1. State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources(North China Electric Power University, Baoding 071001, China; 2. State Grid Tangshan Power Corporation, Tangshan 063000, China

To solve the problem of flexible application of high voltage direct current(HVDC)systems in long distance bulk power overhead line transmission, based on modular multilevel converter(MMC)with double clamp sub-module topology and dual thyristor sub-module topology, a bipolar hybrid MMC based HVDC(MMC-HVDC)transmission system is designed not only to tackle the level of DC voltage, output capacity and DC fault suppression capability, but also to solve the great energy consumption or long DC link fault enduring issues. The equivalent circuit and fault blocking mechanism is analyzed under different DC link faults, and the system restart process is also designed. Finally, the hybrid HVDC is built in PSCAD/EMTDC simulation environment, and a variety of operating conditions and fault ride-through characteristics of the system are studied. Simulation results show that the bipolar hybrid system has not only flexible and diverse steady-state operation characteristics, but also DC fault ride-through and rapid recovery capabilities better suited to large capacity long distance overhead line transmission applications.