Transformerless CHB-Based Converter for Clustered DC Microgrids

Cascaded H-bridge (CHB) multilevel converter presents a promising solution for eliminating the low-frequency transformers (LFTs) traditionally used to connect high-power converters to medium-voltage distribution grids. However, there is a gap in the literature regarding the design considerations for CHB-based transformerless converters with a high number of output voltage levels. This paper conducts a feasibility study to replace the conventional two-level converter connected to a 15 kV grid via an LFT with a 35-level transformerless bidirectional CHB for electric vehicle (EV) charging stations. This innovative structure enables the creation of a clustered DC microgrid with 51 DC links, significantly enhancing system flexibility to integrate renewable energy sources, battery energy storage systems, and additional EV loads. Furthermore, this paper addresses the challenge of achieving full utilization of H-bridges when their number exceeds a specific value. Simulation studies reveal that, with high number of H-bridges (HBs), one or more HBs may become inactive depending on the modulation index and DC-link voltage level. Through the theoretical analysis, this paper unveils the motivation for this issue and recommends its solution.