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International Journal of Science, Engineering and Technology Research (IJSETR), Volume 4, Issue 2, February 2015

417

ISSN: 2278 – 7798 All Rights Reserved © 2015 IJSETR

Abstract— This paper presents reduction of line current

harmonics using Four-leg shunt active power filter with

balanced and unbalanced load condition for three phase

four wire shunt active power filter. The synchronous

reference frame (SRF) method is used to extracting

reference current for four-leg shunt active filter. The

Hysteresis current controller (HCC) is used to generate gate

pulses and applied VSI based four –leg shunt active power

filter. Two control methods were used for four-leg shunt

active power filter one is PI and other one is Fuzzy logic

controller. The PI and Fuzzy logic controller to control,

four leg shunt active power filter to compensate line current

harmonics and neutral current compensation to improve

power quality for three-phase four wire system. The main

aim of this paper is to reduce the total harmonic distortion

(THD) in the line current and neutral current

compensation. The MATLAB/Simulink environment is

used to model for above four leg topology of shunt active

power filter.

Index Terms— Shunt active power filter, SRF method,

Hysteresis current controller, PI and Fuzzy logic controller

1. INTRODUCTION

Modern days, three phase four wire distribution power system

has been broadly employed in residential and office buildings,

manufacturing facilities, schools, etc., to supply low level voltage.

The typical loads connected to the three phase four wire power

system may be reasonable three phase non-linear loads such as

motor drives, power electronics loads, large Uninterruptible power

supplies or single phase non-linear loads such as switch-mode

power supplies in computer equipment, inverter air conditioners and

other power electronic related facilities. The common of these loads

have a nonlinear input or unbalanced characteristic, which may

cause two problems such as high input current harmonics and

extreme neutral current. The survival of current harmonics in power

systems increases losses in the lines, decreases the power factor and

causes timing errors in sensitive electronic equipment. The

harmonic currents produced by balanced three phase non-linear

loads are positive-sequence harmonics and negative-sequence

Harmonics. Nevertheless harmonic currents produced by single

phase non-linear loads which are connected phase to neutral in three

phase four wire system are third order zero-sequence harmonics.

Parimala.V Assistant Professor (SG), Department of PG-ES,

P.A College of Engineering and Technology, Pollachi.

Dr.GaneshKumar.D, Professor,

Department of EEE, P.A College of Engineering and Technology, Pollachi.

Renugadevi.V,Department of PG-ES, P.A College Of Engineering and

Technology, Pollachi

Two primary approaches for humanizing power quality are passive

filter and active filter. Passive filters are broadly worn to eliminate

harmonics in power system for its simplicity and low cost still,

passive filters have a number of drawbacks such as large size, tuning

and risk of resonance problems. Now, the 4-leg active filters have

confirmed to be very effective to solve the troubles of current

harmonics, reactive power, unbalanced load current and excessive

neutral current simultaneously in 3-phase 4-wire system, and can be

a much improved solution than conventional passive filters. The

Synchronous Reference Frame (SRF) theory [7] was generally

applied to calculate the compensating currents assume ideal mains

voltages. But, mains voltage may be unbalanced and/or distorted in

industrial systems [1]. So, the Four leg APF using the p-q theory

does not provide good performance .For humanizing the APF

performance under non-ideal mains voltage conditions, various

improved methods based on Instantaneous reactive power theory

have been future even though superior results have been achieved.

However, Instantaneous reactive power methods for harmonic

detection in the three phase four wire systems need

phase-locked-loop (PLL), low-pass filter and the multiple

coordinate transformation[4] .nevertheless, the conventional PI

controller was used for the generation of a reference current

template. The PI controller requires exact linear mathematical

models, which are not easy to attain and fails to perform

satisfactorily under parameter variations, nonlinearity, load

disturbance, etc. Recently, Fuzzy logic controllers (FLC) have

generate a good deal of interest in certain applications[7] The

benefit of Fuzzy logic controller over conventional controllers are

that they do not need accurate mathematical model can work with

imprecise inputs, can hold non-linearity, and are more strong than

conventional controller.

In this work two control methods are used, one is PI and another one

is Fuzzy logic controller (FLC) of shunt active power filter for the

line current harmonics and neutral current compensation of a

nonlinear load. The synchronous reference frame theory is used for

generate reference current in four legs shunt active filter and

hysteresis controller is used to obtaining a gate pulse for shunt active

power filter. A design criterion is described in the selection of power

circuit components.

Both the control schemed is compared and performance of both the

controllers the fuzzy controller has a less line current harmonics

THD values compared to PI controller A detailed simulation

program of the schemes is developed to predict the performance for

different conditions and Simulink models also has been developed

for the same for different parameters and operating conditions.

Fig.1.1.Four-leg shunt APF with non-linear loads

FOUR-LEG SHUNT ACTIVE POWER FILTER

FOR POWER QUALITY IMPROVEMENT

USING PI AND FUZZY CONTROLLERS

V.Parimala, Dr.D.GaneshKumar, V.Renugadevi


418


2. SHUNT ACTIVE FILTER

2.1. Introduction:

In this case the shunt active power filter operates as a current

source injecting the harmonic components generate by the load but

phase-shifted by 180. The main idea of the 4-leg APF is to

compensate harmonics, reactive power, and neutral current and

unbalanced loads .In 3-phase 4-wire systems, two kinds of VSI

topologies since 4-leg inverter and three leg inverter (split capacitor)

are used. The 4-leg inverter employed 1-leg specially to compensate

neutral current. Fig 1 shows the basic compensation principle of the

shunt APF.

A shunt APF is designed to be connected in parallel with the load,

to detect its harmonic current and to inject into the system a

compensating current, equal with the load harmonic current.

Therefore, the current strained from the power system at the

coupling point of the filter will result in sinusoidal.

2.2. Basic compensation principles:

The active power filter is controlled to deliver the compensating

current if from/to the load to nullify the current harmonics on the AC

side and reactive power flow from/to the source there by making the

source current in phase with source voltage. Figure 3.2 shows the

basic compensation principle of the active power filter and it serve

as an energy storage element to bring the real power difference

between load and source during the transient period. When the load

condition change the real power balance between the mains and the

load will be concerned. This real power distinction is to be

compensated by the DC capacitor.

These adjust the DC capacitor voltage away from the reference

voltage. In categorize to keep suitable operation or the active filter,

the peak value of the orientation, source current must be familiar to

proportionally alter the real power drawn from the source. These

real powers charged/discharged by the capacitor compensate the real

power dissimilarity between the consumed by the load and that of

supplied by the source. If the DC capacitor voltage is improved and

attains the reference voltage, the real power absolute from the source

is imaginary to be equivalent to that consumed by the load again.

Fig .2.1.Basic configuration of shunt active filter.

3. CONTROL STATERGIES

3.1. SYNCHRONOUS REFERENCE FRAME (d-q)

THEORY

In Fig.3.1, the whole reference current generation scheme

has been illustrated. The load currents nd are track

upon which Park‘s transformation is performed to obtain

corresponding d-q axes currents and as given in (3.1), where

ɷ is the rotational speed of synchronously rotating d-q frame.

According to - control strategy, only the average value of

d-axis component of load current should be strained from supply.

Here and indicate the fundamental frequency

component of and . The oscillate components and ,

i.e and are filtered out using low-pass filter.

= - (1)

=

- (2)

The currents and along with are utilized to

make reference filter currents and in d-q coordinates,

followed by inverse Park transformation giving away the

compensation currents , in the four wires as

describe in (2) and (3).

- +

- +

+

-

+

-1

Fig.3.1Reference current generation

= - (3)

= + + - (4)

The reference signals thus obtained are compared with the actual

compensating filter currents in a hysteresis comparator, where the

real current is forced to follow the reference and provides

instantaneous compensation by the APF on account of its easy

implementation and quick overcome over fast current transitions.

Accordingly provides switching signals to trigger the IGBTs inside

the inverter. Ultimately, the filter provides needed compensation for

harmonics in the source current and reactive power unbalance in the

system [11]. One of the compensation of this method is that angle θ

is calculated straight from main voltages and thus makes this method

frequency independent by avoiding the PLL in the control circuit.

Accordingly synchronizing problem. With balanced and unbalanced

conditions of main voltages are also evaded. Thus d-q achieves large

frequency operating limit essentially by the cut-off frequency of

voltage source inverter.

3.2. HYSTERESIS CURRENT CONTROLLER

Hysteresis current controller derives the switching signals of the

inverter power switches (IGBTs).The current controllers of the three

phases are considered to operate separately [7]. Each current

PI /FUZZY

abc to

dqo

transform

ation

(rotating

reference

frame)

LPF

LPF

dqo

to

abc

transf

orma

tion


419


controller determines the switching signals to the inverter bridge

.The switching logic for phase A is formulated as follows

If - HB upper switch is OFF and lower switch is

ON.

If upper switch is ON and lower switch is

OFF

In an identical manner, the switching logic for devices in phase

B and C are derived. The switches are restricted asynchronously to

ramp the current through the inductor up and down so that it follows

the reference. The current ramping up and down linking the two

limits is illustrated in fig.3.3.When the current during the inductor

surpasses the upper hysteresis limit a negative voltage is applied by

the inverter to the inductor. The beginning the current in the

inductor to reduce. Once the current reaches the lower hysteresis

limit a positive voltage is applied by the inverter to the inductor and

this causes the current to boost and the cycle repeats.

Figure.3.2.Hysteresis current waveform

3.3. PI controller:

The control scheme consists of a PI controller, a limiter, and a

three phase sine wave generator for the reference current and

switching signal generation. The real value of the reference currents

is estimated by regulating the DC link voltage. The real capacitor

voltage is compared with a set reference value[7]

The error signal is then processed through a PI controller, which

supply to the zero steady error in tracking the reference current

signal. The output of the PI controller is measured as the peak value

of the supply current (Imax), which is composed of two

components: (a) the fundamental active power component of the

load current, and (b) the loss component of the APF; to maintain the

average capacitor voltage at a constant value. The peak value of the

current (Imax) so attained, is multiplied by the unit sine vectors in

phase with the respective source voltages to obtain the reference

compensating currents. These estimated reference currents (Isa*,

Isb*, and Isc*,Isn, Isn*) and the sensed actual currents ( Isa, Isb, and Isc ,

Isn) are compared to a hysteresis controller, which gives the error

signal for the modulation technique. This error signal decides the

operation of the inverter switches.

3.4 Fuzzy controller:

The control scheme consists of Fuzzy controller, limiter and

three phase sine wave generator for the reference current generation

and generation of switching signals. The crest value of reference

currents is estimated by regulating the DC link voltage. The real

capacitor voltage is compared with a set reference value. The error

signal is then processed during a Fuzzy controller, which supply to

zero steady error in tracking the reference current signal [10].

A fuzzy controller converts a linguistic control strategy into an

automatic control strategy, and fuzzy rules are constructed by expert

experience or knowledge database. Firstly, input voltage Vdc and

the input reference voltage Vdc-ref have been placed of the angular

velocity to be the input variables of the fuzzy logic controller. Then

the output variable of the fuzzy logic controller is presented by the

control Current Imax. To change these numerical variables into

linguistic variables, the subsequent seven fuzzy levels or sets are

chosen as:

NB (negative big), NM (negative medium), NS (negative

small), ZE (zero), PS (positive small), PM (positive medium), and

PB (positive big) as shown in 3.4(a).

The fuzzy controller is characterized as follows:

1) Seven fuzzy sets for each input and output;

2) Fuzzification using continuous universe of discourse;

3) Implication using Mamdani's ‗min‘ operator;

4) De-fuzzification using the ‗centroid‘ method.

( a)

(b)

(c)

Fig.3.3.(a) Input Vdc normalized membership function; (b)

Input Vdc-ref Normalized Membership Function; (c) Output

Imax Normalized Membership Function.

4. SIMULATION RESULTS:

The simulation is carried out with three phase four wire system

with non-linear load. Here the diode rectifier is used as non-linear

load. The Fig-4 shows the circuit diagram without any filter or

controller circuit. From this simulation, source current, voltages are

taken as the output. The THD value of source current under

balanced and unbalanced source condition. The THD value is high

because we won‘t use any controller in this circuit.

Fig 4.1.Simulation for open loop system


420


Figure.4.1(a)waveforms of source voltage in open loop

system

Figure.4.1(b)waveforms of source current in open loopsystem

Fig. 4.1(c).Waveforms of neutral current in open loop system.

Figure.4.1(d).FFT analysis of line current harmonics in open

loop system

The Fig-4.2 and 4.3 shows the closed loop system with PI

controller and fuzzy controller along with the SRF

(Synchronous Reference Frame) theory. Using this SRF theory

three phase system is converted into two phases, and the

reference current is generated from this. This reference current is

given as reference value and the actual current is taken from the

filter; these two are compared in the PI and fuzzy controller and

the error signal is generated. This again gave as reference to the

hysteresis controller to generate the gate signal to the filter

switch. Hence, by turning on and off the devices the curren

waveform has been improved. The system parameters are

shown in Table-1

Table-1 System Parameter

System Parameter Values

Source voltage(Vs) 315Vrms

Source frequency(Fs) 50Hz

Source impedance (Zs) 10mΩ,50µH

Three phase Load

(RL load)

12Ω,20mH

DC link

capacitance(Cdc)

1500 µF

Single phase Diode

Rectifier(RLC load)

15Ω,1mH,470µF

AC side

filter(Rc,Lc),(Rf,Lf)

(0.1Ω,1mH),(2Ω,20µF)

The simulation results of source current, source voltage and

load current are shown in Fig-10 to Fig-11 respectively.

Similar to open loop, the THD vlue of source current is shown

under balanced and unbalanced conditions.

Fig.4.2.Simulation for PI controller

Figure.4.2 (a).waveforms of source voltage and source current

under balance load using PI controller


421


Fig.4.2(b).Waveforms of DC link voltage and neutral current

under balanced load using PI controller

Figure.4.2 (c).FFT analysis of line current harmonics under

balanced load using PI controller

Figure.4.2.(d).waveforms of source voltage and source current

under unbalanced load using PI controller

Fig.4.2.(e).Waveforms of DC link voltage and neutral current

under balanced load using PI controller

Figure.4.2.(f).FFT analysis of line current under unbalanced

load using PI controller

Figure 4.3. Simulation diagram for fuzzy controller

Fig.4.3(a).waveforms of source voltage and source current

under balanced load using Fuzzy logic controller.

Fig.4.3(b).Waveforms of dc link voltage and neutral current

under balanced load using Fuzzy logic controller

Fig.4.3(c).FFT analysis of line current under balanced load

using Fuzzy logic controller.


422


Fig. 4.3(d).waveforms of source voltage and source current

under unbalanced load using Fuzzy logic controller

Fig.4.2.(e).Waveforms of dc link voltage and neutral current

under unbalanced load using Fuzzy logic controller

Fig.4.3(f).FFT analysis of line current in un balanced load under

Fuzzy logic controller

Table.2.Comparision of PI and Fuzzy Controller with balanced

and unbalanced load

Controller Source

current

Balanced

condition

Source

current

Unbalanced

condition

PI 2.70% 4.59%

Fuzzy 1.34% 2.70%

Table.3.Bar Chart For Shunt Active Power Filter

010203040 open loop

Balanced

load

Unbalanced

load

5. CONCLUSION:

In the present paper two controllers are developed and verified

for three phase four wire systems. Even though both of the

presented controllers are capable of compensating line current

harmonics in 3 phase 4-wire systems, it can be seen that the Fuzzy

logic controller has a better dynamic performance than the

conventional PI controller. Hysteresis current control is used for

quick response for generating gate pulses. Additionally, in

contrast to the different control strategies; the d-q method is used

for obtaining the reference currents in the system. This is due to

the verity that the angle ‗θ‘is considered directly from the main

voltage which enables an operation which is frequency

independent. As a result, this technique avoids large number of

synchronization problems. It can also be seen that the DC voltage

regulation system is a stable and steady-state error free system.

Thus with fuzzy logic and the (d-q) approach, a shunt active filter

can be developed. Simulation results are presented to validate the

performance of the shunt active filter.

REFERENCES

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[4]. M Suresh, A. K. Panda, S. S. Patnaik, and S. Yellasiri,

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Bibliography:

V.Parimala has obtained her Bachelor of

Engineering Degree in Electrical and Electronics

Engineering from Madras University. Master‘s

Degree in Power Electronics and Drives from Anna

University. Currently working as Senior Assistant

Professor. Her area of interests includes Power

Quality, Power Electronics, Soft Computing

Techniques and Virtual Instrumentation.

Dr.D.Ganeshkumar has obtained his Bachelor

of Engineering Degree in Electronics and

Instrumentation and Master in Applied

Electronics. He has received his PhD in Vibration

Analysis using Virtual Instrumentation. Principle

investigator for DST, Government of India

funded project or Sound and Vibration Analysis

in Electrical Machines using Virtual

Instrumentation Techniques. His area of interest includes Process

Monitoring and Control in Virtual Instrumentation Systems.

V.RENUGADEVI has obtained her Bachelor‘s

degree in Electrical and Electronics

Engineering from Sri Eshwar College of

engineering and currently pursuing her Master‘s

degree in Power Electronics and Drives from P.A.

College of Engineering and Technology. Her area

of interest includes Power Quality and Drives.

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