An Active-Forced-Commutated Thyristor-Based Multilevel Converter for HVDC Transmission

This paper proposes a thyristor-based hybrid three-level converter (TH3LC) integrated with AC-side full-bridge submodule chain-link (AC-FBCL) for high voltage direct current (HVDC) transmission. The TH3LC makes use of thyristor 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 a quarter of the DC-side voltage, which reduces the converter footprint, the semiconductor count, and the losses. The DSs of the TH3LC are force-commutated by an additional neutral-point-connected, low capacitance commutation FBCL (C-FBCL), enabling low DS switching frequency. The C-FBCL enables stepped-trapezoidal voltage output of the three-level converter for straight-forward synchronization with the AC-FBCL. Simulated 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 significantly smaller footprint due to a reduced number of submodules and less energy storage requirement.