Research On Optimization Of Passivity For LCL-type Active Power Filter

Multi-functional converters with harmonic compensation capability and active power filters(APFs)have been demonstrated to be effective to address the micro-grid harmonic pollution issues.In the micro-grid,the converters can work in a stand-alone state,but sometimes the interaction between the converters is likely to deteriorate the stability and dynamical performance of the whole power system,even making the system unreliable.By improving the passivity of the converter is considered an effective way for maintaining the stability of system.However,there has not been found a detailed analysis in the literature on the passivity of converter under different control strategies that gives an overall understanding of the state-of-art research in this area.The actual micro-grid is composed of a multi-functional converter and an APF with different control modes.Therefore,it is of great concern to analyze the passivity of APF under different control strategies and explore different issues that might come across on the stability of the microgrid.Hence,this paper takes the active power filter as the research object.It focuses on the analysis of passivity and how to optimize the passivity of APF which ensure the stability of system when APF feeds in the micro-grid.The main objectives of this paper are highlighted as follows:Firstly,the stability analysis method of the cascade system is introduced briefly,which will lay a foundation for the stability analysis of the grid-connected APF and the multi-APF parallel system.Then,the influence of the converter passive on the stability of the parallel system is analyzed,and the great significance to analyze the passivity of APF is clarified.After comparing the difference between mathematical models of APF in dq-frame and αβ-frame respectively,we found that the later one exists coupling phenomenon,so we focus on the analysis of passivity in αβ-frame.Secondly,the method of Mason’s method is used to establish the admittance model for the APF under the harmonic current compensation mode.The current control loop of the APF is designed to ensure the stability of the closed-loop system.According to the admittance model of APF,we make an analysis of passivity for APF by bode diagram.It can be seen that reducing the active damping coefficient can reduce or even eliminate the non-passive region of APF admittance,so we can improve the passivity of APF.Then,we study the optimization of passivity for multiple APF parallel systems,and come to a conclusion that APF parallelization through different control parameters can improve the passivity of parallel system,and the simulation verifies the correctness of the above method.Finally,the admittance model is established for the APF under harmonic voltage compensation.The controller design of the fundamental current loop and the harmonic voltage loop of APF is carried out.On the premise of ensuring the stability of the closed loop system,analysis of APF with different control parameters is conducted.It can be seen that for APF under harmonic voltage compensation,the passivity of system is improved when the compensation gain is increased.Therefore,the compensation performance and the passive performance of APF must be taken into account when the compensation gain is selected.The simulation shows the correctness of the result.