Control for a wind turbine

1. A method of controlling a wind turbine comprising a nacelle, a rotor, a rotating hub, a first rotor blade and at least a second rotor blade, both rotor blades being mounted to the hub, the second rotor blade trailing the first rotor blade in a rotor plane of the wind turbine, the method comprising:

measuring a strain in the first rotor blade at a first angular position within the rotor plane using at least one strain measurement device attached to the first rotor blade;

calculating a speed of an impinging airflow at the first angular position based at least on the measured strain in the first rotor blade at the first angular position;

determining a rotational speed of the rotor;

calculating an expected time needed by the second rotor blade to reach the first angular position within the rotor plane, wherein the calculation is carried out based on the determined rotational speed of the rotor and the angle between the first rotor blade and the second rotor blade; and

changing an orientation of the second rotor blade to an optimized orientation within the expected time such that the second rotor blade is in the optimized orientation before reaching the first angular position,

wherein the optimized orientation is selected based on the calculated speed of the impinging airflow at the first angular position to achieve a goal selected from the group consisting of reducing fatigue damage on the second rotor blade, increasing overall performance of the wind turbine, and combinations thereof.

2. The method according to claim 1 , wherein the orientation of the second rotor blade is changed by pitching the second rotor blade.

3. The method according to claim 1 , wherein the orientation of the second rotor blade is changed by yawing the nacelle of the wind turbine.

4. The method according to claim 1 , further comprising:

calculating a second expected time needed by a third rotor blade of the wind turbine to reach the first angular position within the rotor plane, wherein the calculation is carried out based on the determined rotational speed of the rotor and the angle between the first rotor blade and the third rotor blade, and

changing an orientation of the third rotor blade to a second optimized orientation within the second expected time such that the third rotor blade is in the second optimized orientation before reaching the first angular position,

wherein the second optimized orientation is selected based on the speed of the impinging airflow at the first angular position to achieve a goal selected from the group consisting of reducing fatigue damage on the third rotor blade, increasing overall performance of the wind turbine, and combinations thereof.

5. The method according to claim 1 ,

wherein a plurality of strain measurement devices are arranged along the span of the first rotor blade between the root portion of the first rotor blade and the tip portion of the first rotor blade; and

wherein the method comprises

obtaining a plurality of strain measurements along the span of the first rotor blade using the plurality of strain measurement devices;

calculating a speed distribution of the impinging airflow along the first rotor blade at the first angular position based at least on the plurality of strain measurements; and

wherein the optimized orientation is selected based on the calculated speed distribution of the impinging airflow at the first angular position.

6. The method according to claim 1 , wherein the strain in all rotor blades of the wind turbine are measured by respective strain measurement devices attached to the respective rotor blades.

7. A wind turbine, comprising:

a nacelle; a rotor; a rotating hub; a first rotor blade; at least a second rotor blade, both rotor blades being mounted to the hub, the second rotor blade trailing the first rotor blade in a rotor plane of the wind turbine;

at least one strain measurement device attached to the first rotor blade for measuring a strain in the first rotor blade at a first radial position along the first rotor blade and at a first angular position within the rotor plane; and

a controller adapted

to calculate a speed of an impinging airflow at the first angular position based at least on the measured strain in the first rotor blade at the first angular position;

determine a rotational speed of the rotor,

to calculate an expected time needed by the second rotor blade to reach the first angular position within the rotor plane, wherein the calculation is carried out based on the determined rotational speed of the rotor and the angle between the first rotor blade and the second rotor blade, and

to change an orientation of the second rotor blade to an optimized orientation within the expected time such that the second rotor blade is in the optimized orientation before reaching the first angular position,

wherein the optimized orientation is selected based on the calculated speed of the impinging airflow at the first angular position to achieve a goal selected from the group consisting of reducing fatigue damage on the second rotor blade, increasing overall performance of the wind turbine, and combinations thereof.

8. The wind turbine according to claim 7 , wherein the strain measurement device is arranged at the root portion of the first rotor blade.

9. The wind turbine according to claim 7 , wherein a plurality of strain measurement devices are arranged along the span of the first rotor blade between the root portion of the first rotor blade and the tip portion of the first rotor blade.

10. The wind turbine according to claim 9 , wherein the strain measurement devices are connected via optical fibers or wireless with each other.

11. A wind turbine which is controlled by the method according to claim 1 .

12. The wind turbine according to claim 11 , further comprising a controller adapted to control the wind turbine.