Impact of multi-rated submodules on steady-state operating domain in MMC-HVDC systems: co-design of 6.5kV IGBT selection and main circuit parameters

The integration of renewable energy and carbon neutrality objectives demands enhanced operational flexibility in power grids. Modular Multilevel Converter-based HVDC (MMC-HVDC) systems offer superior adaptability for modern power systems. This paper proposes a co-design framework for MMC-HVDC main circuit parameters and steady-state operation domain, integrating submodule capacitance, bridge arm reactors, and coupling transformers. Analytical models establish constrained power flow equations and safety boundaries to construct the steady-state operating region in the PQ complex plane. Validated with the MMC using 6.5kV IGBT, the method achieves <4% parameter design error. Furthermore, IGBT selection critically impacts system economics and operational boundaries: higher rated currents expand the steady-state region and reduce unit capacity costs, while lower rated voltages increase submodule counts. This approach provides a theoretical foundation for parameter configuration in large-scale AC-DC hybrid systems.