Evaluation of Capacitor Voltage Balancing Control Strategies for Multilevel DAB Converters

Dual active bridge converter with multilevel neutral point clamped topology has shown potential to be a viable solution for medium-voltage dc systems. However, it will face a crucial challenge with regard to capacitor voltage balancing. In order to mitigate the negative effects caused by the voltage imbalance, e.g., overvoltage on certain devices, various control strategies for voltage balancing have been developed. To better design and implement the voltage balancing strategies, this article evaluates several voltage balancing approaches, i.e., modified duty cycle (MDC) method, modified phase shift and duty cycle (MPSDC) method, fixed switching state (FSS) method, and complementary switching state (CSS) method. These voltage balancing approaches have distinct characteristics in terms of dynamics, efficiency, robustness, applicability with modulation strategy, and implementation complexity. Evaluation results show that the MDC and MPSDC methods have advantage of extending to various control strategies. However, they are affected by current polarity identification, and have large current and power fluctuations. The FSS method is the most robust and the simplest one, but it also has poor dynamics. On the other hand, the CSS method can deliver smooth dynamics and high efficiency, but it relies on identifying the polarity of inductor current, and it cannot be used to the single phase shift and triple phase shift control strategies. Based on the evaluation results, the suitable applications of different voltage balancing methods are discussed.