Replacing the line commutated converter(LCC)with modular multilevel converter(MMC)at the inverter side is one of the most efficient paths to avoid commutation failure. Firstly

an account is given to the topological structure of LCC-MMC hybrid high voltage direct current(HVDC)system

the basic control scheme

and the power ramp-down problem under AC system fault. Secondly

control strategies of hybrid HVDC transmission system based on maximum modulation index are proposed to maintain direct current under AC system fault by adjusting the modulation index of MMC. Finally

a hybrid HVDC simulation system is built in PSCAD/EMTDC to verify the effectiveness of the proposed control strategies.