Research On Novel Active Power Filter With Low Harmonic Impedance

With the rapid development of power electronics technology,power electronic devices are widely used in distribution networks,resulting in increasingly serious harmonic pollution.In order to improve the power quality of distribution network,active power filter has become an efficient,safe and stable means of governance.However,the traditional shunt active power filter needs harmonic detection and harmonic current compensation,which has high requirements for accuracy and control,and seriously affects the filtering effect.Therefore,it is of great practical significance to carry out research on new active power filters with low harmonic impedance.The traditional shunt active power filter cannot guarantee the filtering effect due to the influence of harmonic current detection accuracy and harmonic current compensation error.In view of this,an active power filter with low harmonic impedance is proposed.By tracking and controlling the fundamental voltage of the common coupling point,the inverter presents a low impedance to the harmonics,and the harmonic current flows into the inverter branch to achieve the purpose of filtering.This method avoids harmonic current detection and reduces the design capacity of the filter inverter.In order to ensure the stable output and low harmonic impedance of the filter inverter,the expression of harmonic impedance of the filter inverter is derived by using the voltage and current double closed-loop control strategy.On this basis,the influence of the inner and outer loop controller on the harmonic impedance is analyzed,and the method of controller parameter design is given.Unbalanced nonlinear load will lead to grid-side current imbalance and grid-side current distortion.Therefore,based on the proposed active power filter with low harmonic impedance,an active power filter suitable for unbalanced conditions is proposed.The filter is composed of a three-phase four-leg inverter.By controlling the filter inverter to track the fundamental positive-sequence voltage of the common coupling point,the fundamental negative-sequence and zero-sequence currents of the grid flow through the inverter branch to solve the grid-side current imbalance.In addition,the harmonic impedance of the control filter inverter is approximately zero,so that the harmonic current flows into the inverter branch to achieve the purpose of filtering.Therefore,in order to realize the symmetrical three-phase voltage output of the filter with low harmonic impedance,a separate control method of the first three bridge arms and the fourth bridge arm is given.The first three arms control the positive and negative sequence components,and the fourth arm controls the zero sequence current.The design method of controller parameters is given.The correctness of the proposed method and parameter design is verified by analyzing the frequency domain characteristics.In order to verify the effectiveness and feasibility of the proposed method,the simulation models of active power filter with low harmonic impedance and active power filter suitable for unbalanced conditions are built by PSCAD / EMTDC simulation software.Compared with the traditional parallel active power filter,the simulation results show that the new active power filter with low harmonics has better filtering performance and does not need harmonic current detection.