Comparative Study between Shunt Active Filter and Hybrid Filter

— This paper presents an comparative study between shunt active filter and shunt hybrid filter based on configuration system and harmonic isolation. The harmonic isolator is based on Self Tuning Filters (STF) (it can be tuned at any frequency). The study of the active filter control is divided in two parts. The first part deals with the harmonic isolator which generates the harmonic reference currents. The second part focuses on the generation of the switching pattern of the IGBTs of the inverter by the modulated hysteresis current controller. The studied hybrid active filter consists of an active filter and three phase tuned LC filters for 7 th harmonic frequency. The nonlinear load is a diode rectifier feeding a (R, C) parallel load. The use of STF's simplifies the control scheme by reducing the number of extraction filters. The use of STFs instead of classical extraction filters allows extracting directly the voltage and current fundamental components. The effectiveness of this study has been verified by computer simulation and comparing the results.


I. Introduction
The power system must be free from harmonics and which will lead to numbers of benefits. A clean network has less strain on appliances and their lifespans are lengthened.
Nowadays with the advancement of technology, the demand for electric power is increasing at an exponential rate. The performance of the end user equipment is heavily dependent on the quality of power supplied to it. But the quality of power delivered to the end user is affected The main affect caused by these problems is the presence of harmonics. This leads to the overheating of the equipment, insulation failure and over speeding of induction motors etc [1,2]. The solution to overcome these problems is to filter out these harmonics.
There are many filters topologies present in the literature. Filters can be classified into three types (passive, active, hybrid) [3,4]. This study presents and compare two types of filters and the control strategy to control the filter in such a way that the harmonics are reduced.
The hybrid filter which is a combination of active filter and shunt passive filter is studied.
While the passive filter mitigates load produced harmonics the active filter helps to enhance filtering properties of passive filter. This ensures a great diminution of the rating of the active filter guiding to an economical practical system.

A. System configuration
The harmonic currents are the source of adverse effects for many types of equipment's such as heating in distribution transformer, perturbation of sensitive control equipment's and resonances with the grid. Many solutions have been studied in the literature to mitigate the harmonic problems, such as filtering (passive, active, and hybrid) with various topologies (shunt, series or both). These solutions have been proposed to improve the power quality of the AC mains.
The passive filter which is the simple way to eliminate the harmonic currents, has many drawbacks such as the series or parallel resonance with the system impedance and it cannot completely eliminate all of the harmonic currents [5].
The active filter which is developed and widely used to overcome to the drawbacks of the passive filters and improve power quality. As known, the performances of the active filter ‫األسمرية‬ ‫الجامعة‬ ‫مجلة‬ : ‫ا‬ ‫والتطبيقية‬ ‫األساسية‬ ‫لعلوم‬ ( ‫المجلد‬ 2 ( ‫العدد‬ ) 2 ‫ديسمبر‬ ،) 2017 ‫م‬ 67 system mostly depend on the accuracy of the harmonic isolation and on the current control technique used to generate the switching patterns for the inverter [6]. Hysteresis and PMW techniques are usually used.
The hysteresis strategy allows faster dynamic response and better robustness according to the variation of the non-linear load. Nevertheless, with this strategy, the switching frequency is not constant. In this paper, to fix the switching frequency, we studied a three-phase non-linear current controller resulting from the hybridization between a PWM current controller and a hysteresis current controller, so called "modulated hysteresis controller". More, a focus is made on the improved harmonic isolator based on Self Tuning Filter (STF) [5,7].

This filter (STF) is used instead of classical harmonics extraction based on High Pass filters or
Low Pass Filters. The effectiveness of the proposed method is verified by computer simulation.  The non-linear load is a three-phase diode rectifier feeding a RL load. This load will generate harmonics currents in the supply system.
The control method is divided in two parts. The first one consists in the harmonic isolator which generates the harmonic reference currents and is implemented into a DSPACE DS1104 development board. The second part is the generation of the switching pattern for the inverter by using an analogue modulated hysteresis current controller.Simulation results

B. System configuration
The cost of active filters in industrial applications could be very high. The power rating of power converter for active filters is very large. For these reasons, the application of active filters for power system is cost limited [8]. In the last few years, many different topologies of hybrid active filters with various control strategies have been proposed in the literature as lower cost alternatives to active filtering for harmonic compensation. Nowadays, hybrid active filters are considered as one of the best solutions for improving power quality [9]. Fig.3 shows the hybrid active filter topology studied in this paper.
The hybrid filter consists in a three-phase LC filter tuned for the 7th harmonic frequency, connected in series with an active filter without any transformer. The passive filter absorbs harmonic currents generated by the load whereas the active filter improves filtering performances of the passive filter. The associated control scheme combines a feedback and feedforwad loop [10,11].

IV. SIMULATION RESULTS
The efficiency of the proposed control scheme has been examined by computer simulation using MATLAB and associated toolboxes "Simulink" and "Power System Blockset". The parameters of the system are given in Table II. The simulated hybrid filter consists in a three-phase LC filter tuned at the 7 th harmonic frequency, connected in series with a three-phase active filter based on MOSFETs power semiconductors. The nonlinear load is a diode rectifier feeding a RC parallel load. Table III shows THD values for different choices of the gain value K for the feedback loop of the active filter. The optimal value is equal to 20 which provides better filtering characteristics. The major aims of this paper were to compare the efficiency of the shunt active filter and hybrid filter. As presented in Table III, the THD of the non-linear load L i is equal to 27.8%

V. CONCLUSION
This paper has discussed the configuration system, control and performances of a shunt active power filter and hybrid filter. The control of the active filter was divided in two parts, a digital one realized by the DSPACE system to generate the reference currents, , the modulated hysteresis current controller used for the switching pattern generation. STF used instead of classical extraction filters for both grid voltages and load currents. The proposed hybrid filter, which is composed of an active filter and three phase tuned LC filters. The simulation results have demonstrated and conforted the major advantages of using HSFs in the filter control.
This paper describes a comparative study between shunt active power filter and shunt hybrid power filter. Simulation results proved that performance of the hybrid filter is much better than the active filter. The DC link voltage of hybrid filter is twice more than that of active filter. The combined system of passive and an active filter has some features as (Source impedance no longer governs the filtering characteristics, the active filter has the ability to dump the parallel and series resonance between the source and the passive filter, the required rating of hybrid filter is much less than a conventional active filter used alone). Table IV summarize some major advantages of hybrid filter over the active filter.