1. State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources(North China Electric Power University), Beijing 102206, China; 2. State Key Laboratory of HVDC(Electric Power Research Institute of China Southern Power Grid Company Limited), Guangzhou 510080, China

The modular multilevel converter(MMC)is applied in the DC transmission system all over the world, while the capacitor voltage balanced control strategy stands out for its difficulty and complexity. This paper presents a sorting algorithm based on a comparison between two groups, called the OF sorting algorithm. The maximum time complexity of the method is N-1, but the algorithm is unlikely to be practical because of its limited range of application. Therefore, a new hybrid sorting algorithm for capacitor voltage balanced control of MMC is proposed to reduce the time complexity and device switching frequency. The effectiveness of the proposed algorithm under various conditions is verified based on the Xiamen MMC-HVDC project. The time complexity of the new hybrid sorting algorithm is 1. 45N, which is better than that of other existing MMC sorting methods for about half reducing the switching frequency compared with the traditional bubble sorting method.