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A high-speed-level gear transmission system model of a wind turbine is presented considering a time-varying wind load and an electromagnetic torque disturbance, along with eccentricity, dynamic backlash, and friction force. The auto-regressive model is employed for simulating the time-varying wind load in the realistic wind field as external excitation. A doubly fed induction generator model of the wind turbine is established to calculate the disturbance quantity of electromagnetic torque. The nonlinear differential equations of the system are strictly deduced using Lagrange equation and solved by the fourth-order Runge-Kutta method. The effect of friction on the dynamic response of the high-speed-level gear transmission system is analyzed with the time-varying wind load and the electromagnetic torque disturbance. These results show that the friction force is critical because frequency amplitude and components can be changed by it. The friction force also enlarges vibration displacement. The low-frequency components in the vertical direction are affected gravely by the friction force without electrical disturbance. In addition, sidebands exist in the vicinity of the low-frequency parts as the electromagnetic torque disturbance appears at the output end. The amplitude of the low-frequency component is further increased because of electromagnetic torque disturbance. This shows the frequency characteristics of the slight gear system fault. The study offers some fresh references into the design and diagnosis of the gear system. [ABSTRACT FROM AUTHOR]