Capacitor Voltage Balancing Algorithm using Voltage Fluctuation Threshold for Modular Multilevel Converters

Suppression of the submodule (SM) capacitor voltage fluctuation with low switching frequency is one of the main control objectives of the modular multilevel converter (MMC). In order to solve this problem, this paper proposes a mark sorting voltage balancing algorithm (MSVBA). The proposed MSVBA marks SMs with two different labels according to the current state and capacitor voltage of each SM. The marking result is also based on the fluctuation threshold which can be adjusted artificially. Therefore the performance of the MSVBA can be controlled flexibly under various conditions. A detailed model of 21-level MMC-HVDC test system is built in PSCAD/EMTDC, and the MSVBA is compared with two previous algorithms in voltage fluctuation and loss distribution. The result proves the validity and the feasibility of the proposed MSVBA. Finally, the performances of the MSVBA with different fluctuation threshold are investigated to give some suggestions on parameter selection.