CHNG:

TNH TON CHNG ST & NI T CHO NH MY.

Tng quan v chng st: St l mt dng phng in tia la trong khng kh vi khong cch rt ln. Qu trnh phng in c th xy ra trong m my ging, gia cc m my vi nhau v gia m my vi t. y ta ch xt s phng in gia my v t. Khi st nh trc tip vo dy dn ca ng dy truyn ti in, cc thit b in hoc vo cng trnh s gy nhng thit hi nh: Gy chy, n, h hi cng trnh. Ph hy thit b, cc phng tin thng tin lin lc. Gy nhiu lon hay ngng vn hnh h thng. Mt d liu hay h d liu. Ngng cc dch v gy tn tht kinh t v cc tn tht khc. Gy cht ngi. Do thit hi do st l rt ln v hu nh khng th d bo trc nn vic phng chng st lun l mi quan tm ca con ngi v tnh ton chng st tr thnh cng vic bt buc ca ngi thit k cung cp in. Cng cn lu rng vic phng chng st khng th t c mc an ton tuyt i m hin nay vic phng chng st ch nhm gim thit hi do st mc thp nht. chng st mt cch ton din v c hiu qu cho mt cng trnh, cn tun theo gii php chng st ton din 6 im nh sau: Thu bt st ti im nh trc to ra kh nng kim sot ng dn st nh xung t. Dn st xung t an ton, khng gy hiu ng phng in th cp trong qu trnh tn st cng nh khng gy nhiu in t cho cc thit b trong vng bo v. Tn nhanh nng lng st vo t vi tng tr ni t nh, tt nht l di 10. ng th cc h thng t, ngn chn s chnh lch in th gia cc h thng t trong qu trnh tn st, khc phc hin tng phng in ngc gy nguy him cho ngi v thit b. Chng st lan truyn trn ng cp ngun, phng h hng cho cc thit b c ni vi chng do qu in p kh quyn hay qu in p ni b. Chng st lan truyn trn ng tn hiu, phng h hng cho cc thit b h thng lin lc nhy cm nh: in thoi, Internet, o lng, iu khin, Hin nay trn th trng c nhiu loi thit b chng st nh kim Franklin, kim phng in sm ESE, kim phng x, tia Laser, Mt s hnh nh ca kim phng in sm :

1. Kim phng in sm ESE:

2. Kim thu st phng in sm ESE (Early Streamer Emission) ca hng Irico Lighting Technologies (Autralia) vi loi kim Dynasphere:

Hnh nh ca kim FRANKLIN :

y l thit b thu st ph bin nht v c l l ni ting nht trong lch s. Kim thu st c t ti mt hoc nhiu im nh cao ca mt cng trnh kin trc. Phm vi bo v ca n c tnh ton nm trong vng trn bn knh tng ng vi chiu cao ca v tr tr t kim so vi mt t. Kim thu st Franklin dng l tng bo v nhng ni m c mt phn cu trc nh hn ln cao trong phm vi cn bo v.

Kh nng bo v ca kim :

Tong ti ny chng ta khng dung nhng phng php dng kim phng in sm ESE m chn phng php kim fanklin : Ni dung ca phng php franklin l khi c m my tch in tch m i qua trn nh ca mt ct thu li (c chiu cao i vi mt t v c in th ca t xem nh bng khng) nh cm ng tnh in th nh ca ct thu li s np mt in tch dng .do nh ct thu li nhn nn cng in trng trong vng ny ln .iu ny s d dng to nn mt knh phng in d dng phng in t u ct thu li n dm19 my tch in m iu ny s to ra mt dng in t m my phng xung t. Phm vi bo v ca ct thu li l hnh nn trn xoay c tit din ngang l nhng ng trn cao hx c bn knh Rx tr s bn knh bo v xc dnh theo cng thc n gin sau:

r x = 1.6 . ha . p .ha l chiu cao hiu dng ca ct thu li p l h s : p=1 khi h 30 p= 5.5/ nu30m < h < 100m b ngang hp nht bx tnh theo cng thc :

2bx = 4. rx .Trong : ha l chiu cao hiu dng ca ct thu li rx l bn knh bo v c ca ct thu li c chiu cao ha trong bi ta c : phn xng scck c cc s liu v kch tht nh sau : di : 33m rng : 11m cao : 5m Vi nhng s liu cu bi a rat a thy khng th p dng phng php mt ct thu li bi v nu chn mt ct thu li th chiu cao hiu dng ca ct thu li Theo tnh ton s b s l 12,8m r rang phng php ny khng tha v chiu cao vt thu li qu quy nh .

Do ta chn phng n dung 4 ct thu li t thnh hnh ch nht c chiu di l 23m v chiu rng l 5m T suy ra: khong cch t mt ct bt k ti cnh di gn ct nht l 3m khong cch t mt ct bt k ti cnh rng gn ct nht l 5m Ta c bn knh ti thiu cn c bo v l Rx : Rx = = 5.83 m

Vi Rx = 5.83 m ta c t l :

=h=

= 0.86 Rx = 5.83

= 1.6 . 5.6 . 1 .

= 6.1 m

Tnh ton nh hng ca 2 ct thu li nm ngang : Hai ct thu li c tc dng tng h khi tha iu kin : a / ha 7 Trong trng hp ny ta c 23 / 5.6 = 4.1 < 7 (tha iu kin tng h ) b ngang hp nht bo v c ca hai ct thu li nm ngang :

2bx = 4. rx .Vi a = 23 m

= 4 . 6.1 .

= 7.13 m

h a = 5.6 m Suy ra bx = 7.13 / 2 = 3.56 m > 3m (khong cch t mt ct thu li n cnh di gn nht ) cao thp nht ca cung trn : ho = h (a/7) = 10.6 ( 23 / 7 ) = 7.31 m > hx = 5m

Tnh ton nh hng ca 2 ct thu li nm dc : Hai ct thu li c tc dng tng h khi tha iu kin : a / ha 7 Trong trng hp ny ta c 5 / 5.6 = 0.9 < 7 (tha iu kin tng h ) B ngang hp nht bo v c ca hai ct thu li nm dc :

2bx = 4. rx .

= 4 . 6.1 .

= 11.2 m

Vi a = 5 m h a = 5.6 m Suy ra bx = 11.2 / 2 = 5.6 m > 5m (khong cch t mt ct thu li n cnh rng gn nht ) cao thp nht ca cung trn : ho = h (a/7) = 10.6 ( 6 / 7 ) = 9.74m > hx = 5m Ngoi ra ta cn kim tra iu kin tng h trong trng hp nhiu ct thu li c th l trng hp 4 ct thu li t thnh hnh ch nht nh trong bi : ta c iu kin nh sau : D 8 . (h hx ) Vi D l khong cch ca hai ct thu li cheo nhau : Trong tng hp ny D = = 23.53 m

Ta c 8 . (h hx ) = 8 . ( 10.6 5 ) = 44.8 m Vy D = 23.53 m < 44.8 m ( tha diu kin ) cao thp nht ca cung trn : ho = h (a/7) = 10.6 ( 23.53 / 7 ) = 7.23 m > hx = 5m

Vy tm li ta chn phng php 4 ct thu li t theo hnh ch nht Chng st cho ng dy dn vo trm bin p phn xng : Do khong cch t ngun vo trm khng xa lm v c th trong l 100m nn ta khng cn s dng DCS (dy chng st ) bo v cho ng dy . cc ng dy trn khng d c c bo v chng st hay khng th cc thit b in c ni vi chng iu chu tc dng ca song st truyn t ng dy n .bin ca qu in p kh quyn c th ln hn in p cch in ca thit b dn n chc thng cch in , ph hoi thit b v mch in b ct ra . do vy bo v cc thit b in trong trm bin p trnh song qu in p truyn t ng dy vo thit b ta phi dung thit b chng st .cc thit b chng st s h thp bin sng qu in p n tr s an ton cho cch in cn c bo v . Trong bi ta s dng 2 CSO (chng st ng ) t trt my ct in ca mi nhnh my bin p v 2 CSV (chng st van ) t thanh gp ca mi my bin p bo v khi cng in p lan truyn do st nh vo ng dy cao hn in p ng li ca CSO .

H thng ni t: 1. Phn tch cc h thng ni t hin i: Theo tiu chun NFC 17-102 ca Php. m bo tn nhanh dng st vo t th in tr ca h thng ni t R 10. nc ta gim in tr ni t thng dng cc bin php n gin nh tng s lng cc, kch thc cc, ci to t bng dng than, mui. Cc bin php ny tuy d lm nhng hiu qu k thut khng cao v gp mt s hn ch. in cc ni t hin nay s dng ph bin nht l cc thp bc ng ng knh 13mm, 16mm, hay 23mm v n c gi thnh r, dn in tt, chng n mn cao do tc ng ca kh quyn hay t bc xung quanh v bn vng trong kt ni v lp t. Lin kt gia cc b phn ni t c th thc hin bng cc c xit v bng mt lp chng thm chng n mn. Gn y cng ngh hn ha nhit CADWELD c s dng ni kt cc b phn ni t vi cht lng cao. Hn ha nhit CADWELD c kh nng to ra mi ni gia cc phn t ng-ng, ng-thp m khng cn nng lng ngoi hay ngun nhit. Cc mi ni hn ha nhit CADWELD c cc c im sau: Tn dng in hiu qu hn dy dn. Khng h hng hay gim cht lng theo thi gian. Mi ni hn ha nhit CADWELD l mi ni phn t nn khng b hng hay n mn. Chu c dng s c lp li, khng i hi k nng c bit thc hin mi hn ha nhit CADWELD. Thit b nh, khng i hi ngun ngoi v khng t tin. t c tr s in tr yu cu trong vng c in tr sut cao lun l mt vn kh khn, yu cu in tr t ca h thng R 10 nu ch s dng n thun cc cc ng, l hoc li nhm c th khng nhn c kt qu nh mong mun. V vy cn phi s dng cc loi ha cht gim in tr t. Cng ty Irico Lighting Technologies cung cp 2 loi ha cht gim in tr t: EEC v GEM. Ha cht gim tr t c cc u im sau: Bn vng v khng cn bo tr. Gi in tr t hng s n nh vi thi gian. Khng b phn hy hay mc ra. Thch hp cho vic lp t ni t kh hay t bn.

Khng ph thuc vo s hin din ca nc duy tr tnh dn in ca n. 2. Tnh ton ni t cho nh my: Ni t chng st : Trong trng hp ny nh my cn bo v c xy dng trn vng t loi t pha st nn in tr sut ca t c gi tr = 0.4 cm.104 nh trong bng sau:Loi t Nc bin Than bn t st Nc sng, ao h t st thnh t v ln dy t 7 10m pha di c hoc dm. t pha st t pha st khong 50% t st, to thnh lp dy t 1 3m trn b mt t: Pha di c dm t en t pha ct Ct in tr sut (cm.104) 0.002 0.01 0.08 0.7 0.1 0.8 0.4 1.5 in tr sut khi thit k s b (cm.104) 0.01 0.02 0.04 0.05 0.07 1

0.098 5.3 v ln hn 1.5 4 v ln hn 4 10 v ln hn

1 2 3 7 10 20 20 40

t vi, vi, ct ht to ln vn, si , vn

m bo cho dng in st tn nhanh vo t chn gi tr in tr ca h thng ni t l R 10 . m bo cho h thng thot st v h thng ni t lm vic trong phn xng . in tr sut ca t = 0.4 cm Tra bng bn di chn h s thay i in tr sut theo ma Km = 1.5 Loi cc ni t t rt t Loi t t t t kh

trung bnh Cc thanh dt nm ngang (in cc ngang) su cch mt dt 0.3m 0.5 m Thanh dt chon su 0.5m 0.8m Cc thng ng ng su cch mt t ln hn 0.8m 6.5 3.0 2.0 5.0 2.0 1.5 4.5 1.6 1.4

Khi tt = . Km = 0.4.104.1,5 = 0.6*104 (cm). Chn phng n ni t l cc v thanh, chn cc thp trn c ng knh dc = 16mm, di l = 2.5m, chn su tc = 0.8m, cc cc chn cch nhau khong a = 5m. Chn thanh ngang bng thp trn, ng knh. 8mm Cch chn cc l chn thnh mach vng in tr ni t ca 1 cc: R1c = Trong tt =0. 6*104 cm: in tr sut ca t dc = 16mm : ng knh ngoi ca cc l = 2,5 m: chiu di ca cc tc = 0,8 m: chn su cc t = tc +l = 2,05m 2

Vy in tr ni t ca mt cc l : R1c = Xc nh s b s cc: ta chn n = 4 cc tra bng 10-3 sch cung cp in 1 ta tm c cc h s sau :

c

= 0,78 h s s dng cc. = 0,55 h s s dng thanh.23.11 = 7.4 () 0.78.4

t

in tr khuch tn ca 4 cc l: Rc = =

Chiu di ca tt c cc cc ni li vi nhau: L = n .a = 20 m. in tr ca thanh ni nm ngang: Rlt = Trong : tt = 0.6. cm. l = 2000 cm: chiu di to nn bi cc thanh ni. dt = 0.8cm: b rng thanh ni. t = 205cm Khi : ttlg

Rlt =Rt =

.0.6.=

lg

= 4.82 ()

in tr tn ca thanh ni:4.82 = 8.76 () 0.55

Vy in tr ni t ca h thng l: Rht =Rc .Rt 7.4 * 8.76 = = 4 () < R = 10 () Rc + Rt 7.4 + 8.76

Nh vy h thng ni t chng st trn tha yu cu. Ni t lm vic cho xng : Ta c in tr ni t trong trng hp ni t lm vic cho phn xng l Rd 4 t trong hku vc phn xng nh chn trn u l t pha st Khi tt = . Km = 0.4.104.1,5 = 0.6*104 (cm). Chn phng n ni t l cc v thanh, chn cc thp trn c ng knh dc = 16mm, di l = 2.5m, chn su tc = 0.8m, cc cc chn cch nhau khong a = 5m. Chn thanh ngang bng thp trn, ng knh. 8mm Cch chn cc l chn thnh mach vng in tr ni t ca 1 cc: R1c = Trong tt =0. 6*104 cm: in tr sut ca t

dc = 16mm : ng knh ngoi ca cc l = 2,5 m: chiu di ca cc tc = 0,8 m: chn su cc t = tc +l = 2,05m 2

Vy in tr ni t ca mt cc l : R1c = Xc nh s b s cc: ta chn n = 4 cc tra bng 10-3 sch cung cp in 1 ta tm c cc h s sau :

c

= 0,78 h s s dng cc. = 0,55 h s s dng thanh.23.11 = 7.4 () 0.78.4

t

in tr khuch tn ca 4 cc l: Rc = =

Chiu di ca tt c cc cc ni li vi nhau: L = n .a = 20 m. in tr ca thanh ni nm ngang: Rlt = Trong : tt = 0.6. cm. l = 2000 cm: chiu di to nn bi cc thanh ni. dt = 0.8cm: b rng thanh ni. t = 205cm Khi : ttlg

Rlt =Rt =

.0.6.=

lg

= 4.82 ()

in tr tn ca thanh ni:4.82 = 8.76 () 0.55

Vy in tr ni t ca h thng l: Rht =Rc .Rt 7.4 * 8.76 = = 4 () < R = 10 () Rc + Rt 7.4 + 8.76

Nh vy h thng ni t lm vict trn tha yu cu.