Method and apparatus for determining loads of a wind turbine blade

Abstract

Method and blade monitoring system for monitoring bending moment of a wind turbine blade. The method comprises obtaining a first sensor set signal indicative of a first bending moment at a first sensor position different from the tip end along the longitudinal axis of the wind turbine blade, and estimating a bending moment at a first estimation position along the longitudinal axis based on the first sensor set signal, wherein the first sensor position is different from the first estimation position along the longitudinal axis. The blade monitoring system comprises a processing unit and an interface connected to the processing unit, the processing unit being configured for performing the method.

Claims

1. A method for controlling a wind turbine comprising a wind turbine blade extending along a longitudinal axis from a root end to a tip end of the wind turbine blade, where the wind turbine blade has a root region, a transition region, and an airfoil region, the method comprising the steps of: a) obtaining a first sensor set signal indicative of a first bending moment at a first sensor position different from the tip end along the longitudinal axis of the wind turbine blade, b) obtaining a second sensor set signal indicative of a second bending moment at a second sensor position along the longitudinal axis, c) estimating a bending moment at a first estimation position along the longitudinal axis based on the first sensor set signal, wherein the first sensor position is different from the first estimation position along the longitudinal axis, wherein the estimation is carried out for the first estimation position being located at the root end of the wind turbine blade, and wherein the estimation is carried out by comparing the first bending moment and the obtained bending moment from step b) or assuming a zero bending moment at the tip end, to an approximation function indicative of the moment distribution along the longitudinal axis of the blade, d) transmitting a calculated bending moment to a control system of a wind turbine, and e) controlling the wind turbine blade via the control system by adjusting one or more operational parameters of the wind turbine blade based on the calculated bending moment.

2. The method according to claim 1, wherein step b) is further carried out assuming a zero bending moment at the tip end.

3. The method according to claim 1, wherein the distance between the first sensor position and the first estimation position along the longitudinal axis is at least 1 m.

4. The method according to claim 1, wherein the first sensor position is located outside the root region of the blade.

5. The method according to claim 1, wherein the first bending moment has a primary component about a first axis perpendicular to the longitudinal axis and a secondary component about a second axis perpendicular to the longitudinal axis.

6. The method according to claim 1, the method comprising obtaining a second sensor set signal indicative of a second bending moment at a second sensor position along the longitudinal axis, and wherein step b) is further based on the second sensor set signal.

7. The method according to claim 6, wherein the second bending moment has a primary component about a first axis perpendicular to the longitudinal axis and a secondary component about a second axis perpendicular to the longitudinal axis.

8. The method according to claim 1, wherein estimating a bending moment comprises estimating a primary component about a first axis perpendicular to the longitudinal axis and a secondary component about a second axis perpendicular to the longitudinal axis at a first estimation position along the longitudinal axis.

9. The method according to claim 1, wherein the bending moment at the first estimation position is estimated by using a first approximation function from the tip end to the first sensor position and a second approximation function from the first sensor position to the first estimation position, and wherein the second approximation function is based on the first approximation function.

10. The method according to claim 9, wherein the first approximation function is selected from a cubic spline function and a polynomial function, and the second approximation function is a linear interpolation.

11. The method according to claim 1, wherein the first sensor position is located in the transition region or the airfoil region of the wind turbine blade.

12. The method according to claim 1, wherein step b) comprises curve fitting.

13. The method according to claim 1, comprising transmitting the estimated bending moment to a control system.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above and other features and advantages of the present invention will become readily apparent to those skilled in the art by the following detailed description of exemplary embodiments thereof with reference to the attached drawings, in which:

(2) FIG. 1 illustrates a wind turbine,

(3) FIG. 2 illustrates a wind turbine blade,

(4) FIG. 3 is a cross section of a wind turbine blade,

(5) FIG. 4 illustrates different views of a wind turbine blade,

(6) FIG. 5 is a flow diagram of an exemplary method according to the invention,

(7) FIG. 6 illustrates a cross section of a wind turbine blade,

(8) FIG. 7 illustrates a cross section of a wind turbine blade,

(9) FIG. 8 illustrates a cross section of a wind turbine blade,

(10) FIG. 9 illustrates a cross section of a wind turbine blade,

(11) FIG. 10 illustrates a cross section of a wind turbine blade,

(12) FIG. 11 illustrates a wind turbine blade with sensor system according to the invention,

(13) FIG. 12 schematically illustrates a first optical fiber and a patch optical fiber,

(14) FIG. 13 illustrates a blade monitoring system,

(15) FIG. 14 illustrates a blade monitoring system, and

(16) FIG. 15 illustrates estimated bending moment with curve fitting.

DETAILED DESCRIPTION OF THE INVENTION

(17) The figures are schematic and simplified for clarity, and they merely show details which are essential to the understanding of the invention, while other details have been left out. Throughout, the same reference numerals are used for identical or corresponding parts.

(18) FIG. 1 illustrates a conventional modern upwind wind turbine according to the so-called Danish concept with a tower 4, a nacelle 6 and a rotor with a substantially horizontal rotor shaft. The rotor includes a hub 8 and three blades 10 extending radially from the hub 8, each having a blade root 16 nearest the hub and a blade tip 14 furthest from the hub 8. The rotor has a radius denoted R.

(19) FIG. 2 shows a schematic view of a first embodiment of a wind turbine blade 10 according to the invention. The wind turbine blade 10 has the shape of a conventional wind turbine blade and comprises a root region 30 closest to the hub, a profiled or an airfoil region 34 furthest away from the hub and a transition region 32 between the root region 30 and the airfoil region 34. The blade 10 comprises a leading edge 18 facing the direction of rotation of the blade 10, when the blade is mounted on the hub, and a trailing edge 20 facing the opposite direction of the leading edge 18.

(20) The airfoil region 34 (also called the profiled region) has an ideal or almost ideal blade shape with respect to generating lift, whereas the root region 30 due to structural considerations has a substantially circular or elliptical cross-section, which for instance makes it easier and safer to mount the blade 10 to the hub. The diameter (or the chord) of the root region 30 may be constant along the entire root area 30. The transition region 32 has a transitional profile gradually changing from the circular or elliptical shape of the root region 30 to the airfoil profile of the airfoil region 34. The chord length of the transition region 32 typically increases with increasing distance r from the hub. The airfoil region 34 has an airfoil profile with a chord extending between the leading edge 18 and the trailing edge 20 of the blade 10. The width of the chord in the airfoil region decreases with increasing distance r from the hub.

(21) A shoulder 40 of the blade 10 is defined as the position, where the blade 10 has its largest chord length. The shoulder 40 is typically provided at the boundary between the transition region 32 and the airfoil region 34.

(22) The blade 10 has different airfoil profiles 41, 42, 43, 44, 45, 46 along the longitudinal axis of the blade.

(23) As illustrated in FIG. 4, the wind turbine blade 10 comprises at least one sensor set including a first sensor set positioned at a first position along the longitudinal axis. The first sensor set comprises a first primary sensor 47A and optionally a first secondary sensor 47B positioned at a first distance d.sub.1 from the root end. The sensor 47A and the sensor 47B may be displaced a distance d.sub.1,12 along the longitudinal direction. The distance d.sub.1,12 may be less than 1 m.

(24) Optionally, the wind turbine blade 10 comprises a second sensor set positioned at a second position along the longitudinal axis. The second sensor set comprises a second primary sensor 48A and optionally a second secondary sensor 48B positioned at a second distance d.sub.2 from the root end. The sensor 48A and the sensor 48B may be displaced a distance d.sub.2,12 along the longitudinal direction. The distance d.sub.2,12 may be less than 1 m.

(25) In the wind turbine blade 10, the sensors are optical sensors in the form of an optical fiber with fiber Bragg gratings embedded in the shell of the wind turbine blade. The sensors of the wind turbine blade may be a part of the same optical fiber and/or be a part of different optical fiber sections coupled by one or more optical connectors.

(26) It should be noted that the chords of different sections of the blade normally do not lie in a common plane, since the blade may be twisted and/or curved (i.e. pre-bent), thus providing the chord plane with a correspondingly twisted and/or curved course, this being most often the case in order to compensate for the local velocity of the blade being dependent on the radius from the hub.

(27) Table 1 below illustrates different suitable combinations of sensor positions (distances from the root end), optionally dependent on the length of the wind turbine blade.

(28) TABLE-US-00001 TABLE 1 Sensor positions. d.sub.1/m d.sub.2*/m d.sub.3*/m d.sub.4*/m d.sub.5*/m L/m 2-L 3-L 10-L 20-L 30-L 40 2-20 3-40 10-45 4-15 5-30 4-10 10-30 6-10 10-15 15-20 5 10 8 12 23 40 50 60 1.5 * d.sub.root 0.9 * d.sub.s 8 d.sub.1 + d.sub.12 8 12 8 23 40 *if present

(29) Sensor position configuration may depend on the number of sensor sets available and estimation position(s). A sensor position near the root end may be desirable, however a sensor position too near the root end is not desirable due to contributions or noise from the pitch bearings.

(30) FIGS. 3 and 4 depict parameters, which may be used to explain the geometry of the wind turbine blade according to the invention.

(31) FIG. 3 shows a schematic view of an airfoil profile 50 of a typical blade of a wind turbine depicted with the various parameters, which are typically used to define the geometrical shape of an airfoil. The airfoil profile 50 has a pressure side 52 and a suction side 54, which during usei.e. during rotation of the rotornormally face towards the windward (or upwind) side and the leeward (or downwind) side, respectively. The airfoil 50 has a chord 60 with a chord length c extending between a leading edge 56 and a trailing edge 58 of the blade. The airfoil 50 has a thickness t, which is defined as the distance between the pressure side 52 and the suction side 54. The thickness t of the airfoil varies along the chord 60. The deviation from a symmetrical profile is given by a camber line 62, which is a median line through the airfoil profile 50. The median line can be found by drawing inscribed circles from the leading edge 56 to the trailing edge 58. The median line follows the centres of these inscribed circles and the deviation or distance from the chord 60 is called the camber f. The asymmetry can also be defined by use of parameters called the upper camber (or suction side camber) and lower camber (or pressure side camber), which are defined as the distances from the chord 60 and the suction side 54 and pressure side 52, respectively.

(32) Airfoil profiles are often characterised by the following parameters: the chord length c, the maximum camber f, the position d.sub.f of the maximum camber f, the maximum airfoil thickness t, which is the largest diameter of the inscribed circles along the median camber line 62, the position d.sub.t of the maximum thickness t, and a nose radius (not shown). These parameters are typically defined as ratios to the chord length c. Thus, a local relative blade thickness t/c is given as the ratio between the local maximum thickness t and the local chord length c. Further, the position d.sub.p of the maximum pressure side camber may be used as a design parameter, and of course also the position of the maximum suction side camber.

(33) FIG. 4 shows other geometric parameters of the blade. The blade has a total blade length L. As shown in FIG. 3, the root end is located at position r=0, and the tip end located at r=L. The shoulder 40 of the blade is located at a position r=d.sub.s, and has a shoulder width W, which equals the chord length at the shoulder 40. The diameter of the root is defined as d.sub.root. The curvature of the trailing edge of the blade in the transition region may be defined by two parameters, viz. a minimum outer curvature radius r.sub.o and a minimum inner curvature radius r.sub.i, which are defined as the minimum curvature radius of the trailing edge, seen from the outside (or behind the trailing edge), and the minimum curvature radius, seen from the inside (or in front of the trailing edge), respectively. Further, the blade is provided with a pre-bend, which is defined as y, which corresponds to the out of plane deflection from a pitch axis 22 of the blade.

(34) FIG. 5 illustrates an exemplary method according to the present invention. The method 100 comprises obtaining 102 a first sensor set signal indicative of a first bending moment at a first sensor position along the longitudinal axis of the wind turbine blade. Further, the method 100 comprises estimating 104 a bending moment at a first estimation position along the longitudinal axis based on the first sensor set signal, wherein the first sensor position is different from the first estimation position along the longitudinal axis. The method may be employed on a wind turbine blade as described herein. Optionally, the method 100 comprises obtaining 106 a second sensor set signal indicative of a second bending moment at a second sensor position along the longitudinal axis, and estimating 104 a bending moment is based on the second sensor set signal. Sensor signals may be obtained serially and/or in parallel.

(35) The first sensor set signal comprises a first primary sensor signal from a first primary sensor (47A) and a first secondary sensor signal from a first secondary sensor (47B). The first primary sensor signal indicates a primary component M.sub.X,1 of the first bending moment and the first secondary sensor signal indicates a secondary component M.sub.Y,1 of the first bending moment.

(36) FIG. 6 and FIG. 7 are cross sections illustrating examples of sensor set positioning on a wind turbine blade.

(37) In FIG. 6, the wind turbine blade 10 comprises a first sensor set with a first primary sensor 47A and a second primary sensor 47B at a first distance d.sub.1 from the root. The first primary sensor 47A lies on a first primary sensor axis 74 extending through the elastic center 70 of the blade cross section (transverse plane at first distance d.sub.1). The first secondary sensor 47B lies on a first secondary sensor axis 76 extending through the elastic center 70 of the blade cross section. In FIG. 6, the elastic center lies on the chord 60 and thus the main axis 72 coincide with the chord 60. The angle .sub.1 between the two sensor axes 74 and 76 is 90. The angle .sub.1 between the main axis 72 and the first primary sensor axis 74 is 90

(38) In FIG. 7, the wind turbine blade 10 comprises a first sensor set with a first primary sensor 47A and a second primary sensor 47B at a first distance d.sub.1 from the root.

(39) The first primary sensor 47A lies on a first primary sensor axis 74 extending through the elastic center 70 of the blade cross section (transverse plane at first distance d.sub.1). The first secondary sensor 47B lies on a first secondary sensor axis 76 extending through the elastic center 70 of the blade cross section. In FIG. 7, the elastic center lies on the chord 60 and thus the main axis 72 coincide with the chord 60. The angle .sub.1 between the two sensor axes 74 and 76 is 90. The angle .sub.1 between the main axis 72 and the first primary sensor axis 74 is 75.

(40) In FIG. 6 and FIG. 7, the sensors are embedded in the shell body 78 of the wind turbine blade.

(41) FIG. 8 is a cross section illustrating an example of sensor set positioning on a wind turbine blade. The wind turbine blade 10 comprises a beam 80 attached to the shell body 78 and the second primary sensor 47B is attached to the beam 80 nearest the leading edge 56.

(42) FIG. 9 is a cross section illustrating an example of first sensor set positioning on a wind turbine blade at a first distance d.sub.1. The wind turbine blade comprises a shell body 78 and the sensors 47A, 47B are mounted on the inner surface of the shell body 78. The angle .sub.1 between sensor axes 74 and 76 is 90.

(43) FIG. 10 is a cross section illustrating an example of second sensor set positioning on a wind turbine blade at a second distance d.sub.2. The wind turbine blade comprises a shell body 78 and the sensors 48A, 48B are mounted on the inner surface of the shell body 78. The angle .sub.2 between sensor axes 74 and 76 is 90.

(44) FIG. 11 illustrates a part of a wind turbine. The wind turbine comprises a hub 8 from which the blades whereof a first wind turbine blade 10 is shown extend substantially in a radial direction when mounted to the hub 8. The wind turbine blade 10 comprises a sensor system 82 with an optical path comprising a first optical fiber 84, a second optical fiber 86 and a patch optical fiber 88. Optical connector 90 couples the first optical fiber 84 and the patch optical fiber 88 and optical connector 90 couples the second optical fiber 86 to the patch optical fiber 88. The optical fibers 84, 86, 88 are SM1500 (4.2/125) fibers. The first optical fiber comprises first primary sensor 47A and second primary sensor 48A in the form of fiber Bragg gratings and optionally first temperature sensor 98A. The second optical fiber comprises first secondary sensor 47B and second secondary sensor 48B in the form of fiber Bragg gratings and optionally second temperature sensor 98B. The first end 85 of the first optical fiber 84 is coupled to a reading unit 92 for reading sensor signals from the sensor system 82. The reading unit 92 provides wavelength values of the sensor signals to a blade monitoring system 94 via a data cable 96. The blade monitoring system is configured for estimating components of the bending moment at the root end of the wind turbine blade based on the sensor signals and configured for transmitting the estimated bending moment to a turbine controller (not shown). The second end 85 of the first optical connector 84 is optically coupled to the first end 89 of the patch optical fiber 88 in connector or connector assembly 90. The second end 89 of the patch optical fiber 88 is optically coupled to the first end 87 of the second optical fiber 86 in connector or connector assembly 90.

(45) FIG. 12 schematically illustrates the optical connectors or connector assemblies 90 between the first optical fiber 84 and the patch fiber 88. The first optical fiber 84 comprises a first core 130 with a first core diameter d.sub.core, 1. The patch optical fiber 88 comprises a patch core 132 with a patch core diameter d.sub.core,p=d.sub.core,1=4.2 m. Fiber cladding material and sheet 134, 136 protect the cores 130, 131, 132. The first optical fiber 84 comprises a first end connector part at the first end (not shown) and a second end connector part 138 (e.g. female E2000 connector) at the second end 85, and the patch optical fiber comprises a first end connector part 140 (e.g. male E2000 connector) for connecting the first optical fiber 84 and the patch optical fiber 88. The connector assembly 90 is formed in the same way as the connector assembly 90 indicated by the reference numbers.

(46) The second optical fiber 86 includes a second core 131 with a second core diameter d.sub.core,2=d.sub.core,p, wherein the second optical fiber extends from a first end to a second end and comprising at least one sensor. The second optical fiber 86 comprises a first end connector part 138 (e.g. female E2000 connector) at the first end 87 and a second end connector part (not shown) at the second end, and the patch optical fiber comprises a second end connector part 140 (e.g. male E2000 connector) for connecting the second optical fiber 86 and the patch optical fiber 88.

(47) FIG. 13 schematically illustrates a blade monitoring system 94. The blade monitoring system 94 comprises a housing 95 accommodating a processing unit 150 connected to an interface 152 and a memory unit 154 via connections 155, 155, respectively. The interface 152 comprises a first connector port 156 and a second connector port 158. The first connector port 156 is configured for connection to a reading unit for receiving data of sensor signals from a sensor system of a wind turbine blade. The second connector port 158 is configured for connection to a turbine controller for transmitting and/or sending data and/or control/alarm signals to a turbine controller.

(48) The processing unit 150 is configured for receiving a first sensor set signal indicative of a first bending moment at a first sensor position of a wind turbine blade extending along a longitudinal axis from a root end to a tip end via the first connector port 156. Further, the processing unit 150 is configured for estimating a bending moment or components thereof at a first estimation position along the longitudinal axis based on the first sensor set signal, wherein the first sensor position is different from the first estimation position along the longitudinal axis.

(49) FIG. 14 schematically illustrates a blade monitoring system 94 wherein a reading unit 92 is integrated in the blade monitoring system and connected to the processing unit 150 via connection 155. The interface 152 comprises a first connector port 156 in the form of a first sensor port 160 for coupling the sensor system, e.g. the first optical fiber 84, (optionally via a patch optical fiber) to the reading unit 92 of the blade monitoring system 94.

(50) FIG. 15 schematically illustrates estimation of a primary component M.sub.X,est,1 of bending moment at the root end (first estimation position) of a wind turbine blade having L=53.2 m. The first distance d.sub.1 is 7 m and the second distance d.sub.2 is 10.5 m. The primary component M.sub.X,est,1 is estimated based on a first primary sensor signal S.sub.11 from a first primary sensor at d.sub.1 and a second primary sensor signal S.sub.21 from a second primary sensor at d.sub.2.

(51) It has been shown that using cubic spline functions from the first primary sensor to the tip end of the wind turbine blade and then using the gradient or derivative of the bending moment at the first distance to perform a linear extrapolation from d.sub.1 to the root end may be preferred. Gradients of the bending moment at the tip end are zero. Further, the bending moment gradient at d.sub.1 may be estimated based on the bending moments at d.sub.1 and d.sub.2, e.g. with a backward Euler method. A large distance between the first and second sensor sets may not be desirable.

(52) The estimation as illustrated in FIG. 15 comprises the following steps using measured bending moment components at d.sub.1 and d.sub.2: A first cubic spline function with correct boundary conditions using the two measurement points and the tip end point where the bending moment is zero is fitted used to perform the interpolation of a point near the end of the wind turbine blade. Here correct means that the first derivatives at the two ends of the interval are correct, the bending moment gradient at d.sub.1 is estimated using the measured sensor data transformed to bending moments at d.sub.1 (M.sub.X,1) and d.sub.2 (M.sub.X,2) with a backward Euler method, and the derivative at the tip end is zero. The three points, i.e. d.sub.1, d.sub.2 and the tip end point are denoted points A in FIG. 15. The interpolated point near the end of the wind turbine blade is denoted point B. Then a second spline function, covering the interval from d.sub.1 to the tip end is constructed using a not-a-knot method where the points A and B are used. Then, the second spline function is extended to the root end with a linear extrapolation denoted app. A between d.sub.1 and the root end.

(53) The not-a-knot method means that the third order derivative at the second and second last point of the domain used for interpolation is the same when looking from each side of the point.

(54) FIG. 15 illustrates estimations of the bending moment. In the graph, app. A represents sensor positions at d.sub.1=7 m and d.sub.2=10.5 m, and app. B represents sensor positions at d.sub.1=7 m and d.sub.2=20 m. As can be seen on FIG. 15, app A provides a better estimation of primary component near the root end while app B provides a better estimation between 10 m and 50 m from the root end.

(55) It should be noted that in addition to the exemplary embodiments of the invention shown in the accompanying drawings, the invention may be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art.

LIST OF REFERENCE NUMERALS

(56) 2 wind turbine 4 tower 6 nacelle 8 hub 10, 10, 10, 10 wind turbine blade 14 blade tip 16 blade root 18 leading edge 20 trailing edge 22 pitch axis 30 root region 32 transition region 34 airfoil region 40 shoulder 41, 42, 43, 44, 45, 46 airfoil profile 47A first primary sensor 47B first secondary sensor 48A second primary sensor 48B second secondary sensor 50 airfoil profile 52 pressure side 54 suction side 56 leading edge 58 trailing edge 60 chord 62 camber line/median line 70 elastic center 72 main axis 74 primary sensor axis 76 secondary sensor axis 78 shell body 80 beam 82 sensor system 84 first optical fiber 85 first end of first optical fiber 85 second end of first optical fiber 86 second optical fiber 87 first end of second optical fiber 87 second end of second optical fiber 88 patch optical fiber 89 first end of patch optical fiber 89 second end of patch optical fiber 90, 90 optical connector 92 reading unit 94 blade monitoring system 94 blade monitoring system 95 housing 96 data cable 98A first temperature sensor 98B second temperature sensor 99 beam splitting/combining unit 130 first core 131 second core 132 patch core 134 fiber cladding material and sheet 136 fiber cladding material and sheet 138 second end connector part of first optical fiber 138 first end connector part of second optical fiber 140 first end connector part of patch optical fiber 140 second end connector part of patch optical fiber 150 processing unit 152 interface 154 memory unit 155, 155,155, 155, 155 connection 156 first connector port 158 second connector port 160 sensor port c chord length d.sub.t position of maximum thickness d.sub.f position of maximum camber d.sub.p position of maximum pressure side camber d.sub.s shoulder distance d.sub.root root diameter f camber L blade length P power output r local radius, radial distance from blade root t thickness v.sub.w wind speed twist, pitch y prebend .sub.1 angle between first primary sensor axis and first secondary sensor axis .sub.2 angle between second primary sensor axis and second secondary sensor axis. .sub.1 angle between first primary sensor axis and main axis .sub.2 angle between second primary sensor axis and main axis d.sub.core,1 first core diameter d.sub.core,2 second core diameter d.sub.core,p patch core diameter