Objectives  The Modular Multilevel Converter (MMC) has better speed regulation performance in AC transmission system because of its unique modularization and low switching frequency. In view of the problem of the capacitor voltage fluctuation of the sub-module and the overlarge common-mode voltage caused by the traditional high-frequency injection method in the marine medium-voltage low-frequency operation mode of the MMC, an asymmetric full-bridge flying-capacitor MMC modified topology was proposed for marine electric propulsion systems.

  Methods  Firstly, the working principle of the improved circuit topology and the cause of capacitor voltage fluctuation under low-frequency working conditions are introduced; then, the corresponding control scheme was designed by combining the square wave injection method and the circulation injection method with cutoff frequency. Finally, based on the Matlab/Simulink simulation platform, a 6 kV/36 MW permanent magnet synchronous propulsion motor model with propeller load was built to simulate the starting condition of the graded forward vehicle of the marine electric propulsion system.

  Results  The simulation results show that this method can effectively reduce the percentage of capacitor voltage fluctuation from 16% to less than 8%, and no common-mode voltage injection.

  Conclusions  The study in this paper can provide reference for the design of the frequency converter for marine medium-voltage direct-current electric propulsion systems.