Quantifying Oscillatory Mode Damping of Power Systems via Frequency-Domain Analysis

A method is introduced to quantify oscillatory mode damping in power systems using only frequency-domain impedance data. In contrast to state-space model based eigenvalue analysis ( in time-domain) or closed-loop transfer function (CLTF) based pole analysis (in s-domain), the approach does not require access to internal converter models and is therefore applicable to black-box inverter-based resources. Mode frequency is identified at the magnitude peak of the modal impedance, the damping sign is determined through combined criteria involving real and imaginary parts and their slopes, and the damping ratio is obtained from resonance bandwidth with an adaptive scheme. Validation on benchmark grids (such as IEEE 5-, 14-, and 39-bus systems) and a simplified MMC-HVDC system shows close agreement with the eigenvalue and CLTF-pole analysis as well as electromagnetic transient simulations. The results demonstrate accurate estimation of poorly damped or diverged modes (| ζ | ≤ 0.1), enabling practical stability assessment in converter-dominated AC/DC networks.