Optimized Capacitor Voltage Balancing Control for Modular Multilevel Converter Based VSC-HVDC System

This paper investigated the capacitor voltage balancing problem of the modular multilevel converter(MMC) and the charge and discharge process of sub-module capacitors.The requirement to reduce the switching frequency of power electronic devices is not satisfied in the conventional capacitor voltage balancing control,since it results in high switching losses.To solve this problem,an optimized capacitor voltage balancing control for MMC based VSC-HVDC transmission system was proposed.The optimized voltage balancing control focuses on the sub-modules whose capacitor voltage exceeds the limits,while the switching states of the other sub-modules are maintained to some degrees by employing the maintaining factor.Therefore,the switching frequency of the power electronic device is reduced.The performance of the optimized capacitor voltage balancing control was evaluated with a time-domain simulation in PSCAD/EMTDC.The simulation results show that the optimized voltage balancing control,compared with the conventional one,can more significantly reduce the switching frequency of power device,without noticeablly increasing the capacitor voltage ripples.