Impact of the Frequency Support Provided by Grid-Forming DFIG-Based Wind Turbine on Power System Ultra-Low Frequency Oscillation

Ancillary frequency controls of wind turbines (WTs) bridge the dynamic interplay between turbine mechanics and the electrical grid, enabling WTs to influence low frequency range dynamics in power systems. This work examines the impact of frequency support by grid-forming (GFM) doubly-fed induction generator (DFIG) based WT on power system ultra-low frequency oscillations (ULFOs). Accounting for different wind speed operating regions, a reduced-order small-signal model is developed to characterize GFM-DFIG's terminal power-frequency response behavior within ULFO frequency band. This model is then integrated into the system frequency response framework. Utilizing complex torque coefficients method, the effects of various control strategies - including virtual inertia, primary frequency control, and turbine blade pitching - on the ULFO mode's frequency and damping characteristics are analytically derived and examined. The results confirm that while the frequency support improves system frequency response, it simultaneously deteriorates ULFO damping, with high primary frequency droop gain posing a risk of instability. Lastly, simulation studies with a modified IEEE 10-machine 39-bus system are conducted to validate the theoretical findings.