corona testing under direct voltage conditions

Partial Discharge TestingUnder Direct VoltageConditions

RENATE S. BEVERUniversity of the District of Columbia

JOHN L. WESTROMNASA Goddard Space Flight Center

DC partial discharge (PD) (corona) testing is performed using a

multichannel analyzer for pulse storing, and data is collected during

increase of voltage and at quiescent voltage levels. Thus high

voltage ceramic disk capacitors were evaluated by obtaining PD

data interspersed during an accelarated life test. Increased PD

activity was found early in samples that later failed catastrophically.

By this technique, trends of insulation behavior are revealed sen-

sitively and nondestructively in high voltage dc components.

Manuscript received February 19, 1981.

This work was supported by a grant made by the NationalAeronautics and Space Administration to the University of theDistrict of Columbia.

Authors' current address: NASA/Goddard Space Flight Center,Greenbelt, MD 20771.

U.S. Government work, not protected by U.S. copyright.

INTRODUCTION

In this paper, some partial discharge (PD) (corona)studies under direct voltage (dc) conditions arediscussed. High voltage components assemblies forspace use mostly see dc impressed voltage. They areused in power supplies and scientific instrumentpackages such as spark chambers and ultravioletdetection equipment for exploration of the energeticparticle and radiation environment of space. Thesecomponents and assemblies should therefore be testedunder dc conditions.

The technique of PD detection has been knownfor some time as attested to by Kreuger [1] and Dakin[2]. However, the majority of the PD work has beencarried out under impressed alternating voltage (ac). Itmay be useful to briefly review the most cogent dif-ferences between dc and ac partial discharge behaviorof a test specimen.

Corona or partial discharges can occur withincavities in solid or liquid systems or at the surface ofexposed conductors or at edges of conductors on soliddielectrics, as essentially a localized gas breakdown [3]according to Paschen's curve. Partial discharges canconceivably also occur at other flaws and in-homogeneities within materials, such as grain boun-daries, but most of the literature deals with gasbreakdown in small cavities. The discharge inceptionvoltage occurs when the gas breakdown stress is justexceeded across the cavity which acts as the dischargesite. This local stress is made up of two components:

1) the externally applied voltage stress, taking intoaccount the voltage distribution due to differentdielectric constants in the system and also the ef-fects of conductor curvature, edges, and points;

2) the internal stress of residual surface charge den-sity on any inner insulating surface lining thecavity or grain boundary due to previousdischarges not having been conducted away.

This residual surface charge and its electric fieldprevent more frequent discharges with applied dcvoltage, even when this far exceeds the breakdownstress across the cavity. All inner insulation surfacespreviously exposed to discharges are very likely tohave some significant surface charge, which may bevery slow to dissipate, similar to the better knowncase of an electret.

The ac-crest and dc breakdown voltages of gaseshave been measured to be closely identical. However,the inception voltage of dc partial discharge is dif-ficult to detect, consisting of only a very few pulsesper minute and will only be "correct" for new, virgin,completely discharged dielectric specimen. As statedpreviously, after the first few pulses of discharge theinterior insulating surfaces lining a cavity becomecharged and thus develop a reverse field whichprevents further discharges until the voltage is raised,

IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS VOL. AES-18, NO. 1 JANUARY 198282

or some of the charge is conducted away. Thus theconductivity is the most cogent factor for dc partialdischarge, and it controls the discharge frequency f ata given quiescent voltage above inception. In fact, ithas been shown to be [4, 5]

f = - cp/yao: ln(1 - N`1) = Ncp/yEEO, for N> 1

where cp is the conductivity of the dielectric (bulk), Eis its permittivity, Eo is the permittivity of free space,N is the ratio of applied voltage V to corona inceptionvoltage V', and

y = [Rc/(Rb + Rc)] [(Cb + Cc)/C,b]

the subscript c referring to cavity and b to the dielec-tric in series with it. The R's and C's are resistancesand capacitances of the lumped parameter circuitmodel, equivalent to the actual cavity in the dielectricfor the dc case shown in Fig. 1.

Thus at constant dc voltage the number ofdischarges per second is controlled by the conductivityof the material and by the amount of overvoltageabove inception. The proportionality betweendischarge pulse frequency and tiny conduction currentthrough the sample has been experimentally shown ingreat detail by Shihab [61.

During the act of voltage rise, or the voltage risetransient, if this is above the threshold voltage,discharges will occur during the voltage step and for ashort time following it, after the voltage has reached asteady value. This is, of course, because the voltagerise is analogous to a quarter cycle of ac, and theblocking space charge is being moved. Observationshave been published that the partial discharge quan-tity does not change appreciably with the frequencyup to about 10 kHz [7]. This is fortunate in that it isthen not necessary to keep the rate of rise of voltageduring a voltage increase absolutely the same everytime.

Two more points need to be kept in mind whichfurther complicate the dc partial discharge behavior:

1) The dc conductivity, so all important in deter-mining the discharge frequency, is in itself decreasingwith time, due to charge trapping at shallow and deeptraps. Thus the dc current Idc can be written as

Idc = ot-m

where t is the time after voltage application, I, is thedc current at t = 0 and is related to the already trap-ped charges, and m is a constant determining thespeed of further charge-trapping.

2) Any real insulation sample has more than onedischarge site, which are excited and reach their incep-tion voltage at random values of increasing appliedvoltage. Thus corona pulse repetition rate and

I

Fig. 1. Lumped parameter circuit model of cavity for dc PD case.

VACUUM CHAMBERJINTERFACE SAMPLE

BIOOLE DC PARTIAL DISCHARGEISYSTEM 2

IFILTER 3

l1-40 SPOWER - [j1 j 5

lKVDC ATIONR CALIBRATIONIP.S. FILTER SIGNAL I

L __ _t0 COUPLER=

PULSE _

ANAYZE HISTOGRAM OF EVENT COUNT & CHARGE

Fig. 2. Test setup for measuring dc PD.

magnitude begin very low just above the firstthreshold voltage, but then increase steadily with in-creasing voltage.

TEST EQUIPMENT AND METHOD

The test equipment consisted basically of 1) a664 000 series ± 40 kV, 3 MA dc partial dischargedetection system with power supply by J.G. BiddleCompany, Plymouth Meeting, Pennsylvania; 2) out-put pulses coupled via a buffer-isolation amplifier cir-cuit to an ND-100 multichannel analyzer made byNuclear Data Corporation, Schaumburg, Illinois.Details of this equipment can be made available; seeFig. 2.

The method of testing was to immerse the ends ofthe insulated corona-strand-shielded test cables fromthe high voltage bushings of the Biddle power supplyunder Fluorinert Dielectric Fluid FC-48 by the MMMCompany, such that the stripped metallic ends withalligator clips and the test sample were all under thefluid. Thus no naked metal at the test end was ex-posed to the gaseous air and thus corona into the sur-rounding air was prevented, as well as surface partialdischarges between the test sample leads across the

BEVER/WESTROM: PARTIAL DISCHARGE TESTING UNDER DIRECT VOLTAGE CONDITIONS 83

iii' P1< Nit

suirface-ontlhe bocl of the tcst. sain-pic OCbviousty. t"icem-ptyv iest leadNc uadler Foinm liquid wets ekN,.'t pa rtial dtschait-cth hi nnpoe ~al't'iii) audi ionic were obta-ined f2 up 'r 3~k\V

Fhe only test. objects that were mieasured while im-mersedi in air were large, m-achine-shiop maniufacturedoflCs that hiad large-sized smioothi term-inals fitt-ed withic"orona balls so as to pres,ent external c-orona intol theair

EXPERIMENTS: CAPACIT'OR STU-DY

Objectives

A\ study of,Clommerciall high voltage, high- dielec-tric conistant ceramic disk capacitoi~_ supplied byseveral manufacturecrs vvas Carried oni. in an effort- to

1) see if partial discharge testing unlder direct voltageconditions carn be uised as a means to select themore reliable cornponlenTS if a chioice i-s avai'lable;

2) see which capacitors ai-e the most,- coronia free,3) hiope to find. som-e soi-t of acceptance and rejectioni

criteria for these capacltors,, based oni dc partialdischarge measurement' alonie;

4) discover what thec relationship between ac -co-fronainicepti-on voitage is andl dc partial dischargebehavior,

To this end., the ped-igree of capacitors for test waspurposefuilly .omnmerciall, firom a variety of nmanufac-turers and qualitv~, it! r',recsi-eened except by themanufacturers themnselves.,

Measurement of Initial Parti'al Discharge

The raw data obtained dutrinig onie tim-re initerval of'ot?servation on- tinree d-iffferent c-apaci'tors is seen in thecphiotographs of Figs. 3(A). (II), andt (C). Each photo-graphi shows a partial dischiar-ge histogramn of thenuimbers of pulses at a given piCOCOuIlomb level versusthat picocoul/ornb level which, occur-red during the timneiniterval of obsei-vationi. Irn the particular examiples theapplied vo!tagc "was chaniged in a linear ramp; otherexamples could equally well be obtained while thevoltage was kept, constant,

Data from countless su-ch hiistograms muist be sumimiarized in tables and finially put- in graphical f'orm.Table I presenits the betiavlior of 1000 pF 10O kV,mnan-ufactui-er A capacitotrs and 8500 pF, 4 kV,manufacturer E capacit-ois from 0 to twice ratedvoItage, '2kV,

Necessarily, such summnfiari-zing destroys some ofth-e detailed information Pf the data. The headingsm-Teat-i that what is given tn eachi casc- is I) the totalnumnber of counits between the calibr-at-ion limits citedall the top of the page: lthen .2) below the underliningis given the range of charege in picocouilombs belowwvhich most~, of' theii c un"wei souInd, p1uc a- fi:uwIhigvher en.ergy straggler-s spelled out individually. ThuIS

58.6pC ± 98.3 1 p means hlat- most of the

., i,-, , " ), i ;. V." .,a- -,, y t. , ,. i. ).-- '. " " u ., ---R( 'W "NSFf"Nis .! () L "IX F 1-1) 8, N (. -), JANI-JARY, 1982'. I-I.:t,(.

TABLE IBehavior of Manufacturer A and E Capacitors 1000 pf, 10 kV Manufacturer A; Same At and Calibration as for

E Below

1/2VR I/2VR-VR VR VR-3/2VR 3/2VR 3/2VR- 2VRN N N N N N

pc pcPC PC PC pc

2VR AC rms

N Incept Extpc kV kV

<4.Spc <7.Opc9 3 15 9

<1 0.6pc <6.6pc <I 8pc+107.5 <14pc+7715

<1 4pc+6019

<1 1.9pc+94+161

1 7 19 8 30 8 251_3.3pc <1I PC <26.1pc <7.8pc <25pc <9.4pc <26.9pc <28.5+58

4.5 4.5

3.5 3.0

0 6 2 4<3.3+1 0 <2.9+270 <4.2

7 12<4+199 <10.2pc <1 1.2

+27+160

9<58.3

8.5 6.0

9/16/77 8500pf, 4KV, Manufacturer E Y5VO-3879-877Observation Time During AV - 10 Sec.: Quiescent Observation Time = 100 Sec.; Calibration: 9-400pc

16<1 IOpc+204

296,325,353,400

16.3pc1600

<200pc+345+400(6)

6 2,786<54pc <400pc

30<39pc

+65,1 15,185pc

1.7

<88.5+328 <23pc+154

0 0 9<41 pc+ 13 7,

176,248I3pc

777 240 2,651 557(1)<1 67pc <1 I6pc <267pc <1 74pc

509(2)<198+302+359

1145<208pc+295pc

15 2,551<96pc <340pc

706(1)<21 1+384422(2)

<1 53pc+1 88,21 3,267,395pc

2.0

2.3

°- VR 1 VRVR VR VR 3VR-2VR VR2V

°-2 VR 2VRVR VR32VR 2VR-2VR 2VR °

Fig. 4. Number of PD counts versus voltage. Manufacturer E,Y5V., 8500 pF, 4 kV dc. Fig. 5. Number of PD pulses voltage. Manufacturer C, Y5F,

I -.1 _r 1_ .co IC -r:- 1000 pF, 4 kV dc.

total number of counts lay below 58.6 pC in a near

Maxwellian distribution, plus one count of 98.3 pCand one at 157 pC.

In addition, some of the data is presented in Figs.4, 5, 6, and 7 as graphs. The types of capacitorsselected for this graph plotting were the ones of which

10 identical units were available from the same

manufacturer, lot, formulation, capacitance, andrated voltage. The same ones were also the ones laterchosen for correlating life tests. The graphs clearlylead one to the following observations:

BEVER/WESTROM: PARTIAL DISCHARGE TESTING UNDER DIRECT VOLTAGE CONDITIONS

01/2VR

NPC

00

H

85

N _200 -

180 -

160 -

140

120

Ono _-

23

15

15

15

0- I VR 2VR VR VR32VR 2 VR-2VR

Fig. 6. Number of PD counts versus voltage. Manufacturer A,Z5R, 1000 pkF. 10 kV dc.

11

13

11

15

1:

14

Fig. 7. Number of PD pulses versus voltage. Manufacturer M,X5R, 1000 pF, 10 kV dc.

1) Much more revealing information as to thestate of the insulation is gathered during the ramp ofchanging voltage, or on A V. Hence data must be ac-quired while changing the applied voltage.

2) The manufacturer M and A capacitors have fewcounts, both at AV and quiescent voltage, and theircharge content is low. The manufacturer C units risesharply in count rate with increasing voltage, and themanufacturer E capacitors' rate of increase of countswith voltage is by far the steepest.

The result of the capacitor study seems to be thatthere is no simple, glaringly obvious rejection criterionthat can easily be applied. Every manufacturer's typeof high voltage capacitor has its own PD characteris-tics, almost like fingerprints. A summary of the dataof the capacitors tested appears in Table II. The sym-bol AV is used for the phrase "change of voltage."

Several monolithic capacitors have been cross sec-tioned with lapidary equipment. The findings are, asseen in Figs. 8 and 9:

1) Every S capacitor that had never been exposedto high voltage by the experimenter, when grounddown, showed delamination cracks running along theoutermost parallel plate electrodes. See the photo-graphs. The cracks finally veered outward and endedat the epoxy coating.

2) S monolithics that had been caused to fail elec-trically by applying 20-30 percent more than ratedvoltage, were ground down in small increments. In-terestingly, what was found was that a shorting crackoccurred, starting from the delamination mentionedabove, inward to the tip end of the next electrode ofopposite polarity. In other words, the short went froma weak spot or fault on the outermost electrode towhere the field strength was greatest at the oppositeelectrode.

An interesting type of space charge and poling ef-fect was observed with all the ceramic capacitors ofthe disk type. This is shown in the data of Table III.

The voltage was raised in steps of 2 VR up to 2 V,the first time and the PD histograms obtained as

usual, the A V steps occurring in about 10 s and thequiescent observations being taken over a 100 s inter-val. The voltage was then reduced quickly to zero(manufacturer C's and E's capacitors had energetic"relaxation" PD pulses, but not so the other types,taking about a total of 40 s to cease). The second timeup the corona pulses were much fewer in number.After reducing the voltage again to zero, the polaritywas then reversed on the capacitor. The third time upat reversed polarity the corona was much strongerthan the very first and certainly than the second time.Another repeat at the same polarity caused a decreasein corona again, whereas polarity reversal brought onstrong corona again, and so on and so forth. This isclearly a polarization effect, but the authors are notsure whether one is observing true gaseous corona incavities or poling of the small crystal domains in theceramic material, one domain at a time.

It can certainly be said that if PD pulses appear on


At constant V30 - t = 100 sec

50 _

20 ,-10 VR 2VR VR 2VR 2 VR

0080 _so

During A V40 - t - tO sec

20 -

00 _80 _60 -

40 _

20 -

n

N _

604020 -

00 - At constant Vt = 100 sec

80 -

60 _

20 -

50°_0VR 2 V VR 2 VR 2 VR

30080 -

60 - During AVt - 10 sec

40 -

20

30 -

60 -

00- 12VR 12VR VR VR-32VR 32VR-2VR

IV

8

6

4

2

86

TAB LE 11

Manitiificttiie-e- Stiiiati-y of' Imilpressli ftiom ('apacitors Tested so Far

A Vers little PD, even tip to 2V',. I ow-lesel enei-gy pUses, ito 'relaxaltilon' PD.

NI Ver-x little Pt), evsncip to 2['f,. L ow-levelciner-ev pulses, ito relaxat1ion FD1). Sorvlesvhatnoistle ithan A.

E Sotme coto-na oni AV' f'roiii [',R to V'f. Corona beeomes piotiotinced and appears Inmiuiltiple counit buit sts tipon AV beycond I/f These bursts self-exinluu1ish thentielseslhowever, uponi -emaining at a quiescent svoltace. The eneerttes as 2 1 is approachedbecoimIe seteraIl 1liundi ecd pitocoulombs. The new, E's wit louI lic Silastcreutndei coat havenio i-elaxation c(tona. Sonie eatiicr E's withi Lunidercoat lhad protioulniced relaxation PD.Soriie otlfer E disks tliat had Uinknown ensirotnmental tests done on tienit previtously,hlad early PD 1pulSes oni A L' alt-eadv fitomii 2,[t) Vf, atid -elaxat cor1on)-()Iia amd bi-okedowii completely by pIttI-e li1-1,toul1i1 the1 epoxy c)atine neat- tlhe edee ofthic disk to thetadial lead, adjacenti.

C These hlave a laII-geC liUmber oft etiergetic PD pLlses, begitnlic) ali-cadv on A,' f'otio 0 to2 V[f. As A V goes iota 2 ['ftto 2[' ttI e energies ot obsers ed pillses rante inito tltetiousands of' picocoulhmbs eveti bevyod 8000 pC. Tlhet-e is equallvs ctietgetic relaxatitorcorotia. Yct tionie of thicse fiavet expetlenced hicakdow'n, evceI at 21,/.

Mntolihichts FIrot 2 , to V/f, tftI-c is altaclr y corona,increasinE o11(re sia-plvaip beviid . All ilieS S's tested so tat- break dstown at abouit 20-30 percent above ['f. Tlis sCtllls to be ti-tuc

wlietilie- tle capacitotrs are of receint maIlLufcitucire or I}litee years olId, atid reuaIcdIless ofcapacitance saziLIC or voltage r-atiltig.

AV below V'R that one should be alert for unreliableperformance and possible breakdown. But this is notnecessarily going to happen, as illustrated by themanufacturer C's disks. The results of planned lifetests, accelerated by elevated temperature, need to beevaluated to see if there is a general correlation be-tween corona upon AV below VR on one hand andpossible short life duration on the other. If so, thenearly corona below VR upon A V could be used as a re-jection criterion.

LIFE TESTING ON HIGH VOLTAGECAPACITORS FOR WHICH THERE IS ALREADYINITIAL PARTIAL DISCHARGE DATA L A

(A)Test objects were, with one lead wire identified as

the + side:

a) 5 of manufacturer E, Y5VO, 8500 pF, 4 kV dcrated, ceramic disk capacitors;

b) 5 of manufacturer A, Z5R, 1000 pF, 10 kV dcrated ceramic disk capacitors;

c) 5 of manufacturer M, X5R, 1000 pF, 10 kV dcrated ceramic disk capacitors;

d) 5 of manufacturer C, Y5F, 1000 pF, 4 kV dcrated ceramic disk capacitors.

1) Before the life test began, measured at roomtemperature, for each capacitor were

a) the capacitance of I Volt ac rms, 1000 Hz; Lb) the dissipation factor, similarly; (B)c) the insulation resistance, at rated voltage dc; Fig. 8. Two monolithic capacitors by manufacturer S, in cross sec-

polarity was observed. It was noted tion. No high voltage applied by experimenter.

BEVER/WESTROM: PARTIAL DISCHARGE TESTING UNDER DIRECT VOLTAGE CONDITIONS

IXTABLI" I'l1IlffilcS- I,) 'N (cChal-n2 1110 2 TIKulltnd Ro-er 100, (4 Ploartk-r0

Mantil ;r~C RF 345( N 5E (.-apacm '), 10(30 pl1' 4 k\,. Ratcd. DX OPtDon

-, -4

C6, i C L

Zr

PG 4 ) fc / ~ pc

C4p

1. 'i .... 1. .;

1.. ... (3 " Pt,

4i~( 4 F7~ Z. Io

100 , nalhbrato

4N

U~~~~~~~~~~~~dp

46(10

at what time after- applicationi of' voltage the resistance

reading was taken. The capacitor leads were reversed

and measurements were repeated. Surge currents were

limited to less thani 50 MA iii all the cases above as

well as subsequenit cases

2) Survivability test: A\ll test objects were heatedt

gradually (over I h) to 85 0C under Fluorinert liquid

FC 48 and the applied voltage was raised to each.

capacitor at the rate of and not exceeding 500 V/s~to

rated voltage and left there fortIh, In case of

failure, a fusistor was mounted in series with eachi in-

dividual capacitor- After I h- of cooling, step was

repeated.

3) Now began the lonig-termr life testing at 85

again heating up slowly and applying only 21 ratedivoltage dc (rate of rise not exceeding 500 V/s), The

life test time period was a total of 1000 h, divided up

into four parts. During the first two 250 h periods

only 23 rated voltage was applied; thus 6 kV do was

applied to the parts rated at 4 kV, and 15 kMV dc was

applied to the parts rated at 10 kV.

4) The test was interrupted at 250 h. All of step I

was repeated and the uinits were partial discharge

tested.

5) Sceps 3 anid 4 were repeated Ohree niore times

except foi the thirdt and fouthjt- tinmes, 2 VC wa\ an-

plied to the capacitors,

DISCUSSION

io f' h h0 t1 20 i-c

k', ( i-;i a cd

c b 0 N iPh o wr411''

00ar Spadce 111, n-iI

The results of the life test are shlown in Tables IV

through IX.

I) The most obvious event was that all but onie of

the manufacturer Vs 8-500 pE, 4 kV rat-ed capacitor-s

failed. The first indication to' weakness was, of

c-ourse, tLhatL these units had copious corona, in multi-

F [ " 'i , -- -) .,N'T. -'R () N P" 'SY ST I AMS''. ;. .- lz. A (' T I ( ) N.- A, 1, R ( -1 N D V () 1. A L'- .. 'N C). -.1 A NNO -. A R Y 19 8 -7I I , ", 1- 1-

o I/ ""

CC. p

C'

'IC

r .,V(

vy"

TABLE IVPartial Discharge History During Life Testa

Manufacturer OKV Ol/t12VR 1/2VR 1/2VR-IVR VR VR43/2VR 3/2VR 3/2VR-32VR 2VRE N/pc N/pc N/pc N/pc N/pc N/pc N/pc N/pc N/pc

Y5VO - 3879 - 877, 8500 pf, 4KV rated #18

OriginalMeasurement 0 0 5 7 16 6 109

Lk49pc &.20pc 444pc 20pc 4177pc222pc 105pc

Bursts Bursts

Life Test:After 850C, 0 1 0 2,705 1 7168 1444 7592 Breakdown250 hrs at 20. 9pc 314T.7pc 16-Tpc ToT40pc+ A.220+ To 400pc+3/2VR Multiple Bursts 322+ Bursts

Bursts 341pcat 3.5KV Bursts

ManufacturerE

Y5VO - 3879 - 877, 8500 pf, 4KV rated #15

OriginalMeasurement 0 0 1 3 1 416 19 2896 951

10'.Opc _22+16pc 10?ZFpc 4 208pc L70pc £.316pc e170+234 pcBursts Bursts Bursts

Life Test:After 850C, 5 5 7 1906 18 11,955 1310250 hrs at 4 27pc e 22pc 17c .4 233pc 4 +128 T327pc 4202pc3/2VR Bursts +241pc Bursts Bursts

Start at3.5KV

After 850C, 6 4 8 426 171 6985 2938 Breakdown500 hrs at 43pc 49pc 444pc 4243pc e52+198 i.352pc 436tpc During Next 250 hrs at3/2VR Bursts +275pc Bursts Bursts 2VR, 850C

at 3.5KV Bursts

aObservation time during AVX 10 s; quiescent time = 100 s; calibration: 8 400 pC.

TABLE VPartial Discharge History During Life Testa

Manufac- OKV 0-pl/2VR 1/2VR 1/2VRKVR VR VR-P3/2VR 3/2VR 3/2-%VR 2VRturer A N/pc N/pc N/pc N/pc N/pc N/pc N/pc N/pc N/pc

10OOpf, 1OKV rated, #1; 5038-25R

Original #1 2 3 7 9 3 15 9 15 19Measurement e 2. lpc &4.5pc L 7pc Illpc s6. 6pc 18pc (14pc x14pc 4 12pc

+107pc +77pc +60pc +94+161pc

After 250 hrs 1 0 1 1 0 2 24 8 19at 850, 3/2VR 4.5pc 2.5pc 1.7pc 9+25pc z 32+77pc .36pc 36pc

After 500 hrs 1 0 0 2 0 5 0 2 6at 850C, 3/2VR 2.7pc 4 7+29pc c43pc 17445 .7.3pc+368pc

After 250 hrs 0 0 0 1 2 4 2 2 18more, 850C, 38.6pc 2.8,3.7pc 48.7+36pc e 3. 7pc 20426pc . 9+133pc2VR

Original #2 2 1 7 19 8 30 8 25 11Measurement 2.1pc 3.3pc 11pc e.26pc o 7.8pc .25pc .49.4pc -27pc 4 28+58pc

After 250 hrs 0 0 2 27 0 32 16 28 12at 850C, 3/2VR 43.7pc -24.5pc L 8.9pc <.14.3pc C9+51pc llpc

After 500 hrs 0 u u 19 0 19 0 21 4at 850C, 3/2VR C15pc Z 14.5 442pc 413pc

After 250 hrs 0 1 1 16 0 27 2 25 4more, 850C, 2VR 2.8pc 3.6pc c18.6pc < 20pc 3.3pc c8.7pc Z 30+259pc

After another 0 1 4 30 0 44 8 41 10250 hrs at 2pc 3.3pc L12.7pc cl5pc i7.8pc I11.4pc e.2.8+74+222pc+307pc850C, 2VR +85pc+236pc

aObservation time during AV ' 10 s; quiescent observation time = 100 s; calibration: 2 - 460 pC.


TABLE VIPartial Discharge History During Life Testa

RF 345C Y5F, 10OOpf, 4KV rated

Manufac-turer C

OriginalMeasurement,#7

After 250 hrsat 850C, 3/2VR

After 500 hrsat 850C, 3/2VR (E

After 250 hrsmore at 850C, _!2VR

After 250 hrsmore at 850C,2VR

OKV 0-.*1/2VR 1/2VR

0 49 4j140pc l5pc

0 49 0,404pc

6 38 53.7pc 4420pc 3.3pc

_0 53 105.lpc To 460+pc 6B.2pc

0 40 1,437pc 1.Opc

1/2VRg-VR

169To 460+pc

29278pc

61,473+

107+282 pc

85L104+

4 to 360pc

89To 460+pc

VR VR9-3/2VR13 53731pc To 460+pc

0 159To 460+pc

6 16563.7+21 To 460+pc

11 305* 6pc To 460+pc

1 3141.5pc To 460+pc

Original 0 39 1 54 2 149 4 386 29Measurement, To 460+pc 2.8pc Lll7pc L3.7pc L218pc 5pc _440pc -26+7 to 224pc

#5

After 250 hrs 1 60 0 72 5 111 7 218 13at 850C, 3/2VR 2.4pc To 460+pc t4lOpc tlOpc L383pc 454+413 To 460+ 440+316pc

After 500 hrs 3 60 6 73 7 118 10 226 33

at 850C, 3/2VR t3.3pc To 430 .9.lpc L393 A.14 To 460+ c83pc To 460+ C 43

After 250 hrs 6 57 8 108 7 213 4 522 21more at 850C, 44.6pc To 460+ 4.2pc To 440+ 414 4 343 L5.lpc L440 33+120pc2VR

After 250 hrs 0 27 0 89 0 98 0 107 6

more at 850C, L395 To 460+ L_277+396 r460+ <36.6pc

2VR

aObservation time during AV\. 10 s; quiescent observation time = 10 s; calibration: 2 - 460 pC.

TABLE VIIPartial Discharge History During Life Testinga

Date&tHistory Manufacturer E 848 (6KV) X5T lOOOpf, 6KV rated

#21 7/17/78 New 0 0-1-1/2VR 1/2VR 1/2VR-VR VR VRg-3/2VR 3/2VR 3/2VR-*3P-2VR 2VR 2VRg+O

0 22 3 588 23 2398 14 8339 11 76i33pc L_4.2pc 4307pc C66pc 4179+305(2) L55pc 4120pc+ 421pc z78+

+180pc 248(5) +49(2)pc 440(7)pcBursts

8/2/78 After 250 0 0 0 117 0 50 3 19 4 22hrs more at 4307+78+85 L55+121pc .4 9pc < 18+60+ t7+ 493pc8/6VR, 850C +112pc 177+220+390 21Opc

8/18/78 After 0 0 0 36 0 39 2 45 4 14250 hrs at -68+120pc 9 36pc 4 7. 3pc cw 253pc lOpc C 108pc2VR, 85°C

#22 7/17/78 New 0 14 18 309 73 3048 8 11,260 1649 913pc 427pc .£114+164 L22+69pc &139pc 4 lOpc 4200pc 4 90 4l9.5pc

+348pc +194(2) +258(2)+320 +160(2)+362pc Bursts +210pcBursts Bursts

8/2/78 After 0 0 0 35 8 28 13 81 10 6250 hrs at c 1llpc+ e 5pc 4102pc z llpc C 63+ C 16pc 34pc8/6VR, 850C 163+176 134(3)

6/2/78 Imme- 0 12 2 60 13 215 27 1773 37 5diate polarity 107pc z.4pc 494+ L29pc (65+136pc z30pc 4139 44 e-21pcreversal 350(5)pc Burst at 8KV +267(4) Bursts +173pc

8/18/78 After 0 0 0 12 0 21 0 35 137 3250 hrs more at i-19+58 4 160pc 4114pc u31pc 412pc2VR, 850C +232pc Burst

aObservation time during AV\, 10 s; quiescent observation time = 100 s; calibration: 2 - 460 pC.

IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS VOL. AES-18, NO. JANUARY 1982

3 /2VR7870+174+34 7pc

14a26+90pc

11cl6+9lpc

6912 1+300pc

3,17.2pc

3/2VR-*2VR

1153To 460+pc

168s 263+45Opc

305To 460+

459To 460+

236,.371

2VR

359L 128+

212+220+283pc

48L61+5 to 460

52,69+5 to 392

70t75+187+317pc

2437pc

90

TABLE VIIIOriginal C, DF, Insulation R (4/25/78).

General Radio Capacitance Bridgeat 4V , 1 Khtz , + 0.2%

M lOOOpfX5R , 10KV

1513121411

AZ5R

12345

CY5F

95761

EY5V0

2018161411

C,pf

10461028106610501050

lOOOpf10KV

979982980972968

lOOOpf4KV

982984974980992

8500pf4KV

7260-7248,69757115-70957226-72047448

DF

0.01720.01680.01770.01790. 0174

HP 4329A HR meter

Insul R in .aat VR; 3 minsafter VR appl.

43

3.33.3

2

93.33.3

32.53.3

0.01180 .01190.01200.01200.0120

0.00910.00900 .00880.00880.0093

.0065- .0061

.0067

.0066

.0066

.0071

111

1.31.3

.4

.8

.3

.4

.3

x i0I2x 1012x 1012x 1012x 102

Insul R,.O5Lat 1000Vafter 3 mins after 6 mins

2.5 x 1012 4 x 1012

2 x 1012

7.5 x 10126 x 1i12

3.8 x10125.5 xlO1

x 1012x 1012x 1i12

x 1012x 10

x 1012x 1012x 1012x 1012x 1012

1.5 x 1012 2.5 x 1o12

1 x 1012 2.7 x 1012

12 12.6 x 10 .8 x 10

.9 x 10 1.4 x 101

x1012

x1012x 01

ple bursts, originally. As can be seen in Table IV,numbers 15, 18, and 16 had markedly increasedcorona already after the first 250 h at 3 V, and 85°.Whereas originally multiple bursts of partial dischargepulses began to occur just above VR when the voltagewas increased, now it occurred upon AV below VR.Whereas originally there were no multiple bursts atquiescent voltage except at 2 V., multiple bursts up toalmost 400 pC now occurred at quiescent VR and 3 V.Units 14 and 19 seemed to improve after the first 250h, but then also worsened as the life test continued,14 showing degradation after a total of 500 h at 32VRand 19 after an additional 500 h at 2 VR. Except for19, which survived, 15, 18, 16, and 14 failed whenthey were raised to 2 VR, either to do PD testing or tocontinue the last half of the life testing.

This is not to say that all of manufacturer E'scapacitors will worsen or fail under this regime of ac-celerated life testing, but only that this particularbatch and type failed. In fact, Table VII shows thatwith a different type andformulation of E'scapacitors, namely, 1000 pF, 6 kV, no worsening andin fact a decrease in corona was obtained after 250 hat 8 VR and 250 h more at 2 VR (848 - (6 kV) - X5T -1000 pF).

2) It is interesting to note in Tables VIII and IXthat of the usually standard quantities measured,

namely, capacitance, dissipation factor, and insulationresistance, only the dissipation factor showed adefinite degradation from about 0.0066 to 0.0099after the first 500 h at 32 VR. Techniques for measur-ing the insulation resistance (IR) of capacitors in asatisfactory manner are a subject in themselves. Suf-fice it to say at this point that the instrumentationavailable for that parameter determination, for thisstudy, was only good enough for an order ofmagnitude measurement and did not give an indica-tion of degradation before failure.

3) Certainly one can say that the particular A, M,and C capacitors all survived the 1000 h life test. Onecan begin to see that after 1000 h: a) roughly speak-ing, the A and the M capacitors stayed just about thesame as originally, perhaps a few more high chargecounts at 2 VR than originally, and b) the C capacitorsseemed to first get worse, but then during the last 250h at 2 VR seemed to improve in corona behavior.These C capacitors are somewhat of a puzzle, in thatthey exhibit some extremely energetic single coronapulses, level to 8000 pC, originally. Yet they survivedthe life test to 2 VR. The question that has not beenanswered to where the corona is coming from in allthese samples. Dissection of the capacitors isnecessary for this. Some lapidary work shows that thesize of the electrodes over the disks and the way of


important idea is that all the measurements in a giventest series be carried out in the same, consistent way.3V5, 85 C (6/21/78)

M , lOOOpfX5R , 10KV

C pf

15 102413 102412 104514 102611 1028

Down

A I lOOOpfZ5R , 10KV

962964960957953

Down

C lOOOpfY5F 4KV

9 Removed5 9927 9836 Removed1 Removed4 New 10223 New 9972 New 991

Up

E 8500pfY5VO 4KV

20 Catastrophic11 Catastrophic

16 779014 784513 New 7940Empty .38Clips

19 New 750015 New 774817 New

Insulation R, in IL,at VR; 3 Minsafter VR applied.DF

.0169

.0173

.0174

.0176

.0167

.0123

.0118

.0120

.0116

.0119

.0094

. 009 3

2X10122.2x10121. 4x1012

2x10123xlO122xlo12

1x1lO121X1012

1. 3X1012.9X10121x1012

.0099

.0097

.0098Up

.0098

.0095

.0108

.0005

.0099

.0099

.48x1012 Subsequently dropped & chipped.4xlO1

.27X1012

12.3x102

.53xlO11

.22x10

9% increase 50% increase

dressing out the metal leads is different for the dif-ferent manufacturers and different for a givenmanufacturer from time to time (E). Both A and Mseem to extend their electrodes precisely to the edge ofthe capacitor disks. They offset the leads slightly fromthe body of the disk near the rim so as to permit theepoxy coating to get in between the lead and the disk.Some voids were seen there, but do not matterbecause the disk is metal coated to the edge. The par-ticular C's and E's tested have electrode diameterssignificantly smaller than the disk diameter. The par-ticular C's leads were pressed right down close to thebody of the ceramic with no epoxy coating betweenbody and lead. The 8500 pF E's had the leads offsetnear the rim and seemingly coated with a soft innercoating and then the epoxy on top of that. More workneeds to be done on this sectioning.

4) There is a question as to whether one should,for at least 24 h before PD testing, ground thecapacitor leads or just leave them open circuited. Theunits in this study were left open circuited since this isthe experience they will have in service. Experimentshows that grounding for 24 h before testing will in-deed decrease the number of counts somewhat, butdoes not change the essential nature of the type ofcorona (bursts, or singles, very energetic or not). The

CONCLUSIONS

1) Direct voltage PD measurement is a non-

destructive, yet revealing technique as to how a deviceintended for direct voltage use will behave in actualservice. Especially informative as to the state of theinsulation are data acquired during the raising ofvoltage or upon AV. Data must be acquired over in-tervals of time ranging from about 10 s to 200 s, andtherefore the testing is time consuming.

2) Rather than only looking for rejection and ac-

ceptance criteria of high voltage dc devices, the user

should also look for his own rating criteria. Forcapacitors, from the work done so far, the followingguidelines emerge:

a) Make PD measurements at 0, 0 VR, VR,1

V. VR. VR,VR VR, VR, and possibly3

2- VR 2VE, 2VR. Here VR is the manufacturer's

rated voltage. If already below VR a sharp increase inthe total number of pulses and in their charge con-

tent, going above 100 pC, is observed during data ac-

quisition upon AV, then derate the device to one-halfof the manufacturer's rating (C). If no such sharp in-crease is observed, then use the device at the manufac-turer's rating (A, M). If multiple bursts of corona are

observed, reject the device (the 8500 pF E's).b) Furthermore, expose the devices to 250 h of ac-

celerated life tests, at 85 °C and 3 V After the 250 h2 R,.Afeth25

again take PD data. If the corona has become more

numerous and energetic, then reject the devices (the8500 pF E's). Space charge injection during the 250 hand the resulting polarization should normally makethe measured corona less numerous and energetic afterthe 250 h, even after a 24 h waiting period. If despitethis, the corona has become worse, it is a definite in-dication of insulation damage.

c) The measurement technique must be consistent,that is, always apply high voltage in the same direc-tion to the sample if several sequential measurementsare made; always go through the same time sequenceof raising voltage; always wait 24 h after the last ap-plication of high voltage before doing PDmeasurements. During this waiting time the terminalscan be grounded or ungrounded, as long as the same

is done every time.

3) The relationship between ac and dc corona hasbeen partially explored, but more work needs to bedone on this. So far, a definite conclusion is that dccorona will not be found below ac inception voltage.There is no rhyme or reason among the variousmanufacturers as to how they rate their dc capacitorsfor dc use from their measured ac inception voltage.Some manufacturers give dc ratings only 20 percentabove the ac corona inception (A); others give dc


TABLE IXAfter 500 h at

12345

92

ratings that are as much as three times the ac incep-tion voltage (C) at 100 pC. The 100 pC limit on the accriterion is probably a good experiential number. Itcould possibly be made more stringent at no PDpulses above 50 pC for dc use.

4) A database needs to be established on differentgroups or types of insulation systems, e.g., highvoltage cables or transformers or potted traveling-wave tubes. This report, which is mostly a ceramiccapacitor study, must not be misused to judge, for in-stance, high voltage cable quality from a singlemeasurement of the PD signature of a particularcable.

5) With the aid of such sets of databases, it is theopinion of the authors that PD testing, especially dur-ing a change of voltage gives a better, more detailed,"microscopic" insight into the state of electrical in-sulation than the more customary "macroscopic"quantities like capacitance, dissipation factor, and in-sulation resistance. PD testing can be used to detecttrends before seemingly sudden, unanticipatedcatastrophic failures occur.

REFERENCES

(11 Kreuger, F.H. (1964)Discharge Detection in High Voltage Equipment,New York: American Elsevier, 1964.

[2] Dakin, T.W. (1978)Partial discharges with dc and transient high voltages.Proceedings of the National Aerospace Electronics Con-ference, Dayton, OH, May 1978.

[31 Dakin, T.W. (1968)Corona discharges in dc and partially rectified ac insula-tion systems.Proceedings of the 8th Electrical NEMA-IEEE InsulationConference, Los Angeles, CA, Dec. 1968.

[41 Melville, D.R.G., Salvage, B., and Steinberg, N. (1965)Discharge detection measurement under dc voltage condi-tions.Proceedings of the IEE, Sept. 1965, 112, 1815-1817.

[51 Densley, J. (1977)Partial discharges under direct voltage conditions.National Research Council of Canada, Ottawa, Ontario,Canada.

[6] Shihab, S. (1972)Partial discharges in voids in polymer insulatingmaterials, using high voltage dc.Ph.D. dissertation, High Voltage Institute, University ofBraunschweig, Germany, 1972.

[7] Benett, A.I. (1972)Frequency dependence of partial discharges and measure-ment of void content in insulation.Presented at the NAS-NRC Conference on Electrical In-sulation, 1974, paper 2, session B.

[81 Densley, J., and Sudershan, T.S. (1977)Some results of partial discharge measurements duringthe growth of electrical trees.Conference on Electrical Insulation and DielectricPhenomena, Colonie, NY, 1977.

Renate S. Bever was born in Stuttgart, Germany. She received the M.S. degree inphysics from Cornell University, Ithaca, N.Y., in 1951.

She has taught physics at several colleges and universities, among themAmerican University and the University of the District of Columbia. While there,she did research on high voltage insulation systems for space use. She has con-tinued this activity since joining the staff of NASA/Goddard Space FlightCenter, Greenbelt, Md., in 1978, as an aerospace engineer in the Space PowerApplications Branch.

John L. Westrom received the B.S.E.E. degree from the University of Illinois,Urbana, in 1960.

Since 1966 he has designed instrument power supplies and distribution systemsat the NASA/Goddard Flight Center, Greenbelt, Md. He is head of the Instru-ment Power and Payload Interface Section. He also is a consultant on highvoltage design for instrument systems.


corona testing under direct voltage conditions

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