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A novel dynamic phasors method to model voltage sources converter based HVDC (VSC-HVDC) transmission system is proposed in this paper. This approach is based on the time-varying Fourier series coefficients of the system variables, and focused on the dynamics of the Fourier coefficients. Followed by the time-domain converter station model for VSC-HVDC described by switch function, detailed analysis of the VSC-HVDC dynamic phasors model is presented in this paper. The VSC-HVDC model is simplified by keeping important system state variables that corresponding to the time-varying Fourier series, which include the converter station switching function considering both the DC component and basic frequency component, and the DC transmission line considering only the DC component. Therefore, high frequency switching process is greatly simplified. Typical simulation results show that this method can ensure simulation accuracy and reduce computational cost at the same time.