A Comparative Study of Modulation Methods of the Modular Multilevel Converter for Ship Propulsion

A modular multilevel converter (MMC) which can unlimitedly increase its voltage and power levels in the same condition of structured complexity has great potential in the frequency modulation and speed regulation of a ship high-power electric propulsion system. According to the requirement of application, two pulse width modulation (PWM) methods, or carrier phase shift PWM (CPS-PWM) and submodule unified PWM (SUPWM), are selected, and the differences in harmonic level, submodule (SM) switching frequency and program complexity caused by the two methods have been compared. The theoretical and simulation results show that when the number of sub-modules and the carrier frequency meet the requirements of ship propulsion, both of them are equal in harmonic level. And the difference in CPS-PWM SM switching frequency is smaller than that in SUPWM, and the complexity of SUPWM program is lower than that of CPS-PWM program.