The modular multilevel converter (MMC) is a new type of voltage source converter (VSC) for medium or high voltage direct current (HVDC) power transmission. This paper proposes the installation of a new HVDC system, HVDC#3, by using MMC systems to transfer power between mainland and Jeju island power systems in Korea in 2020 beside the operation of two old line-commutated converter (LCC)-based HVDC systems. The control strategies of MMC-HVDC will be first analyzed. The operation of MMC-HVDC system for stabilizing Jeju power system will be then tested in two cases. First case is the operation of MMC for stabilizing power when wind power is variable. Other case is the control of MMC since a fault occurs in one of two old LCC-HVDC systems. The simulation results using PSCAD simulation program will confirm the effectiveness of the proposed method.