US 2003/0091436 A1 discloses a method for regulating a windmill and an apparatus for the use of said method, wherein a device for regulating the airflow around a blade of a windmill comprises a spoiler which is provided with a cavity. The spoiler is mounted to a surface of the blade and can change the airflow around the blade by assuming different forms. When the spoiler is in an activated form, the airflow is changed in that the spoiler no longer follows the contours of the blade and creates a discontinuity or at least changes the profile of the blade in such way that the airflow conditions are changed. It is possible to regulate the speed of rotation of the rotor on which the blade is mounted.
Conventionally, a wind turbine rotor speed may be controlled using a pitch system. The pitch system may rotate the entire turbine blade in order to keep the power output close to constant, in particular for wind speeds above rated wind speed. The effect of the blade rotation may be a change in the angle of attack (of the wind) and thus a change of lift of the blade and ultimately the torque of the rotor.
In the conventionally known methods and systems, controlling the speed of a wind turbine rotor may be difficult and problematic.
Thus, there may be a need for a method and an arrangement for controlling a rotational speed of a rotor of a wind turbine which is reliable, safe, which reduces wear of components of the wind turbine and which is in particular applicable at wind speeds above rated wind speeds.