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Chng 1: Sng hi v cht lng in nng

Phm Chnh HngTng Ha 4K50 Page 2

Tuy nhin, c mt nguyn nhn m trong bo co khng chra, l nhhng ca cc loi sng hi pht tn trn li ang ngy cng ra tng.

2.Mo ( nhim) sng hi (harmonic distortion):Vi s pht trin ngy cng nhanh ca cc loi ti phi tuyn, cc b bin iin t cng sut, mt vn ny sinh i vi cht lng in nng (electric

Power Quality) l s pht tn cc thnh phn sng hi ln li in.

Mt v d cho thy snh hng ca ti phi tuyn i vi dng in:

Hnh1.2 Dng mo do tc ng ca ti phi tuynNh ta bit, mt sng tun hon bt k u c th phn tch thnh tng

cc sng sin, trong c thnh phn c bn (bc 1) v cc thnh phn bc cao. (1.1)

Hnh 1.3 phn tch Fourier ca mt sng tun hon

2.1.Ngun gc sng hi:2.1.1.Cc ngun pht sng hi trong ti thng mi(commercial loads):a)Cc b ngun switching:

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Hnh 1.4 switch mode power supply

Do s phng np ca t, m dng in c dng nh sau:

Hnh 1.5 (a) dng chy trong b SMPS v (b) bin cc thnh phn sng hi

b)n hunh quang:

Cc loi n hunh quang chim t40 n 60% cc loi ti thng mi, tuynhin, do y l loi n phng in trong cht kh nn cn c mt chn lu nhmcung cp in p ln nhm to ra dng h quang khi khi ng, do vy sng hi

pht sinh ra rt ln.

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Hnh 1.6 n hunh quang vi (a) dng sng dng in trong chn lu in tv (b) Phn tch phdng in

c)Cc biu chnh tc :ng dng ph bin ca cc biu chnh tc (adjustable-speed drives

ASDs) trong ti thng mi l trong cc h thng iu khin ng c thang my,cc h thng bm, qut gi Mt b ASD bao gm mt b bin i cng sut

bin i in p xoay chiu vi bin v tn s cnh thnh in p vi bin v tn sthay i. Sthay i ca in p v tn s scho php thay i tc ng c ph hp vi yu cu cng ngh. H thng ASD cng c th tm thy rtnhiu trong cc ti cng nghip (industrial loads).

2.1.2.Ngun pht sng hi trong ti cng nghip (industrial loads):a)Cc b bin i ba pha:

Gm c cc b chnh lu ba pha cu i xng, khng i xng, nghch lu(DC bus l chnh lu ba pha)...

Mt u im ca cc b bin i ba pha so vi cc b bin i mt pha lchng khng sinh ra thnh phn dng in bc ba (chim ch yu thnh phn hidng in). Tuy nhin, chng vn l mt ngun pht thi sng hi ln.

Di y l dng sng dng in v phn tch cc thnh phn sng hi camt b chnh lu cu Thyristor ba pha vi gc mbng 30o:

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(a)

(b)

Hnh 1.7 Dng sng dng in ca b chnh lu cu ba pha v tr s cacc thnh phn sng hi

aDng sng dng in

bBin cc thnh phn hi

b)Cc thit b c khnng bo ha:

Bao gm bin p v cc thit bin t c li st, k c cc loi ng c.Sng hi sinh ra khi cc thit bny ri vo vng bo ha ca li st.

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Hnh 1.8 c tnh t ha my bin p

Mc d dng kch thch ca my bin p lc cha nhiu thnh phn snghi, trong thc t n vn nhhn mt phn trm gi tr ca dng in khi y ti.Tuy nhin, nh hng ca chng c th thy r khi trong h thng truyn ti cnhiu my bin p. Ta c th thy ln ca sng hi bc ba thng tng ln ngk trong thi im bui sng, khi m cc my bin p lm vic non ti v in ptng. Khi , dng in kch thch tng ln, do tr ssng hi cng tng ln.Hin tng in p mo do s qu kch thch ca bin p ch xut hin khi trngthi lm vic l non ti.

Cc ng c cng sinh ra cc hi dng in khi b qu kch thch.c)Cc thit b h quang:

Bao gm cc thit bnh l h quang, my hn h quang, cc thit b chiusng vi chn lu in t. c tnh VoltAmpere ca dng h quang l phi tuyn,sau khi hquang c sinh ra, in p gim, trong khi dng in h quang li tngln v b gii hn ch bi trkhng ca ngun.

Thnh phn sng hi trong cc thit b hquang cng tng tnh trongtrng hp chn lu in t (hnh 1.6). Tuy nhin, trong trng hp thit b hquang ba pha, ta c ththay i t ni dy ca bin p kh cc thnh phn bc

ba.

2.2.Cc thng snh gi mc sng hi trn li in:C hai thng s quan trng nh gi mc nhim sng hi trn ng

dy.

2.2.1.Total harmonic distortion(THD):THD xc nh mc nh hng ca thnh phn sng hi trong mt sng

mo. THD c tnh nh sau:

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THD = .100% (1.2)

Trong :

M1, ..., Mh l gi tr hiu dng ca thnh phn sng hi bc h.THD thng c s dng nh gi mo in p.

2.2.2.Total demand distortion:Khi nh gi mc mo dng in, cc nh nghin cu xut mt thng

skhc, l TDD. TDD c xc nh nh sau:

TDD =

.100% (1.3)

Trong :IL l gi tr ln nht ca thnh phn bc mt ca dng ti yu cu tnh ti

im kt ni.

3.nh hng ca nhim sng hi i vi cht lng in nng:3.1.Pht nng thit b:Do nh hng ca hiu ng mt ngoi, in trca vt dn gim xung khi

tn s tng, khi , tc dng nhit I2R tng lm cho vt dn b lm nng nhanhchng. Nu trong iu kin tn ti sng hi vi tr s ln, thit b vn c th b qunhit ngay ckhi mang dng nh mc. V tc dng nhit m khi thit k ch to,ta cn phi tnh d tr nhit ln hn, do lm tng gi thnh v kch thc.

Ngoi ra, do s pht x nhit ln hn so vi nh mc m lm cho tn haocng sut cng tng ln.

3.2.Dy trung tnh li ba pha:

Hnh 1.9 H thng ba pha cc ti phi tuyn mt pha, cn bng

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Thnh phn sng hi bc ba gy ra nhng vn ln trong h thng ba phabn dy nh trn hnh 1.9. Khi cc ti trn mi pha l cc ti tuyn tnh cn bng,tng dng in trn mi pha ti im trung tnh n bng khng, do khng cdng chy trong dy trung tnh. Khi cc ti tuyn tnh khng xc nh, trong dy

trung tnh s xut hin dng in, tuy nhin tr sdng in ny cng khng thvt qu gi trdng in trn mi pha.Tuy nhin, khi ti phi tuyn, trn mi pha s xut hin cc thnh phn iu

ha bc cao. i vi cc ti cn bng, thnh phn sng bc mt v cc thnh phnbc cao hn ba s trit tiu nhau ti im trung tnh n. Tuy nhin, nu tn tithnh phn bi ba, chng s cng li trong dy trung tnh. Khi , dng in chytrong dy trung tnh c th ln hn c thnh phn sng c bn trong mi pha. iuny cc k nguy him bi dy trung tnh c th b ph hy bi qu nhit do khngc bo v bi cc thit bng ct.

tnh dng in chy trong dy trung tnh, ta bit rng tt c cc thnhphn sng hi th t thun v nghch trn ba pha s trit tiu nhau ti im trungtnh, trong khi , thnh phn bi ba s cng li ti im trung tnh: (1.4)

T, ta thy rng dng chy trong dy trung tnh bng ba ln gi tr hiudng ca cc thnh phn bi ba tn ti trong mt pha. iu ny cho thy dngchy trong dy trung tnh c khnng ln hn dng chy trong tng pha.

3.3.Tc hi i vi my bin p:Dng in chy trong cun dy v t thng chy trong li st gy nn tn

tht cng sut ca my bin p. Cc sng hi bc cao ca dng in s gy thmcc tn hao trn vt dn m chng i qua, k c trn cun dy ca my bin p.Dng iu ha chy trong cun dy ssinh ra dng iu ha t thng chy trongli thp, t gy ra tn hao t tr v tn hao dng Phu c trong li thp. Tn haot tr t l vi tn s ca t thng, trong khi tn hao dng Phu c t l vi bnh

phng ca tn s. Do vy, dng iu ha bc cao c th gy ra tn hao ng ktrong li bin p. Tn hao cng thm ny c th gy ra qu nhit v lm hng cchin ca bin p. Mun khc phc, ngi ta phi tnh d tr bin p ln hn, do lm tng chi ph.

3.4.Tc hi i vi my in quay:

Sng hi gy ra thm cc tn hao t trng trong li thp ca ng c.Ging nh vi bin p, trong my in quay, tn hao t tr v tn hao dng Phu ctrong li tng khi c cc thnh phn sng hi bc cao t vo cun dy ng c.Tn hao dng Phu c tng t l vi bnh phng tn s v tn hoa t trtng vitn s. Dng hi cng gy ra thm tn hao trong cun dy ng c.

Mt nh hng khc, v c l l nghim trng hn, l sdao ng momen v snghi. Hai trong s nhng sng hi thng gp trn li in l hi bc nm v bc by.Sng hi bc nm l sng hi th t nghch, dn ti ttrng quay ngc chiu vi t

trng c bn vi tc bng nm ln tc c bn. Hi bc by l hi th t thun, ttrng quay cng hng ttrng c bn vi tc bng by ln c bn. T dn ti

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stng tc ttrng v dng in cm ng trn rotor to ra sdao ng ca trc ngc. Nu tn sdao ng trng vi tn s t nhin ca nhng thnh phn c, nhng thithi nghim trng c th xy ra. Srung ng v ting n qu mc trong hot dng cang c trong mi trng sng hi nn c kim tra ngn chn nhng thit hi.

Nhng ng c lm vic trong mi trng nhim sng hi cn c thit kcbit, cho php hot ng vi cc thit b ASD.

4.Cc tiu chun nh gi cht lng in nng:Hin nay trn th gii c hai tiu chun ph bin nhm khng ch mc

pht thi sng hi ln li.a)Tiu chun IEEE 5191992:

b)Tiu chun IEC 61000325:

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Khi nc ta ch n vn cht lng in nng, hon ton c th pdng c hai tiu chun ny.

5.Cc bin php nhm hn chsng hi trn li in:

[2]

-Passive filter.

-Active power filter.-Hybrid active power filter.

Di y ta s tm hiu ln lt phn tch u, nhc im v hn ch catng bin php.

5.1.Passive filter (lc thng):B lc th ng sng hi th ng c xy dng t nhng phn t th

ng (in tr, cun khng, tin) hn ch nhng tc hi ca sng hi. Nhngb lc ny thch hp cho h thng phn phi ba pha bn dy. Chng nn c tcng gn nhng ti cn lc cng tt, tt nht l ti im chia t ba pha thnh mt

pha ca li phn phi. Sng hi c thc gim thiu ch cn 30% khi dng ccb lc thng.

Lc thng c thc phn loi thnh lc song song v lc ni tip. Lcsong song c phn ra thnh hai loi cng hng ni tip v kiu by vi trkhng thp ti tn sc la chn. c mc gn vi ngun pht sinh nhiu, lcsong song loi b cc thnh phn sng hi khi ngun cp. y l loi lc ph bin

nht.Lc ni tip c phn ra thnh cng hng song song v kiu chn vi tr

khng cao ti tn sc la chn. N khng thng dng bi v c th gy moin p ti.

5.1.1.Lc thng ni tip:y l cu hnh thng dng nht cho ng dng mt pha vi mc ch lm

nh nht hi bc ba. Tn s c iu chnh ring khc cng x th c lc.Hnh1.5 ch ra cu trc c bn ca b lc thng ni tip.

Hnh 1.10 B lc thng ni tip

u im:

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-Cung cp in khng cao iu chnh tn s-Khng to ra bt k s cng hng h no trong h thng-Khng a thm cc thnh phn iu ha vo li-Ci thin h s cng sut dch chuyn v h s cng sut tng

Nhc im l:

-Phi chu ton b dng ti danh nh-Ch c hiu qu duy nht i vi tn ssng hi c chn-Ch c th cung cp cho ti phi tuyn5.1.2.Lc thng song songLc thng song song cng c khnng lc ring tng tn s sng hi

c iu chnh nh bc 5, bc 7, bc 11, Hnh 1.6 chra s thng thngca lc song song.

Hnh 1.11 B lc thng song song

u im:

-Cung cp in khng thp iu chnh tn s-Cung cp thnh phn hi ring ti ti hn l n t ngun-Ch yu cu dng hi qua v khng l ton b dng ti-Ci thin h s cng sut dch chuyn v h s cng sut tng

Nhc im l:

-Ch lc c mt tn sc iu chnh

-C th to ra cng hng h thng-C th thm vo sng hi t nhng ti phi tuyn khc-Nhiu b lc c yu cu m bo gii hn sng hi mong mun

5.1.3.in khng vo xoay chiu ni tip (Series Passive AC Input Reactor)Cu hnh c bn c ch ra trong hnh 1.7. Loi ny lc tt c tn s

sng hi, bng cch thay i ln.

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Hnh 1.12 in khng vo xoay chiu ni tip

u im ca bin khng ni tip l:

-Gi thnh thp-Nng cao h s cng sut thc

-Kch thc nh-B lc khng to ra cng hng h thng-Bo v chng li nhng nhiu lon trn ng dy

Nhc im l:

-Phi chu ton b dng ti danh nh-Ch c th ci thin mo dng hi ti 3040% l tt nht-Ch gim t h s cng sut dch chuyn5.1.4.Lc thng thp (Lc di rng)

Cu hnh c bn c ch ra trong hnh 1.8. N c khnng loi b ttc tn s sng hi trn tn s cng hng.

Hnh 1.13 B lc thng thp

u im ring ca b lc thng thp l:

-Gim thiu tt c cc tn s sng hi-Khng to ra bt k s cng hng h thng no-Khng to thm sng iu ha t nhng ngun khc-Ci thin h s cng sut thc

Nhc im l:

-Chu ton b dng ti danh nh

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-Ch dng cho ti phi tuyn5.2.Active Filter (Lc tch cc)Cng ngh lc tch cc vi vic s dng cc phn t bn dn tuy mi pht

trin t thp nin 80 ca th k20 nhng mang li nhng kt qu kh quan vivic loi b tc hi sng hi xung di 5%, n s trnh by v l thuyt lc tchcc v thc thi b lc tch cc trn quy m phng th nghim v kt qubc ut c.

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Chng 2: B lc tch cc


CHNG 2: B LC TCH CC

L thuyt lc tch cc v ng dng trong b sng hi bt u t nhng nm1970 [Bird nm 1969; Gyugyi v Strycula nm 1976; Kawahira nm 1983]. c

bit, vi s pht trin nhanh chng ca cng ngh tin v vic nng cao tc chuyn mch ca cc van bn dn nh IGBT to iu kin cho php ng dngrng ri cc b lc tch cc trong lnh vc bin i nng lng. Cng vi ,nhng k thut mi trong lnh vc in t cng sut cng gp phn vo vic hinthc ha cc b lc tch cc. Nht Bn hin nay, c hn mt nghn b lc tchcc nghch lu ngun p s dng IGBT hoc GTO hot ng. [2]

Lc tch cc trong lnh vc bin i nng lng thc hin nhng chc nngsau:

-B cng sut phn khng-B sng hi, cch ly sng hi, gim sng hi,

-B cc thnh phn th t nghch ca in p v dng in-iu chnh in p.

L thuyt c p dng nhiu thc thi b lc tch cc l l thuyt vcng sut tc thi do Hirofumi Akagi xut, sau y ta si tm hiu v l thuytcng sut tc thi, v ng dng ca l thuyt ny trong thc thi b lc tch cc.

2.1. C sl thuyt:

L thuyt cng sut tc thi (l thuyt p-q) trong h thng ba pha hay cngi l l thuyt Akagi-Nabae c cng b vo khong nhng nm 1983, 1984.Hnh 2.1 l hnh nh ca mt h thng ba pha ba dy tng qut, trong khng cthnh phn in p th t khng.

Hnh 2.1 H thng ba pha ba dy

S dng php chuyn i Clark, ta c th chuyn t h thng dng in-inp ba pha abc sang h thng dng in-in p hai pha

tng ng nh sau:

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Chng 2: B lc tch cc


in p:

[]

(2.1)

Dng in:

[]

(2.2)L thuyt p-q da trn nh ngha cng sut tc thi trong min thi gian. V

th, n khng ch c hiu lc vi ch bn vng m cn chqu .

Nhng khi nim truyn thng khc v cng sut c c trng bi vic x

l h thng ba pha nh ba mch mt pha. L thuyt p-q ban u bin i in pv dng in trong trc ta abc sang h trc , v ri nh ngha cng sut tcthi trn h trc ny. T, l thuyt ny lun xem xt h thng ba pha nh mtn v, khng xp chng hay tng ba pha ring bit.

Nhng cng sut tc thi - cng sut thc tc thip, cng sut o tc thi q,c nh ngha tin p pha v dng in dy tc thi trn h trc nh sau :

*+ [ ] [] (2.3)Trong h ba pha ba dy thnh phn dng th t khng khng xut hin nni0 = 0, v th ch c cng sut tc thi c nh ngha trn trc l tn ti. T

, cng sut thc tc thi p biu din tng nng lng chy qua mt tit dinthng trong mt n v thi gian trong thnh phn . Trong trng hp nyp3=

p. Cng sut o q l t lilng ca nng lng m t trao i gia cc phatrong h thng. N khng i din cho nng lng trao i gia ngun v ti.

T ta c thtnh c theop, q nh sau:

[]

[ ] *

+

(2.4)

Vi nh ngha ny, h thng ba pha ba dy c xem xt trong trng thin nh c c im chung, nh chu k, in p, dng in. Chng c th gmthnh phn c bn cng nh cc thnh phn hi. Ngoi ra, mi nhm ba pha ca

phasor trong tn sc a ra c th khng cn bng, ngha l n c th gmthnh phn th t thun, nghch, theo l thuyt thnh phn i xng. Chu k chungin p v dng in c th phn tch thnh chui Fourier: (2.5)

(2.6)

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Chng 2: B lc tch cc


y n ch bc ca hi. Cng thc trn c th vit trong s hng ca phasor,bao gm thnh phn c bn ( n= 1) v phasor hi nh sau : (2.7)

(2.8)Cui cng, php bin i thnh phn i xng c ng dng mi nhmhi a-b-c ca phasor in p hay dng in xc nh thnh phn th t khng,thun, nghch, l :

(2.9)

T, vit li s hng ca thnh phn i xng trong min thi gian: (2.10) [ ( )] (2.11) [ ( )] (2.12)

Tng t, dng in dy tc thi cng c th vit li: (2.13) [ ( )] (2.14) [ ( )] (2.15)

S khai trin thnh cc thnh phn i xng trn cho php phn tch h

thng ba pha khng cn bng nh l tng ca hai h thng ba pha cn bng. Hiin p v dng in trong cc s hng ca thnh phn i xng c th thay th

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Chng 2: B lc tch cc


tng ng. Nu php bin i c ng dng, biu thc tip theo cho in pv dng in chung bin i trong trc chun c th nhn c:

(2.16)

(2.17)

V: (2.18) (2.19)C th thy rng thnh phn th t thun v nghch ca in p v dng in

trc v c tch ra. Quan h gia cng sut trong khi nim thng thngv cng sut mi c nh ngha trong l thuyt p-q c thy r hn nu cngsutp, qc chia thnh cc thnh phn trung bnh ca chng v thnh phn dao

ng:

Cng sut thc: Cng sut o: Trong , l thnh phn cng sut trung bnh, , l thnh phn cng

sut dao ng. Chng c vit li nh sau: (2.20)

(2.21)

{ } (2.22)

{ } (2.23)Nhng biu thc cng sut chung ny lm sng t mi quan h gia cng

sut tch cc v phn khng ca khi nim tc thi v thng thng. C th thyrng thnh phn c bn cng sut tch cc ba pha thng thng ( P = 3VIcos ) lmt s hng ca cng sut thc trung bnh , cn cng sut phn khng ba pha ( Q

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Chng 2: B lc tch cc


= 3VIsin ) bao gm trong cng sut o trung bnh . Tt csng hi in p vdng in c thng gp vo cng sut trung bnh bnh v nu chng c cngtn s v c cng thnh phn th t ( thun hay nghch ). S c mt ca nhiu hnmt thnh phn tn s hi v/hoc thnh phn th t to ra cc thnh phn

v

.

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Chng 2: B lc tch cc


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2, The Power Electronics Handbook

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