Operation Analysis of Thyristor-Based Front-to-Front Active-Forced-Commutated Bridge DC Transformer in LCC and VSC Hybrid HVDC Networks

The active-forced-commutated (AFC) bridge employs a symmetrical thyristor-bridge with auxiliary self-commutated full-bridge chain-link circuit to assist its soft transition and forced commutation. This combination can form a thyristor-based voltage source converter (VSC) with significantly reduced on-state losses and dc-fault blocking capability. Due to the full topological symmetry of the AFC bridge, either current direction or dc-link voltage polarity can be reversed for power flow reversal as for the full-bridge modular multilevel converter. Thus, the AFC-bridge is compatible with both line-commutated-converter (LCC) and VSC terminals in a multiterminal high-voltage direct current network. This paper investigates its front-to-front (F2F) dc-dc application for matching the regional dc grids in an LCC and VSC hybrid HVDC network. Simulation studies are carried out to demonstrate its potentials as a high-efficiency multifunctional solution for dc-dc conversion.