New Hybrid Thyristor-Based Multilevel Converter With DC Fault Blocking Capability, for HVDC Applications

High voltage direct current systems are the most suitable solution for distant high-power transmission, where modular multilevel converters (MMCs) are now utilized due to their controllability, modularity, redundancy, and scalability. Dc fault blocking capability in MMCs is normally achieved by using full-bridge submodules (FB-SMs) in the arms. However, using FB-SMs results in a high semiconductor count, which increases power losses and the overall cost. In this article, a new hybrid thyristor-based multilevel converter (HTMC) with dc fault blocking capability is proposed that uses a low number of FB-SMs with a majority of half-bridge submodules and antiparallel thyristor valves. The theory of operation is detailed including the function of each element and thyristor valve commutation. Full parameter analysis is provided for the proposed converter. The claims of the article are verified using a MATLAB Simulink model and an experimental test rig. Additionally, a detailed comparison with other viable converters is provided, which establishes that the proposed HTMC converter offers a low number of IGBTs, and lower cost and losses, with dc fault-blocking capability.