Multiphases PWM and Capacitor Voltage Fluctuation of Modular Multilevel Converter

In order to perform high speed power control between a renewable energy source such as sunlight or wind power and the power system, a large capacity self-extinguishing power conversion device is necessary. However, it faces a technical problem to connecting the self-extinguishing devices in series. To solve the above-mentioned problem, MMC (Modular Multilevel Converter) is expected as a means to realize it. In order to operate the MMC stably, it is necessary to suppress the voltage fluctuation of the capacitor of cell capactance. Therefore, the choice of the PWM method and capacitor capacity become important. In this paper, the PWM method to suppress the capacitor voltage fluctuation is selected, and clarified the relationship between the voltage fluctuation and the current, the duty, and the load power factor of the capacitor of cell capactance. In addition, in the selected PWM method, the THD of each output current when the capacitor capacity was changed to find the suitable capacitor capacity was obtained. Furthermore, the PWM method and capacitor of cell capactance to suppress capacitor voltage fluctuation are confirmed by the simulation of MATLAB-Simulink and using a single-phase 3-level MMC circuit, the simulation and the actual machine test were compared.