Analysis of pulsation concentration effect related to sorting-algorithm-based voltage-balancing strategy in an MMC system

The sorting-algorithm-based voltage-balancing strategy in modular multilevel converter (MMC) changes the submodule operating state to regulate the charging and discharging time of submodule capacitors and to balance the capacitor voltages. However, these changes and regulations can lead to the pulsation concentration effect in the switching drives, thus significantly decreasing the efficiency of the system and severely deteriorating the system operating performance. Therefore, this study first proposes the mechanism of pulsation concentration effect in the voltage-balancing strategy. A quantitative analysis of the relations between the pulsation concentration effect, the voltage-balancing strategy and corresponding deteriorations is then provided, indicating the root cause of such effect and types of associated deteriorations, such as switching drive redistributions and switching frequency fluctuation in the MMC system. Based on the above analysis, an optimal voltage-balancing strategy is then provided to effectively suppress the pulsation concentration effect; and a code restructuration method is presented at the same time to further avoid the extra switching actions and decrease the switching loss. The deteriorations caused by the pulsation concentration effect and effectiveness of proposed optimal voltage-balancing strategy are finally demonstrated via experiments.