Wind turbine control method

1. A control method of a wind turbine which comprises a series of blades, and a blade pitch angle control system, the method comprising the following steps:

calculating a value indicative of a misalignment φ of the wind turbine on a basis of at least one signal indicative of wind direction,

determining whether the value indicative of the misalignment φ of the wind turbine is above a first predefined misalignment threshold value;

adapting a value of a blade pitch angle β, wherein the step of adapting the value of the blade pitch angle β further comprises:

adapting the value of the blade pitch angle β at least on a basis of the value indicative of the misalignment φ of the wind turbine if the value indicative of the misalignment φ of the wind turbine is determined to be above the first predefined misalignment threshold value; and

adapting the value of the blade pitch angle β without the value indicative of the misalignment φ of the wind turbine if the value indicative of the misalignment φ of the wind turbine is determined not to be above the first predefined misalignment threshold value, and

rotating at least one of the blades of the wind turbine about its longitudinal axis on a basis of the adapted value of the blade pitch angle β.

2. The control method according to claim 1 wherein the step of adapting the value of the blade pitch angle β at least on the basis of the value of the misalignment φ further comprises:

calculating an initial set point of the blade pitch angle β,

modifying the initial set point of the blade pitch angle β if the initial set point of the blade pitch angle β is less than a minimum blade pitch angle β MIN so that a blade pitch angle final set point is greater than or equal to the minimum blade pitch angle β MIN ; and

rotating at least one of the blades of the wind turbine about its longitudinal axis on the basis of the blade pitch angle final set point value;

wherein the minimum blade pitch angle β MIN is calculated on the basis of at least the value indicative of the misalignment φ.

3. The control method according to claim 2 , wherein the calculation of the minimum blade pitch angle β MIN comprises carrying out a comparison of a signal indicative of wind speed with a curve or table comprising a correlation between the minimum blade pitch angle β MIN , which defines a stalling threshold, and each value of the signal indicative of wind speed.

4. The control method according to claim 3 , wherein the signal indicative of wind speed comprises a blade tip speed ratio λ, defined as a quotient between a linear speed of the blade tip and the wind speed:

λ

=

blade

⁢

tip

⁢

linear

⁢

speed

wind

⁢

speed

5. The control method according to claim 4 , wherein the signal indicative of wind speed is a filtered measurement of the blade tip speed ratio λ obtained by means of applying a filter F 1 to the blade tip speed ratio λ.

6. The control method according to claim 5 wherein the filter F 1 comprises a time constant τ which is configurable.

7. The control method according to claim 6 wherein the filter F 1 is a moving mean.

8. The control method according to claim 7 further comprising calculating the moving mean with a configurable number of used points.

9. The control method according to claim 5 wherein the step of calculating the minimum blade pitch angle β MIN comprises a modification of a filtration time constant τ for the filter F 1 applied to the blade tip speed ratio λ.

10. The control method according to claim 9 further comprising comparing the value indicative of the misalignment φ with the first predefined misalignment threshold value and in that the modification of the filtration time constant τ for the filter F 1 is related to the value indicative of the misalignment φ; in such a way that the filtration time constant τ for the filter F 1 is greater when the value indicative of the misalignment φ is greater than the first predefined misalignment threshold value and lower when the value indicative of the misalignment φ is lower than the first predefined misalignment threshold value.

11. The control method according to claim 9 wherein the modification of the filtration time constant τ for the filter F 1 applied to the blade tip speed ratio λ is carried out in a zone of partial production.

12. The control method according to claim 9 wherein if after a predefined time interval ΔT from a change in misalignment triggered when a rate of change of misalignment with respect to time is above a second predefined misalignment threshold value the wind speed is maintained, then the filtration time constant τ of the filter F 1 regains its original value τ.

13. The control method according to claim 3 , wherein the calculation of the minimum blade pitch angle β MIN comprises a step of adding an additional term Δβ MIN to the minimum blade pitch angle β MIN obtained through comparing the signal indicative of wind speed with the curve or table comprising the correlation between the minimum blade pitch angle β MIN , which marks a stalling threshold for each value of the signal indicative of wind speed, when an existence of misalignment is determined on the basis of the value indicative of the misalignment φ.

14. The control method according to claim 13 wherein the additional term Δβ MIN is predetermined or dependent on the value indicative of the misalignment φ.

15. The control method according to claim 13 wherein if after a predefined time interval ΔT from a change in misalignment triggered when a rate of change of misalignment with respect to time is above a second predefined misalignment threshold value the wind direction is maintained, then the additional term Δβ MIN is canceled.

16. The control method according to claim 2 wherein the step of calculating the initial set point of the blade pitch angle β is performed on the basis of at least one value related to an error in a speed of rotation of a rotor of the wind turbine.

17. A control method of a wind turbine which comprises a series of blades, and a blade pitch angle control system, the method comprising the following steps:

calculating a value indicative of a misalignment φ of the wind turbine on a basis of at least one signal indicative of wind direction,

determining whether the value indicative of the misalignment φ of the wind turbine is above a first predefined misalignment threshold value;

if the value indicative of the misalignment φ of the wind turbine is determined to be above the first predefined misalignment threshold value, adapting a value of a blade pitch angle β at least on a basis of the value indicative of the misalignment φ of the wind turbine; and

if the value indicative of the misalignment φ of the wind turbine is determined not to be above the first predefined misalignment threshold value, the value of the blade pitch angle β is not adapted; and

rotating at least one of the blades of the wind turbine about its longitudinal axis on the basis of the value of the blade pitch angle β.