Control of wind turbines in response to wind shear

1. A method of controlling a wind turbine having a rotor and a plurality of rotor blades, comprising:

obtaining a wind speed value;

determining, using one or more processors, blade loading, comprising determining blade bending moments;

estimating a component of vertical wind shear across a rotor plane based on the wind speed value, an estimate of main bearing tilt moment derived from the blade bending moments, a measure of blade pitch, and azimuth angle;

estimating a wind velocity over the rotor plane based on the component of vertical wind shear;

determining a tip speed ratio based on a speed of rotation of a tip of the rotor blades, the wind speed value and the estimated wind velocity over the rotor plane;

selecting a curve of tip speed ratio versus blade pitch angle based on the estimated vertical wind shear component;

selecting a blade pitch angle based on the selected curve and the tip speed ratio;

automatically adjusting a blade pitch of the wind turbine in accordance with the selected blade pitch angle; and

operating the wind turbine using the adjusted blade pitch.

2. A method according to claim 1 , wherein the vertical wind shear is further estimated from a measure of yaw error.

3. A method according to claim 1 , wherein the wind speed value is the wind speed determined at a nacelle of the wind turbine.

4. A method according to claim 1 , wherein the blade pitch angle is a collective blade pitch angle applied to adjust all of the plurality of rotor blades.

5. A method according to claim 1 , wherein the blade pitch angle is an individual blade pitch angle and the selection of the blade pitch angle is performed for each of the plurality of rotor blades.

6. A method according to claim 5 , wherein tip speed ratio is determined at a plurality of positions on the rotor plane and the blade pitch angle selected for each of the plurality of positions.

7. A method of controlling a wind turbine having a rotor and a plurality of rotor blades, comprising:

obtaining a wind speed value;

determining, using one or more processors, blade loading comprising determining blade bending moments;

estimating a component of horizontal wind shear across a rotor plane based on an estimate of main bearing yaw moment derived from the blade bending moments, azimuth angle, and blade pitch position;

estimating a wind velocity over the rotor plane based on the component of horizontal wind shear;

determining a tip speed ratio based on a speed of rotation of a top of the rotor blades, the wind speed and the estimated wind velocity over the rotor plane;

selecting a curve of tip speed ratio versus blade pitch angle based on the estimated horizontal wind shear component;

selecting a blade pitch angle based on the selected curve and the tip speed ratio;

automatically adjusting the blade pitch in accordance with the selected blade pitch angle; and

operating the wind turbine using the adjusted blade pitch.

8. A method according to claim 7 , wherein the horizontal wind shear is estimated from the yaw moment, the wind speed and a measure of inflow angle.

9. A control system for a wind turbine having a rotor and a plurality of rotor blades, the control system comprising:

a blade load sensor;

an azimuth position sensor;

a blade pitch position sensor;

a processor;

a memory storing a program which, when executed on a processor, performs an operation comprising:

determining flapwise blade bending moments based on signals from the blade load sensors;

determining an estimate of main bearing tilt moment based on the flapwise blade bending moments and a blade pitch position and azimuth position;

estimating a component of vertical wind shear based on the main bearing tilt moment and a wind speed;

estimating a wind velocity over a rotor plane based on the component of vertical wind shear;

determining a tip speed ratio based on a speed of rotation of a tip of the rotor blades, the estimated wind velocity over the rotor plane, and a set of curves of tip speed ratio and blade pitch reference for a given vertical wind shear; and

determining a blade pitch angle from one of the set of curves based on the estimated vertical wind shear component and the tip speed ratio; and

a controller operable to adjust a pitch angle of at least one of the rotor blades in accordance with the determined blade pitch angle.

10. A control system according to claim 9 , comprising a yaw error sensor, wherein the processor further estimates vertical wind shear based on sensed yaw error.

11. A control system according to claim 9 , wherein the wind speed is measured by a sensor located at a nacelle of the wind turbine.

12. A control system according to claim 9 , wherein the controller is operable to generate a collective pitch angle reference to adjust the pitch angle of all of the plurality of rotor blades.

13. A control system according to claim 9 , wherein the controller is operable to generate an individual pitch angle reference to adjust the pitch angle of each of the rotor blades individually.

14. A control system according to claim 13 , wherein the operation further comprises:

determining the tip speed ratio of a plurality of positions on the rotor plane; and

determining a pitch angle for each of the plurality of positions.

15. A control system according to claim 14 , wherein the plurality of positions on the rotor plane comprises top, bottom and hub positions of the rotor plane.

16. A control system for a wind turbine having a rotor and a plurality of rotor blades, the control system comprising:

a blade load sensor;

an azimuth position sensor;

a blade pitch position sensor;

a processor;

a memory storing a program which, when executed on the processor, performs an operation comprising:

determining bending moments based on signals from the blade load sensors;

determining an estimate of main bearing yaw moment based on the blade bending moments, the azimuth position and the blade pitch position;

estimating a component of horizontal wind shear based on the yaw moment, a sensed wind speed and a measure of inflow angle;

estimating a wind velocity over a rotor plane based on the component of horizontal wind shear;

determining a tip speed ratio based on a speed of rotation of a tip of the rotor blades, a measure of wind speed, the estimated wind velocity over the rotor plane, and a set of curves of tip speed ratio and blade pitch reference for a given wind shear; and

determining a blade pitch angle from one of the set of curves based on the estimated horizontal wind shear component and the tip speed ratio; and

a controller operable to adjust a pitch angle of at least one of the rotor blades in accordance with the determined blade pitch angle.

17. A wind turbine comprising a rotor, a plurality of rotor blades and a control system according to claim 9 .