Research On Distributed PV System Based On Modular Multilevel Converter

The global fossil energy crisis intensified,promoting the development of photovoltaic power plants in China.However,when the commonly used photovoltaic inverter is used in large-capacity distributed photovoltaic power plant,there are many problems such as the number of inverters running in parallel being too big,the whole system structure being too complex and so on.In this paper,the modular multi-level converter(MMC)is applied to distributed photovoltaic system,which achieves good grid-connected effect.Firstly,the development status of distributed photovoltaic generation is introduced,and the application of MMC converter in photovoltaic generation is summarized.The output characteristics of the photovoltaic cell are analyzed,and the advantages and disadvantages of the commonly used maximum power tracking algorithm are introduced.Then,the dual-mode MPPT algorithm is developed,which has the advantages of fast tracking speed and high steady-state precision.Secondly,the topology structure,working principle and mathematical model of the MMC converter are introduced,and the production mechanism of the internal circulation is studied.The advantages and disadvantages of the common modulation method of MMC are analyzed.It is proved that the n+1 level CPS-SPWM modulation method is helpful to reduce the MMC Circulation.Then,the topology structure of the modular multi-level PV system is introduced,and the voltage equalization control of the sub-module capacitance of the MMC converter is improved to make it possible to effectively suppress the inter-phase circulation of the MMC converter and balance the capacitance voltage of each sub-module of the MMC.The PI control,quasi PR control and bridge arm current control are studied deeply,and the grid control strategy of MMC converter is worked out.Finally,the simulation model of the whole system is built by Matlab/Simulink,and the main parameters of each part of the system are calculated.The validity of the dual-mode MPPT algorithm,the MMC's capacitance voltage equalization control strategy and its grid-connected control strategy are simulated,the simulation results show the effectiveness of the modular multi-level PV system topology,and the grid-connected control strategy based on the bridge arm current control has better control effect in grid-connected current and internal circulation.