Abstract: To make the voltage source converter based high voltage direct current（VSC-HVDC） system extend to long distance bulk power overhead line transmission is the inherent requirement of energy source distribution in China and future trend of power grid development.However,the challenges associated with the state-of-the-art VSC-HVDC system mainly include:a) difficulty to handle dc fault,b) less transmission capacity compared with the traditional HVDC systems,and c) difficulty to select grounding point.For the first challenge,this paper analyzes vulnerability of the existing topologies against dc faults,focusing on an overview of the topologies with dc fault ride-through capability.According to the characteristics of dc fault processing,these topologies can be divided into three basic types.Furthermore,advantages and defects of these typical topologies are discussed.For the second and third challenges,to achieve a high voltage and power level,a bipolar VSC-HVDC topology with assembly converter is proposed.In this topology,each pole consists of several basic converter units（BCU） in series or in parallel;between the positive pole and the negative pole is the neutral bus which is connected to the ground electrode.Furthermore,the advantages and shortcomings of the possible grounding modes are summarized.At last,according to the unidirectional energy flow characteristics of China,two basic VSC-HVDC structures are analyzed.