Patent ID: 12196180

DETAILED DESCRIPTION OF EMBODIMENT(S)

FIG.1aillustrates, in a schematic perspective view, an example of a wind turbine1. The wind turbine1includes a tower2, a nacelle3at the apex of the tower, and a rotor4operatively coupled to a generator housed inside the nacelle3. 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 turbine1.

The rotor4of the wind turbine includes a central hub5and a plurality of blades6that project outwardly from the central hub5. It will be noted that the wind turbine1is the common type of horizontal axis wind turbine (HAWT) such that the rotor4is mounted at the nacelle3to rotate about a substantially horizontal axis defined at the centre at the hub5. While the example shown inFIG.1has three blades, it will be realised by the skilled person that other numbers of blades are possible.

When wind blows against the wind turbine1, the blades6generate a lift force which causes the rotor4to rotate, which in turn causes the generator within the nacelle3to generate electrical energy.

On each rotor blade6there is at least one lightning receptor7or 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 blades6an electrically powered component8which 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 turbine1may 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.1bschematically illustrates, an embodiment of the wind turbine1and illustrates the inside of the nacelle3. The nacelle3comprises a nacelle frame13which structurally supports the nacelle3and the components within the nacelle3. The wind turbine1comprises rotor blades6which are mechanically connected to an electrical generator10via gearbox9. In direct drive systems, and other systems, the gearbox9may not be present. The electrical power generated by the generator10is injected into a power grid via an electrical converter (not shown). A main shaft11is mechanically attached to the hub5at a front end. A bearing housing12is mechanically attached to the nacelle frame13and is configured to rotatably support the main shaft11such that the bearing housing12supports the hub5and the plurality of blades6to allow them to rotate relative to the nacelle3. The main shaft11extends through the bearing housing12and into the gearbox9(or electrical power generator10in a direct drive system) at a rear end.

The lightning protection system is also illustrated inFIG.1b. The lightning protection system provides an electrical conduction path suitable for conducting lightning from the rotor4to electrical ground. The lightning current protection system includes a shroud14forming part of the electrical conduction path.FIG.1billustrates the shroud14in contact with the bearing housing12at a rear end, and the rotor4, specificallyFIG.1billustrates the shroud in connection to the main shaft11at a front end. Alternatively, the front end of the shroud14may be in contact with the hub5, any intermediate component (not shown), or any combination of blades6, the hub5, the main shaft11, the bearing housing12, and the nacelle frame13. The shroud14is arranged around the front end of the main shaft11. The shroud14may partially or fully enclose, or surround the circumference, the front end main shaft11between where the shroud makes contact with the hub5and/or the front end of the main shaft11, and the bearing housing12.

The arrangement of the shroud14around the front end of the main shaft11promotes the skin effect such that, when lightning current passes along the electrical conduction path via the shroud14, a bearing arrangement of the bearing housing12is not damaged in a lightning strike, or, such that a greater proportion of lightning current passes via the shroud than the bearing arrangement. The nacelle3may contain the bearing housing12and the shroud14may be enclosed within the nacelle3.

FIGS.2aand2billustrates the shroud14connection to the front end of the main shaft11and to the bearing housing12in more detail. The bearing housing12may comprise bearing races16(e.g. front and rear bearing races), a cover17, and structural supports18mechanically attached to the nacelle frame13. For structural strength the bearing housing12may comprise: steel, an alloy, an alloy comprising steel, a suitable composite, or a combination of these. It is of note that the bearing housing12may have an associated electrical conductivity depending on the material selection.

FIG.2billustrates the main shaft11, the shroud14, and bearing housing12ofFIG.2awithout the structural supports18and the nacelle frame13and with some of the cover17, bearing races16, shroud14, and main shaft11removed to show the inside of the bearing housing12. The main shaft is visible passing through the bearing housing12. A bearing arrangement20which may comprise bearings21is shown inside the bearing races16. The bearing arrangement20provides a mechanical connection between the main shaft11and the bearing housing12to allow the main shaft11to rotate within the bearing races16. The bearings21may be cylindrical, spherical, or any functional shape. The bearings21may produce a low coefficient of friction between the main shaft11and the bearing races16.

The front end of the main shaft11may have a flared end which attaches to the hub5. This may be beneficial for mechanically attaching the main shaft11to the hub5. The shroud14may be, as shown inFIGS.2aand2b, in contact with the bearing housing12and the flared end of the main shaft11. The shroud14may electrically couple the flared end of the main shaft11to the bearing housing12.

FIG.3illustrates a schematic side view of the arrangement ofFIGS.2aand2bwith the hub5shown attached to the main shaft11. An electrical ground24may be electrically coupled to the bearing housing12. A lightning strike point23is shown as a representation of the point where the lightning initially strikes the wind turbine1.FIG.3shows two electrical conduction paths for the lightning current to take. A first electrical conduction path comprises the lightning strike point23, the hub5, the main shaft11, the bearing arrangement20, the bearing housing12and the electrical ground24. A second electrical conduction path comprises the lightning strike point23, the hub5, the shroud14, the outside of the bearing housing12and the electrical ground24. Both electrical conduction paths may further comprise a path from the bearing housing12to electrical ground24via the nacelle frame13and the tower2.

FIG.4illustrates a schematic diagram of the circuit paths from the lightning strike point23to the electrical ground24. A first electrical conduction path26is from the rotor4to electrical ground24via the bearing arrangement20. Specifically, lightning may strike a blade6at the lightning strike point23and travel along the blade through the hub5, the main shaft11, the bearing arrangement20, the bearing housing12, and then the nacelle frame13to electrical ground24. However, the bearings21of the bearing arrangement20may become damaged (for example, fused to surrounding components) if a large current passes through them.

The shroud14electrically couples the rotor4to the bearing housing14via a short circuit path. This provides an electrical conduction path via the shroud14which may bypass the bearing arrangement20. A short circuit path bypassing the bearing arrangement20may be a second electrical conduction path28which electrically couples the rotor4to the bearing housing12and to electrical ground24. This reduces the current passing through the bearing arrangement20and reduces the risk of damage to the bearings21.

In reality, if an electrical source is connected to the lightning strike point23the current will “flow” through the first electrical conduction path26and the second electrical conduction path28in 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 path26and28will depend on the electrical impedance of each path. The impedance associated with the first electrical conduction path26is Z1. The impedance associated with the second electrical conduction path28is Z2. Z1is likely to be relatively small due to the large cross sectional area of conducting material comprising the main shaft11. Thus, it is important that the shroud14is made from a material with a low resistivity to reduce Z2. The shroud14may 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 Z2is much larger than Z1to reduce the current through the bearing arrangement20. In reality, this is not always practical and surprisingly, not even necessary. Thus, the preferred electrical conduction path from the rotor4to electrical ground24for a DC current may comprise the main shaft11and/or the bearing arrangement20.

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 hub5and the main shaft11before traveling though (and potentially damaging) the bearings21. The addition of the shroud14electrically connected to the current dense area on the hub5and/or the front of the main shaft11provides: (i) the benefit of a large surface area in the second electrical conduction path28; and, (ii) the extra benefit of a short length conduction path from the current dense area (i.e. the outside surface of the hub5and/or the (outside surface of the largest diameter section of the) front of the main shaft11) contacting the shroud14to electrical ground24. A short length conduction path may minimise (or at least reduce) resistance in the second electrical conduction path28(since resistance is a function of conductor length). Both of these effects increase the amount of current “flowing” through the second electrical conduction path28when the wind turbine is struck by lightning.

The benefits of the lightning protection system comprising the shroud14results 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 hub5to electrical ground24. The electrically powered components in the blade, such as electrically powered component8, are protected from lightning strike damage due to the lightning protection system. Specifically, the shroud14offers an alternative current path which avoids a large voltage drop across such electrically powered components.

The shroud14may allow the hub5and/or the main shaft11to rotate freely about the bearing housing12while still being in electrical contact with the hub5and/or the front end of the main shaft11and the bearing housing12.

The shroud14may have a diameter greater than the main shaft11diameter at the contact with the bearing housing12and/or the main shaft11diameter between the front end electrical connection and the rear end electrical connection of the shroud14. For a shroud14which fully envelops the circumference of the main shaft11and with a diameter greater than the main shaft11, the second electrical conduction path28will have a larger effective cross sectional area than the first conduction path26. This is because the current density is concentrated near the outside diameter of each conductor (i.e. the shroud14and the main shaft11) and the shroud14has a greater outside diameter than the main shaft11.

Thus due to the skin effect, for transient lightning currents the preferred electrical conduction path from the rotor4to electrical ground24comprises the shroud14(i.e. the second electrical conduction path28) because of the shroud's comparatively large effective cross sectional area in comparison to the electrical conduction path which comprises the main shaft11and the bearing arrangement20. 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 50% of the total lightning current at an average transient lightning current, or, conduct enough of the total lightning current that the bearing arrangement20does not require repair after an average lightning strike.

The shroud14may alternatively have a greater diameter than the bearing housing12at the contact with the bearing housing12. The shroud14may in addition or alternatively have a greater diameter than the hub5and/or the front end of the main shaft at the contact with the hub5and/or the front end of the main shaft. The shroud14may in addition or alternatively have a diameter at the contact with the hub5and/or the front end of the main shaft11, which is greater than the diameter of the shroud14at the contact with the bearing housing12. The front end of the main shaft11in all of these variants may be flared, such that the flared end of the main shaft11may have a diameter at the contact with the shroud14which is greater than a diameter of the bearing housing12at the contact with the shroud14. All of these variants may be advantageous as it may: reduce the electrical impedance of the shroud14; create a large shroud14surface area; and, increase the ease of manufacturing and functionality.

The shroud14may have a relatively small thickness. The shroud14may comprise a thickness of less than 10 mm for example. A thinner shroud14in comparison to a thicker shroud14with the same diameter, will have advantages such as a greater surface area and reduced weight.

The shroud14may make sliding or rolling contact with either the bearing housing12, or the hub5and/or the front end of the main shaft11. The sliding or rolling contact may provide electrical coupling. For a rolling contact the shroud14may comprise spherical or cylindrical bearings or any functional shape to facilitate the mechanical connection while allowing the hub5and/or the front end of the main shaft11to rotate freely about the bearing housing12and without causing excessive friction. Any other form of suitable electrical coupling may be used.

The shroud14may make sliding or rolling contact with either the bearing housing12, or the hub5and/or the front end of the main shaft11, at a plurality of discrete contact points around the circumference of the shroud14. In operation of the wind turbine1, the hub5and main shaft11will rotate, this may cause some vibrations in the components of the wind turbine1. Specifically, the shroud14and/or components in contact with the shroud14may vibrate and result in partial air gap(s) at certain points of the mechanical connection between the shroud14and/or components in contact with the shroud14, 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.

FIGS.5aand5billustrate an example of a suitable shroud14awith a plurality of discrete contact points around the circumference of the shroud14a. The shroud14amay comprise a plurality of metal leaves30a,30b,30c, etc. biased in contact (such that the leaves are flexed) with either the bearing housing12, or the hub5and/or the front end of the main shaft11. The metal leaves may be biased by the material's own elasticity or outside force. Biasing the metal leaves allows the shroud14to be in electrical contact with the either the bearing housing12, or the hub5and/or the front end of the main shaft11, while still allowing the hub5and/or the front end of the main shaft11to rotate freely. The metal leaves may be in sliding or rolling contact with either the bearing housing12, or the hub5and/or the front end of the main shaft11. 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 hub5and/or the front end of the main shaft11to rotate freely.

The shroud14may be electrically coupled by at least one fixed contact at either the front or rear end of the shroud14. The front end of the shroud14may be electrically connected to the hub5and/or the front end of the main shaft11. The rear end of the shroud14may be electrically connected to the bearing housing12. The shroud14may be electrically coupled by at least one sliding or rolling contact at either the rear or front end of the shroud14.

The shroud14may comprise a plurality of sections around the circumference of the shroud14. These sections may be electrically connected to each other. These sections may be physically touching or separate from each other.

FIG.6aillustrates an example of a shroud14bwhich may have 12 sections. Specifically, the sections are metal leaves30a,30d,30g, etc. which only partially enclose the main shaft11, or are only partially around the main shaft11, or partially surround the circumference of the main shaft11, such that the main shaft would be visible in use. This shroud14bis a trade-off between a reduced weight/material, and increasing the proportion of transient lighting current in the first electrical conduction path26. However, it may still reduce the proportion of transient lighting current in the first electrical conduction path26enough not to damage the bearings21. The shroud14bis purely illustrative and the shroud14may have any number of metal leaves and/or contact points to be effective. Due to the effect of vibrations, even though a shroud14may 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 shaft11, bearing housing12, and/or hub5. In this case, the lightning protection system may still operate correctly, and operate with redundancy, since each section of the shroud14may be individually biased.

FIG.6billustrates an example of a shroud14cwhich may also have 12 metal leaves30a,30b,30c, etc. which only partially enclose the main shaft11, or are only partially around the main shaft11, or partially surround the circumference of the main shaft11, such that the main shaft would be visible in use.

Although, shroud14chas the same number of metal leaves as shroud14b, it has been found experimentally that shroud14cconducts a lesser proportion of the transient lightning current than the shroud14b. Thus, it is advantageous for the shroud14to be equally distributed around the circumference of the main shaft11. Put another way, the mass of the shroud14may be equally distributed around the circumference of the main shaft11. If the shroud14comprises sections then the sections may be equidistant form each neighbouring section of the shroud14.

FIG.7illustrates an example of a shroud14d. The shroud14dmay alternatively comprise a metal mesh, grid or net. The shroud14dmay further comprise a band or hoop32for biasing the shroud in sliding contact with either: (i) the hub5and/or the front end of the main shaft11; or (ii) the bearing housing12. In the example ofFIG.7, the shroud14bis mechanically fixed to the bearing housing12and is biased by the band or hoop32to be in sliding contact with the hub5.

FIG.8a-dillustrates non-limiting examples of alternatives to the shroud14.

FIG.8aillustrates an example with a shroud14e(which may be similar to shroud14a,14b, or14cin that it may comprise sections), which is mechanically fixed to the hub5and is biased to be in rotary contact with the bearing housing12. Alternatively, the shroud14eis mechanically fixed to the front end of the main shaft11.

FIG.8billustrates an example with a shroud14f(which may be similar to shroud14a,14b, or14cin that it may comprise sections), which is mechanically fixed to the bearing housing12and is biased to be in rotary contact with the hub5. Alternatively, the shroud14fis biased to be in rotary contact with the bearing housing12.

FIG.8cillustrates an example with a shroud14g(which may be similar to shroud14a,14b, or14cin that it may comprise sections), which is mechanically fixed to the hub5and is biased to be in sliding contact with the bearing housing12. Alternatively, the shroud14gis mechanically fixed to the front end of the main shaft11.

FIG.8dillustrates an example with a shroud14h(which may be similar to shroud14din that it may comprise a metal mesh, grid, or net), which is mechanically fixed to the hub5and is biased to be in sliding contact with the bearing housing12. Alternatively, the shroud14gis mechanically fixed to the front end of the main shaft11. 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 shroud14, the shroud may comprise of two sections. A first section at a first end may be mechanically fixed to the bearing casing12and a second section may be mechanically fixed at a first end to either the hub5and/or main shaft11. 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 shroud14described above, the shroud14may comprise a third mechanical and/or electrical connection between the first connection point at the hub5and/or the main shaft11and the second connection point at the bearing housing12. This third contact point may not be designed for carrying the weight of the shroud14, but may be designed as another conduction path to carry electrical current away from the main shaft and to the bearing housing12.

In other non-limiting examples of an alternative to the shroud14, the rolling contact the shroud14may make with either the bearing housing12, or the hub5and/or the front end of the main shaft11, may comprise wheels that roll along either the bearing housing12, or the hub5and/or the front end of the main shaft11. Alternatively, rolling contact may comprise bearings, such as ball bearings and/or sacrificial bearings.

Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.