Wind turbine with lightning current transfer system

Abstract

The present invention refers to a wind turbine comprising a tower, a nacelle mounted on the tower and a rotor connected to the nacelle, wherein the rotor comprising a rotor hub and a plurality of blades connected to the rotor hub, and the rotor hub is coupled to a main shaft which is rotatably supported in the nacelle by a bearing arrangement According to the invention, the wind turbine further comprises a plurality of lightning current transfer systems, each lightning current transfer system being coupled to a corresponding blade and comprising a down conductor extending between a first end and a second end, wherein the down conductor extends at least within a root portion of the corresponding blade, within the rotor hub and the main shaft, and the down conductor is guided from an interior to an exterior of the main shaft. The first end of the down conductor is located at the exterior of the main shaft and is coupled with a first contact element that is configured to provide an electrical connection with a second contact element connected to the nacelle, in such a way that, lightning current can be transferred from the corresponding blade to the second contact element by means of the down conductor.

Description

[0001] Wind turbine with lightning current transfer system

[0002] Technical Field

[0003] The invention relates to a wind turbine comprising a lightning current transfer system.

[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] The lightning current transfer units for wind turbines known at the state of the art typically have complex structures and are positioned on the outside of the wind turbines.

[0008] 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.

[0009] It is an object of the present invention to provide an alternative lightning current transfer system with a simplified current path. Summary of Invention

[0010] The present invention is based on the idea of providing a wind turbine with a lightning current transfer system with a down conductor that extends inside a portion of the wind turbine, at least within a root portion of a blade, the rotor hub and the main shaft, and wherein the down conductor is guided from an interior to an exterior of the main shaft.

[0011] According to the present invention, a wind turbine is provided, which comprises the following elements: a tower; a nacelle mounted on the tower; a rotor connected to the nacelle, the rotor comprising a rotor hub and a plurality of blades connected to the rotor hub, the rotor hub being coupled to a main shaft, which is rotatably supported in the nacelle by a bearing arrangement; a plurality of lightning current transfer systems, each lightning current transfer system being coupled to a corresponding blade and comprising a first contact element and a down conductor extending between a first end and a second end, the first contact element being coupled with the first end, wherein the down conductor extends at least within a root portion of the corresponding blade, within the rotor hub and the main shaft, and the down conductor is guided from an interior to an exterior of the main shaft, and the first end is located at the exterior of the main shaft and the first contact element is configured to provide an electrical connection with a second contact element connected to the nacelle, in such a way that, lightning current can be transferred from the corresponding blade to the second contact element by means of the down conductor.

[0012] This solution is advantageous because the lightning current transfer system efficiently transfers the lightning current from a blade of the wind turbine to the tower, so that the lightning current can be discharged to the ground without damaging the mechanical components of the wind turbine, such as the bearings. Since each blade is coupled to a corresponding lightning current transfer system and is able to discharge the lightning current, the wind turbine can efficiently transfer the lightning current to the tower and then to the ground, irrespective of the blade hit by lightning. Moreover, as the lightning current transfer system is located inside the blades and inside the rotor hub and the main shaft, it avoids that the lightning current may accidentally flow on the exterior of the wind turbine.

[0013] The down conductor may be for instance a cable, a rod, or a combination thereof.

[0014] The main shaft may be rotatably supported by a bearing arrangement directly in the nacelle.

[0015] According to an embodiment of the first aspect of the present invention, a wind turbine is provided, wherein the down conductor is guided from an interior to an exterior of the main shaft by means of an aperture formed in the main shaft. Accordingly, the aperture allows lightning current to exit the interior of the main shaft.

[0016] Preferably, the aperture is formed in the main shaft in correspondence with the central axis passing through a diameter of the tower.

[0017] According to another embodiment of the first aspect of the present invention, a wind turbine is provided, wherein the bearing arrangement comprises a first bearing and a second bearing formed at opposite ends of the main shaft, and the aperture is formed on a portion of the main shaft located between the first bearing and the second bearing.

[0018] The advantage of this configuration is that the down conductor may be guided from an interior to an exterior of the main shaft without contacting the bearings, thus avoiding damaging the bearings when transferring the lightning current.

[0019] According to alternative embodiments of the present invention, the bearing arrangement may comprise one, three, four or more bearings, and the aperture may be formed on a portion of the main shaft away from the bearings, to avoid damaging them with the lightning current.

[0020] According to another embodiment, a wind turbine is provided, wherein the first contact element is configured to be in sliding electrical contact with the second contact element during a rotation of the rotor.

[0021] The advantage of this configuration is that it provides an electrical connection when the first contact element contacts the second contact element during rotation of the rotor of the wind turbine.

[0022] As the blade pointing upward has the highest probability of being struck by a lightning, a lightning current is guided in this case to the sliding contact closest to the tower such that the blade pointing upward provides the most direct path to the tower. According to another embodiment the first contact element comprises sliding means, for example a brush element, and the second contact element comprises a ring-shaped conductor element at least partially surrounding the main shaft along an angular direction of a cylindrical coordinate system.

[0023] The advantage of this configuration is that the brush, which is indirectly connected to the rotor, forms a rotating contact that is configured to establish an electrical connection with the ring shaped conductor element forming a stationary contact on the nacelle.

[0024] For example, if the second contact element comprises a ring-shaped conductor partially surrounding the main shaft, when the sliding means (e.g. the brush) reaches the proximity of the ring-shaped conductor during rotation of the rotor, they can establish the electrical connection.

[0025] According to another embodiment of the first aspect of the present invention, a wind turbine is provided, wherein the second contact element is a ring-shaped conductor element entirely surrounding the main shaft along an angular direction of a cylindrical coordinate system.

[0026] This solution is advantageous because the second contact element forms a continuous stationary contact, which can provide a stable electrical connection with any first contact elements connected to the corresponding down conductors during rotation of the rotor. In this way, the lightning current can be directly and efficiently transferred to the ground.

[0027] According to another embodiment, a wind turbine is provided, wherein the second contact element comprises sliding means, for example a brush element, and the first contact element comprises a ring-shaped conductor element at least partially surrounding the main shaft along an angular direction of a cylindrical coordinate system.

[0028] The advantage of this configuration is that the first contact element formed around the main shaft provides a rotating contact which can provide an electrical connection with the second contact element fixed on the nacelle, during rotation of the rotor.

[0029] For example, if the first contact element comprises a ring-shaped conductor partially surrounding the main shaft, when the ring-shaped conductor reaches the proximity of the sliding means (e.g. the brush) during rotation of the rotor, they can establish the electrical connection.

[0030] For example, if the first contact element comprises a ring-shaped conductor entirely surrounding the main shaft, an electrical connection may be continuously established during rotation of the rotor.

[0031] The second contact element may be part of the main shaft housing. According to another embodiment of the first aspect of the present invention, a wind turbine is provided, wherein the main shaft is rotatably supported in a main shaft housing by the bearing arrangement, the main shaft housing being fixed to the nacelle and the second contact element is formed on an inner portion of the main shaft housing, preferably at a base of the main shaft housing nearest the tower.

[0032] The advantage of this configuration is that the lightning current can be efficiently transferred to the tower and then to the ground through the second contact element formed on an inner portion of the main shaft housing. This solution avoids that the lightning current potentially reaches other components of the wind turbine.

[0033] Preferably the second contact element is formed on an inner portion of the main shaft housing at the base thereof nearest the tower. This solution provides the shortest path for the lightning current, as it avoids that it flows on the sides of the main shaft housing away from the tower.

[0034] According to another embodiment, a wind turbine is provided, wherein the rotor rotates about a substantially horizontal axis and the first contact element is arranged on an opposite side of the horizontal axis with respect to the corresponding blade accommodating the second end of the down conductor, preferably the first contact element and the corresponding blade being arranged at substantially 180 degrees from each other about the horizontal axis.

[0035] The advantage of this configuration is that the down conductor thus provides the shortest path for the lightning current. Providing the shortest path for the lightning current reduces the voltage drop along the down conductor. This solution reduces the risk of lightning current flashing over to other components in the wind turbine.

[0036] According to another embodiment, a wind turbine is provided, wherein each of the lightning current transfer systems further comprises a junction coupled to an inner portion of the corresponding blade, preferably within a root of the corresponding blade, and the junction is electrically connected to the second end of the down conductor and to a down conductor in the corresponding blade.

[0037] The junction may be advantageously used to fix the one or more down conductors to a predefined portion of the blade, preferably within the root of the blade, thus guiding the one or more down conductors within the blade.

[0038] According to another embodiment the junction comprises a mounting bracket fixed within the corresponding blade, wherein the mounting bracket is preferably made of an insulating material. The mounting bracket may be advantageously used to fix the one or more down conductors to a predefined portion of the blade, preferably within the root of the blade, thus guiding the one or more down conductors within the blade. The mounting bracket may be advantageously made of an isolating material to avoid unwanted transfer of the lightning current to other inner portions of the blade.

[0039] According to another embodiment, a wind turbine is provided, wherein each blade is rotationally coupled to the rotor hub by means of a pitch bearing and a portion of the down conductor runs through an opening in the pitch bearing.

[0040] The advantage of this configuration is that the blade is free to rotate around the rotor hub thanks to the pitch bearing. The down conductor is advantageously guided through the opening in the pitch bearing to avoid that the lightning current accidentally reaches the pitch bearing and damages it. This solution hence increases the lifetime of the pitch bearing.

[0041] According to another embodiment, a wind turbine is provided, wherein the down conductor runs through a central axis of the pitch bearing.

[0042] This solution is advantageous as it maximizes the distance of the down conductor from the pitch bearing and it allows the down conductor to twist about its own axis when the blade pitches.

[0043] According to another embodiment of the first aspect of the present invention, a wind turbine is provided comprising three blades and three corresponding lightning current transfer systems, wherein the first contact elements of the three lightning current transfer systems are arranged around the main shaft at 120° from each other.

[0044] The advantage of this configuration is that the lightning current can be efficiently transferred to the tower of the wind turbine and to the ground irrespective of the blade hit by lightning. In fact, each blade is coupled to a corresponding lightning current transfer system and is able to discharge the lightning current to the ground.

[0045] Short description of the Figures

[0046] 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:

[0047] Fig. 1 schematically illustrates a front view of a wind turbine;

[0048] Fig. 2 illustrates a schematic view of a powertrain of the wind turbine of Fig. 1 , including a rotor arrangement; Fig. 3 schematically illustrates a cross-section of a powertrain of the wind turbine of Fig. 1 , including a rotor arrangement, according to an embodiment of the present invention;

[0049] Fig. 4 schematically illustrates a longitudinal section view of the main shaft of the wind turbine with the main bearings, according to an embodiment of the present invention;

[0050] Fig. 5 illustrates a schematic view from the front of the wind turbine according to an embodiment of the present invention, showing the features at different cross-sectional planes;

[0051] Fig. 6 schematically illustrates the working principle of the wind turbine according to an embodiment of the present invention.

[0052] Detailed Description of embodiments

[0053] 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.

[0054] 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 4b (not visible), which is rotatably supported in the nacelle 3.

[0055] 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.

[0056] Fig. 2 illustrates a schematic view of a powertrain in the nacelle 3 of the wind turbine 1 of Fig. 1 , including the rotor 4. This general system architecture is conventional and would be well understood by the skilled person. However, it provides useful context to the invention, hence it will be briefly described in the following.

[0057] As visible in Fig. 2, the rotor 4 includes the rotor hub 4a coupled to the main shaft 4b, which is rotatably supported in a main shaft housing 30 by a bearing arrangement. In the embodiment of Fig. 2, the bearing arrangement comprises a forward bearing 20 and a rear bearing 20. However, it should be understood that the bearing arrangement may comprise any number of bearings, such as one, three, four, or more. The forward and rear bearings 20 are situated between the main shaft 4b and main shaft housing 30, at forward and rearward positions, respectively, along the length of the shaft 4b. The forward and rear bearings 20 together enable the main shaft 4b to rotate freely with respect to the main shaft housing 30 during operation of the wind turbine, about a rotor axis R that extends through the centre of the main shaft 4b.

[0058] The hub 4a is connected to a plurality of rotor blades 5, generally three blades in a horizontal axis wind turbine.

[0059] The rear portion of the shaft 4b provides input drive to the gearbox 40. The gearbox 40 steps up the rotational speed of the main rotor shaft 4b via internal gears (not shown) and drives a highspeed gearbox output shaft 42. The high-speed output shaft 42 in turn drives the generator 50, which converts the rotation of the high-speed output shaft 42 into electricity. The electrical energy generated by the generator 50 may then be converted by other components (not shown here) as required before being supplied to the grid, for example, or indeed any electrical consumer. So- called “direct drive” wind turbines that do not use gearboxes are also known. The gearbox 40 may therefore be considered optional.

[0060] The main shaft housing 30 is supported on a base frame 32. Although not shown here, the base frame 32 may be coupled to a yaw drive at the upper part of the wind turbine tower 2 to enable the base frame 32 and, thus, the entire nacelle 3 to yaw with respect to the tower 2 so as to enable the direction of the hub 4a to be adjusted with respect to the wind direction.

[0061] The base frame 32 is typically a cast component, for example of steel or iron, and has the function to transfer the loads from the main shaft 4a, through the bearings 20, the main shaft housing 30, and the base frame 32, and into the wind turbine tower 2.

[0062] Fig. 3 schematically illustrates a cross-section of a powertrain of the wind turbine of Fig. 1 , including a main rotor arrangement, according to an embodiment of the present invention.

[0063] Even if in Fig. 3 only one blade 5 is shown, it is to be understood that the same arrangement is provided also in the other blades 5’, 5” of the wind turbine 1 , as shown in Fig. 5.

[0064] The wind turbine 1 is provided with a lightning current transfer system 10 for transferring a lightning current from the blade 5 to the tower 2 and finally to the ground. The lightning current transfer system 10 may be isolated from rotor hub 4a and the wind turbine nacelle 3 such that lightning current flows through the rotor hub 4a and the wind turbine nacelle 3 but not on the structure of the rotor hub 4a or the wind turbine nacelle 3. The lightning current transfer system comprises a down conductor 11 that extends within a root portion of the blade 5, within the rotor hub 4a and within the main shaft 4b, and is then guided from an interior to an exterior of the main shaft 4b through an aperture 9. The down conductor 11 has a first end and a second end. The first end is located at the exterior of the main shaft 4b and is coupled with a first contact element 14. The first end with the first contact element 14 is arranged on the opposite side of the rotor axis A of the rotor with respect to the blade 5. The second end may be coupled to a junction 13 and is fixed within the root of the blade 5.

[0065] The configuration of the down conductor 11 within the main shaft 4b is shown in detail in Fig. 4, where it is illustrated that the down conductor 11 is guided through the aperture 9 formed in the main shaft 4b. The aperture is formed on a portion of the main shaft 4b comprised between the front bearing 20 and the rear bearing 20. The down conductor 11 is then connected to the first contact element 14, which is grounded, as explained in detail below. This configuration ensures that the lightning current flows through the down conductor 11 and is transferred to the ground without damaging the front and rear bearings 20, which are positioned along the main shaft 4b away from the down conductor 11 and the aperture 9.

[0066] In the configuration of Fig. 3, the down conductor 11 is a cable, for example a high voltage cable. The down conductor cable 11 may be routed and supported within the rotor hub 4a by means of appropriate support elements of the type known in the state-of-the-art (not shown in Fig. 3). For instance, the down conductor cable 11 may be fixed to inner portions of the rotor hub 4a in several positions. According to alternative embodiments (not shown), the down conductor 11 may be a rod, or it may comprise a rod connected to a cable, such as a high voltage cable.

[0067] With continued reference to Fig. 3, it should be appreciated that the first contact element 14 is electrically connected to a second contact element 16 formed on an inner portion of the main shaft housing 30, which is located at the bottom of the main shaft housing 30, near the tower 2. The first and second contact elements 14 and 16 advantageously provide a sliding electrical contact. As schematically shown in Fig. 3, the first contact element 14 comprises sliding means, such as a brush structure, and the second contact element 16 comprises a slip-ring / commutator fixed on the housing 30. The first contact element 14 is configured to slide along the second contact element 16 during rotation of the rotor 4.

[0068] In the configurations shown in Figs. 3, 5, and 6, the second contact element 16 is a ring-shaped conductor element partially surrounding the main shaft 4b along an angular direction of a cylindrical coordinate system. However, the second contact element 16 may be also configured as a ring conductor element or sliding track entirely surrounding the main shaft 4b along an angular direction of a cylindrical coordinate system. According to an alternative embodiment (not shown), the second contact element 16 may comprise sliding means, such as a brush structure, and the first contact element 14 may comprise a slip-ring / commutator formed around the main shaft 4b.

[0069] In the configuration of Fig. 3, the aperture 9 is formed in correspondence with the central axis of the tower 2 and is equally spaced from the front bearing 20 and the rear bearing 20. The down conductor 11 with the first contact element 14 is thus connected to the second contact element 16 in correspondence with the central axis of the tower 2.

[0070] With continued reference to Fig. 3, the second end of the down conductor 11 is guided through the center of the pitch bearing 7 which is used to connect the blade 5 to the rotor hub 4a. This configuration avoids that the down conductor 11 directly or indirectly contacts the pitch bearing 7 of the blade 5, thus avoiding damage of the pitch bearing, during transfer of lightning current.

[0071] The junction 13 may be coupled to the second end of the down conductor 11 and is positioned within the root of the blade 5. The junction 13 is preferably made of an electrically conducting material which connects the down conductor 11 to the down conductors 12a / 12b. The junction 13 may comprise a mounting bracket of an isolating material to avoid unwanted transfer of the lightning current to other inner portions of the blade 5. For instance, the cable interconnector 13 may comprise a mounting bracket made of fiberglass.

[0072] As shown in Fig. 3, the junction 13 is further connected to two down conductors 12a and 12b that extend within the blade 5. The two down conductors 12a and 12b are connected to opposite portions of the blade 5 to efficiently transfer the lightning current to the root of the blade 5.

[0073] The lightning current transfer system 10 ensures that, when a lightning current reaches a blade 5, it can be easily and promptly transferred to the first contact element 14 and the second contact element 16, and then to the nacelle 3 and to the tower 2, by flowing through the down conductor 11 , without damaging the pitch bearing 7 and the main bearings 20.

[0074] In particular, the arrangement of the lightning current transfer systems 10 in the wind turbine 1 ensures that, in the event of a lightning, the current is transferred to the nacelle 3 and to the tower 2 by flowing through the most direct path. This concept will be apparent from the schematic configurations of Fig. 5 and 6.

[0075] Fig. 5 schematically illustrates a top portion of a wind turbine 1 comprising three lightning current transfer systems 10 as the ones described above. Fig. 5 illustrates the top portion of the wind turbine from the front, showing the features at different cross-sectional planes. As can be seen in Fig. 5, the wind turbine 1 comprises three lightning current transfer systems 10, one for each blade 5. Accordingly, three down conductors 11 , 11’, and 11” are routed within the rotor hub 4a, in order to connect the main shaft housing 30 and the nacelle 3 to each blade 5. It should be understood that the three down conductors 11 , 11’, and 11” extend in different portions of the rotor hub 4a and are not entangled to each other. As the rotor 4 rotates, the down conductors 11 , 11’, and 11” rotate together with the blades 5 and the rotor hub 4a and substantially maintain their relative orientation with respect to each other.

[0076] Each down conductor 11 , 1 T, and 11" is connected, on one end, to a corresponding first contact element 14, 14', and 14" and may be connected on the opposite end, to a corresponding junction 13, 13', and 13".

[0077] As can be seen in Fig. 5, each first contact element 14, 14’, and 14” is arranged at substantially 180° with respect to the corresponding blade 5, 5’, and 5” about the rotor axis R.

[0078] The working principle of the wind turbine 1 of Fig. 5 during a lightning will be now described with reference to Fig. 6.

[0079] When lightning hits the wind turbine 1 , it is more likely to hit the blade 5 pointing upwards. For example, with reference to the specific configuration of Fig. 6, the lightning will most likely hit the top blade 5. The lightning current transfer system 10 provided in the blade 5 thus ensures that the lightning current can flow through the down conductors 12a and 12b and then through the down conductor 11 with the first contact element 14. Since the first contact element 14 is arranged at substantially 180° with respect to the corresponding blade 5 about the rotation axis of the rotor 4, when the blade 5 points upwards, the first contact element 14 points downwards, with reference to the orientation of Fig. 6.

[0080] The second contact element 16 may extend partially or entirely around the main shaft 4b. In the illustrative configuration of Fig. 6 it extends only partially on the main shaft housing 30 around the main shaft 4b. When the first contact element 14 points downwards, it contacts the second contact element 16 and provides an electrical connection. The lightning current is then transferred through the main shaft housing 30, the nacelle 3 and then the tower 2 to be discharged to the ground along the direction D shown in Fig. 6. In other words, the lightning current can be discharged to the ground by flowing through the direct path formed by the down conductors 12a and 12b and the down conductor 11 providing a shortest path for lightning current discharged to ground.

[0081] It should be understood that the lightning current transfer system 10 ensures that the lightning current is transferred to the ground even in the event that the lightning strikes the non-upwardly pointing blade. With reference to the configuration of Fig. 6, for example, if the lightning hits the lateral blade 5' on the right side, the lightning current reaches the down conductors 12a'-12b', then the down conductor 1 T and the first contact element 14'. Since the first contact element 14’ is arranged at substantially 180° with respect to the corresponding blade 5’ about the rotation axis of the rotor 4, when the blade 5’ points to the right side, the first contact element 14’ points to the left side, with reference to the orientation of Fig. 6.

[0082] The second contact element 16 may extend partially or entirely around the main shaft 4b. In the illustrative configuration of Fig. 6 it extends only partially on the main shaft housing 30 around the main shaft 4b. Hence, the lightning current flows through the arched portion C of the main shaft housing 30 before reaching the second contact element 16. In case of lightning, it is not necessary to have a stable and precise electrical contact, since a lightning current is capable of bridging small irregularities and gaps, hence the current can be transferred to the second contact element 16 via the arched portion C.

[0083] The second contact element 16 then ensures that the lightning current can flow towards the tower 2. Once the lightning current has reached the tower 2, it can be discharged to the ground along the direction D. This connection ensures an efficient discharge of the lightning current through the substantially straight down conductor 11 and the arched portion C.

[0084] The same holds in the event that the lightning hits the lateral blade 5"on the left side of Fig. 6. The lightning current reaches the down conductors 12a"-12b", then the down conductor 11" and the first contact element 14". Since the first contact element 14” is arranged at substantially 180° with respect to the corresponding blade 5” about the rotation axis of the rotor 4, when the blade 5” points to the left side, the first contact element 14” points to the right side, with reference to the orientation of Fig. 6.

[0085] The second contact element 16 may extend partially or entirely around the main shaft 4b. In the illustrative configuration of Fig. 6 it extends only partially on the main shaft housing 30 around the main shaft 4b. Hence, the lightning current flows through the arched portion B of the main shaft housing 30 before reaching the second contact element 16. In case of lightning, it is not necessary to have a stable and precise electrical contact, since a lightning current is capable of bridging small irregularities and gaps, hence the current can be transferred to the second contact element 16 via the arched portion B.

[0086] The second contact element 16 then ensures that the lightning current can flow towards the tower 2. Once the lightning current has reached the tower 2, it can be discharged to the ground along the direction D. This connection ensures an efficient discharge of the lightning current through the substantially straight down conductor 11 and the arched portion B. 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.

[0087] Finally, those fields considered known to the skilled person have not been described to avoid unnecessarily covering the described invention.

[0088] REFERENCES:

[0089] 1 : wind turbine

[0090] 2: tower

[0091] 3: nacelle

[0092] 4: rotor

[0093] 4a: rotor hub

[0094] 4b: rotor shaft

[0095] 5, 5’, 5”: blade

[0096] 7: pitch bearing

[0097] 8: opening

[0098] 9: aperture

[0099] 10: lightning current transfer system

[0100] 11 , 1 T, 11”: down conductor

[0101] 12a, 12b, 12a’, 12b’, 12a”, 12b”: conductors

[0102] 13, 13’, 13”: junctions

[0103] 14, 14’, 14”: first contact element

[0104] 16: second contact element

[0105] 20: bearing 30: main shaft housing

[0106] 32: base frame

[0107] 40: gearbox

[0108] 42: gearbox output shaft 50: generator

[0109] A, B, C: arched portions

[0110] D: lightning current direction

[0111] R: rotation axis

Claims

CLAIMS1. A wind turbine (1) comprising the following elements: a tower (2); a nacelle (3) mounted on said tower (2); a rotor (4) connected to said nacelle (3), said rotor (4) comprising a rotor hub (4a) and a plurality of blades (5) connected to said rotor hub (4a), said rotor hub (4a) being coupled to a main shaft (4b), which is rotatably supported in said nacelle (3) by a bearing arrangement (20); a plurality of lightning current transfer systems (10), each lightning current transfer system (10) being coupled to a corresponding blade (5) and comprising a first contact element (14) and a down conductor (11) extending between a first end and a second end, the first contact element (14) being coupled with the first end, wherein said down conductor (11) extends at least within a root portion of said corresponding blade (5), within said rotor hub (4a) and said main shaft (4b), and said down conductor (11) is guided from an interior to an exterior of said main shaft (4b), and said first end is located at the exterior of said main shaft (4b) and said first contact element (14) is configured to provide an electrical connection with a second contact element (16) connected to said nacelle (3), in such a way that, lightning current can be transferred from said corresponding blade (5) to said second contact element (16) by means of said down conductor (11).

2. The wind turbine (1) of claim 1 , wherein said down conductor (11) is guided from an interior to an exterior of said main shaft (4b) by means of an aperture (9) formed in said main shaft (4b).

3. The wind turbine (1) of claim 2, wherein said bearing arrangement (20) comprises a first bearing (20) and a second bearing (20) formed at opposite ends of said main shaft (4b), and said aperture (9) is formed on a portion of said main shaft (4b) located between said first bearing (20) and said second bearing (20).

4. The wind turbine (1) of any of claims 1 to 3, wherein said first contact element (14) is configured to be in sliding electrical contact with said second contact element (16) during a rotation of said rotor (4).

5. The wind turbine (1) of any of claims 1 to 4, wherein said first contact element (14) comprises sliding means, for example a brush element, and said second contact element (16) comprises a ring-shaped conductor element at least partially surrounding said main shaft (4b) along an angular direction of a cylindrical coordinate system.

6. The wind turbine (1) of any of claims 1 to 5, wherein said second contact element (16) is a ring-shaped conductor element entirely surrounding said main shaft (4b) along an angular direction of a cylindrical coordinate system.

7. The wind turbine (1) of any of claims 1 to 4, wherein said second contact element (16) comprises sliding means, for example a brush element, and said first contact element (14) comprises a ring-shaped conductor element at least partially surrounding said main shaft (4b) along an angular direction of a cylindrical coordinate system.

8. The wind turbine (1) of claims 1 to 7, wherein said main shaft (4b) is rotatably supported in a main shaft housing (30) by said bearing arrangement (20), said main shaft housing being fixed to said nacelle (3) and said second contact element (16) is formed on an inner portion of said main shaft housing (30), preferably at a base of said main shaft housing (30) nearest said tower (2).

9. The wind turbine (1 ) of any of claims 1 to 8, wherein said rotor rotates about a substantially horizontal axis and said first contact element (14) is arranged on an opposite side of said horizontal axis with respect to said corresponding blade (5) accommodating said second end of said down conductor (11), preferably said first contact element and said corresponding blade being arranged at substantially 180 degrees from each other about said horizontal axis.

10. The wind turbine (1) of any of claims 1 to 9, wherein each of said lightning current transfer systems (10) further comprises a junction (13) coupled to an inner portion of said corresponding blade (5), preferably within a root of said corresponding blade (5), and said junction (13) is electrically connected to said second end of said down conductor (11) and to a down conductor in said corresponding blade (5).

11. The wind turbine (1) of claim 10, wherein said junction (13) comprises a mounting bracket fixed within said corresponding blade (5), wherein said mounting bracket (13) is preferably made of an insulating material.

12. The wind turbine (1) of any of claims 1 to 11 , wherein each blade (5) is rotationally coupled to said rotor hub (4a) by means of a pitch bearing (7) and a portion of said down conductor(11) runs through an opening (8) in said pitch bearing (7).

13. The wind turbine (1) of claim 12, wherein said down conductor (11) runs through a central axis of said pitch bearing (11).

14. The wind turbine (1) of any of claims 1 to 13 comprising three blades (5) and three corresponding lightning current transfer systems (10), wherein said first contact elements(14, 14’, 14”) of said three lightning current transfer systems (10) are arranged around said main shaft (4b) at 120° from each other.

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