Detection of oscillating movement of a wind turbine Field of Invention

The present invention relates to the field of wind turbines, in particular to detection of oscillating wind turbine move ment. More specifically, the present invention relates to a system for determining an amount of oscillating movement of a wind turbine, a wind turbine comprising such a system, and a method of determining an amount of oscillating movement of a wind turbine .

Art Background

Modern wind turbines are built upon towers of ever increasing heights. These large structures can be excited by the normal operation of the turbine influenced by the operational envi ronment. Thereby, substantial oscillating movements may oc cur, in particular in the tower structure. Large motion of the tower may cause structural damage.

An accelerometer may be used in the tower top to detect se vere oscillations, such that the wind turbine control system can stop the wind turbine in a safe manner. However, with this approach only movement of the tower top is captured. Thus, more complex patterns of movement where the tower top does not move as significantly as other parts of the struc ture cannot be detected.

Accordingly, there is a need for a system that can detect such complex patterns of oscillating movement in order to ef fectively protect the wind turbines against structural dam age . Summary of the Invention

This need may be met by the subject matter according to the independent claims. Advantageous embodiments of the present invention are described by the dependent claims.

According to a first aspect of the invention, there is pro vided system for determining an amount of oscillating move ment of a wind turbine, the wind turbine comprising a tower, a non-rotating upper part supported by the tower, a rotor having a rotor axis, and a generator for generating electric power. The system comprises (a) a sensor unit adapted to pro vide a rotor speed signal indicative of a rotational speed of the rotor relative to the non-rotating upper part, (b) a fil tering unit adapted to, based on the rotor speed signal pro vided by the sensor unit, provide a filtered signal compris ing information associated with an oscillating movement of the wind turbine, and (c) a processing unit adapted to deter mine the amount of oscillating movement based on the filtered signal provided by the filtering unit.

This aspect of the invention is based on the idea that the rotational speed of the rotor relative to the non-rotating upper part as measured by the sensor unit will be influenced by movement of the wind turbine and that this influence (or at least a selected part thereof) can be determined by fil tering the rotor speed signal from the sensor unit. More spe cifically, since the rotation of the rotor relative to earth (i.e. the true rotational speed of the rotor) will not be in fluenced by tower movement, any movement in the wind turbine structure, which causes a rolling movement of the non rotating upper part of the wind turbine, will cause corre sponding variations in the rotor speed measured by the sensor unit. In other words, oscillating movements in the tower structure will cause corresponding oscillations in the rotor speed signal from the sensor unit. By filtering out these os cillations from the rotor speed signal, the amount of oscil lating movement (i.e. the amplitude of the oscillating move- ment at a particular location, such as a selected position along the tower) can be determined.

According to an embodiment of the invention, the sensor unit comprises a sensor, in particular an optical and/or magnetic sensor, adapted to detect a predetermined pattern on the ro tor .

The sensor is preferably mounted on the non-rotating upper part of the wind turbine in position suitable for detecting the predetermined pattern on the rotor.

According to a further embodiment of the invention, the sen sor unit comprises a frequency sensor adapted to detect a frequency of electric power generated by the generator.

The frequency of the electric power generated by the genera tor depends on the rotational speed of the rotor (relative to the non-rotating part of the wind turbine) . Thus, by measur ing this frequency, a measure of the rotational speed rela tive to the non-rotating part is obtained in a simple manner.

According to a further embodiment of the invention, the wind turbine further comprises a gearbox and a high speed coupling arranged between the generator and the rotor, and the sensor unit comprises a sensor adapted to detect a rotational speed of the high speed coupling relative to the non-rotating upper part .

The rotational speed of the high speed coupling will be re lated to the rotational speed of the rotor and will therefore contain the same information associated with oscillating movement of the wind turbine as the rotor speed since it is measured relative to the non-rotating upper part of the wind turbine .

According to a further embodiment of the invention, the sys tem further comprises (a) a further sensor unit adapted to provide a further rotor speed signal indicative of the rota tional speed of the rotor relative to ground, and (b) a sub traction unit adapted to subtract the further rotor speed signal from the rotor speed signal to thereby provide a roll signal indicative of an angular roll speed of the non rotating upper part. The filtering unit is adapted to provide the filtered signal comprising information associated with the oscillating movement of the wind turbine based on the roll signal.

In this embodiment, a further rotor speed signal is provided by a further sensor unit. The further rotor speed signal rep resents the true rotational speed of the rotor, i.e. the ro tational speed relative to ground or earth. By subtracting the further rotor speed signal from the rotor speed signal, a roll signal is obtained which comprises information on the angular roll speed of the non-rotating upper part of the wind turbine, i.e. the part of the rotor speed signal which is caused by wind turbine movement (s) causing side-to-side move ment of the non-rotating part. By filtering this roll signal, which is derived from the rotor speed signal and the further rotor speed signal, the magnitude of a particular oscillating movement in the wind turbine structure can be obtained.

According to a further embodiment of the invention, the fur ther sensor unit comprises an inertial sensor adapted to be arranged at the rotor.

The inertial sensor may preferably comprise an accelerometer and/or a gyroscopic sensor arranged in a hub of the rotor.

According to a further embodiment of the invention, the fil tering unit comprises a bandpass filter centered on a funda mental frequency of the tower.

By utilizing a bandpass filter centered on a fundamental fre quency of the tower, information relating to an oscillating movement (in particular the magnitude thereof) corresponding to the fundamental frequency can be obtained.

The fundamental frequency may in particular denote an eigen- frequency of the tower or.

According to a further embodiment of the invention, the fun damental frequency of the tower corresponds to a second or higher order fundamental mode of the tower.

By filtering out the higher order modes (i.e. the second fun damental mode, the third fundamental mode, etc.) the amount of movement corresponding to these particular modes of oscil lating movement can be obtained. Accordingly, excessive mag nitudes of movement contained in complex patterns of movement can be detected even in cases where the upper part of the wind turbine itself does not move very much, for example when a mid section (or one or more other sections) of the tower between ground and the upper part is swinging from side to side .

According to a further embodiment of the invention, the pro cessing unit is adapted to utilize a mathematical model of the tower to determine the amount of oscillating movement.

The mathematical model describes the physical properties of the tower and includes relevant physical parameters (e.g. tower height, tower stiffness, and tower-top mass) of the wind turbine .

According to a further embodiment of the invention, the math ematical model of the tower provides a relation between tower acceleration and the angular roll speed of the non-rotating upper part.

According to a further embodiment of the invention, the sys tem further comprises a warning unit adapted to compare the determined amount of oscillating movement with a threshold value and output a warning signal if the determined amount of oscillating movement exceeds the threshold value.

The warning signal may simply indicate that dangerous oscil lating movement occurs. Alternatively, the warning signal may also indicate the amount of movement such that a wind turbine control system can choose an appropriate action, such as a load reduction, rotor speed reduction, or emergency shut down .

According to a second aspect of the invention, there is pro vided a wind turbine. The wind turbine comprises (a) a tower, (b) a non-rotating upper part supported by the tower, (c) a rotor having a rotor axis, (d) a generator for generating electrical power, and (e) a system according to the first as pect or any of the above embodiments.

This aspect of the invention relates to a wind turbine fitted with an advantageous system according to the first aspect (or one of the above described embodiments) . Accordingly, the wind turbine is capable of detecting the amount of oscilla tion movement to thereby protect itself in cases of danger ously large oscillating movements.

According to a third aspect of the invention, there is pro vided a method of determining an amount of oscillating move ment of a wind turbine, the wind turbine comprising a tower, a non-rotating upper part supported by the tower, a rotor having a rotor axis, and a generator for generating electri cal power. The method comprises (a) providing a rotor speed signal indicative of a rotational speed of the rotor relative to the non-rotating upper part, (b) providing a filtered sig nal based on the rotor speed signal, the filtered signal com prising information associated with an oscillating movement of the wind turbine, and (c) determining the amount of oscil lating movement based on the filtered signal. This aspect of the invention is based on essentially the same idea as the first aspect described above.

It is noted that embodiments of the invention have been de scribed with reference to different subject matters. In par ticular, some embodiments have been described with reference to method type claims whereas other embodiments have been de scribed with reference to apparatus type claims. However, a person skilled in the art will gather from the above and the following description that, unless otherwise indicated, in addition to any combination of features belonging to one type of subject matter also any combination of features relating to different subject matters, in particular to combinations of features of the method type claims and features of the ap paratus type claims, is part of the disclosure of this docu ment .

The aspects defined above and further aspects of the present invention are apparent from the examples of embodiments to be described hereinafter and are explained with reference to the examples of embodiments. The invention will be described in more detail hereinafter with reference to examples of embodi ments. However, it is explicitly noted that the invention is not limited to the described exemplary embodiments.

Brief Description of the Drawings

Figure 1 shows a schematic illustration of roll motion of an upper part of a wind turbine caused by simple tower sway.

Figure 2 shows a schematic illustration of roll motion of an upper part of a wind turbine caused by 2 ^nd mode tower sway.

Figure 3 shows a schematic illustration of an upper part of a wind turbine equipped with a rotor speed sensor.

Figure 4 shows a system according to an exemplary embodiment of the present invention. Figure 5 shows a system according to a further exemplary em bodiment of the present invention.

Detailed Description

The illustration in the drawing is schematic. It is noted that in different figures, similar or identical elements are provided with the same reference numerals or with reference numerals which differ only within the first digit.

Figure 1 shows a schematic illustration of roll motion of an upper part of a wind turbine caused by simple tower sway or side-to-side movement, i.e. 1 ^st mode tower sway. More specif ically, Figure 1 shows a wind turbine comprising a tower 1 mounted to the ground 2, an upper non-rotating part 3 housing a rotor 4 with rotor blades 5. The left-hand part of Figure 1 shows a state where the tower 1 has swayed to the right and the right-hand part of Figure 1 shows a state where the tower 1 has swayed to the left. The dashed line 6 is horizontal and the dashed line 7 shows the plane of the bottom of the non rotating upper part 3 (also referred to as a nacelle) of the wind turbine. As indicated by arrow 6a, the swaying movement of tower 1 causes sideways movement of the upper part 3. Fur thermore, as indicated by arrow 6b, the swaying movement of tower 1 also causes a corresponding angular roll movement of the upper part 3. In other words, in this case the maximum lateral movement is in the tower top and may thus be detected by an accelerometer.

Figure 2 shows a schematic illustration of roll motion of an upper part of the wind turbine caused by 2 ^nd mode tower sway. More specifically, Figure 2 shows that an oscillating move ment of a midsection of the tower 1 is taking place as indi cated by arrow 10. As can be seen, the midsection moving from side to side in this manner also causes roll motion of the upper part 3 of the wind turbine as indicated by arrow 6b, whereas in this case no significant sideways movement of the upper part 3 takes place. In other words, there is no or in significant lateral movement of the tower top. This means that an accelerometer in the tower top is not able to detect the oscillating movement indicated by arrow 10. Tower motion due the second fundamental tower mode can build up in large oscillations which may have a severe load impact on the tower structure .

Figure 3 shows a schematic illustration of the upper part 3 of the wind turbine shown in Figure 1 and Figure 2 equipped with a rotor speed sensor 8. The rotor speed sensor 8 is mounted on surface 9, which is fixed to the top of the tower 1. The rotor speed sensor 8 may e.g. be an optical sensor or a magnetic sensor, capable of detecting a predetermined pat tern on the surface of the rotor axis 4a. Referring again to Figures 1 and 2, it can be seen that the rolling motion of upper part 3 caused by the tower sway will influence the ro tor speed detected by rotor speed sensor 8 (i.e. the rotor speed relative to the non-rotating upper part 3) but not the true rotor speed (relative to ground 2) .

Since the fixed surface 9 where the rotor speed sensor 8 is mounted is also fixed to the tower top 3, then as the tower top 3 inclines side-to-side this sensor 8 has a rotational velocity aligned with the roll motion of the tower top. This roll motion, therefore, impacts the measurement of the rotor speed by causing a cyclic oscillation in the relative angular velocity between the fixed sensor 8 and the rotating shaft 4a. This introduces an error in the rotor speed measurement relative to what would be observed from a truly fixed frame of reference (such as the ground, for example) .

Figure 4 shows a system 400 according to an exemplary embodi ment of the present invention. More specifically, the system 400 comprises a sensor unit 408, a filtering unit 415, funda mental frequency data 417, and a processing unit 420. The sensor unit 408 may e.g. correspond to the rotor speed sensor 8 shown in Figure 3, which is adapted to detect a ro tational speed of the rotor 4 relative to the non-rotating upper part 3. Alternatively, the sensor unit 408 may be adapted to detect a frequency of electric power generated by the wind turbine generator (not shown) and thereby the rota tional speed of the rotor 4 relative to the non-rotating up per part 3. As a further alternative, the sensor unit 408 may detect a rotational speed of another part of the drivetrain, in particular the rotational speed of a high speed coupling between a gearbox and the generator. The detected rotor speed signal is provided to the filtering unit 415.

The filtering unit 415 is adapted to obtain or generate a filtered signal based on the rotor speed signal, in particu lar by utilizing the fundamental frequency data 417. In par ticular, the filtering unit 415 may comprise or be a bandpass filter adapted to filter the rotor speed signal around a fun damental frequency included in fundamental frequency data 417, such as a around a fundamental frequency corresponding to a first mode, a second mode or a higher mode of oscillat ing tower movement .

The processing unit 420 receives the filtered signal and is adapted to determine the amount of oscillating movement based thereon, e.g. by applying a mathematical model of the wind turbine tower 1 with the non-rotating upper part 3 and rotor 4 in order to determine the magnitude of the oscillating movement .

The system 400 may furthermore comprise a warning unit (not shown) adapted to compare the determined amount of oscillat ing movement with a threshold value and to output a warning signal if the determined amount of oscillating movement ex ceeds the threshold value. The warning signal may be used by a wind turbine controller to determine an appropriate action for protecting the wind turbine, e.g. by reducing load or by shutting down. The warning unit may be implemented as part of the processing unit 420 may

Figure 5 shows a system 500 according to a further exemplary embodiment of the present invention. More specifically, the system 500 comprises a sensor unit 508, a further sensor unit 511, a subtraction unit 512, a roll signal 513, a filtering unit 515, fundamental frequency data 517, and a processing unit 520.

The sensor unit 508, filtering unit 515, and fundamental fre quency reference data 517 are similar to the corresponding units shown in Figure 4 and will therefore not be described in further detail again.

The further sensor unit 511 preferably comprises an inertial sensor, such as an accelerometer or a gyroscopic sensor, ar ranged in the hub of the rotor 4 and adapted to provide a further rotor speed signal indicative of the rotational speed of the rotor 4 relative to ground 2, i.e. the true rotational speed of rotor 4.

The subtractor 512 receives the rotor speed signal from sen sor unit 508 and the further rotor speed signal from the fur ther sensor unit 511, and calculates the corresponding dif ference by subtracting the latter from the first and thereby producing a roll signal 513 which is indicative of the angu lar roll speed of the non-rotating upper part 3.

The roll signal 513 is filtered by filtering unit 515 in a similar way as described above in conjunction with Figure 4, i.e. by applying a bandpass filter centered on a fundamental tower frequency included in the fundamental frequency data 517 in order to extract the part of the signal that corre sponds to a particular mode of movement, i.e. the first, sec ond or any higher order mode. The processing unit 520 processes the filtered signal and ap plies a suitable mathematical model of the tower to determine the magnitude of the oscillating movement. It is noted that the term "comprising" does not exclude other elements or steps and the use of the articles "a" or "an" does not exclude a plurality. Also elements described in as sociation with different embodiments may be combined. It is further noted that reference signs in the claims are not to be construed as limiting the scope of the claims.