5.1 Nguyn tc hot ng5.2 Bo v dng in cc i (cp III)5.2.1 Bo v dng in cc i 5.2.2 Bo v dng in cc i c kim tra in p5.3 Bo v dng in ct nhanh ( cp I v cp II)5.4 Tng kt: bo v dng in 3 cp5.5 nh gi bo v qu dng in5.6 Bi tp mu

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Nguyn tc hot ng: BVD l loi bo v tc ng khi dng in i qua ch t thit b bo v ln hn gi tr nh trc.BVD c phn thnh: Bo v dng in cc i Bo v dng in ct nhanh

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5.2.1 Bo v dng in cc i 5.2.2 Bo v dng in cc i c kim tra in p*

5.2.1.1 Dng khi ng 5.2.1.2 nhy5.2.1.3 Thi gian tc ng5.2.1.4 Vng bo v5.2.1.5 S BI*

Kat: h s an ton 1,2Ktv: h s tr v 0.85Kmm: h s m my 1,3 n 1.8Ilvmax : dng lm vic cc i qua thit b c bo vnBI : t s bin dngKsd : h s s

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Knh > 1.1 - 1.3 khi lm bo v d trKnh > 1.5 - 1.8 khi lm bo v chnhINMmin : l dng NM nh nht qua ch t bo v khi nm cui vng bo v

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R le lm vic vi thi gian khng i khi dng in vt qu gi tr khi ng th gi l t tnh c lpR le lm vic vi thi gian xc nh no khi dng in vt qu gi tr khi ng th gi l c tnh thi gian ph thuc, gm c:c tnh thi gian c dc chunc tnh thi gian rt dcc tnh thi gian rt dcR le c c tnh ph thuc khi ng khi dng in vt qu gi tr dng khi ng, thi gian tc ng ph thuc vo dng in qua r le. Dng in qua r le cng ln th thi gian tc ng gim.

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c tnh thi gian c dc chun: Loi ny lm vic theo c tnh thi gian ph thuc khi dng in NM nh v c tnh thi gian c lp khi dng in NM ln. (Ni cch khc, khi dng in NM nh hn khong 10 n 20 ln dng in nh mc th c tnh l c tnh thi gian ph thuc. Khi dng in NM ln hn khong trn th c tnh l c tnh l ng thng). Thng dng bo v rng ri li phn phi

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c tnh thi gian rt dc: Loi ny c dc dc hn dc chun. c dng thay th c tnh c dc chun khi dc chun khng m bo tnh chn lc

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c tnh thi gian cc dc: Loi ny c dc ln nht, thch hp dng bo v my pht, my bin p ng lc, my bin p ni t nhm chng qu nhit. *

Nguyn tc: bo v pha trc c thi gian tc ng bng thi gian tc ng ca bo v k sau n cng vi khong thi gian

*Khong t bao gm (theo tiu chun IEC 255-4 khong 0.3 0.5s)Thi gian tc ng v tr v ca r leThi gian tc ng ct ca my ctSai s thi gian ca r le nh thi gianThi gian d tr

Cch chn c tnh ph thuc:Chn c tnh ca BV B. V c tnh raXc nh dng NM ln nht ngay st BV B (N2) IN2max ng vi c tnh BV B suy ra thi gian tc ng ca BV B (tB1). Vy tB1 l thi gian tc ng ca BV B khi NM ti N2. m bo tnh chn lc th thi gian BV A khi c NM ti N2 phi ln hn tB1: t A1 t + t B1 Xc nh c im A 1 trn c tuyn ca BV A . *Chn c tnh trong cataloge sao cho tho mn t A1 t + t B1 vi mi dng NM b hn IN2maxLu v cc c tnh phi cng cp in p

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Vng bo v qu dng cc i: khi dng in qua r le ln hn dng in khi ng.*

S sao (dng mng NTT)S sao thiu (dng mng KNTT)S s tm (ch chng chm pha, khng dng bo v MBA u sao tam gic v r le khng tc ng khi c NM 2 pha B-C pha th cp*

V d*

5.2.2.1 Dng khi ng 5.2.2.2 nhy5.2.2.3 Thi gian tc ng5.2.2.4 Vng bo v5.2.2.5 S BI*

phn bit gia NM v qu ti ng thi nng cao nhy ca BVD C, ngi ta dng s BV dng in cc i c kim tra p.Khi NM th dng in tng v in p gim xung nn c r le dng in v r le in p u khi ng ( BV ch tc ng khi c r le dng in v r le in p tha mn)Dng khi ng ca BV c tnh: *Trong biu thc khng c Kmm v sau khi ct NM, ngoi cc ng c t khi ng nhng khng lm in p gim nhiu nn cc r le khng tc ng cR rng khi khng c Kmm th nhy s tng. V dng khi ng nh

Yu cu ca r le gim p:R le gim p khng c tc ng i vi in p lm vic ti thiuR le gim p phi tr v trng thi bnh thng sau khi loi b NMin p khi ng c chn sao cho r le khng khi ng khi in p min v r le tr v ngay sau khi ct NM*Kat = 1.2Ktv = 1.25Ksd = 1 nu BU u sao n v sao n Ksd = 1.732 sao n tam gicUlvmin = 0.9Udm

*UN max l in p NM cc i khi c NM ( ch min)ti cui vng bo vKnh > 1.1 - 1.3 khi lm bo v d trKnh > 1.5 - 1.8 khi lm bo v chnhINMmin : l dng NM nh nht qua ch t bo v khi nm cui vng bo v

*Tng t nh 5.2.1.3

Vng bo v qu dng cc i: khi dng in qua r le ln hn dng in khi ng.*

S sao (dng mng NTT)S sao thiu (dng mng KNTT)S s tm (ch chng chm pha, khng dng bo v MBA u sao tam gic v r le khng tc ng khi c NM 2 pha B-C pha th cpThm BU*

*5.3.1 Bo v ct nhanh tc thi (cp I, khng c tnh nhy)5.3.2 Bo v ct nhanh c tr hon (cp II)

*5.3.1.1 Dng khi ng 5.3.1.2 Thi gian tc ng5.3.1.3 Vng bo v

Khi c 1 ngun cung cp: Dng in khi ng BV ct nhanh bng h s an ton nhn vi dng in ngn mch ln nht cui vng bo v*Vng bo v

Khi c 2 ngun cung cp: Dng in khi ng BV ct nhanh 2 pha phi ging nhau v bng h s an ton nhn vi dng in ngn mch ln nht cui vng bo v no ln nht*V d:

Khi c 2 ngun cung cp: trng hp tn ti vng khng bo v c (vng cht)*V d: Vng cht

*Bng zero

*Mun tm vng bo v ta gii phng trnh

*5.3.2.1 Dng khi ng 5.3.2.2 nhy5.3.2.3 Thi gian tc ng5.3.2.4 Vng bo v

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*Bng t

*Mun tm vng bo v ta gii phng trnh

V d*

Cp I : ct nhanh tc thiCp II : ct nhanh c tr honCp III : bo v dng cc i

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