A Power-Dense and Fault-Tolerant Power Electronic Converter Architecture for High-Voltage DC (HVDC) Systems

High-Voltage DC (HVDC) systems are an effective technology for long-distance and subsea transmission and crucial for large-scale integration of renewable generation into bulk power systems. While the inherent modularity, redundancy, and performance have made Modular Multilevel Converters (MMC) the dominant power converter architecture for HVDC systems, they suffer from challenges in power density, substation footprint, and fault-tolerance. This paper presents a converter architecture that aims to improve these aspects. The proposed iso-MMC topology features modular high-frequency transformers in every submodule that replace the line-frequency transformers of MMC-HVDC stations. Further, the submodules are integrated between the three-ac phases reducing circulating currents and eliminating the need of bulky submodule capacitors. The proposed circuit has no uncontrolled conduction pathways thus improving the dc fault ride-through capabilities. Lastly, the net semiconductor die rating is smaller for designs with high dc to ac voltage transfer ratio, a more desirable operating space for HVDC systems. The analytical results are verified using simulation and comparative studies.