Analysis, Limitations, and Opportunities of Modular Multilevel Converter-Based Architectures in Fast Charging Stations Infrastructures

As fast and ultrafast charging stations (FCS/UFCS) increase their degree of penetration in the grid, it becomes beneficial to connect them directly at the medium-voltage (MV) level, as this reduces volume, losses, and cost. Recent solutions propose the replacement of the typical combination of the low-frequency transformer plus low-voltage rectifier by a solid-state transformer (SST), which can be composed, among other topologies, by a modular multilevel converter. This article proposes and discusses the application of the modular multilevel converter (MMC) in FCS infrastructures, utilized as an active front-end rectifier, either as part of an SST or for direct connection to the second stage, comprised of isolated dc/dc converters. Focus is given on the advantages and limitations of the converter operating as a step-down rectifier, where
$U_\text{DC}< \hat{U}_\text{AC}$
. As different architectures are presented, first the MMC step-down constraints and optimum operation points are explored and discussed, then the architectures are evaluated in terms of key performance indicators, such as size, efficiency, and reliability. At the end, experimental results for a full-scale MV MMC are presented, validating the analysis carried throughout this article.