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.
