High power Modular Multilevel DC-DC Converter based on dual-loop CPS modulation

Modular Multilevel Converter-Bidirectional DC-DC Converter (MMC-BDC) is suitable for high voltage and high power application. This paper designs a kind of six-module-cascaded MMC-BDC applied to mass rail transit. Based on carrier phase-shifted (CPS) modulation, a dual-loop control strategy consisting of a common current loop and an individual voltage loop is proposed. It is able to realize both bidirectional power flow and real time voltage balancing online. Regarding the designing of this strategy, this paper reveals a series of contradictions that needs to be properly compromised, e.g. large voltage loop gain results in large inductor ripple current, which means more power losses, while smaller gain leads to long voltage balancing time. And it's found that the ripple current is larger in unbalancing mode than that of in balancing one, which is important for the parameters choice. The design method to trade off the contradictions was given and the dual-loop control strategy was successfully implemented. Analysis and simulation waveforms verify the correctness and the effectiveness.