Measurements of tower deflections on full-scale wind turbines using an opto-mechanical platform

Abstract

In this study tower deflections of a full-scale turbine are measured with an opto-mechanical platform. This measurement approach is advantageous as no modification to the turbine is required. The tower deflections are measured simultaneously with measurements of wind speed and turbulence upstream of the turbine and of turbine parameters (blade pitch angle, rotor torque and rotor speed). Measurements show that tower deflections during normal operation are sensitive to the yaw misalignment of the rotor, and the tower deflections with negative yaw misalignment are seen to be larger than with positive yaw misalignment. The tower deflections during power cut-off and normal operation are, respectively, modeled with free and forced vibration single-degree-of-freedom models to calculate the damping ratio. The damping ratio during power cut-off, 6.8%, is in the lower range of damping ratios that are measured during normal operation, 5.5%–13.2%. These findings suggest that blade pitch and rotor torque can be used to alleviate the unsteady structural loads caused by the lightly damped oscillations during transition to power cut-off. Additionally, positive yaw misalignments can be used as a strategy for load alleviation during normal operation of wind turbines.