Capacitor and Switch Size Comparisons on High-Power Medium-Voltage DC–DC Converters With Three-Phase Medium-Frequency Transformer

This article compares the capacitor requirement of selected isolated dc-dc converters for high-power and medium-voltage applications. A numerical comparison is made for a range of switching frequencies and transition times, considering not only the submodule capacitors of the converters but also the dc-link capacitors. Likewise, a comparison regarding the semiconductor requirement is performed for the converters. Selected inverter topologies to form the dual-active-bridge (DAB) dc-dc converter include the two-level converter (2LC), the modular multilevel converter (MMC), the transition arm converter (TAC), the modular TAC (MTAC), and the controlled transition bridge (CTB) converter. The results show that conventional DAB requires the minimum capacitor size among the converters. Moreover, in the appropriate operation region, the MTAC-DAB and the CTB-DAB provide the possibility of up to 40% and 60% reductions in the amount of required energy storage compared with the MMC-DAB, respectively. It is also shown that the size of the dc-link capacitors becomes comparable with the chain-links' capacitors when the percentage of the transition times per period is reduced to be below 5%. The comparison regarding the semiconductor requirement revealed that the MMC-DAB requires the smallest installed switch power among the multilevel converters.