A simple fuzzy logic based robust active power filter for harmonics minimization under random load variation
Introduction
The increased use of power electronic controlled equipments and non-linear electronic devices in power systems has given rise to a type of voltage and current waveform distortion called as ‘harmonics’ [1].
Harmonics causes various problems in power systems and in consumer products, such as distorted voltage waveforms, equipment overheating, blown capacitor fuses, transformer overheating, excessive neutral currents, low power factor, etc. [2], [3], [4]. The research has been underway for last three decades to have a control over the harmonics and to supply the consumers with reliable and clean fundamental frequency, sinusoidal electric power that does not represent a damaging threat to their equipments.
To reduce harmonic distortion, both passive and active compensation techniques (filters) can be implemented. Passive filters have the drawback of bulky size, component ageing, resonance and fixed compensation performance. These provide either over- or under-compensation of harmonics, whenever a load-change occurs [5]. Hence, active compensation known as active power filter (APF) is preferred over passive compensation. APF's are an up-to-date solution to power quality problems, which allow the compensation of current harmonics and unbalance together with power factor improvement.
Artificial intelligence is one of the major fields developed since past four decades, and is popular due to its ability to handle complex problem at difficult situations. These tools of artificial intelligence (fuzzy logic, artificial neural network, genetic algorithms, wavelet theory, optimization methods) are used for improving the power quality effectively since 1980s and produce good performances [6]. Fuzzy logic is one of the alternatives to artificial intelligence originated by Zadeh [7]. Use of fuzzy logic for minimization of harmonics and improvement of power quality is not a new issue rather various authors have introduced some innovative methodologies using these tools [8]. In [9], Dell’Aquila et al. have developed shunt active filter with fuzzy logic estimation of the power devices duty cycle via current control of PWM inverter. This scheme was tested for compensating the real line current drawn by an AC induction motor drive using virtual instrumentation and found that the scheme is able to compensate the harmonics pollution. In [10], authors have proposed a Takagi–Sugeno fuzzy model, derived from input–output data by means of product space fuzzy clustering, to predict the future harmonics compensating current. The developed control model is applied to compensate the harmonics produced by the variable non-linear load. An important point of the work is the use of optimization method to minimize the computational steps, which makes the overall scheme complex. Refs. [11], [12], [13] are devoted to design the conventional PI and fuzzy logic based PI controller, which is part of the active filter. In [14], the work is focused to minimize the harmonics for unbalanced and voltage sag compensation via fuzzy logic for series APF.
The most important observation from the work reported by various researchers for power quality improvement is the design of active power filter under ‘fixed load’ condition, however, in practical life the loads are variable. Hence, there is the need to design an active power filter, which is capable of maintaining the THD well within the IEEE norms [15], under variable load conditions. This paper, therefore, presents a simple, robust fuzzy logic based active power filter to control the harmonics under variable load conditions.
Section snippets
Overview
The basic principle of the shunt active filter was originally presented by Sasaki and Machida [16]. Various types of active power filters have been proposed [17], [18], [19] and are classified based on the type of converter, different configurations used, control methodologies, the economic and technical considerations and selection for specific applications [20], [21]. Among the various topologies of APF developed so far, shunt APF based on the current controlled voltage source type PWM
The proposed fuzzy controller
A fuzzy logic controller (FLC) is designed to take over the work of variable limiter based on look-up table. The new topology of APF with FLC based limiter is shown in Fig. 6(a and b). It has been discussed in previous section that the limiter value varies with change in load current, however, the nature of variation is highly non-linear, and it is difficult to relate it by mathematical expression. Also in practice, the nature of input power to load is not pure sinusoidal rather stochastic in
Case I
Variable load is a seven-step increasing load (with four-stages), connected through circuit breakers (CB). Initial load is (R + jωL), R = 52 Ω, L = 2.5 mH. The load is varied in random steps, (in four-stages) to test the effectiveness of the proposed APF. The APF starts working at 0.1 s. In stage-1 ‘another similar load’ is connected in six steps. Loads are increasing at 0.3, 0.5, 0.7, 0.9, 1.1, 1.3 s and decreasing at 1.5, 1.7, 1.9, 2.1, 2.3 and 2.5 s. In stage-2, the ‘double of the basic load’ is
Conclusions
A simple fuzzy logic based robust active power filter for harmonic minimization under random load variations is presented in this paper. The proposed control technique is found extremely satisfactory to mitigate harmonics and reactive power components from utility current. Proposed APF topology is tested rigorously for non-linear varying loads in different steps and various modes, and it is found that system has a fast dynamic response for such a randomly varying load, and is found effective to
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