School of Electrical and Information Engineering, Zhengzhou University of Light Industry, Zhengzhou 450002, China

The active frequency response capability of wind turbines has become a prerequisite for the wind power integration in the future, but the improper switching of control strategies between the frequency support stage and the speed recovery stage may cause a secondary frequency drop (SFD). Aiming at this problem, this paper proposes a flexible frequency response strategy for wind turbines. Firstly, the nonlinear model of wind turbines is linearized at the initial operation point, and the impact of the initial operation point on the output characteristics of wind turbines is analyzed. Secondly, based on the linearized model of wind turbines, the frequency response transfer function for the power system composed of wind power and thermal power is derived, and the impact of system parameters on the dynamic characteristics of frequency response is studied. Then, the contradiction between SFD and rotational speed recovery is studied, and a participation factor of auxiliary power is constructed by using piecewise function and Logistic function. The electromagnetic power output of the wind turbine is adjusted through shape coefficients. Finally, the comparative simulation study is conducted on the MATLAB/Simulink platform. Simulation results show that the proposed strategy can eliminate the SFD while guaranteeing a satisfactory rotational speed recovery.