selective compensation of power-quality problems through active power filter by current...

792 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 23, NO. 2, APRIL 2008

Selective Compensation of Power-QualityProblems Through Active Power Filter

by Current DecompositionBhim Singh, Senior Member, IEEE, and Vishal Verma, Member, IEEE

Abstract—Active power filters (APFs) are used to compensateharmonics, reactive current and negative sequence fundamentalfrequency current of load current drawn by nonlinear loads. Theflexibility of selective compensation, if provided in the controlscheme, makes APF versatile device for compensation of reactivepower, harmonic currents and unbalance in source currentsand their combinations, depending upon the limited rating ofthe voltage-source inverter employed as an APF. The proposedscheme decomposes the load current into positive and negativesequence fundamental frequency active components, reactivecomponent and harmonic components in synchronous referenceframes. The control scheme performs with priority-based scheme,which respects the limited rating of the APF. The simulated resultsin MATLAB environment and experimental results of the devel-oped prototype of APF are presented to validate the effectivenessof the proposed control scheme. Digital signal processor basedimplementation of control scheme is also detailed for selectivecompensation of power-quality problems with details of developedprototype.

Index Terms—Active power filter (APF), harmonic compensa-tion, reactive power, synchronous reference frame (SRF), unbal-ance.

I. INTRODUCTION

THE ac power system has always been susceptible to prob-lems regarding reactive power and unbalance from the very

beginning. It has worsened with the increased use of power elec-tronic converters as some of these converters not only increase re-active currents, but also generate harmonics in the source current[1]–[12]. The increased reactive power, harmonics, and unbal-ance cause an increase in line losses, instability, and voltage dis-tortion when harmonics travel upstream and produce drop acrossthe line impedance, which corrupts the power system. Conven-tionally, shunt passive filers are used to suppress the harmonics,and their application provides limited fixed reactive power com-pensation [1]. Moreover, passive filters do not provide any so-lution for unbalance, and variable reactive power compensation.The appropriate solution for all the aforesaid problems can be re-alized by a shunt active power filter (APF) [2]–[5].

The performance of APF is dependent on how the referencecompensating signals are estimated. Instantaneous reactive

Manuscript received March 14, 2005; revised April 19, 2007. Paper no.TPWRD-00139-2005.

B. Singh is with the Department of Electrical Engineering, Indian Institute ofTechnology Delhi, New Delhi-110016, India (e-mail: [email protected]).

V. Verma is with the Department of Electrical Engineering, Delhi College ofEngineering, Delhi-110042, India (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TPWRD.2007.911108

power theory [3], [7], modified p-q theory [4], [7], synchronousreference frame (SRF) theory [6], [7], instantaneoustheory [5], [7], and method for estimation of current reference bymaintaining the voltage of dc link [8] and subtracting the positivesequence fundamental current from the load current for compre-hensive compensation of power-quality (PQ) problems by APFare generally reported in literature. These control schemes lookvery attractive for their simplicity and ease of implementation,but lack in providing adequate solution under extreme or severecondition of harmonics, reactive power and unbalance or theircombinations with limited power rating of the voltage-sourceinverter (VSI), employed as APF. In such cases to safeguard theAPF hardware, the protection scheme isolates the APF, whichleaves the system to the mercy of unwanted disturbances.

Some reported literature have used selective compensationfor different topologies of APF by estimating the derived com-ponents using pq theory [3], modified pq theory [4], FBD theory[9], orthogonal decomposition [10], Lagrange multiplier-baseddecomposition [11], and estimation based on neural networks[12] etc., which utilizes complex calculations and most of theschemes are not employed for undistorted mains conditions.Some schemes [11], [13] have used notch filter and adaptivetechniques to compute decomposition of current.

In this paper, a simple SRF-based scheme is used todecompose load current into four parts; positive sequence fun-damental frequency active current , positive sequencefundamental frequency reactive current , current atharmonic frequencies and negative sequence fundamentalfrequency current . With these current components, se-lective compensation of combinations of them can be madewhich respects the rating of the APF used. The proposedscheme is easily implemented on digital signal processor(DSP). The sine and cosine reference signals synchronizedwith ac mains are generated by a digital phase-locked loop(PLL) synthesized by DSP through zero crossing of the filteredvoltage signal of the one phase. Moreover, indirect currentcontrol in present study eliminates the need of any feedforwardcompensation. The effectiveness and dynamics of the schemewith selective compensation under varying load conditions isdemonstrated through simulation in MATLAB environmentusing power system blockset (PSB) toolboxes and experimentalresults obtained by real-time implementation of the proposedscheme using DSP.

II. SYSTEM CONFIGURATION

Fig. 1 shows the schematic diagram of a power conditionersystem employing shunt APF. The composite load includes a

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SINGH AND VERMA: SELECTIVE COMPENSATION OF POWER-QUALITY PROBLEMS 793

Fig. 1. Block diagram of shunt active power filter with composite load.

three-phase diode rectifier, used as front-end converter for vari-able frequency ac motor drives. The ac side inductance is oftensufficiently provided by the connected transformer. The unbal-ance in the present study is created by connecting resistive loadbetween two phases. However, severe unbalance is created byopening one of the load terminals at point of common coupling(PCC). The proposed SRF-based reference current signal gen-erator decomposes the load current into different components.These desired decomposed component or the combination ofcomponents are fed as reference source currents to the hysteresiscontroller of the VSI, which forces the source current to followthe reference current. The APF acts as a harmonic mitigator, re-active power compensator for loads demanding reactive powerand balances the supply side current through compensation ofnegative sequence component in the supply current. It is con-trolled, as a current controlled current source through injectionof compensating current which compensate harmonics, mit-igates demanded reactive power by nonlinear load, and compen-sates unbalance due to a negative sequence component of theload current.

The dc link capacitor of APF is selected on the basis ofallowable change on the dc bus voltage, and change in supplycurrent. When the dc bus voltage of APF changes from to

, the energy released or absorbed by the dc link capacitor is. This energy is being exchanged with energy released or

absorbed by filter inductor. Hence, is

(1)

where “ ” represents the maximum ripple allowed in dc busvoltage. For the present study with 3% voltage ripple, as700 V and current change of 15 A to 75 A, the computed valueof capacitor is 2314 , which is taken as 2350 .

III. CONTROL THEORY

The proposed control of APF depends on decompositionscheme applied to net load current, selectivity of differentpower-quality components based on their individual weigh-tages and indirect control of source current. Following sectiondeals with the basic scheme of decomposition of load current

and the overall control scheme adopted for selectivity of PQcomponents and indirect current control of APF.

A. Basic Theory

It is proposed to use SRF theory to decompose the loadcurrents instantaneously into active and reactivecomponents of positive and negative sequence of currents

at fundamental frequency and harmonic frequencies. The SRF isolator extracts the fundamental component

of the load current by transformation of , and to d-qreference frame. In the synchronously rotating reference frame,the positive sequence components at fundamental frequency

, are transformed to dc quantities and all harmonic andnegative frequency components undergo a frequency shift of

(2)

(3)

The SRF isolator extracts the dc quantities by low-pass fil-ters (LPF) for each and , realized by moving averager at100 Hz since the waveform has half wave symmetry in steadystate. The averager running at 100 Hz can compute and detectthe deviation from steady state condition within half cycle. Theextracted dc components and are transformedback into first frame and then into a-b-c coordinates to ob-tain net positive sequence fundamental components as follows:

(4)

Whereas the real and the reactive decomposition ofthe positive sequence fundamental frequency current

can be easily made from the d-q frame,thus the a-b-c coordinates of real and reactive components ata fundamental frequency can be evaluated as detailed in thefollowing:

(5)

(6)

(7)

(8)

Similarly, for a negative sequence, a fundamental componentcan be extracted by rotating the frame in the opposite direction,i.e., after transformation to two phase stationary frame [(2)],executing the following transformation:

(9)


Fig. 2. Control scheme for selective compensation.

and then extracting dc quantities by a LPF in the similar fashion.The dc quantity would amount to the negative sequence compo-nent of current due to unbalanced voltage at PCC or unbalancedload condition. The dc components so extracted, and

are transformed back into and then into a-b-ccoordinates to obtain the negative sequence fundamental com-ponents as follows:

(10)

(11)

Thus, for selective compensation, the above decomposedcomponents can easily be used. Particularly when APF isemployed with limited capacity, to justify the compensationcosts, it is very important, that in case of both reactive powercompensation and unbalance compensation or any of these lim-ited compensation may be provided depending on the allowablerating of APF to keep continuing its operation.

B. Control Scheme

The control scheme for the proposed system is using the SRFbased current decomposition, discussed in the previous section.Fig. 2 shows the flow of various control signals and controlscheme based on the decomposed components. The controlscheme depicted in Fig. 2 also incorporates the command formaintaining the average dc bus voltage at the VSI constant orto control it within given limit values.

The command for desired compensation is de-rived from taking difference of the load current and the otherdecomposed components which needs to be compensated. Theflexibility in selectivity is provided by the priority resolver andgain scheduler block. This block is programmed to resolve thepriority of the compensation desired by the system. The currentcomponent which fully needs to be compensated would be as-signed a gain of “0”, and the components which need not becompensated would be assigned the gain “1”. The level of pri-ority decides the depth of compensation desired by the system,or to circumvent the overloading of APF by adjustment of thegains, , , in reverse order, i.e., the component with

higher priority for its compensation would have lower gain inratio of its priority. The gains are being assigned value less than“1”. This scheme facilitates the control of conditioner for selec-tive compensation by indirect current control through hysteresiscarrier-less PWM current controller.

A PI controller is used also to regulate the dc bus voltage toits reference value and compensates for the inverter losses. Alow pass filter is used to filter the ripples in the feedback path ofthe dc link voltage. The filtering of dc voltage ensures that powertransfer between the dc bus of the inverter and supply takes placeonly at fundamental frequency and not as a result of harmonicfrequency. To minimize the inverter losses and to maintain thedc bus voltage constant or within limits, the demanded current isadded to positive sequence fundamental frequency active com-ponent of load current, as shown in the Fig. 2.

The PWM gating pulses for the IGBTs in VSI of APF are gen-erated by indirect current control using hysteresis current con-troller over reference supply currents and sensedsupply currents . The switching frequency is lim-ited to 20 kHz, by limiting clock frequency of the flipflop atthe output end to avoid the controller entering into limit cycleswhile simulation and kept near fixed at 12.8 kHz by processing256 samples per cycle amidst changing frequency at ac mainswhile implementation on DSP. Such switching frequency is se-lected for low power applications. The controlled compensationcurrent is injected such that the supply current follows the ref-erence current.

C. Priority Resolver and Gain Scheduler

This section deals with the calculation of gainsaccording to priority assigned to the components needs to becompensated. The level of priority decides the depth of compen-sation desired by the system, in order to avoid the overloadingof APF by adjustment of the gains , , and in reverseorder, i.e., the component with higher priority for its compensa-tion would have lower gain in ratio of its priority.

The priority of compensation is assigned on the basis ofgravity of ill effects of these decomposed components on thepower system. In the proposed scheme, the first preferencehas been given to the harmonics and subsequently the gainsof negative sequence and reactive components are computedconsidering the interests of both consumer and utility.

If it is assumed that the compensating capacity of the APF belimited to 40% of the total load power, then maximum valueof the sum of different rms load current components, can becompensated are given as:

Maximum ofwhere, is rated load current.

Case I: When the power required for desired compensationlies within the rating of APF, it compensates all the com-ponents fully, i.e.,If , then

Case II: When the power required for desired compensa-tion is higher than the compensating capacity of APF thenlimited compensation is provided. Giving priorities in theorder: first the harmonics, followed by unbalance and then


Fig. 3. Main block of proposed control scheme with APF under MATLAB (ref. Fig. 1).

Fig. 4. Decomposition of load current for balanced and unbalanced nonlinear load conditions.

the reactive power compensation, as per the following con-straints.• The negative sequence currents withstand capability of

generator is 10% of full load current.• The power factor (pf) of the system be greater than 0.85.

Keeping the maximum limit for unbalance to be 5% of thefull load current before any part compensation of reactive poweris attempted. Then, it is proposed to compensate the reactivepower till pf is maintained 0.85, beyond that both unbalance andreactive power compensation is employed with weightages inthe ratio 4:1, respectively, until one of them is fully compen-sated. Thus, the reference current used in hysteresis controllerblock can be computed by accumulation of the product of eachof the decomposed components with their respective gains.

IV. MATLAB-BASED SIMULATION

The power source, rectifier fed equivalent resistive loadwith input side inductive filter, three phase resistive load,

circuit breaker and APF are modeled in MATLAB using PowerSystem Blockset. Fig. 3 depicts the setup used to estimate theperformance of the APF with proposed control scheme throughsimulation. The source block consists of a three-phase voltagesource “infinite bus” with series impedance representing the rel-evant short-circuit impedance of the supplying grid to gauge theperformance of the APF with proposed scheme under specifiedconditions. The considered loads to evaluate the effectivenessof the proposed scheme are diode rectifier with inductive filteron source side and star connected resistive (R) load connectedto ac mains through circuit breaker as depicted in Fig. 3. Theunbalance has been created by opening one of the phases atload terminals. The control signal tags in Fig. 3, viz. C-F, K,L1, L2, Q, TT are connecting blocks (global /local) and someof them are terminating in block “DUMP” to record the results.The rectifier fed load has been modeled as diode rectifier withsmoothing capacitance of 2000 F at dc bus feeding resistiveload (38.6 kW). The star connected resistive load consists of


Fig. 5. Dynamic response of APF for harmonic and unbalance compensation with diode rectifier load. Single phasing is done from 0.14 s to t = 0:3 s. Harmonicalone has been compensated from t = 0:06 s to t = 0:18 s and from t = 0:3 s to t = 0:36 s, and, compensated harmonic together with negative sequencecompensation from t = 0:18 s to t = 0:3 s.

TABLE IPARAMETERS OF THE CONSIDERED SYSTEM

10 in each branch. The unbalance has been created eitherby opening the circuit breaker of phase “c” at R-load or phase“a” for diode rectifier. The source side inductor for each phaseof diode rectifier is taken as 2 and 0.5 mH, respectively, forthe results shown in Figs. 4 and 5. The simulated results havebeen studied to compute the performance of APF with selectivecompensation and analysis through THD of source and loadcurrent. The parameters are shown in Table I.

V. PERFORMANCE OF THE SYSTEM

The proposed decomposition scheme and selective compen-sation by APF is simulated under MATLAB environment withdiode rectifier. The load consists of which generates large har-monic spectra, and also absorbs a little reactive power due topresence of inductive filter. The unbalance has been created ei-ther by opening the circuit breaker of phase “c” at R load orphase “a” for diode rectifier (source of negative sequence fun-damental current). The results in Fig. 4 clearly demonstrate thatthe scheme is able to successfully decompose the load currentinto fundamental frequency positive sequence real and reactivecurrents, fundamental frequency negative sequence current and

harmonic frequency currents under balanced source conditions.A 38.6-kW diode rectifier load is under operation fromto , representing a balanced loading condition. Theload current during this time is composed of harmonics and pos-itive sequence real and reactive components only. Atwhen a balanced resistive load (15.87 kW) is switched in thescheme detects increase in only positive sequence real compo-nent. Results in Fig. 4 up to show decompositionof current into harmonic , real and reactive componentsof the fundamental positive sequence components ( and

), whereas negative sequence component stays at zero forbalanced load. At , unbalance has been created byopening the circuit breaker of phase “c” at R load. It can be ob-served that negative sequence component has marked itspresence besides decrease in , and there has been no in-fluence of such load on , validating the effectiveness andtruthfulness of the proposed current decomposition scheme.

The diode rectifier with dc side smoothing capacitor has awide harmonic spectrum of ac side input current. The load alsoretards the response of APF for dynamic change of the load inpresence of large capacitor on the dc side, thus this may be con-sidered an appropriate measure to gauge the dynamics of theproposed scheme for change of load. Fig. 5 shows the dynamicresponse of APF for harmonic compensation under balancedand unbalanced rectifier load conditions. Unbalance caused dueto single phasing depicts both unbalance and varying load con-dition. At , the APF is started to provide harmoniccompensation for aforesaid rectifier load. Due to the inception ofAPF at the dc bus which has earlier been maintainedby diode bridge connected in antiparallel with the switches inthe VSI of the APF rises to the desired level of 700 V. In turn this


Fig. 6. (a)–(c) Harmonic spectrum of rectifier load current and source current with harmonics, unbalance and reactive power compensation (H + U +X : C).

TABLE II%THD AND RMS VALUE OF LOAD CURRENT AND SOURCE CURRENT

FOR FULL COMPENSATION (SIMULATION STUDY)

causes rise in current drawn from the source. It may be observedfrom Fig. 5 that such rise in source current is curbed when thedc bus is maintained to its desired level. At , the loadterminal of phase “a” has been opened, thus creating unbalance.From to APF continues to compensateharmonics only leaving behind the source current unbalanced.At the compensating signal has been appended withnegative sequence component, to compensate the negative se-quence component together with harmonic component, therebybalancing the source currents besides making it sinusoidal. At

when the connection of the phase “a” has been restoredat load terminals, the APF quickly returns to compensation ofharmonics alone depicting the fast dynamics of the proposedscheme. It can also be seen from Fig. 5 that scheme also as-sists the APF to self-support its dc bus, under various dynamicsof the load. The harmonic spectrum of load and source currenthas been shown in Fig. 6(a)–(c) to assess the compensation pro-vided by APF with the proposed scheme. The reduction in fun-damental frequency component of source currents reflects com-pensation of reactive power as shown Fig. 6(a)–(c). It can beobserved that various power-quality compensations have beensuccessfully achieved respecting the IEEE 519 standard. Thedetail of the rms currents of load and source along with %THDhas been presented along with harmonic spectrum of respectiveloads and phases in the recordings and in Table II. It can beobserved that %THD remains less than 4.5% for rectifier load,which is well within IEEE 519 standard.

VI. HARDWARE IMPLEMENTATION

A three-phase experimental prototype of the APF for selec-tive compensation has been fabricated in the laboratory to verifythe performance of proposed scheme. The VSI has been builtwith IGBT switches with interface inductor and filter ca-pacitor . The switching signals for the VSI have been gen-erated by hysteresis current controller. The nonlinear load has

Fig. 7. Experimental results showing source currents for selective compensa-tion with small unbalanced load. i(H : C)) harmonics compensation alone;i(H + U : C) ) harmonics and unbalance compensation; i(H + U + R :C)) harmonics, unbalance, and reactive power compensation.

been realized by a three-phase diode rectifier with some sourceside inductor (1.5 mH) at each load terminals. The self-sup-ported dc bus has been maintained at 385 V both for load havingunbalance due to opening of supply connection at phase “c” ter-minals of R load (57.4 between phases a–b), and for singlephasing by opening of supply connection at phase “a” terminalsof diode rectifier load (feeding 39.2 resistive load at dc side).The source voltage is kept at 110 V (rms). A 3-ph 15-kVA au-totransformer has been used to feed the load.

The control algorithm has been implemented in real time onDSP (ADMC401). The voltage signals are sensed by CV-3-1500 LEM voltage sensors and current signals by CT 100-shall effect current sensors. The signals are sampled in simul-taneous sampling mode by inbuilt analog-to-digital converters(ADCs). The loop for the real time implementation has beenrealized through interrupt routine synchronized with samplingsignal to ADC. The correction in sampling rate is incorporatedin accordance with changing frequency of the source voltage.Number of samples over a cycle has been held constant at 256.The loop time thus become nearly fixed at 78.125 to realizea near constant frequency hysteresis controller at 12.8 kHz. Theout of phase unit template for phase “a” has been derived from“b” phase voltage input signals filtered by second-order Cheby-shev filter with appropriate delay. Experimental results of theproposed scheme have been obtained with load dynamics in-cluding unbalance through single phased resistive load (57.4


Fig. 8. (a)–(c) Recorded harmonic spectrum of load currents of the three phases with unbalanced load.

Fig. 9. (a)–(c) Recorded harmonic spectrum of source currents of the three phases for full compensation with proposed scheme.

Fig. 10. Experimental results showing source current for selective compensa-tion by APF with single phasing of load.

between phases “a”–”b”) and severe unbalance through singlephasing of diode rectifier to validate the simulation results in thescaled down form.

Fig. 7 shows the compensation performance of the APF forselective compensation of harmonics, unbalance in the sourcecurrent and reactive power for all the three phases. For first twodivisions of time (nearly 40 ms), no compensations are pro-vided; just the dc bus is maintained at 385 V. The three sourcecurrents shown in Fig. 7 comprise of, in majority the load cur-rents. After the second to fifth division of time, only harmoniccompensation are done thus current become near sinusoidal butit is still unbalanced and the load still draws reactive power fromsource. After the fifth to seventh division of time, besides har-monic compensation, the unbalance in the source current is also

compensated, thus it can be observed that currents become si-nusoidal and balanced during this period. The reactive powercompensation along with harmonic and unbalance compensa-tion is attempted beyond the seventh division of time. It maybe observed in Fig. 7 that there is an instantaneous phase jumpin all the three phase source currents, and APF is able to com-pensate all the current related power-quality problems with se-lectivity. The harmonic spectrum of load and source currentsof all the three phases has been shown in Figs. 8 and 9 re-spectively. The harmonic spectra are recorded on FLUKE 43B power-quality analyzer. It can be observed from a recordedharmonic spectra of load and source currents harmonics have re-duced drastically from 42% to 4.2%. Further, investigation withsevere unbalanced load has been depicted in Fig. 10 with com-pensations provided in similar sequence and for same durationand instances of time divisions. It may be observed that APFwith proposed scheme is able to balance the current from sourceside even when the load is not drawing current from phase “a”.The self-support of the dc bus may be observed in Fig. 11. Thesnapshot of the oscilloscope (Fig. 11) shows phase voltage, loadcurrent and source current for phase “b”, besides the voltage atthe dc bus. It may be observed from Fig. 11 that the dc bus isable to self support during compensation of unbalance and re-active power.

It may be observed that experimental results also validate theeffectiveness of the scheme for selective compensation of un-balance and harmonics in the load current in line with simula-tion results. The details of the rms currents of load and sourcealong with %THD has been presented for different combina-tions of selective compensation in Table III. The results obtained


Fig. 11. Experimental results showing load current and source current of theother phase (load terminal not opened) for selective compensation with singlephasing of the load along with the self-supported dc bus voltage.

TABLE III%THD AND RMS VALUE OF LOAD CURRENT AND SOURCE CURRENT

FOR FULL COMPENSATION (EXPERIMENTAL STUDY)

by experimentation based on the proposed scheme conforms toIEEE-519 standard.

VII. CONCLUSION

A new current decomposition technique based on SRFtheory with indirect current control for prioritized selectivecompensation of power-quality components has been investi-gated for APF. The observed performance of the active filterhas demonstrated the ability of the proposed control techniqueto selectively compensate the current harmonics, unbalancedloading and reactive power, based on priority to respect the lim-ited power capacity of the VSI. An experimental study has beenconducted and has validated the effectiveness of the proposedscheme. It has also been observed that the proposed scheme hasa fast response and is able to maintain near sinusoidal sourcecurrent for harmonic compensation conforming to IEEE-519standard and is able to self support the dc bus. The scheme hasthe advantage of flexibility of selection of the power-qualityindices for which references may be computed. The schemetogether with indirect current control has offered automaticcompensation of phase lag, which would have otherwise oc-curred due to passive ripple filters etc. The scheme in generalmay be applicable to other topologies of active and hybridpower filters/conditioners. In a nutshell it is recommendedthat under limited power capacity of the VSI of APF/activepower conditioner with indirect current control with prioritizedscheme compensation provided by the proposed scheme offersbefitting solution to uncertainties in power system due tocurrent quality problems.

REFERENCES

[1] J. Schlabbach, D. Blume, and T. Stephanblome, Voltage Quality inElectrical Power Systems, ser. PEE Series. New York: IEE Press,2001.

[2] L. Gyugyi and E. C. Strycula, “Active AC power filter,” in Proc. IEEEIAS Annu. Meeting, 1976, pp. 529–529.

[3] H. Akagi, Y. Kanazawa, and A. Nabae, “Generalized theory of the in-stantaneous reactive power in three-phase circuits,” in Proc. IEEE andJIEE IPEC, 1983, pp. 821–827.

[4] Y. Komatsu and T. Kawabata, “Experimental comparison of pq andeextended pq method for active power filter,” in Proc. EPE, 1997, pp.2.729–2.734.

[5] V. Soares, P. Verdelho, and P. D. Marques, “Active power filter con-trol circuit based on instantaneous active and reactive current i � i

method,” in Proc. IEEE PESC, 1997, pp. 106–101.[6] S. Bhattacharya, D. M. Divan, and B. Banerjee, “Synchronous frame

harmonic isolator using active series filter,” in Proc. EPE, 1991, pp.3030–3035.

[7] G. D. Marques, “A comparison of active power filter control methodsin unbalanced and non-sinusoidal conditions,” in Proc. IEEE IECON,1998, pp. 444–449.

[8] B. Singh, K. Al-Haddad, and A. Chandra, “A new control approach tothree-phase active filter for harmonics and reactive power compensa-tion,” IEEE Trans. Power Syst., vol. 13, no. 1, pp. 133–138, Feb. 1998.

[9] M. Depenbrock and V. Staudt, “The FBD-method as tool for compen-sating total non-active currents,” Proc. IEEE Harmonics and Qualityof Power, pp. 320–324, 1998.

[10] L. S. Czarnecki, “Orthogonal decomposition of the currents in a3-phase nonlinear asymmetrical circuit with a non-sinusoidal voltagesource,” IEEE Trans. Instrum. Meas., vol. 37, no. 1, pp. 30–34, Mar.1998.

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[12] B. Singh, V. Verma, and J. Solanki, “Neural network-based selectivecompensation of current quality problems in distribution systems,”IEEE Trans. Ind. Electron., vol. 54, no. 1, pp. 53–60, Feb. 2007.

[13] H. Karimi, M. Karimi-Ghartemani, M. R. Iravani, and A. R. Bakhshai,“An adaptive filter for synchronous extraction on harmonics and dis-tortions,” IEEE Trans. Power Del., vol. 18, no. 4, pp. 1350–1356, Oct.2003.

Bhim Singh (SM’99) received the B.E. (electrical)degree from the University of Roorkee (U.O.R),Roorkee, India, in 1977 and the M.Tech. and Ph.D.degrees from the Indian Institute of Technology(IIT), New Delhi, in 1979 and 1983, respectively.

He was a Reader in the Department of ElectricalEngineering, U.O.R, then joined the Department ofElectrical Engineering, IIT Delhi, where he becamean Associate Professor in 1994 and Full Professor in1997. His fields of interest include power electronics,electrical machines and drives, active filters, static

VAR compensators, and analysis and digital control of electrical machines.Dr. Singh is a Fellow of INAE, IE (I), and IETE, and is a Life Member ISTE,

SSI, and NIQR.

Vishal Verma (M’04) received the B.Tech. degreefrom G.B. Pant University, Pantnagar, India, and theM.Tech. and Ph.D. degrees from the Indian Instituteof Technology, New Delhi.

His field of interest includes power electronics,drives, active filters, and power-quality issues.In 1991, he joined the Department of ElectricalEngineering, G.B. Pant University, as a AssistantProfessor and then in 2004, he joined Delhi Collegeof Engineering, Delhi.

Dr. Verma is a member of ISTE and a Life Memberof CES(I).

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