Active Damping Research Based On Grid Impedance Estimation And Notch Filter For LCL Filters

With the development of the human society,the world energy trend has gradually changed from the fossil energy system to sustainable energy era based on renewable energy system.In order to achieve the connection between the renewable energy and the actual power grid,grid connected inverter has an important role in the grid connected systems.At present,the research focus of grid connected inverter is focused on the topology,control strategies and filtering methods.In this paper,the influence of the grid impedance on the stability of the system with LCL filter is studied.Because of the resonance peak of LCL,an active damping method based on an adaptive notch filter and grid impedance is proposed.At first,the three-phase inverter mathematical model in the static coordinate system and the rotating coordinate are derived,and performance of LCL filter and basic design process of LCL filter.Moreover,three harmonic injection modulation algorithm is introduced and applied in this paper.In addition,a control principle based on three connected inverter is proposed,and the influence of LCL filter parameters and controller parameters on the stability of the system is studied,and the concept of critical frequency in LCL filter system is introduced,which has a very important significance in the system stability,and this concept is also the foundation of designing the controller.A grid-connected inverter system is also built in MATLAT/Simulink.Besides,to solve the potential resonance problem because of the varying grid impedance,the two identification methods of grid impedance are proposed.In this paper,based on the grid impedance model,the recursive least squares method and the extended Kalman filter method are used to identify the grid impedance.After getting the identification result,active damping method based on notch filter is also introduced.The simulation results by MATLAB showed that the proposed method is effective.Finally,a three-phase inverter experimental platform based on dSPACE is built.