Analysis on Circulating Current Frequency of Chain-link Modular Multilevel DC-DC Converters for Low Step-Ratio High-Power MVDC Applications

The direct chain-link modular multilevel dc converter has raised great interest recently in medium/high-voltage dc-dc conversion due to the high power device utilization and lower power losses than the front-to-front configurations of modular multilevel converter. This paper introduces a single-phase chain-link modular multilevel buck-boost converter for medium voltage dc applications and presents an analysis methodology on its internal circulating current frequency. Its connection and comparison with the classic dc-ac modular multilevel converter are given first, and its dc and ac components are analyzed in the respective equivalent circuits. Then, the derivation methodology for the proper circulating current frequency with lowest internal reactive power is provided, which could minimize the current stress and decrease the power losses. Also, this method can be directly applied in the derivative topologies and further configurations to satisfy various conversion requirements. The theoretical analysis is verified by a set of full-scaled simulations and further verified against experimental tests on a down-scaled prototype.