Research On Energy Storage Type Photovoltaic Grid-connected Inverter System Based On MMC

With the vigorous development of photovoltaic energy and the continuous development of power grid to high-voltage and high-power,modular multi-level converters have been widely used in high-voltage grid-connected systems with their advantages of easy cascade expansion and high modularity.In addition,photovoltaic power is very sensitive to weather conditions,directly connected to the grid will seriously affect the safe and stable operation of the power grid,in order to reduce the instability of photovoltaic power generation on the power grid active impact,install energy storage device in the grid connection process,thereby reducing the rate of change of active power.Based on this background,this paper proposes a photovoltaic grid-connected inverter based on supercapacitor energy storage MMC,and the main research contents are as follows:1.Analyze the status of MMC photovoltaic inverters and energy storage devices,and the topology of the energy storage type photovoltaic grid-connected inverter based on MMC is proposed,which includes two parts: MMC grid-connected inverter and LLC-SC energy storage module.In order to improve the energy storage efficiency of the inverter,the LLC resonant converter is used to charge the supercapacitor,which can not only realize soft switching,reduce switching losses,but also realize the bidirectional flow of energy between the supercapacitor and the MMC inverter,which helps to improve the stability of grid connection.2.By establishing the mathematical model of MMC photovoltaic grid-connected inverter and LLC-SC energy storage module,the basic principles of the system are analyzed.The principle of the LLC-SC energy storage module storing excess energy when the MMC inverter is connected to the grid,and the influence of the energy storage module on the MMC grid-connected inverter are studied,which lays the foundation for the research of system control strategy.3.In the section of system control strategies,the carrier phase shifting control strategy is designed to ensure the three-phase alternate required for grid connection,the suspension capacitor voltage sharing control strategy of the MMC photovoltaic grid-connected inverter is designed to ensure the balance of capacitance voltage of each sub-module;the dual-loop PID charging control strategy is designed for the LLC-SC energy storage module,to ensure the charging current is tracked by the charging voltage to the reference value within the defined threshold,which protects the energy storage module circuit.4.In MATLAB/Simulink,a 15-level MMC-SCES simulation model is built,according to the simulation results,when the system is stable,the system can stably output the three-phase alternating current for grid connection;the supercapacitor voltage in the LLC-SC energy storage device is stable,and the LLC resonant converter realizes soft switching,which improves the energy storage efficiency of the system,and preliminarily verifies the feasibility of the grid-connected inverter system topology and control strategy.5.Completed the prototype design of the four-stage MMC-SCES grid-connected inverter system,including the main circuit design of key hardware circuits,parameter calculation,device selection,and STM32-based program design.The hardware circuit part mainly includes the design of the main circuit of MMC,the design of the LLC-SC energy storage module and the drive control circuit.The software part includes the overall design of the system program,the program design of carrier phase shift control,and the program design of dual-loop PID control.The key hardware circuits and outputs of the system are tested,and the results show that all parts of the system are working normally,which further verifies the feasibility of the proposed scheme proposed in this paper.