Patent Description:
Wind turbines may be struck by lightning in a lightning storm due to their height. The electric current within a typical negative cloud to ground lightning discharge rises very quickly to its peak value in <NUM>-<NUM> microseconds, then decays more slowly over <NUM>-<NUM> microseconds. This electric current when striking the blade of a wind turbine travels to ground via the lowest impedance path. Without any lightning protection, this path generally comprises the bearings of a wind turbine which allows the hub and blades to rotate freely. Such a large electric current through these bearings can cause damage to the bearings.

To protect the bearings of a wind turbine many protection systems have been developed. One such protection system is achieved by mounting a spark gap device on the nacelle so that when electric current travels along or down the blade (after being struck by lightning), the high electric current ionises the surrounding air across the spark gap to produce a current path which avoids the bearings. One purpose of the spark gap is to not impede rotation of the wind turbine, between the hub and the nacelle. However, this produces Radio Frequency interference as the spark gap is active which can be undesirable, moreover, the proportion of the total electric current which takes the spark gap route can be unpredictable and bearings may still be damaged.

Lightning protection systems that have a direct connection between the blade and the nacelle exist, such as in <CIT>. This describes a lightning current transfer unit (LCTU) which comprises at least two sliding contacts adapted to connect a blade and the nacelle or tower. The LCTU further comprises at least one electric conductor establishing a dedicated connection between surfaces that the two sliding contacts are contacting. The sliding contact on the blades is typically in contact with a blade band which is a metallic band surrounding the circumference of a blade root end.

It is important for the blade band to be at least a minimum distance away from metallic parts joining the blade root to the hub, so that the preferred current path is via the LCTU. The trend is for larger and longer blades and as blade length increases the location of this blade band may be outside the extremities of the front face of the nacelle. Therefore, the LCTU solution places design constraints on the physical components of the blade and the nacelle to prevent arcing between parts that are at different voltages. At a certain blade length physical limits will be reached for the LCTU solution and an alternative is needed.

<CIT>, <CIT>, <CIT> and <CIT> all describe lightning protection systems for wind turbines where lightning current is transferred from a rotating blade to a stationary nacelle.

A first aspect of the invention provides a wind turbine comprising: a rotor comprising a hub and a plurality of blades that project outwardly from the hub; a main shaft attached to the hub at a front end and extending to a gearbox or generator at a rear end; a bearing housing for rotatably supporting the main shaft via a bearing arrangement; and a lightning current protection system providing an electrical conduction path suitable for conducting lightning from the rotor to electrical ground, wherein the bearing housing is electrically connected to electrical ground, the lightning current protection system includes a shroud forming part of the electrical conduction 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 and/or the front 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 around the front end of the main shaft.

The shroud may have a greater diameter than the main shaft at the contact with the bearing housing.

The wind turbine of the first aspect, may further comprise a nacelle containing the bearing housing, wherein the shroud is enclosed within the nacelle.

The shroud may make a sliding or rolling contact with either: (i) the hub and/or the front end of the main shaft; or (ii) the bearing housing, and the sliding or rolling contact provides electrical coupling.

The shroud may make a sliding contact with either: (i) the hub and/or the front end of the main shaft; or (ii) the bearing housing at a plurality of discrete contact points around the circumference of the shroud.

The shroud may be electrically coupled by at least one fixed contact at either the front or rear end of the shroud and electrically coupled by at least one sliding or rolling contact at either the rear or front end of the shroud.

The shroud may comprise a plurality of metal leaves biased in contact with either the bearing housing or the hub and/or the front end of the main shaft. The shroud may alternatively comprise a metal mesh, grid or net.

The shroud which may comprise a metal mesh, grid or net, may further comprise a band or hoop for biasing the shroud in sliding contact with either: (i) the hub and/or the front end of the main shaft; or (ii) the bearing housing.

The shroud may have a relatively small thickness. Preferably, the thickness of the shroud is less than <NUM>.

The front end of the main shaft may have a flared end which attaches to the hub; the shroud may be in contact with the bearing housing and the flared end of the main shaft; and the shroud may electrically couple the flared end of the main shaft to the bearing housing. The flared end of the main shaft may have a diameter at the contact with the shroud which is greater than a diameter of the bearing housing at the contact with the shroud.

The lightning current protection system may avoid an air gap, or spark gap, in the electrical conduction path for conducting lightning from the hub to electrical ground.

At least one of the plurality of blades may include an electrically powered component which receives electrical power via the hub.

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

The preferred electrical conduction path from the rotor to electrical ground for a DC current may be via the main shaft and/or the bearing arrangement.

The preferred electrical conduction path from the rotor to electrical ground for a transient lightning current may be via the shroud.

These and other aspects, features and/or advantages of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

<FIG> illustrates, in a schematic perspective view, an example of a wind turbine <NUM>. The wind turbine <NUM> includes a tower <NUM>, a nacelle <NUM> at the apex of the tower, and a rotor <NUM> operatively coupled to a generator housed inside the nacelle <NUM>. In addition to the generator, the nacelle houses miscellaneous components required for converting wind energy into electrical energy and various components needed to operate, control, and optimize the performance of the wind turbine <NUM>.

The rotor <NUM> of the wind turbine includes a central hub <NUM> and a plurality of blades <NUM> that project outwardly from the central hub <NUM>. It will be noted that the wind turbine <NUM> is the common type of horizontal axis wind turbine (HAWT) such that the rotor <NUM> is mounted at the nacelle <NUM> to rotate about a substantially horizontal axis defined at the centre at the hub <NUM>. While the example shown in <FIG> has three blades, it will be realised by the skilled person that other numbers of blades are possible.

When wind blows against the wind turbine <NUM>, the blades <NUM> generate a lift force which causes the rotor <NUM> to rotate, which in turn causes the generator within the nacelle <NUM> to generate electrical energy.

On each rotor blade <NUM> there is at least one lightning receptor <NUM> or covering mesh, positioned at a desirable location for lightning to strike the wind turbine. Further, there may be on at least one of the plurality of blades <NUM> an electrically powered component <NUM> which receives electrical power via the hub, such as a de-icing unit, lighting unit, etc. Electrically powered components in the blades require protection from lightning strikes, in part due to their power connection via the hub.

The wind turbine <NUM> may be included among a collection of other wind turbines belonging to a wind power plant, also referred to as a wind farm or wind park, that serve as a power generating plant connected by transmission lines with a power grid. The power grid generally comprises a network of power stations, transmission circuits, and substations coupled by a network of transmission lines that transmit the power to loads in the form of end users and other customers of electrical utilities.

<FIG> schematically illustrates, an embodiment of the wind turbine <NUM> and illustrates the inside of the nacelle <NUM>. The nacelle <NUM> comprises a nacelle frame <NUM> which structurally supports the nacelle <NUM> and the components within the nacelle <NUM>. The wind turbine <NUM> comprises rotor blades <NUM> which are mechanically connected to an electrical generator <NUM> via gearbox <NUM>. In direct drive systems, and other systems, the gearbox <NUM> may not be present. The electrical power generated by the generator <NUM> is injected into a power grid via an electrical converter (not shown). A main shaft <NUM> is mechanically attached to the hub <NUM> at a front end. A bearing housing <NUM> is mechanically attached to the nacelle frame <NUM> and is configured to rotatably support the main shaft <NUM> such that the bearing housing <NUM> supports the hub <NUM> and the plurality of blades <NUM> to allow them to rotate relative to the nacelle <NUM>. The main shaft <NUM> extends through the bearing housing <NUM> and into the gearbox <NUM> (or electrical power generator <NUM> in a direct drive system) at a rear end.

The lightning protection system is also illustrated in <FIG>. The lightning protection system provides an electrical conduction path suitable for conducting lightning from the rotor <NUM> to electrical ground. The lightning current protection system includes a shroud <NUM> forming part of the electrical conduction path. <FIG> illustrates the shroud <NUM> in contact with the bearing housing <NUM> at a rear end, and the rotor <NUM>, specifically <FIG> illustrates the shroud in connection to the main shaft <NUM> at a front end. Alternatively, the front end of the shroud <NUM> may be in contact with the hub <NUM>, any intermediate component (not shown), or any combination of blades <NUM>, the hub <NUM>, the main shaft <NUM>, the bearing housing <NUM>, and the nacelle frame <NUM>. The shroud <NUM> is arranged around the front end of the main shaft <NUM>. The shroud <NUM> may partially or fully enclose, or surround the circumference, the front end main shaft <NUM> between where the shroud makes contact with the hub <NUM> and/or the front end of the main shaft <NUM>, and the bearing housing <NUM>.

The arrangement of the shroud <NUM> around the front end of the main shaft <NUM> promotes the skin effect such that, when lightning current passes along the electrical conduction path via the shroud <NUM>, a bearing arrangement of the bearing housing <NUM> is not damaged in a lightning strike, or, such that a greater proportion of lightning current passes via the shroud than the bearing arrangement. The nacelle <NUM> may contain the bearing housing <NUM> and the shroud <NUM> may be enclosed within the nacelle <NUM>.

<FIG> illustrates the shroud <NUM> connection to the front end of the main shaft <NUM> and to the bearing housing <NUM> in more detail. The bearing housing <NUM> may comprise bearing races <NUM> (e.g. front and rear bearing races), a cover <NUM>, and structural supports <NUM> mechanically attached to the nacelle frame <NUM>. For structural strength the bearing housing <NUM> may comprise: steel, an alloy, an alloy comprising steel, a suitable composite, or a combination of these. It is of note that the bearing housing <NUM> may have an associated electrical conductivity depending on the material selection.

<FIG> illustrates the main shaft <NUM>, the shroud <NUM>, and bearing housing <NUM> of <FIG> without the structural supports <NUM> and the nacelle frame <NUM> and with some of the cover <NUM>, bearing races <NUM>, shroud <NUM>, and main shaft <NUM> removed to show the inside of the bearing housing <NUM>. The main shaft is visible passing through the bearing housing <NUM>. A bearing arrangement <NUM> which may comprise bearings <NUM> is shown inside the bearing races <NUM>. The bearing arrangement <NUM> provides a mechanical connection between the main shaft <NUM> and the bearing housing <NUM> to allow the main shaft <NUM> to rotate within the bearing races <NUM>. The bearings <NUM> may be cylindrical, spherical, or any functional shape. The bearings <NUM> may produce a low coefficient of friction between the main shaft <NUM> and the bearing races <NUM>.

The front end of the main shaft <NUM> may have a flared end which attaches to the hub <NUM>. This may be beneficial for mechanically attaching the main shaft <NUM> to the hub <NUM>. The shroud <NUM> may be, as shown in <FIG>, in contact with the bearing housing <NUM> and the flared end of the main shaft <NUM>. The shroud <NUM> may electrically couple the flared end of the main shaft <NUM> to the bearing housing <NUM>.

<FIG> illustrates a schematic side view of the arrangement of <FIG> with the hub <NUM> shown attached to the main shaft <NUM>. An electrical ground <NUM> may be electrically coupled to the bearing housing <NUM>. A lightning strike point <NUM> is shown as a representation of the point where the lightning initially strikes the wind turbine <NUM>. <FIG> shows two electrical conduction paths for the lightning current to take. A first electrical conduction path comprises the lightning strike point <NUM>, the hub <NUM>, the main shaft <NUM>, the bearing arrangement <NUM>, the bearing housing <NUM> and the electrical ground <NUM>. A second electrical conduction path comprises the lightning strike point <NUM>, the hub <NUM>, the shroud <NUM>, the outside of the bearing housing <NUM> and the electrical ground <NUM>. Both electrical conduction paths may further comprise a path from the bearing housing <NUM> to electrical ground <NUM> via the nacelle frame <NUM> and the tower <NUM>.

<FIG> illustrates a schematic diagram of the circuit paths from the lightning strike point <NUM> to the electrical ground <NUM>. A first electrical conduction path <NUM> is from the rotor <NUM> to electrical ground <NUM> via the bearing arrangement <NUM>. Specifically, lightning may strike a blade <NUM> at the lightning strike point <NUM> and travel along the blade through the hub <NUM>, the main shaft <NUM>, the bearing arrangement <NUM>, the bearing housing <NUM>, and then the nacelle frame <NUM> to electrical ground <NUM>. However, the bearings <NUM> of the bearing arrangement <NUM> may become damaged (for example, fused to surrounding components) if a large current passes through them.

The shroud <NUM> electrically couples the rotor <NUM> to the bearing housing <NUM> via a short circuit path. This provides an electrical conduction path via the shroud <NUM> which may bypass the bearing arrangement <NUM>. A short circuit path bypassing the bearing arrangement <NUM> may be a second electrical conduction path <NUM> which electrically couples the rotor <NUM> to the bearing housing <NUM> and to electrical ground <NUM>. This reduces the current passing through the bearing arrangement <NUM> and reduces the risk of damage to the bearings <NUM>.

In reality, if an electrical source is connected to the lightning strike point <NUM> the current will "flow" through the first electrical conduction path <NUM> and the second electrical conduction path <NUM> in differing proportions. The amount of current "flowing" through each conduction path will depend on two characteristics: the impedance (a function of reactance and resistance); and, the 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.

If the current is in a steady state i.e. direct current (DC) (it has no frequency component), then the only characteristic relevant is the impedance (equivalent to resistance at DC). The proportion of current "flowing" in each path <NUM> and <NUM> will depend on the electrical impedance of each path. The impedance associated with the first electrical conduction path <NUM> is Z1. The impedance associated with the second electrical conduction path <NUM> is Z2. Z1 is likely to be relatively small due to the large cross sectional area of conducting material comprising the main shaft <NUM>. Thus, it is important that the shroud <NUM> is made from a material with a low resistivity to reduce Z2. The shroud <NUM> may comprise: copper, an alloy, an alloy comprising copper, a suitable composite, or a combination of these. It is also advantageous for the shroud to be light weight. Although, ideally Z2 is much larger than Z1 to reduce the current through the bearing arrangement <NUM>. In reality, this is not always practical and surprisingly, not even necessary. Thus, the preferred electrical conduction path from the rotor <NUM> to electrical ground <NUM> for a DC current may comprise the main shaft <NUM> and/or the bearing arrangement <NUM>.

The transient nature of the current within a lightning flash results in several phenomena that need to be addressed in the effective protection wind turbine structures. Rapidly changing currents tend to travel on the surface of a conductor, in what is called the skin effect, unlike direct currents, which "flow-through" the entire cross sectional area of a conductor (like water through a hose). Hence, typically conductors used in the protection of facilities tend to be multi-stranded, with small wires woven together. This increases the total bundle surface area in inverse proportion to the individual strand radius, for a fixed total cross-sectional area.

More specifically, the skin effect describes magnetic field effects that force current onto the outer most multiple concentric, conductive elements. There are three dominant 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 the mutual inductance. The mutual inductance forces the current onto the outermost conductive part; it causes the skin effect. A result of the skin effect is that the current density through a cross sectional area of a conductor is not uniform at AC frequencies. At higher current frequencies there is a greater current density around the edge of the cross sectional area (i.e. the surface of the conductor, or, at the outside diameter of the conductor), which diminishes exponentially towards the centre of the cross sectional area. Even for a hollow circular conductor with a high frequency current, there is a greater current density around the edge of the cross sectional area, which diminishes exponentially towards the inner edge of the cross sectional area.

Thus, without a lightning protection system a transient lightning current conducting through a wind turbine will likely travel on the surface of the hub <NUM> and the main shaft <NUM> before traveling though (and potentially damaging) the bearings <NUM>. The addition of the shroud <NUM> electrically connected to the current dense area on the hub <NUM> and/or the front of the main shaft <NUM> provides: (i) the benefit of a large surface area in the second electrical conduction path <NUM>; and, (ii) the extra benefit of a short length conduction path from the current dense area (i.e. the outside surface of the hub <NUM> and/or the (outside surface of the largest diameter section of the) front of the main shaft <NUM>) contacting the shroud <NUM> to electrical ground <NUM>. A short length conduction path may minimise (or at least reduce) resistance in the second electrical conduction path <NUM> (since resistance is a function of conductor length). Both of these effects increase the amount of current "flowing" through the second electrical conduction path <NUM> when the wind turbine is struck by lightning.

The benefits of the lightning protection system comprising the shroud <NUM> results in no need for air gaps or spark gaps, this reduces an RF interference caused by a lightning strike and allows for the full benefits of the skin effect to be utilised. Thus, the lightning protection system avoids an air gap, or spark gap, in the electrical conduction path for conducting lightning from the hub <NUM> to electrical ground <NUM>. The electrically powered components in the blade, such as electrically powered component <NUM>, are protected from lightning strike damage due to the lightning protection system. Specifically, the shroud <NUM> offers an alternative current path which avoids a large voltage drop across such electrically powered components.

The shroud <NUM> may allow the hub <NUM> and/or the main shaft <NUM> to rotate freely about the bearing housing <NUM> while still being in electrical contact with the hub <NUM> and/or the front end of the main shaft <NUM> and the bearing housing <NUM>.

The shroud <NUM> may have a diameter greater than the main shaft <NUM> diameter at the contact with the bearing housing <NUM> and/or the main shaft <NUM> diameter between the front end electrical connection and the rear end electrical connection of the shroud <NUM>. For a shroud <NUM> which fully envelops the circumference of the main shaft <NUM> and with a diameter greater than the main shaft <NUM>, the second electrical conduction path <NUM> will have a larger effective cross sectional area than the first conduction path <NUM>. This is because the current density is concentrated near the outside diameter of each conductor (i.e. the shroud <NUM> and the main shaft <NUM>) and the shroud <NUM> has a greater outside diameter than the main shaft <NUM>.

Thus due to the skin effect, for transient lightning currents the preferred electrical conduction path from the rotor <NUM> to electrical ground <NUM> comprises the shroud <NUM> (i.e. the second electrical conduction path <NUM>) because of the shroud's comparatively large effective cross sectional area in comparison to the electrical conduction path which comprises the main shaft <NUM> and the bearing arrangement <NUM>. In addition, the shroud's impedance should be low enough to conduct the transient lightning current effectively to realise the benefits of the skin effect. The Preferred electrical conductor may, conduct over <NUM>% of the total lightning current at an average transient lightning current, or, conduct enough of the total lightning current that the bearing arrangement <NUM> does not require repair after an average lightning strike.

The shroud <NUM> may alternatively have a greater diameter than the bearing housing <NUM> at the contact with the bearing housing <NUM>. The shroud <NUM> may in addition or alternatively have a greater diameter than the hub <NUM> and/or the front end of the main shaft at the contact with the hub <NUM> and/or the front end of the main shaft. The shroud <NUM> may in addition or alternatively have a diameter at the contact with the hub <NUM> and/or the front end of the main shaft <NUM>, which is greater than the diameter of the shroud <NUM> at the contact with the bearing housing <NUM>. The front end of the main shaft <NUM> in all of these variants may be flared, such that the flared end of the main shaft <NUM> may have a diameter at the contact with the shroud <NUM> which is greater than a diameter of the bearing housing <NUM> at the contact with the shroud <NUM>. All of these variants may be advantageous as it may: reduce the electrical impedance of the shroud <NUM>; create a large shroud <NUM> surface area; and, increase the ease of manufacturing and functionality.

The shroud <NUM> may have a relatively small thickness. The shroud <NUM> may comprise a thickness of less than <NUM> for example. A thinner shroud <NUM> in comparison to a thicker shroud <NUM> with the same diameter, will have advantages such as a greater surface area and reduced weight.

The shroud <NUM> may make sliding or rolling contact with either the bearing housing <NUM>, or the hub <NUM> and/or the front end of the main shaft <NUM>. The sliding or rolling contact may provide electrical coupling. For a rolling contact the shroud <NUM> may comprise spherical or cylindrical bearings or any functional shape to facilitate the mechanical connection while allowing the hub <NUM> and/or the front end of the main shaft <NUM> to rotate freely about the bearing housing <NUM> and without causing excessive friction. Any other form of suitable electrical coupling may be used.

The shroud <NUM> may make sliding or rolling contact with either the bearing housing <NUM>, or the hub <NUM> and/or the front end of the main shaft <NUM>, at a plurality of discrete contact points around the circumference of the shroud <NUM>. In operation of the wind turbine <NUM>, the hub <NUM> and main shaft <NUM> will rotate, this may cause some vibrations in the components of the wind turbine <NUM>. Specifically, the shroud <NUM> and/or components in contact with the shroud <NUM> may vibrate and result in partial air gap(s) at certain points of the mechanical connection between the shroud <NUM> and/or components in contact with the shroud <NUM>, such as in one or more of the plurality of discrete contacts. The degree of electrical contact may therefore need to be maintained above a minimum to ensure good electrical conductivity in the current path through the shroud.

<FIG> illustrate an example of a suitable shroud 14a with a plurality of discrete contact points around the circumference of the shroud 14a. The shroud 14a may comprise a plurality of metal leaves 30a, 30b, 30c, etc. biased in contact (such that the leaves are flexed) with either the bearing housing <NUM>, or the hub <NUM> and/or the front end of the main shaft <NUM>. The metal leaves may be biased by the material's own elasticity or outside force. Biasing the metal leaves allows the shroud <NUM> to be in electrical contact with the either the bearing housing <NUM>, or the hub <NUM> and/or the front end of the main shaft <NUM>, while still allowing the hub <NUM> and/or the front end of the main shaft <NUM> to rotate freely. The metal leaves may be in sliding or rolling contact with either the bearing housing <NUM>, or the hub <NUM> and/or the front end of the main shaft <NUM>. For a rolling contact the metal leaves may comprise spherical or cylindrical bearings or any functional shape to facilitate the mechanical connection while allowing the hub <NUM> and/or the front end of the main shaft <NUM> to rotate freely.

The shroud <NUM> may be electrically coupled by at least one fixed contact at either the front or rear end of the shroud <NUM>. The front end of the shroud <NUM> may be electrically connected to the hub <NUM> and/or the front end of the main shaft <NUM>. The rear end of the shroud <NUM> may be electrically connected to the bearing housing <NUM>. The shroud <NUM> may be electrically coupled by at least one sliding or rolling contact at either the rear or front end of the shroud <NUM>.

The shroud <NUM> may comprise a plurality of sections around the circumference of the shroud <NUM>. These sections may be electrically connected to each other. These sections may be physically touching or separate from each other.

<FIG> illustrates an example of a shroud 14b which may have <NUM> sections. Specifically, the sections are metal leaves 30a, 30d, <NUM>, etc. which only partially enclose the main shaft <NUM>, or are only partially around the main shaft <NUM>, or partially surround the circumference of the main shaft <NUM>, such that the main shaft would be visible in use. This shroud 14b is a trade-off between a reduced weight/material, and increasing the proportion of transient lighting current in the first electrical conduction path <NUM>. However, it may still reduce the proportion of transient lighting current in the first electrical conduction path <NUM> enough not to damage the bearings <NUM>. The shroud 14b is purely illustrative and the shroud <NUM> may have any number of metal leaves and/or contact points to be effective. Due to the effect of vibrations, even though a shroud <NUM> may have many sections and/or connection points, in operation one or more of those sections and/or connections points may not be in electrical or mechanical connection with the main shaft <NUM>, bearing housing <NUM>, and/or hub <NUM>. In this case, the lightning protection system may still operate correctly, and operate with redundancy, since each section of the shroud <NUM> may be individually biased.

<FIG> illustrates an example of a shroud 14c which may also have <NUM> metal leaves 30a, 30b, 30c, etc. which only partially enclose the main shaft <NUM>, or are only partially around the main shaft <NUM>, or partially surround the circumference of the main shaft <NUM>, such that the main shaft would be visible in use.

Although, shroud 14c has the same number of metal leaves as shroud 14b, it has been found experimentally that shroud 14c conducts a lesser proportion of the transient lightning current than the shroud 14b. Thus, it is advantageous for the shroud <NUM> to be equally distributed around the circumference of the main shaft <NUM>. Put another way, the mass of the shroud <NUM> may be equally distributed around the circumference of the main shaft <NUM>. If the shroud <NUM> comprises sections then the sections may be equidistant form each neighbouring section of the shroud <NUM>.

<FIG> illustrates an example of a shroud 14d. The shroud 14d may alternatively comprise a metal mesh, grid or net. The shroud 14d may further comprise a band or hoop <NUM> for biasing the shroud in sliding contact with either: (i) the hub <NUM> and/or the front end of the main shaft <NUM>; or (ii) the bearing housing <NUM>. In the example of <FIG>, the shroud 14b is mechanically fixed to the bearing housing <NUM> and is biased by the band or hoop <NUM> to be in sliding contact with the hub <NUM>.

<FIG> illustrates non-limiting examples of alternatives to the shroud <NUM>.

<FIG> illustrates an example with a shroud 14e (which may be similar to shroud 14a, 14b, or 14c in that it may comprise sections), which is mechanically fixed to the hub <NUM> and is biased to be in rotary contact with the bearing housing <NUM>. Alternatively, the shroud 14e is mechanically fixed to the front end of the main shaft <NUM>.

<FIG> illustrates an example with a shroud 14f (which may be similar to shroud 14a, 14b, or 14c in that it may comprise sections), which is mechanically fixed to the bearing housing <NUM> and is biased to be in rotary contact with the hub <NUM>. Alternatively, the shroud 14f is biased to be in rotary contact with the bearing housing <NUM>.

<FIG> illustrates an example with a shroud <NUM> (which may be similar to shroud 14a, 14b, or 14c in that it may comprise sections), which is mechanically fixed to the hub <NUM> and is biased to be in sliding contact with the bearing housing <NUM>. Alternatively, the shroud <NUM> is mechanically fixed to the front end of the main shaft <NUM>.

<FIG> illustrates an example with a shroud <NUM> (which may be similar to shroud 14d in that it may comprise a metal mesh, grid, or net), which is mechanically fixed to the hub <NUM> and is biased to be in sliding contact with the bearing housing <NUM>. Alternatively, the shroud <NUM> is mechanically fixed to the front end of the main shaft <NUM>. An example of biasing the shroud may be with a bungee or hoop elastically biased to be in sliding contact.

In another non-limiting example of an alternative to the shroud <NUM>, the shroud may comprise of two sections. A first section at a first end may be mechanically fixed to the bearing casing <NUM> and a second section may be mechanically fixed at a first end to either the hub <NUM> and/or main shaft <NUM>. The two sections may then form an electrical and mechanical connection at their second ends via a sliding or rolling contact as described previously.

In another non-limiting example of an alternative to the shroud <NUM> described above, the shroud <NUM> may comprise a third mechanical and/or electrical connection between the first connection point at the hub <NUM> and/or the main shaft <NUM> and the second connection point at the bearing housing <NUM>. This third contact point may not be designed for carrying the weight of the shroud <NUM>, but may be designed as another conduction path to carry electrical current away from the main shaft and to the bearing housing <NUM>.

In other non-limiting examples of an alternative to the shroud <NUM>, the rolling contact the shroud <NUM> may make with either the bearing housing <NUM>, or the hub <NUM> and/or the front end of the main shaft <NUM>, may comprise wheels that roll along either the bearing housing <NUM>, or the hub <NUM> and/or the front end of the main shaft <NUM>. Alternatively, rolling contact may comprise bearings, such as ball bearings and/or sacrificial bearings.

Claim 1:
A wind turbine (<NUM>) comprising:
a rotor (<NUM>) comprising a hub (<NUM>) and a plurality of blades (<NUM>) that project outwardly from the hub;
a main shaft (<NUM>) attached to the hub at a front end and extending to a gearbox (<NUM>) or generator (<NUM>) at a rear end;
a bearing housing (<NUM>) for rotatably supporting the main shaft via a bearing arrangement (<NUM>); characterised by
a lightning current protection system providing an electrical conduction path suitable for conducting lightning from the rotor (<NUM>) to electrical ground (<NUM>), wherein the bearing housing (<NUM>) is electrically connected to electrical ground, the lightning current protection system includes a shroud (<NUM>) forming part of the electrical conduction 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 (<NUM>) and/or the front end of the main shaft (<NUM>), and the shroud electrically couples the rotor to the bearing housing via a short circuit path (<NUM>) that bypasses the bearing arrangement, wherein the shroud is arranged around the front end of the main shaft.