WIND TURBINE WITH ELECTRICAL SWIVEL

Technical field

The invention relates to a turbine driven electric power production system with a closed loop hydraulic transmission system for the transfer of mechanical energy from a wind turbine to an electric generator. As opposed to conventional wind turbine systems comprising mechanical speed-up gears where the generator is arranged in the nacelle of the wind turbine power production system, the generator in the present invention is arranged on the ground or close to the ground. The location and weight of the drive train and the generator is becoming increasingly important for the installation and maintenance as the delivered power and the size of the wind turbine is increasing.

Considering that about 30 % of the downtime for a conventional wind turbine is related to the mechanical gearbox, the weight of a 5MW generator and the associated mechanical gear is typically 50 000 to 200 000 kg and that the centre of the turbine extends 100 to 150 m above the ground or sea level, it is easy to understand that the deployment and maintenance of conventional systems with mechanical gears and generator in the nacelle is both costly and difficult.

In addition, when the generator is arranged in the nacelle the power cables from the electric generator to the termination point in the bottom of the tower may become twisted when the wind turbine rotor and nacelle has to be pointed into the wind. After some turns in one direction the turbine has to be brought back to its initial position. This may require a planned, costly production stop and restart.

Wind turbine systems with a variable gear based on a hydrostatic transmission system between the wind turbine rotor and the electric generator are known from background art. The use of a hydrostatic transmission system allows for more flexible placing of the electric generator in the wind turbine power production system than with conventional technology where the placement of the components is restricted by the placement of the mechanical drive train and the gear box.

When the generator is located and arranged static on the ground or close to the ground, the problem related to twisting of electric power cables is solved. However, at least some of the components in the nacelle require electric control signals to operate properly. Likewise, the signals from a number of sensors arranged in the nacelle and

turbine rotor should be transmitted to the supervisory or control unit that may be arranged in the base of the tower. The present invention allows for heavy weight components, such as the generator, most of the hydraulic components including the fluid reservoir, pumps, and the hydraulic motor to be arranged near the ground or close to the ground. The present invention solves the problem of transferring electrical power and control and measurement signals between the nacelle and the bottom of the tower by implementing an electrical swivel that is arranged for transferring electric signals between the components in the nacelle above the swivel and the components below the swivel in a power production system where the rotor is directed into the wind. The swivel may rotate any number of turns about a vertical axis, and no twist counter and stop and start of the turbine rotor is required during normal operation for bringing the turbine back to its initial position.

Background art

In conventional wind turbine power production systems the energy from the wind is transferred mechanically, either directly or by a rotational speed-up gear to an electric generator.

The generator must rotate at a nominal speed to be able to deliver electricity to the grid or network connected to the power production system. If, during low wind speed conditions, the turbine is not supplying an appropriate level of mechanical torque to the system it will fail to deliver energy and instead the generator will act as an electric motor and the net will drive the generator and turbine through the mechanical gear.

On the other hand, if the wind is too strong the rotational frequency of the wind turbine rotor may become too high for the generator to operate properly or the mechanical apparatus could break down due to the strong forces.

Several solutions exist for overcoming the problems related to varying wind conditions. The most obvious solution is to stall and/or brake the turbine or pitch the turbine blades when the wind is too strong. Manual brakes and pitch control of the turbine blades are in use today, however, this solution may lower the efficiency of the system.

A well known solution from background art is the use of inverters to convert the output frequency of the electric generator to a desired frequency. The generator driven by the turbine will then be allowed to run at a variable rotational frequency depending on the wind speed. The use of inverters may be costly and may reduce the overall efficiency of

the system.

It is known from background art that mechanical transmission systems based on planetary gears with variable gear ratio can be employed to maintain the generator rotational speed close to a desired value during varying wind conditions.

In US patent application 2005/194787 and international patent application WO- 2004/088132 a wind turbine where the transfer of energy from the turbine to the generator is mechanically gear driven is described. The gear ratio can be varied by varying the rotational speed and direction of the outer ring of the planetary gear. In these applications a hydrostatic transmission system is used for controlling the planetary gear.

It has been proposed in several publications to use a hydrostatic transmission system comprising a hydraulic pump and a hydraulic motor for transferring energy from the turbine to the generator. By employing a hydraulic pump and/or motor with variable displacement, it is possible to rapidly vary the gear ratio of the hydraulic system to maintain the desired generator speed under varying wind conditions.

Japanese patent application JP 11287178 by Tadashi, describes a hydraulic transmission system used for the transfer of energy from a wind turbine rotor to an electric generator where the generator speed is maintained by varying the displacement of the hydraulic motor in the hydrostatic transmission system.

Hydrostatic transmission systems allow more flexibility regarding the location of the components than mechanical transmissions.

The relocation of the generator away from the top portion of the tower in a wind turbine power production system removes a significant part of the weight from the top portion of the tower. Instead the generator may be arranged on the ground or in the lower part of the tower. Such an arrangement of the hydrostatic motor and the generator on the ground level will further ease the supervision and maintenance of these components, because they may be accessed at the ground level.

International patent application 94/19605A1 by Gelhard et al. describes a wind turbine power production system comprising a mast on which is mounted a propeller which drives a generator. The power at the propeller shaft is transmitted to the generator hydraulically. The propeller preferably drives a hydraulic pump which is connected by hydraulic lines to a hydraulic motor driving the generator. The hydraulic transmission

makes it possible to arrange the very heavy generator in a machinery house on the ground. This reduces the load on the mast and thus makes it possible to design the mast and its foundation to be lighter and cheaper.

A trend in the field of so-called alternative energy is that there is a demand for larger wind turbines with higher power. Currently 5MW systems are being installed and 10 MW systems are under development. Especially for off-shore installations far away from inhabitated areas larger systems may be environmentally more acceptable and more cost effective. In this situation the weight and maintenance access of the components in the nacelle of the wind turbines is becoming a key issue. Considering that about 30 % of the downtime for a conventional wind turbine is related to the mechanical gearbox, the weight of a 5MW generator and the associated mechanical gear is typically 50 000 to 200 000 kg and that the centre of the turbine stretches 100 to 150 m above the ground or sea level, it is easy to understand that the deployment and maintenance of conventional systems with mechanical gears and generator in the nacelle is both costly and difficult.

When a generator is located in the nacelle as described for background art above, the power cables from the generator have to pass through the tower and down to a termination point near the base of the tower. Since the generator is rotated with the nacelle to point the turbine rotor into the wind, power cables must be flexible to allow twisting when the turbine rotates about a vertical axis. After some turns in one direction the turbine has to be brought back to its initial position. This requires a planned, costly production stop and restart. In background art twist counters are often used to count the number of rotations from the initial position of the nacelle and indicates when it is time to unwind the cables.

Electrical swivels are known in background art from other technical fields. In European patent application EP1443678 by Moser an inductive swivel is described. The swivel has a casing and a shaft that fits opposite the casing so as to rotate or swivel. It also has first and second coils as well as first and second halves of a split core. The first coil is fixed to the casing and the second coil to the shaft.

US 1903534 (Rime), US2106557 (Putnan), US6127739 (Appa), US6407900 (Shirakawa et al), US2004/0247437 (Otaki et al) and EP1340910 (Cavaliere) all propose the use of electrical swivels for the transfer of electrical power from an electrical generator in a rotating nacelle to the ground.

In US2007/0024058 (McClintic) slip rings are described for providing electrical power to a yaw drive, where hydraulic pressure provided by a hydraulic pump coupled to the blade assembly is stored in a high pressure storage tank and a hydraulic motor driving an electric generator is connected to the high pressure tank.

Slip rings used in wind turbine systems according to prior art for the transfer of electrical power may become large and may themselves contribute to added weight in the top of the tower. In addition slip-rings according to prior art are exposed to harsh operating conditions, especially off-shore and near-shore, where salt and water may reduce the lifetime and required service intervals of the slip rings. Thus, a solution according to background art may not contribute to the goal of the present invention to reduce the number of heavy-weight components in the nacelle and reduce the need for service and maintenance in the top of the tower.

Thus, there is a need for reducing the weight and the number of critical and heavyweight components in the nacelle in wind turbine power production systems with a new and innovative solution that reduces the number of components in the nacelle and reduces the need for service and maintenance in the top of the tower while still allowing operation and control of all components in the system for efficient transfer of energy from the turbine rotor to the electrical generator.

It can be seen from WO2007053036 (Chappie et al) that the energy transfer between a turbine rotor and an electric generator may be made more efficient by controlling the displacement of the hydraulic pump or motor based on one or more speed measurements, such as the wind speed and/or turbine speed. Thus, there is a need for a control system or a displacement actuator where the hydraulic motor is located, i.e. near the ground or close to the ground to receive signals from sensors or gauges measuring e.g. wind speed, turbine rotational speed, oil pressure and temperature, in the nacelle. Likewise, components in the nacelle, such as actuators, valves etc. may need to be controlled by the control system in the tower base.

Short summary of the invention

A wind turbine power production system with a closed loop hydrostatic transmission system comprising a closed loop with a pump and a motor connected by tubes or pipes for the transfer of mechanical energy from a wind turbine rotor to an electric generator arranged on the ground or near the ground and arranged for being driven by said motor,

wherein said wind turbine rotor is arranged for rotating about a vertical axis above an electrical swivel, wherein said swivel is arranged for transferring electrical signals, said electrical signals comprising one or more measurement signals from one or more sensors above said swivel to a control system below said swivel.

The electrical signals transferred by the swivel may also comprise electric power to electric power consumption components above said swivel. Likewise, the electrical signals may comprise signals from a control unit to one or more controlled units or components above said electrical swivel.

Brief description of drawings

The invention is illustrated in the attached drawing figures meant to illustrate preferred and alternate embodiments of the invention. The drawings shall not be construed to limit the scope of the invention which shall solely be limited by the attached claims.

Fig. 1 illustrates a simplified vertical section of a wind turbine power production system according to background art where a mechanical gear box and a generator are arranged in the nacelle, and the power cables and signal cables extend from the nacelle to the bottom of the tower.

Fig. 2 illustrates in a similar fashion to Fig. 1 , a section of a wind turbine power production system according to background art where a hydrostatic transmission system and a generator are arranged in the nacelle and the hydrostatic transmission system is used as a variable gear. As in Fig. 1 the power cables and signal cables extend from the nacelle to the bottom of the tower.

Fig. 3 and 4 illustrate schematic vertical sections of a wind turbine power production system according to the invention wherein the generator is located in the base of the tower or near the ground and an electrical swivel or union is arranged to allow the nacelle to rotate freely about a vertical axis without twisting the electrical cables and thereby allowing the wind turbine rotor to be directed into the wind continuously.

Fig. 5 illustrates a simplified cross-section of an electrical swivel with cylindrical core. Fig. 5a illustrates an electrical swivel with coils and inductive transfer, while Fig. 5b shows an electrical swivel with electrical transfer based on slip rings and brushes. The lower part of Fig. 5a and 5b show an axial view of the electrical swivel.

Fig. 6 illustrates a simplified cross-section of a disk-shaped electrical swivel. Fig. 6a illustrates an electrical swivel with coils and inductive transfer, while Fig. 6b shows an electrical swivel with electrical transfer based on slip rings and brushes. Fig. 6c illustrates an axial view of the disk-shaped electrical swivel with coils.

Fig. 7 illustrates examples of integrated electrical/hydraulic swivels with cylindrical core. Fig. 7a illustrates an integrated swivel with coils and inductive transfer, while Fig. 7b shows an integrated swivel with electrical transfer based on slip rings and brushes. The lower part of Fig. 7a and 7b shows axial views of the electrical swivels.

Fig. 8a illustrates how a disk-shaped electrical swivel and a hydraulic swivel may be arranged to provide common features for the transfer of hydraulic fluid and electrical signals. In Fig. 8b an integrated electrical/hydraulic swivel is illustrated. In this example the integrated swivel is disk-shaped, comprising coils or brushes for the transfer of electrical signals arranged around the hydraulic fluid channels.

Embodiments of the invention

The invention will in the following be described referring to the attached figures and will describe a number of embodiments according to the invention. It should be noted that the invention should not be limited to the embodiments described in this disclosure, and that any embodiments lying within the spirit of this invention should also be considered part of the disclosure.

Referring firstly to Fig. 1 of the drawings in which is shown a cross section view of a wind turbine power production system (1) according to background art. The wind power production system (1) comprises a wind turbine rotor (2) with a mechanical gear box (30) and an electric generator (20) for the transfer of mechanical energy from the wind turbine rotor (2) to electric energy from the generator (20). The gear box (30) and the generator (20) are arranged in a nacelle (3) on the top of a tower (4) of known design. The nacelle is arranged on a rotating bearing (5) so that wind turbine rotor (2) and nacelle (3) can pivot at the top of the tower (4), where the yaw of the nacelle is controlled by a yaw control system (6). The main task of the yaw control system (6) is to continuously point the wind turbine rotor (2) into the wind (or away from the wind). The electric power from the generator (20) is transported by the power cables (21 ) between the generator (20) and the electrical power terminations (22). The system may also comprise electric signal cables (63) that furnish control signals and power from a base

control unit (62) to a nacelle control unit (61) or directly to the components of the nacelle and electric signal cables (63) that furnish measurements signals from the nacelle control unit (61) or directly from the components of the nacelle to the base control unit (62).

Fig. 2 illustrates a vertical section of a wind turbine power production system (1 ) with a hydrostatic transmission system (10) used as a variable gear according to background art, for the transfer of mechanical energy from the wind turbine rotor (2) to electric energy from the generator (20). Similar to Fig. 1 the nacelle is arranged on a rotating bearing (5) with a vertical axis so that the wind turbine rotor (2) and nacelle (3) can pivot at the top of the tower (4), where the yaw of the nacelle is controlled by a yaw control system (6). The main task of the yaw control system (6) is to continuously point the wind turbine rotor (2) into the wind (or away from the wind). The system may also comprise electrical signal and power cables as shown in Fig. 1.

It is well known by the persons skilled in the art that the downtime of the mechanical gearbox used in systems according to background art as depicted in figure 1 may constitute as much as 30 % of the downtime for a conventional wind turbine. In addition the weight of a 5MW generator and the associated mechanical gear is typically 50 000 to 200 000 kg. When the centre of the turbine extends 100 to 150 m above the ground or above sea level, in the case of off-shore or near shore installations, it is understood by a person skilled in the art that the construction, deployment and maintenance of conventional systems with mechanical gears and generator in the nacelle is both costly and difficult.

A major issue with the systems according to background art as shown in Figs. 1 and 2 is that the wind turbine should preferably be continuously pointed into the wind by the yaw control system (6). The power cables (21) and signal cables (63) may then become more and more twisted if the turbine keeps rotating in the same direction for some time. After some turns in one direction the turbine has to be brought back to its initial position. A twist counter (64) will indicate to the control system (62) when it is time to unwind the cables. This may require a planned production stop and restart.

Figures 3 and 4 illustrate a vertical section of a wind turbine power production system (1 ) according to an embodiment of the invention with a closed loop hydrostatic transmission system (10) comprising a closed loop with a pump (11) and a motor (12) connected by

tubes or pipes (13,14), for the transfer of mechanical energy from a wind turbine rotor (2) to an electric generator (20) where the generator is arranged on the ground or near the ground and arranged for being driven by the motor (12). The electrical signals may comprise one or more measurement signals from one or more sensors above the swivel (7e) to a control system below the swivel (7e)

In an embodiment of the invention the wind turbine rotor (2) is arranged for rotating about a vertical axis (8) above an electrical swivel (7e) where said swivel (7e) is arranged for transferring electrical power and/or electrical control and measurement signals.

A wind turbine power production system with a hydrostatic transmission system and an electrical swivel according to the invention allows for the relocation of the generator to the base of the tower. This may significantly reduce the weight of the top portion of the tower.

The weight of a 5MW generator and the associated mechanical gear is typically 50 000 to 200 000 kg. When the centre of the turbine extends 100 to 150 m above the ground or sea level, installation of such systems may become a critical issue. In order to mount the heavy components in the nacelle, large cranes capable of lifting the heavy weight components up to the nacelle may be needed. This problem can be solved by the present invention wherein the heavy weight components may be arranged anywhere in the tower or external to the tower, above or below the tower foundation (or above or below the sea level for off-shore or near shore installations). For near-shore or off-shore installations this is particularly advantageous because of the reduced problems related to the stability of both the crane and the wind turbine power production system that are depending on varying environmental conditions.

It is understood by a person skilled in the art that the weight of a 5 MW turbine, generator and the associated gear and support system at the height of the turbine center which may extend 100 to 150 m above ground or sea level, is the most important factor for dimensioning the tower construction and the foundation or floating support of the tower and turbine. According to the present invention the generator and/or gearbox may be mounted on or below ground or sea level to reduce the weight at the turbine center. The dimensions and associated costs of the tower and the supporting system may therefore be reduced accordingly.

The arrangement of the generator near the ground or sea level will further significantly ease the accessibility and thereby the supervision and maintenance of these components. The downtime of the mechanical gearbox used in systems according to background art as depicted in figure 1 may constitute as much as 30 % of the downtime for a conventional wind turbine. Manual inspection and supervision in the nacelle is difficult and has proven dangerous during power production. However, more scheduled maintenance work may be carried out if the components are located on the ground as illustrated in Fig. 3 for the present invention. Repairs and installation of spare parts may also be significantly simpler when the generator is easily accessible near the ground (or near sea level). This becomes increasingly important with increasing nominal power delivered from the power production system and thus increasing diameter of the turbine and increasing weight of the generator and components in the nacelle.

In this embodiment of the invention the problems related to continuously pointing the turbine into the changing wind direction without having to turn the turbine back to an initial position after a rotational angle limit are solved by arranging the generator on the ground. In the background art the turbine has to be rotated back to its initial position after some turns in one direction, in order to unwind the power cables, which requires a planned and costly production stop and restart.

Slip rings used in wind turbine systems according to prior art for the transfer of electrical power may become large and may themselves contribute to added weight in the top of the tower. In addition slip-rings according to prior art are exposed to harsh operating conditions, especially off-shore and near-shore, where salt and water may reduce the lifetime and required service intervals of the slip rings. Thus, a solution according to background art may not contribute to the goal of the present invention to reduce the number of heavy-weight components in the nacelle and reduce the need for service and maintenance in the top of the tower.

Fig. 5 and Fig. 6 illustrate simplified electrical swivels that may be used in the invention. In an embodiment of the invention the electrical swivel comprises coils (90, 94) for inductive transfer of electrical signals. As can be seen in Fig. 5a and Fig 6a, said electrical swivel (7e) comprises a first member (71 ) and a second member (72) wherein said first member (71) and said second member (72) comprise coils (90, 94) for inductive transfer of electrical signals between said coils (90) arranged on said first member (71) and said coils (94) arranged on said second member (72). One of said

members (71 , 72) may be arranged fixed to the nacelle (3) and the other of said members may be arranged fixed to the tower (4). Inductive transfer of electrical signals may provide a wear-less electrical connection between the nacelle and the tower.

The use of inductive transfer of electrical signals in the swivel according to an embodiment of the invention has a number of advantages compared to slip-ring swivels used for the transfer of electrical power, such as reduced weight, less wear due to encapsulation of all parts and longer service intervals. In most practical situations the inductive swivel is suited for the transfer of small signals, e.g. signals from sensors in the nacelle to a control system in the base of the tower, control signals from the control system to components in the nacelle, and also the transfer of power to components in the nacelle with low power requirements.

Further, as can also be seen in Rg 5a, in an embodiment of the invention said first member (71) is arranged to rotate inside said second member (72). In this embodiment the diameter of the electrical swivel may be small due to the cylindrical arrangement of the coils (90, 94) on the first member (71) and the second member (72). In an embodiment of the invention the electrical components and/or control system in the nacelle may be connected to the electrical wires (73) connected to the coils (90) of the first member (71) and the electrical components and/or the control system in the tower or external to the tower may be connected to the electrical wires (74) connected to the coils (94) of the second member (72).

In an embodiment of the invention said first member (71) is arranged to rotate above or below said second member (72) as can be seen in Fig. 6a. In this disk-shaped embodiment the centre of the electrical swivel may be open, to allow other components, such as tubes, pipes, other swivels, etc. to occupy this room. In this embodiment of the invention the coils (90, 94) may also be arranged close to, or integrated with the rotating bearing (5) seen in Fig. 1. In an embodiment of the invention the electrical components and/or control system in the nacelle may be connected to the electrical wires (92) connected to the coils (90) of the first member (71 ) and the electrical components and/or the control system in the tower or external to the tower may be connected to the electrical wires (96) connected to the coils (94) of the second member (72).

The coils (90, 94) may comprise cores (91, 95) of ferromagnetic material as is obvious to a person skilled in the art.

Small signal slip rings arranged for transferring signals may be used in some wind turbine power production systems, especially when the requirements for long servie intervals are less stringent. These slip rings should preferably be much lighter than their counterparts described in background art.

In an embodiment of the invention, as can be seen in Fig 5b and Fig. 6b, the electrical swivel (7e) comprises a first member (71 ) and a second member (72) wherein one of said members (71 , 72) comprises slip rings (75) and said other member (71 , 72) comprises brushes (76) for transfer of electrical signals between said slip rings (75) and said brushes (76). One of said members (71 , 72) may be arranged fixed to the nacelle (3) and the other of said members may be arranged fixed to the tower (4).

Further, as can also be seen in Fig 5b, in an embodiment of the invention said first member (71 ) is arranged to rotate inside said second member (72). In this disk-shaped embodiment the diameter of the electrical swivel may be small due to the cylindrical arrangement of the coils (90, 94) on the first member (71) and the second member (72). In an embodiment of the invention the electrical components and/or control system in the nacelle may be connected to the electrical wires (73) connected to the coils (90) of the first member (71 ) and the electrical components and/or the control system in the tower or external to the tower may be connected to the electrical wires (74) connected to the coils (94) of the second member (72).

In an embodiment of the invention said rotating member (71 ) is arranged to rotate above or below said second member (72) as can be seen in Fig. 6 b. In this embodiment the centre of the electrical swivel may be open, to allow other components, such as tubes, pipes, other swivels, etc to occupy this room. In this embodiment of the invention the slip rings (75) and the brushes (76) may be arranged close to, or integrated with the rotating bearing (5) seen in Fig. 1. In an embodiment of the invention the electrical components and/or control system in the nacelle may be connected to the electrical wires (92) connected to the slip rings (75) of the first member (71 ) and the electrical components and/or the control system in the tower or external to the tower may be connected to the electrical wires (96) connected to the brushes (76) of the second member (72).

As will be obvious to a person skilled in the art the swivel according to the invention may be arranged up-side-down with the first member (71) fixed to the tower, and the outer second member (72) fixed to the nacelle.

In an embodiment of the invention the power production system (1) comprises a hydraulic swivel (7h) arranged for transferring fluid in the closed loop of the hydrostatic transmission system (10) as shown in Fig. 8a. The hydraulic swivel (7h) comprises an outer part (52) and an inner part (51) where the inner part can rotate inside the outer part. One of the outer part (52) or the inner part (51 ) of the hydraulic swivel (7h) is fixed to the tower (4), and the other part (51, 52) is fixed to the nacelle (3). Tubular members (53) are arranged for continuously transferring the fluid via circumferential channels (55, 56) through the swivel (7h) when the inner part of the swivel (51 ) rotates in the outer part (52) of the swivel. The dimensions and number of tubular members in the swivel depends on the application as will be obvious to a person skilled in the art.

In an embodiment of the invention the hydraulic fluid swivel (7h) is arranged as an integrated part of said electrical swivel (7e) as illustrated in Fig. 7a, 7b and 8b. In the combined electric/hydraulic swivel embodiment one of the first member (71) or second member (72) of the electrical swivel may be integrated with the inner part (51 ) of the hydraulic swivel, and the other member (71 , 72) of the electrical swivel (7e) may be integrated with the outer part (52) of the hydraulic swivel (7h).

The integrated swivel may be arranged with coils or slip rings and brushes for the transfer of electrical signals.

In an embodiment of the invention the electrical swivel (7e) is arranged inside a hydraulic swivel (7h).

In an embodiment of the invention the coils (90, 94) or brushes (76) and slip rings (75) and circumferential channels (55, 56) of the integrated electrical/hydraulic swivel (7e, 7h) may be arranged cylindrically as shown in Fig. 7a and 7b).

In an embodiment of the invention the integrated swivel is disk-shaped and the circumferential channels (55, 56) may be arranged in the centre of the disk, and the coils (90, 94) or brushes (76) and slip rings (75) may be arranged around the circumferential channels (55, 56) of the integrated swivel as shown in Fig. 8b.

For modern wind turbines both on-shore and off-shore the energy efficiency of the power production system is important for the successful deployment of such systems. As pointed out earlier the energy transfer between a turbine rotor and an electric generator may be made more efficient by controlling the displacement of the hydraulic pump or motor based on one or more speed measurements, such as the wind speed and/or

turbine speed. In an embodiment of the invention a gear ratio of the hydrostatic transmission system (10) is controlled by the control system (62) arranged for controlling a variable displacement (d) of the hydraulic motor (12).

In an embodiment of the invention the electrical coils or windings are arranged in such a way that they do not add any torque to the rotating parts of the swivel.

In an embodiment of the invention voltage equalizing coils, slip rings or any other means to avoid problems related to leak currents through the hydraulic transmission system are arranged between the nacelle and the tower as understood by a person skilled in the art. The same means may be used as elements of a lightning system. In addition appropriate earthing of the tower and the components in the system may be performed as understood by a person skilled in the art.

In an embodiment of the invention the electrical signals comprise electric power to electric power consumption components above said swivel.

When the generator has been relocated to the ground or close to the ground according to the present invention the components in the nacelle driven by electricity may be fed from the power available in the base of the tower. In such a case the electrical swivel may be arranged to transport required electric power from the base of the tower to the components in the nacelle. The number of conductors and the design of the slip rings and brushes will be obvious to a person skilled in the art.

In an embodiment of the invention the electrical signals are control signals from a control unit (62) in the base of the tower to one or more controlled units (61) above said electrical swivel (7e). The control system of the power production system may be arranged as a centralised control and supervision system or a distributed control and supervision system. When the centralised control unit is arranged in the base of the tower, control signals may be sent to actuators and other components in the nacelle for actuating or controlling the components. As an example control signals for the pitch control actuator, brake actuator, hydraulic pump, displacement actuator etc. may be transferred by the electrical swivel.

In an embodiment of the invention the electrical signals are one or more measurement signals from one or more sensors above said swivel (7e) to a control system (62) below said swivel (7e). The control and supervision system of the power production system may be arranged as a centralised control and supervision system or a distributed control

and supervision system. When the centralised control unit is arranged in the base of the tower, measurement signals may be sent from sensors in the nacelle to the centralised control and supervision system. As an example, measurement signals for the pitch angle of the rotor blades, wind speed, oil pressure, turbine speed etc. may be transferred by the electrical swivel.

The control and measurement signal may be transferred by any kind of analog or digital signal protocol such as RS232, RS422, RS485, CANbus etc. as will be understood by a person skilled in the art.

The present invention can be applied in all types of wind turbine production systems with a hydrostatic transmission system, such as in onshore, near-shore and offshore power production plants.