The characterization of a LDPE affected by different levels of water tree degradation using absorption current measurements

2001 IEEE 7 h In e na ional Con e ence on Solid Dielec ics, June 25-29,2001, Eindho en, he Ne he lands 508 THE CHARACTERIZATION OF A LDPE AFFECTED BY DIFFERENT LEVELS OF WATER TREE DEGRADATION USING ABSORPTION CURRENT MEASUREMENTS M. I. F. F u d, J.C. Filippini3, A. Jimknez', J.A. P6 ez-Gbmez' and A. Jad aque' I) Depa amen o de Fisica Aplicada I, Fac. In o ma ica, Uni . de Se illa, A da. Reina Me cedes s/n 41012, Spain. 2, Dkpa emen de Gknie Physique e de Gknie des Ma e' iaux, Ecole Poly echnique de Mon kal, 2900 Bod Edoua d- Mon pe i , C.P. 6079, Succ. Cen e-Ville, Mon kal, QuLbec, H3C 3A7, Canada. 3' Labo a oi e d'Elec os a ique e Ma e' iaux Diklec iques de G enoble, CNRS-UJF, 25 A enue des Ma y s, B.P. 166, 38042 G enoble, F ance. INTRODUCTION One o he mos impo an applica ions o polyme dielec ics (EPR, HDPE, XLPE, LDPE ...) is hei ex ended use as powe cable insula ions. Conduc ion phenomena a e o g ea in e es o bo h use s and manu ac u e s o cables [l]. In his wo k, in o de o cha ac e ize he conduc ion p ocess in polyme dielec ics, we will ocus ou in e es on he abso p ion cu en h ough a LDPE. As ou cu en measu emen s we e pe o med on labo a o y specimens, we will ho oughly desc ibe: a) labo a o y specimens, b) expe imen al condi ions and c) esul s o measu emen s. Ac ually, we ha e expe imen ally conside ed he in luence o he deg ee o deg ada ion o deg aded wid h pe cen age and pola iza ion ol age on he measu emen s o abso p ion cu en and capaci ance. F om he poin o iew o accele a ed ageing, he ypical elec o-chemical deg ada ion p ocess o se ice powe cables known as "wa e eeing" was simula ed a labo a o y. Then we measu ed abso p ion cu en s o specimens wi h a di e en deg ee o eed deg ada ion and inally, a co ela ion be ween he deg ee o ageing and he i ing pa ame e s o cu en expe imen al da a o he Cu ie- on Schweidle law was p oposed. EXPERIMENTAL Plane-plane LDPE specimens we e ob ained om shee s p epa ed wi h a "Ca e " hea ing p ess a LEMD- CNRS. A p ecise me hod o he ab ica ion o hose shee s was de eloped which allowed us o cu disks o g ea homogenei y whose hickness/diame e we e espec i ely &SO0 pm (Ak10 pm) and @=35 mm. A g anula ed mass o LDPE (PElS om Bo ealis), m=l g, was inse ed in each o he 12 ci cula holes o a b ass ma ix and exposed o successi e selec ed condi ions o p essu e, empe a u e and wai ing pe iods. In his way we could a ain he necessa y homogenei y and quasi- uni o m wid h in ou expe imen al specimens o ensu ing a good (i) eDea abili y / (ii) emoducibili y in elec ic measu emen s which a e de ined as: (i) quali y in he epe i ion (i e a ion) o a measu emen on a unique specimen; (ii) quali y in he epe i ion o a measu emen on se e al specimens -iden ical specimens in p inciple-. F om he poin o iew o accele a ed ageing, he ypical elec o-chemical deg ada ion o cable polyme ic insula ions known as "wa e eeing" was simula ed a labo a o y using a plane-plane elec ode con igu a ion, a 0.1 M 'NaC1 solu ion and an ageing ol age o 5 kV, a 1500 Hz. Once he labo a o y specimens we e assembled om disks, and p e iously o he measu emen o abso p ion cu en wi h a Kei hley 6517, he e olu ion o capaci ance [2] and g6 was also con olled o he same scheduled pe iods o accele a ed ageing. In pa allel wi h hese expe iences, a des uc i e mic oscopic analysis o he wa e ee aged specimens was also pe o med, which allowed us (once he wa e ee kine ics o he LDPE unde s udy was comple ed) o es ablish a con inued co ela ion be ween he e olu ion o he abo e elec ical a iables and he ac ual eed hickness. Such a co ela ion pe mi s he usage o he ela ed a iables as an ins umen o deg ada ion diagnosis o any specimen unde es . G ow h and cha ac e iza ion o wa e ees In o de o ob ain a mul i ude o wa e ees we p oduced a wide dispe sion o ee incep ion poin s in one o he sides o he cu ed disks o LDPE. We applied a p essu e o 372 ba , du ing an in e al o 2 min wi h an ab asi e pape P400 (used and eplaced a e i s applica ion) and we epea ed his p ocess wice o each disk [3]. This ab asi e-pape me hod, a leas in LDPE, e ealed as less agg essi e and mo e ep oducible han he me hod o sandblas ing [4], especially because we could choose a g ain wid h which p oduces a kind o de ec deep enough o ini ia ing wa e ees wi hou in ol ing a di ec isk o dielec ic b eakdown when applying he ageing ol age. The labo a o y specimens [2] (see Fig. 1) we e placed on a me allic ecipien which ac ed bo h as g ound elec ode and as a con aine o a small quan i y o silicon oil. I Figu e 1. Specimen con igu a ion: (1) pla inum elec ode, (2) silicon cap, (3) sal solu ion, (4) PE cylinde , (5) LDPE disk, (6) sil e pain . 0-7803-6352-3/01/$10.00 0 2001 IEEE 509 helps o a oid pa ial discha ges in he ai by ul illing he occasional small ca i ies ha usually appea be ween he LDPE disk and he me allic plane o he g ound elec ode. As LDPE esul s much mo e a ec ed by pa ial discha ges han =PE, we go se e al e y impo an ad an ages om he usage o he oil echnique.- (a) The e is a d as ic diminu ion o he isk o dielec ic b eakdown o sho ageing pe iods - o hin laye s o wa e ees-. I means no only a educ ion in he numbe o los es objec s bu he ac ha he p ocess o ee g ow h o he es o specimens is much mo e con inued and eliable, as wa e ee e ac ions a ibu ed o pe iods o absence o applied elec ic ield a e mainly a oided [5]. (b) The possibili y o applying a s onge ageing elec ic ield (Eoildz10 kV/m s KO,- oileds3 kV/mm). Consequen ly, deepe and hicke deg aded laye s a e de eloped o much sho e ageing pe iods. I pe mi s us hei de ec ion h ough e y signi ican changes o he alues o elec ical a iables in measu emen s { C,I( ), gG}. Once he ageing pe iod is inished, he ac i e pa o he es objec (LDPE disk wi h wa e ees) is ex ac ed and s ained wi h odhamine du ing 48 h a a empe a u e o 60°C. A e wa ds, i e 2OOpm-wid h slices a e cu om each disk wi h a mic o ome and hei colou ed wa e ees can al eady be isualized by using an op ical mic oscope. Wa e ee a e age leng h was de e mined o each scheduled ageing ime om equa ion (l), Whe e li and xi a e espec i ely he leng h and wid h o each indi idual wa e ee, n is he o al numbe o ees pe slice and x,, is he o al leng h o each slice. Expe imen al p ocedu es o elec ic measu emen s Abso p ion cu en echnique The LEMD expe imen al se -up o abso p ion cu en measu emen is shown in Fig.2. Cu en measu emen and DC s abilized ol age 6 8. ll.l....l.l..ll..“ “...“ .... ”..“ ..I*.I-.............” z “j Figu e 2. Expe imen al se -up o cu en measu emen s ((1)-(8) componen s desc ibed in he ex ). supply we e bo h pe o med by using a Kei hley 6517 (3 and 4 espec i ely). Da a acquisi ion was done by means o a GPIB cable (2) and a IEEE-488 ca d inse ed in a PC586 (1) wi h a so wa e de eloped om Kei hley Tes poin packe . A special goal o his kind o e y low cu en measu emen s consis s in minimizing noise. Elec omagne ic noise could be educed by p o iding a especially designed es cell (3), iaxial connec ions (43) be ween Kei hley and es cell, a me al box o he p o ec ion esis ance (6) and an addi ional con inuous Cu Fa aday cage which con ined all he p e ious elemen s ). The physical componen s o he es cell a e schema ically d awn in Fig.3. A special a en ion mus be paid o he design o i s uppe elec ode (7), which mus make con ac a a e y well de ined a ea on e e y LDPE specimen. A i s sc ewed e sion o his 47 2 3 4 Figu e 3. Tes cell: (1) PE cylinde , (2) gua d ing, (3) LDPE sample, (4) lowe elec ode, (5) p o ec i e sc een, (6) PTFE insula o , (7) uppe elec ode, (8) con ac sp ing, (9) Cu wi e. uppe elec ode was subs i u ed by a mo e weigh ed one whose con ac was simply done by i s weigh . In his way, epea abili y and ep oducibili y we e g ea ly imp o ed because o he cons an p essu e a he con ac and by a oiding he gene a ionlbuild-up o cha ges by a kind o mechanical s i ing on he su ace o he polyme [6]. Analy icaVg aphical p ocessing o expe imen al da a was pe o med wi h Kaleidag ph 3.07 and Sigmaplo 5.0 p og ams. Capaci ance measu emen s An I lab appa a us (model LDTRP-2) and a 1621 Gene al Radio b idge successi ely swi ched o he specially designed cell men ioned abo e we e used o ge he alues o capaci ance, C, and dissipa ion ac o o e e y scheduled ageing pe iod. Pa asi ic e ec s Be o e each measu emen , he sal solu ion is emo ed and he sample su ace is d ied ou . Ne e heless, a e his ope a ion, a wai ing pe iod , bz20 min, is absolu ely equi ed o a oiding a huge a ia ion in capaci ance (and he es o elec ical a iables) wi h he ins an chosen o pe o ming he measu emen -pa icula ly in hick eed laye s-. Conce ning abso p ion cu en measu emen s, esidual cu en s can be measu ed in he absence o applied pola iza ion ol age, Vm when he ageing pe iod is o e . They can endu e o pe iods anging om 10 min up o 1 h depending on he LDPE olume a ec ed by wa e ees. An adequa e me hod o minimizing hese 5 10 34 esidual cu en s was applied by sho -ci cui ing he en i e es objec suppo -once he ageing pe iod is inished- du ing a pe iod o abou , &,=30 min. 22.3 1 4.11S2.10-10 I 1.0566 EXPERIMENTAL RESULTS 64 146 250 The g ow h kine ics and he capaci ance o wa e ees 34 3.5889. lo-’’ 1 a893 56.3 1.2905.10-9 1.2491 70.7 6.1215*10-’0 1.3001 The e olu ion o wa e ees wi h he ageing ime in a LDPE is shown bo h g aphically and om he a ia ions o he a e age wa e ee leng h -wa e ee kine ics- in Fig.4. 60 . 40- 20 - 0- % (a) (b) Figu e 4. Wa e ee e olu ion in LDPE (a) h ee pho og aphs aken a e ageing imes ( om op o bo om) o 4 h, 31 h and 272 h; (b) a e age wa e ee leng h dependence on ageing ime. A quasi-linea dependence o he a ia ions o he a e age capaci ance o he samples wi h he ageing ime is plo ed in FigS. 19 18 17 g 16 U” 15 14 13 J 0 50 100 150 200 250 300 ,”(h) Figu e 5. Elec ic capaci ance s ageing ime. Using an equi alen plane capaci o model o he wa e eed samples and aking in o accoun he a ia ions o capaci ance, C, and wa e ee leng h, 1, wi h he ageing ime, he a e age ela i e pe mi i i y o wa e ees, can be de i ed [2] om, whe e ~~d.3 is he ela i e pe mi i i y o non- deg aded PE and Ci is he non-deg aded capaci ance o he sample. A maximum alue, ~~~d.1, was a ained. Cu en measu emen s E olu ion o Iab.( ) wi h deaada ion Fig.6 p esen s he abso p ion cu en expe imen al da a o a measu ing pe iod o 10 min, a pola iza ion ol age Vp300 V and di e en pe iods o accele a ed ageing. Se ies K , V&OO V 10’0 1 . . h I 4 1013 - 10 100 lo00 (d Figu e 6. Abso p ion cu en measu emen s o di e en wa e ee ageing pe iods and a pola iza ion ol age, Vp300V (a e aged alues om samples o Se ies K): (1) Oh, (2) 15h, (3) 34 h, (4) 64 h, (5) 146 h, (6) 250 h. The expe imen al da a we e i ed o he po en ial Cu ie- on Schweidle law, I=IO -“ . Thei co esponding pa ame e s &,m} and deg aded wid h pe cen age, g, we e summa ized in Table 1. We can ob ain a i s e sion o diagnos ic me hod -es ima ion o he deg ee o deg ada ion om a unique Iabs( ) measu emen - by simply plo ing and adjus ing he dependence g=g(m), which is shown in Fig.7. Table 1. Fi ing pa ame e s { Io,m} o each ageing ime and co esponding deg aded wid h. en (h) I g (%) I IOW I 0 0 I h.2S1.10-’2 1 80 1 1 I ....... I 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 n Figu e 7. The dependence g=g(m) as a i s e sion o diagnos ic ool o LDPE eed samples. 51 1 E olu ion o Iah9( ) wi h pola iza ion ol age Fig.8 shows he abso p ion cu en measu emen s in aged samples (Se ies J) as a unc ion o he pola iza ion ol age, V,. A simila s udy was pe o med o new samples (Se ies M) and hei co esponding i ing pa ame e s { 10,m) a e summa ized in Table 2. Se ies J IO" 3 10" ! I I 10 100 1000 (s) Figu e 8. Abso p ion cu en measu emen s and hei linea cu e i ing o di e en alues o he pola iza ion ol age, V, (a e aged alues om aged samples o Se ies J, g=63%): (1) 100 V, (2) 300 V, (3) 500 V, (4) IOOOV. Table 2. Abso p ion cu en i ing pa ame e s { bm} new samples (Se ies M) and aged samples (Se ies J) as a unc ion o VDc. A p e ailing end o sa u a ion in he e olu ion o exponen m wi h pola iza ion ol age, V,, o bo h aged and new samples is obse ed. DISCUSSION The ela i e pe mi i i y o wa e ees in LDPE con inuously inc eased om i s alue o non-deg aded PE, c2=2.3, up o a maximum alue qm,=4.1, o an ageing pe iod o 272 h and an a e age wa e ee leng h o 373 pm (g=74.6%); i supplies a pe mi i i y ampli ica ion ac o A=1.8, a li le bi highe han he alue ound o XLPE in [2], A=1.6 . F om abso p ion cu en measu emen s we could de e mine he e olu ion o coe icien s {Io,m} om he Cu ie- on Schweidle law wi h he ageing ime. A p og essi e inc ease was ob ained o bo h coe icien s: 9897% o Io and 95% o m, when he deg aded wid h g ew om g=O% (new sample) up o g=70.7% (sample wi h a high le el o deg ada ion). We could es ablish, as V, go om 100 V up o IO00 V: (Io) 2137% inc ease in new samples and 202% inc ease in aged samples; (m) 63% inc ease in new samples, 25% dec ease in aged samples and in bo h cases appea a ema kably end o sa u a ion wi h VDC. CONCLUSIONS A LDPE (PElS -base polye hylene o compounds o cable insula ion wi h an ioxidan s and s abilize s- om Bo ealis) wi h di e en le els o wa e ee deg ada ion was cha ac e ized by means o measu emen s o wa e ee kine ics, capaci ance and abso p ion cu en . The consecu ion o an accep able le el o epea abili y/ ep oducibili y in elec ical measu emen s depends g ea ly on he op imiza ion o he expe imen al p ocedu es: om he ab ica ion o LDPE shee s / wa e ee incep ion poin s and me hod o accele a ed ageing o he echniques o minimizing he pa asi ic e ec s. Pa icula ly, an adequa e design o an op imal coupling be ween he es cell and he labo a o y specimen should be aken in o accoun . Combining wa e ee kine ics and capaci ance measu emen s, he maximum a e age alue o he ela i e pe mi i i y o wa e ees in LDPE was calcula ed, clm=4.1 . Abso p ion cu en measu emen s we e i ed o he po en ial Cu ie- on Schweidle law in o de o s udy he e olu ion o i s pa ame e s { bm) wi h ageing ime and pola iza ion ol age. Such a s udy is he base o wo possible me hods o LDPE-wa e ee deg ada ion diagnosis om measu emen s o abso p ion cu en s. REFERENCES [l] J.M. U ubi, G. Reolid, Ibe d ola, Bilbao, Spain, J. Sa da, Asinel, Mad id, Spain, F. Valls, Cables Pi eIli, Ba celone, Spain, "The beha iou o d y insula ion medium ol age cables in a mois en i onmen ", Jicable'95, pp. 366-369, 1995. [2] I. Radu, M. Acedo, J.C. Filippini, P. No inghe and F. F u os, "The E ec o Wa e T eeing on he Elec ic Field Dis ibu ion o XLPE: Consequences o he Dielec ic S eng h", IEEE T ans. on EI, Vol. [3] F. Ciup ina, A. I im, P.V. No inghe , G. Ma in and J.C. Filippini, "Abso p ion Cu en in I adia ed Polye hylene wi h Wa e T ees", MmdE'97, Bucha es (Romania), June 9-1 1, pp. 241-244, 1997. [4] S.S. Bamji, A.T. Bulinski and Y. Chen, "The mally S imula ed Cu en Technique o E alua e Polyme Deg ada ion due o Wa e T eeing", IEEE T ans. on [5] R Pa sch and P. Rome o: "Elec ical Signals om Wa e T ees. Pola iza ion and Depola iza ion Cu en s". 6 h IEE DMMA, Manches e , U.K. (1992), e no. 55. [6] C. Ku and R. Liepins, "Elec ical P ope ies o Polyme s. Chemical P inciples". Chap e 5, pp 223- 229. Hanse Publishe s, 1987. 7, NO. 6, pp. 860-868,2OOO. El, Vol. 28, NO. 2, pp. 299-302, 1993.