‘frequency characteristics of hybrid filter systems

8/3/2019 Frequency characteristics of hybrid filter systems

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FREQUENCY CHARACTERISTICS OF HYBRID FILTER SYSTEMSFausto B. Libano, Domingos S. L. Simonetti and Javier Uceda

Universidad Politkcnica de MadridDivisi6n de Ingenieria ElectrhicaC / Jos6 Gutienez Abascal, 2 - 28006 - Madrid - SpainE.mail: fausto @upmdie.upm.esphone: 34-1-336 31 91 fax: 34-1-564 59 66

Abstract: Hybrid filter topologies are becoming moreimportant to improve harmonic performance in industrialsystems. Hybrid filters are composed by active and passivefilter associations taking advantage of both filter schemes.As a consequence of their complexity the analysis anddesign requires an optimization process to set up the mostimportant parameters.

In this paper the most important filterconfigurations are analyzed in the frequency domain. Theanalysis helps in the selection process of the mainparameters, specially the value of the equivalent activefilter impedance, which affects strongly the filter behavior.This study provides a good support in the analysis anddesign of hybrid filters.

I . INTRODUCTIONThe extensive application of nonlinear loads hascontributed to increase the harmonic pollution into power

systems. Besides, the quality degradation of the suppliedpower can be caused by different reasons like unbalancedthree phase currents, sub-synchronous frequency currents,magnetic saturation of electrical machines, etc. Nowadays,hybrid power line conditioners have been researched anddeveloped to compensate the reactive current at thefundamental frequency and to suppress harmonicsgenerated by nonlinear loads.Utilities are recently beginning to considerharmonic standards such as IEEE 51 9 [ l ] (recommendedpractices and requirements for harmonic control inelectrical power systems), EN 60555-2 (harmonic currentvalue limits for household appliances and similar electricalequipment with less than 16 A per phase), EN 60555-4 (forequipment with line currents above 16 A per phase -presently in development phase).

A general rule in the electric power distribution, isthat individual low-power end users and high-powerconsumers are responsible for limiting the harmoniccurrents caused by power electronics equipment or anyother type o f nonlinear loads, injected into the powersystem, while electric utility companies are responsible forlimiting harmonics voltage distortion at the point ofcommon coupling, by making sure system resonantconditions do not cause excessive magnification of theharmonic levels in power transmission and distributionsystems [2-31.

Hybrid filter topologies consist of both active andpassive filters in different structures [2-81. The hybrid

active filters improve the compensation characteristics ofthe passive filters having as main function to provideharmonic compensation. By improving the compensationcharacteristics of the passive filters, hybrid active filters geta reduction in the rating of the active filter [4,6,9,10].Another motivation to develop hybrid structures is based ontheir suitability to work on high voltage networks.

The control strategies of the power hybrid filtersare in continuous development. The first approach wasproposed on the basis of the Instantaneous Reactive PowerTheory [ 1 1- 121. Later, contro l schem es based onSynchronous Reference Frame approach were proposed [9-101. This paper presents a frequency-domain analysisfor some hybrid topologies with special attention toassociations employing series active filtering. The focus ison the influence of the active filter impedance on theharmonic voltage and current performance.

11. HYBRID ACTIVE ILTERSTo overcome some of the problems shown byactive and passive filters, hybrid topologies have been

proposed. An hybrid filter is an association of active andpassive filters. The aim is to combine passive filterrobustness with active filter performance, imp roving systemreliability. Also, as a practical result most viable and cost-effective hybrid active filter topologies enable the use ofsignificantly small rated active filters (4% A rating)compared to pure parallel or series active filter solutions[191. Additionally, they offer several value-added featuressuch as line voltage regulation, reactive powercompensation and harmonic isolation.Som e topologies and their main characteristics aresummarized in table I using single line diagram. In thistable, the comparison is always taken respect to a classicalpure shun t active filter.

111. FREQUENCYHARACTERISTICSAll systems will be analyzed based on the single-phase equivalent circuit considering balanced three-phase

nonlinear load. Let us assume that the active filter is anideal controllable voltage source Vc or V A F depending ofthe topology) by applying a gain K in the measuredmagnitude (current or voltage). The load is a current sourceIL , Zs is the source impedance and Z, is the equivalent totalimpedance of the passive filter cells.

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The filtering characteristics are presented normalized ona per-phase base, with basis on the load power (Sb=20 kVA),system voltage (Vb=200 V) and frequency (fb=50 Hz). Atfundamental frequency the active filter do not act. In the analysis,etc) account for the harmonic content of these voltages andcurrents.all the variables with the subscript "h" ( V Sh , V C h , Ish, I Fh , V T h , ILh ,

3.1 - Hybrid series active filter structure ( 2 )Let us consider the single-phase equivalent circuit

shown in figure 1, where the voltage sources Vch (K*Ish),account for the behavior of active filters, proportional to sourceharmonic current IStl

. . . . . . . . . . . . . . .SOURCE, ... . . . . . . . . . . .. .. ........................ ,.............H Y B R I D FILTER LOAD

Fig. I - Eq. circuit or harmonic requen cies.From the circuit we can obtain:I = (1 )From equation ( I ) , assuming vsll=O,he ratio ISh/ILh is

plotted in figure 2 . In the same way, assuming IL h =o thefunction ISl,NSI,s also plotted in figure 3. Both families ofcurves for the typical Ls=0,02 pu and different values of K areshown.

Figures 2 and 3 the show filtering characteristics bymeans of the source harmonic current Ish as a function of theload harmonic current ILll nd the source harmonic voltage V S h .It can be seen that for values of K>>IZ,I , the load currentharmonics are constrained to flow into the shunt passive filter.Th e possibilities of resonance are strongly damped.

' f / fb lo 100Fig. 2 - Ratio Is,/ILl,,

h=0.02 pu15 -10 -

5 -

0.1 E/fb 100

l o t

h=0.02 pui=0.05 pu0.1 100

Fig.3 - Ratio Is IJvs I , .Following the same procedure with equations (2) an d

(3) as it was done with ( I ) , we can plot a new family of curves,shown in figures 4,5 ,6 and 7.

* t 'I\' u f b l o.1

Fig.4 - Ratio VTIJILII.

0. f / fbFig. 5 - Ratio VTIJvsl,.

From figures 4 an d 5, it can be seen that if K is muchlarger than the source impedance IZ,l and IZJ, it will exist avoltage drop in the passive filter, VTI,,mainly proportional toI L h . A large value of K in some cases increases Total HarmonicDistortion (THD) of the passive filter terminal voltage, whichmay exceed the IEEE-519 voltage TH D standards.

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I I

4 -

Figures 6 and 7 , show the required harmoniccompensation voltage V cll, in the series active filter togenerate t h e complete compensation. For K>>IZs+Ztl, t isnecessary to generate a short voltage peak to compensate theload harmonics. This peak is very difficult to be implementedpractically.3.2 - Hybrid parallel active filter structure {3}

Considering the single-phase equivalent circuit shown infigure 8, where the voltage source Vcll(K*Isll), accounts for thebehavior of the active filter, as a function of the source harmoniccurrent Isll. . . . . . . . . . . . . . . . .

: I,,,

. . . . . .SOIIRCE ........................................I ~ Y R K I D LLTER

. . . . . . . . . . .

..................LOADFig. 8 - E q. circuit o r harrnonic, frequencies.For this configuration o f hybrid filter, eq uation s for Ishand Vcllare equal to the equations ( 1 ) and ( 3 ) , nd consequentlysame characteristics are obtained. However, the harmonicvoltage, VT h, hows a different equation producing the followingcharacteristics: Z +K

(4)fzv =- S f . I +?h K+Zs+Z Lh K+Zs+Z "bz.f f

L I I 1I I l l l i0. f/fh lo

Fig. 9 - Ratio V T/ / I L~ , .I V T I f l S h l I II

Ls=0.02 puI0 ! 10 I 1 f /fb ") l ( X )

Fig. I O - Ratio VT,Jv,yi,.From these figures, it can be seen that for values of

K>>IZs*Zfl he harmonic voltage VTll s practically equ al to thesource voltage Vsll.

3.3 - Hybrid series and parallel active filters structure {4 }Let us to consider the single-phase equivalent circuit

shown in figure 1 1 , where the voltage sources V AF l K1*Isl,)andVAF2K2*V Fh), ccount for the behavior of tw o active filters, as afunction of sourc e harmonic current I,,I, and the so urce harmo nicvoltage Vsll, espectively.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .SOUKCE IlYBRiD FILTER LOAD

Fig. I I - Eq . circuit fo r harmonic frequencies.From the circuit we can obtain:

(1 K 2 Iz 1Ish = - . I I h + ."Sh ( 5 )

From equation ( S ) , assuming Vsll=O, the ratio ISh/ILh isplotted in figure 12. In the same way, assuming ILI,=O hefunction ISh/VShs also plotted in figure 13. Both families ofcurves for Ls= 0,02 pu and K2= 0,9S pu (approximately the idealvalue) and different values of K, re shown.

Z S + K I + ( I - K2 fZ Z ~ + K ~ + ( ~ -~ 1 . z ~

1 1 4 5



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1 5 h=0.02 pu

il

Fig 12 - Ratio l s i~ lL i l .

I

Fig 13 - Ratio Isi/vsil.From figures 12 and 13 the filtering characteristics in

terms of the source harmonic current Is,,as a function of theload harmonic current 1~1, nd the source harmonic voltageVsl, show that for Kz-l and K,>>IZ,I the load currentharmonics are constrained to flow into the shunt passive filter.Th e possibility of resonance is strongly damped.

Assuming K , = 2 pu (practical value to avoidinstability), L,=0,02 pu and different values of K2 , the samefunctions IsI,/ILl,nd ISl,NShre plotted again in figures 14 andIS .

KI=2 u

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an d K2 is taking values smaller than unity, there is a tendencyof series resonance between the sou rce and the passive filters,mainly considering the harmonic current load.

Following the same procedure with the equations (6),(7) and (8) as it was done with (5), some new family of curvescan be shown in figures 16, 17, 18 , 19, 20, 21, 22, 2 3, 24, 25,

1.5c, puu

Fig. 17 - Ratio VrisV,i,.2 I I

Fig. 19 - Ratio Vri/vsi,.1146



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The value of ZvILh s very small, and disappears onthe load bu s for K I higher than 1 and K p l . In this case, theactive filter VA FZ rovides a zero impedance path to theharmonic current and no harmonic current is injected into thepower system.

The active filter 1 (VAFI),rom the point of view ofthe load harmonic current, for higher values K I needs togenerate fast variations of low magnitude voltages in the mainresonant harmonic frequencies.

K2.(Zsf K1).Zfa--] + ( l -Kd.Zf

K2.ZfZS f l + ( I - K d. Zfv , . I B - .VYt (8)K/ . (1 Kd.Zf Klv&q = . I f i + .V% (7)

a + K / + ( I -Kd.Zf a+K]+(l-d.Z j

L.5a.02 puiK ~ a . 9 5 d\

I X

15

Il l

5

I1

Fig. 20 - Ratio VA~ -~ / IL~ , .

I \ !4=0 .02 puK1=2 PU

Fig. 27 - Ratio VA&s, , .Fig. 23 - Ratio V A F ~ N ~ , , .

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The voltage of active filter 2 ( V A F ~ )ill be able tocancel the harmonic voltage appearing across the passivefilter, thus providing a compensation for the harmonic v oltageat the load terminals. Both active units are low nominal VArate.

Iv. DISCUSSIONF RESULTSThe choice of the impedance presented by the seriesactive filter is fundamental for the harmonic performance

presented by the hybrid filtering system. The ideal value of theK parameter is straightforward from the equations and curvespresented. For association { 2 ) in table I, a very high K valueis desired. In this situation, all the harmonic content of theload current passes through the passive filter. Besides, voltagesource harmonics are not present at load voltage. Therefore,the harmonic content of the bus voltage is caused by the loadharmonic current circulating by the passive filter.

The ideal solution for association (3 ) in table I isagain a high K value. The harmonic current of the load doesnot circulate through the line, but all harmonics of the voltagesource are present at the bus load. Despite this fact, harmoniccirculation between voltage source harmonics and passivefilter does not occur.

The association ( 4 ) presents two different ideal Kchoices. The ideal K2 value is 1 . This value assures no loadharmonic current circulating by the line. It also eliminates busload distortion caused by the passive filter and sourceharmonics. On the other hand, a very high K , value assures noharmonic current generation caused by source harmonics.Unfortunately, the use of ideal K values usually leadsto system instability [5,7]. Therefore, different K values mustbe used, and the harmonic performance of the filtering systemchanges. For associations {2] nd { 3 } , hoosing a K valuebetween 1 an d 3 keeps a good harmonic performance withlimited harmonic circulation on the line side. But a specialattention must be paid on the resonance that occurs at someharmonics keeping adequate values.A similar con clusion is av ailable for association { 4 ) .No ideal choices of K , an d K2 values can keep a goodharmonic performance without dangerous resonance effect.This way, K2 alue must be between 0.8 an d 1. K2 valueslower than 0.8 can generate undesired resonance andharmonic contents, as can be seen in figures for association{ 4 ) . By the analysis of figures presented we see that K , valuemust be higher than I to avoid undesired harmonic currentand voltage in the system.

V. CONCLUSIONSThe use of active-passive filters association improvescurrent and voltage harmonics performance of t h e s y s te m . The

active filter acts as an impedance inserted in the system,changing its harmonic behavior.The ideal impedance value is straightforward

obtained from harmonic equations, but unfortunately it usuallyleads to system instability. Th erefore , no ideal values m ust beused, worsening h armonic performance.The analysis presented in this paper shows theharmonic behavior of active filtering associations under

variation of the active-filter harmonic-impedance. Mainly it

was shown that there are no-ideal K values which allow tokeep a good harmonic performance with limited harmonicgeneration. The study is of importance in the harmonicanalysis of an hybrid filters, specially in the selection of themain operation parameters.

REFERENCES[ I ] S. Bhattachaiya, D. M. Divan and B. Banejee, "Active Filter Solutionsfor Utility Interface", IEEEIISIE 95, Vol. I , pp. 53-61, 1995.[2] H. Akagi, "Trends in Active Power Line Conditioners", IEEE/PESC, pp.12-24, 1992.[3] H. Akagi."New Trends in Active Filters", EPE95, Vol. 0. pp. 017-026,199s .[4] H. Fugita, H. Akagi and A Nabae, " A Combined System of Shunt Passiveand Series Active Filters - an Alternative to Shunt Active Filters", EPE91,Vol. 3, pp. 12-17, 1991.[5] T. Tanaka and H. Akagi, "A New Combined System of Series Active andShunt Passive Filters Aiming at Harmonic Compensation for h r g e CapacityThyristor Conveiters", IEEEhECON, pp. 723-728, 1 991.[6] H. Fugita and H. Akagi, "A Practical Approach to HarmonicCompensation in Power Systems - Series Connection of Passive and ActiveFilters", IEEE Trans. on Ind. App., Vol. 27. No. 6, pp. 1020-1025,November/December 199[7] H. Akagi and H. Fujita, "A New Power Line Conditioner for HarmonicCompensation in Power Systems", IEEEKHPS VI, pp. 236-24 , September1994.[ 8 ] S. Bhattacharya and D. M. Divan, "Design and Implementation of aHybrid Series Active Filter Systems", IEEE/PESC, Vol.1, pp . 189.195, 1995.[9] S . Bhattachaiya. D. M. Divan and B. B. Banerjee, "Synchronous FrameHarmonic Isolator Using Active Series Filter", EPE, Vol. 3, pp. 30-35, 1991.[IO] S. Bhattachaiya, D. M. Divan and B . B. Banwjee, "Control andReduction of Terminal Voltage Total Harmonic Distortion (THD) in a HybiidSeiies Active and Parallel Passive Filter System", IEEE/PESC, pp. 779-786,1993.[I I] H. Akagi, Y.Kanazawa and A. Naba e, "Generalized Theory of theInstantaneous Reactive Power in Three-phase Cii-cuits",Pro.JIEE-IPEC-Tokio, pp. 13-75,1983.1121 H. Akagi, Y . Kanazawa a nd A. Nabae, "Instantaneous Reactive PowerCompensators Comprising Switching Devices without Energy StorageComponents", IEEE Trans. on Ind. App., vol. IA-20, No. 3, pp. 625-630,May/June 1984.1131 M . Takeda, K. lkeda and Y. Tominaga, "Ha rmonic CurrentCompensation With Active Filter", IEEE/PESC, Vol. I , pp. 808-815, 1987.[ 141S. Fukudn and T. Endoh, "Control Method and Characteristics of ActivePower Filters", EPE9 3, Vol.l,pp.139 -144.[I51 N. Balbo, R. Penzo, D. Sella, L. Mnlesani, P. Manttavelli and A.Zuccato, "Hybrid Active Filter for Parallel Harmonic Compensation",EPE93, Vol. I, p. 133-138, 1993.1161 N. albo, K . Penzo, D. Sella, L. Malesani, P. Manttavelli and A.Zuccato, "Simplified Hybiid Active Filters for Hal-monic Compensation inlow Voltage lndustiial Applications", IEEE/ICHPS VI , pp . 263-269,September 1994.[ 171 M. Rastogi, N. Mohan and A. Edris, "Filtering of Harmonic CuiTentsand Damping of Resonances in Power Systems with a Hybrid-Active Filter",IEEE/APEC'95, pp. 607-612, 1995.[ l8 ] J . Janczak and J. K . Feinro, "Voltage Waveshape Improvement byMeans of Hybrid Active Power Filter", IEEEKHPS VI, pp. 250.255,September 1994.1191 P. T. Cheng, S. Bhattacharya and D. M . Divan, "Hybrid Solutions forImproving Passive Filter Performance In High Power Applications",APEC'96, Vol. II, pp. 91 1-917, 1996.Mr. Fausto Bastos Libano is in his doctoral work at UPM witha scholarship from the Brasilian Foundation C APE S.Mr. Fausto Bastos Libano is with the Dep. Eng. ElCtrica -Escola PoiitCcnica - Pontificia Universidade Ca tdi ca d o RioGrande do SUI. Porto A legre, A V. Ipiranga 6681, predio 30sala 150 - Caixa Postal 1429 -CEP: 90619-900 - Brasil.

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‘frequency characteristics of hybrid filter systems

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