New hybrid parallel active filter configuration minimising active filter size

J.-HSung, S.Park and K.Nam

Abstract: l h e mthors propose 11 new topology for a hybrid pai-allel active filter that can rwdicr rediice the active filter (inverter) power rating. The basic idea is to install the active filter in parallcl with the passive filter inductor. One iidvaiit;igc of this is that the passive fitter ciin operirte by itselr if the inverter is in the high-impedance mode. This nieans that the passive filter CRII operatc without Ihc aid of the active filter. Another adviintage is that, because the significant portion of fundamental and harmonic components flows through the passive filter, the current loading to the inverter is rcduccd greatly. This contributes to reducing the cost of the expensive inverter, and, hence, this makcn tlic application of a hybrid active power filter more feasible. Through computer siinulations and experiments, the authors liave verified the effectiveness of the proposed topology.

1 Introduction

Nonlinear loads, such as diode and thyristor rectifjers and uninterruptible power systems, proliferate in various areas. Without prowr compensation, the quality of power in transmission/distrihution systems can deteriorxte. Passive filters liave hcen broadly used to climinate the Iiarmonics in utilities, due to their low cost and high crficiency. I-Iowcvcr, thcy have soinc drawbacks, such as strong dcpendance on sniircc iinpcdancc, undcsirablc rcsonanccs with source and loads, and the variation of liltcr characteristics with time. Active power filters (APFs) have been developed to c o m pmsatc the problcms of passivc filteis, but the constriiction and operiition costs of pure active filters ;ire too high. Combining the iirivmtages of passive tilters und tictive filtcrs, hybrid aclivc liltcr topologics liavc been dcvcloped [1-7]. They arc cost-crrcctivc, whilc offci-ing line voltage regulation and harmonic isolation bctwccn supply and load. They offcr easy protcction, bcciusc possiblc faiturcs in llic activc fil~cr do not af lcc~ Ihc load scction much [5].

As for a control method for the hybrid pslrallel active filter (HPAF), impedance variation techniques have been studied by many rasewrchers. Fujita el ni. [6] have proposed an idca of‘ supprvssing Iiannonics by incrcasirig the effective source impedance selectively to the hatmonic components. On the other hand, Divan et d. [7, 81 have proposed an idea of utilisiiig the multiple synchronous ieference frames (SRF) in extracting the desired harmonic signals. This paapcr prcscnts a ncw topological structure for the HI’AF. I-Iowcvcr, for Ihc control method, 11ic authors will also utilisc thc impcdancc variation mctliod.

2 Problem stataments

Fig. I shows a n cxisting HPAF lopoiogy anc l its singtc- phaw cquivalcnt circuit. Nole (hat the inverter is connected lo 11ic sliunl passivc Lillcrs through w transfortner [C;, 1. The ~ransfoimcr plays a rolc OI‘ inakhiiig current and v o h g wilh the ratings of the switching devices in the inverter. Such a coilfiguration requires a relativcly sinall invcrlcr compared with the piire active filter, bccausc ~Iic filter capacilors cl-eatc a large vdtdge drop for the filndamentsl component. Flowcvcr, a drmback of Ltiis configuration is that the

passive filter docs not work if the inverter fails. Another h c t concerning the existing HPAF is that, because the invcrtcr is contmted in serias with the passive filter, all the currait flowing through the passive filter also flows through tlic scmiconductor switchcs ol’ the inverter. Hciicc, the inverter cirrrcnt rating i s still Iiigli.

We lhcrcrorc proposc a licw ropologicai structure For R W A F as shown in Pig. 2. Note that the matching trans- rorincr is coimwtcd in pni~llcl with the filter inductor. Hence the passive filter works even if the active filter goes into the high-iniped;iiice stilte. Furthermore, since the aclivc liltcr opcratcs in parallcl with tlic filter inductor, thc current load of the itivcrtcr is rcduccd correspondingly. Thc inverter rating of the proposed scheme is about hall’ of that of’ the cxisting HPAF for it given load, tls showii below.

3 Filtering characteristics of proposed topology

Fig. 7 shows the proposed single-phase equivalent circuit for liariiionics with the notation of impedauces, where Z,, Z,, Z , i 1 ~ 1 Z , denote the source impedance, the iinped- a i m of the passive filter capacitor, the impedance of the passive fil ler inductor and thc impedance of the inverter- side inductor, respectively. The invertcr i s dcnolcd by the vokagc sourcc V,. Assuming that the voltage soui‘ce also gcnerates liannanics, wc dcnolc il by yy,,,

Choosing thc ciirrcnt dircctiotis tis shown i n Fig. 3, the overiill equations in the steady state arc obtained as Follows:

93

.... . . .. ,::. :I I ' t

I : [

I ' . I

j : t !

. . .

. . L

. .

: .i .......... f

- eauivalent circuit

L . I j ' l .::: _'LU . ''

. . . . . . . .. . . . .. . . . . . . . : . . . . . . . . : . . . . . . :.

'Fa I iFb I !onlinear . . . . ioa!

equivalent circuit

d

Ish = {Lh + 11%

VI = - 2 I I I h 4- &Y(JVh - I J d

(2 1 (3)

where I,,, I,,, and 1, denote the source harmonic cur- i w t , 1hc load harmonic current, thc lillcr capacitor har- monic current and lhc inverter harmonic currcnl, respectively.

'S h

Fig.3

Similar to previous work [6], we assume (ha1 lhc inverter output vollagc i s controlled such that

Then the source harmonic ciurrcnl I,,, can he obtained from eqns. 1 and 2 as

eauivalent circuit

fT+ t

vs fb 'VI

squivatsnt circuit

VSh K f 2.5 -I- xire t

(5 1 From eyns. 2 4 , lhc rollowing is obtained:

Thus, il follows from eqns. 5 and 6 that

Z r + Z F 1

IC + ZI 4- (Zr + ZVI)(ZS + Z F c ) f S h =

Note thal I,, and Y,, are independent of VI, i.c. they are tixcci cvcn as K increases. Hence, wc can scc from eqn. 7 Lhal decreases as K increases. IT K is sulYiciently large, all the harmonic currcnts produced by the load a id source wotild sink into the passive filter. This resull is the same as one prwented in Fujita's work [GI,

In the extreme case where K i s inliiiilc, f.yjl = 0 and it fol- lows tha I

VI = Z F c I L h . - Z I ~ I h + h'i~ (8) ,7

(9)

Note, on the other hand, lhal Ihc fundamental component of the filter curxent satislics

IM Prdr.-Ekcfr. Poiser Alij?L Vol. 147 No. 2. M w r h 2000 94

where 1, and Jro denote Lhc rundanicntal currents flowing through the invertcr and thc passive filter, resptively. From eqns. 8-1 0, the upper bound of the appii.cni powr of the inverter can be cslimatcd as follows:

( V 4 1 5 3Ivr l ( l~m f I I Ihl)

(11) wlicrc Z,,. = [Z,(Z,, + ZI;!) + Zj,$&Zl + Z,). Note, on tlic other hand, Ihai tlic uppcr bound of the appiirent power of the existing I-IPAF is givcn by Fujita [6], such thnt

(VAja 5 3lz;Ir>h + lfshl(~I;*~ $. l h l ) (12) where 2;. = ZFc: + 2, ~ t i d Ym is the flui~damcntal conipo- ncnt of the current passing through the invcrtcr.

At a first glaiicc at cqns. I 1 and 12, it can be noted that ilie upper bound or tkc proposed HPAF has the rzlctor l.ZFj(ZI t. &)I, while the cxisling W A F has not. Further, ZF, I , are less than Z>, I',,,, rcspcctivcly, as shown below. The reason for such rediction m n w G+orn the fact that the inductors Z,, Z,, are coniicclod in parallcl in the proposed HPAF, thus the filter capacitor currcnl is divided.

Note that V, = 0 for Ihc ruundarncncal component. For example, if 2, = Z>,, then the inverter fulidsnicnlal currcnt or llic proposed HPAF is about R h ~ l f of the ca% of thc existing HPAF, it. rAD 5 OSrl,, , Further, because Z, = Z,, -t- 0.5Zp, with Z, = Z&7, thc authors have deduced that Zrs Z$ = Z , + Zn al- a harmonic rrequency. In such ii ctise, eqns. 11 and 12 may be rewritten as

n

bound of (V& Hence, the proposed topology requires a much smaller power rating. Similar iesults will follow for the cases where Z, # ZFp

4 Control strategy

Fig. 4 shows llic ovcrdl control block diagram Ibr. the har- inoiiic elimination fiom the source arid UC link voltage wnlrol of lhc invcrtcr. All controllers arc dcsigiied h s e d on thc synchronous rotatiiig frame (SRF). Note that the sowce current is a complex valued variable, i.e. i& = z& + ,j&, . The superscript c denotes the variable descriked in the SliF, while the superscript s denotes that in the sta- tioiiary frame. Note, on the other hand, that i,& is also decomposed spectrally such that i,zlT = i&(l0 + i,&, wherc i&, , i,Galhj denote the fiindiimentd arid harmonic conipo- iicnts, raqxctively. The same rule applies ta the other vari- xblcs.

4. 'I Harmonic elimination As the fundilmental componcnt ol' lhe source ciarcnt appears as UC quantities in the SRF, it cm be easily extracted from the sourw current by xpplying ii lowpass filter. The harmonic components i&a ;ire abt2iined by sim- ply suhlracting Ihc fiindaincnlal coinponcnt from the souIce current in the SRF. Multiplying the mensured har- monic coinponents by the constant gain K, the tiutlors obtain the following voltage cornmand for harmonic elimi- nation:

wherc &h, dciiotcs tlic volhgc command and L P F denotes the transfcr function or' :I lowpass fillcr. Thcn transrorming ~ j , ( , q inlo the oik rchcncc Ihnc, Ihc authors have obtained the inverter voltage command BS shown in Fig. 4.

4.2 lnvertec DC-link voltage control A DC link voltage control scheme is shown in Fig. 4. Tlic eiror between the voltage rel'crcncc Y,,; mi the mcasured

0 5

DC link voltage V,, is fed into a PI regulator. It should bc noted ihat V,), is controlled by thc fundamental COlnp0- nent such thal

(16) G o d s

?God, = [ P O (Vih - 1'nc)l IOdq

whcrc Pi denotes a PI controller, and ~ h / ~ and &,,, denote the voltage command for DC link voltagc rcgulation arid the fundamental coinponent of the inverter currcnt in SRF, respectively. Nott: that ihIli I is iiitroduccd 10 supply voltage it1 phasc with the current, i.e. the voliagc generating a real power for tlic DC link [RI.

Table 1: Parameters used in simulation and experiment

Source voltage V , 220 v,, 5th filter capacitance C, 1100~iF 5th filter inductance Lh 297pH

Source parasitic inductance LS 20pH

DC link capacitance C,, 45001~F DC link voltage 250V

lovertar filter inductance Lc 300~1~

5 Simulation and experimental results

Thc proposed scheme is simulatcd by a commercial tool, Simplorer, with the paranictcrs shown in Table 1. Wc asmnc that the 5th harmonic filicr is inistuned at the 4.61h. As a nonlineiir lotd, we haw uscd a three-phkise diodc rcc- tifier haviiig ;1 IOW resistor load. Figs. 5 and 6 shows the simulahn results with only passivc filter. Owing to tlic mistuned effect of the passive filter and a m a l l valuc or the soui'ce impedance, tala1 Iiannonic distortioii (THD) of sourcc current amounts to 10%. This can be checked froin the spectra shown in Fig. 6. Figs. 7 and 8 show currents plots and thcir spcctra of the existing HPAF [6] and the proposed HPAF. In both simulations, the passive filter is composed of only the 5 ~ h passive filter. The poiiil cvhcn the inverter starts harmonic clirniliation control from the: m'o vector state is iiidicalcd in Figs. 7 and 8. Tlic Tl-ID of the soiircc current decreases to bdow 1% in both cases, which meets the IEEE 519 hmionic current limits (5%). Judging from the source current harmonics, both HPAFs have sim- ilar filtering characteristics. However, there is R striking dif- ference that the jnverlci: curmiit of the proposed HPAF (Fig. 8) is about 50% of the inverter current of the existiiig HPAF (Fig. 7) as was estimated in cqns. I 3 and 14.

100

10

4

100

10

I

0.1 100

10

1

a i

time, ms

Experiments were coadiicted undcr thc same condition as in tlic simulation, i.e. the authors construclcd a IOkVA expcrimcntal sysiem with the components having the parameters listed in 'lkble 1. The iiivcrlcr was made with

lhc 32 bit DSP TMS32OC3 I and intelligent power niodulcs (75A, 6(10V). TIE inverkr switching frequency was selected to be 4.2kI-Iz. Figs. 9 and IO show the experimental results \villi only ti passive filter sild thc I IPAF. I n thc forincr. case, current dislorlion was observed in tlie soiirce current, atid this is caused by the filtcr misniatcli. Comparing tlic source currents of the two cases, the authors note n signifi- cant reduction in 1iat.nionics with the proposecl I-IPAF. I t should also bc no~cd that the magnitude of the inverter c~ir- rent is nearly half of the passive filtcr currcn~. Fig. 1 I shows thc cxpcrimcntal w:tveform from which it can be secn that the par-allcl I%SOII~IICC generated between source and pas- sive filter is suppressed a f lw tlic ; d v c filter starts. Fig. 12

phaso-a load current

phase-a sourCB current

I phase-afiltercurrent- f I I

phase -a inverter current

phase a load current

r

I I

... I phase-asource currenl I I + I

1

1

phmso-a filter current

I phase-a inverter current I

shows ]IOW tlie red DC link voltage tracks the DC link wltage coiiitnrind with the proposcd W A F .

-I. I

phase-a filter current

trigger signal APF start

6 Concluding remarks

A new topology for a hybrid parallel aciivc power filtcr hiis been proposed. With the proposcd W A F , the power ixi- ing of the active filter is cotisidcmbly rtduccd by ;illowing a major portion to flow through tlic passivc filter nt any tinie. The cf€xtiveness of the proposcd I-IPAF Iias bccn confirmed by the rewlts of simulatioris atid expeinentul work.

References

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6 8

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