A Multiphase Generator Grid Connection Topology Based on MMC Access and Its Control Method

Modular Multilevel Converter (MMC) is a high voltage source converter that can be used in the high-power grid-connected wind turbine system. However, the sub-module capacitors of MMC are suspended. The voltage of the sub-module capacitor has once and twice the fluctuation of the grid frequency. If the fluctuation is larger, the output characteristics of MMC will be affected. For this purpose, a topology structure combining multiphase motor, PWM rectifier, DC-DC isolated bidirectional active bridge (IBAB), and MMC is designed. The corresponding control strategies are proposed for each structure of the system. Under the bidirectional power flow ability of IBAB, the capacitor voltage of the MMC sub-module can be directly controlled. The coordinated control strategy of PI and PR is adopted to suppress the fluctuation of sub-module capacitor voltage. The simulation model of MMC with the Multiphase Permanent Magnet Synchronous Generator (M-PMSG) is built in MATLAB/Simulink. The simulation results show that this method can suppress the fluctuation of the capacitor voltage of the sub-module and improve the voltage level of the system.