M PHNG QU IN P KHI QUYN TRONG H THNG IN

Phm Ngc Cng ty TNHH MTV Th nghim in min Trung

Dch t bi bo Simulation of Lightning Overvoltages in Electrical Power Systems ca H. Schmitt & W. Winter.Tm tt: St nh vo ng dy ti in trn khng l nguyn nhn gy ra qu trnh lan truyn sng dc theo ng dy v truyn vo trm v y chng gy ra qu in p m c th gy ra mi nguy him cho bt c thit b no kt ni vo trm, nh l cp lc hoc cc my bin p. Vi vic s dng cc chng st van oxit kim loi, nhng qu in p ny s c gim xung ti mt bin an ton, l di mc cch in ca cc thit b m n t vo. Bi bo ny m t cc mi quan h c bn cho vic tnh ton cc qu in p khi quyn gy ra, cc m hnh dng cho vic chn on, kho st v a ra mt v d minh ha t vic m phng trc quan bng vic s dng phn mm NETOMAC ca hng SIEMENS [1].

I. Cc loi st nh:

Trong vic tnh ton qu in p m xy ra sau khi st nh vo ng dy truyn ti, cn mt s phn bit c thit lp t 3 loi ny. o la:A. St nh t xa:

St nh t xa khng xy ra ngay lp tc ti vng ln cn ca thit b ng ct. St nh t xa gy ra mt sng lan truyn m c m phng bi 1 in p xung st vi thi gian u song l 1,2 s v thi gian chiu dai song l 50 s. Cc sng lan truyn ti thit b ng ct, v n c cho rng gi tr bin thp hn in p phng in cua cch in ca ng dy trn khng. V th, st nh t xa khng dn n bt c mt phng in no cch in ng dy ngay lp tc gn ni thit b ng ct. Bin ca cc st ny (90% gi tr in p chu ng) c tnh ton t 50% gi tr in p phng in ca cch in, ly gi tr lch chun = 0,03 i vi cc in p xung st. [2]

U90 = U50 *(1+1,3.) (1)

V d m phng ca vic st nh t xa vi bin l 1600kV i vi h thng 230kV c minh ha trong hnh 1a.

B. nh vo cc ct:

Khi st nh trc tip vo ct lm gia tng in th ca ct v ph thuc vo mc in tr bc v mc cch in ca ng dy trn khng, dn n phng in ngc li t ct ti ng dy. Nhng phng in ngc ny l do cc sng lan truyn dc qua cc dy dn ti thit b ng ct. M phng ca st nh ny c dng sng lm trong khi tng v c m t trong [3] vi thi gian ti gi tr na xp x 100 s. i vi bin ca dng st, gi tr ln nht l 200kA khi st nh vo mc t s c cho l trng hp t nht [3]. Hnh 1b m t dng st vi gi tr l 200kA.

C. Xung quanh khu vc st nh vo cc vt dn ca ng dy trn khng:Bin dng ca st m nh vo vt dn trn ng dy b nh hng ln ca hnh dng ca ct v nh hng ca dy chng st trn khng. Cc ct cao cng vi khong cch vt dn rng v gc bo v t dy chng st trn khng s dn ti cc dng st nh trc tip ln dy dn s cao hn. Do , bin ca dng in st cng c tnh nh l mt hm ca hnh dng ct i vi cc ng dy trn khng. Thng thng, dng in st dao ng trong khong t 10kA n 60kA. Tng t nh st nh vo ct, dng sng lm cng xy ra trong khi tng, vi thi gian ti gi tr na xp x l 100 s. Mt v d ca dng sng c cng dng l 30kA c m t nh hnh 1c.

II. Dng in st ln nht trong trng hp st nh trc tip vo dy dn:

Bin ln nht khi st nh trc tip vo dy dn c nh hng rt ln n hnh dng ca ct. Cng thc tnh ton dng in ny c da vo m phng hnh hc trong hnh 2 [3,4].

Ph thuc vo thng s ca ct, nh l chiu cao tnh t mt t ca dy dn v dy chng st, v vng ca dy dn so vi dy chng st, mt th parabole c v nhm thay th cho cc cung cong ca cc khong cch bng nhau t dy dn v t t. V th, khi st nh vo bn phi parabole ny th s nh vo t, v st nh vo bn tri ca parabole th s nh vo ct. xc nh s phn chia khu vc m lin quan n st nh trc tip vo dy dn trn khng, ng trung trc ca ng thng ni dy chng st (E) v ng dy trn khng (L) c v. Ch khu vc c gch cho c to ra bi ng trung trc v parabole c th c xem nh l khu vc st c th nh trc tip vo dy dn. Dng in st ln nht c th I trong trng hp st nh trc tip vo dy dn c th c tnh ton theo quan h sau:

hE = 8*I0,65 (2)

Trong hE c xc nh bi chiu di ca phn ng dy trn khng (L) v giao im (i) ca ng trung trc vi parabole.

III. M hnh phng in ca cch in:

c tnh phng in ca cch in c m t vi in p phng in 50%. Hnh 3 m t c tnh c bn ca in p phng in 50% nh l mt hm ca thi gian. Theo , in p cao m trong khong thi gian ngn hoc in p thp m trong khong thi gian di dn n phng in i tng th nghim mt na ca xung bao gm c vi in p ny. i vi vic m phng c trng phng in ca cch in, ch tiu thit k Kind [5] c p dng cho vic tnh ton vi NETOMAC. Khi in p hin ti cch in vt qua gi tr in p tham kho, c suy ra t in p phng in 50% ca cch in, vic tnh ton ca khu vc thi gian-in p c bt u:

U = U + (3)Khi gi tr gii hn xc nh in p-thi gian ca khu vc F b vt qua, th s phng in trn cch in xy ra.

IV. M hnh sng lan truyn ca thit b:

Trong tnh ton qu in p do st gy ra, cn thit phi cn c s tnh ton thi gian lan truyn ca cc qu in p ny, s phn x li im cui khi h mch, v s khc x trn s chuyn tip ti cc tng tr sng xung ca cc bin khc nhau. V th, m phng ca qu in p do st yu cu phi m phng cc b phn ca cc thit b in, nh l cc ng dy truyn ti trn khng, cc dy chng st, cc ct v thit b ng ct cng nh cc sng lan truyn trn vt dn. Trong trng hp ca m hnh sng lan truyn, cc b phn ca thit b in khng c m phng nh l cc in khng tp trung, nhng c m t bng cc in khng xung st cua chng Z, tc lan truyn v v chiu di l. Sng truyn trong mi trng khng kh, cch in bng kh vn tc bng vn tc nh sng c (v=300m/s). Trong trng hp ca cc phn vt dn vi cch in l ra ngoi nh l cp th tc lan truyn thp hn (XLPE, vn tc khong 200 s, cp c giy ngm du cch in khong 160 s), ph thuc vo hng s quan h in mi R

v= (4)

in khng xung st m t ch tn s cao ca cc b phn ca thit b v c tnh ton vi quan h sau:

Z= 1/v.C (5)Trong : Z= v = C: l in dung trn 1km.

L: in cm trn 1km.

in khng xung st ph thuc vo mc in dung ca pha so vi t, m ch yu b nh ca cao so vi mt t. i vi cc ng dy trn khng v dy chng st, in khng xung st tng hp c tnh ton theo quan h nh sau:

Z = 60*ln (6).

y h c m t l chiu cao treo dy so vi mt t, v r l bn knh ca vt dn. n gin trong m phng cn c kh y cc yu t chnh xc trong cc trng hp kho st sut in p st trn thit b, cc sng truyn trn vt dn c th c m phng trong NETOMAC vi vic s dng m hnh Marti [6]. y, gim nh hng ca cc cp pha vi nhau gia cc pha trong 1 h thng v gim vic ph thuc tn s ln ng sut st cng c tham gia tnh ton. in khng xung st ca cc thit b c th c tnh ton theo cng thc (5). Trong vic tnh ton qu in p st, cc yu t ca thit b, nh l cc my bin dng, my bin in p, my ct...khng m t mt cch ring l. Khi c s thay i nh ca cc c th ny c nhn ra trong c tnh khc x ca s lan truyn sng ti, chng cng ch c nh hng nh trong kt qu tnh ton. Tng phn vi hin tng qu rt nhanh, nh l vic ng ct cc thit b, c cho l khi tnh ton cc qu in p ny s dng mt in khng xung st trung bnh cho cc b phn chc chn, nh l cc thanh ci hoc cc xut tuyn. Vic tnh ton cc in khng xung cho cc ct l c suy ra t quan h trong cng thc (6), ni m bn knh trung bnh trng lng rav trn chiu cao nguyn ca ct h c mang vo tnh ton theo [7].

Zav = 60. (7).

V. o c sa cha i vi qu in p khi quyn:

Bin dng st khi nh trc tip vo vt dn ng dy trn khng b nh hng ln ca hnh dng ct v nh hng ca ng dy chng st. V th, c mt qui lut chung nht l cc ct cao vi khong cch dy dn rng v gc bo v ca dy chng st ln dn ti dng st nh trc tip vo vo dy dn trn khng cng cao. c bit, trong khu vc nhy vi ging bo, th c th la chn phng n thit k ct sao cho bin dng in st nh vo ct l cao nht c th gim ng k vic st nh trc tip vo dy dn trn khng ( v d nh thit k cc dy chng st gn nh thng ng vi dy dn trn khng).

St nh vo ct dn n vic tng in th ca ct, m iu ny c xc nh bi in tr bc ca ct. Nu in th ny vt qu mc chu ng ca cch in, th s c phng in ngc li dc theo cch in, c bit khi chng xy ra gn khu vc thit b ng ct, c th gy ra nguyn nhn lm qu in p tng cao v qu in p vi tc thay i ln. y, vic lin kt cc ct cui cng ti h thng ni t nh l mt phng php hiu qu trong vic gim ng k in tr bc ca ct v ngn cn s phng in ngc dc theo cch in ca cc ct ny. Nhng v l do kinh t nn iu ny c hn ch v ch thc hin i vi cc ct chnh v u tin t pha trm m thi. Vic s dng sng (hay l m) phng in trn cc cch in cng c nh hng n c tnh phng in. Nu c vi h thng cng nm chung trn mt ct th cch in ca mt trong cc h thng ny nn c trang b sng phng in m vic phng in s d xy ra hn trn cc h thng ny iu ny ph thuc vo khe h ca sng khi lp t. Trong khu vc c xc sut ging bo cao, vic o c cn c thc hin, k t khi tr c ra i, vic phng in ngc s c hn ch ti mt h thng ca ng dy truyn ti in v tng s c ca h thng s c giam ang k.

Thm vo mt s kt cu khc cp trc y, vic s dng chng st van ng dy ci thin cht lng st trn ng dy hoc trnh s mt in ca mch kp ang gia tng trong nhng nm gn y. S lng v v tr lp t ca cc chng st van trn cc ng dy n ph thuc vo nhiu yu t nh l: mi trng yu cu, hot ng st trong khu vc lp t, cu trc ng dy, in tr bc ca ct, c tnh ca chng st.

Bn cnh cc tnh nng c bit ny, vic lp t chng st van trong trm cn l mt phng php hu hiu trong vic gii hn qu in p qu , v d nh vic qu in p khi ng ct cc thit b hoc cc qu in p do st [8].

Ngy nay, cc chng st van oxit kim loi c s dng nhiu hn ch qu in p xy ra bi cc tc dng ca cc c tnh phi tuyn (c tnh in p d). Cc tiu ch kch thc cho vic la chn chng st, thm vo l in p vn hnh ln nht ca h thng Um, cch thc ni im trung tnh ca h thng v h s s c t tng hp. Da trn cc c s d liu ca h thng, chng ta c th la chn chng st bi in p vn hnh lin tc ca chng Uc, in p nh mc Ur v c tnh in p d. c nhng tnh cht bo v ti u ca chng st, iu ny l tht s cn thit c bit trong s tnh ton cng st, lp t chng thng bo v cc thit b nh l cc MBA hoc cc khng in. bo v cc thit b ng ct t cc qu in p truyn vo, chng st c lp t pha ti hoc i ca trn cc xut tuyn ca ng dy trn khng. S chuyn tip t ng dy qua cc cp lc cng c bo v bi chng st van ngn cn cc qu in p khng cho php bi v cc sng phn x t im chuyn tip t cp ( in khng xung thp) n cc ng dy trn khng (in khng xung cao).

Nu vn hnh ng dy ch khng ti hoc vn hnh hn hp ng dy trn khng / xut tuyn cp (u cui h mch) trong mt h thng trong khong thi gian ko di, vic lp t chng st cui im lin kt cng cn thit hn ch cc qu in p bi phn x ca sng lan truyn cui ng dy h mch.

VI. Tnh ton cc qu in p do st s dng phn mm NETOMAC:

Theo cc kt qu phn tch pha trn, phn ny s m t cc kt qu ca mt tnh ton qu in p vi chng trnh m phng trc quan NETOMAC, s dng v d ca trm GIS 230kV (GIS Trm c cch in bng kh).

Giao din trm v ng dy trn khng c minh ha bng NETCAD. NETCAD l mt cng c mnh, nhanh trong vic phc ha, ko th, hiu chnh v thit k cc h thng in v cc cu trc iu khin. Thm vo , NETCAD c mt th vin m hnh phong ph cho cc chc nng c bit trong vic tnh ton ca qu in p do st.

Thm vo k nng bit nhiu cc chc nng ca CAD nh l ct dn, ko th, xoay, thu phng ..., h thng cng cung cp mt th vin ln cc k hiu bao gm cc yu t dnh cho chng trinh NETOMAC. Ngi dng cn chun b cc s mng v cc khi s thng qua cc th vin ha ca n. Hnh 4 cho thy mt s c to ra t cc khi ca th vin phn mm.

Thng s c nhp vo t cc nhn m mi i tng c m phng v cc ch thch km theo mt cch ngn gn. Cng c nhng cng c c trng cho vic kt ni cc nhm quan h li vi nhau to ra mt sn phm mi, t cc biu tng ban u trong cc dng m hnh chng ta c th thm chng vo trong cc th vin chng trnh hoc trong cc th vin ngi dng. V d, kt ni cc cu trc iu khin phc tp hoc cc mng con, bao gm mt s lng ln cc phn khc nhau ca thit b, vo trong cc biu tng mi. Nh l mt kt qu ca tnh bo mt trong tha k, ph thuc vo cc yu cu v vic s dng d liu ging nhau, h thng cho php mt s la chn to thnh ty vo mc phc tp nh th no hoc tnh rnh mch ca mt h thng c thay th.

Cc b phn ring l c th c kch hot v kh kch hot v lin kt ti bt c im no trong h thng. V vy n c th ch r v m t cc cu trc khc nhau ca ct cng nh cc m hnh sng lan truyn bt c ni no trong h thng.

Th vin NETCAD dng cho vic tnh ton cc qu in p do st cha vi m hnh ca cc loi ct, dn ct, chng st, MBA, ng dy trn khng, cp lc hoc cc b phn ca trm khc nhau.

i vi vic m phng cc dng in st hoc cc hnh dng sng in p xung, m hnh ca dng v ngun in p cng bao gm trong th vin m hnh. Hnh 5 ch cho thy v d vi m hnh ng dy trn khng trong th vin ny. Hnh 6 ch cho thy vi b phn ca mt thit b ng ct. Tt c cc thit b nh l thanh ci, MBA, my pht hoc cp lc u c m hnh ha cng nh cc m hnh sng truyn.

Hnh 7 m t cc biu tng ca ca ngun dng v p cho vic m phng cc dng in st v dng sng in p xung. Ph thuc vo loi hnh ca ngun (ngun p, ngun dng), v tr s c, cu trc ca mng, th c cc phng php tnh khc nhau. Th vin cng bao gm cc thit b o lng c bit. Hnh 7 m t dng, p v nng lng ca chng st van gy ra bi st nh.

Giao din ngi dung NETCAD l mt h thng m. Do c th xy dng cc m hnh mi trong cc dng ha m rng th vin tng bc mt. Vic s dng th vin ha lm cho chng ta d dng to c cc m hnh h thng chi tit v a ra cc vn d c ch khc nhau nh l:

M hnh t ng ca ngun dng v p cho vic m phng cc dng in st v hnh dng ca in p xung st.

M hnh cc cu trc khc nhau ca ct cng nh l m hnh sng lan truyn.

M phng cc phng in ca cch in.

Trong cc trng hp c bit (nh hng qua li ca 2 ng dy trong h thng 3 pha hoc vn gim ph thuc tn s, m hnh Marti c m rng trong NETOMAC.

Hnh 8 m t mt s thit b n gin ca h thng 230kV. M phng ct u tin trong h thng 3 ct ca ng dy trn khng v ni vo trm bng cp. Trong trm, bao gm cc khong i ra t MBA, cng c m phng theo c tnh sng lan truyn. Cc tuyn ra t MBA cng tham gia vo trong vic tnh ton dung lng ca st gia pha cao p vi t. bo v cc thit b, chng st van c lp t im lin kt gia ng dy trn khng v cp v pha u cc MBA bn 230kV.

Mc ch ca vic kho st l tnh ton cc qu in p, theo sau st nh vo ng dy trn khng, xy ra cc v tr khc nhau trong trm, cc v tr ni cp vo ng dy, pha cao ca cc u cc MBA. Kt qu dng la chn CSV thch hp cho vic bo v cc thit b t cc qu in p khng mong mun. Trong nhng tnh ton ny c 3 loi st nh khc nhau nh m t nh ban u (nh t xa, nh trn ct v nh trc tip vo ng dy) c m t, c tnh ton trong cc kt cu khng thun li ca trm. Cc ng sut cao nht xy ra khi trm ang vn hnh trong tnh trng trm ct, c ngha l 1 ng dy ch cung cp cho 1 MBA..

Hiu qu ca CSV c tnh ton, kim tra, so snh vi in p ln nht xy ra vi gi tr cch in cho php ca thit b (BIL), c tnh ton vi gi tr bin (S) an ton theo tiu chun c bit [2].

Tnh ton ng sut st i vi cc thit b ng ct, cp lc v MBA khi c st nh trc tip l 30kA ln ng dy trn khng khu vc ct 1 nh trong hnh 9. i vi trm, m hnh trm ct nh hnh 7 cng c tham gia vo trong tnh ton ny. Mt v d minh ha cho vic hot ng ca CSV, hnh 10 m t ng sut dng v p ln CSV ca MBA khi c st nh trc tip ln ng dy cng c m t.

Nh v d m t, mc cch in c bn BIL = 1050 kV c xc lp i vi cc thit b ng ct v cp, v BIL = 900kV vi u cc pha cao ca MBA. Kt qu ch ra rng, vi CSV thit k hin ti, ng sut in p c xc nh l BIL/S = 1050/1,15 = 913kV i vi thit b ng ct v cp lc v BIL/S = 900/1,15 = 783kV i vi cc u cc pha cao ca MBA c duy tr.

VII. Tng kt:

Vic tnh ton cc qu in p l cn thit cho vic m phng cc thit b in, nh l vic m phng cc ng dy trn khng, cp lc, ct, cc cc trm cng nh m hnh sng lan truyn. M phng st nh t xa lu li in p st tng ng vi in p xung tiu chun 1,2.50 s, trong khi dng st trong trng hp nh vo ct hoc trc tip vo ng dy trn khng c quyt nh bi dng hnh hc ca ct v nh hng ca ng dy chng st, v c quyt nh ring l cho mi trm.

c tnh phng in ca cch in c m t bi biu in p-thi gian, theo tiu ch Kind. Vic nh hng ln nhau gia cc pha n trong h thng 3 pha, cng nh l gim ph thuc tn s, c th c m phng trong cc trng hp c bit bng vic s dng m hnh Marti.

i vi tnh ton ca cc qu in p bng phn mm NETOMAC, cc modul chnh c s dng, bng nhng ng dng n gin, c th thc hin d dng vi cc trng hp khc nhau tng i rng, nh l trong vic thit kt trm, pht trin cc thit b mi, hoc trong h thng vn hnh ca b chuyn i nng lng (HDVC). Da vo cc kt qu ca vic tnh ton, v d, xc nh s lng v v tr ti u ca CSV trong vic thit k trm, hoc tnh ton cc ng sut in p ca thit b v xc nh cc thit b o thch hp ni vt qu gi tr gii hn.

VIII. Tham kho:

[1]: Kulicke, B.:Digitalprogramm NETOMAC zur Simulation elektromechanischer und magnetischer Ausgeichsvorgnge in Drehstromnetzen, Elektrizittswirtschaft, Heft 1 (1979), pp.18-23.

[2]: International Standard IEC 71-2: Insulation co-ordination Part 2: Application guide, 1996.

[3]: CIGRE WG 33.01: Guide to procedures for estimating the lightning performance of transmission lines, CIGRE Technical Brochure 63 (1991).

[4]: IEEE Working Group on lightning performance of transmission lines: A simplified method for estimating lightning performance of transmission lines, IEEE Trans PAS 104 (1985), pp.919-927.

[5]: CIGRE WG 33.02: Guidelines for representation of network elements when calculating transients, CIGRE Technical Brochure 39 (1990).

[6]: J.R. Marti: Accurate modelling of frequency- dependent transmission lines in electromagnetic transient simulations, IEEE Trans PAS 101 (1982), pp.147-155.

[7]: W. A. Chisholm, Y.L. Chow, K.D. Srivastava: Travel time of transmission towers, IEEE Trans PAS 104 (1985), pp.2922-2928.

[8]: J. Elovaara, K. Foreman, A. Schei, O. Vlcker: Temporary overvoltages and their stresses on metal oxide surge arresters, Electra 128 (1990), pp.114-125.