1 PG Student, Power Electronics and Drives Deivision, VIT University, Vellore, India, E-mail:anbu_vit2010@yahoo.com

2 Senior Assistant Professor, Control System Division, VIT University, Vellore, India, E-mail:rashmiranjandas@vit.ac.in

A Simplified Indirect Control Scheme for Shunt Active Power Filters

M. ANBARASAN1 AND RASHMIRANJANDAS2

Abstract: This paper presents a simplified indirect control scheme for shuntactive power filters for compensating harmonic currents and improving powerfactor. The control is based on the simple method for generation of the referencecompensating current. The reference compensating current is obtained bysubtracting load current from reference source current. PI controller is used toadjust the amplitude of the reference source current that regulates the DCcapacitor voltage. Hysteresis band current controller is used to control the shuntactive power filter output current with less switching ripples. The effectivenessof the proposed control scheme for the single phase and three phase shunt activepower filters are analyzed through simulations using Matlab/Simulink software.

Index Terms: Active power filter, harmonic currents, reference source current,hysteresis band current controller

I. INTRODUCTION

Rapid advancements in the power electronics technology have resulted in the usageof various power electronics equipments for both industrial and commercialapplications. This widespread use of power electronics equipments pollutes thepower system with harmonic currents due to its nonlinear nature. The harmoniccurrents causes many adverse effects such as low power factor, overheating ofpower system components,EMI problems[1,2] .Conventionally shunt passive LCfilters are used to compensate harmonic currents and improvement of power factor.However they offer many disadvantages like large size, possibility of resonanceand fixed compensation characteristics [3,4].

Shunt active power filters are considered as the effective tool for solvingproblems caused due to the harmonic currents. Shunt active power filters arebroadly classified into two types namely single phase and three phase shunt active

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56 M. Anbarasan and Rashmiranjandas

power filters. Single phase shunt active power filters are limited to low powerapplications and three phase shunt active power filters are widely used in highpower industrial applications[5]. The control of shunt active power filters arebasically divided into direct current control[6]­[8] and indirect currentcontrol[9]­[11].

For the three phase shunt active power filters “p­q theory” is widely used,but itfails to work when supply voltage is distorted or unbalanced[15]. In this paper asimple indirect control scheme is proposed, which can be applied for both singlephase and three phase shunt active power filters topology.The performance of theproposed control scheme is analyzed through simulations using Matlab/Simulinksoftware.

Fig. 1. shows the simple representation of shunt active power filter. Thecompensation is achieved by injecting the compensating current at the point ofcommon coupling (PCC). The compensating current is characterized by equalamplitude but opposite in direction to that of load current harmonics component.

Figure 1: Simple Configuration of Shunt Active Power Filter

A Simplified Indirect Control Scheme for Shunt Active Power Filters 57

Many research papers published for single phase shunt active powerfilters[12]­[14] are seems to be complicated.

II. GENERATION OF REFERENCE COMPENSATING CURRENT 1 1At instance consider single phase shunt active power filter for referencecompensating current generation. In this control, supply voltage is assumed to besinusoidal and it is given by

The nonlinear load current is given by

1

( ) sin ( )nn

iL t In n t∞

=

= ω + θ∑ (2)

Eqn(2) contains fundamental components and harmonics component.

Therefore, the instantaneous load power can be expressed as

PL (t) = vs(t) iL(t)= I1Vm sin2(ωt) cos θ1+

I1Vm sin(ωt) cos(ωt) sin θ1 +

2

sin( ) sin( )m n nn

V t I n t∞

=

ω ω + θ∑= pa(t) + pr(t) + ph(t) (3)

In eqn(3) pa(t) is the active power demanded by the load, pr(t) is the reactivepower demanded by the load and ph(t) is the harmonic power. For compensationShunt active power filter should provide only reactive power and suppress harmonicpower.

The active fundamental current component is given by

ia(t) = I1cos θ1sin(ωt) (4)

Source is expected to supply only active fundamental current component withcompensation.

Reference source current to be generated is given by

i*sm(t) = I*

smsin(ωt) (5)

In eqn(5) I*sm represents the peak value of the active fundamental current

component.

I*sm = I1cos θ1 (6)

For the generation of reference compensating current, initially reference sourcecurrent has to be generated. Generation of reference source current is achieved bymaintaining the DC capacitor voltage of the shunt active power filter as constant.

58 M. Anbarasan and Rashmiranjandas

Control of DC capacitor voltage is done by controlling the amplitude of the sourcecurrent,this is done by the following method as follows

The Source current with compensation can be given as

is(t) = I*smsin(ωt) (7)

The active power supplied from the source is given by

ps(t) = vs(t) is(t)

* *1 1cos(2 ) ( )

2 2sm smsm sm s sV I V I t P p t= − ω = + (8)

In eqn(8). Ps is the DC component, and ( )sp t is the AC component. Theinstantaneous power consumed by the load is given by

pL(t) = vs(t) iL(t) = PL + ( )Lp t (9)

Where PL is the DC component and is given by

1 1cos2

mL

V IP

θ= (10)

( )Lp t is the AC component and it is given by

11

2

( ) cos(2 )2

(cos(( 1) )2

cos(( 1) ))

mL

m nn

n

n

V Ip t t

V In t

n t

∞

=

−= ω + θ

+ − ω + θ

− + ω + θ

∑

(11)

The compensating power generated by the shunt active power filter is given by

pc(t) = ps(t) – pL(t)

= ( ) ( )s s L LP p t P p t+ − −

= ( )c cP p t+ (12)

In eqn(12). Pc represents the DC component of the compensating power, and( )cp t represents the AC component of the compensating power. If the power loss of

the shunt active power filter is assumed to be a constant and it is represented asPloss, the magnitude of Pc can be given by

*1 1

1( cos )

2smc m mP V I V I= − θ (13)

A Simplified Indirect Control Scheme for Shunt Active Power Filters 59

The energy balance in the DC capacitor is given by

21( )

2 c c lossC V P P t∆ = − ∆ (14)

Thus the average voltage of the DC capacitor is given by

*1 1( cos 2 )smm m loss

c

V I V I P tV

C− θ − ∆

∆ = (15)

The average voltage of the DC capacitor can be regulated by adjusting theamplitude of the reference source current I*

sm.

Figure 2: Generation of Reference Source Current

Fig. (2) shows the generation of reference source current. In this method thereference sinusoidal signal is obtained by sensing the supply voltage and dividingit by the peak value of the supply voltage. The difference in source and load activepower is reflected in terms of the average voltage across the capacitor,based on thedifference in actual and reference voltage (referred as voltage error), PI controllergives output thereby adjusting magnitude of the reference source current. I*

sm isobtained by PI controller as

* ( ( )) ( ( ))sm p cref c i cref cI K v v t K v v t dt= − + −∫ (16)

The reference compensating current is obtained by subtracting the load currentfrom the reference source current,and it is given by

i*c(t) = i*

sm(t) – iL(t) (17)

III. HYSTERESIS BAND CURRENT CONTROLLER

The power circuit of the shunt active power filter consist of voltage sourceinverter(VSI) operated in current mode. Hysteresis band current controller providesthe switching pulses for the switches of the shunt active power filter.The referencecompensating current i*c(t) and actual compensating ic(t) are compared within the

60 M. Anbarasan and Rashmiranjandas

hysteresis band.Hysteresis band current controller reduces the ripples in the outputwaveform.Hysteresis band current controller helps the shunt active power filter toproduce compensating current that exactly tracks the reference compensatingcurrent.

Figure 3: Hysteresis band Current Controller

Switching logic is formulate as follows:

If ic > (i*c – HB) upper switch is OFF and lower switch is ON for the same leg.

If ic > (i*c + HB) upper switch is ON and lower switch is OFF for the same leg.

HB is the hysteresis bandwidth

IV. MATLAB/SIMULINK MODELS

Figure 4: Proposed Indirect Control Scheme for Single Phase Shunt APF

A Simplified Indirect Control Scheme for Shunt Active Power Filters 61

Figure 5: Simulink Model for Single Phase Shunt APF with Control

Figure 6: Proposed Indirect Control Scheme for three Phase Shunt APF

62 M. Anbarasan and Rashmiranjandas

As seen from the Fig. (6) for the three phase shunt active power filter all thisprocedures are applied individually to their phases, but only one PI controller isenough for producing ‘ism’ corresponding to each phase.

Figure 7: Simulation Results and Analysis

A Simplified Indirect Control Scheme for Shunt Active Power Filters 63

Table 1System Parameter Values for Single Phase APF

Sytem parameters Values

Supply peak voltage 325VLs, Rs 1mH,0.1ohmLc 2mHLl,Rl 10mH,5ohmAPFcapacitor 4000 microfaradDC capacitor voltage 500VProportional constant (kp) 0.1integral constant (ki) 20Hysteresis band (HB) 1

Table IISystem Parameter Values for three Phase APF

Sytem parameters Values

Supply ph­ph rms voltage 380 VLl,Rl 10 mH, 30 OhmLca=Lcb=Lcc 3.5 mHAPF capacitor 2200 microfaradDC capacitor voltage 700 VProportional constant (kp) 0.08integral constant(ki) 0.5Hysteresis band(HB) 1

Figure 8: Simulated Waveforms of Single Phase System with APF

64 M. Anbarasan and Rashmiranjandas

Fig. 8. shows the simulated waveforms of source current after compensation,compensating current and load current. It is known that source current is same asthat load current before compensation.

Figure 9: Harmonic Spectrum of Source Current without Compensation

Fig. 9. shows the source current without compensation has the THD of 29.51%.It is clear that odd harmonic currents are present in the source current withoutcompensation.

Figure 10: Harmonic Spectrum of Source Currentwith Compensation

Fig. 10. shows after compensation the source current THD is reduced to 2.34%and power factor is improvedtounity.

A Simplified Indirect Control Scheme for Shunt Active Power Filters 65

Fig. 11. shows the source current before compensation, which is also same asthat of load current.

Figure 11: Simulated Wave form of Source Current Showing Phase a, b, cwithout Compensation

Figure 12: Simulated Wave form of Source Currents Showing Phase a, b, cwith Compensation

Figure 13: Simulated Wave form of Compensating Currents

66 M. Anbarasan and Rashmiranjandas

Fig. 14. shows the source current without compensation has the THD of 27.38%.

Figure 14: Harmonic Spectrum of Source Current without Compensation

Figure 15: Harmonic Spectrum of Source Current with Compensation

Fig. 15. shows after compensation the source current THD is reduced to 3.58%and power factorisim proved to unity.

VI. CONCLUSIONThe proposed indirect control scheme can be applied effectively for both singlephase and three phase shunt active power filters. The proposed scheme is easy forthe implementation.The performance of the proposed control is analysed throughsimulations and founded that the proposed control scheme is very much effectivein compensating the harmonic currents present in the supply current and improvesthe power factor near to unity

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A Simplified Indirect Control Scheme for Shunt Active Power Filters 67

[2] C. Y. Hsu and H. Y. Wu, “A New Single­phase Active Power Filter with ReducedEnergy­storage Capacity,” IEE Proc. Electr. power appl, 143(1), 1996, 25­30.

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[12] Doval, J., Nogueiras, A., Penalver, C. M., and Lago, A, “Shunt Active Power Filter withHarmonic Current Control Strategy,” Proc. PELS Power Electronics Specialist Conference(PESC), Fukuoka, Japan 1998, pp. 1631–1635.

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