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Wind turbine, yaw system controller and yaw system for a wind turbine and method of reducing the loads on such a yaw system. A method for reducing the loads acting on a wind turbine yaw system (19) due to yawing moments which are induced to the yaw system (19) by a rotor which comprises at least one

Wind turbine, yaw system controller and yaw system for a wind turbine and method of reducing the loads on such a yaw system. A method for reducing the loads acting on a wind turbine yaw system (19) due to yawing moments which are induced to the yaw system (19) by a rotor which comprises at least one rotor blade with a pitch control system is provided. In the method, the yawing moment which is induced to the yaw system (19) by the rotor is determined and the pitch of the at least one rotor blade is set based on the detected yawing moment such that the determined yawing moment is reduced.

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1. A method of reducing loading acting on a wind turbine yaw system due to yawing moments induced to the yaw system by a rotor having a rotor blade with a pitch control system, comprising: determining the yawing moment induced to the yaw system by the rotor; andsetting a pitch of the rotor blade bas

1. A method of reducing loading acting on a wind turbine yaw system due to yawing moments induced to the yaw system by a rotor having a rotor blade with a pitch control system, comprising: determining the yawing moment induced to the yaw system by the rotor; andsetting a pitch of the rotor blade based on the detected yawing moment such that the determined yawing moment is reduced,wherein the yawing moment is determined from a yaw system observable,wherein the yaw system is aligned by providing torques by a plurality of yaw drives where the torques act in opposite directions, and the yawing moment induced to the yaw system is determined by determining a difference between the torques provided by the yaw drives in order to keep the yaw system aligned. 2. The method as claimed in claim 1, wherein the pitch of each rotor blade of the rotor is individually set to reduce the determined yawing moment. 3. The method as claimed in claim 1, wherein the pitch of the rotor blade is set based on the detected yawing moment only when the detected yawing moment exceeds a predefined value. 4. The method as claimed in claim 1, wherein the plurality of yaw drives are electrical drives and the torques provided by the yaw drives are determined from the electrical power needed by the yaw drives for providing the torques. 5. The method as claimed in claim 1, wherein the plurality of yaw drives are hydraulic drives each provided with a hydraulic fluid input and a hydraulic fluid output and the torques provided by the yaw drives are determined by determining for each yaw drive the difference in the hydraulic pressure at the hydraulic fluid input to the hydraulic pressure at the hydraulic fluid output which is needed by the respective yaw drive for providing the respective torque. 6. A method of reducing loading acting on a wind turbine yaw system due to yawing moments induced to the yaw system by a rotor having a rotor blade with a pitch control system, comprising: determining the yawing moment induced to the yaw system by the rotor; andsetting a pitch of the rotor blade based on the detected yawing moment such that the determined yawing moment is reduced,wherein the yawing moment is determined from a yaw system observable,wherein the yaw drive provides a retaining torque for keeping the yaw system aligned and the yawing moment induced to the yaw system is determined from the retaining torque which is necessary to keep the yaw system aligned. 7. The method as claimed in claim 6, wherein the yaw drive is an electrical drive and the retaining torque provided by the yaw drive is determined from the electrical power needed by the yaw drive for providing the retaining torque. 8. The method as claimed in claim 6, wherein the yaw drive is a hydraulic drive provided with a hydraulic fluid input and a hydraulic fluid output and the retaining torque provided by the at least one yaw drive is determined from the difference in the hydraulic pressure at the hydraulic fluid input to the hydraulic pressure at the hydraulic fluid output which is needed by the yaw drive for providing the retaining torque. 9. A method of reducing loading acting on a wind turbine yaw system due to yawing moments induced to the yaw system by a rotor having a rotor blade with a pitch control system, comprising: determining the yawing moment induced to the yaw system by the rotor; andsetting a pitch of the rotor blade based on the detected yawing moment such that the determined yawing moment is reduced,wherein the yawing moment is determined from a yaw system observable,wherein the yaw system is kept aligned by providing a retaining friction and the yawing moment induced to the yaw system is determined by determining the retaining friction necessary to keep the yaw system aligned. 10. The method as claimed in claim 9, wherein the retaining friction is provided by an electromagnetic friction brake and the retaining friction provided by the electromagnetic friction brake is determined from the electrical power needed by the electromagnetic friction brake to provide the retaining friction. 11. The method as claimed in claim 9, wherein the retaining friction is provided by a hydraulic friction brake and the retaining friction provided by the hydraulic friction brake is determined from the hydraulic pressure needed by the hydraulic friction brake to provide the retaining friction. 12. A yaw system controller for a wind turbine having a rotor with a rotor blade and a pitch drive for setting the pitch of the rotor blade, a pitch controller and a yaw system with at least one yaw drive, comprising: an error signal input for receiving an error input signal that represents a yawing moment induced to the yaw system by the rotor, anda yaw drive load controller output where the yaw drive load controller determines and outputs a horizontal pitch signal through the yaw drive load controller output to the pitch controller of a wind turbine, wherein the horizontal pitch signal is based on the error input signal and represents a pitch to be set by the pitch drive such as to reduce a yawing moment induced to the yaw system by the rotor,an error signal deriving device that derives the error signal from a yaw system observable,a yaw drive controller that determines and outputs a drive torque signal representing a torque to be provided by the yaw drive,the yaw drive controller comprising a yaw speed controller which comprises a yaw speed controller output and which determines and outputs a torque reference signal through the yaw speed controller output,the yaw drive controller further comprises a yaw torque controller connected to the yaw speed controller output to receive the torque reference signal and which is adapted to determine the drive torque signal based on the torque reference signal, andwherein the error signal input of the yaw drive load controller is connected to the yaw speed controller output for receiving the torque reference signal as the error input signal. 13. A yaw system controller for a wind turbine having a rotor with a rotor blade and a pitch drive for setting the pitch of the rotor blade, a pitch controller and a yaw system with at least one yaw drive, comprising: an error signal input for receiving an error input signal that represents a yawing moment induced to the yaw system by the rotor, anda yaw drive load controller output where the yaw drive load controller determines and outputs a horizontal pitch signal through the yaw drive load controller output to the pitch controller of a wind turbine, wherein the horizontal pitch signal is based on the error input signal and represents a pitch to be set by the pitch drive such as to reduce a yawing moment induced to the yaw system by the rotor,an error signal deriving device that derives the error signal from a yaw system observable,a friction brake controller for controlling the friction provided by a friction brake of a yaw system with a friction brake controller output, where the friction brake controller determines and outputs a friction signal through the friction brake controller output to a friction brake, the friction signal representing a friction to be provided by the friction brake, andwhere the error signal input of the yaw drive load controller is connected to the friction brake controller output for receiving the friction signal as the error input signal. 14. The yaw system controller as claimed in claim 13, wherein the error signal input of the yaw drive load controller receives a yaw speed signal from a yaw speed detector of the yaw system as the error input signal. 15. The yaw system controller as claimed in claim 14, wherein the yaw drive load controller comprises a PI-controller.