A Bidirectional Current-Fed Isolated MMC with Zero-Current Switching for High Step Ratio DC-DC Applications

To effectively integrate distributed energy resources and non-traditional loads into the grid, medium-voltage direct current (MVDC) systems have gained attention in recent years. A key enabling technology for MVDC distribution is high step ratio dc-dc converters with bidirectional capability, performing a role similar to ac distribution transformers. The current shaping modular multilevel converter (CS-MMC) has been proposed for this type of application because of its ability to operate at high-effective frequencies by eliminating internal string inductors, thereby minimizing module capacitance, size, and cost. Until now, published CS-MMC work has focused on unidirectional, voltage-fed operation. To enable bidirectional operation, a novel current-fed operational approach is proposed in this paper that is fundamentally different from earlier works. The circuit topology, operating principles, and experimental results are presented. A peak measured efficiency of 95.8% is achieved when transferring 2.2 kW from 100V to 1kV for a 6 module, 2.7 kW, 1kV/100V prototype.