design of apf2

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DESIGN AND IMPLEMENTATION OF A HYBRID SERIES ACTIVE FILTER SYSTEMSubhashish Bhattacharya Deep& DivanDepartment of Electrical and Computer Engineering

University of Wisconsin- Madison1415 Johnson DriveMadison,W 53706Te1:608-265-38 15/608-262-5702 Fax:608-262-1267email:[email protected]/[email protected] - A Hybrid Series Active Filter system hasbeen designed, built and installed at Beverly PumpStation in New England Electric Power Companyutility for 765kVA, 48OV adjustable speed driveload to comply with IEEE 519 recommendedharmonic standards. The series active filter has asmall rating - 35kVA which is 4% of the load kVA -and is controlled by a Synchronous Reference Frame(SRF) based controller. The hybrid series activefilter system is controlled to act as an harmonicisolator between the supply and load. This paperdiscusses the SRF controller implementation issues,design considerations of the series couplingtransformer and protection issues of the small ratingseries active filter inverter. Operation of the seriesactive filter under off-tuned passive filter conditionsis investigated. Effectiveness of the series activefilter to provide harmonic damping and the use ofsimpler and low cost passive filter structures such aspower factor correction capacitors is demonstratedby laboratory experimental results. Field installationresults demonstrate the practical and economicviability of hybrid series active filter systems forharmonic compensation of large non-linear loads tocomply with IEEE 519 recommended harmonicstandards.

I. INTRODUCTIONThe incidence of harmonic related problems in theutility-load interface is increasing with the proliferation ofpower converters in industrial applications and intransmssi onl di stri buti on systems. A rapid increase in theinstalled capacity of power electronics loads, a prerequisitefor realizing energy efficiency and productivity benefits, hasbrought utilities to crossroads. Utilities on the one handarepromoting the use of non-linear Adjustable Speed Drive

(ASD) loads for significant energy savings to the customerand on the other hand, more frequently encounter harmonicrelated problems including substantially higher transformerand line losses due to harmonics, required derating ofdistribution equipment and severe harmonic interactionsbetween customers or between the utility supply and load.The user achieves energy efficiency at the expense ofincreased system losses and reduced system stability and safeoperating margin for the utilities. To alleviate harmonicrelated problems, utilities are also beginning to enforceI EEE519 recommended harmonic standards for industrial andlarge commercial customers by rebate programs. Thisincreases the need for cost-effective and practically viable

approaches to harmonic filtering problem for large non-linear loads to meet IEEE 519 recommended harmonicstandards.Passive filters have traditionally been used to absorbharmonics generated by large industrial loads, primarily dueto their simplicity, low cost and high efficiency. The supplyimpedance strongly influences the compensationcharacteristics of the passive filter and they are highlysusceptible to series and parallel resonance with the supply.Passive filters are sensitive to L-C component tolerances.Tuned passive filters have the caveat of attracting harmoniccurrents from ambient loads and are susceptible to load andline switching transients. In particular, for industrial loadsconnected to stiff supply, it is difficult to design passivefiltersso that it diverts a significant part of the load harmoniccurrent and hence its effectiveness deteriorates for stiffsupply systems61Active filter solutions were developed to mitigate theproblems of the passive filters and consistof pure active filtersolutions [l-41and hybrid active filter solutions [5-71. Theoptimal active filter solution is application and utilityinterface specific,as will be illustrated by the following fieldinstallation case study. Parallel active filters are a viablesolutionif the peak harmonic current is limted and there aredisplacement power factor constraints under light loadconditions. They require large rating for highpeak harmoniccurrent, such as diodecapacitor rectifiers. and the consequentcost penalties. It is difficult to implement a largekVA ratedPWM inverter with high current bandwidth. High currentbandwidth requires high switching frequency which is deviceand power level limited. The higher order harmonicsgenerated by PWM inverters can flow into other passivefilters in the system. Inverter output filters are highlysusceptible to utility line interactions and require activedamping. Active damping requires high bandwidth PWMinverters and hence is not a cost effective solution for stiffsupply systems.

n.HYBRID SERIES ACTIVE FILTER SYSTEMHybrid active filter topologies consist of both activefilters and passive filters in different configurations. Hybridactive filters effectively address and mitigate the problems ofboth passive filter and pure active filter solutions and providea cost-effective and practical harmonic compensationapproach, particularly for high power non-linear loads.Hybrid active filters improve the compensation characteristicsof the passive filters and thus realize a reduction in the ratingof the active filter [5-71.

0-7803-2730-6/95$4.000 1995 IEEE 189
mailto:email:[email protected]:[email protected]:[email protected]:[email protected]:email:[email protected]


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-wNONLINEAR

SERIES ACTIVE LLE L INVERTER zTSHUNTPASSIVE FI LTERFig. 1. Hybrid Series Active Filter SystemAmong the hybrid active filter solutions the HybridSeries Active Filter system is particularly attractive andconsists of a small rated series active filter and tuned L-Cpassive filtersas shown in Fig. 1. The active filter has a smallrating - typically 5% of the load kVA rating - and can becontrolled to actas a "harmonic isolator" between the supplyand load. This prevents supply-load interaction and resonanceproblems. The harmonic isolation feature reduces the needfor precise tuning of the passive filters and allows theirdesign to be insensitive to supply impedance and eliminatespossibility of filter overloading due to ambient voltageharmonics and/or ambient harmonic loads. The passive filterscanbe tuned to dominant load current harmonics and canbedesigned to achieve unity displacement power factor. Thepassive filter can also be implemented with low cost powerfactor correction capacitorsas shown in [9] and also validatedby experimental results in section VIII. For multiple. large

and diverse types of harmonic producing loads in anindustrial plant; a single hybrid series active filter can beinstalled at the point of common coupling with the utility andprovides a very cost effective approach. This approachcanalso provide limited line voltage regulation and is easilyamenable for retrofit applications - a cost effective solutionfor ASDs with input power factor correction capacitors.However, series active filters require adequate protection.

In.HYBRID SERIES ACTIVE FILTER SYSTEMINSTALLATIONITEThe single line diagram of the hybrid series activefilter installation site at the Beverly Pump Station ( BPS)ofSouth Essex Sewerage District (SESD) in the New EnglandElectric Power CompanyOISEEpc)utility isshown inFig. 2.The utility 4160V/480V step-down transformer is rated 750kVA with 5.9% mpedance (18.1 mQ and45uH) on750kVAbase and 480V. Since NEEPC owns the step-downtransformer, the point of commoncoupling for application ofI EEE 519 recommended harmonic limits is defined on thelow voltage480V side,This isan important point sinceI EEE519 is applicableonlyatthepoint of common coupling to theutility and it is not an equipment specification. The ShortCircuit Ratio (SCR) at the utility transformer is calculated tobe 26.38 and the corresponding IEEE 519 recommendedTotal Harmonic Distortion (THD) limit is 8.0%. The utility

short circuit capacity is95MVA and impedance is 2.4mRand hence the short circuit fault current atBus A is 13.5 kA.TheBPS is an example of a facility where the load isdomnated by ASDs. The BPS sitehas two buses- Bus B has a200 hp constant speed induction motor with 60kVAR ofpower factor correction capacitor. for backup and isdisconnected under normal operating conditions. The hybridseries active filter system is installed on Bus A. The35kVAseries active filter has a 1UX)A bypass circuit breaker (Fig.2). The passive filter consists of 133kVAR of 5th and65kVAR of 7th L-C filters. Bus A has four ASDs whichsupply four 200 hp induction motors for pump driveapplication. The ASDs have front-end full wave dioderectifiers with 1.5% at 480V input line reactors (6OuH) andlarge dc link capacitors. TheASDs have 1kH z sine trianglemodulated PWM inverters for variable speed induction motorpump drives.The supply currentTHD without theac side input linereactors of the ASDs exceeded 100%. The total supply lineinductance seen by eachASD including the input line reactorsis around 1lOuH. The 1.5% input line reactors help reducethe supply currentTHD to 50%and allow implementation ofcost effective active filtering approaches.

UTILITYSUPPLYlXA

Fig.2.Single L ine Diagramof Beverly Pump Station - SESD, Salem, MASiteof Hybrid Series Active Filter Installation- MaximumL oad on Bus A =765kVAI90


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Table 1 gives measured supply harmonic currents atthe nominal load of 425 kVA and the correspondingrecommendedI EEE519 harmonic limits, The supply currentTHD is 46.47% and it exceeds theI EEE519 limts except forthe 19th harmonic current. The system response is domnatedby the utility transformer impedance without any passivefilters. Since the transformer impedance is very small, itprevents voltage distortion problems due to the ASDharmonics. Initially 5th and 7th tuned passive filters weredesigned to meet I EEE 519 harmonic limits. Since diodefront-end ASDs have high displacement power factor (DPF),a maximum of 85 kVAR is required to achieve unity DPF atrated load. With small kVAR passive filters needed for suchapplications, it is difficult to achieve the required tuning toabsorb significant percentages of ASD harmonic currents.Passive filters cannot be tuned exactly to harmonicfrequencies because they can be overloaded due to supplyvoltage distortions and/or ambient harmonic currents.Measured supply voltage distortions at the utility transformerare 1.4% of 5th and 0.8%of 7th and total voltage distortion(THD) of 1.65%. Note that the measured voltageTHD iswithin theIEEE519 voltageTHD limts of 5% atthepoint ofcommon coupling.Table 1 shows that with a combined200 kVAR (133kVAR of 5th and 65 kVAR of 7th) 5th and 7th tuned passivefilters, the 5th harmonic supply current exceeds I EEE 519limit. Component tolerances for the passive filter capacitorsare 0% to +o% and hence the inductance values of thereactors are assumed to have a tolerance of 0%to -10%.Estimated voltage distortion at 480V Bus A based onharmonic load currents given in Table 1 without passivefilters is 10.23% and with the passive filters is 3.39% withmaximum tolerances. This case study illustrates thelimitations of passive filters for harmonic filtering of diodefrontend ASDs with highDPF andthepotential problems ofdesigning effective passive filters for stiff utility systems andin the presence of utility supply voltage distortions. This alsodemonstrates the need for implementation of an effectiveactive filtering approach to meet I EEE 519 harmonic

based on&e maximum tolerances for the filter components.High harmonic currentTHD of 46.47% render parallelactive filter solution uneconomical. This site presents aperfect application for hybrid series active filter, where theseries active filter can be controlled to act as a harmonicisolator between the supply and load and thereby, all the loadharmonic currents areconstrained to flow into the passive

filters. This approach also allows tuning and rating of thepassive filters to domnant 5th and 7th harmonics. The worstcase rating of theseries active filter is given by the arithmetics u m of the passive filter terminal voltage distortion andsupply voltage distortion (or background voltage distortion).Other hybrid active filter approaches [6-71 such asdifferent configurations of hybrid parallel active filters wereevaluated for this site. Effectiveness and small rating ofhybrid parallel active filters is sensitive to theprecise tuningof the passive filters to dominant load harmonics and lineparameters. For a fairly stiff supply, as at thi s site, it isdifficult to design small kVAR passive filters with extremelylow impedances at 5th and 7th harmonic frequencies and thisresults in increased rating of the hybrid parallel active filter.Also measured supply voltage distortion levelsof 1.4% at 5thand0.8%at 7th at this site, adversely affect the rating of theparallel hybrid active filter if it is to provide damping orharmonic isolation, and hence does not provide a cost-effective harmonic filtering approach for thissiteapplication.

Iv. SERIES ACTIVE FILTER IMPLEMENTATIONThe Series Active filter inverter is implemented by aPassive Clamped Resonant DC Link Inverter (PC-RDCLI)with a resonant frequency of 70lrHz, asshown inFig. 3.Theseries active filter inverter is 35kVA which is 4% of the totalload kVA (765 kVA). The dc bus voltage Vdc is 32SV. Theseries coupling transformer has a nomnal turns ratio of 1:20with lOOOA rated onthesupply side (primary winding) and50A on the inverter side (secondary winding). The outputfilter inductor L f is 1.9 mH. The series active filter iscontrolledas a current controlled harmonic voltage source asexplained in section V. The RDCL inverter is current

regulated by an adjacent state Current Regulated DeltaModulator (CRAM) [CRAM is a Discrete Pulse Modulation@PM) based scheme for RDCLI] with zero ball (hysteresis)around the zero state, which allows the use of zero stateforonly one resonant cycle, Theuse of zero state always ensuresadjacent state switching which reduces ther ms ripple currentin the dc capacitor, resonant inductor and resonant capacitor.This strategy maximizes the bandwidth of the currentregulator with the constraint of adjacent state switching. Fieldinstallation results, show that harmonic filtering is achievedup to 25th harmonic which givesan effectiveRDCL invertercurrent bandwidth of 1.5 kHz and demonstrates theeffectivenessof this new adjacent state CRAM. The currentbandwidth of the inverter determines the frequency limtsforharmonic isolation.The design considerations for the series couplingtransformer are an integral part of the series active filtersystem design and are unlike conventional transformerdesigns. The leakage inductance of the series couplingtransformer is the critical design parameter and has beendesigned to be low. Transformer leakage inductance entailsfundamental voltagedropand fundamental VA, which has tobe supported by the inverter. This effectively reduces theseries active filter inverter rating available for harmonicisolation. The measured leakage inductance is 1.625uH,(0.2%),and magnetising inductance on the primary side is1OmH (1.22 pu).

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Series Coupling ASD LOAD765 LVA

Series Active Filter Implemented byPassive Clamped Resonant DC L ink Inverter - 35 kV AFig.3. Implementation of Hybrid Series Active Filter System

v.SYNCHRONOUS REFERENCE FRAME CONTROLL ERIMPLEMENTATIONTheSRFcontroller [ 6, 9] s shown inFig. 3, usesinstantaneous supply currents and Phase Lock Loop (PLL)

signal from passive filter terminal voltage for generationoffundamental frequency unit vectors for transformations.

In the synchronously rotating de- qe reference frame,the components at the fundamental frequency, aretransformed to dc quantities and all the harmonics aretransformed to non-dc quantities and undergo a frequencyshift of 60Hz in the spectrum. SRF controller extracts the dcquantities by alow pass filter (LPF)and hence it is insensitive

to phase errors. This is a significant advantage of theSRFcontroller since most other controllers will introducesignificant phase errors at fundamental and at harmonicfrequencies. The dc components of the supply current in thede- qe reference frame, iesqsdc and iedsdc, aretransformedto fundamental frequency components in the stationaryreference frame:

The series active filter is current controlled wth the three-phase fundamental supply Current references i*sa. i*sb. i*sc.TheSRF controller realizes the series active filter inverter asa current controlled harmonic voltage source which injectsonly harmonic voltages and zero fundamental frequencyvoltage into the supply line and consequently does not handleany fundamental VA. This constrains all the load harmoniccurrents into the passivefilters.

t o L owPass t osbi s c 2Phase * de-qe

Aeasured StationarySupply Currents RefFrame

is& Filtert ods - qsLPF

Synchronous SynchronousRef Frame Ref Frame rtrom PLL cossin ve- ~

Fig. 4.Synchronous Reference Frame Controller For Series Active

I -StationaryRefFrame

Filter

ReferenceSupply Currents

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TheSRF controller is insensitive to background supplyvoltage distortions and to system parameters such as supplyline inductance, leakage inductance of the series couplingtransformer, load and passive filter unbalances and off-tunedpassive filter conditions.The SRF controller is implemented by analog anddigital hardware as opposed to aDSP based implementationfor ease of debugging in the field installation. The SRFcontroller implementation is sensitive to dc offsets and gains.This requires a flat or constant gain of theLPF up to the cut-off frequency. Effective heterodyning is possible due to wideseparation of the fundamental frequency (at dc) and thelowest 5th harmonic in the synchronous reference frame andreduces the sensitivity of the LPF to phase and amplitudeerrors. Amplitude accuracy is achieved by maximally flatsecond orderButterworth LPF realized by switched capacitorfilters (SCF) which are sampled systems. The samplingfrequency of the SCF determines the highest harmonicfrequency possible that can be filtered without aliasingerrors.This is an important consideration and a design trade-off for SCF applications since the cut-off frequency of theLPF determines the limit of the maximum harmonicfrequency and vice-versa. A fifth order LPF (two cascaded2nd order Butterworth LPF and 1st orderLPF) with a cut-off frequency fo of 30 Hz has been implemented by SCF(MAX280) and achieves attenuation of less than 1% for the5th harmonic frequency. The sampling frequency of the SCFis 3M z 1OOfo) and this allows hamonic filtering up to1500 Hz (25th harmonic) without aliasing errors. It isimportant to note that SCFs are not phase and gain sensitiveto component tolerances, unlike continuous time analogfilters.A modified SRF controller [9] enables reduction ofpassive filter voltage distortion by controlled injection of load

current harmonics into the supply within I EEE 519 limts.This achieves a further reduction in the series active filterrating. This also enables the use of simpler passive filterstructures such as power factor correction capacitors asvalidated by laboratory experimental results in section VIII.VI . PROTECTION AND SEQUENCING OF SERIES

ACTIVE FILTERSince the active filter is coupled in series with thesupply line and it has a small rating, it requires asophisticated protection and sequencing scheme. In theinstallation unit, the series coupling transformer has anti-parallel thyristors and mechanical contactor to short thesecondary of the transformer in the events of system faults,large transients and voltage overload conditions. A statemachine based sequencing scheme has been implemented forstart-up, shut-down, fault coordination and by-pass of theseries active filter inverter under appropriate conditions.

vrr. SIMULATION RESULTS WITH OFF-TUNEDPASSIVE FILTERSTable2 gives the5th and 7th passive filtercomponentvalues for perfectly tuned filters (Case 1) and withcomponent tolerances designed for theBPS installation site.Filter capacitor tolerances are0% +o% and hence the filter

inductors are designed to have a tolerance of 096, -10%. Case2 andCase3represent maximum off-tuning above and belownomnal tunedfrequencies respectively.able 2: 5th and 7th Tuned Passive Fllter Component Values

with Tnlernnces

Tuned5th &7thpassi ve

Table 3 shows the simulation results of the effect ofoff-tuned passive filters on the hybrid series active filtersystem without any supply voltage distortion. For the sameload condition, higher supply current and passive filtervoltageTHD results for off-tuned filters above nomnal (case2) comparedtobelow nomnal (case 3).Thisis expected sincethe filter impedance to the domnant5th and 7th load currentharmonics is higher for the off-tuned filter above nomnalthan off-tuned filter below nomnal. Note that the I EEE 519THD limit of 8% is met in all three cases. Operation ingeneral with off-tuned passive filters hasan adverse impacton the rating of the series active filter inverter, compared tocase 1. Therating of the series active filter inverter s 4% ofthe total load kVA and for a load current of 0.5 pu (as inTable 3). the current controller of the series active filterinverter is saturated for case2 and case 3 and hence results inhigher supply currentTHD compared to case1.VnI. EXPERIMENTAL RESULTS

Fig. 5 shows laboratory setup and experimental resultswith a three phase diode bridge rectifier-capacitor load. Thesame 35kVA series active filter inverter and the seriescoupling transformer installed at the BPS site were used. Thepassive filter was implemented by a low cost power factorcorrection capacitor CFof 25uF in per phase wye connection.To limit the r ms harmonic load current in the filter capacitorCF, an inductor LR was used in series with the diode rectifier.The supply inductance LS was varied to excite resonanceconditions with the filter capacitor CF, to demonstrate theharmonic damping capability of the SRF controlled seriesactive filter system. Experimental results show theeffectiveness of the series active filter to provide harmonicdamping and achieve sinusoidal supply currents with a 7thharmonic resonance condition (Ls=6.OmH, f0=410Hz, 6.83tuned Ls-CF filter). SRF controlled series active filterconstrains all load harmonic currents in the filter capacitor

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Cp. The fdta capacitor voltageVcpp iusdistortions due t othe relatively hi gh impadmce of the filter capacitor Qatlower (34th 1 th) harmonic h q w ~ i e s .he lineacutrplvoltsneof t he series active filter invata s also shown.

v. & v I

The designed 5th and 7th passive filter componentvalues are Ls=O.l91mH, C5=1475uF and L7-0.195mH.C7=74OuF. In t he site installation measured 5th passive filter

component values are L5=0.22omH, Cpl586uF and hencesiteitwallatior~nsultsat t hepointof unnmm couplingof t heutility transformcr. Theser ies active filmhas a small ratingand is not &signed to operateu n k krgeoff-tunedpassivefilter conditions. Fig. 6 shows thesupplyCMcllfvoltageaudt he supply c\nrrntTHD without the hybrid des ctive filtersystemoperati ng at 115kVA 0 9 . 3 kW, e35.9 kVAR)kro&Thesupplycurrent THD is44.08 .Id t excetdl IEEE519 limits ami has very high crest factor of 1.8. Theindividual harmoni cs which cb not mea IEEE519 limts are375% of 5th 19.3%of 7th, 4.6% of 11th. 4.0% of 13thmd1.3%o 23rd.Table 4 shows the insta lati on results at 220 kVA(P=l05kW, w193.3 LVAR) load wi t h the passive filters,red t the utility trsndfocreer and at the passive f i lm.M wurcmenU without the series active filter clearlydemoastR tetbcoff-amingofthe~vefi ltenrraditisreenthat a large percentage of the 5th harmonic 1 0 4 curtentf lows into t he supply. Measuremenu with t he seri es wivefilter demonstrate the harmonic isolation feature of theseri es d v e ilter within its rssing. Ofi-tuning of the passivef i l moverloedr t he small rated (4% o the load LVA) #riaactive filter inverter. Lhe supply current THD is reducedf mm44.0% t o 8.% and t he crest faax is reduced from1.8t o 1.4 by t he hyhid seri es active filmOpaation. The8.1%5th harmonic distortion is t hc ma& component of 8.9%supply current THD and the remaining supply currentharmonics meet I EEE 519 individual harmonic limits.Effectivenessof tbe Mia active filter system to pr ovi dehumonic isolation even under off-tuned passive filterconditionsandwith 5thand7th supply voltagedistortions hasbeea demonstrptedby fgld instplluiaaresults.Fild installationexpaienCe suggats that hybrid seri esllctive filter system is a col t effectivesd viable hari cfilteringsolution for multiple and diverseharmonic lords.The series wive filmprovides 'harmonic isolation at thepoint of co01l couplingand providesan effectivesolutionto my posoible

t he 5th film k off-hmed t o 4.5 (htdf 5.0). due toOut+f-speCificrrtiOn reraOr. Figs. 6 & 7 &OWhe mcpIwrtd

DPFcoaditiansdue to loaddiversity.

I Pll#r.

controlkr for8erie-s active filter achieves harmonic isolationbetween t he supply and t he load under norma and pbnonnalconditions such as off-tunedpassive filter conditions andA hybrid series active filter system has been presence of supply voltage d storti ons. Installation resul tsshowthatseriesactivefilter implemented with a Remnant DCLink Inverter provides current bandwidth in excess of 1.5* isolation betweencHz (Zthharmonic) to achi eve harmonw:the supplyand load. Effectivenessof the series active filter toprovide harmonic damping and the use of simpler and lo w

X. CONCLUSIONS

successfully installed and demonstrated at the BPS site inNEEPC utility f ar adjustablespeed drive loadsto meet IEEE519 r ecommended hannonic st andar ds. The series activefilm has a small rati ng - 35 kVA which is 4% of the loadrating. The developed Synchronous ReferenceRmebased194


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cost power factor correction capacitor passive filters has beenexperimentally demonstrated. Synchronous Reference Framecontroller issues have been addressed. Protection andsequencing issues of the series active filter have beenidentified and addressed in thei nstal l ati onunit. Impact of off-tunedpassive filters on the operationof the series active filterhas been investigated.Thi s i s the first field installation of Hybrid SeriesActive filter and demonstrates its cost effectiveness andpractical viability as a harmonic filtering approach to meetI EEE 519 harmonic standards for large non-linear powerelectronic loads and for multiple and diverseharmonicloads.

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0 . 0F W O l bt h 3lndP W I C A CVIREKT MPL IYUDC I PCCI I UI8C1 X R l I J % Aua 3 1 1993 I l f i 111EREPENCES_ -. -. --1 L. Gyugyi, E.C. Snycula. "Active AC Power Fi l ters", Roc. EFX- IASAnnual Meeting, pp 529,1976.2 N. Mohan. et d, "Active Filters for A C Harmonic Supprcssion",I EEupEsWinterMeeting 1977, A770268.3.M Takda, ct al. "HnmonicCurrent Compensation with ActiveFiltcf',E m S . nnual M n g , p 808,1987.4 S. M m , A Lint Vol e Regulator/Conditioner for Harmonic-SensitiveLopd I sdPtion".&S Annual Meeting,pp 947-951,1989.5 F.Z. Peng, H. Akagi. A. N a b . " A N ew Approach T o HarmonicC omp ed on n Power Systems", [EEE/ I ASRecord,pp874-880,1988.6 S. Bhattacharya, D.M . Divan, B. Bancrjec, "Synchronous ReferenctFrame Harmonic Isolator Using Series Active Filter", Roc. 4th EPE,Rorencc,1991, vol. 3,pp 030-035.7 H. Fyita, H Akagi, "A pncticalApproach to Harmonic Compensationin Power Systems - Series Connection of Passive and Active Filtm".IEEWlA SAnnualMeeting.pp 1107-1112,1990.8 D. M Di van, S. BhanPchrya, R. Znvadil, ct. al, "Desi gn of an ActiveSerics/Passive Parallel Hannonic Fil ter for ASD Loadsa I WastewaterTrutmcnt Plant",Roc of Sccond Intl. PQA 92, Atlanta, USA, 1992.9. S. Bhattacharya, D.M . Divan, B. BpmJ et, "Control and Reduction ofTerminal Voltage T d armonic Distortion W D ) n I Hybrid SeriesAcfiveandParallcl PassiveFiltaSystcm", PESCCo d Recard 1993.

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