Line commutated converter（LCC） is adopted on the rectifier side of hybrid HVDC system

and hybrid Modular Multilevel Converter based on full bridge sub-module and half bridge sub-module（FH-MMC） is adopted on the inverter side. DC Pole-to-ground fault is the main fault type of HVDC system. When DC Pole-to-ground fault occurs

the hybrid HVDC system needs to switch operation mode. Bipolar operation is transformed to monopole operation on the LCC side

and the AC voltage DC bias and fault current are eliminated by outputting negative level voltage on the FH-MMC side. The power flow characteristics of FH-MMC arms under this operation mode are analyzed. Energy imbalance occurs between upper and lower arms. the sub-module capacitance voltage of the fault arm keeps rising

which affects the safe operation of switching devices. Therefore

an energy balance strategy based on fundamental frequency circulating current injection is proposed in this paper. The proposed control strategy ensures that the normal pole can still transmit half of the rated power under DC pole-to-ground fault

and realize uninterrupted operation of HVDC system. Finally

a hybrid HVDC simulation model is built in PSCAD/EMTDC

and the simulation results verify effectiveness of the proposed control strategy.