Abstract: In order to solve the problems of high frequency and uneven device loss in the existing sub-module equalization control methods of module multilevel converters(MMC), a multi-objective voltage equalization control strategy combining capacitance-voltage equalization and on-time equalization is proposed, and a cost function is designed as a criterion for the selection of sub-modules in the next control cycle, so as to improve the comparability between different targets through normalization processing. Meanwhile, local adjustments are made based on the switching state of the sub-module in the previous cycle, which significantly reduces unnecessary switching actions and achieves smoother sub-module selection. For the voltage measurement delay, a capacitive voltage prediction model based on the switching state is constructed to further improve the voltage equalization accuracy. The method ensures the voltage equalization effect, effectively reduces the switching frequency of the MMC sub-module, makes the on-time of the sub-module more balanced, and optimizes the device loss distribution. Finally, an 11-level MMC model is built on MATLAB/Simulink, and the simulation results verify the effectiveness of the proposed method.

Key words: modular multi-level converters, capacitance voltage balancing, switching frequency, conduction duration balancing, cost function