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 submodule (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 article, 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 a 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.