A Capacitor Voltage Balancing Strategy Based on Improved Group-Sort Algorithm for MMC

The balance of sub-module capacitor voltage is the necessary guarantee for the normal operation of modular multilevel converter (MMC). With the nearest level modulation (NLM), an improved group-sort algorithm is proposed in this paper. Based on the average value of arm sub-module capacitor voltages, the sub-modules are divided into two groups, the higher value group and the lower value group. According to the direction of arm current and the number of on-state sub-modules given by NLM, partial sorting in one group is enough to decide the states of the sub-modules in the next cycle, time complexity of group-sort algorithm is reduced obviously and the same equalization effect is achieved. Finally, simulation is carried out based on a 23-level MMC on MATLAB/SIMULINK. The efficiency and correctness of the group-sort algorithm are proved.