Wind turbine comprising a lightning protection system

Abstract

The invention provides a wind turbine comprising a lightning protection system for providing an electrically conductive path suitable for conducting lightning from the rotor to an electrical ground. The lightning protection system comprises a shroud electrically coupled to a wind turbine bearing housing and a front end of a wind turbine hub and / or main shaft. The shroud forms part of the electrically conductive path to electrically couple the wind turbine rotor to the bearing housing via a short circuit path that bypasses the bearing arrangement.

Description

TECHNICAL FIELD

[0001] The present invention is in the field of wind turbines and in particular in the field of lightning protection systems for wind turbines. BACKGROUND

[0002] Wind turbines can be struck by lightning in a lightning storm due to their height. The current within a typical negative cloud-to-ground lightning discharge rises very rapidly to its peak within 1-10 microseconds and then decays slowly over 50-200 microseconds. This current travels to ground via the path of least impedance when striking a blade of a wind turbine. Without any lightning protection, this path typically includes the bearings of the wind turbine that allow the hub and blades to rotate freely. This large current through these bearings can cause damage to the bearings.

[0003] To protect the bearings of a wind turbine, a number of protection systems have been developed. One such protection system is implemented by installing a spark gap device on the nacelle such that when the current travels along or down the blade (after being struck by lightning), the high current ionizes the surrounding air across the spark gap to create a current path that bypasses the bearings. One purpose of the spark gap is to not impede the rotation of the wind turbine between the hub and the nacelle. However, this creates radio frequency interference as the spark gap is active, which can be undesirable, in addition, the proportion of the total current that takes the spark gap route can be unpredictable, and the bearings can still be damaged.

[0004] There exists lightning protection systems with a direct connection between the blade and the nacelle, for example in WO 2005 / 050008 Al. This describes a lightning current transfer unit (LCTU) comprising at least two sliding contact points adapted to connect a blade and a nacelle or tower. The LCTU further comprises at least one electrical conductor establishing a dedicated connection between surfaces where the two sliding contact points touch. The sliding contact point on the blade is typically in contact with a blade band, which is a metal band around the circumference of the blade root end. It is important that the blade band is at least a minimum distance from the metal parts that join the blade root to the hub, so that the preferred current path goes through the LCTU. There is a trend towards larger and longer blades, and as the blade length increases, the position of the blade band can be outside the end of the front surface of the nacelle. Thus, the LCTU solution imposes design constraints on the physical parts of the blade and nacelle that prevent arcing between parts at different voltages. At a certain blade length, the LCTU solution will reach physical limits and alternative solutions are needed. SUMMARY

[0005] A wind turbine according to a first aspect of the invention comprises: a rotor comprising a hub and a plurality of blades projecting outwardly from the hub; a main shaft attached at a forward end to the hub and extending at a rearward end to a gearbox or generator; a bearing housing for rotatably supporting the main shaft via a bearing arrangement; and a lightning current protection system providing an electrically conductive path adapted to conduct lightning from the rotor to an electrical ground, wherein the bearing housing is electrically connected to the electrical ground, the lightning current protection system comprises a shroud forming part of the electrically conductive path, wherein a rearward end of the shroud is in contact with the bearing housing and a forward end of the shroud is in contact with the hub and / or the forward end of the main shaft, and the shroud electrically couples the rotor to the bearing housing via a short circuit path that bypasses the bearing arrangement, wherein the shroud is arranged to surround the forward end of the main shaft.

[0006] The shroud can have a larger diameter than the main shaft at the contact with the bearing housing.

[0007] The wind turbine of the first aspect can further comprise a nacelle housing the bearing housing, wherein the shroud is enclosed within the nacelle.

[0008] The shroud can form a sliding or rolling contact with one of: (i) the hub and / or the forward end of the main shaft; or (ii) the bearing housing, and the sliding or rolling contact provides the electrical coupling.

[0009] The shroud can form a sliding contact with one of: (i) the hub and / or the forward end of the main shaft; or (ii) the bearing housing, at a plurality of discrete contact points around a circumference of the shroud.

[0010] The shroud can be electrically coupled by at least one fixed contact point at the forward or rearward end of the shroud and electrically coupled by at least one sliding or rolling contact point at the rearward or forward end of the shroud.

[0011] The shroud can comprise a plurality of metal sheets that are biased into contact with the bearing housing or the forward end of the hub and / or the main shaft. The shroud can alternatively comprise a metal mesh, grid or net.

[0012] The shroud, which can comprise a metal mesh, grid or net, can further comprise a band or ring for biasing the shroud into sliding contact with one of: (i) the hub and / or the forward end of the main shaft; or (ii) the bearing housing.

[0013] The shroud can have a relatively small thickness. Preferably, the thickness of the shroud is less than 10mm.

[0014] The forward end of the main shaft can have a flared end attached to the hub; the shroud can be in contact with the bearing housing and the flared end of the main shaft; and the shroud can electrically couple the flared end of the main shaft to the bearing housing. The diameter of the flared end of the main shaft at the contact with the shroud can be greater than the diameter of the bearing housing at the contact with the shroud.

[0015] The lightning current protection system can avoid air gaps or spark gaps in the electrical conduction path for conducting lightning from the hub to the electrical ground.

[0016] The shroud can comprise copper.

[0017] The bearing housing can comprise steel.

[0018] At least one of the plurality of blades can comprise a power component that receives power via the hub.

[0019] There can be at least two electrical conduction paths from the rotor to the electrical ground, one via the shroud and one via the main shaft and / or bearing arrangement.

[0020] The preferred electrical conduction path for DC current from the rotor to the electrical ground can be via the main shaft and / or bearing arrangement.

[0021] For transient lightning current, the preferred electrical conduction path from the rotor to the electrical ground can be via the shroud.

[0022] These and other aspects, features and / or advantages of the present application will become apparent to those persons skilled in the art upon reading the following detailed description. BRIEF DESCRIPTION OF DRAWINGS

[0023] Embodiments of the present application will now be described, by way of example only, with reference to the accompanying drawings in which:

[0024] Figure 1a An example of a wind turbine is illustrated in a schematic perspective view.

[0025] Figure 1b An example of a wind turbine according to the present application is illustrated in a schematic side view.

[0026] Figure 2a An example of a main shaft, bearing housing and shroud is illustrated in a schematic perspective view.

[0027] Figure 2b The bearing arrangement behind the bearing housing is illustrated in a partial cross-sectional view. Figure 2a

[0028] An example of an arrangement with a shroud and conduction paths is illustrated in a schematic side view. Figure 3

[0029] The impedance of the two paths that current can take, through the shroud and through the bearing arrangement, is illustrated in a circuit diagram. Figure 4

[0030] An example of a shroud is illustrated in a schematic isometric view. Figure 5a

[0031] ​Figure 5b An example of a shroud is shown in a schematic side view. Figure 3

[0032] Figure 6a An example of a shroud contact point is shown in a schematic isometric view.

[0033] Figure 6b An example of a shroud contact point is shown in a schematic isometric view.

[0034] Figure 7 An example of a device with a shroud and a conduction path is shown in a schematic side view.

[0035] Figures 8a to 8d An example of an alternative shroud connection device is shown in a schematic side view. DETAILED DESCRIPTION

[0036] Figure 1a An example of a wind turbine 1 is shown in a schematic perspective view. The wind turbine 1 comprises a tower 2, a nacelle 3 at the top of the tower, and a rotor 4 operatively coupled to a generator housed within the nacelle 3. In addition to the generator, the nacelle houses various components required to convert wind energy into electrical energy and various components required to operate, control, and optimize the performance of the wind turbine 1.

[0037] The rotor 4 of the wind turbine comprises a central hub 5 and a plurality of blades 6 protruding outwardly from the central hub 5. It will be noted that the wind turbine 1 is of the common type of horizontal axis wind turbine (HAWT), such that the rotor 4 is mounted at the nacelle 3 so as to rotate about a substantially horizontal axis defined at the center of the hub 5. Although the example shown has three blades, those skilled in the art will recognize that other numbers of blades are possible. Figure 1a

[0038] When wind blows towards the wind turbine 1, the blades 6 generate lift forces that cause the rotor 4 to rotate, which in turn causes the generator within the nacelle 3 to generate electrical energy.

[0039] On each rotor blade 6, there is at least one lightning receptor 7 or a lightning cover positioned at a likely location for a lightning strike to the wind turbine. In addition, on at least one of the plurality of blades 6, there can be an electrical component 8, such as a de-icing unit, a lighting unit, etc., that receives electrical power via the hub. The electrical components in the blades need protection from lightning strikes, in part due to their electrical power connection via the hub.

[0040] ​​The wind turbine 1 can be comprised in a collection of other wind turbines belonging to a wind power plant, also referred to as a wind farm or wind park, which serves as a power plant connected to an electric power network via transmission lines. The electric power network typically comprises a network of power plants, transmission circuits and substations linked by transmission lines, which transmit electric power to loads in the form of end users and other customers of the electric utility.

[0041] Figure 1b An embodiment of the wind turbine 1 is schematically illustrated and the interior of the nacelle 3 is shown. The nacelle 3 comprises a nacelle frame 13 which structurally supports the nacelle 3 and components within the nacelle 3. The wind turbine 1 comprises rotor blades 6 which are mechanically connected to a generator 10 via a gearbox 9. In direct drive systems and other systems, the gearbox 9 can be absent. The electric power generated by the generator 10 is injected into the electric power network via an electric converter (not shown). A main shaft 11 is mechanically attached at a front end to the hub 5. A bearing housing 12 is mechanically attached to the nacelle frame 13 and is configured to rotatably support the main shaft 11 such that the bearing housing 12 supports the hub 5 and the plurality of blades 6 to allow them to rotate relative to the nacelle 3. The main shaft 11 extends through the bearing housing 12 and enters the gearbox 9 (or the electric generator 10 in direct drive systems) at a rear end.

[0042] In Figure 1b A lightning protection system is also shown. The lightning protection system provides an electrically conductive path adapted to conduct lightning from the rotor 4 to an electric ground. The lightning current protection system comprises a shroud 14 forming part of the electrically conductive path. Figure 1b The shroud 14 is shown in contact with the rotor 4 and at the rear end with the bearing housing 12, precisely, Figure 1b The shroud 14 is shown in contact with the rotor 4 and at the rear end with the bearing housing 12, precisely,

[0043] The arrangement of the shroud 14 around the front end of the main shaft 11 facilitates a skin effect such that when a lightning current flows through the shroud 14 along the electrically conductive path, the bearing arrangement of the bearing housing 12 is not damaged when hit by lightning or a larger proportion of the lightning current flows through the shroud compared to through the bearing arrangement. The nacelle 3 can house the bearing housing 12 and the shroud 14 can be enclosed within the nacelle 3.

[0044] Figure 2a and Figure 2bThe connection to the front end of the spindle 11 and to the shroud 14 of the bearing housing 12 is shown in more detail. The bearing housing 12 may include bearing races 16 (e.g., front and rear bearing races), a cover 17, and structural support members 18 mechanically attached to the nacelle frame 13. For structural strength, the bearing housing 12 may comprise steel, alloys, steel-containing alloys, suitable composite materials, or combinations thereof. It should be noted that the bearing housing 12 may have associated electrical conductivity depending on the material selection.

[0045] Figure 2b Demonstrated without structural support member 18 and cabin frame 13 Figure 2a The spindle 11, housing 14, and bearing housing 12 are shown, with some of the cover 17, bearing race 16, housing 14, and spindle 11 removed to reveal the interior of bearing housing 12. The spindle passing through bearing housing 12 is visible. A bearing assembly 20, which may include a bearing 21, is shown located inside bearing race 16. Bearing assembly 20 provides a mechanical connection between spindle 11 and bearing housing 12 to allow spindle 11 to rotate within bearing race 16. Bearing 21 can be cylindrical, spherical, or any functional shape. Bearing 21 can generate a low coefficient of friction between spindle 11 and bearing race 16.

[0046] The front end of the spindle 11 may have a flared end for attachment to the hub 5. This is advantageous for mechanically attaching the spindle 11 to the hub 5. Figure 2a and Figure 2b As shown, the protective cover 14 can contact the bearing housing 12 and the flared end of the spindle 11. The protective cover 14 can electrically connect the flared end of the spindle 11 to the bearing housing 12.

[0047] Figure 3 Showing Figure 2a and Figure 2b A schematic side view of the device, in which hub 5 is shown attached to main shaft 11. Electrical grounding part 24 can be electrically connected to bearing housing 12. Lightning strike point 23 is shown as the point where lightning initially struck wind turbine 1. Figure 3 Two electrical conduction paths of lightning current are shown. The first electrical conduction path includes the lightning strike point 23, hub 5, main shaft 11, bearing assembly 20, bearing housing 12, and electrical grounding part 24. The second electrical conduction path includes the lightning strike point 23, hub 5, shield 14, the exterior of bearing housing 12, and electrical grounding part 24. These two electrical conduction paths may further include a path from bearing housing 12 via nacelle frame 13 and tower 2 to electrical grounding part 24.

[0048] Figure 4A schematic of the circuit path from the lightning strike point 23 to the electrical ground 24 is shown. The first electrical conduction path 26 is from the rotor 4 via the bearing arrangement 20 to the electrical ground 24. Specifically, lightning can strike the blade 6 at the lightning strike point 23 and travel along the blade, through the hub 5, the main shaft 11, the bearing arrangement 20, the bearing housing 12 and then the nacelle frame 13 to the electrical ground 24. However, if there is a large current passing through, the bearings 21 of the bearing arrangement 20 can be damaged (e.g. fused to the surrounding components).

[0049] The shroud 14 electrically couples the rotor 4 to the bearing housing 14 via a short circuit path. This provides an electrical conduction path via the shroud 14 that can bypass the bearing arrangement 20. The short circuit path that bypasses the bearing arrangement 20 can be a second electrical conduction path 28 that electrically couples the rotor 4 to the bearing housing 12 and the electrical ground 24. This reduces the current flow through the bearing arrangement 20 and reduces the risk of damaging the bearings 21.

[0050] In practice, if a power source is connected to the lightning strike point 23, current will "flow" through the first electrical conduction path 26 and the second electrical conduction path 28 in different proportions. The amount of current that "flows" through each electrical conduction path will depend on two properties: impedance (a function of reactance and resistance); and surface area. Resistance is a function of the resistivity of the material, the length of the material and the cross-sectional area of the material.

[0051] If the current is in a steady state, i.e. direct current (DC) which has no frequency component, the only relevant property is impedance (equivalent to resistance at DC). The proportion of current that "flows" in each path 26 and 28 will depend on the electrical impedance of each path. The impedance associated with the first electrical conduction path 26 is Z1. The impedance associated with the second electrical conduction path 28 is Z2. Z1 can be relatively small due to the large cross-sectional area of the electrically conductive material that includes the main shaft 11. It is therefore important that the shroud 14 is made of a material with low resistivity to reduce Z2. The shroud 14 can comprise: copper, an alloy, an alloy including copper, a suitable composite material or a combination of these. It is also advantageous that the shroud is light in weight. However, ideally Z2 is much greater than Z1 to reduce the current through the bearing arrangement 20. In practice, this is not always practical and surprisingly not even necessary. Therefore, the preferred electrical conduction path for DC current from the rotor 4 to the electrical ground 24 can include the main shaft 11 and / or the bearing arrangement 20.

[0052] The transient nature of the current in a lightning strike leads to several phenomena that need to be addressed in effectively protecting wind turbine structures. Unlike direct current that "flows through" the entire cross-sectional area of a conductor (like water through a hose), rapidly changing current tends to travel on the surface of a conductor (in the so-called skin effect). Consequently, the conductors typically used to protect equipment tend to be multi-stranded, woven together from small wires. This increases the total bundle surface area, which is inversely proportional to the radius of a single strand, for a fixed total cross-sectional area.

[0053] More specifically, the skin effect describes the effect of a magnetic field that forces current onto the outermost of a plurality of concentric conductive elements. There are three main factors that affect the current distribution in a conductive element. The resistance and inductance (i.e., reactance) of each element; these are part of the complex impedance as described above. There is also an interaction through mutual inductance. Mutual inductance forces current to the outermost conductive parts; it induces the skin effect. As a result of the skin effect, the current density through the cross-sectional area of a conductor is not uniform at AC frequencies. At higher current frequencies, there is a greater current density around the edges of the cross-sectional area (i.e., the surface of the conductor, or at the outer diameter of the conductor), which decreases exponentially toward the center of the cross-sectional area. Even for a hollow circular conductor of high frequency current, there is a greater current density around the edges of the cross-sectional area, which decreases exponentially toward the inner edges of the cross-sectional area.

[0054] Consequently, without a lightning protection system, the transient lightning current conducted through the wind turbine will likely travel on the surface of the hub 5 and the main shaft 11 before traveling through (and potentially damaging) the bearing 21. The addition of the current-dense region electrically connected to the hub 5 and / or the shroud 14 of the forward portion of the main shaft 11 provides: (i) the benefit of a large surface area in the second electrically conductive path 28; and, (ii) the additional benefit of a short length electrically conductive path from the current-dense region of the shroud 14 (i.e., the outer surface of the hub 5 and / or the outer surface of the forward portion of the main shaft 11 (which is the largest diameter portion)) to the electrical ground 24. The short length electrically conductive path can minimize (or at least reduce) the electrical resistance in the second electrically conductive path 28 (as electrical resistance is a function of the length of the conductor). Both of these effects increase the amount of current that "flows" through the second electrically conductive path 28 when the wind turbine is struck by lightning.

[0055] The benefits of the lightning protection system including the shroud 14 result in the elimination of air or spark gaps, which reduces RF interference caused by lightning strikes and allows the full benefits of the skin effect to be utilized. Thus, the lightning protection system avoids air or spark gaps in the electrical conduction path for conducting lightning from the hub 5 to the electrical ground 24. The electrical components in the blades, such as the electrical component 8, are protected from lightning strikes due to the lightning protection system. In particular, the shroud 14 provides an alternative current path that avoids large voltage drops across these electrical components.

[0056] The shroud 14 can allow the hub 5 and / or the main shaft 11 to rotate freely about the bearing housing 12 while still being in electrical contact with the front end of the hub 5 and / or the main shaft 11 and the bearing housing 12.

[0057] The diameter of the shroud 14 at the point of contact with the bearing housing 12 can be greater than the diameter of the main shaft 11, and / or the diameter between the front electrical connection and the rear electrical connection of the shroud 14 can be greater than the diameter of the main shaft 11. For a shroud 14 that completely encloses the circumference of the main shaft 11 and has a diameter greater than the main shaft 11, the second electrical conduction path 28 will have a larger effective cross-sectional area than the first electrical conduction path 26. This is because the current density is concentrated near the outer diameter of each conductor (i.e., the shroud 14 and the main shaft 11), and the shroud 14 has a larger outer diameter than the main shaft 11.

[0058] Thus, due to the skin effect, the preferred electrical conduction path from the rotor 4 to the electrical ground 24 for the transient lightning current includes the shroud 14 (i.e., the second electrical conduction path 28) because the effective cross-sectional area of the shroud is relatively large compared to the electrical conduction path including the main shaft 11 and the bearing arrangement 20. Furthermore, the impedance of the shroud should be low enough to effectively conduct the transient lightning current to realize the benefits of the skin effect. A preferred electrical conductor can conduct more than 50% of the total lightning current at the average transient lightning current, or, conduct enough of the total lightning current such that the bearing arrangement 20 does not need to be repaired after an average lightning strike.

[0059] Alternatively, the diameter of the shroud 14 at the point of contact with the bearing housing 12 can be greater than the diameter of the bearing housing 12. Additionally or alternatively, the diameter of the shroud 14 at the point of contact with the front end of the hub 5 and / or the main shaft can be greater than the diameter of the front end of the hub 5 and / or the main shaft. Additionally or alternatively, the diameter of the shroud 14 at the point of contact with the front end of the hub 5 and / or the main shaft 11 can be greater than the diameter of the shroud 14 at the point of contact with the bearing housing 12. In all of these variations, the front end of the main shaft 11 can be flared such that the diameter of the flared end of the main shaft 11 at the point of contact with the shroud 14 can be greater than the diameter of the bearing housing 12 at the point of contact with the shroud 14. All of these variations can be advantageous because it can: reduce the electrical impedance of the shroud 14; create a large shroud 14 surface area; and, increase ease of manufacturing and functionality.

[0060] The shroud 14 can have a relatively small thickness. The shroud 14 can include a thickness of less than 10 mm, for example. A thinner shroud 14 will have advantages such as a greater surface area and reduced weight compared to a thicker shroud 14 having the same diameter.

[0061] The shroud 14 can form a sliding or rolling contact with the bearing housing 12 or the front end of the hub 5 and / or the main shaft 11. The sliding or rolling contact can provide the electrical coupling. For a rolling contact, the shroud 14 can include a spherical or cylindrical bearing or any functional shape to facilitate the mechanical connection while allowing the front end of the hub 5 and / or the main shaft 11 to freely rotate about the bearing housing 12 and not cause undue friction. Any other form of suitable electrical coupling can be used.

[0062] The shroud 14 can form a sliding or rolling contact with the bearing housing 12 or the front end of the hub 5 and / or the main shaft 11 at a plurality of discrete contact points around the circumference of the shroud 14. In operation of the wind turbine 1, the hub 5 and the main shaft 11 will rotate, which can cause some vibration in the components of the wind turbine 1. In particular, the shroud 14 and / or the components in contact with the shroud 14 can vibrate and cause local air gaps at certain points of the mechanical connection between the shroud 14 and / or the components in contact with the shroud 14, such as in one or more of the plurality of discrete contact points. It can therefore be desirable to maintain the degree of electrical contact above a minimum value to ensure good electrical conductivity in the current path through the shroud.

[0063] Figure 5a and Figure 5b An example of a suitable shroud 14a is shown having a plurality of discrete contact points around the circumference of the shroud 14a. The shroud 14a can include a plurality of metal sheets 30a, 30b, 30c, etc. that are biased into contact with the bearing housing 12 or the front end of the hub 5 and / or the main shaft 11 (such that the sheets are flexed). The metal sheets can be biased by the elasticity of the material itself or by an external force. The biasing of the metal sheets allows the shroud 14 to be in electrical contact with the bearing housing 12 or the front end of the hub 5 and / or the main shaft 11 while still allowing the front end of the hub 5 and / or the main shaft 11 to freely rotate. The metal sheets can be in sliding or rolling contact with the bearing housing 12 or the front end of the hub 5 and / or the main shaft 11. For a rolling contact, the metal sheets can include a spherical or cylindrical bearing or any functional shape to facilitate the mechanical connection while still allowing the front end of the hub 5 and / or the main shaft 11 to freely rotate.

[0064] The shroud 14 can be electrically coupled at the front or rear end of the shroud 14 by at least one fixed contact point. The front end of the shroud 14 can be electrically connected to the front end of the hub 5 and / or the main shaft 11. The rear end of the shroud 14 can be electrically connected to the bearing housing 12. The shroud 14 can be electrically coupled at the rear or front end of the shroud 14 by at least one sliding or rolling contact point.

[0065] The shield 14 may include multiple segments surrounding the circumference of the shield 14. These segments may be electrically connected to each other. These segments may be physically in contact with or separated from each other.

[0066] Figure 6a An example of a shroud 14b with 12 segments is shown. Specifically, these segments are metal plates 30a, 30d, 30g, etc., which only partially surround, or partially encircle, the spindle 11, or partially encircle its circumference, making the spindle visible in use. This shroud 14b is a trade-off between reduced weight / material and increased proportion of transient lightning current in the first electrical conduction path 26. However, it is still sufficient to reduce the proportion of transient lightning current in the first electrical conduction path 26 to the point of not damaging the bearing 21. The shroud 14b is illustrative only, and the shroud 14 can have any number of effective metal plates and / or contact points. Due to the effects of vibration, even if the shroud 14 may have many segments and / or connection points, one or more of these segments and / or connection points may not be electrically or mechanically connected to the spindle 11, bearing housing 12, and / or hub 5 during operation. In this situation, the lightning protection system can still operate correctly and redundantly because each section of the shield 14 can be individually biased.

[0067] Figure 6b An example of a shield 14c is shown, which may also have 12 metal plates 30a, 30b, 30c, etc., which partially surround or partially encircle the circumference of the spindle 11, so that the spindle will be visible in use.

[0068] Although shield 14c has the same number of metal plates as shield 14b, it has been experimentally found that shield 14c conducts a smaller proportion of transient lightning current than shield 14b. Therefore, it is advantageous for shield 14 to be uniformly distributed around the circumference of main axis 11. In other words, the mass of shield 14 can be uniformly distributed around the circumference of main axis 11. If shield 14 comprises multiple segments, these segments can be formed equidistantly from each adjacent segment of shield 14.

[0069] Figure 7 An example of a shield 14d is shown. Shield 14d may alternatively include a metal mesh, grille, or net. Shield 14d may further include a band or ring 32 for biasing the shield into sliding contact with one of: (i) the front end of the hub 5 and / or the spindle 11; or (ii) the bearing housing 12. Figure 7 In the example, the shield 14b is mechanically fixed to the bearing housing 12 and biased by the belt or ring 32 so as to be in sliding contact with the hub 5.

[0070] Figures 8a to 8d A non-limiting example of an alternative to the shield 14 is shown.

[0071] Figure 8a An example is shown with a shield 14e (which can be similar to the shields 14a, 14b, or 14c in that it can comprise multiple segments) that is mechanically fixed to the hub 5 and biased into rotational contact with the bearing housing 12. Alternatively, the shield 14e is mechanically fixed to the front end of the spindle 11.

[0072] Figure 8b An example is shown with a shield 14f (which can be similar to the shields 14a, 14b, or 14c in that it can comprise multiple segments) that is mechanically fixed to the bearing housing 12 and biased into rotational contact with the hub 5. Alternatively, the shield 14f is biased into rotational contact with the bearing housing 12.

[0073] Figure 8c An example is shown with a shield 14g (which can be similar to the shields 14a, 14b, or 14c in that it can comprise multiple segments) that is mechanically fixed to the hub 5 and biased into sliding contact with the bearing housing 12. Alternatively, the shield 14g is mechanically fixed to the front end of the spindle 11.

[0074] Figure 8d An example is shown with a shield 14h (which can be similar to the shield 14d in that it can comprise a metal grid, lattice, or mesh) that is mechanically fixed to the hub 5 and biased into sliding contact with the bearing housing 12. Alternatively, the shield 14g is mechanically fixed to the front end of the spindle 11. An example of biasing the shield can be with a plug or ring that is resiliently biased into sliding contact.

[0075] In another non-limiting example of an alternative to the shield 14, the shield can comprise two segments. A first segment at a first end can be mechanically fixed to the bearing housing 12 and a second segment can be mechanically fixed to the hub 5 and / or spindle 11 at the first end. The two segments can then form an electrical and mechanical connection at their second ends via sliding or rolling contact as previously described.

[0076] In another non-limiting example of an alternative to the shield 14 described above, the shield 14 can comprise a third mechanical and / or electrical connection between a first connection point at the hub 5 and / or spindle 11 and a second connection point at the bearing housing 12. This third contact point can not be designed to carry the weight of the shield 14, but can be designed as another conduction path to carry current away from the spindle and to the bearing housing 12.

[0077] In other non-limiting examples of alternative versions of the shroud 14, the rolling contact that the shroud 14 can make with the bearing housing 12 or the forward end of the hub 5 and / or the spindle 11 can include a wheel that rolls along the bearing housing 12 or the forward end of the hub 5 and / or the spindle 11. Alternatively, the rolling contact can include a bearing, such as a ball bearing and / or a sacrificial bearing.

[0078] While the application has been described above with reference to one or more preferred embodiments, it will be appreciated that different changes or modifications can be made without departing from the scope of the application as defined in the appended claims.

Claims

1. A wind turbine (1) comprising: a rotor (4) comprising a hub (5) and a plurality of blades (6) projecting outwardly from the hub; a main shaft (11) attached at a front end to the hub and extending at a rear end to a gearbox (9) or generator (10); a bearing housing (12) for rotatably supporting the main shaft via a bearing arrangement (20); and a lightning current protection system providing an electrically conductive path adapted to conduct lightning from the rotor (4) to an electrical ground (24), wherein the bearing housing (12) is electrically connected to the electrical ground, the lightning current protection system comprising a shroud (14) forming part of the electrically conductive path, wherein a rear end of the shroud is in contact with the bearing housing and a front end of the shroud is in contact with the hub (5) and / or the front end of the main shaft (11), and the shroud electrically couples the rotor to the bearing housing via a short circuit path (28) that bypasses the bearing arrangement, wherein the shroud is arranged to surround the front end of the main shaft, wherein, for DC current, the electrically conductive path from the rotor (4) to the electrical ground (24) is via the main shaft (11) and / or the bearing arrangement (20), and / or, wherein, for transient lightning current, the electrically conductive path from the rotor to the electrical ground is via the shroud (14).

2. A wind turbine according to claim 1, wherein, The shroud (14) has a larger diameter than the main shaft (11) at the contact with the bearing housing (12).

3. The wind turbine according to claim 1, further comprising a nacelle (3) housing the bearing housing (12), wherein The shroud (14) is enclosed within the nacelle.

4. The wind turbine of claim 1, wherein, The shroud (14) forms a sliding or rolling contact with one of: (i) the hub (5) and / or the front end of the main shaft (11); or (ii) the bearing housing (12), and the sliding or rolling contact provides the electrical coupling.

5. A wind turbine according to claim 4, wherein, The shroud (14) forms a sliding contact with one of: (i) the hub (5) and / or the front end of the main shaft (11); or (ii) the bearing housing (12) at a plurality of discrete contact points around a circumference of the shroud.

6. The wind turbine of claim 1, wherein, The shroud (14) is electrically coupled at the front or rear end of the shroud by at least one fixed contact point and at the rear or front end of the shroud by at least one sliding or rolling contact point.

7. The wind turbine of claim 1, wherein, The shroud (14) comprises a plurality of metal sheets biased into contact with the bearing housing or the hub and / or the front end of the main shaft.

8. The wind turbine according to any of claims 1 to 6, wherein, The shroud (14) comprises a metal mesh, grid or net.

9. A wind turbine according to claim 8, wherein, The shroud (14) further comprises a band or ring for biasing the shroud into sliding contact with one of: (i) the hub and / or the front end of the main shaft; or (ii) the bearing housing.

10. The wind turbine of claim 1, wherein: the front end of the main shaft (11) has a flared end attached to the hub (5); the shroud (14) is in contact with the bearing housing (12) and the flared end of the main shaft; and the shroud (14) is in contact with the bearing housing (12) and the flared end of the main shaft; and The shroud electrically couples the flared end of the main shaft to the bearing housing.

11. A wind turbine according to claim 10, wherein, The diameter of the flared end of the main shaft (11) at contact with the shroud (14) is greater than the diameter of the bearing housing at contact with the shroud.

12. The wind turbine of claim 1, wherein, The lightning current protection system avoids an air gap or spark gap in the electrical conduction path for conducting lightning from the hub to electrical ground.

13. The wind turbine of claim 1, wherein, The shroud (14) comprises copper.

14. The wind turbine of claim 1, wherein, The bearing housing (12) comprises steel.

15. The wind turbine of claim 1, wherein, At least one blade of the plurality of blades (6) comprises an electrical power component that receives electrical power via the hub (5).

16. The wind turbine of claim 1, wherein, There are at least two electrical conduction paths (26, 28) from the rotor to electrical ground, one via the shroud (14) and one via the main shaft (11) and / or the bearing arrangement (20).

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