Abstract:  ABSTRACT：  Modular multilevel converter （MMC） has a widely application and research because of its advantages of high output level, low switching frequency and good waveform quality, etc. The voltage balance of sub-module capacitor is one of the key research directions of MMC. While traditional voltage balancing method will greatly increase the frequency loss of switching element and controller operation with the increasing of the number of sub-modules. This paper puts forward an optimized capacitor voltage balancing strategy for MMC based on improved quick sort algorithm, which can trigger or maintain control scheduling module by real-time monitoring the voltage of sub-module capacitor, setting the discrete degree index of sub-modules voltage. At the same time, through the optimization of the sorting algorithm, the calculation efficiency of controller in multi-modules is greatly increased and the hardware requirements is reduced. Finally, this paper builds the MMC-HVDC model in PSCAD/EMTDC simulation platform to simulate and verify the algorithm. The simulation results show that the improved voltage balancing control method can significantly improve the running speed and reduce the switching frequency of sub-module without noticeablly influencing the system performance. 

Key words:  modular multilevel converter （MMC）, high-voltage direct current, capacitor voltage balancing, switching frequency, quick sort