07 - quoc_le kim hung-r

TP CH KHOA HC V CNG NGH, I HC NNG - S 2(25).2008

NGHIN CU BN KNH CP IN HIU QU LI IN PHN PHI 22KV 3 PHA 3 DY V 3 PHA 4 DYSTUDY ON DIFFERENT EFFECTIVE ZONES OF 22KV THREE PHASE THREE WIRE AND THREE PHASE FOUR WIRE DISTRIBUTION NETWORKL KIM HNG i hc Nng V NH QUC Cng ty in lc 3TM TT Li in phn phi (LPP) 22kV Vit Nam hnh thnh v pht trin vi 2 dng cu trc khc nhau, l: LPP 22kV vi cu trc 3 pha 3 dy (c s dng nhiu khu vc pha Bc v min Trung) v vi cu trc 3 pha 4 dy (c s dng nhiu khu vc pha Nam). Do cu trc li khc nhau, nn vic s dng cc my bin p (MBA), bn knh cp in hiu qu, mt cc trm bin p ngun cng c nhiu im khc nhau cn phn tch. Bi bo ny nghin cu s khc nhau c bn gia LPP 22kV trung tnh trc tip ni t vi cu trc 3 pha 3 dy v 3 pha 4 dy v mt bn knh cp in hiu qu, thng qua vic xem xt nh hng ca dng in ngn mch chm t trong mng 3 pha 3 dy v mng 3 pha 4 dy. ABSTRACT The 22kV distribution network was formed and developed into two different forms: three phase three wire (mainly used in Northern and Central Vietnam) and three phase four wire (mainly used in Southern Vietnam). Due to different network configurations, different use of transformers especially one-phase transformers, different effective zones, the density of source substations are different and needs analyzing. To clarify this issue, this article studies the basic differences between 22kV neutrally earthed three phase three wire and three phase four wire distribution network regarding effective zones through considering earth faults effects on three phase three wire and three phase four wire networks.

1. t vn Do lch s li, LPP Vit Nam ni chung v min Trung ni ring hin vn cn tn ti nhiu cp in p (nh 6, 10, 15, 22, 35kV), cng vi ch lm vic im trung tnh ca LPP cng khc nhau, c th: LPP 6, 10, 15, 35kV lm vic ch im trung tnh ngun cch t; LPP 15, 22kV lm vic ch im trung tnh ngun ni t trc tip. LPP c nhiu cp in p, gy kh khn trong cng tc vn hnh nh vic kt ni h tr ngun in ln nhau khi s c, gy tn km trong cng tc d phng cc thit b in x l s c... thng nht LPP Vit Nam, B nng lng

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(nay l B Cng thng) ban hnh quyt nh s 1867/KHKT ngy 12/5/1994 v vic qui nh cc tiu chun k thut cp in p 22kV [1], trong nu r: + Trung tnh ca mng 22kV trc tip ni t (ni t hiu qu). + ng dy 22kV c th 3 pha 3 dy (s dng t lm dy trung tnh) hoc 3 pha 4 dy (dy trung tnh i ring hoc s dng chung dy trung tnh ca ng dy h th). + i vi cc li in xy dng mi phi thit k theo tiu chun cp in p 22kV, i vi li in c phi ci to dn sang cp in p 22kV. Tuy thng nht v cp in p nhng cu trc LPP 22kV vn cha c s thng nht v quan im. LPP 22kV khu vc pha Nam th s dng cu trc 3 pha 4 dy, LPP 22kV khu vc min Trung v pha Bc th li s dng cu trc 3 pha 3 dy. V vy, vic nghin cu vn bn knh cp in hiu qu ca tng cu trc LPP l cn thit. nghin cu vn ny ta cn xem xt th no l ni t hiu qu v nh hng ca kt cu li n bn knh cp in hiu qu. 2. H thng ni t hiu qu a- Theo tiu chun Vit Nam 4756-89 v vic ni t, ni khng cc thit b in nh ngha [4]: Mng in trung tnh ni t hiu qu l mng in c in p pha ln hn 1.000V v h s qu in p khi ngn mch mt pha khng ln hn 1,4. H s qu in p khi ngn mch chm t trong mng in 3 pha l t s gia in p pha khng b s c ti im ngn mch chm t v in p pha ti im trc khi c ngn mch. b- Theo ch dn ca VDE 0111/12.66 [3]: Mt h thng hay mt phn ca h thng in p cao c xem l ni t hiu qu, nu khi ngn mch mt pha chm t t s in p pha trn cc pha khng b s c vi in p lm vic cc i cho php khng vt qu 0,8; h thng nh vy c th lp t vi cch in gim nh. tho mn iu kin ny th t s gia tng tr th t khng (Z0) trn tng tr th t thun (Z1) vi 1-0 = 0 (Trng hp ngn mch mt pha tnh ton trn l thuyt vi in tr ni t ti ngun bng khng v ngn mch trc tip) phi c gii hn nh hn 5. Vi gi tr 1 - 0 15o hoc 30o xy ra trong thc th th Z0/Z1 khng c ln hn 3 (1-0 l gc lch pha Z1Z0). c- i vi cc h thng ln, b qua in tr tc dng lc theo tiu chun AIEE mc 32-1.05 v tiu chun ANSI C62.92 [2]: Mt h thng hay mt phn ca h thng in p cao c xem l ni t hiu qu khi tt c cc im trn h thng hay mt phn xc nh ca h thng c t l in khng th t khng (X0) i vi in khng th t thun ca n (X1) khng ln hn 3 v t s gia in tr th t khng (R0) i vi in khng th t thun (X1) khng ln hn 1 trong bt k iu kin vn hnh no vi bt k dung lng ngun pht nh A th no. B Nhn xt: C - Ba nh ngha v h thng ni t hiu qu trn, ZE tuy khc nhau v cch thc nh ngha nhng li ging N1 nhau v ni dung; c th l: khi c s c ngn mch mt Hnh 1. S ngn mch mt pha chm pha, in p cc pha khng b t mng trung tnh ni t hiu qu 45


s c tng khng c vt qu 1,4 ln in p pha trc s c ( 0,8 ln in p dy). - Ngoi ra, nh ngha b v c cho ta thy r hn nh hng ca kt cu li khi xem xt tnh hiu qu ca ni t h thng in thng qua t s Z0/Z1 v gc lch pha 1 0. xem xt nh hng ca kt cu li n tnh hiu qu ni t h thng, ta xt trng hp ngn mch mt pha trong li ni t (hnh 1): Khi xy ra ngn mch mt pha chm t ti im N1, th: UNC = 0; INA = 0; INB = 0 Lc , s dng phng php thnh phn th t ta c:

3 I 1 = I NC + a I NB + a 2 I NA = I NC ; 3 I 2 = I NC + a 2 I NB + a I NA = I NC 3 I 0 = I NC + I NB + I NA = I NC ; U C = U 1 + U 2 + U 0 = 0U 1 = E I 1 Z 1 ; U 2 = I 2 Z 2 ; U 0 = I 0 Z 0 T ta suy ra:

I1 = I 2 = I 0 =

E Z1+ Z 2 + Z 0

; I k 1 = I NC =

3 E Z1+ Z 2 + Z 0

U NA = E (a 2

a2 Z1+ a Z 2 + Z 0 Z1+ Z 2 + Z 0

) (1)

) (2) Z1+ Z 2 + Z 0 Vi: INA, INB, INC: Dng in ngn mch ca pha A, B, C; I1, I2, I0: Dng in th t thun, th t nghch, th t khng; Ik1: Dng ngn mch chu k ban u; UNA, UNB, UNC: in p ca pha A, B, C khi xy ra ngn mch; U1, U2, U0: in p th t thun, th t nghch, th t khng; E: Sc in ng siu qu ; a: ton t thnh phn th t. nh hng ca tng tr Z0 n in p cc pha khi b ngn mch c th hin bi th cc quan h (1) v (2) theo Z0/Z1 v 1 - 0 [3] (hnh 2 v hnh 3). Qua hnh 2 v 3 ta thy: + Vi Z0 = 0 th in p pha trn hai pha khng b s c bng 0,5. 3.E" . Khi Z0 tin ti v cng (li trung tnh cch t), tt c cc ng cong tim cn n gi tr 3.E" ngha l in p pha khng b s c chm t tr nn bng in p dy. + khi ngn mch mt pha chm t t s in p pha trn cc pha khng b s c vi in p lm vic cc i cho php khng vt qu 0,8 (tng ng vi h s qu in p 1,4) th yu cu Z0/Z1 5 ng vi 1- 0 = 0 v Z0/Z1 3 ng vi 1- 0 = 30o.

U NB = E (a

a Z1+ a2 Z 2 + Z 0

46


UA 3E 2.0 E

A B C

UB 3E 2.0 E

A B C

1.5

1- 0

1800 1500 1200 900 600 300 00

1.5 1500 1.0 00 0.5 600 900 0 1 2 3 4 5 6 300

1- 0

1800

1.0

1200

0.5

0

2

4

6

8

Hnh 2. Biu din in p UA theo in p pha v in p dy khi chm t mt pha

Z0 Z1

7

8

Hnh 3. Biu din in p UB theo in p pha v in p dy khi ngn mch mt pha

Z0 Z1

3. nh hng ca dy dn trung tnh trong mng ni t hiu qu thy r nh hng ca dy trung tnh trong mng ni t hiu qu ta xem xt tng tr th t khng i vi tng kt cu li [3]: a- i vi mng 3 pha 3 dy tng tr th t khng c tnh ton theo cng thc sau: + ng dy 3 pha 3 dy mch n: Z0 = RL+3RE+j10-4(6ln Trong : = 1650

3

rA 2

+0,5) (/km/dy dn)

(3) (4) (5)

+ ng dy 3 pha 3 dy i xng hai mch: Z00 =Z0 + ZM0 (/km/dy dn/h thng hai mch) Trong : ZM0 l tng tr tng h th t khng ca hai mch. ZM0 = 3RE+j10-4.6ln sau: + ng dy 3 pha 4 dy mch n:

(m) vRE= .10-4(/km) 2

AL

(/km)

b- i vi mng 3 pha 4 dy tng tr th t khng c tnh ton theo cng thc

Z 12E Z0E = Z0 - 3 (/km/dy dn) ZE

(6)

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y Z1E tng tr tng h gia dy dn v dy t: Z1E = RE+j10-4.2ln

AE

(/km)

(7)

v ZE tng tr th t khng ca dy t: ZE = RN+ RE+j10-4.(2ln

rE

+0,5) (/km)

(8) (9)

+ ng dy 3 pha 4 dy mch kp: Z00E = Z0E + ZMOE (/km/dy dn/h thng hai mch) Trong ZMOE l tng tr tng h ca hai mch bao gm c dy t. ZMOE = ZMO - 3

Z 12E ZE3

(10)

Trong cc cng thc trn: - A = 3 a12 a 23 a13 ; A = - A =3

a1'2' a 2'3' a1'3' ;3

a11' a 22 ' a 33' ;AE =

a1S a 2 S a 3S ;AL =

3

A' 2 A" ;

- 1, 2, 3 cc dy dn ca mch I; 1, 2, 3 cc dy dn ca mch II; a12, a1S: ln lt l khong cch gia hai dy dn (m), dy dn vi dy trung tnh (m); rE bn knh dy trung tnh; - RL, RE, RN: ln lt l in tr dy dn, t, dy trung tnh (/km); dn su trong t (m); in tr sut ca t (m); t thm tng i (1 i vi Cu v Al, 5090 i vi thp). c- i vi mng 3 pha 4 dy, mt phn dng th t khng i qua dy trung tnh (dy t), do dng chy qua t b gim i. H s gim tng ng bng: f = 1 - Z1E/ZE. (11) Tng t thnh phn dng th t khng chy qua t bng: 1 - f = Z1E/ZE. (12) Nhn xt: + Mng in ni t hiu qu th t s Z0/Z1 3, m Z0 ph thuc vo: kt cu t ( in tr sut ca t), kt cu mng in (3 pha 3 dy hay 3 pha 4 dy, mch n hay mch kp v kch thc dy dn trung tnh...). Cng vi mt kt cu mng in, nhng vng c in tr sut ln s c Z0 ln, t s Z0/Z1 cao, dn n gim tnh hiu qu ca vic ni t, tc l cng vi mt kt cu mng in c vng l ni t hiu qu, c vng ni t khng hiu qu. + i vi cu trc 3 pha 3 dy, khi ti bt i xng cng ln dng in tr v qua in tr ni t trm ngun cng ln, dn n mc di trung tnh ca MBA ngun 22kV cng ln v in p ging trn in tr ni t ca trm ngun cng ln nn tnh khng an ton i vi con ngi cng cao. + i vi cu trc 3 pha 4 dy, do c dy trung tnh kt hp vi cc c cu ni t lp li nn dng in tr v qua in tr ni t trm ngun gim i, dn n mc di trung tnh ca MBA ngun 22kV gim xung, ng thi in p ging trn in tr ni t trm ngun 22kV gim xung, nn tnh an ton i vi con ngi c nng cao.

48


4. Tng hp kt qu tnh ton ngn mch vii cc cu trc li in phn phi khc nhau thy r hn nh hng ca kt cu li n tnh hiu qu ca ni t h thng, ta xt v d tnh ton ngn mch ti mt mng 22kV n gin nh hnh v 1 vi cc gi thit: - Dy dn s dng dy AC-95. - B qua in tr ca dy dn. - in khng ngn mch th t thun, th t nghch v th t khng qui v thanh ci 22kV trm ngun: X1 = X2 = 0,00089, X0 = 0,00028. - Khong cch hnh hc gia cc pha 1,43 mt. - Khong cch dy pha - dy trung tnh: 1,219 mt. - Khong cch dy pha - t: 10 mt. - in tr sut ca t: 100 ohm.mt. - Cc dy pha c b tr trn mt phng nm ngang, dy trung tnh nm pha di cc dy pha. Kt qu tnh ton ngn mch c thc hin trn chng trnh tnh ton ngn mch PSS/ADEPT [5] nh sau: a- Cu trc LPP 3 pha 3 dy b- Cu trc LPP 3 pha 4 dy (dy trung tnh AC70)L (km) 0 5 10 15 20 25 30 35 40 45 50 In(1) (kA) 3,822 1,279 0,766 0,548 0,427 0,350 0,297 0,258 0,229 0,206 0,187 In (1,1) (kA) 3,728 1,792 1,309 1,026 0,841 0,712 0,617 0,544 0,487 0,440 0,402 In(2) (kA) 2,554 1,698 1,240 0,970 0,794 0,672 0,582 0,513 0,459 0,415 0,379 In(3) (kA) 2,949 1,960 1,432 1,120 0,917 0,776 0,672 0,593 0,530 0,480 0,438 R0/X1 X0/X1 0,000 0,519 0,820 1,028 1,187 1,318 1,430 1,530 1,619 1,702 1,779 0,315 2,651 3,860 4,591 5,074 5,410 5,653 5,833 5,966 6,066 6,139 L (km) 0 5 10 15 20 25 30 35 40 45 50 In(1) (kA) 3,821 1,741 1,100 0,802 0,631 0,521 0,443 0,386 0,342 0,308 0,280 In (1,1) (kA) 3,727 2,001 1,376 1,049 0,846 0,710 0,611 0,536 0,478 0,432 0,393 In(2) (kA) 2,553 1,698 1,240 0,970 0,794 0,672 0,582 0,513 0,459 0,415 0,379 In(3) (kA) 2,948 1,961 1,432 1,120 0,917 0,776 0,672 0,593 0,530 0,480 0,438 R0/X1 X0/X1 0,000 0,752 1,152 1,403 1,576 1,702 1,799 1,874 1,934 1,983 2,022 0,315 1,254 1,742 2,038 2,234 2,373 2,474 2,551 2,610 2,656 2,691

c- Ta c th biu din kt qu tnh ton mi quan h gia t s X0/X1 v chiu di ng dy theo th hnh 4. Nhn xt: + Kt qu tnh ton ngn mch i vi c dy AC-95 cho thy dng ngn mch gim nhanh (theo chiu di ng dy) chng t in khng v in tr ca li 22kV (khi s dng dy dn c kch thc 95mm2) tng i ln, iu ny s gy st p v tn tht cng sut tng i ln, ng thi gy tr ngi cho vic chnh nh bo

Hnh 4. Quan h gia X0/X1 v chiu di ng dy 49


v rle. Thc t cn c mt s tuyn ng dy cn ko i xa hn v tit din dy cn b hn na. Mt trong nhng gii php nn p dng l phn on ng dy bng cu ch t ri, my ct hoc thit b t ng ng lp li phn vng bo v. + Kt qu tnh ton ngn mch vi kt cu li 3 pha 3 dy v 3 pha 4 dy cho thy li 3 pha 4 dy c bn knh cp in hiu qu ln hn li 3 pha 3 dy. Kt qu tnh ton cng nh thc th cho thy mng trung tnh ni t hiu qu vi kt cu 3 pha 3 dy ch thch hp vi cc mng c cng sut ln, ph ti tp trung, tit din dy ln v bn knh cung cp in b. Mng 3 pha 4 dy kt hp vi ni t lp li dy trung tnh c nhiu ngha trong thc t i vi li in nng thn, min ni khu vc min Trung v Ty Nguyn. Vi kt cu ny n ci thin c tnh hiu qu ca ni t do lm tng phm vi v bn knh cung cp in ca mng. m bo tnh kinh t ca mng nh chn cc thit b in c h s qu p tm thi b.5. Kt lun - LPP 22kV vi cu trc 3 pha 4 dy c bn knh cp in hiu qu ln hn cu trc 3 pha 3 dy, do s gim c vn u t TBA trung gian v cc thit b bo v v phn on hn so vi li in 3 pha 3 dy. - Vi cu trc 3 pha 4 dy (c dy trung tnh ko theo mng in) kt hp vi cc c cu ni t lp li nn dng in tr v qua in tr ni t trm ngun gim i, do vy in p ging trn in tr ni t trm ngun 22kV gim xung, nn tnh an ton i vi con ngi cao hn LPP 22kV vi kt cu 3 pha 3 dy. - Mng trung tnh ni t hiu qu vi kt cu 3 pha 3 dy ch thch hp vi cc mng c cng sut ln, ph ti tp trung, tit din dy ln v bn knh cung cp in b. - Mng 3 pha 4 dy kt hp vi ni t lp li dy trung tnh c nhiu ngha trong thc t i vi li in nng thn, min ni khu vc min Trung v Ty Nguyn. Vi kt cu ny n ci thin c tnh hiu qu ca ni t do lm tng phm vi v bn knh cung cp in ca mng. Ngoi ra: V mt tin cy cung cp in, nu li in 3 pha 4 dy s dng cc MBA 1 pha hoc t hp 3 MBA 1 pha thnh 3 pha kt hp vi thit b t ng ng li 1 pha, tin cy cung cp in s tng cao. Cu trc LPP 22kV 3 pha 4 dy vi kt cu dy trung tnh cao h th i chung ti cc khu vc ng dn c, dy trung tnh i ring cc khu vc c mt dn c thp s kt hp hon ton c hai mng trn. Thay vo , nu kt hp s dng MBA 1 pha v thit b t ng ng lp li 1 pha s rt hiu qu v cc mt kinh t v tin cy cung cp in. V mt bo v chng qu p kh quyn: i vi nhng vng c mt st cao, nu s dng dy trung tnh kt hp lm dy chng st s gim c sut s c do qu in p kh quyn. Mng 22kV c th c ch trung tnh ni t hiu qu vi cc ng dy cng sut kh ln v vi khong cch trung bnh, ng vi c dy > 95 mm2 v bn kch cung cp in L < 40km. Lc ny c th m bo tnh kinh t nht ca li nh chn cc thit b vi qu p tm thi b. i vi cc li c c dy b hn 95mm2 r rng cn phi quan tm n vic chn li cc thit b in khc vi h s qu p tm thi ln hn, v ng ngha vi vic mt dn tnh kinh t ca li c trung tnh ni t hiu qu.

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TI LIU THAM KHO[1] B Nng lng (1994), Quyt nh s 1867 NL/KHKT ngy 12 thng 9 nm 1994 ban hnh qui nh cc tiu chun k thut cp in p trung th 22kV, H Ni. [2] L Vn (1998), Nghin cu ch ni t trung tnh li phn phi, ti nghin cu khoa hc cp trng, Trng i hc k thut- i hc Nng, Nng. [3] Richard Roeper (1996), Ngn mch trong h thng in, Nxb khoa hc v k thut, H Ni. [4] TCVN 4756-1989, Quy phm ni t v ni khng thit b in, Nxb Xy dng, H Ni. [5] Cng ty in lc 3 (2005), Hng dn s dng chng trnh PSS/ADEPT, Nng.

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07 - quoc_le kim hung-r

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