The present invention relates to a wind turbine comprising a rotor with at least two wind turbine blades and a rotor hub connected to a generator unit via a main shaft, wherein the generator unit is further connected to a torque dampening unit configured to dampen the generator torque.

The present invention relates to a method of dampening a generator torque in a drivetrain of a wind turbine, comprising the steps of connecting a generator unit to a rotor with at least two wind turbine blades via a main shaft and connecting the generator unit to a torque dampening unit positioned on a mainframe.

Background of the Invention

It is known that wind turbines comprise a drivetrain (also called powertrain) having a gearbox unit mounted between the rotor and the generator for increasing the rotational speed of the rotor into a more suitable rotational speed for the generator. Such a gear box unit forms a large and expensive component which requires periodic maintenance, or even replacement, to avoid a failure. Such a maintenance or replacement process is both time consuming and expensive and it increases the downtime of the wind turbine. Another solution is to use a direct drive in which the gearbox unit is omitted and the rotor is connected directly to the generator via a rotation shaft. This reduces the total costs of the wind turbine and saves space and weight in the nacelle.

It is known to design the rotor shaft to transfer both rotational and bending forces from the rotor to the mainframe located in the nacelle. Such a large and heavy main shaft is typically supported by one or two main bearing units where the bending moments and the rotational torque are concentrated at these main bearing units. Such a solution is disclosed in US 8710693 B2 in which the front main bearing unit is further outfitted with a rotor braking mechanism. The front main bearing unit is connected to the mainframe via two hydraulic cylinders that act as dampers for dampening the angular movement of this bearing unit due to the torque applied by the rotor during braking. Only the front main bearing unit is used to transfer loads from the drivetrain to the mainframe, thereby causing an imbalance in the load transfer. In order to compensate for this imbalance, additional material must be added to the wind turbine tower to achieve the required structural strength. EP 2003362 A2 discloses a direct drive wind turbine comprising a bearing unit having an elastomeric damper mechanism. The damper mechanism comprises a spring element made of an elastic rubber material in which a central chamber is formed. The chamber is filled with a hydraulic fluid supplied by a hydraulic drive unit. The drive unit is used to apply a pre-tension force to the damper mechanism. In this solution, the generator unit is arranged between the rotor and the mainframe, thereby concentrating all the weight on one side of the wind turbine tower. The bearing unit is connected to the generator unit and to an outer surface of the main frame. EP 2732157 Al discloses a similar solution for a direct drive wind turbine where the bearing unit containing the elastomeric damper mechanism is connected to the generator unit and directly to the mainframe located in the nacelle. Two additional main bearing units are arranged at the opposite end of the rotor shaft for support of the main shaft. The counter torque generated by the generator is transferred to the mainframe via the bearing unit.

DE 102010009863 Al discloses a wind turbine comprising a generator unit coupled to a gearbox unit via a clutch coupling. The gearbox unit has two opposite facing torque arms each of which is connected to two torque damping units which in turn are connected to a mainframe of the nacelle. The generator unit is connected to the mainframe via individual support elements. The generator torque is transferred via the clutch coupling to the gearbox unit and further to the mainframe via the torque damp- ening damping units. The torque dampening units are placed in a non-optimal position for transferring torque to the mainframe which in turn means that additional material is required for the mainframe in order to provide sufficient structural strength.

Therefore, there is a need for an improved dampening unit that solves the above- mentioned drawbacks of the present solutions.

Object of the Invention

An object of the invention is to provide a wind turbine that allows the torque in the drivetrain to be dampened. An object of the invention is to provide a wind turbine that allows for an optimal transfer of forces from the drivetrain to the rest of the wind turbine.

An object of the invention is to provide a wind turbine where the configuration of the generator unit is to be altered without modifications of the remaining components of the drivetrain.

Description of the Invention

An object of the invention is achieved by a wind turbine comprising a rotor with at least two wind turbine blades and a nacelle provided on top of a wind turbine tower, the rotor comprises a rotor hub connected to a generator unit via a main shaft, the nacelle comprises a mainframe rotatably connected to the rotor hub via at least one main bearing unit, the generator unit is further connected to at least one torque dampening unit configured to dampen the generator torque, the torque dampening unit is further connected to the mainframe, wherein the torque dampening unit is aligned with the wind turbine tower, e.g. a central axis of the wind turbine tower.

This provides an optimal transfer of the generator torque to the mainframe and further to the wind turbine tower during power production. The torque dampening unit is preferably centred relative to the central axis of the wind turbine tower, e.g. the yaw bearing unit. This saves material and weight of the wind turbine tower, and optional a mainframe with a less structural strength is required. This eliminates the need for damper elements placed at the generator unit, or optionally at the gearbox unit. The term "damper element" refers to any element capable of actively or passively dampen the counter-torque or resulting angular movement generated in the generator unit.

This wind turbine configuration is suitable for applications in a low wind speed area. The 'low wind speed area' may be determined as an area having an annual average wind speed of no more than 7,5 ^m / _s , e.g. IEC class III or NREL class III. According to one embodiment, the at least one torque dampening unit is connected to the generator unit via a flange tube, wherein the main shaft is partly arranged inside the flange tube. This allows the torque dampening unit to act as a support unit for the generator unit where the generator unit is connected to the mainframe via the flange tube. The generator unit is mounted on the main shaft, thus no support structure is placed at the generator unit. This saves space in the nacelle, thus allowing the workers to move more freely around in the nacelle while providing better access to the nacelle. This eliminates the costs and complexity of conventional support elements and saves installation time as no alignment process is required. Furthermore, any significant vibrations or shocks in the generator unit, e.g. due to temporary slips in the magnetic coupling between the stator and rotor, wind guests, or deceleration/acceleration of the rotor, is transferred to the torque dampening unit, thereby minimizing the torsion forces in the main shaft and the generator unit compared to conventional generator unit resting directly on the mainframe.

This configuration forms a flexible interface, e.g. from the torque dampening unit and towards the mounting interface for the generator unit, that allows for a simple and easy modification or replacement of the generator unit. This flexible interface allows the use of different types of generators, e.g. a direct-drive generator or a high-speed generator and gearbox. It also allows the configuration of the generator unit to be modified, e.g. modification of the stator-rotor arrangement in the generator unit or replacement of the generator unit with another larger or smaller generator unit of the same type. No significant modifications of the mainframe or the main bearing unit are required.

This in turn also reduces the resonant frequency of the generator unit, e.g. to about 2Hz, compared to conventional generator unit which has a significantly higher resonant frequency. This configuration allows the control system controlling the power converter to dampen the electrical voltage and/or current oscillations (grid oscillations) in the power output by operating the generator unit in anti-phase. This is not possible in conventional wind turbines as this would cause mechanical oscillations in the generator unit. Instead, conventional wind turbines dampen these electrical oscillations by pitching the wind turbine blades out of the wind.

The flange tube has a generator end, e.g. a flange, that forms a mounting interface for the generator unit. The opposite end of the flange tube, e.g. the rotor end, comprises at least two projecting elements, e.g. arms or fingers, extending outwards from the outer side surface of the flange tube. These projecting elements may be arranged in a plane defined by the rotor end and/or the central longitudinal axis of the flange tube. The projecting elements are distributed evenly along the periphery of the rotor end. The flange tube, e.g. the intermediate tube section between the two ends, may be shaped as a cylindrical tube or cone-shaped tube. The flange tube extends along a part of the main shaft, e.g. from the torque dampening unit to the generator unit. This allows the flange tube to at least transfer torque forces from the generator unit to the torque dampening unit.

According to one embodiment, the main bearing unit comprises a first bearing part connected to the mainframe and a second bearing part connected to the rotor hub, wherein the first bearing part is configured to rotate relative to the second bearing part, and wherein the main shaft is mounted to at least a mounting flange of the rotor hub or the second bearing part.

This provides a drivetrain in which the main shaft is supported by at least one main bearing unit placed at a first opening, e.g. a rotor opening, of the mainframe. The main bearing unit may be a moment bearing configured to transfer the bending moment of the rotor, e.g. the wind turbine blades and the rotor hub, to the mainframe. The main bearing comprises at least two bearing parts rotatably connected to each other. The rotor hub comprises a mounting interface for mounting to the main bearing unit and/or the main shaft. This allows the bending moment of the rotor to be transferred directly to the mainframe via the main bearing unit, thus no significant bending moment is transferred to the main shaft. The size and weight of the rotor hub can therefore be optimized as no shaft extends through the rotor hub.

The rotor hub comprises a first mounting flange or mounting surface for mounting to the second bearing part. The main shaft comprises a first end, e.g. a rotor end, and a second end, e.g. a generator end, for mounting to the rotor and the generator unit respectively. The rotor end of the main shaft is mounted directly to the rotor hub, e.g. the mounting surface or a second mounting flange. This tube-shaped main shaft has a constant diameter, e.g. outer diameter, between the rotor end and the generator end. The side walls of this main shaft has a predetermined thickness, e.g. of several centi- metres, to achieve sufficient structural strength to absorb the various forces and moments. Alternatively, the rotor end is mounted to the second bearing part and thus the main shaft has a decreasing diameter from the rotor end towards the generator end along at least a part of the longitudinal length of the main shaft. This cone- or funnel- shaped main shaft allows to better absorb the forces and moment of inertia, thus the wall thickness is reduced compared to that of the tube-shaped main shaft.

This configuration allows the distance between the rotor and the generator unit to be increased compared to other conventional drivetrains. In conventional wind turbines, the generator unit is preferably placed as close as possible to the wind turbine tower to reduce the bending moment and torque generated in the mainframe. The generator unit is placed at a predetermined distance, e.g. between 2 to 8 metres or 3 to 6 metres, from the main bearing unit so that the bending moment generated by the generator unit substantially counteracts the bending moment of the rotor. This reduces the re- suiting bending moment acting on the main bearing unit, thereby allows it to be subjected to increased wind loads. The size and weight of the mainframe may also be reduced as the generator unit is not supported directly by the main frame.

According to one embodiment, the at least one torque dampening unit comprises at least a first damper element and a second damper element, wherein at least one projecting element of the flange tube is arranged between the first and second damper elements.

The torque dampening unit comprises two or more damper elements arranged in one or more pairs. Each pair forms a sandwich structure around each projecting element, thereby allowing this sandwich structure to dampen the torque in different directions. This sandwich structure is arranged on a supporting seat on the mainframe, thus allows the torque and other forces to be transferred to the mainframe and into the wind turbine tower. Each damper element is mounted to the adjacent projecting element or has a contact surface for contacting a matching contact surface of that projecting element. Each pair is arranged in the same plane as the rotor end and/or the central longitudinal axis of the flange tube. According to one embodiment, the at least one torque dampening unit comprising at least a first set of damper elements and a second set of damper elements, wherein the first and second sets of damper elements are arranged on opposite sides of the main shaft.

Alternatively or additionally, the damper elements of the torque dampening unit are arranged in one or more sets, where each set comprises at least one damper element or a pair of damper elements. Each individual set may be connected to or contact the same or individual projecting elements of the flange tube. Each set is arranged in the same plane as the rotor end and/or the central longitudinal axis of the flange tube. This allows the torque loads and other forces to be distributed over the sets of damper elements, thereby providing a more accurate control of the dampening effect.

According to one embodiment, the first and second damper elements and/or the first and second sets of damper elements are interconnected by means of a cross- connection.

One or more damper elements, e.g. in the same pair or set, are interconnected via one or more cross-connections so that they are operated in opposite modes, e.g. one is compressed and one is expanded when activated. Alternatively, at least two different pairs or sets of damper elements are interconnected by means of at least one cross- connection. The cross-connection may be a hydraulic, electrical, pneumatic or another suitable connection. The damper elements are connected to at least one control unit configured to control the communication between the individual damper elements and optionally drive each damper element by means of a suitable energy source, e.g. a hydraulic, electrical, or pneumatic drive unit. Alternatively, the control unit may be omitted and the damper elements may be connected directly to each other. This allows for a better control of the dampening effect. One or more sensors, e.g. vibration sensors, torque sensors, or angular displacement sensors, may be arranged in the drivetrain for measuring various control parameters. These sensors may be connected to the control unit and/or the control system of the wind turbine. The wind turbine control system may use these measured data to control the operation of the drivetrain and the pitching of the wind turbine blades. The control unit may use these measured data to control the operation of the damper elements. Alternatively, the control unit may use these measured data to adjust the dampening properties of each damper element, e.g. set a pre-determined tension or compression force.

The damper element may be an actively driven element, e.g. a hydraulic, electrical or pneumatic driven damper, connected to a suitable energy source as mentioned above. The damper element may comprise at least one moveable element connected or contacting the projecting element, which element is configured to move relative to at least one internal chamber. The damper element may alternatively be a passive element, e.g. a spring element or an elastic deformable element. The damper element is activated by the control unit and/or when torque is applied to the flange tube.

According to one embodiment, the mainframe comprises at least one opening facing the generator unit, wherein at least one of the flange tube and the main shaft extends through the at least one opening, where the inner dimensions of the at least one opening substantially corresponds to the outer dimensions of the flange tube or main shaft.

The mainframe further comprises a second opening, e.g. a generator opening, through which the main shaft and the flange tube extend. The torque dampening unit is located between the first and second openings of the mainframe. The dimensions of the first opening substantially correspond to the dimensions of the main bearing unit, e.g. the mounting interface for the rotor hub. The dimensions of the second opening substantially correspond to the outer dimensions of the flange tube. The second opening is placed in an acute angle relative to the first opening and/or the yaw bearing of the wind turbine. This allows the stiffness of the main frame to be increased as the second opening does not have to fit a gearbox unit.

Furthermore, the flange tube comprises an opening at the rotor end and the generator end for the main shaft. An air gap is formed between the opening at the rotor end and the rotating main shaft to allow for any movements of the main shaft during rotation or bending.

According to one embodiment, the wind turbine is a direct-drive wind turbine. The drivetrain of the wind turbine is preferably configured as a direct-drive system in which the gearbox unit is omitted. The generator unit is resisting on the main shaft by means of at least one supporting bearing unit, e.g. a moment bearing. Preferably, two bearing units are arranged between the generator unit and the main shaft. Alternative- ly, the generator unit comprises a generator shaft on which the generator unit is resting. The generator shaft is connected to the main shaft by means of a mounting interface, e.g. matching mounting flanges. This saves space and weight in the nacelle, thereby reducing the costs of the wind turbine and the maintenance costs. This configuration allows for the use of a direct-drive generator unit having a reduced size and weight compared to that of a high-speed generator.

According to one embodiment, the generator unit and at least a second generator unit are supportively arranged on the main shaft or a generator shaft mounted to the main shaft.

Two or more direct-drive generators may be arranged on the main shaft in an exemplary embodiment. These at least two direct-drive generators may each be configured for different operating ranges, e.g. a first wind speed range and a second wind speed range. This allows one generator unit to produce the power output at low wind speeds, while the other generator unit is switched off, i.e. not producing power. As the wind speed increases, the other generator unit is switched in so that it produces the power output while the one generator unit is switched off. A switch arrangement may be connected to both generator units where the switching is controlled by the control system. The generators may be connected in parallel to obtain an increased current out- put, alternatively in series if desired.

The generator unit may be a high-speed generator connected to a gearbox unit via an intermediate rotatable shaft. Alternatively, the gearbox unit may be integrated into the high-speed generator so they form a single unit. The high-speed generator and gear- box unit may both be arranged on the main shaft.

At least one of these generators, e.g. all the generators, is mounted directly on the main shaft. This allows for an optimal transfer of the bending moment from the generator units to the rotor hub or main bearing. Alternatively, one or more of these genera- tors, e.g. all the generators, is mounted on the generator shaft, e.g. individual generator shafts or a common generator shaft, which is mounted to the main shaft. This allows for a simple and easy replacement of the generator unit. This configuration allows multiple smaller and lighter generator types to be combined to form the required generator specifications instead of a single large and heavy generator. The generators may have the same megawatt [MW] rating or different MW ratings. In example, two 1,8 MW generators may be combined for a 3,6 MW wind turbine, three 2 MW generators may be combined for a 6 MW wind turbine, or a 2 MW generator may be combined with two 1 MW generators for a 4 MW wind turbine. This saves space in the nacelle and allows the effect to be increased without having to redesign the generator. Furthermore, this reduces the transport costs as smaller and lighter components are used, and it reduces the installation costs as less heavy lifts are required.

According to one embodiment, the generator unit comprises a stator arrangement and a rotor arrangement configured to rotate relative to the stator, wherein at least the rotor or the stator comprises superconductive coils configured to operate at a cytostatic operating temperature.

The direct-drive generator may be a permanent magnet (PM) generator, a high- temperature superconducting (HTS) generator, or another suitable direct-drive generator. The PM-generator may comprise a plurality of stator coils and/or rotor coils made of a rare earth material, e.g. neodymium. This allows for a higher energy yield, re- duced weight, and a higher power to weight ratio than with a conventional high-speed generator having the same megawatt rating. However, rare earth material is expensive.

The generator unit is preferably a HTS -generator, e.g. a partly or fully superconductive HTS -generator. The rotor coils and/or the stator coils of the HTS -generator are superconductive coils configured to be operated at a nominal cytostatic operating temperature. The nominal operating temperature may be between 1 to 100°K, e.g. 5 to 50°K. The superconductive coils are arranged in a cryostat connected to a cooling system configured to circulate a coolant to the coil arrangement. The coolant may be liquid helium or liquid nitrogen. This allows for a further reduction in size and weight of the generator unit, while eliminating the need for rare earth materials.

An object of the invention is also achieved by a method of dampening torque in a drivetrain in a wind turbine, the wind turbine comprising a rotor with at least two wind turbine blades and a nacelle provided on top of a wind turbine tower, the rotor comprises a rotor hub connected to a generator unit via a main shaft, the nacelle comprises a mainframe rotatably connected to the rotor hub via at least one main bearing unit, wherein the method comprises the steps of:

- mounting the main shaft to the rotor at one end so that a rotational torque of the rotor is transferred to the generator unit,

mounting the generator unit on the main shaft at the other end,

connecting the generator unit to at least one torque dampening unit positioned on the mainframe, wherein the at least one torque dampening unit is aligned with the wind turbine tower so that a generator torque is transferred to the mainframe at a position over a central axis of the wind turbine tower.

This provides an optimal transfer of the generator torque to the mainframe and further to the wind turbine tower as the torque dampening unit is centred relative to the cen- tral axis of the wind turbine tower, e.g. of the yaw bearing unit. This saves material and weight of the wind turbine tower and the mainframe as less structural strength is required. This also eliminates the need for damper elements placed at the generator unit, or optionally at the gearbox unit. According to one embodiment, the method further comprises the step of:

mounting the generator unit to a flange tube at one end and connecting the at least one torque dampening unit to the flange tube, e.g. at least one projecting element, at the other end, wherein the main shaft is partly arranged inside the flange tube.

This eliminates the need for expensive and complex support structure at the generator unit, thereby saving space in the nacelle and providing better access to the nacelle. This means that the workers are able to move more freely around in the nacelle. The flange tube enables any significant vibrations or shocks in the generator unit to be transferred to the torque dampening unit. This minimizes the torsion forces generated in the main shaft and the generator unit.

According to one embodiment, the step of mounting the main shaft to the rotor com- prises at least:

mounting the rotor hub to a bearing part of the at least one main bearing unit, and mounting the main shaft directly to the rotor hub, or

mounting the rotor hub to a bearing part of the at least one main bearing unit, and further mounting the main shaft to the bearing part.

This allows the bending moment of the weight of the rotor to be transferred directly to the mainframe via the main bearing unit. The main bearing unit is positioned at the first opening of the mainframe, thus enabling the generator unit to be suspended on the main shaft. By moving the generator unit further backwards relative to the main baring unit, the bending moment of the generator unit can be used to counteract the corresponding bending moment of the rotor. This allows the main shaft to be mounted directly to the rotor hub or to the main bearing unit. The main shaft is preferably a rigid shaft with good properties against axial bending moments. The flexible interface allows for an easy and simple method of modifying the configuration of the generator unit as defined above. The generator unit can be modified by: disengaging the electrical connections, the inlet and outlet of the cooling system, and other connections of the generator unit,

demounting the generator unit from the main shaft and optionally from the flange tube,

removing the generator unit and moving a new generator unit into position relative to the main shaft,

mounting the new generator unit on the main shaft and optionally mounting the flange tube to the new generator unit, and

- reengaging the electrical connections, the inlet and outlet of the cooling system, and other connections of the generator unit.

This allows the wind turbine to be outfitted with different types of generator units, such as a high-speed generator with a gearbox, a PM-generator, or a HTS -generator. The nacelle housing or cover is opened and the various couplings of the old generator unit are disengaged. The old generator unit is then demounted and removed, e.g. moved parallel relative to the central axis of main shaft. The net generator unit is positioned relative to the main shaft and moved into position, e.g. moved parallel relative to the central axis of main shaft. The new generator is then mounted and the various couplings are re-established. Finally, the nacelle housing or cover is then closed. No significant modifications of the mainframe or the main bearing unit are required, thus reducing the costs for modifying the wind turbine and the downtime of the wind turbine.

The length of the flange tube may be adapted to the selected type and/or size of generator unit. If a high-speed generator and a gearbox are selected, then both units may be mounted on the main shaft. Likewise, two or more generators, e.g. HTS -generators, may be mounted on the main shaft near the generator end. An intermediate flange tube may be mounted between the gearbox and the high-speed generator or the two generators to transfer the generator torque to the torque dampening unit.

Description of the Drawing

The invention is described by example only and with reference to the drawings, wherein:

Fig. 1 shows an exemplary embodiment of a wind turbine,

Fig. 2 shows a cross-section of the nacelle and rotor shown in fig. 1 according to a first embodiment,

Fig. 3 shows a cross-section of the nacelle and rotor shown in fig. 1 according to a second embodiment,

Fig. 4 shows a cross-section of the nacelle and rotor shown in fig. 1 according to a third embodiment,

Fig. 5 shows an exemplary embodiment of the flange tube according to the invention, and

Fig. 6 shows a cross-section of an exemplary embodiment of the torque dampening unit with a cross connection between the damper elements. In the following text, the figures will be described one by one and the different parts and positions seen in the figures will be numbered with the same numbers in the dif- ferent figures. Not all parts and positions indicated in a specific figure will necessarily be discussed together with that figure. Reference list

1 Wind turbine

2 Wind turbine tower

3 Nacelle

4 Wind turbine blades

5 Rotor hub

6 Generator unit

7 Main shaft

8 First end, rotor end

9 Second end, generator end

10 Mounting flange

11 Main bearing unit

12 Mainframe

13 First opening, rotor opening

14 Supporting bearing unit

15 Flange tube

16 Torque dampening unit

17 Central axis

18 Second generator unit

19 Generator shaft

20 Stator arrangement

21 Rotor arrangement

22 Second opening, generator opening

23 Projecting elements

24 First damper element

25 Second damper element

26 Cross connection Detailed Description of the Invention

Fig. 1 shows an exemplary embodiment of a wind turbine 1 comprising a wind turbine tower 2, a nacelle 3 arranged on top of the wind turbine tower 2, and a rotor rotatably connected to the nacelle 3, e.g. a generator unit (not shown). The rotor comprises at least two wind turbine blades 4, here three are shown, mounted to a rotor hub 5.

Fig. 2 shows a cross-section of the nacelle 3 and rotor where the nacelle cover housing is removed. The rotor hub 5 is connected to at least one/first generator unit 6 via a rotatable main shaft 7 according to a first embodiment. The main shaft 6 has a first/rotor end 8 and a second/generator end 9. In this embodiment, the main shaft 6 has a constant diameter along the longitudinal length of the shaft as shown in fig. 2. The first end 8 is connected to a mounting flange 10 of the rotor hub 5. The second end 9 is connected to the generator unit 6, e.g. to a generator shaft thereof.

The rotor hub 5 is further connected to a main bearing unit 11, e.g. via another mounting flange as shown in fig. 2. The main bearing unit 11 comprises a first bearing part 11a and a second bearing part l ib. The first bearing part 11a is connected to a mainframe 12 of the nacelle 3. The second bearing part 1 lb is connected to the rotor hub 5. The main bearing unit 11 is arranged in a first/rotor opening 13 of the mainframe 12.

The generator unit 6 is resting on the main shaft 6 via at least one supporting bearing unit 14, thus no support on the mainframe 12 is needed. The generator unit 6 is further placed at a predetermined distance, e.g. between 2 to 8 metres, from the main bearing unit 11 so that the bending moment of the generator unit 6 is used to counteract the bending moment of the rotor.

Fig. 3 shows a cross-section of the nacelle 3 and rotor where the nacelle cover housing is removed according to a second embodiment. In this embodiment, the main shaft 7' has a decreasing diameter along its longitudinal length seen from the first end 8' towards the second end 9'. Furthermore, the first end 8' of the main shaft 7' is connected to the second bearing part l ib instead of the mounting flange 10. This allows the main shaft to absorb greater moments of inertia compared to the main shaft of fig. 2. In both figs. 2 and 3, the generator unit 6 is further connected to a flange tube 15 at one end, e.g. a generator end. The flange tube 15 is at the other end, e.g. the rotor end, connected to at least one torque dampening unit 16. The flange tube 15 is configured to transfer the generator torque and other torque forces from the generator unit 6 to the torque dampening unit 16. These torque forces are then dampened as described below. The torque dampening unit 16 is arranged on a seat located on the mainframe 12. The torque dampening unit 16 and thus the seat are further aligned with the wind turbine tower 2, e.g. positioned over the central axis 17 of the wind turbine tower 2, to provide an optimal transfer of forces to the wind turbine tower 2.

The main shaft 7, 7' is arranged inside the flange tube 15 and extends through openings in the flange tube to allow for connection to the generator unit 6 and the rotor hub 5. The flange tube 15 has a cone-shaped structure where the diameter of the generator end is larger than the diameter of the rotor end as shown in figs. 2 and 3.

Fig. 4 shows a third embodiment of the nacelle 3 and the rotor where at least two generator units 6, 18 are connected to the main shaft 7. The first generator unit 6 and the second generator unit 18 are arranged relative to each other and supportively mounted on a common generator shaft 19 via individual bearing units 14. The front generator unit 6 is connected to the flange tube 15 and the second generator unit 18 is here shown as directly mounted to the first generator unit 6.

The generator unit 6, 18 comprises a stator arrangement 20 arranged relative to a rotor arrangement 21. The stator 20 comprises a plurality of stator coils (not shown) configured to interact with a plurality of rotor coils (not shown) located in the rotor 21. At least one of the generator units 6, 18 is a HTS-generator having superconductive coils forming the stator coils and/or rotor coils. The two generators 6, 18 have the same MW -rating, thus forming a wind turbine with twice the MW-rating.

Fig. 5 shows the flange tube 16 with a portion of the tube removed for illustrative purposes. The flange tube 15 comprises at least two projecting elements 23, e.g. arms, at the rotor end. The projecting elements 23 are arranged on opposite sides of the main shaft 7, 7' as shown in fig. 5. The projecting elements 23 further extend outwards from an outer surface of the flange tube in a plane defined by the rotor end of the flange tube 15.

The flange tube 15 has an outer diameter which substantially corresponds to the inner diameter of a second/generator opening 22 (shown in fig. 4).

Fig. 6 shows an exemplary embodiment of the torque dampening unit 16 seen from the rotor end of the flange tube towards the generator unit 6, 18. The generator 6, 18 is not shown for illustrative purposes. The torque dampening unit 16 comprises at least a first 24 and a second 25 damper element configured to actively dampen the generator torque, here four damper elements are shown.

The first and second damper elements 24, 25 are arranged in pair so that they form a sandwich structure around each of the projecting elements 23 as shown in fig. 6. A first set of damper elements 24, 25 is connected to a second set of damper elements 24,25 by means of a cross connection 26. In example, hydraulic fluid may be transferred from one damper element to another damper element via this cross connection. This enables the damper elements 24, 25 to dampen the generator torque in a clockwise and counter-clockwise direction.

The present invention is not limited to the illustrated embodiment or the described embodiments herein, and may be modified or adapted without departing from the scope of the present invention as described in the patent claims below.