Aging of Porcelain Suspension Insulators Under Mechanical and Electrical Stress on EHV AC Lines

8/16/2019 Aging of Porcelain Suspension Insulators Under Mechanical and Electrical Stress on EHV AC Lines

1/7

ESMO-95 CP-08

Agi ng of Porcel ai n Suspensi on I nsul at or sUnder Mechani cal and El ect r i cal St r ess On EHV AC Li nes

A. S J agt i ani ( Non- Member)

Seni or Engi neerAEP Ser vi ce Cor p. - Col umbus

J . R. Booker ( Member )

Seni or Engi neerAEP Servi ce Corp. Col umbus

Abst r act

A smal l number of f ai l ur es of st andard porcel ai n suspensi on i nsul ators used onAmeri can El ect r i c Power CO s AEP) 765 kV l i nes were f ound w t h t he porcel ai nshed br oken away f romt he cap and pi n assembl y, al l ow ng a l l donut l l f por cel ai nt o f al l t o the har dwar e bel ow

The i nvest i gat i on of t hese f ai l ur es was appr oached f r ommany angl es t o det erm net he sequence of event s l eadi ng t o t he f ai l ur e. Addi t i onal l y, i t was desi r ed t odet erm ne i f t hi s phenomena was t he resul t of agi ng por cel ai n under el ect r i caland/ or mechani cal st r ess.

1. Introduction

AEP's ol der 765 kV l i nes ar e desi gned usi ng 3- phase, 4- conduct or bundl esof 9 5 4 MCM 45/ 7 Rai l conduct or s st r ung i n hor i zont al conf i gur at i on w t ht wo 7#8 al um numcl ad st eel gr ound w r es suppor t ed at t he top. The newerl i nes use a bundl e of 1351.5 MCM 45/ 7 ACSR D pper conduct or s and t wo 7#8al umnum cl ad st eel gr ound w r es. The suppor t i ng st r uct ur es ar e f our -l egged sel f - suppor t i ng st eel t ower s and l at t i ce t ype guyed V al um numt ower s w t h cent er gri l l age f oundat i on and four g u y s . The t ypi cal span i s1400 t o 1500 f t . i n f l at t er r ai n.

65'+/- l 38 +/-

Typi cal Self Supporting Steel Tower Typi cal Guyed V Al um num Tower

80-7803-2506-0/95/ 4.00 01 995 IEEE


2/7

The 1 8 cent er- t o- cent er squar e bundl e of f our conduct or s i s suppor t ed bya VII st r i ng i nsul at or har dwar e assembl y usi ng ei t her si ngl e or doubl est r i ngs ( dependi ng on l oad) i n each l eg of t he 8fVrT t r i ng. The out si dephase i nsul ator hardware assembl i es use t hi r t y 1OVqx Xr1 , 25 ,000 l bs . or3 6 , 0 0 0 l bs. M&E st r engt h bal l - and- socket por cel ai n suspensi on i nsul at or s.The mddl e phase assembl y uses t hi r t y- t wo 1 f l x 5%11 nsul at or s. No corona

r i ngs are used on suspensi on assembl i es.The dead- end i nsul at or har dwar e assembl i es use f our hor i zont al st r i ngs oft hi r t y- f our 1O x 5%11 36,000 l bs. st r engt h por cel ai n bal l - and- socketsuspensi on i nsul at or s f or 1f Rai 181 onduct or , and t hi r t y- wo l l %r f x 6Ur15 0 0 0 0 l b. st r engt h f or Di pper conduct or. Both t hese dead- end hardwareassembl i es i ncl ude cor ona r i ngs.

The st udy on t he donut t ype f ai l ur e has been under way f or 2% years att hi s wr i t i ng, begi nni ng af t er t he di scover y of a br oken i nsul at or by al i ne crew dur i ng t hei r r egul ar mai nt enance i nspect i on of AEP syst emsGavi n- Mar ysvi l l e 765 kV l i ne. Al t hough t here was no i nst ance of phaseconduct or ( s) dr oppi ng t o the gr ound, t he ver y natur e of t hi s t ype off ai l ur e was i nt r i gui ng and pr ompt ed AEP's t r ansm ssi on l i ne engi neer s t oi nvest i gat e t he cause ( s ) .

To keep thi ngs i n pr oper per spect i ve, t here ar e appr oxi mat el y 2 0 0 0 ml esof 7 6 5 kV l i nes on t he AEP Syst em of whi ch appr oxi matel y 1 2 0 0 m l es havebeen i n ser vi ce f or 2 0 - 2 4 years. There ar e appr oxi matel y1, 400, 000 10f l x 3Art bal l - and- socket por cel ai n suspensi on i nsul at or s i n useon these ol der l i nes.

So f ar 2 1 i nsul at or s out of 1, 400, 000 have exhi bi t ed thi s pr obl emt o onedegr ee or another. On a percent age basi s t hi s woul d amount t o about.0015% of t he total popul at i on of i nsul at or s on t hese st r i ngs.

2 . TESTS

A test pr ogr amconsi st i ng of var i ous el ect r i cal and mechani cal t est s wasst ar t ed i n cooper at i on w t h t he i nsul at or manuf act ur er s. Bot h dest r uct i veand non- dest r uct i ve t est s were undert aken. Some cont i nue as of thi swri t i ng.

2a. The El ect r i cal Test s

Appr oxi matel y 1100 sampl e i nsul at or s of 2 5 , 0 0 0 l bs. and 36 ,000 l bs. ( M&E)r at i ng were r emoved f r omboth out si de and mddl e phases of '765 kV l i nes.Af t er t he i nsul ators were r emoved, al l were mar ked to i ndi cat e t he t owert hey wer e r emoved f r om t he dat e of t hei r r emoval , and thei r l ocat i on i nt he str i ng, w t h I11l1 ndi cat i ng t he t op uni t next t o the t ower , and 1130 ,1131 , and s 3 2 f 1 cl oser t o t he ener gi zed conduct or . Al l t he i nsul at or s wer esent t o AEP S John E. Dol an Engi neer i ng Laborat or y i n Gr ovepor t , Ohi o.

When t he i nsul at or s wer e r ecei ved at t he Dol an Labor at or y, t he i nsul at orswere vi sual l y i nspect ed f or any obvi ous manuf act ur i ng def ects or anydamage f r omhandl i ng.

Al l t he i nsul at or s wer e f l ashed over at 60 Hz t o det ect any cracks orpunct ur es i n t he porcel ai n. Thr ee hundr ed i nsul ator s wer e t hen sel ectedf or an oi l punct ur e t est per f or med as per ANSI C 2 9 . 1 St andard. Thepunct ur e vol t age was r ecor ded.

Test dat a has been gr ouped i n t he f ol l ow ng manner f or di spl ay: I nsul at or s1 t hrough 2 8 wer e t r eat ed as one l ar ge gr oup whi l e dat a f r om i nsul at ornumber s 29 , 30 , 31 , and 32 were anal yzed by posi t i on. Thi s gr oupi ng wasdone t o show t he di ver gence of t he char act er i st i cs of t he i nsul at or s neart he ener gi zed conduct or f r omt hose of t he remai ni ng uni t s.


3/7

180

170a-

-2 160

2 150

140a,

m

>"

LT

n=24

n = size of sample

I I I I30

I28 #29 #30 #31 #32

Position inString

Graph 1

Gr aph 1 shows t he oi l punct ur e vol t age of gr ouped i nsul ators . Not e t hedecr ease i n punct ur e vol t age t oward t he ener gi zed end of t he st r i ng.

2B. D ssi pat i on Factor

D ssi pat i on f act or was measur ed on sever al i nsul at or st r i ngs. Gr aph 2Ashows t he resul t i ng dat a f or one compl et e V st r i ng, demonst r at i ng ani ncr ease i n di ssi pat i on f act or at both the ener gi zed and the gr ound ends.Si m l ar measurement s were recor ded at one manuf act ur er' s l aborat ory. Forr ef erence, t he di ssi pat i on f act or was al so measur ed on ei ght i nsul at or sf r om st ock, whi ch were of si m l ar age, t ype and manuf act ur er, but hadnever been energi zed. Gr aphhese uni t s t est ed i n t he 2 . 8 t o 3. 0% r ange.2B shows t he t ypi cal change i n di ssi pat i on f act or w t ht emper at ure on one sampl e.

1 16 32

Position in StringGraph 2A

r espect t o

82


4/7

20 40 8 1M 120 140-a

TEMPERATURE n O

Graph 2B

2C. Punct ur ed I nsul at or Det ect i on ( PI D) Test

A PI D i nst r ument was used t o l ocat e punct ur ed or part i al l y cr ackedi nsul at or s on i n- ser vi ce, ener gi zed l i nes. The t ower s wher e thi s devi cewas used wer e pi cked part i al l y at r andomand par t i al l y near r oads f or easy

access. The devi ce i s at t ached t o a hot st i ck and moved f i r st down andt hen back up t he energi zed i nsul ator st r i ng whi l e t he oper ator st ands ont he t ower. As i t i s moved, an i nt er nal comput er r ecor ds t he el ect r i cf i el d magni t ude each t i me an i nsul ator shed passes. The dat a i s l aterpl ot t ed t o show t he rel at i ve f i el d magni t ude f or each i nsul at or on bot ht he downward and upward t r aver ses. The nor mal cur ve l ooks l i ke a smoothl l neckl acel l , t h a hi gh at t he energi zed end, l owt owar d the cent er and al esser hi gh at t he gr ound end of t he st r i ng. I f t her e i s a punct ur edi nsul ator , t he cur ve makes a shar p downward di scont i nui t y at t hat poi nt ,i ndi cat i ng t he change i n t he el ect r i c f i el d. Usi ng t hi s devi ce on j ustunder 5, 000 i nsul at or s, one el ect r i cal l y f ai l ed uni t was l ocat ed andr emoved and subsequent l y examned w t h t he ot her f ai l ur es.


5/7

Graph 3

Gr aph 3 shows t he curves gener at ed usi ng the P I D devi ce. Note t hedi scont i nui t y f or t he i nsul at or i n t he 17 l ocat i on. Due t o physi cali nt er f er ence, t he f i r st and l ast i nsul at or s i n t he str i ng ar e not i ncl udedi n the measurement .

2 D . X- Ray Exam nat i ons

As par t of t he i nvest i gat i on, s even i nsul at or s wer e X- r ayed. One of t heseven uni t s was previ ousl y subj ect ed to t he oi l - punct ur e t est , t wo uni t swere f i el d punct ur ed uni t s and t he r emai ni ng f our wer e bel i eved t o behi ghl y st r essed uni t s. The X- r ay showed t hat t he punct ur e pat h di d notf ol l owa st r ai ght l i ne bet ween t he met al cap on the out si de sur f ace t o t hemet al pi n on t he i nsi de surf ace of t he por cel ai n. Thi s punct ur e pat hi ndi cat es t hat t he punct ur es occur r ed over a per i od of t i me f ol l ow ng apath of l east r esi st ance whi l e crossi ng t hr ough t he por cel ai n body. Therewer e no ot her cr acks i n the por cel ai n.

MECHANICAL LOAD TESTS

Over 8 0 0 i nsul at or s of 25, 000 l bs. and 36, 000 l bs. ( M E ) st r ength wer et ested mechani cal l y t o f ai l ur e. As was done f or oi l punct ur e t est dat a,t he f ai l ur e l oad val ues of t he i nsul at or s f r om 1 t hr ough 2 8 ( posi t i on i nt he i nsul ator st r i ng) were averaged as a gr oup and the test val ues f ori nsul ator numbers 2 9 30, 31, and 32 were aver aged separat el y. The t estval ues were t hen pl ott ed on Gr aph 4 .

84


6/7

xu)

U

L

46 -

44 -

42 -

40 -

38 -

36 -

34

48

n=size of sample

46 -

44 -

42 -

40 -

38 -

36 -

34

=25=size of sample

n=26

n=33

I I

n=33

I I

I28 #29 #30 #31 #32

Position in String

Graph 4

Gr aph 4 shows a downward tr end f r omi nsul ator 29 t oward the energi zed end.

CONCLUSI ON

The f ol l ow ng concl usi ons ar e based on el ect r i cal and mechani cal t est s, X- r ayexamnat i ons and obser vat i ons on appr oxi mat el y 1100 porcel ai n suspensi oni nsul at or s over t he per i od of 2% year s. Test s were conduct ed at t womanuf actur er s' l abor atori es and at AEP's John E. Dol an Engi neer i ng Labor ator y.I t i s concl uded t hat t he punct ur e pr ocess i s t he r esul t of a combi nat i on ofsever al f actor s.

1.

2.

3 .

D el ectr i c l osses i n i nsul at i ng mat er i al s occur as a resul t of conduct i on,pol ar i zat i on and i oni zat i on. Losses i ncrease as t he t emper at ur e of t hedi el ectr i c mat er i al i ncreases. I ncreased l osses i n t ur n pr oduce f ur t heri ncr eases i n t he t emper at ur e gi vi ng r i se to the possi bi l i t y of t her malr unaway and shor t - t er mpunct ur e br eakdown. For t he i nsul ator t ypes underconsi der at i on, t hi s i s known to occur at subst ant i al l y hi gher vol t agest han have been shown t o exi st across i ndi vi dual i nsul at or s i n servi ce.

The punct ur es exam ned dur i ng thi s s t udy occur r ed i n areas of maxi mumel ect r i cal s t r ess, nor mal l y f r omt he r i mof t he met al cap t o t he pi n. Thei nsul ators were under l ower cont i nuous vol t age t han i s known t o causeshor t - t ermbr eakdown, but had been i n servi ce f or 20 year s or mor e. I t i spostul ated t hat when l ocal i zed st r ess exceeds a cert ai n val ue t hatdamagi ng di schar ges occur t hat can event ual l y resul t i n puncture[' ' .Whet her t hi s i s cont i nuous f or t he ser vi ce l i f e, or happens i n spi kesdur i ng t i mes of hi gh st r ess due t o sur f ace cont amnat i on and moi st ur e i snot known. I t woul d, however, be dependent on anomal i es i n t he i nsul atorconst r uct i on whi ch cause l ocal i zed i nt ensi f i ed st r ess such as eccent r i cpi n posi t i on or a shar p f l ashi ng on t he cap edge f r omt he cast i ng pr ocess.

Once the i nsul ator i s punct ur ed, mechani cal l oad, f r eezi ng of moi st ure anddi ggi ng due t o cont i nued arci ng appear s t o l ead t o c i r c um er e nt i al

cr acki ng. Thi s pr ocess, however , was not ver i f i ed i n t he l abor at or ytes t s .

85


7/7

Aging of Porcelain Suspension Insulators Under Mechanical and Electrical Stress on EHV AC Lines

Documents

Transcript of Aging of Porcelain Suspension Insulators Under Mechanical and Electrical Stress on EHV AC Lines