1

M U

I. L do chn ti:

hn 10 nm, sau khi B trng B Nng lng (c) c quyt nh s

1867NL/KHKT ngy 12/05/1994 v vic ''Ban hnh quy nh cc tiu chun k

thut cp in p trung th 22kV". Cc Cng ty in lc cng nh cc n v hot

ng trong cc lnh vc quy hoch, thit k, ch to thit b, qun l vn hnh v

ang nghin cu cc gii php kinh t k thut p ng yu cu thng nht cp

in p trung th cho c nc. Song song vi qu trnh i mi ca Vit Nam ; tc

GDP khng ngng tng trng th tc pht trin ph ti in nc ta cng

tng nhanh chng. Vic thng k, tnh ton hiu qu kinh t - k thut trong u t

ci to, nng cp, xy dng mi li in 22kV theo Quyt nh trn khng phi

lc no cng c ch trng.

Hin nay, c mt s cng trnh nghin cu v ti ni t cho cc trm bin

p trung gian (TBATG) 110/22kV hoc 35/22kV v gn y, vo cc ngy 18-

19/03/2004 Tng Cng ty in lc Vit Nam t chc hi tho v '' St v chng st

trn h thng ng dy ti in Vit Nam''. Tuy nhin, cc nghin cu v ni t

v chng st tp trung ch yu li in truyn ti t 110kV tr ln, cn li in

phn phi (LPP), th cha c quan tm ng mc. Vi tc pht trin khoa

hc k thut nhanh chng nh hin nay th mt s tnh ton k thut in, quan

im, quy trnh - quy phm trang b in trc y c th hoc dng nh khng

cn ph hp na. Cu hi t ra khi nghin cu p dng Tiu chun k thut LPP

trung p vo trong thc tin l: C nht thit trm bin p phn phi (TBAPP) no

cng phi c mt b chng st van (CSV) t trc my bin p (MBA) khng?

Nht l ti nhng thnh ph ln nh TP.H Ch Minh, H Ni, Hi Phng,

Nng... ni c rt nhiu nh cao tng th xc sut st nh trc tip vo ng dy

hu nh khng c. Theo s liu thng k s c LPP ca cc in lc ti mt s

tnh, thnh ph khu vc min Trung cc nm 2001, 2002, 2003 v 2004 th MBA b

s c qu in p (QA) do st nh l rt t, thm ch khng c nh trung tm

2

thnh ph Hu, Nng, Quy Nhn ; tuy nhin nhng s liu thng k trn ch

mang tnh tham kho v cn phi phn tch nh gi mt cch khoa hc.

Hnh I.1 : B tr CSV cp ngm Hnh I.2: B tr CSV trc MBAPP

Nu gii quyt vn kinh t bng cch b CSV pha trung p MBA phn phi

th vi phm iu IV.2.155 trong quy phm trang b in l ''trm phn phi c ni

vi ng dy trn khng phi c bo v bng CSV t thanh ci hoc

MBA". Mt vn na cn t ra l: Theo iu 11 ti quyt nh s 1867NL/KHKT

ngy 12/05/1994 ca B Nng lng (c) th in tr ni t khng vt qu 10 i vi TBA cng sut n 100kVA v khng vt qu 4 i vi TBA cng sut trn 100kVA, vn ch ranh gii gia 4 v 10 qu ln nh vy liu c mt tr s in tr ni t trung gian 5, 6... cho cc TBA 160kVA, 250kVA... 1000kVA hay khng?

Nghin cu nhm gii quyt hai vn nu trn l l do chn ti v tnh kinh t v m quan th khi a vo p dng.

3

II. i tng v phm vi nghin cu:

i tng nghin cu : Sng qu p trn ng dy v TBAPP, trn cch in

v thit b chng st ; cc vn lin quan n gii php ni t v in tr ni t

TBAPP.

Theo iu IV.2.134 v iu IV.2.155 Quy phm trang b in Phn IV _Thit

b phn phi v TBA _Tiu chun ngnh 11TCN-21-84 th ''Khng cn bo v

chng st nh thng i vi TBA in p 20-35kV ngoi tri c MBA cng sut

mi my n 1000kVA". Mt khc, theo iu II.5.57 Quy phm trang b in Phn

II _H thng ng dn in _Tiu chun ngnh 11TCN-19-84 th ''ng dy trn

khng in p t 22kV tr xung khng yu cu nhng bin php c bit trnh qu

in p kh quyn". Nh vy, theo Quy phm ca trang b in ta c th nghin cu

ni t LPP 22kV trong phm vi ng dy trung p 22kV khng treo dy chng

st v TBA khng cn phi bo v chng st nh thng.

Phm vi nghin cu : Tp trung vo cc gii php lin quan n ni t v cch

in ca thit b nh MBA, CSV, s cch in ... t ra cc gii php kinh t,

k thut trong ni t v chng st ti cc TBAPP 22kV thuc khu vc min Trung,

Ty Nguyn ; l ni ang rt kh khn v kinh t v kh t v tr s tip a khi

tin hnh ni t an ton.

III. Mc tiu v nhim v nghin cu:

Hin nay c mt s ti nghin cu v tnh ton, chn la CSV v cc

nghin cu v tnh ton sut ct ng dy, tnh ton in p bc, in p tip xc

cho cc TBATG 35/22kV, 110/22kV... Do , ti ny s khng i su vo cc

vn c nghin cu nu trn m ch tp trung vo tnh kh thi v gii php

k thut-kinh t khi b tr CSV trong LPP 22kV v in tr ni t ca cc

TBAPP. Cn ni thm rng, mc d gi tr kinh t ca mt thit b hay mt b ni

t TBAPP khng ln nhng s lng TBAPP li rt ln v a dng ; tit kim u

t vi triu ng cho mt TBA nhng vi s lng hng ngn TBAPP th nn quan

tm.

4

Mc tiu ti: Nghin cu v mt l thuyt v thc tin cc vn lin quan n

bo v chng st v gii php ni t cho LPP 22kV khu vc min Trung. T

, ra gii php k thut ph hp vi thc t, cho php y nhanh tin thi

cng v nng cao hiu qu u t cho LPP.

Nhim v nghin cu:

1, Cc vn v ni t, chng st ng dy v TBAPP.

2, Tnh ton gi tr in tr ni t v b tr chng st van ti TBAPP.

3, Hiu qu kinh t trong vic ng dng ti vo thc t.

IV. ngha khoa hc v tnh thc tin ca ti:

ngha khoa hc:

1, H thng cc yu cu k thut v chng st van v ni t ca LPP 22kV

2, xut gi tr in tr ni t cc TBAPP.

3, xut gii php b tr chng st van cho cc TBAPP.

Tnh thc tin ca ti:

1, Gim vn u t do chn gi tr in tr ni t ph hp vi tng gam cng

sut t MBA ; t y nhanh tin thi cng, sm a cng trnh vo s dng.

2, Gim vn u t do b tr hp l chng st TBAPP ti mt s thnh ph ln

in hnh ; hn ch thi gian mt in do phi lp t, bo dng nh k ng thi

lm tng v m quan thnh ph.

V. Tn ti:

Xut pht t cc vn nu, ti c t tn l : "Nghin cu, tnh ton ni t v chng st li in phn phi 22kV nng cao hiu qu kinh t". VI. B cc lun vn:

Lun vn c bin ch thnh bn chng : M u

Chng 1: Li in phn phi trung tnh ni t.

Chng 2: Ni t trm bin p phn phi.

Chng 3: Qu in p ng dy v trm bin p phn phi.

Chng 4: Hiu qu kinh t trong vic ng dng ti vo thc t.

5

CHNG 1

LI IN PHN PHI TRUNG TNH NI T

1.1 Hin trng li in phn phi :

1.1.1_ng dy trung p:

Theo thng k ca cc in lc khu vc min Trung - Cng ty in lc 3,

quy m ng dy trung p li in phn phi tnh n 28/02/2005 nh sau :

Bng 1.1 : Thng k chiu di li in trung p min Trung

STT in lc Chiu di li in phn phi (km)

22kV 15kV 10kV 6kV ...

1 Qung Bnh 357,8 445,1 365,1

2 Qung Tr 34,9 987,0 46,1

3 Tha Thin-Hu 321,7 296,8 388,4 199,2

4 Nng 316,0 211,0 47,6

5 Qung Nam 494,4 1.335,3

6 Qung Ngi 830,3 700,7

7 Ph Yn 1.124,1 143,6

8 Bnh nh 708,9 87,9 676,7 19,5

9 Khnh Ho 459,2 685,8 126,6

10 Gia Lai 1.950,2 168,6 44,7 9,0

11 Kon Tum 1.088,6 5,2

12 k Lk 1.673,2 438,6

13 k Nng 440,4 211,1

Tng cng 9.799,7 3.634,9 3.191,6 813,1

Qua bng thng k trong thi gian qua ti khu vc min Trung ni ring,

vic thc hin chuyn i, ci to, xy dng mi li in phn phi thnh cp in

p thng nht 22kV c hnh thnh r rt. i vi cc d n ci to, xy dng

6

mi LPP sau cc TBATG 110/22kV, 35/22kV... th bt buc thit k theo cp

in p 22kV ; i vi cc ng dy sau cc TBATG 35/6kV, 35/10kV, 35/15kV

th c thit k theo cp in p 22kV d phng cho vic chuyn i cp in

p sau ny. Do vy, mt s ng dy trung p ang vn hnh cp in p 6kV,

10kV hoc 15kV nhng thc cht c thit k lp t theo cp in p 22kV

trung tnh ni t trc tip.

Dy dn ng dy trung p ph bin l loi AC50/8, AC-70/11, AC-95/16,

AC-120/19... Mt s thnh ph ln min Trung thng s dng cp nhm, bc

cch in XLPE-12,7kV hoc cp ngm 22kV rut ng.

1.1.2_Trm bin p phn phi :

Theo thng k ca cc in lc khu vc min Trung - Cng ty in lc 3,

quy m trm bin p phn phi tnh n 28/02/2005 nh sau :

Bng 1.2 : Thng k trm bin p phn phi min Trung

STT in lc S trm S MBA Cng sut t (MVA)

1 Qung Bnh 962 968 168,77

2 Qung Tr 868 868 118,53

3 Tha Thin-Hu 1.257 1.288 273,64

4 Nng 1.546 1.666 535,35

5 Qung Nam 1.500 1.510 257,26

6 Qung Ngi 1.156 1.184 174,62

7 Ph Yn 1.042 1.043 154,54

8 Bnh nh 1.712 1.712 320,29

9 Khnh Ho 1.997 2.049 342,65

10 Gia Lai 1.555 1.571 200,81

11 Kon Tum 711 711 66,88

12 k Lk 1.636 1.656 256,69

13 k Nng 462 462 59,22

Tng cng 16.404 16.692 2.929,34

7

Cng vi ng dy trung p 22kV, cc TBAPP cng c ci to, xy

dng mi theo tiu chun 22kV. Mt s MBA ph ti sau cc TBATG_35/15kV,

35/10kV, 35/6kV... khi ci to v xy dng mi u c 2 cp in p 15(22)kV,

10(22)kV, 6(22)kV.

Cc TBAPP khu vc th, thnh ph ph bin l loi MBA 3 pha ; cc

khu vc nng thn vng su, vng xa th ph bin loi MBA 1 pha.

Hin nay, do cc Cng ty in lc cha thc s quan tm thng nht gam

cng sut t MBA nn trn li in vn cn rt nhiu chng loi khc nhau :

MBA 3 pha c gam 31,5kVA ; 37,5kVA ; 50kVA ; 75kVA ; 100VA ; 125kVA ;

160kVA ; 180kVA ; 200kVA ; 250kVA ; 320kVA ; 400kVA ... MBA 1 pha th c

gam cng sut 5kVA ; 10kVA ; 12,5kVA ; 15kVA ; 20kVA ; 25kVA ; 31,5kVA ...

Vic MBA c nhiu gam cng sut khc nhau s dn n c nhiu t in khc

nhau ; kt cu TBA khc nhau v do trong tnh ton, nghin cu cng nh trong

qun l vn hnh s phc tp, kh khn hn.

ng dy 22kV

Dy qun pha s cp MBA mng

22kV c trung tnh ni t trc tip

c th l dy qun 3 pha hoc 1 pha. FCO

Dy qun 3 pha c th u Yo, Y

hoc . Nu mc mt i xng ti khng vt qu 15%, cho php dy MBA CSV

qun u Y hoc ; khi khng cn to v ni t trung tnh.

Dy qun Yo l cn thit nu pha th AT

cp c ti 1 pha chim u th v mc H p

mt i xng ti vt qu 15%.

Khi , trung tnh ni t trc tip Hnh 1.1: S nguyn l TBAPP

vi in tr ni t cho php t 4 n 10. iu ny khng qu khc khe v c th t c, nhng i vi a cht khu vc min Trung th rt cn quan tm

8

nghin cu m rng gii hn cho php ny. Vn ny s c tnh ton nghin

cu trong Chng 2.

Hin nay, ch yu li in phn phi trung p do ngnh in qun l.

Khi nghin cu, tnh ton li in trung p trong n ny cng ch hn

ch mang tnh in hnh vi cp in p 22kV trung tnh ni t trc tip v vi

nhng gam cng sut MBA ph bin.

1.1.3_ng dy h p :

ng dy h p Vit Nam ch yu l 3 pha 4 dy, trung tnh ni t trc

tip. Dy dn ph bin l loi dy nhm, dy nhm li thp, dy ng ; cc khu

vc th, thnh ph th s dng dy bc AV hoc cp vn xon ABC m bo

m quan v hnh lang cho cy xanh th. Tit din dy dn thng chn thp nht

l 3550 mm2 khu vc ng dn c nh huyn l, trung tm x, th x, th trn, thnh

ph th ch yu do ngnh in qun l li in h p n tn h tiu th. Do vy,

tiu chun k thut thng c m bo v an ton cho ngi s dng in.

cc khu vc nng thn, min ni th do a phng qun l. Do vy, li

in h p thng c nt vi nhiu quy cch k thut khc nhau, d mt an ton ;

kh nng ti u t v ci to, nng cp km.

Trong thi gian t nm 1998 n nay, ngnh in c nhiu c gng u t

li in h p n tn h tiu th khu vc nng thn, min ni theo ch trng

xo i, gim ngho ca Chnh ph bng ngun vn khu hao c bn hoc vn vay

u i t Ngn hng th gii WB.

1.2 Vn bo v ni t v chng st li phn phi 22kV:

Theo Quy phm trang b in 11 TCN-18-84 n 11 TCN-18-21 c ban

hnh ti quyt nh s 507L/KT ngy 22/12/1984 ca B in lc (c) v quyt

nh s 1867NL/KHKT ngy 12/9/1994 ca B Nng lng, ta c th tm lc cc

quy nh k thut hin hnh i vi li phn phi 22kV trung tnh ni t trc tip

nh sau :

9

Hnh 1.2 : Recloser vi 2 b CSV 2 u Hnh 1.3 : B tr 3 b CSV trn Ct thp

on DK vo TBA khng cn bo v bng dy chng st (iu IV.2.155).

Khng cn bo v chng st nh thng i vi TBA ngoi tri c cng sut mi

MBA n 1000kVA (iu IV.2.134).

Tr s in tr ni t khng vt qu tr s 10 i vi MBA n 100kVA v khng vt qu 4 i vi MBA trn 100kVA (iu I.7.37 hoc iu 11). TBAPP c ni vi DK phi c bo v bng CSV t thanh ci hoc MBA.

Nu pha h p ni vi DK th phi dng CSV c pha h p (iu IV.2.155).

Xut pht t nhng quy nh trong cc quy phm nu trn, vic tnh ton, b

tr CSV u phi tun th nh th hin thc t trn hnh I.1, I.2, 1.2 v 1.3 ti

thnh ph Nng, c th nh sau :.

Hnh I.1 th hin b tr 1 b CSV cho cp ngm trung p ti ng Phan

Chu Trinh. Hnh I.2 th hin b tr 1 b CSV trc MBA phn phi ti ng L

Li. Hnh 1.2 th hin v tr t 2 b CSV ti 2 u my ct recloser trn ng L

Li v hnh 1.3 th hin b tr 3 b CSV cho 3 tuyn cp ngm trung p ti mt ct

st trn ng Trng N Vng.

10

S phn b CSV trn mt xut tuyn cng tun th theo quy phm, nh Hnh

1.4 cho mt on ca xut tuyn 471 E11 di 1300 mt b tr t ng t ng L

Thng Kit n Ng Nm Phan Chu Trinh Hong Diu. trn ng L Li, i

trong ni th thnh ph Nng vi s lng l 12 b, tng ng vi 36 CSV.

H.Phng L Lai/Ba nh

Ng 5

P.C.Trinh. 50 190 120 100 110 120 230 45 250 40 45

v tr ct

48 49 53 55 57 61 64 68 69 77 78 79

H.Vng L Dun Q.Trung L.T.Trng L.T.Kit

Hnh 1.4 : Hin trng b tr CSV trn on tuyn 471 E11 Nng

Vic b tr CSV ti xut tuyn trn cho ta suy ngh n s cn thit t n

12 b CSV bo v MBA v my ct recloser hay khng? cng nh mc nh

hng n v m quan ca thnh ph nh th no?

Khi xem xt h thng tip a hin trng, ta thy i vi ng dy trung p

22kV th tip a thc hin ton tuyn trong trng hp s dng li 3 pha 3 dy

ly t lm dy trung tnh v tip a lp li i vi li 3 pha 4 dy [4]. Dng ni

t thng l tia-cc hn hp hoc dng tia tu theo c im a hnh, a cht

cng trnh.

Dng ni t TBA thng l tia-cc hn hp, b tr trong phm hnh lang an

ton ng dy. ng dy h p th b tr tip a lp li, cc v tr r nhnh v v

tr no cui.

11

1.3 Li in phn phi c trung tnh ni t :

1.3.1_Li in trung p 22kV trung tnh ni t :

Li in trung p c trung tnh trc tip ni t ph bin l cc loi sau :

Ni t trc tip c ko dy trung tnh ra mng (M, Canada, c) ; s dng

ph hp cho khu vc c mt ph ti thp trong mt khu vc rng ln.

Ni t trc tip khng ko dy trung tnh ra mng (Anh) ; s dng ph hp

cho khu vc c mt ph ti tp trung hoc c d kin pht trin ph ti 3 pha.

Ti Vit Nam, do mi a vo thng nht cp in p 22kV trung tnh trc

tip ni t nn li in trung p cn tn ti theo thi quen trc y ; c th

min Nam s dng li in trung p 3 pha 4 dy (c ko dy trung tnh ra mng),

min Bc v min Trung s dng ch yu li in trung p 3 pha 3 dy, s dng

t lm dy trung tnh.

Tuy nhin, ni chung li in trung p trung tnh trc tip ni t c nhng

u im sau :

- n nh c in th trung im, loi tr c kh nng xut hin h

quang duy tr cng vi nhng hu qu ca chng.

- Tit kim c vn u t nh gim c s lm vic ca cch in khi

chm t v khi c qu trnh qu .

- Nng cao tin cy lm vic ca thit b do c d tr cch in ln hn so

vi cc ch ni t trung im khc.

- Thun li cho s lm vic ca h thng r le.

Nhc im chnh ca h thng ny l :

- Tnh lin tc trong cung cp in gim, c th khc phc bng cch trang b

thm cc thit b t ng ng li hoc t thm h thng d phng hn ch thi

gian ct in, song vic lm s lm gia tng vn u t.

- Vn u t cng tng do phi chi ph thm cho h thng bo v r le, bo

v cc ng dy thng tin.

12

- Dng in ngn mch 1 pha c th t n gi tr rt ln (vt qu dng

in ngn mch 3 pha) gy ra lc in ng ph hoi lan truyn trn phn ln h

thng, ph hoi s lm vic ng b ca h thng.

- Gy nguy him cho ngi v sc vt v in p bc v in p tip xc c

th rt ln khi ngn mch 1 pha chm t.

- Chi ph cho h thng ni t gia tng ng k.

1.3.1.1_Li in 22kV ni t trc tip c ko dy trung tnh:

Dy trung tnh c ko ring cng dy pha hoc s dng dy trung tnh

ng dy h p v c tip a lp li ; mc cch in l in p pha. Trc chnh

l 3 pha 4 dy ; nhnh r l 3 pha 4 dy hoc 2 pha 3 dy hoc 1 pha 2 dy.

S pht trin ph ti mt pha phi c tnh ton ton b h thng c

cn bng ba pha, trnh gy hng hc cho cc MBA ba pha hoc my in quay ba

pha. Khi ny, in th trung tnh lun bng in th t v phi thc hin ni t lp

li dy trung tnh dc theo chiu di ng dy. Trng hp l tng th tng tr

ri theo chiu di ca dy trung tnh bng khng v khng lm tng th trung tnh.

Trong thc t, khi xt im xa ngun th tng tr ca dy trung tnh khc khng

v dng s c s c phn ra gia dy trung tnh v rt nhiu im ni t. in

p xut hin dy trung tnh rt kh tnh ton v ph thuc vo in tr sut ca

t, l thng s rt kh kim tra c.

Thng thng, ngi ta kt hp vic s dng cc nhnh r 1 pha gim chi

ph u t ban u cho vic xy dng mng ng dy trung h p (THA). Dng

in s c mt pha theo t tr v MBA s gim i nhiu v phn ln dng in s

c s tr v theo dy trung tnh nh m mc nguy him do in p bc hay

in p tip xc cng gim i.

1.3.1.2_ Li in 22kV ni t trc tip khng ko dy trung tnh :

Nh trng hp nu trn, in th dy trung tnh bng in th t. Tuy

nhin, trung tnh ca mng ch c ni t chc chn ti cc TBA. Mc cch

in ca thit b trn mng ch c gii hn in p pha. Trc chnh l 3 pha 3

dy ; nhnh r l 3 pha 3 dy hoc 2 pha s dng in p dy.

13

i vi mng ny, s c mt pha bao gm c s c qua in tr trung gian

d c pht hin bng cch kim tra dng s c tr v qua t v trung im

MBA. Bo v phn ng theo dng th t khng nn n gin v tin cy.

nh gi tnh kinh t ca m hnh li in trung p 22kV_3 pha 4 dy

hay 3 pha 3 dy ly t lm trung tnh ta cn phi nghin cu, so snh, nh gi tht

c th mi c kt lun a vo ng dng trong thc t.

1.3.2_Li in h p trung tnh ni t :

Phn ln cc thit b, kh c in c ni n li in ba pha c trung tnh

ni t, v li in ny rt kinh t v v mt k thut c nhiu u im hn l cc

loi li in mt pha. Cc LPP cho cc khu dn c , khu cng nghip ring bit

cng l li in ba pha ; nhng mng in mt pha t cc khu dn c l cc nhnh

ca li in ba pha.

Do li in lm vic vi im trung tnh c ni vi t nn thng quan

tm n in p dy Ud v in p pha Uf.

Li in h p thng gn lin vi sinh hot thng xuyn ca con ngi

khi tip xc vi cc kh c in cm tay, nn vn an ton in khi con ngi tip

xc vi in thng nhiu ngi c quan tm, nghin cu.

Tm li, khi nghin cu LPP hin trng cng nh nhng quy trnh, quy

phm hin hnh v nhng u im ca LPP ba pha trung tnh ni t trc tip ta

c th thy rng xu th p dng Tiu chun k thut cp in p trung p 22kV v

li in h p 3 pha 4 dy l ph bin. Trong phm vi ti ; vi vic ch nghin

cu cc gii php k thut ni t v chng st cho LPP 22kV ni ring, c th hy

vng gp phn nh trong vic h thng ho quy nh k thut cho LPP. Kt qu

nghin cu mi ch l s xut m rng nhng quy nh c p dng trong c

nc.

14

CHNG 2

NI T TRM BIN P PHN PHI

2.1 Cc gii php ni t:

Tc dng ni t l tn vo t dng in s c (r cch in ngn mch,

chm t hoc dng in st) v gi cho in th trn cc phn t c ni t thp.

Theo chc nng ca n, ni t trong h thng in c ba loi:

2.1.1_Ni t lm vic : m bo s lm vic ca trang thit b ni t trong

cc ch bnh thng v s c theo cc ch quy nh.

i vi LPP 22kV trung tnh ni t trc tip, dng ngn mch mt pha

thng rt ln, c th gy nn h hng thit b v nh hng ln n iu kin an

ton cho con ngi trong phm vi s c. Do vy, khi chm t mt pha trong LPP

22kV c xem l s c v phi c cch ly ra khi li in. Yu cu ca ni t

lm vic l dng in chm t t im s c n im ni t ca trung tnh

MBA, phi lin tc, n nh, c kh nng chu c dng ngn mch, c tng

trwotr nh khng gy phng in cho thit b c bo v.

2.1.2_Ni t an ton : m bo an ton cho ngi khi cch in ca trang

thit b b h hng gy r in.

Ni t an ton ti cc TBA l ni v kim loi ca thit b vo h thng ni

t, lm gim in p xut hin trn v thit b khi c h hng cch in v

chnh lch in th trn thit b t trn hai chn con ngi n mt gi tr an ton

m con ngi co th chu c.

H thng ni t an ton khng tham gia vo s lm vic ca li in. Hiu

qu ca ni t an ton ph thuc vo kiu li in, in tr dng in i qua,

ng i ca dng in xung t...

Tiu chun chnh xc nh hiu qu ca h thng ni t an ton l gi tr

in p bc v in p tip xc khi s c.

15

2.1.2.1_in p tip xc Utx :

in p tip xc Utx l phn in p t ln thn ngi khi tip xc gin tip.

Trong qu trnh tip xc vi thit b in nu c mch in khp kn qua ngi th

in p ging trn ngi ln hay b tu thuc vo cc in tr khc mc ni tip vi

thn ngi nh gng, ng, thm cch in, nn nh ... Trong thc t, in p tip

xc lun b hn in p ging trn vt ni t.

Ngi ta khng tiu chun ho in p tip xc m ch tiu chun ho in

p cho php i vi t. y l in p ng vi dng in chm t tnh ton i qua

t trong bt c thi gian no trong nm u khng vt qu tr s 250V i vi

in p trn 1000V v 40V i vi thit b in p di 1000V [12].

Gi tr dng in chy qua c th con ngi khi tip xc vi phn t c in

p, phn ln ph thuc vo li in.

2.1.2.2_in p bc Ubc :

in p bc Ubc l in p m con ngi phi chu khi chn tip xc ti hai

im trn mt t hay trn sn, nm trong phm vi dng in chy trong t do c

s chnh lch in th. in p bc c th xut hin gn cc cc tip t v gia

cc cc ny c th c dng in chy, hoc xut hin gn v tr dy ang dn in

b ri xung t.

Trong k thut an ton, khi tnh in p bc thng xt hai im trn mt

t vng c in th ln nht v c khong cch gia hai im t nht l 0,81 mt.

Ngi ta khng tiu chun ho in p bc cho php. Dng in i qua hai

chn ngi t nguy him hn v n khng i qua c quan h hp, tun hon nhng

vi tr s in p bc khong 100250V th cc bp c ca ngi b co git, lm ngi b ng xung v gy nguy him v khi dng in s i t chn qua tay.

Khi ngn mch mt pha ti u cc MBA th dng in ngn mch mt pha

phi nm trong gii hn cho php, m bo an ton cho ngi vn hnh ; tc l in

p bc Ubc v in p tip xc Utx phi b hn gii hn cho php.

Theo tiu chun IEEE/ANSI 80-1986, Dalziel nghin cu v cng b [7] :

16

Sgh = I2gh x t (2.1)

Trong : Sgh l hng s kinh nghim lin quan n nng lng do in git

c xc nh theo thng k.

Igh : dng in gii hn tn s cng nghip khng lm rung tim

trong khong thi gian 0,03 3 giy lin quan n nng lng in c hp th. t l thi gian duy tr dng in qua ngi, cng l thi gian xy ra s

c v c tnh bng thi gian ct bo v ln nht.

Kt qu nghin cu ca Dalziel s ngi c trng lng t 50kg v 70kg

khng b t vong do rung tim l :

Sgh,50 = 0,0135 cho ngi nng 50kg.

Sgh,70 = 0,0272 cho ngi nng 70kg.

Dng in gii hn tn s cng nghip khng lm rung tim i vi ngi

c trng lng 50kg v 70kg l :

t

I gh116,0

50, = (2.2)

t

I gh157,0

70, = (2.3)

ng vi dng in gii hn ny, in p tip xc Utx v in p bc Ubc ln

nht cho php c tnh nh sau :

i vi ngi nng 50kg:

t

CU sstx

)5,11000(116,050,

+= (2.4)

tC

U ssbc)61000(116,0

50,+= (2.5)

i vi ngi nng 70kg:

t

CU sstx)5,11000(157,0

70,+= (2.6)

tC

U ssbc)61000(157,0

70,+= (2.7)

Trong : 1000 [] l tr s in tr ngi

17

S [.m] l in tr sut ca lp dm ri trn mt nn TBA, thng ly S=20003000.m

CS l h s quy i in tr sut, c tnh ton theo cng thc ca

Jackson-Several nh sau :

s

Ss h

C2106,0

1106,01 +

=

(2.8)

l in tr sut tnh ton ca lp t bn di lp dm hs l chiu dy ca lp dm, thng thng l 0,10,15 m

Trong tnh ton, in p tip xc Rch/2 = 1,5CS S (2.9) Trong tnh ton, in p bc 2.Rch = 6 CS S (2.10) in p tip xc ln nht Umax Utx,50_70 (2.11) in p bc ln nht Umax Ubc,50_70 (2.12)

Khi tnh ton dng in chy qua c th con ngi, cn thy rng : in git

ch c th xy ra nu c th con ngi tip xc ng thi vi hai phn t dn in

tt, v c s chnh lch in th gia chng ; v d tip xc vi hai phn t ca

mch in ang lm vic on cch in b hng, tip xc vi mt dy dn on

cch in b hng ng thi ngi ng trn nn t, tip xc vi v ca thit b c

in p do cch in b hng ng thi li tip xc vi phn t kim loi ni vi t,

s tip xc ca hai bn chn hai v tr trn mt t c in th khc nhau, hoc

ng gn ng i ca dng in trong t ...

Khi ch tip xc vi mt cc ca li in, in p m con ngi phi chu

l in p i vi t ti ch xy ra tai nn v s b hn in p gia hai dy pha

cn li.

2.1.2.3_in p cho php:

Do in tr ngi l mt hm s c nhiu bin s m mi bin s li ph

thuc vo nhiu yu t khc nhau nh trng thi vui, bun ca con ngi, s thay

i kh hu, thi tit...Nn vic xc nh gii hn an ton cho ngi khng da vo

dng in an ton m phi theo in p cho php.

18

V phng din k thut phn bit in p cao v in p thp nhng khng

c ngha l in p thp l an ton cho con ngi.

Tiu chun in p cho php mi nc mi khc nhau, nhng Vit Nam

th khng thy xy ra hin tng cht ngi in p 40V nn thng s dng

trong tnh ton kim tra an ton. Yu cu in tr thit b ni t an ton l :

D

cpD I

Ur (2.13)

2.1.3_Ni t chng st:

Ni t chng st tn dng st vo t, nhm gim bin ca qu in

p kh quyn n gi tr c th ; gi cho in th ca cc phn t c ni t

khng qu cao hn ch phng in ngc t cc phn t n cc b phn

mang in hoc khng gy nh thng cch in ca cc thit b in khc.

Ni t chng st bao gm ni t kim chng st bo v chng st nh

thng cho cc TBAPP c cng sut trn 1000kVA v ni t cc chng st van

Trong nhiu trng hp, cng mt h thng ni t ng thi thc hin hai

hoc ba nhim v ni trn.

Cc loi ni t thng thng c thc hin bng mt h thng nhng cc

thp hoc ng ng vo t hoc nhng thanh ngang bng cng mt vt liu chn

trong t.

Dng in I chy qua cc cc tn vo t, to nn trong t quanh n mt

in trng. Mi im ca in trng c mt in th nht nh.

in tr ca cc ni t

d

dd I

Ur = (2.14) I l dng chm t mt pha ti ni t ni t.

U l in p trn cc ni t.

in tr r gm in tr ca bn thn in cc v in tr tn trong t. in

tr tn trong t ph thuc nhiu vo kch thc, hnh dng, s lng, cch b tr

cc in cc...

19

i vi ni t chng st, cn phn bit r in tr hay tng tr tn xung vi

in tr tn n nh tn s cng nghip.

2.2 in tr tn ni t tn s cng nghip:

Theo quy phm trang b in i vi LPP 22kV : in tr tn ni t an

ton cho php khng vt qu 4 10 i vi ni t ca trm v khng qu 5 30 i vi ni t ng dy.

in tr tn n nh tn s cng nghip ca cc in cc ni t dng n

gin, trong mi trng t ng nht, c th xc nh bng gii tch.

m bo an ton cho ngi vn hnh, h thng ni t phi c thit k

sao cho in p tip xc v in p bc trong mi iu kin khng vt qu tr s

nguy him cho ngi.

2.3 in tr ni t xung kch ca ni t chng st:

Vi h thng ni t bo v chng st, tr s in tr ni t yu cu l tr s

xung kch, cn gi l in tr ni t xung kch hay in tr tn xung Rx. Mun c

tr s ny phi xc nh tr s in tr ni t tn s cng nghip r, sau hiu

chnh vi iu kin xung kch qua h s xung kch x. Nu b phn ni t l mt s cc hoc thanh kt hp vi nhau, in tr ni

t trong trng hp xung c tnh [9] bng cng thc:

= ,..nrR

x

dxx

(2.15)

Trong : n l s lng cc hoc thanh.

x l h s xung kch. x l h s s dng xung kch.

Nu cc cc b tr gn nhau, dng in c st i vo cc cc s c hin tng

mn chn sinh ra. Dng in st i qua cc cc, tn vo trong t s b cn tr mc

d in th trn u cc cc l nh nhau, lm cho cng in trng b gim.

20

Khi , b mt tn dng in st ca t b thu hp li, lm cho h s s dng cc b

xu i, in tr ca h thng ni t tng ln.

H s s dng xung kch ph thuc vo chiu di ca cc, khong cch gia

cc cc v kch thc, hnh dng ca mng ni t.

in tr tn xung khng ph thuc vo kch thc hnh hc ca in cc m

ch ph thuc vo cc c tnh ca t v bin dng in, gy nn phng in tia

la trong t.

in tr tn xung ca in cc hnh bn cu [13, tr.97]:

sdpddtx IER 2/. )(= (2.16) t l in tr sut ng tr Ep () l cng in trng phng in trong t.

Is l bin dng in st.

in tr tn xung ca c t hp ni t xc nh theo [13, tr.98]:

xxtxc

xtxcx nRR

RRR )(.

+= (2.17)

Trong :

Rxc l in tr tn xung ca cc.

Rxt l in tr tn xung ca thanh.

2.4 Tnh ton in tr ni t ca TBAPP:

Ly in p cho php i vi ngun in xoay chiu pha h p l 40V tho

mn iu kin in p tip xc v in p bc, th sau khi tm c tr s dng

in ngn mch mt pha IN ta s tm c in tr ni t r ca TBA tng ng

cng sut t MBA.

Khi tnh ton, ta xt dy qun pha trung p ch trung tnh ni t trc

tip tc cun dy u Yo. Trng hp cun dy u Y hay ch mng in s nh hn v khng cn xem xt.

Cc h thng cung cp in xa ngun v cng sut b so vi li in quc

gia nn cho php tnh ngn mch n gin.

21

2.4.1_Pha trung p :

Nh trnh by Chng 1, cc TBATG cp ngun 22kV c th l 35/22kV

hoc 110/22kV vi cc gam cng sut khc nhau. Li in trung p 22kV c th l

3 pha 4 dy hoc 3 pha 3 dy ly t lm trung tnh.

i vi LPP trung p 22kV 3 pha 4 dy th cun th cp pha ngun u Y

hoc (TBATG 35kV) hoc Yo (TBATG110kV), pha cun dy s cp MBA phn phi u Yo. Khi ny dy trung tnh c ni t lp li. in tr tip t thc hin

ni im gia MBA c pha ngun ln pha ti.

i vi LPP trung p 22kV 3 pha 3 dy ly t lm trung tnh th cun th

cp pha ngun u Y hoc (TBATG 35kV) hoc Yo (TBATG110kV), pha cun dy s cp MBA phn phi u Y hoc . Khi ny, tt c cc ct in trung p 22kV u c ni t an ton [4, tr.35].

Trong thc t, li in trung p min Trung s dng 3 pha 3 dy nhng ni

t lp li. iu ny, kh ph hp vi iu kin khc nghit ca vng i ni min

Trung ; ni vn c in tr sut ca t tng i cao v khi bo v chm t

trong nhiu trng hp khng tc ng.

i vi LPP 22kV trung tnh ni t trc tip th dng in chm t ln

thit b bo v tc ng v ct ch b s c.

Theo Tiu chun Vit Nam TCVN5064-1994, in tr mt chiu ro 20oC

ca mt s loi dy nhm li thp thng dng nh sau : Dy dn AC50/8 ; AC70/11

; AC95/16 ; AC120/19 c ro tng ng l 0,5951/km ; 0,4218/km ; 0,3007/km ; 0,2440/km. : TBATG DK (L_km) TBAPP

TC22kV HT RH+jXH RD+jXD

TC-22kV Hnh 2.1 : S tnh ton ngn mch pha thanh ci 22kV TBAPP

22

Tng tr ng dy (2.18) = ,.LrR oDin khng ng dy (2.19) = ,.LrX oDTng tr DK theo bn knh cp in L v loi dy dn nh Ph lc 1 :

in khng ca h thng c tnh theo [2, tr.58] hoc tra trong s tay k

thut in :

= ,10.%.2

N

dmNH S

UUX (2.20)

Do in tr ca h thng RH b so vi in khng XH nn c th b qua trong

tnh ton. Khi tng tr ZH t h thng ti im ngn mch l :

++= ),( HDDH XXjRZ (2.21) Tng tr ZH tng ng vi tit din dy dn v bn knh cp in nh ph

lc 4a, b, c.

Dng ngn mch mt pha xc nh theo cng thc :

kA

ZUI

H

dmN ,3

= (2.22)

Cng sut MBA h thng ly theo mc ph bin l 10MVA, 16MVA,

25MVA, 40MVA v 63MVA. Thng s k thut MBA 3 pha 2 cun dy 110/22kV

ly theo s liu tham kho vi UN%=10,5 nh Ph lc 3.

Khi , ng vi bn knh cp in L (km), dy dn nhm li thp AC ta s

c kt qu tnh ton dng in ngn mch mt pha IN (kA) nh cc Bng 2.12.3 : Bng 2.1 : Kt qu tnh ton IN (kA) ng vi bn knh cp in L=10km

STT SH (MVA) AC50/8 AC70/11 AC95/16 AC120/19

1 10 0.089 0.089 0.089 0.089

2 16 0.140 0.140 0.140 0.140

3 25 0.211 0.212 0.212 0.212

4 40 0.324 0.325 0.326 0.327

5 63 0.476 0.482 0.485 0.486

23

Bng 2.2 : Kt qu tnh ton IN (kA) ng vi bn knh cp in L=15km

STT SH (MVA) AC50/8 AC70/11 AC95/16 AC120/19

1 10 0.087 0.088 0.088 0.088

2 16 0.136 0.137 0.137 0.137

3 25 0.203 0.204 0.205 0.205

4 40 0.304 0.308 0.309 0.309

5 63 0.432 0.442 0.448 0.447

Bng 2.3 : Kt qu tnh ton IN (kA) ng vi bn knh cp in L=20km

STT SH (MVA) AC50/8 AC70/11 AC95/16 AC120/19

1 10 0.086 0.086 0.086 0.086

2 16 0.133 0.134 0.134 0.134

3 25 0.195 0.197 0.198 0.198

4 40 0.286 0.291 0.294 0.295

5 63 0.394 0.408 0.415 0.418

Kt qu tnh ton cho thy, ng dy cng di th dng in ngn mch

cng b v khi tng tit din dy dn th dng ngn mch tng khng ng k.

i vi li in cao p trung tnh ni t, do v gi tr ca dng in chm

t ln nn rt kh m bo yu cu v in p bc v in p tip xc. in tr

ni t pha 110kV ly theo quy phm trang b in R 0,5 mc d trn thc t kh c kh nng p ng, nht l cc khu vc i ni.

2.4.2_Pha h p :

2.4.2.1 Dng in ngn mch :

Khi ngn mch mt pha cui ng dy th dng in ngn mch mt pha

phi ln ptmt hoc my ct h p tc ng.

Tng t tnh ton ngn mch phn trung p, im ngn mch cng gn th

dng in ngn mch mt pha cng ln. Trong tnh ton, an ton ngi ta thng

tnh ngn mch ti tram bin p.

24

Cun dy 22kV MBA ng dy h p

rB + jxB rh + jxh

IN

r r1 r2 r3 I

Hnh 2.2 : S ngn mch cui ng dy h p

Hnh 2.2 th hin im ngn mch cui ng dy h p, pha cun dy

trung p u Y hoc . ; ph hp vi iu kin thc t ti khu vc min Trung.

Cp lc (6m)

MBA AT

RB XB RC XC RAT XATHT IN

Hnh 2.3 : S chi tit tnh ton ngn mch ti TBA

Hnh 2.3 th hin s tnh ton dng ngn mch ti thanh ci trm bin p.

Trong tnh ton, c th coi gn ng trm bin p l ngun.

Ta c cng thc tnh tng tr : 22

max )()( ATCBATCBN XXXRRRIUZ +++++== (2.23) in tr, in khng ca cp lc v ptmt c xc nh bng cch tra cu

trong s tay k thut in.

25

Tuy nhin, do tr s in tr, in khng b so vi MBA (xem Ph lc 2) nn

c th b qua trong tnh ton.

in tr v in khng my bin p tra bng hoc c xc nh theo cng

thc gn ng [2, tr.57], [2, tr.58]:

= ,10. 322

dm

dmNB S

UPR (2.24)

= ,10.%.2

dm

dmNB S

UUX (2.25)

Trong : PN (kW) l tn tht ngn mch Um (kV) l in p nh mc.

Sm (kVA) l cng sut nh mc MBA

Tng tr MBA l : BBB jXRZ += (2.26)

Dng in ngn mch mt pha : 3B

dmN Z

UI = (2.27)

Kt qu tnh ton ngn mch ti TBA nh bng 2.4 v 2.5

Bng 2.4 : Kt qu tnh ton IN v r cho mt s TBAPP 3 pha thng dng nh sau:

STT Sm (KVA) PN (kW) uN% IN(kA) r () 1 31.5 0.6 4.5 0.970 10.00 2 50 1 4 1.672 10.00 3 75 1.4 4 2.520 10.00 4 100 1.75 4 3.369 10.00 5 125 2.05 4 4.217 9.00 6 160 2.35 4 5.413 7.00 7 180 2.8 4 6.009 6.00 8 200 3.15 4 6.624 5.00 9 250 3.25 4 8.321 4.00 10 315 4.85 4 10.097 4.00 11 400 5.75 4 12.593 4.00 12 500 7 4 15.326 4.00 13 630 8.2 4 18.729 4.00 14 800 10.5 5 19.262 4.00 15 1000 13 5 23.046 4.00

26

Bng 2.5 : Kt qu tnh ton IN v r cho mt s TBAPP 1 pha thng dng nh sau:

STT Sm (KVA) PN (kW) uN% IN(kA) r () 1 10 0.2 4 0.581 35 2 12.5 0.3 5 0.586 35 3 15 0.3 6 0.616 35 4 20 0.4 7 0.713 35 5 25 0.5 8 0.786 35 6 31.5 0.6 9 0.886 35 7 37.5 0.6 10 0.957 35 8 40 0.7 11 0.929 35 9 50 1 12 1.060 30 10 75 1.4 13 1.464 25 11 100 1.75 14 1.807 20

rB + jxHB

IN

NI

r r

Hnh 2.4 : S tnh ton ngn mch pha h p MBA

Kt qu tnh ton cho php kim tra iu kin an ton theo in p bc v

in p tip xc ; l nhng khi nim lin quan n bo v trnh tai nn in git

qua s tip xc gin tip. V phng din tip xc gin tip, ta c cc tnh hung

tip xc vi cc phn t nh ro chn, v hay phn kim loi g gi cc thit b hoc

tip xc trc tip vi trang thit b in, m chng c in p do chm v ...

hoc l s tip xc vi cc phn t c in p cm ng, do nh hng in t hay

tnh in nh trng hp ng dn nc hay ng dn kh di t gn mt s tuyn

ng st, t chy bng in xoay chiu mt pha hay mt s ng dy truyn ti

in ba pha ch cn bng.

27

Ly in p cho php an ton i vi con ngi khi ngun in xoay chiu

pha h p l 40V, tho mn iu kin in p tip xc v in p bc, th sau khi

tm c tr s dng in ngn mch mt pha IN ta s tm c in tr ni t r

ca TBA tng ng cng sut t MBA.

2.4.2.2_Dng in s c :

li in ba pha ni t, im trung tnh ca ngun cung cp c ni t

qua h thng tip t lm vic, in tr h thng ni t lm vic l Ro.

Nu mt thit b in c v c tip t, th khi s c cch in b hng,

dng in s chy qua in tr rc ca pha b s c, qua in tr tip t bo v thit

b r, ng thi qua in tr ngi Rng, qua in tr h thng ni t lm vic Rov

ng kn qua ngun cung cp. Dng in s c s l :

ngD

ngDocd

S

RrRr

RrI

+++

.Uf

C (2.28)

Uf l in p pha nh mc ca li in.

Dng in chy qua h thng ni t bo v r l :

DngSD rR

II +=ng

C

R. (2.29)

T (2,15) v (2.16), ta c :

ngDngDocd

ngfD RrRrRr

UUI

.)).((. +++= (2.30)

Do rD

28

trm bin p. Khi : oD

fD Rr

UI += (2.32)

Trong thc t, in p i vi t ca pha b s c s b hn Uf, v dng in

s c chy qua h thng ni t lm vic Ro, tc lm tng in th trung tnh ca

li in.

in p m con ngi phi chu khi tip xc vi v ca thit b s c l :

oD

DfDDDtx Rr

rUrIUU +=== .. (2.33)

Mt ngi, tay tip xc vi phn t mang in, chn tip xc vi h thng

ni t lm vic c th chu in p tip xc :

Do

ofoDxt rR

RURIU += ..* (2.34)

Tng hai in p tip xc s bng in p pha : ftxtx UUU =+ *Theo [10], mt h thng ni t bo v t yu cu ch cn nu c mt in

tr chy trong t b hn :

40

40= fotxf

txoD U

RUU

URr (2.35)

Cun dy 22kV MBA ng dy h p

rB + jxB rh + jxh

IN

IN

IN

r

Hnh 2.5 : S dng bo v ni dy trung tnh

29

Tr s 40V l in p ging trn v thit b nu xy ra chm v. in p ny

c th xam l an ton cho ngi lao ng khng phi l ngi c ngh in.

Khin xy ra chm v thit b, in p ca hai pha cn li c th tng n tr

s khng cho php :

UUUUU )40()40(' 22 ++= (2.36) m bo an ton, dng dy dn ni v thit b vi dy trung tnh. Khi ,

s c chm v thit b tr thnh ngn mch mt pha bo v lm vic ct nhanh

s c v quay li tr li trng hp xt mc 2.4.2.1

Kt qu tnh ton ph lc 5 v ph lc 6 cho ta kin ngh nh sau :

a) i vi tram bin p phn phi 3 pha, th :

i vi TBA phn phi c cng sut S 250kVA, yu cu r 4 i vi TBA phn phi c cng sut 250kVA> S 200kVA, yu cu r 5 i vi TBA phn phi c cng sut 200kVA> S 180kVA, yu cu r 6 i vi TBA phn phi c cng sut 180kVA> S 160kVA, yu cu r 7 i vi TBA phn phi c cng sut 160kVA> S 100kVA, yu cu r 10 b) i vi trm bin p phn phi 1 pha, th :

i vi TBA phn phi 1 pha c 40kVA S, yu cu r 35 i vi TBA phn phi 1 pha c 40kVA

30

ni t r v tn ti rt lu. Khi , in p i vi t ca c 4 dy ca mng in

h p u tng ln v v thit b ni vi dy trung tnh s tng ln. Bin php bo v

trong trng hp ny l chn r th no lc xy ra tng in p pha h p cch

in khng b h hng v m bo an ton cho ngi lc chm vo v thit b.

Trung p H p

C

B

A

O

r r1 r2 r3 I

Hnh 2.5 : S m t xm nhp in p cao n in p thp

Theo quy trnh th :

dd I

r 125 (2.37)

y I l dng in lc chm t pha trung p.

2.6 Cc nh hng bn ngoi n in tr ni t:

tnh ton thit k h thng ni t, cn thit phi bit in tr sut ca t

t. Tuy nhin, t ph thuc vo kh nng gi m ca t cng nh nh hng ca iu kin mi trng xung quanh v thi tit.

Trong nm do iu kin thi tit kh tng thay i, lm cho nhit ca

t, hm lng ca m trong t v bo ha ca chng cc tng t khc nhau

31

cng thay i. Do , t bin i trong mt phm vi rng, tr s trong ma ma v ma kh c th khc nhau rt xa.

in tr sut tnh ton ca t tt = km. (2.38) in tr sut ca t o c [.m] km h s ma ph thuc vo loi ni t, loi in cc v chn su.

Trong tnh ton ly tr s km b nu khi o t kh ro v ly km ln nu khi

o t m t.

Ni t an ton v ni t lm vic pht huy tc dng vo bt k lc no

trong nm. Cn ni t chng st ch pht huy tc dng vo ma st, tc ma ma

dng, t m t nn ch cn h s ma km b hn.

Trong tnh ton s b, c th dng nhng tr s t trong sch tra cu k thut cho tng loi t, chng hn nh t ct t l 300m ; t tht l 100m ; t st l 60m ...

2.7 Cc yu cu v kinh t -k thut ca h thng ni t:

H thng ni t c in tr tn b th tn tt dng in s c. Tuy nhin, s

dn n chi ph cho vt t v nhn cng tng. Vic nh gii hn cho tr s in tr

tn v vic la chn cc phng n ni t phi hp l v kinh t -k thut.

Khi thit k, lp t h thng ni t cho LPP 22kV cn lu cc im

sau:

a) i vi ni t lm vic, tr s in tr ni t cho php quyt nh bi yu

cu ca tnh trng lm vic ca tng thit b c th.

b) i vi ni t an ton, tr s in tr ni t cho php phi chn sao cho cc

tr s in p bc v in p tip xc trong mi trng hp khng c vt qu

tr s cho php, gy nguy him cho ngi vn hnh.

c) Trong cc nh my in v TBA, ni t lm vic v ni t an ton cc

cp in p khc nhau thng c ni thnh h thng chung ; khi phi thit k

theo tr s in tr tn cho php b nht.

32

d) Khi thit k h thng ni t ca trm v ng dy cn ch tn dng cc

hnh thc ni t c sn nh ng ng kim loi chn trong t, ng dn nc,

kt cu kim loi ca b tng ct thp, mng ct ...

33

CHNG 3

QU IN P NG DY V TRM BIN P

c gii php bo v qu in p (QA) tc ng ln ng dy v TBA

22kV, ta nghin cu chi tit cch in cc phn t ca ng dy v MBA phi

hp cch in mt cch hiu qu v kinh t v k thut ; l s iu ho gia QA

tc dng ln cch in v kh nng chu ng QA ca cch in.

cch in c an ton th kh nng chu QA phi cao hn QA tc

dng ln cch in v do vy phi : hoc tng kh nng chu QA hoc gim QA

tc ng ln cch in.

i vi cch in ngoi, do l cch in t phc hi nn mc tiu ca phi

hp cch in s l gim s ln s c ti mc hp l cho php.

i vi cch in trong, do l cch in khng t phc hi nn mc tiu ca

phi hp cch in l khng cho php xy ra bt k mt ln s c cch in no.

3.1 Cch in ca li phn phi 22kV:

3.1.1_Cc c tnh in:

3.1.1.1_in p th nghim tn s cng nghip:

a) in p phng in kh Uk : l tr s in p b nht, xy ra phng

in mt, t l vi khong cch phng in lk.

Uk = Ek.lk (3.1)

Vi Ek = 3 5 kV/cm l cng trng trung bnh dc theo ng phng in lk, ty thuc vo loi cch in.

b) in p phng in t U : l tr s in p b nht, xy ra phng

in mt khi b mt cch in chu tc dng ca ma chun [13].

)7601(5,0 ,

pUU chuu += (3.2) Vi U,ch l tr s in p phng in t iu kin p sut chun.

P l p sut khng kh lc th nghim (mmHg)

34

in p phng in t hu nh khng ph thuc vo nhit . Thng

thng, in p phng in t b hn 20% 30% so vi in p phng in kh. Tr s U tn s cng nghip l c s tnh chn cch in theo yu cu ca qu

in p ni b.

3.1.1.2_in p th nghim xung:

Kh nng chu tc dng ca QA kh quyn c biu th bi c tnh Vn -

giy ca cch in. Bin ca in p th nghim xung c quy nh trong Tiu

chun Vit Nam.

3.1.2_Cc c tnh c:

i vi cch in treo: phi xc nh bn chu ko.

i vi cch in ng v xuyn: phi xc nh bn chu un.

3.1.3_Chn mc cch in ca h thng in:

3.1.3.1_in p lm vic lu di ln nht cho php: cn chn cao hn tr s

nh mc khong t 5-20% ty theo cp in p.

i vi li in 22kV trung tnh ni t trc tip th chm t mt pha

tng ng vi ngn mch mt pha, dng in ngn mch chm t ln, i hi

phi ct in nhanh phn t b s c. Do , in p lm vic ln nht cho php

trong trng hp ny ch cn chn 5% 15% cao hn so vi tr s nh mc ca in p pha.

3/)15,105,1(max_ dmlv UU = (3.3) 3.1.3.2_ Qu in p ni b: xy ra khi trong h thng c s thay i ch

lm vic nh khi ng ct cc phn t ca h thng hoc khi c s c t dy, chm

t, ngn mch....

Qu in p ni b c biu th bng bi s ca in p pha ca h thng:

Uqanb = k.Ulv_max (3.4)

Tr s qu in p ni b ph thuc vo phng thc lm vic ca im

trung tnh ca h thng. i vi h thng li in 22kV c im trung tnh ni t

trc tip th bi s k thp hn h thng c trung tnh cch in.

35

3.1.3.3_ Qu in p kh quyn:

Qu in p kh quyn xut hin trn ng dy theo hai kh nng : hoc do

st nh vo ng dy, hoc do st nh gn ng dy gy nn QA cm ng.

Sng QA khng ch gy nn phng in trn cch in ng dy, m cn c th

truyn theo ng dy vo trm gy nguy him cho cch in ca cc thit b trong

trm.

3.1.3.4_ Chn cch in ng dy:

QA kh quyn xy ra khi st nh thng vo ng dy hoc nh gn v

gy cm ng trn ng dy. i vi cch in th QA gy ra do st nh thng

vo ng dy l nguy him nht v c la chn lm iu kin xc nh mc

cch in xung v tr s in p th nghim xung.

in p tc dng ln cch in ca dy dn do st gy nn l mt i lng

thng k. Theo thng k th gn 70% cc trng hp st nh gy nn in p vt

100kV tc dng ln cch in ng dy. Do c th thy rng khng phi cch

in ca bt c ng dy ti in no cng c th chu c tr s in p xung

ny

Theo tiu chun TCVN-5849-1994, TCVN-5850-1994, TCVN-5851-1994,

IEC 60305, IEC 60383, IEC 60471, IEC 60720, IEC 1109 th cch in ng v

cch in treo LPP 22kV c chn nh sau :

a) Cch in ng ng dy 22kV (Pin type):

in p chu ng xung 1,2/50s 125 (150)kVpeak in p chu ng tn s cng nghip kh trong 1 pht 75 (85)kVrms in p chu ng tn s cng nghip t trong 1 pht 55 (60)kVrms in p nh thng tn s cng nghip 160 (160)kV S ghi trong ngoc (...) p dng i vi cch in thu tinh.

b) Chui cch in treo ng dy 22kV :

in p chu ng xung 1,2/50s 190 (170)kVpeak in p chu ng tn s cng nghip kh trong 1 pht 130 (105)kVrms in p chu ng tn s cng nghip t trong 1 pht 100 (60)kVrms

36

in p nh thng tn s cng nghip 160kV Tr s ghi trong ngoc (...) s dng cho chui lin Composit

Hin nay, Cng ty in lc 3 ang th nghim s dng cch in ng

22kV loi Pin Post v Line post trn LPP khu vc min Trung, nhng cha c kt

lun c th v hiu qu s dng.

Trong thi gian gn y th chui cch in composit thng c lp t

trn LPP 22kV min Trung nh gn nh, gi thnh r m vn p ng c cc

yu cu v in.

3.1.3.5_ Chn cch in TBA:

Sng QA lan truyn t ni st nh v hai pha ca ng dy, cui cng

n TBA v tc dng ln cch in ca cc trang thit b trong trm. V h hng

cch in trm l s c trm trng hn nhiu so vi h hng cch in ng dy,

cho nn phi c bin php bo v chng st truyn vo trm, nhm hn ch h hng

cch in trm bng cch b tr CSV ti nhng v tr thch hp. Khi sng QA

truyn n trm vt qu mc cch in xung ca trm th chui khe h b phng

in xuyn thng, dng in st chy qua in tr phi tuyn ca CSV v tn vo

t. in p tc dng ln cch in ca TBA gim xung cn bng in p ging

trn in tr phi tuyn, gi l in p d ca CSV.

bn in xung ca cch in trm phi c chn cao hn in p d ca

CSV khong 2025%. i vi LPP 22kV trung tnh ni t trc tip th mc cch in xung c th chn thp hn so vi h thng c im trung tnh cch in.

a) Mc cch in ca MBA :

in p chu ng xung 1,2/50s 125kVpeak in p chu ng tn s cng nghip kh trong 1 pht 50kVrms.

b) S xuyn u 22kV MBA :

in p chu ng xung 1,2/50s 125kVpeak in p chu ng tn s cng nghip kh trong 1 pht 50kVrms.

c) S xuyn u h p MBA :

in p chu ng xung 1,2/50s 40kVpeak

37

in p chu ng tn s cng nghip trong 1 pht 10kVrms. Nh vy, yu cu cch in ca MBA thp hn yu cu cch in ng dy.

3.1.3.6_Chn chng st van :

V c tnh Volt -giy v Volt -Ampere ca CSV l khng ng thng, nn

n gin ngi ta thng dng phng php th kho st cho c hai trng

hp nu trn.

Theo tiu chun IEC 60099, IEC 60071, IEC 60270, IEC 60815, IEC-61109,

ANSI C62-11-1993 th chng st van 22kV c chn nh sau :

- in p vn hnh cc i ca h thng Um=1,1*22=24kV

H s chm t Ke = 1,4 i vi li 22kV trung tnh ni t trc tip.

- Gi tr qu in p tm thi T.O.V :

kVUKU meVOT 42,19324.4,1

3... ===

Cn la chn UT.O.V chng st UT.O.V liThng thng QA tm thi xut hin khi c s c chm t, hoc do sa

thi ph ti. Tuy nhin, mt s kt cu li c th xy ra QA cng hng, iu

ny cng c th xy ra khi cc cc my ct tc ng khng ng thi. Qu in p

cng hng khng c dng lm c s tnh chn T.O.V ca CSV.

Trong mt vi trng hp vn hnh, gim dng s c ngi ta ch chn

ni t trung tnh ca mt s MBA, khi c th xy ra kh nng mt s b phn

ca h thng c th tr thnh mt tc dng i vi h thng ni t. Nu khng d

phng cho vic ny th c th dn n T.O.V cao hn v lm hng chng st. Tuy

nhin, him khi xy ra trng hp ny nn ngi ta chp nhn nguy c hng chng

st thay v chn CSV c T.O.V cao hn.

- in p vn hnh lin tc ca chng st :

kVUU mC 87,13324

3==

Tra bng, chng st van c chn c cc thng s k thut nh sau :

- in p chu ng xung 1,2/50s 125kVpeak

38

- in p chu ng tn s cng nghip kh trong 1 pht 50kVrms - in p d ng vi sng 8/20s, dng phng nh mc 10kA : 43,4kV - in p lm vic lin tc cc i (MCOV) : UC=15,3kVrms

- H s qu in p tm thi (TOV) : 27,13,1542,19.. ===

C

VOT

UUT

Da vo c tuyn qu in p tm thi ca CSV, theo tiu chun IEC kim

tra h s T tho mn iu kin thi gian chu QA tm thi t10sec i vi li in trung tnh ni t trc tip.

3.1.3.7_Phi hp cch in :

Qua nghin cu chi tit cc phn t cch in ca li phn phi 22kV, ta c

th tm tt vic phi hp cch in nh sau :

- Do cch in ca ng dy trn khng DK l cch in ngoi, nn

nhim v ca phi hp cch in l :

Bo v chng st nh trc tip cho cc ng dy.

Chn mc cch in ng dy c kh nng chu c phn ln QA do st

v QA do thao tc.

Thit k ni t ct in gim s ln phng in ngc trn b mt cch

in ng dy nhm gii hn sut ct do QA st v QA thao tc ti mc hp l

cho php.

- Do cch in ca TBA l cch in trong, nn nhim v ca phi hp cch

in l :

Bo v QA do st truyn t DK vo TBA.

Bo v QA tm thi TOV xut hin khi c ngn mch chm t 1 pha.

Vic tng kh nng chu ng ca cch in ti mc cao hn cc QA ni

trn s rt tn km, do vic phi hp cch in y l gim QA tc dng ln

cch in ti TBA bng cc CSV ; m ph bin l s dng CSV khng khe h, loi

in tr phi tuyn Oxyt km ZnO.

39

3.2 Bo v chng st cho ng dy trung p:

ng dy phn phi trn khng l phn t di nht trong h thng in ; c

ng dy bng qua cc i ni vi mt st ln nh khu vc tnh Gia Lai, Kon

Tum, Qung Nam ; c ng dy bng qua khu th vi nhiu dy nh cao tng

nh thnh ph H Ni, thnh ph H Ch Minh, Nng. L phn t di nht trong

h thng in nn ng dy thng b st nh trc tip v thng chu qu in

p cm ng do st nh gn ng dy..

Trn quan im kinh t-k thut, chng ta khng th ngn c st nh trc

tip vo ng dy trung p. Hn na, QA do st trn cc ng dy trung p

thng dn n phng in qua b mt cch in v gy ngn mch chm t

ng dy. QA do st thng tn ti trong thi gian rt ngn nn my ct ng

lp li thng lm vic thnh cng nu cch in ng dy khng b v. Tnh ton

hp l tin cy bo v cho ng dy trung p phi hp vi la chn cch in

ca ng dy l iu cn quan tm.

3.2.1_ St

Dng in st c dng mt sng xung, trung bnh trong khong vi ba micro

giy, dng in tng nhanh n tr s cc i to nn phn u sng v sau gim

dn trong khong 20-100s, to nn phn ui sng. Is (kA) Ismax

0,5 Imax

t (s) s

Hnh 3.1 : Dng in st ghi trn my hin sng

40

S lan truyn sng in t to nn bi dng in st gy nn QA trong h

thng in, do cn phi bit nhng tham s ch yu ca n :

a. Bin dng in st vi xc sut xut hin ca n.

b. dc u sng dng in st vi xc sut xut hin ca n.

c. di sng dng in st.

d. Cc tnh dng in st.

St vi bin ln, dc cao, xut hin vi xc sut b nn s khng hp

l khi chn cch in ng dy tho mn qu in p ln nht c th xy ra, m

ch chn theo mt mc hp l v kinh t v k thut.

3.2.2_Yu cu chung i vi bo v chng st ng dy :

ng dy ti in l phn t di nht trong li in, nn thng b st

nh v chu tc dng ca QA kh quyn.

Qu in p khng ch gy ra phng in trn cch in ng dy, m cn

truyn sng vo TBA gy nn h hng cch in trong trm. Do vy, BVCS ng

dy phi c s kt hp cht ch vi vic BVCS TBA, c bit nu st nh vo

on ng dy gn TBA th sng QA c gi tr rt ln, gy nguy him trn cch

in v thit b ca TBA.

3.2.3_St nh trc tip vo ng dy phn phi 22kV:

St nh trc tip vo ng dy l trng hp nguy him nht, v phn ln

cc trng hp u a n phng in trn cch in ng dy.

Phng php gn ng xc nh sut ct ng dy do st nh trc tip

vo dy dn c tnh nh sau [13, tr.122]

Gi thit ton b s ln st nh u vo dy dn (b qua kh nng st nh

vo ct, chim khong 20% st nh vo ng dy).

Ti ni st nh, tng tr ca khe st Zs ni tip vi tng tr tng ng

ca ng dy Zdd/2. Dng in chy qua khe st bng:

2dd

s

sS ZZ

ZII+

= (3.5)

41

Is/2

Zs Is/4 Is/4

Zdd Zdd Hnh 3.2 : Phn b dng st khi st nh vo dy dn

Zdd l tng tr sng ring ca dy dn.

Zs l tng tr sng ring ca khe st, c gi tr Zs = 200 Nu xem Zdd =400 th Idd =Is/4 v in p trn dy dn s l:

Udd = Idd. Zdd = 100 Is (3.6)

Nu tr s ca QA ny ln hn mc cch in xung ca cch in ng

dy : Udd U0,5 th s gy nn phng in. Trong : U0,5 l in p phng in xung kch ca chui s.

i vi ng dy dng ct thp hoc ct b tng ct thp, th cch in ca

ng dy ti ct in ch l chui s v cc khong cch khng kh gia dy dn

v ct. iu kin ny ng vi khi dng st vt qu mc bo v chng st ca

ng dy :

IS Ibv = U0,5/100 (3.7) Trong cch tnh gn ng, iu kin phng in ch ph thuc vo bin

ca dng st m khng ph thuc dc u sng ca n.

Tt c cc trng hp st nh vi bin IS Ibv u dn n phng in trn cch in ng dy, nn ngi ta cho rng xc sut phng in p cng chnh l xc sut xut hin dng st Is

26bv

bv

i

ip e

== (3.8) i vi ng dy phn phi 22kV khu vc min Trung ch yu dng ct

btng ct thp v ct thp th :

42

Udd = 100 Is U0,5 kAII bv 25,1100125

100U0,5

S ===

Xc sut phng in trn cch in ng dy :

95.02625,1

==

ep Xc sut hnh thnh h quang n nh v kh nng ct in ng dy ph

thuc vo phng thc ni dy trung tnh ca h thng.

i vi li in 22kV th sut ct in nc c tnh theo [13]:

- i vi ng dy c dy dn b tr hnh tam gic:

nc = 0,6.htb.nngs.m.vp. (3.9) Vi htb l treo cao trung bnh ca dy dn trn cng, tnh theo m

m : mt st trung bnh trong mi ngy c dng st.

nngs : s ngy st trung bnh hng nm/1km2 trong khu vc c ng dy i

qua.

l xc sut hnh thnh h quang n nh. Khi ng dy i qua vng st hot ng mnh, th sut ct ng dy cng

ln. Chng hn, ta tnh vi nngs = 100 ngy/km2/nm ; m = 0,1 ln/ngy ; = 0,7 ; chiu cao treo dy htb=10 m vi xc sut phng in c tnh trn th sut ct

ng dy s l :

nc = 0,6.10.0,1.100.0,95.0,7 = 39,9 ln/nm

Nu s ngy st trong nm b th sut ct ng dy khng ng k.

- i vi ng dy c dy dn b tr trn mt phng nm ngang:

nc = (6.htb+s).nngs.m.vp..10-1 (3.10) Vi s l khong cch gia hai dy dn ngoi cng.

3.2.4_Ch tiu chng st ca ng dy ti in :

Trng hp st nh thng ln ng dy l nguy him nht, v khi

ng dy chu ton b nng lng ca phng in st. V tr s ca QA kh

quyn rt ln nn ch c th chn mc cch in ng dy trn mc hp l v

43

kinh t-k thut ; gim s c n mc gii hn hp l. iu ny cng c ngha l

cn tnh ton gim s ln ct in ng dy do st.

3.2.4.1_ S ln st nh vo ng dy DK:

Xc sut st nh vo ng dy ph thuc vo cao treo dy dn trn

cng.

ng dy c cao treo dy trung bnh ca dy trn cng l htb th trong

phm vi b rng B=2.htb v mi pha ca ng dy, ton b s ln st s nh vo

dy dn.

Khi B>2.htb th xc sut st nh vo dy dn s gim dn v khi B 5.htb th ton b s ln st nh u xung t.

Ngi ta kt lun rng khi B 3.htb v mi pha ca dy dn th ton b st nh vo dy dn. Ni mt cch khc, dy dn thu ht v pha mnh cc phng

in ca st trong di t c chiu rng B 6.htb c v hai pha ca n.

%

100

80

60

40

20

B/htb 0 1 2 3 4 5

Hnh 3.3: Xc sut st nh ph thuc vo khong cch B

Vi chiu di ca ng dy L th din tch ca khu vc st nh l 6.htb.L.

S ln st nh thng ln ng dy mt nm l : 310....6 = ngstb nmLhN (3.11)

Trong : L l chiu di ng dy, tnh theo km

44

3.2.4.2_ S ln phng in ca cch in ng dy:

Phng in trn cch in ch xy ra khi QA kh quyn c tr s cao hn

hoc bng mc cch in xung (U0,5) ca ng dy.

Xc sut phng in vp trn cch in ng dy:

vp = P{is ibv} = P{uqa u0,5} (3.12) V tham s ca phng in st bin v dc ca dng st c th c

nhiu tr s khc nhau, do khng phi tt c cc ln st nh ln ng dy du

dn n phng in trn cch in.

c phng in, QA kh quyn phi c gi tr ln hn mc cch in

xung kch ca ng dy, kh nng ny c biu th bi xc sut phng in (vp)

v nh vy s ln xy ra phng in trn cch in ng dy trong mt nm l : 310.....6. == pdngstbpdpd vnmLhvNN (3.13)

Do thi gian tc dng ca QA kh quyn khong , trong khi thi gian

lm vic ca bo v r le khng b hn nn y cha phi l s ln nhy my

ct in do st hng nm.

s410

s210

3.2.4.3_ S ln ct in ng dy do st:

Khng phi bt c ln phng in no trn cch in ng dy cng dn

n ct in, m ct in ng dy ch xy ra khi phng in tia la xung chuyn

thnh phng in h quang n nh, duy tr bi in p lm vic ca ng dy.

Xc sut hnh thnh h quang n nh ph thuc vo gradien ca in p dc theo ng phng in v c xc nh bng thc nghim.

Bng 3.1 : Xc sut hnh thnh h quang theo gradien in p

Elv = Ulv/lp [kV/m] 50 30 20 10

0,6 0,45 0,25 0,1

S ln ct in do st hng nm ca ng dy c tnh theo [20]: 310.....).9,06,0( = pdngscd vLnhN (3.14)

45

so snh kh nng chu st ca cc ng dy c tham s khc nhau, i

qua nhng vng c cng hot ng ca st khc nhau thng tnh tr s sut ct

ng dy tc s ln ct in ng dy do st trn chiu di 100km.

.....6,0 pdngscd vnmhn = (3.15) Nh vy, gim s ln ct in do st cn gim vp hoc gim . gim xc sut phng in vp trn cch in c th thc hin bng cch

treo dy chng st v tng cng cch in ng dy hoc gim in tr ni t

ct in gim in p tc dng ln cch in..

gim xc sut hnh thnh h quang c thc hin bng cch gim cng in trng dc theo ng phng in, nh dng ct x g tng chiu

di ng phng in. Trong trng hp ny gradien in p lm vic c th gim

ti khong 0,1-0,2kV/m v xc sut hnh thnh h quang ch cn 10-20%. S dng

ct x g cn c tc dng lm tng mc cch in xung kch ca ng dy nn xc

sut phng in p cng c gim. Xut pht t iu kin an ton cung cp in, s ln ct in cho php trong

mt nm c tnh s b theo:

Nc.cp = Ncp (1-TL) (3.16) Trong : Ncp l s ln ngng cung cp in cho php trong nm.

TL l h s t ng li thnh cng. 3.2.4.4_ Ch tiu chng st ng dy:

cn

1=M (3.17) Trong M l khong thi gian trung bnh gia hai ln ct in ng dy lin tip

do st gy nn.

3.2.4.5_ tin cy BVCS nh trc tip vo ng dy :

ng dy trung p (DTA) 22kV c chiu di v khong ct nh hn so

vi ng dy cao p 110kV, 220kV v ng dy siu cao p 500kV nhng trong

cng mt thi gian th sut ct in do st gy nn li cao hn nhiu ln, gy thit

46

hi ng k cho kinh t. Vic nh gi mc nguy him do st gy nn cn phi

xem xt tnh c th ca LTA. c trng ca DTA 22kV l khng tn ti dy

chng st, do ng dy mt in l do st nh trc tip vo dy dn pha. S

xut hin QA ti im st nh c bin cao hn bn phng in xung kch

ca cch in pha LPP, khi xc sut ct ng dy s ln hn. Mc nguy

him nht l khi st nh vo nh ct, v khi d xy ra phng in gia cc pha

v gia cc pha vi t.

3.2.5_Qu in p cm ng:

Khi st nh gn ng dy, in t trng ca dng st to nn gn mt

t, tc dng ln dy dn ca ng dy, s gy nn QA cm ng.

3.2.5.1_ Thnh phn in p cm ng in U c

Ngay t khi bt u giai on phng in ch yu, in tch dng rng buc

trn dy dn c gii phng chy v hai pha ca dy dn to nn thnh phn in

ca QA cm ng [14].

stb

dstbcu

d ibhvk

vi

bhvfkU )(.)(. == (3.18)

b l khong cch ngn nht t ni st b n dy dn ca ng dy.

k l h s t l.

htb l treo cao trung bnh ca dy dn.

k(v) = k.f(v)/v gim khi v tng vi v l tc ca phng in ch yu ;

v = (0,1 0,3).C trong C =300.000km/s 3.2.5.2_ Thnh phn in p cm ng t Utc

Di tc dng ca t trng ca dng st trong giai on phng in ch

yu, trong mch vng kn dy dn-t xut hin thnh phn in p cm ng t.

Isb

hvkU tbtcu

t )(= (3.19) Trong kt() l h s tng theo tc phng in ch yu .

3.2.5.3_ in p cm ng ln nht

Tr s cc i ca QA cm ng Ucmax

47

Ucmax = Ucmax + Utcmax = stb Ib

h30 (3.20)

3.2.5.4_ S ln phng in trn cch in ng dy do QA cm ng:

Sut phng in trn cch in ng dy l s ln phng in ng vi

100km chiu di ng dy bng [21]:

260

5,0

5,0

.156U

tbp eU

hnmn

= (3.21)i vi ng dy 22kV, do U0,5 b nn sut phng in np ln, cn c

bin php hn ch sut phng in do QA cm ng.

Cn lu mt c im ca QA cm ng l n xut hin ng thi c ba

pha, in p gia cc pha do khng chnh lch nhau nhiu v khng nguy him

n cch in gia cc pha.

3.2.5.5_ tin cy bo v QA cm ng vo ng dy :

DTA 22kV ct in do QA cm ng t phng in st gn ng dy

cng nh trong trng hp st nh trc tip. QA cm ng do phng in gia

cc pha xy ra khi c phng in mt trong cc pha xung t. Cc QA khi xy ra

phng in hai hoc ba pha c tr s bng [20]:

dd

ddccu ZK

ZRUU.5,0).1(

.5,0

12%502

+= (3.22)

41.).1(

41

12312

12

%503 KZK

KZRUU

dd

ddc

cu +

++=

(3.23)

S lng QA cm ng Nc2 vt qu mc cho php Uc2 c xc nh

theo [20]:

2602

2

2

...36,9 cuU

cu

tbgiocu eU

hTN

= (3.24) Tng t, xc nh c s lng QA cm ng Nc3 vt qu mc cho

php Uc3.

48

Sut ct ng dy do QA cm ng c xc nh theo cng thc sau :

nc = [Nc3.3hq + (Nc2-Nc3).2hq ].(1-TL) (3.25) Kt qu tnh ton tin cy cho bit sut ct ng dy trung p 22kV tnh

cho 100km chiu di v 100 gi st nh sau :

Bng 3.2 : Sut ct cho 100km chiu di ng dy v 100 gi st

Loi ct Ct g Ct btng (thp)

Sut ct do st nh trc tip 3,6 13

Sut ct do QA cm ng

49

- QA khi ng ng dy di, c bit cc my ct c b phn t ng

ng li.

- QA khi ct nhng dng in in cm b, c bit khi ct MBA khng

ti, ng c khng ng b v my b.

- QA khi chm t bng h qunag khng n nh trong li c cch in v

trung tnh ni t cng hng.

3.2.6.2_Qu in p cng hng :

Xy ra do nhng dao ng cng hng trong h thng. Bao gm :

- QA cng hng tn s lm vic.

- QA cng hng tn s cao.

- QA cng hng tn s thp hn tn s ngun.

- QA cng hng tham s xy ra do s thay i chu k tham s ca mch.

Ni chung, QA ni b l nhng s kin m s xut hin v din bin ca

n chu s chi phi ca nhiu yu t ngu nhin, nn cc thng s ca n ch mang

tnh thng k. QA ni b c bit nguy him i vi cch in ca cc h thng

siu cao p. Khi tnh chn CSV cho li in 22kV trung tnh ni t trc tip,

thng khng xt n yu t QA ni b do nh hng khng ng k v mc

nguy him khng ln.

3.3 Bo v chng st truyn vo trm bin p phn phi :

3.3.1_Bin php v yu cu i vi bo v chng st truyn vo trm:

TBA phi c bo v vi an ton rt cao, chng st nh thng bng h

thng thu st. Cc DK d c c BVCS hay khng th cc thit b u ni vi

chng u chu tc dng ca sng st truyn t ng dy n. Bin ca QA

kh quyn c th ln hn in p cch in ca thit b, dn n cch in b chc

thng, lm hng thit b v mch in b ct ra. Do vy, trm cn phi c bo v

chng sng QA do st gy ra trn ng dy truyn vo trm bng cc thit b

chng st m ch yu l CSV v bin ca n thng ln hn cch in xung ca

trm.

50

Mun cho CSV bo v c mt thit b no th c tnh Volt -giy ca

thit b c bo v v in p d trn CSV hoc trn thit b hn ch QA phi

nh hn in p th nghim xung ca cch in trong ca thit b c bo v.

Thit b c CSV bo v ch an ton khi thc hin c hai iu kin sau:

- Khong cch gia CSV v thit b phi nm trong gii hn cho php v mun

m rng phm vi bo v ca CSV phi c bin php gim nh dc ca sng

truyn vo trm.

- Tr s dng in xung chy qua CSV khng c vt tr s nh mc ca n.

3.3.1.1_nh hng ca khong cch gia CSV v thit b c bo v n

in p tc dng ln cch in:

Trong cc TBA phn phi, do s lng CSV t c hn nn thc t theo

hng sng truyn thng c mt phn cc thit b c bo v nm trc CSV

nh dao cch ly, my ct ... cn mt phn nm sau CSV nh MBA. Hnh 3.4 th

hin cch b tr CSV thng thng trc MBA phn phi.

U = a.t LCSV-TB, v, = l/v Uo

A B s b Z CSV Up C

Hnh 3.4 : S tnh LCSV-TB bo v chng st trm bin p

U = a.t l sng ti c dng xin gc vi thi gian u sng s v gi s dc u sng a = Uo/s l mt hng s (Uo =U50% ca cch in ng dy on gn trm)

LCSV-TB l khong cch gia CSV v thit b c bo v.

in p xung ln nht tc dng ln cch in c bo v UB UtnxUtnx l in p th nghim xung BIL ca cch in thit b c bo v.

51

Up l in p phng in, in p d ca CSV

in p cc i trn cch in UB c tr s bng :

tnxTBCSV

pB UvbLaUU ++= )(.2 (3.26)

Vi =300m/s Theo kinh nghim, bin hn bo v yu cu t nht l 20%, nn quan h gia

BIL ca thit b bo v ti B v in p UB l :

2,1BU

BIL (3.27)

T (3.26) v (3.27), ta c :

=+ PTBCSV UBILabL 2,1.2)( (3.28)

Do 20% l bin hn bo v, nn gi tr (LCSV-TB+b) (3.28) l gi tr gii hn

khong cch t CSV n thit b c bo v.

= PUBILaL 2,1.2max (3.29)

Mun tnh L phi bit dc a, vic ny rt kh khn v :

- Trong thc t a khng phi l mt hng s m thay i theo thi gian.

- 2dtdi

Za

s

= , vi Z l tng tr sng ca ng dy v is l dng in st, s

bin thin ca is ph thuc vo yu t hnh hc ca ng dy.

- Cha tnh n nh hng tc thi ca in p tn s cng nghip.

- Cha tnh n nh hng ca in dung thit b ni t

- Cha tnh n ph thuc ca dng, p CSV vi tn s.

Trn nguyn tc chiu di dy ni ca CSV n ng dy v thit b, cng

nh t CSV n t cng ngn cng tt.

Mt phng php khc n gin hn c cp, bng cch tnh in p ri

trn chiu di dy dn theo thi gian ca mt sng v in p phng ca chng st.

Tng hai gi tr in p l in p cm ng t trn cch in ca thit b c bo

v.

52

Bng 3.3 cho gi tr in p kV/m chiu di dy dn i vi cc sng c thi

gian mt sng khc nhau. C th dng bng 3.3 tnh in p ri trn dy dn nhm

kim tra bin hn bo v ca CSV.

Vi MBA 22/0,4kV v CSV chn mc 3.1.3.5 v mc 3.1.3.6 vi gi

thit chiu di dy dn t CSV n MBA l 5 mt ; xung st 10kA thi gian mt

sng 10s. Ta c in p ri trn dy dn l:

Udd = 1,33*5*10 = 66,5kV

Bin hn bo v trong trng hp ny l :

%4,9%100*14,43*1,15,66

125%100*11,1*2

=

+=

+= Cdd UUBILMP

Khi rt ngn khong cch xung cn 2 mt th Udd=26,6kV v

%1,68%100*14,43*1,16,26

1252 =

+=MP

R rng khong cch CSV n thit b cng b cng m rng bin hn bo

v.

Bng 3.3 : Gi tr in p ri (kV/m) trn dy dn ni

Thi gian 5kA 10kA 20kA 40kA 65kA mt sng (s)

0,1 66,6 133,33 266,66 533,33 866,66 0,5 13,33 26,66 53,33 106,66 173,33 1 6,60 13,33 26,66 53,33 86,66 2 3,33 6,66 13,33 26,66 43,33 4 1,66 3,33 6,66 13,33 21,66 8 0,83 1,66 3,33 6,66 10,83 10 0,66 1,33 2,66 5,33 8,66

Mun tng phm vi bo v cho php ca CSV cn phi gim dc ca

sng truyn vo trm. thc hin iu ny th cn phi c bin php hn ch s

xut hin QA trn dy dn ca ng dy gn TBA vi bin v dc ln.

53

Nu trm u vi n ng dy, trong sng truyn theo mt ng dy

vo trm th tng tr sng Z.(n-1) song song vi CSV, c tc dng lm gim in

p tc dng ln cch in ca thit b trm, l nhn t thun li cho vic hn ch

QA cho trm.

3.3.1.2_nh hng ca dng in xung qua CSV n tr s in p d:

Dng in xung qua CSV ln qu, s gy nn trn in tr lm vic ca

CSV mt nhit nng vt qu gii hn cho php v c th lm hng CSV. Mt

khc, lm cho in p d ca CSV tng cao, nguy him cho cch in ca thit b

c bo v.

Khi CSV lm vic: 2Ut = I CSVZ + UdTrong : Ut l bin sng QA truyn ti trm.

Ud l in p d trn CSV khi dng in xung qua n bng qu tr s

dng in phi hp (5 14kA) ICSV l dng in xung qua CSV.

Z l tng tr sng ng dy.

Khi , dng in xung qua CSV s l :

ZUUI dutCS

= 2V (3.30) Nu ng dy khng c tng cng bo v on ti trm th khi st nh

gn ni t CSV, dng in st Is s phn b gia in tr ni t ca ct b st

nh v qua CSV nh sau :

c

duSCSV R

UII = (3.31)

in p d trn CSV c tnh nh sau :

12

2 ..

kt

xcpxdu I

IUICU == (3.32) Trong : Ucp l in p lm vic ln nht cho php. Trong li in 22kV, CSV

c th chn theo in p lm vic ln nht cho php bng 0,8Um.

54

(3.33) 1. ktcp ICU =Ikt : dng in km theo tn s cng nghip qua in tr khng ng thng

ca CSV.

1: h s khng ng thng ca in tr lm vic ca CSV trong phm vi dng in b, tn s cng nghip.

2: h s khng ng thng ca in tr lm vic ca CSV trong phm vi dng in xung tr s ln.

Ut A

CSV

Z A

2Ut Ud ICSV

Hnh 3.5 : S bo v trm xt dng qua CSV

Vi chng st van LA-18 chn tng ng vi li in 22kV ni t trc

tip th Ud = 43,4kV tng ng Ixm = 10kA, dng st Is =100kA, Rc = 10 th dng qua CSV l :

kA01I7,9510

43,4-100 xdmSV C =>>== kAI (3.34)

Nh vy, khi c sng st vi cng 100kA th h hng CSV l iu

khng trnh khi.

55

3.3.2_Tham s tnh ton ca sng st truyn vo trm:

3.3.2.1_Bin ca sng truyn vo trm: ly bng mc cch in xung ca

cch in ng dy trong khong cch c tng cng bo v Uo = U50% .

3.3.2.2_ dc u sng: chn theo kh nng nguy him nht c th xy ra.

Thi gian dc u sng s = B.Uo.x/C (3.35) dc u sng = Uo/s = C/B.x (3.36)

Trong : B l h s bin dng u sng di tc dng ca vng quang xung, tnh

theo 1/kV.

Uo l bin ca sng, bng U50% ca cch in ng dy trong khong

cch c tng cng bo v, tnh theo kV.

x l khong cch gia ni b st nh v trm, km

C = 0,3 km/s hay 300.000km/s l tc nh sng. l dc u sng, kV/s

3.3.3_Tnh ch tiu chng st ca trm do sng truyn vo:

Khi xc nh c v tr t ca CSV, ta xc nh c chiu di ti hn ca

khong cch ti trm phi c tng cng bo v:

xth = C/(cp.B) (3.37) Nu st nh ngoi khong cch ti hn, th sau khi truyn qua khong cch

xth vo trm, sng b vng quang lm gim dc u sng v khng cn nguy

him cho cch in ca thit b trong trm na. Cn mi trng hp lm xut hin

in p trn dy dn trong khong cch ti hn ny u xem nh nguy him v n

a vo trm sng c dc vt tr s cp. 3.3.4 Tnh ton qu in p sng truyn t DK vo TBA:

Vic tnh ton QA trn cch in ca MBA do sng QA truyn t DK

vo trm c thc hin nh p dng phng php trnh by trn. Cc phng n

tnh ton khc nhau nh sau :

* Thay i loi CSV.

* Chng st van c dng l loi in tr phi tuyn ZnO. Mt khc, theo

kinh nghim vn hnh sau mt thi gian lm vic BIL ca cc thit b in ch cn

56

khong 85%. Do , trong nghin cu c xt n yu t ny n tin cy BVCS

cho thit b trm.

* Thay i khong cch gia MBA v CSV (LMBA-CSV)

3.3.5_in p trn cch in ca trm:

Trong cc TBAPP, theo hng sng truyn c th c mt phn ca thit b

c bo v nm trc CSV nh dao cch ly, my ct ng dy cn mt phn t

sau CSV nh MBA. Vic kho st cc s n gin s cho thy nh hng ca

khong cch gia CSV v thit b c bo v v dc ca sng n in p tc

dng ln cch in ca thit b c bo v.

in p trn cch in c dng dao ng xung quanh tr s in p d ca

CSV vi chu k T = 4. = 4.LCSV-TB/v. in p cc i trn cch in c tr s bng: Ucdmax = Up + 2.a. (3.38)

3.4 Bo v chng st trm bin p:

Khi chn cch in ca TBA thng chn mc cch in trong cao hn mc

cch in ngoi, nhng qua thi gian vn hnh th mc gi ci ca cch in

trong din ra rt nhanh ; s phi hp cch in c th b ph hoi di tc ng ca

QA v c th phng in chc thng cch in trong ca thit b.

Bo v chng st (BVCS) bao gm BVCS nh thng v bo v chng QA

lan truyn t ng dy vo TBA. i vi cc TBA c BVCS nh thng gn

nh c tin cy rt cao. Trong khi tng hp cc gii php chng sng QA do

st lan truyn t ng dy vo trm cng khng th m bo d tin cy tuyt i

cho thit b TBA ; vn l chn gii php k thut hp l nhm nng cao tin

cy v hon thin phng n bo v, c chi ph b nht v u t xy dng, qun l

vn hnh thun tin cng nh hn ch tn tht do QA st gy ra.

3.4.1_S bo v chng st TBA :

Ngi ta thng s dng cc CSV bo v thit b trong LPP, vi s

lng v v tr t tu thuc vo yu cu BVCS, m ch yu v bo v cho MBA.

Tc dng bo v ca CSV ch yu da vo tnh cht gi cho s thay i in p ti

57

im t n nm trong phm vi hp, khi in p sng ti v dng in qua n thay

i trong phm vi rt rng. in p trn cch in cng ln khi khong cch gia

CSV v MBA cng xa v cng l c tnh quan trng cn nghin cu trong s

bo v MBA.

S chnh lch gia in p trn cch in MBA v in p d trn CSV

c xc nh bi s st p trn dy dn Udd v hiu in th gia cc im ni t ca CSV v thit b (Udcsv v Udtb)

Ucd = UdCSV + Udd + (Udcsv - Udtb) (3.39) Trong tnh ton thng chp nhn Udcsv - Udtb = 0. Ni t ca CSV v

thit b cn c bo v c b tr theo khong cch ngn nht.

Khi t CSV trc tip gn st thit b cn bo v hoc MBA th Ucd = UdCSV

. Trong trng hp my nu Ud < Ucp (Ucp l in p cho php QA kh quyn i

vi cc in ca thit b cn bo v) th vn BVCS cho thit b tr nn n gin.

Tuy nhin, trong thc t cn phi xt n hiu qu kinh t k thut cao nht m vn

m bo yu cu tin cy ca BVCS. Mt thnh phn quan trng trong s

BVCS cho TBA l phi bo v on ng dy u v cui trm bng cch b tr

cc chng st ng gim bin dng in i qua CSV.

Nh vy vai tr th nht ca on u trm l gim dng st i qua CSV, sau

ng vai tr lm gim dc dng st ; lm thoi mt u sng QA ca st

chy t ng dy vo trm.

Kt qu nghin cu cho thy, dng sng QA ti u MBA ph thuc rt

t vo bin ca sng ti v thng t gi tr cc i ti na chu k u tin ca

thnh phn chu k, tc l trng vi cc i u tin ca sng QA.

Khi thay i mt u sng ti s khng gy nn s thay i ln v dng sng QA, nhng thay i rt nhiu v bin sng QA trn u MBA.

Khi khng thay i bin v dc sng ti, nhng thay i khong cch

gia CSV v MBA hoc thay i cc c tnh ca CSV, QA trn MBA s thay

i. Khi tng Ip v U(t) th bin QA trn MBA cng tng.

58

3.4.2_Tnh ton in p trn cch in TBA:

3.4.2.1_Quy tc Petecxen:

Khi thay i mi trng truyn sng th s c hin tng phn x v khc x

ca sng ti im nt (im thay i mi trng) s c thnh phn khc x sang

mi trng mi v thnh phn phn x tr v mi trng c. Phng trnh in p

l :

Ut + Uf = Uk (3.40)

Trong : Ut l sng ti t mi trng 1.

Uk : sng khc x sang mi trng 2.

Uf : sng phn x tr v mi trng 1.

Cc khi nim c a ra nh sau :

H s khc x t

k

UU= (3.41)

H s phn x t

f

UU= (3.42)

Quan h gia 2 h s trn l : = 1 + . (3.43) S dng quy tc Petecxen tnh ton truyn sng gia 2 mi trng.

Sng truyn t mi trng c tng tr sng Z1 sang mi trng c tng tr sng Z2

di v tn (khng c phn x t u cui Z2).

Gi Uk l sng khc x sang mi trng Z2, ta c :

2Ut = Uk + Ik.Z1 (3.44)

2Ut = Ik.(Z1+Z2) (3.45)

It A Ik A

U A Z1 Z1

2Ut Z2 2Ut Z2(p)

Hnh 3.7 : S sng ti v sng phn x.

59

Trng hp tng qut khi Z2 l s phc th phi vit dng ton t v gii

theo quy lut ca ton t.

3.4.2.2_Quy tc sng ng tr:

Trong thc t, c nhiu phn t ng dy, cc tham s tp trung R, L, C

cng ni vo mt nt. cc phn t ny c tng tr sng Z1, Z2,... Zn v dc theo

chng c cc dng sng bt k U1x U2x,... Unx truyn v im nt. Ti im nt c

ghp phn t Zx (tham s tp trung). Gia cc phn t ny khng pht sinh ngu

hp vi nhau, quy c chiu dng in i v pha im nt l chiu dng, ta c

cc phng trnh sau :

Ux = Ix .Zdt = 2.Udt (3.46)

=

=n

mmx

m

dtdt UZ

ZU1

(3.47)

=

= nm m

dt

Z

Z

1

11 (3.48)

tnh in p ti im nt c th thay th cc tham s phn b bng tham

s tp trung hnh thnh mch bao gm Zdt v Zx ghp ni tip ngun e = 2.Udt c tr

s xc nh nh biu thc trn.

3.4.3_Tnh ton bo v chng st TBA:

tnh ton vi mt trm c th ta thay i bin v dc s ca sng st t ngay u trm, khi xc nh c Us v s c QA ti cch in ca thit b cn nghin cu c gi tr UQA = Ucp(t)

Vi nhng cp thng s khc nhau Us v s ta xc nh c ng cong sng nguy him i vi cch in thit b in trong trm. Trong trng hp c th

ta tnh ng cong sng nguy him i vi cch in MBA.

3.4.4_ tin cy ca s bo v chng st TBA:

tin cy ca s BVCS TBA phi c c trng bng s ln QA

trong mt nm i vi cch in ca thit b in. V QA do st v cc tham s c

lin quan vi s ln QA trong nm mang tnh ngu nhin nn chng ta phi ni

60

n tnh cht xc sut ca cc i lng ny. N ch c th c khng nh sau

nhiu nm nghin cu kho st.

i vi MBA, nh gi tin cy ngi ta thng dng ch tiu M=1/ - s nm MBA lm vic m khng xut hin QA nguy him do st i vi cch

in v l s ln xut hin QA nguy him do st trong trm. Kt qu nghin cu cho thy:

- i vi cc TBA ph ti 22kV b tr cc khu vc trng c kh nng b

QA do st truyn vo trm, khi gim khong cch gia MBA v CSV (LMBA-CSV)

th chiu di ti a on u trm cng gim dn. Mt khc, vi cng mt khong

cch LMBA-CSV nu dng CSV loi ZnO th Lngh-max ngn hn so vi cc loi CSV

khc, do tin cy ca s bo v MBA cng tng ln. Ngoi ra khi tng

cng cch in ca ng dy on u trm cng nh khi mc chu QA do st

ca MBA gim, chiu di Lngh-max phi c tng thm. Nh vy, i vi cc vng

c in tr sut cao m khng thc hin c in tr ni t ca ct Rct ta cn

xem xt tng cng cch in on ng dy dn vo trm

- Khi cch in ca thit b (MBA) gim, s ln QA nguy him tng ln v

s nm lm vic an ton v mt BVCS b gim xung. tng tin cy BVCS

cho MBA cng nh cc thit b trong trm cn phi gim khong cch LMBA-CSV - tng tin cy ca cc phng n bo v cho MBA cn phi gim s

ln QA nguy him khi st nh vo ct v sau l qu trnh phng in qua cch

in xung dy dn (gim ) trn on ng dy dn vo trm. tng tin cy bo v MBA (gim ) c th thc hin theo phng n sau :

a, Tng cng cch in ng dy on u trm bng cch tng thm s

lng cch in 1 hoc 2 bt. i vi cc vng c in tr sut ca t cao th bin

php ny cn c xem xt.

b, S dng ct g tng cng mc cch in ca ng dy, nht l on

u trm.

61

S dng loi CSV c ng c tnh Vn-ampe (in p d) thp hn

bo v cho thit b in. Vic s dng chng st van ZnO s lm gim chiu di ti

a on u trm v tng tin cy bo v MBA.

d, Gim in tr tip t on ng dy dn vo trm.

e, i vi cc TBA ph ti 22kV b tr cc thnh ph ln, c nhiu nh

cao tng vi khong cch bo v ngang tuyn bng 6 ln chiu cao treo dy dn th

khng phi b tr CSV ti u cc MBA.

Kt lun :

Qu in p kh quyn xut hin trn ng dy hoc l do st nh trc tip

hoc do st nh gn gy QA cm ng.

Khi st nh trc tip vo ng dy phn phi 22kV, th khng c cch

in no chu ng c QA v dng st. Do tnh kinh t-k thut, ngi ta

thng chp nhn m khng phi u t thit b chng st nh trc tip vo

ng dy v trm bin p phn phi 22kV.

Khi st nh gn ng dy phn phi 22kV, cn phi b tr thit b chng

st bo v ng dy v thit b, nht l on ng dy i qua khu vc c mt

st ln.

Khi ng dy phn phi 22kV i qua khu vc th, c nhiu nh cao tng

; l ni c khong cch ln hn 3 ln chiu cao treo dy trung bnh th cn xem xt

ch b tr chng st van trn on ng dy dn vo thnh ph.

62

CHNG 4

NH GI HIU QU KINH T

TRONG VIC NG DNG TI VO THC T

4.1 Khi lng v chi ph hng mc cng trnh :

4.1.1 Khi lng u t :

4.1.1.1 Tnh ton h thng tip t TBA :

H thng tip t TBA l cc cc hoc cc thanh kim loi c hnh dng, kch

thc khc nhau chn trong t v c lin h v in vi t, thng thng cc cc

v thanh ny khng c sn, qut bitum, tt nht l em m thic hoc m km.

Khi tnh chn dy dn s dng trong h thng tip t ngi ta cc iu

kin sau :

in tr phi b nht sao cho n khng gp phn lm tng in tr ca ton

b h thng tip t.

Phi c bn c kh v chng n mn kim loi.

Phi lin tc chu ng c dng in i qua sao cho nhit ca dy dn

khng c vt qu 150oC nu c lp t ni d trng thy v 95oC nu c

chn su xung t. Bng 8 cho ta nhng s liu v dng in ln nht cho php,

i vi dy dn ca h thng tip t sao cho vn m bo nhit khng qu

150oC [11, tr.171].

Bng 4.1 : Dng in ln nht cho php i vi dy dn ni t (A)

Tit din Dng in lu di Dng in cc i lu di cho php dy dn cho php trong thi gian di 1 giy (mm2) Thp ng Thp ng

16 - 150 - 2.500 25 - 200 - 4.000 35 - 250 - 5.500 50 100 350 3.000 8.000 70 175 - 4.500 11.500 100 200 - 6.000 16.000 200 300 - 12.500 32.500

63

Tit din nh nht ca dy dn ni t v cch thc hin cc mi ni cho phn

trung p cng thc hin ging nh phn h p. i vi li in c trung tnh ni

t trc tip, th bt c mt s chm t no dng in thc t cng u l dng

in ngn mch ; do vy, cc dy dn chnh hoc dy dn nhnh ca h thng tip

t c tnh ton vi ton b gi tr ca dng in chm t.

Cc mi ni tip xc nu khng c bo m tt th khi chm t mt pha nh

hng ca n khng th hin r, nhng thc t cc mi ni c th b yu hon ton

khi chm t kp. Nu cc mi tip xc xu khng c lu v khc phc, th

chnh cc mi tip xc y s l ngun gy nguy him cc ln chm t v sau ; v

khi in tr cc mi tip xc s ln v s to nn in p tip xc vt qu gi

tr cho php v tr nn nguy him. Nu ta thc hin cc mi ni tip xc bng cch

hn chc chn, th s m bo cc mi tip xc tt, in tr tip xc b. Tic rng,

vic ny khng c thc hin trong thc t do phc tp khi kim tra nh k v tr

s tip t cng nh khi o tr s tip t trc khi ng in

Cc ni t thng c ch to bng cc loi thp nh : thp trn, thp gc,

thp ng.

Thanh ni t thng c ch to bng cc thp dt, thp trn.

Kch thc nh nht ca cc cc v thanh thp khi chn trong t c quy nh

nh sau :

Thp trn c : ng knh 6mm. Thp gc v thanh thp dt : tit din 48mm2 v chiu dy cnh 4mm. ng thp : chiu dy thnh ng 3,5mm.

B phn ni t cng c th s dng ng (Cu) nhng gi thnh cao, d b mt

cp. Mt khc, tu trng hp c th cha chc tt hn thp v ng c th to

nn s in phn trong t nu nh bn cnh n l cc vt kim loi chn ngm khc

nh ng nc, v cp ... v do vy ng t c s dng lm b phn ni t.

Ring nhm (Al) th khng c s dng.

64

Cc ni t l thp c, thp gc hoc ng thp chn thng ng thng c s

dng rng ri nht v chng c nhng u im sau :

D ng su xung lp t c tr s n nh t b thay i theo nhit v m, chiu su ny thng ln hn 60cm.

bn c hc tt, c th c gii ho c vic ng xung t.

Ngoi ra, khi tin hnh tnh ton thit k h thng ni t cn phi lu cc vn

sau :

Vic tng ng knh hoc tit din ca cc ni t s khng kinh t v n nh

hng khng ng k n vic gim tr s in tr ni t m lm tng lng kim

loi s dng.

Trong mt h thng ni t nu s cc cng nhiu, khong cch gia cc cc

cng b th tr s in tr ni t c gim khng ng k do c hin tng mn

chn tng h gia cc cc trong mt h thng ni t.

Hin nay, nht l khu vc min Trung : Cc nh t vn thit k h thng tip a

TBAPP thng s dng h thng tia cc hn hp bao gm cc cc L63x6 m km

nhng nng, di 2,5m/cc v c b tr cch nhau bng 2 ln chiu di cc ; cc

cc c lin kt vi nhau bng cc thp dt 40x4 ; chn su ca cc v dy

lin kt thng l 0,8m. Tu theo a hnh, a cht cng trnh m s lng cc v

tia c khc nhau. Trong phm vi ti, ch minh ho tnh ton cho trng hp h

thng tip a TBAPP b tr trn nn t c cc lp a cht ng nht vi t = 300 .m ng vi yu cu in tr ni t R khc nhau. Cch tnh nh sau :

in tr ni t ca mt cc ni t chn thng ng [11]:

11

111

1 .4.4lg

21.2lg

.366,0LtLt

dL

Lr dc

++= (4.1)

65

in tr ton b thanh ni t t nm ngang c hoc khng c kt hp vi

cc cc chn thng ng [11]:

2

22

2 ..2lg

.366,0tbL

Lr dt

= (4.2)

Trong : t l in tr sut ca t [.m] L1 l chiu di mt cc [m].

L2 l ton b chiu di thanh ni [m].

d l chiu rng ca 1 cc [m]

b l chiu rng thanh ni [m].

t1 l su t mt t n tm ca cc [m].

t2 l chn su ca thanh ni [m].

in tr ni t R ca h thng tip a tia-cc hn hp :

cttc

tcd nrr

rrR ...

.+= (4.3)

Trong :

t l h s s dng ca thanh ni t nm ngang theo dng ng thng c kt hp vi cc cc chn thng ng.

c l h s s dng ca cc cc ni t chn thng ng b tr theo dng ng thng hoc mch vng.

t, c ph thuc vo khong cch gia cc cc k tip v chiu di cc ; ng thi ph thuc vo s lng cc n v xc nh bng cch tra bng.

n l s lng cc cc chn thng ng.

66

.....

....

2500

...... 5000 5000 5000 5000 5000 5000 5000 5000 5000 5000

.....

Mt t

t2 b

-40x4 t1 L2 L1

L63x6

d Hnh 4.1 : S b tr h thng tip a TBA thng dng

Kt qu tnh ton khi lng u t st thp theo gi tr yu cu ca in tr

h thng ni t r ca TBA, vi gi thit nu trn nh bng sau :

Bng 4.2 : chiu di v s cc theo in tr ni t yu cu

STT r TBA theo =500.m Ch.di thanh 40x4 (m) s n cc L63x6 1 10 30 6 2 8 40 8 3 7 50 10 4 6 60 12 5 5 70 16 6 4 100 20

67

4.1.1.2 Chng st van :

Nh phn tch Chng 3, i vi cc TBA nm su trong cc thnh ph

ln c cc nh cao tng th QA do st truyn t DK vo TBA l khng c. Do

vy, cc TBAPP thuc khu vc ny khng cn phi b tr CSV u cc MBA v

khi ta gim c u t 1 b CSV (3 ci), tng ng cho 1 TBA.

Vic gim mua sm 1 b CSV ng ngha vi vic khng phi t chc lp t, vn

chuyn, th nghim v qun l vn hnh, bo dng nh k.

4.1.2 Ch ph u t :

Ch ph u t c tnh theo n gi vt t, thit b hin hnh trong nc

km thng bo gi ca S Ti chnh-Vt gi hoc kt qu u thu thit b nhp

ngoi trong thi im mi nht. C s thit lp ch ph u t nh sau :

n gi xy dng c bn XDCB chuyn ngnh xy dng ng dy ti in

v trm bin p ban hnh km theo Quyt nh s 285/Q-NLDK, s 286/Q-

NLDK ngy 23/02/2004 ca B Cng nghip.

n gi XDCB chuyn ngnh th nghim hiu chnh ng dy v trm bin

p ban hnh km theo quyt nh s 85/1999/Q-BCN ngy 24/12/1999 ca B

Cng nghip.

Bng tng hp ly theo vn bn s 1664/CV-EVN-KTDT ngy 06/04/2005

ca Tng Cng ty in lc Vit Nam v vic hng dn iu chnh d ton cng

trnh XDCB chuyn ngnh in.

Vn bn s 1727/CV-NLDK ngy 05/04/2005 ca B Cng nghip v vic

hng dn iu chnh d ton cng trnh XDCB chuyn ngnh in theo Thng t

03/2005/TT-BXD ca B Xy dng.

4.1.2.1 H thng tip a TBA :

Chi tit nh ph lc 7 v ph lc 8 km theo

68

Bng 4.3 : Khi lng v chi ph u t theo in tr ni t.

STT R TBA Khi lng u t (kg) Chi ph u t (VN)

1 10 190.5 4.756.460 2 8 254.0 6.153.722 3 7 317.5 7.550.984 4 6 381.0 8.948.247 5 5 450.8 10.899.403 6 4 635.0 14.537.297

4.1.2.2 Chng st van :

Chi tit nh ph lc 9 km theo

Gi tr mt b CSV 22kV l : 2.950.823 VN

4.2 Phm vi ng dng v hng pht trin ca ti:

Phm vi nghin cu : Qua nghin cu, tnh ton b tr CSV hp l trn

LPP 22kV trung tnh ni t trc tip th phm vi ng dng ca ti l ti cc

thnh ph ln, ni c nhiu nh cao tng m st khng trc tip nh vo ng

dy v ct in c. Cn phn gi tr in tr ni t TBA theo cng sut MBA th

c th ng dng trn li phn phi 22kV trung tnh ni t trc tip.

Hng pht trin ca ti : D kin s tip tc nghin cu QA do sng

truyn t on ng dy 35kV dn vo cc TBATG. Trn c s , tm ra mc

cn thit phi b tr cc on tuyn c treo dy chng st i trong cc thnh ph ln

; ni c nhiu khu dn c cao tng. Ring v gi tr in tr ni t, c th m rng

nghin cu nh hng ca tr s ni t n cng sut TBATG 35/22kV c trung

tnh ni t trc tip hoc qua in tr ho