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CONTROL OF SHUNT ACTIVE POWER FILTER IN

A POWER SYSTEM USING MATLAB/SIMULINK

M.Chakravarthy, Dr.S.N.Saxena, Dr.B.V.Sanker Ram

ABSTRACT

This paper describes a three-phase shunt active power filter using a conventional three leg IGBT converter, without the need of power supply at DC bus. Two approaches have been discussed in this paper to control the active power filter. The first approach discussed is voltage source instantaneous power control technique and the second is carrier-based control technique. Simulation results from a complete model of shunt active power filter are presented to validate and compare the control strategies.

Performance of the modeled shunt active power filter is illustrated by a six pulse fully-controlled bridge rectifier connected to power system.

Modeling and analysis of shunt active power filter have been done by MATLAB/SIMULINK providing a graphical user interface for building models as block diagrams. The approach explained in this paper provides an insight into how a model is organized and how its parts interact.

I. INTRODUCTION

An active power filter can be considered as a modified voltage source inverter. Fig.1 shows the detailed circuit diagram of an active power filter.

Fig. 1 Schematic diagram of Active Filter connecting with Non Linear Load

Power electronic devices normally used as converters cause various harmonics to be produced and also cause low power factor in the system. Earlier passive filters were used to reduce harmonics and power factor, but their disadvantages are that they are not responsive to changes in load and contribute to unwanted resonance in the circuit. In this chapter, various control strategies are discussed and their comparisons are made.

II. CONTROL TECHNIQUES

i. Voltage Source Instantaneous Power Control Technique

The three voltages Va, Vb and Vc from

the voltage transducers are given to the controller, which converts them to Vo, Vα and

Vβ using the following equations.

a b c

0

V

V

V

3

1

V

……… (1)         2 V 2 V V 3 2

V b c

a

……… (2) M Chakravarthy and Dr. S N Saxena are with

Department of Electrical & Electronics Engineering – Gokraju Rangaraju Institute of Engineering & Technology, Hyderabad, India.

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     

 b Vb

2 3 V 2 3 3 2 V ……… (3)

Similarly, the controller is fed with current signals from the current transducers, which are converted to i0, iα and iβ using the

following equations.

2

i

2

i

2

i

3

2

i

a b c

0 ……… (4)

2

i

2

i

i

3

2

i

b c

a ……… (5)      

 b ic

2 3 i 3 3 3 2

i ……… (6)

The real and reactive powers are obtained from the following equations.

0 0 0

V

I

P

……… (7)

p = VαIα + VβIβ ……… (8)

q = -VβIα + VαIβ ……… (9)

This calculation is represented diagrammatically in Fig. 2.

Fig.2. Block diagram representation of calculation of reference currents

The active power signal p has a steady component and a fluctuating component. The

Fig.3. High pass and low pass filter characteristics

steady component only should be drawn from the grid, whereas the fluctuating component should be drawn from the active power filter. If this can be done, the grid will not be affected by harmonic currents. If q can be supplied entirely from the active power filter, the power factor will also be improved. If reference currents can be calculated with this in view, the shunt active power filter will contribute in a great way to improve the power quality. The power signal p is passed through a high pass filter. A diagrammatic representation of this scheme is shown in Fig.4.

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po (zero sequence power signal) is

passed through low pass filter and the active power signal is passed through high pass filter. The output of low pass filter is steady component of zero sequence power. In high pass filter, output is fluctuating component of power as shown in Fig.4.This difference is is used to calculate the reference currents using the following expressions.

icα ref = – + Vβ q] ..(10)

icβ ref = – + Vα q] ..(11)

These reference currents in the α-β frame are converted to a-b-c frame, using the reverse transformation given below.

icaref= [ +icαref] ……… (12)

icbref = [ - (icαref)+ icβref ] .…….(13)

iccref= [ - (icα)- icβ ] …..…….(14)

This scheme involves selection of two levels of current.

Fig.5. Control algorithm for hysteretic band current controller

First one is slightly above the reference current and other slightly below the reference current. Feedback signal from the actual current is taken. When the actual current is below lower value, the MOSFETs are switched on, and when the current crosses the upper value, the MOSFETs are switched off. As a result, the actual current remains within the upper and lower bands of the current reference. The scheme is explained diagrammatically in Fig. 5.

ii. Carrier-based Control Technique

This control scheme requires less computational efforts than the other schemes. It is formed by a DC voltage regulator and reference current generation. Closed-loop PWM is used for generating switching signals to active power filter to obtain the desired currents. The compensating currents of active power filter are calculated by sensing the load currents, DC bus voltages, peak voltage of AC source and zero crossing of voltage source. The peak voltage and zero crossing of AC source voltage are used for calculation of instantaneous voltages of AC source.

Vsa(t) = Vsm sin(ωt) ………(15)

Vsb(t) = Vsm sin(ωt - 2π/3) .…..….……(16)

Vsc(t) = Vsm sin(ωt - 4π/3) .…..….……(17)

The basic function of the proposed shunt active filter is to eliminate and to compensate unbalanced current. After compensation, the AC source feeds fundamental active power component of load current and losses of inverter for regulating DC capacitor voltage.

The instantaneous power of the load at kth sample can be obtained as below:

PL(k) = Vsa(k)iLa(k)+Vsb(K)iLb(k)+vsc(k)iLc(k)

.…..….……(18)

The average power of load is obtained from the following equation

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n = T fs; T=1/f .…..….……(20)

f - is the fundamental system frequency fs - is the sampling frequency To compensate for the current harmonics and reactive power, the average active power of AC Source should be same as load average power. Considering unity power factor, average active power of source is given below: Ps = 3/2 VsmI*smp = PLav .…..….……(21)

Component of AC side current is given by I*smp = 2/3 PLav/Vsm .…..….……(22)

The desired peak current of AC Source is given by I*sm= I*smp + I*smd .…..….……(23)

The desired source currents are obtained by finding the product of peak source current to unit sinusoid current as given below: iua = vsa/ Vsm .…..….……(24)

iub = vsb/ Vsm .…..….……(25)

iuc = vsc/ Vsm .…..….……(26)

i*sa = I*sm iua .…..….……(27)

i*sb = I*sm iub .…..….……(28)

i*sc = I*sm iuc .…..….……( 29)

i*ca= I*sa - iLa .…..….……(30)

i*cb= I*sb - iLb .…..….……(31)

i*cc= I*sc - iLc .…..….……(32)

The control block diagram is shown in Fig.6.

III. SIMULATIONS

The block diagram shown in Fig. 7 presents the piecewise blocks of the Voltage

Source Instantaneous Power Control

Technique.

Fig.6. Control system of shunt active power filter.

In order to decrease the complexity of the design, a feature called “subsystem” is utilized. A “subsystem” is defined as a block with terminals and which represents a sub network. The design consists of the following subsystems:

1. Source

2. Non-linear Load 3. Active filter

4. PWM pulse generation 5. P-Q theory

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Fig. 7 Simulation design for Voltage Source Instantaneous Power Control technique

Implementation of Three-Phase to Two-Phase Transformation Block

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The Voltage Source Instantaneous Power Control Technique implementation used by the active filter controller is shown in the Fig.9.

Fig.9. Voltage source Instantaneous Power Control technique implementation block

Fig.10. Subsystem block for P-Q-Valpha1Vbeta1 transformation

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All Rights Reserved © 2012 IJARCSEE Fig.11. Subsystem block for Icalpha and Icbeta

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All Rights Reserved © 2012 IJARCSEE Fig.12. Subsystem block for Ica, Icb, Icc

The diagram in Fig.12 implements the following equations:

Voltage Source Inverter Block

Fig.13 shows the active filter controller block, which is supplied with the line currents and line voltages. This block generates the reference currents using the P-Q Theory.

Fig. 13 Voltage Source Inverter

Dynamic Hysteretic Current controller

The reference currents generated by the active filter controller are added and subtracted with constants to form a bandwidth. The higher value and the lower value of the bandwidth are compared using a

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Fig.14. Dynamic hysteresis current controller

Implementation of Carrier-based Control Technique using MATLAB/SIMULINK:

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All Rights Reserved © 2012 IJARCSEE The block diagram in Fig.15 explains the

generation of firing pulses for carrier-based control technique. This technique is based on switching semiconductor switches to produce output voltage waveform with low harmonics. The advantage of this technique is that it allows independent and easy control of active and reactive power components. This PWM technique is used for tracking the computed

currents by the active filter. In this technique, the difference of reference and measured currents is applied to a PI controller and its output signal is given to a conventional carrier based PWM controller.

Fig.16 shows the simulation design block of shunt active power filter for carrier-based control technique.

Fig. 16 Simulation design block of shunt active power filter for carrier- based control technique

IV. RESULTS

Voltage Source Instantaneous Power Control Technique

Fig.17 shows the active filter controller block, which is supplied with the line currents and line voltages. The reference currents, complementary to the harmonics that are produced in the line currents and generated by this active filter controller.

Fig.18 shows the bandwidth created for the reference currents generated by the hysteresis current controller block. Fig.19 shows the currents before Compensation.

Fig.20 shows the source currents of the system connected to a non-linear load after compensation.

Fig.21 shows the FFT analysis of source currents after compensation.

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All Rights Reserved © 2012 IJARCSEE Fig. 18 Bandwidth for the reference currents

generated

Fig. 19 Source currents before compensation

Fig.20 Compensated source currents after compensation

Fig. 21 FFT Analysis of the source currents after Compensation

Results of Carrier-based Control Technique The carrier-based control technique, when fed with distorted source mains with THD of 24.97%, reduces the source THD to 1.27%, thus conveying the significant working of the control technique.

Fig.22 Source currents before

compensation. Fig.23 FFT analysis of source currents before compensation.

Fig. 24 FFT analysis of source currents after compensation. Fig.25. shows source currents after compensation.

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All Rights Reserved © 2012 IJARCSEE Fig.23 FFT analysis of source currents before

compensation using carrier-based control technique

Fig. 24 FFT analysis of source currents after compensation using carrier-based control

technique

Fig.25 Source currents after compensation using carrier-based control technique

Conclusions

Two control techniques for a three-phase shunt active power filter employing a conventional three-leg converter were developed and a critical comparison between

both approaches was carried out.

Experimental results using both the control strategy were successfully obtained and reported in this paper. In conclusion the carrier-based control technique provides lower values of THD to source currents during non-linear load when compared to voltage source instantaneous control technique.

V. REFERENCES

1. D.Sutanto and M.Bou-Rabee, “Active filter

with reactive power compensation

capability,” in Int.Power Eng. Conf., Singapore, Mar 1993, pp 73-78

2. H.Akagi, Y.Kanazawa, and A.Nabae, “ Generalized theory of the instantaneous reactive power in three-phase circuits,” in IPEC’83- Int Power Elec Conf, Tokyo, Japan, 1983, pp 1375-1386.

3. Silcy George, Vivek Agarwal, “ A DSP Based Optimal Algorithm for shunt Active filter Under Non-Sinusoidal supply and and unbalanced load conditions”, in IEEE transactions on Power Elec. Vol-22, Mar 2007

4. C.A.Quinn and N.Mohan, and H.Mehta,”A four-wire, current-controlled converter provides harmonic neutralization in three-phase, four-wire systems,” in APEC’93-Applied Power Elec. Conf, 1993, pp. 841-846 5. M.Aredes, J.Hafner, and K.Heumann, “A three-phase four-wire shunt active filter using six IGBT’s,”inEPE’95-ur.Conf.Power

Elec.Appl.Sevilla,Spain,Sept.1995,vol.1, pp 1.874-1.879

Figure

Fig. 1 Schematic diagram of Active Filter  connecting with Non Linear Load  Power  electronic  devices  normally  used  as  converters  cause  various  harmonics  to  be  produced  and  also  cause  low  power  factor  in  the system
Fig. 7 Simulation design for Voltage Source Instantaneous Power Control technique  Implementation of Three-Phase to Two-Phase Transformation Block
Fig. 13 Voltage Source Inverter  Dynamic Hysteretic Current controller
Fig. 15 Simulation design block of firing pulses for carrier based control technique
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References

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