Method and arrangement for controlling a wind turbine

1. A method of controlling a wind turbine having a rotor to which at least one rotor blade is connected, the method comprising:

operating the rotor according to a first rotational speed reference;

operating the rotor according to a second rotational speed reference above the first rotational speed reference if a fluctuation of an actual rotational speed of the rotor decreases below a fluctuation threshold and/or if a blade pitch angle of the rotor blade increases above a blade pitch angle threshold;

wherein changing of operating the rotor according to the first rotational speed reference to operating the rotor according to the second rotational speed reference comprises:

supplying a control signal indicative of the second rotational speed reference to a turbine controller;

determining a decrease of an actual pitch angle based on a relationship between a power coefficient of the rotor blade versus blade pitch angle; and

decreasing the blade pitch angle according to the determined decrease of the blade pitch angle.

2. The method according to claim 1 , wherein the rotor is operated according to the first rotational speed reference if the fluctuation of the actual rotational speed of the rotor increases above another fluctuation threshold which is above the fluctuation threshold and/or if the blade pitch angle of the rotor blade decreases below another blade pitch angle threshold which is below the blade pitch angle threshold.

3. The method of claim 2 , wherein the rotor is operated according to the first rotational speed reference if the fluctuation of the actual rotational speed of the rotor increases above the another fluctuation threshold which is above the fluctuation threshold and/or if the blade pitch angle of the rotor blade decreases below the another blade pitch angle threshold which is below by 1° to 3° the blade pitch angle threshold.

4. The method according to claim 1 , wherein the blade pitch angle threshold lies above an optimal blade pitch angle to which an optimal power coefficient is associated.

5. The method according to claim 4 , wherein a decrease of the power coefficient with increasing blade pitch angle is smaller at the optimal blade pitch angle than at the blade pitch angle threshold.

6. The method according to claim 4 , wherein an aerodynamic thrust coefficient is larger at the optimal blade pitch angle than at the blade pitch angle threshold.

7. The method according to claim 1 , wherein controllability of the rotor is better at blade pitch angles above the blade pitch angle threshold than at blade pitch angles below the blade pitch angle threshold.

8. The method according to claim 1 , wherein the second rotational speed reference is determined based on the actual blade pitch angle.

9. The method of claim 8 , wherein the second rotational speed reference is determined based on a deviation of the blade pitch angle from the blade pitch angle threshold.

10. The method according to claim 1 , wherein operating the rotor according to the second rotational speed reference is performed if the blade pitch angle of the rotor blade is in a blade pitch angle range comprising blade pitch angles above the blade pitch angle threshold and below a further blade pitch angle threshold.

11. The method according to claim 1 , wherein operating the rotor according to a first rotational speed reference is performed in a wind speed range, where the actual rotational speed is substantially maintained constant according to the first rotational speed reference, wherein within the wind speed range the blade pitch angle is adjusted by increasing the blade pitch angle with increasing wind speed.

12. The method according to claim 11 , wherein operating the rotor according to a second rotational speed reference is performed in a further wind speed range above the wind speed range.

13. The method according to claim 1 , wherein, when operating the rotor according to the second rotational speed reference, power output of the wind turbine is increased relative to when operating the rotor according to the first rotational speed reference.

14. The method according to claim 13 , further comprising:

monitoring at least one electrical component of the wind turbine for an operational condition;

increasing, when operating the rotor according to the second rotational speed reference, power output, if the monitored operational condition of the electrical component allows increase of the power output.

15. The method according to claim 14 , wherein the electrical component of the wind turbine comprises at least one of:

a converter;

a transformer;

a cable;

a capacitor;

an inductor; and

a filter,

wherein the operational condition of the electrical component comprises at least one definition of:

a temperature of the electrical component;

a cooling of the electrical component; and

an electrical load of the electrical component.

16. An arrangement for controlling a wind turbine having a rotor to which at least one rotor blade is connected, the arrangement comprising:

a speed-blade pitch controller adapted to operate the rotor according to a first rotational speed reference,

wherein the speed-blade pitch controller is adapted to operate the rotor according to a second rotational speed reference above the first rotational speed reference if a fluctuation of an actual rotational speed of the rotor decreases below a fluctuation threshold and/or if a blade pitch angle of the rotor blade increases above a blade pitch angle threshold,

wherein changing of operating the rotor according to the first rotational speed reference to operating the rotor according to the second rotational speed reference comprises:

supplying a control signal indicative of the second rotational speed reference to a turbine controller;

determining a decrease of an actual pitch angle based on a relationship between a power coefficient of the rotor blade versus blade pitch angle; and

decreasing the blade pitch angle according to the determined decrease of the blade pitch angle.