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Voltage source converter (VSC) based high voltage direct current (HVDC) technology, abbreviated as VSC-HVDC, is a new and attractive transmission technology. With switch device having turn-on and turn-off capability, such as IGBT, VSC-HVDC has special characteristics comparing to conventional HVDC. Some of these characteristics are: passive ac load can be supplied; active and reactive power can be controlled separately and simultaneously; no requirement for fast communication between VSC stations. Based on the method of current injection at the ac bus of voltage source converter, the detailed dynamic model for VSC- HVCD is developed. This model includes three parts, i.e. the quasi-steady model for the ac side of voltage source converter, the dynamic model for VSC-HVDC inner dc system and the model for VSC-HVDC control system. With the help of User Program Interface function provided by Power System Analysis Software Package (PSASP/UPI), the transient simulation algorithm for VSC-HVDC is designed. Simulation results for multi-machine system with VSC-HVDC has verified the developed VSC-HVDC dynamic model and the designed transient simulation algorithm. Simulation results also show that with the active and reactive power control capability, not only the steady-state power flow can be controlled, but the system damping characteristic of low-frequency oscillation can be improved.