According to the existing optimal voltage balancing algorithm based on capacitor voltage sorting

a precision control method of sub-module switching frequency for modular multilevel converters(MMCs)is proposed. First

by introducing the double holding factors

unnecessary repeated switching of the insulated gate bipolar transistor(IGBT)is avoided and the switching frequency is significantly reduced

which decreases the switching losses of converters. Secondly

a proportional-integral(PI)controller is proposed

which is able to accurately control the switching frequency of sub-modules while decreasing the frequency. Third

to validate the effectiveness of the proposed method

a 101-level point-to-point MMC simulation model is developed in PSCAD/EMTDC. The simulation results show that the method is able to accurately control the switching frequency of sub-modules on the premise of guaranteeing stable operation of the system

which proves the rationality and effectiveness of the proposed method. This work is supported by National Natural Science Foundation of China(No. 51177042)and National High Technology Research and Development Program of China(863 Program)(No. 2013AA050105).