Improved Control of Capacitor Voltage Balancing in Modular Multilevel Converter Submodules

Abstract

An improved incremental capacitor voltage balancing control strategy is proposed to solve the problems of large switching frequency, high complexity of the bubble method sorting algorithm, and exceeding the limit of the capacitor voltage fluctuation rate in the traditional capacitor voltage balancing control of modular multilevel converter. First, the radix sort algorithm is used instead of the traditional bubble sorting algorithm to sort the submodule capacitor voltage to improve the sorting speed. Second, the incremental capacitor voltage balancing algorithm is used instead of the traditional voltage balancing control strategy to reduce the switching frequency. Individual submodules have capacitive overvoltages when power quality requirements are high and the unbalance threshold is set high. Therefore, based on the incremental voltage compensation control strategy, the upper and lower limit control is introduced to reduce the voltage deviation during incremental switching and to solve the problem of the capacitor voltage fluctuation rate exceeding the limit. Finally, the simulation verifies the effectiveness of the control strategy and realizes the stable operation of the system at a low switching frequency.

Keywords

• Base sequencing
• MMC equalization control
• MMC mathematical model
• optimized incremental pitching
• upper and lower limit control