In order to improve the transmission capability of power grids and mitigate the congestion of power flow，an idea about conversion of existing alternating current（AC）lines to multi-terminal direct current（MTDC）system is proposed.According to an analysis of the tripole high voltage direct current（HVDC）system，it is indicated that the tripole HVDC is not suitable for the MTDC system.Then，the three-wire bipole structure based extended MTDC system is proposed.Firstly，the structure of the extended MTDC system is described.Secondly，the operating principle and mechanism of power expansion of the three-wire bipole structure are analyzed in particular，as is its key equipment，the current regulation controller，whose operating characteristics and feasible topologies are elaborated.Finally，by use of the PSCAD/EMTDC simulation software，the designed model is simulated.The simulation results verify the feasibility of the application of three-wire bipole structure in the MTDC system，and the effectiveness of the corresponding control strategies.Meanwhile，the feasibility of the full-bridge modular multilevel converter acting as the current regulation controller is established.