کاربرد فیلتر توان فعال سری هیبرید (HSAPF) قوی در بهبود کیفیت توان

ABSTRACT Improvement of Power Quality Using a Robust Hybrid Series Active Power Filter

Over the past few years, the enormous increase in the use of non-linear loads, arises many power quality issues like high current harmonics, voltage distortion and low power factor etc. on electrical grid [1]. Hence the proliferation of non-linear load in system generates harmonic currents injecting into the AC power lines. This distorted supply voltage and current causes malfunction of some protection devices, burning of transformers and motors, overheating of cables. Hence it is most important to install compensating devices for the compensation of harmonic currents and voltages produced due to nonlinear load. Traditionally, passive power filters have been used as a compensating device, to compensate distortion generated by constant non-linear loads. These filters [2] are designed to provide a low impedance path for harmonics and maintaining good power quality with an simplest design and low cost. However, passive filters have some demerits like mistuning, resonance, dependence on the conditions of the power supply system and large values of passive component that leading to bulky implementations.

For high-quality power requirements, numerous topologies of active filters i.e. APF connected in series or in parallel (series active filters and shunt active filters) to the nonlinear loads with the aim of improving voltage or current distortion. These filters are the most widely used solution, as they efficiently eliminate current distortion and the reactive power produced by non-linear loads.

But they are generally expensive and have high operating losses [3] [4]. Henceforth to overcome these drawbacks and to improve the compensation performance with reduced cost of the APFs, a novel HAPF topologyIII is introduced by Peng et al. in 1988 [5], in which APF is connected in series with the source as well as non-linear load and PPF connected in parallel with the load, which behaves as power factor correction capacitor is proposed. This topology [6] attracted much more attention to endure high load currents and works as a harmonic isolator between source and non-linear load.

The control strategy is important to enhance the performance of HSAPF. In reality, many articles for hybrid power filter have already proposed advanced techniques to reduce current harmonics created by these non-linear loads. In [7], a linear feedback-feed-forward controller is designed for hybrid power filter. But this controller is not easy for getting both steady-state and transient state performances with the linear control strategy because the dynamic model of HSAPF system contains multiplication terms of control inputs and state variables. Due to the non-linear characteristics of HSAPF, a sliding mode controller is presented in [8].

The sliding mode control is known as an appropriate control technique for controlling non-linear systems with uncertain dynamics and disturbances due to its order reduction property and low sensitivity to disturbances and plant parameter variations, which reduces the burden of the requirement of exact modeling. Furthermore, this sliding mode control also diminishes the complicacy of feedback control design by means of decoupling the system into individual subsystems of lower dimension.

Because of these given properties, the implementation of sliding mode control can be found in the areas of power electronic switching devices. The principle of sliding mode control is defined as to enforce the sliding mode motion in a predefined switching surfaces of the system state space using discontinuous control. The switching surfaces should be selected in such a way that sliding motion would maintain desired dynamics of motion according to certain performance criterion. The conventional control methods, such as Linear-quadratic regulator (LQR) [9] or Linear quadratic Gaussian (LQG) servo controller [10] for linear systems, are required to choose proper switching surfaces. Then, the discontinuous control needs to be chosen such that any states outside of the discontinuity surface are enforced to reach the surface at finite time. Accordingly, sliding mode occurs along the surface, and the system follows the desired system dynamics. The main difficulty of hardware implementation of classical sliding mode control method is chattering.

Chattering is nothing but an undesirable phenomenon of oscillation with finite frequency and amplitude. The chattering is dangerous because the system lags control accuracy, high wear of moving mechanical parts, and high heat losses occurs in electrical power circuits. Chattering occurs because of unmodeled dynamics. These unmodeled dynamics are created from servomechanisms, sensors and data processors with smaller time constants.

In sliding mode control the switching frequency should be considerably high enough to make the controller more robust, stable and no chattering because chattering reduces if switching frequency of the system increases. The application of sliding mode controller in power converter systems for example in HSAPF, a natural way to reduce chattering is increasing switching frequency.

However, it is not possible in case of power converters because of certain limitations in switching frequency for losses in power converters, for which it results in chattering. Therefore, this chattering problem cannot blame sliding mode implementation since it is mainly caused by switching limitations. In [11], it is shown that the chattering exponentially tends to zero if the relative degree of the system with actuators or sensors is two.

The relative degree of HSAPF system is two. Because of this relative degree of HSAPF system and also for these obstacles in classical sliding mode controller, this research paper proposed a new controller i.e. sliding mode controller-2. This proposed controller suppressed chattering and enhance the performance of HSAPF. This controller is completely new for this topology of HSAPF system. The recent research paper [12] focuses on carrier based PWM (CBPWM) for HSAPF topology. But in some cases the CBPWM based HSAPF may not be completely measurable in most of the real-world situations.

In case of CBPWM, power system perturbations have not been taken into consideration and also the presence of a time delay at the reference tracking point gives rise to a slow response of the overall system. Thus, tracking error is not reduced effectively and stability of the system is minimally improved. To overcome this, a SMC- 2 controller is proposed for voltage source converter (VSC). The idea behind this controller is to achieve gain stability, perfect tracking and distortion free current and load voltage. In view of above mentioned issues, we give more emphasis on the development of robust controller with a faster reference tracking approach in HSAPF, which permits all perturbations such as load voltage distortion, parametric variation of load, source current distortion and supply voltage unbalance so that compensation capability of the HSAPF system can be enhanced.

This paper is organized as follows. In section II, the description of schematic of system topology and hardware modules of three phase HSAPF model is explained. Section-III depicts the averaged modelling of HSAPF system. Section-IV disclose the controller design for HSAPF. Section-V depicts the simulation as well as experimental results for harmonic compensation using HSAPF. Section-VI presents the conclusions of this work.