Device and method of controlling blade instabilities of a wind turbine to avoid blade fluttering

A method of controlling blade instabilities of a wind turbine, the wind turbine comprising a rotor having a plurality of rotor blades, the rotor being mounted to a nacelle to rotate about a rotation axis with a rotor speed, wherein each blade is configured to be pitched by a pitch angle about a pitch axis of the blade, the method comprising:

• defining at least one preliminary overspeed threshold value;
• defining a fluttering rotor speed at and above which a predetermined fluttering of at least one of the blades occurs, the fluttering rotor speed is defined as a function of the pitch angle and/or as a function of the wind speed;
• setting a final overspeed threshold value to be equal to or smaller than a minimum rotor speed of the at least one preliminary overspeed threshold value and the fluttering rotor speed at an actual pitch angle and/or at an actual wind speed;
• and
• controlling the rotor speed to not exceed the final overspeed threshold value.