Dissertation Defence: 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
March 19 at 1:00 pm - 5:00 pm
Saeed Sharifi, supervised by Dr. Liwei Wang, will defend their dissertation titled “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” in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Electrical Engineering.
An abstract for Saeed Sharifi’s dissertation is included below.
Examinations are open to all members of the campus community as well as the general public. Registration is not required for in-person exams.
Abstract
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 converters face fundamental limitations: their restricted AC-voltage synthesis capability increases internal circulating currents, elevates semiconductor losses, and forces large submodule capacitor size, especially under unbalanced or reactive-power-dominant conditions such as STATCOM operation. In addition, existing MMC designs typically employ lossy IGBT-based valves or thyristor-based ones that often struggle with reliable forced commutation, complex energy balancing, and limited AC voltage range. These challenges motivate the development of new converter structures that can achieve higher output voltages, lower losses, reduced capacitive energy storage, and more robust operation across MV and HVDC applications.
This dissertation introduces a unified set of solutions addressing these shortcomings. First, it proposes the Flying-Stack Modular Multilevel Converter (FS-MMC), a new architecture that uses flying stacks to synthesize higher AC voltages than traditional MMCs while reducing capacitor ripple and circulating energy. The FS-MMC provides enhanced AC-side voltage boosting, improved performance under unbalanced conditions, and reduced semiconductor currents, making it highly effective for STATCOM and MV/HV wide voltage range applications. A boosting-mode operational strategy, including an overlapping interval method, is further developed to expand the AC-voltage capability of the FS-MMC without increasing semiconductor stress, enabling efficient high-voltage operation, improved voltage utilization, along with fault-blocking/ride-through and black-start capabilities.
To reduce semiconductor losses in hybrid MMCs, the dissertation introduces the Thyristor-Based Director Switch (TB-DS), which achieves low-loss, smooth commutation of high-power thyristors through a commutation chain-link. This enables reliable forced turn-OFF, bidirectional current control, and significantly lower switching loss compared with IGBT-based solutions. Building on this concept, the thesis further presents the Thyristor-Based Submodule (TBSM), in which thyristors conduct the main arm current while a compact IGBT chain-link handles only brief commutation intervals. This hybrid submodule structure reduces total semiconductor losses by approximately 40% relative to conventional IGBT HBSMs while maintaining stable capacitor balancing and high controllability.
Collectively, the FS-MMC, its boosting-mode operation, TB-DS, and TBSM form a comprehensive advancement in modular converter design. They demonstrate substantial improvements in efficiency, voltage scalability, energy balancing, and operational robustness. The contributions of this thesis provide a practical pathway toward next-generation MV and HVDC converter systems with reduced losses, smaller passive components, and enhanced suitability for future grid-support and DC-transmission applications.
