Determining the effect of sun on wind turbine tower inclination using tower top accelerometers

Abstract

Systems, methods, and computer program products for determining a tilt of a wind turbine tower (12) based on acceleration measurements from an accelerometer (50) operatively coupled to a nacelle (14) of a wind turbine (10). Acceleration data is collected from the accelerometer (50) configured to sense acceleration along accelerometer axes (x, y, z) while the nacelle (14) is in each of a plurality of yaw positions. The nacelle (14) is rotated stepwise to each yaw position and stopped for a period of time. Acceleration data is collected while the nacelle (14) is stopped and a static acceleration level along the accelerometer axes (x, y, z) caused by gravity (28) is determined. Once acceleration data has been collected at each position, minimum and maximum acceleration levels can be identified. A tilt of the tower (12) is then determined based on the minimum and maximum acceleration levels.

Description

Technical Field

[0001] This invention generally relates to wind turbines, and more particularly to methods, systems, and computer program products for determining the tilt of a wind turbine tower using a tower-top accelerometer. Background Technology

[0002] When a new wind turbine is erected, there is a risk of slight tilting of the tower due to its non-vertical construction. This tilt can be caused by one or more factors, such as differential settlement of the foundation, solar radiation heating of the tower, and tower defects. According to current standards from the International Electrotechnical Commission (IEC) and the German Institute for Building Technology (DIBt), the maximum permissible tilt of the tower is determined by adding an additional bending moment to the limit tower bending moment provided by aerospace elastic simulations. Currently, this standard is based on the assumption that the tower has a tilt of 8 mm / m under design load.

[0003] It is necessary from time to time to determine the actual tower tilt and understand the influence of various factors on tower tilt. A known procedure for determining tower tilt is to use a 3D laser scanner to measure the degree to which the tower deviates from vertical. Using scanner equipment requires site visits and specialized equipment, which makes tower tilt measurement expensive.

[0004] Therefore, there is a need to improve systems, methods, and computer program products to determine the tower tilt of wind turbine systems. Summary of the Invention

[0005] In one embodiment of the invention, a method is provided for measuring the tilt of a wind turbine tower including a nacelle mounted on the tower. The method includes collecting acceleration data from an accelerometer operatively coupled to the nacelle and configured to sense acceleration along an accelerometer axis at each of a plurality of yaw positions. The method determines the level of acceleration along the accelerometer axis due to gravity at each yaw position to generate a plurality of acceleration levels, and determines the tilt of the tower based on the plurality of acceleration levels. The method further includes the steps of: collecting a first dataset of acceleration data during a first time period; collecting a second dataset of acceleration data during a second time period; determining a first tilt of the tower from the first dataset; determining a second tilt of the tower from the second dataset; and determining a time-dependent effect of solar radiation on the tilt of the tower based on the first and second tilts of the tower.

[0006] In another embodiment of the invention, the cabin rotates about a rotation axis, and the accelerometer is configured such that the accelerometer axis is normal to the rotation axis.

[0007] In another embodiment of the application, the plurality of yaw positions encompasses a full rotation of the nacelle about the rotation axis.

[0008] In another embodiment of the application, determining the inclination of the tower based on the plurality of acceleration levels comprises determining a maximum acceleration level of the plurality of acceleration levels, determining a minimum acceleration level of the plurality of acceleration levels, and determining the inclination of the tower based on the maximum acceleration level and the minimum acceleration level.

[0009] In another embodiment of the application, determining the inclination of the tower based on the maximum acceleration level and the minimum acceleration level comprises determining a first inclination of the accelerometer based on the maximum acceleration level, determining a second inclination of the accelerometer based on the minimum acceleration level, and determining the inclination of the tower based on a difference between the first inclination of the accelerometer and the second inclination of the accelerometer.

[0010] In another embodiment of the application, collecting acceleration data from the accelerometer while the nacelle is in each of the plurality of yaw positions comprises stopping the nacelle at each yaw position, collecting acceleration data over a time period while the nacelle is stopped, and restarting yaw of the nacelle after the time period.

[0011] In another embodiment of the application, the method further comprises discarding a first portion of the acceleration data collected during a beginning portion of the time period, optionally discarding a second portion of the acceleration data collected during an ending portion of the time period, and low-pass filtering the remaining portion of the acceleration data that is not discarded to produce filtered acceleration data, wherein the acceleration level for each yaw position is determined based on the filtered acceleration data.

[0012] In another embodiment of the application, the method further comprises determining an average of the acceleration levels for each yaw position based on the filtered acceleration data, and detrending the filtered acceleration data based on the average of the acceleration levels to remove any bias and possible signal drift.

[0013] In another embodiment of the application, the method further comprises generating an acceleration plot comprising the acceleration level for each yaw position plotted against the yaw position, comparing the acceleration plot to a sinusoidal plot, and discarding the acceleration data if the acceleration plot does not match the sinusoidal plot.

[0014] In another embodiment of the application, the acceleration data is collected while the wind turbine is in a low wind speed idle mode or a parked mode.

[0015] In another embodiment of the present application, the acceleration data is collected when the wind speed is below a wind speed threshold selected from the group consisting of a cut-in wind speed and a service wind speed.

[0016] In another aspect of the present application, a wind turbine is provided. The wind turbine includes a controller in communication with an accelerometer and configured to implement the method of measuring the inclination of the tower of the wind turbine.

[0017] In another aspect of the present application, a computer program product is provided. The computer program product includes a non-transitory computer readable storage medium, and program code stored on the non-transitory computer readable storage medium, which program code, when executed by one or more processors, causes the one or more processors to implement the method of measuring the inclination of the tower of the wind turbine.

[0018] In another aspect of the present application, a controller for a wind turbine is presented. The controller includes one or more processors, and a memory coupled to the one or more processors and including program code, which program code, when executed by one or more processors, causes the controller to implement the method of measuring the inclination of the tower of the wind turbine.

[0019] The above summary of the present application briefly summarizes some embodiments of the present application to provide a basic understanding of some aspects of the present application discussed herein. The summary of the present application is not intended to provide an extensive overview of the present application nor is it intended to delineate the scope of the present application. The sole purpose of the summary of the present application is merely to present some concepts in a simplified form as an introduction to the detailed description of the embodiments presented below. BRIEF DESCRIPTION OF DRAWINGS

[0020] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate various embodiments of the present application and together with the general description of the present application given above, and the detailed description of the embodiments given below, serve to explain the embodiments of the present application.

[0021] Figure 1 is a perspective view of an exemplary wind turbine in accordance with an embodiment of the present application, including a nacelle having a nacelle reference frame.

[0022] Figure 2 is a perspective view of a portion of the wind turbine of Figure 1 with the nacelle partially disassembled to expose the structure including one or more accelerometers housed inside the nacelle.

[0023] Figure 3 is a schematic diagram of a control system that can be used to control the wind turbine of Figure 1 and Figure 2 .

[0024] Figure 4 is a schematic diagram of a controller that can be used to implement Figure 3 a control system.

[0025] Figure 5 is a schematic diagram of a wind turbine comprising a tower that is not vertical, such that there is a tilt angle between the rotational axis of the nacelle and the z-axis of a fixed reference frame.

[0026] Figure 6 is a schematic diagram of the relationship between the accelerometer reference frame and the fixed reference frame of a wind turbine Figure 1 Figure 2 and Figure 5 in each of several operating conditions, as the nacelle is yawed about the rotational axis,

[0027] Figure 7 is a flowchart of a process for determining the tilt angle based on data received from the accelerometers of Figure 6

[0028] Figure 8 is a graphical view of exemplary accelerometer data versus yaw position for a wind turbine according to one embodiment of the invention.

[0029] It should be understood that the attached drawings are not necessarily drawn to scale, can be somewhat schematic in nature, and can give a somewhat simplified representation of various features for the sake of clarity and ease of understanding. The specific design features of the operational sequences disclosed herein (including, e.g., the specific sizes, orientations, positions, and shapes of various illustrated components) can be determined in part by the particular intended application and use environment. Certain features of the illustrated embodiments can have been enlarged or distorted relative to others to facilitate visualization and clear understanding. DETAILED DESCRIPTION

[0030] Embodiments of the invention relate to systems, methods, and computer program products that estimate the tilt of a wind turbine tower based on the static contribution of the earth's gravity to the output of a top-of-tower accelerometer. Embodiments of the invention exploit the principle that if a wind turbine tower has a tilt, then that tilt will cause the static contribution of gravity to the accelerometer output to change in a determinable way as the nacelle is rotated about its rotational axis. By yawing the turbine, the tower tilt can be estimated based on the differences in the acceleration signals at different yaw positions. By using existing hardware in a wind turbine system, embodiments of the invention can be implemented at a low incremental cost by simply updating the software in the wind turbine system.

[0031] ​​Embodiments of the present application can also be used to provide information about long term changes in tower inclination, such as can be caused by differential foundation settlement occurring over a long period of time. For example, differential foundation settlement due to clay settlement can occur over a period of years. Periodic measurement of tower inclination for a large number of wind turbines can provide a statistical distribution of tower inclination. This statistical distribution can then be used to support optimization of the inclination criteria considered in current design loads, for example by reducing the assumed tower inclination contribution. Information from these tests can also be used as input to the wind turbine controller to optimize operating parameters by taking tower inclination into account, as well as to gain knowledge about tower inclination caused by the effects of solar radiation. In this way, the effects of solar radiation can be singled out in order to better understand the causes and mitigation of tower inclination.

[0032] The present application relates to collecting acceleration data from an accelerometer operatively coupled to the nacelle and configured to sense acceleration along an axis of the accelerometer when the nacelle is in each of a plurality of yaw positions.

[0033] However, as a background understanding, GNSS based sensors can also be applied for the same purpose, but with appropriate modifications.

[0034] For example, a high precision GPS sensor can achieve a precision of 10 mm, which is sufficiently precise for measuring the displacement of the center of the tower top cross section relative to the tower base cross section. To establish the center point of the tower just above the foundation, static GPS measurements are taken at a number of (e.g. four) evenly distributed places around the perimeter of the tower. To ensure that the GPS is able to receive a signal of appropriate strength, this can be done on the outside of the tower. The measurement points are fitted into a circle using, for example, a least squares approximation, and the center point of the tower base is established as the center of the fitted circle.

[0035] To find the position of the tower top, a GPS is placed on top of the nacelle. To compensate for the fact that the GPS is not placed directly on the center of the tower top, as well as to compensate for the effect of the overhang moment, the turbine is yawed 360 degrees while the position is tracked with the GPS. To establish the center of the tower top cross section, the GPS data measured during the yaw sweep is fitted into a circle, defining the center of the circle. Once the centers of the base cross section and the top cross section are estimated, the tower misalignment can be determined as the distance between these two center points.

[0036] One disadvantage of using GPS / GNSS sensors is that they tend to drift over time. To avoid drift, one possible mitigation is to place a ground station in a fixed position, which communicates with the sensor placed in the nacelle. But this would incur additional cost.

[0037] As an alternative to using a ground station, monitoring for drift and recalibrating the sensors can be employed. Once the GPS / GNSS is installed in the nacelle, a baseline measurement is made. This is done by doing a 360 degree yaw of the turbine at low wind speed, with the turbine at idle / stationary. By fitting a circle to the data and finding the centre of this, the centre of the tower can be established. This procedure can be done at fixed time intervals. Any change in the centre point of the fitted circle can be attributed to drift. By doing a yaw round, it will further be possible to disregard any tilt of the tower due to the overhang moment. The procedure of a yaw round can be programmed into the controller so that it takes place at fixed time intervals. However, since fast response can not be necessary, the implementation can be performed so that a yaw round only takes place when the wind speed is below the cut-in point, so no AEP is lost as a result. More importantly, the yaw round is only performed at night, so any temperature effects on the tower can be disregarded.

[0038] Figure 1 An exemplary wind turbine 10 according to one embodiment of the present application is illustrated. The wind turbine 10 includes a tower 12 extending upward from a foundation, a nacelle 14 disposed at the top of the tower 12, and a rotor 16 with a plurality of blades 20 operatively coupled with a generator within the nacelle 14. In addition to the generator, the nacelle 14 typically houses various components needed to convert wind energy into electrical energy, as well as to operate and optimize the performance of the wind turbine 10. The tower 12 supports the loads presented by the nacelle 14, the rotor 16, and other wind turbine components housed within the nacelle 14. The tower 12 of the wind turbine 10 elevates the nacelle 14 and rotor 16 to an elevation above ground level that allows the rotor 16 to freely rotate in air currents that often occur at higher elevations with lower turbulence and higher velocities.

[0039] The rotor 16 includes a hub 18 and one or more (e.g., three) blades 20 attached to the hub 18 at locations distributed around the perimeter of the hub 18. The blades 20 extend radially outward from the hub 18 and are configured to interact with passing airflow to create a rotational force that causes the hub 18 to rotate about its longitudinal axis 22. This rotational energy can be transferred to a generator housed within the nacelle 14 and converted to electrical energy. To optimize the performance of the wind turbine 10, the pitch of the blades 20 can be adjusted by a pitch system in response to wind speed and other operating conditions.

[0040] An exemplary fixed reference frame 24 includes x, y, and z axes that define a three-dimensional coordinate space according to the right-hand rule. The z axis of the fixed reference frame 24 is parallel to the gravitational field 28 of the Earth and is oriented such that the z axis points upward away from the center of the Earth. The x and y axes of the fixed reference frame 24 are orthogonal to the z axis and to each other and define a horizontal plane. The fixed reference frame 24 thus provides a frame of reference that is fixed relative to the Earth.

[0041] The nacelle reference frame 30 includes a longitudinal axis (l-axis), a transverse axis (t-axis), and a normal axis (n-axis) that define another right-handed three-dimensional coordinate space that is fixed relative to the nacelle 14. The l-axis of the nacelle reference frame 30 can be aligned with the longitudinal axis 22 of the rotor 16. The t-axis of the nacelle reference frame 30 can be aligned with a transverse axis 40 of the nacelle 14 that is generally orthogonal to the longitudinal axis 22 of the rotor 16. The n-axis can be aligned with a yaw axis 42 of the nacelle 14 about which the nacelle 14 yaws. The yaw axis 42 can be generally aligned with the longitudinal axis of the tower 12, which can be aligned with the z axis of the fixed reference frame 24 in the absence of any tower tilt. As used herein, the yaw angle of the nacelle 14 can be considered to be zero degrees when the projection of the l-axis of the nacelle reference frame 30 onto the horizontal plane is parallel to the x axis of the fixed reference frame 24.

[0042] Figure 2 A perspective view is presented in which the nacelle 14 is partially disassembled to expose the structures housed within. A main shaft extending from the rotor 16 into the nacelle 14 can be held in place by a main shaft bearing support 44 that supports the weight of the rotor 16 and transfers loads on the rotor 16 to the tower 12. The main shaft can be operatively coupled to a gearbox 46 that transfers its rotation to a generator 48. Electrical power generated by the generator 48 can be supplied to an electrical grid (not shown), or to an energy storage system (not shown) for later release to the electrical grid, as understood by those of ordinary skill in the art. In this way, the kinetic energy of the wind can be harnessed by the wind turbine 10 for electrical power generation.

[0043] The nacelle 14 can also house one or more accelerometers 50 configured to detect the level of acceleration of the nacelle 14. Each accelerometer 50 can be a single-axis accelerometer that senses acceleration along a single axis, or (more typically) a multi-axis accelerometer that senses acceleration along multiple axes, such as three orthogonal axes. Each accelerometer 50 can be sensitive to linear acceleration (e.g., due to tower 12 sway or nacelle 14 rotation) and to the local gravitational field 28.

[0044] The weight of the nacelle 14 (including components housed therein) can be carried by the load bearing structure 52. The load bearing structure 52 can include the outer housing of the nacelle 14 and one or more additional structural components such as a frame or lattice, and a gear clock that operatively couples the load of the nacelle 14 to the tower 12 through a yaw bearing (not shown). The yaw bearing can be configured to allow the nacelle 14 to be rotated by a yaw system about its rotational axis 42 to keep the wind turbine 10 pointed into the wind.

[0045] Figure 3 An exemplary control system 54 that can be used to control the wind turbine 10 is illustrated. The control system 54 includes a wind turbine controller 56 in communication with the accelerometer 50, a pitch system 58, a yaw system 60, and a supervisory controller 62. The supervisory controller 62 can be configured to implement a system-wide control strategy for a group of wind turbines 10 (e.g., a wind farm) that optimizes the collective performance of the wind turbines 10 to, for example, maximize power production and minimize overall maintenance for the group. The yaw system 60 can be used by the wind turbine controller 56 to control the pointing direction of the nacelle 14 and can include one or more yaw controllers, drive systems, position sensors, etc. configured to implement yaw command signals received from the wind turbine controller 56. The pitch system 58 can be configured to collectively or independently adjust the pitch of the blades 20 in response to a set of pitch command signals received from the wind turbine controller 56.

[0046] The wind turbine controller 56 can be configured to monitor the speed of the rotor 16 and adjust the pitch of the blades 20 in response to prevailing wind conditions to control the operation of the wind turbine 10. When the wind speed is below the cut-in speed of the wind turbine 10, the wind turbine 10 can be in an idle mode during which the rotor 16 does not rotate or rotates slowly. As the wind speed exceeds the cut-in speed, the rotor 16 of the wind turbine 10 can begin to rotate, which can allow the wind turbine 10 to begin generating power. As the wind speed increases further from the cut-in speed to the rated wind speed, the wind turbine controller 56 can be configured to set the blade pitch to maximize the conversion of aerodynamic energy to rotational energy of the generator 48. Once the rated wind speed is reached, the wind turbine 10 can generate power at its maximum rated output power. From this point, as the wind speed increases further, the wind turbine controller 56 can adjust the blade pitch to maintain the speed and torque applied to the generator 48 at the rated level. Finally, when the wind speed reaches the cut-out speed, the wind turbine controller 56 can feather the blades to prevent damage to the wind turbine 10. Typically, maintenance can only be performed on the wind turbine 10 when the wind is below a service wind speed. When maintenance is performed, the wind turbine controller 56 can place the wind turbine 10 in a parked mode by feathering the blades 20 and applying brakes to prevent the rotor 16 from rotating.

[0047] Figure 4 An exemplary controller 64, such as the wind turbine controller 56 or supervisory controller 62, that can be used to provide one or more components of embodiments of the application is illustrated. The controller 64 can include a processor 66, a memory 68, and an input / output (I / O) interface 70. The processor 66 can include one or more devices that operate on data based on internal logic or operating instructions stored in the memory 68. The memory 68 can include a single storage device or multiple storage devices capable of storing data. Computer program code embodied as one or more computer software applications, such as an application 72 resident in the memory 68, can have instructions executed by the processor 66. One or more data structures 74 can also be resident in the memory 68 and can be used by the processor 66 or the application 72 to store or manipulate data. The I / O interface 70 can provide a machine interface that operatively couples the processor 66 to other devices and systems, such as one or more of the accelerometers 50, the wind turbine controller 56, the pitch system 58, the yaw system 60, and the supervisory controller 62. Thus, the application 72 can cooperate with external devices and systems to provide various features, functions, applications, processes, or modules including embodiments of the application by communicating through the I / O interface 70.

[0048] Figure 5 A wind turbine 10 is illustrated in accordance with one embodiment of the application in which the tower 12 has an offset from vertical that results in a non-zero tilt angle Θ between the vertical axis z of the fixed reference frame 24 and the normal axis n of the nacelle reference frame 30. Wind turbine towers often have a slight tilt angle Θ due to the tower not being perfectly vertical. The tilt angle Θ can be caused by different reasons, such as differential settlement of the foundation, uneven thermal expansion of different sides of the tower 12 due to solar radiation, tolerances of the joints between sections of the tower 12, and other tower imperfections.

[0049] Figure 6 Exemplary paths 76-78 of a respective accelerometer reference frame 82-84 of a three-axis accelerometer 50 radially offset from the rotation axis 42 are illustrated for each of three different cases 88-90 as the nacelle 14 is rotated. In each case 88-90, as the accelerometer 50 is run around the rotation axis 42, the accelerometer reference frame 82 rotates around its z-axis as observed from the fixed reference frame 24. As a result, the orientation of the x- and y-axes of the accelerometer reference frame 82-84 also rotates.

[0050] In the zero tilt case 88, the n-axis of the nacelle frame 30 is parallel to the z-axis of the fixed frame 24 (i.e., the tower 12 is vertical). Because the nacelle's axis of rotation 42 is parallel to the z-axis of the fixed frame 24, the accelerometer frame 82 follows a circular path 76 in the horizontal plane as the nacelle 14 rotates. In the zero tilt case 88, the inclination of the x, y, and z-axes of the accelerometer frame 82 (i.e., the angle relative to the z-axis of the fixed frame 24) does not vary with the yaw angle Φ of the nacelle 14.

[0051] In the non-zero tilt case 89, the n-axis of the nacelle frame 30 is not parallel to the z-axis of the fixed frame 24, i.e., the tower 12 has a non-zero tilt angle Θ. Because the nacelle's axis of rotation 42 is not parallel to the z-axis of the fixed frame 24, the accelerometer frame 83 follows a circular path 77 that is inclined relative to the horizontal plane. Thus, as the nacelle 14 rotates, the inclination of the x- and y-axes of the accelerometer frame 83 varies sinusoidally as a function of the yaw angle Φ, as viewed from the fixed frame 24.

[0052] Case 90 is a non-zero tilt case in which the accelerometer frame 84 is not aligned with the nacelle frame 30, e.g., there are mechanical tolerances or installation errors that cause the accelerometer frame 84 to be rotated relative to the nacelle frame 30 about one or more of its axes x, y, z, e.g., the y-axis. As in the non-zero tilt case 89, the n-axis of the nacelle frame 30 is not parallel to the z-axis of the fixed frame 24, i.e., the tower 12 has a non-zero tilt angle Θ. In addition, due to the exemplary error rotation about the y-axis, the x- and z-axes of the accelerometer frame 84 are not parallel to the l- and n-axes of the nacelle frame 30, which introduces respective measurement error angles ψ x and ψ z .

[0053] Because the nacelle's axis of rotation 42 is not parallel to the z-axis of the fixed frame 24, in the non-zero tilt case 90 the accelerometer frame 84 also follows a circular path 78 that is inclined relative to the horizontal plane. Thus, as the nacelle 14 rotates, the inclination of the x- and y-axes of the accelerometer frame 83 varies sinusoidally as a function of the yaw angle Φ of the nacelle 14, as viewed from the fixed frame 24. However, the measurement error angles ψ x and ψ z between the x- and z-axes of the accelerometer frame 84 and the l- and n-axes of the nacelle frame 30 do not vary as a function of the yaw angle Φ. Thus, any measurement error angles ψ x , ψ y , and ψ z due to the misalignment between the accelerometer frame and the nacelle frame 30 are constant with respect to the yaw angle Φ. The error angles ψ x , ψ yand ψ z Only an offset is introduced that cancels itself out and can be ignored.

[0054] An example of a trigonometric calculation that can be used to determine the tilt angle is provided below.

[0055] Assuming no measurement error angle ψ x , ψ y , ψ z , the output of the three-axis accelerometer 50 as the nacelle rotates about its axis of rotation 42 can be provided by the following equations (Eqn.):

[0056]

[0057]

[0058] a z = g x cos(θ) Eqn. 3

[0059] where a x , a y , and a z are the respective outputs of the x, y, and z axes of the accelerometer, g is the output level of the respective accelerometer axis when subjected to a level of acceleration equivalent to one earth's gravity, is the yaw angle of the nacelle, and θ is the tilt of the tower. The value of g in equations 1-3 can be determined for a fixed three-axis accelerometer as:

[0060]

[0061] Vector algebra can be used to calculate the change in angle of the apparent gravity vector between any two accelerometer readings a and b using the following equation:

[0062]

[0063] As the nacelle 14 rotates, the output of the x axis of the accelerometer can peak at yaw angles Φ = 0 and Φ = π, while the output of the y axis of the accelerometer can peak at yaw angles Φ = π / 2 and Φ = -π / 2. At these yaw angles, equations 1 and 2 provide the following peaks:

[0064] Φ = 0; a x = g x sin(θ) Eqn. 6

[0065] Φ = π; a x = -g x sin(θ) Eqn. 7

[0066]

[0067]

[0068] Accordingly, the tilt angle Θ can be calculated by determining the peak output value of one or more axes of one or more of the accelerometers 50 and applying one of the following equations to the output of the accelerometers:

[0069]

[0070]

[0071] where Φ P is the yaw angle at which the output of the respective accelerometer reaches a peak value.

[0072] Figure 7 A flowchart depicting an exemplary process 100 that can be used to determine tower tilt according to one embodiment of the present application is illustrated. In block 102, the process 100 can cause the wind turbine to enter an idle or parked mode such that the blades 20 of the rotor 16 are not rotating.

[0073] Once the wind turbine 10 is in the idle / parked mode, the process 100 can proceed to block 104 and yaw the nacelle 14 to a starting position (e.g., zero degrees) and then proceed to block 106 to begin collecting and processing accelerometer data. While the nacelle 14 is in the yawed position, the process 100 can collect accelerometer data over a period of time (e.g., one minute). The process 100 can determine the starting point of the data collection period based on a signal from the yaw motor indicating that the yaw has stopped, and thus the nacelle 14 is in a fixed yawed position. In one embodiment of the present application, a predetermined amount of data can be discarded from the beginning of the period (e.g., the first 10 seconds). Discarding this initial data can eliminate dynamic effects (e.g., tower oscillations) resulting from stopping or starting the yaw motor. In some embodiments, a predetermined amount of data can also be discarded from the end of the period (e.g., the last few seconds). In cases where the controller collecting the data does not control when the yaw system 60 is started, and thus is merely responsive to detection of a yaw start, data from the end of the period can be discarded. In any case, the process 100 can filter the remaining data using a filtering algorithm that preserves the static contribution of gravity to the output of the accelerometers (e.g., a low pass filtering algorithm) and determine the average value of the filtered data. The process 100 can then store this average value for use in determining tower tilt.

[0074] Once the process 100 has collected and processed acceleration data, the process 100 can proceed to block 108 and determine whether acceleration data has been collected and processed for each of a plurality of yaw positions (e.g., 10 positions evenly spaced in a 360 degree rotation). If acceleration data has not been collected for each yaw position (the "No" branch of decision block 108), the process 100 can proceed to block 110, yaw the nacelle 14 to the next yaw position, and return to block 106 to begin collecting and processing acceleration data for that yaw position. If acceleration data has been collected and processed for each yaw position (the "Yes" branch of decision block 108), the process 100 can proceed to block 112.

[0075] In block 112, the process 100 can detrend the processed acceleration data from each yaw position. Detrending can remove effects due to the position of the accelerometer 50 being offset from the axis of rotation 42.

[0076] In block 114, the process can determine whether the acceleration measured at each yaw position matches an expected profile. For example, the acceleration levels can be plotted against the yaw position and compared to a sinusoidal function. The comparison can include fitting the plotted function to a sinusoidal function (e.g., scaling the amplitude and period, and adding a phase offset to the plotted function), determining the mean squared difference between the plots, and comparing the mean squared difference to a threshold. If the acceleration data does not match the profile (the "No" branch of decision block 114), the process 100 can proceed to block 116, discard the data, and terminate. For example, if the mean squared difference is above a predetermined threshold, indicating that the data is corrupted or otherwise rendered unreliable, the process 100 can determine that the acceleration data does not match the profile.

[0077] If the acceleration data matches the profile (the "Yes" branch of decision block 114), the process 100 can proceed to block 118 and determine the tower inclination. The process 100 can determine the tower inclination by determining the minimum and maximum acceleration levels a min , a max measured over the yaw positions. The minimum and maximum acceleration levels a min , a max may be normalized to 1g, a sinusoidal function applied to each normalized value, and the difference between the results determined according to the following equation:

[0078]

[0079] The process 100 can then determine the tower inclination by dividing the result of equation 18 by 2. Once the tower inclination has been determined, the inclination angle Θ, the acceleration data, or both the inclination angle Θ and the acceleration data can be stored in a memory (e.g., in a central database), and the process 100 terminates.

[0080] Figure 8 Graphs 120, 130 are depicted, which illustrate experimental results obtained by processing acceleration data obtained from four accelerometers in the nacelle of a wind turbine according to one embodiment of the present application. Graph 120 includes plots 122-125 of the output of each accelerometer versus the yaw angle in a polar coordinate system. Graph 130 includes plots 132-135 of the output of each accelerometer versus the yaw angle in a Cartesian coordinate system. Acceleration measured in the y-direction from the four distributed control nodes is used for the analysis.

[0081] Advantageously, the process of determining the tower inclination can be fully automated, so embodiments of the present application can be implemented to collect tower inclination data without the need for human interaction. The collected data can be automatically stored in a database and used to determine long-term trends in tower behavior (e.g., diurnal, seasonal, and aging-related behavior). Furthermore, the system can be configured to collect acceleration data only when the wind speed is below cut-in, to avoid loss of energy production. Since the data is collected from multiple yaw angles, the process can also compensate for tower misalignment due to the overhung moment of the nacelle.

[0082] Having established an embodiment of obtaining the inclination of the tower, the time-dependent effect of sunlight on the inclination of the tower can be determined by collecting a first data set of acceleration data during a first time period, and collecting a second data set of acceleration data during a second time period. With these two data sets, a first inclination of the tower can be determined from the first data set, and a second inclination of the tower can be determined from the second data set. Based on the first inclination of the tower and the second inclination of the tower, the presence or absence of a time-dependent effect of sunlight on the inclination of the tower is determined. The first time period and the second time period are time periods in which the level of sunlight on the tower is different. In one embodiment, this can be done by comparing the tower inclination level during the day with the night. In another embodiment, light level sensors can be used to establish the first time period and the second time period as time periods of different sunlight levels. Other, less direct measures of establishing different sunlight levels can be information such as cloud cover at the site of the wind turbine.

[0083] In general, features, functions, applications, processes or modules performed in connection with implementing embodiments of the application can be referred to herein as "computer program code" or simply "program code". The program code typically comprises computer-readable instructions that, when read and executed by one or more processors of a computing device, cause the computing device to perform operations necessary to execute embodiments of the application. Program code embodied in any of the aspects of the application described herein can be distributed as a computer program product in a variety of forms. In particular, the program code can be distributed using non-transitory computer-readable media having computer-readable program instructions thereon. Computer-readable program instructions can also be downloaded to a computing device from a computer-readable storage medium or to a computing device from a computer network, and stored in a computer-readable storage medium.

[0084] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of embodiments of the application. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and "comprising", when used in this specification, specify the presence of stated features, integers, actions, steps, or components, but do not preclude the presence or addition of one or more other features, integers, actions, steps, components, or groups thereof. Additionally, use of the term "including", "having", "with", "composed of", or "comprises" or variations thereof herein is intended to be open-ended, meaning that additional items or components can be added to those items or components recited.

[0085] While all applications have been illustrated by description of various embodiments and while these embodiments have been described in some detail, it is not the intention that the application be limited or restricted in any way to the details of construction, operation, or working examples. Additional advantages, modifications, and substitutions are contemplated, and further attendant upon the practice of the application. Therefore, the application is not limited to that precise details shown and described. Accordingly, it is intended that the scope of the application be defined by the scope of the appended claims rather than the description of specific embodiments.

Claims

1. A method of measuring a tilt of a tower (12) of a wind turbine (10) and determining a time-dependent effect of sunlight on the tilt, the wind turbine (10) comprising a nacelle (14) mounted on the tower (12), the method comprising: collecting acceleration data from an accelerometer (50) operatively coupled to the nacelle (14) and configured to sense acceleration along accelerometer axes (x, y, z) when the nacelle (14) is in each of a plurality of yaw positions; determining a level of acceleration along the accelerometer axes (x, y, z) due to gravity (28) at each yaw position to generate a plurality of acceleration levels; and determining a tilt of the tower (12) based on the plurality of acceleration levels, the method further comprising the steps of: collecting a first data set of the acceleration data during a first time period; collecting a second data set of the acceleration data during a second time period; determining a first tilt of the tower (12) from the first data set; determining a second tilt of the tower (12) from the second data set; and determining that there is or is not a time-dependent effect of sunlight on the tilt of the tower (12) based on the first tilt of the tower (12) and the second tilt of the tower (12). the nacelle (14) rotates about a rotation axis (42), and the accelerometer (50) is configured such that the accelerometer axes (x, y, z) are normal to the rotation axis (42).

2. The method of claim 1, wherein, the plurality of yaw positions encompass a full azimuth rotation of the nacelle (14) about the rotation axis (42).

3. The method of claim 2, wherein, determining a tilt of the tower (12) based on the plurality of acceleration levels comprises:

4. The method of any one of claims 1-3, wherein, determining a maximum acceleration level of the plurality of acceleration levels; determining a minimum acceleration level of the plurality of acceleration levels; and determining a tilt of the tower (12) based on the maximum acceleration level and the minimum acceleration level. determining a tilt of the tower (12) based on the maximum acceleration level and the minimum acceleration level comprises:

5. The method of claim 4, wherein, determining a first tilt of the accelerometer (50) based on the maximum acceleration level; determining a second tilt of the accelerometer (50) based on the minimum acceleration level; determining a tilt of the tower (12) based on a difference between the first tilt of the accelerometer (50) and the second tilt of the accelerometer (50). collecting acceleration data from the accelerometer (50) when the nacelle (14) is in each of the plurality of yaw positions comprises, at each yaw position:

6. The method of any one of claims 1-3, wherein, stopping the nacelle (14) at the yaw position; collecting acceleration data over a time period while the nacelle (14) is stopped; and restarting yaw of the nacelle (14) after the time period.

7. The method of claim 6, further comprising: discarding a first portion of acceleration data collected during a beginning portion of the time period; ​ discarding a second portion of the acceleration data collected during an end portion of the time period; and low-pass filtering the remaining portion of the acceleration data that was not discarded to produce filtered acceleration data, wherein the acceleration level for each yaw position is determined based on the filtered acceleration data.

8. The method of claim 7, further comprising: determining an average of the acceleration level for each yaw position based on the filtered acceleration data; and de-trending the filtered acceleration data based on the average of the acceleration level.

9. The method of any of claims 1-3, further comprising: generating an acceleration plot (132) comprising the acceleration level for each yaw position plotted against the yaw position; comparing the acceleration plot (132) to a sinusoidal plot; and discarding the acceleration data if the acceleration plot (132) does not match the sinusoidal plot.

10. The method of any one of claims 1-3, wherein, collecting the acceleration data while the wind turbine (10) is in a low wind speed idle mode or a parked mode.

11. The method of any one of claims 1-3, wherein, collecting the acceleration data when the wind speed is below a wind speed threshold, the wind speed threshold selected from the group consisting of a cut-in wind speed and a service wind speed.

12. A wind turbine (10), comprising: a controller (64) in communication with an accelerometer (50) and configured to implement the method of any of claims 1-11.

13. A computer program product, comprising: a non-transitory computer readable storage medium; and program code stored on the non-transitory computer readable storage medium that, when executed by one or more processors (66), cause the one or more processors (66) to implement the method of any of claims 1-11.

14. A controller (64) for a wind turbine (10), the controller (64) comprising: one or more processors (66); and a memory (68) coupled to the one or more processors (66) and comprising program code that, when executed by the one or more processors (66), cause the controller (64) to implement the method of any of claims 1-11.

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