Electrical Design and Evaluation for a 10 kV SiC Power Module-Based Submodule of a Modular Multilevel Converter

Modular multilevel converter (MMC) is a highly suitable converter for medium- and high-voltage applications. Its structure is based on the series connection of many submodules (SMs). While the introduction of 10 kV SiC MOSFET modules has enabled higher voltage and switching frequencies for each SM using simple configurations, this also exacerbates operational challenges, including electrical stresses. Consequently, the design and evaluation of SMs under these conditions are crucial for ensuring reliable operation. This paper focuses on the electrical design and evaluation for a 10 kV SiC power module-based SM in a medium-voltage MMC with middle SMs. First, stray inductance of the PCB busbar with different connections are evaluated, providing guidance to select proper configuration with smaller stray inductance. Then, surface electric field intensity on the customized PCB busbar under various voltage stresses are evaluated to prevent partial discharge as much as possible. Finally, to evaluate the electric field distribution in the SM, electric fields of the entire core area of a SM, including the power module, gate drivers, PCB busbar, heatsink, and copper bars, are analyzed under full dc voltage and realistic switching stress with high dv/dt. The stray inductance and partial discharge characteristics of the PCB busbar are verified experimentally, which align with the design and evaluation results very well. The functionalities of all the individual SMs are experimentally verified by 55-min continuous operation under full voltage and rated output current. The functions of the SM were further confirmed in full-power tests of a downscaled 250 kVA MMC.