DK201970419A1 - A system for damping oscillations in a structure - Google Patents

Abstract

A system for damping torsional oscillations in a structure, the system comprising: a first unbalanced mass arrangement rotatable about a first rotation axis to generate a first rotating unbalance vector; a second unbalanced mass arrangement rotatable about a second rotation axis to generate a second rotating unbalance vector; and control means configured to rotate the first and second unbalanced mass arrangements at the same frequency; wherein the first and second unbalanced mass arrangements are configured such that the first rotating unbalance vector is opposed to the second rotating unbalance vector, thereby generating a torsional moment that opposes the torsional oscillation of the structure.

Description

DK 2019 70419 A1 1

A SYSTEM FOR DAMPING OSCILLATIONS IN A STRUCTURE

TECHNICAL FIELD Aspects of the invention relate to damping oscillations in stationary structures such as wind turbine towers.

BACKGROUND — Wind turbines are used to produce electrical energy using by converting kinetic energy from the wind into electrical power. A horizontal-axis wind turbine generally includes a tower, a nacelle located at the top of the tower, and a rotor having a plurality of blades and supported in the nacelle by a shaft. The shaft couples the rotor either directly or indirectly with a generator, which is housed inside the nacelle. Consequently, as wind forces the blades to rotate, electrical energy is produced by the generator. Torsional oscillations, if allowed to increase to unacceptable levels for prolonged timescales, can cause mechanical stresses in the tower and/or the nacelle, which may accelerate wear and thereby impact service life. When such torsional oscillations occur in structures which accommodate people, those oscillations may cause discomfort to those people within the structure. Accordingly, there is a need to improve structures which are susceptible to such torsional oscillations.

SUMMARY OF THE INVENTION According to an aspect of the invention there is provided a system for damping torsional oscillations in a structure, the system comprising: a first unbalanced mass arrangement rotatable about a first rotation axis to generate a first rotating unbalance vector; a second unbalanced mass arrangement rotatable about a second rotation axis to generate a second rotating unbalance vector; and control means configured to rotate the first and second unbalanced mass arrangements at the same frequency; wherein the first and second unbalanced mass arrangements are configured such that

DK 2019 70419 A1 2 the first rotating unbalance vector is opposed to the second rotating unbalance vector, thereby generating a torsional moment that opposes the torsional oscillation of the structure.

Each unbalanced mass arrangement is configured with respect to the other to produce a set of substantially opposing rotating unbalance vectors, thereby generating a torsional moment which is able to oppose torsional oscillations in a structure. By controlling the frequency at which the unbalanced mass arrangements are rotated, the torsional damping system conveniently provides a configurable means of matching the damping moment generated by the damping system to the resonant torsional vibrations which are to be damped. The first unbalanced mass arrangement may be configured to rotate within a range of 160° to 200° out of phase with the second unbalanced mass arrangement. The range of 160° to 200° defines a range of phase angle offset values which conveniently generates a pair of respective rotating unbalance vectors that are substantially opposed to one another. The first unbalanced mass arrangement may be configured to rotate 180° out of phase with the second unbalanced mass arrangement, which represents the optimum phase angle offset for a damping system comprising two rotating unbalanced mass arrangements. The system may comprise at least one additional unbalanced mass arrangement (s), wherein a phase angle offset (¢) between the unbalanced mass arrangements is determined according to the equation: ¢ = (360°)/N, where N equals the total number of unbalanced mass arrangement(s) in the system. The phase angle offset is conveniently determined according to the above relationship such that the combined inertia of the two or more unbalanced mass arrangements produces a torsional oscillation that is in the same direction as the torsional oscillation which is to be damped. The control means may comprise an actuator arranged to rotate the first unbalanced mass arrangement, and a coupling means arranged to rotatably couple the first

DK 2019 70419 A1 3 unbalanced mass arrangement to at least one of the other unbalanced mass arrangement(s). The coupling means may comprise a first gear rotatably coupled to the first unbalanced mass arrangement, a second gear rotatably coupled to at least one of the other unbalanced mass arrangement(s) and a coupling gear configured to rotatably couple the first gear to the second gear. The coupling gear may comprise a ring having teeth extending radially inwards to — mesh with outwardly extending teeth of the first and second gears. The coupling means may comprise a belt or a chain. The coupling means may comprise a second actuator rotatably coupled to at least one of the other unbalanced mass arrangement(s), the second actuator being configured to rotate the at least one other unbalanced mass arrangement at the same frequency as the first unbalanced mass arrangement. The control means may be configured to rotate the first and second unbalanced mass arrangements at the same frequency. The system may comprise a means of determining the frequency of the torsional oscillations in the structure, wherein the control means is configured to rotate the unbalanced mass arrangements based on the determined frequency.

The system may comprise a means of determining the amplitude of the torsional oscillations in the structure, wherein at least one of the unbalanced mass arrangements has a controllable moment of inertia about its axis of rotation and the control means is configured to control the moment of inertia based on the determined amplitude of the torsional oscillations in the structure. The control means may be configured to receive a signal from a sensor arranged to measure the torsional oscillations in the structure, the signal being indicative of the frequency of the torsional oscillations.

DK 2019 70419 A1 4 The unbalanced mass arrangements may be mountable to the structure such that their rotation axes are arranged substantially in parallel with a torsional oscillation axis of the structure.

According to a further aspect of the invention there is provided a structure that can undergo torsional oscillations comprising a system according to any one of the previous paragraphs, wherein the system is mounted at a position where the torsional oscillations to be damped are largest.

The system may be arranged such that the first and second axes of rotation are arranged either side of the torsional axis of the structure. The structure may comprise a wind turbine tower.

The controller is arranged to perform the method of the above aspect. It will be appreciated that the foregoing represents only some of the possibilities with respect to the particular subsystems of a torsional damping system which may be included, as well as the arrangement of those subsystems with the controller. Accordingly, it will be further appreciated that embodiments of a torsional damping system which include other or additional subsystems and subsystem arrangements remain within the scope of the present invention. Additional sub-systems may include, for example, systems relating to the operation of a wind turbine.

The set of instructions (or method steps) described above may be embedded in a computer-readable storage medium (e.g. a non-transitory storage medium) that may comprise any mechanism for storing information in a form readable by a machine or electronic processors/computational device, including, without limitation: a magnetic storage medium (e.g., floppy diskette); optical storage medium (e.g., CD-ROM); magneto optical storage medium; read only memory (ROM); random access memory (RAM); erasable programmable memory (e.g., EPROM ad EEPROM); flash memory; or electrical or other types of medium for storing such information/instructions.

DK 2019 70419 A1 Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all 5 embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a front view of a wind turbine coupled to a system for damping torsional oscillations, shown schematically; Figure 2 is a schematic view of an exemplary system for damping torsional oscillations having two unbalanced mass arrangements which are rotatable about respective rotation axes; Figures 3a to 3h are schematic representations of the torsional moments generated by the system of Figure 2, as the unbalanced mass arrangements are rotated about their respective rotation axes; Figures 4 is a side view of the system of Figure 2, illustrating a means of controlling the rotation of the first and second unbalanced mass arrangements; Figure 5 is a side view of the system of Figure 2, showing an alternative means of controlling the rotation of the first and second unbalanced mass arrangements;

DK 2019 70419 A1 6 Figure 6 is a view from above of an exemplary means of coupling the first and second unbalanced mass arrangements of the system of Figure 2; Figure 7 is a front view of a multi-rotor wind turbine system coupled to a system for damping torsional oscillations, shown schematically; and Figure 8 is a schematic view of an exemplary system for damping torsional oscillations having three unbalanced mass arrangements which are rotatable about respective rotation axes.

DETAILED DESCRIPTION Referring to the figures, and to Fig. 1 in particular, an exemplary horizontal-axis wind turbine 10 generally includes a tower 12, a nacelle 14 disposed at the apex of the tower 12, and a rotor 16 operatively coupled to a generator (not shown) housed inside the nacelle 14. In addition to the generator, the nacelle 14 houses miscellaneous components required for converting wind energy into electrical energy and various components needed to operate, control, and optimize the performance of the wind turbine 10. The tower 12 supports the load presented by the nacelle 14, the rotor 16, and other components of the wind turbine 10 that are housed inside the nacelle 14. The tower 12 further operates to elevate the nacelle 14 and rotor 16 to a height above ground level or sea level, as may be the case, at which faster moving air currents of lower turbulence are typically found. The rotor 16 of the wind turbine 10 serves as the prime mover for the electromechanical system. Wind exceeding a minimum level will activate the rotor 16 and cause rotation in a substantially perpendicular direction to the wind direction. The rotor 16 of wind turbine 10 includes a central hub 20 and a plurality of blades 22 that project outwardly from the central hub 20 at locations circumferentially distributed thereabout. While the exemplary rotor 16 shown herein includes three blades 22, various alternative quantities of blades may be provided. The blades 22 are configured to interact with the passing air flow to produce lift that causes the rotor 16 to spin generally within a plane defined by the blades 22.

DK 2019 70419 A1 7 The wind turbine 10 may be included among a collection of similar wind turbines belonging to a wind farm or wind park that serves as a power generating plant connected by transmission lines with a power grid, such as a three-phase alternating current (AC) power grid. The power grid generally consists of a network of power stations, transmission circuits, and substations coupled by a network of transmission lines that transmit the power to loads in the form of end users and other customers of electrical utilities. Under normal circumstances, the electrical power is supplied from the generator 18 to the power grid as known to a person having ordinary skill in the art. With reference to Fig. 1, the wind turbine 10 is provided with a torsional damping system 24 for damping torsional oscillations in the tower 12. The torsional damping system 24 is coupled to an interior wall of the tower 12. This coupling may be achieved by any suitable means, for example by welding or by bolts, and/or by mounting the torsional damping system 24 to a flange joint joining two tower sections together.

Referring to Fig. 2, the damping system 24 comprises a first unbalanced mass arrangement 28 which is rotatable about a first rotation axis A1, and a second unbalanced mass arrangement 32 which is rotatable about a second rotation axis A2. The first and second unbalanced mass arrangements 28, 32 each comprise a respective mass M1, M2 mounted to a corresponding rotatable element. In this example, the rotatable element may be a disc, but it could take other configurations, for example an arm. Each mass M1, M2 is radially displaced from the rotational axis of its respective disc such that, upon rotation of the unbalanced mass arrangement 28, 32, a corresponding rotating unbalance vector is generated. A control means, in the form of a controller 26, is operable as a command and control interface between the various components of the torsional damping system 24. In this way, the controller 26 forms part of a central control system of the torsional damping system 24. It will be appreciated by the skilled person that the controller 26 may be incorporated into any number of computer based control systems of the wind turbine 10, such as a control unit for the generator, for example.

DK 2019 70419 A1 8 The controller 26 is arranged to control the operation of actuator(s) that control the rotation of the unbalanced mass arrangements 28, 32 during operation of the torsional damping system 24. To achieve this, the controller 26 comprises a computer system for carrying out suitably prescribed processes and strategies.

The torsional damping system 24 also includes two oscillation sensors for sensing oscillations in perpendicular horizontal directions of the wind tower 12. The oscillation sensors comprise accelerometers Ax and Ay in these embodiments, although it will be appreciated that other sensing approaches may be achieved to derive suitable torsional oscillation data. Torsional oscillations may also be determined based on a measurable operating parameter of the wind turbine, such as the rotational frequency of the rotor 16. In particular, torsional oscillations may be measured for a range of rotor frequencies. The results of which may be then used to generate a model which predicts the frequency and the amplitude of future torsional oscillations, as would be readily understood by the skilled person. Accordingly, it will be appreciated that the torsional damping system 24 described herein may be adapted to incorporate any number of suitable means of sensing the torsional vibrations, without diverting from the scope of the invention.

It will be appreciated that there are a number of alternative means of providing a rotatable unbalanced mass arrangement suitable for use in a torsional damping system, as described herein. In alternative embodiments, such an unbalanced mass may be defined by a respective hole being formed in a rotatable disc at a position — which is radially offset from its rotational axis, for example. With reference to Fig. 3a, a first unbalance vector V1 is produced by the rotation of the first unbalanced mass arrangement 28, and a second unbalance vector V2 is generated by the rotation of the second unbalanced mass arrangement 32. The first unbalanced mass arrangement 28 is configured to rotate 180° out of phase with the second unbalanced mass arrangement 32, as will be described in more detail below. Accordingly, the first rotating unbalance vector V1 is opposed to the second rotating unbalance vector V2, thereby generating a torsional moment that is able to be suitably configured to oppose the torsional oscillation of the tower 12, as shown in Fig. 3b.

DK 2019 70419 A1 9 During operation of the torsional damping system 24, the two unbalanced mass arrangements 28, 32 are set into rotation at a determined frequency in order to counteract a specific torsional oscillation of the wind turbine 10.

Advantageously, when the unbalanced mass arrangements 28, 32 are of equal moments of inertia and are rotated at the same frequency in the same direction, the resulting motion produces a torsional oscillating force that is applied to the tower by virtue of the connection of the torsional oscillation system 24 to the tower. By changing the rotational frequency of both mass arrangements 28, 32, the direction of the resulting equivalent torsional moments will change correspondingly. By virtue of the damping effect of the unbalanced mass arrangements, therefore, the severity of the torsional oscillation of the tower at a predetermined frequency is controlled to remain within acceptable levels. Typically, the predetermined rotational frequency will be the (or one of the) resonant torsional frequency of the tower. The first and second rotation axes A1, A2 are arranged with respect to a torsional oscillation axis of the wind turbine 10. The torsional axis of the tower 12 is aligned, typically, with the longitudinal axis (i.e. vertical axis) of the tower 12. Accordingly, it is envisaged that the most effective configuration will be with the first and second unbalanced mass arrangements 28, 32 being mounted to the tower 12 such that the first and second rotation axes A1, A2 are arranged substantially in parallel to the torsional axis of the tower 12. Accordingly, the first and second rotation axes A1, A2 are also arranged to be substantially parallel to each other. In this way, the torsional damping system 24 is arranged to optimally dampen the torsional oscillations in the tower 12. The unbalanced mass arrangements 28, 32 are connected to a control means of the torsional damping system 24, which is configured to control the rotation of the first and second unbalanced mass arrangements 28, 32 in order to counteract the torsional oscillations in the wind turbine 10. As described above in relation to Fig. 2, the phase angle offset between the unbalanced masses is set at 180° so that their combined inertia produces a torsional

DK 2019 70419 A1 10 oscillation that is in the same direction as the torsional oscillation which is to be damped. Figs. 3a, 3b, 3c, 3d, 3e, 3f, 3g and 3h show a progression of the unbalanced mass arrangements 28, 32 as they are each rotated in a clockwise direction through one complete revolution.

The two unbalanced mass arrangements 28, 32 have a phase angle offset of 180°, which provides the optimum damping effect on torsional oscillations. The change in the direction of each rotating unbalance vector V1, V2 is shown for each respective arrangement as they rotate about their respective rotation axes A1, A2.

The rotating unbalance vector V1, of the first unbalanced mass arrangement 28, is resolved into a transverse force FA and a torsional force FO, as shown in Fig. 3b. The transverse and torsional forces Fa, Fo are shown as being aligned with X and Y axes, respectively. The torsional forces of the first and second unbalanced mass arrangements 28, 32 combine to generate a transient clockwise torque T which may be exerted by the damping system 24 upon the wind turbine 10. However, it should be appreciated that the opposing transverse forces Fa cancel each other so that there is no overall transverse force between the first and second unbalanced mass arrangements 28, 32.

Fig. 3a and Fig. 3e each depict instances in which the respective rotating unbalance vectors V1, V2 comprise a minimal torsional component. This is because the respective unbalanced vectors V1 and V2 each act along a common plane in opposite directions. Thus, the respective transverse forces Ta are at a maximum but cancel out, similarly to Fig 3b, to produce no overall transverse force.

Therefore, the unbalanced vectors do not contain a torsional component in the angular positions indicated in Figs 3a and 3e.

The arrangements shown in Figs. 3d, 3f and 3h all depict points in the rotation of unbalanced masses 28, 32 in which the respective unbalance vectors V1, V2 comprise both torsional and transverse components Fo, Fa. In Figs. 3d, the corresponding force vectors V1, V2 result in a clockwise torque, whereas the arrangements shown in Figs. 3f and 3h generate anti-clockwise torques. As with Fig 3b, the opposing transverse

DK 2019 70419 A1 11 forces cancel each other in each of these arrangements meaning that there is no resultant transverse or lateral force exerted on the tower structure. Figs. 3c and 3g both depict orientations of the unbalanced masses in which the torsional forces Fo are at a maximum and the transvers forces Fa are at a minimum. The arrangement shown in Fig. 3c produces a clockwise torque whereas the arrangement in Fig. 3g generates an anticlockwise torque. From the arrangements shown in Figs. 3a to 3h, it will be appreciated that a single rotation of the unbalanced mass arrangements 28, 32 leads to the sequential generation of first a clockwise torque and then an anti-clockwise torque. This process repeats itself with every revolution of the unbalanced mass arrangements 28, 32 to produce an oscillating torsional damping force which dampens the torsional oscillations in the wind turbine 10.

Although the torsional damping system 24 as described herein is arranged such that the phase angle offset between the first and second unbalanced mass arrangements 28, 32 is 180°, the phase angle offset may comprise a range of other suitable angles, whilst still being able to exert a damping force, as required by the present invention. It should be appreciated that a precise 180° angular phase shift would be most effective, but that a useful result would still be achieved with angular phase shifts less than this ideal value. It has been determined that a torsional damping system 24 in which the unbalanced mass arrangements 28, 32 are arranged with a phase angle offset which diverges from 180° by +10% would result in a torsional moment that is 0.38 % smaller than the optimum value (i.e. that which is achieve for a phase angle offset of 180°). Furthermore, a phase angle offset which diverges from 180° by +20° has been determined to reduce the torsional moment by 1.5%.

Any deviation from 180° in the offset angle will also produce an oscillating lateral force in the damping system 24 which is exerted upon the structure of the wind turbine 10. However, it is believed that such lateral forces are unlikely to generate instabilities in the wind turbine structure unless the torsional frequency is substantially similar to a

DK 2019 70419 A1 12 resonant bending frequency of the wind turbine component. For example, such lateral forces should be avoided if the torsional frequency was within an order of magnitude of the bending frequency, for example.

Accordingly, the unbalanced mass arrangements 28, 32 may be arranged with a phase angle offset which is between 160° and 200° whilst still being configured to exert an effective damping force to counteract the torsional oscillations within the wind turbine 10. From this, therefore, it will be noted that the force vectors should generally be opposed to one another during rotation, but that a precise out of phase relationship is not required.

During operation of the wind turbine 10, the controller 26 is arranged to control the operation of the torsional damping system 24 according to a torsional damping control strategy, as will now be explained in more detail. The torsional damping control — strategy commences with the controller 26 receiving a signal which is indicative of a demand for torsional damping to be applied to the wind turbine 10. The torsional damping demand signal includes information relating to determined torsional oscillations which have been measured by the accelerometers Ax, Ay.

According to an exemplary embodiment, the torsional damping demand signals comprise sensed accelerations in the X and Y directions. The damping demand signals received by the controller 26 where they are processed to determine the relevant properties of the determined torsional oscillations of the wind turbine 10. Such properties include, for example, the frequency and amplitude of the torsional oscillations.

The torsional properties of a range of frequencies may be analysed in order to determine which specific frequencies cause the greatest load on the structure of the wind turbine 10, and which should therefore be damped by the torsional damping system 24. Typical frequencies to be damped are the fundamental frequency of the wind turbine structure and its harmonics. For each particular torsional vibration to be damped, the actual frequency and/or amplitude are measured by the accelerometers Ax and Ay of the damping system 24.

DK 2019 70419 A1 13 The controller 26 is arranged to calculate, in dependence on the input signals from the accelerometers, the required moments of inertia of the two unbalanced mass arrangements 28, 32 which will suitably counteract the specific torsional oscillations in the tower 12. The controller 26 then controls the unbalanced mass arrangements 28, 32 to rotate at a frequency corresponding to the desired moment of inertia. In this way, the controller 26 is configured to rotate the first and second unbalanced mass arrangements 28, 32 based on the determined frequency of the torsional oscillations in the wind turbine 10.

According to an exemplary embodiment, the controller 26 is arranged to control the operation of the damping system 24 to match the amplitude of a particular torsional oscillation within the wind turbine 10. In particular, the accelerometers Ax, Ay are arranged to measure the amplitude of the torsional oscillations in the wind turbine structure. The controller 26 then adjusts the radial position of each mass M1, M2 to — control the moments of inertia of the first and second unbalanced mass arrangements 28, 32 about their respective axes of rotation, based on the determined amplitude of the torsional oscillations in the structure. By controlling the moment of inertia of the rotating masses, this allows the system to match the amplitude of the torsional oscillations. It also gives the possibility to engage and disengage the torsional damper at the correct frequency instead of waiting for the inertias to accelerate to the correct frequency and phase.

The moments of inertia of the unbalanced mass arrangements 28, 32 can be varied in several ways. For example the two masses M1 and M2 can be moved to different distances from the respective axes of rotation A1, A2, as illustrated by the arrows shown in Fig. 2. Alternatively, the masses M1, M2 can each comprises a substance such as a liquid or a granular solid substance that can be pumped and distributed in separate chambers, or compartments, of the respective unbalanced mass arrangement. By moving the masses M1, M2 or changing their radial distributions, the respective moments of inertia of the first and second unbalanced mass arrangements 28, 32 can be controlled to a desired calculated value, in order to counteract the torsional oscillations within the wind turbine 10.

DK 2019 70419 A1 14 With reference to Fig. 4, the first and second unbalanced mass arrangements 28, 32 are coupled together by a coupling means 34 which is arranged to rotatably couple the first and second unbalanced mass arrangements 28, 32 together, so that they both rotate at the same frequency. The coupling means 34 comprises a belt 35, or a chain, which rotatable couples a first drive shaft 38 arranged to drive the first unbalanced mass arrangement 28, to a second drive shaft 40 which drives the second unbalanced mass arrangement 32. An actuator 36 is rotatably coupled to the first drive shaft 36 of the first unbalanced mass arrangement 28, and is controlled to rotate the first and second unbalanced mass arrangements 28, 32 at the desired frequency by the controller 26. The actuator comprises an electric stepper motor, which is particularly useful for controlling and maintaining the rotational frequency of the first and second unbalanced mass arrangements 28, 32.

Referring to Fig. 6, an exemplary embodiment of the invention is shown in which the coupling means 134 comprises a first gear 150 rotatably coupled to the first unbalanced mass arrangement 128 and a second gear 152 rotatably coupled to the second unbalanced mass arrangement 132. A coupling gear 154, in the form of an annular gear, is arranged to rotatably couple the first gear 150 to the second gear 152. The coupling gear 154 comprises a ring having teeth 144 extending radially inwards to mesh with the outwardly extending teeth 146 of the first and second gears 150, 152. The above described geared arrangement may be arranged within the interior 148 of a tower 12, as shown in Fig. 6. The coupling gear 154 is centred around the torsional axis AT of the tower 12, such that the first and second unbalanced mass arrangements 128, 132 are positioned on either side of the torsional axis AT. In this way, the first and second axes of rotation A1, A2 are arranged either side of the torsional axis AT of the structure such that the damping system 124 is best positioned to directly counteract the torsional oscillations which are exerted thereabouts. Furthermore, by using an annular coupling gear a central, or axial, portion of the tower interior 148 remains substantially clear to allow through passage of essential componentry of the wind turbine 12, such as power cabling which connects the

DK 2019 70419 A1 15 generator in the nacelle 14 to an external power network. The space provided between the first and second gears 150, 152 may, alternatively, be utilised to accommodate an access ladder to allow maintenance workers to climb up through the interior of the tower 12. Advantageously, the above described gearing arrangement allows damping — of torsional oscillations in the tower 12, whilst reducing any disruption to the normal operations of the wind turbine 10. According to an alternative embodiment of the invention, each of the first and second unbalanced mass arrangements 228, 232 may be rotated by a separate actuator, as shown in Fig. 5. A first actuator 236 is configured to rotate the first unbalanced mass arrangement 228 via the first drive shaft 238 and a second actuator 242 is arranged to rotate the second unbalanced mass arrangement 232 via the second drive shaft 240. Each of the first and second actuators 236, 242 are controlled by the controller 228 to rotate at a desired frequency and phase. In particular, the second actuator 242 is controlled by the controller 228 to rotate the second unbalanced mass arrangement 232 at the same frequency as the first unbalanced mass arrangement 228 so as to define a coupling means 234 of the torsional damping system 224. Although the present invention is described in relation to a wind turbine 10, the torsional damping system 24 could be applied to any other structure which is subjected to torsional loads. The wind turbine 10 in Fig. 2 comprises a free standing tower 10 as would be commonly understood by the skilled person. It will be appreciated, however, that the torsional damping system 24 may also be applied to a wind turbine 10 having a cable-stayed tower, in which the torsional oscillations may be even more prominent.

The system may be applied to any component of a wind turbine 10 which may be subjected to torsional oscillation, including the tower 12 and the blades 22. In addition, the system may also be adapted to dampen resonant torsional vibrations of the nacelle 14 caused due to yaw stiffness. In this way, the torsional damping system 24 may be configured to dampen resonant torsional vibrations of a point mass of a wind turbine structure. The torsional damping system 24 may be mounted in a number of different locations within the structure of the wind turbine 10. In embodiments, the damping system 24

DK 2019 70419 A1 16 may be installed towards the top of the tower 12, for example. The damping system 24 can be mounted to an underside surface of the nacelle 14, at the interface between the nacelle 14 and tower 12. The torsional damping system 24 may also be adapted to dampen torsional vibrations in the arms of a multi-rotor wind turbine, as will now be described with reference to Fig. 7, which shows an exemplary multi-rotor wind turbine 310 comprising four turbine assemblies 308, which may also be referred to as rotor nacelle assemblies or ‘RNAS’ in the art. Note that the term ‘turbine assembly’ is used here to refer mainly to the generating components of the wind turbine system and as being separate to the support structure of the wind turbine 310. The four turbine assemblies 308 are mounted to a tower 312 in two pairs, each pair including two turbines assemblies 308 that are mounted to the tower 312 by a support — arm arrangement 306. Each support arm arrangement 306 comprises a pair of support arms 302 which extend from a mount portion 304 and carry a respective turbine assembly 308. As such, each of the support arms 302 includes an inner end connected to the mount portion 304 and an outer end that is connected to a turbine assembly 308.

Each turbine assembly 308 includes a rotor 316 that is rotatably mounted to a nacelle 314 in the usual way. The rotor 316 has a set of three blades 322 in this embodiment. Three-bladed rotors are the most common rotor configuration, but different numbers of blades are also known. Each rotor 316 is operatively coupled to a generator (not shown) housed inside the nacelle 314. Accordingly, the tower 312 supports the load presented by the nacelle 314, the rotor 316, and other components of the wind turbine 310 that are housed inside the nacelle 314. During operation of the wind turbine 310, the mass of the nacelle 314 and the rotation of the rotor 316 can cause the wind turbine assembly 308 to tilt forwards and backwards about the horizontal axis H of the support arm 302, as shown by the arrows in Fig. 7. A torsional damping system 324 is mounted within each support arm 302, of the support arm assembly 306, and is configured to counteract the tilt contribution from the nacelles 314 during the operation of the wind turbine 310.

DK 2019 70419 A1 17 According to this embodiment, the torsional axis is aligned with the longitudinal axis (i.e. horizontal axis) of the support arm 302. Accordingly, the most effective configuration of the torsional damping system 324 will be with the first and second unbalanced mass arrangements being mounted to the support arm 302 such that the first and second rotation axes are arranged substantially in parallel to the horizontal axis H of the support arm 302. Otherwise, the operation of the torsional damping system 324 is as described above, with respect to the previous embodiments. It should also be noted that, more generally, where the support arms 302 are mounted in an attitude that is not horizontal, it is envisaged that the unbalanced mass arrangements would be mounted to the support arms accordingly so that the rotational axes thereof coincide with the major longitudinal axis of the support arm. The torsional axis for a tall structure, such as the tower of a wind turbine, is typically aligned in parallel with the longitudinal (i.e. vertical) axis of the structure. The torsional damping system 24 may be optimally configured so that the first and second unbalanced mass arrangements 28, 32 are arranged such that their rotational axes A1, A2 are aligned in parallel with the longitudinal axis of the structure, in order to optimise the torsional damping effect.

In order to dampen torsional oscillations in the wind turbine 10, the rotational axes A1, A2 may be arranged at any angle to the torsional axis of structure, provided that at least a component of the respective first and second rotating unbalance vectors V1, V2 are configured to act in substantially the same plane as the torsional oscillations. Accordingly, the rotational axes A1, A2 of the unbalanced mass arrangements may comprise any angle less than 90° with respect to the torsional axis of the component of the wind turbine in which the torsional oscillations are to be damped. A typical wind turbine tower 12 is subjected to a variety of static and dynamic forces due to the cantilevered mass of the nacelle 14 and the rotation of the rotor blades 22, respectively, which can affect the direction of the torsional axis of the tower 12. Accordingly, the torsional damping system 24 may be configured to be adjustable, relative to wind turbine, in order to obtain the desired direction of the resulting equivalent torsional harmonic motion. In particular, the damping system 24 may be

DK 2019 70419 A1 18 arranged in a first arrangement in which the first and second rotational axes A1, A2, are substantially aligned with the torsional axis of the tower 12. Then, at a later time, the torsional damping system 24 may be adjusted such that the first and second rotational axes A1, A2 are angled, relative to the torsional axis of the tower 14, in order to generate a damping force which acts in a direction that more closely matches the direction of the torsional oscillations in the tower 12. The damping system 24, as shown in Fig. 2, comprises two unbalanced mass arrangements 28, 32. However, the damping system 24 can be made with two or more unbalanced mass arrangements, provided that the unbalanced mass arrangements are all equally distributed about a geometrically centred focal point, and provided that their corresponding phase angles are offset so as to produce a torsional oscillating motion.

An exemplary embodiment of a torsional damping system 424 comprising three unbalanced mass arrangements, 428, 432, 434 is shown in Fig. 3. The three unbalanced mass arrangements, 428, 432, 434 are equally distributed about a central point C, such that the angle between their respective rotational axes A1, A2, A3 are all 120°. Furthermore, the phase angle offset between each of the three unbalanced mass arrangements, 428, 432, 434 is also 120° such that each of the rotating unbalance vectors V1, V2, V3 are all substantially opposed to each other.

From this, therefore, it will be noted that the phase angle offset between the unbalanced mass arrangements of the damping system may be determined by the following equation: D = 360°

N Where N defines the number of rotatable unbalanced mass arrangements, and ¢ represents the phase angle offset between each of the respective unbalanced mass arrangements. During operation of such torsional damping systems, each of the unbalanced mass arrangements may be controlled to rotate at substantially the same frequency as each other. It will also be appreciated that the control systems and

DK 2019 70419 A1 19 methods, as described previously in relation to damping systems 24 comprising two unbalanced mass arrangements, may equally be applied to a torsional damping system comprising one or more additional unbalanced mass arrangements.

In Fig. 1 the damping system 24 is shown as being mounted inside the tower 12 of the wind turbine 10. The damping system 24 can be produced as a unit for retrofitting into existing wind turbine towers 12, and other structures, or it can be installed in the tower 12 from the beginning.

The torsional damping system 24 can be provided with its own controller 26, as illustrated in Fig.2, or it can be controlled by a controller of the wind turbine 10. The damping system 24 is mounted to a platform welded to an internal wall of the tower 12. While the present invention has been illustrated by the description of various embodiments thereof, and while the embodiments have been described in considerable detail, it is not intended to restrict or in any way limit the scope of the appended claims to such detail. The various features discussed herein may be used alone or in any combination. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the scope of the general inventive concept.

Reference Numerals Turbine 10, 310 Tower 12, 312 Nacelle 14, 314 Rotor 16, 316 Central hub 20 Blades 22, 322 Damping system 24, 124, 224, 324 Controller 26 First unbalanced mass arrangement 28, 128, 228, 428 First rotational axis A1

DK 2019 70419 A1 20 Second unbalanced mass arrangement 32, 132, 232, 432 Second rotational axis A2 First mass M1 Second mass M2 Coupling means 34, 134, 234

Belt 35 Actuator 36 First shaft 38 Second shaft 40

— First actuator 236 Second actuator 242 Tower wall 142 Inward teeth 144 Outward teeth 146

— Tower interior 148 Support arm 302 Support mount 304 Support arm arrangement 306 Wind turbine assembly 308

Horizontal axis H Third unbalanced mass arrangement 434

Claims (17)

DK 2019 70419 A1 21 CLAIMS

1. A system for damping torsional oscillations in a structure, the system comprising: a first unbalanced mass arrangement (28; 128; 228; 428) rotatable about a first rotation axis (A1) to generate a first rotating unbalance vector (V1); a second unbalanced mass arrangement (32; 132; 232; 432) rotatable about a second rotation axis (A2) to generate a second rotating unbalance vector (V2); and control means configured to rotate the first and second unbalanced mass arrangements (28; 128; 228; 428, 32; 132; 232; 432) in the same direction; wherein the first and second unbalanced mass arrangements (28; 128; 228; 428, 32; 132; 232; 432) are configured such that the first rotating unbalance vector (V1) is opposed to the second rotating unbalance vector (V2), thereby generating a torsional moment (T) that opposes the torsional oscillation of the structure.

2. A system according to claim 1, wherein the first unbalanced mass arrangement (28) is configured to rotate within a range of 160° to 200° out of phase with the second unbalanced mass arrangement (32).

3. A system according to claim 2, wherein the first unbalanced mass arrangement (28) is configured to rotate 180° out of phase with the second unbalanced mass arrangement (32).

4. A system according to claim 1, comprising at least one additional unbalanced mass arrangement (434), wherein a phase angle offset (¢) between the unbalanced mass arrangements (428; 432; 434) is determined according to the equation: == where N equals the total number of unbalanced mass arrangements (428; 432; 434) in the system.

5. A system according to any one of claims 1 to 4, wherein the control means comprises an actuator (36) arranged to rotate the first unbalanced mass arrangement (28), and a coupling means (34; 134; 234) arranged to rotatably couple the first

DK 2019 70419 A1 22 unbalanced mass arrangement (28) to at least one of the other unbalanced mass arrangement(s).

6. A system according to claim 5, wherein the coupling means (134) comprises a first gear (150) rotatably coupled to the first unbalanced mass arrangement (128), a second gear (152) rotatably coupled to at least one of the other unbalanced mass arrangement(s) and a coupling gear (154) configured to rotatably couple the first gear (150) to the second gear (152).

7. A system according to claim 6, wherein the coupling gear (154) comprises a ring having teeth (144) extending radially inwards to mesh with outwardly extending teeth (146) of the first and second gears (150, 152).

8. A system according to claim 7, wherein the coupling means (34) comprises a — belt (134) or a chain.

9. A system according to claim 8, wherein the coupling means (234) comprises a second actuator (242) rotatably coupled to at least one of the other unbalanced mass arrangement (232), the second actuator (242) being configured to rotate the at least one other unbalanced mass arrangement at the same frequency as the first unbalanced mass arrangement (228).

10. A system according to any preceding claim, comprising a means of determining the frequency of the torsional oscillations in the structure, wherein the control means is configured to rotate the unbalanced mass arrangements (28; 128; 228; 428, 32; 132; 232; 432) based on the determined frequency.

11. A system according to any preceding claim, comprising a means of determining the amplitude of the torsional oscillations in the structure, wherein at least one of the unbalanced mass arrangements has a controllable moment of inertia about its axis of rotation and the control means is configured to control the moment of inertia based on the determined amplitude of the torsional oscillations in the structure.

DK 2019 70419 A1 23

12. A system according to any preceding claim, wherein the control means is configured to rotate the first and second unbalanced mass arrangements at the same frequency.

13. A system according to any preceding claim, wherein the control means is configured to receive a signal from a sensor (Ax, Ay) arranged to measure the torsional oscillations in the structure, the signal being indicative of the frequency of the torsional oscillations.

14. A system according to any preceding claim, wherein the unbalanced mass arrangements (28; 128; 228; 428, 32; 132; 232; 432) are mountable to the structure such that their rotation axes are arranged substantially in parallel with a torsional oscillation axis of the structure.

15. A structure that can undergo torsional oscillations comprising a system (24; 124; 224: 324: 424) according to any one of claims 1 to 14, wherein the system is mounted at a position where the torsional oscillations to be damped are largest.

16. A structure according to claim 15, wherein the system is arranged such that the first and second axes of rotation (A1, A2) are arranged either side of the torsional axis (AT) of the structure.

17. A structure according to claim 15 or claim 16, wherein the structure comprises a wind turbine tower (12).