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Chng 3

BAO VE CHONG SET ANH TRC TIEP CHO HE THONG IEN

3.1 KHAI NIEM CHUNG Set anh trc tiep vao day dan cua ng day tai ien, vao cac thiet b

va bo phan mang ien cua tram phan phoi va nha may ien se gay nen qua ien ap nguy hiem lam ngan mach, cham at cac pha, lam h hong cach ien cua cac thiet b, gay gian oan s cung cap ien cho cac ho tieu thu, lam thiet hai ln cho nen kinh te quoc dan. V vay, he thong ien phai c bao ve mot cach co hieu qua chong set anh trc tiep.

Viec bao ve chong set anh trc tiep thng c thc hien bang cac cot thu set hoac day thu set.

ay la nhng ket cau gom: bo phan thu set, bo phan noi at va bo phan dan dong ien set noi lien ve ien hai bo phan tren vi nhau (H.3.1).

Bo phan thu set (1) cua cot thu set lam bang thep ong hoac thep thanh (co tiet dien khong nho hn 100mm2) at thang ng, c goi la kim thu set hoac bang day thep cang ngang gia cac cot trong trng hp day chong set. Bo phan dan dong ien set (2) c tao thanh bi ban than ket cau thep cua cot thep hoac be tong cot thep, hay bang day thep co tiet dien khong nho hn 50mm2 trong trng hp

Hnh 3.1 Nguyen ly cau tao

cua cot thu set

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kim thu set at tren ket cau cong trnh bang vat lieu khong dan ien nh ong khoi, cot go, mai nha. Bo phan noi at (3) c tao thanh bi mot he thong coc va thanh bang ong hoac thep noi lien nhau, chon trong at, co ien tr tan be e dong ien set tan mot cach de dang trong at.

nh cua bo phan thu set vt cao tren tat ca cac thiet b va bo phan mang ien can c bao ve.

Tac dung bao ve cua cot thu set (hoac day chong set) dien ra trong giai oan phong ien tien ao cua set. Dong tien ao phat trien theo phng co cng o ien trng ln nhat. Khi con tren cao, cach xa mat at th phng nay ch do ban than ien trng cua au dong tien ao xac nh, phong ien phat trien theo con ng thuan li nhat cho ieu kien ion hoa khong kh, hoan toan ngau nhien. Nh vay, cac vat tren mat at thc te khong co anh hng g en phan ln ng i cua khe tien ao (H.3.2a).

Hnh 3.2 Phng phat trien cua khe set khi con tren cao (a) va t o cao nh hng (b)

Nhng bat au t mot o cao H nao o, goi la o cao nh hng cua khe set, do co s tch tu ien tch cam ng trai dau vi mat o cao nhng ni co o dan ien cao mat at, nh cac ket cau kim loai, cay cao b ma t... trng cua dong tien ao b bien dang. Phng co cng o ien trng cao luc nay se la gia au dong tien ao va nh cua cac vat dan nho cao tren mat at (cot thu set, day chong set, cot ien, cot anten...) (H.3.2b).

Do o, dong tien ao se phat trien hng ve nh cac vat dan nay (cot, DCS). Nh vay, kha nang set anh vao nh cot (hoac day chong set) se tang va t co kha nang set anh vao cac vat thap xung quanh cot. Neu cot thu set vt qua mot gii han nao o so vi o cao cua vat can c bao ve at gan no, th hau nh toan bo cac lan set anh eu vao nh cot, cac vat se c bao ve an toan. Khu vc an toan o c goi la pham vi bao ve cua cot thu set.

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Pham vi bao ve nay phu thuoc vao nhieu yeu to: chieu cao, so lng, cach bo tr cac cot thu set, chieu cao nh hng cua set va cac ieu kien a chat thuy van cua ni at he thong thu set.

3.2 XAC NH PHAM VI BAO VE CUA COT THU SET - MO HNH A. KOPIAN Pham vi bao ve cua cot thu set c

xac nh bang thc nghiem tren mo hnh va x ly so lieu theo nguyen ly thong ke. Phong ien set c mo phong bang phong ien tia la xung trong khoang cach khong kh ln gia mot ien cc thanh (1), ac trng cho au dong tien ao va mot ien cc thanh (2), ac trng cho cot thu set at tren mot tam kim loai noi at tot, ac trng cho mat at (3) theo s o hnh 3.3.

Cot thu set (2) at tren tam kim loai (3) co o cao h. ien cc thanh (1) c at o cao nh hng H (theo ty le a chon cua mo hnh) so vi mat at.

Gia o cao nh hng H va o cao cua bo phan thu set h co quan he /k H h= . Ty le nay cung c xac nh bang thc nghiem k = 20 oi vi cot

thu set h 30m, khi h > 30m th o cao nh hng hau nh khong phu thuoc vao h, bang khoang H = 600m.

k = 10 oi vi day chong set, khi o treo cao day chong set 30CSh m ,

khi 30csh m> th o cao nh hng cua set khong thay oi va bang khoang H = 300 m.

Tren ien cc thanh (1) cho tac dung mot ien ap xung chuan dng co bien o bang ien ap phong ien xung be nhat U50% cua khoang cach kh gia au cc thanh (1) va tam kim loai (3). Phai dung song cc tnh dng (mac du phan ln set anh xuong mat at co cc tnh am) e cho phong ien tien ao xuat phat t ien cc thanh (1) ch khong phai t cot thu set. Nh vay, kch thc mo hnh be hn, ngoai ra se co mot o d tr nhat nh trong viec xac nh pham vi bao ve cua cot (hoac day) thu set (trong ien trng rat khong

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ong nhat phong ien xuat phat t ien cc thanh dng va ien ap phong ien xuyen thung khoang cach kh be hn hai lan so vi khi cc thanh am). Cot thu set (2) gi co nh, thay oi v tr cua ien cc (1) tren mat phang ngang tng ng vi o cao nh hng H va xac nh xac suat set anh vao cot thu set phu thuoc vao v tr ien cc ( moi v tr cua ien cc, cho phong ien nhieu lan, v mc o tan man cua phong ien trong khoang cach ln rat cao).

Ket qua thc nghiem cho thay khi R 3,5h (H.3.4) th toan bo so lan phong ien eu tap trung vao nh cot thu set, khu vc nay c goi la khu vc co xac suat 100% set anh vao cot. Khi R > 3,5h th co mot so lan phong ien xuong at.

Hnh 3.4 Khu vc co xac suat 100% set anh vao cot thu set

Hnh 3.5 Xac nh xac suat set anh vao cot thu set

Khi R tang th so lan phong ien xuong at cang nhieu. Do cot thu set lam bien dang trng cua dong tien ao nen ni o bo cua set mat at b lech ve pha chan cot mot khoang cach be nhat ro 1,6 h. Nh vay khoang cach be nhat ro la ban knh cua pham vi bao ve ngay tren mat at (H.3.5).

e xac nh pham vi bao ve cho mot vat co o cao hx, ngi ta dung mot thanh kim loai co o cao hx (theo ty le a chon cua mo hnh) at cach cot thu set khoang cach rx (H.3.6). ien cc (1), cot thu set (2) va vat c bao ve (4) cung nam trong mot mat phang vuong goc vi mat at (3).

moi v tr cua ien cc (1) xe dch vat c bao ve (4) ra xa dan cot thu set (2) tc tang dan rx cho en khi xay ra phong ien vao vat c bao ve. Va moi v tr tng ho o cua chung tien hanh phong ien nhieu lan. T o,

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xac nh c khoang cach gii han rx ma vat co o cao hx khong b phong ien. rx chnh la ban knh cua pham vi bao ve cua cot thu set o cao hx.

So lan phong ien moi v tr cua ien cc cang ln th o tin cay cua pham vi bao ve cang cao.

Cach xac nh pham vi bao ve cua mot he thong cot thu set va cua day chong set cung c tien hanh tng t.

T phng phap thc nghiem tren mo hnh o, a xac nh c pham vi bao ve rx theo hx nh sau:

3.2.1 Pham vi bao ve cua cot thu set Vi o tin cay 99% th pham vi bao ve cua cot thu set co o cao xh la

mot hnh chop tron xoay co ng sinh dang hyperbol xac nh theo:

1 6, xxx

h hr h p

h h

=

+ (3.1)

vi: p = 1 khi h 30 m

/30 5 5,p hh

= = khi 30 m < h 60 m.

Hnh 3.6: a) Cach xac nh pham vi bao ve cua mot cot thu set b) Pham vi bao ve cua mot cot thu set

o cao vt len tren vat c bao ve cua cot thu set

a xh h h= goi la o cao hieu dung cua cot thu set

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Thc te van hanh cung cho thay rang oi vi cot thu set co o cao ln hn 60 m th set khong ch anh vao nh kim thu set ma con anh ca vao mot phan than cot gan nh. V vay chieu cao cua pham vi bao ve cua cot thu set co o cao t 60250 m giam con h = h h.

- oi vi cot thu set cao t 60100 m th h tnh theo 0 5 60, ( )h h = (3.2a)

- oi vi cot thu set cao t 100250 m th h tnh theo h = 0,2h (3.2b)

Hnh 3.7 Pham vi bao ve cua mot cot thu set

co o cao 60 250m h m

Neu o cao hieu dung ha cua cot thu set bang hay ln hn ( )ah h h th pham vi bao ve cua cot thu set c tnh theo (3.1), trong o h c thay bang 'h h h= .

Neu o cao hieu dung nho hn ( )ah h h < th cot thu set khong con pham vi bao ve na.

Trong thiet ke, e n gian, ngi ta thng thay the ng sinh dang hyperbol gii han khu vc bao ve bi hai oan thang (H.3.8).

Hnh 3.8 Pham vi bao ve cua mot cot thu set (phng phap n gian hoa)

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oan ab noi lien nh cot thu set a vi iem a tren mat at cach chan cot oan 0,75ph.

oan bc noi lien iem c co o cao 0,8h tren cot thu set vi iem c tren mat at cach chan cot 1,5ph (o cao cua iem b bang /2 3h ), nh vay co ngha la neu vat c bao ve co o cao hx /2 3h th pham vi bao ve c xac nh bi:

1 5 10 8

, ( ),x

x

hr hp

h= (3.3)

va neu 2 th 0 75 13

, ( )xx xh

h h r hph

> = (3.4)

3.2.2 Pham vi bao ve cua hai cot thu set

1- Hai cot thu set co cung o cao

Nh tren a trnh bay, khi ien cc (1) v tr R 3,5h tren o cao inh hng th 100% so lan set anh se vao cot thu set (2). Nh vay neu hai cot thu set cung chieu cao h cach nhau a = 2R = 7h th moi iem tren mat at gia hai cot thu set se khong b set anh.

T o suy ra, neu hai cot thu set at cach nhau a < 7h th chung co the bao ve c mot vat co o cao ho at gia chung, vi ho xac nh theo:

hay 7 7o oa a

h h h hp p

= = (3.5)

Hoac noi mot cach khac, e bao ve mot o cao ho gia hai cot thu set th khoang cach a gia hai cot thu set phai thoa ieu kien:

7 ( )oa p h h

Hnh 3.9 trnh bay cach xac nh pham vi bao ve cua hai cot thu set bang phng phap n gian hoa: Tren mat chieu ng, pham vi bao ve pha ngoai hai cot thu set c xac nh nh oi vi moi cot rieng le. Khu vc gia hai cot c gii han bi mot cung tron qua hai nh cot 1 2,A A va iem B co o cao ho gia khoang cach hai cot (H.3.9a).

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Hnh 3.9 Pham vi bao ve cua hai CTS cung chieu cao

Hnh 3.9b la mat chieu bang cua pham vi bao ve o cao hx. Be rong be nhat rox gia hai cot c xac nh nh la co mot cot thu set o cao ho at gia khoang a, bao ve cho vat co o cao hx (H.3.9c).

2- Hai cot thu set co o cao khac nhau

- Neu hai cot thu set co o cao khac nhau, v du 1 2h h> th pham vi bao ve cua chung c xac nh nh sau (H.3.10):

- Pham vi bao ve pha ngoai hai cot thu set giong nh pham vi bao ve cua tng cot rieng le.

- Pham vi bao ve gia hai cot co c bang cach qua nh cot thap ( 2h ) ve mot ng thang ngang, no cat ng sinh cua pham vi bao ve cua cot cao

1h iem 3, iem nay c xem nh la nh cua mot cot thu set gia tng

1 2h h = va khu vc bao ve gia cot thap 2 1 va hh cach nhau a nh a trnh bay tren gia hai cot co o cao bang nhau.

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Hnh 3.10 Pham vi bao ve cua hai cot thu set co o cao khac nhau

3.2.3 Pham vi bao ve cua nhieu cot thu set

Khi cong trnh can bao ve chiem mot dien tch rong ln th ngi ta thng dung mot he thong nhieu cot thu set e bao ve. e xac nh pham vi bao ve, ngi ta chia he thong cot thu set thanh tng nhom ba hoac bon cot thu set gan nhau. Mat bang cua pham vi bao ve cua ba cot thu set khong nam tren cung mot ng thang trnh bay hnh 3.11a va cua bon cot thu set at bon goc cua mot hnh ch nhat trnh bay hnh 3.11b.

Ben ngoai dien tch cua a giac i qua chan cac cot thu set (H.tam giac hoac hnh ch nhat) pham vi bao ve c xac nh nh gia tng oi cot thu set vi nhau. Con tat ca cac thiet b co o cao ln nhat hx at trong dien tch cua hnh tam giac hay dien tch hnh ch nhat se c bao ve an toan neu ieu kien sau c thoa man:

8( )xD h h p (3.6)

vi p = 1 khi h 30 m

5 5,ph

= 30m < h 60 m

D la ng knh vong tron ngoai tiep cua hnh tam giac hay hnh ch nhat chay qua nh cac cot thu set.

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Hnh 3.11 Mat bang cua pham vi cua ba cot thu set (a) va bon cot thu set (nam tren bon nh cua mot hnh ch nhat)

Neu cac cot thu set co o cao tren 60m, th pham vi bao ve ven chu vi gia tng oi cot, theo chieu cao (mat cat ng cua pham vi bao ve) phai giam i h ke t nh, con pham vi bao ve ben trong dien tch cua a giac van gi nguyen theo (3.6). h cung c tnh theo (3.2a) hoac (3.2b).

Neu so cot thu set nhieu hn ba va bo tr bat ky th phan ra tng nhom ba cot gan nhau va kiem tra ieu kien bao ve theo (3.6).

3.3 PHAM VI BAO VE CUA DAY CHONG SET

Phng phap xac nh pham vi bao ve cua day chong set cung tng t nh doi vi cot thu set. ien cc (1) ( o cao nh hng) va vat can bao ve (4) i chuyen trong mat phang thang goc vi day chong set (2). Ket qua thc nghiem cho thay, khu vc co xac suat 100% set anh vao day chong set ve moi ben cua day chong set co chieu rong bang B = 2h va ngay tren mat at ve moi ben cua day chong set no bao ve c mot giai co chieu rong b = 1,2h vi h la o treo cao day chong set (B tng ng vi R trong trng hp cot thu set, b tng ng vi r ).

3.3.1 Pham vi bao ve cua mot day chong set

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Hnh 3.12 Pham vi bao ve cua mot day chong set

Cach ve gii han pham vi bao ve cua mot day chong set trnh bay trong hnh 3.12. Neu vat can bao ve co o cao hx th pham vi bao ve vi o tin cay 99% c xac nh theo:

1- Vi day chong set co o treo cao h 30m

1 2, xxx

h hb h

h h

=

+ (3.7)

hoac theo phng phap n gian.

Khi 23x

h h> : 0 6 1, ( )xxh

b hh

= (3.8)

23x

h h : 1 2 10 8

, ( ),x

x

hb h

h= (3.9)

2- Vi day chong set co o treo cao 30m < h < 250m

Pham vi bao ve theo chieu cao (mat cat ng) giam mot khoang h tnh t nh. Vi h tnh theo:

30 100 0 29 (h 30),m h m h< = (3.10a)

100 250 0 2,m h m h h< < = (3.10b)

3.3.2 Pham vi bao ve cua hai day chong set

Khi hai day chong set at cach nhau S = 2B = 4h th moi iem tren mat at nam gia hai day chong set se c bao ve an toan. Neu S < 4h th gia hai day chong set co the bao ve c mot o cao:

4oS

h h= (3.11)

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Khi day dan ba pha cua ng day tai ien cung nam tren mot mat phang ngang th ieu kien e day gia vi o cao hDD c bao ve la khoang cach S gia hai day chong set phai thoa ieu kien:

4 ( )DCS DDS h h< (3.12)

Gii han pham vi bao ve (mat cat ng) pha ngoai hai day chong set cung giong nh oi vi tng day chong set rieng le, con khu vc bao ve gia hai day chong set c gii han bi cung tron ve qua hai iem treo day chong set va iem gia co o cao ho (H.3.13a).

Hnh 3.13 Pham vi bao ve cua hai day chong set

Tren mat chieu bang (H.3.13b), be rong cua pham vi bao ve cho vat co o cao xh bang 2 xS b+ .

3.3.3 Cach xac nh pham vi bao ve cua day chong set cua ng day tai ien cao ap

Trong thc te, day chong set thng c dung e bao ve cac day dan cua ng day tai ien cao ap. o treo cao trung bnh cua day dan thng ln hn

/2 3h , do o trong trng hp nay viec ve toan bo pham vi bao ve cua day chong set la khong can thiet va thong thng ch can xac nh goc bao ve la u.

la goc tao thanh gia ng thang noi lien cac iem treo day chong set va day dan va ng thang vuong goc vi mat at qua day chong set (H.3.14).

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Hnh 3.14 Goc bao ve cua day chong set

cang be th xac suat set anh vao day dan v cang be:

490

lg ch

v

= (3.13)

vi hc chieu cao cua cot ien, m; - goc bao ve, tnh theo o.

trng hp gii han 23DD DCS

h h= th day chong set co the bao ve

c mot goc gh bang: 31 0 6( , )o

gh ghtg = = .

Song trong thc te, e tang mc an toan, tc la giam xac suat set anh vong qua day chong set vao day dan, ngi ta thng chon = 2025o cho cac ng day tai ien quan trong.

ng day co chieu dai ln, cac cap sieu cao ap co the en hang ngan km, mot mua set co the chu en hang tram lan set anh, viec tang mc an toan (chon goc be) cua ng day se lam giam xac suat set anh vao day dan mot cach ang ke.

3.4 CAC YEU CAU KY THUAT KINH TE KHI DUNG HE THONG COT THU SET E BAO VE SET ANH THANG CHO TRAM BIEN AP VA NHA MAY IEN

cac tram bien ap va nha may ien trong nhng ieu kien nhat nh, cot thu set co the at ngay tren cac ket cau cong trnh cua tram va nha may nh cot xa, mai nha, ong khoi, cot en pha chieu sang... nh li dung cac o cao san co nay cua tram ma tan dung pham vi bao ve va giam c gia thanh xay dng cot thu set. Nhng khi set anh vao cot thu set, dong ien set truyen qua than cot tan qua ien tr noi at co the gay nen phong ien ngc t cac ket cau cong trnh (xa, cot) cua tram en cac bo phan mang ien (day dan,

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thanh gop) neu ien ap giang xung tren he thong noi at vt qua mc cach ien xung cua tram. Do o, viec at cot thu set tren ket cau cong trnh cua tram ch cho phep khi tram co mc cach ien xung cao va ien tr noi at be. oi vi tram 110kV tr len, cac yeu cau nay de dang thoa man. Con oi vi tram 35kV ch cho phep at cot thu set tren cac ket cau cong trnh cua tram (tr xa may bien ap), trong cac ieu kien, khi ien tr noi at cua ket cau co at cot thu set khong vt qua 4 trong pham vi co ban knh 20m, neu ien tr suat cua at 500m va trong pham vi ban knh 30m neu ien tr suat cua at > 500m.

Chan cua cac ket cau co at cot thu set phai c noi theo ng ngan nhat vao he thong noi at cua tram tai giao iem cua cac thanh can bang the, ong thi tai ni at cot thu set e tan dong set thuan li, noi at cua tram c tang cng bang cach bo sung them mot so tia hoac coc hoac to hp tia coc noi at tuy tr so ien tr suat cua at be hay ln, c goi la noi at bo sung.

Khi cac ieu kien tren khong thoa man, cung nh khi ien tr suat cua at > 1000m th cot thu set c at cach ly vi ket cau cong trnh cua tram va noi at cua cot thu set khong noi chung vao mach noi at an toan cua tram. Khi at cot thu set cach ly th khoang cach t than cot, thu set en cac vat can bao ve cua tram, trong khong kh, cung nh khoang cach trong at gia noi at cua cot thu set va noi at an toan cua thiet b, phai u ln e bao am khong co phong ien ngc t pha cot thu set sang thiet b c bao ve (H.3.15).

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Hnh 3.15 Cot thu set at cach ly

Khi set anh vao cot thu set ien the cua iem A tren than cot (tng ng vi o cao ln nhat cua vat c bao ve) c tnh theo:

( )sA s x A tbdi

U I R Ldt

= + (3.14)

va ien ap giang xung tren he thong noi at cot thu set c tnh theo:

s xU I R= (3.15)

vi: Is - bien o dong ien set, kA

LA - ien cam cua phan cot thu set t mat at en iem A, H Rx - ien tr tan xung cua noi at cot thu set,

( )s tbdi

dt - o doc au song trung bnh cua dong ien set, kA/s.

Trong tnh toan thiet ke chong set cho cac tram co cong suat ln, theo qui pham chong set hien hanh e co o an toan cao thng lay

150 30, /ssdi

I kA kA sdt

= = .

oi vi cot thu set bang thep kieu mang li, tr so ien cam theo n v chieu dai khoang 1 7,oL H m / .

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T o 150 30 1 7,A x AU R l= + .

Muon khong co phong ien t cot sang vat c bao ve, ro rang phai bao am sao cho o ben ien xung cua khoang cach khong kh cao hn ien ap xung tac dung len khoang cach khong kh o, tc la:

.KK Axx k A k K

x

UU E S U S

E= > > (3.16)

vi: KxU - ien ap phong ien xung be nhat cua khoang cach khong kh Sk, kV KxE - o ben ien xung trung bnh cua khong kh, kV/m

Con ien ap tren bo phan noi at cua cot thu set bang:

xx

x

UU E S U S

E= > > (3.17)

trong o xE la o ben ien xung trung bnh cua at, kV/m

o ben ien xung cua khong kh trung bnh khoang 500 kV/m va cua at khoang 300 kV/m. T o suy ra khoang cach an toan trong khong kh:

150 50

0 3 0 1500

, ,x Ak x AR l

S R l+

> = +

va trong at: 150

0 5300

,x xR

S R> =

Co the xuat phat t ieu kien an toan nay e xac nh tr so ien tr noi at can phai at c cua he thong thu set hoac khi a biet ien tr noi at th can c vao o e xac nh hoac kiem tra khoang cach an toan.

V du ve bo tr va tnh toan o cao he thong thu set

Hnh 3.16 trnh bay mat bang cua mot tram phan phoi ngoai tri 110 kV va thiet ke he thong bao ve chong set anh thang. Chieu cao ln nhat can c bao ve cua tram la o cao cua day dan hx = 11m.

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Hnh 3.16 V du ve bo tr va tnh toan chieu cao cua he thong cot thu set e bao ve tram bien ap

e bao ve chong set anh trc tiep vao tram se dung he thong kim thu set at tren ket cau cong trnh cua tram, v tr cua chung anh so t 1, 2,... 7, 8 (tren tru xa cong va cac xa trung gian).

C s e tnh toan ay la he thong cot thu set phai co pham vi bao ve trum kn toan bo cac thiet b va bo phan mang ien cua tram, mat khac phai co, o cao hieu dung va phai, khoang t (0,50,6) o cao cua cong trnh, khong gay tr ngai cho s van hanh bnh thng cua tram va bao am my quan cho cong trnh.

Chia mat bang cua tram thanh cac khu vc t I en V va lan lt tnh o cao hieu dung cua cac cot thu set trong tng khu vc, ket qua cho trong bang sau:

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Khu vc D (m) ha (m) hx (m) htt (m)

I 55 6,9 11 17,9

II 48,3 6,0 11 17,0

III 49,6 6,2 11 17,2

IV 49,6 6,2 11 17,2

V 53,8 6,7 11 17,7

La chon tat ca cac cot cung o cao hieu dung la ha = 7m, nh vay o cao cua tat ca cac cot thu set 7 11 18a xh h h m= + = + = (chu y: cac xa tram

co at kim thu set co o cao 11m).

T hnh ve thay rang cac oan thanh gop gan xa A va B co o cao 8,2m nam ngoai gii han khu vc III va IV, nen can kiem tra lai kha nang bao ve oi vi cac oan thanh gop nay.

Ve pham vi bao ve cua oi cot 4 7 va 6 8 18 8 2

1 6 1 6 18 10 818 8 2

,, , ,

,xh hx

r h mh hx

= = =+ +

45 818 18 6 6 11 4

7 7

,, ,o

ah h m= = = =

11 4 8 2 3 21 6 1 6 11 4 1 6 11 4 3

11 4 8 2 19 6

, , ,, , , , ,

, , ,o x

ox oo x

h hr h m

h h

= = = =+ +

Hnh ve cho thay cac oan thanh gop gan cac xa A va B eu nam trong pham vi bao ve cua cac cot thu set tng ng.

Ket luan: Toan bo trang thiet b va bo phan mang ien cua tram eu c bao ve kn chong set anh thang.

3.5. LY THUYET MO HNH IEN HNH HOC

3.5.1. C s ly thuyet

Nh a biet, tac dung bao ve cua he thong thu set dien ra trong giai oan phong ien tien ao cua cu set au tien. Khi kenh tien ao tien cang gan ve pha mat at th cng o ien trng pha mat at cang tang, ac biet nh cac vat the dan ien nho cao tren mat at. Va khi au kenh tien ao (K) cach cac nh nay (M, N, P...) khoang t vai ba chuc met en mot vai tram

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met, th cng o ien trng cac nh vat the nay a u cao e gay ion hoa khong kh va tao nen nhng dong plasma hng ve pha au kenh tien ao (K), ay chnh la nhng tien ao ngc va qua trnh nay c goi la qua trnh phong ien on set. Kenh phong ien on set nao en gan kenh tien ao (K) t may xuong nhat se c ket noi va nh vat v tr set o bo a c xac nh (H.3.17).

Hnh 3.17 Qua trnh phong ien on set.

Khoang cach t au kenh tien ao (K) ma set bat au nh hng en iem khi au cua kenh phong ien on set c ket noi (N) c goi la khoang cach phong ien on set hay la khoang cach phong ien cuoi cung ( )cr

(con co tai lieu goi la ban knh hap thu).

R.N.Golde (nha bac hoc Anh) la ngi au tien e xuat ly thuyet nay (1963), tiep theo la nhieu cong trnh cua cac nha khoa hoc khac bo sung, phat trien hoan thien va a no vao ng dung thc te.

Tat ca cac cong trnh nghien cu nay eu nhan thay rang khoang cach phong ien cuoi cung cr phu thuoc bien o dong set maxsi va cr cang ln khi

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maxsi cang cao theo dang:

maxn

c sr ki= (3.18)

nhng lai co s khac biet ang ke ve he so k va so mu n v du:

Bang 3.1

Tac gia k n Golde 6,72 0,8

Wagner 6,72 2/3 Brown 7,1 0,75 Link 3 1 Gary .........

9,4 ........

1/3 ..........

Tren c s quan trac va nghien cu set rong rai nhieu nam cua nhieu chuyen gia trong lanh vc, CIGRE e ngh thong nhat dung mot quan he chnh xac hn:

= + 6 82 30 1 max / ,max ( )si

c sr i e (3.19)

vi cr tnh bang met.

maxsi la tr so cc ai cua cu set am au tien, kA.

Theo cac quan he max( )c sr f i= th khoang cach phong ien cuoi cung cr cua dong set co bien o be nho hn so vi cr cua dong set co bien o ln.

ieu o co ngha la au kenh tien ao (K) cua dong set be se tien en gan nh vat the hn la au kenh tien ao cua dong set ln, tc la maxsi be th cr

ngan hn so vi maxsi ln.

1 1 2 2max max( ) ( )c s c sr I r I< neu 1 2max maxs sI I<

Nh vay neu oi hoi he thong thu set co hieu qua bao ve cang cao, o tin cay cang ln, th khi thiet ke can phai chu y en dong set co bien o cang be.

Theo tieu chuan cua CIGRE cac mc oi hoi o tin cay tng ng vi khoang cach phong ien cuoi cung va bien o dong set nh sau:

Bang 3.2

Mc o tin cay oi hoi

( )cr m maxsi (kA) Xac suat V max( )si Theo 1 Theo 2 Theo 3

Rat cao 20 3,7 98% 86,76% 99,9%

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Cao 30 6,1 95% 79,13% 99% Bnh thng 45 10,6 90% 66,6% 92%

Mc o tin cay oi hoi tng ng vi tam quan trong cua cong trnh oi vi nen kinh te, an ninh, quoc phong va chnh tr cua at nc. Theo bang 2 th:

1. Phan bo xac suat bien o dong set am au tien theo CIGRE (3.19) .

2. Phan bo xac suat xuat hien bien o dong set am au tien 26 1/ ,sI

isv e

= (theo Qui pham CS hien hanh).

3. Phan bo xac suat xuat hien bien o dong set am au tien theo cong bo cua Viet Nam (Trung tam NC set Gia Sang 1997).

3.5.2 Pham vi bao ve cua cot thu set (CTS)

1. Trng hp chieu cao h cua cot thu set be hn hoac bang khoang cach phong ien cuoi cung ch r .

T nh H cua CTS lam tam ve mot hnh cau (A) co ban knh bang cr . Hnh cau (A) cat mat phang ngang (B) cach mat at khoang cach bang cr theo

ng tron M. T mot iem M bat ky tren ng tron M lam tam ke mot cung tron (C) ban knh cr th cung tron nay va tiep xuc vi CTS tai nh H va

tiep xuc vi mat at. Cung tron (C) la gii han cua pham vi bao ve cua CTS; trong khong gian ba chieu pham vi bao ve tao thanh mot hnh chop tron xoay quanh CTS (H.3.18a) (co ng sinh la mot cung tron ban knh cr , tam di

chuyen tren ng tron M).

Khi tien ao set cham vao mat cau (A) th set se anh vao nh H cua CTS. Khi tien ao set cham vao mat phang (B) th set se anh xuong at.

Pham vi bao ve ln nhat ng vi = ch r .

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Hnh 3.18a Trng hp ch r<

Hnh 3.18b Trng hp ch r>

Ban knh ay cua pham vi bao ve bang:

100

20 2 2( )c cr OT r h h h r h= = = khi ch r= 0 cr r h= = pham vi

bao ve ln nhat.

Bieu thc nay cho thay pham vi bao ve cua cot thu set va phu thuoc vao chieu cao h cua CTS va phu thuoc vao dong ien set ( )cr f= max( )si .

Bien o dong set cang ln th cr cang ln, do o 0r cang ln, tc pham

vi bao ve cang rong, nhng o tin cay cang thap. (v xac suat xuat hien dong set co bien o cang cao cang be

1 2s si iv v< neu 2 1s si i> ).

2. Trng hp > ch r (H.3.18b)

T nh H cua CTS lam tam vi ban knh cr ve ban cau (A) va hnh tru

tron (D) ban knh cr nhan CTS lam truc. Hnh tru (D) cat mat phang ngang (B) cach mat at cr theo ng tron M. T mot iem M tren ng tron ve mot cung tron (C) ban knh cr . Cung tron (C) tiep xuc vi CTS tai iem H va

tiep xuc vi mat at tai T. Trong khong gian ba chieu no tao thanh mot hnh chop tron xoay quanh CTS co nh la H va ban knh ay la

0 cr OT r OH= = = . ay chnh la trng hp co pham vi bao ve ln nhat.

Nhng cung la trng hp hieu qua bao ve cua CTS kem, v phan CTS

cHH h r = khong co tac dung bao ve: khi tien ao set cham ban cau (A)

set anh vao nh H cua CTS, khi tien ao set cham vao mat phang ngang B th set se anh xuong at, nhng khi tien ao set cham mat tru (D) th set se anh vao phan than cot thu set.

cHH h r =

Tom lai trng hp chieu cao h cua cot thu set ln hn khoang cach phong ien cuoi cung cr th ch co phan cot co chieu cao ch r = co tac dung

bao ve va pham vi bao ve trong trng hp nay la ln nhat. Con phan than cot t nh H en H ( )cHH h r = co the b set anh vao, tc khong co tac

dung bao ve.

3.5.3. Pham vi bao ve cua day chong set

a) Trng hp o cao cua day chong set < ch r (H.3.19)

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Hnh 3.19: Pham vi bao ve cua DCS trng hp ch r< .

Cung vi cach ve tng t, pham vi bao ve cua day chong set (DCS) trong trng hp nay, trong khong gian hai chieu (tren mot mat phang vuong goc vi truc DCS) c gii han bi cac cung tron (C) co tam tai M ban knh bang cr .

Khi tien ao set cham vao mat tru A th set se anh vao DCS, con khi tien ao set cham vao mat phang B th set anh xuong at.

Pham vi bao ve ln nhat khi ch r= .

b) Trng hp > ch r (H.3.20)

Khi 2c cr h r< < pham vi bao ve thu hep ch con mot vung nam di day

chong set c gii han bi cung tron (C) co tam tai M va ban knh cr (H.3.20a).

h cang ln hn nhieu so vi cr th pham vi bao ve cang b thu hep va khi 2 ch r th pham vi bao ve khong con na (H.3.20b).

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Hnh 3.20: Pham vi bao ve cua day chong set, trng hp h > rc

c) Pham vi bao ve cua hai DCS.

Xet trng hp tong quat: hai DCS co o cao khac nhau, 1 2 2 ch h r< < .

Hnh 3.21 trnh bay cach xac nh pham vi bao ve trong mot mat phang vuong goc vi ng day.

T cac iem 1 2,H H tren cung mot mat phang vuong goc vi cac DCS1 va DCS2 lam tam ve cac cung tron 1 2,A A ban knh cr . Cac cung tron nay cat mat phang (B) cach mat at cr tai cac iem 1 2,M M va cat nhau tai 3M .

Hnh 3.21a: Trng hp h1 < h2 < Rc

Hnh 3.21b: Trng hp Rc < h1< h2

Hnh 3.21 Cach xac nh pham vi bao ve cua 2 DCS co chieu cao khac nhau.

T cac iem 1 2,M M va 3M lam tam ve cac cung tron (C) ban knh cr .

Cac cung tron (C) chnh la gii han cua pham vi bao ve cua hai day chong set 1 va 2.

IV. PHAM VI BAO VE CUA LI KIM LOAI

Pham vi bao ve cua mot li kim loai nam ngang co the xac nh t pham vi bao ve cua hai day thu set song song at cach nhau d bang canh cua o li hnh vuong (hoac la bang canh be nhat cua o li hnh ch nhat).

Tren c s cua mo hnh ien hnh hoc gia hai day thu set (nh la mot phan t cua li) pham vi bao ve c gii han bi mat cau (C) (H.3.22).

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Hnh 3.22 Pham vi bao ve cua mot o li

Trong ap dung thc te, e n gian thng thay the mat cau (C) bi mat phang ngang tiep xuc vi mat cau ( chnh gia chom cau). Mat phang gii han pham vi bao ve nay nam thap hn chieu cao cua li mot oan ; khong phu thuoc vao o cao cua li tren mat at.

Vi cho trc th be rong o li d bang:

2( )cd r= , m (3.21)

Neu cho trc be rong o li d th bang: 2 22( / )c cr r d = , m (3.22)

V du: Vi o li co kch thc 5m 10m (H.3.23) th canh be nhat cua o li bang d = 5m.

Hnh 3.23 V du xac nh v tr cua mat phang gii han () theo be rong be nhat cho trc (d) cua o li bao ve.

104

Neu cong trnh can c bao ve theo mc o tin cay cao th tng ng vi khoang cach phong ien cuoi cung 30cr m= th:

2 230 30 5 2 0 104 10 4( / ) , ,m cm = = = Neu cong trnh can c bao ve vi mc o tin cay bnh thng, tc

tng ng vi 45cr m= th:

2 245 45 5 2 0 0695 6 95( / ) , ,m cm = = =

V. DAY CHONG SET BAO VE CHO DAY DAN CUA NG DAY TAI IEN CAO AP Vi mc bao ve nao o, xac nh c khoang cach phong ien cuoi

cung ( )ci sir I tng ng vi bien o dong set siI . Tren mat phang vuong goc vi truc ng day lay truc DCS lam tam ve cung tron (Ai) ban knh cr va lay

truc DD lam tam ve cung tron (Bi) ban knh cir , 2 cung tron (Ai) va (Bi) cat ng thang (Di) cach mat at cir va cat cung tron (Bi) tai Mi. Neu au tien

ao set cham vao cung tron (Ai) set se anh vao DCS, neu au tien ao set cham vao cung tron MiNi cua cung tron (Bi) set se anh vao DD, con neu tien ao set cham vao ng thang Di set se anh xuong at (H.3.24).

Hnh 3.24 S phu thuoc cua pham vi bao ve cua day chong set vao bien o dong set

105

Vi tien ao dong set Ij > Ii th cung MjNj se ngan hn, co ngha la kha nang set anh vao DD se giam bt. Khi bien o dong set at en Ik th khong con vong cung (Bk) na, ng tron (Ak) cat trc tiep ng thang (Dk) cach mat at ckr tai Mk, co ngha la kha nang set anh vao DD la be nhat. Nh

vay kha nang set anh vao DD ch xay ra oi vi nhng dong set co bien o Is Isk. Isk chnh la bien o ti han cua dong set, tng ng vi khoang cach phong ien cuoi cung. 6 82 30 1 / ,( )skIck skr I e= + . Vi mot ng day tai

ien cao ap co cac kch thc hnh hoc a cho th khi set anh vi bien o dong set ln hn hoac bang gia tr ti han s skI I> th day dan se khong b set

anh tc la day dan nam trong pham vi bao ve cua day chong set. Con cac dong set co bien o skIs I eu co kha nang anh vao DD, va Is cang be th

xac suat set anh vao DD cang cao.

3.5.4. Phng phap qua cau lan va ng dung

a. Xac nh pham vi bao ve cua mot he thong thu set bat ky Phng phap lan qua cau set

C s tren ly thuyet mo hnh ien hnh hoc mot phng phap tong quat a c xay dng phng phap lan qua cau set e kiem tra xem vi he thong thu set a thiet ke, cong trnh can c bao ve co nam trong pham vi bao ve vi mc o tin cay oi hoi hay khong.

Ban knh cua mo hnh qua cau set c chon bang khoang cach phong ien cuoi cung cr tng ng vi mc o tin cay yeu cau theo mot ty le nao

o, v du t 1/100 en 1/500. Tam K cua mo hnh qua cau set mo phong au tia tien ao set, khi bat au nh hng ve pha he thong thu set bao ve cong trnh hoac ve pha mat at co cung khoang cach phong ien cuoi cung cr .

Xay dng mo hnh cua cong trnh c bao ve cung vi he thong thu set theo ty le a chon.

Bay gi lan qua cau set theo moi hng co the co cua cong trnh: - Xung quanh ngoai vi cua cong trnh. - Theo moi hng mai, noc, v.v... cua cong trnh.

Cong trnh coi nh c bao ve chong set anh thang vi mc o tin cay yeu cau neu:

- Khi lan qua cau set xung quanh ngoai vi cong trnh, qua cau set ch cham at va cham vao he thong thu set ma khong cham vao cong trnh c bao ve.

106

- Khi lan qua cau set theo moi hng mai, noc,... qua cau set ch cham vao he thong thu set ma khong cham vao cong trnh theo cac hng nay.

Can lu y, neu cong trnh co mai bang kim loai, co lp giap kim loai hay cot thep (cua cau truc be tong, cot thep) tng vach ngoai vi cong trnh va c noi at tot th co the coi chung nh mot bo phan cua he thong thu set.

Neu qua cau set cham vao mot phan nao o cua cong trnh th s bao ve cho phan nay cua cong trnh la khong at, can phai cai tien, tang cng he thong thu set cho en khi qua cau set khong con cham vao cong trnh bat c mot ni nao.

Trng hp cong trnh co cau truc n gian co the khong can thiet phai xay dng mo hnh ma thong qua trc giac (suy ngh, can nhac hoac tnh toan tren c s mo hnh ien hnh hoc) co the i en ket luan ve kha nang bao ve cua cong trnh.

e v du, hnh 3.25 trnh bay cach xac nh pham vi bao ve cua mot nhom 4 kim thu set co cung o cao h bao ve cho mai bang cua mot cong trnh kch thc X Y. qua cau set vi tam K, ban knh cr , tng ng vi mc o

tin cay yeu cau, tiep xuc vi nh cua 4 kim thu set va chom cau cach mat bang mai mot oan h. Nh vay, mai cong trnh nam hoan toan trong pham vi bao ve cua 4 kim thu set.

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Hnh 3.25 Pham vi bao ve cua 4 kim thu set

b. ng dung phng phap qua cau lan e xac nh v tr set anh tren cau truc cao co hnh dang phc tap

Khi lan mot qua cau vi ban knh bang khoang cach phong ien qua mot cau truc t moi hng, iem nao tren cau truc cham vao be mat qua cau th iem o co kha nang b set anh vao vi bien o dong set tng ng vi khoang cach phong ien bang ban knh qua cau. Ro rang la iem nao tiep xuc cang lau vi be mat qua cau khi lan eu th xac suat b set anh vao iem o cang cao. ta noi rang be mat nho ra cua iem o ln. Be mat nay chnh la quy tch nhng iem tam cua qua cau khi lan (xoay) xung quanh iem o. Ta nh rang tam cua qua cau chnh la au tien ao cua cu set. Do o oi vi mot iem nhon, quy tch cac iem tien ao tao thanh mat cau, be mat nho ra cua mot iem nam tren mat phang ch la mot iem, khi lan qua cau th be mat qua cau lap tc i qua iem khac. Ngha la xac suat set anh vao mot iem tren mat phang la nho nhat.

1. Xac suat set anh vao cac cau truc co hnh dang n gian

Ap dung phng phap mo ta tren, ta co the anh gia xac suat tia tien ao se cham vao mot cau truc n gian nh la hnh tru thon thang ng, mot cau truc mong hnh ch nhat dng ng, mot cau truc hnh khoi, mot cau truc hnh tru hoac mot hnh chop.

oi vi hnh tru thon thang ng (H.3.26) phan tren cung co xac suat tia tien ao cham en ln nhat (quy tch au tien ao la hnh ban cau). Tiep en la phan di cua cot (quy tch la cac ng tron).

108

Hnh 3.26 Quy tch cac iem au tien ao tai phan tren cua cau truc hnh tru nhon thang ng

oi vi mot cau truc mong hnh ch nhat nh bc tng (H.3.27) cac iem co xac suat tia tien ao cham en cao la hai iem goc cua bc tng (mot na hnh tron).

Hnh 3.27 Quy tch cac iem au tien ao tai cac bo phan khac nhau cua cau truc

mong hnh ch nhat

oi vi cau truc hnh khoi (H.3.28), cac iem co xac suat cao nhat la bon goc cua cau truc (mot phan tam mat cau) tiep en la cac iem tren cac canh ng va ngang (mot phan t ng tron).

Hnh 3.28 Quy tch cac iem au tien ao cua cau truc hnh khoi

oi vi hai trng hp bc tng mong va cau truc hnh khoi, dng nh xac suat tien ao set cham vao cac iem tren canh nam ngang bang vi xac

109

suat oi vi cac iem tren canh ng. Nhng cac iem tren canh nam ngang co xac suat tia tien ao cham vao cao hn do trong thc te set anh vao iem theo hng nam ngang khong pho bien bang cu set anh t hng nghien hoac thang ng.

oi vi cau truc hnh tru tron, o canh cung la hnh tron (hnh 3.29), cac iem tren canh tron co xac suat set anh cao nhat (Quy tch la mot phan mat cau).

Hnh 3.29 Quy tch cac iem au tien ao cua cau truc hnh tru

oi vi cau truc co hnh chop kim t thap (H.3.30), ta co the coi nh cau truc la s ket hp cua mot tru bo tr truc cua hnh khoi. iem co xac suat cao nhat nam ngon cua kim t thap (quy tch la mat cau) tiep en la cac goc cua ay hnh chop va sau o la cac canh xien. ieu nay giai thch tai sao set anh vao cac cau truc hnh chop thng xay ra ngon hoac rat gan ngon, va set van anh vao cac canh xien di ngon.

Hnh 3.30 Quy tch cac iem au tien ao cua cau truc hnh chop

2. Xac suat set anh vao cac cau truc co hnh dang phc tap

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Mot cau truc phc tap co the c coi nh la s ket hp cua cac cau truc n gian a c xem xet tren. Cac iem tren cau truc vi xac suat b set anh cao co the xac nh bang phng phap mo ta tren oi vi cac cau truc n gian thanh phan cua cau truc phc tap.

Ap dung phng phap nay oi vi cau truc co hnh dang phc tap (H.3.31) se nay sinh van e phu hoac giao nhau cua cac quy tch do viec gan ung cua cac cau truc n gian khac nhau.

Khi o, quy tch ngoai cung se mang tnh troi hn so vi quy tch ben trong. Do o, xac suat tia tien ao cham vao oi vi cac iem se t le vi kch thc (ngha la dien tch va chieu dai) cua quy tch troi hn nhng iem khac.

Hnh 3.31 Cau truc co hnh dang phc tap

Phan tch mot cau truc phc tap co dang bac thang cho thay rang xac suat set anh vao canh cua bac tren cung (A) va di cung (B) cao nhng thap oi vi canh cua bac gia do o ln cua cac quy tch tng ng. Trong thc te, ngi ta quan sat c set anh vao canh cua bat di cung cua cau truc co dang bac thang nh vay.

Hnh 3.32 Quy tch cac iem au tien ao oi vi cac iem A, B, C cua cau truc co hnh dang phc tap

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Vi phng phap tren, khi thiet ke he thong bao ve chong set, ta co the xac nh v tr lap at kim thu set nhng iem nhay cam vi set e co c he thong bao ve hieu qua va kinh te.