Research On Grid-connected Photovoltaic Energy Storage System Based On Virtual Synchronous Generator

With the gradual increase of the permeability of distributed generation units such as photovoltaic power generation in the power grid,the stability of the power system caused by its randomness and intermittency becomes more and more prominent.Virtual synchronous generators(VSG),which can simulate the mechanical and external characteristics of synchronous generator make the distributed power sources have the characteristics of synchronous generator on the large power grid,thus effectively reducing the impact of their access on the large power grid.In this thesis,the grid-connected system of photovoltaic-energy storage based on virtual synchronous generator is systematically studied.The main research work is summarized as follows:(1)Based on the analysis of the structure characteristics of the existing VSG,a two-stage photovoltaic energy storage grid-connected system is built,which is composed of DC subsystem and AC subsystem.The DC subsystem includes photovoltaic array,Boost converter,battery and bidirectional DC / DC converter.The AC subsystem consists of three-phase grid-connected inverter and LC filter.It is a grid connected control structure based on VSG.(2)The VSG modeling and controller design are completed on the gridconnected inverter by deeply analyzing the grid-connected characteristics of the synchronous generator and combining with the DC subsystem.The modeling of VSG is based on the second-order mathematical model of the synchronous generator.At the same time,two important parameters,virtual damping coefficient and virtual moment of inertia,are introduced and the control system principle of the synchronous generator is used for reference.(3)Based on the model,the negative sequence voltage compensation is added to the traditional VSG control,and an improved VSG power-current coordinated control strategy based on the negative sequence voltage command is proposed.The negative sequence voltage command generation module is the core of the negative sequence voltage compensation link.The negative sequence voltage obtained from the negative sequence voltage compensation link and the positive sequence voltage obtained from the traditional VSG control link jointly act on the grid connected inverter to achieve the coordinated control of power and current.Which can effectively improve the operation performance of VSG under the unbalanced state of grid voltage.(4)The simulation experiment is carried out on MATLAB/Simulink platform.The results show that the system can effectively stabilize the DC bus voltage and suppress the output fluctuation of photovoltaic power supply;realize the dynamic adjustment of the grid frequency and voltage,quickly stabilize the frequency and voltage of the system;effectively improve the output power quality of the inverter under the unbalanced state of the grid,and realize the independent or coordinated control of the power and current.