Method for estimating a wind speed at a wind turbine

1. A method for estimating a wind speed at a wind turbine, the wind turbine comprising a rotor carrying a set of wind turbine blades, each wind turbine blade having a variable pitch angle, the method comprising:

obtaining a rotational speed of the rotor;

obtaining a pitch angle of the wind turbine blades;

deriving a minimum tip speed ratio based on the obtained pitch angle, the minimum tip speed ratio defining an upper limit of a stable control region;

selecting an initial tip speed ratio that is greater than the minimum tip speed ratio;

deriving an initial estimated wind speed based on the initial tip speed ratio and the obtained rotational speed;

iteratively deriving an estimated wind speed based on the obtained rotational speed and the obtained pitch angle, and using the derived initial estimated wind speed as a starting point; and

using the estimated wind speed to control the wind turbine, wherein stability of the control region is based on a stability of the estimated wind speed.

2. The method according to claim 1 , further comprising controlling the wind turbine in accordance with the estimated wind speed.

3. The method according to claim 1 , wherein deriving a minimum tip speed ratio further comprises using a minimum tip speed ratio look-up table comprising interrelated values of pitch angle and minimum tip speed ratio.

4. The method according to claim 1 , wherein the selected initial tip speed ratio exceeds the minimum tip speed ratio at least by a predefined amount.

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

calculating an estimated tip speed ratio based on the obtained rotational speed and the estimated wind speed;

setting a flag, indicating that the estimated wind speed is unreliable, when the estimated wind speed results in the estimated tip speed ratio being lower than the minimum tip speed ratio; and

controlling the wind turbine in accordance with an alternate estimated wind speed, the alternate estimated wind speed corresponding to the minimum tip speed ratio and in accordance with the flag.

6. The method according to claim 5 , further comprising:

removing the flag and controlling the wind turbine in accordance with the estimated wind speed when the estimated tip speed ratio increases above the minimum tip speed ratio.

7. The method according to claim 1 , wherein iteratively deriving an estimated wind speed further comprises using a look-up table comprising interrelated values of rotational speed, wind speed, pitch angle, and power coefficient.

8. The method according to claim 1 , wherein iteratively deriving an estimated wind speed further comprises:

estimating a power output of the wind turbine and forming a power error signal by comparing the estimated power output to a measured power output of the wind turbine.

9. The method according to claim 8 , further comprising adjusting the power error signal by applying a gain factor and integrating the adjusted power error signal to form a new estimated wind speed feedback.

10. The method according to claim 1 , wherein the defined limit is such that when the wind turbine is operated at tip speed ratios below the minimum tip speed ratio, the estimated wind speed is unstable, and when the wind turbine is operated at tip speed ratios above the minimum tip speed ratio the estimated wind speed is stable.

11. The method according to claim 1 , wherein the upper limit of the stable control region is determined in part by where, for a given pitch angle, a partial derivative of a power output with respect to the wind speed is positive.

12. A wind turbine comprising: a rotor carrying a set of wind turbine blades, each wind turbine blade having a variable pitch angle; and a controller configured to implement an operation for estimating a wind speed at a wind turbine, the operation comprising:

obtaining a rotational speed of the rotor;

obtaining a pitch angle of the wind turbine blades;

deriving a minimum tip speed ratio based on the obtained pitch angle, the minimum tip speed ratio defining a boundary between a stable control region and an unstable control region;

selecting an initial tip speed ratio that is greater than the minimum tip speed ratio;

deriving an initial estimated wind speed based on the selected initial tip speed ratio and the obtained rotational speed; and

iteratively deriving an estimated wind speed based on the obtained rotational speed and the obtained pitch angle and using the derived initial estimated wind speed as a starting point;

wherein stability of the control region is based on a stability of the estimated wind speed.

13. A computer program product comprising computer executable code on a non-transitory computer readable storage medium which, when executed by one or more processors, implements an operation for estimating a wind speed at a wind turbine, the wind turbine comprising a rotor carrying a set of wind turbine blades, each wind turbine blade having a variable pitch angle, the operation comprising:

obtaining a rotational speed of the rotor;

obtaining a pitch angle of the wind turbine blades;

deriving a minimum tip speed ratio based on the obtained pitch angle, the minimum tip speed ratio defining an upper limit of a stable control region;

selecting an initial tip speed ratio that is greater than the minimum tip speed ratio;

deriving an initial estimated wind speed based on the initial tip speed ratio and the obtained rotational speed;

iteratively deriving an estimated wind speed based on the obtained rotational speed and the obtained pitch angle, and using the derived initial estimated wind speed as a starting point; and

using the estimated wind speed to control the wind turbine, wherein stability of the control region is based on a stability of the estimated wind speed.