Unbalanced control strategy featuring inconsistent sub-module voltage for modular multilevel converter–bidirectional DC–DC converter

Modular multilevel converter–bidirectional DC–DC converter (MMC–BDC) is a prospective solution for the bidirectional DC–DC power conversion between medium-voltage DC (MVDC) bus and distributed energy storage system. Researchers have studied its application in the shipboard power system. In face of distributed energy storage devices with discrepant state of charge (SOC), the sub-modules (SMs) of MMC–BDC should work under inconsistent power to unify the SOC level. The existing unbalanced SM power control strategies fail to consider unbalanced operation range and efficiency. Therefore, this study proposes a novel unbalanced control strategy based on the feedforward compensation method, which is characterized by inconsistent SM voltage. Comparisons are made to demonstrate the advantages of the proposed strategy over traditional ones, and the dynamic characteristics are analyzed. Simulation and experiment results illustrate the effectiveness of the proposed unbalanced operation strategy.