Active Power Filters with Pulse Modulationto Switching …eprints.qut.edu.au/8925/1/8925.pdf · Keywords Active power filter, ... Shunt filters are connected in parallel with ...

2006 International Conference on Power System Technology

Active Power Filters with Unipolar Pulse Width

Modulation to Reduce Switching Losses

Sasan Zabihi and Firuz Zare, Member, IEEE

Abstract Due to sensitivity of consumers on powerquality and advancement in power electronics, Active PowerFilters (APF) continue to attract considerable attention. APFtechnology is the most efficient way to compensate reactivepower and cancel lower order harmonics generated bynonlinear loads. This paper presents a hysteresis currentcontrol technique based on unipolar Pulse Width Modulation(PWM) with time and magnitude errors control to reduceswitching losses and to improve the quality of output current.The validity of proposed method and achievement of desiredcompensation are shown by simulation results.

Keywords Active power filter, hysteresis current control,PWM, voltage source converter

I. INTRODUCTION

Increasing demand of electric and electronic instrumentsincluding non-linear loads (e.g. computers, powerelectronic inverters and induction furnace) necessitatesinvestigations about stability maintenance, continuousenergy serving and power quality considering economicconcerns. Non-linear loads result in voltage and currentharmonics in distribution network.

Harmonics provide main problems in network likepower losses and excess heat. Thus, harmonic eliminationseems to be vital. Nowadays, active power filters (APF)play effective role in distortion recognition andelimination. These filters are classified with respect todistortion determination strategy, inverter controltechniques, inverter topologies and their connection typesto the grid. Shunt filters are connected in parallel withdistribution networks. They recognize current distortionsby sampling the line current and compensate distortedcurrent components to maintain sinusoidal source current.

In most recommended grids, the reference signal can beextracted by sampling and analyzing the input voltages andload currents for harmonic and reactive powercalculations. These control methods require rapid and real-time calculating blocks like high-speed digitalmicroprocessors and capable A/Ds, so cost increment,circuit complexity and system stability reduction would beinevitable.

Current control technique based on unipolar PWMprovides better stability and compensation with lossreduction; efficiency increment and dc offset eliminationcompare to bipolar PWM technique. This paper presents ahysteresis current control technique based on unipolarPulse Width Modulation (PWM) with time and magnitudeerrors control to reduce switching losses and to improvethe quality of output current.

Dr. Firuz Zare is a lecturer at the School of Engineering Systems,Queensland University of Technology, Gardens Point Campus, 2 GeorgeSt, GPO Box 2434 Brisbane, QLD 4001, Australia. (fzuteduau)

Sasan Zabihi is a postgraduate student at Mazandaran University inIran. (s bihi ).

II. DISTORTION DETECTION BY REFERENCE FRAME D-Q-O

Several methods are used to recognize and extractharmonic distortions which are classified as frequency,time and time-frequency approaches. Fast FourierTransformer (FFT)[2] and adaptive neural network [1] infrequency domain, synchronous reference frame theory d-q-o (SRF) [3] and instantaneous active and reactive powertheory (pqr)[4] in time domain and the other methods suchas small wave technique and one-cycle control orseparation with suitable digital or analogue filters[5] havewide applications.

In this approach, reference frame algorithm is used dueto simplicity in calculation and implementation.

Having measured three-phase currents in a-b-corientation, transformed to d-q-o by park equation:

Cos| cos(9 )IT cos(9 )4;T3 3 F,id 2T 4;T la~ 1

Liq sin sin(9 ) sin(9 ) ilb~1 1L'T

L

vCiO=00+Jo tdt (2)

Reference frame rotates synchronous with fundamentalcurrents. Therefore, time variant currents with fundamentalfrequencies would be constant after transformation.However, harmonics with different speeds remain timevariant in this frame. Thus, currents would be separatesimultaneously to DC and AC parts.

ld-ld+ld (3)

iq = Iq+ (4)

AC part of d axis and whole current in q axis are usedfor harmonics elimination and VAR compensation. Zerocurrent is produced due to a three-phase voltage imbalanceor waveform distortions which have not been considered inthis paper. Finally, compensated currents are determinedby adverse park application on d and q axis to be injectedto the network after tracing and reconstruction as shown inFig. 1.

lCa~

ICb

L'c_

CosOcos(O - )33

Cos(O 4)r3

sin 0

sin(0 -- L iqd + Iq(5)

IL = Ig + iapf (6)Conduction and switching losses of diodes and IGBTs

in inverters increase voltage ripple in DC-link which

1-4244-0111-9/06/$20.00 c2006 IEEE.

affects the performance of the filter. These effects are

controlled by a feedback loop where PI regulator compares

the DC-link voltage with a reference voltage to extract dcomponent of current.

1Cabc

Fig. 1: Synchronous d-q-o reference frame based compensation algorithm

III. HYSTERESIS BAND CURRENT CONTROL APPROACH

Reference currents are generated by DC to ACconverters using a current control technique such as a

hysteresis control. The hysteresis band is used to controlload currents and determine switching signals for invertersgates. Suitable stability, fast response, high accuracy,

simple operation, inherent current peak limitation and loadparameters variation independency make the hysteresiscurrent control as one of the best current control methodsof voltage source inverters. In this approach the currenterror, (difference between the reference and invertercurrents) is controlled in hypothetical control bandsurrounding reference current as shown in Fig.2.

e=ic Iapf (actual) (7)

Switching pulses

1< t~~1ysteresisFig.2: Conventional hysteresis band current controller

When the load current exceeds the upper band, thecomparator output activated so the output voltage ischanged in such a way to decrease the load current andkeep it between the bands and deactivated at lower limit.Switching frequency varies with respect to distancebetween upper and lower band. The other parameters likeinverter-network inductance and DC link voltage affectsignificantly on the switching frequency.

Adaptive band definition considered as proper solutionin this problem to fix the switching frequency [6]. Thehighest switching frequency is as follows [7]:

f_Vd= (8)

w(max) 9 HB.L

Where, HB and L are hysteresis band and loadinductance, respectively.

IV. COMPARISON OF BIPOLAR AND UNIPOLAR CONTROL

Fig.3 shows a full bridge single-phase inverterschematic that is connected in parallel to a non-linear load.The inverter can be controlled in unipolar or bipolar PWMmethod.

arml arm)

Fig.3: Typical shunt APF connected in parallel with load

A. Bipolar controlIn bipolar control, switches turn on and off

simultaneously and diagonally (i.e.S1, S3 or S2, S4) andeach leg activates supplementary (i.e. Si, S2 and S3, S4).

In fact there are two switching states for the inverter, infirst half cycle (positive voltage supply), error signalmoves in descending slope, having contact with thehysteresis lower band, SI and S3 connect simultaneously,VPN reach to +Vo and result in ascending error slope.dit

I

( Vg +Vo) (9)

Having current contact the upper hysteresis band, S2 andS4 connected and produced -Vo between P and N. Voabsolute value is bigger than Vg, inductance resultantvoltage would be negative and produce descendentvariations slope by error redirection.

di I(/Vg/==V0) (1 0)dt L /

It is obvious that applied voltage difference to inductoris 2Vo in two half switching cycles.

"Vbipolar =/Vg /+Vo -(+/Vg /-Vo)= 2Vo (11)

Error slope variation is shown in Fig.4.

0

-5

-lo

+20

0

-200

002 0025 0 03 0035 0&04

Fig.4: Current and voltage waves with hysteresis band current control(for APF under bipolar control mode)

B. Unipolar control

There are four switching states in this approach. Inunipolar control, when Vg>0 (reference current inascendant slope dicJdt>0), S3 is on in the reference currenthalf cycle, SI and S2 turn on and off periodically inswitching cycles. Thus Vo and zero voltages producebetween P and N so error signal slope is negative inhysteresis band and periodically decreases and increase.Originated current variations in reference current halfcycle is as follows:

/. if:.. z. X~~~~~~/

.W.,< 'V

Vdo... z\.

-di 1d' L (/Vg +VI) First half cycle of switching (12)

dic = (V ) Second half cycle of switching (13)dt LgAs the output voltages have three levels, +Vo, -Vo and

zero, the two switching states (similar to bipolar) can notcontrol the load current sufficiently. In this case morebands are required to achieve different switching statescorresponding to different output voltages. As shown inFig.5, the reference current has a positive diJ/dtfrom 0- 90° and the load current can follow the referencecurrent based on two voltage levels, +Vo and zero volts.But when the reference current has a negative diJ/dt,(90- 270°), the output voltage of inverter has to bechanged in such a case to generate negative diJ/dt for theload current, thus more band are required to change thevoltage level from +Vo&O to -Vo&O. Current variation inthe second half cycle of the reference current is:

dic IVg

1-VO) First half cycle of switching (14)

dt L

di =I(- V |) Second half cycle of switching (15)dt LVPN has unit polarity in each half cycle of the reference

current, so this approach called unipolar control.

A\Vjnjpoiar (I Vg +Vo)- Vg

--Vg ( Vg I-VO)=Vo (16)

It is shown that the current control based on unipolarPWM has a low switching losses or better performancecompare to the bipolar PWM due to less number ofswitching transient per switching cycle.

02nd upper band

/1 st upper band

b . Load cuentloWer bandlblower band

Lio ~ ~ ~ v+Vdc ~ ~ Vd

6.62 0.025 0.0 35 6.064Fig.5: Current and voltage waves with hysteresis band current control (for

APF under unipolar control mode)

V. UNIPOLAR BASED ON TIME AND MAGNITUDE ERRORSCONTROL

In hysteresis current control based on unipolar PWM, thereare two upper bands and lower bands in order to changethe slop of inverter output current based on there levelvoltages, +Vo, 0 and -Vo. The idea is to keep the currentwithin the main area but the second upper and lower bandsare to change the voltage level in order to increase ordecrease the di,/dt of inverter output current. As shown inFig.6.(a) Al cannot be very small as the noisy signalchanges the switching time due to instantaneouscomparison between the load and the reference currentsand increases the switching losses; and it cannot be big asthe total harmonic distortion may be increased.

In APF, load current has several different slops within onecycle and to have a fast current tracking, the controlalgorithm in unipolar current control has been definedbased on magnitude and time errors control as shown inFig.6.(b). In this case, the second upper or lower bandvalues can be big enough in order to remove the noiseissue of the inverter output current but the second decisionto change the level is based on time error. For example,when the load current exceeds the first upper band at t4, theoutput voltage of inverter is change from +Vo to 0. Thecontroller waits for At, if the inverter output current doesnot cross the second upper band within this period, thenthe controller changes the output voltage from zero to -Voat t5. In this case, when the slop of reference current isclose to the slop of inverter output current, then the timeerror control improves the quality of the APF and pushesthe inverter current into the main area.

upper bands

Load current with noisee - -

Main Area

(a)

Load Current

.second upper bandfirst upper bandfirst lower bandsecond lower band

Am

ti A t

(b)Fig.6: (a) noisy load current with upper and lower bands (b) unipolar

current control based on time and magnitude error control

For magnitude error control, two switches in inverterlegs are controlled with a main band and two other ones bythe second band in a unipolar PWM. Control signalsoriginated by the unipolar command circuit based on thehysteresis current control are shown in Fig.7.

TransportDelay

Fig.7: Proposed unipolar control command circuit

In the first positive half cycle (Vg>0), S3 is permanentlyon. Inductance voltage calculated as follows:VL(ON) =Vg 0<t<dTs (17)S1: OFF, S2: ON

VL (OFF) = Vg - Vo dTs < t < Ts(

SI :ON,S2 :OFFFor constant-frequency operation and quasi-steady-state

analysis, the average inductor voltage is approximately

t I 0

balanced during each switching cycle, that is:

VL (ON)AdTs+ VL (OFF).(1-d)Ts = 0Substituting (17) and (18) in (19):

Vg.d + (Vg-Vo)(l-d) =O When Vg > 0

(19)

(20)

Vg.d+(Vg+V0)(l-d)=0 When Vg <0 (21)Combining above equations provide relationship

between duty ratios of switches and input ac voltage aswell as dc bus voltage ofAPF inverter, that isVge =VO (]-d) (22)

Vge = V Vg > (23)ge-V Vg < 0

VI. SIMULATION RESULTS

Simulations are performed using Matlab and Simulinkplatform for a non-linear load (a full bridge diode rectifierwith resistive load) and a single-phase voltage sourceinverter produce and cancel the distorted currents as anAPF with the specifications shown in table 1.

TABLE IDESIGN SPECIFICATIONS AND CIRCUIT PARAMETERSParameters QuantitySwitching frequency of inverter under variablebipolar or unipolar mode controlFundamental frequency 50 HzSupply voltage AC 150 VInverter DC bus voltage 200 VRectifier load resistance 5 Qtransmission line inductance 1 mHInverter inductance 2 mHCdc capacitor 1000 RF

According to simulation results (Fig.8, Fig.9 andFig. 10), the main part of load required VAR is supplied byfilter and only ten percent is extracted from supplyconfirming a suitable power factor correction. Smallamount of active power is consumed as switching losses infilter. The required load active power is provided by thesupply.Load current, inverter injected current and compensated

supply current are shown in Fig. 1 .(a, b, c) respectivelyand the supply current is relatively harmonic free.

1 2OO0 1]~ ~ ~ ~ ~ ~

c 1 0000DX

< 5000CX5

b)[ 2 1

X 104>3$n

LO

__j

15000

10000

0 -

58000O O.2 0.A &06 08

10000

5000

0 02 0A4 6 01

Fig.9: Inverter active and reactive power

15000

.-...A . IW.

c 10000L

5n 000LII0o o2 04 .0.6 08 1

_0 9ii1000

2000

0' 0'2 04 05. 05

Fig. 10: Source active and reactive power

-50

054 055 066 &7 0.58 05940

6406C1.4 H2 5 &256 I]5;7 01.8 0 9

6.54 0.55 H66 0576V.58 0.59

Fig. I 1: (a) Load current; (b) Compensating current; (c) Supply currentwaveforms after harmonic compensation

Comparing the load and compensated currents with twomethods, THD reduced from 22.76% in the load current to0.36% and 0.39% in supply current for bipolar andunipolar respectively. As shown in Fig.12, Fig.13, andFig.14 the main point is the THD of compensated supplycurrent for both methods are almost same (0.36% and0.39%) while the switching losses of unipolar modulationis 50% less than bipolar modulation.

Simulation results show suitable filter capability fordesign objectives and acceptable efficiency because of lossreduction.

Fig.8: Load active and reactive power

i I0 0~2 0~4 05s 08 1

.........

sm-.A

A

3E:

bU

rn

40

30

20

10

n

Fundamental (5OHz) = 51.22, THD= 22.75%

.-6

O 5 10 15Harmonic order

Fig. 12: Harmonic spectrum of the nonlinear load current

Fundamental (5OHz) = 48 85, THD= 0.36%

56

40

30

20

10

6 5 10 15 20Harmonic order

Fig. 13: Harmonic spectrum of the source current after compensation withthe bipolar current controller

60

s0

40

30

20

10

n

Fundamental (5OHz) 48. , THD= 0.39%

I_

[3] M. C. Benhabib and S. Saadate, "New Control approach for four-wire active power filter based on the use of synchronous referenceframe," Elsevier Electric power systems Research 73, 2005, 353-362.

[4] H. Akagi, Y. Kanazawa, and A. Nahae, "Instantaneous reactivepower compensators comprising switching devices without energystorage components," IEEE Trans. Ind. Appl. 1984, 20, pp. 625-630.

[5] T. Kwan, and K. Martin, "Adaptive detection and enhancement ofmultiple sinusoids using a cascade of IIR filters," IEEE Trans.Circuits Syst. 1989, 36, pp. 936-947.

[6] K. Murat, 0. Engin, "An adaptive hysteresis band current Controllerfor shunt active power filter," Elsevier Electric Power SystemsResearch 73(2005) pp.1 13-119.

[7] M. Rukonuzzaman, and M. Nakaoka, "Single-phase shunt activepower filter with harmonic detection," IEE proc. Electr. PowerAppl. Vol. 149, No.5, September 2002.

IX. BIOGRAPHIES

Dr Firuz Zare (M' 1997) was born inIran in 1967. He holds a PhD degree inElectrical Engineering with distinction inPower Electronics from QueenslandUniversity of Technology in Australia. Hehas recently joined as a Lecturer at theschool of engineering systems of QUT.His research interests are power electronicapplications, pulse-width modulation

techniques, renewable energy systems and electromagneticInterferences. He has worked as a development engineer and aconsultant within industrial and power electronic areas for severalyears.

u0 5 10 15 20

Harmonic orderFig. 14: Harmonic spectrum of the source current after compensation with

the unipolar current controller

VII. CONCLUSION

This paper proposed a unipolar PWM used in hysteresiscurrent control for active power filters based on time andmagnitude errors control in order to improve the quality ofoutput current and switching losses. The simulation resultsshow that for a same THD, unipolar modulation has lowerswitching losses. In bipolar modulation the voltage stressacross the load is 2Vo while in unipolar modulation is Voand the switching losses of bipolar method is twice ofunipolar modulation as two pair switches are switched onand off. Power quality improvement can be achieved in adistribution network using this control approach in activepower filters.

VIII. REFERENCES[1] A. Ametani, "Harmonic reduction in thyristor Converters by

harmonic current injection,".IEEE Trans8.Power, Appar.Syst. 1976,95, pp.441-449.

[2] M. Rukonuzzaman and M. Nakaoka, "An advanced active powerfilter with adaptive neural network based harmonic detectionscheme," IEEE Power Electronics Specialist cascade, PESC,Vancouver Canada, 2001, pp. 1602 - 1607.

F,I.

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