This paper proposes a non-model based
method formulated as an alternative to conventional PI control.
This method has been applied to the problem of control of
a shunt active power filter designed for reactive and harmonic
current compensation. The amplitude of the current is controlled
by the fuzzy system, which processes the error between the
DC link voltage and a reference set voltage. The performance
of the proposed method is examined by MATLAB simulations
and is found to have better dynamic behavior than PI control.
The increase in loads consuming non-sinusoidal current,
referred to as non-linear loads has resulted in the generation
of harmonics in the power supply system. The major sources
of harmonic currents are power electronic converters and
controllers so also the Switch Mode Power Supplies (SMPS)
in computers, computer terminals, data processors and other
office equipments. The switching action of these loads draws
a non-sinusoidal current from the supply. This current consists
of the fundamental frequency component and the harmonic
components. The harmonic components generate noise in the
form of harmonic voltages and currents at audio/radio frequencies,
which are inductively or capacitatively, coupled into the
communications and data lines. This noise is picked up by
computer networks, communication equipment, telephone systems
and other sensitive equipment. These systems get more and
more affected as the speed of computer networks increases.
Other effects of harmonics include heating of conductors,
malfunctioning of relays, circuit breakers and sensitive
electronic equipment. Active Power Filter (APF) is basically
designed on the concept of injecting equal but opposite
distortion to a system in order to compensate harmonic distortion.
Shunt Active Power Filters, using different control strategies
have been widely investigated to provide a viable solution
to the problems created by non-linear loads. A number of
methods exist for determining the reference switching current
for the APF (Grady et al., 1990; El-Habrouk et al., 2002;
and Hirofumi, 2005). In this paper, we have considered the
control strategy based on regulation of the DC capacitor
voltage (Jou et al., 1994; and Kishore et al., 1999). The
scheme requires only one current sensor and the compensation
process is instantaneous. DC bus regulation is achieved
using conventional PI algorithm. However, in this scheme,
the non-linear model of APF system is assumed to be linear
and the PI controller design is based on a mathematical
model of the linearized system. A set of equations that
describe the stable equilibrium state of the control surface
is developed by root locus or some other method, and coefficients
are assigned to the proportional and integral aspects of
the system. The PI controller applies the mathematical model
to a given input and produces a specific output from the
mathematical algorithm. The PI model may seem to be simple
and economical for a set of designed PI parameters and the
harmonic compensation achieved by the APF and the response
to step change in load may be satisfactory, but there may
still exist a tendency to overshoot the set value, while
compensating large errors. Further, for the same set of
parameters the system may lack the capacity to adjust satisfactorily
to large fluctuations, and hence fine-tuning of the designed
parameters becomes necessary. Practically, the fine-tuning
of PI parameters is mostly done by trial and error, which
is a time-consuming process. Thus, it is desirable to develop
a reliable auto-tuning method in order to automate this
process. The existing auto-tuning methods can be classified
into model-based techniques and non-model or soft computing
techniques. The model-based technique uses a test signal
to identify the process model and determine the PI parameters.
The problem with this method is that the performance of
the controller depends mainly on the accuracy of the identified
model and the test signal can introduce disturbance into
the process. The soft computing technique, on the other
hand, determines the PI parameters directly according to
the rules and from the features extracted from the input/output
signals of the closed loop system. With this method the
PI parameters can be tuned without external signals during
process operation. In view of the same, an intelligent alternative
method for DC voltage regulation using fuzzy logic is found
to be more suitable for the switching of APF. Juan et al.
(1997) have applied fuzzy logic control to three-phase APF
control earlier, but only standard triangular Membership
Functions (MF's) have been used and the suitability of other
membership functions has not been investigated. Triangular
functions have been the most popular MF shape, but they
are not the best choice (Pedrycz, 1994) for approximating
non-linear systems.
An APF is a power electronic converter that is switched
to maintain the mains current sinusoidal and in phase with
the mains voltage irrespective of the load current. The
APF configuration is shown in Figure 1. It consists of a
single-phase voltage source inverter with an energy storage
capacitor Cd at DC bus. This APF is connected in shunt with
the load through a filtering inductor Lf. Due to the non-linear
load, the load current iL consists of a sinusoidal fundamental
component il and harmonic components ih. The APF is used
to prevent power system pollution due to these harmonic
currents. The active power filter produces a current if
that is equal to the harmonic current required by the load.
|
