DESC:FIELD OF THE INVENTION

The present invention is directed to the technical field of wind turbines, in particular to wind turbines of the horizontal type, which means that the wind turbines comprise a horizontal rotor axis and a rotor being directed against the wind.

Such wind turbines generally comprise a nacelle incorporating a drive train. The nacelle is mounted to a tower. A rotor with a number of rotor blades, particularly with three rotor blades, is connected to the drive train via a hub, to which the rotor blades are mounted. The rotor rotates around its rotational axis. In general, the drive train comprises at least a generator device. According to one type of wind turbines the rotor blades are adjustably mounted to the hub. This is realized by means of respective pitch drives, said pitch drives being part of a pitch system. The pitch system, which is generally known in the prior art, participates in the control of the rotor speed to given set points. In general, the rotor speed is controlled by the load and by the pitch angle. By means of the pitch drives, the rotor blades may be moved about rotor blade axes into different pitch positions, said rotor blade axes extending in an axial direction of the rotor blades.

The present invention relates to a method of reducing tower torsion oscillations in a wind turbine. Furthermore, the present invention relates to a device for setting the pitch of the rotor blades of a rotor of a wind turbine, as well as to a wind turbine itself.

BACKGROUND OF THE INVENTION

During operation of a wind turbine, the different elements of the wind turbine, the tower and/or the nacelle and/or the rotor for example, may undergo undesired oscillations which may result in fatigue damages and extreme loads. This is a serious problem, in particular since wind turbines become increasingly higher. For example, tall and slim towers tend to show torsional oscillations. This excites blade oscillations which significantly increased blade loads, mainly edgewise.

Furthermore, in the constructions of wind turbines it may come to a coupling of the tower-torsion-natural frequency with the rotor blade-natural frequency, in particular with the edgewise rotor blade-natural frequency. This leads to increased bending vibrations at the rotor blades.

In the past, wind turbines have become taller and taller. Not seldom, modern wind turbines reach an overall height of more than 100 metres. Typically, wind turbines are slender structures. The nacelle including the rotor with its rotor blades is provided at the top of the tower. Therefore, the wind turbine provides a lot of contact surface for the wind. If the wind hits the wind turbine, the wind turbine, especially the tower, the rotor with its rotor blades and the nacelle are exposed to vibrations. Such vibrations can have high amplitudes. However, these vibrations negatively influence the lifetime, the design and the performance of the wind turbine. Additionally, negative vibrations are also generated by forces induced on the nacelle and the tower by the rotating rotor blades.

Therefore, there is a need to reduce such negative vibrations and oscillations of the wind turbine, in particular of the tower and/or of the rotor and/or of the nacelle.

Since the existence of such vibrations and oscillations is a long-known phenomenon, a lot of different solutions have been provided in order to compensate such vibrations and oscillations.

EP 1 269 015 B1 for example discloses a solution for operating wind turbines based on the frequency of their towers. According to this known solution the critical natural bending frequency of the wind turbine is determined. The rotor speed range is determined, in which there is an excitation of the wind turbine in the range of the critical natural bending frequency. In order to avoid any problems, the critical rotor speed range is rapidly passed.

For wind turbine comprising a pitch system as described further above it has been proposed in the prior art that such a pitch system can be used in order to reduce negative vibrations and oscillations by use of an individual pitch-offset for the rotor blades.

US 2014/0003936 A1 discloses a system and a method to reduce tower oscillations in a wind turbine, which makes use of a pitch system. This known solution includes obtaining a rotor velocity as well as obtaining one or more parameters associated with the tower of the wind turbine. According to this known solution, a modified rotor velocity based on the one or more tower parameters is determined. Based on the modified rotor velocity a first pitch angle is determined and the rotor blades of the wind turbine are pitched based on the first pitch angle in order to reduce tower oscillations.

A similar solution is disclosed in US 2008/0206051 A1. According to this known solution, an accelerometer is used which is attached to the nacelle of the wind turbine. This accelerometer detects the acceleration due to vibrations of the nacelle. Furthermore, this known solution comprises an active damping unit for calculating a pitch angle of the rotor blades of the wind turbine for generating a thrust on the rotor blades so as to cancel out the vibrations of the nacelle on the basis of the acceleration detected with the accelerometer.

US 9,710,493 B2, from which the present invention departs, describes a method for dampening oscillations in a wind turbine. According to this known solution, oscillations of the tower of the wind turbine in the direction of the X-axis are monitored. Furthermore, a compensating torque to be applied by the rotor to the tower about a Y-axis is determined. For each rotor blade an adjustment of the pitch angle suitable to generate said compensation torque is determined and the pitch angle of the rotor blades are adjusted accordingly.

The aforementioned solutions of the prior art have in common, that parameters of the tower or of the nacelle have to be determined. This requires additional sensor elements and measuring devices.

OBJECTIVE OF THE PRESENT INVENTION

Starting from the aforementioned prior art, it is the object of the present invention to provide a solution in the field of wind turbines having a pitch system, which is capable of reducing tower torsion oscillations of the wind turbine, in particular without having the need of providing additional sensor elements which are allocated to the tower or to the nacelle of the wind turbine.

According to the present invention, the object is solved by the method of reducing tower torsion oscillations in a wind turbine , which is the first aspect of the invention, by the device for setting the pitch of the rotor blades of a rotor of a wind turbine, which is the second aspect of the invention, and by the wind turbine, which is the third aspect of the invention.

Further features and details of the invention become apparent from the claims, from the description as well as from the drawings. Therein, features and details which are described in connection with one aspect according to the invention apply with respect to their disclosure in their entirety also to the other aspects according to the invention, so that any statements made with respect to one aspect of the invention also apply to their full extent to the other aspects of the invention, and vice versa.

SUMMARY OF THE INVENTION

The underlying concept of the present invention is that oscillations of the wind turbine are reduced or compensated by means of an individual pitch control.

According to the present invention the term “torsional oscillations” includes any kind of oscillations and vibrations which lead to a torsion. In particular, the term “torsional oscillations” as mentioned in the specification represents the combined torsional oscillation of the tower and the rotor. Therefore, the solution according to the present invention is preferably provided to reduce the combined torsional oscillation of the tower and the rotor with the effect of a reduction of blade moments as well as of tower torsional moments. According to the present invention torsional oscillations of the tower of the wind turbine are reduced. In this case the present invention is particularly directed to solutions of tower torsion balancing via individual pitching.

Since the wind turbine already comprises a pitch system with a pitch control for individually adjusting the rotor blades, this pitch system can be extended by an additional term, said term considering tower oscillations, for example a coupled tower torsion oscillation/rotor blade oscillation, in particular an edgewise rotor blade oscillation.

The present invention according to its three aspects is directed to the technical field of wind turbines, in particular to the technical field of horizontal wind turbines. Such wind turbines are generally known in the prior art.

According to a preferred embodiment, the wind turbine comprises a nacelle which incorporates a drive train. The drive train transmits the rotor speed to the generator where it is converted into electric energy. Therefore, the wind turbine comprises a rotor being connected to said drive train, said rotor preferably comprising a hub and a number of rotor blade, preferably three rotor blades, said rotor blades being mounted to said hub. The rotor is pivotally mounted around a rotational axis to the drive train. In order to transform the rotational energy of the rotor into electric energy, the drive train, to which the rotor of the wind turbine is mounted, comprises a number of different components. One of these components is a generator device. The generator device generates electric energy from the rotational energy which is provided by the gear device. For this purpose, the generator device preferably comprises a stator component and a rotor component, said rotor component being coupled to the generator shaft. The nacelle is mounted at the top of a tower of said wind turbine. At its lower base end, the tower is anchored to the ground by means of a foundation.

The rotor blades are adjustably mounted on the hub. This is realized by means of a pitch drive, said pitch drive being part of a pitch drive unit, which is part of a pitch system. The pitch system, which is generally known in the prior art, participates in the control of the rotor speed to given set points. In general, the rotor speed is controlled by the load and by the pitch angle. By means of the pitch-drives, the rotor blades may be moved about rotor blade axes into different pitch positions, said rotor blade axes extending in an axial direction of the rotor blade. In particular, each rotor blade can be adjusted individually from the other rotor blades.

A rotor blade having an adjustable pitch can be particularly understood in such a manner, that the angle of attack of the rotor blade, which may be defined as the pitch angle, can be adjusted or is provided in an adjustable manner, during a pitch-offset for example.

With the present invention it is possible to reduce or compensate torsion oscillations, in particular combined torsional oscillations of the tower and of the rotor, of the wind turbine. However, it is not required to use any sensor element being provided with the tower or with the nacelle. Since the wind turbine according to the present invention comprises a pitch system with regard to the rotor blades of the rotor, such components of the pitch system, which per se are provided by the pitch system, are used. For example, bending sensor elements are used, which are provided with the rotor blades and which are used for individually adjusting the rotor blades by means of a pitch adjustment.

It is the merit of the inventors of the present invention, that they have found out, that a strong correlation between the resulting rotor moment in yaw direction and the tower torsion moments exists, in particular both for the amplitude and for the phase. The inventors have verified that the resulting moment of the rotor is a fingerprint of the tower torsion. This will be explained in combination with Figure 6 further below. Therefore, at this point, full reference is also made to the disclosure of Figure 6 and the description thereof.

It is one preferred feature of the present invention that sensor elements are evaluated in order to get an angle-dependent evaluation of the moments. By means of an individual adjustment of the rotor blades, a counter moment can be brought in into the rotor.

The present invention will now be explained in detail and with reference to the different aspects of the present invention.

According to the present invention, different axes with different orientations are considered. One axis is the so-called yaw-axis, which can be defined as the Z-axis in a spatial coordinate system. According to the present invention, the yaw-axis corresponds in its alignment to the longitudinal axis if the tower. In particular the yaw-axis is a vertical axis. Another axis is the so-called tilt-axis, which can be defined as the Y-axis in the spatial coordinate system. According to the present invention the tilt-axis is perpendicular to the X-axis, said X-axis corresponding in its alignment to the axis around which the rotor rotates. In particular, the tilt-axis is a horizontal axis.

According to the first aspect of the present invention, the object is solved by the method of reducing tower torsion oscillations in a wind turbine.

According to this first aspect, a method of reducing tower torsion oscillations in a wind turbine is provided, said wind turbine comprising a tower and a rotor being mounted at the top of the tower, said rotor comprising a number of rotor blades with adjustable pitch, preferably three rotor blades. In particular the method is performed in connection with a wind turbine as described in the general explanations of the invention further above.

The method according to the first aspect of the invention is characterized by the following steps:

In a first step a resulting rotor moment Myaw is determined, which leads to torsion in yaw-direction around a yaw-axis. Preferred embodiments of how this resulting rotor moment Myaw is determined, calculated for example, are described in more detail further below.

In the next step, a counter moment Mcounter, is determined, which counter-acts to the resulting rotor moment Myaw. Preferred embodiments how this counter moment Mcounter is determined, calculated for example, are described in more detail further below.

In the next step, the counter moment Mcounter gets partitioned into the individual contributions of each rotor blade. In the last step, the resulting rotor moment Myaw gets damped by applying the counter moment Mcounter on the rotor. According to the present invention, this is achieved via an individual pitch-offset of at least one rotor blade, in particular of all rotor blades. This particularly means that the counter moment Mcounter is brought in into the rotor via an adjustment of the at least one rotor blade during the pitching procedure. Preferred embodiments thereof are described in more detail further below.

As described further above, the pitch of a rotor blade is particularly defined as the orientation of the rotor blade in relation to the wind having a specific angle of attack, the pitch-angle for example. A pitch-offset therefore particularly means that the angle of attack and therefore the orientation of the rotor blade in relation to the wind gets adjusted.

According to the present invention, the damping step comprises, that the counter moment Mcounter, which counter-acts to the resulting rotor moment Myaw, gets added to the resulting rotor moment Myaw, such that the resulting rotor moment Myaw in sum gets reduced. The add-on of the counter moment Mcounter is achieved by performing a pitch-offset, which means that the pitch-angle of at least on rotor blade, in particular of all rotor blades, gets changed, until the existing resulting rotor moment Myaw is reduced by a factor which corresponds to the counter moment Mcounter. Therefore, since there is a strong correlation between the resulting rotor moment Myaw and the torsional oscillations of the tower of the wind turbine, the tower torsion oscillations can be reduced or compensated by means of applying the counter moment Mcounter to the resulting rotor moment Myaw via the additional pitch offset.

In the following it is described in more detail, how the resulting rotor moment Myaw gets determined, calculated for example.

According to a preferred embodiment, the resulting rotor moment Myaw is generated from individual blade bending moments of the rotor blades, in particular via vector summation of the individual blade bending moments. In general, the rotor blades comprise bending sensor elements anyway such that the rotor blades do not have to be equipped with additional such sensor elements in order to be capable of performing the method according to the present invention. In particular, the bending sensor elements evaluate the bending moments in edgewise and flapwise direction to calculate resulting out-of-plane blade bending moments perpendicular to the rotor plane for any pitch angle. In such a case it is preferably provided, that these individual blade bending moments of the rotor blades are determined from signals derived from blade bending sensor elements being provided for each rotor blade. For example, it is preferably provided, that the oscillations of the rotor blades can be detected or evaluated by means of a frequency analysis of the, in particular edgewise, rotor blade sensor signals. These oscillations can be considered as a counter-oscillation in the determination, the calculation for example, of the individual pitch-offset procedure.

According to a preferred embodiment, the rotor of the wind turbine comprises three rotor blades. In this case the resulting rotor moment Myaw considering all of the three rotor blades, can be determined, calculated for example, by the following equation

This equation and its elements are described in more detail in connection with Figure 4 further below. Therefore, at this point, full reference is made to the disclosure of Figure 4 and the description thereof as well.

According to a preferred embodiment, when performing the method of reducing tower torsion oscillations by generating and applying counter moment Mcounter, it can be provided, that a resulting rotor torsion moment Mtilt is additionally determined and considered, which leads to loads in tilt-direction around a tilt-axis. In such a case the counter moment Mcounter is preferably determined such that it considers the effects of the resulting rotor torsion moment Mtilt as well. In this case the moment Mtilt is also incorporated in the individual pitch-offset of the at least one rotor blade.

In case that the rotor comprises three rotor blades, the resulting rotor torsion moment Mtilt can be determined by the following equation:

This equation and its elements are described in more detail in connection with Figure 4 further below. Therefore, at this point, full reference is made to the disclosure of Figure 4 and the description thereof as well.

According to a preferred embodiment, the counter moment Mcounter is determined from the resulting rotor moment Myaw. Preferably, the counter moment Mcounter is determined by filtering the resulting rotor moment Myaw. According to a preferred embodiment, the filtering procedure can be performed by a band pass filter.

According to a preferred embodiment, the counter moment Mcounter is determined according to the equation:

Mcounter = -Myaw (filtered).

In particular the counter moment Mcounter is determined such that it focusses on oscillation with tower torsional natural frequency (eigenfrequency) only. This filtering procedure is described in more detail in connection with Figure 5 further below. Therefore, at this point, full reference is made to the disclosure of Figure 5 and the description thereof as well.

According to a preferred embodiment, the counter moment Mcounter is individually partitioned to each rotor blade, whereby the partitioned counter torsion moment Mcounter for each rotor blade is applied via an individual pitch-offset of each rotor blade. According to a preferred embodiment, the rotor comprises three rotor blades. In this case the counter moment Mcounter is individually partitioned to each of the three rotor blades. The partitioning of the counter moment Mcounter is described in more detail in connection with the drawings further below. Therefore, at this point, full reference is also made to the disclosure of the detailed description of the drawings.

According to yet another preferred embodiment, the step of determining the counter moment Mcounter and/or the step of damping the resulting rotor moment Myaw by applying the counter moment Mcounter on the resulting rotor moment Myaw via the individual pitch-offset of the rotor blades is fine tuned by incorporating additional modules for controlling the wind turbine. This “fine tuning”-procedure is described in more detail in connection with the drawings further below. Therefore, at this point, full reference is also made to the disclosure of the detailed description of the drawings.

According to a preferred embodiment, the method according to the first aspect of the invention is performed by a device for setting the pitch of the rotor blades. Such a device is provided according to the following second aspect of the invention. Therefore, at this point, full reference is made to the disclosure of the second aspect of the invention as well.

According to the second aspect of the present invention the object is solved by a device for setting the pitch of the rotor blades of a rotor of a wind turbine, as also referred to as the setting-device.

With regard to the configuration, to the performance and to the function of the setting-device, full reference is also made to the general description of the invention further above, to the description of the first aspect of the invention as disclosed further above, to the description of the third aspect of the invention as disclosed further below as well as to the drawings and the description of the drawings following further below.

The setting-device comprises a first determination component, said first determination component being provided for determining a resulting rotor moment Myaw, which leads to torsion in yaw-direction around a yaw-axis.

Furthermore, the setting-device comprises a second determination component, said second determination component being provided for determining a counter moment Mcounter to the resulting rotor moment Myaw.

Moreover, the setting-device comprises an appliance, said appliance being provided for applying the counter moment Mcounter on the resulting rotor moment Myaw via an individual pitch-offset of the rotor blades.

The different determination components as well as the appliance can be configured as electronic devices, as logic devices, as software components or as combinations thereof. In a preferred embodiment, the determination components and the appliance are parts of a computing device, said computing device interacting with a pitch drive unit.

Furthermore, the setting-device comprises a pitch drive unit, which is provided for setting the pitch of the rotor blades in dependence on the counter moment Mcounter being applied to the resulting rotor moment Myaw. The pitch drive unit may comprise one or more pitch drives, each pitch drive being allocated to a respective rotor blade.

According to a preferred embodiment, the first determination component determines, calculates for example, the resulting rotor moment Myaw, by evaluating respective signals of sensor elements being allocated to the rotor blades for example. Based on this resulting rotor moment Myaw, the counter moment Mcounter is determined, calculated for example, in the second determination component. For example, this can be achieved by filtering the resulting rotor moment Myaw. In relation to the counter moment Mcounter, a suitable pitch-offset is determined by use of the appliance. Based on the determined pitch-offset, the pitch drives of the itch drive unit is actuated accordingly.

The setting-device according to the second aspect can be the pitch control device or can be part of the pitch control device of the wind turbine. In such a case the pitch control device can be extended by an additional function which considers, via the resulting rotor moment Myaw, the torsional oscillations of the tower, for example the coupled tower torsion/edgewise rotor blade oscillation. By means of this additional function, the resulting rotor moment Myaw and therefore the torsional oscillations of the tower of the wind turbine can be damped by adjusting the rotor blades via an individual pitch-offset of the rotor blades by adjusting the pitch, which is the angle of attack (the pitch-angle) of the rotor blades, in dependence on the counter moment Mcounter.

Preferably the setting-device comprises means for performing the method according to the first aspect of the invention.

According to the third aspect of the invention, the object is solved by a wind turbine.

With regard to the configuration, to the performance and to the function of the wind turbine, full reference is also made to the general description of the invention further above, to the description of the first and second aspects of the invention as disclosed further above, as well as to the drawings and the description of the drawings following further below.

The wind turbine which is preferably configured as described in detail further above, comprises a tower, a rotor being mounted at the top of the tower, said rotor comprising a number of rotor blades with adjustable pitch, and a device for setting the pitch of the rotor blades. The wind turbine is characterized in that the device for setting the pitch of the rotor blades is provided according to the second aspect of the invention. As an alternative and in case that the method according to the first aspect of the invention is not performed by the setting-device according to the second aspect of the invention, the wind turbine comprises a suitable device being provided for performing the method according to the first aspect of the present invention.

The present invention according to its three aspects has a number of advantages, which are particularly as follows:
- Reduction of loads and therefore, reduction of costs, in particular in connection with
the tower and the rotor blades;
- Reduction of blade moments, mainly edgewise loads;
- Reduction of tower torsional oscillations.

BRIEF DESCRIPTION OF DRAWINGS
The invention will now be explained in more detail with respect to exemplary embodiments with reference to the enclosed drawings, wherein:

Figure 1 shows a schematic view of a wind turbine;

Figure 2 shows a partial view of the wind turbine depicted in Figure 1;

Figure 3 depicts the course of the method of reducing tower torsion oscillations according to the invention;

Figure 4 schematically depicts the determination of the resulting rotor moment Myaw; and

Figure 5 shows the correlation between the resulting rotor moment Myaw and the tower torsion oscillations.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 and 2 depict a wind turbine 10 of the horizontal type with a tower 12 and a nacelle 11. Nacelle 11 is rotatable mounted to the top of tower 12. At its lower base end, the tower 12 is anchored to the ground by means of a foundation 13. Nacelle 11 incorporates a drive train 27, said drive train 27 being mounted inside nacelle 11 and being connected to a rotor 14. Rotor 14 comprises three rotor blades 16, 17, 18 which are mounted to a hub 15. Hub 15 of rotor 14 is connected to a drive shaft 28 of the drive train 27. The rotor blades 16, 17, 18 are adjustably mounted on the hub 15. This is realized by means of pitch drives 16.1, 17.1, 18.1, said pitch drives 16.1, 17.1, 18.1 being part of a pitch system. The pitch system controls the rotor speed to given set points. By means of the pitch drives 16.1, 17.1, 18.1, the rotor blades 16, 17, 18 may be moved about rotor blade axes such that the pitch 16.2, 17.2, 18.2 of the rotor blades 16, 17, 18 can be changed. This means that the pitch-angles of the rotor blades 16, 17, 18 can be changed such that the orientation of the rotor blades 16, 17, 18 can be varied.

The rotor 14 is rotatable connected to the drive train 27 via its rotational axis. The drive train 27 transmits the rotor speed to a generator device 20, where it is converted into electric energy. In order to transform the rotational energy of the rotor 14 into electric energy, the drive train 27 comprises a gear device 19. At its entrance side, the gear device 19 is connected to the rotor 14 via drive shaft 28, a slow running drive shaft for example. The rotor 14 is connected to said drive shaft 28 via its rotor hub 15. On its exit side, the gear device 19 comprises an output shaft 29, a fast running output shaft for example. The gear device 19 has the function to transform the low speed or low revolution and the high torque of the drive shaft 28 into a high speed or high revolution and a small torque of the output shaft 29. The gear device 19 is connected to the generator device 20. In particular the output shaft 20 of the gear device 19 is coupled to a generator shaft of the generator device 20. The generator device 20 generates electric energy from the rotational energy which is provided by the gear device 19. For this purpose, the generator device 20 preferably comprises a stator component and a rotor component, said rotor component being coupled to the generator shaft.

The wind turbine 10 depicted in Figures 1 and 2 may reach an overall height of more than 100 metres. Since the wind turbine 10 comprises a slender structure, and since the nacelle 11 including the rotor 14 with its rotor blades 16, 17, 18 is provided at the top of the tower 12, the wind turbine 10 provides a lot of contact surface for the wind. If the wind hits the wind turbine 10, the wind turbine 10, especially the tower 12, the rotor 14 with its rotor blades 16, 17, 18 and the nacelle 11 are exposed to vibrations, in particular to torsional oscillations. Such vibrations can have high amplitudes. However, these vibrations negatively influence the lifetime, the design and the performance of the wind turbine 10 and lead to damages.

The pitch system comprises a device 21 for setting the pitch of the rotor blades 16, 17, 18. This device 21 is referred to as the setting-device 21 hereinafter. First of all, setting-device 21 comprises a pitch drive unit comprising the pitch drives 16.1, 17.1, 18.1. Furthermore, setting device 21 comprises a computing device 22 including a first determination component 23 and a second determination component 24. First determination component 23 is provided for determining a resulting rotor moment Myaw, which leads to torsion in yaw-direction around a yaw-axis. Second determination component 24 is provided for determining a counter moment Mcounter to the resulting rotor moment Myaw. Furthermore, setting-device 21 comprises an appliance, said appliance being provided for applying the counter moment Mcounter on the resulting rotor moment Myaw via an individual pitch-offset of the rotor blades 16, 17, 18. Preferably, the aforementioned appliance is part of the computing device 22 as well. By means of the pitch drives 16.1, 17.2, 18.2, which are part of the pitch drive unit of the pitch system, the pitch 16.2, 17.2, 18.2 of the rotor blades 16, 17, 18 is adjusted in dependence on the counter moment Mcounter being applied to the resulting rotor moment Myaw. This will be explained in more detail with regard to Figures 3 to 6 in the following.

According to the present invention, different axes with different orientations are considered. One axis is the so-called yaw-axis 25. According to the present invention, the yaw-axis 25 corresponds in its alignment to the longitudinal axis 30 of the tower 12. In the embodiment, the yaw-axis 25 is a vertical axis. Another axis is the so-called tilt-axis 26, which is the y-axis and which is perpendicular to the X-axis 32. The X-axis 32, which is the roll axis, corresponds in its alignment to the axis 31 around which the rotor 14 rotates. In the embodiment, the tilt-axis 26 is a horizontal axis.

According to the present invention solutions are provided which are capable of reducing tower torsion oscillations in a wind turbine. A method of reducing tower torsion oscillations in accordance with the present invention is depicted in Figure 3, according to which the method comprises the following steps:
In a first step 100, a resulting rotor moment Myaw is determined, which leads to torsion in yaw-direction around yaw-axis 25. Preferred embodiments of how this resulting rotor moment Myaw is determined, calculated for example, are described in more detail in connection with Figure 4 further below.

In the next, second step 200, a counter moment Mcounter, is determined, which counter-acts to the resulting rotor moment Myaw. Preferred embodiments of how this counter moment Mcounter is determined, calculated for example, are described in more detail in connection with Figure 5 further below.

In the next, third step 300, the counter moment Mcounter gets partitioned inti the individual contributions of each rotor blade. During the final step 400, the transformation from counter moment to the pitch angle takes place, including any fine-tuning issues. According to the present invention, this is achieved via an individual pitch-offset of at least one rotor blade 16, 17, 18, in particular of all rotor blades 16, 17, 18. This particularly means that the counter moment Mcounter is brought in into the rotor 14 via an adjustment of the at least one rotor blade 16, 17, 18 during the pitch-off procedure.

In the following and in particular with reference to Figures 4 and 5, it will now be explained in detail, how torsion oscillations of the tower 12 of the wind turbine 10 can be reduced.

The present invention provides a solution for performing a tower torsion balancing via an individual pitching of the rotor blades. Tall and slim towers tend to show torsional oscillations which are associated to a rotational movement of the whole rotor around yaw-axis 25. This excites rotor blade oscillations which significantly increase blade loads, mainly edgewise. The resulting rotor moment Myaw around yaw-axis 25 can be derived from all three rotor blades, in particular from blade bending sensors being allocated thereto, via vectorial summation of the individual blade bending moments (Figure 4). The resulting rotor moment Myaw gets damped with counter-acting blade moments via individual pitching (Figure 5). This can reduce the combined torsional oscillation of the tower 12 and the rotor and leads to the effect of a reduction of blade moments, mainly of edgewise loads, as well of a reduction of tower torsional oscillations.

In a first step, as also shown in Figure 3 as step 100, the resulting rotor moment Myaw around yaw-axis 25, and additionally a resulting rotor torsion moment Mtilt around tilt-axis 26 is determined. This will now be explained in detail in connection with Figure 4.

With regard to Figure 4, the mathematical relations are explained in more detail. Figure 4 depicts a wind turbine 10 with a tower 12 and a rotor 14, said rotor 14 having three rotor blades 1, 2, 3. For mathematical reasons, the rotor blades have been renumbered in comparison to Figures 1 and 2. In comparison to Figures 1 and 2, rotor blade 1 of Figure 4 corresponds to rotor blade 16 in Figures 1 and 2. In similar way, rotor blade 2 corresponds to rotor blade 17 and rotor blade 3 corresponds to rotor blade 18.

Figure 4 shows how the resulting rotor moment Myaw, which leads to torsion in yaw-direction 25.1 around yaw-axis 25, and the resulting rotor torsion moment Mtilt, which leads to torsion in tilt-direction 26.1 around tilt-axis 26, are determined.

The resulting rotor moment Myaw around yaw-axis 25 is derived from blade bending sensor signals, said bending sensors being allocated to rotor blades 1, 2 and 3.

In Figure 4, MBF1, MBF2 and MBF3 represent flat, out-of-plane moments of rotor blades 1, 2 and 3. The symbol ??represents the rotor position. 0° means that rotor blade 1 points to the bottom.

Each rotor blade moment MBF is split up in a yaw component and in a tilt component. Therefore, moment MBF1 is split up in horizontal component M1horz and in vertical component M1vert. Moments MBF2 and MBF3 are split up in the same way.

Thus, the vertical and the horizontal components of the different moments are as follows:

MBF1

MBF2

MBF3

The resulting rotor moment Myaw, which leads to torsion in the yaw-direction 25.1, can be calculated from the following equation:

The resulting rotor moment Mtilt, which leads to torsion around the tilt-axis 26, can be calculated from the following equation:

The inventors have found out that a strong correlation between the bending moments of the rotor blades and the tower torsion moments exists, in particular both for the amplitude and for the phase. The inventors have verified that the resulting rotor moment Myaw is a fingerprint of the tower torsion. This is shown in Figure 6.

Figure 5 depicts a number of different diagrams showing the correlation of the resulting rotor moment Myaw, which has been determined according to the principles described in connection with Figure 4, versus the tower oscillation which is represented by the tower top torsion moment. All different diagrams show the respective curves for the moments Myaw and tower top torsion moment in comparison to each other.

Next, an appropriate counter moment Mcounter is determined, which is subsequently applied via an individual pitch-offset of the rotor blades. The determination of counter moment Mcounter will now be explained in detail in connection with Figure 5. According to a preferred embodiment, counter moment Mcounter is determined, calculated for example, by filtering of the resulting rotor moment Myaw. Thereby, the balancing of the resulting rotor moment Myaw gets focussed on the oscillation with natural frequency (eigenfrequency) of the tower torsion only. Filtering is performed by use of notch-filtering.

The tower oscillations are exactly determined with regard to the phase position and to the amplitude. The applied oscillations are subtracted out.

The torsional oscillation gets damped by adding to the resulting rotor moment Myaw a counter moment Mcounter which is

Mcounter = -Myaw (notch-filtered)

The counter moment Mcounter is realized via an additional pitch-offset, which will be described next.

According to the preferred embodiment, the counter moment Mcounter is partitioned to each rotor bade individually via an additional pitch-offset. Preferably the desired counter moment Mcounter is realized by additional rotor blade bending moments ?MF1,??MF2,?MF3, which must fulfil the following boundary conditions:
I

?MF1 * sin(f1) + ?MF2 * sin(f2) + ?MF3 * sin(f3) = Mcounter

The sum of all yaw-components shall result in the required counter moment Mcounter.

II

The resulting counter moment around the tilt-axis shall be untouched:

?MF1 * cos(f1) + ?MF2 * cos(f2) + ?MF3 * cos(f3) = 0

III

Mleft = -Mright,

Where Mleft contains the additional blade bending moments with phi=[0 180[ and Mright with phi=[180 360[. Thus, the counter moment Mcounter shall be distributed equally to left and right rotor side.

After solving the system of equations ?MF1, ?MF2, ?MF3, the additional individual pitch offset ? of blade i is proportional to ?MFi

wherein f in [rad/Nm] is a free parameter to adjust and fine tune the algorithm.

Since the system of equation must be solved for all three blades the one individual pitch offset is not independent from the other two.

When the counter moment Mcounter is applied via an individual pitch-offset, additional necessary modules can be considered as well. During such a procedure, which can be described as a fine tuning-procedure, additional necessary modules can be considered. For example, an incorporation of time delays due to signal filtering, pitch movement limitations, aerodynamic reactions and the like can be realized or considered. This can for example result in a phase shift corrected counter moment Mcounter, which can be, according to a preferred embodiment, be shifted about a halve period in a time-delayed pitch-offset, or the like. The proportional factor f can be fine-tuned according to the specific turbine operation, for example to the wind condition and range of power.

List of Reference Numerals

10 Wind turbine
11 Nacelle
12 Tower
13 Foundation
14 Rotor
15 Hub
16 Rotor blade
16.1 Pitch drive
16.2 Pitch
17 Rotor blade
17.1 Pitch drive
17.2 Pitch
18 Rotor blade
18.1 Pitch drive
18.2 Pitch
19 Gear device
20 Generator device
21 Device for setting the pitch of the rotor blades (setting-device)
22 Computing device
23 First determination component
24 Second determination component
25 Yaw-axis
25.1 Yaw-direction
26 Tilt-axis
26.1 Tilt-direction
27 Drive train
28 Drive shaft
29 Output shaft
30 Longitudinal axis of the tower
31 Axis around which the rotor rotates
32 X-axis (Roll axis)

100 First step of the method
200 Second step of the method
300 Third step of the method
400 Fourth step of the method

,CLAIMS:WE CLAIM:

1. A method of reducing tower torsion oscillations in a wind turbine (10), said wind turbine (10) comprising a tower (12) and a rotor (14) being mounted at the top of the tower (12), said rotor (14) comprising a number of rotor blades (16, 17, 18) with adjustable pitch (16.2, 17.2, 18.2), said method being characterized by the following steps:
- Determining a resulting rotor moment Myaw, which leads to torsion in yaw direction (25.1) around yaw-axis (25);
- Determining a counter moment Mcounter, which counter-acts to the resulting rotor moment Myaw;
- Damping the resulting rotor moment Myaw by applying the counter moment Mcounter on the resulting rotor moment Myaw via an individual pitch-offset of at least one rotor blade (16, 17, 18).

2. The method according to claim 1, characterized in that the resulting rotor moment Myaw is generated from individual blade bending moments of the rotor blades (16, 17, 18), in particular via vector summation of the individual blade bending moments.

3. The method according to claim 2, characterized in that the individual blade bending moments of the rotor blades (16, 17, 18) are determined from signals derived from blade bending sensor elements being provided for each rotor blade (16, 17, 18).

4. The method according to anyone of claims 1 to 3, characterized in that the rotor (14) comprises three rotor blades (16, 17, 18) and that the resulting rotor moment Myaw is determined by the following equation:

5. The method according to anyone of claims 1 to 4, characterized in that a resulting rotor torsion moment Mtilt is determined, which leads to torsion in tilt-direction (26.1) around a tilt-axis (26).

6. The method according to claim 5, characterized in that the rotor comprises three rotor blades (16, 17, 18) and that the resulting rotor torsion moment Mtilt is determined by the following equation:

7. The method according to anyone of claims 1 to 6, characterized in that the counter moment Mcounter is determined by filtering the resulting rotor moment Myaw, in particular by notch-filtering the resulting rotor moment Myaw.

8. The method according to claim 7, characterized in that the counter moment Mcounter is determined according to the equation Mcounter = -Myaw (filtered), and that in particular the counter rotor torsion moment Mcounter is determined such that it focusses on oscillation with tower torsional natural frequency only.

9. The method according to anyone of claims 1 to 8, characterized in that the counter moment Mcounter is individually partitioned to each rotor blade (16, 17, 18) and that the partitioned counter moment Mcounter for each rotor blade (16, 17, 18) is applied via the individual pitch-offset of each rotor blade (16, 17, 18).

10. The method according to anyone of claims 1 to 9, characterized in that the step of determining the counter moment Mcounter and/or the step of damping the resulting rotor moment Myaw by applying the counter moment Mcounter on the resulting rotor moment Myaw via the individual pitch-offset of the rotor blades (16, 17, 18) is fine tuned by incorporating additional modules for controlling the wind turbine (10).

11. The method according to anyone of claims 1 to 10, characterized in that this method is performed by a device (21) for setting the pitch (16.2, 17.2, 18.2) of the rotor blades (16, 17, 18).

12. A device (21) for setting the pitch (16.2, 17.2, 18.2) of the rotor blades (16, 17, 18) of a rotor (14) of a wind turbine (10), said device (21) comprising a first determination component (23), said first determination component (23) being provided for determining a resulting rotor moment Myaw, which leads to torsion in yaw-direction (25.1) around a yaw-axis (25), a second determination component (24), said second determination component (24) being provided for determining a counter moment Mcounter to the resulting rotor moment Myaw, an appliance, said appliance being provided for applying the counter moment Mcounter on the resulting rotor moment Myaw via an individual pitch-offset of the rotor blades (16, 17, 18), and a pitch drive unit, said pitch drive unit being provided for setting the pitch (16.2, 17.2, 18.2) of the rotor blades (16, 17, 18) in dependence on the counter moment Mcounter being applied to the resulting rotor moment Myaw.

13. The device according to claim 12, characterized in that the device (21) comprises means for performing the method according to anyone of claims 1 to 10.

14. A wind turbine (10), said wind turbine (10) comprising a tower (12), a rotor (14) being mounted at the top of the tower (12), said rotor (14) comprising a number of rotor blades (16, 17, 18) with adjustable pitch (16.2, 17.2, 18.2), and a device (21) for setting the pitch (16.2, 17.2, 18.2) of the rotor blades (16, 17, 18), characterized in that the device (21) for setting the pitch of the rotor blades is provided according to anyone of claims 12 or 13 or that the wind turbine (10) comprises a device being provided for performing the method according to anyone of claims 1 to 10.