WO2021121503A1

WO2021121503A1 - Damping control of a wind turbine with hinged blades

Info

Publication number: WO2021121503A1
Application number: PCT/DK2020/050357
Authority: WO
Inventors: Thomas S. Bjertrup Nielsen; Søren DALSGAARD
Original assignee: Vestas Wind Systems A/S
Priority date: 2019-12-20
Filing date: 2020-12-14
Publication date: 2021-06-24
Also published as: EP4077918A1

Abstract

The invention relates to method for damping tower oscillations of a wind turbine with a variable rotor area, the wind turbine comprises a tower, a rotor with one or more rotor blades which are arranged hinged at an adjustable pivot angle, where the pivot angle is adjustable dependent on a variable pivot force or pivot moment provided by a pivot actuator, the method comprises - detecting a tower oscillation, and - dependent on the detected tower oscillation, decreasing or increasing the pivot angle to achieve a reduction of the detected oscillation.

Description

DAMPING CONTROL OF A WIND TURBINE WITH HINGED BLADES

FIELD OF THE INVETION

The invention relates to methods for controlling a wind turbine having a rotor wherein blades are hinged to provide a variable rotor area, particularly to damping structural oscillations by controlling the rotor blades.

BACKGROUND OF THE INVENTION

Wind turbines provided with wind turbine blades which are connected to a blade carrying structure via hinges allows a pivot angle defined between the wind turbine blades and the blade carrying structure to be varied. Thereby, the diameter of the wind turbine rotor and consequently the rotor area can be varied.

Accordingly, the rotor area can be increased at low wind speeds to increase and optimize power production and decreased at high wind speeds where the wind energy may be sufficient for production of a nominal wind turbine power so as to decrease the rotor thrust.

The flexibility of the wind turbine to adapt to different wind speeds implies that the same type of wind turbines with the same rotor type can be used at different locations with different wind conditions.

Due to the limited stiffness of the tower structure, the wind turbines can be excited by the wind to oscillate in at least fore-aft and lateral directions.

While such oscillations are substantially inevitable, large oscillations may reduce the lifetime of wind turbine components or could lead to an instantaneous failure. Accordingly, there is a need for methods which can provide damping of such oscillations.

SUMMARY

It is an object of the invention to improve turbines having variable rotor diameter such as improving the control of the pivot angle of the rotor blades. Particularly, it is an object to improve control of such wind turbines in order to be able to reduce or provide damping of fore-aft and lateral oscillations. In a first aspect of the invention there is provided a method for damping tower oscillations of a wind turbine with a variable rotor area, the wind turbine comprises a tower, a rotor with one or more rotor blades which are arranged hinged at an adjustable pivot angle, where the pivot angle is adjustable dependent on a variable pivot force or pivot moment provided by a pivot actuator, the method comprises

- detecting a tower oscillation, and

- dependent on the detected tower oscillation, decreasing or increasing the pivot angle to achieve a reduction of the detected oscillation.

Due the hinged rotor blades, the rotor area can be adjusted. For example, the rotor area can be increased at low wind speeds to maximize power production. Advantageously, the capability of adjusting the rotor area can be utilized for damping rotor oscillations by varying the rotor area via the pivot angle adjustments to increase damping in a given direction, e.g. increasing the rotor area to increase damping in the fore-aft direction and decreasing the rotor area to increase damping in the lateral direction. According to an embodiment, detecting the tower oscillation comprises determining an oscillation direction value indicative of the oscillation direction of the tower oscillation. In a further embodiment, a determination to decrease or increase the pivot angle is dependent on the oscillation direction value. Advantageously, the oscillation direction may provide precise information about the direction of the most significant oscillation, so that this information can be used to determine how the pivot angle should be adjusted.

According to an embodiment, a determination to decrease or increase the pivot angle is dependent on a correlation between the oscillation direction value with predetermined relationships between oscillation direction values and pivot angle changes. In a further embodiment, a magnitude of the decrease or increase of the pivot angle is dependent on the correlation. Advantageously, predetermined information between oscillation direction values pivot angle changes can be used for determining variations of the pivot angle to provide a simple but reliable determination of the pivot angle.

According to an embodiment, detecting the tower oscillation comprises determining an oscillation magnitude, wherein the method further comprises determining a magnitude of the decrease or the increase of the pivot angle dependent on the oscillation magnitude.

Advantageously, the magnitude of the changes of the pivot angle can be adapted dependent on the oscillation amplitude, e.g. so that large oscillation amplitudes are counteracted with large pivot angle variations.

According to an embodiment, the decrease or the increase of the pivot angle is performed so that a mean value of the pivot angle, over a period of time, is decreased or increased, respectively.

The properties of the hinged blades provides advantageous possibilities for damping tower oscillations by simply changing the mean value of the pivot angle. In a further embodiment, the mean value of the pivot angle is controllable by means of adjusting the pivot force or equivalently the pivot moment.

For example, the mean pivot angle may be increased to provide increased damping in the lateral direction. Oppositely, the mean pivot angle may be decreased to provide increased damping in the fore-aft direction.

According to an embodiment, a decision to start decreasing or increasing the pivot angle such as the mean pivot angle is dependent on an actual pivot angle. In a further embodiment, decreasing of the pivot angle such as the mean pivot angle is conditioned on that the actual pivot angle is above an upper pivot angle threshold, and increasing of the pivot angle or mean pivot angle is conditioned on that the actual pivot angle is below a lower pivot angle threshold. Advantageously, since the damping effect of the changing the pivot angle depends on the actual pivot angle, a possible change of the pivot angle or mean pivot angle can be based on the actual pivot angle.

According to an embodiment, decreasing or increasing of the pivot angle or mean pivot angle is restricted to be performed within a maximum period. Advantageously, by limiting the period where pivot angle control is used for damping purposes mechanical loading of the wind turbine can be limited.

In a further embodiment, decreasing or increasing of the pivot angle or mean pivot angle is restricted to be performed only when a measured or estimated load of the wind turbine, such as a tower load, are below or above a maximum load threshold.

According to an embodiment, decreasing or increasing the pivot angle or mean pivot angle comprises changing the pivot angle by a predetermined amount. Advantageously, be changing the actual pivot angle by a predetermined amount, e.g. to provide a predetermined change of the mean pivot angle, predetermined damping effects may be obtained.

According to an embodiment, the method comprises adjusting the pivot angle by adjusting the pivot force or the pivot moment.

According to an embodiment, the method further comprises adjusting the pivot force or pivot moment to provide a variation of the pivot angle to impose an oscillating force onto the tower to counteract the tower oscillation.

According to an embodiment, the variation of the pivot angle is superposed on the decreased or increased mean value of the pivot angle. Advantageously, the combination of a changed mean pivot angle and cyclic variation of the pivot angle can improve the damping.

According to an embodiment, the variation of the pivot angle is applied with a same phase or substantially the same phase to all rotor blades. The synchronous variation of the pivot angle provides damping of at least fore-aft oscillations. The collective, i.e. the synchronous, pivot angle variation may be in phase with the tower oscillation velocity in order to increase the effective aerodynamic damping. Similarly, the collective, i.e. the synchronous, pivot force variation may be in phase with the tower oscillation acceleration.

According to an embodiment, the pivot angle variations are determined dependent on a rotor angle and dependent on a phase of the determined oscillation so that the pivot angle of one or more of the rotor blades varies as a function of the rotor angle. Advantageously, by adjusting the pivot angle cyclic and dependent on the rotor angle, damping of lateral, i.e. side-to-side, motion can be damped. For example, the variation of the pivot angle may be applied as variations with different phases to all rotor blades.

A second aspect of the invention relates to damping control system arranged to perform the steps according to the first aspect.

A third aspect of the invention relates to a wind turbine comprising a rotor with a variable rotor area, where the rotor comprises one or more rotor blades which are arranged hinged at an adjustable pivot angle, where the variable rotor area in a plane perpendicular to a rotor shaft direction depends on the pivot angle, and where the pivot angle is adjustable dependent on a variable pivot force provided by a pivot actuator, and where the wind turbine comprises the damping control system according to the second aspect.

A fourth aspect of the invention relates to a computer program product comprising software code adapted to control a wind power plant when executed on a data processing system, the computer program product being adapted to perform the method of the first aspect.

In general, the various aspects and embodiments of the invention may be combined and coupled in any way possible within the scope of the invention.

These and other aspects, features and/or advantages of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which Figs. 1 and 2 show a wind turbine comprising hinged rotor blades,

Fig. 3 shows a detailed view of a blade hinged to the arm of the blade carrying structure of the rotor,

Fig. 4A shows a control system for determining a damping control signal, and Fig. 4B shows a relationship between pivot angle and damping in lateral and fore- aft directions.

DESCRIPTION OF EMBODIMENTS

Figures 1 and 2 show an example of a wind turbine 100 (WTG) comprising a tower 101 and a rotor 102 with at least one rotor blade 103, such as three blades. Fig. 1 shows a front view with the blades facing the wind and Fig. 2 shows a side view seen perpendicular to the wind direction 110. The blades 103 are connected with the hub 105 which is arranged to rotate with the blades. The hub 105 comprises a blade carrying structure 106 which may be configured as a structure with arms, one per blade, extending radially relative to the main shaft axis of the hub to end- portions of the arms. The rotation axis of the main shaft axis is indicated with reference 111. The blades 103 are connected to the blade carrying structure 106, such as the arms thereof, via a hinge 108.

The rotor is connected to a nacelle 104 which is mounted on top of the tower 101 and is adapted to drive a generator situated inside the nacelle via a drive train comprising the main shaft axis 111. The rotor 102 is rotatable by action of the wind. The wind induced rotational energy of the rotor blades 103 is transferred via a shaft to the generator. Thus, the wind turbine 100 is capable of converting kinetic energy of the wind into mechanical energy by means of the rotor blades and, subsequently, into electric power by means of the generator. The generator is connected with a power converter, such as a power converter configured with a generator side converter and a line side converter where the generator side converter converts the generator AC power into DC power and the grid side converter converts the DC power into an AC power for injection into the power grid. The generator and the power converter is part of the power generating system of the wind turbine.

The wind turbine 100 is configured so that in a normal power producing operation, the rotor 102 is arranged on the lee side of the tower 101, i.e. as illustrated with the wind direction 110, the rotor is located to the right of the tower 101.

The blades may be hinged at a location between an outer blade tip 113 and an inner blade tip 114 so that the blade 103 comprises an inner blade portion 103a extending between the hinge location and the inner blade tip 114 and an outer blade portion 103b extending between the hinge location and the outer blade tip 113. During normal operation, the inner blade portion 103a extends from the hinge location towards the main shaft axis and the outer blade portion 103b extends outwards away from the main shaft axis, at least for a range of pivot angles. As is seen in Fig. 3, the inner blade portion 103a extends towards the main shaft axis 111 for pivot angles from 0 to 80 degrees, assuming that the acute angle between the longitudinal extensions of the inner and outer blade portions is 10 degrees. At the 90 degrees pivot angle, the inner blade portion 103a points away from the main shaft axis 111.

Due to the hinged connection, the wind turbine blades 103 are able to perform pivot movement relative to the blade carrying structure 106. The pivot angle a is defined as the angle between the longitudinal axis of the outer blade portion 103b axis and a plane normal to the main shaft axis. A pivot angle of 0 degrees means that the outer blade is normal to the main shaft axis and maximal rotor area occurs at this angle.

The rotor area is defined as the area within the outer blade tips 113 in a plane perpendicular to the main shaft axis. The actual swept area swept by the rotor blades is the area between the inner and outer blades tips 113, 114 in a plane perpendicular to the main shaft axis.

The rotor area varies as a function of pivot angle in such a manner that the rotor area is at a maximum when the pivot angle is at a minimum, and at a minimum when the pivot angle is at a maximum. The wind turbine 100 may be excited to oscillate via resonant tower bending. The resonant bending can be in any directions in a plane perpendicular or substantially perpendicular to the longitudinal direction of the tower. For example, the direction of the tower oscillation may be in a lateral direction 191 perpendicular to the main shaft axis 111 or in fore-aft direction 291 parallel to the main shaft axis 111. The tower may oscillate in more than one direction, e.g. so that the tower top oscillates in circular, elliptic or other non-linear paths. Fig. 3 shows a more detailed view of one arm of the blade carrying structure 106 with the blade 103 hinged to the arm.

The rotor 102 is designed to carry blade loads through the pivot hinge 108 and the pivot actuator 301 to the arm structure 106. This design allows the blades 103 to pivot around the hinge axis.

As illustrated, the pivot actuator 301 may be hydraulic actuator such as a hydraulic cylinder. For example as illustrated in Fig. 3, the position of the piston in the hydraulic cylinder is mechanically connected with the inner blade portion 103a, e.g. near the inner blade tip 114. The pivot actuator or the pivot system comprising the pivot actuator and the mechanical connection may be configured as an elastic or flexible connection so that the resulting pivot angle a is partly determined by the pivot force/moment provided by the pivot actuator 301 and partly by the force acting on the rotor blade 103 such as the aerodynamic forces. The mechanical connection may comprise an elastic member 302 such as a spring. Alternatively, the elastic property of the pivot actuator may be achieved by controlling the position of the piston dependent on a measured piston force, e.g. so that the position is controlled according to Hookes law. Fig. 3 illustrates the orientation of the outer blade portion 103a for different wind levels.

The pivot angle a can be adjusted by a variable pivot force F or variable pivot moment M provided by a pivot actuator 301. Thus, the pivot actuator 301 may be a force actuator arranged to generate a displacement via an applied force, or may be a torque actuator arranged to generate an angular displacement via an applied torque. The pivot actuator 301 may be configured to be able to generate a desired pivot force F or pivot moment M. For example, the pivot actuator may comprise a feed-back control system arranged to control the pivot actuator to generate the desired pivot force or pivot moment. Herein, the pivot force F and pivot moment M are equivalent and the pivot actuator may be configured to provide a desired force or equivalently a desired moment. The relationship between the pivot force and the pivot moment is given by the distance between the hinge where the moment acts or is applied and a location on the inner blade portion 103a where the pivot force acts or is applied.

As will be clear from the description, adjusting the pivot angle a by use of the pivot actuator does not necessarily mean that the pivot angle a is controlled to approach a desired pivot angle. Adjusting the pivot force merely means that the actual pivot angle can be affected by the pivot force, but where the resulting pivot angle depends on a force equilibrium between the pivot actuator force generated by the pivot actuator 301, a wind load force generated due to the rotor thrust and elastic properties of the pivot actuator. This is particularly the case when the pivot system or pivot actuator is configured with an elastic/flexible connection.

The rotor thrust is the load on the rotor 102 generated by the incoming wind and dependent on the aerodynamic properties of the blades 103.

Thus, in general the resulting pivot angle is obtained dependent on a balance between at least the generated pivot force and a wind load force generated in response to the wind load on the rotor 102. Other forces generated due to the elastic properties of the pivot actuator, centrifugal forces and/or aerodynamic forces are also included in the equilibrium and thereby affects the resulting pivot angle a.

For example, with a given set-point for the actuator force, the force equilibrium implies that an increased wind speed and thereby increased wind thrust leads to an increase of the pivot angle a. This has the advantage that the rotor area may decrease in response to a wind gust. Additionally, centrifugal forces and/or aerodynamic forces acting on the wind turbine blades 103 cause the wind turbine blades to pivot towards larger pivot angles a for increasing wind speeds. Accordingly, at any given wind speed, the wind turbine blades will find an equilibrium pivot angle which balances the various forces acting on the wind turbine blades. The higher the wind speed, the larger the equilibrium pivot angle will be.

Fig. 4A shows a damping control system 400, in short control system 400, for damping tower oscillations of a wind turbine 100. The control system 400 generates a control signal 401 for controlling or modifying the control of the pivot actuator 300. The control system 400 receives a tower oscillation signal 402 obtained based on monitoring and detecting the tower oscillations. The tower oscillation signal may be a signal indicating a magnitude of the oscillation. For example, the tower oscillation signal may be, or be based on, an oscillation power signal, an acceleration signal from an accelerometer mounted on the tower, or tower state, e.g. position, velocity or acceleration or a combination thereof. The tower oscillation signal could also be based on determined variations in the power output from the power generator or could be based on measured variations in the pivot angle a.

The damping control system 400 may be comprised by a wind turbine control system 411 or arranged to operate with the wind turbine control system by providing the control signal 401 to the wind turbine control system. The wind turbine control system 411 comprises functions for controlling the pivot angle a dependent on e.g. wind speeds and power references and is configured to control the pivot angle dependent on the damping control signal 401.

The tower oscillation signal 402 need not specify a direction of tower oscillation. For example, the tower oscillation signal 402 may be generated or be non-zero when a magnitude of the tower oscillation signal 402 exceeds a given threshold, or the control system 400 may be configured to compare the tower oscillation signal 402 with a threshold. Accordingly, the control signal 401 may only be generated when the tower oscillation signal 402 exceeds a given threshold or is non-zero. The control signal 401 generated in response to the tower oscillation signal 402 instructs the pivot actuator 300 to increase or decrease the pivot angle a.

The control signal 401 may be input to another main pivot controller which is responsible for controlling the pivot angle to achieve a desired power production, the control signal may be combined with other pivot control signals or the control signal 401 may be used in other combinations to achieve the desired increase or decrease the pivot angle a.

The control signal 401 may be configured so that the value thereof corresponds to the desired increase or decrease of the pivot angle a at a given point in time. Accordingly, the control signal 401 may be a time-varying signal or a signal which is constant or substantially constant, at least in a period of time.

The desired increase or decrease of the pivot angle a may be in the form of cyclic alternating increases and decreases of the pivot angle a, e.g. in the form of a sinusoidal variation of the angle a or other time varying signals such as a square wave signal. The cyclic variation of the pivot angle may include a constant off-set pivot angle a0 so that the mean value of the cyclic variation is non-zero. Alternatively, the mean value of the cyclic variation of the pivot angle may be zero or substantially zero.

The control system 400 or other control system adjusts the pivot force F to provide the desired variation of the pivot angle. The variation of the pivot angle may be determined so as to impose an oscillating force onto the tower which counteracts the tower oscillation. Accordingly, when the tower oscillation signal 402 includes information about the tower oscillation frequency and phase, the control signal 401 can the determined to generate variations in the pivot angle to achieve a reduction of the detected oscillation.

The control signal 401 may be determined to generate variations of the pivot angle so that the pivot angle variations of all blades 103 of the rotor 102 has the same phase or substantially the same phase, i.e. so that the pivot angle variation is substantially the same, as a function of time, for all rotor blades 103. Further, the amplitude of all pivot angle variations may be equal or substantially equal for different rotor blades 103 - and as a function time. The synchronously varying pivot angles, which may be obtained dependent on a phase of the oscillation signal 402, will counteract fore-aft oscillations. That is, by using the phase of the oscillation signal 402, which at least includes a fore-aft oscillation component, the phase of the control signal 401 can be determined to achieve a damping of the fore-aft oscillations.

The principle for damping fore-aft motions comprises: When the tower with nacelle 104 and rotor 102 moves upwind the pivot angle a is decreased by increasing the pivot force F to increase the thrust force, while the pivot angle a is increased by decreased pivot force F to decrease the thrust force when the tower with nacelle and rotor moves downwind. The variation in pivot force is performed cyclic and simultaneously for all blades 103, where frequency of the cyclic variation may correspond to the frequency of the oscillation, e.g. by following a sinusoid force variation curve where the phase is adapted to so that the force variation generated in response to the variation of the pivot angle counteracts the direction of the motion of the tower. However the pivot force F can also be according to other curves, such as a stepped sinusoidal curve or a square wave curve.

In an example, due to the hinged blades 103, if the tower moves forward, the pivot angle should be increased to move the center of mass of a blade in the same forward direction, since the reaction force is in opposite direction to the direction of the center of mass movement. Accordingly, the pivot angle should may be changed in phase, i.e. with zero-phase, relative to the fore-aft tower movement, to provide fore-aft tower damping. This applies when the center of mass is located on the outer blade portion 103b.

In an example, the cyclic variation of the pivot angle for damping tower movements may be used during shut down of the wind turbine. During a shut down the blades pivot towards a barrel position, i.e. where the pivot angle a is increased towards a maximum. As the blades pivot towards larger pivot angles a, the thrust on the rotor is significant reduced which may result in larger tower movements, particularly fore-aft movements. In such a case the pivot angle control may be used to damp the tower movements by applying a cyclic variation of the pivot angle a on top of the ramping of the pivot angle towards a maximum, or shut-down, pivot angle a.

Alternatively, the control signal 401 may be determined to generate variations of the pivot angle so that the pivot angle variations of different blades 103 has different phases. In this case, the pivot angle variations and the responsible control signal 401 are determined dependent on a rotor angle, i.e. the azimuth of the rotor 102, and dependent on a phase of the determined oscillation. By coordinating the pivot angle variations as a function of the rotor angle, the blades 102 can be controlled via the responsible pivot force variations to generate force variations acting in the lateral direction 191 to counteract lateral oscillations.

The following example illustrates the principle for damping lateral motion of the tower: The wind turbine is seen from upwind. When the nacelle moves towards the right side, the blade that is found at about 12 to 4 o'clock, more precise at 1 to 3 o'clock receives an increased pivot force F resulting in a decreased pivot angle a, a larger blade area in the wind, a larger local relative wind speed and as such a force from the wind that counteracts the lateral tower movement towards the right. Additionally or alternatively, the blade that is found at 6 to 10 o'clock, more precise at 7 to 9 o'clock, receives a decreased pivot force F resulting in an increased pivot angle a, a smaller blade area in the wind, a lower local relative wind speed and as such a lower force from the wind that by its reduction counteracts the lateral tower movement towards the right. Accordingly, the damping of lateral motion of the tower may be achieved by cyclic varying the pivot angle of one or more blades 103 of the rotor 102.

Alternatively or additionally, the pivot angle a is increased or decreased so that a mean value of the pivot angle is decreased or increased, over a period of time, as compared to the mean value of the pivot angle prior to that period of time. For example, the mean value of the pivot angle may be increased based on a applying a decreased pivot force F over a period of time. For example, the pivot force may be decreased or increased with a constant amount over a period of time, such as over a period of time in the range from 1-100 seconds or in a range from 1-10 minutes. Decreasing the pivot angle, leads to an increase of the rotor area and thereby an increased damping in the fore-aft direction 291. Increasing the pivot angle leads to a decrease of the rotor area, but an increased projected length of the rotor blades along the fore-aft direction 291, and thereby an increased damping in the lateral direction 191.

Fig. 4B shows the relation between pivot angle a and the lateral damping 452 and the fore-aft damping 451. An intermediate pivot angle such as a pivot angle around 45 degrees provides good damping properties in both the lateral and the fore-aft direction as compared to the more extreme pivot angles where there is less damping in one of the directions.

The change of a mean value of the pivot angle may be applied in addition to the cyclic variation of the pivot angle or as an alternative to the cyclic variation of the pivot angle. Accordingly, the cyclic variation of the pivot angle may be superposed on the decreased or increased mean pivot angle.

Thus, the mean pivot angle a can be changed dependent on the direction of the oscillations, e.g. whether the monitored tower oscillation signal 402 indicates fore- aft or lateral oscillations.

For example, during power production in relative low wind speeds the pivot angle a is relatively low in order to maximize the rotor area. In this case, tower lateral movements or accelerations may be high. In order to dampen the lateral oscillations, the mean pivot angle a is increased to increase the lateral damping. The increase of the mean pivot angle may be upheld in a given period, e.g. up to 1-10 minutes or until the lateral oscillation amplitudes or acceleration values have decreased below a certain threshold.

In another example, the wind turbine produce power at high wind speeds, e.g. above the nominal wind speed, with relative high pivot angles a, in order not to exceed the nominal power capacity of the wind turbine. The high pivot angles and, thereby, the low rotor area provides only a low damping of the tower movement in the fore-aft direction 291. If the fore-aft tower oscillation accelerations becomes too high, the mean pivot angle a can be reduced to increase the damping in the tower fore-aft direction. Again, the reduction of mean pivot angle a can be upheld in a predetermined period of time or until the fore-aft oscillation amplitude have decreased to an acceptable level. Clearly, the decrease of the mean pivot angle increases the rotor area and, thereby, the wind induced loads. Accordingly, a time limit of the decreased mean pivot angle may be set during high wind condition to reduce the impact of the increased structural loads. Thus, in general, a change of the pivot angle or mean pivot angle may be restricted to be performed within a maximum period. As an alternative or in addition to the time restriction, decreases of the mean pivot angle in high wind speeds, may be invoked dependent on direct or indirect predictions of wind induced loads and only invoked as long as the wind induced loads do not exceed certain threshold maximum loads.

Since the available effect of increased damping depends on the actual pivot angle, i.e. if the pivot angle is already low only a small additional fore-aft damping effect is available, a the controller system 400 may determine to invoke an increase or a decrease of the mean pivot angle dependent on an actual pivot angle. Thus, a decrease of the pivot angle or mean pivot angle may only be allowed if the actual pivot angle is above an upper pivot angle threshold and, correspondingly, an increase of the pivot angle may only be allowed if the actual pivot angle is below a lower pivot angle threshold.

The amount of the desired decrease or increase of the pivot angle a or mean pivot angle a may be determined according to predetermined amount, e.g. dependent on the oscillation amplitude, as determined from the tower oscillation signal 402, or a correlation of the oscillation amplitude with and the actual pivot angle. Additionally, the amount of the desired decrease or increase of the pivot angle or mean pivot angle may be determined dependent on the oscillation direction.

Furthermore, due to the relationship between the damping effect on different oscillation directions and the direction of the change of the mean pivot angle, the control system 400 may be configured to invoke a change of the pivot angle or mean pivot angle dependent on a correlation between the oscillation direction determined from the tower oscillation signal 402 with predetermined relationships between oscillation directions and changes of the pivot angle or mean pivot angle. Additionally, the control system 400 may be configured to determine a magnitude of the change of the pivot angle or mean pivot angle dependent on this correlation.

The control system 400 may be configured to invoke a change of the mean pivot angle dependent on predetermined preferences for a damping direction.

The embodiments and examples described above for damping oscillations based on cyclic pivot angle variations and/or change of the mean pivot angle can be initiated and the control signal 401 determined based on the tower oscillation signal 402. Specifically, the tower oscillation signal 402 may be determined so that it comprises an oscillation magnitude. In this case the magnitude of the cyclic pivot angle variations and/or the magnitude of the change of the mean pivot angle may be determined based on the oscillation magnitude.

Additionally, the tower oscillation signal 402 may be determined so that it comprises an oscillation direction value indicative of the oscillation direction of the tower oscillation.

The oscillation direction value may be direction of the oscillation or a direction of the largest oscillation amplitude. For example, the direction may be determined from the output of a 3D accelerometer mounted on the wind turbine such as the tower or nacelle, i.e. an accelerometer configured with at least two accelerometers measuring the acceleration in the two directions in the horizontal plane. In another example, the direction may be determined based on position or velocity measurement or estimations of the tower top.

The direction of the oscillation may also be obtained from the spectral content of the oscillation signal, e.g. by comparing the frequency of the largest oscillation amplitude with known eigenfrequencies of the tower. Since the tower may have different eigenfrequencies in different directions, the comparison of the frequencies can provide a direction of e.g. the largest oscillation amplitude. Accordingly, the oscillation direction may be determined based on correlating an oscillation frequency component of the tower oscillation signal 402 with tower eigenfrequencies. The oscillation direction value could also be a value determined from the oscillation frequency such as a correlation value between the oscillation frequency and eigenfrequencies.

In another example, the oscillation direction value may be determined based on the energy in lateral and fore-aft directions of the tower oscillations.

Based on the oscillation direction value, the control system may be configured to determine whether to increase or decrease the mean pivot angle and/or to invoke a cyclic variation of the pivot angle for damping lateral or fore-aft oscillations. Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is to be interpreted in the light of the accompanying claim set. In the context of the claims, the terms "comprising" or "comprises" do not exclude other possible elements or steps. Also, the mentioning of references such as "a" or "an" etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.

Claims

1. A method for damping tower oscillations of a wind turbine (100) with a variable rotor area, the wind turbine comprises a tower (101), a rotor (102) with one or more rotor blades (103) which are arranged hinged at an adjustable pivot angle (a), where the pivot angle is adjustable dependent on a variable pivot force (F) or pivot moment (M) provided by a pivot actuator (301), the method comprises

- detecting a tower oscillation, and

2. A method according to claim 1, wherein detecting the tower oscillation comprises determining an oscillation direction value indicative of the oscillation direction of the tower oscillation.

3. A method according to claim 2, wherein a determination to decrease or increase the pivot angle is dependent on the oscillation direction value.

4. A method according to any of claims 2-3, wherein a determination to decrease or increase the pivot angle is dependent on a correlation between the oscillation direction value with predetermined relationships between oscillation direction values and pivot angle changes.

5. A method according to claim 4, wherein a magnitude of the decrease or increase of the pivot angle is dependent on the correlation.

6. A method according to any of the preceding claims, wherein detecting the tower oscillation comprises determining an oscillation magnitude, and wherein the method further comprises determining a magnitude of the decrease or the increase of the pivot angle dependent on the oscillation magnitude.

7. A method according to any of the preceding claims, wherein a decision to start the decreasing or increasing of the pivot angle is dependent on an actual pivot angle.

8. A method according to claim 7, wherein the decreasing of the pivot angle is conditioned on that the actual pivot angle is above an upper pivot angle threshold, and wherein the increasing of the pivot angle is conditioned on that the actual pivot angle is below a lower pivot angle threshold.

9. A method according to any of the preceding claims, wherein the decreasing or increasing of the pivot angle is restricted to be performed within a maximum period.

10. A method according to any of the preceding claims, wherein the decreasing or increasing of the pivot angle comprises changing the pivot angle by a predetermined amount.

11. A method according to any of the preceding claims, wherein the decreasing or the increasing of the pivot angle comprises changing the pivot angle so that a mean value of the pivot angle, over a period of time, is decreased or increased, respectively.

12. A method according to claim 11, wherein the mean value of the pivot angle is controllable by means of the pivot force (F) or the pivot moment (M).

13. A method according to any of the preceding claims, wherein the decreasing or the increasing of the pivot angle comprises changing the pivot angle to provide a cyclic variation of the pivot angle.

14. A method according to any of the preceding claims, further comprising adjusting the pivot force (F) or pivot moment (M) to provide a variation of the pivot angle to impose an oscillating force onto the tower to counteract the tower oscillation.

15. A method according to any of claims 13-14, wherein the variation of the pivot angle is superposed on the decreased or increased mean value of the pivot angle.

16. A method according to any of claims 13-15, wherein the variation of the pivot angle is applied with a same phase or substantially the same phase to all rotor blades.

17. A method according to any of claims 13-16, wherein the pivot angle variations are determined dependent on a rotor angle and dependent on a phase of the determined oscillation so that the pivot angle of one or more of the rotor blades (102) varies as a function of the rotor angle.

18. A damping control system (400) arranged to perform the steps according to the method of claims 1-17.

19. A wind turbine (100) comprising a rotor (102) with a variable rotor area, where the rotor comprises one or more rotor blades (103) which are arranged hinged at an adjustable pivot angle (a), where the variable rotor area in a plane perpendicular to a rotor shaft direction depends on the pivot angle, and where the pivot angle is adjustable dependent on a variable pivot force (F) provided by a pivot actuator (301), and the damping control system (400) according to claim

17.

20. A computer program product comprising software code adapted to control a wind power plant when executed on a data processing system, the computer program product being adapted to perform the method of any of the claims 1-17.