WATER TREES INFLUENCE ON THE POWER CABLES INSULATION BREAKDOWN DURING OPERATION

Transcription

1 Joual of Scece ad Ats Yea 10, No. 1 (12), pp , 2010 WATER TREES INFLUENCE ON THE POWER CABLES INSULATION BREAKDOWN DURING OPERATION CRISTINA STANCU 1, PETRU V. NOTINGHER 2, MIHAI PLOPEANU 2, RADU SETNESCU 1, 3, ADRIAN MANTSCH 1, TANTA SETNESCU 1, 3, MADALINA DUMITRU 1 1 INCDIE ICPE CA, , Buchaest, Romaa 2 Uvesty Poltehca of Buchaest, , Buchaest, Romaa 3 Valaha Uvesty of Tagovste, Scece ad Ats Faculty,130024, Tagovste, Romaa Abstact: I ths pape a study egadg the fluece of wate tees o the values ad epatto of the electc feld powe cables sulatos s peseted. Expemets wee caed out o flat samples ude the ccula cow shape, take fom some hgh voltage cables sulatos. I these samples wate tees wee developed ad themal paametes, wate cotet fom teed aeas, dmesos ad wate tees desty wee measued. Usg the dmesos of wate tees, wate cotet teed aeas ad themal popetes, the electc feld computato was doe. The esults show that the electc feld values ae stogly modfed due to the developmet of wate tees. Keywods: polyethylee, cables sulato, wate tees, oc space chage, electc feld computato. 1. INTRODUCTION Dug sevce, the sulato of powe cables, ae submtted to electcal, themal, mechacal, evometal stesses etc. that cotbute to the tato ad developmet of ceta degadato pocesses. Ude the fluece of the electcal feld ad the pesece of wate, polymec sulatos wate tees develop [1-2]. Wate tees ae wate flled mco-cavtes lked by vey th chaels (of mcos ode). They appea egos wth tese electcal felds, lke the teface sulato/coducto (veted tees) o the vcty of cavtes ad mputes (bow te tees) ad stat to develop fom the aeas whee the electcal feld s moe tese towads the aeas whee the electcal feld s less tese [2]. Studes made o low desty polyethylee as well as o coss-lked polyethylee hghlght the exstece of postve ad egatve os, both sde ad outsde the wate tees, thus costtutg the oc space chage [3-5]. The space chage plays a mpotat ole the degadato pocess by locally tesfcato the electcal feld, facltatg the gowth of wate tees ad causg the appeaace of electcal tees, the pelmay phase sulato beakdow [6-7]. Reseaches made so fa show that the pesece of wate tees lead to the woseg of the sulatos electcal popetes whee t develop: the decease of the cepto voltage of patal dschages [7], the cease of the wate cotet [8], the electcal feld tesfcato the vcty of teed aeas [6], a decease of the beakdow voltage [7] etc. O the othe had, t was show that, eve afte the voltage swtchg off (V = 0), the esdual electcal feld takes mpotat values [6]. The esdual electcal feld poduced by the oc space chage ovelaps the electcal feld ceated by the electodes whose value may lead to the sulatos pematue beakdow by cepto of electcal tees at the tp of wate tees, at lowe values tha the omal voltage [9]. I [9] a sees of aspects egadg the fluece of wate tees o the epatto ad values of the electcal feld, as well as the beakdow voltage, usg the Taguch method [10] o the themal step method [11] whch the wate cocetato the teed aea s assumed to be kow ae peseted. ISSN:

2 60 Wate tees fluece o the powe cables. Csta Stacu et al. a) b) Fg. 1. a) Schematc epesetato of samples; b) Hgh voltage powe cable stuctue. I the peset pape the esults of a expemetal ad theoetcal study egadg the fluece of wate tees o the epatto ad values of the electcal feld, by takg to accout the wate cocetato the teed aea ae peseted. 2. EXPERIMENTS The expemets wee caed out o flat ccula cow shaped coss lked polyethylee samples, wth the e damete D = 23 mm, the oute damete D o = 57 mm ad thckess g = 0.5 mm (Fg. 1.a). The samples wee slced fom the sulato of a 110 kv hgh voltage powe cable (Fg. 1.b). The samples, whose e ad oute sem coductg layes wee emoved, wee aged ude the combed acto of the electcal feld ad wate, wth the am to develop wate tees. Afte ageg, the themal coductvty, specfc heat, wate cocetato, wate tees dmesos ad desty wee detemed WATER TREES GROWTH Wate tees wee developed fo τ = h, electc felds wth the stegth E = 4 kv/mm ad fequecy f = 5 khz, usg the setup peseted Fg. 2 [6] SPECIFIC HEAT AND THERMAL CONDUCTIVITY Detemato of specfc heat was doe wth a DSC type 131 Evo maufactued by Setaam (Face), at T = 30 C wth a gowth ate of tempeatue β = 4 ºC/m (Fg. 3). Fo specfc heat c(t) ad themal coductvty computato λ wee used the elatos: c T HFs HFe, dt S T ms dt α γ c 1 λ (1) Fg. 2. Setup used fo wate tees developmet: 1 Sgal geeato, 2 Amplfe, 3 Tasfome, 4 Sample holde, 5 Pobe, 6 HP Scope.

3 Wate tees fluece o the powe cables. Csta Stacu et al. 61 Fg. 3. Vaato of tempeatue tme fo detemato of heat capacty whee HF s epesets the heat flow sde the sample, HF e the heat flow whe the cucble (measuemet cell) s empty, m s sample mass measued mg, S(T) sesblty ad dt/dt vaato of tempeatue tme, whee α, γ ad c epeset the themal dffusvty, the desty, the specfc heat of sample ad the umbe of samples [12] WATER CONTENT OF TREED AREAS Fo wate cotet detemato fom teed aeas (c a ), the tal mass m, volume V ad desty γ = m /V of each sample ad the aveage values m, V ad γ wee detemed: m V m 1 ; V 1 ; γ 1. (2) Afte the wate tees developmet, teed aeas wee cut ad the ew values of mass ad desty of each sample was detemed, usg the elatos: γ a a 1 1 m a m a ; γ (3) whee m a = γ a V epesets the teed sample mass befoe cuttg ad γ a desty of teed sample. The volume occuped by wate tees a sample was computed usg the elato: πd 2 m Va Lmaxcwtm Aes, (4) 4 whee D m ad c wtm epesets the aveage damete ad cocetato of tees, L max the legth of geatest tee ad A es the sample aea. Supposg that the sample volume s ot modfed by wate tees fomato, the pecetage cocetato was detemed usg the elato: γa γ c a 100 (5) γ a 2.4. DIMENSIONS AND DENSITY OF TREES Wate tees dmesos wee caed out usg the setup peseted Fg. 4, fo each value of ageg tme τ moe slces fom tested sample wee cut ad the total umbe of tees N w, legth L k ad damete D k of each sample wee detemed. ISSN:

4 62 Wate tees fluece o the powe cables. Csta Stacu et al. Fg. 4. Measuemet of wate tees dmesos: 1 - Sample; 2 - Vdeo Camea; 3 - PC; 4 - Moto. The aveage damete D m, legth L m ad tees desty c wtm wee computed usg the elatos: N w N 1 1 w N w Lm Lk ; Dm Dk ; cwtm, (6) N N 2π w k 1 whee epesets the umbe of take slces. 3. ELECTRIC FIELD COMPUTATION w k INSULATION AT NOMINAL VOLTAGE U N Let us cosde the sulato of a cable havg the legth geate tha the damete whee a cotuous wate tee of legth l a was developed cotag a space chage laye of thckess l s (l s l a) the vcty of the e semcoducto (Fg. 1.b). The computato doma D (Fg. 5) s dvded thee sub-domas: D 1 (of vaable pemttvty ε 1 () = ce -d ), bouded by cyldcal sufaces S 1 (of adus 1 ) ad S 2 (of adus 1 + l a ), D 2 (of pemttvty ε 2 ()= 2.2 ε 0 ), bouded by sufaces S 2 ad S 3 (of adus 1 +l s ) ad D 3 (of vaable pemttvty ε 3 () = 2.2 ε 0 ), bouded by cyldcal sufaces S 3 ad S 4 (of adus 2 ). Sub-domas D 1 ad D 2 cota a chage laye of vaable desty ρ v () = ρ v0 (a 2 + b + e), the expessos of ρ v0, a, b ad e wee peseted [6]. Fg. 5. Electc feld computato doma fo cable sulato. Fg. 6. Computato doma fo esdual electc feld.

5 Wate tees fluece o the powe cables. Csta Stacu et al. 63 Fo computatos the followg values wee cosdeed: l a = 500 μm, l s = 1000 μm, ρ v0 = 0.1 C/m 3 ad V 1 = U =110 kv. The potetal V(P) ad the electc feld E(P) a pot P D 1,2,3 wee computed wth the equatos ΔV = -ρ v /ε ad E = - dv/d. The bouday codtos ae: V1 P V2 P, P S 2 dv1 P dv2 P, P S 2 (7) d d V2 P V3 P, P S 3 dv2 P dv3 P, P S 3, d d whee V 1 (P), V 2 (P) ad V 3 (P) epesets the potetal values pots P S 2,3 [6] RESIDUAL ELECTRIC FIELD COMPUTATION (V = 0) Resdual electc feld s computed usg the expemetal themal step cuet measued by Themal Step Method [11]. It s cosdeed that cyldcal sulato of a cable (delmted by cyldcal sufaces of ad ad e ad geeato G >> e ) wee developed fom e suface cotuous tees (ude the shape of cyldcal shell of thckess l a ) ad a oc space chage laye wth the same shape ad thckess l s > l a. As the electc feld has a adal symmety, fo the computato t was cosdeed a flat doma D, costtuted fom 4 sub-domas D 1 4 delmted by the sufaces S 1 S 5 (Fg. 6). - sub-doma D 1 (delmted by sufaces S 1 ad S 2 ) - coespodg to the wate teed ego of legth l a (ε (, T) = (1+α (T - T 0 ))ce -d ) (T 0 - the tempeatue of the sulato befoe applyg the themal step ad α ε pemttvty vaato facto wth the tempeatue). Ths ego s supposed to cota a space chage laye of thckess l a ad of desty ρ v () = ρ v0 (a 2 +b+e); - sub-doma D 2 (delmted by sufaces S 2 ad S 3 ) coespodg to the ego placed fot of wate tees ad whch cotas a space chage laye of thckess l s l a ad of desty ρ v () = ρ v0 (a 2 +b+e); - sub-doma D 3 (delmted by sufaces S 3 ad S 4 ) coespodg to the ego wthout wate tees ad wthout space chage (ε (T) = ε PE (1+α (T - T 0 ))); - sub-doma D 4 (delmted by sufaces S 4 ad S 5 ) - coespodg to the ego themal dffuse/sample of thckess y 0, whee the electc feld s ull. The computato of the electc feld ad the tempeatue s doe wth the Comsol Multphyscs softwae, cosdeg that y 0, l a, l s, a, b, e, εpe, εw, c, c 1 d, d 1 ad g as kow ad the Posso ΔV = -ρ v /ε ad Foue ( γc T / t dvλgadt ) equatos, whee ρ v epesets the oc space desty, ε absolute pemttvty, γ desty, c specfc heat ad λ themal coductvty of mateal. The mposed bouday codtos ae: - V() = 0 o S 1, S 4 ad S 5 - D1 D2 0 o S 2 ad S 3 - T(, t) = T 1 o S 1 ad T(, t) = T 5 o S 5. whee epesets the omal ut vecto, D s the electc dsplacemet, T 1 ad T 5 the tempeatues values o the semcoducto layes sufaces (Fg. 5). The mateal costats c ad d fom the expesso of pemttvty coespodg to D 1 (ε () = ce -d ) wee detemed wth the expessos: ε l a ε ; ; la PE ε d ε mla εm caεm 1 ca ε PE, (8) ISSN:

6 64 Wate tees fluece o the powe cables. Csta Stacu et al. whee c a s the aveage wate cocetato the wate tees, ε m ad ε PE - the elatve pemttvty of wate ad polyethylee ad ε m - the aveage pemttvty teed ego (subdoma D 1 ). The values of ε wee deduced o the bass of the pemttvty vaato cuves fo wate ad polyethylee at tempeatues betwee T 5 = -5 C ad T 1 = 25 C [13]. It was obtaed εpe = K -1 fo polyethylee ad εw = K -1 fo wate. Because the elatve pemttvty deceases expoetally wth fom S 1 ad S 2, a vaato law smla to the pemttvty was cosdeed fo ε : - d1 α c, (9) ε 1e the values of c 1 ad d 1 have bee obtaed fom the expessos: α ε l a α ; ; l a εpe αε d αε αε m caαε w 1 ca αεpe 4. RESULTS AND DISCUSSIONS 4.1. WATER TREES DEVELOPMENT ml a. (10) I Fg. 7 ad Table 1 dffeet developmet tme of wate tees fo E = 4 kv/mm ad f = 5 khz ae peseted. It ca be see that, wth the ageg tme cease τ, the dmesos ad desty of tees cease. Ths s due to the os peetato esulted by salt dssocato fom wate (ude the electc feld acto), o oe had, ad cease of wate cotet wo by the os the dsplacemet, o the othe had [14]. Fg. 7. Wate tees XLPE samples fo τ = 48 h (a), τ = 72 h (b) ad τ = 96 h (c) (E = = 4 kv/mm, f = 5 khz). Table 1 Dmesos ad desty of wate tees Ageg tme [h] L m [μm] D m [μm] c wtm [wt/mm 2 ] Table 2 Dffusvty, specfc heat ad themal coductvty values Sample τ [h] α [m 2 /s] c λ L max c a [%] [J/kgK] [W/mK] [µm] U-teed Teed

7 Wate tees fluece o the powe cables. Csta Stacu et al THERMAL DIFFUSIVITY, SPECIFIC HEAT AND THERMAL CONDUCTIVITY The values of themal dffusvty α, specfc heat c ad themal coductvty λ ae peseted Table 2. Values of α, c ad λ ae close to the values obtaed by othe authos [15-16] ad ae geate o the teed samples elatve to the u-teed oes. Ths s explaed by the fact that, wth the cease of ageg tme τ the wate cotet fom teed aeas ceases ad λ ad c values each to the values fo wate oes [17] WATER CONTENT Values obtaed o teed samples afte τ = 30 h ae peseted Table 2. Fo a ease computato, was supposed that u-teed sample do ot cota wate, although t was show [7] the wate exstece a small cocetato (about 0.01 %), appeaed dug fabcato, due to wate coolg. The obtaed values fo τ = 30 h ae elatve small, but cease wth wate tees developmet ELECTRIC FIELD COMPUTATION Electc feld computato was doe the followg codtos: l a = 500 μm, l s = 1000 μm, ρ v0 = 0.1 C/m 3 (Fg. 8) ad the esults ae peseted fgue 8. It ca be see that wate tees deteme a mpotat vaato of the electc feld fot of them ( coodate pots 1 + l a, cuve 2, Fg. 8). Electc feld value these pots s 5.3 MV/m, 43 % geate tha the absece of wate tees. Space chage ceases at ts tu the electc feld values (cuve 3, Fg. 8). Also, eve afte the voltage swtchg off, the electc feld does t dop to zeo, beg 1.3 MV/m. Ths value of the electc feld (close to the ecessay oe fo wate tees cepto (3 MV/m)) makes ease degadato pocesses cotuato, eve the voltage absece, by ew wate tees appeaace ad electcal tees cepto fot of them, whch lead to pematue beakdow of cable sulato. Fgue 8. Vaato of electc feld E wth the coodate the absece of wate tees (1), pesece of a wate tee (2), wate tee ad space chage (V = 110 kv) (3) ad wate tee ad space chage (V = 0) (4) (l a = 500 µm, l s = 1000 µm, ρ v0 = C/m 3 ). 5. CONCLUSIONS The cease of the samples ageg tme the pesece of wate leads to a cease of wate tees dmesos (L m, D m ) ad aveage cocetatos (c wtm ). Wate tees developmet detemes a cease of the themal popetes values of the teed aeas. The pesece of wate tees detemes a chage of the electc feld epatto, moe mpotat whe dmesos, cocetato ad wate cotet cease. ISSN:

8 66 Wate tees fluece o the powe cables. Csta Stacu et al. The esdual electc feld values (fo U = 0) ae stll hghe ad make ease the electcal ageg pocesses (cepto of electcal tees) ad pematue beakdow of cable sulatos. REFERENCES [1] Fake, E.A., IEEE Tasactos o Electcal Isulato, EI-12, 218, [2] Stees, E.F., Keuge, F.H., IEEE Tasactos o Electcal Isulato, 25, 989, [3] Stacu, C., Notghe, P.V., Cupa, F., Notghe j, P., Agel, S., Castello, J., Touelle, A., IEEE Tas. Id. Appl., 45, 30, [4] Ross, R., IEEE Tasactos o Delectcs ad Electcal Isulato, 5, 660, [5] Vsata, O., Ifluece des aboesceces d eau su les popétés délectques des polymèes, PhD Thess, UPB-UJF, Buchaest, [6] Stacu, C., Caactésato de l état de vellssemet des solatos polymèes pa la mesue d aboesceces et de chages d espace, PhD Thess, UPB-UM2, [7] Radu, I., Behavo of some sulatg mateals hgh electc felds, PhD Thess, UPB, [8] Meye, C.T., IEEE Tasactos o Electcal Isulato, 18, 28, [9] Boggs, S., Desley, J., Jbo, K., IEEE Tasactos o Powe Delvey, 13, 310, [10] Km, C. et all, Euopea Tasactos o Electcal Powe, publshed ole at [11] Castello, J., Maleu, S., Touelle, A., Wate Teeg Detecto 15 kv Cables usg the Themal Step Method, Poceedgs of Iteatoal Cofeece o Isulated Powe Cables, 845, [12] Setaam Scetfc& Idustal Equpmet, Détemato de Cp, [13] Alg, I., Dudk, S., Jege, W., Mazatowcz, M., Pecolato pctue, Polyme, 47, 1722, [14] Aucklad, D., Coope, R., IEEE Delectc Mateals, Measuemets ad Applcatos, 122, 860, [15] Wudelch, B., Themal Aalyss of Polymec Mateals, Spge, [16] Wübbehost, M., IEEE Tasactos o Delectcs ad Electcal Isulato, 5, [17] Mat, L.H., Lag, K.C., The Themal Coductvty of Wate, Poceedgs of the Physcal Socety, 45, 523, Mauscpt eceved: Accepted pape: