Abstract: Million watt scale directly-driven wind turbines are chosen as the study object and its dynamic characteristics are analyzed at the condition of a blade fracture accident happening. By integrating the wind rotor loads sub-model, electromechanical coupling sub-model and pitch control sub-model, the system models of wind turbines are obtained. In the wind rotor loads sub-model, not only the aerodynamic force but also the inertia force and gravity force are considered; in the pitch actuator sub-model, the dynamic characteristics are considered. According to the established wind turbine models, the corresponding numerical simulation models are constructed in Simulink; the simulation is carried out assuming that wind turbines have yet to be stopped after a few seconds of blade fracture. The research results show that when a blade fracture happening, the wind rotor mechanical rotational torque mutates. Since the generator electromagnetic torque tracks quickly the wind rotor mechanical rotational torque, the wind rotor rotating speed remains near the given value. Furthermore, the energy capture and conversion of wind turbines is out of balance; the DC capacitor voltage, the active power and reactive power injected into the grid have dramatic fluctuations which has impact on the grid. Through the research, the dynamic characteristics of wind turbines are obtained when a blade fracture happening which can provide useful reference for wind turbine structure design, operation control, etc.

Key words: Directly-driven wind turbines, Dynamic characteristics, Fracture accident, Wind turbines blade