Unbalanced Operation Principle and Fast Balancing Charging Strategy of a Cascaded Modular Multilevel Converter–Bidirectional DC–DC Converter in the Shipboard Applications

In the vessel integrated power system (IPS), several common events, such as replacement of the aging batteries, isolation of fault modules, and launching of pulse loads, can make the state-of-charge (SOC) of energy storage elements (ESEs) inconsistent, which, consequently, requires different charging or discharging power commands for the ESEs to balance their SOC. Modular multilevel converter–bidirectional dc–dc converter (MMC-BDC) can be utilized in the vessel IPS to connect ESEs with the medium-voltage (MV) bus for its outstanding characteristics. However, the degree of imbalance in the module power of MMC-BDC is limited inherently, which can destabilize the system, but is usually neglected by the researchers. Therefore, this article investigates the unbalanced operating principle of a cascaded MMC-BDC. The imbalance boundary is studied. A control strategy is designed for the unbalanced operation to optimize the voltage stress of the components. A method of allocating unbalanced module power to the submodules (SMs) is raised on the basis of an imbalance boundary. A novel fast balancing charging strategy is proposed consequently. Finally, the proposed theory and strategies are verified through simulation and a 7.2-kW prototype.