Optimization of Capacitor Voltage Control Strategy for MMC Sub-module

Abstract

Modular Multilevel Converter (MMC) is widely studied and used because of its own advantages such as high output level number, low switching frequency and good waveform quality. The voltage equalization problem of sub-module capacitors is one of the key research directions of MMC. In the paper, a sorting method of inter-group comparison in two groups, called OF sorting method, is proposed first. The maximum time complexity of this method of sorting is N-1, but the use of this method is limited. Therefore, a new MMC hybrid sorting voltage equalization algorithm is proposed on this basis. This paper proposes a capacitor voltage equalization control strategy based on the combination of improved equalization sorting algorithm and improved insertion sorting algorithm, which reduces the computational effort by monitoring the sub-module capacitance voltage in real time, setting the dispersion index between sub-module voltages, and then controlling the opening and holding of the sorting module to reduce the number of elements to be ranked one by one. Finally, the MMC model is built in Matlab/Simulink. The simulation results show that the improved equalization control strategy can effectively reduce the computational effort and reduce the switching frequency of sub-modules on the basis of better maintaining the capacitor voltage balance.