For solving the problem of commutation failure of the traditional HVDC transmission system

a kind of flexible LCC scheme with cascaded H-bridge connected in series between the line commutation converter（LCC）and the converter transformer is proposed. The cascaded H-bridge as a controllable voltage source generates capacitive alternating current voltage

can achieve adjustment of actual commutation voltage of converter valve

thus the probability of commutation failure is reduced. In this paper

the quasi steady-state model of the flexible LCC system is set up in accordance with the phase relationship of the secondary side of the converter transformer. The control relationship of DC busbar voltage of the system and the actual extinction angle is studied further

proving that the flexible LCC has characteristic which can achieve power factor adjustment of power grid. After that

a controllable voltage source and LCC coordinated control strategy under different power grid conditions angle is proposed so that the system can

in case of fault condition

meet simultaneously the requirements of DC power transmission and anti-commutation-failure. Finally

it is proved by PSCAD/EMTDC simulation that the proposed scheme can

in case of AC voltage sag

effectively maintain system stability and suppress the commutation failure more effectively over the traditional LCC scheme.