GB2466200A

GB2466200A - A Detection System of an Angle of Attack of Air Flow over a Wind Turbine Rotor Blade

Info

Publication number: GB2466200A
Application number: GB0822518A
Authority: GB
Inventors: Carsten Hein Westergaard
Original assignee: Vestas Wind Systems AS
Current assignee: Vestas Wind Systems AS
Priority date: 2008-12-10
Filing date: 2008-12-10
Publication date: 2010-06-16
Also published as: GB0822518D0

Abstract

A wind turbine is control method comprises sensing a direction of airflow in front of a leading edge of a rotor blade 10. At least one wind vane 16, preferably a pair of wind vanes 16, 18 are each mounted in front of the leading edge of the blade preferably towards the blade tip. An imaging device 12 such as a camera is mounted for rotation with the blade and acquires images of the wind vanes. Successive images are processed to determine movement of the wind vane between images and therefore wind direction can be determined. This image processing is used to produce a control signal for controlling the wind turbine. Preferably the said signal is used to the pitch angle of the blade or the position of a trailing edge flap or the rotor speed.

Description

CONTROL OF WIND TURBINES

This invention relates to wind turbines and particularly but not exclusively to wind turbines having blades which may be individually controlled to change the loading on the blade.

It is well known to provide wind turbine blades with a pitch control system which varies the aerodynamic properties of the blades by changing the angle at which the blade is presented to the oncoming wind. The pitch may be controlled to reduce fatigue loads, to reduce extreme loading during high winds and also to reduce the risk of tower strike by the blades.

A variety of methods are known for sensing the loading of the blade or the oncoming wind to enable the blade position to be optimised.

WO 2004/074681 of Forskningscenter Ris� describes a known method of controlling blade pitch angle by use of a 5-hole pitot tube mounted in front of the blade at a distance from the hub, The 5-hole pitot tube can determine both the velocity and angle of attack of wind in front of the blade. WO 2004/07468 1 also discloses the use of a trailing edge flap on a wind turbine blade to change the aerodynamic profile of the blade thereby to alter the loading on the blade. Measurements of wind speed and direction made using the 5-hole pitot tube may be used to control the flap position, that is the relative angle of the flap to the blade. WO 2004/074681 also discloses the use of other pressure measurement devices and a sonic anemometer placed in front of the blade to provide a source of data for control of pitch angle or flap angle.

All the devices proposed in WO 2004/074681 for measuring the properties of the oncoming wind suffer from the disadvantage of being complex and unreliable. Wind turbines commonly have to operate with minimum maintenance, often in very hostile environments. A high premium is placed on reliability and longevity of components and it is highly desirable to avoid any unscheduled maintenance. Pitot tubes in particular tend to become obstructed and their performance degrades over time.

An aim of the invention therefore is to provide a method and system for controlling the aerodynamic load on a wind turbine which is more reliable

and simplethan the prior art described above.

According to the invention there is provided a system for control of a wind turbine, comprising a wind vane mounted for indicating airflow in the region of the leading edge of a rotor blade of the wind turbine, an imaging device mounted for rotation with the blade and arranged to acquire successive images of the wind vane, and a processor for processing the successive images of the vane to determine therefrom a control signal for controlling the wind turbine.

Embodiments of the invention may provide control for the pitch angle of the blade, the position of a control flap such as a trailing edge flap on the blade, or the rotor speed. Embodiments of the invention provide a very simple, cost effective and robust system for controlling the blade which has the advantage of being suitable for deployment in hostile and remote areas where wind turbines are often sited. Embodiments of the invention require little maintenance, yet may provide effective control.

Embodiments of the invention may also be used to control other variables that affect the aerodynamics of the turbine system, including the blade, such as leading edge flaps, spoilers, slats, tip brakes, tower movements, nacelle yawing etc..

The imaging device may be mounted on the leading edge of the blade or may be mounted on the spinner. Mounting the imaging device on the spinner has the advantage that the imaging device may be easily protected against weather and is also close to electrical components with which it must communicate, such as power supply and processors.

The wind vane is preferably mounted on a rigid structure attached to the blade towards the tip of the blade and extending in front of the leading edge of the blade to a point where airflow is not disturbed by the blade.

The wind vane is preferably a symmetrical airfoil and is preferably pivotally attached at one end to the rigid structure.

In one preferred embodiment two or more wind vanes are provided in front of the leading edge of the blade.

The invention also provides a wind turbine having a rotor comprising a plurality of rotor blades at least one of which comprises a control system as defined above.

The invention further provides a method for controlling a wind turbine comprising: acquiring with an imaging device mounted for rotation with a wind turbine rotor the blade, successive images of a wind vane mounted near the leading edge of the blade; comparing the successive images of the wind vane to derive a control signal indicative of movement of the wind vane between images; and applying the control signal to the wind turbine.

The invention also provides a method for collection of wind data comprising acquiring, with an imaging device mounted for rotation with a wind turbine rotor blade, successive images of a wind vane mounted near the leading edge of the blade; comparing the successive images of the wind vane to collect data over the plane of the wind turbine rotor derived from a measure of the angle of attack of the wind vane; and storing the collected data with other operation data related to the wind turbine. The data collected may be derived from a derivative with respect to time of the angle of attack of the wind vane.

Embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings in which: Figure 1 is a schematic view of a wind turbine blade embodying the invention; Figure 2 illustrates images acquired by the system of claim I and shows how data may be extracted from these images; Figure 3 shows an alternative embodiment of the invention in which a camera is mounted on the rotor spinner; Figure 4 shows the design of an individual wind vane and how the angle of the wind vane varies according to the angle of incident air; and Figure 5 is a flow chart showing the main steps of the image acquisition and processing process.

Figure 1 shows an embodiment of the invention. A wind turbine blade is shown schematically at 10. This blade may be a single piece blade or of modular construction. It may also be provided with one or more trailing edge flaps (not shown). An imaging device such as a camera 12 is mounted on a rigid pole at the root end 14 of the blade. The camera is mounted on the leading edge side of the blade and is within a protective housing. The camera is preferably a CCD or CMOS video camera which takes successive images at a desired frame rate. This rate may be anything from a few frames per second, for example, 30 fps, the normal range of video cameras, up to very high speeds, for example, 1000 fps, the typical speed of CMOS image sensors. In addition some sensors offer the option to acquire very closely spaced images, for example in the order micro-seconds at a reduced bit rate. The method is found in CMOS mobile phone cameras described as "burst modes".

A pair of wind vanes 16, 18 are each mounted on a respective rigid structure such as a pole 20, 22 towards the tip of the blade. The poles are positioned such that the wind vanes are near the leading edge of the blade, preferably a short distance in front of the leading edge of the blade and preferably near the tip. This distance may be in the order of 1 metre in front of the leading edge and is preferably sufficiently in front of the leading edge to avoid the airflow over the wind vanes being affected by the leading edge. As is shown in Figure 1, the camera 12 has a field of view which images the wind vanes 16, 18. Although two wind vanes are shown in this embodiment, a single wind vane, or more than two wind vanes could be used, It is preferred, but not essential, that the wind vanes are mounted towards the tip of the blade as this is the region where air conditions in front of the blade most affect blade performance. It is preferred to mount the camera near the root so that it is accessible for maintenance.

The wind vanes 16, 18 enable wind direction to be determined and an analysis in the change of the angle of the wind vanes over time can be used to control a blade variable such as flap position, pitch position or rotor speed.

Figure 2A shows one of a succession of images of the wind vanes 16, 18 taken by the camera 12. Successive images are processed using pattern recognition algorithms from which changes of position of each vane can be determined over time. The pattern recognition algorithms identify the position of the rigid rods 20, 22 as this is a position that will move due to variations in blade loading and flexing. The tip of the rod will also vary due to loading, particularly where the wind turbine is blade pitch regulated.

Once these two points are known, pattern recognition is used to identify the vane and the angle of the vane. The reliability of image processing may be enhanced by referencing the time track of past detections from which a measure of predictability of movement may be gained so enabling rogue results to be filtered out.

In Figure 2B, plots against time are shown for the (i) blade pitch angle P; (ii) wind vane angle a and (iii) blade tip deflection ö. The pitch angle is the difference between the tip and the anchor point of the rod; the angle of the wind vane is the angle of attack; and the blade tip deflection is the position of the anchor point of the rod on the blade.

From each processed image, a data point can be produced which is consistent with the position of the rod, taking into account any deflection of the blade, the pitch of the blade, if being used on a pitched blade system, and the angle of the wind vane. This data may then be used to determine the optimum position of a blade variable such as the trailing edge flap, the blade pitch angle, or the rotor speed. The data derived is fed back to a control system, which may be part of the overall wind turbine control system to generate the required control signals to vary the blade pitch, trailing edge flap position or rotor speed. Alternatively, the data may be used to control the position of any other control surface, such as a leading edge flap, for example.

Where a high speed CMOS camera is used to obtain images, for example operating in burst mode as referred to above, both the angle of attack of the wind vane and the derivative of the angle of attack with respect to time may be determined. That derivative is the rate at which the angle of attack changes. This offers completely new options for control of the wind turbine.

The data collected by the imaging device need not be used directly to control the wind turbine, but could be stored with other operational data from the turbine. The embodiment enables collection for storage of wind data over the plane of the wind turbine rotor which is derived from a measure of the angle of attack of the wind vane, for example the actual angle or a derivative with respect to time of that angle.

Figure 5 illustrates the method of blade pitch angle, or trailing edge flap angle control, other turbine aerodynamic parameter control described above. At step 100 the camera 12 acquires images of the wind vanes 16, 18. At step 102 a processor processes these images to fix the rod and tip position and at 104 determines values for P, a and ô and thus the angle of the wind vanes. At step 106 a controller evaluates successive measurements from the processor and determines whether the data requires either the blade pitch angle or the flap position to be varied. If it does, it sends the necessary control signals.

Figure 3 is an alternative arrangement to that shown in Figure 1. In Figure 3 the imaging device is mounted on the wind turbine rotor spinner 24. This provides a more practical mounting position for the camera where it may be readily weather proofed. In Figure 3 the imaging device is the same as that used in the embodiment of figure 1 and is shown mounted within a weather proofing housing 26. A spinner mounted camera is also more convenient for supplying the necessary cabUng to the camera for power and data transfer than a camera mounted on the rotor blade. It will be appreciated that in both of the embodiments of Figures 1 and 3, the camera rotates with the rotor blades making it simpler to image the wind vanes constantly than with, for example, a nacelle mounted camera.

Figure 4 shows one of the wind vanes in greater detail. Here the wind moves from a position Wi to a position W2 and the wind vane moves from a position 16(1) to a position 16(2). The wind vane is pivotable about the end of rod 20 with the pivot being at the front of the vane. The vane is preferably a symmetrical airfoil. The length of the rod 20 is variable.

However, it is desirable to choose this length in order to minimise the disturbance in airflow over the vane induced by the wind turbine blade.

The size and weight of the vane may be chosen according to the sensitivity of the response required. A longer vane chord length will minimise the effect of small movements and an increased weight of vane will smoothen out the effect of movements. The size and weight of vane selected may be linked to the image processing speed. Where a slower camera frame rate is used, for example, in the order of 30 fps, it is desirable to use a heavier, slower responding vane. However, for high speed use, for example, in the region of 1000 fps, a small and light vane may be used which is highly sensitive. Clearly, a high speed arrangement can provide a more finely controlled system.

In an alternative embodiment the wind vane may be a flat plate that deflects. This may be realised as a plate mounted on a bearing which may optionally be spring loaded.

The embodiments of the invention described provide a simple, cheap and highly reliable source of data from which the position of a wind turbine blade flap or the pitch angle of a wind turbine blade may be controlled.

The combination of a camera and a wind vane is inexpensive, requires very little maintenance and can function reliably even in the most hostile conditions.

Various modifications and alternatives to the embodiments described are possible and will occur to those skilled in the art. For example, the system described may be used to control other variables having effects on the aerodynamics of the turbine system, including the blade, such as leading edge flaps, spoilers, slats, tip brakes, rotational speed, tower movements, nacelle yawing etc.

Claims

Claims 1. A system for control of a wind turbine, comprising a wind vane mounted for indicating airflow in the region of the leading edge of a rotor blade of the wind turbine, an imaging device mounted for rotation with the blade and arranged to acquire successive images of the wind vane, and a processor for processing the successive images of the vane to determine therefrom a control signal for controlling the wind turbine.
2. A system according to claim 1, wherein the blade comprises a control flap and the control signal controls the position of the flap.
3. A system according to claim 2, wherein the flap is a trailing edge flap.
4. A system according to claim 1, 2 or 3, wherein the pitch of the blade is variable and the control signal comprises a pitch variation signal.
5. A system according to claim 1, 2, 3 or 4, wherein the control signal comprises a rotor speed signal.
6. A system according to any of claims 1 to 5, wherein the imaging device is mounted in front of the leading edge of the blade.
7. A system according to claim 1 to 5, wherein the imaging device is mounted on the rotor spinner.
8. A system according to any of claims 1 to 7, wherein the wind vane is mounted on a rigid structure extending from the leading edge of the blade.
9. A system according to any of claims I to 8, wherein the wind vane comprises a symmetrical airfoil.
10. A system according to claim 8 and 9, wherein an end of the symmetrical airfoil is pivotally connected to the rigid structure.
11. A system according to any of claims 1 to 8, wherein the wind vane comprises a flat plate.
12. A system according to any preceding claim, comprising at least two wind vanes each arranged near the leading edge of the blade and wherein the imaging device is arranged to acquire images of the at least two wind vanes.
13. A system according to any preceding claim, wherein the processor determines a derivative of the angle of attack of the wind vane.
14. A wind turbine having a rotor comprising a plurality of blades, at least one of the blades having a control system according to any preceding claim.
15. A method for controlling a wind turbine comprising: acquiring with an imaging device mounted for rotation with a wind turbine rotor the blade, successive images of a wind vane mounted near the leading edge of the blade; comparing the successive images of the wind vane to derive a control signal indicative of movement of the wind vane between images; and applying the control signal to the wind turbine.
16. A method according to claim 15, wherein the blade comprises a control flap and the control signal is applied to control the position of the flap with respect to the blade.
17. A method according to claim 16, wherein the control flap is a trailing edge flap.
18. A method according to claim 15, 16 or 17, wherein the pitch of the blade is variable and the control signal is applied to vary the blade pitch.
19. A method according to claims 15 to 18, wherein the control signal is applied to vary the rotor speed.
A method for collection of wind data comprising acquiring, with an imaging device mounted for rotation with a wind turbine rotor blade, successive images of a wind vane mounted near the leading edge of the blade; comparing the successive images of the wind vane to collect data over the plane of the wind turbine rotor derived from a measure of the angle of attack of the wind vane; and storing the collected data with other operation data related to the wind turbine.
21. A method according to claim 20, wherein the data collected is derived from a derivative with respect to time of the angle of attack of the wind vane.
22. A system for control of a wind turbine, substantially as herein described with reference to Figures 1 to 5 of the accompanying drawings.
23. A wind turbine having a rotor comprising a plurality of blades and a system for controlling the wind turbine substantially as herein described with reference to Figures 1 to 5 of the accompanying drawings.
24. A method of controlling a wind turbine rotor substantially as herein described with reference to the accompanying drawings.