WO2022057990A1

WO2022057990A1 - Lightning protection fabric

Info

Publication number: WO2022057990A1
Application number: PCT/DK2021/050285
Authority: WO
Inventors: Anton Bech
Original assignee: Vestas Wind Systems A/S
Priority date: 2020-09-21
Filing date: 2021-09-17
Publication date: 2022-03-24

Abstract

A wind turbine blade having a spanwise direction extending from a root end to a tip end, and a chordwise direction. A spar cap extends in the spanwise direction and includes electrically conductive material. A lightning protection layer extends over the spar cap. The lightning protection layer includes a fabric web and a plurality of electrically conductive strands upon a surface of the fabric web. The electrically conductive strands are located adjacent an outer surface of the blade. The lightning protection layer has electrically conductive stitching joining the electrically conductive strands to the fabric web. The electrically conductive stitching contacts the spar cap and the electrically conductive strands to electrically connect the spar cap and the electrically conductive strands.

Description

LIGHTNING PROTECTION FABRIC

FIELD OF THE INVENTION

The present invention relates to a wind turbine blade having a lightning protection layer, a method of manufacturing a wind turbine blade having a lightning protection layer, and an electrically conductive hybrid fabric for lightning strike protection of a wind turbine blade.

BACKGROUND OF THE INVENTION

Wind turbines are susceptible to lightning strikes. The blades are particularly susceptible to lighting strikes. This is especially the case for wind turbine blades with a carbon fibre spar cap near the outer surface of the blades.

As a result, it is common for a wind turbine to include a lighting protection system that electrically couples the wind turbine components to the ground. This lightning protection system may include lightning receptors and conductors that are electrically connected, through the tower and nacelle, to ground. For instance, a metal mesh or foil lightning protection layer may be applied at the outer surface of the blade, in front of the carbon fibre layers, to protect the carbon fibre spar cap. The carbon fibre spar cap may also be electrically coupled to the metal mesh to avoid flash over between the lightning protection layer and the spar cap.

SUMMARY OF THE INVENTION

A first aspect of the invention provides a wind turbine blade having a spanwise direction extending from a root end to a tip end, and a chordwise direction, the blade comprising: a spar cap extending in the spanwise direction and comprising electrically conductive material; a lightning protection layer over the spar cap and including a fabric web and a plurality of electrically conductive strands upon a surface of the fabric web, the electrically conductive strands located adjacent an outer surface of the blade; and the lightning protection layer further comprising electrically conductive stitching joining the electrically conductive strands to the fabric web, the electrically conductive stitching contacting the spar cap and the electrically conductive strands to electrically connect the spar cap and the electrically conductive strands. A further aspect of the invention provides a method of manufacturing a wind turbine blade, comprising: providing a lightning protection layer having a fabric web, a plurality of electrically conductive strands and electrically conductive stitching to join the electrically conductive strands to the fabric web; laying up the lightning protection layer in a blade mould with the electrically conductive strands located adjacent a surface of the blade mould; and laying up a spar cap comprising electrically conductive material over the lightning protection layer such that the electrically conductive stitching forms an electrically conductive path between the conductive material of the spar cap and the electrically conductive strands of the lightning protection layer.

A further aspect of the invention provides an electrically conductive hybrid fabric for lightning strike protection of a wind turbine blade, comprising: a fabric web; a plurality of electrically conductive strands arranged across a surface of the fabric web, wherein the electrically conductive strands are joined to the surface of the fabric web by electrically conductive stitching through the fabric web.

With this arrangement, voltage rises and flash over between an electrically conductive spar cap and a lightning protection layer disposed adjacent the outer surface of the blade can be prevented. The electrically conductive stitching provides multiple electrical paths across lightning protection layer spread across the surface of the lightning protection layer. By using strands, as opposed to a mesh or other sheet material, the manufacturability and reparability of the lightning protection layer is improved.

Optionally, wherein the electrically conductive stitching comprises a course of stitches of an electrically conductive thread extending across and in contact with at least two of the electrically conductive strands.

Optionally, wherein the course of stitches has a running stitch.

Optionally, wherein the electrically conductive thread has a diameter less than 0.5mm, preferably around 0.3mm.

Optionally, wherein a plurality of stitches of the electrically conductive stitching provide multiple points of electrical contact with the spar cap. Optionally, wherein the electrically conductive strands have undulations across the surface of the fabric web.

Optionally, wherein the undulations have an amplitude of less than 10mm, preferably less than 7mm.

Optionally, wherein the electrically conductive strands have a diameter less than 3mm, preferably less than 1.5mm, preferably around 1mm.

Optionally, wherein the electrically conductive strands each extend generally in the spanwise direction of the blade.

Optionally, wherein the lightning protection layer extends substantially the full spanwise length of the blade.

Optionally, wherein the electrically conductive strands extend beyond an end of the fabric web of the lightning protecting layer and converge at an electrical terminal, preferably wherein the electrical terminal is towards a root end of the blade.

Optionally, wherein the lightning protection layer has a chordwise extent in the blade chordwise direction which is wider than a width of the spar cap.

Optionally, wherein the conductive material of the spar cap includes carbon fibre, preferably pultruded carbon fibres extending in the blade spanwise direction.

Optionally, wherein the electrically conductive strands and/or electrically conductive stitching comprise metal, preferably Copper.

Optionally, wherein the fabric web of the lightning protection layer includes glass and/or carbon fibres.

Optionally, wherein the fabric web of the lightning protection layer includes a plurality of plies, preferably the plies are non-woven plies, preferably the fabric web is a biaxial or triaxial material. Optionally, wherein the plurality of plies of the fabric web are stepped to taper in a thickness direction of the lightning protection layer.

Optionally, wherein the electrically conductive strands have undulations across the surface of the fabric web.

Optionally, the method further comprising abrading the spar cap prior to laying up the spar cap to reveal the conductive material of the spar cap.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

Figure 1 shows a wind turbine;

Figure 2 shows a wind turbine blade;

Figure 3 shows a schematic planform view of a wind turbine blade with lightning protection features;

Figure 4 shows a schematic of a cross-section of the skin of the wind turbine blade;

Figure 5 shows a perspective view of the wind turbine blade skin;

Figure 6 shows a schematic of a lightning protection layer viewed from a spanwise direction of the wind turbine blade;

Figure 7 shows a schematic of the lightning protection layer viewed from a chordwise direction of the wind turbine blade;

Figure 8 shows a schematic of a lightning protection layer according to a second example;

Figure 9 shows a planform view of the lightning protection layer;

Figure 10 shows a planform view of the lightning protection layer according to an alternative example;

Figure 11 shows a planform view of the lightning protection layer according to a further example;

Figure 12 shows the lightning protection layer connected to an electrical terminal.

DETAILED DESCRIPTION OF EMBODIMENT(S)

In this specification, terms such as leading edge, trailing edge, pressure surface, suction surface, thickness, chord and planform are used. While these terms are well known and understood to a person skilled in the art, definitions are given below for the avoidance of doubt.

The term leading edge is used to refer to an edge of the blade which will be at the front of the blade as the blade rotates in the normal rotation direction of the wind turbine rotor.

The term trailing edge is used to refer to an edge of a wind turbine blade which will be at the back of the blade as the blade rotates in the normal rotation direction of the wind turbine rotor.

The chord of a blade is the straight line distance from the leading edge to the trailing edge in a given cross section perpendicular to the blade spanwise direction.

A pressure surface (or windward surface) of a wind turbine blade is a surface between the leading edge and the trailing edge, which, when in use, has a higher pressure than a suction surface of the blade.

A suction surface (or leeward surface) of a wind turbine blade is a surface between the leading edge and the trailing edge, which will have a lower pressure acting upon it than that of a pressure surface, when in use.

The thickness of a wind turbine blade is measured perpendicularly to the chord of the blade and is the greatest distance between the pressure surface and the suction surface in a given cross section perpendicular to the blade spanwise direction.

The term spanwise is used to refer to a direction from a root end of a wind turbine blade to a tip end of the blade, or vice versa. When a wind turbine blade is mounted on a wind turbine hub, the spanwise and radial directions will be substantially the same.

A view which is perpendicular to both of the spanwise and chordwise directions is known as a planform view. This view looks along the thickness dimension of the blade.

The term spar cap is used to refer to a longitudinal, generally spanwise extending, reinforcing member of the blade. The spar cap may be embedded in the blade shell, or may be attached to the blade shell. The spar caps of the windward and leeward sides of the blade may be joined by one or more shear webs extending through the interior hollow space of the blade. The blade may have more than one spar cap on each of the windward and leeward sides of the blade. The spar cap may form part of a longitudinal reinforcing spar or support member of the blade. In particular, the first and second spar caps may form part of the load bearing structure extending in the longitudinal direction that carries the flap-wise bending loads of the blade.

The term shear web is used to refer to a longitudinal, generally spanwise extending, reinforcing member of the blade that can transfer load from one of the windward and leeward sides of the blade to the other of the windward and leeward sides of the blade.

Figure 1 shows a wind turbine 10 including a tower 12 mounted on a foundation and a nacelle 14 disposed at the apex of the tower 12. The wind turbine 10 depicted here is an onshore wind turbine such that the foundation is embedded in the ground, but the wind turbine 10 could be an offshore installation in which case the foundation would be provided by a suitable marine platform.

A rotor 16 is operatively coupled via a gearbox to a generator (not shown) housed inside the nacelle 14. The rotor 16 includes a central hub 18 and a plurality of rotor blades 20, which project outwardly from the central hub 18. It will be noted that the wind turbine 10 is the common type of horizontal axis wind turbine (HAWT) such that the rotor 16 is mounted at the nacelle 12 to rotate about a substantially horizontal axis defined at the centre at the hub 18. While the example shown in figure 1 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 10, the blades 20 generate a lift force which causes the rotor 16 to rotate, which in turn causes the generator within the nacelle 14 to generate electrical energy. Figure 2 illustrates one of the wind turbine blades 20 for use in such a wind turbine. Each of the blades 20 has a root end 22 proximal to the hub 18 and a tip end 24 distal from the hub 18. A leading edge 26 and a trailing edge 28 extend between the root end 22 and tip end 24, and each of the blades 20 has a respective aerodynamic high pressure surface (i.e. the pressure surface) and an aerodynamic low pressure surface (i.e. the suction surface) extending between the leading and trailing edges of the blade 20.

Each blade has a cross section which is substantially circular near the root end 22, because the blade near the root must have sufficient structural strength to support the blade outboard of that section and to transfer loads into the hub 18. The blade 20 transitions from a circular profile to an aerofoil profile moving from the root end 22 of the blade towards a "shoulder" 30 of the blade, which is the widest part of the blade where the blade has its maximum chord. The blade 20 has an aerofoil profile of progressively decreasing thickness in an outboard portion of the blade, which extends from the shoulder 30 to the tip end 24.

As shown schematically in figure 3, the blade 20 includes one or more lightning receptors and one or more lightning 'down conductors' which form part of a lightning protection system for the wind turbine. The lightning receptors attract the lightning strike and the down conductors conduct the energy of the lightning strike down the blade 20 via the nacelle 14 and tower 12 to a ground potential. The lightning conductor may take a variety of forms, such as a lightning protection layer 40 on the outer surface of the blade and/or a cable 38, e.g. running through the interior of the hollow blade. The lightning receptors may include the lightning protection layer 40, and/or a solid metal tip 36 or metal coated laminate (e.g. a copper cap) provided conformal with the shape of the blade nearest the tip end 24. The metal tip 36 may be electrically connected to the lightning conductor(s).

The majority of the outer surface of the blade 20 may be covered with the lightning protection layer 40. The lightning protection layer 40 serves to shield conductive material in the blade from a lightning strike, and it may act as either a lightning receptor, a down conductor, or both. The down conductor may extend substantially the full length of the blade. Where the majority of the outer surface of the blade 20 is covered with the lightning protection layer 40, the cable 38 may connect to the lightning protection layer 40 adjacent the tip end 24 of the blade and adjacent the root end 22 of the blade, with no cable 38 along the majority of the length of the blade covered with the lightning protection layer 40. The lightning protection layer 40 may extend from root to tip in which case there may be no need for cable 38. The lightning protection layer 40 may extend in sections along the length of the blade with cable sections between the lightning protection layer 40 sections. Cable 38 may alternatively extend under the lightning protection layer 40 (inside the blade) so that the cable 38 and lightning protection layer 40 are electrically connected in parallel.

At the root end 22 of the blade 20, the down conductor 38 may be electrically connected via an armature arrangement to a charge transfer route via the nacelle 14 or hub 18 and tower 12 to a ground potential. Such a lightning protection system therefore allows lightning to be channelled from the blade to a ground potential safely, thereby minimising the risk of damage to the wind turbine 10.

The wind turbine blade 20 includes an outer blade shell defining a hollow interior space with a shear web extending internally between upper and lower parts of the blade shell.

Figure 4 shows a chordwise cross-section of the blade shell adjacent a suction surface of the wind turbine blade 20, viewed along a spanwise direction of the blade 20. Although it will be clear that the features of the blade shell adjacent the pressure surface of the wind turbine blade 20 may be substantially the same.

The blade 20 includes a spar cap 50 where the shear web (not shown) meets the blade shell. The spar cap 50 is incorporated into the blade shell. The spar cap 50 is an elongate reinforcing structure extending substantially along the full length of the blade 20 from the root end 22 to the tip end 24.

A core 54, such as a foam, balsa, or honeycomb core, may be positioned either side of the spar cap 50. One or more layers or plies 53 may be provided on an inner side of the spar cap 50, for example glass fibre composite plies or carbon fibre composite plies, which form the inner surface 51 of the blade 20. The spar cap 50 includes conductive material, such as carbon fibres. For example, the spar cap may include pultruded fibrous strips of material such as pultruded carbon fibre composite material or other carbon fibre reinforced plastic material.

The spar cap 50 is equipotentially bonded to the lightning protection layer 40 to ensure that there is no build-up of charge in the spar cap, or a large voltage difference between the lightning protection layer 40 and the spar cap 50 in the event of a lightning strike. The equipotential bonding also prevents arcing between the lightning protection layer

40 and the spar cap 50 which may damage the blade.

The lightning protection layer 40 is a conductive layer located adjacent the outer surface 52 of the blade 20. The lightning protection layer 40 includes a plurality of electrically conductive strands 41 attached to a surface of a fabric web 43 by electrically conductive stitching 45. The lightning protection layer 40, comprising the fabric web 43, the conductive strands 41 and the conductive stitching 45, is provided as a unit. The unitary lightning protection layer 40 is pre-stitched with the conductive stitching 45 such that the lightning protection layer is a complete unit prior to layup in a mould used to form the blade shell. The lightning protection layer 40 can be placed into the blade mould as a unitary piece.

The lightning protection layer 40 incorporates the conductive strands 41 , which effectively replace the conventional mesh or other sheet material used in the blade for lightning protection. The fabric web 43 provides support to the conductive strands 41. This helps avoid damage to the conductive strands 41 during layup, as compared with the conventional thin mesh. The conductive stitching 45 retains the conductive strands

41 in position on the fabric web 43, and provides electrical pathways between the conductive strands 41 and through the fabric web 43 to electrically connect the conductive strands 41 to other conductive components of the blade 20. This improves the manufacturability of the blade 20, as the lightning protection layer is pre-stitched to form it into a unit that can be placed into the blade mould as a complete lightning protection layer 40. The arrangement also improves and increases the electrical pathways through the lightning protection layer 40, as the stitching 45 can physically loop over one or more electrically conductive strands 41 to maximise the electrical contacts between the respective components. The strands may extend in a generally spanwise direction of the blade 20. The strands 41 may have a diameter less than 3mm, preferably less than 1.5mm, and more preferably around 1 mm. The strands 41 may be metal strands, for example copper strands. The strands may be coated with an electrically conductive surface treatment, such as nickel or tin, to improve or provide electrical conductivity. The material of the strands 41 may be selected to prevent or reduce galvanic corrosion between the materials, such as any metal-to-metal interfaces and any metal-to-carbon interfaces. The strands 41 may be spaced less than 10mm apart, and preferably approximately 5mm apart.

The strands 41 may be exposed on top of the fabric web 43 along their entire length, such that the strands do not weave or extend through the fabric web 43 or through any layers of the fabric web 43. This allows the electrically conductive strands 41 to maximise their protection of the blade 20, whilst minimising any discontinuities in the fabric web 43. The fabric web 43 may be a composite layer including glass and/or carbon fibres. Figure 4 shows the fabric web 43 including three layers of material, although it will be appreciated that the fabric web 43 may include more or less layers, for example one, two, or four or more layers. The fabric web 43 may be biaxial or triaxial material having reinforcing fibres in at least two directions.

Electrically conductive stitching 45 attaches the strands 41 to a surface of the fabric web 43. The stitching 45 may be a course of stitches. The stitches may be formed of an electrically conductive thread. The thread may have a diameter of less than 0.5mm, and preferably a diameter of around 0.3mm. The stitching may be any suitable electrically conductive material, such as metal or having a metal coating. The conductive stitching may comprise copper, and/or may include a surface coating such as nickel or tin.

The stitching 45 may have a running stitch, in which the course of stitches passes through the fabric web 43 over at least one strand 41 and is passed back through the fabric web 43. A running stitch does not tie, lock, or otherwise form a knot to attach each stitch. In alternative examples, each stitch of the course of stitches may be tied, locked, or knotted to reduce relative movement between the strands 41 and the fabric web 43. The course of stitches may be spaced at regular intervals. The stitching 45 electrically contacts the strands 41 and extends through the fabric web 43. The stitching 45 may loop back through the fabric web 43 towards the strands 41. Each of these loops of the stitching may electrically contact the spar cap 50, thereby providing multiple points of electrical contact with the spar cap 50. The strands 41 and spar cap 50 may therefore be electrically connected through multiple paths. Whilst some of the loops of the stitching 45 may not directly connect to the conductive material of the spar cap 50, many points of direct electrical contact can be made by providing a course of stitches spread over the lightning protection layer 40. For example, over a thousand points of electrical contact may be formed in the lightning protection layer. This is preferable to providing one or two discrete conducting paths.

The fabric web 43 is laid next to one or more layers or structural plies 55 of the blade shell. These plies 55 are positioned adjacent to an outer surface 52 of the blade 20. In some examples, the plies 55 may be glass fibre composite plies. The plies 55 may be substantially the same thickness as the lightning protection layer 40 so as to minimise any discontinuities on the outer surface of the blade 20.

In the example shown in Figure 4, the plies 55 and the fabric web 43 include overlapping portions that assist the mechanical connection between the plies 55 of the blade shell and the fabric web 43. The plies 55 and the fabric web 43 may include one or more ply drops 56 (ply terminations) and/or ply ramps 57. The plies of the fabric web 43 may be stepped to taper in a thickness direction of the lightning protection layer 40.

The electrically conductive strands 41 are located adjacent an outer surface 52 of the blade 20, however it will be seen from Figure 4 that the blade 20 may include one or more of: a fleece layer 58, and a gelcoat and/or paint layer 59. For example, a fleece layer 58 and a gelcoat layer 59 may be located between the lightning protection layer 40 and the outer surface 52 of the blade 20.

The wind turbine blade 20 is manufactured in a blade mould (not shown). The outer layers, such as the fleece layer 58, may be positioned in the mould first. The lightning protection layer 40, as a unit that includes the strands 41 attached to a surface of a fabric web 43 by the electrically conductive stitching 45, and the plies 55 may then be laid into the mould, with the electrically conductive strands 41 located face down towards the mould (i.e. adjacent the surface of the blade mould). The spar cap 50 may then be laid over the lightning protection layer 40 such that the electrically conductive stitching 45 forms an electrically conductive path between the conductive material of the spar cap 50 and the electrically conductive strands 41 of the lightning protection layer 40. In order to improve the electrical connection between the lightning protection layer 40 and the spar cap 50, the spar cap 50 may be abraded prior to being laid on the mould in order to reveal the conductive material of the spar cap 50. Alternatively, the conductive material of the spar cap 50 may be sufficiently revealed without abrading the spar cap 50, for example the conductive material may be revealed when a peel ply is removed after a composite spar cap is manufactured.

Figure 5 shows a portion of the blade 20 in which the inner layers 53, the core 54, and a spanwise portion of the spar cap 50 are removed to reveal an inner surface of the lightning protection layer 40. In this example, the lightning protection layer 40 has a chordwise extent in the chordwise direction of the blade 20 which is wider than the width of the spar cap 50. This ensures the spar cap 50 is protected by the lightning protection layer 40 across its entire width. The lightning protection layer 40 may be more than 50mm wider than the spar cap 50, but is preferably more than 100mm wider than the spar cap 50. In some examples, the spar cap 50 may have a constant width across at least a portion of its length. The lightning protection layer 40 may have a width that is a percentage of the width of the spar cap 50, for example approximately 110%, or a minimum percentage of the width of the spar cap, for example at least 105%.

The lightning protection layer 40 is pre-made prior to being inserted into the blade mould, as shown in Figure 6. The lightning protection layer 40 is an electrically conductive hybrid fabric having a fabric web 43, and a plurality of electrically conductive strands 41 arranged across a surface of the fabric web 43. The fabric web 43 may be a dry fibre preform, for example a bi-axial or tri-axial composite fibre preform. The electrically conductive strands 41 are joined to the surface of the fabric web 43 by electrically conductive stitching 45 through the fabric web 43, as previously discussed, such that a plurality of electrical paths are formed through the fabric web 43 from the strands 41. The strands 41 are positioned on the fabric web 43 prior to applying the stitching 45. It will be understood that many of the electrical contacts formed by the stitching 45 of pre-made lightning protection layer 40 may be revealed/exposed more clearly when the lightning protection layer 40 is consolidated during manufacture of the blade. In some examples, the stitching 45 may not be proud of the surface of the fabric web 43 until the lightning protection layer 40 is consolidated in the blade shell in the mould.

Figure 7 shows the stitching 45 extending through the fibres 44 of the fabric web 43. The stitching 45 loops over the strand 41 to form a direct electrical connection between the strand 41 and the stitching 45. At the opposite side of the fabric web 43, the stitching 45 forms a loop 46 that lays on top of the fabric web 43 so as to form an electrical connection with a spar cap 50.

In some examples, the stitching 45 may be adapted in various ways in order to improve the electrical contact between the stitching 45 and the spar cap 50, for instance the stitching 45 may be tied or knotted 46a adjacent the surface of the fabric web 43, such as shown in Figure 8. This may increase the extent of stitching 45 that protrudes from the surface of the fabric web 43. Alternatively, the course of stitches may be formed with some relative slack in the stitching 45 such that the loop folds and/or flattens against the surface of the fabric web 43, thereby increasing the electrical contact area of the stitching 45 with a spar cap 50.

The strands 41 typically extend in a generally spanwise direction of the blade 20, although some strands may extend in a generally chordwise direction or an oblique angle therebetween. In use, the wind turbine blade 20 will undergo significant forces that bend the blade 20, particularly in the flapwise direction. The bending strains that result from this bending are carried by the structure of the blade 20. As shown in Figure 9, the strands 41 may have undulations (alternatively referred to as ‘corrugations’) across the surface of the fabric web 43. The undulations may extend in a generally chordwise direction of the blade, parallel to the surface of the blade 20 and perpendicular to the extent of the strands 41 . The undulations may reduce the strains resulting from any spanwise bending, increasing fatigue resistance, particularly fatigue under high strains, and thereby protecting the strands 41. The undulations typically have a peak-to-peak amplitude of approximately 5mm, although the undulations may have an amplitude as large as 8mm or more in some examples and as small as 3mm or less in other examples.

In the examples shown in relation to Figures 4 to 8, the stitching 45 extends perpendicular to the strands 41 , in a generally chordwise direction of the blade 20, across the spanwise extending strands 41. In the example shown in Figure 9, the undulations allow the stitching 45 to extend in a substantially spanwise direction of the blade 20, parallel to the strands 41 , whilst attaching each strand 41 to the fabric web 43. Each strand 41 may contact a separate course of stitches 45a, 45b, 45c, although it will be clear that the stitches 45a-c may be continuous or otherwise joined at one or more spanwise locations.

In some examples, the stitching 45 may extend across a plurality of strands 41. This improves the electrical network, forming an increased number of conductive paths through which the lightning protection layer 40 can be connected to a spar cap 50. Figure 10 shows an example in which each course of stitches 45i, 45j extends across three strands 41 , although the stitching 45 may extend across and electrically contact any number of strands 41 .

The lightning protection layer 40 may include a plurality of chordwise extending strands 41 b in addition to a plurality of spanwise extending strands 41a, for example, as shown in Figure 11. This may help to increase the chordwise conductivity of the blade 20. As the bending strains carried by the structure of the blade 20 are predominantly spanwise strains, in some examples the spanwise strands 41a may have undulations and the chordwise strands 41b may be substantially straight. The presence of undulations, or the amplitude and extent of any undulations, may be decided based on the strains experienced by any section of the blade 20. For example, a strand 41 may have a first portion that has undulations and a second portion that is substantially straight.

The strands 41 of the lightning protection layer 40 may act as both a lightning receptor, which attracts a lightning strike, and a lightning conductor, which safely conducts the lightning strike through the structure towards a ground potential. At least a portion of the strands 41 may extend along the spanwise direction of the blade 20, from a tip end 24 to a root end 22 of the blade 20. The strands 41 may be generally evenly spaced in a chordwise direction, across the width of the spar cap 50. In order to provide a convenient conductive path towards ground, the strands 41 may converge towards an electrical terminal 60.

In this way, the strands 41 may have an advantage over a conventional metal mesh lightning protection layer as it may be easier to converge a number of discrete strands 41 together towards a terminal than it would be to connect a continuous aluminium mesh to a terminal. A series of discrete strands 41 may also be easier to handle than a single large mesh, and are therefore easier to integrate into the blade 20 structure during manufacture. Additionally, it may be easier to repair discrete strands 41 than it is to repair a continuous aluminium mesh. If an aluminium mesh were used to form a conductive layer it would typically be very thin in order to reduce weight. A thin aluminium mesh layer may be more susceptible to damage than a series of strands 41.

Figure 12 shows an example in which, towards the root end 22 of the blade 20 beyond an end of the fabric web 43, the strands 41 converge towards an electrical terminal 60. In this example, or in alternative examples, the strands 41 may additionally or alternatively converge towards the tip end 24 of the blade 20. For example, the strands may converge towards the metal tip 36. The strands 41 may extend from the metal tip 36, continuously, to the root end of the blade 20. In some examples, the blade 20 may be a split blade formed of two or more blade portions (not shown). In this case, electrical terminals 60 may be formed at an end of each blade portion such that the electrical terminals 60 can be connected when connecting the blade portions. The electrical terminals 60 may be connected by any means, for example a bolt or similar arrangement.

In the examples shown, the strands 41 may be substantially straight solid electrically conductive strands 41. In alternative examples, each strand 41 may be formed by braiding a plurality of yarns together. Typically, the lightning protection layer 40 includes a single layer of strands 41 upon a surface of the fabric web 43, such that the strands 41 are positioned adjacent the outer surface 52 of the blade 20. The strands 41 may not overlap and/or directly contact, except at an electrical terminal 60, so as to reduce any discontinuities on the outer surface 52 of the blade 20. In some examples, the outer layer of the fabric web 43 may be an electrically conductive carbon fibre layer or carbon fleece layer, which may increase the electrical contact between the strands 41 and the stitching 45.

In alternative examples, the lightning protection layer 40 may have a plurality of layers of strands 41 , for example a second layer of strands may be positioned on an opposing side of the fabric web 43 adjacent the spar cap 50. In an alternative example, an electrically conductive layer, such as a conductive fleece, may be positioned on an opposing side of the fabric web 43 to the strands 41 , such that the conductive fleece forms electrical contacts between the spar cap 50 and the conductive stitching 45.

The stitching 45 in the examples shown is a course of stitching using a running stitch using a single length of thread. In alternative examples, the stitching 45 may include two or more lengths of thread, for example two lengths of thread may be interlocked to create a series of lock stitches tying the fabric web 43 and the strands 41 together.

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.

Claims

1. A wind turbine blade having a spanwise direction extending from a root end to a tip end, and a chordwise direction, the blade comprising: a spar cap extending in the spanwise direction and comprising electrically conductive material; a lightning protection layer over the spar cap and including a fabric web and a plurality of electrically conductive strands upon a surface of the fabric web, the electrically conductive strands located adjacent an outer surface of the blade; and the lightning protection layer further comprising electrically conductive stitching joining the electrically conductive strands to the fabric web, the electrically conductive stitching contacting the spar cap and the electrically conductive strands to electrically connect the spar cap and the electrically conductive strands.

2. A wind turbine blade according to claim 1 , wherein the electrically conductive stitching comprises a course of stitches of an electrically conductive thread extending across and in contact with at least two of the electrically conductive strands.

3. A wind turbine blade according to claim 2, wherein the course of stitches is a running stitch.

4. A wind turbine blade according to claim 2 or claim 3, wherein the electrically conductive thread has a diameter less than 0.5mm, preferably around 0.3mm.

5. A wind turbine according to any preceding claim, wherein a plurality of stitches of the electrically conductive stitching provide multiple points of electrical contact with the spar cap.

6. A wind turbine blade according to any preceding claim, wherein the electrically conductive strands have undulations across the surface of the fabric web.

7. A wind turbine blade according to claim 6, wherein the undulations have an amplitude of less than 10mm, preferably less than 7mm.

8. A wind turbine blade according to any preceding claim, wherein the electrically conductive strands have a diameter less than 3mm, preferably less than 1.5mm, preferably around 1mm.

9. A wind turbine blade according to any preceding claim, wherein the electrically conductive strands each extend generally in the spanwise direction of the blade.

10. A wind turbine blade according to any preceding claim, wherein the lightning protection layer extends substantially the full spanwise length of the blade.

11. A wind turbine blade according to any preceding claim, wherein the electrically conductive strands extend beyond an end of the fabric web of the lightning protecting layer and converge at an electrical terminal, preferably wherein the electrical terminal is towards a root end of the blade.

12. A wind turbine blade according to any preceding claim, wherein the lightning protection layer has a chordwise extent in the blade chordwise direction which is wider than a width of the spar cap.

13. A wind turbine blade according to any preceding claim, wherein the conductive material of the spar cap includes carbon fibre, preferably pultruded carbon fibres extending in the blade spanwise direction.

14. A wind turbine blade according to any preceding claim, wherein the electrically conductive strands and/or electrically conductive stitching comprise metal, preferably Copper.

15. A wind turbine blade according to any preceding claim, wherein the fabric web of the lightning protection layer includes glass and/or carbon fibres.

16. A wind turbine blade according to any preceding claim, wherein the fabric web of the lightning protection layer includes a plurality of plies, preferably the plies are nonwoven plies, preferably the fabric web is a biaxial or triaxial material.

17. A wind turbine blade according to claim 16, wherein the plurality of plies of the fabric web are stepped to taper in a thickness direction of the lightning protection layer. 19

18. An electrically conductive hybrid fabric for lightning strike protection of a wind turbine blade, comprising: a fabric web; a plurality of electrically conductive strands arranged across a surface of the fabric web, wherein the electrically conductive strands are joined to the surface of the fabric web by electrically conductive stitching through the fabric web.

19. An electrically conductive hybrid fabric according to claim 18, wherein the electrically conductive strands have undulations across the surface of the fabric web.

20. A method of manufacturing a wind turbine blade, comprising: providing a lightning protection layer having a fabric web, a plurality of electrically conductive strands and electrically conductive stitching to join the electrically conductive strands to the fabric web; laying up the lightning protection layer in a blade mould with the electrically conductive strands located adjacent a surface of the blade mould; and laying up a spar cap comprising electrically conductive material over the lightning protection layer such that the electrically conductive stitching forms an electrically conductive path between the conductive material of the spar cap and the electrically conductive strands of the lightning protection layer.

21. A method according to claim 20, comprising abrading the spar cap prior to laying up the spar cap to reveal the conductive material of the spar cap.