When operating a wind turbine it is sometimes desirable to be able to estimate the wind speed prevailing at the wind turbine, more particularly the wind speed experienced by the wind turbine blades of the wind turbine. When the wind speed is measured, this is often done at a point behind the rotor of the wind turbine. Thereby the measured wind speed is affected by the impact on the wind by the rotor, and therefore it does not reflect the wind speed at a position in front of the rotor. Furthermore, since the wind speed is typically measured in a single point, it will not reflect variations in the wind speed across an area defined by the rotor. Accordingly, controlling the wind turbine on the basis of such a measured wind speed may lead to inaccurate control of the wind turbine.
Therefore, various attempts have previously been made in order to provide an estimate for the wind speed at a wind turbine.
U.S. Pat. No. 5,155,375 discloses a controller and a method for operating a variable speed wind turbine to better track wind speed fluctuations for greater efficiency in conversion of wind energy to electrical energy. The rotor speed is controlled with a wind speed supplied by a wind observer which predicts the average wind speed at a subsequent point in time over the cross section presented to the wind by the wind turbine. The wind speed is predicted as a function of the present (previously predicted) wind speed and correction terms including net torque and the difference between the predicted and actual rotor speed.