Stability Analysis Of Grid-Connected Converter Under Weak Grid And Research On Feedforward Control Strategy Of Grid Voltage

The application of grid-connected converters in new energy generation,direct current transmission and power quality management is increasingly widespread.However,due to the existence of grid impedance,there have been many cases of instability oscillation caused by the interaction between grid-connected converters and the grid in recent years.This problem has become one of the important factors affecting the stable operation of the grid-connected converter system.In this paper,the influence of grid voltage feedforward on the stability of the grid-connected converter and the stability of the system with multiple parallel machines are studied.Aiming at the influence of grid voltage feedforward on the stability of grid-connected converter under weak grid,Firstly,the impedance model of grid-connected converter with grid voltage feedforward is established,and then the influence mechanism of grid voltage feedforward on the stability of grid-connected converter under weak grid is analyzed by impedance ratio criterion.It is pointed out that the fundamental reason that feedforward control in weak grid will affect the stability of the system is the delay introduced by voltage sampling filter and digital control.Aiming at the problem of low adaptability of grid-connected converter to weak grid with traditional grid voltage feedforward,The paper proposed a grid voltage feedforward method based on all-pass filter,By introducing all-pass filter into the feedforward channel,the feedforward voltage is corrected in advance to reduce the adverse effect of delay on the stability of the system,and the adaptability of the grid-connected converter to the weak grid is significantly improved.Furthermore,on the basis of analyzing the influence law of the parameters of all-pass filter on the weak grid adaptability of the system,the selection method of the parameters of all-pass filter is given.The stability of multiple parallel grid-connected converters is studied.Based on the impedance model of the grid-connected converter system,the corresponding equivalent characteristic equation is derived.By analyzing the influence of filter parameters and the number of shunt units on the equivalent characteristic equation of the system,the relationship between the inductance of the filter and the number of shunt units of the converter and the weak grid adaptability of the grid-connected converter system is studied quantitatively.The results show that under the same design principle of controller parameters,the weak grid adaptability of the grid-connected converter system is proportional to the ratio of the filter inductance and the number of parallel converters.Finally,the simulation model under PLECS and the experimental prototype are built,the simulation and experimental results verify the correctness of the theoretical analysis and the effectiveness of the proposed method.