Thyristor-Embedded Hybrid Modular Multilevel Converter: A New Voltage-Source Converter for MV and HV Applications

Compared with line-commutated converters (LCC) technology, voltage-source converters (VSCs), exemplified by modular multilevel converters (MMCs) have emerged as a superior alternative for HVdc applications, offering enhanced controllability, functionality, and modular design. However, VSC-MMC has relatively higher losses and energy storage requirements than LCC. Addressing these intrinsic drawbacks of VSC-MMCs, there has been a recent demand for converters that combine ultralow conduction losses and high power density from thyristor-based valves in LCC with VSC-MMCs' modularity and controllability. This article presents the architecture and operational strategy of one such converter: Thyristor-embedded hybrid MMC (SCR-HMMC). SCR-HMMC distinguishes itself by utilizing fewer switches, reducing arm energy storage requirements, and achieving lower losses compared with other state-of-the-art solutions. The bidirectional converter is adept at managing high-ac/low-dc and low-ac/high-dc voltage conversions while also boasting black-start functionality, modular design, and scalability. Its modularity and scalability combined with reduced expenses, losses, and size render SCR-HMMC a viable proposition for both high-voltage and medium-voltage (MV) applications. A 20-kW MV SCR-HMMC prototype has been fabricated for the laboratory setting, demonstrating successful operation in both inverter and rectifier modes, marking one of the first such in-depth practical demonstrations of a thyristor-embedded bidirectional multilevel VSC.