A two-stage frequency response method for DFIGs under variable wind speeds

•The proposed stage I scheme can fully provide frequency support power to mitigate the rate of change of frequency and frequency nadir while reducing power deficit when wind turbine generators start to restore rotor speed.•The proposed stage II scheme can reduce the secondary frequency dip and suppress rotor speed oscillation of wind turbine generators under variable wind speeds.•The control strategy for switching wind turbine generators from stage I to stage II is presented to mitigate the adverse impact on system frequency. Under variable wind speeds, a key issue is how to design frequency control strategies to adaptively adjust the frequency control power of wind turbine generators according to variable wind speeds, thereby comprehensively improving the frequency nadir, the rate of change of frequency and the secondary frequency dip. To counter this issue, a two-stage frequency response method for doubly-fed induction generators is proposed, which consists of the stage I scheme and the stage II scheme. In the stage I scheme, the frequency control gain is adaptively adjusted according to real-time rotor speed of wind turbine generators and rate of change of frequency, which can enable wind turbine generators to fully provide frequency support power to mitigate the rate of change of frequency and frequency nadir while coordinating stage I and stage II to reduce the power deficit when the rotor speed starts to recover. In the stage II scheme, a novel power recovery form for wind turbine generators is designed to smooth the power injection of wind turbine generators in the rotor speed recovery stage and adaptively adjust the duration of speed recovery control according to different power deficits under variable wind speeds, which avoids a serious secondary frequency dip. Simulation results show the effective performance of proposed method under variable wind speeds.