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.