| With the development of society,restricted by energy shortage and environmental pollution,distributed power generation based on renewable energy is gradually becoming popular.At the same time,it also brings new problems of distributed energy grid connection:the long-distance energy transmission causes the grid impedance to increase,and the strength of the connection to the main grid is weak;weak grid connection generates a variety of harmonic interference,frequency deviation,voltage flicker and other problems As a result of harmonic distortion,grid-connected power quality cannot be guaranteed.In order to make the grid-connected inverter system more suitable for weak grid environments,targeted optimization and improvement of the inverter is required.First,the background of new energy distributed power generation and the structure and research status of grid-connected inverters are introduced;the characteristics of weak grids and their impact on grid-connected inverter systems are explained;the key technologies and hot issues of grid-connected VSC are discussed Explain;establish the impedance model of the grid-connected VSC system,model and analyze each link of the grid-connected VSC,introduce the principle of the impedance criterion,and derive the stability criterion of the grid-connected VSC system;The influence of the phase-locked loop bandwidth,damping coefficient and LCL filter parameters on the stability of the inverter is analyzed.Secondly,in order to solve the problems of increased harmonic interference and voltage and frequency fluctuations leading to PLL output distortion under weak grid connection conditions,a PLL based on lead compensator and frequency estimation is proposed: a prerequisite for maintaining good filtering performance of MAF-PLL Next,the phase advance compensator is added and the frequency estimation loop is introduced,so that the improved PLL still has good phase-locking accuracy when the frequency fluctuates in a weak grid.In view of the large impedance and characteristics of the power grid under a weak grid,an improved Design program with significant filtering effect under changes: This program discusses the performance of the LCL filter when using active lag network and capacitor current feedback for active damping,and analyzes passive damping.This method enables the LCL filter to achieve enhanced stability and robust results by selecting the switching and resonant frequencies,the grid and converter inductance,and the ratio between the filter capacitance and the total inductance.An estimation method for total harmonic distortion(THD)of grid current is proposed.Simulation and experiments verify the effectiveness of the method.Finally,in order to verify the applicability of the optimized and improved gridconnected VSC system in the weak grid,the stability analysis of the improved VSC system is performed according to the Nyquist stability criterion,and according to the optimization scheme proposed earlier,Select the appropriate LCL filter parameters and add an improved PLL to the grid-connected system.An experimental platform was built,and the effectiveness of the proposed scheme under weak grid conditions was proved through comparative experiments. |
