Detecting blade weight anomaly of wind turbine

Abstract

A first aspect of the invention provides a method of monitoring a blade of a rotor of a wind turbine, the method comprising: obtaining a bending value by measuring bending of the blade as the rotor rotates; obtaining an azimuth value associated with the bending value by measuring an azimuth angle of the blade; determining a cosecant of the azimuth angle; determining a gravity component on a basis of the bending value and the cosecant of the azimuth angle; and using the gravity component to determine whether a blade weight anomaly exists and generate a corresponding output.

Description

[0001] DETECTING BLADE WEIGHT ANOMALY OF WIND TURBINE

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a method of monitoring a blade of a rotor of a wind turbine; a method of controlling a wind turbine; apparatus for monitoring a blade of a rotor of a wind turbine; and a wind turbine system.

[0004] BACKGROUND OF THE INVENTION

[0005] US2009246019 A1 discloses a method of detecting the formation of ice on the blades of a wind turbine. The wind turbine has at least one turbine blade mounted to a rotor and provided with at least a first strain sensor for measuring mechanical strain of the turbine blade. The method comprises detecting changes in an output signal of the strain sensor due to changes in the mass of the turbine blade caused by the formation of ice on the turbine blade. A relationship between the output value of the first strain sensor and the bending moment due to the mass of the turbine blade is determined by reference to the peak-to-peak amplitude of the output signal.

[0006] SUMMARY OF THE INVENTION

[0007] A first aspect of the invention provides a method of monitoring a blade of a rotor of a wind turbine, the method comprising: obtaining a bending value by measuring bending of the blade as the rotor rotates; obtaining an azimuth value associated with the bending value by measuring an azimuth angle of the blade; determining a cosecant of the azimuth angle; determining a gravity component on a basis of the bending value and the cosecant of the azimuth angle; and using the gravity component to determine whether a blade weight anomaly exists and generate a corresponding output.

[0008] Optionally the gravity component is a weighted gravity component determined by: obtaining a weighting parameter based on the azimuth angle of the blade, wherein the weighting parameter is zero when the cosecant of the azimuth angle is infinite, and determining the weighted gravity component on a basis of the bending value, the cosecant of the azimuth angle and the weighting parameter. Optionally the weighting parameter varies between a maximum when the cosecant of the azimuth angle is + / - 1 and a minimum when the cosecant of the azimuth angle is infinite.

[0009] Optionally the weighting parameter varies continuously with the azimuth angle of the blade.

[0010] Optionally the weighting parameter varies sinusoidally with the azimuth angle of the blade.

[0011] Optionally the weighting parameter varies at twice the frequency of the azimuth value.

[0012] Optionally the rotor has a plurality of blades, and the method comprises: for each blade, obtaining a bending value by measuring bending of the blade as the rotor rotates, obtaining an azimuth value associated with the bending value by measuring an azimuth angle of the blade; determining a cosecant of the azimuth angle, and determining a gravity component on a basis of the bending value and the cosecant of the azimuth angle; combining the gravity components of the plurality of blades to obtain a combined gravity component; and using the combined gravity component to determine whether a blade weight anomaly exists and generate a corresponding output.

[0013] Optionally combining the gravity components comprises determining a sum or mean of the gravity components.

[0014] Optionally the bending value and the azimuth value are obtained as the rotor is rotating and substantially no power is being captured from the rotor.

[0015] Optionally the bending value and the azimuth value are obtained as the rotor is rotating at a rate below 10 RPM or below 2 RPM.

[0016] Optionally the gravity component is compared with an output from a trained model, and the determination whether a blade weight anomaly exists is based on the comparison.

[0017] Optionally the blade comprises a sensor; and the bending of the blade is measured by the sensor. A further aspect of the invention provides a method of controlling a wind turbine, the method comprising monitoring a blade of a rotor of the wind turbine by a method according to the preceding aspect; and controlling the wind turbine based on the output.

[0018] A further aspect of the invention provides apparatus for monitoring a blade of a rotor of a wind turbine, wherein the apparatus is configured to: receive a bending value indicative of bending of the blade as the rotor rotates; receive an azimuth value which is associated with the bending value and indicative of an azimuth angle of the blade; determine a cosecant of the azimuth angle; determine a gravity component on a basis of the bending value and the cosecant of the azimuth angle; and use the gravity component to determine whether a blade weight anomaly exists and generate a corresponding output.

[0019] A further aspect of the invention provides a wind turbine system comprising: a rotor comprising a blade; a bending sensor configured to obtain a bending value by measuring bending of the blade as the rotor rotates; an azimuth sensor configured to obtain an azimuth value associated with the bending value by measuring an azimuth angle of the blade; and apparatus according to the preceding aspect configured to receive the bending value and the azimuth value and monitor the blade accordingly.

[0020] BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

[0022] Figure 1 is a front view of a wind turbine;

[0023] Figure 2 is a side view of the wind turbine;

[0024] Figure 3 shows a blade of the wind turbine

[0025] Figure 4 shows strain gauges in the root of the blade;

[0026] Figure 5 shows apparatus for monitoring the blades;

[0027] Figure 6 shows an edge bending signal;

[0028] Figure 7 is a schematic front view of the rotor showing a gravity component for one of the blades;

[0029] Figure 8 is a graph showing variation of cosecants with 0; Figure 9 is a graph showing variation of a weighting parameter with 0;

[0030] Figure 10 is a graph showing variation with 0 of the product of cosecant© and the weighting parameter;

[0031] Figure 11 is a flow diagram showing a method of monitoring a blade of the wind turbine; Figure 12 shows three weighted gravity components; and

[0032] Figure 13 shows a combined weighted gravity component.

[0033] DETAILED DESCRIPTION OF EMBODIMENT(S)

[0034] Figures 1 and 2 show a wind turbine 1 including a tower 3 mounted on a foundation and a nacelle 4 disposed at the apex of the tower. The wind turbine 1 depicted here is an onshore wind turbine such that the foundation is embedded in the ground, but the wind turbine 1 could be an offshore installation in which case the foundation would be provided by a suitable marine platform.

[0035] A rotor is operatively coupled via a gearbox to a generator (not shown) housed inside the nacelle 4. The rotor includes a central hub and a plurality of rotor blades 2a-c, which project outwardly from the central hub. It will be noted that the wind turbine 1 is the common type of horizontal axis wind turbine (HAWT) such that the rotor is mounted at the nacelle to rotate about a substantially horizontal axis defined at the centre at the hub. The example shown has three blades, but it will be realised by the skilled person that other numbers of blades are possible.

[0036] When wind blows against the wind turbine 1, the blades generate a lift force which causes the rotor to rotate, which in turn causes the generator within the nacelle 4 to generate electrical energy.

[0037] All of the blades 2a-c are substantially identical, and an exemplary one of the blades 2a is shown in Figures 3 and 4. Each of the blades has a root end 6 proximal to the hub and a tip end 7 distal from the hub. A leading edge 5a and a trailing edge 5b extend between the root end 6 and the tip end 7, and each of the blades has a respective aerodynamic high pressure surface (i.e. the pressure surface) and an aerodynamic low pressure surface (i.e. the suction surface) extending between the leading and trailing edges of the blade. Each blade has an edgewise direction 14 (between the leading edge 5a and the trailing edge 5b) and a flapwise direction 15 (at right angles to the edgewise direction).

[0038] Ice accretion on the blades can impact both performance and safety of the wind turbine. A clear indication of the presence of ice is important to determine reasons for performance losses, changes to control strategy, or legal requirements. From a safety standpoint, ice presence can lead to ice throw which can pose risk if a turbine is placed near people, roads, or animals. Improved ice detection can also lead to bill of materials (BOM) savings, reducing the need for further material, making the wind turbine 1 lighter, cheaper, and easier to transport.

[0039] As shown in Figure 4, the blade includes a blade shell 10 defining a hollow interior space 11. Strain gauges 12a, 13a are adhered to the interior blade shell, or embedded within the blade shell, at the root end 6 of the blade.

[0040] Each strain gauge 12a, 13a measures linear extension of the blade root. An example of a suitable strain gauge is an optical strain gauge such as a fibre Bragg grating (FBG) as described in EP3317531 B1. Such an optical strain gauge comprises equally spaced reflection points in the core of the optical fibre that reflect difference wavelengths of light under different levels of strain. Such a sensor is well known to the skilled person.

[0041] A first one of the strain gauges is an edge strain gauge 12a which is positioned to measure bending of the blade in the edgewise direction 14. When the blade bends one way in the edgewise direction 14, the edge strain gauge 12a extends and generates a positive edge bending signal. When the blade bends the other way in the edgewise direction 14, the edge strain gauge 12a contracts and generates a negative edge bending signal. In this case there is only a single edge strain gauge 12a, but in other embodiments there may be two edge strain gauges positioned on opposite sides of the blade root 6.

[0042] A second one of the strain gauges is a flap strain gauge 13a which is positioned to measure bending of the blade in the flapwise direction 15. When the blade bends one way in the flapwise direction 15, the flap strain gauge 13a extends and generates a positive flap bending signal. When the blade bends the other way in the flapwise direction 15, the flap strain gauge 13a contracts and generates a negative flap bending signal. In this case there is only a single flap strain gauge 13a, but in other embodiments there may be two flap strain gauges positioned on opposite sides of the blade root 6.

[0043] Figure 5 is a schematic diagram of apparatus 20 for monitoring the blades 2a-c of the rotor of the wind turbine 1. All elements of the apparatus 20 may be part of the wind turbine 1, or some elements may be located remotely from the wind turbine 1 (for instance at a central wind park controller).

[0044] Each blade has a pair of strain gauges 12a, 13a; 12b, 13b; 12c, 13c which generate bending signals which are input into a computer-implemented processing system 21.

[0045] As the rotor rotates, each blade rotates through a series of azimuth angles. The azimuth angle may be defined in any way, but in this specification we define the azimuth angle as 0 when the blade is pointing down, pi when the blade is pointing up, and pi / 2 or 3pi / 2 when the blade is pointing left or right.

[0046] An azimuth sensor 22 measures an azimuth angle of the rotor, from which an azimuth angle of each blade is determined by the processing system 21. The azimuth sensor 22 may comprise an encoder in the rotor hub, a magnetic ring or a gyroscope for example.

[0047] When the blade is vertical (i.e. at its highest point so it is pointing up, or at its lowest point so it is pointing down) the weight of the blade will not be bending the blade in the edgewise direction, so the magnitude of the edge bending signal is low. When the blade is horizontal (i.e. pointing left or right) the weight of the blade will be maximally bending the blade in the edgewise direction, so the edge bending signal is at a maximum or a minimum. Hence the edge bending signal follows a sinusoidal profile as shown in Figure 6, with a maximum at pi / 2 and a minimum at 3pi / 2.

[0048] Note that the rotor axis may be tilted up slightly from the horizontal, as shown in Figure 2, so the blade 2a pointing down is not precisely vertical but is slightly tilted. Figure 7 is a schematic view of the rotor, showing the weight force acting on the blade 2a. The blade 2a in Figure 1 has an azimuth angle of 0 and so the edgewise bending value for that blade is zero. In Figure 7 the rotor has rotated so that the blade 2a has an azimuth angle of 0, and a centre of mass with a gravity component w acting downwardly at a radius r from the rotor axis.

[0049] The gravity component w can be expressed as a force wsin0 acting at right angles to the radial direction, and a force wcos0 acting in the radial direction. The force wsin0 at right angles to the radial direction bends the blade, so this is proportional to the edge bending value from the edge strain gauge 12a. Denoting the edge bending value as B, the force wsin0 is proportional to B; and hence the gravity component w is proportional to B / sin0.

[0050] The parameter 1 / sin0 can also be denoted cosecant©, and hence w is proportional to Bcosecant©. So the gravity component w can be determined on a basis of the edge bending value B and the cosescant of the azimuth angle 0. Note that the edge bending value B could be a strain value from the edge strain gauge 12a, a bending moment based on a measurement by the edge strain gauge, or a blade load value obtained in some other way.

[0051] Figure 8 is a graph showing how the cosecant of the azimuth angle varies with azimuth angle, with a singularity (going to infinity) when the blade is pointing up or down at azimuth angles of 0, pi, 2pi, etc.

[0052] This presents a problem with using the relation w=Bcosecant0, because it also goes to infinity when the blade is pointing up or down.

[0053] A solution to this problem is to use a weighted gravity component in which the weighting is zero when the blade is pointing up or down. The weighted gravity component is determined by: obtaining a weighting parameter based on the azimuth angle of the blade, wherein the weighting parameter is zero when the cosecant of the azimuth angle is infinite, and determining the weighted gravity component on a basis of the bending value, the cosecant of the azimuth angle and the weighting parameter. The weighting parameter typically varies between a maximum when the cosecant of the azimuth angle is + / - 1 and a minimum when the cosecant of the azimuth angle is infinite.

[0054] In its most simple form, the weighting parameter may be based on a discontinuous step function which takes a value of zero when the blade is pointing up or down, and a value of one for all other azimuth angles.

[0055] In a more preferred form, the weighting parameter varies continuously with the azimuth angle of the blade. An example is shown in Figure 9 in which the weighting parameter varies sinusoidally with the azimuth angle of the blade, at twice the frequency of the azimuth value. Hence the weighting parameter of Figure 9 varies continuously between maxima 30 when the cosecant of the azimuth angle is + / - 1 and zero when the cosecant of the azimuth angle is infinite.

[0056] Optionally the weighting parameter of Figure 9 may be given by the function:

[0057] function weight = weightFunction(theta)

[0058] adjustedTheta = theta * 2 - pi / 2;

[0059] weight = (sin(adjustedTheta) / 2 + 0.5) . / ((sin(adjustedTheta) I + sin(adjustedTheta + 2 / 3*pi) / 2 + sin(adjustedTheta + 4 / 3*pi) / 2) + 1.5);

[0060] end

[0061] where Theta is the azimuth angle.

[0062] Note that the weighting parameter of Figure 9 is not a pure sine wave, but it does vary sinusoidally in the sense that it has the form of a sine curve (i.e. with a regular smooth repeating pattern).

[0063] Figure 10 shows how the product of the weighting parameter and the cosecant of the azimuth angle varies with azimuth angle. Note that the weighting parameter varies smoothly with no singularities when the blade is pointing up or down. It goes to zero when the cosecant of the azimuth angle is infinite (i.e. when the blade is pointing up or down) and has a maximum 31 or minimum 32 where the cosecant of the azimuth angle is + / - 1 (i.e. when the blade is pointing horizontally left and right). This continuous weighting parameter improves signal-to-noise ratio because it reduces the contribution of the signal when the blade is pointing up or down and noise would be amplified the most.

[0064] The method described above may be applied to only a single blade 2a, but more typically the apparatus 20 is configured to perform the method shown by the flow diagram of Figure 11, in which the method is applied to all three blades 2a-c based on a combined gravity component.

[0065] For each blade 2a, 2b, 2c a series of edgewise bending data points is obtained at 40 from its respective edge strain gauge(s) 12a, 12b, 12c; and an azimuth value is obtained for each data point from the azimuth sensor at 41. Hence for each blade, a bending value is obtained by measuring bending of the blade as the rotor rotates, and an azimuth value is obtained which is associated with the bending value by measuring an azimuth angle of the blade.

[0066] At 42 a weighted gravity component is obtained for each blade on a basis of the bending value, the cosecant of the azimuth angle and the weighting parameter. Figure 12 shows exemplary weighted gravity components cmp1; cmp2; cmp3 of the three blades 2a-c over two revolutions of the rotor.

[0067] The weighted gravity components cmp1; cmp2; cmp3 of the three blades are then combined at 43 (for instance by determining a sum or mean) to obtain a combined weighted gravity component, shown in Figure 13. The combined weighted gravity component is then used at 44 to determine whether a blade weight anomaly exists and generate a corresponding output. In the example of Figure 13, the combined weighted gravity component is a sum of cmp1 , cmp2, and cmp3 which is approximately constant and proportional to the combined weight of the blades.

[0068] The combined weighted gravity component is then compared at 48 with the combined outputs from trained models, such as Kalman filters 23a-c, and a determination whether a blade weight anomaly exists is based on the comparison.

[0069] The Kalman filter 23a-c for each blade is trained when a temperature sensor senses that the ambient temperature is above a threshold (for example 5 degC). Hence the output of each Kalman filter 23a-c provides an estimate of the weighted gravity component for its respective blade in the absence of ice on the blade.

[0070] Each trained model determines its output based on input comprising wind speed, environmental temperature and blade loads, for example the bending values from the strain gauges.

[0071] A Kalman filter 23a-c is an example of a trained model for predicting the weighted gravity component, but other models are possible.

[0072] The combined weighted gravity component of Figure 13 correlates with the combined weights of the blade, so if the combined weighted gravity component is higher than the combined outputs from the Kalman filters, then this indicates that a blade weight anomaly exists, most likely due to accretion of ice or other material on the surface of the blades.

[0073] The processing system 21 determines whether a blade weight anomaly exists and generates a corresponding output 24 which is indicated in Figure 5. This output 24 may simply be an indication on a display device, or in a computer memory, that a blade weight anomaly does or does not exist. Alternatively, the output 24 may be fed to a wind turbine control system 25 which controls the wind turbine 1 based on the output 24. For example, referring to Figure 11, if the output 24 indicates a blade weight anomaly then the wind turbine control system 25 may initiate ice adaption measures 49 such as activating heaters on the blades 2a-c, pitching out the blades 2a-c to enter idle operation, suspending calibration procedures, or changing a control strategy of the wind turbine to extract more power from the rotor. Similarly, if the output 24 indicates that there is no blade weight anomaly, then such ice adaption measures may be stopped at 50 so the wind turbine returns to normal operation.

[0074] The detection method of Figure 11 may be performed when the wind turbine is in idle mode, or any similar mode in which the rotor rotates at a lower rate than nominal. For example the rotor may be rotating at a rate below 10 RPM, below 2 RPM or below 1 RPM. Optionally substantially no power is being generated by the generator, or a small amount of power may be being generated to charge batteries or to keep the wind turbine running. The idle mode operation may have been triggered by a detection of ice during normal operation in which the generator is generating power. In this case, the rotor will remain in idle mode until a blade weight anomaly is no longer detected at 48, and the pitch of the blades is adjusted at 50 to return the rotor to normal operation.

[0075] Checks may be done to ensure the quality of the weighted gravity components. Examples of checks include, but are not limited to, positive rotor speed for at least a minimum period of period time, valid status of signals, enough datapoints per azimuth position, etc.

[0076] Checks may also be done to ensure the quality of the outputs of the Kalman filter(s) 23a-c, or other model(s). Examples of checks include, but are not limited to, residuals, model error, means / variance of residuals, etc.

[0077] The information on blade condition provide by the combined weighted gravity component could enable advanced control strategies, that can extract more power from the rotor. Instead of traditional mixed profile assumptions, the control strategy could use the estimated blade condition to define the control strategy.

[0078] Typically the blade weight anomaly detected at 48 is an ice-related blade weight anomaly, i.e. an indication that ice has accreted on the blade. The method may also be used to indicate a blade weight anomaly caused by accretion of another material on the blade, or a low blade weight anomaly caused by degradation of the blade.

[0079] The embodiments above provide a method of monitoring a blade of the rotor of a wind turbine based on a bending value obtained by measuring bending of the blade as the rotor rotates. The bending value could be a strain value from one or more edge strain gauges, from one or more flap strain gauges, or from a combination of both flap and edge strain gauges. Other embodiments of the invention may obtain the bending value from other sensors, for example a camera, lidar sensor or accelerometer. The bending value may be obtained directly from a specific sensor or from a fusion of different sensors. The bending value may be obtained by directly measuring bending of the blade, or by indirectly measuring bending of the blade using an engineering model. The bending value for a particular blade may also originate from a model based on the other two blades of the rotor.

[0080] Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims

Claims

CLAIMS1. A method of monitoring a blade of a rotor of a wind turbine, the method comprising:obtaining a bending value by measuring bending of the blade as the rotor rotates; obtaining an azimuth value associated with the bending value by measuring an azimuth angle of the blade;determining a cosecant of the azimuth angle;determining a gravity component on a basis of the bending value and the cosecant of the azimuth angle; andusing the gravity component to determine whether a blade weight anomaly exists and generate a corresponding output.

2. A method according to claim 1 , wherein the gravity component is a weighted gravity component determined by: obtaining a weighting parameter based on the azimuth angle of the blade, wherein the weighting parameter is zero when the cosecant of the azimuth angle is infinite, and determining the weighted gravity component on a basis of the bending value, the cosecant of the azimuth angle and the weighting parameter.

3. A method according to claim 2, wherein the weighting parameter varies between a maximum when the cosecant of the azimuth angle is + / - 1 and a minimum when the cosecant of the azimuth angle is infinite.

4. A method according to claim 2 or 3, wherein the weighting parameter varies continuously with the azimuth angle of the blade.

5. A method according to any of claims 2 to 4, wherein the weighting parameter varies sinusoidally with the azimuth angle of the blade.

6. A method according to any of claims 2 to 5, wherein the weighting parameter varies at twice the frequency of the azimuth value.

7. A method according to any preceding claim, wherein the rotor has a plurality of blades, and the method comprises: for each blade, obtaining a bending value by measuring bending of the blade as the rotor rotates, obtaining an azimuth value associated with the bending value by measuring an azimuth angle of the blade; determining a cosecant of the azimuth angle, and determining a gravity componenton a basis of the bending value and the cosecant of the azimuth angle; combining the gravity components of the plurality of blades to obtain a combined gravity component; and using the combined gravity component to determine whether a blade weight anomaly exists and generate a corresponding output.

8. A method according to claim 7, wherein combining the gravity components comprises determining a sum or mean of the gravity components.

9. A method according to any preceding claim, wherein the bending value and the azimuth value are obtained as the rotor is rotating and substantially no power is being captured from the rotor.

10. A method according to any preceding claim, wherein the bending value and the azimuth value are obtained as the rotor is rotating at a rate below 10 RPM.

11. A method according to any preceding claim, wherein the gravity component is compared with an output from a trained model, and the determination whether a blade weight anomaly exists is based on the comparison.

12. A method according to any preceding claim, wherein the blade comprises a sensor;and the bending of the blade is measured by the sensor.

13. A method of controlling a wind turbine, the method comprising monitoring a blade of a rotor of the wind turbine by a method according to any preceding claim; and controlling the wind turbine based on the output.

14. Apparatus for monitoring a blade of a rotor of a wind turbine, wherein the apparatus is configured to: receive a bending value indicative of bending of the blade as the rotor rotates; receive an azimuth value which is associated with the bending value and indicative of an azimuth angle of the blade; determine a cosecant of the azimuth angle; determine a gravity component on a basis of the bending value and the cosecant of the azimuth angle; and use the gravity component to determine whether a blade weight anomaly exists and generate a corresponding output.

15. A wind turbine system comprising: a rotor comprising a blade; a bending sensor configured to obtain a bending value by measuring bending of the blade as the rotor rotates; an azimuth sensor configured to obtain an azimuth value associated with the bending value by measuring an azimuth angle of the blade; and apparatus according to claim 14 configured to receive the bending value and the azimuth value and monitor the blade accordingly.

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