Abstract: In order to maintain the advantages of low loss, high reliability and efficiency of traditional line commutated converter (LCC) and avoid the risk of commutation failure, a novel topology of controllable line commutated converter (CLCC) with controllable turn-off capability was proposed. The topology is based on a serial-parallel of fully-controlled and semi-controlled devices. During the commutation process, current through the thyristor valve transfers to the auxiliary branch at first by the fully controlled devices, then after the forward blocking capability is restored, the current is shut off by the auxiliary branch which also contained fully controlled devices, completing commutation between bridge arms. The CLCC can work in two operation modes, one is natural commutation and the other is controllable commutation. The working principle of the two operation modes and the internal control strategy of the converter are investigated. A UHVDC transmission system simulation model is built to analysis the performance of CLCC converter under various conditions, including steady state, transient state and fault conditions. The simulation results show that CLCC converter not only had the same system operation performance with LCC converter, but also completely solved the commutation failure problem, which was conducive to improve the safety and stability of power grid and improve the power acceptance capacity in multi-infeed HVDC systems.