There are a number of issues associated with the current high voltage direct current（HVDC） transmission systems， such as commutation failures， difficulties in handling DC faults， resonance between smoothing reactor at rectifier and the DC capacitor at inverter， and inability to facilitate reverse power flow. A new hybrid HVDC transmission system is proposed to overcome such issues associated with current HVDC systems. The system composes a line commutated converter（LCC） at rectifier and current source converter（CSC） at inverter based on fully controlled power electronic devices. This is suitable to solve communication failure caused by LCC. This paper focuses on setting up the low frequency and steady state mathematical mode of the CSC. Corresponding control strategies and the controller are designed as well. A hybrid CSC-HVDC based on Ge-Nan HVDC transmission system is realized by PSCAD/EMTDC. Firstly， start up and power reversal are described in detail. Secondly， transient responses and recovery characteristic under three-phase ground fault at the inverter are experimented. Furthermore， the ability to handling DC fault and re-start up strategy are also studied. The simulation results illustrate that the hybrid CSC-HVDC has a promising potential for the bulk power transmission as an improvement scheme for HVDC.