Research On Control Strategy Of Inverter Based On Virtual Synchronous Generator Under Unbalanced Grid

Inverter plays an importa nt role in the stable operation of a new energy distributed generation system.Because the power electronic device,traditional control method makes inverter have fast response and poor anti-interference ability,lacking the inertia of maintaining voltage and frequency and the damping characteristic of dealing with disturbance.To avoid the disadvantages of traditional control method,this paper established the virtual synchronous generator(VSG)control model to solve the difficulty for the load or grid vo ltage frequency effective support.As for the common VSG control strategy causing the shock and distortion of current when the grid is unbalanced,the improved VSG control strategy is put forward.This paper focuses on the VSG control strategy of the grid connected inverter.As for the shortcomings of the traditional control method of the inverter,the inertia and damping characteristics of the synchronous generator are applied to the VSG control method to build VSG model.The main structure of VSG controller is established using the two order mathematical equation of synchronous generator,while the virtual exciter and the virtual governor are constructed using the adjustment for the voltage amplitude and frequency of synchronous generator.The small signal model is established to analyze the influence of the main circuit parameters and the inertial parameters on the output power of the inverter.In order to design the parameters of VSG system,the circuit constraints is used to design the parameters of filter,and the small signa l equation is used to design the parameters of controller.Because of the current distortion and power oscillation caused by unbalanced grid,the positive sequence current controller is put into the traditional VSG control strategy to eliminate the negative sequence component of the output current,so the current can follow with positive sequence current and the oscillation amplit ude of output power is decreased.The VSG control system is based in the Matlab/Simulink platform to make the inverter run stability when it departs from grid or connects grid.the response characteristics of VSG control are studied to verify the effect of virtual damping and virtual inertia in maintaining system stability.C omparing the output characteristic of the tradit ional VSG control and improved VSG control while the grid is unbalanced,the positive sequence current control strategy is analyzed with the improvement effect of the output power quality.Finally,based on the experimental platform of three-phase grid connected inverter,the VSG control strategy is tested under different conditions to prove the validity and correctness of the proposed control strategy.