Modular Multilevel Converters Enabling Multibus DC Distribution

The modular multilevel converter (MMC) is a promising topology for enabling hybrid grids, which includes different medium- and low-voltage (MV and LV) ac and dc grids. Owing to its capability to break dc current and work in buck mode, especially, MMCs have been widely studied for MVdc applications in ac-dc hybrid grids. It has been adopted as a frontend bidirectional ac-dc converter or a dc-dc converter employing medium frequency transformers to enable ultra-fast charging stations. Furthermore, MMC can provide different levels of LVdc by expanding the topology with an additional converter, which can help to realize a multibus dc distribution grid. Finally, this property can be used to realize a solid-state transformer (SST) for hybrid grids. However, it is required to identify and characterize the merits and demerits of various MMC structures available to form multibus dc distribution systems. Therefore, in this paper, MMC-based converters proposed in the literature are classified according to the ac-dc and dc-dc conversions. In addition, expected challenges and further possibilities in using them as a multibus dc enabler are also discussed in this paper.