Capacitor Voltage Increment Suppression Control for CTB-HMMCs With Bypassed UFB-SMs Under DC Fault

The crossing thyristor branches (CTBs) based hybrid modular multilevel converter (HMMC) can effectively protect high-voltage direct current system under dc-line short-circuit fault with advantages of low percentage of unipolar full-bridge (UFB) submodules (SMs), short dc current interruption time, and high efficiency. However, the faulty UFB-SMs being bypassed would affect dc fault current interruption performance of the CTB-HMMC, which may cause large capacitor voltage increment of UFB-SMs and deteriorate CTB-HMMC. This article first analyzes the capacitor voltage increment of the CTB-HMMC with bypassed UFB-SMs under dc-line short-circuit fault. And then, this article proposes a capacitor voltage increment suppression control (CVISC) for the CTB-HMMC with bypassed UFB-SMs under dc-line short-circuit fault, where the capacitor voltage increment of UFB-SMs during dc-line short-circuit fault can be significantly suppressed through bypassing suitable number of UFB-SMs according to the proportion of faulty UFB-SMs being bypassed in each arm, and therefore the proposed CVISC effectively protects the CTB-HMMC under dc-line short-circuit fault. The simulation and experiment are conducted, and their results verify the effectiveness of the proposed CVISC.