Determining wind turbine rotor speed
A method of determining rotor speed of a wind turbine. The method involves receiving angular velocity sensor data indicative of an angular velocity of the wind turbine rotor hub, provided e.g., by a gyroscope and receiving acceleration sensor data indicative of an acceleration of the rotor hub in at least one radial direction relative to an axis of rotation of the rotor hub. The method further involves determining first and second estimated rotational speeds of the rotor hub based on the respective angular velocity sensor data and acceleration sensor data. The method also involves determining a correction value based on a difference between the first and second estimated rotational speeds, and applying the correction value to the first estimated rotational speed to determine the true current rotor speed of the wind turbine.
1. A method of determining a rotor speed of a wind turbine, the method comprising:
receiving angular velocity sensor data, from an angular velocity sensor located in a rotor hub of the wind turbine, indicative of an angular velocity of the rotor hub across a time interval;
receiving acceleration sensor data, from an acceleration sensor located in the rotor hub, indicative of an acceleration of the rotor hub in at least one radial direction relative to an axis of rotation of the rotor hub across the time interval;
determining a first estimated rotational speed of the rotor hub across the time interval based on the received angular velocity sensor data;
determining a second estimated rotational speed of the rotor hub across the time interval based on the received acceleration sensor data;
determining a correction value based on a difference between a mean value of the first estimated rotational speed for the time interval and a mean value of the second estimated rotational speed for the time interval;
applying the correction value to the first estimated rotational speed to determine the rotor speed of the wind turbine; and
controlling the wind turbine based on the determined rotor speed.
2. The method according to claim 1 , wherein determining the first and second estimated rotational speeds comprises transforming the received angular velocity sensor data and the received acceleration sensor data into a rotational coordinate system that rotates with the rotor hub.
3. The method according to claim 1 , wherein applying the correction value to the first estimated rotational speed to determine the rotor speed of the wind turbine occurs at a timestep, and wherein the method further comprises: retrieving a rotor speed determined at a previous timestep, and wherein determining the second estimated rotational speed comprises applying an offset to the acceleration of the rotor hub in the at least one radial direction to compensate for a centripetal force contribution included in the received acceleration sensor data, the offset being determined based on the determined rotor speed from the previous timestep.
4. The method according to claim 1 , wherein the acceleration of the rotor hub in the at least one radial direction perpendicular to the axis of rotation of the rotor hub is a sinusoidal signal, and wherein the second estimated rotational speed is determined based on a determined period of the sinusoidal signal.
5. The method according to claim 4 , wherein determining the second estimated rotational speed comprises converting the sinusoidal signal to a logic signal, and basing the determination of the second estimated rotational speed on a determined period of the logic signal.
6. The method according to claim 5 , wherein converting to the logic signal comprises assigning a true value when the sinusoidal signal moves above a first threshold value and assigning a false value when the sinusoidal signal moves below a second threshold value.
7. The method of claim 6 , wherein the first threshold value is +0.5 g and the second threshold value is −0.5 g.
8. The method according to claim 1 , wherein determining the correction value comprises determining a representative value of the correction value over the time interval, and wherein applying the correction value comprises applying the representative value of the correction value to the first estimated rotational speed to determine the rotor speed of the wind turbine, wherein the representative value is a mean value.
9. The method of claim 8 , wherein; and the time interval corresponds to a time taken for the rotor hub to complete a prescribed number of full rotations.
10. The method according to claim 1 , wherein the at least one radial direction includes two different radial directions relative to the axis of rotation of the rotor hub, the two different radial directions being perpendicular to each another.
11. The method according to claim 1 , wherein the correction value is determined at a prescribed trigger point, wherein the prescribed trigger point is upon each completed full rotation of the rotor hub.
12. The method according to claim 1 , further comprising generating an error signal if the determined correction value is greater than a prescribed threshold correction value.
13. The method according to claim 1 , wherein the angular velocity sensor and the acceleration sensor are part of a single sensor unit located in the rotor hub, wherein the angular velocity sensor is a multi-axis gyroscope and/or wherein the acceleration sensor is an accelerometer.
14. The method according to claim 1 , wherein the angular velocity sensor and the acceleration sensor are embedded in a safety control system of the rotor hub.
15. The method of claim 1 , wherein; the time interval corresponds to a time taken for the rotor hub to complete a prescribed number of full rotations.
16. The method according to claim 1 , wherein controlling the wind turbine based on the determined rotor speed comprises performing a controlled shutdown of the wind turbine based on the determined rotor speed.
17. A control system for a wind turbine, the system comprising an angular velocity sensor and an acceleration sensor each located in a rotor hub of the wind turbine, and the system comprising a computer processor configured to:
receive angular velocity sensor data, from the angular velocity sensor, indicative of an angular velocity of the rotor hub across a time interval;
receive acceleration sensor data, from the acceleration sensor, indicative of an acceleration of the rotor hub across the time interval in at least one radial direction relative to an axis of rotation of the rotor hub;
determine a first estimated rotational speed of the rotor hub across the time interval based on the received angular velocity sensor data;
determine a second estimated rotational speed of the rotor hub across the time interval based on the received acceleration sensor data;
determine a correction value based on a difference between a mean value of the first estimated rotational speed for the time interval and a mean value of the second estimated rotational speed for the time interval;
apply the correction value to the first estimated rotational speed to determine a rotor speed of the wind turbine, and controlling the wind turbine based on the determined rotor speed.
18. The control system according to claim 17 , wherein controlling the wind turbine based on the determined rotor speed comprises performing a controlled shutdown of the wind turbine based on the determined rotor speed.
19. A wind turbine, comprising:
a tower;
a nacelle disposed on the tower;
a rotor extending from the nacelle and having a rotor hub with plurality of blades extending therefrom; and
a control system, comprising:
an angular velocity sensor;
an acceleration sensor each located in the rotor hub of the wind turbine; and
a computer processor configured to:
receive angular velocity sensor data, from the angular velocity sensor, indicative of an angular velocity of the rotor hub across a time interval;
receive acceleration sensor data, from the acceleration sensor, indicative of an acceleration of the rotor hub across the time interval in at least one radial direction relative to an axis of rotation of the rotor hub;
determine a first estimated rotational speed of the rotor hub based on the received angular velocity sensor data;
determine a second estimated rotational speed of the rotor hub based on the received acceleration sensor data;
determining a correction value based on a difference between a mean value of the first estimated rotational speed for the time interval and a mean value of the second estimated rotational speed for the time interval, wherein determining the correction value comprises determining a representative value of the correction value over the time interval;
apply the correction value to the first estimated rotational speed to determine a rotor speed of the wind turbine, wherein applying the correction value comprises applying the representative value of the correction value to the first estimated rotational speed to determine the rotor speed of the wind turbine, wherein the representative value is a mean value; and
control the wind turbine based on the determined rotor speed.