireTEw._|CXITION _ ^m TEpe_

UNIT ..a, _ SEFii^i ha _ H"IoiTy

93iji,)ai:;=i,aJ"^ ISETJlT®

§¥5

Ih,'Cf'O.

^L'P5

PH^SET2I.T5

MIcf' a-^neS

•

PH^SET'I.T,

W'CcO-^WPSI

=-=.i.ii' a 20 JO

"T^CE (XV O-C)

Te8tkes,T¥.±or;A,EjffE3e+¥#gecques

win .;fafre#Ti&

:;g:E;:gt::?:::

When the 10 min reading hasrrfere~figiv+takeninmediately increase the voltof the second step. ¥<

`This will occur at some poiri't.culation but should not be neglecteqd; other-

: wise, the test results will be invalida.tedr?M

IEREStd 95.1977

2 to 3 Inin once one becomes pro-

±C±§-:tt£'og'n;9f?i::trib:es:£e°dfud,:'#fa:a:.deter-;i#:=¥faThe test shd`uld be continued through the

;:;i;,3:3i;i;:oil:eg!s::pe:pi:c.:f,at:o:t;.:s.!i::g::a:ilce:v!:indicated for the end of th.e second ste-p aha._{LD*` fi5.!ri to the ideal of instantaneous voltage ap-

If the elapsed time approaches the p6iiod -`d

the IV has not been calculated, choose Some ar-bitrary value for IV, such as 5, and follow thattime schedule until the calculation is coln-pleted.

th£::dnoefctef:as::pC%]|:C#::t¥::obme:¥::ii~g:£€:%:the calculation. Generally, this calculation

&ig;,;:a.egi:-gi::;Yo:toar:

plication.t- The behavior of the current should be ob-served continually to allow for the variationsdireL£-tot line-voltage Swings. By interpolating

accurate results may be ob-

Test results are plotted in Fig A3.

IEEE RECOMMENDED PRACTICE FOR lh'SULATIO\. TESTI\`G 0F

Table AIElapsed Tine at the Conclusion of Each Voltage Step

IEEEStd 95.1977

Absorption FLatici IV

56

(min) (6) (rpJn)

Volt.gePe,cent

of Fin. Step

NOTES:

(i)#oTte.#:nncdre°:::,::'!#rlc°en=i:"rs"top.

26

(Table continued on Page 27)