Method for preventing rotor overspeed of a wind turbine

1. A method for preventing a possible overspeed of a rotor of a wind turbine, comprising:

detecting a yaw misalignment of the rotor; and,

imposing a restriction on changes of a pitch angle towards a fully-powered position of at least one rotor blade mounted to the rotor after detecting the yaw misalignment.

2. The method of claim 1 , wherein imposing the restriction comprises limiting changes of the pitch angle to changes in a direction towards a feathered position of the at least one rotor blade.

3. The method of claim 2 , wherein limiting changes of the pitch angle comprises at least one of preventing a further decrease of a first pitch angle of a primary pitch system of the at least one rotor blade, preventing a further decrease of a second pitch angle of a secondary pitch system of the at least one rotor blade, and limiting a speed of the pitch angle.

4. The method of claim 1 , wherein the restriction remains imposed during the yaw misalignment.

5. The method of claim 1 , further comprising preventing an increase of a power intake of the rotor by changing an active flow control and/or by changing an angle of a rotor blade flap.

6. The method of claim 1 , wherein detecting the yaw misalignment comprises at least one of:

determining an actual value of a yaw angle; and,

comparing the actual value of the yaw angle with a set-point of the yaw angle.

7. The method of claim 6 , wherein the restriction on the changes of the pitch angle is imposed when a difference between the actual value of the yaw angle and the set-point of the yaw angle is larger than a first threshold, and/or when an increase of a difference between an actual value of the yaw angle and a set-point of the yaw angle per time is larger than a speed threshold.

8. The method of claim 1 , further comprising, when the yaw misalignment lasts longer than a time threshold, at least one of continuously changing the pitch angle in a direction towards a feathered position of the at least one rotor blade, and realigning the rotor.

9. The method of claim 1 , further comprising:

detecting a difference between an actual value of a yaw angle and a set-point of the yaw angle which drops below a second threshold; and,

limiting a speed of the pitch angle.

10. The method of claim 1 , further comprising:

detecting a difference between an actual value of a yaw angle and a set-point of the yaw angle which drops below a third threshold; and,

allowing yaw-independent control of the pitch angle.

11. The method of claim 1 , wherein the restriction is imposed during active yaw control of the wind turbine.

12. A wind turbine, comprising:

rotor;

at least one rotor blade mounted to the rotor; and,

a control system comprising a pitch system configured to rotate the at least one rotor blade about a pitch axis and a yaw system configured to position the rotor with respect to a wind direction in accordance with a yaw angle set-point and comprising a yaw sensor configured to detect an actual yaw angle, the control system being configured to determine a difference between the actual yaw angle and the yaw angle set-point, and to determine based on the difference if a rotation of the at least one rotor blade in a direction to a fully-powered position is to be limited.

13. The wind turbine of claim 12 , wherein the control system is configured to operate the yaw system as an active yaw system.

14. The wind turbine of claim 12 , wherein the control system comprises a pitch control system connected to the pitch system and a generator mechanically coupled to the rotor, the pitch control system being configured to determine a pitch angle set-point for the pitch system in accordance with a set-point of the generator, wherein the yaw system comprises a yaw control system connected to the yaw sensor, wherein the yaw control system is configured to prevent decreasing the pitch angle set-point when the difference between the actual yaw angle and the yaw angle set-point is larger than a pre-defined first threshold.

15. The wind turbine of claim 14 , wherein the wind turbine comprises a turbine controller and wherein at least one of the pitch control system and the yaw control system is provided by the turbine controller.

16. A control system for controlling a wind turbine, comprising:

an input module configured to receive an actual value of a yaw angle of a yaw system of the wind turbine and an actual value of a status of an electrical system of the wind turbine;

a pitch module connected with the input module and being configured to determine a pitch angle set-point so that the status of the electrical system and a set-point of the status of the electrical system substantially matches; and,

a yaw module connected with the input module and the pitch module, the yaw module being configured to determine a yaw angle set-point and a difference between the yaw angle set-point and the actual value of the yaw angle, the yaw module being configured to prevent, depending on the difference between the yaw angle set-point and the actual value of the yaw angle, a decrease of the pitch angle set-point.

17. The control system of claim 16 , further comprising an output module connected with the yaw module and the pitch module and configured to issue the pitch angle set-point to a pitch system of the wind turbine.

18. The control system of claim 16 , wherein the yaw module is configured to prevent the decrease of the pitch angle when the difference between the yaw angle set-point and the actual value of the yaw angle is larger than a given first threshold.

19. The control system of claim 16 , wherein the yaw module is configured to prevent the decrease of the pitch angle when a function of a difference between the yaw angle set-point and the actual value of the yaw angle is larger than a given second threshold, and/or when a difference between the yaw angle set-point and the actual value of the yaw angle is increasing faster than a given speed threshold.

20. The control system of claim 16 , wherein the electrical system comprises at least one of a generator, a converter connected to the generator, and an inverter connected to the generator.