A Low-Loss Thyristor-Based Hybrid Three-Level and Modular Multilevel Converter With DC Fault Blocking Capability for HVDC Transmission

This paper proposes a thyristor-based hybrid three-level converter (TH3LC) equipped with an AC-side full-bridge submodule chain-link (FBCL) for voltage-source converter (VSC) high voltage direct current (HVDC) applications. The TH3LC uses thyristor-based director switches (DSs) to conduct the bulk of transferred power while the AC-FBCL enables a multilevel voltage output. Third-order harmonic voltage injection is used by the AC-FBCL to limit its total blocking voltage to less than or equal to a quarter of the DC-side voltage, reducing the converter footprint, semiconductor count, and losses. The DSs of the TH3LC are force-commutated by an additional neutral-point-connected, low capacitance commutation FBCL (C-FBCL). The C-FBCL enables a stepped-trapezoidal voltage at the output of the three-level converter for straight-forward synchronization with the AC-FBCL and zero-voltage switching for the DSs. Case studies show that the TH3LC provides similar converter efficiency to the half-bridge MMC (HB-MMC) while possessing DC fault blocking capability and a smaller footprint due to a reduced number of submodules.