Doubly-fed induction generator (DFIG)-based wind turbines with virtual inertia control can contribute to the primary frequency regulation and relieve the lack of system inertia caused by high penetration of large-scale wind farms. However

DFIG-based wind turbines will exit frequency regulation if the rotor speed falls below a definite value since the kinetic energy stored in the rotor is limited. This procedure and the subsequent rotor speed recovery will lead to a secondary frequency drop. The characteristics of virtual inertia control and available kinetic energy released by DFIG-based wind turbines under different wind speed conditions are quantitatively analyzed. Then an active rotor speed protection strategy is proposed to make DFIG-based wind turbines gradually release the kinetic energy to avoid the secondary frequency drop. In the proposed strategy

a negative feedback control is adopted to release rotor kinetic energy

and its proportional and integrated parameters are set adaptively according to the operational states and capacity of the DFIG-based wind turbines. The validity and effectiveness of the proposed protection strategy is illustrated by case studies.This work is supported by National Natural Science Foundation of China (No. 51477093

No. 51277112).