7/27/2019 00969390

http://slidepdf.com/reader/full/00969390 1/10

1118 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 48, NO. 6, DECEMBER 2001

Harmonic Filtering of High-Power 12-Pulse RectifierLoads With a Selective Hybrid Filter System

Duro Basic, Victor S. Ramsden, and Peeter K. Muttik  , Member, IEEE 

 Abstract—Current distortion of 12-pulse rectifier loads is sig-nificantly lower compared to six-pulse rectifier loads. However, inpassive filtering of the lowest anddominant characteristic 11th and13th harmonics the use of 5th and 7th filters is often required inorder to prevent possible parallel and series resonance betweenpassive filter and source impedance which can be excited by sourcebackground distortion or by load current residual noncharacter-istic harmonics at the 5th and 7th harmonic frequencies. In hy-brid filter systems, an active filter (AF) can be added in series withthe passive filter in order to isolate the source and load. In mostproposed hybrid filter systems, AF control is based on the detec-tion of total current distortion and high-frequency inverters. Witha selective AF control system and voltage-controlled inverter, theAF can be controlled to isolate the load at the critical frequen-cies only while at all other frequencies the passive filter functionis preserved so that lower switching frequency and AF rating is re-quired. In this paper, we present a selective AF filter control systemand simple hybrid filter topology suitable for the compensation of high-power 12-pulse rectifier loads. Harmonic current controllersbased on the second-order infinite-impulse response digital reso-nant filters are used, as they can be considered as simple digitalalgorithms for more complex double cascaded synchronous-refer-ence-frame-based proportional plus integral controllers. They arecentered to the targeted harmonic frequencies by using an adap-tive fundamental frequency tracking filter. This approach givesgood results, even if the reference waveform (in our case, a loadvoltage) is highly distorted or unbalanced and no separate phase-locked loop is required. Test results for a laboratory model of thissystem and stability analysis are presented and the importance of 

delay-time compensation is discussed.

 Index Terms—Active filters, harmonics, hybrid filters.

I. INTRODUCTION

T HE lowest harmonics in the source current spectrum of a

12–pulse rectifier are theoretically the 11th and 13th har-

monics, but some residual noncharacteristic 5th and 7th har-

monics can be present. Normally, filtering of 11th and 13th

characteristic harmonics is required to reduce voltage distortion

at the point of common coupling. However, 5th and 7th har-

monic filters are often required in order to prevent possible par-

Manuscript received February 17, 1999; revised June 1, 2001. Abstract pub-lished on the Internet October 24, 2001.

D. Basic is with the Centre for Electrical Machines and Power Elec-tronics, University of Technology, Sydney, N.S.W. 2007, Australia (e-mail:duro.basic@eng.uts.edu.au).

V. S. Ramsden, retired, was with the Electrical Engineering Group, Facultyof Engineering, University of Technology, Sydney, N.S.W. 2007, Australia.He is now at 13 Bareena Rd., Avalon, N.S.W. 2107, Australia (e-mail:vicr@eng.uts.edu.au).

P. K. Muttik is with Transmission and Distribution Systems, ALSTOMAustralia Ltd., Milperra, N.S.W. 2214, Australia (e-mail: peeter.muttik@crn.al-stom.com).

Publisher Item Identifier S 0278-0046(01)10280-7.

allel and series resonance between the passive filter and source

impedance which can be excited by the source background dis-

tortion or by load noncharacteristic 5th and 7th harmonics.

Hybrid filters with a shunt passive filter and a small-rating

active filter (AF) in series with the supply [1] or in series with a

passive filter [2], [3] have been proposed for harmonic isolation

of large rectifier loads with a simple control strategy based on

a proportional controller and detection of total source current

distortion (obtained after subtraction of the fundamental com-

ponent). In both cases, the AF behaves as a resistor at harmonic

frequencies in series with the supply providing harmonic iso-

lation. A proportional controller, however, cannot provide sat-isfactory attenuation of source current harmonics if the passive

filter is not properly tuned at the dominant load harmonics, and

a broad-band high-frequency AF inverter is required.

For large 12–pulse rectifiers, a selective AF control system

has been proposed [4] with full isolation at 5th and 7th har-

monic frequencies achieved with square-wave voltage injection

into dominant harmonic (11th and 13th) passive filters. For the

detection and control of 5th and 7th harmonics, low-pass fil-

ters and proportional plus integral (PI) controllers were applied

in reference frames rotating synchronously with corresponding

harmonic space vectors. This technique was successfully used in

vector-controlled ac drives for many years and later applied for

AFs [5]. However, a single synchronous reference frame (SRF)is appropriate for balanced three-phase systems only because

it can track only positive- or negative-sequence vectors. For

tracking both sequence harmonic vectors in unbalanced three-

phase systems, double cascaded SRFs have been proposed [6],

resulting in complex AF control systems, especially if tracking

several spectral components is required.

In this paper, we propose and examine a selective hybrid filter

system with a voltage-controlled inverter suitable for the har-

monic isolation of high-power 12-pulse rectifier loads at the

critical frequencies. The AF is connected in parallel with the

load through a simple tuned passive filter created by a power-

factor-correction capacitor and the AF matching transformer

leakage inductance. The selective AF control system is based onsource current detection and second-order infinite-impulse re-

sponse (IIR) digital notch and resonant filters. These filters can

be considered as a simple digital algorithm for double cascaded

SRF notch filters or PI controllers [7], [12] and they are suitable

for tracking multiple harmonics [8]. Estimation of the funda-

mental and targeted harmonic frequencies is based on an adap-

tive notch filter so that an additional phase-locked loop (PLL)

is not required. Experimental results and stability analysis are

presented and the importance of delay-time compensation is dis-

cussed.

0278–0046/01$10.00 © 2001 IEEE

7/27/2019 00969390

http://slidepdf.com/reader/full/00969390 2/10

BASIC et al.: HARMONIC FILTERING OF HIGH-POWER 12-PULSE RECTIFIER LOADS 1119

Fig. 1. (a) System configuration. (b) Equivalent block diagram of the system.

II. SYSTEM CONFIGURATION AND PRINCIPLE OF OPERATION

A simplified drawing of the system configuration is shown in

Fig. 1(a). A series combination of a single tuned filter and AF

filter is connected in parallel with a 12-pulse rectifier load. If the

nonlinear load can be considered as a harmonic current source

generator, thesystem of Fig. 1(a) can be represented at harmonic

frequencies by the block diagram shown in Fig. 1(b), where

is voltage background distortion, and is AF voltage injec-

tion.

The load and passive filter can be isolated from the source at

targeted harmonic frequencies by using a closed-loop system

which will shape the AF voltage harmonic injection to pro-

duce AF current exactly equal to the targeted load cur-

rent harmonics . The source current will be zero even in

presence of disturbance . Thus the load current harmonics

should be detected ( ) and used as the current reference .

This reference is compared with detected AF current ( )

and the error is corrected by the controller ( ) and AF in-

verter ( ) as shown in Fig. 2(a). This scheme can be usedwith a simple hysteresis controller resulting in a current con-

trolled voltage inverter or with P controller and voltage-con-

trolled inverter [9]. Instead of separate detection of the load and

AF currents, the error signal can be directly retrieved from the

sourcecurrentdistortion ( ) asshownin Fig.2(b).The block 

diagram of Fig. 2(b) can be rearranged as shown in Fig. 2(c)

so that the contributions to the source current distortion of the

load current and source voltage are clearly visible. It is obvious

than the source current harmonics with the AF will be lower

by the factor than with the passive filter only,

where is the transfer function of the AF control system

[see Fig. 2(c)].

Fig. 2. Block diagrams showing closed-loop control of the AF current. (a)With separate detection of the load and AF currents.(b) With direct error signal(source current) detection. (c) Modified block diagram of Fig. 2(b).

As mentioned earlier, we have selected a system based on

the source current detection and voltage-controlled AF inverter

[Fig. 2(c)]. This method requires fewer current sensors, and theselective AF harmonic voltage injection targets several critical

source current harmonics only, while at all other frequencies the

passive filter function is mostly preserved.

III. AF CONTROL SYSTEM

Two banks of harmonic controllers in Fig. 3 track corre-

sponding harmonics in the source current and adjust AF voltage

until their full cancellation is achieved. Only two controller

banks are necessary in a three-wire three-phase power system

and they can be designed in the phase domain or, as in ourcase, in the domain (block ). Load voltage signals

and an adaptive notch filter are used to track the fundamental

frequency. The retrieved voltage fundamental components are

used to estimate the passive filter fundamental current neces-

sary for the AF inverter dc voltage control by balancing the

total active power flowing into the AF dc-link capacitor. From

the estimated fundamental frequency the frequencies of 

the targeted harmonics are calculated and transferred to the

harmonic current controllers. Load voltage harmonics can be

retrieved also and used as a feedforward compensation of the

disturbance signals in Fig. 2(c). Delay-time compensation is

introduced at the harmonic controller outputs to stabilize the

7/27/2019 00969390

http://slidepdf.com/reader/full/00969390 3/10

1120 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 48, NO. 6, DECEMBER 2001

Fig. 3. Selective active filter control system with tracking and controlling four critical harmonics.

AF control system. All blocks in Fig. 3 will be described in

more detail.

 A. Fundamental Frequency Notch Filter 

In nonselective AF control systems based on the detection of 

total current distortion [1]–[3], the removal of the fundamental

componentis practically theonly and most importantsignal pro-

cessing task. However, a selective control system may not be

sensitive to the fundamental component and the fundamental

component notching can be omitted., but, to provide the pos-

sibility of using a proportional gain ( ), useful if a broader

range of harmonics should be attenuated, a fundamental com-

ponent notch filter has been implemented. A notch filter can be

constructed by using SRF notch filters [5] or by using the

theory [1]–[3]. However, neither of these methods can provide

full notching of the fundamental component in an unbalanced

situation. SRF-based notch filters will completely pass the neg-

ative-sequence components and -based notch filters will in-

troduce on top of that an additional harmonic distortion (mainly,3rd harmonic). To solve these problems, two cascaded SRFs can

be used for separate notching of the positive- and negative-se-

quence components (Fig. 4). The transfer function of double

SRF notch filters given in (1) can be transformed by using the

bilinear transformation into a discrete form that will result in an

IIR second-order digital notch filter [7], [12] (2)

(1)

(2)

7/27/2019 00969390

http://slidepdf.com/reader/full/00969390 4/10

BASIC et al.: HARMONIC FILTERING OF HIGH-POWER 12-PULSE RECTIFIER LOADS 1121

Fig. 4. Double-cascaded-SRF-based notch filters.

where ( , and are sampling time and

frequency), and

(3)

The notch width is dependent on the cutoff frequency of the

SRF-based low-pass filter. If the location of the poles in (2) isvery close to unity , a very narrow notch filter can be

created. The center frequency of the notch is determined by the

center frequency parameter .

 B. Harmonic Controllers

A PI controller (4) is commonly used for the tracking of 

dc signals because it produces zero steady-state error in a

closed-loop system

(4)

Three-phase current components at arbitrary frequency can

be converted into dc signals by filtering the currents transformed

into SRF coordinates ( , block in Figs. 4 and 5). Thus,

SRF-based PI controllers can be used for tracking sinusoidal

currents [Fig. 5(a)] and this technique is in common use in ac

motor drives. Our simulation results showed that, because of 

an additional phase shift introduced by the low-pass filters in

Fig. 5(a), small PI gains are required for a stable closed-loop

system, causing a poor transient response. However, these fil-

ters can be omitted because of the low-pass nature of a PI con-

troller (in this case, the fundamental frequency notch filter is re-

quired). A single SRF PI controller of Fig. 5(a) can be used for

controlling either a positive- or a negative-sequence componentat synchronous frequency. In unbalanced three-phase three-wire

systems, both sequences are present and a PI controller based on

a single SRF cannot provide full compensation of critical har-

monics, which can be a problem because even a very small un-

compensated harmonic current can be amplified or can create

a high-voltage distortion in a resonant power system. To over-

come this problem, we can use double cascaded SRF controllers

[Fig. 5(b)].

In this case, the resultant transfer function is

(5)

Fig. 5. (a) Single and (b) double cascaded SRF PI controllers.

This transfer function can be transformed into a digital form

by using the bilinear transformation [7], [12], giving a simple

second-order IIR digital resonant filter (6)

(6)

The filter parameters are related to PI controller parameters as

follows:

(7)

The controller bandwidth depends on the P and I gains ( and

) of the analog prototype. If the location of the zeros in (6) is

very close to unity , a narrow bandwidth controller can

be created. At the center frequency, this resonant filter produces

infinite gain and no phase shift.

C. Frequency Tracking

An adaptive IIR notch tracking filter is used for fundamental

frequency component estimation. By this technique, an addi-

tional PLL is not necessary. All other harmonically related com-

ponents can be retrieved if the fundamental frequency is known.

The direct notch filter form (2) can be transformed into latticeform as a numerically reliable alternative [10]

(8)

with the complementary (band-pass) transfer function

(9)

7/27/2019 00969390

http://slidepdf.com/reader/full/00969390 5/10

1122 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 48, NO. 6, DECEMBER 2001

Fig. 6. Adaptive lattice second-order IIR notch filter for fundamentalfrequency tracking.

The center frequency is defined by the parameter

(10)

The parameter defines the filter bandwidth. It is related to

as follows:

(11)

The filter algorithmis shown in Fig. 6 in lattice form where

is the filter input, and are the filter state variables,

and and the notch filter output and the complemen-

tary band-pass filter output, respectively. This notch filter can

be used to track the fundamental frequency. The simplest way

is to use an adaptive algorithm which seeks a minimum point

of the cost function (expected value of the squared filter outputs

) by changing the parameter in the negative gradient di-

rection (gradient-descent algorithm). To minimize the noise-in-duced term of the cost function, a narrow bandwidth is required.

In this situation, the gradient-descent adaptive algorithm can ex-

hibit very slow convergence and a simplified adaptation algo-

rithm for the lattice form can improve the convergence speed

[10]. This version of the lattice algorithm is

(12)

where instead of using and calculating the gradient ,

the filter internal state is used. The minimum is normally

achieved when the fundamental component is notched

and, thus,

(13)

Center frequency parameters for all harmonics can be directly

calculated from (14)

(14)

To reduce the computation burden, we adopted a recursive

technique. This technique is based on the fact that a dis-

crete value of a cosine function at sampling instants

can be found recursively

from two last samples and and the angular

increment

(15)

Combining(14) and (15), a recursive equation can be derived for

the center frequency parameters of all harmonics

(16)

 D. Delay-Time Compensation

Because of synchronous sampling of the inputs and updating

of the outputs, a pure delay of a sample is introduced. Be-

cause of zeroth-order sample–hold ( delay) and AF in-

verter deadtime ( ), an additional delay of approximately

is introduced so the total time delay in-

troduced in the closed loop of the system in Fig. 5 is nearly .Phase shift caused by the system delay can be considerable at

the harmonic frequencies and, as will be shown in Section V, it

can lead to instability if the compensation of higher order har-

monics is required. For example, for a sampling frequency of 

kHz, the phase lag at the 11th and 13th harmonics will

be approximately 45 and 117 .

In a selective AF control system, it is possible to predict an

individual cosinesignal one samplein advance by using (15), as-

suming that the amplitude of the signal will stay approximately

the same

(17)

From (17), the prediction two samples in advance will be

(18)

Thus, with the correction given in (19) applied at outputs of 

all harmonic controllers (delay time compensation blocks), it

is possible to compensate for the phase lag at the targeted fre-

quencies introduced in the closed loop due to system delay time

(19)

 E. AF Inverter DC Voltage Control

AF dc capacitor voltage is maintained at the reference

value by a separate PI controller which corrects the AF ref-

erence so that AF filter introduces an additional voltage at fun-

damental frequency in phase with the passive filter fundamental

current

(20)

7/27/2019 00969390

http://slidepdf.com/reader/full/00969390 6/10

BASIC et al.: HARMONIC FILTERING OF HIGH-POWER 12-PULSE RECTIFIER LOADS 1123

Fig. 7. Impedance seen from source side (Z  ), load side ( Z  ), and currentmagnification factor ( K  ) for system of Fig. 1(a) with the passive filter only.

The total active power flow into the AF is controlled in both di-

rections at the fundamental frequency with the regulator output

so that the dc capacitor voltage remains constant without anadditional active source at the dc side. The filter fundamental

current is estimated from the retrieved voltage signals and the

passive filter impedance [(20)].

IV. PERFORMANCE ANALYSIS OF AN EXAMPLE SYSTEM

Firstly, we will analyze the performance of a laboratory

model of the system of Fig. 1 without the AF and with a single

tuned passive filter. In our experimental setup, the system

impedance was 4.5% ( mH, V, A).

The passive filter tuned frequency was adjusted approximately

to the 11th harmonic. The passive filter capacitance was 90

F and the tuning inductor was the AF matching transformer( ) leakage inductance with the AF side short circuited.

For these system parameters, the impedance seen from the load

side ( ) and source current magnification ( )

(21)

are shown in Fig. 7. They have a parallel resonance peak be-

tween 5th–7th harmonics that can cause an excessive voltagedistortion due to load current harmonics. The impedance seen

from the load side ( , Fig. 7) has a series res-

onance minimum between 5th–7th harmonics that can cause an

excessive source current flowing into the passive filter due tobackground source voltage distortion.

Several experiments were carried out to illustrate problems

related to the applications of passive compensation of 12-pulse

loads. The voltage and current waveforms and source current

spectrum with the load only are shown in Fig. 8(a). The most

emphasized harmonics of the load current are characteristic 11th

and 13th harmonics, but small residual 5th and 7th harmonics

are present [Fig. 8(a)]. The load voltage waveform is highly

distorted with typical notches caused by the thyristor commu-

tation. In the following test, only the passive filter was con-

nected without the load. The passive filter current [Fig. 8(b)]

is highly distorted because of the series resonance and back-

ground voltage distortion. In our laboratory the dominant back-

ground distortion (Table I) was at 3rd and 7th harmonics. Un-

expectedly, the 5th harmonic was relatively small and, thus, the

harmonic spectrum of the passive filter current in Fig. 8(b) in-

dicates strong 7th harmonic component only. The source cur-

rent and voltage waveforms and source current spectrum with

the load and passive filter connected are shown in Fig. 8(c). In

comparison to the previous results, the source current is nowmore distorted because of the additional distortion created by

the magnification of the residual 5th and 7th load current har-

monics. All higher frequency components, including the domi-

nant characteristic 11th and 13th harmonics of the load current,

are attenuated well and, consequently, the commutation notches

on the voltage waveform have disappeared.

From these results, we can conclude that although the most

emphasized characteristic harmonics are attenuated well, we

cannot apply passive filters for characteristic harmonics only

without additional 5th and 7th harmonic filters in order to pre-

vent parallel resonance. In this case, a selective AF filter can be

used very effectively to prevent the problems at 5th and 7th har-

monics and to improve the passive filter performance at the 11thand 13th harmonics.

Finally, we will show the results when the AF is applied. For

the AF control system a digital signal processor (DSP) board

with TMS320C32-60Mz processor was used. Two source

currents, two load, and dc capacitor voltages were detected by

on-board 16-bit A/D converters. The AF inverter was controlled

directly through on-board digital outputs because space-vector

pulsewidth modulation (PWM) was implemented as a part of 

the AF control program. The sampling frequency was set at

kHz and the AF control system was programmed to

target 5th, 7th, 11th, and 13th harmonics. All necessary signal

processing tasks were executed by second-order IIR digital

filter blocks (22)

(22)

The parameters of the fundamental frequency notch and

tracking filters and harmonic controllers are given in Table II.

The source current and voltage waveforms shown in Fig. 8(d)

are considerably improved because all targeted harmonics are

reduced to negligible levels and the resonance phenomena at 5th

and 7th harmonics are prevented. The AF current and voltage

waveforms are also shown in Fig. 8(d). It can be noticed that

the AF voltage is much lower than the load voltage. High-fre-quency harmonics are filtered out by the passive filter so that the

required AF frequency band is restricted to the 13th harmonic.

Thus, the AF voltage rating and switching frequency in this hy-

brid filter topology can be much lower than with a shunt AF of 

similar performance.

V. STABILITY ANALYSIS

For the stability analysis, the block diagram of Fig. 9 will be

used. The source current is sampled and processed by the AF

digital controller [transfer function ]. Because of syn-

chronous sampling of the inputs and outputs the control signal

7/27/2019 00969390

http://slidepdf.com/reader/full/00969390 7/10

1124 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 48, NO. 6, DECEMBER 2001

(a)

(b)

(c)

(d)

Fig. 8. Current and voltage waveforms and harmonic spectra. (a) With load only. (b) With the passive filter only. (c) With load and passive filter. (d) With load,passive, and active filters.

7/27/2019 00969390

http://slidepdf.com/reader/full/00969390 8/10

7/27/2019 00969390

http://slidepdf.com/reader/full/00969390 9/10

1126 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 48, NO. 6, DECEMBER 2001

Fig. 9. Block diagram of the AF closed-oop system with digital controller used in the stability analysis.

Fig. 10. (a) Modulus and argument and (b) polar plot of the AF controllertransfer function G  ( j !  ) .

Without the compensation, the system is not stable at higher

frequencies for inductive , while in the region where is

capacitive, the compensation may not be required. An expanded

view of the rectangle in Fig. 12(a) is shown in Fig. 12(b), con-

firming that the closed-loop system with the delay-time com-

pensation is stable as the Nyquist diagram does not encircle

the point ( 1, ). However, the stability margin may be low

if low sampling frequency is used and can be considerably

improved if higher sampling frequency is used.

Fig. 11. (a) Modulus and argument of the AF control loop pulse transferfunction G  ( j !  ) without the delay time compensation and (b) Nyquist plotin this case.

VI. CONCLUSION

A hybridfilter systemwith selective AF control allows theuse

of a low-rating AF with reduced switching frequency that is par-

ticularly advantageous in high-power applications. The effec-

tiveness of a small-rating AF connected in series with a low-cost

7/27/2019 00969390

http://slidepdf.com/reader/full/00969390 10/10

BASIC et al.: HARMONIC FILTERING OF HIGH-POWER 12-PULSE RECTIFIER LOADS 1127

Fig. 12. (a) Nyquist plot for the AF system with the delay time compensationand (b) an expanded view around the critical point ( 0  1, j 0  ).

power-factor-correction capacitor is experimentally verified in

the filtering of the dominant harmonics of a 12–pulse rectifier

load and preventing series and parallel resonance conditions

by targeting several critical harmonics. Harmonic current con-

trollers based on IIR second-order digital resonant filters are

centered to the targeted harmonic frequencies by using an adap-

tive fundamental frequency tracking filter. This approach gives

good synchronization, even if the reference waveform (in our

case, a load voltage) is highly distorted and no separate PLL is

required. Stability analysis was carried out and it showed the

importance of the system delay-time compensation.

REFERENCES

[1] F. Z. Peng, H. Akagi, and A. Nabae, “A new approach to harmonic com-pensation in power systems—A combined system of shunt passive andseries active filter,” IEEE Trans. Ind. Applicat., vol. 26, pp. 983–990,Nov./Dec. 1990.

[2] H.Fujita andH. Akagi, “Apracticalapproachto harmonic compensationin power systems—Series connectionof passive and active filters,” IEEE Trans. Ind. Applicat., vol. 27, pp. 1020–1025, Nov./Dec. 1991.

[3] , “Design strategy for the combined system of shunt passive andseries active filters,” in Conf. Rec. IEEE-IAS Annu. Meeting, vol. 1,Sept./Oct. 1991, pp. 898–903.

[4] P. T. Cheng, S. Bhattachaarya, and D. M. Divan, “Application of domi-nant harmonic active filter system with 12 pulse nonlinear loads,” IEEE Trans. Power Delivery, vol. 14, pp. 642–647, Apr. 1999.

[5] S. Bhattacharya and D. Divan, “Synchronous frame based controllerimplementation for a hybrid series active filter system,” in Conf. Rec.

 IEEE-IAS Annu. Meeting, vol. 3, Oct. 1995, pp. 2531–2540.[6] J. Hafner, M. Aredes, and K. Neumann, “A shunt active power filter

applied to high voltage distribution lines,” IEEE Trans. Power Delivery,vol. 12, pp. 266–272, Jan. 1997.

[7] D. Basic, V. S. Ramsden, and P. Muttik, “Digital implementation of thesynchronous reference frame controller for a selective hybrid filter con-trol system,” in Proc. AUPEC/EECON’99, Sept. 1999, pp. 473–478.

[8] W. Zhang, A. J. Isaksson, and A. Ekstorm, “Analysis on the controlprinciple of the active DC filter in the Lindome converter station of the Konti–Skan HVDC link,” IEEE Trans. Power Syst., vol. 13, pp.374–381, May 1998.

[9] F. Z. Peng, H. Akagi, and A. Nabae, “Compensation characteristics of the combined system of shunt passive and series active filter,” IEEE Trans. Ind. Applicat., vol. 29, pp. 144–152, Jan./Feb. 1993.

[10] A. Regalia, Adaptive IIR Filtering in Signal Processing and Con-trol. New York: Marcel Dekker, 1995.

[11] C. L. Phillips and H. T. Nagle, Digital Control System—Analysis and  Design. Englewood Cliffs, NJ: Prentice-Hall, 1990.

[12] D. Basic, V. S. Ramsden, and P. K. Muttik, “Hybrid filter controlsystem with adaptive filters for selective elimination of harmonics andinterharmonics,” Proc. IEE—Elect. Power Applicat., vol. 147, no. 4,pp. 295–303, July 2000.

Duro Basic received the Dipl.Eng. degree fromthe University of Novi Sad, Novi Sad, Yugoslavia,the M.E. degree from the University of Belgrade,Belgrade, Yugoslavia, and the Ph.D. degree fromthe University of Technology, Sydney, Australia, in1981, 1993, and 2001, respectively, all in electricalengineering.

He is currentlya Research Officerworkingon con-trol of electrical drives at the Centre for ElectricalMachines and Power Electronics (CEMPE), Univer-sity of Technology. His research interests are power

electronics, active filters, power quality, and control of electrical drives.

Victor S. Ramsden graduated in electrical engi-neering in 1964 and received the Master’s degreein 1965 from Melbourne University, Melbourne,Australia, and received the Ph.D. degree from theUniversity of Aston, Birmingham, U.K.

He spent one year with ASEA in Sweden and oneyear with GEC Stafford. In 1972, he joined the Uni-versity of Technology, Sydney, Australia (UTS), ob-taining a Professorship in Electrical Engineering in1993. Beginning in 1988, he lead a collaboration onpermanent-magnet machine design between UTS and

CSIRO Telecommunicationsand Industrial Physics, where he worked part time.His research interests include ac motor control, electrical machine design, ironlosses, renewable energy, and medical applications. He retired in 2000 and re-mains an Emeritus Professor with UTS.

Peeter K. Muttik (S’78–M’79) received the B.Sc.,B.E. (Hons.), and Ph.D. degrees from the Universityof Adelaide, Adelaide, Australia, in 1973, 1974, and1980, respectively.

He currently holds the position of Chief Engineer,Transmission and Distribution Systems, in theproject sector of ALSTOM Australia Ltd., Milperra,Australia, which he joined in 1980. He has wideexperience in power system analysis, static varcompensators and other high-power electronicsturnkey projects, and in harmonic filter design,

commissioning, and testing.Dr. Muttik is a member of the Institution of Engineers, Australia.