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Conference Record of the 1990 I International ymposium on Electrical Insula tion, Toronto, Canad a, June 3-6, 1990

N E W T R E N D S I N I N T E R P R ETAT I O N OF PA RT I A L D I S C H A R G E M E A S U R E M E N T S

J . L . Tr i pi er * , P. Maur i n, D. Lef evr e

38 Avenue Kl Bber 7 5 7 9 5 PARI SG E C ALSTHOM

ABSTRACT

A mul t i - channel numeri cal sys t em f or t he 2. P E R S P E C T I V E S B R O U G H T U P B Y N U M E R I C A Lmeasur ement of par t i al di schar ges hasbeen devel oped i n order t o i mprove the

MEA SUR I G S l S T M S

abi l i t y t o i dent i f y PD sour ces and t oseek new cri t eri a f or PD har m ul ness.Thi s system i s bei ng used dur i ng thet es t i ng of t he f ol l ow ng ac tualequi pment s or l arge- scal e r epr esent at i vemodel s: l ar ge al t ernat ors , HV and MVsw t chgear , t r ansf ormers, and on- boardpower el ectr oni c syst ems.

1 INTRODUCTION

Manuf acturer s anxi ous t o conti nuei mpr ovi ng t he qual i t y of t hei r pr oduct sspend a gr eat deal of ef f ort s i n order t obui l d i nsul at i ons f r ee of par t i aldi scharge. Never t hel ess , t he no f aul t

l evel i n t hi s r espect has not beenr eached. Manufact ur i ng or operat i ngcondi t i ons may i nduce par t i al di schar ges,whi ch must be di agnosed i n or der t oanswer t he f ol l ow ng quest i on asaccur at el y as possi bl e: Ar e t he obser vedpar t i al di scharges harm ul t o t he l i f eexpectancy of t he devi ce? Bei ng abl e toanswer t hi s quest i on i s t he obj ect i ve ofmuch resear ch cur r ent l y i n pr ogr ess.

Fr o m a r e gul at o r y poi nt o f v i ew,harm ul ness i s eval uat ed i n t er ms of at hreshol d val ue of t he appar ent charge,whi c h i s der i ved f r o m t he s t at i s t i c alevi dence t hat devi ces whose di schar gel evel i s beneat h t hi s l i m t val ue behavewel l i n ser vi ce. For exampl e, 5 0 0 pC hasbeen chosen as t he P D l i m t dur i ng 1. 5 Unt esti ng of power t r ansf ormers.

The acceptance of such a cr i t eri onenabl es t o consi der a devi ces t at i s t i c al l y f i t f or s er v i c e, but al l owsno i nt erpretat i on i n t erms of har m ul nessf or t he case consi dered. Theref ore , i t i si mport ant t o f i nd new t ool s capabl e ofdi agnosi ng harm ul ness on a case- by- casebasi s . To t hi s end, t he P D energy l evelcar r i es some hope, al r eady conf i r med byr esearches show ng the r el ati on betweenPD energy and physi cal degradat i ons ofi nsul at i ng mate r i al s .

J L . T RI P I E R D E P U T Y M A N A G E RR E S E A R C H A N D D E V E L O P M E N T L A B O R ATO RY P O W E RTRANSFORMER GROUP ELECTRICAL EQUIPMENTDIVISION GEC ALSTHOM SA 25 R U E D E SB AT E L I E RS 9 3 4 0 4 S A I N T- O U E N - F R A N C ETELEPHONE : 49.45.70.4 3 FAX 49.45.71.72

Numeri cal t echni ques cont r i but e t o ani mprovement of t he di agnosi s. They enabl et o eradi cat e some l i m t at i ons Ofconventi onal syst ems and t o devel op,t hrough numeri cal cal cul ati ons, newharm ul ness cri t er i a whi ch coul d not beobt ai ned economcal l y w t h anal ogt echni ques.

They car r y t hr ee maj or advant ages :compl et e recor di ng of event s, savi ng oft he r esul t s, and mul t i - channel dataac qui s i t i on.

Compl et e r ecor di ng of t he phenomenaenabl es t o sel ect a p os t e r i o r i t hesequences most char acter i st i c of t hedi schar ge act i vi t y, an especi al l yval uabl e f eat ur e i n case of sporadi cact i vi t y, or dur i ng test s whi ch cannot beprol onged w t hout endanger i ng t hei nt egr i t y of t he equi pment .

The savi ng of t he r esul t s enabl es t o keepa compl ete and obj ect i ve r ecor d of a t ests es si on. I t g r eat l y f ac i l i t a t es t heaccess t o previ ous cases f or compar i sonpurposes, t he cr eat i on of data bases of s i gnat ur es charact e r i s t i c of spec i f i cmateri al s or devi ces, and t hechar acter i zat i on of a si t uat i on and i t svar i at i on w t h t i me.

Thi s abi l i t y t o memori ze data provi des anaccurate and ef f i c i ent t ra i ni ng t ool f ort es t t echni c i ans , par t i cul ar l y i n t he

case of l arge appar at uses or syst ems f orwhi ch actual t es t i ng i s i nf requent .Fi nal l y, i t enabl es t o per f orm any andal l f or ms of post- processi ng, such asf i l t er i ng, source separa t i on, o rdet erm nat i on of i ndi rect quant i t i es( e. g. energy, mean cur r ent , quadr ati cr at e) , on al l or par t of t he recordedevent s.

Mul t i pl e measur ement channel s of f er t hef ol l ow ng advant ages: i f t he channel s areused i ndependentl y on sever al poi nts of asi ngl e obj ect under t est, such as t hevar i ous t er m nal s of a power t r ansf ormer ,an accur ate el ect r i cal l ocati on i s madepossi bl e by anal yzi ng t he ampl i t udes oft he same i mpul se obser ved at di f f erentpoi nt s . I n t he case of rout i ne test s orqual i t y i nspect i on surveys, mul t i pl exi ng

al l ows t he si mul t aneous t est i ng ofsevera l obj ec t s , w th a poster ior idi agnosi s of any obj ect exper i enci ngdi schar ges.

CH2727-6/90/0000-111 1 .MI Q 1990 I

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noi se l evel on i nput 2 p Vi nput i mpedance 50 nout put i mpedance 100 nf r equency r esponse 20 KHz 1 MHz 5 3

dBi nput l evel 5 4 p V t o 175 p Vmaxi mum gai n 9 4 dBat t enuat or 3 x 20 dBdynam c r ange 25 dB ( 50 dB under

devel opment )

The charac t e r i s t i c s o f t hese ampl i f i er senabl e t o opt i m ze t he s i gnal - t o -noi ser at i o fo r each t ype of t es t : f o rexampl e, an appar ent c har ge of 1 pC on a800 KV t es t i ng l oop 25 meters i n gi r t hcan easi l y be det ect ed, and a 0. 1 pCapparent char ge i s easi l y det ected on a100 V l oop 1 met er i n gi r t h.

Thi s measur i ng syst em i s dr i ven by asoft ware pr ogr am desi gned t o i ni t i ate andcont r ol al l dat a acqui s i t i on andprocessi ng operat i ons. Si nce one mai nchar ac t e r i s t i c of t he sys t em i s t he veryl arge number of r ecor ds t o pr ocess, eachone i nvol vi ng many dat a, i t seemedl ogi cal t o bui l d the dr i vi ng programar ound a dat a base management pr ogr am t owhi ch speci f i c r out i nes are added t ocr eat e and process t he data.

The user i nt er f ace w t h the dr i vi ngprogram f ol l ows t he s i mpl e schemepr esent ed on Fi gur e 3, wher e t hedi f f er e nt f unc t i ons of t he s ys t e m ar eor gani zed on 2 l evel s .

MENU

7C Q U I S I T I O Nt t C A L I B R AT I O NM E A S U R E M E N T2 D M O D E3 D M O D E

D ATA S H E E T

S E L E C T I O NE R A S E

F IG .3 S O F T WA R E O R G A N I S AT I O N

The mai n f unct i ons on t he f i r s t l evel arer e al - t i me ac qui s i t i on, t h en ( o f f - l i ne)condi t i oni ng, di spl ay, and pr ocessi ng.

The acqui s i t i on f unct i on enabl es t oadm ni st r ate t he di r ect sampl es col l ect ed

on t he obj ect s under t est , and t oaccompl i sh t he r eal - t i me acqui s i t i on. Thel at t er i s t r i ggered by typi ng i n ar equest , and ter m nated when a condi t i on,such as t he occur r ence of a gi ven numberof i mpul ses on a measuri ng channel , i smet .

I n order t o be abl e to ut i l i ze t her esources of t he dat a base sof t waref ul l y, some i nf or mat i on rel at ed t o themeasured data i s r ecor ded w t h t he dat ai tsel f : t he date and t i me of t he t es t , aswel l as t he measur i ng channel s, t estcode, sampl i ng r ef er ence, comment s madebef ore and af t er t he r eco rd, e t c . . . Al lt hi s i nf ormat i on can be used l a ter , f orexampl e to search f or r ecor ds accordi ngto a gi ven c r i t er i on.

The di spl ay capabi l i t y enabl es t or epr esent t he obser ved phenomena i n 2D or3D mode.

I n 2D mode, t he records ar e pl ott ed asapparent charge ampl i t ude f unct i on oft i me. Li ne compressi on or expansi oncapabi l i t i es ar e provi ded i n or der t oenabl e wor ki ng on a par t i cul ar i nt er valof a gi ven r ecor d. An osci l l oscopef eat ur e al l ows t o vi sual i ze the record ont he scr een one peri od at a t i me.

I n 3D mode, a gr aph of t he r esul t s of apar t i cul ar r ecor d i s obt ai ned w t h t hephase angl e, t he ampl i t ude, and t henumber of di schar ges as t he t hree axes.Ot her 3D r epr esent at i ons of i nt er est canal so be i ncl uded i n thi s par t of t hepr ogr am

The pr ocessi ng f unct i on enabl es t oc al c ul at e , on al l or par t of a r ec or d ,i ndi rect quant i t i es such as energy,quadr ati c out put , mean curr ent , as wel las any quant i t y of i nt erest , e .g . 3rd and4t h order st ati st i cal moment s of t heampl i t ude spect r um

Fi nal l y, a l i br a r y f unc t i on pr o vi des al lc l as s i cal s el ec t i on and sor t i ng t ool s f ort he recor ds.

I t can be seen t hat v i r t ual l y al lmanagement operat i ons i n t he syst em ar eper f ormed by s of t war e, whi ch combi nes t henecessary r i gor w t h s i mpl i c i t y.

4 E X P E R I M E N T S I N P R O G R E S S

An exper i ment al pr ogr am has been set upand i s bei ng conduct ed i n order a ) t over i f y t he apt i t ude of t he sys t em atper f or m ng appar ent char ge measur ement sw t h t he chosen numeri cal t echni que, and( b) t o conf i r m some recent work on PDi dent i f i cat i on and harm ul nessassessment .

Thi s pr ogr am i nvol ves t h e f ol l ow ngcases :

Al t ernat or bars. Measurement s on a l arge-scal e model on whi ch bot h capaci t i ve andr esi s t i ve par t i al di schar ge sour ces coul dbe act i vat ed i ndependentl y haveest abl i shed t hat t he behavi ors of t he t wodi s charg e f am l i es ar e di f f er e nt . F i gure 4 shows t hat capaci t i ve di schar ges have aphase l ocat i on and an asymmet r yc oef f i c i ent qui t e di f f er e nt f r o mres i s t i ve di scharges .

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3 THE R E S U LT I N G M E A S U R I N G S Y S T E M

The measur i ng syst em descr i bed her e hasbeen desi gned t o t ake advant age of t hebenefi t s of numeri cal t echni quesment i oned above, w t h part i cul ar emphasi son t he fol l ow ng obj ect i ves: new t ool sf or di schar ge harm ul ness eval uati on, andi dent i f i cat i on of t he sources.

The pr i nc i pl e of t he s ys t em i s t h ef o l l ow ng:

From 1 t o 7 i ndependent measur i ngchannel s ar e used t o ampl i f y and ref or matt he di scharge i mpul ses col l ect ed at t het er m nal s of c l ass i cal measur i ngi mpedances, e. g. 50 n.

An addi t i onal channel sampl es and f or mat st he ref er ence vol t age of t he tes t .Dependi ng on t he chosen conf i gur ati on,bet ween 2 and channel s ar e i nput t o amul t i pl exer capabl e of a r a te of 200, 000measur ement s per second. At t he out put oft he mul t i pl exer , each s i gnal i s convert edt o a 12 bi t di gi t al val ue, t hen s t or edt empor ar i l y i n t he hi gh- capaci t y R M 4MO of a personal comput er ( see Fi gur e1) .

HL

Fol l ow ng t he measuri ng sequence, t hesi gnal s ar e tr ansf err ed permanent l y t o anext ernal mass s t or age devi ce so t hey canbe accessed l ater f or processi ng anddi spl ay.

I n or der t o operate as c l osel y as

poss i bl e t o t r ue r eal - t i me , mul t i - channelacqui s i t i on, t he f ol l ow ng organi zat i onof t he acqui si t i on pr ocess has beenr etai ned : An acqui s i t i on round col l ect st he val ues of a l l s i gnal s present on t heact i ve channel s.

Channel 1 i s reser ved f or t he r ef erencevol t age, wher eas Channel s 2 - 8 areassi gned t o t he measur ement ofdi schar ges. An acqui s i t i on i s i ni t i at edwhen one of t he t wo f ol l ow ng event soccur s: t here i s a pul se r epr esent ati veof a di schar ge on at l east one of t he P Dchannel s, or t her e i s a vol t age peak onChannel 1. Thus, as can be seen on Fi gur e2, t he val ues on output of t hemul t i pl exor acqui r ed i n t hi s asynchr onousmodel can be st ored i n matr i x f orm I nt hi s matr i x, each l i ne repr esent s a roundof acqui si t i ons, and each col umn ameasuri ng channel .

M0 0 0

G 6 G G

I f t he acqui si t i on was tr i gger ed by t heoccur r ence of a di schar ge on one of t hemeasur i ng channel s, t hen t he f i r st col umnr ecei ves t he i nst ant aneous vol t age of t he

r efer ence, and t he others r ecei ve theampl i f i ed val ues of t he si gnal s pr esenton al l acti ve channel s. On t he otherhand, i f t he acqui s i t i on was i ni t i at ed bya peak r efer ence vol t age, a zer o val ue i sassi gned t o t he other c hannel s, whi chenabl es t o i dent i f y thi s peak whenr econst r uct i ng t he acqui s i t i ons i n t het i me domai n. Si nce t hi s acqui si t i on of avol t age peak i s made ever y hal f - per i od,i t f orms a count er. I t i s syst emat i cal l yf ol l owed by a speci al acqui si t i on wher e,i nstead of vol t age and appar ent char geval ues, codes corr espondi ng t o theposi t i ons of t he i nput a t t enuat or s of t her ef erence vol t age r educer and of t he DPampl i f i er s a r e r ecorded. I n th i s way, i tbecomes possi bl e t o sw t ch l evel s at anyt i me dur i ng t he tes t .

Ampl i f i er s whose speci f i cat i ons arel i st ed bel ow ar e i mpl ement ed on each PDmeasur i ng channel , bef ore t hecondi t i oni ng modul es

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FIGURE 4 PA RT I A L DISCHARGES WA SUR EWN T ON ROEBLL BAR MOlWIL

I

On- boar d power el ectr oni cs. Si gnat uresof st andard def ect s i n subset s orcomponent s such as i mpul se t r ansf or mer sand condenser s have been obt ai ned. Thesesi gnatures can cont r i but e to a r api ddi agnosi s of t he f aul t y el ement dur i ngt esti ng of t he systems.

di schar ges at t he HV pot ent i al coul d t husbe f ol l owed t hrough a prol onged on- l oadt est . The anal ysi s of i ndi r e ct quant i t i escal cul at ed f rom t he r ecords r eveal s achange i n t he asymmetr y coef f i ci ent swhi ch coul d be i nter pret ed as si gns of apr ogr essi ve degradat i on of t he domai nsubm t t ed t o t he di schar ges. ( Fi gur e 5)

HV and MV sw t chgear i nsul ator s.Si gnatures of t ypi cal def ect s have beenobt ai ned. Thei r var i ati on w t h t i me CONCLUSI ONdur i ng agei ng t e st s w l l al so be s tudi ed.

Thi s numeri cal par t i al di schar qe anal ysi ssyst em whi ch- associ at es r eal - t i meacqui s i t i on and of f - l i ne process i ng ofr esul t s, has been shown t o be a val uabl et ool f or t est i ng a var i ety of HV and MV

Power and di st r i but i on t r ansf ormers.Si gnatures of def ect s have been obt ai nedon r epr esent at i ve model s of c as t - r e si nt r ansf or mer s . t hese enabl e , f or i nst ance,t he di scr i m nati on between suDerf i ci a1 equi pment s.

di scharges and i nt ernal di scharges. Test sof a l arge- scal e model of a ver y hi gh-vol t aqe w ndi na have been moni t ored w t h

I t s capabi l i t i es enabl e a mor e ref i nedi dent i f i cat i on of di schar ge sour ces andprovi de access t o new har m ul nesscr i t er i a t hr ough numer i cal cal cul at i ons

egul ar PD anal yses a t var i ous t es tvol t ages beyond t he PD i ni t i at i ont hreshol d. One sour ce of par t i al on t he r ecorded data.

Test dur ati onMax App. Char geNumber of di schar ges

percycl eEner gy per secondI ndi vi dual EnergyMean Cur r entSkewness on posi t i ve

gr oupSkewness on negat i ve

g r o u p

RECORDA

1

3000

6

0 0 270

0, 3

1, 4

1, 60

RECORDB

2

3000

700, 12

322

0 8

0 , 9 5

C

3

3700

60 0 2

701

0, 23

0 , 9 4

67600

260, 15114

5

- 0, 43

0 90

FI GURE 5 : CALCULATI ONS FROM RECORDS ON A H I G H VOLTAGE TEST ONTRANSFORMER W NDI NG.

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