Fuzzy Logic Controlled Shunt Active Power Filter for ... - IEEE Xplore

2009 International Conference on Advances in Computing, Control, and Telecommunication Technologies

Fuzzy Logic Controlled Shunt Active Power Filter for Mitigation of Harmonics with Different Membership Functions T Narasa Reddy

M V Subramanyam

Department of Electrical Engineering CVR College of Engineering Hyderabad, India [email protected]

Department of Electrical Engineering CVR College of Engineering Hyderabad, India [email protected]

Abstract—The simulation study of a Fuzzy Logic controlled, single phase Active Power Filter to improve power quality by compensating harmonics and reactive power required by a nonlinear load is presented. The advantage of fuzzy control is that, it does not require accurate mathematical model, can work with imprecise inputs, can handle nonlinearity, and are more robust then conventional nonlinear controllers. The compensation process is based on sensing line currents only, an approach different from conventional methods, which require knowledge of harmonics or reactive volt-ampere requirement of load

II.FUZZY CONTROL SCHEME

Fig 1 shows the basic principle of fuzzy controlled active power filter. Active power filter draw /supply a compensating current IC from/to the utility, so that it cancels current harmonics on AC side and make the source current in phase with source voltage. To implement the control algorithm of a shunt power active filter in closed loop, the DC side capacitor voltage is sensed and then compared with a reference value. The obtained error e(Vdref- Vd) and change of error signal are used as inputs for the fuzzy processing or fuzzy controller. Output of Fuzzy controller is reference current which is fed to PWM pulse generator.

Keywords—Active Filter, power Quality, Fuzzy Logic (key words)

I.

INTRODUCTION

Conventionally, the major part of electrical power was consumed by linear loads, which carry the sinusoidal current in proportion to supply voltage. However majority of these linear loads is phase displaced with respect to the supply voltage resulting in low power factor. In recent years, the application of power electronics has grown tremendously. These power electronics systems offer highly non-linear characteristics and draw non sinusoidal current from utility, causing harmonic pollution into supply system. Increase in such non-linearity results in undesirable features such as distortion of supply voltage, low system efficiency and a poor power factor. They also cause disturbance to other consumers and interference in near by communication networks. To overcome these problems, active power filters have been developed. Several control strategies have been reported to improve its performance. Akagi et al.[1] proposed the instantaneous power theory; Divan et al.[2] proposed the synchronous reference frame theory and flux controller techniques to generate the reference signal for the control of shunt active power filter. These techniques require large number of computation. The next breakthrough in active power filter development has resulted from the microelectronics revolution. Now it is possible to implement complex algorithms such as Fuzzy Logic [3], Fuzzy Genetic Algorithms [10] and Neural Nets [4][8].Several direct [3, 4] and indirect[5,6] active power filter techniques are reported. Indirect method is simple, requires less hardware and fast computation method [7].In this paper, a new approach for indirect control of active power filter using fuzzy PI control is attempted.

IS

IL

Non Linear Load

Ic

Is PWM Pulse generator

Vdcref

Fuzzy Logic

VdcController

Fig. 1 Functional block diagram of shunt active power filter Isref

Pulses IS

Comparator

Triangular carrier wave

Fig2. PWM pulse generator 978-0-7695-3915-7/09 $26.00 © 2009 IEEE DOI 10.1109/ACT.2009.157

616

The switching signals for PWM converter are obtained by comparing the actual source currents with the reference current templates shown in fig. 2.

Formation of fuzzy PI controller is shown in figure 4.

Fuzzy

d dt

Fuzzy Logic Controller: Fuzzy logic unlike Boolean or crisp logic, deal with problems that have vagueness, uncertainty or imprecision and uses membership functions with values varying between 0 and 1. Figure 3 shows a schematic block diagram of fuzzy inference system or fuzzy controller

Logic

∫

Controller

Plant Fig. 4 Fuzzy PI Controller

Data base control Rule base

Design of control rules: Fuzzification Interface

Defuzzification Interface

The fuzzy control rule design involves defining rules that relate the input variables to the output model properties. As fuzzy logic controller is independent of system modal, the design is mainly based on the intuitive feeling for, and experience of, the process.The rules are expressed in english like language with syntax such asIf {error e is A and change of error Δe is B} then {control output is C}For better control performance finer fuzzy petitioned subspaces( NL, NM, NS, ZE, PS, PM, PB ) are used, and summarized in table 1. These seven membership functions are same for input and output and characterized using triangular membership functions.

Decision making logic

Controlling system Process

Fig. 3 Fuzzy Inference System

(de/dt)/e NL NM NS Z PS PM PL

It consists of blocks • Fuzzification Interface • Knowledge base • Decision making logic • Defuzzification As it is a two dimensional fuzzy control, a fuzzy logic controller should posses proportional integral control effects. An integral action is normally needed to achieve the best performance in practical situation. A conventional PI control method is

NM NL NL NL NM NS Z PS

NS NL NL NM NS Z PS PM

Z NL NM NS Z PS PM PL

PS NM NS Z PS PM PL PL

PM NS Z PS PM PL PL PL

PL Z PS PM PL PL PL PL

Table 1. Control rule base for fuzzy PI controller III.Modeling and Simulation of the System:

y (t ) = K P ⋅ e(t ) + K I ∫ e(t ) ⋅ dt (1)

Simulation is carried out for single phase shunt active filter using MATLAB/Simulink. The simulations are performed for two different non linear loads viz., diode bridge rectifier with RC load and Diode bridge rectifier with RL load.

In digital implementation, its velocity form is used

y K +1 = y K + Δy K +1 Δy K +1 = K P ⋅ Δe K + K I ⋅ e K

NL NL NL NL NL NM NS Z

(2)

Design of power circuit:

(3) If eK and Δek are fuzzy variables (3) becomes a fuzzy controller. Therefore, a practical fuzzy PI controller is extended to (4); A signifies the fuzzy function that acts on the rules given in the form of a look up table

The design of the power circuit essentially consists of the calculation of dc-link capacitor and filter inductor. It also includes filter device selection. Calculation of dc-link capacitor is based on the energy balance principle. DC-link capacitor shall supply/absorb the energy whenever there is a sudden change in the active power demand by the load [6]. Energy stored in capacitor= Energy demand of the load during transients

y K +1 = y K + Δy K +1 Δy K +1 = A{E K , ΔE K } = A{K 1 ⋅ e K , K P ⋅ Δe K } ..(4)

617

RMS supply voltage (VS) Supply frequency (f) DC link capacitor (Cdc) Carrier frequency (fC) Carrier amplitude (A) Filter inductance (Lf)

1 T ⋅ C dc ⋅ (VO2 − VO2min ) = VS ⋅ I S ⋅ 2 2 Where Cdc = DC-link capacitance VO = DC-link voltage VOmin = Desired minimum capacitor voltage Vs = RMS supply voltage Is = Maximum rms load current during transient T = Time period of supply voltage. In practice, a higher capacitance value than the calculated should be selected to take care of the capacitor losses.Filter inductor calculation is based on the current control technique used for generation of the switching pulses for the converter switches. In the triangularization of error control technique used here, the switching pulses are generated by comparison of current with the triangular error.Value of filter inductor, Lf is calculated with the constraint that for a given switching frequency the maximum slope of the inductor current shall be smaller than the slope of the triangular carrier waveform. In this way the intersection between the current error signal and the triangular waveform will always exist The slope of triangular wave λ is

λ = 4 ⋅ A ⋅ fC

Where A = peak of triangular carrier wave fC = frequency of triangular carrier wave The maximum permissible slope of the inductor is given by

dI L VS + VO = dt Lf IL = Inductor current VS = RMS supply voltage VO = DC-link capacitor voltage Lf =Filter inductance Since the slope of inductor current has to be smaller than the slope of the carrier triangular wave.

Lf ≥

VS + VO 4 ⋅ A ⋅ fC

230 Volts 50 Hz 5000μF 20 KHz 1 Volt 1mH

Load parameters: 1. Non-linear load: Diode bridge rectifier with RC load Resistance (R) 10 and 30 Ω Capacitance (C) 1000 μF Diode bridge rectifier with RL load Resistance (R) 10 and 30 Ω Inductance (L) 100mH 2. Linear load (power factor = 0.37) Resistance (R) Inductance (L)

27 Ω 35 mH

Simulation circuit is shown in fig. 5. Fig. 6 shows the simulated response of the 1-phase active filter with combination of linear and non linear loads. Switching timings for different load is given below. 0-40 msec: RL (30Ω, 100mH) load with diode bridge rectifier. At t=40 msec active filter is turned on. 40-186 msec: RL(30Ω, 100mH) load with diode bridge rectifier. 186-327 msec RL(10Ω, 100mH) load with diode bridge rectifier. 327-538 msec RC(30Ω, 2000 μF) load with diode bridge rectifier. 538-764 msec RC(10Ω, 2000 μF) load with diode bridge rectifier. 764-1000 msec RC(10Ω, 2000 μF) load with diode bridge rectifier and linear load (R=27 Ω, L=35mH)

The selection of semiconductor switches [11] depends on state voltage, current and voltage rating, operating frequency, energy losses and device cost. IGBT’s have some advantages of MOSFET, BJT and GTO combined, such as IGBT is voltage driven device with high input impedance like MOSFET, small on state voltage drop like BJT and similar to GTO large negative voltage blocking capacity. IV. Simulation results: The simulation are carried out for single phase shunt active power filter having following specification

618

(c)

(d)

Fig. 5: Matlab simulation circuit of Fuzzy Logic controlled active power filter

(e) Fig.6 Frequency spectrum for time interval 40 msec- 240 msec (a) Load current, Supply current for membership functions, (b) Triangular, (c) Trapezoidal, (d) Gbell, (e) Gauss membership function Table:1. THD with different membership functions Without Filter

Triangular

Harmonic No.(3)

33.22

0.0287

THD

93.11%

619

Gauss

0.035

0.035

0.0281

5.54%

7.2%

4.29%

oidal

(a)

(b)

Gbell

Trapez

4.54%

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[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9] [10] [11]

[12]

[13] [14] [15]

Conclusion: The simulation results show that the source voltage is in phase with supply voltage and settling time is 2-3 cycle. Capacitor voltage is varying between ±10% of Vdc. The total harmonic distortion is within the norms prescribed by IEEE-519 standard, i.e. less than 5%.

620

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