Chống sét & Nối đất

CNG BI GING CUNG CP IN

TRNG HSPKT VINH

Chng 10 CHNG ST V NI T 10.1. QU IN P THIN NHIN V C TNH CA ST 1. Khi nim chung. ch lm vic bnh thng, in p tc dng ln cch in ca cc thit b in thng l dao ng xung quanh tr s in p nh mc. Song c nhiu khi do nhng nguyn nhn khc nhau c th gy ra in p tng cao v rt cao so vi tr s nh mc tc dng ln cch in gy nguy him thit b in. Hin tng tng cao in p nh vy gi l qu in p hay qu in th. C nhiu cch phn loi qu in p. Theo kt cu ca mng ngi ta phn ra 2 loi: - Qu in p ngang: L hin tng tng in th gia hai im trn hai pha khc nhau hoc gia pha vi t. - Qu in p dc: L hin tng tng in p gia hai im ca cng mt pha (v d gia cc vng dy ca mt cun dy). Qu in p dc thng xy ra cun dy my bin p, my pht in hoc ng c in. Theo nguyn nhn pht sinh ngi ta cng chia qu in p ra lm 2 loi: Qu in p ni b: Thng xut hin khi c s thay i t ngt thng s ca mng in, do cng hng hoc do cc thao tc ng ct. V d khi ng ct nh my in, ng ct my bin p hoc ng dy truyn ti in, cng nh khi s c chm t cc pha, ngn mch, t dy ... v c phn bit thnh hai loi chnh l: +) Qu in p thao tc: Thng xut hin khi c s dao ng nng lng in t bn trong h thng in khi thay i s vn hnh mt cch t ngt. Qu in p loi ny c c tnh dao ng chu k, ko di trong thi gian t my phn trm micr giy n vi giy. +) Qu in p cng hng thng xut hin khi tn s dao ng ring ca mng 0 gn bng tn s xoay chiu ca ngun in, hoc di dng cng hng thng s khi mt trong cc thng s ca mng in thay i ph hp. V d thay i in cm ca cc my in quay theo chu k. Qu in p cng hng cng c tnh cht dao ng chu k. Khi c qu in p ni b in p c th tng ti (3 4)Um v thi gian tn ti khong 10 ms hoc ln hn, kh nng xut hin nhiu. Qu p thin nhin: Pht sinh khi c phng in st trc tip vo cc cc thit b ngoi tri nh cc ng dy ti in, thit b phn phi ngoi tri, nh ca hay cc cng trnh khc; Hoc l khi c s phng in vo mt i tng no gn ri cm ng ln mng in (gi l qu in p cm ng). Qu in p do st nh trc tip l nguy him nht. c im ca qu in p thin nhin l thi gian tc ng ngn khong (20 60)s, bin in p rt cao v c c tnh xung. Ta bit rng i vi mi thit b in c mt cp in p nh mc, cp ny xc nh tr s in p th nghim c trng cho kh nng cch in ca n gi l mc cch in. Nu dng mc cch in qu cao s lm tng gi thnh ca trang thit b in, nu h thp mc cch in c th dn n nhng s c nng n. V vy mc cch in phi c xc nh tu theo c tnh v tr s qu in p c th c v cc tham s ca thit b dng in hn ch qu in p. Kh nng cch in chu c qu in p ni b xc nh bng in p th nghim tn s cng nghip cn kh nng cch in chu c qu in p thin nhin c xc nh bi in p th nghim xung kch.V Tin Trung - Khoa in Trang 139

TRNG HSPKT VINH


Vy qu in p l mt hin tng khng bnh thng trong h thng in, n c th gy ra nhng hiu ng nguy him cho h thng. Bin ca sng qu in p uy hip cch in ca thit b, c bit l cc my in quay. dc ca sng qu p uy hip cch in gia cc vng dy vi nhau ca cc thit b in. Tu theo thi gian tc dng, bin ca sng, hiu ng c nhit ca qua p s lm h hng cc trang thit b in cc mc khc nhau thm ch c th gy ra chy, n cc thit b in. Chnh v nhng hu qu nghim trng m phi c cc bin php bo v thch ng bo v qu in p. Nguyn l chung ca bo v qua p l: Cc thit b dng bo v qu in p bt k loi no u phi tho mn iu kin: c tnh phng in (vn - giy) ca thit b bo v phi nm di c tnh phng in ca cch in ca thit b in cn bo v t nht l 20% (hnh 8- 1). Phm vi gio trnh ny ny ch trnh by phn bo v qu p thin nhin. bo v qu p thin nhin ngi ta p dng ng thi c hai bin php l: - Ngn nga khng cho phng in st trc tip vo mng in. - S dng cc thit b tho st nhm h thp bin hoc trit tiu hon ton sng st xm nhp vo mng in trc khi n kp phng in trn cch in ca cc thit b in. 2. Qu in p thin nhin v tham s ca st a. S hnh thnh st.

1

1 2

1 2 3

1 3

is

is

is

is

ta) b)

tc)

td)

t

Qu trnh hnh thnh ca sta) Hnh thnh my ging vi nhng vng mang in tch tri du. b) Dng tin o pht trin (0,005 0,01)s. c) Phng in ch yu (50 100)s. d) Lo sng cui st (0,03 0,05)s.Trang 140 V Tin Trung - Khoa in


TRNG HSPKT VINH

St l s phng in trong kh quyn gia cc m my mang in v t m tp trung mt qung in tch tri du vi m my hay gia cc m my mang in tch tri du khong cch gn. Cc m my mang in l do kt qu ca s phn li cc in tch tri du v s tp trung chng trong cc phn khc nhau ca m my. i vi khng kh khi cng in trng t (25 30) kV/cm th khng kh b ion ho rt mnh v khng kh bt u tr thnh dn in to iu kin cho hnh thnh phng in st. Ta nghin cu s phng in t m my vo t nh sau. Qu trnh phng in ca st chia thnh ba giai on: - Giai on th nht: S phng in gia m my v t c bt u bng s xut hin mt dng sng pht trin xung t chuyn ng tng t vi tc (100 1000)km/s gi l tia tin o to ra in p rt ln ti u cc ca n (khong triu vn), tia tin o l mt dng plasma. Giai on ny gi l giai on tin o tng bc. - Giai on th hai: Khi dng tin o pht trin gn n t hay cc vt dn in ni vi t th s to ra mt in trng cc ln vi t lm ion ho mnh lit khng kh gia tia tin o v mt t khi giai on th hai bt u, giai on phng in ch yu ca st. Trong giai on ny cc in tch dng ca t (sng in tch) di chuyn hng t t theo dng tin o vi tc ln (6.104 5.106) km/s chy ln v trung ho cc in tch m ca dng tin o. Khng kh trong dng phng in c nung nng n nhit khong 10.0000C v gin n rt nhanh to thnh sng m thanh. S phng in ca st c m t (hnh 8-3).

Xung th nht

Xung th hai Tin o mi tn

Xung th ba

a)

Tin o tng bc

b b)Dng tin o 0,005 0,01s

Dng phng in ch yu 50 100 s

Lo sng cui st 0,03 0,05 s

S phng in ca st.a.Hnh nh v s lo sng ca phng in st nhn c trong bung nh. b.S bin thin theo thi gian ca dng in st.

- Giai on th ba ca phng in st: l giai on kt thc s di chuyn cc in tch ca my m t bt u c s phng in v s lo sng dn dn bin mt. Thng phng in st gm mt lot phng in k tip nhau do s dch chuyn in tch t nhng phn khc ca m my. Tin o ca nhng ln phng in sau i theo dng ion ho ban u. V vy chng pht trin lin tc v c gi l tin o dng mi tn.

V Tin Trung - Khoa in

Trang 141

TRNG HSPKT VINH


b. Cc tham s c bn ca st. Hnh dng ca phng in st ghi c trn cc my hin sng cc nhy c dng nh hnh v. Khi nghin cu hay tnh ton chng st ngi ta quan tm n mt s cc thng s c bn sau y: Is (kA) Us (kV) 2a a

Imax m

Umax

1 Umax/2

t (s)

ds

s

Sng dng in st v sng in p st tng ngng 1 l dng in st ghi c trn my hin sng - ng 2 dng in st tnh ton

*). Thi gian tn ti sng ca sng st. Hnh nh ca mt phng in st c dng xung kch nh hnh v trn, ta c th phn bit thnh hai na l sn trc v sn sau ca dng in st. Cc thi gian tng ng quy nh nh sau: - Thi gian ton sng: s c tnh t khi xut hin st n khi bin in p st vt qua nh ri suy gim n cn li tr s bng 0, 5 tr s bin nh Umax Thi gian tn ti ca mt ln st khng ln thng ch t 20 n 80s v th nng lng ca mt phng in st cng khng ln (tng ng nng lng lm nng 1 tn nc thm 20oc) - Thi gian u sng: s c gn ng l k t khi xut hin st n khi t bin nh Umax . hay l khong thi gian ca sn trc sng st thng khong 2 n 4s. V th c Is (kA) tnh xung ca sng st l rt dc. Do 100 c tnh xung kch ca in p sng st vi bin nh rt ln v thi gian tn ti rt ngn nh vy nn khi xm nhp cun dy my in s gy qu p mnh m cho nhng vng 90 dy phn u rt nguy him. *). Bin ca phng in st Imax v Umax 80 Bin cc i ca dng st l gi tr bin nh ca dng in st I max, n c th t khong 150 kA hoc hn na, nhng rt t trng hp dng in st c bin ln hn 150 kA 70 (khong 2% tng s ln phng in st). V vy trong tnh ton thng ly dng in st bng 60 (50 150) kA. S ph thuc bin dng in st v xc sut xut hin n c biu din trn Is (hnh 8-6). (kA) s 50ng thi sut hin vi dng in st l sng in pIst v n cng c hnh dng tng t nh sng dng in st. Bin nh ca sng in p st Umax rt ln, hng chc, hng trm 40 220 amax kV. ln sng st ph thuc ch yu vo s tch ly in tch ca m my v s t tp in tch tri du di mt t khi phng in st nh hng vo . 30 180a 20Trang 142

140 0 10 20 30 40 50 60 70 80 % 100V Tin Trung - Khoa in

10

0

0,4

0,8

1,2

1,6

%

H Hnh 8-6. Cc ng cong xc sut bin v dc dng in st


TRNG HSPKT VINH

*) dc u sng ca sng st a dc u sng ca sng dng in st l tc thay i bin dng in st trong thi gian u sng, di a= s dt dc u sng ln nht ca dng in st thng t khi vt qu 50 kA/s. Bin dng in st ln th dc u sng cng ln. V vy vi dng in st tnh ton 150 kA v ln hn thng ly dc u sng trung bnh 50 kA /s v khi dng in st tnh ton nh hn 150 kA th ly dc u sng khong 30 n 10 kA /s. dc u sng ln nh vy s rt nguy him cho cc cun dy my in khi sng st xm nhp n, n c th gy phng in xuyn kch gia cc vng dy. Xc sut xut hin ca st c bin cng ln th cng t, qui lut ca n c biu din bng s (hnh 8-6). *). Ch tiu ngy st nng.s Cng hot ng ca st c biu th bng ch s ngy st trong nm (n ngs) mi a phng. Nc ta vo vng kh hu nhit i, nng v m rt thun li cho vic hnh thnh my ging v st do cng hot ng ca st khong nngs = (40 100) ngy /nm. Trn th gii c nhng vng khng c st nh Nauy, Ailen c nhng vng ngy no cng c st nh o Java ca Innsia c nngs = 365 ngy /nm. nngs l mt ch tiu rt quan trng, cn phi c cp n khi tnh ton thit k cung cp in m bo cho h thng hot ng an ton. 3. Hiu ng ca st a. Hiu ng nhit v cV Tin Trung - Khoa in Trang 143

TRNG HSPKT VINH


St c hiu ng nhit v c, ti tm ng truyn ca dng st c nhit rt ln khong 100000C, n thiu chy tt c cc vt th ni dng st i qua. Khi st nh vo cc vt liu cha m lm bc hi cc k mnh lit gy n v ph v vt th. Tuy nhin n ng lng st sinh ra khng ln, v tuy dng in st Is ln nhng thi gian tn ti ngn. b. Hiu ng in t Hiu ng in t c c th ho bng sng qu p trn ng dy trn khng. Qu in p cm ng sinh ra trn dy dn ca cc ng dy ti in trn khng khi st nh trc tip vo ng dy cng nh khi nh gn ng dy. Do s bin thin rt nhanh ca trng in t ca dng in st trong giai on phng in ch o gy ra in p cm ng. Qu in p cm ng gm c hai thnh phn: - Qu p cm ng in (hay tnh in), sinh ra do s bin thin rt nhanh ca in trng. - Qu p cm ng t, sinh ra do s bin thin nhanh chng ca t trng. Ti thi im phng in ch o ca st. Nu st nh gn ng dy th trn ng dy s cm ng mt sc in ng rt ln v to ra sng qu p trn ng dy. Nu st nh trc tip vo ng dy s c sng dng in v sng in p truyn theo c hai hng ca ng dy. Cc sng ny lan truyn vo trm, uy hip cch in ca cc thit b in trong trm. Tm li: St c th gy ra nhng hu qu nguy him v nghim trng. hn ch s nguy him ca st, ta phi dng cc phng php phng chng st. 10.2. BO V QU IN P CHO NG DY TI IN. Trong vn hnh, s c ct in do sng st xm nhp vo cc ng dy ti in trn khng chim t l ln (80%) trong ton b s c ca h thng in. Bi vy, bo v chng st cho ng dy c tm quan trng rt ln trong vic m bo vn hnh an ton v cung cp in. 1. Bo v chng st nh trc tip vo ng dy trn khng. bo v chng st cho ng dy trn khng tt nht l treo dy chng st trn ton b tuyn ng dy. Song bin php ny rt tn km, v vy n ch c dng cho cc tuyn ng dy (110 220) kV. ng dy ti in trn khng in p t 35kV tr xung ct st hay ct b tng ct st t c bo v bng dy chng st ton tuyn. tng cng kh nng chng st cho nhng ng dy ny c th t chng st ng hoc tng thm bt s nhng ni cch in yu, nhng ct vt cao, ch giao cho vi ng dy khc, nhng on ti trm. Nhng mng in i hi tnh lin tc cung cp in rt cao th tt nht l dng ng dy cp. Dy chng st: Tu theo cch b tr dy dn trn ct, c th treo mt hoc hai dy chng st. Cc dy chng st c treo trn ng dy ti in sao cho dy dn ca c ba pha u nm trong phm vi bo v ca cc dy chng st. Biu din phm vi bo v ca mt v hai dy chng st (hnh 8-7a,b). Phm vi bo v ca mt dy chng st l mt khng gian hnh mi nh che kn ton b cc kt cu ca ng dy trn khng bn trong. Mt ct ngang ca n c dng hnh tam gic nhng hai cnh bn l ng cong lm, trong tnh ton cho php tuyn tnh ha thnh ng

Trang 144



TRNG HSPKT VINH

gy khc to bi 2 an thng v qua cc im c bit c ta nh sau: (0,6h-h) v (1,2h0,8h) nh hnh v. Phm vi bo v ca mt dy chng st c tnh ton qua hai thng s l hx v bx 2 h h x > h b x = 0,6h 1 x P cao 3 h cao h 2 h x < h b x = 1,2h 1 x P 3 0,8h

Trong : - h l chiu cao ca dy chng st. - hx l chiu cao ca i tng c bo v. - P l h s. Nu h 30m th P = 1. Nu h > 30m th P = 30 5,5 h hh = a/4R

h = a/40,2 h

0,2 h

oh

oh a

h2h/3hx0,6 h 0,6 h a 0,6 h0

2h/3

h

hx0,6 h 0,6 h 0,6 h

0,6 h

0,6 h

2bx

bx

a

bx

Gc bo v v phm vi bo v ca mt dy chng st

Gc bo v v phm vi bo v ca hai dy chng st

Phm vi bo v ca hai dy chng st song song cng xc nh nh trng hp mt dy chng st. Ring phm vi bo v pha trong gia hai dy c gii hn bi mt cung trn i a . 4 Bng cch khc ngi ta biu th phm vi bo v ca dy chng st qua gc bo v . i vi cc ct in thng thng, dy dn s c bo v chc chn nu gc bo v khng qu 300. Gim gc bo v s lm gim xc sut st nh vo dy dn nhng li lm tng gi thnh qua cc dy chng st v im gia c cao h = v phi tng chiu cao ca ct. i vi ng dy thng ly t s 300. 2. Bo v chng tc hi ca sng st khi xm nhp vo ng dy trn khng. Khi sng st xm nhp vo ng dy trn khng, n s e da cc cch in ca ng dy, gy nn cc phng in gia pha vi t hay gia cc khong vt s c th dn nV Tin Trung - Khoa in Trang 145

a < 4 vi iu kin 200 h

TRNG HSPKT VINH


BVRL tc ng lm ngng cung cp in. Mc d trong tnh ton chn cch in dy c k ti kh nng cho php phng in b mt ca cch in ca n, nhng y khng phi l gii php tt nht. V vy cn thit phi trang b cc thit b tho st ct bt bin nh hc tho hon ton sng st xung t mt cch an ton. Nh vy s gim nh cho iu kin cng tc ca cc cch in ng dy v m bo tnh lin tc cung cp in cao hn. Cc thit b tho st l cc khe h phng in hay chng st ng v hon ho nht hin nay l chng st van, iu ny c bn ngnh qun l mng in quy nh bt buc.

10.3. BO V CHNG ST CHO TRM BIN P. 1. Bo v chng st nh trc tip. a. Ct thu st (ct thu li). bo v st nh trc tip cho cc thit b in v cc cng trnh khc t trong trm bin p ngi ta dng ct thu st (cn gi l ct thu li). Ct thu st c tc dng thu ht phng in st vo mnh v dn xung t bo v cc cng trnh trong khu vc khi b st nh trc tip. Ban u tia tin o pht trin theo hng ca in trng, khi tia tin o phng in n cao H so vi mt t, v u kim thu st c hiu ng mi nhn, in trng ln hn so vi xung quanh do iu kin phng in rt thun li, v vy tia tin o pht trin v pha kim thu st v phng in vo n. H gi l cao nh v, nu h l cao ca ct thu st th H = (10 20) h. Nh vy ct thu st cng cao (h cng ln) th vic thu st cng tt, song khng th cao bao nhiu cng c, phi m bo iu kin kinh t, an ton v kh nng thi cng. Ct thu st gm c kim thu st, dy dn st xung t cng vi trang b ni t. Trong thc t gi l b phn thu st, b phn dn st v b phn ni t. B phn thu st l mt thanh thp trn c 12mm u nhn c trng km. B phn dn st l dy thp c 8mm ni t b phn thu st ti b phn ni t. B phn ni t l in cc ni t c th phn bit thnh hai loi: - in cc n hay gi l in cc tp chung: L mt cc thp trn hay thp gc hoc mt tm thp bn chn su di t, loi ny rt thch hp cho vic tn nhanh sng st vo trong t. - in cc h thng; L nhng cc bng thp v cc thanh thp hn vi nhau theo s li cc - tia hn hp, chn di t phn tn dng in st vo t ng thi gim c tr s in p bc trong khu vc gn. Kch thc, cc s liu k thut ca cc b phn trn tra cu phn ph lc. b. Phm vi bo v ca ct thu st. Khng gian xung quanh gn ct thu st m i tng c bo v t trong rt t kh nng b st nh gi l phm vi bo v ca ct thu st. Cho n nay ch c mt cch duy nht l xc nh phm vi bo v bng thc nghim trn m hnh. Tuy cn nhiu nhc im nhng qua mt thi gian kh di c kim nghim trong thc t, kt qu ca m hnh c th chp nhn c vi tin cy ln. Theo quy phm thit k chng st cho cc cng trnh kin trc, phm vi bo v ca cc ct thu st c xc nh nh sau: *) Phm vi bo v ca mt ct thu st ng ring r.Trang 146 V Tin Trung - Khoa in


TRNG HSPKT VINH

Phm vi bo v ca mt ct thu st ng ring r l mt hnh nn cong trn xoay nh trng vi nh kim, y l mt hnh trn c bn knh bng 1, 50 ln chiu cao ca ct (r 0 = 1,50 h). Mt ct ca phm vi bo v qua ct thu st l mt tam gic nh chng vi nh kim, y nm trn mt t, hai cnh bn l ng cong lm nhng c tuyn ha thnh ng gy khc to bi hai an thng v qua cc im c bit c ta nh sau: (0,75h-h) v (1,5h-0,8h) nh (hnh 8-8).

0,2 h h 2h/3 hx 0,75 h 0,75 h 0,75 h

0,75 h

Rx

Phm vi bo v ca mt ct thu st cao hx bt k tng ng c bn knh bo v ca ct thu st l Rx c xc nh bng cc biu thc sau y: 2 h h x > h R x = 0,75h 1 x P cao 3 h cao hx < 2 h 3 h R x = 1,5h 1 x P 0,8h h > 30m th P =

30 h Trng hp bit Rx v hx th chiu cao ca ct thu st c xc nh bng cc cng thc sau: hx R + 1,9h x 2,67 th h = x Nu Rx 1,5 Vi h 30m th P = 1; Nu hx R + 0,75h x > 2,67 th h = x Rx 0,75

*) Phm vi bo v ca ct thu st kp, to bi hai ct thu st cao bng nhau. Phm vi bo v hai bn xc nh nh trng hp 2 ct thu st ng ring r; Phm vi gia hai ct c gii hn trn l mt cung trn i qua hai nh kim v tm cung nm trn ng trung trc ca on thng ni lin gia hai nh kim v c cao H bng 4 ln chiu cao ca ct thu st (H = 4h). Bn knh ca cung trn xc nh bng cng thc:V Tin Trung - Khoa in Trang 147

TRNG HSPKT VINH


R = H - h0 = 4h - h0. Trong : - h0 l chiu cao ti im thp nht ca cung v c xc nh bng cng thc: h0 = 4h -

9h 2 + 0,25a 2

Khi bit a v h0 th chiu cao ca ct thu st c xc nh bng cng thc:

h = 0,571h 0 + 0,183h 2 0 + 0,0357 a 2Mt ct ngang ca phm vi bo v ti im thp nht gia hai ct thu st hon ton ging nh phm vi bo v ca mt ct thu st ng ring r c chiu cao l h 0. Mt ct ny cho php xc nh c b rng ca phm vi bo v trn mt bng ti im gia hai ct thu st theo cc cng thc sau y: 2 h hx > h Ro x = 0,75h 1 x P cao 3 ho h Ro x = 1,5h 1 x P 0,8ho Ch : hnh thnh vng bo v ca hai ct thu st th chiu cao ct l h v khong cch cao 2 hx < h 3 gia hai ct l a phi tho mn i kin:

a 28 cho bx > 0. h

R

h0,2 h

h2h/3 hx 0,75h

h0

h

hh0 0,75h0,75ho 1,5ho

0,75h

a

0,75h

Rx 2bx Rx

Phm vi bo v ca hai ct chng st cao bng nhau

*) Phm vi bo v ca nhiu ct thu st kt hp. Hnh v sau ch r phm vi bo v ca ba v bn ct thu st kt hp.Trang 148 V Tin Trung - Khoa in


TRNG HSPKT VINH

2 bx12 a12 1 rx1 bx31 D a31 a23

rx2

bx23

3 rx3

Phm vi bo v ca 3 ct chng st

bx12 1 rx1 bx41 a41 a12 D

2

rx2

bx23 a23 3 a34 bx34 4 rx3

rx4

Phm vi bo v ca 4 ct chng st Phm vi bo v pha ngoi tam gic hoc a gic (do v tr cc ct to thnh) xc nh nh trng hp hai ct thu st phi hp tng i mt. Phm vi pha trong tam gic hoc a gic hon ton c bo v vi iu kin: D 8 (h - hx ) vi h 30m. D (h - hx )P vi h > 30m. Trong : - D l ng knh vng trn ngoi tip tam gic hoc ng cho di nht ca a gic. - h cao ct thu st. - hx cao cn c bo v. c. Mt s im cn ch khi bo v chng st nh trc tip. K thut bo v chng st cho cc loi thit b in khc nhau c nu r trong cc quy trnh quy phm ca Nh nc y ch nhc ti my im c bn nht.V Tin Trung - Khoa in Trang 149

TRNG HSPKT VINH


Ct thu st c th t c lp hoc t ngay trn cc thit b cn bo v. Nhng ct c lp lm bng thp ng, nu cao ln hn 20 mt th lm bng ct hn khung mt co. Nu dng ct b tng ct thp th r hn, thm ch c th dng ct bng tre hoc g. Nu l ct thp th cho php kt hp dng ngay n lm b phn dn sng st xung t; Nu dng ct tre, g th phi dng dy dn sng st xung t. bo m bn lu di th tit din ca dy khng c nh hn 50 mm2. trnh hin tng mang in th cao ra nhng vng t xu th khng c dng cc dy no gi cc ct thu li. bo m tip xc tt, ni chung cc im tip xc phi hn, nu dng bulng bt gi th phi to mt tip xc tt, m chng g v phi c tit din ln hn hai ln tit din dy. Cc dy dn c sn hoc trng km trnh han g. Phi nh k kim tra mng li chng st nht l vo nhng k trc ma ma. Ngoi ra cng cn ch khong cch cn thit gia ct thu st v vt c bo v. Ni chung tt c nhng vt c bo v phi nm trn vn trong phm vi bo v ca ct thu st, nhng ng thi chng phi cch ct thu st mt khong nht nh. (hnh 2-13).

Ct thu st

H Skk S

TB

Khong cch gia ct thu st v vt c bo v iu kin cn thit chn khong cch trong khng kh v trong t gia ct thu st v vt c bo v l: Skk 0,3.Rxk + 0,1. S 0,5.Rxk Trong : - Skk l khong cch ti thiu trong khng kh. - Rxk l in tr ni t xung kch ca thu st. - l cao ca vt c bo v. - S l khong cch ti thiu trong t (gia cc cc ni t ca ct thu st v vt c bo v). 2. Bo v chng st lan truyn t ng dy vo trm. Cc ng dy trn khng d c c bo v chng st hay khng th kh nng cc thit b in ni vi chng vn c th phi chu tc dng ca sng st chy t ng dy n. Bin Trang 150 V Tin Trung - Khoa in


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ca qu p thin nhin c th ln hn mc cch in ca thit b in dn n chc thng cch in gy ngn mch, ph hoi thit b v mng in c th b ngng cung cp in. V vy bo v trm bin p cn phi t cc thit b chng st. Cc thit b chng st ny s h thp bin sng qu p n tr s an ton cho cch in ca cc thit b c bo v. Thit b chng st ch yu cho trm bin p l chng st van kt hp vi chng st ng v khe h phng in. a. Khe h phng in. Khe h phng in l thit b tho st n gin nht gm hai in cc bng thp ng knh 10mm dng sng d, mt in cc c ni vi dy dn, in cc cn li c ni xung t qua khe h ph S2 (hnh 8-14). Khong cch S1 gia 2 in cc gi l khong cch bo v, khong cch ny ph thuc vo in p nh mc ca mng in, tham kho (bng 8-1). Ba sng v pha gn vo ct (hoc x) c tip a chung. phng chim u hay con vt khc gy kn mch, ngi ta lm thm khe h ph S2 trn ng dy tip a. Khe h bo v t trn cao gn vi ng dy trn khng.1

S1

S2

2

S mc khe h chng st kiu sng1- Dy pha; S1 - Khe h bo v; S2- Khe h ph; 2- Dy tip a

Khi mng in lm vic bnh thng cc khe h cch ly nhng phn t mang in (dy dn) vi t. Khi c sng qu p xm nhp trn ng dy, khe h phng in s b chc thng ni tt ng dy vi t tho sng st xung t. Khi khe h tho st to ra mt sng ct rt dc gy nguy him cho cc cun dy my in. Mt khc do khe h khng c bung dp h quang nn h quang chy trong khe h ri b tt ngu nhin c th l khi cha tho ht sng st hoc l ko di sau khi ht sng st v vn tip tc gy chm t ko di rt nguy him: Gy qu p cho cch in ng dy, c th lm cho bo v Rle tc ng ct mng in. u im ca khe h phng in l n gin, r lp t, bn v tc ng kh tin cy, nhng cng nhiu nhc im nn khe h phng in thng ch c ng dng cho nhng mng in n gin hoc dng lm bo v ph cho mt s cc bo v chng st.

Khong cch khe h bo v hnh sng. in p nh mc ca trm (kV) Khe h bo v (mm)V Tin Trung - Khoa in

3 8 20

6 15 40

10 25 50Trang 151

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Khe h ph (mm) b. Chng st ng. (CSO hay PT).

5

10

15

Chng st ng l mt thit b tho st c cu to n gin nh hiu qu tho st rt tt, c dng tho sng st lan truyn trn ng dy xung t, bo v cho cc thit b trong trm. *) Cu to:lp l x cao th np kn

DDK 2 1 2S2

4

2S1

3

h quang

Cu to ca chng st ng1- ng sinh kh; 2 - in cc thanh; 3- in cc hnh xuyn; 4- Dy tip a.

Cu to ca chng st ng gm: - ng (1) ch to bng vt liu cch in nh thu tinh hu c, php, baklit... Trong ng ph mt lp cht sinh kh khi b chy nhit cao. - Trong ng t mt in cc kiu thanh (2) v hai in cc ph 2, 2. - Chng st ng c 2 lp 3 v 4, np 4 gn vi thit b ni t, np 3 c c l thot kh ra ngoi (khi pht sinh h quang). - Khe h trong S1, c to nn bi np 3 v in cc 2 c nhim v dp h quang, khe h ngoi S2 to nn bi np 3 (2) v in cc 2 c nhim v cch ly thn ca chng st ng ra khi in p cao, nu khng vt liu lm ng s b in p lm gi ho v gy nn dng in r. Thc cht y l mt loi khe h phng in c t trong bung dp h quang. *) Nguyn l lm vic. Khi c sng qu in p tc dng, cc khe h trong v ngoi u b chc thng sng st c tho xung t. Nhng khi ht sng qu in p, di tc dng ca in p lm vic vn c h quang cc khe h. Di tc dng ca h quang in, vt liu sinh kh trong ng b t chy lm sinh kh rt mnh, p sut trong ng tng ln n hng chc t mt phe, thi mnh h quang ra ngoi ming ng v dp tt h quang theo ng thi im mong mun (chm dt dng k tc). *) c im ca chng st ng. Kh nng dp h quang ca chng st ng ph thuc vo khe h trong v kch thc ca ng. ng vi mt khong cch khe h nht nh, mt ng knh ng nht nh, ch c th dp c h quang ca mt dng in st nht nh. Nu dng in st qu nh lng kh sinh ra t khng dp h quang, ngc li nu dng in qu ln s c th ph hu chng st ng. V vy trong s liu k thut ca chng st ng c cho gii hn dng in. 35 PT V d: Chng st ng do Lin X ch to c k hiu: 0,5 3Trang 152 V Tin Trung - Khoa in


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K hiu trn c ngha l chng st ng c in p nh mc 35kV, dng in dp c nh nht Imin = 0,5 kA, dng in dp c ln nht Imax = 3 kA. u im ca chng st ng l n gin, tng i r tin. Nhc im c bn ca chng st ng l khi n lm vic (tho st) th thit b c bo v phi chu mt sng ct vi dc ln, sng ny rt nguy him i vi cc thit b c cun dy nh cc my bin p, my pht, ng c vv. V vy bo v my bin p ngi ta dng chng st van c nhiu u vit hn. Khi chn v s dng chng st ng cn ch : +) Chn chng st ng theo in p nh mc ca mng: Um cs = Um mng +) Tnh ton tm hai tr s ca dng in ngn mch ln nht I N max v IN min ti ni t chng st ng. +) c tnh Vn - giy ca chng st ng phi thp hn c tnh Vn - giy ca thit b c bo v. +) Phi theo di hnh dng ngoi v s ln tc ng ca chng st ng. +) Khi lp t chng st ng phi m bo nghing thch hp chng ng nc mu v ch ti khu vc thot kh, khng c chng ngi vt trn ng thot kh. c. Chng st van. (CSV hay PB) Chng st van l mt thit b chng st hon ho nht dng bo v cho trm bin p, trm phn phi v cc my in khc. *) Cu to. Cu to ca chng st van gm: - Bn ngoi l mt ng s hay cht do cch in c hnh dng v kch thc ty thuc cp in p nh mc s dng (hnh 8-16a). - Bn trong ng cha hai phn t chnh l khe h phng in v in tr phi tuyn. +Khe h phng in bao gm nhiu cp khe h ghp ni tip phn chia nh h quang gip cho vic dp tt h quang c r hn. Mi cp khe h c ch to bi 2 a ng mng dp nh hnh gia l mt tm m Mca hoc ba cch in dy khong 1mm to c khe h phng in ng thi ghp song song vi khe h l 1 in tr r hoc coi nh 1 in dung c no lm cho h quang s phn u trn cc cp khe h. Mi chng st van c s cp khe h phng in ty theo nh ch to thit k.1 2

r2 1

r r

3

R a) b) c)

Cu to v s nguyn l ca chng st van.


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+ in tr phi tuyn gm cc tm in hnh tr trn ghp ni tip, mi tm c ng knh t 3 n 6cm cao t 3cm n 4cm. in tr phi tuyn c th l Vilit hoc Tirit hac ZnO...vv V d tm in tr Vilt c ch to bng cc ht Ccborun k thut in (SiC). Bn thn ht Ccborun c in tr sut nh khong 1 /cm, trn b mt ca cc ht c mt lp mng mng khong 10-6cm l nhng phn t xit Silic. Khi in p nh, in tr sut ca lp mng mng rt ln (106 108).cm. Khi in p t ln ln in tr ca lp s gim xung v s chuyn ng v s lng cc in t t do tng ln. Tnh cht ca in tr vilit c th hin c tuyn V -A, l quan h gia dng in xung kch i vi in p trn in tr U; v c tuyn (-V) nh hnh v. * c tuyn (V-A) th hin qua biu thc: U = C.i Trong : - l h s van ca vilit (0,2 0,22). - C l hng s. * c tuyn (-V) biu din s ph thuc ca gi tr in tr Vilit vo in p t ln n: Khi in p t ln Vilt tng cao th gi tr in tr ca n gim v ngc li khi in p gim xung th in tr s tng ln nhanh chng.

U= 0

= 1 0 < < 1

R0 < < 1

i Hnh 8-17. c tnh V -A ca in tr Vilit = 1 - in tr thng thng = 1 - in tr l tng 0 < < 1 - in tr Vilit

U Hnh 8-18. c tnh R = f(U) ca in tr Vilit

*) Nguyn l lm vic. Do in tr vilit c tnh cht c bit nn khi c qu in p t ln chng st van, in tr ca CSV nhanh chng h thp xung to iu kin cho tho ht sng st qua n xung t, n khi t ln CSV ch cn li l in p mng th in tr ca CSV li tng ln rt ln chm

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dt dng k tc vo thi im thch hp nht. ng thi trong khi tho st in p d trn CSV cng c gi tr nh, iu ny s m bo an ton cho thit b c bo v. Chng st van c c tnh tc ng tng i bng phng cho nn chng st van khng nhng h thp c bin ca sng st m cn lm gim dc ca sng st. Chnh v th, n c th bo v chng c hin tng xuyn kch gia cc vng dy trong cng pha ca cc my in. in tr vilit d b nhim m s thay i c tnh in v lm mt tc dng ca chng st van do cn c bin php chng nhim m cho in tr Vilit. Cc loi chng st van thng gp trong thc t. - Loi PBC dng cho nh my in. - Loi PB dng cho trm bin p. - Loi PBBM dng cho cc my pht c cc cp in p 3, 6, 10 kV. Ngoi cc loi chng st van thng thng trn thc t cn c loi chng st van t, cc khe h phng in ca chng st van c t trong t trng ca mt nam chm vnh cu. Khi tho st th h quang trong khe h c dp tt nhanh hn do hiu ng t thi. Chng st van c ch to vi cc cp in p di 35kV thnh tng modul. Khi s dung cho mng in c cp in p ln hn ngi ta ghp nhiu modul li vi nhau. Khi thit k, s dng chng st van cn quan tm v tr t so vi thit b c bo v. Cc thng s k thut ca cc loi chng st van tra cu phn ph lc. *) Cc s bo v s dng chng st van bo v cho my bin p. - Trng hp chng st van t gn my bin p (hnh 8-19), in p t ln chng st van cng chnh l in p t ln my bin p. T s thay th tng ng ta c: 2Ut = Zs.Icsv + Ucsv. in p t trn chng st van: Ucsv = Icsv.Rcsv.

U(t) 2BA

ZS

2

UPB U2

Trng hp chng st van t gn my bin p

-

l Trng hp chng st van t xa my bin p (hnh 8-20). 2 PB (CSV) MBA

U(t)


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My bin p t cch chng st van mt khong l no th in cm ca thanh dn gia chng st van v my bin p cng vi in dung ca thanh dn v my bin p to thnh mt mch dao ng L - C. Trong in p d trn chng st van s kch thch dao ng lm tng tr s qu in p mt lng U. (hnh 8-21)

Uu sng ti U 1 Updxk Utd 2

i

Cc sng qu in p trn my bin p1 - Khi chng st van t trn u my bin p 2 - Khi chng st van t xa my bin p

Bin ln nht ca sng xung kch tc ng nhiu ln ln cc thit b, my bin p v s cch in khng lm cho cch in b chc thng hay phng in mt ngoi, gi l mc bo m bn v in ca cch in. Mc bo m cn phi cao hn tr s in p d ca chng st van. Ub = (1,1 1,15)Utd in p th nghim xung kch xc nh mc cch in phi ln hn mc bo m trn v trong qu trnh th nghim bn v in ca cch in c b gim xung v sai s do th nghim. i vi cch in ca my bin p ly Utn = 1,15Ub. i vi s cch in, khe h khng kh ly Utn = (1,15 1,2)Ub. S phi hp c tnh ca cch in c bo v vi cc c tnh ca chng st van (U t) thng qua vic qui nh mt khong cch cn thit gia chng gi l s phi hp cch in. Khi chng st van t xa my bin p th s c sng phn x nhiu ln gia chng st van v my bin p, lm cho dc ca sng tng ln v in p t ln my bin p ln hn in p t ln chng st van. Nu khong cch cng di th s chnh lch ny cng ln.

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3. Bo v chng st lan truyn t ng dy vo trm bin p (35 110)kV Bo v chng st lan truyn t ng dy vo trm thng dng nht l chng st van kt hp vi dy chng st. Do yu cu cch in ca ng dy v vic tng mc cch in ca ng dy n gin hn so vi tng mc cch in ca trm. V th trm c mc cch in yu nht trong mng in. Nhng sng truyn t ng dy c th gy nguy him cho trm v bin ca chng c th ln hn cng cch in ca trm. V vy khng th cho st nh vo gn trm. thc hin c iu c th dng dy chng st cho ton tuyn nhng khi n gn trm ng dy phi c bo v tt hn. Chng hn, gc bo v ca vng chng st nh hn, in tr tip a ct nh hn ... Nu khng c dy chng st ton tuyn th on ng dy ti trm di khong (1 2) km cng phi c dy chng st. C nh vy mi trnh c st nh gn trm ng thi gim c dc ca sng st ti trm.TBA

DK

l = (1 2) km

MC

CSV1

CSV2 CSV3 C

S bo v qu in p thin nhin cho trm my bin p (35 110) kV Trn hnh v trnh by s bo v cho trm bin p (35 110)kV. ngn nga st nh trc tip vo ng dy khng c dy chng st, on gn ti trm c bo v bng dy chng st khong 2 km. Chng st ng (CSV1) t u on ng dy gn trm nhm hn ch bt bin nh ca sng st. Nu ng dy c bo v bng dy chng st ton tuyn th khng cn t CSO1. Chng st ng 2 (CSV2) dng bo v my ct khi n h mch, v lc sng my ct phn x cng du v c th chc thng cch in ca my ct. c tnh vn - giy ca chng st ng 2 cn phi thp hn c tnh vn - giy ca my ct t nht (20 25)% ng thi yu cu CSV 2 khng c tc ng khi my ct ng v sng ct khi CSV 2 tc ng c dc rt ln uy hip cch in gia cc vng dy ca my bin p. Mt khc CSV 2 tc ng s gim thp bin in p ca sng st mt ln na lm cho chng st van trong trm bin p c th khng tc ng hoc tc ng khng tin cy. Mun vy c tnh vn - giy ca CSV 2 phi cao hn c tnh vn - giy ca chng st van. Cc trm bin p (3 10) kV c bo v n gin hn, khng cn t dy chng st on ng dy gn trm m ch cn t CSV1 cch trm mt khong 200 mt. Trn thanh gp ca trm hay st my bin p t chng st van CSV2 (hnh 8-23a).


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i vi mng in nh (nh mng in nng thn) dng ngn mch nh, cng sut trm nh c th dng khe h hnh sng (hnh 2-23b, c). U(t) 200 m MBA CSV2

CSV1

a)Trm tng p Trm gim p

b)

c)

Hnh 8- 23. S bo v qua in th thin nhin cho trm my bin p cng sut nht, in p (6 10)kV S bo v chng sng st lan truyn vo trm bin p Vi nhng trm bin p trung gian 35 kV c cc my bin p cng sut ln t 1000 n 40.000 kVA, c bit l cung cp cho nhng h ph ti quan trng nh nh my xi mng, cn phi c bo v chng st c bit hn, bi l gi tr MBA l kh ln v nu s c MBA s dn n thit hi ln v kinh t do mt in phi ngng sn sut. Khi phi cao vic bo v qu p thin nhin cho trm bin p quan trng, ngi ta khng ni trc tip ng dy trn khng n trm bin p, nh vy rt nguy him khi c sng st truyn ti m c th p dng dng s c bn theo quy phm TCVN-IV.2.157 nh sau: ng dy trn khng c th c treo dy chng st trn an ngn (300m 600m) c v hai pha a v b ca im u ni r vo trm bin p, Pha hai u a v b ca on ng dy ny lp 2 b chng st ng dy CSVa1 v CSVb1 kt hp vi ni t phi m bo tr s in tr ni t cng nh cng tt (Rn 4) loi tr nhanh phn bin nh ca nhng sng st lan truyn t ngoi ng dy n khu vc trm bin p. Ti u ni cp vi ng dy trn khng lp chng st van CSV1, Trn thanh ci trm bin p lp CSV2 v t in C . an cp ngm c chiu di (50 100)m, cp phi dng loi 3 pha c cng v bc ngoi bng ai thp, chn ngm su 80 cm vi nn t c in tr sut nh, tng t nh cc tia ni t, khng c lun cp trong cp ng nha gy nn cch in ln gia v cp vi t. S ni nh hnh v v nguyn l bo v nh sau:Trang 158 V Tin Trung - Khoa in


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DK 35 kV CSV a1

Dy chng st(300 600)m CSV b1

CSV1

(50-100)m Cp ngm v ai thp TBA C CSV2

- Dy chng st c tc dng khng cho st nh trc tip vo an ng dy gn trm, - CSVa1 v CSVb1 c tc dng tho sng st lan truyn t ng dy trn khng votrm.

- Trng hp st nh gn hoc st c bin ln, CSVa1 hoc CSVb1 s ct bt bin nh, phn cn li tc dng ln CSV1 lm chng st van CSV1 tc ng tho sng st xung t ng thi trong thi gian ny nh c dy ni t hai u v cp v ni t v MBA nn in p t ln cch in ngang ca my bin p c gi tr rt nh v CSV1 tc ng s ni tt rut cp vi v cp lm sng st truyn n MBA bng hai ng song song: + ng th nht sng st truyn qua v cp n ni t v MBA do hiu ng mt ngoi nn phn ln sng st truyn theo v cp, ng thi trn ng truyn qua v cp sng st s c tn nhiu vo trong t. + ng th hai sng st truyn theo rut cp vo dy cun MBA. Nh vy nu nh tng tr sng ca hai ng truyn bng nhau th chnh in p t ln cch in ngang ca MBA s nh di mc cch in ca MBA rt nhiu. iu ny m bo cho cch in ca MBA khng b sng st phng in. tng cng an ton cho MBA p ngi ta mc thm b chng st van CSV2 trn thanh ci u vo trm bin p. v nguyn tc CSV1 c nng lc tho st cao hn v c in p tc ng ln hn CSV2 m bo CSV2 c nhy tho ht sng qu p cn li; Mt khc CSV2 c tc dng d phng cho nhng trng hp hat ng ca CSV1 c th khng c hon ho. hp thu ht phn nh nhn ca sng st truyn n trm (nu c) ngi ta mc thm t in C c gi tr khong (0,25 0,5)F. 10.4. TNH TON TRANG B NI T. 1. Cch thc hin ni t. C hai loi: Ni t t nhin v ni t nhn to.


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Ni t t nhin l s dng cc ng dn nc v cc ng bng kim loi khc t trong t (tr cc ng dn kim loi lng v kh d chy), cc kt cu kim loi ca cng trnh nh ca c ni t, cc v kim loi ca cp t trong t... lm trang b ni t. Khi xy dng trang b ni t cn phi tn dng cc nguyn vt liu t nhin c sn. in tr ni t ny c xc nh bng cch do thc t ti ch hay da theo cc ti liu tnh gn ng. Ni t nhn to thng c thc hin bng cc thp, thanh thp dt hnh ch nht hay hnh thp gc di t 2 3m ng su xung t sao cho u trn ca chng cch mt t khong 0,5 0,7 (ti liu 13). chng n mn kim loi, cc ng thp v cc thanh thp dt hay thp gc c chiu dy khng nn b hn 4mm. Dy ni t cn c tit din tho mn bn c kh v n dnh nhit, chu c dng in cho php lu di. Dy ni t khng c b hn 1/3 tit din dy dn pha, thng dng thp c tit din 120mm2, nhm 35 mm2 hoc ng 25 mm2. in tr tip t ca trang b in ni t khng c ln hn cc tr s quy nh trong cc quy phm. i vi li in trn 1000V c dng in chm t ln, ngha l trong cc mng c im trung tnh trc tip ni t hay ni t qua mt in tr nh (mng in 110KV tr ln) th khi xy ra ngn mch, bo v r le tng ng s ct b phn b hng, thit b in b s c ra khi mng in. S xut hin in th trn cc trang b ni t khi ngn mch chm t ch c tnh cht tm thi.Xc xut xy ra ngn mch chm t ng thi ti thi dim ngi tip xc vi v thit b in c mng in p rt nh nn quy phm khng quy nh in p ln nht cho php m ch i hi bt k thi gian no trong nm ca trang b ni t cng tho mn R 0,5. Trong mng in c dng chm t ln, buc phi ni t nhn to trong mi trng hp khng ph thuc vo ni t t nhin, in tr ni t nhn to khng c ln hn 1. li in c in p ln hn 1000V, trung tnh khng ni t trc tip hoc ni t qua cun dp h quang, thng bo v rle khng tc ng ct b phn hay thit b c dng chm t mt pha. Do vy chm t mt pha c th ko di, in p UN trn thit b chm t cng s tn ti lu di lm tng xc xut ngi tip xc vi thit b c in p. Do quy phm quy nh din tr ca trang b ni t ti thi im bt k trong nm nh sau: Khi dng trang b ni t trung cho c in p di v trn 1000V: 125 Rd Id Khi dng ring trang b ni t cho cc thit b c in p trn 1000V: 250 Rd Id y 125 v 250 l in p ln nht cho php ca trang b ni t. I d dng in tnh ton chm t mt pha. Trong c hai trng hp, in tr ni t khng vt qu 10Trang 160 V Tin Trung - Khoa in


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i vi mng in c in p di 1000V, in tr ni t ti mi thi im trong nm khng c vt qu 4 (ring i vi thit b nh, cng sut tng ca my pht in v my bin p khng qu 100KVA cho php n 10 ). Ni t lp li ca dy trung tnh trong mng 380/220V phi c in tr khng c qu 10. i vi thit b in p cao hn 1000V c dng in chm t b v cc thit b c in p 100V nn s dng ni t t nhin c sn. i vi ng dy ti in trn khng, cn ni t cc ct btng ct thp v ct st ca tt c cc ng dy ti in 35 KV, cn cc ng dy3-20 KV ch cn ni t khu vc c dn c. Cn ni t tt c cc ct btng ct thp, ct st, ct g ca tt c cc loi ng dy mi cp in p khi c t thit b bo v chng st hay dy chng st. in tr ni t cho php ca ct ph thuc vo in tr sut ca t v bng 10-30 . Trn cc ng dy ba pha, in p 380/220V c im trung tnh trc tip ni t, cc ct st, x st cc ct btng ct thp cn phi c ni vi dy trung tnh. Trong cc mng in c in p di 1000V, c im trung tnh cch in, cc ct st v btng ct thp cn c in tr ni t khng qu 5. in tr ni t ch yu c xc nh bng in tr sut ca t, hnh dng kh thc ca in cc v chn su trong t. in tr sut ca t ph thuc vo thnh phn, mt , m v nhit ca t v ch c th xc nh chnh xc bng o lng. Cc tr s gn ng ca ca in tr sut ca t t tnh bng [cm] nh sau: t st , t st ln si ( dy ca lp dt st 1 3m) t vn, t rung t bn Ct t ln ct 1.104 0,4. 104 0,2. 104 (7 10). 104 (3 5). 104

in tr sut ca t khng c nh trong nm m thay i do nh hng ca m v nhit t. Do vy in tr ca trang b ni t cng thay i. V vy trong tnh ton ni t phi dng in tr sut tnh ton l tr s ln nht trong nm. tt= K. do ca t (10-10) y K l h s tng cao, ph thuc vo iu kin kh hu ni xy ra trang b ni t: Bng 10-1 cho ta h s K hiu chnh tang cao in tr sut ca t: Loi cc ni t t rt t Cc thanh dt nm ngang(in cc ngang) t su cch mt t 0,3 0,5m Thanh dt chn nm ngang t su cch mt t 0,5 0,8m Cc ng thng ng ng su cch mt t 0.8mV Tin Trung - Khoa in

6,5 3,0 2,0

Loi dt t t trung bnh 5,0 2,0 1,5

t kh 4,5 1,6 1,4

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TRNG HSPKT VINH


2 Tnh ton ni t nhn to. in tr ni t nhn to c thc hin khi ni t t nhin o c khng tho mn in tr cho php ln nht [R]max ca trang b ni t khi in tr ni t nhn to c tnh theo cng thc sau: [ R ] max .R tunhien R nhantao = R tunhien [ R ] max Kiu ni t Chn thng ng, lm bng thp trn, u trn tip xc vi mt t Chn thng ng, lm bng thp trn, u trn nm su cch mt t mt khong Chn nm ngang, lm bng thp dt, nm su cch mt t mt khong Cch t cc Cng thc tnh () in Rd = 0,366 4l tt . lg l d Ch thch

l>d

Rd =

0,366 2l 1 4t + l tt lg + lg l d 2 4t l

l>d

R ng =

0,366 2l 2 tt lg l b.t

Tm chn thng ng, su cch mt t mt khong Vnh xuyn lm t thp dt, t nm ngang, su cch mt t mt khong

R d = 0,25

tt ab

R ng

tt 8D 2 = 2 ln bt 2 .D

l 0,25 2t b-chiu rng ca thanh dt, nu in cc trn c ng knh d th b=2d a v b l kch thc di rng ca tm b- chiu rng ca cc t

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