Optimal Phase-Shifted Modulation Method for Suppressing the Current Ripple of Modular Multilevel Converter -Bidirectional DC-DC Converter under Unbalanced Operation

Modular multilevel converter -bidirectional dc-dc converter (MMC-BDC) can be used in the second generation of vessel integrated power system (IPS) to interconnect the medium voltage dc (MVdc) bus and distributed energy storage elements (ESEs) for its outstanding characteristics. When MMC-BDC operates with sub-module (SM) power well balanced, traditional phase-shifted modulation (TPSM) reduces the current ripple at the MVdc side of MMC-BDC for the frequency multiplication effect. However, when it comes to the unbalanced operation conditions, TPSM can result in large current ripple. In this paper, the relationship of the MVdc side current ripple and the switching signal patterns is revealed. An optimal phase-shifted modulation (OPSM) method based on offline genetic algorithm (GA) is proposed to suppress the MVdc side current ripple by adjusting the phase-shifted ratios of the driving signals of each SM. Combining the proposed modulation method with an proposed online stepwise energy management strategy, all unbalanced conditions can be covered with limited sets of optimal phase-shifted ratios. Simulation and experiment results show that the MVdc side current ripple of MMC-BDC is remarkably reduced with the proposed OPSM.