Control method for a wind turbine

1. A method of controlling a wind turbine, the wind turbine comprising at least one wind direction sensor for measuring a wind direction relative to the wind turbine, a number of pitch-adjustable rotor blades, a yawing system, and a control system for yawing a rotor of the wind turbine relative to the wind and for changing the pitch of the rotor blades and/or a generator torque, the method comprising:

receiving, for a first wind direction interval of a plurality of wind direction intervals, a power that the wind turbine is expected to produce for the first wind direction interval;

receiving measured power produced by the wind turbine at relative wind directions that fall within a second wind direction interval of the plurality of wind direction intervals, the second wind direction interval is different from the first wind direction interval;

determining an average power produced by the wind turbine for the second wind direction interval by averaging the measured power produced by the wind turbine at the relative wind directions that fall within the second wind direction interval;

upon determining that the average power produced by the wind turbine for the second wind direction interval is equal to the power that the wind turbine is expected to produce for the first wind direction interval, determining a difference between the first wind direction interval and the second wind direction interval;

receiving a wind direction measured by the wind direction sensor;

upon determining that the wind direction falls within the second wind direction interval, adjusting the wind direction based on the determined difference; and

applying the adjusted wind direction in the controlling of the wind turbine.

2. The method of controlling according to claim 1 , further comprising calibrating the wind direction measured by the wind direction sensor as a function of a predetermined calibration parameter such that a calibrated measured wind direction of 0 degrees corresponds to a relative wind direction of 0 degrees.

3. The method of controlling according to claim 1 , wherein the wind direction measured by the wind direction sensor is adjusted by an interpolation function between determined differences of different wind direction intervals.

4. The method of controlling according to claim 1 , wherein the power produced by the wind turbine is determined by measuring a grid power or estimating a generator output power.

5. The method of controlling according to claim 1 , further comprising determining a blade load by determining a root moment of one or more of the rotor blades.

6. The method of controlling according to claim 1 , wherein the power that the wind turbine is expected to produce for the first wind direction interval is estimated based on a cos x function of the first wind direction interval, wherein x is a constant.

7. The method of controlling according to claim 1 , further comprising obtaining a number of pre-defined wind speed intervals and obtaining a group of data sets and a second difference for each wind speed interval.

8. The method of controlling according to claim 7 , wherein the wind direction measured by the wind direction sensor is adjusted as a function of the respective second difference of the respective wind speed interval comprising a wind speed at a time of adjusting.

9. The method of controlling according to claim 7 , where the group of data sets is determined over a predetermined time period.

10. The method of controlling according to claim 7 , where the group of data sets is determined based on a predetermined number of data sets in the group of data sets.

11. The method of controlling according to claim 7 , where the steps of obtaining the group of data sets and determining the respective second differences are repeated at time intervals.

12. The method of controlling according to claim 1 , where adjusting the wind direction measured by the wind direction sensor comprises subtracting the difference times a gain factor smaller than one.

13. A control system for a wind turbine comprising at least one wind direction sensor for measuring a wind direction relative to the wind turbine, a number of pitch-adjustable rotor blades, and a yawing system, the control system being configured for yawing a rotor of the wind turbine relative to the wind and for changing the pitch of the rotor blades and/or a generator torque, and where the control system is configured to perform the steps of:

receiving, for a first wind direction interval of a plurality of wind direction intervals, a power that the wind turbine is expected to produce for the first wind direction interval;

receiving measured power produced by the wind turbine at relative wind directions that fall within a second wind direction interval of the plurality of wind direction intervals, the second wind direction interval is different from the first wind direction interval;

determining an average power produced by the wind turbine for the second wind direction interval by averaging the measured power produced by the wind turbine at the relative wind directions that fall within the second wind direction interval;

upon determining that the average power produced by the wind turbine for the second wind direction interval is equal to the power that the wind turbine is expected to produce for the first wind direction interval, determining a difference between the first wind direction interval and the second wind direction interval

receiving a wind direction measured by the wind direction sensor;

upon determining that the wind direction falls within the second wind direction interval, adjusting the wind direction based on the determined difference; and

applying the adjusted wind direction in controlling the wind turbine.

14. A wind turbine, comprising:

a tower;

a nacelle disposed on the tower;

a rotor extending from the nacelle;

a plurality of blades disposed on the rotor;

one or more actuators mechanically coupled to the plurality of blades and configured to control a respective pitch of the plurality of blades;

a wind direction sensor configured to measure a wind direction relative to the wind turbine;

a yawing system configured to yaw a rotor of the wind turbine relative to the wind direction; and

a control system configured to issue commands to the yawing system and the one or more actuators, and where the control system is configured to perform an operation comprising:

receiving, for a first wind direction interval of a plurality of wind direction intervals, a power that the wind turbine is expected to produce for the first wind direction interval;

receiving measured power produced by the wind turbine at relative wind directions that fall within a second wind direction interval of the plurality of wind direction intervals, the second wind direction interval is different from the first wind direction interval;

determining an average power produced by the wind turbine for the second wind direction interval by averaging the measured power produced by the wind turbine at the relative wind directions that fall within the second wind direction interval;

upon determining that the average power produced by the wind turbine for the second wind direction interval is equal to the power that the wind turbine is expected to produce for the first wind direction interval, determining a difference between the first wind direction interval and the second wind direction interval;

receiving a wind direction measured by the wind direction sensor;

upon determining that the wind direction falls within the second wind direction interval, adjusting the wind direction based on the determined difference; and

applying the adjusted wind direction in the controlling of the wind turbine.