Abstract: Through virtual inertia control, doubly fed induction generator (DFIG)-based wind turbines (WTs) can participate into frequency regulation of the power system by increasing the output active power and releasing the kinetic energy stored in the rotor for a period of time. However, it will cause secondary frequency drop of the system when the WT terminates frequency regulation. In this paper, the characteristics of the WT's rotor speed trajectory during virtual inertia control are investigated. Then a dynamic rotor speed protection (DRSP) strategy is proposed to guarantee the stable operation of the WT during frequency regulation and avoid the secondary frequency drop. Simulations under different wind speed conditions are conducted in DIgSILENT/PowerFactory and the results validate the effectiveness of the proposed strategy.