A Bidirectional Current-Fed Isolated MMC With Partial Soft-Switching for High Step Ratio DC-DC Applications

Medium-voltage direct current (MVDC) systems offer many advantages in a future of renewables and dc loads including reducing the number of required power conversion stages. An enabling technology for MVDC distribution systems will be high step ratio dc-dc converters with bidirectional power transfer 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. This converter family operates at high effective frequencies, enabled by the elimination of internal string inductors. While foundational work on the CS-MMC focused on unidirectional, voltage-fed operation, recent work has demonstrated the bidirectional capabilities of the CS-MMC when operated as a current-fed converter driven by the low-voltage side. This article proposes an improved modulation for the bidirectional CS-MMC that: 1) ensures zero-voltage switching of the secondary-side switches in forward operation; and 2) minimizes the reverse body diode conduction time to improve efficiency. This article is also the first to detail a balancing mechanism between the VSM strings and between the dc-blocking capacitors of the bidirectional CS-MMC. Peak measured efficiencies of 97.0% in forward operation and 96.3% in reverse operation are achieved for a 6 module, 3 kW, 1 kV/100 V prototype.