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Bi lm
Cu 1: Ni dung v mi lin h gia cc phn ca mn hc
Ni dung mn hc c chia lm 3 phn:
- H thng pht in cao p v dng in ln. Nghin cu nguyn l, cu to, u im, nhc im v mt s ng dng ca cc h thng pht in cao p mt chiu (mch bi p, mch bi p ni tng, my pht in tnh), h thng pht pht in cao p xoay chiu (my bin p th nghim n tng, my bin p th nghim ni tng, my bin p cng hng), h thng pht in xung (my pht xung n tng, my pht xung a tng, my pht xung dng).
- Cc thit b o lng in p cao v dng in ln. Nghin cu nguyn l, cu to, u im v nhc im ca mt s thit b o in p cao v dng in ln ca h thng mt chiu (o bng in tr cao v in k, vn met quay), h thng xoay chiu v in p xung (in k ni tip in dung, mch chubb, phn p in tr - nhnh cao p, nhnh h p, phn p in dung, my bin p in dung), o dng in ln (bng in tr shunt, dng hiu ng cm ng cun Rogowski, dng cc cm bin Hall), o in trng (s dng cc cm bin in quang, cm bin Pockels, cm bin Kerr).
- Cc phng php th nghim, gim st v chn on cch in. Nghin cu nguyn nhn, hin tng ca suy gim cch in, ng thi nu ra cc phng php trong vic o lng, th nghim, gim st v chn on cch in ca vt liu.
Nh vy ni dung gia cc phn c lin h vi nhau. Sau khi nghin cu nguyn l to ra in p cao v dng in ln c ng dng rng ri trong thc t, ta s c c s vng chc nghin cu, ch to ra cc thit b o lng cao p o c cc i lng in p cao, dng in ln c chnh xc cao v chi ph thp. T a ra cc phng php chn on, th nghim cch in ca vt liu cao p, gim st cch in ca vt liu phc v cho qu trnh sn xut, vn hnh t hiu qu nht.
Cu 2: Chn on suy gim cch in ca cao su propylene ethylene (EPR) bng cch s dng k thut pht x m thanh
1. Mc ch ca nghin cu: Tm ra mi lin h gia cch in ca cao su propylene ethylene (EPR), c s dng lm vt liu cch in cho cp (cch in XLPE) v phng in cc b (PD). T a ra phng php chn on s suy gim cch in bng k thut pht x m thanh.
2. Ni dung ca phng php:
- K thut pht x m thanh ch yu gm c cm bin p in c t trn b mt ca vt liu cn kim tra. Cc cm bin ny d dng pht hin cc sng n hi gy ra bi phng in cc b (PD). T vic phn tch cc tn hiu thu c t cm bin, ta s bit c s xut hin ca phng in cc b (PD).
- i vi cp in, phng in cc b thng xy ti nhng ch ni hoc u cc ca cp. t in p cao vo cp 22 kV cch in XLPE v gy phng in PD bng cch t mt in p 18kV ln cp. Mt in cc kim vi ng knh 2mm c a vo im ni ca cp 22kV, tip mt khong trng nh vi chiu di nh hn 1 mm c to ra. Tuy nhin, s kim sot kch thc khong trng rt kh. V vy, cc th nghim c thc hin trn cch in cao su propylene ethylene (EPR) dng tm bi v xu hng suy gim ca sng n hi ca EPR dng tm gn nh ging vi cch in XLPE ca cp 22kV thc t. Trn c s kt qu th nghim, chng ti a ra mt trnh t chn on bng cch s dng k thut AE.
- Hnh 1 ch ra cch thc ci t cho th nghim.
Hnh 1: Phng php ci t th nghim
Cc thit b chnh bao gm: Cch in EPR dng tm (kch thc 200mmx200mm) chiu dy 10mm, trng lng ring 1,19g/cm3, hng s lng cc 3,0 v in tr sut 5x1015mm, v c mt l c su 9,3mm, ng knh 1,2mm trn tm EPR. Mt in cc kim vi ng knh 6mm c t vo cho n su 9mm, tc l mt khong trng 0,3 mm c to ra trong tm EPR. Ngoi ra, mt in cc a trn vi ng knh 30 mm v dy 5 mm c b tr di cng ca tm EPR. Phng in PD trong khong trng c to ra bng cch t in p xoay chiu 13,3 kV vi mt tn s 60 Hz vo in cc kim.
Cm bin pht x m thanh (AE) c ng knh 10 mm v nhy tn s c phm vi rng t 20 kHz n 1000 kHz c b tr mt sau ca tm EPR v cch tm EPR 30mm. Cc tn hiu c pht hin bi cm bin AE c khuch i bi mt b khuch i u c gi tr 20 dB (ti a t c 40 dB) vi di tn s 2 kHz -1,2 MHz v b khuch i chnh 40dB (ti a t c 40dB) vi di tn s10 kHz - 2 MHz, ngha l khuch i tng t c l 60 dB. Cc tn hiu t h thng khuch i c gi n mt my hin sng k thut s vi mt tn s ly mu l 100 MHz hoc 500 kHz v sau ghi li bng my tnh.
- Trnh t thc hin th nghim c ch ra hnh 2 di y:
Hnh 2: Trnh t thc hin th nghim
3. Kt qu thu c:
- Khi tin hnh th nghim nh trn, s phn b in trng quanh in cc kim c ch ra hnh 3, gi tr in trng ln nht khong 8x107V/m.
Hnh 3: S phn b in trng quanh in cc kim
- Mi lin h gia in tch phng in cc b (Electrical charge) v ln cc tn hiu (Signal intensity) thu c t cc cm bin AE c ch hnh 4.
Hnh 4: Mi lin h gia in tch phng in v ln ca tn hiu AE
Ta thy rng khi in tch phng in cng ln th ln ca tn hiu thu c t cc cm bin cng r. Cn c vo mi quan h tuyn tnh, chng ti c th c tnh c mc suy gim cch in.
- Khi in p t vo thay i theo thi gian, cc tn hiu AE thu c khi tn s ly mu c t 500kHz c ch ra hnh 5.
Hnh 5: Dng sng in hnh ca cc tn hiu AE
T hnh v ta thy, cc tn hiu thu c t cc cm bin AE xut hin vo khong 8ms tng ng vi mt na chu k ca in p t vo, cc phng in cc b (PD) thng xy ra mnh na chu k dng v gim na chu k m.
- Hnh 6 ch ra dng sng in hnh ca cc tn hiu thu c ti thi thi gian t=30h (hnh 6a), v t=50h (hnh 6b)
Hnh 6: Dng sng in hnh ca tn hiu AE ti thi gian t=30h v t=50h
Ph tn s thu c bng php phn tch chui fourie c dng nh hnh 7.
Hnh 7: Ph tn s thu c khi phn tch fourier
Cc tn hiu ny thu c khi t tn s ly mu ca my oscilloscope 100 MHz. S lng xut hin ca phng in cc b trong mi giy ti thi gian t = 30h v t = 50h tng ng l 0,11 v 0,25. Cc sng n hi trong cch in EPR c gy ra bi cc sng b nn trong mt khong khng nht nh. Do rng xung ca phng in in cc b khong 20 ns, nn phm vi tn s ca sng nn trong khong khng c th vt qu 50 MHz. Mc d mt vi dng sng trong cch in EPR c c kt hp li v pht ra, tn s ca tn hiu c pht hin ph thuc vo loi vt liu, nhy ca cc cm bin AE v c im ca sng n hi. rng xung ca phng in cc b (PD) ph thuc vo s lng phng in v khng thay i ngay c khi mu cch in EPR b h hng. Cc sng nn gy ra bi phng in cc b giai on cui khi cch in EPR b ph hng l ln, cc thnh phn tn s xung quanh 90 kHz c xem nh l ch ln ca phng in cc b.
- S bin i theo thi gian ca in tch phng in v cc tn hiu thu c ca AE c ch ra hnh 8.
Hnh 8: Thay i theo thi gian ca in tch phng in v tn hiu AE
in tch phng in quan st c thng qua s kt hp ca cc xung dng in phng in cc b c pht hin. in tch v cc tn hiu thu c tng i ln trong khong thi gian t = 0 n 2h, cc in tch v tn hiu ny tng ln theo thi gian t. Tuy nhin cc in tch v tn hiu ny khng tng mt cch u u.
- S bin i theo thi gian ca tn hiu t khong thi gian t = 40 n 44h c ch ra hnh 9.
Hnh 8: Thay i theo thi gian ca in tch phng in v tn hiu AE trong khong thi gian t=40h n t=44h
Ta thy gi tr trung bnh ca cc tn hiu AE thu c l tng i ln v chng thay i mt cch lin tc. Cc in tch v tn hiu ny tng mt cch t t trong chu k tng gim lin tc ca n. iu ny ch ra rng khi thi gian chn on ngn c th dn n kt qu sai trong vic chn on s suy gim cch in.
S bin i theo thi gian ca cc thnh phn tn s ca cc tn hiu c ch ra hnh 10.
Hnh 10: Thay i theo thi gian ca cc thnh phn tn s ca tn hiu AE trong khong thi gian 50h
m bo s thay i theo thi gian ca cc thnh phn tn s c pht hin l rt c ch cho qu trnh phn bit cc tn hiu AE c gy ra do phng in (PD) hay do cc thnh phn n khc gy ra, trong th nghim ny t ln ca cc thnh phn tn s 80-120 kHz l rt ln trong sut qu trnh th nghim. Mc suy gim ca cc sng n hi tn s thp l nh. iu cng c th thy rng cc thnh phn tn s s rt ln thi gian t=25h. Trong th nghim chn cc thnh phn tn s 80-120kHz v m s lng cc tn hiu vi cng ln hn 0,005V.
S thay i s lng cc tn hiu thu c trong mt pht (hnh 10a) v trong 10 pht (hnh 10b)
Hnh 10: S thay i s lng cc tn hiu AE m c
Trong khong thi gian t 0h 2h khng c xem xt n bi v trong khong thi gian cc tn hiu AE thu c cn chu nh hng ca b mt bn trong khong trng. S lng cc tn hiu m c trong khong thi gian t 2h14h l rt ln. Sau khong thi gian l 15h, s lng cc tn hiu AE thu c tng ln theo thi gian.
Nh vy, kt qu thc nghim cho thy rng khng ch c cng ca tn hiu AE, m s lng cc tn hiu AE m c ca cc thnh phn tn s c trng l rt hu ch cho vic chn on cch in ca EPR.
Nguyn Vn Thng - CB110576Page 1