Lightning current transfer unit with sliding electrical contacts and wind turbine comprising same

Abstract

The present invention relates to a lightning current transfer unit for a wind turbine comprising a conductive element suitable for coupling the lightning current transfer unit to a first component of the wind turbine, a pair of sliding-contact pads biased against the conductive element and towards each other so as to clamp the conductive element, and one or more electric conductors electrically connected to the pair of sliding-contact pads The conductive element has a ring-shape with a central axis and is arranged in such a way that its central axis coincides with the axis of rotation of the first component on which is mounted with respect to a second component of the wind turbine that is rotatably connected to the first component. Moreover, the conductive element has a first surface and a second surface, opposite to the first surface, and a first sliding-contact pad is configured to establish a sliding electrical connection with the first surface and a second sliding-contact pad is configured to establish a sliding electrical connection with the second surface during the rotation. In the lightning current transfer unit according to the invention, lightning current can be transferred between the conductive element and the one or more electric conductors via the sliding-contact pads. The present invention also refers to a wind turbine comprising such a lightning current transfer unit.

Description

[0001] Lightning current transfer unit with sliding electrical contacts and wind turbine comprising same

[0002] Technical Field

[0003] The invention relates to the field of lightning current transfer units for wind turbine and wind turbine comprising same.

[0004] Description of the Related Art

[0005] Due to their size, wind turbines are prone to lightning strikes. The height of wind turbines overshoots by far surrounding trees and buildings in order to efficiently convert wind power into electric energy. Without a lightning protection system, if lightning strikes the tip of a blade of a wind turbine, the current flows in an uncontrolled manner through the blade, the blade bearings, the hub, the hub bearings, the generator shaft, the generator and the tower into the ground. Due to the high current values, there is a high risk that the bearings and electrical generators are damaged by the high voltages and currents flowing through the bearings and the generators. Therefore, efforts have been made to avoid damages in the turbine generators and the generator shaft bearings by offering a current path with lower impedance around the sensitive areas.

[0006] Lightning current transfer units are commonly used in the state of the art to transfer lightning current from a blade to the nacelle of the wind turbine and to reduce and / or avoid damage in the turbine generators and the generator shaft bearings of wind turbines.

[0007] For example, document WO 2005 / 050008 A1 discloses a lightning current transfer system for a wind turbine that comprises at least two contact means suitable for connecting a rotating and a stationary part of the wind turbine, at least one electric conductor establishing an electrical and mechanical connection between the two contact means, and force transferring means for ensuring a continuous adapting of the two contact means to the changing positions of the components of the wind turbine. The lightning current transfer system of WO 2005 / 050008 are firmly mounted between the blades and the nacelle, for instance by means of bolts or other mechanical means.

[0008] The lightning current transfer systems known in the state of the art lack redundancy. Therefore, in the case of a failure on any part of the system, the turbine is exposed to risk of lightning current passing through the pitch bearings or main bearing.

[0009] Moreover, the system design is difficult to scale with increasing sizes of the turbine rotor. It is therefore an object of the present invention to provide a lightning current transfer unit that overcomes one or more of the disadvantages discussed above.

[0010] Summary of Invention

[0011] The present invention is based on the idea of providing a lightning current transfer unit that collects lightning current from two, opposite surfaces of a conductive element used to mount said lightning current transfer unit to a component of the wind turbine, in order to ensure redundancy for current transfer in case of failure of a component.

[0012] According to a first aspect of the present invention, a lightning current transfer unit for a wind turbine is provided, which comprises:

[0013] - a conductive element having a ring-shape with a central axis and being configured to couple the lightning current transfer unit to a first component of the wind turbine such that the central axis coincides with the axis of rotation of the first component with respect to a second component of the wind turbine that is rotatably connected to the first component, the conductive element having a first surface and a second surface opposite to the first surface;

[0014] - a pair of sliding-contact pads biased against the conductive element and towards each other so as to clamp the conductive element, wherein a first sliding-contact pad is configured to establish a sliding electrical connection with the first surface and a second sliding-contact pad is configured to establish a sliding electrical connection with the second surface during the rotation; and

[0015] - one or more electric conductors electrically connected to the pair of sliding-contact pads, so that lightning current can be transferred between the conductive element and the one or more electric conductors.

[0016] The advantage of this configuration is that the lightning current transfer unit transfers lightning current through two, opposite surfaces of the conductive element, thereby providing at least two alternative electric paths for transfer of lightning current. Therefore, this configuration enhances the lifetime and reduces the stress on all current carrying components and improves the reliability of the entire unit. In particular, the lightning current through each sliding contact will be halved such that the sliding contacts are subjected to less electrical wear (e.g., caused by vaporisation of material) on each slider provided by each sliding contact. In fact, failure of one component, such as a single sliding-contact pad, will not result in the failure of the entire lightning current transfer unit, as lightning current is able to flow along an alternative electric path.

[0017] Furthermore, the lightning current transfer unit according to the invention is modular and can be used in various possible configurations and positions for lightning transfer. In the present disclosure, it is to be understood that the expression “ring-shape” may indicate a ring extending a full 360 degrees about an axis, or a ring arc extending less than 360 degrees about an axis.

[0018] According to the present invention, the conductive element having a ring-shape is advantageously mounted on the first component of the wind turbine in such a way that its central axis coincides with the axis of rotation of the first component with respect to a second component of the wind turbine that is rotatably connected to the first component. In this way, during rotation of the first component, a sliding connection can be established between the conductive element and the pair of sliding-contact pads clamping the conductive element. Preferably, the conductive element is sandwiched between the pair of sliding-contact pads so as to establish a stable sliding connection.

[0019] According to preferred embodiments, two electric conductors may be respectively electrically connected to the pair of sliding-contact pads and may be arranged in the lightning current transfer unit so that lightning current flows in the same direction in both electric conductors. In this way, the two electric conductors will experience an attractive electromagnetic force, and this attractive electromagnetic force will complement the biasing of the pair of sliding-contact pads against the conductive element, thereby rendering the entire lightning current transfer unit more compact and stable.

[0020] Preferably, the electric conductors may be electric cables.

[0021] According to an embodiment of the first aspect of the present invention, a lightning current transfer unit is provided, further comprising an electrode configured to establish a spark gap connection with the conductive element.

[0022] The advantage of this configuration is that the lightning current transfer unit is provided with another additional electrical path for transferring lightning current. Accordingly, the lightning current has three redundant paths for being discharged to the ground, i.e. two electrical paths via the sliding-contact pads and the electric conductors and one electrical path via the spark gap.

[0023] If the lightning current transfer unit is further provided with the spark gap connection, the majority of the lightning current is transferred via this spark gap, since it has a lower impedance (with respect to a lightning strike) than the sliding-contact pads. As noted above, the lightning current through each sliding contact will be halved such that the sliding contacts are subjected to less electrical wear (e.g., caused by vaporisation of material) on each slider provided by each sliding contact. Furthermore, the spark gap, when present, shares the current. Higher frequency part of a waveform of a lightning current typically takes the spark gap route and the remaining part would be divided among the two other paths such that the lightning current through each sliding contact is further reduced by the spark gap.

[0024] According to preferred embodiments, two electric conductors may be respectively electrically connected to the pair of sliding-contact pads and the two electric conductors and the spark gap connection may be arranged in the lightning current transfer unit so that lightning current flows in the same direction in both electric conductors and in the spark gap connection. In this way, the two electric conductors will experience an attractive electromagnetic force between each other and with the spark gap electrode, thereby rendering the entire lightning current transfer unit more compact and stable.

[0025] According to another embodiment of the first aspect of the present invention, a lightning current transfer unit is provided, wherein the conductive element has the shape of a ring or a ring arc.

[0026] The advantage of this configuration is that the conductive element can adapt to the range of rotation of the component on which is mounted.

[0027] For example, if the conductive element is mounted on a blade that is rotatably connected to the rotor hub by means of a pitch bearing, the conductive element is adapted to the pitch range and does not completely extend around the full perimeter of the blade.

[0028] For example, if the conductive element is mounted on the nacelle that is rotatably connected to the rotor hub, the conductive element is adapted to the rotational range of the rotor and extends a full 360 degrees, around the rotation axis.

[0029] According to another embodiment of the first aspect of the present invention, a lightning current transfer unit is provided, wherein the conductive element comprises a fixing part configured to be fixed on a surface of the first component and an angled part projecting from the fixing part, preferably forming an angle equal to or greater than 90°.

[0030] The advantage of this configuration is that the angled part protrudes from the fixing part and from the component on which the conductive element is mounted and provides two projecting surfaces that can be easily clamped by the sliding-contact pads and used to form the redundant sliding connections.

[0031] Preferably, the angled part forms an angle equal to or greater than 90° so that it can be easily clamped by the sliding-contact pads and used to form the redundant sliding connections. According to another embodiment of the first aspect of the present invention, a lightning current transfer unit is provided, wherein the first surface and the second surface of the conductive element are opposite surfaces of the angled part.

[0032] The advantage of this configuration is that the two opposite surfaces of the angled part can be easily sandwiched between the sliding-contact pads and used to form the redundant sliding connections.

[0033] According to another embodiment of the first aspect of the present invention, a lightning current transfer unit is provided, wherein the angled part is configured to establish the spark gap connection with the electrode.

[0034] The advantage of this configuration is that the angled part protrudes from the fixing part and from the component on which the conductive element is mounted and can be positioned at a predefined distance from the electrode to form a spark gap connection.

[0035] According to another embodiment of the first aspect of the present invention, a lightning current transfer unit is provided, wherein the first and second sliding-contact pads are kept in sliding electrical contact with the first and second surfaces, respectively, by means of a first flexible arm and a second flexible arm.

[0036] The advantage of this configuration is that the flexible arms can bias the sliding-contact pads against each other and against the two surfaces for establishing the sliding connection.

[0037] Preferably, the two flexible arms are arranged on opposite sides of a support element coupled to the conductive element.

[0038] According to second aspect of the present invention, a wind turbine comprising: a tower; a nacelle mounted on the tower; a rotor rotatably connected to the nacelle, the rotor comprising a hub and a plurality of blades rotatably connected to the hub; and one or more lightning current transfer units as the ones described above, wherein the conductive element of each lightning current transfer unit is arranged on the rotor or the nacelle so that the central axis of the conductive element coincides with the axis of rotation of the rotor with respect to the nacelle, or with the axis of rotation of the blade with respect to the hub. The advantage of this configuration is that the one or more lightning current transfer units receive and collect lightning current from two, opposite surfaces of the conductive element, thereby providing at least two alternative electric paths for transferring the lightning current from a blade to the tower and then to the ground. Therefore, this configuration enhances the lifetime and reduces the stress on all current carrying components and improves the reliability of the entire unit. In fact, failure of one component, such as a single sliding-contact pad, will not result in the failure of the entire lightning current transfer unit, as lightning current is able to flow along the alternative electric path.

[0039] Furthermore, the lightning current transfer unit according to the invention is modular and can be used in various possible configurations on the wind turbine. In order to ensure a stable sliding connection, the conductive element of each lightning current transfer unit is advantageously arranged on the blade, on the rotor hub, or the nacelle so that the central axis of the conductive element coincides with the axis of rotation of the rotor with respect to the nacelle, or with the axis of rotation of the blade with respect to the hub.

[0040] According to an embodiment of the second aspect of the present invention, a wind turbine is provided, comprising a plurality of lightning current transfer units, wherein the conductive element of each lightning current transfer unit extends at least partially around a surface of a root portion of a corresponding blade, so that each lightning current transfer unit is coupled to the corresponding blade.

[0041] The advantage of this configuration is that lightning current can be transferred from a blade on which the lightning current transfer unit is mounted, for instance a blade hit by a lightning, to the tower of the wind turbine and then discharged to the ground. The presence of the lightning current transfer unit ensures that lightning current is transferred from the blade in a controlled way, without affecting or damaging the pitch bearing.

[0042] According to preferred embodiments, a wind turbine is provided, wherein the conductive element extends around an external surface of the root portion of the corresponding blade.

[0043] The advantage of this configuration is that the lightning current transfer unit is mounted on the outside of the blade and can be easily accessed from outside.

[0044] According to other preferred embodiments, a wind turbine is provided, wherein the conductive element extends around an internal surface of the root portion of the corresponding blade.

[0045] The advantage of this configuration is that the lightning current transfer unit is mounted on the outside of the blade and is protected from adverse weather conditions. According to another embodiment of the second aspect of the present invention, a wind turbine is provided, comprising a lightning current transfer unit coupled to the nacelle and comprising a conductive element arranged on a surface of the nacelle, for example an external surface of the nacelle, wherein the central axis of the conductive element coincides with the axis of rotation of the rotor.

[0046] The advantage of this configuration is that lightning current can be transferred from the lightning current transfer unit mounted on the nacelle to the tower of the wind turbine and finally discharged to the ground. The presence of the lightning current transfer unit ensures that lightning current is transferred from the nacelle in a controlled way, without affecting or damaging the bearing of the main shaft.

[0047] According to another embodiment of the second aspect of the present invention, a wind turbine is provided, comprising: a first lightning current transfer unit with a first conductive element coupled to the blade so that the central axis of the first conductive element coincides with the axis of rotation of the blade with respect to the hub, and a second lightning current transfer unit with a second conductive element coupled to the nacelle so that the central axis of the second conductive element coincides with the axis of rotation of the hub with respect to the nacelle, wherein the first lightning current transfer unit is electrically connected to the second lightning current transfer unit, so that lightning current can be transferred from the blade to the nacelle.

[0048] The advantage of this configuration is that lightning current can be received from a blade, for instance a blade hit by a lightning, then transferred to the nacelle through the lightning current transfer unit mounted thereon and finally transferred to the tower of the wind turbine to be discharged to the ground. The presence of the lightning current transfer units ensures that lightning current is transferred from the blade to the nacelle in a controlled way, without affecting or damaging the bearings formed thereon, such as the pitch bearing and the main shaft bearing.

[0049] According to another embodiment of the second aspect of the present invention, a wind turbine is provided, wherein the first lightning current transfer unit is electrically connected to the second lightning current transfer unit by means of a cable.

[0050] The advantage of this configuration is that lightning current can be easily transferred from the first lightning current transfer unit to the second lightning current transfer unit. For example, if the first lightning current transfer unit is coupled to a blade and the second lightning current transfer unit is coupled to the nacelle, lightning current can be transferred from the blade to the nacelle (and then to the tower) by means of the two lightning current transfer units electrically connected to one another.

[0051] According to another embodiment of the second aspect of the present invention, the wind turbine is provided, wherein the first lightning current transfer unit is electrically connected to the second lightning current transfer unit by a portion of the hub.

[0052] The advantage of this configuration is that lightning current can be easily transferred from the first lightning current transfer unit to the second lightning current transfer unit without the need to add additional electrical conductors. In fact, the hub is generally made of a conductive material and can be used to transfer current from the first lightning current transfer unit, for instance mounted on a blade, and the second lightning current transfer unit, for instance mounted on the nacelle.

[0053] According to another embodiment of the second aspect of the present invention, a wind turbine is provided, wherein a down conductor is routed inside a blade and is electrically connected to the conductive element of the lightning current transfer unit.

[0054] The advantage of this configuration is that the down conductor can be used to transfer lightning current from a blade, for instance a blade hit by a lightning, to the conductive element of the lightning current transfer unit mounted on the blade, which can then transfer current to the nacelle and the tower, in order to discharge it to the ground.

[0055] According to another embodiment of the second aspect of the present invention, a wind turbine is provided, which comprises a lightning current transfer unit mounted on one of the blades and on the hub to transfer lightning current from the blade towards ground, wherein the conductive element is mounted to the blade or the hub and the pair of sliding-contact pads is connected to the other of the blade or the hub.

[0056] The advantage of this configuration is that it provides a stationary part and a rotating part for establishing a sliding electrical connection (in fact, when considering only the pitching movement of the blade, the hub may be considered as a stationary part).

[0057] According to another embodiment of the second aspect of the present invention, a wind turbine is provided, which comprises a lightning current transfer unit mounted on the hub and the nacelle to transfer lightning current to the nacelle, wherein the conductive element is mounted to the nacelle or the hub and the pair of sliding-contact pads is connected to the other of the nacelle or the hub. The advantage of this configuration is that it provides a stationary part and a rotating part for establishing a sliding electrical connection (in fact, when considering the rotation movement of the rotor, the nacelle may be considered as a stationary part).

[0058] Short description of the Figures

[0059] In the following, embodiments, examples, advantages and implementations of the invention will be explained in more detail by means of the accompanying figures, in which:

[0060] Fig. 1 schematically illustrates a front view of a wind turbine;

[0061] Fig. 2 schematically illustrates a cross-sectional view of a lightning current transfer unit according to an embodiment of the present invention;

[0062] Fig. 3 schematically illustrates the working principles of a lightning current transfer unit according to an embodiment of the present invention;

[0063] Fig. 4 schematically illustrates a cross-sectional view of a single lightning current transfer unit mounted on a wind turbine, according to an embodiment of the present invention;

[0064] Fig. 5 schematically illustrates a detail of a blade provided with a conductive element for a lightning current transfer unit according to an embodiment of the present invention;

[0065] Fig. 6 schematically illustrates a cross-sectional view of a pair of lightning current transfer units according to an alternative embodiment of the present invention;

[0066] Fig. 7 schematically illustrates a cross-sectional view of a pair of lightning current transfer units according to an alternative embodiment of the present invention;

[0067] Fig. 8 schematically illustrates a cross-sectional view of a pair of lightning current transfer units according to an alternative embodiment of the present invention;

[0068] Fig. 9 schematically illustrates a cross-sectional view of a pair of lightning current transfer units according to an alternative embodiment of the present invention;

[0069] Fig. 10 schematically illustrates a cross-sectional view of a pair of lightning current transfer units according to an alternative embodiment of the present invention.

[0070] Detailed Description of embodiments In the following, the present invention is described with reference to particular embodiments, as is illustrated in the enclosed figures. However, the present invention is not limited to the particular embodiments described in the following detailed description and shown in figures. Instead, the described embodiments simply exemplify the different features of the present invention, the scope of which is defined in the claims. Further modifications and variations of the present invention will be clear to the skilled person.

[0071] Fig. 1 illustrates a wind turbine 1 with a tower 2, a wind turbine nacelle 3 positioned on top of the tower 2, and a wind turbine rotor 4. The wind turbine rotor 4 comprises a rotor hub 4a and three wind turbine blades 5. The rotor hub 4a is coupled to the main shaft (not visible), which is rotatably supported in the nacelle 3.

[0072] When a wind beyond a certain level hits the wind turbine 1 , the wind activates the rotor 4 due to the lift induced on the blades 5 and allows it to rotate in a perpendicular direction to the wind. The rotation movement is converted to electric power, which is then supplied to the utility grid.

[0073] As described in detail with reference to Figs. 6-10, the wind turbine 1 may be provided with one or more lightning current transfer units 50 in order to discharge lightning current to the ground.

[0074] Fig. 2 schematically illustrates a cross-sectional view of a lightning current transfer unit 50 according to an embodiment of the present invention.

[0075] The lightning current transfer unit 50 comprises the conductive element 10, a pair of sliding- contact pads 16A and 16B, and the support element 40 which supports the lightning cables 30A and 30B and the electrode 44.

[0076] The conductive element 10 is a ring-shaped element that is configured to couple the lightning current transfer unit 50 to the blade 5, the rotor hub 4a, or the nacelle 3 of the wind turbine 1 in such a way that the central axis of the ring coincides with the axis of rotation of the blade 5, the rotor hub 4a, and / or the nacelle 3 respectively. The conductive element 10 comprises a fixing part 12 configured to fix the conductive element 10 to a component of the wind turbine 1 and an angled part 14 projecting from the fixing part 12, so as to form an angle preferably equal to or greater than 90°. In the configuration of figure 2, the fixing part 12 is in the form of a ring and the angled part 14 forms a continuous protruding edge at one end of the ring. According to alternative configurations (not shown), the fixing part 12 may consist of or comprise any other means suitable for fixing the angled part 14 to the blade, such as means for coupling bolts and threaded holes, or the like. A pair of sliding-contact pads 16A and 16B is mechanically biased against the angled part 14 of the conductive element 10 by means of respective flexible arms 20A and 20B fixed on the support element 40. The pair of sliding-contact pads 16A and 16B sandwiches the angled part 14 of the conductive element 10 in order to establish sliding electrical connections with the opposite surfaces 14A and 14B of the angled part 14 during rotation of the component of the wind turbine 1 to which the lightning current transfer unit 50 is mounted. The sliding-contact pads 16A and 16B may be made of a conductive or semi-conductive material.

[0077] Even if in figure 2, two flexible arms 20A and 20B fixed on a support element 40 are shown to keep the sliding-contact pads 16A and 16B in electrical contact with the angled part 14 of the conductive element 10, it is to be understood that any other means may be used to mechanical bias the sliding-contact pads 16A and 16B against the angled part 14. Moreover, the flexible arms 20A and 20B may be fixed on opposite surfaces of the support element 40 or on the same surface. The support element 40 may be for instance a mounting bracket. The support element 40 may be made of a conductive material, or a non-conductive material with conductive pins mounted thereon.

[0078] A pair of conductive pins 18A and 18B is mounted on the arms 20A and 20B, respectively, and is further connected to the lightning cables 30A and 30B. The lightning cables 30A and 30B are fixed to the support element 40 by means of the pins 42A and 42B and are configured to transfer the lightning current to other electrical components mounted on the wind turbine 1 and then to the ground, as will be described below.

[0079] Finally, the lightning current transfer unit 50 comprises an electrode 44 mounted on the surface of the support element 40 facing the conductive element 10. The angled part 14 of the conductive element 10 and the electrode 44 are arranged so as to define a spark gap there between. The spark gap has a width that allows a defined flash over between the angled part 14 of the conductive element 10 and the electrode 44, independent of the rotational position of the hub 4a and / or the blade 5 in the case of a lightning strike into the blade and the associated high voltage at the blade root (the high voltage being between the blade lightning protection system and the voltage potential at the hub).

[0080] Fig. 3 schematically illustrates the working principles of the lightning current transfer unit 50 of Fig. 2.

[0081] If a lightning strikes a blade, the lightning current is guided, for example through a cable within the blade (not shown in Fig. 3), to the conductive element 10. The conductive element 10 is mounted to a stationary part or to a rotating part of the wind turbine and the sliding-contact pads 16A and 16B are connected to the complementary rotating part or stationary part of the wind turbine. For example, when considering the pitching rotation of the blade, the blade may be considered as the rotating part and the hub may be considered as the complementary stationary part. For example, when considering the rotation of the rotor, the hub may be considered as the rotating part and the nacelle may be considered as the complementary stationary part. In this way, the lightning current transfer unit 50 ensures that, during rotation of one component of the wind turbine, the conductive element 10 and the sliding-contact pads 16A and 16B rotate with respect to each other and form a sliding electrical connection.

[0082] The flexible arms 20A and 20B press the sliding-contact pads 16A and 16B against the first surface 14A and the second surface 14B of the angled part 14 of the conductive element 10, respectively, so as to ensure a conductive sliding-connection on each surface of the angled part 14. In this way, the mechanism utilizes both surfaces 14A, 14B of the angled part 14 of the conductive element 10 and ensures redundancy of the system. The lightning current is then transferred from the sliding contacts 16A and 16B to the lightning cables 30A and 30B.

[0083] At the same time, the high voltages of the lightning lead to a flashover at the spark gap so that an additional electrical connection between the conductive element 10 and the electrode 44 is formed. Since the spark gap connection is the connection with the lowest impedance in the lightning current transfer unit 50 of Fig. 3, the majority of the lightning current is transferred via this spark gap in this configuration. In this way, the lightning current can flow through three redundant paths, i.e. one electrical path along direction D1 via the spark gap and two electrical paths via the sliding-contact pads 16A and 16B and the lightning cables 30A and 30B along directions D2 and D3. Since the current flows in the same direction in the lightning cables 30A and 30B and in the spark gap, the lightning cables 30A and 30B experience an attractive electromagnetic force with one another, which helps in retaining the sliding-contact pads in their positions.

[0084] It should be understood that, according to alternative configurations (not shown), a lightning current transfer unit may be also provided, which comprises two sliding-contact pads 16A and 16B formed on opposite surfaces of the angled part 14 of the conductive element 10, but that does not comprise any spark gap connection. In this configuration, redundancy of the system is ensured by the two sliding connections and current is divided in a substantially equal way between the two sliding contacts 16A and 16B and the two lightning cables 30A and 30B, which have similar (low) impedances. Fig. 4 schematically illustrates a cross-sectional view of a lightning current transfer system 100 comprising a single lightning current transfer unit 50 mounted on the wind turbine 1 .

[0085] The lightning current transfer unit 50 is mounted on an internal surface of the blade 5 and is connected to a down-conductor 7.

[0086] The blade 5 is connected to the hub 4a by means of the pitch bearing 6 and is rotatable around its longitudinal axis A1 relative to the hub 4a. This rotational movement of the blade 5 is referred to as pitching. The rotational movement of the blade 5 is limited in most cases to less than 150 degrees. Accordingly, the conductive element 10 preferably does not completely extend on the internal surface of the blade 5, but is adapted to the pitch range.

[0087] The ring of the conductive element 10 is positioned within the blade 5 so that its central axis coincides with the central axis of the root of the blade 5. This ensures that, upon rotation of the blade 5 around the axis A1 , a sliding connection is established between the conductive element 10 and the sliding-contact pads 16A and 16B sandwiching the angled part 14. In this way, a lightning current can be transferred from the down conductor 7 mounted within the blade 5 to the lightning cables 30A and 30B via the sliding-contact pads 16A and 16B and the spark gap connection. The lightning current is then transferred to the electric cable 62 connected to the lightning cables 30A and 30B. The electric cable 62 may be, for instance, routed within the rotor hub 4a and connected to the nacelle 3 to finally discharge the lightning current to the ground.

[0088] The lightning current transfer unit 50 may be further supported by a non-conductive mounting structure 61 , e.g., an arm or a rod or rail or stick or bracket etc., fixed, such as bolted, to the rotor hub 4a, in order to improve its mechanical stability during pitching of the blade 5 and to electrically isolate it from the rotor hub 4a. The non-conductive mounting structure 61 serves to mechanically support the lightning current transfer unit 50 on the rotor hub 4a and electrically isolate the lightning current transfer unit 50 against the rotor hub 4a.

[0089] Fig. 5 schematically illustrates an alternative configuration, wherein the conductive element 10 is fixed around an external surface of the root of the blade 5. In the configuration of Fig. 5, the fixing part 12 is a blade band arranged around the external surface of the root of the blade 5. In preferred configurations, the blade 5 is connected to the hub 4a by means of the pitch bearing 6 and is rotatable around its longitudinal axis A1 relative to the hub 4a. Accordingly, the conductive element 10 preferably does not completely extend completely around the external surface of the blade 5, but is adapted to the pitch range.

[0090] Fig. 6 schematically illustrates a cross-sectional view of a lightning current transfer system 102 comprising a pair of lightning current transfer units 50 and 50’ mounted on the wind turbine 1 . The configuration of the lightning current transfer unit 50 of Fig. 6 is identical to the configuration of the lightning current transfer unit 50 described with reference to Fig. 4. The lightning current transfer unit 50 is mounted on an internal surface of the blade 5 and connected to the down conductor 7 and is configured to transfer lightning current to the electric cable 62 via the two sliding connection paths and the spark gap connection.

[0091] In the lightning current transfer system 102 of Fig. 6 the electric cable 62 is connected to a sliding contact of another lightning current transfer unit 50'.

[0092] The lightning current transfer unit 50' is mounted on the surface of the nacelle 3 facing the rotor hub 4a. The ring of the conductive element 10' is formed on the nacelle 3, around the rotor hub 4a, so that the central axis of the ring coincides with the rotational axis A2 of the rotor 4 with respect to the nacelle 3. The conductive element 10' extends a full 360° around the axis A2. This configuration ensures that a sliding connection is established between the angled part 14' of the conductive element 10' and the sliding-contact pads 16A' and 16B' during rotation of the rotor hub 4a.

[0093] In the lightning current transfer system 102, the lightning current of the electric cable 62 coming from the lightning current transfer unit 50 fixed to the blade 5 is split between the lightning cables 30A' and 30B' and the spark gap connection and is directed towards the conductive element 10'. Since the conductive element 10' is in electrical contact with the nacelle 3, the lightning current can be transferred to the nacelle 3 and then to the tower 2 and finally to the ground.

[0094] Even if in Fig. 6 a single blade 5 is shown, the wind turbine 1 preferably comprises three blades 5 and each blade 5 is coupled to a corresponding lightning current transfer unit 50. With continued reference to fig. 6, if three lightning current transfer units 50 are fixed to corresponding blades 5, it should be understood that three electric cables 62 are directed towards the lightning current transfer unit 50'. In this configuration, each electric cable 62 is connected to a corresponding pair of lightning cables 30A' and 30B' mounted on a corresponding support element 40' and connected to a corresponding pair of sliding-contact pads 16A and 16B'. Each pair of sliding-contact pads 16A and 16B' is in sliding contact with the same ring of the conductive element 10'. In other words, in this configuration, the lightning current transfer unit 50' fixed to the nacelle 3 comprises a single ring extending 360° around the rotation axes A2 and three sets of lightning cables 30A' and 30B' and sliding-contact pads 16A and 16B'. This ensures that lightning current can be efficiently transferred to the nacelle 3 and to the ground irrespective of the blade 5 which is hit by the lightning.

[0095] With continued reference to Fig. 6, it can be seen that each lightning current transfer unit 50 and 50' is mounted to the rotor hub 4a by means of a non-conductive mounting structure 61 and 6T, in order to ensure mechanical stability during rotation of the rotor and electrical isolation between each lightning current transfer unit 50 and 50' and the rotor hub 4a. The non-conductive mounting structure 61 serves to mechanically support the lightning current transfer unit 50 on the rotor hub 4a and electrically isolate the lightning current transfer unit 50 against the rotor hub 4a.

[0096] Fig. 7 schematically illustrates a cross-sectional view of a lightning current transfer system 104 comprising a pair of lightning current transfer units 50 and 50’ mounted on the wind turbine 1 .

[0097] The configuration of the lightning current transfer system 104 of Fig. 7 is similar to the configuration of the lightning current transfer system 102 of Fig. 6 and it differs from the latter in the orientation of the lightning current transfer unit 50 coupled to the blade 5.

[0098] As visible in Fig. 7, the lightning current transfer unit 50 is arranged within the blade 5 so that the support element 40 is mechanically connected to an internal surface of the root of the blade 5 by means of a non-conductive mounting structure 63, e.g., an arm or a rod or rail or stick or bracket etc., and the conductive element 10 is connected to the rotor hub 4a by means of a non- conductive mounting structure 61. The non-conductive mounting structure 63 serves to mechanically support the support element 40 of the lightning current transfer unit 50 on the blade 5 and electrically isolate the support element 40 against the blade 5. Even if the lightning current transfer unit 50 of Fig. 7 has a different orientation from the lightning current transfer unit 50 of Fig. 6, it should be appreciated that the ring of the conductive element 10 is still oriented so that is central axis coincides with the rotation axis A1 of the blade 5. This ensures that a sliding connection can be established between opposite surfaces of the conductive element 10 and the pair of sliding-contact pads 16A and 16B.

[0099] In the lightning current transfer system 104 of Fig. 7, lightning current is transferred from the down conductor 7 to the lightning current transfer unit 50 through the lightning cables 30A and 30B and / or the electrode 44 of the spark gap connection. Therefore, during rotation of the blade 5, the sliding connections between the sliding-contact pads 16A and 16B and the angled part 14 of the conductive element 10 and / or the spark gap connection between the electrode 44 and the angled part 14 ensure transfer of current from the down conductor to the conductive element 10.

[0100] As shown in Fig. 7, the conductive element 10 is connected to the electric cable 62, which is then connected to the lightning current transfer unit 50' fixed to the nacelle 3. The configuration of the lightning current transfer unit 50' of Fig. 7 is identical to the configuration of the lightning current transfer unit 50' of Fig. 6. Accordingly, the lightning current transfer system 104 of Fig. 7 ensures transfer of current from the blades 5 to the nacelle 3 and then to the ground. Fig. 8 schematically illustrates a cross-sectional view of a lightning current transfer system 106 comprising a pair of lightning current transfer units 50 and 50’ mounted on the wind turbine 1 .

[0101] The configuration of the lightning current transfer system 106 of Fig. 8 is similar to the configuration of the lightning current transfer system 102 of Fig. 6 and it differs from the latter in that it does not comprise any electrical cable 62 connecting the lightning current transfer unit 50 and the lightning current transfer unit 50’.

[0102] In the lightning current transfer system 106 of Fig. 8, lightning current is transferred from the lightning current transfer unit 50 mounted on the blade 5 to the lightning current transfer unit 50' mounted on the nacelle 3 by flowing through conductive mounting structures 65, 65’ and a portion of the rotor hub 4a. As visible in Fig. 8, the lightning current transfer unit 50 is mounted on the rotor hub 4a by means of a mounting structure 65, which is fixed, such as bolted, to the hub 4a. In some illustrative examples, the mounting structure 65 may be an arm or a rod or rail or stick or bracket etc. fixed to the hub 4a. In a similar way, the lightning current transfer unit 50' is mounted on the rotor hub 4a by means of another mounting structure 65’. In some illustrative examples, the mounting structure 65 and / or 65’ may be an arm or a rod or rail or stick or bracket etc. mounted to the hub 4a. Both mounting structures 65, 65’ are made of a conductive material. Since the rotor hub 4a is also made of a conductive material, the lightning current transfer unit 50 of the blade 5 is electrically conducted to the lightning current transfer unit 50' of the nacelle 3.

[0103] When a lightning current is transferred to the lightning current transfer unit 50 through the down conductor 7, lightning current reaches first the conductive element 10 and then the lightning cables 30A and 30B via the two sliding contacts paths and the spark gap path, as explained above. In the lightning current transfer system 106 of Fig. 8, the lightning cables 30A and 30B are electrically connected to the mounting structure 65, so that current can be transferred from the down conductor 7 to the mounting structure 65 via the lightning current transfer unit 50. The lightning current then flows from the mounting structure 65 of the lightning current transfer unit 50 of the blade 5 to the mounting structure 65’ of the lightning current transfer unit 50' of the nacelle 3 via the conductive portion of the rotor hub 4a. Once the lightning current reaches the lightning cables 30A' and 30B' of the lightning current transfer unit 50' of the nacelle 3, it follows a reversed path with respect to the lightning current transfer unit 50 of the blade 5 and reaches first the lightning cables 30A’ and 30B’ and then the conductive element 10' via the two sliding-contact paths and the spark gap path. The conductive element 10' is electrically connected to the nacelle 3, so that the lightning current can be finally transferred to the nacelle 3 and then to the ground.

[0104] Fig. 9 schematically illustrates a cross-sectional view of a lightning current transfer system 108 comprising a pair of lightning current transfer units 50 and 50’ mounted on the wind turbine 1 . The configuration of the lightning current transfer system 108 of Fig. 9 is similar to the configuration of the lightning current transfer system 104 of Fig. 7 and it differs from the latter in that it does not comprise any electrical cable 62 connecting the lightning current transfer unit 50 and the lightning current transfer unit 50’.

[0105] The working principle of the lightning current transfer system 108 of Fig. 9 is similar to the one of the lightning current transfer system 106 of Fig. 8. In the lightning current transfer system 108 of Fig. 9, lightning current is transferred from the lightning current transfer unit 50 mounted on the blade 5 to the lightning current transfer unit 50' mounted on the nacelle 3 by flowing through conductive mounting structures 65, 65’ and a portion of the rotor hub 4a. As visible in Fig. 9, the lightning current transfer unit 50 is mounted on the rotor hub 4a by means of a mounting structure 65, which is fixed, such as bolted, to the hub 4a. In a similar way, the lightning current transfer unit 50' is mounted on the rotor hub 4a by means of another mounting structure 65’. Both mounting structures 65, 65’ are made of a conductive material. Since the rotor hub 4a is also made of a conductive material, the lightning current transfer unit 50 of the blade 5 is electrically conducted to the lightning current transfer unit 50' of the nacelle 3. In some illustrative examples, the mounting structure 65 and / or 65’ may be an arm or a rod or rail or stick or bracket etc. mounted to the hub 4a.

[0106] When a lightning current is transferred to the lightning current transfer unit 50 through the down conductor 7, lightning current reaches first the lightning cables 30A and 30B and then the conductive element 10 via the two sliding contacts paths and the spark gap path, as explained above. In the lightning current transfer system 108 of Fig. 9, the lightning cables 30A and 30B are electrically connected to the mounting structure 65, so that current can be transferred from the down conductor 7 to the mounting structure 65 via the lightning current transfer unit 50. The lightning current then flows from the mounting structure 65 of the lightning current transfer unit 50 of the blade 5 to the mounting structure 65’ of the lightning current transfer unit 50' of the nacelle 3 via the conductive portion of the rotor hub 4a. Once the lightning current reaches the lightning cables 30A' and 30B' of the lightning current transfer unit 50' of the nacelle 3, it follows a similar path as the lightning current transfer unit 50 of the blade 5 and reaches first the lightning cables 30A’ and 30B’ and then the conductive element 10' via the two sliding-contact paths and the spark gap path. The conductive element 10' is electrically connected to the nacelle 3, so that the lightning current can be finally transferred to the nacelle 3 and then to the ground.

[0107] Fig. 10 schematically illustrates a cross-sectional view of a lightning current transfer system 110 comprising a pair of lightning current transfer units 50 and 50’ mounted on the wind turbine 1 . The configuration of the lightning current transfer system 110 of Fig. 10 is similar to the configuration of the lightning current transfer system 102 of Fig. 6 and it differs from the latter in that the electric cable 62 connecting the two lightning current transfer units 50 and 50’ is replaced by two electric cables 62. Accordingly, a first lightning cable 30A of the first lightning current transfer unit 50 is connected to a corresponding first lightning cable 30A’ of the second lightning current transfer unit 50’ through a first cable 62 and a second lightning cable 30B of the first lightning current transfer unit 50 is connected to a corresponding second lightning cable 30B’ of the second lightning current transfer unit 50’ through a second cable 62. Since current flows in the same directions in the first set of cables 30A, 62, and 30A’ and in the second set of cables 30B, 62, and 30B’, the cables are attracted to each other and keep the structure stable and compact.

[0108] Moreover, as visible in Fig. 10, the two lightning current transfer units 50 and 50’ are mounted on corresponding mounting arms 60 and 60’ that are fixed on the same stand 60A and form a predefined angle with each other. The stand 60A is preferably bolted to the hub 4a. This configuration is flexible and easily adaptable to the rotation of the components of the wind turbine.

[0109] Even if the present invention has been described with reference to the embodiments described above, it is clear to the skilled person that it is possible to apply different modifications, variations and improvements of the present invention in light of the teachings described above and the field, and within the scope of the enclosed claims, without departing from the scope and purpose of the present invention.

[0110] Finally, those fields considered known to the skilled person have not been described to avoid unnecessarily covering the described invention.

[0111] REFERENCES:

[0112] 1 : wind turbine

[0113] 2: tower

[0114] 3: nacelle

[0115] 4: wind turbine rotor

[0116] 4a: rotor hub

[0117] 5: blade

[0118] 6: pitch bearing 7: down conductor

[0119] 10, 10’: conductive elements

[0120] 12, 12’: fixing parts

[0121] 14, 14’: angled parts

[0122] 14A: first surface of conductive element

[0123] 14B: second surface of conductive element

[0124] 16A, 16B: sliding-contact pads

[0125] 18A, 18B, 18A’, 18B’: conductive pins

[0126] 20A, 20B, 20A’, 20B’: flexible arms

[0127] 30A, 30B, 30A’, 30B’: lightning cables

[0128] 40, 40’: support elements

[0129] 44, 44’: electrodes

[0130] 50, 50’: lightning current transfer units

[0131] 60, 60’: pair of mounting arms

[0132] 62: conductive cable

[0133] 61, 6T, 64, 64’: non-conductive mounting structure

[0134] 65, 65’: conductive mounting structure

[0135] 100, 102, 104, 106, 108: lightning current transfer systems

[0136] A1 , A2: rotation axes

[0137] D, D1 : lightning current directions

Claims

CLAIMS1. A lightning current transfer unit (50) for a wind turbine comprising: a conductive element (10) having a ring-shape with a central axis and being configured to couple said lightning current transfer unit (50) to a first component of said wind turbine such that said central axis coincides with the axis of rotation of said first component with respect to a second component of said wind turbine that is rotatably connected to said first component, said conductive element (10) having a first surface (14A) and a second surface (14B) opposite to said first surface; a pair of sliding-contact pads (16A, 16B) biased against said conductive element (10) and towards each other so as to clamp said conductive element (10), wherein a first sliding- contact pad (16A) is configured to establish a sliding electrical connection with said first surface (14A) and a second sliding-contact pad (16B) is configured to establish a sliding electrical connection with said second surface (14B) during said rotation; and one or more electric conductors (30A, 30B) electrically connected to said pair of sliding- contact pads (16A, 16B), so that lightning current can be transferred between said conductive element (10) and said one or more electric conductors (30A, 30B).

2. The lightning current transfer unit (50) of claim 1 , further comprising an electrode (44) configured to establish a spark gap connection with said conductive element (10).

3. The lightning current transfer unit (50) of claim 1 or 2, wherein said conductive element (10) has the shape of a ring or a ring arc.

4. The lightning current transfer unit (50) of any of the previous claims, wherein said conductive element (10) comprises a fixing part (12) configured to be fixed on a surface of said first component and an angled part (14) projecting from said fixing part (12), preferably forming an angle equal to or greater than 90°.

5. The lightning current transfer unit (50) of claim 4, wherein said first surface (14A) and said second surface (14B) of said conductive element (10) are opposite surfaces of said angled part (14).

6. The lightning current transfer unit (50) of claim 4 or 5 in combination with claim 2, wherein said angled part (14) is configured to establish said spark gap connection with said electrode (44).

7. The lightning current transfer unit (50) of any of previous claims, wherein said first and second sliding-contact pads (16A, 16B) are kept in sliding electrical contact with said first (14A) and second (14B) surfaces, respectively, by means of a first flexible arm (20A) and a second flexible arm (20B).

8. A wind turbine (1) comprising: a tower (2); a nacelle (3) mounted on said tower (2); a rotor (4) rotatably connected to said nacelle (3), the rotor comprising a hub (4a) and a plurality of blades (5) rotatably connected to said hub (4a); and one or more lightning current transfer units (50) according to any of the previous claims, wherein said conductive element (10) of each lightning current transfer unit (50) is arranged on said rotor (4) or said nacelle (3) so that the central axis of said conductive element (10) coincides with the axis of rotation (A2) of said rotor (4) with respect to said nacelle (3), or with the axis of rotation (A1) of said blade (5) with respect to said hub (4a).

9. The wind turbine (1) of claim 8 comprising a plurality of lightning current transfer units (50), wherein said conductive element (10) of each lightning current transfer unit (50) extends at least partially around a surface of a root portion of a corresponding blade (5), so that each lightning current transfer unit (50) is coupled to said corresponding blade (5).

10. The wind turbine (1) of claim 9, wherein said conductive element (10) extends around an external surface of said root portion of said corresponding blade (5).

11. The wind turbine (1) of claim 9, wherein said conductive element (10) extends around an internal surface of said root portion of said corresponding blade (5).

12. The wind turbine (1) of any of claims 8 to 11 , comprising a lightning current transfer unit (50’) coupled to said nacelle (3) and comprising a conductive element (10’) arranged on a surface of said nacelle (3), for example an external surface of said nacelle (3), wherein the central axis of said conductive element (10’) coincides with the axis of rotation (A2) of said rotor (4).

13. The wind turbine (1) of any of claims 8 to 12, comprising:a first lightning current transfer unit (50) with a first conductive element (10) coupled to said blade (5) so that the central axis of said first conductive element (10) coincides with the axis of rotation (A1) of said blade (5) with respect to said hub (4a), and a second lightning current transfer unit (50’) with a second conductive element (10’) coupled to said nacelle (3) so that the central axis of said second conductive element (1 O’) coincides with the axis of rotation (A2) of said hub (4a) with respect to said nacelle (3), wherein said first lightning current transfer unit (50) is electrically connected to said second lightning current transfer unit (50’), so that lightning current can be transferred from said blade (5) to said nacelle (3).

14. The wind turbine (1) of claim 13, wherein said first lightning current transfer unit (50) is electrically connected to said second lightning current transfer unit (50’) by means of a cable (62).

15. The wind turbine (1) of claim 13, wherein said first lightning current transfer unit (50) is electrically connected to said second lightning current transfer unit (50’) by a portion of said hub (4a).

16. The wind turbine (1) of any of claims 8 to 15, wherein a down conductor (7) is routed inside a blade (5) and is electrically connected to said conductive element (10) of said lightning current transfer unit (50).

17. The wind turbine (1) of any of claims 8 to 16, comprising a lightning current transfer unit (50) mounted on one of said plurality of blades (5) and on said hub (4a) to transfer lightning current from said blade (5) towards ground, wherein said conductive element (10) is mounted to said blade (5) or said hub (4a) and said pair of sliding-contact pads (16A, 16B) is connected to the other of said blade (5) or said hub (4a).

18. The wind turbine (1) of any of claims 8 to 17, comprising a lightning current transfer unit (50’) mounted on said hub (4a) and said nacelle (3) to transfer lightning current to said nacelle (3), wherein said conductive element (10’) is mounted to said nacelle (3) or said hub (4a) and said pair of sliding-contact pads (16A, 16B) is connected to the other of said nacelle (3) or said hub (4a).

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