The influence of turbulence on the aero-elastic instability of wind turbines

Modern multi-megawatt wind turbines are designed with longer and slender blades using new composite materials and advanced fabrication methods. The trend towards lighter and more flexible blades may lead to aeroelastic instability of wind turbines under certain circumstances, thus resulting in rapid destructive failure or limit-cycle oscillations of the wind turbine components. Turbulence, which is ignored in the existing stability analysis tools, actually plays an important role in the aeroelastic instability since the angles of attack along the blade are highly dependent on the turbulence. This paper investigates the influence of turbulence on the aeroelastic instability of wind turbines based on simulated time histories. Focus is on the high-performance wind turbines operating close to stall, where the increased turbulence intensity may trigger off aeroelastic instability due to negative aerodynamic damping. A 13-degree-of-freedom (13-DOF) wind turbine model is developed using Euler-Lagrange equations, which includes the couplings of the tower-blade-drivetrain vibration, the quasi-static aeroelasticity and a collective pitch controller. Numerical simulations are carried out using data calibrated to the NREL 5 MW baseline wind turbine. Aeroelastic stability of the wind turbine system has been evaluated for various values of the rated generator torque, the rated rotational speed of the rotor, the mean wind speed and the turbulence intensity. Critical turbulence intensity, at which the wind turbine shifts from a stable state into an instable state, is determined in different cases. Results show that turbulence intensity has significant influence on the aeroelastic stability of high-performance wind turbines operating close to stall, and the stability of the wind turbine might be changed due to the increase of the turbulence intensity.