The Study Of MMC Grid Connected Control In Photovoltaic System

Modular multilevel converter(MMC)has many advantages due to its modular topology.such as easy expansion,good redundancy,low output harmonic content and low switching frequency of power devices which makes it become a research hotspot in recent years.After more than ten years of development and research,MMC has been well applied in the fields of high-voltage flexible HVDC transmission and new energy grid-connected.It also has broad application prospects in the fields of new energy grid-connected,motor-driven,and island transmission.With the expansion of the application field and the inherent characteristics of MMC,there are still many problems to be solved,such as the bridge arm energy balance control,independent control of the sub-module capacitance voltage,the energy storage ratio of the active module and so on.This paper first analyzes and compares several modulation strategies and sub-module equalization control algorithms commonly used by MMC.And an improved staircase wave modulation algorithm is proposed to make it suitable for independent control of sub-modules.After that,the grid-connected control of Photovoltaic-MMC system is studied.Firstly,the topology structure and working principle of the Photovoltaic-MMC system are analyzed and an equivalent model is established.Then,the power and energy flow of the Double-Output system are analyzed,and the power allocation strategy is established,too.Finally,a Double-Output MMC photovoltaic grid-connected control system capable of simultaneously outputting energy to the DC grid and the AC grid is proposed.In order to verify the correctness of the theoretical analysis,a three-phase MMC experimental platform based on layered ideas is designed and built,the design of MMC hardware system and software system are included.The hardware system includes the main circuit design and the design of its main controller(DSP),bridge arm controller(FPGA)and sub-module controller(CPLD).The software system design includes the software design of DSP-FPGA-CPLD.Based on this experimental platform,the grid-connected experiment of the three-phase photovoltaic MMC Double-Output and the experiment of the independent control of the sub-module are conducted.The experimental results show the feasibility of the scheme.