Oscillation Measurement on Wind Turbine Blade
One of the main challenges of inspecting a wind turbine blade on site, using a laser-based technique like shearography is coping with the oscillation of the blade in a windy environment. Relative movement between the blade and the shearography camera may render most of the images acquired by the inspection technique useless when the movement exceeds the rigid body motion tolerance of the shearography. One solution is to attach the shearography unit to the blade during inspection, thus essentially cancelling the relative movement between the shearography camera and the blade. Therefore characterising the oscillation of the blade under inspection conditions is of crucial importance in designing an on-site shearography inspection system.
No data was found in the academic or technical literature about the amplitude and frequency of the oscillation of large wind turbine blades, measured directly on the surface of the blade and under maintenance/inspection conditions. Alfonso Montellano López from the London South Bank Innovation Centre in collaboration with Xabier Echarte (Gamesa) and Javier Porres (from the wind turbine maintenance and inspection company Comantur S.L.) designed, set up and carried out a test session measuring the deflection of the blade at three different radii (distance from the nacelle along the surface of the blade) of a G90 (2MW) blade in Barchín del Hoyo (Spain) wind farm. The conditions of the test were representative of an inspection session given that the blade was stationary at the four possible pitch angles used in a G90 blade, the test lasted for about four hours and the wind speed at the farm was between 1 and 6 m/s, being 10 m/s the maximum wind speed allowed for safety reasons for inspection and maintenance work on site.
Real-time data on the oscillation of the blade was obtained using IMU sensors and post-processed using Octave. Results on the amplitude and frequency of the oscillation showed maximum peak amplitude of 94.3 mm at the tip of the blade and a maximum frequency of 0.35 Hz both for the in plane and out of plane direction, the reference plane being defined by the surface of the blade.