Design-oriented DC-side stability analysis of VSC-HVDC systems based on dominant frequency analysis

This paper derives analytical DC-side stabilizing conditions of voltage-source-converter-based high voltage DC (VSC-HVDC) links for control design. The case of a single modular multilevel converter (MMC) connected to an infinite DC bus system is first explored to determine the frequency of the dominant mode (dominant frequency) in the DC link. Then, the small-signal model of a generic two-terminal HVDC system is formulated. Since stability discrimination only focuses on the behaviors of the dominant mode, nondominant modes are collapsed in a way that leaves their dynamic properties nearby the dominant frequency intact. Accordingly, concise DC-side stabilizing conditions based on key parameters of physical systems and controllers can be derived, which indicates that the instability mechanism of VSC-HVDC links under DC-side perturbations is that the DC-voltage-controlled converter and the transmission line cannot provide sufficient positive damping to offset the negative damping induced by the power-controlled inverter. It is also interesting to find that increasing the proportional gain of the DC voltage control diminishes the damping of the converter, thereby endangering system stability. The maximum value of the proportional gain is further determined to guarantee sufficient system stability margins under various operating points. Finally, simulation studies validate the proposed DC-side stabilizing conditions for two-terminal VSC-HVDC systems.