DEVICE FOR DETERMINING THE DISTANCE BETWEEN A WIND TURBINE BLADE AND ITS WIND TURBINE TOWER AT PASSING

Abstract

The invention relates to a method of determining a tip-to-tower clearance of a wind turbine, the wind turbine comprising a wind turbine tower, where a distance sensor unit is arranged on at least one wind turbine blade of the wind turbine and comprises at least a transmitter and a receiver, wherein the method comprises the steps of: transmitting a signal from the distance sensor unit toward the wind turbine tower, measuring a signal reflected from the wind turbine tower, determining a distance between the wind turbine tower and the at least one wind turbine blade based on the transmitted signal and the reflected signal, wherein the method further comprises the step of correcting the measured distance based on at least one of an actual pitch angle and a deflection angle of the at least one wind turbine blade at the location of the distance sensor unit.

Claims

1-95. (canceled)

96. A method of determining a tip-to-tower clearance of a wind turbine, the wind turbine comprising a wind turbine tower, a nacelle arranged on top of the wind turbine tower, a rotatable rotor with at least one wind turbine blade arranged relative to the nacelle, where a distance sensor unit is arranged on the at least one wind turbine blade and comprises at least a transmitter and a receiver, wherein the method comprises the steps of: transmitting a signal from the distance sensor unit toward the wind turbine tower, measuring a signal reflected from the wind turbine tower, determining a distance between the wind turbine tower and the at least one wind turbine blade based on the transmitted signal and the reflected signal, characterised in that the method comprises the step of correcting the measured distance based on at least one of an actual pitch angle and a deflection angle of the at least one wind turbine blade at the location of the distance sensor unit.

97. A method according to claim 96, characterised in that at least one distance profile indicative of at least one pitch angle of the at least one wind turbine blade is established, wherein the actual pitch angle is determined based on the at least one distance profile.

98. A method according to claim 96, characterised in that the method further comprises the step of measuring a rotational speed of the at least one wind turbine blade, wherein the actual pitch angle is estimated using a predetermined correlation between the actual pitch angle and at least the rotational speed.

99. A method according to claim 96, characterised in that the method further comprises the step of waking up the distance sensor unit prior to the at least one wind turbine blade passing the wind turbine tower, where the distance sensor unit goes to sleep after the at least one wind turbine blade has passed the wind turbine tower.

100. A method according to claim 96, wherein the step of correcting the measured distance is performed to obtain the tip-to-tower clearance.

101. A method according to claim 96, wherein the method further comprises a step of determining an angular position of the at least one wind turbine blade.

102. A method according to claim 96, wherein the step of correcting the measured distance is further based on a tilting angle of the wind turbine.

103. A method according to claim 96, wherein the method further comprises a step of measuring one or more blade eigenfrequencies of the at least one wind turbine blade.

104. A method according to claim 96, wherein the method further comprises a step of providing a measurement quality factor based on the one or more blade eigenfrequencies.

105. A distance sensor unit for determining a tip-to-tower clearance of a wind turbine, the wind turbine comprising a wind turbine tower, a nacelle arranged on top of the wind turbine tower, and a rotatable rotor with at least one wind turbine blade arranged relative to the nacelle, wherein the distance sensor unit is arranged to be located on the at least one wind turbine blade, wherein the distance sensor unit comprises a transmitter and a receiver, wherein the transmitter is configured to transmit a signal toward the wind turbine tower and the receiver is configured to measure a signal reflected from the wind turbine tower, wherein the distance sensor unit further comprises a processor configured to determine a distance between the wind turbine tower and the at least one wind turbine blade based on the transmitted signal and the reflected signal, characterised in that the processor is further configured to correct the measured distance based on at least one of an actual pitch angle and a deflection angle of the at least one wind turbine blade.

106. A wind turbine comprising a wind turbine tower, a nacelle arranged on top of the wind turbine tower, a rotatable rotor with at least one wind turbine blade arranged relative to the nacelle, and a distance sensor unit arranged on the at least one wind turbine blade, wherein the distance sensor unit comprises a transmitter and a receiver, wherein the transmitter is configured to transmit a signal toward the wind turbine tower and the receiver is configured to measure a signal reflected from the wind turbine tower, wherein the distance sensor unit further comprises a processor configured to determine a distance between the wind turbine tower and the at least one wind turbine blade based on the transmitted signal and the reflected signal, characterised in that the processor is further configured to correct the measured distance based on at least one of an actual pitch angle and a deflection angle of the at least one wind turbine blade at a location of the distance sensor unit on the at least one wind turbine blade.

107. A wind turbine according to claim 106, wherein the distance sensor unit is installed at least 1 meter from a tip of the at least one wind turbine blade.

108. A wind turbine according to claim 106, wherein the distance sensor unit is installed at least 0.5 meter from a receptor located in the at least one wind turbine blade.

109. A method for determining a deflection of a wind turbine blade of a wind turbine, the method comprising the steps of: measuring at least one sensor acceleration in at least one acceleration direction relative to a sensor unit location on the wind turbine blade, wherein the sensor unit location has a radial position relative to a rotation axis of a rotatable rotor of the wind turbine; and calculating the deflection based on the at least one sensor acceleration.

110. A method according to claim 109, wherein the step of measuring the at least one sensor acceleration is performed discontinuously within a roundtrip of the wind turbine blade.

111. A method according to claim 109, wherein the method comprises a step of calculating the centripetal acceleration at an undeflected radial position based on the angular velocity.

112. A method according to claim 109, wherein the method comprises incorporating a compensation for gravitational acceleration in the step of calculating the deflection.

113. A method for monitoring a wind turbine blade comprising the steps of: measuring one or more sensor accelerations in one or more acceleration directions relative to a sensor unit location on the wind turbine blade, wherein the sensor unit location has a radial position relative to a rotation axis of a rotatable rotor of the wind turbine, wherein the one or more acceleration directions are respectively associated with the one or more sensor accelerations, wherein the step of measuring the one or more sensor accelerations is performed continuously in a measurement time period to obtain an acceleration data sample; and analysing the acceleration data sample to obtain a frequency composition of the acceleration data sample, wherein the frequency composition is indicative of one or more blade eigenfrequencies of the wind turbine blade.

114. A method according to claim 113, wherein the method comprises a step of evaluating the one or more blade eigenfrequencies.

115. A method for monitoring the actual pitch angle of a wind turbine blade of a wind turbine, the method comprising the steps of: transmitting a signal from a distance sensor unit towards a wind turbine tower of the wind turbine, wherein the distance sensor unit is located in a sensor unit location on the wind turbine blade; measuring a signal reflected from the wind turbine tower to obtain a measured signal, wherein the signal reflected from the wind turbine tower is based on the step of transmitting a signal; and determining an actual pitch angle at the sensor unit location.

Description

DESCRIPTION OF THE DRAWING

[0247] The invention is described by example only and with reference to the drawings, wherein:

[0248] FIG. 1 shows an exemplary wind turbine,

[0249] FIG. 2 shows a wind turbine with a distance sensor unit and a receiving device,

[0250] FIG. 3 shows an exemplary configuration of the distance sensor unit and the receiving device

[0251] FIG. 4 shows the wind turbine with the distance sensor unit integrated into the blade body,

[0252] FIG. 5 shows the tip section of the wind turbine shown in FIG. 4,

[0253] FIG. 6 shows a cross-sectional view of the tip section shown in FIG. 5,

[0254] FIG. 7 shows a top view of the wind turbine tower and two measured distance profiles at different pitch angles,

[0255] FIG. 8 shows a distance measurement between the wind turbine blade and the wind turbine tower with a pitch angle,

[0256] FIG. 9 shows a distance measurement between the wind turbine blade and the wind turbine tower with a deflection angle, and

[0257] FIG. 10 shows an exemplary series of distance measurements from which a distance profile may be determined.

[0258] In the following text, the figures will be described one by one and the different parts and positions seen in the figures will be numbered with the same numbers in the different figures. Not all parts and positions indicated in a specific figure will necessarily be discussed together with that figure.

REFERENCE LIST

[0259] 1 Wind turbine

[0260] 2 Wind turbine tower

[0261] 3 Nacelle

[0262] 4 Rotor

[0263] 5 Wind turbine blades

[0264] 6 Hub

[0265] 7 Distance sensor unit

[0266] 8 Receiving device

[0267] 9 Radar measuring system

[0268] 9a Transmitter

[0269] 9b Receiver

[0270] 10 Processor

[0271] 11 Accelerometer

[0272] 12 Battery

[0273] 13 Photovoltaic cells

[0274] 14 Gyroscope

[0275] 15 Radio transceiver

[0276] 16 Radio transceiver

[0277] 17 Controller, local controller

[0278] 18 Recess

[0279] 19 Distance profiles

[0280] 20 Pitch angles

[0281] 21 Chord line

[0282] 22 Rotor plane

[0283] 23 Deflection angle

[0284] 24 Longitudinal direction

[0285] 25 Centripetal force

[0286] 26 Gravity force

[0287] 27 Tilting angle

[0288] 28 Distance measurement

[0289] D Distance

DETAILED DESCRIPTION OF THE INVENTION

[0290] FIG. 1 shows an exemplary wind turbine 1 with a rotor assembly. The wind turbine 1 comprises a wind turbine tower 2, a nacelle 3 arranged on top of the wind turbine tower 2. A yaw system comprising a yaw bearing unit is arranged between the wind turbine tower 2 and the nacelle 3. A rotor 4 is arranged relative to the nacelle 3 and is rotatably connected to a drive train (not shown) arranged inside the nacelle 3. At least two wind turbine blades 5, here three are shown, are mounted to a hub 6 of the rotor 4.

[0291] Each wind turbine blade 5 comprises an aerodynamically shaped body extending from a blade root to a tip end and further from a leading edge to a trailing edge. The wind turbine blades are here shown as full-span pitchable blades, alternatively fixed full-span blades may be used instead. A pitch system comprising at least a pitch bearing unit is arranged between the hub 6 and the blade root of the wind turbine blade 5.

[0292] FIG. 2 shows a wind turbine 1 with a distance sensor unit 7 and a receiving device 8. The distance sensor unit 7 is installed on the wind turbine tower 2 and configured to measure the distance, D, between one wind turbine blade 5 as it passes the wind turbine tower 2 in the lowermost position using a non-contact measuring technique.

[0293] The receiving device 8 is configured to communicate with the distance sensor unit 7 via a wireless communications link. The receiving device 8 is preferably arranged at the hub 6. However, the receiving device 8 may also be arranged in other locations on the wind turbine 1, e.g. at the top of the wind turbine tower 2, or at a location separate from the wind turbine 1.

[0294] FIG. 3 shows an exemplary configuration of the distance sensor unit 7 and the receiving device 8. The distance sensor unit 7 comprises a radar measuring system 9 having a transmitter 9a and a receiver 9b. The transmitter 9a is configured to transmit a signal, e.g. a radar beam, with a measuring field. The receiver 9b is configured to receive a reflected signal, e.g. a reflected radar beam.

[0295] The distance sensor unit 7 further comprises a processor 10 configured to determine an actual distance based on the transmitted signal and the reflected signal, e.g. using a Doppler shift or a time-of-flight measurement. The processor 10 is further configured to determine an actual pitch angle of wind turbine blade 5 at the sensor location.

[0296] An accelerometer 11 is built into the distance sensor unit 7 and an acceleration signal is inputted to the processor 10. The processor 10 analyses the acceleration signal to determine the angular position of each wind turbine blade 5. When one wind turbine blade 5 is in a first angular position, the distance sensor unit 7 wakes up and the distance sensor unit 7 performs a distance measurement. When the one wind turbine blade 5 is in a second angular position, the distance sensor unit 7 is powered down.

[0297] The distance sensor unit 7 comprises its own power source. Here, the power source is a rechargeable battery 12 or a super capacitor connected to photovoltaic cells 13. The distance sensor unit 7 is hence shaped as a small compact sensor that is self-powered.

[0298] A gyroscope 14 is further built into the distance sensor unit 7. The gyroscope 14 is configured to measure the rotational speed of the wind turbine blade 5 and input the measured rotational speed to the processor 10. The measured rotational speed may be used to determine the actual distance between the wind turbine blade 5 and the wind turbine tower 2, for example in combination with any of the acceleration, rotational speed, radial position, and angular position. For example by using the rotational speed to estimate the centrifugal/centripetal force. The gyroscope 14 may also be used in combination with the accelerometer 11 to determine the angular position and/or the rotational speed.

[0299] The distance sensor unit 7 may further comprise a radio transceiver 15 configured to communicate with a radio transceiver 16 of the receiving device 8. The radio transceivers 15, 16 are able to exchange data via radio signals. The radio transceiver 16 of the receiving device 8 is further connected to a local controller 17. The controller 17 (also referred to as an external data processor) may instead be implemented as part of the local wind turbine controller used to control the operation of the wind turbine 1. Thus, processing, if any, may also be performed externally from the distance sensor unit.

[0300] In this embodiment, the distance sensor unit thus determines and corrects a distance. In some other embodiments of the invention, the distance sensor unit measures a distance and transmits this measured distance to a receiving device, and subsequently, the receiving device corrects the measured distance based on an actual pitch angle and/or deflection angle.

[0301] In some embodiments of the invention, the method is further based on storing data in a memory unit, for example a memory unit located in the distance sensor unit for storing measurements and corrections. A memory unit may for example be a digital storage associated with the distance sensor unit or a data process external to the distance sensor unit, a data processor which may communicate with the distance sensor unit and perform or assist in performing the calculation of the actual distance, actual pitch angle, rotor speed, etc.

[0302] FIG. 4 shows the wind turbine 1 with the distance sensor unit 7′ integrated into the body of the wind turbine blade 5. Here, the distance sensor unit 7′ is arranged in the tip section of the wind turbine blade 5. Typically, the distance sensor unit is located on the suction side of the blade. Typically, the distance sensor unit is located closer to the tip than to the root of the blade.

[0303] The transmitted signal and/or the reflected signal are preferably stored in a memory unit in the distance sensor unit. Further, the measured distance, the measured rotational speed, the actual distance and/or the actual pitch angle are preferably also stored in the memory unit. Once the distance sensor unit 7′ is activated, the processor 10 transmits all or some of the stored or computed data to the local controller 17 via the respective radio transceivers 15, 16.

[0304] FIG. 5 shows the tip section of the wind turbine 1 where the top of the distance sensor unit 7′ has a smooth curved surface so that it has a minimal aerodynamic impact on the local airflow over the blade surface. In typical embodiments, the distance sensor unit is flushed with the surface of the blade to not disturb the aerodynamics of the blade.

[0305] FIG. 6 shows a cross-sectional view of the tip section of the wind turbine blade 5, wherein a recess 18 is formed in the blade surface. The majority, if not all, of the distance sensor unit 7′ is concealed within the volume of the recess 18. The top of the distance sensor unit 7′ is thereby substantially aligned with the blade surface, as indicated in FIG. 6.

[0306] FIG. 7a and FIG. 7b shows a top cross-sectional view of the wind turbine tower 2 and two measured distance profiles 19, 19′ at different pitch angles 20, 20′, respectively. The two FIGS. 7a, 7b corresponds to two different measurements performed under different conditions, resulting in different actual pitch angles and consequently different distance profiles. In both cases, the processor 10 scans the measuring field and takes multiple distance measurements which together form a distance profile 19 at a certain pitch angle 20. A first distance profile 19 is indicative of a first pitch angle 20. A second distance profile 19′ is indicative of a second pitch angle 20′. The processor 10 uses the first and/or the second distance profile 19, 19′ to determine an actual pitch angle of the wind turbine blade 5 at the sensor location. The measuring field at least covers an area in front of the distance sensor unit in which the tower is or is going to be reflected.

[0307] The horizontal direction may be interpreted as a position axis, indicating the position/angular position in which measurements of the distance profile were performed. The dashed lines between the tower 2 and the respective distance profiles 19, 19′ indicate the angle at which the distance sensor unit performs its distance measurement, which depends on the pitch angle. In this exemplary illustration, the first distance profile 19 is based on measurements performed at a small pitch angle 20 of, whereas the second distance profile 19′ is based on measurements performed at a larger pitch angle 20′. Note that the distance profile is a representation of the tower reproduced by reflections of the signals sent out by the radar measuring system 9.

[0308] Note particularly how the shape of the first distance profile 19 approximates an arc of the circular cross-sectional shape of the wind turbine tower 2. In contrast, the second distance profile 19′ is skewed. The smallest distance D of the second distance profile 19′ is shifted towards the left as a result of the pitch angle 20′. Further, the horizontal position of the second distance profile 19′ is shifted towards the right, in comparison with the first distance profile 19. Furthermore, the horizontal extend of the second distance profile 20′ is enlarged, in comparison with the first distance profile 19.

[0309] Any of these above mention features of the distance profiles 19,19′ (e.g. skewness, smallest distance, horizontal position, horizontal extend) may alone or in combination with each other be utilized to approximate a pitching angle of the wind turbine blade 5. For example, the first (and/or last) pick up of a reflection represents an indication of the actual pitch angle.

[0310] As illustrated in FIGS. 8-9, the processor 10 is configured to compensate for the influence of the pitch angle and a deflection angle (shown in FIG. 9) so that it determines the actual distance i.e. the shortest distance between the wind turbine blade 5 and the wind turbine tower 2.

[0311] FIG. 8 shows a distance measurement between the wind turbine blade 5 and the wind turbine tower 2, where the wind turbine blade 5 is positioned in a pitch angle 20″ perpendicularly to the wind turbine tower 2 in the horizontal plane. As illustrated, in this position the chord line 21 of the wind turbine blade 5 is pitched into an oblique angle relative to the rotor plane 22.

[0312] The distance sensor unit 7 measures a distance D which is influenced by the pitch angle 20″. The processor 10 uses the principle explained in relation to FIG. 7 to determine the pitch angle 20″. The processor 10 then uses trigonometry to calculate the actual distance D′ between the wind turbine blade 5 and the wind turbine tower 2 based on the measured distance D and the pitch angle 20″.

[0313] FIG. 9 shows a distance measurement between the wind turbine blade 5 and the wind turbine tower 2 according to an embodiment of the invention, where the wind turbine blade 5 is positioned in a bend condition so that the distance sensor unit 7 is positioned in a deflection angle 23 in the vertical plane. The distance sensor unit 7 measures a distance D″ between the wind turbine blade 5 and the wind turbine tower 2.

[0314] The processor 10 measures the acceleration in the longitudinal direction 24 of the wind turbine blade 5 via the accelerometer. The processor 10 uses the measured signal from the gyroscope 14 to determine the centripetal force, for example using the radial position which may be known from commissioning at installation, or which may be measured or determined. The centrifugal force 25 acting on an arbitrary segment of (e.g. the distance sensor unit) the wind turbine blade 5 parallel to the rotor plane 22 and the gravity force 26 acting on the wind turbine blade 5 in the vertical plane are projected onto a tangent line at the sensor location using the tilting angle 27 of the rotor 4. The projected force components are summed to indicate the magnitude of the acceleration component in the longitudinal direction, which is measured by the accelerometer.

[0315] The processor 10 then determines the difference between the measured acceleration in the longitudinal direction 24 and the sum of the estimated projected force component of the centripetal force 25 and the projected gravity force 26. The processor 10 uses trigonometry to calculate the actual distance D″′ between the wind turbine blade 5 and the wind turbine tower 2 based on the above difference.

[0316] Note that in other embodiments of the invention, a deflection may be calculated using alternative approaches based on measuring the acceleration as outlined within the disclosure. Note also the invention is not restricted to any particular convention regarding directions of centrifugal force, centripetal force, and gravity. For example, an accelerometer may measure gravity as upwards, and calculations for determining a deflection or a tip-to-tower distance may be performed accordingly.

[0317] Thus, in some embodiments, the distance sensor unit 7 is then able to compensate for both the influence of the pitch angle 20 in the horizontal plane and the influence of the deflection angle 23 in the vertical plane. In some other embodiments, the deflection is established as outlined above, but without measuring the distance via a transmitted and reflected signal.

[0318] FIG. 10 shows an exemplary series of distance measurements 28 obtained, e.g. from the radar, from which a distance profile 19 may be determined. The measurements and distance profile are shown in a coordinate system. The vertical axis represents distance. The horizontal axis illustrates position of the distance sensor unit as it passes by the wind turbine tower. Alternatively, the axis may equivalently represent angular position of the wind turbine blade or time as it passes by the wind turbine tower. In the context of data analysis of the invention, any parameter (e.g. time, angular position, spatial position) may be used as a variable which distance measurements are performed based upon, e.g. as a parameter on the horizontal axis as illustrated in FIG. 10.

[0319] In the illustrated set of measurements, a total of five distance measurements 28 have been performed, and a distance profile 19 is obtained based on these measurements 28. Note that the invention is not limited to a particular number of measurements. A distance profile may be obtained from as few as one, two, or three measurements. In such scenarios, further information may be utilized to establish an accurate distance or distance profile. For example, if the exact angular position of the wind turbine blade is known at the time of measurement, this can be used to estimate an actual pitch angle and, optionally, correct the measured distance.

[0320] In some embodiments, a number of signals are transmitted from the distance sensor unit, but only some of the signals are measured, e.g. since some of the signals were successfully reflected from the wind turbine tower.

[0321] To establish a distance profile 19, the measurements 28 may for example be compared to a lookup table of various trial distance profiles, and one of these may be selected, for example based on minimizing residuals between the trial distance profiles and the measurements 28. Similarly, a fit may be performed, for example based on a mathematical or numerical function which is representative of the distance profile 19. A fit or trial distance profiles may also rely on other inputs, e.g. tilting angle, deflection angle, angular position of the wind turbine blade etc.

[0322] Thus, a distance profile 19 can be established based on distance measurements 28. The obtained distance profile 19 may then be indicative of the tip-to-tower clearance, an actual pitch angle, the deflection angle etc. Thus, obtaining a distance profile 19 based on distance measurements 28 may be an example of how to correct a measured distance.

[0323] Note that, in practice, measurements may not necessarily provide single well-defined data points in illustrated in FIG. 10. A radar measurement may for example provide an angular array of measurement data points. However, such more complex data may similarly be used to obtain a distance, correct a measured distance, or obtain a distance profile 19, e.g. by fitting the data.

[0324] In an exemplary embodiment, an actual pitch angle of the wind turbine blade is determined, at least partly, based on measurements of the acceleration in the distance sensor unit. In some embodiments, an actual pitch angle may even be determined independently from transmitting and receiving a signal. The pitch angle of the wind turbine blade may affect the directions/orientations in which the accelerations are measured, for example relative to gravity, and/or relative to the longitudinal direction of the blade. Thus, based on measured accelerations in the blade, the pitch angle may be determined.

[0325] In an exemplary embodiment, the measured distance is determined based on transmitting and measuring a signal by the distance sensor unit 7. Particularly, the transmitter 9a and the receiver 9b performs a series of radar measurements, for example based on pulses or modulation or radio waves or microwaves. The series of radar measurements is performed as the distance sensor unit mounted on a wind turbine blade passes the wind turbine tower. This series of measurements is basis for a distance profile.

[0326] A measured distance may be directly derived from the distance profile, e.g. the smallest distance in the series of measurements may be understood as an actual distance. However, this distance may be inaccurate in comparison with an actual distance due to a non-zero pitch angle, a non-zero deflection angle, and/or non-zero tilting angle.

[0327] Further steps are then taken to correct the measured distance. For example, corrections may be performed which takes into account tilting angle, the pitch angle, and/or the deflection angle.

[0328] A correction of the measurement error due to a non-zero pitch angle may for example be performed based on distance profiles. They may also be based on separate measurements of the pitch angle, e.g. a measurement at the bearing system of the wind turbine. The correction may also be based on modelling of the pitch angle, e.g. for example a wind-speed dependent actual pitch angle. Alternatively, the actual pitch angle may also be based on an accurate measurement of the angular position of the wind turbine blade. For example, if a distance measurement performed while the wind turbine blade is exactly in a downwards angle (or another accurately determined angle), the pitch angle (or correspondingly, the actual distance) is derivable based on the measured distance and the angular position.

[0329] In an exemplary calculation, a series of distance measurements are performed while a wind turbine blade rotates past the wind turbine tower. The measurements yield a minimal distance of 3 meters. Through other processing, a pitching angle of 15 degrees is determined. The actual distance may then be approximated, for example using a sine relation of a triangle sin(A)=opposite/hypotenuse, where A is an angle of 75 degrees, i.e. a right angle minus the pitching angle. The opposite of the triangle corresponds to the actual distance, whereas the hypotenuse corresponds to the measured distance. Thus, the actual distance may be calculated to be approximately 2.9 meters. This example is merely meant to illustrate how an actual distance may be approximated using trigonometrical principles. In embodiments of the invention, the actual distance may be calculated without the use of trigonometry, and/or by performing further calculations, e.g. taking into account the cross-sectional shape of the wind turbine tower, the tilting angle, the deflection angle etc.

[0330] A correction of the measurement error due to a non-zero tilting angle may for example be based on a separate measurement or calculation of the tilting angle. The tilting angle may typically be known by design but may alternatively be separately measured or calculated at the nacelle or the wind turbine.

[0331] In an exemplary calculation of a correction due to a non-zero tilting angle, a wind turbine blade length is 80 meters with a tilting angle of 2.5 degrees. Here, the correction to the distance may be approximated, for example using a sine relation of a triangle sin(A)=opposite/hypotenuse, where A is an angle of 2.5 degrees, i.e. the tilting angle. The opposite of the triangle corresponds to the correction, whereas the hypotenuse corresponds to the length of the wind turbine blade. Thus, the correction may be calculated to be approximately 3.5 meters. This example is merely meant to illustrate how correction to the measured distance may be approximated using trigonometrical principles. In embodiments of the invention, the actual distance may be calculated without the use of trigonometry, and/or by performing further calculations, e.g. taking into account that the angle of the distance measurement is also affected by the tilting angle.

[0332] A correction of the measurement error due to a non-zero deflection angle may for example be based on a separate measurement or calculation of the deflection angle. The calculation or measurement of the deflection angle may for example be based, at least partly, on rotational speed off the wind turbine.

[0333] Any of the deflection angle and the tilting may change the angle in which the distance measurement is performed. For example, if the deflection and tilting angle are both zero, the distance measurement may be performed approximately in a horizontal plane when the blade passes the wind turbine tower. A deflection angle or a tilting angle may then affect the angle at which the distance sensor unit performs is distance measurement, such that it deviates from in a horizontal plane.

[0334] In an exemplary calculation of a correction due to a non-zero deflection angle, the combined deflection angle and tilting angle results in a deviation of 8 degrees from by the measurement angle from a horizontal plane. The measurements yields a minimal distance of 3 meters. The actual distance may then be approximated, for example using a sine relation of a triangle sin(A)=opposite/hypotenuse, where A is an angle of 85 degrees, i.e. a right angle minus the deviation from the horizontal plane. The opposite of the triangle corresponds to the actual distance, whereas the hypotenuse corresponds to the measured distance. Thus, the actual distance may be calculated to be approximately 2.97 meters. This example is merely meant to illustrate how an actual distance may be approximated using trigonometrical principles. In embodiments of the invention, the actual distance may be calculated without the use of trigonometry, and/or by performing further calculations. In embodiments of the invention, the distance sensor unit may also be arranged to perform measurements at a certain angle, which may also be taken into account.

[0335] The present invention i.e. the above described method and system is advantageous in that, in contrary to prior art distance measuring systems, the present invention determines the actual distance i.e. accounting for the angle in which the distance sensor sends/receives e.g. radar beams. More specifically, accounting for the reflected signals (e.g. from a radar) based on which the distance is calculated is different depending on the angle of the distance sensor relative to the tower. Further, the invention allows measuring the deflection via accelerations, which is also indicative of the tip-to-tower distance. Moreover, the invention allows monitoring the state of the wind turbine blade based on eigenfrequencies of the wind turbine blade.

[0336] The invention is not limited to the embodiments described herein, and may be modified or adapted without departing from the scope of the present invention as described in the patent claims below.