Performance Comparison of Modulation and Sorting Methods for Modular Multilevel Converter

The modulation strategy of modular multilevel converter (MMC) needs to be combined with the sub-module (SM) redistribution strategy to maintain the stability of the SM capacitor voltage while working properly. The submodule (SM) switching frequency and the voltage balancing performance have been compared between two pulse width modulation (PWM) methods, i.e. carrier phase shift PWM (CPS-PWM) and submodule unified PWM (SUPWM), combined with two kinds of sorting methods, i.e. modulation wave compensation method and improved module selection method. The theoretical and simulation results show that when the number of sub-modules and the carrier frequency meet the requirements of ship propulsion, it is difficult for modulation wave compensation method to maintain good performance in different output frequency conditions. Using improved module selection method will increase the switching frequency of the device, but it has a higher performance in voltage balancing.