Anemometer measurement correction in wind turbines

Abstract

A method and system for determining a wind at a wind turbine (1) are provided. The method comprises a step of receiving an anemometer signal representative of the wind, a step of receiving a rotor azimuth signal representative of an orientation of a rotor (4) of the wind turbine (1), and a step of determining the wind based on at least the anemometer signal and a correction. The correction used for determining the wind speed is based on at least the azimuth signal.

Description

[0001] ANEMOMETER MEASUREMENT CORRECTION IN WIND TURBINES

[0002] TECHNICAL FIELD

[0003] The invention relates to a method of determining a wind at a wind turbine. The invention further relates to a computer program product and a system for determining a wind, and to a wind turbine comprising such a system.

[0004] BACKGROUND

[0005] Wind turbines as known in the art include a wind turbine tower supporting a nacelle and a rotor. The rotor comprises a rotor hub with a number of, typically three, rotor blades mounted thereto. In many wind turbines, especially those of the HAWT (Horizontal Axis Wind Turbine) type, a pitch angle of the rotor blades is continuously adjusted to allow for optimal power output at different wind speeds. At wind speeds above those where maximum power output is reached, the rotor blades often ‘pitch out’ to prevent damage to the wind turbine. Wind speed measurements can also be important for other purposes than controlling the power output during operation. For example, wind speed measurements can indicate when the wind speed reaches a minimum value for the power production to start, or when the wind speed is sufficiently low for allowing an engineer to safely ascend the wind turbine for maintenance operations.

[0006] Wind measurements are made by wind sensors, often called anemometers. These anemometers are typically positioned on top of the nacelle, where they are close to the rotor and can be used for measuring wind speeds and directions indicative of a wind at the rotor. However, because of their position on the nacelle and behind the rotor, the measurements performed by these anemometers are often influenced by airflow disturbances caused by the rotor blades that move in front of the nacelle.

[0007] In, for example, US patent application US 2007 / 0125165 A1 , a correction algorithm is proposed to compensate for the effect of the moving rotor blades of a rotating rotor on the air flow on the wind flow directly behind the rotor blade plane where the anemometer is placed. The applied correction is dependent on the current power output of the wind turbine. In US patent US 7,861 ,583 B2 the same correction algorithm is further developed by calculating a suitable angular position of the rotor for making wind speed measurements and then measuring the wind speed when the rotor is in this specific angular position only. While such measures may partly overcome the wind measurement inaccuracies introduced by the rotating rotor blades of a wind turbine in production, this does not solve all the problems that these rotor blades may cause for the accuracy of the wind measurements.

[0008] It is against this background that the present invention is set.

[0009] SUMMARY OF THE INVENTION

[0010] According to an aspect of the invention there is provided a new method of determining a wind at a wind turbine. This new method comprises a step of receiving an anemometer signal representative of the wind as measured by an anemometer, a step of receiving a rotor azimuth signal representative of an orientation of a rotor of the wind turbine relative to the wind turbine, and a step of determining the wind based on at least the anemometer signal and a correction. The correction used for determining the wind speed is based on at least the azimuth signal

[0011] Advantageously, the correction provided herewith compensates for the effect of the anemometer being partly or fully shielded by one of the rotor blades. Such shielding affects the anemometer signal and leads to variations in this anemometer signal, even when the wind in the vicinity of the wind turbine remains perfectly constant. By taking into account the orientation of the rotor of the wind turbine relative to the wind turbine, and thus also the orientation of the rotor blades relative to the anemometer, an appropriate correction can be determined and the accuracy of the wind measurement is improved.

[0012] Wind is characterised by a speed (m / s) and a direction of the wind, which together define a wind velocity. Wind measurement or wind velocity measurement is therefore to be understood as a measurement of how fast the wind is blowing, the direction of the wind, or the combination of both. The shielding effect of the rotor blades may affect both aspects of the wind to different extents. Accordingly, the correction may thus be used to improve the accuracy of one or both of the speed and the direction of the wind measurement.

[0013] In embodiments of the method according to the invention, the correction is further based on the anemometer signal. The effect of the rotor blades on the wind speed or direction as measured by the anemometer may depend on the wind speed or direction itself. For low speed winds, for example, this effect may be smaller than for high speed winds. Similarly, for winds impinging straight onto the rotor blades, the shielding effect may differ from that of winds impinging under a different angle. By applying a correction that is dependent on the uncorrected wind speed and / or direction as measured by the anemometer, the accuracy of the correction and the corrected wind measurement are improved.

[0014] Furthermore, the correction may be based on a pitch signal representative of a pitch angle of at least one rotor blade of the wind turbine. The shielding effect of the rotor blades may depend on the exact orientation of the rotor blade or blades relative to the anemometer. This exact orientation of the rotor blades depends not only on the rotational position of the rotor as represented by the azimuth signal, but also by the rotor blade’s pitch angle. Where the azimuth signal defines an angle (between 0 and 360 degrees, or 0 and 2*pi radians) of rotation around the rotational axis of the rotor, the pitch signal defines a rotation around a pitch axis of the rotor blade that is generally aligned with or coincidental with its longitudinal axis. When pitching out the rotor blade, its frontal surface area decreases and the shielding effect on the anemometer may reduce. Accordingly, by applying a correction that is dependent on the pitch angle, the accuracy of the correction and the corrected wind measurement are improved.

[0015] In some embodiments, the correction may further be based on a weather parameter. The rotor blade’s shielding effect may not just depend on the rotor blade geometry, position and orientation, but on variable external circumstances too. For example, the air temperature, humidity level, or density may affect the aerodynamics of the wind hitting the rotor blade. Apart from the direct effect on the aerodynamics, weather conditions may influence the surface texture and geometry of the rotor blades too. For example wet rotor blade surfaces may have a different effect on the wind blowing over them than dry rotor blade surfaces. Ice forming in cold and humid conditions may change not just the surface roughness, but also the geometry of the rotor blade surface. Other parameters, not directly related to the weather, may affect the wind measurements too. For example, leading edge erosion may influence how the rotor blade affects the wind speed and / or direction as measured by the anemometer. By intermittently or continuously monitoring such leading edge erosion and taking this into account for determining the correction, the accuracy of the wind measurements may be improved even further.

[0016] In exemplary embodiments of the invention, the correction is determined using a correction function comprising a trigonometric function of the azimuth signal. The rotation of the rotor hub and the rotor blades mounted thereto gives the shielding effect a periodic nature. Accordingly, a periodic function, for example in the form of a trigonometric function may be able to provide accurate corrections for the anemometer measurements.

[0017] The trigonometric function may, for example, take the form of a sine or cosine function such as y(x) = A * sin(2 * pi * x / T + f) + B, wherein y(x) is the correction, x is the orientation of the rotor, A is a correction signal amplitude, T is a correction signal period, f is a correction signal phase shift, and B is a correction signal offset.

[0018] The inventors have found that the now proposed correction may be particularly accurate and useful when the wind turbine is in a stop or idling configuration. Accordingly, this new method of measuring wind speed and / or direction is preferably performed while a rotational speed of the rotor is below 5 rpm, i.e., with the rotor blades not moving or rotating at a relatively low speed.

[0019] In some embodiments, the correction may further be based on an operational parameter, representative of an internal state of the wind turbine. For example, the operational parameter may represent a current power output of the wind turbine or a rotational speed of the rotor of the wind turbine.

[0020] According to another aspect of the invention, a system is provided for measuring a wind at a wind turbine. The system comprises an anemometer, configured to generate an anemometer signal representative of the wind, and an azimuth sensor, configured to generate a rotor azimuth signal representative of an orientation of a rotor of the wind turbine relative to the wind turbine. The system further comprises a controller, operatively coupled to the anemometer and the azimuth sensor to respectively receive the anemometer signal and the azimuth signal therefrom, and to determine the wind based on at least the anemometer signal and a correction, wherein the correction is based on at least the azimuth signal.

[0021] The azimuth sensor may, for example, be provided in the form of an electromechanical encoder that directly measures a rotational position of the rotor. Alternatively, the azimuth sensor may derive the rotational position of the rotor from signals generated by one or more movement sensors, such as an accelerometer or gyroscope, provided in the rotor hub or rotor blades. Other sensor technology, such as the use of LiDAR to track the position of one or more of the rotor blades may be used as a basis for the rotor azimuth signal too. According to yet another aspect of the invention, a wind turbine is provided comprising a nacelle, a rotor with a plurality of rotor blades, the rotor being rotatably mounted to the nacelle, and a system for measuring the wind speed and / or direction at the wind turbine as described above, wherein the anemometer of the system is mounted to the nacelle.

[0022] BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Examples of the invention will now be described with reference to the accompanying drawings, in which:

[0024] Figure 1 schematically illustrates a wind turbine.

[0025] Figure 2 shows a block diagram of an embodiment of a system according to the invention.

[0026] DETAILED DESCRIPTION

[0027] Figure 1 illustrates, in a schematic view, an example of a wind turbine 1 . The wind turbine 1 includes a tower 2, a nacelle 3 disposed at the apex of, or atop, the tower 2, and a rotor 4 operatively coupled to a generator housed inside the nacelle 3. In addition to the generator, the nacelle 3 houses other components required for converting wind energy into electrical energy and various components needed to operate, control, and optimise the performance of the wind turbine 1 . The rotor 4 of the wind turbine 1 includes a central hub 5 and three rotor blades 6 that project outwardly from the central hub 5. An anemometer 1 10 is installed on top of the nacelle 3 to measure the wind speed or direction nearby the rotor 4 of the wind turbine 1 . Various types of anemometers 110 may be used. In preferred embodiments, the anemometer 110 may be configured to measure both wind speed and wind direction. Alternatively, separate sensors are used for measuring the speed and the direction of the wind.

[0028] The anemometer 1 10 is placed on the nacelle and behind the rotor, i.e. downstream of the rotor relative to the wind direction when the rotor is yawed against the wind direction. Due to this placement of the anemometer, the anemometer is exposed to shielding affects which leads to variations in the anemometer signal caused by the passing of the rotor blades. Figure 2 shows a block diagram of an embodiment of a wind measurement system 100 according to the invention. The system 100 comprises the anemometer 110 that is configured to generate an anemometer signal representative of the wind. The system 100 further comprises an azimuth sensor 120 that is configured to generate a rotor azimuth signal representative of an orientation of a rotor 4 of the wind turbine 1 relative to the wind turbine 1 . The azimuth signal defines an angle (between 0 and 360 degrees, or 0 and 2*pi radians) of rotation around the rotational axis of the rotor 4. Thus, the azimuth signal defines an angular position of the rotor measured around the rotational axis of the rotor 4.

[0029] Optionally, the system 100 further comprises and makes use of a blade pitch sensor 140 that is configured to generate a pitch signal that defines a rotation around a pitch axis of the rotor blade 6 that is generally aligned with or coincidental with its longitudinal axis. When, like in the example shown in Figure 1 , the wind turbine 1 comprises more than one rotor blade 6, separate blade pitch sensors 140 may be provided for each rotor blade 6. Additional sensors 150 may be provided for measuring, for example, weather parameters such as the air temperature, humidity level, or density. Other parameters that may be measured by the additional sensors include an amount and / or geometry of ice formed at an outer surface of one or more of the rotor blades 6.

[0030] All sensors 110, 120, 140, 150, are operatively coupled to a controller 130 that is configured to receive all the sensor signals and to determine the wind speed and / or direction based on at least the anemometer signal and a correction. The correction is based on at least the azimuth signal from the azimuth sensor 120. The azimuth sensor 120 may, for example, be provided in the form of an electromechanical encoder that directly measures a rotational position of the rotor 4. Alternatively, the azimuth sensor 120 may derive the rotational position of the rotor 4 from signals generated by one or more movement sensors, such as an accelerometer or gyroscope, provided in the rotor hub 4 or rotor blades 6. Other sensor technology, such as the use of LiDAR to track the position of one or more of the rotor blades 6 may be used as a basis for the rotor azimuth signal too.

[0031] Preferably, the sensor signals from the other sensors 110, 140, 150 are used for determining the correction too. In some embodiments, the correction may further be based on an operational parameter, representative of an internal state of the wind turbine 1 . For example, the operational parameter may represent a current power output of the wind turbine 1 or a rotational speed of the rotor 4 of the wind turbine 1 . Advantageously, the correction provided by this wind measurement system 100 compensates for the effect of the anemometer 110 being partly or fully shielded by one of the rotor blades 6. Such shielding affects the anemometer signal and leads to variations in this anemometer signal, even when the wind in the vicinity of the wind turbine 1 remains perfectly constant. By taking into account the orientation of the rotor 4 of the wind turbine relative to the wind turbine 1 , and thus also the orientation of the rotor blades 6 relative to the anemometer 1 10, an appropriate correction can be determined and the accuracy of the wind measurement is improved.

[0032] The correction may further be based on the anemometer signal itself. The effect of the rotor blades 6 on the wind speed or direction as measured by the anemometer 110 may depend on the wind speed too. For low speed winds, for example, this effect may be smaller than for high speed winds. Similarly, for winds impinging straight onto the rotor blades, the shielding effect may differ from that of winds impinging under a different angle. By applying a correction that is dependent on the uncorrected wind speed and / or direction as measured by the anemometer 110, the accuracy of the correction and the corrected wind measurement are improved.

[0033] Furthermore, the correction may be based on a pitch as received from the pitch sensor 140. The shielding effect of the rotor blades 6 may depend on the exact orientation of the rotor blade or blades 6 relative to the anemometer 1 10. This exact orientation of the rotor blades 6 depends not only on the rotational position of the rotor 4 as represented by the azimuth signal, but also by the rotor blade’s pitch angle. When pitching out the rotor blade 6, its frontal surface area decreases and the shielding effect on the anemometer 110 may reduce. Accordingly, by applying a correction that is dependent on the pitch angle, the accuracy of the correction and the corrected wind measurement are improved.

[0034] In some embodiments, the correction may further be based on a weather parameter. The rotor blade’s shielding effect may not just depend on the rotor blade geometry, position and orientation, but on variable external circumstances too. For example, the air temperature, humidity level, or density may affect the aerodynamics of the wind hitting the rotor blade 6. Apart from the direct effect on the aerodynamics, weather conditions may influence the surface texture and geometry of the rotor blades 6 too. For example wet rotor blade surfaces may have a different effect on the wind blowing over them than dry rotor blade surfaces. Ice forming in cold and humid conditions may change not just the surface roughness, but also the geometry of the rotor blade surface. As a further example of weather parameters that may be considered, the direction of the wind may influence the effect of the rotor blade 6 on the wind measurements performed by the anemometer 110 too.

[0035] Other parameters, not directly related to the weather, may affect the wind measurements too. For example, leading edge erosion may influence how the rotor blade 6 affects the wind speed and / or direction as measured by the anemometer 1 10. By intermittently or continuously monitoring such leading edge erosion and taking this into account for determining the correction, the accuracy of the wind measurements may be improved even further

[0036] In exemplary embodiments of the invention, the correction is determined using a correction function comprising a trigonometric function of the azimuth signal. The rotation of the rotor hub 5 and the rotor blades 6 mounted thereto gives the shielding effect a periodic nature. Accordingly, a periodic function, for example in the form of a trigonometric function may be able to provide accurate corrections for the anemometer measurements.

[0037] The trigonometric function may, for example, take the form of a sine or cosine function such as y(x) = A * sin(2 * pi * x / T + f) + B, wherein y(x) is the correction, x is the orientation of the rotor 4, A is a correction signal amplitude, T is a correction signal period, f is a correction signal phase shift, and B is a correction signal offset.

[0038] The inventors have found that the now proposed correction may be particularly accurate and useful when the wind turbine 1 is in a stop or idling configuration. In a stop configuration the rotor may be completely stopped or move at a speed lower than an idling rotation speed. Accordingly, this new method of measuring wind is preferably performed while a rotational speed of the rotor 4 is below 5 rpm, i.e., with the rotor blades 6 not moving or rotating at a relatively low speed. The method of measuring the wind may be used only when the wind turbine 1 is in a stop or idling configuration, or for any operational mode of the wind turbine 1 where the rotor 4 moves, i.e. when the wind turbine 1 is in a stop, idling, or production configuration. In the production configuration the wind turbine 1 produces power e.g. under partial load or full load operation.

Claims

CLAIMS1 . A method of determining a wind at a wind turbine (1 ), the method comprising: receiving an anemometer signal representative of the wind as measured by an anemometer (1 10), wherein the anemometer is placed on the nacelle and behind the rotor, receiving a rotor azimuth signal representative of an orientation of a rotor (4) of the wind turbine (1 ) relative to the wind turbine (1 ), wherein the azimuth signal defines an angle of rotation around a rotational axis of the rotor (4), determining the wind based on at least the anemometer signal and a correction, wherein the correction is based on at least the azimuth signal.

2. A method of determining wind as claimed in claim 1 , wherein the anemometer signal is representative of a wind speed of the wind, and wherein determining the wind comprises determining the wind speed based on at least the anemometer signal and the correction.

3. A method of determining wind as claimed in claim 1 or 2, wherein the anemometer signal is representative of a wind direction of the wind, and wherein determining the wind comprises determining the wind direction based on at least the anemometer signal and the correction.

4. A method of determining a wind as claimed in any one of the preceding claims, wherein the correction is further based on the anemometer signal.

5. A method of determining a wind as claimed in any one of the preceding claims, wherein the correction is further based on a pitch signal representative of a pitch angle of at least one rotor blade (6) of the wind turbine (1 ).

6. A method of determining a wind as claimed in any one of the preceding claims, wherein the correction is further based on a weather parameter.

7. A method of determining a wind as claimed in claim 6, wherein the weather parameter is representative of at least one of: a temperature; a humidity level; and an air density.

8. A method of determining a wind as claimed in any one of the preceding claims, wherein the correction is determined using a correction function comprising a trigonometric function of the azimuth signal.

9. A method of determining a wind as claimed in claim 8, wherein the correction function is of the form: y(x) = A * sin(2 * pi * x / T + f) + B, wherein y(x) is the correction, x is the orientation of the rotor,A is a correction signal amplitude,T is a correction signal period, f is a correction signal phase shift, and B is a correction signal offset.

10. A method of determining a wind as claimed in any one of the preceding claims, wherein the method is performed while a rotational speed of the rotor (4) is below 5 rpm.1 1. A method of determining a wind as claimed in any one of the preceding claims, wherein the correction is further based on an operational parameter, representative of an internal state of the wind turbine (1 ).

12. A method of determining a wind as claimed in claim 1 1 , wherein the operational parameter represents a current power output of the wind turbine (1 ) or a rotational speed of the rotor (4) of the wind turbine (1 ).

13. A computer program product comprising software code adapted to, when executed on a data processing system, perform a method as claimed in any one of the preceding claims.

14. A system (100) for determining a wind at a wind turbine (1 ), the system (100) comprising: an anemometer (110), configured to generate an anemometer signal representative of the wind, wherein the anemometer is placed on the nacelle and behind the rotor,an azimuth sensor (120), configured to generate a rotor azimuth signal representative of an orientation of a rotor (4) of the wind turbine (1 ) relative to the wind turbine (1 ), wherein the azimuth signal defines an angle of rotation around a rotational axis of the rotor (4), a controller (130), operatively coupled to the anemometer (1 10) and the azimuth sensor (120) to respectively receive the anemometer signal and the azimuth signal therefrom, and to determine the wind based on at least the anemometer signal and a correction, wherein the correction is based on at least the azimuth signal.

15. A wind turbine (1 ) comprising a nacelle (3), a rotor (4) with a plurality of rotor blades (6), the rotor (4) being rotatably mounted to the nacelle (3), and a system (100) for determining the wind at the wind turbine (1 ) as claimed in claim 14, wherein the anemometer (1 10) of the system (100) is mounted to the nacelle (3).

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