TRNG I HC IN LC

MN HC

NGN MCH TRONG H THNG IN

Qu in t 2

NI DUNG CHI TIT

Chng 1:Khi nim chung v ngn mch v dng in ngn mch trong h thng in

Chng 2: Thit lp s tnh ton ngn mch h thng in

Chng 3: Tnh ton ngn mch ba pha duy tr

Chng 4: Qu trnh qu in t v cc thng s ca my pht in khi ngn mch ba pha

Chng 5: Tnh ton dng in ngn mch qu

Chng 6: Ngn mch khng i xng

n tp

Qu in t 3

Chng 1:Khi nim chung v ngn mch v dng in ngn mch trong h thng in

1.1 Nhng khi nim v nh ngha c bn

a. Ngn mch v chm t mt pha:

- Ngn mch: hin tng cc dy dn pha chm chp nhau hoc chm chp dy trung tnh. Khi xy ra ngn mch th tng trca h thng gim xung, dng in chy trong h thng tngcao gi l dng ngn mch.

- Ngn mch mt pha (chm t) trong mng c trung tnh ni t trc tip l hin tng chm t ca mt pha xung t vdng ngn mch chy qua im trung tnh l kh ln.

Qu in t 4

- Chm t mt pha trong mng in c trung tnh khng ni t hay ni t qua cun dy dp h quang l hin tng m ti ni chm t dng in chy qua rt b v chy qua cc in dung k sinh tr v im chm t, thng rt b nn khng th c xem l dng ngn mch

- Tng tr ngn mch l tng tr trung gian ti ch ngn mch, tr s ca n ph thuc vo tip xc, mc pht h quang, cht liu Trng hp nguy him nht l ngn mch qua tng tr bng 0 gi l ngn mch trc tip

b. Cc dng ngn mch:

- Ngn mch ba pha i xng (k hiu N(3), 3PH): c nh ngha l ngn mch xy ra ng thi c 03 pha, tuy khng thng xuyn xy ra nhng y l loi s c nng n nht

Qu in t 5

- Cc dng ngn mch khng i xng l trng hp dng ngn mch khng cn bng gia cc pha

+ Ngn mch chm t 01 pha (k hiu N(1), 1LG)

+ Ngn mch 02 pha khng chm t (k hiu N(2), L-L)

+ Ngn mch 02 pha chm t (k hiu N(1,1) 2LG)

c. Nguyn nhn v hu qu:

- Nguyn nhn: nguyn nhn chung v ch yu ca ngn mch ldo cch in b h hng, m tc nhn gy h hng cch in cth l: b gi ci do thi gian lm vic qu lu, chu tc ng vmt c kh (nh o t, th diu, xe c va qut ), hay do cc loi vt (chim chc, rn, th vt ) hoc do gi bo, sm st. hoc ngn mch xy ra c th do thao tc ng ct nhm

Qu in t 6

- Hu qu:

Pht nng cc b rt nhanh gy chy n, gi ci cch in

Sinh ra lc c kh ln lm h hng cc thit b xung quanh

Gy st p li nh hng n sn xut

Gy mt n nh h thng nh hng n an ninh mng

To cc phn t gy nhiu t cc dng in bt i xng nh hng n cht lng in nng.

Lm gin on cung cp in cho cc h tiu th

d. Mc ch ca vic tnh ton ngn mch:

La chn cc trang thit b ph hp

Tnh ton hiu chnh cc phn t bo v cho h thng

Qu in t 7

La chn cc s h thng thch hp cho vn hnh

La chn cc thit b hn ch dng in ngn mch.

Nghin cu cc hin tng qu in t trong h thng

Nghin cu n nh h thng

1.2. Dng in ngn mch, ln v s bin thin theo thi gian:

1. Ngn mch vi ngun p khng i (ngn mch xa ngun):

a. Qu trnh qu khi ngn mch 03 pha mng in n gin,xt mch in n gin, vi cc ngun p c dng sau:

)120sin(

)120sin(

)sin(

0

0

++=+=

+=

tUutUutUu

mC

mB

mA

Qu in t 8

- V ngun l 03 pha i xng nn c th tch ring thnh tng pha nghin cu. Xt mch tng ng vi pha A:

Phng trnh cn bng p ch qu :

Trong :

l thnh phn chu k

l thnh phn t do

(Ta = L/R - c trng cho tc suy gim ca thnh phn t do)

)sin( += tUu mA

)()(.)sin()( titieCtZ

UtidtdiLRiu aCK

tLR

Nm +=++=+=

)sin()sin()( NCKmNm

CK tItZUti +=+=

Taa

tLR

a eieCti1

0 ..)( ==

Qu in t 9

Qu in t 10

Qu in t 11

Nhn xt:

9 C th tnh ton dng in ngn mch theo hai thnh phn: thnh phn chu k (xoay chiu) v thnh phn t do (mt chiu)

9 Thnh phn dng in chu k hon ton c th xc nh c bi s mch v sc in ng ngun sau thi im xy ra ngn mch

9 Thnh phn dng in t do mang c tnh ngu nhin, phthuc rt nhiu yu t, trng thi mch trc khi s c, tnh cht ph ti v thi im xy ra ngn mch Thnh phn t do xut hin mang tnh ngu nhin nhng c th bit c dng bin thin l hm m vi hng s thi gian Ta = L/R

9 V phng in phng php tnh th vic xc nh thnh phn chu k c ngha quan trng hn

Qu in t 12

b. Dng in ngn mch xung kch:

- Lun lun tn ti mt gi tr cc i i vi tr s tc thi ca dng in ngn mch gi l tr s xung kch ca dng ngn mch hay cn gi l dng in ngn mch xung kch.

- Dng ngn mch xung kch cng xut hin gn lin vi s tn ti ca thnh phn dng in t do, thnh phn t do t tr scc i th dng ngn mch xung kch cng s c gi tr cc i.

- Tr s ca dng xung kch ixk ng vi trng hp thnh phn tdo xut hin ln nht, vi ia0 = iamax = iCKm (ngn mch lc khng ti) v t = 0,01s:

+=+=+= TaCKmTaCKmCKmTaaCKxk eIeIIeiii

01,001,001,0

0 1..)01,0(

Qu in t 13

- Ngi ta t h s: , gi l h s xung kch

- Tu theo gi tr ca Ta, h s xung kch nm trong phm vi:

1 kxk 2- Tr s xung kch ca dng in ngn mch rt cn thit khi

tnh ton kim tra tc dng lc ca dng in ln cc trang thit b lc s c xy ra.

c. Tr s hiu dng ca dng in ngn mch ton phn:

- Tr s hiu dng ti mt thi im t no c nh ngha:

Taxk ek

01,0

1+=

222/

2/

21atCK

Tt

TtNt IIdtiT

I +== +

Qu in t 14

vi: T l chu k thi gian ca dng in xoay chiu

l tr s hiu dng ca thnh phn dng ngnmch chu k

l tr s hiu dng ca thnh phn bc 0, ly bng tr s ca thnh phn t do ia(t) ti thi im tnhton t

- Tr s Iat c th xc nh c theo biu thc chung ca thnh phn dng in t do (ng vi lc xut hin ln nht):

2CKm

CKII =

)(tiI aat =

CKxk

CKmxkCKmxka

TaCKm

Taaaat

Ik

IkiiieIeitiI

.2)1(

)1()01,0( ..)(

11

0

====

===

Qu in t 15

Thay vo ta c:

Do 1 kxk 2 ta c: chnh l phm vi thay i ca tr s hiu dng

cc i dng in ngn mch ton phn

d. Cng sut ngn mch:

- Cng sut ngn mch c nh ngha l:

trong :Utb in p dy trung bnh ca phn mng in c dng in

ngn mch trc khi xy ra ngn mchINt tr s hiu dng ca dng in ngn mch tnh ti thi im t

[ ] 222 )1(212)1( +=+= xkCKCKxkCKxk kIIkII31

CK

xk

II

NttbNt IUS .3=

Qu in t 16

- Cng sut ngn mch mang ngha sau:

Khi tnh cho dng in ngn mch qua my ct ta s nhn c cng sut ln nht sinh ra gia 02 cc tip im ca my ct. Do my ct cn phi c ch to sao: Sct SNt Khi tnh cho dng in ngn mch tng, tr s cng sut c s l cng sut tng h thng cung cp n im ngn mch

2. Ngn mch gn my pht in ng b ang vn hnh:

HT

tbNttbNt Z

UIUS2

.3 ==

Qu in t 17

Chng 2: Thit lp s tnh ton ngn mch h thng in

2.1. Nhng gi thit c bn:

Tn s h thng khng thay i: gi thuyt ny khng gy sai s nhiu v lm gim ng k cc php tnh, v d nh lc cc in khng s bng hng s.

B qua bo ho t: n gin coi mch t khng bo ho, khi in cm ca phn t c xem l hng s v mch in l tuyn tnh.

Thay ph ti bng tng tr hng: sai s mc phi nm trong phm vi cho php khi coi ph ti l hng s.

Qu in t 18

B qua cc i lng nh ca mt vi thng s ca mt sphn t: trong mt s cc bi ton tnh ngn mch khng i hi tnh chnh xc cao ta c th b qua cc i lung:

B qua dung dn k sinh ca cc ng dy in thp p

B qua mch khng ti ca cc MBT

B qua in tr ca cun dy my pht, MBT v in tr ng dy

Sc in ng ba pha ca ngun l i xng: thc t s bt i xng ca cc sc in ng l khng ng k.

Qu in t 19

2.2. H n v tng i:

1. Tr s tng i:

- S dng h n v tng i trong nhiu trng hp lm n gin ho rt nhiu cc php tnh v t gy nhm ln hn so vi cc h n v khc.

- Tr s tng i ca mt i lng c hiu l t s gia trs ca i lng trong h n v c tn vi mt i lng c bn c chn trc trong cng h n v.

- Trong h thng in c cc i lng c bn nh in p (U), sc in ng (E), dng in (I), cng sut (S), tng tr (Z), do ta c cc i lng trong n v tng i tng ng:

cbcb U

UU =)*(cb

cb UEE =)*(

cbcb I

II =)*(cb

cb SSS =)*(

cbcb Z

ZZ =)*(

Qu in t 20

Trong :

Ucb, Ecb, Icb, Scb, Zcb l cc i lng c bn c chn trc,

U, E, I, S, Z l cc i lng trong h n v c tn cn chuyn sang h n v tng i.

U*(cb), E*(cb), I*(cb), S*(cb), Z*(cb) l cc i lng trong h n v tng i

- Tu theo yu cu bi ton ta c th chn cc i lng c bn ph php, thng thng chn Scb v Ucb l c th tnh c cc i tng khc trong h n v tng i:

cb

cbcb S

UII .3.)*( = 2)*( .cb

cbcb U

SXX = 2)*( .cb

cbcb U

SRR = 2)*( .cb

cbcb U

SZZ =

Qu in t 21

- Sau khi thc hin cc php tnh trong h n v tng i, ta c th thc hin vic chuyn i ngc li sang h n v c tn nh sau:

cbcb UUU .)*(=

cbcb UEE .)*(=cb

cbcbcbcb U

SIIII.3

.. )*()*( ==

cb

cbcbcbcb S

UZZZZ2

)*()*( .. ==2. i h c bn:

- Trong mt s trng hp tr s tng c cho theo nhiu h c bn khc nhau, do trc khi tnh ton cn qui i vcng mt h c bn chn theo cng thc sau:

cb

dmdmcb U

UUU .)*()*( =cb

dmdmcb U

UEE .)*()*( =

Qu in t 22

cb

dmdmcb S

SSS .)*()*( = dm

cb

cb

dmdm

cb

dmdmcb S

SUUI

IIII ... )*()*()*( ==

dm

cb

cb

dmdmcb S

SUUZZ ..

2

)*()*(

=

dm

cb

cb

dmdcb S

SUUXX ..

2""

)*(

=

- V gi tr Sm v Um ca cc thit b khc nhau l khc nhau nn vic chuyn v h n v c bn chung lun lun cn thit

- Khi biu din trong h n v tng i th in p dy v in p pha c tr s bng nhau

Qu in t 23

3. H n v tng i trong tnh ton mng in c nhiu cp in p:

- Khi tnh ton mch in c MBA, thit lp c s tnh ton trong h n v tng i cn qui i cc thng s mch in v cng mt cp in p chn trc gi lcp in p c s.

- Cng thc bin i:

XkkkX

Ikkk

I

EkkkEUkkkU

n

n

n

n

221

021

0

210

210

)....(

....1....

....

====

Qu in t 24

Trong :

U, I, X l thng s ca on mch ang xt

U0, I0, X0 l thng s sau khi qui i v cp c s

ki - t s MBT tnh theo mt hng t in p c s n cp in p tip theo

1

01 U

Uk = ; 1

'1

2 UUk = ; ;

n

nn U

Uk'

1=

2.3. S thay th v thng s tnh ton ca cc phn t trong h thng in

1. ng dy:

Qu in t 25

a. ng dy trn khng (U < 35KV): mi on dy c ththay th bng mt tng tr Z (b qua in dung k sinh ca ng dy)

Trong h n v c tn, tng tr c xc nh:

()

()

()

- Trong h n v tng i, tng tr c xc nh:

Ucb - in p c bn bng cp in p ca mng c ng dy ang xt

lrR o.=lxX o.=

jXRZ +=

2002)*( .).(cb

cb

cb

cbcb U

SljxrUSZZ +==

Qu in t 26

b. ng dy cp v ng dy trn khng (66KV < U < 330KV):

- Trong h n v c tn, tng tr c xc nh:

()

()

- Trong h n v tng i, tng tr c xc nh:

ljxrZ o ).( 0+=lClbB o ... 0==

2002)*( .).(cb

cb

cb

cbcb U

SljxrUSZZ +==

202)*( ...cb

cb

cb

cbcb U

SlCUSBB ==

Qu in t 27

c. ng dy siu cao p (U > 400KV):

- Phng trnh ca mng 02 dng hn hp tng ng ca ng dy di:

+ M hnh theo s hnh :

=

=

1

.

.

1

1

.

.

1

2

.

.

2 .D B

. .cosh .sinh

.sinh .cosh

I

UCA

I

U

YZYZYZ

YZYZYZ

I

U

+=

+=

6.1

6.1

ZYYY

ZYZZ

Qu in t 28

Trong :

2. Cc my bin p:

a. My bin p 02 cun dy:

- Trong h n v c tn, t cc thng s do nh sn xut cung cp ta c th xc nh c cc i lng tr, khng ca my bin p:

() () ()

() ()

ljbglYYljxrlZZ

o

o

).(.).(.

00

00

+==+==

dm

dmNB S

UUX2

.100

%=2

.

=

dm

dmCuB S

UPRFe

dm

QUX =

2

0

dm0

dm

SU

IZ

2

0 .%100= 20200 XZR =

Qu in t 29

- Trong tnh ton ngn mch, cc thng s tn hao (RB, X0, R0) thng c b qua v chng khng nh hng ln n dng ngn mch. Khi s tng ng ch cn li in khng XB v my bin p l tng ( ).

- Chuyn sang h n v tng i, chn in p c bn l pha bn cao ca my bin p ta c:

H

C

UUk =

dm

cbCcb

dmNCcb

cb

dm

dmNcbB S

SUUU

US

SUUX ..

100%

)(..

100%

2

2

2

)(*

==

b. My bin p 03 cun dy:- B qua cc tn hao my bin p, ta c c s thay th c

dng n gin vi cc in khng:

Qu in t 30

()

()

()

- Chuyn sang h n v tng i, chn in p c bn l pha bn cao ca my bin p ta c:

dm

dmCNC S

UUX2

.100

%=

dm

dmTNT S

UUX2

.100

%=

dm

dmHNH S

UUX2

.100

%=

dm

cbCcb

dmCNCcb

cb

dm

dmCNcbC S

SUUU

US

SUUX ..

100%

)(..

100%

2

2

2

)(*

==

Qu in t 31

dm

cbCcb

dmHNCcb

cb

dm

dmHNcbH S

SUUU

US

SUUX ..

100%

)(..

100%

2

2

2

)(*

==

dm

cbCcb

dmTNCcb

cb

dm

dmTNcbT S

SUUU

US

SUUX ..

100%

)(..

100%

2

2

2

)(*

==

3. Khng in v t in:a. Khng in phn on:- Trong h n v c tn:

()

- Trong h n v tng i:dm

dmKK I

UXX.3

.100

%=

dm

cb

cb

dmK

cb

cb

dm

dmKcbK I

IUUX

UI

IUXX ..

100%.3.

.3.

100%

)(* ==

Qu in t 32

b. Khng in b ngang:- Trong h n v c tn:

()

- Trong h n v tng i:

c. T b dc:- Trong h n v tng i:

Kdm

dmK Q

UX2

=

2

2

)(* .cb

cb

Kdm

dmcbK U

SQUX =

2)(* .cb

ccbc USXX =

cb

Qu in t 33

4. Ph ti in:- Ph ti c thay th bng mt tng tr c nh Z:

()

()

()

- Trong h n v tng i:

jXRZ +=cos..

2

2

2

SUP

SUR ==

sin..2

2

2

SUQ

SUX ==

22

2

)(* ..cb

cb US

SUPR =

cb

22

2

)*( ..cb

cbcb U

SSUQX =

Qu in t 34

5. My pht in:

- S thay th tng ng ca my pht c th c biu thbng mt sc in ng EF v mt in khng XF.

- Cc gi tr ny thng c cho trc v php bin i sang h n v tng i ging nh phn trnh by trn.

2.3. Bin i ng tr s : Nhm n gin ho s trong vic tnh ton dng ngn mch tng hp, chng ta thng phi s dng cc php bin i s di dy:

1. Ghp song song cc nhnh c ngun:

=

==++++++= n

ii

n

iii

n

nndt

Y

YE

YYYYEYEYEE

1

1

21

2211.

.........

Qu in t 35

=

=+++=n

iindt YYYYY

121 ...

2. Bin i Sao Tam gic:a. Bin i Sao thnh Tam gic:

Qu in t 36

3

212112

.X

XXXXX ++=

1

323223

.X

XXXXX ++=

2

313113

.X

XXXXX ++=

b. Bin i Tam gic thnh Sao:

Qu in t 37

231312

13121

.XXX

XXX ++=

231312

23122

.XXX

XXX ++=

231312

13233

.XXX

XXX ++=

c. Bin i Sao - li:

Qu in t 38

= YXXX nmmn ..trong :

- Xm , Xn l in khng ca nhnh th m v n trong hnh sao.

- Y l tng in dn ca tt c cc nhnh hnh sao.

Php bin i ny s dng tin li trong tnh ton ngn mch khi c mt nt l im ngn mch v tt c cc nt cn li lcc nt ngun. Nu cc ngun l ng th th in khng tng h gia cc ngun c th b qua, lc s s tr nn rt n gin. V d, t s li hnh di khi cc nt 1, 2, 3, 4 c ngun ng th v nt 5 l im ngn mch

Qu in t 39

d. Tch ring cc nhnh ti im ngn mch:

Nu ngn mch trc tip 3 pha ti im nt c ni mt snhnh (s di), th c th tch ring cc nhnh ny ra khi vn gi u mi nhnh cng ngn mch nh vy.

Qu in t 40

S nhn c lc ny khng c mch vng s d dng bin i. Tnh dng trong mi nhnh khi cho ngn mch ch trn mt nhnh, cc nhnh ngn mch khc xem nh ph ti c sc in ng bng khng. Dng qua im ngn mch l tng cc dng tnh cc nhnh ngn mch ring r. Phng php ny thng dng khi cn tnh dng trong mt nhnh ngn mch no .

Qu in t 41

e. Li dng tnh cht i xng ca s :

Li dng tnh cht i xng ca s ta c th ghp chung cc nhnh mt cch n gin hn hoc c th b bt mt snhnh m dng ngn mch khng i qua.

Qu in t 42

Qu in t 43

Chng 3: Tnh ton ngn mch ba pha duy tr

3.1 Khi nim chung:

- Tnh trng ngn mch 03 pha duy tr c nh ngha l tnh trng ngn mch lu di, khi m tt c cc thnh phn t do xut hin trong qu trnh qu tt gn n gi tr 0.

- Thng thng rt t xy ra tnh trng ngn mch duy tr, bi vcc thit b bo v t ng c lp cc im ngn mch ra khi h thng. Tuy nhin, vn phi xt n ngn mch duy tr nh gi trng thi ngn mch nng n v pht nhit ca thit b trong tnh trng s c ko di.

Qu in t 44

3.2 My pht in trong trng thi ngn mch duy tr:

- Di tc ng ca b t ng iu chnh kch t (TK), c 02 trng hp xy ra cn phn bit:

+ Ngn mch xa my pht, TK vn gi c c in p u cc my pht tr s nh mc.

+ Ngn mch gn my pht, TK tng dng in kch t n tr s gii hn trong khi in p u cc my pht vn thp hn gi tr nh mc.

- Khi tnh ton ngn mch duy tr, s tng ng ca cc trng thi ni trn c xc nh nh sau:

Qu in t 45

+ Trong trng hp u my pht c xem nh l mt thanh ci c in p khng i (U = Um) m khng cn quan tm n in khng v sut in ng bn trong.

+ Trong trng hp sau, my pht c xem nh mt sut in ng Eqgh ni tip mt in khng ng b Xd.

3.3 Tnh ton dng in ngn mch duy tr khi my pht khng c TK:

- Khi khng c b TK th sut in ng ca my pht trc v sau thi im ngn mch l khng thay i (do dng kch t If khng i), c th xc nh c qua cng thc:

200

20 ).sin.()cos.( dq XIUUE ++=

Qu in t 46

Trong : U0, I0, cos tr s in p, dng in v h scng sut ca my pht trng thi xc lp trc khi xy ra s c.

- S bin i tng ng ca my pht ch n gin lsut in ng Eq v in khng Xd. Nh vy thc cht ca vic tnh ton ngn mch duy tr y ch l gii mch in tuyn tnh thng thng. 3.4 Tnh dng in ngn mch duy tr xt n nh hng ca TK:

- Lc u ngn mch xa ngun pht, Xng ln, my pht lm vic trng thi nh mc. Khi Xng gim dn (im ngn mch cng gn ngun pht) st p trn Xd trong my pht tng ln, gi in p u cc khng i TK tng dng kch t - tng sut in ng Eq.

Qu in t 47

- n khi Xng = Xth (in khng ti hn) no th Eng = Eqgh, l trng thi ti hn, nu Xng gim na th in p u cc sthp hn nh mc m khng iu chnh c na.

- Ta c th kt lun khi c ngn mch duy tr:

ghngd

qghN

dm

qghq

thng

IXX

EI

UUEE

XX

>+=