Research On Current Control Technologies Of Grid-tied Inverter

Distributed energy resource has received extensive attention due to its low pollution and regeneration.As the most important part in distributed generation system,inverter affects power quality of the grid.Therefore,the quality of output current of the grid-tied inverter is strictly restricted and the current control strategy of grid-tied inverter has become a research hotspot.LCL filter has a good ability to suppress high frequency harmonics,of which the resonance peak affects stability of control system.Active damping method suppresses the resonance peak by state feedback,but sampling time and PWM calculation time affect the performance of active damping.In this paper,the influence of control delay on active damping is analyzed,and the equivalent of control delay is added to the discrete state space equation.Using observed value instead of actual value as the feedback of the active damping,a discrete state observer is designed.The influence of delay on the active damping is compensated by using the lead compensation observer,and a current-sensor is omitted.Among current control methods of grid-tied inverter,split-capacitor method is an indirect current control structure which simplified the controller design.Basic principle of split-capacitor method is introduced in this paper.The resonance problem of split-capacitor method is analyzed,and the passive damping strategy is studied.UDE(Uncertainty-Disturbance-Estimator)control strategy is proposed in the paper.Basic principle and control model is introduced.UDE control model based on equivalent control object of split-capacitor is derived.The parameter design method of UDE is studied,and it is pointed out that the control parameters of UDE directly correspond to the closed-loop control bandwidth,which solved the problem that parameters of traditional PI control are difficult to adjust.In order to solve the problem of low frequency harmonics caused by grid voltage in the case of weak grid,full voltage feed-forward is derived.As for control methods of grid current,repetitive control and internal model principle is introduced.Compensation strategy of internal model and controlled object is studied for stability problem of repetitive control in this paper.The stability and anti-interference of repetitive control with compensation are analyzed.Aiming at the dynamic performance deficiency of repetitive control,compound control of PI control and repetitive control is discussed.The series and parallel structure of compound control are introduced,of which the steady-state performance is analyzed.Finally,simulation and experiments are carried out on the MATLAB/Simulink simulation modeland the 2kW grid-tied inverter experimental platform.The simulation and experimental results verify the correctness of the theoretical analysis.