Optimization control of sub-module capacitor voltage balance for modular multilevel converter based on the capacitor voltage estimation and segmented design

With the increase of voltage levels and transmission capacity of the modular multilevel converter based HVDC, the capacitor voltage balance control will become more difficult. To deal with the problem, segmented design for each arm is introduced. Combining the segmented design and capacitor voltage sorting based balance control strategy, we analyzed the mechanisms of voltage balance among different sub-modules in each segment and among different segments. Then, we proposed a voltage balance control strategy among segments. The equivalent switching frequency of MMC is divided into two parts. Thereby, the influence factors for different switching frequency are then analyzed. Thereafter, an optimization control strategy for capacitor voltage balance is proposed to decrease the equivalent switching frequency. The simulation results, based on the PSCAD/EMTDC software environment, indicate that the capacitor voltage balancing is well guaranteed while the equivalent switching frequency reduce by more than 25%, verifying the correctness of the relative theory analysis and validity of the proposed control strategy.