Modular Modeling Method for Transfer Function of VSC Grid-Connected System

This paper provides a detailed introduction to a modular modeling approach for the transfer function of the VSC grid-connected system. The obtained results can accurately reflect the small-disturbance stability level of VSC grid-connected systems. These results are based on the VSC outer-loop reference signal and the corresponding feedback signal, constituting a two-input two-output system. The modeling process involves three steps. Initially, only the dynamic processes of the VSC inner-loop control and PLL control modules are considered. Subsequently, the relevant modules of the VSC outer-loop control are included. Finally, it is condensed into a concise form. By utilizing the basic property of linearity in systems that adhere to the superposition principle, it can be converted into an equivalent single-input single-output system. This enables the application of the Nyquist stability criterion for stability analysis. All results presented in this paper have been validated through PSCAD/EMTDC simulation.