To address the issue of commutation failure (CF) in conventional HVDC transmission systems caused by AC system faults

this paper proposes a controllable capacitor converter valve topology and control strategy based on hybrid series connection of semi-controlled and fully-controlled devices. First

a hybrid series converter valve topology is developed to enhance the system’s ability to withstand CF. Based on commutation process analysis

it shows that capacitor insertion can significantly increase the commutation voltage-time area

thereby reducing CF risk. Moreover

the mechanisms by which capacitor capacity and insertion timing affect commutation performance are revealed. Considering the voltage and current stress characteristics of fully-controlled devices

the sizing and selection of capacitors and turn-off devices are determined. Subsequently

an adaptive control strategy based on instantaneous line voltage magnitude detection is designed. By monitoring AC system voltage variations in real time and adaptively adjusting the turn-off timing of fully-controlled devices

the suppression of CF is further improved. Finally

a simulation model built in PSCAD/EMTDC verifies the effectiveness of the proposed scheme. The results demonstrate that the proposed topology and control strategy can effectively suppress CF across a wide range of fault severities from mild to severe.