An MMC optimal balancing control strategy based on SMs’ Capacitor Voltage Characteristics

For the half-bridge modular multilevel converter (MMC), we proposes a new kind voltage optimized balance method to reduce the sub-modules’(SMs’) switching frequency according to the law of SMs’ capacitor voltage change during MMC operation. Firstly, the MMC operation principle is analysed to clarify the limitation of the SMs’ capacitance voltage fluctuation and the optimization objectives such as switching frequency and switching loss. Secondly, according to the SMs’ capacitance voltage level, the SMs are classified into low, middle and high categories, when the bridge arm current is positive, the SMs of the high voltage category should be removal as far as possible, the SMs of the middle voltage category are adopted as much as possible to maintain the original status, and the SMs of the low voltage category are adopted as much as possible to put in which can reduce the switching frequency. Finally, based on the MMC simulation model which is built in MATLAB, the correctness and effectiveness of the strategy introduced in this paper are verified.