Design, simulation, and hardware evaluation of novel hybrid modular multilevel converters and semiconductor valves for highly efficient and fault-tolerant power conversion in medium/high-voltage direct current transmission systems

Modern high-voltage direct current (HVDC) transmission systems increasingly rely on modular multilevel converters (MMCs) to integrate renewable energy and support weak AC grids. However, conventional MMCs face key limitations, including restricted AC-voltage synthesis capability, increased circulating currents, elevated semiconductor losses, and large submodule capacitor requirements, particularly under unbalanced or reactive-power-dominant conditions. In addition, existing designs based on IGBTs or thyristors suffer from challenges such as high losses, complex energy balancing, and limited operational flexibility. This dissertation presents a unified set of converter topologies and building blocks to address these challenges. First, the Flying-Stack Modular Multilevel Converter (FS-MMC) is introduced as a novel architecture capable of synthesizing higher AC voltages than conventional MMCs while reducing circulating energy and capacitor ripple. The FS-MMC enables enhanced voltage utilization, improved operation under unbalanced conditions, and reduced semiconductor stress. A boosting-mode operation, incorporating an overlapping interval strategy, is further developed to extend the AC-voltage operating range without increasing device stress, enabling efficient operation across both medium- and high-voltage applications. To improve efficiency, a thyristor-based director switch (TB-DS) is proposed, enabling low-loss current conduction with controlled commutation through auxiliary IGBT-based chain-links. This approach achieves reliable turn-off capability and significantly reduces switching losses compared to conventional IGBT-based director switch designs. Building on this concept, a thyristor-based submodule (TBSM) is developed, where thyristors conduct the main current while IGBTs operate only during brief commutation intervals. This hybrid structure reduces total semiconductor losses by approximately 40% relative to conventional MMC submodules while maintaining controllability and capacitor voltage balancing. The proposed FS-MMC topology, along with the TB-DS and TBSM building blocks, is validated through detailed simulations and experimental studies. Results demonstrate improved efficiency, enhanced AC-voltage flexibility, and reduced energy storage requirements. These contributions provide a practical pathway toward next-generation MVDC and HVDC converter systems with higher efficiency, improved scalability, and enhanced operational robustness.