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.