Research on Strategy for Capacitor Voltage Balancing of Modular Multilevel Converter

In modular multilevel converter, the balancing of capacitor voltage in each modular is urgent to be solved. On the basis of analyzing the topology and operation principle of MMC, a strategy based on PI controllers for capacitor voltage balancing is proposed in this paper by using carrier phase-shifting SPWM. The aim of control strategy is to balance capacitor voltage and to control loop bridge-arm current. By adjusting the waveform of the modulation signal, the charge/discharge time of each capacitor is modified in order to make capacitor voltage keep consistent and track the given value. No need of sequencing measured capacitor voltage, the control strategy decrease switching frequency of IGBT and system loss. In the end, the MMC system is verified on Matlab /Simulink simulation platform, and the results show that the control strategy is correct and effective.