A Switching Times Reassignment-Based Voltage Balancing Strategy for Submodule Capacitors in Modular Multilevel HVDC Converters

The capacitor voltage balancing algorithm (VBA) is an important issue to maintain the regular operation of modular multilevel converter (MMC). However, it requires a higher switching frequency to obtain a better balancing performance. In this paper, the VBA based on the reassignment of switching times is proposed to improve the balancing performance of capacitor voltages with a low operating frequency. The mechanism of the reduced switching frequency voltage balancing algorithm (RSFVBA) is presented. Based on that, the relationship among switching times, switching frequency, and capacitor voltage deviation is deduced. And then, the voltage deviations in the positive and negative intervals of the arm current are analyzed in detail. The RSFVBA proposed in this paper adjusts the switching times of SMs in different intervals of arm current to reduce the voltage deviation. Moreover, the proposed RSFVBA can also provide a low operating frequency because the reassignment of switching times does not change the total switching times in a line cycle. The feasibility of the proposed strategy is verified by simulation results in the professional tool Matlab/Simulink.