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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
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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.
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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
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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
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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
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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
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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
>+=