A method implemented in a power grid including a DFIG wind turbine is provided. The method includes: assuming the DFIG wind turbine to be operated in a MPPT mode; generating a simplified transient model of the DFIG wind turbine, the simplified transient model including an equivalent circuit model, and an equivalent rotor motion model in a nonlinear form; linearizing the equivalent rotor motion model in the nonlinear form to be an equivalent rotor motion model in a linear form with respect to a steady-state operating point of the DFIG wind turbine; and determining a first contribution and a second contribution of the DFIG wind turbines to a post-disturbance frequency of the power grid, in a center of inertia (COI) frequency frame and in a frequency spatial variation frame, respectively, by incorporating the simplified transient model of the DFIG wind turbine into a frequency dynamics analysis.