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.