This paper suggests an operation and control strategy of an offshore wind farm based on a high-voltage DC (HVDC) transmission, where the variable-speed permanentmagnet synchronous generator (PMSG) wind turbine system is adopted. The HVDC transmission system consists of a hybrid of a twelve-pulse diode rectifier and a voltage-source converter (VSC) on offshore side, whereas a VSC is employed on AC grid side (GSVSC). For this topology, the twelve-pulse diode rectifier is capable of delivering a part of wind farm power, the rest of which is absorbed naturally by the wind farm VSC (WFVSC) since the WFVSC is controlled and operated as a voltage source of a constant frequency. At grid sags, a lowvoltage ride-through of HVDC transmission is achieved, in which the HVDC link voltage is controlled by the WFVSC, and the wind farm output power is reduced by increasing the generator speeds. By utilizing the uncontrolled rectifier, a cost of HVDC converter is reduced compared with a case of the fully rated VSC, while the performance is the same. PSCAD/EMTDC simulation results for a 500-MW offshore wind farm have been shown to verify the validity of the proposed scheme.