Analysis And Design Of Improved Repetitive Control On LCL-type Grid-tied Inverter System

With the development of renewable energy,grid-tied inverter has become a key interface between distributed power generation system and the grid.With more and more solar and wind power generation system connected to the grid,more attentions are paid to harmonics suppression and system stability of grid-tied inverters.In this thesis,the effects of delay on the system stability of an LCL-type grid-tied inverter with digital control and active damping are introduced.Under a weak grid,the variation of grid inductance leads to the variation of resonance frequency.Thus,it is necessary to restrict the variation of resonance frequency.A compromise is made between resonance frequency and system open loop cutoff frequency under constraints on the resonance frequency.Under these constraints,considering the stability and parameter design of repetitive controller,the parameters of LCL filter,active damping coefficient,and PI controller are designed in the first place.The harmonics components in the grid voltage and the non-linearity,e.g.the dead time,are the main disturbances that deteriorate the quality of injected current.Internal model principle based repetitive control is applied with PI controller,where a plugin structure is used to eliminate the harmonics of injected current.Considering the variation of grid frequency,an improved method based on phase weighting is proposed to keep the harmonics rejection ability without the side effects of detected frequency fluctuation from phase-locked loop.The parameters of repetitive controller,especially the gain of repetitive controller,are optimized to ensure system stability considering the grid inductance variation under weak grid condition.Finally,simulation model by MATLAB/Simulink software and DSP controlled experiment setup have been built to verify the effectiveness of integrated parameters design and phase weighting based repetitive controller.The results validate the frequency adaptation capability of proposed improvement for repetitive controller and the robustness of designed system parameters under weak grid condition.