Flexible Nearest Level Modulation for Modular Multilevel Converter

Modular multilevel converter (MMC) is one of the most promising converter topologies for medium/high-voltage applications, where the nearest level modulation (NLM) is widely applied. However, the capacitor voltage balancing methods under this modulation mode exist some inefficient actions, which are useless for capacitor voltage balancing but lead to unnecessary switching of power semiconductor devices. In this article, the disturbance and unbalance mechanisms of capacitor voltages in MMC under NLM are analytically revealed and solved. The disturbing metrics for capacitor voltage are digitally extracted to identify the accurate time interval in which the switching actions have immediate effects on capacitor voltage balancing. Then, a demand-oriented scaling factor is introduced to adjust the balancing strength by changing the range of the time intervals which are highly efficient with respect to capacitor voltage balancing, thus flexible tradeoff between the switching frequency of power semiconductor devices and the balancing level of capacitor voltages can be achieved. Finally, a dedicated mission-profile-emulator for the testing of MMC is designed and built with comprehensive validations to verify the effectiveness of the proposed method.