Determine the frequency of the swaying motion of a wind turbine tower

Abstract

A method (200) for determining the frequency of the sway motion of a wind turbine tower, which supports a nacelle and a generator including a stator and a rotor, and the sway motion causes a rolling motion of the nacelle, is described. The method includes obtaining (210) a first signal indicative of the rotor speed relative to the nacelle, obtaining (220) a second signal indicative of the rotor speed relative to the ground, determining (230) a first amplitude spectrum (311) based on the first signal, determining (240) a second amplitude spectrum (312) based on the second signal, determining (250) a difference function (414) based on the first amplitude spectrum and the second amplitude spectrum, and determining (260) the frequency of the sway motion of the wind turbine tower as the frequency (416) corresponding to a peak (415) in the difference function. In addition, a device for determining the frequency of the sway motion of a wind turbine tower, a wind turbine including such a device, and a computer program are described.

Description

Field of the Invention

[0001] The present invention relates to the field of wind turbines, and more particularly to a method for determining the frequency of the swaying motion of a wind turbine tower, which carries a nacelle and a generator including a stator and a rotor, and the swaying motion causes a rolling motion of the nacelle. The invention also relates to a device for determining the frequency of the swaying motion of a wind turbine tower, a wind turbine including such a device, and a computer program. Background Art

[0002] A wind turbine includes many different structural components, all of which have specific frequencies at which they naturally sway. These are referred to as natural frequencies or frequency modes.

[0003] When controlling a wind turbine, it is very useful to know these frequencies, for example in order to (a) avoid exciting these structural modes, for example thereby causing excessive structural loads, and (b) actively dampen modes that are known to cause problems related to loads or other control objectives.

[0004] Natural frequencies can be estimated based on a mathematical model of the wind turbine structure. However, such an estimate suffers from the need for approximations and assumptions. In addition, the estimate cannot take into account that some conditions and parameters may change during the lifetime of the wind turbine. For example, due to changing environmental conditions (such as extreme heat or cold), the composition of the ground may change, or the mechanical properties of the wind turbine structural elements may change or vary.

[0005] Therefore, there may be a need for a method for determining the current frequency of the swaying motion of a wind turbine tower in a simple and accurate manner. Summary of the Invention

[0006] This need can be met by the subject matter of the independent claims. The dependent claims describe advantageous embodiments of the invention.

[0007] According to a first aspect of the present invention, there is provided a method for determining the frequency of the swaying motion of a wind turbine tower, which carries a nacelle and a generator including a stator and a rotor, and the swaying motion causes a rolling motion of the nacelle. The method includes (a) obtaining a first signal indicative of the rotor speed relative to the nacelle, (b) obtaining a second signal indicative of the rotor speed relative to the ground, (c) determining a first amplitude spectrum based on the first signal, (d) determining a second amplitude spectrum based on the second signal, (e) determining a difference function based on the first amplitude spectrum and the second amplitude spectrum, and (f) determining the frequency of the swaying motion of the wind turbine tower as the frequency corresponding to the peak in the difference function.

[0008] This aspect of the invention is based on the idea that the rotor speed measured relative to the nacelle will contain a superimposed wobble caused by the rolling motion of the nacelle, which wobble will not be present in the rotor speed measured relative to the ground. Thus, when a difference function is formed based on the amplitude spectra of two rotor speed measurements, the frequency of the superimposed wobble will cause a corresponding peak in the difference function. Thus, the frequency of a given wobble can be determined as the frequency at which the difference function exhibits a peak.

[0009] In this context, the term "rolling motion" can specifically denote the motion of the nacelle in the rotor disk plane. In other words, when the nacelle rolls in the direction of rotation of the rotor (i.e., together with the rotor), the rolling motion will cause a decrease in the rotor speed value represented by the first signal. Similarly, when the nacelle rolls in the direction opposite to the direction of rotation of the rotor (i.e., against the rotor), the rolling motion will cause an increase in the rotor speed value represented by the first signal.

[0010] The invention enables an accurate determination (substantially in real time) of the actual wobble frequency based on two rotor speed measurements, i.e., without relying on mathematical models and assumptions / estimations of mechanical properties. Advantageously, the two rotor speed measurements (or at least the data required to derive the measurements) are already available in a typical wind turbine. Thus, the invention can be easily implemented without the need for additional hardware.

[0011] According to an embodiment of the invention, the wobble motion of the wind turbine tower is a second-mode tower wobble motion.

[0012] The second mode is a mode that causes the strongest rolling motion of the nacelle (together with the lateral displacement of the nacelle), and thus it is the easiest (and most important) to detect by means of the present invention.

[0013] According to another embodiment of the invention, the difference function is determined as the absolute difference between a first amplitude spectrum and a second amplitude spectrum as a function of frequency.

[0014] By calculating the absolute differences between the corresponding values of the first and second amplitude spectra, the determination of the peaks in the difference function becomes simpler.

[0015] According to another embodiment of the invention, the method further includes applying smoothing to each of the first and second amplitude spectra before determining the difference function.

[0016] By smoothing the amplitude spectra, any negative effects from measurement errors and other outliers can be significantly reduced.

[0017] According to another embodiment of the invention, the first signal is obtained by a rotor speed sensor arranged in the nacelle and adapted to measure the rotor speed.

[0018] In other words, the rotor speed sensor is a device used by any wind turbine to provide an input to the wind turbine control system, which uses this input as a feedback parameter to control the rotor speed to a desired value, for example, by adjusting the pitch angle of the rotor blades, setting the generator torque, applying brakes, etc.

[0019] According to another embodiment of the present invention, the rotor speed sensor is an optical or magnetic sensor adapted to detect when a corresponding part of the rotor passes by the first sensor.

[0020] In other words, the rotor speed sensor detects when certain regions or parts of the rotor circumference pass by the rotor speed sensor by sensing corresponding changes in optical or magnetic properties. The said certain regions or parts are equally spaced along the rotor circumference.

[0021] According to another embodiment of the present invention, the second signal is obtained by a monitoring device arranged at the hub of the wind turbine.

[0022] The monitoring device may in particular be part of a safety system that monitors various operating parameters of the wind turbine and is not included in the normal control of the wind turbine. The information collected by the monitoring device can in particular be used to detect faults and malfunctions during operation and to schedule maintenance of the wind turbine. In some cases, the monitoring device may not directly measure the rotor speed, but instead obtain various data related to the rotor movement, which allows the rotor speed to be easily derived.

[0023] According to another embodiment of the present invention, the monitoring device includes at least one inertial sensor.

[0024] The at least one inertial sensor may be particularly useful for obtaining information about the movement of the rotor relative to the ground (i.e., relative to the Earth's gravitational field).

[0025] According to another embodiment of the present invention, the at least one inertial sensor includes a gravity sensor and / or a gyroscope.

[0026] According to another embodiment of the present invention, determining the first amplitude spectrum and determining the second amplitude spectrum include applying a sliding discrete Fourier transform to the first signal and the second signal, respectively.

[0027] In this way, the sway frequency is always determined based on the most recent data, such as data corresponding to the last second, last 10 seconds, last minute, or any other predetermined time interval.

[0028] According to another embodiment of the present invention, the method further includes, if the peak of the difference function exceeds a predetermined threshold, confirming the determined frequency as the frequency of the sway movement of the wind turbine tower.

[0029] By requiring that the peak exceeds a predetermined threshold, it is ensured that random differences between spectra do not lead to incorrect frequency determination.

[0030] According to a second aspect of the invention, there is provided an apparatus for determining the frequency of the swaying motion of a wind turbine tower, the wind turbine tower carrying a nacelle and a generator including a stator and a rotor, the swaying motion causing a rolling motion of the nacelle. The described apparatus comprises: (a) an interface configured to (aa) receive a first signal indicative of the rotor speed relative to the nacelle, and (ab) receive a second signal indicative of the rotor speed relative to the ground; and (b) a processing unit configured to (ba) determine a first amplitude spectrum based on the first signal, (bb) determine a second amplitude spectrum based on the second signal, (bc) determine a difference function based on the first amplitude spectrum and the second amplitude spectrum, and (bd) determine the frequency of the swaying motion of the wind turbine tower as the frequency corresponding to the peak in the difference function.

[0031] This aspect of the invention is substantially based on the same idea as the first aspect described above.

[0032] According to a third aspect of the invention, there is provided a wind turbine comprising the apparatus according to the second aspect.

[0033] The wind turbine benefits from the advantages described above in connection with the first and second aspects.

[0034] According to a third aspect of the invention, there is provided a computer program comprising computer-executable instructions which, when executed by a processor, are adapted to carry out the steps of the method according to the first aspect or any of its above-described embodiments.

[0035] The computer program can in particular run on a wind turbine controller. Thus, it can also be installed as a software upgrade on an existing turbine.

[0036] Note that embodiments of the invention have been described with reference to different subject matters. In particular, some embodiments have been described with reference to method-type claims, while other embodiments have been described with reference to apparatus-type claims. However, those skilled in the art will infer from the above and the following description that, unless otherwise stated, any combination of features related to different subject matters, in particular any combination of features of method-type claims and features of apparatus-type claims, in addition to any combination of features belonging to one type of subject matter, is also part of the disclosure of this document.

[0037] The above and other aspects of the invention will be apparent from and will be elucidated with reference to the examples of embodiments described hereinafter. The invention will be described in more detail hereinafter with reference to the examples of embodiments. However, it is expressly noted that the invention is not limited to the described exemplary embodiments. Description of the Drawings

[0038] Figure 1 Shows different types of oscillating tower motions.

[0039] Figure 2 Shows a flowchart of a method according to an embodiment of the present invention.

[0040] Figure 3 Shows a graph of two normalized amplitude spectra determined in connection with an embodiment of the present invention.

[0041] Figure 4 Shows a graph of a difference function determined in connection with an embodiment of the present invention.

[0042] Figure 5 Shows a functional block diagram of a device according to an embodiment of the present invention. Detailed Description

[0043] The illustrations in the drawings are schematic. Note that in different figures, similar or identical elements are provided with the same reference numerals or reference numerals that differ only in the first digit.

[0044] Figure 1 Shows different types of oscillating tower motions. More specifically, Figure 1 Shows a schematic representation of a wind turbine tower mounted to the ground 5 and carrying a nacelle at its top. The wind turbine is viewed from the front such that the rotor axis extends perpendicular to the plane of the drawing. When no oscillating (lateral) motion occurs, the tower is stationary and straight up, as indicated by the solid line 10. When the first mode of oscillating motion occurs, the tower with the nacelle 22 will bend from side to side, as indicated by the dashed line 20 and the arrow 24. When the second mode of oscillating motion occurs, the central section of the tower with the nacelle 32 (midway between the ground 5 and the nacelle 32) will move from side to side as indicated by the arrow 34. In this mode, the nacelle will not exhibit much lateral motion but will exhibit a roll motion (about an axis parallel to the rotor axis), as indicated by the arrow 36. Thus, when the rotational speed of the rotor is measured by a sensor fastened to the nacelle, the roll motion will affect the measured rotor speed. Therefore, when such rotor speed measurements are used to set the pitch reference value in the wind turbine control system, it may result in increased pitch activity and pitch travel as well as increased fore and aft tower loads. By making an exact knowledge of the second mode frequency (and possibly further higher mode frequencies of the nacelle motion that cause roll) available, these drawbacks can be mitigated.

[0045] Figure 2 Shows a flowchart 200 of a method according to an embodiment of the present invention.

[0046] At 210, a first signal indicative of the rotor speed relative to the nacelle is obtained. The first signal is preferably obtained directly from a rotor speed sensor mounted within the nacelle.

[0047] At 220, a second signal indicative of the rotor speed relative to the ground is obtained. The second signal is preferably obtained directly or indirectly from a monitoring device arranged at the hub of the wind turbine, which is part of the safety system.

[0048] Then, at 230, a first amplitude spectrum is determined based on the first signal, and at 240, a second amplitude spectrum is determined based on the second signal. Preferably, the two amplitude spectra are determined by applying a discrete Fourier transform (DFT) to the first and second signals, in particular a sliding DFT considering only a predetermined amount of the most recent signal values (such as the last second, last 10 seconds, last minute, etc.).

[0049] It should be noted that steps 210, 220, 230, 240 do not need to be performed in the order described above.

[0050] Next, at 250, a difference function is determined based on the first amplitude spectrum and the second amplitude spectrum. The difference function can in particular be calculated as the absolute difference between the first and second amplitude spectra as a function of frequency.

[0051] Finally, at 260, the frequency of the swaying motion of the wind turbine tower is determined as the frequency corresponding to the peak in the difference function. Here, to ensure the robustness of the frequency determination, only if the peak exceeds a predetermined threshold, the frequency corresponding to the peak in the difference function can be output as a representative of the swaying frequency. In addition, it may also be required that, based on the knowledge of the mechanical properties of the wind turbine structure, the determined frequency is within a certain predetermined interval.

[0052] Figure 3 Graph 300 shows two normalized amplitude spectra determined in connection with an embodiment of the present invention. More specifically, graph 300 shows a first normalized amplitude spectrum 311 corresponding to a first signal indicative of the rotor speed relative to the nacelle, and a second normalized amplitude spectrum 312 corresponding to a second signal indicative of the rotor speed relative to the ground, for example, as determined in steps 230 and 240 of method 200 discussed above. As can be seen, the spectra 311, 312 are normalized such that the normalized value of the first spectrum 311 is 1 at a normalized frequency of one, the latter corresponding to the frequency of the second tower mode. Figure 2 As can be seen, the spectra 311, 312 are normalized such that the normalized value of the first spectrum 311 is 1 at a normalized frequency of one, the latter corresponding to the frequency of the second tower mode.

[0053] As can be seen, for most frequencies, the amplitude spectra 311, 312 are very similar. However, at a normalized frequency of approximately 1.0, the first spectrum 311 has a value significantly greater than that of the second spectrum 312, namely 1.0. Additionally, at normalized frequencies of approximately 2.8 and 3.5, the first spectrum 311 has greater values than the second spectrum 312.

[0054] Figure 4 illustrates an embodiment incorporating the present invention, such as the difference function 414 determined in step 250 of method 200 discussed above in connection with Figure 2 the graph 400 of the difference function 414 determined in step 250 of method 200 discussed above in connection with Figure 3 the absolute difference between the first normalized amplitude spectrum 311 and the second normalized amplitude spectrum 312 shown and discussed above.

[0055] Note that the difference function 414 shows a significant peak 415 at a normalized frequency 416 of approximately 1.0. Additionally, the difference function 414 shows additional and smaller peaks 417 and 418 at a normalized frequency of approximately 2.8 and another normalized frequency of slightly approximately 3.5.

[0056] The main peak 415 will be interpreted as corresponding to the second tower mode, since this mode is the first mode involving nacelle roll motion. The other peaks 417 and 418 can be interpreted as corresponding to higher order modes, but will generally be ignored due to their insignificant amplitudes.

[0057] Figure 5 illustrates a functional block diagram 500 of a device according to an embodiment of the present invention. The functional blocks of the device include a sliding DFT block 510, a smoothing block 520, a difference calculation block 530, an absolute value calculation block 540, and a peak tracking block 550.

[0058] The sliding DFT block 510 receives a first rotor speed signal 501 and a second rotor speed signal 502 and outputs corresponding first 511 and second 512 amplitude spectra to the smoothing block 520. The smoothed spectra are input to the difference calculation block 530, and the calculated difference is passed to the absolute value calculation block 540. The absolute difference between the smoothed first and second spectra is then input to the peak tracking block 550, which identifies one or more peaks in the difference function as discussed above and outputs the corresponding frequency (or frequencies) 555, particularly the frequency of the second tower mode.

[0059] Physically, the device may be implemented as an interface and a processing unit. The interface is configured to receive a first signal 501 indicating the rotor speed relative to the nacelle and to receive a second signal 502 indicating the rotor speed relative to the ground. The processing unit is configured to perform the functions of function blocks 510, 520, 530, 540, 550, that is, to determine a first amplitude spectrum 511 based on the first signal 501, to determine a second amplitude spectrum 512 based on the second signal 502, to determine an absolute difference function based on the first amplitude spectrum 511 and the second amplitude spectrum 512, and to determine the frequency of the swaying motion of the wind turbine tower as the frequency 555 corresponding to the peak in the absolute difference function. The function blocks 510, 520, 530, 540, 550 may advantageously be implemented as software.

[0060] By continuously knowing the swaying frequency, in particular the frequency of the second tower mode, many advantages can be obtained, such as

[0061] - reducing the fore-aft tower loads

[0062] - reducing the pitch bearing loads

[0063] - reducing the pitch actuation

[0064] - enabling active damping of the swaying of the second tower mode, in particular

[0065] - filtering out a specific frequency in the measured values of the wind turbine controller will prevent the controller from reacting to that frequency and thus reduce the loads

[0066] - reducing the loads on existing and new turbines

[0067] It should be emphasized that all these advantages can be obtained without additional hardware, in particular without additional sensors.

[0068] Note that the term "comprising" does not exclude other elements or steps, and the use of the article "a" or "an" does not exclude a plurality. Elements described in connection with different embodiments may also be combined. It should also be noted that the reference signs in the claims should not be construed as limiting the scope of the claims.

Claims

1. A method (200) for determining the frequency of the swaying motion of a tower of a wind turbine, wherein the tower of the wind turbine carries a nacelle and a generator including a stator and a rotor, and the swaying motion causes a rolling motion of the nacelle, the method comprising obtaining (210) a first signal indicative of the rotor speed relative to the nacelle, obtaining (220) a second signal indicative of the rotor speed relative to the ground, determining (230) a first amplitude spectrum (311) based on the first signal, determining (240) a second amplitude spectrum (312) based on the second signal, determining (250) a difference function (414) based on the first amplitude spectrum and the second amplitude spectrum, and determining (260) the frequency of the swaying motion of the tower of the wind turbine as the frequency (416) corresponding to a peak (415) in the difference function.

2. The method according to claim 1, wherein The swaying motion of the tower of the wind turbine is a second-mode tower swaying motion.

3. The method according to claim 1 or 2, wherein The difference function is determined as the absolute difference between the first amplitude spectrum and the second amplitude spectrum as a function of frequency.

4. The method according to claim 1 or 2, further comprising applying smoothing to each of the first and second amplitude spectra before determining the difference function.

5. The method according to claim 1 or 2, wherein The first signal is obtained by a rotor speed sensor disposed within the nacelle and adapted to measure the rotor speed.

6. The method according to claim 5, wherein The rotor speed sensor is an optical or magnetic sensor adapted to detect when a corresponding part of the rotor passes a first sensor.

7. The method according to claim 1 or 2, wherein The second signal is obtained by a monitoring device disposed at the hub of the wind turbine.

8. The method according to claim 7, wherein, The monitoring device includes at least one inertial sensor.

9. The method according to claim 8, wherein, The at least one inertial sensor includes a gravity sensor and / or a gyroscope.

10. The method according to claim 1 or 2, wherein, Determining the first amplitude spectrum and determining the second amplitude spectrum include applying a sliding discrete Fourier transform to the first signal and the second signal, respectively.

11. The method according to claim 1 or 2 further comprises: If the peak of the difference function exceeds a predetermined threshold, the determined frequency is confirmed as the frequency of the swaying motion of the tower of the wind turbine.

12. A device for determining the frequency of the swaying motion of a tower of a wind turbine, wherein the tower of the wind turbine carries a nacelle and a generator including a stator and a rotor, and the swaying motion causes a rolling motion of the nacelle, the device comprising an interface configured to receive a first signal (501) indicative of the rotor speed relative to the nacelle, and receive a second signal (502) indicative of the rotor speed relative to the ground, and a processing unit configured to determine a first amplitude spectrum (511) based on the first signal, determine a second amplitude spectrum (512) based on the second signal, determine a difference function based on the first amplitude spectrum and the second amplitude spectrum, and determine the frequency of the swaying motion of the tower of the wind turbine as the frequency (555) corresponding to a peak in the difference function.

13. A wind turbine comprising the device according to claim 12.

14. A computer program comprising computer-executable instructions which, when executed by a processor, are adapted to perform the steps of the method according to any one of claims 1 to 11.

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