US Patent 9719493 Method for dampening oscillations in a wind turbine

A method is provided for dampening oscillations in a wind turbine, the wind turbine having a tower 1, a rotor 3 arranged on the tower 1 such as to be able to rotate about an X axis, and a plurality of blades 4 with adjustable pitch mounted on the rotor. The method includes monitoring the fore-aft oscillation of the tower 1 in the direction of the X axis, at the 2nd or a higher bending mode frequency; determining a compensating torque to be applied by the rotor 3 to the tower 1 of the wind turbine about a Y axis, which is horizontal and at right angles to the X axis, for at least partly dampening the oscillation at the 2nd or higher bending mode frequency; determining for each rotor blade 4 of the wind turbine an adjustment of the pitch angle suitable to generate the compensating torque about the Y axis; and adjusting the pitch angle β1, β2, β3 of at least one of the individual blades 4 to generate at least part of the compensating torque.