Research On Active Damping Method Based On High Pass Filter In LCL Grid Connected Inverters

The excessive use of fossil fuels has brought about many problems,such as energy security,environmental pollution.Therefore,renewable energy,represented by solar energy and wind energy,has been drawing more and more attentions.LCL grid connected inverters are often used as the interface between distributed power generation systems units and power system.In order to solve the resonance problem of LCL filter,an active damping method based on capacitor current proportional feedback is often used.When using this damping method,if the resonance frequency of the system is higher than 1/6 sampling frequency,the system will become a non-minimum phase system,which will bring troubles to the parameter design.Also,the robustness of the system using this damping method is not good to the variation of the grid impedance.In order to solve these porblems,we investigated the active damping method based on high pass filter.The mathematical model of single-phase LCL grid connected inverter is established in this thesis,the influence of digital control delays on system stability is analyzed.When using digital single current loop control,the stability range of the system is given.When the active damping method based on capacitor current proportional feedback is used,the physical meaning of active damping method is analyzed from the the perspective of the impedance.The boundary condition for the system to be a non-minimum phase system is obtained,and the constraint conditions on the system stability are given for the minimum phase system and non-minimum phase system,respectively.In order to reduce the possibility that the system is a non-minimum phase system,the HPF active damping method is studied.The analysis of the frequency characteristics of the virtual impedance shows that both the positive and negative dividing frequency of the virtual impedance can be improved by adjusting the cutoff frequency of the HPF,whether the active damping feedback quantity is the capacitor current or the grid inductor current.The influence of control parameters on the stability of the system is analyzed by the discrete root locus method.On this basis,a method of designing the control parameters from the root locus is presented.When considering the robustness of system,the impedance stability criterion of LCL grid connected inverter is given,the influence of the grid impedance on the stability of the system is analyzed.The results show that when the HPF active damping method is used,the robustness of the system to the variation of the grid impedance is better.The control model is built in MATLAB/Simulink,and the simulation results verify the correctness of the theoretical analysis.