Damping Characteristic Analysis of Ultra-Low Frequency Oscillations in High Proportional Hydropower Systems Considering Wind Power Integration

Hydropower-dominated systems can experience ultra-low-frequency oscillations (ULFOs, below 0.1 Hz) associated with hydro-governor dynamics and the water hammer effect. This paper investigates how wind power equipped with auxiliary frequency control modifies ULFO damping. A reduced frequency-to-active-power model of the doubly-fed induction generator (DFIG) is derived and incorporated into a conventional coherent-frequency aggregation framework. The model retains rotor-speed dynamics, the local aerodynamic power slope, deloading, and auxiliary frequency control. Complex torque analysis distinguishes the hydro and wind damping contributions. In the studied cases, wind penetration, droop gain, deloading, and hardware-dependent rotor inertia materially affect damping, while virtual-inertia and outer-power-loop gains have comparatively small effects. The reported virtual-inertia sweep slightly reduces the modal damping ratio. Existing EMT comparisons support the reduction for the tested fixed-wind operating points with inactive pitch control. The results provide a basis for interpreting wind frequency support as a complement to hydro-governor tuning, subject to coherency, operating-point, and physical-limit assumptions.