1.Electric Power Research Institute，China Southern Power Grid，Guangzhou510080，China;2.State Key Laboratory of Control and Simulation of Power System and Generation Equipments，Tsinghua University，Beijing100084，China

The modular multilevel converter（MMC）based on the high voltage direct current（HVDC）system has become one of the most competitive alternatives for the voltage source converter based HVDC（VSC-HVDC）system.When the ac-line voltage is relatively low and does not match the dc-line voltage，an optimal design methodology can be used to reduce the series-connected sub-modules in an arm.Moreover，the MMC can be directly integrated into the ac grid without a voltage matching transformer.The optimal design methodology also provides more practical options for the VSC-HVDC system.This paper presents various possible design solutions for the demonstrative±160 kV/200 MW three-terminal VSC-HVDC system.The analysis and comparison for these solutions are also presented.The design selection principles for the MMC in the VSC-HVDC system are proposed.