Research On Grid-connected Inverter Control Strategy Considering The Influence Of SOGI-PLL In Weak Grid Case

With the increasing penetration of renewable energy into large grid,power quality and stability issues have attracted more attention.Generally,distributed generation is scattered far away from the load center,and the equivalent impedance of grid can not be ignored,so the grid pre sents the characteristics of weak grid.Grid-connected inverter is the energy conversion interface between renewable energy generation unit and grid,which mainly relies on phase locked loop(PLL)to obtain phase information,so PLL is the key to achieve r eliable grid connection.Due to the access of nonlinear devices(near the common coupling point of grid-connected inverter),background harmonics exist in the grid voltage.Therefore,this paper taked single-phase LCL type grid-connected inverter as the research object,and from the point of view of grid-connected system stability and voltage synchronization technology,putted forward corresponding improvement strategy to ensure the stable operation of grid-connected inverter in weak grid.Firstly,the mathematical models of single-phase LCL grid-connected inverter and two kinds of typical PLLs were established,and the influence of two kinds of typical PLLs on grid-connected inverter system in weak weak grid was analyzed by using impedance stability criterion.Based on the above analysis,the influence of each parameter of PI regulator in PLL based on second order generalized integrator(SOGI-PLL)on the stability of grid-connected system was studied by drawing Bode diagram and Nyquist diagram.Then,the simulation model built in Simulink was used to verify,and it was proved that the theoretical analysis was consistent with the simulation results;At the same time,according to the analysis results,a stability based PLL parameter optimization method was pr oposed.The theoretical and simulation results showed that the method can meet the phase margin requirements of the system on the basis of considering the dynamic performance of PLL.The grid impedance of the weak grid can not be neglected,this paper analyzed the suppression ability of grid voltage feed-forward control strategy to grid current distortion,and focused on the influence mechanism of introducing typical proportional feed-forward strategy to grid-connected system stability.On the basis of considering the adaptability to the grid impedance,combined with the characteristics of SOGI,the feed-forward was modified,that is,an improved proportional feed-forward control strategy based on PCC point voltage correction was proposed to improve the robustness of the system to the grid impedance changes.Based on the problem of harmonics in voltage under the condition of weak grid,this paper analyzed the influence of voltage waveform distortion on the phase-locked accuracy of traditional SOGI-PLL,optimized its structure,and proposed an improved SOGI-PLL based on CDSC algorithm.Finally,the correctness of the theoretical analysis and the effectiveness of the improved control strategy were verified by the analysis of the simulation model.