Research On Current Control Strategy Of LCL Grid-connected Inverter Based On Repetitive Control

In the face of the increasingly serious global fossil energy crisis and environmental pollution,new energy has received a lot of attention,and distributed power generation system has become a research hotspot all over the world.As a bridge between power grid and distributed generation unit,grid-connected inverter has the responsibility to provide highquality AC power to power grid,which needs to be realized by current control strategy.The repetitive control method based on internal model principle stands out among other current control strategies because of its superior control precision and robustness.In this paper,the three-phase LCL grid-connected inverter is taken as the research object,with the aim of improving system stability,effectively suppressing harmonics and improving power quality,the current control strategy based on repeated control under different complex conditions is carried out in-depth research.Firstly,the grid-connected inverter system is designed in this paper.On the basis of the topology structure of the three-phase three-bridge grid-connected inverter based on LCL filter,the mathematical model is established in different coordinate systems,and the main parameters of the inverter are designed and verified according to the relevant constraints of the grid-connected system.Under the premise of stable frequency and no load,aiming at the compensation problem of system delay in digital control system,the grid-connected inverter’s deadbeat repetitive control strategy is designed based on weighted average current(WAC).The WAC method is used to simplify the inverter model with LCL filter from the third order to the first order to facilitate the design of current controller.Based on the reduced order model,the inverter control scheme is designed with the advantages of fast response without beat control and high tracking accuracy of repetitive control.The influence of control delay on system stability is analyzed in depth.The phase advance compensation is designed for the control delay.In the case of frequency fluctuation of the grid,a frequency adaptive repetitive control strategy of grid-connected inverter based on Lagrange interpolation is designed to solve the problem that the traditional repetitive control scheme cannot adapt to the frequency fluctuation of the grid quickly and accurately.In this paper,the defects of repetition control,such as the decrease of tracking accuracy and the weakening of harmonic suppression ability,are analyzed when the frequency fluctuates in the power grid.The polynomial based on Lagrange interpolation method is used to approximate the fractional time delay in the repetition internal model.Therefore,a frequency adaptive repetition controller is designed which can accurately track and suppress all the harmonic frequencies.Based on this control scheme,detailed parameter design is carried out to ensure the accurate control of gridconnected current and effective suppression of harmonic current under the condition of frequency fluctuation of the grid.In the case of distributed power generation system with a large number of nonlinear loads,the harmonic compensation control strategy of grid-connected inverter is designed to solve the problem that the quality of grid-connected current decreases under the traditional repetitive control scheme.The error of weighted current control as an indirect control method with load is analyzed,and a harmonic compensator is designed by deducing the actual relationship between grid-connected current and virtual current.On this basis,the harmonic compensation controller is designed and the parameters are calculated,which ensures that the grid-connected current output of the inverter has no obvious distortion under various working conditions with nonlinear load and harmonic increase,and can ensure the high stability of the system.Based on the above research,the simulation of 10 k W three-phase LCL grid-connected inverter on Matlab/Simulink platform and the experimental study of low power simulated grid-connection of three-phase LCL inverter are carried out.Through analysis and comparison,it is proved that the control strategy of three-phase LCL grid-connected inverter proposed in this paper based on repeated control can ensure system stability in various environments,make the inverter complete rapid and accurate tracking control,and feed high quality electric energy into the grid.