DK201570797A1 - Improvements relating to lightning protection systems for wind turbine blades - Google Patents

Abstract

A wind turbine blade comprising a blade shell having a lightning protection layer; a down conducting component located within the blade; and a connector element that extends through the blade shell and which is configured to make an electrical connection between the down conducting component and the lightning protection layer. A contact face of the connector element opposes a contact region of the lightning protection layer and an adjustment element is located between the contact face of the connector element and the contact region of the lightning protection layer. Aspects of the invention also relate to a method of assembling a wind turbine blade.

Description

IMPROVEMENTS RELATING TO LIGHTNING PROTECTION SYSTEMS FOR WIND TURBINE BLADES

Technical field

The present invention relates to wind turbine blade structures and associated fabrication processes for improving the resilience of wind turbine blades to lightning strikes.

Background

Wind turbines are vulnerable to being struck by lightning; sometimes on the tower, nacelle and the rotor hub, but most commonly on the blades of the turbine. A lightning strike event has the potential to cause physical damage to the turbine blades and also electrical damage to the internal control systems of the wind turbine. Wind turbines are often installed in wide open spaces which makes lightning strikes a common occurrence. Accordingly, in recent years much effort has been made by wind turbine manufacturers to design wind turbines so that they are able to manage effectively the energy imparted to them during a lightning strike in order to avoid damage to the blade and the cost associated with turbine down-time during blade replacement.

In general, lightning protection systems for wind turbine blades are known. In US2011/0182731, a wind turbine blade includes a conductive layer that is laid over the outer surface of the blade so as to make contact with a series of discrete receptor elements. The conductive layer increases the area of the blade that can receive lightning, thereby increasing the rate at which the receptor elements can capture lightning strikes safely. Although a conductive layer used in this way can be said to increase the capability of the lightning protection system to intercept lightning strikes, such a system can be complex to manufacture since the conductive layer must be added to the blade after fabrication. The discrete receptor elements are time consuming to install and the conductive layer requires an additional time-consuming manufacturing step thereby increasing assembly time and cost. A different conductive layer approach is seen the Applicant’s pending International patent application WO2015/055215, which describes a wind turbine blade including a surface protection layer embedded in the blade surface. The blade includes a series of receptor elements that are exposed at the blade surface, establish an electrical contact with the surface protection layer and extend through the blade shell to connect to a down conducting system. Such a protection system is cost effective to manufacture and effective in operation. However, there is a need to ensure that the system is robust to a large number of lightning strikes over its lifetime, and it is against this context that the invention has been devised.

Summary

Against this background, embodiments of the invention provide a wind turbine blade comprising a blade shelf having a lightning protection layer; a down conducting component located within the blade; and a connector element that extends through the blade shell and which is configured to make an electrical connection between the down conducting component and the lightning protection layer. A contact face of the connector element opposes a contact region of the lightning protection layer and an adjustment element is located between the contact face of the connector element and the contact region of the lightning protection layer.

In one embodiment, the adjustment element is shaped to complement the contact face of the connector element and, in particular, the adjustment element may define a concave seating surface for engaging with the contact face of the connector element. Accordingly, the contact face of the connector element may be convex so as to mate with the seating surface. In this sense, the contact face of the connector element may have a radius of curvature substantially the same as a radius of curvature of the concave seating surface of the adjustment element.

In embodiments where the connector element is in the form of a bolt, for example providing a receptor element that is exposed at the surface of the wind turbine blade, the contact face of the connector element may be provided by a head of the connector element.

In one embodiment, the adjustment element may be annular or ring-like in form, for example a washer that fits onto the connector element.

In another aspect, embodiments of the invention provide a method of assembling a wind turbine blade comprising a blade shell having a lightning protection layer, the method comprising: forming a bore through the blade shell between an exterior surface and an interior surface thereof; inserting a connector element into the bore, so that a contact face thereof opposes a contact region of the lightning protection layer; and locating an adjustment element between the contact face of the connector element and the contact region of the iightning protection layer, so that an electrical connection is formed between the lightning protection layer and the connector element.

The adjustment element may be fitted on the connector element before inserting the connecting element in the bore.

The connector element may be engaged into a down conducting component provided in the wind turbine blade. This may include driving the connector element against the down conducting component with a pre-determined force so that the adjustment eiement adjusts its position relative to the connector eiement and the contact region of the lightning protection layer.

Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individuai features thereof, may be taken independently or in any combination. That is, alE embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that matter.

Brief description of the drawings

For a more complete understanding of the invention, some embodiments of the invention will now be described with reference to the following drawings, in which:

Figure 1 is a plan view of a wind turbine blade equipped with a lightning protection system;

Figure 2 is an enlarged view of a region of the turbine blade in Figure 1, that illustrates a surface protection iayer of the iightning protection system;

Figure 3 is a section through a leading edge region of the turbine blade in Figure 2 along the line C-C;

Figure 4 is an enlarged perspective view of the surface protection layer in exploded format;

Figure 5 is an enlarged view of a region of Figure 3 marked as ‘A’ in which a self-adjusting connector assembly is shown establishing an electrical contact between the surface protection layer and a down conducting system of the blade;

Figure 6 is a stylised view, similar to that of Figure 5, showing the principle of operation of the self-adjusting connector;

Figure 7 is a perspective view of a self-adjusting connector assembly of Figures 5 and 6, whereas Figure 8 and 9 are perspective views, respectively, of a connector element and an adjusting element of the self-adjusting connector assembly;

Figure 10 is a section view of the connector element of Figure 8;

Figure 11 is a section view of the adjusting element of Figure 9; and

Figure 12 is a plan view of a wind turbine blade equipped with a lightning protection system in accordance with another embodiment of the present invention.

Detailed description

With reference to Figure 1, a wind turbine blade 2 incorporates a lightning protection system 3. The blade 2 is formed from a blade shell 4 having two half-shells. The halfshells are typically moulded mainly from glass-fibre reinforced plastic (known as ‘GFRP’ or, simply ‘GRP’) that comprises glass fibre fabric embedded in a cured resin matrix. The precise construction of the blade shell 4 is not central to the invention and so further detailed description is omitted for clarity.

The blade 2 comprises a root end 6, at which the blade 2 would be attached to a rotor hub of a wind turbine, a tip end 8, a leading edge 10 and a trailing edge 12. A first outer surface 14 of the blade 2 defines an aerodynamic profiled surface that extends between the leading edge 10 and the trailing edge 12. The blade 2 also includes a second surface also extending between the leading edge 10 and trailing edge 12, which is not shown in the plan view of Figure 1, but which is indicated as reference numeral 16 in Figures 3 and 4, for example.

When the blade 2 is attached to a rotor hub of a wind turbine, airflow strikes the surface 16 of the blade 2 and for this reason the surface 16 is also referred to as a ‘pressure side’ or ‘windward side’ in the art. Conversely, the surface 14 is referred to as the ‘suction side’ or leeward side’.

Turning to the lightning protection system 3, this is based on a ‘zoning’ concept in which the blade 2 is demarcated in a longitudinal or 'span-wise' direction into regions or ‘zones’ depending on the probability of receiving a lightning strike in that region. A similar principle is described in WO2013/007267.

Here, the blade 2 is divided into three zones for the purposes of lightning protection -these are illustrated In Figure 1 as zones A, B and C. The lightning protection apparatus that is used in each of the zones is selected based on a set of lightning strike parameters, such as peak current amplitude, impulse current, specific energy, impulse shape and total charge that the blade 2 is expected to withstand in each of the zones. A brief explanation of the different zones now follows, by way of example.

Zone A extends from the root end 6 of the blade to approximately 60% of the blade length in the span-wise direction. In this zone, the blade 2 has a low risk of a lightning strike and so will be expected to receive a low incident of strikes at low current amplitudes, and low total charge transfer, which is acceptable for blade structural impact. In this embodiment, the blade 2 is not equipped with any external lightning protection within this zone.

Zone B extends from the end of zone A to approximately 90% of the blade length in a span-wise direction. In this zone the blade 2 has a moderate risk of lightning strike and is expected to withstand moderately frequent direct lightning strike attachments having increased impulse current, peak current and total charge transfer. Accordingly, the blade 2 is provided with a first lightning protection sub-system in the form of a surface protection layer 20.

Finally, zone C extends from the end of zone B to the tip end 8 of the blade 2. In this zone the blade 2 is subject to a high likelihood of lightning strikes and is expected to withstand peak current amplitudes of in excess of 200kA and total charge transfer in excess of 300 coulomb and, moreover, a high incident of strikes. To provide the required ievel of protection for the blade, zone C includes two further lightning protection subsystems. Firstly, there is provided an array of receptors (hereinafter ‘receptor array’) 22 and, secondly, there is provided a blade tip assembly 24. Both the receptor array 22 and the blade tip assembly 24 are electrically connected to a down conducting system 26, comprising first and second down conductors 28, 30 running along the length of the blade 2 from the tip end 8 to the root end 6, generally being arranged adjacent the leading edge 10 and trailing edge 12 of the blade 2, respectively. Although an overview of the receptor array 22 and the blade tip assembly 24 has been provided here for completeness, they are not central to the inventive concept and so further explanation will be omitted.

Detailed discussion will now turn to the surface protection layer 20. As has been mentioned, the surface protection layer 20 is in zone B and comprises an electrically conductive layer that is integrated into both the upper half-shell and the lower half-shell of the blade 2. The conductive layer may be a metallic screen or mesh, and preferably a mesh/screen in the form of an expanded metal foil that acts to attract lightning strikes over a large area of the blade and which is connected to the down conducting system 26 in a manner that will be described. The thickness of the conductive layer is such that the aerodynamic profile of the blade 2 is unaffected and so it is preferred that the conductive layer is less than 5mm in thickness. It is currently envisaged that the conductive layer is less than 1mm in thickness, preferably 0.3mm. In principle, an expanded foil of any metallic material is acceptable as long as it provides the necessary current-carrying and charge dissipation capability, although aluminium and copper foils are currently preferred. As is seen in Figure 2, the surface protection layer 20 is connected to the down conductors 28, 30 by a plurality of connector arrangements 40. Four connector arrangements 40 are shown in this view of the surface of the blade 2, two being adjacent the leading edge 10 of the blade and two being adjacent the trailing edge 12 of the blade 2. Other arrangements are possible.

Figure 3 shows a leading edge connector arrangement 40 in more detail. The connector arrangement 40 includes a biock-Sike connector component 42 that forms part of the down conducting system and which is shaped to fiii the volume in the relatively deep profile of this region of the blade 2 and provide an electrical connection to a first connector element 44 associated with the leeward surface 14 and a second connector element 46 associated with the windward surface 16.

The connector component 42 comprises first and second connector bases 48, 50 that are encapsulated by an insulating member 52 that is generally annular in form. The insulating member 52 is moulded directly to the connector bases 48, 50 and so serves to suppress the initiation of ionization and streamers during highly charged environmental conditions, which thereby guards against a lightning strike directly onto the connector bases 48, 50 rather than on a connector element 44, 46. The insulating member 52 is formed of a suitable polymer having a high dielectric strength, and it is envisaged that the insulating member 52 will be polyurethane for its good dielectric properties and Sow cost, although other insulating materials are acceptable.

In more detail, the insulating member 52 is generally C-shaped, and is defined by first and second arm portions 52a, 52b that extend from each end of a yoke portion 52c. Each of the connector bases 48, 50 is encapsulated by a respective one of the arm portions 52a, 52b and in this manner the connector bases 48, 50 are located in a predetermined position against a respective leeward 14 and windward surface 16 of the blade 2. The connector bases 48, 50 are conductive, preferably brass for its high conductivity, corrosion resistance, and suitability for drilling, although other metals or alloys would be acceptable.

The first connector element 44 electrically couples the surface protection layer 20 on the leeward surface 14 to the first connector base 48. Similarly, the second connector element 46 couples the surface protection layer 20 on the windward surface 16 to the second connector base 50. The connector elements 44, 46 are identical so only one of them shall be described in detail. The first connector element 44 is in the form of a bolt having a head 44a and a shank or stem 44b. Stainless steel is currently the preferred material for the bolt, although other conductive materials, particularly metals, are also acceptable. The shank 44b extends through the blade 2 and engages with the first connector base 48 by way of cooperating screw threads, and the head 44a is arranged to lie flush with the surrounding surface of the surface protection layer 20. The connector element 44 is installed in the blade after the blade has been fabricated and removed from its mould. As will become apparent, holes or drillings are formed through the blade skin from the exterior surface to the interior surface and into the receptor base 48. The holes are formed manually and are preferably perpendicular to the blade surface. A suitable jig or tool can be used to help ensure that the hole for the connector element 44 is formed perpendicularly. Following formation of the hole, the connector element 44 is then inserted into the hole and screwed into or otherwise engaged with the receptor base 48 to make the electrical connection.

An identical arrangement is provided to couple the surface protection layer 20 on the windward surface 16 to the second connector base 50.

Connection between the connector component 42 and the down conducting system 26 may be made by welding the second connector base 50 to a corresponding down conductor, which as illustrated is the first down conductor 28 near the leading edge 10 of the blade 2. For efficient assembly, the conductive link 56 and the down conductor 28 may be arranged in a predetermined pattern with respect to the first and second connector bases 48, 50 and connected thereto by, for example, exothermic welding to ensure the electrical integrity of the connection prior to casting the instating member 52 around the components. In this way, the connector component 42 becomes installable as a unit together with the down conducting system 26.

The insulating member 52 is sandwiched between an interior of the leeward surface 14 and an interior of the windward surface 16. A layer of suitable engineering adhesive, such as epoxy resin, (not shown) is located between the insulating member 52 and the interiors of the leeward and windward surfaces 14, 16 to bond the insulating member to the interior of the blade 2. It should be appreciated that Figure 3 shows a cross section through the insulating member 52 and that the first connector base 48, the second connector base 50 and the conductive link 56 are fully encapsulated by the insulating member 52. The insulating member 52 may have a width in the span-wise direction of around 15 cm.

As has been mentioned above, the head of the connector element 44 defines an electrical coupling or interface between the surface protection layer 20 and the respective connector component 42. In particular, the connector element electrically coupies the surface protection layer 20 at the outer surface of the blade to the down conductor 28 in the interior of the blade.

Note that the upper surface of the head 44a is exposed at the blade surface and so can also serve as a receptor element for lightning strikes. The surface protection layer 20 and, in particular, the electrical connection between it and the connector element 44 will now be described with reference to Figures 4 and 5.

In overview, the surface protection layer 20 incorporates a conductive layer currently envisaged to be expanded aluminium foil, in Figure 4, the surface protection layer 20 is shown in exploded view for clarity against a blade mould surface portion 80. The surface protection layer 20 includes three main components: an outer insulating layer 82, an inner insulating layer 84 and a conductive layer 86 sandwiched between the insulating layers 82, 84.

Both the outer insulating layer 82 and the inner insulating layer 84 are glass fibre fabric. The outer insulating layer 82 becomes the outer surface or skin of the blade 2 once the blade 2 is fully fabricated. Therefore it is preferred that the outer insulating iayer 82 is a relatively lightweight fabric, for example less than 200gsm, so as not to inhibit the formation of ieaders from the conductive iayer 86 during lightning conditions. The thin outer insulating iayer 82 also reduces the risk of surface damage during a strike. Conversely, since it is desirable to insulate in-board from the surface protection iayer 20, the weight of the inner insulating iayer 84 is heavier, for example around 600gsm, although these values should not be considered limiting.

In order to promote a good electricai contact between the conductive iayer 86 and the connector element 44, the conductive iayer includes reinforced zones, identified in Figure 4 generally as ‘90’. The reinforced zones 90 serve to strengthen the conductive layer 86 in localised regions by thickening the metal foil. For example, the conductive iayer 86 may undergo a soldering or casting process to solidify the expanded foil in iocaiised regions. Aiternativeiy, one or more conductive elements in the form of piates, discs or the like are bonded to the conductive layer 86 in the required zones. Bonding may be by way of brazing for example. It should be appreciated, however, that although the reinforced zone 90 may improve the robustness of the conductive layer 86 where it connects to the connecting element 44, it is not essentia! to the inventive concept.

In each reinforced zone 90, a forming element 92 is applied to the outer insulating layer 82 prior to the lay down of the conductive layer 86. The forming element shapes the conductive layer 86 during blade fabrication to provide a recess for receiving a respective connector element 44 so that the surface protection layer 20 can be electrically connected to the internal components of the down conducting system 26, as outlined above.

In establishing the electrical connection between the surface protection layer 20 and the down conducting system 26, it is important that the connector elements 44 extend through the blade shell at an angle that is perpendicular to the surrounding region of the surface protection layer 20 in order that the head 44a of the connector element 44 establishes a robust electrical connection with the surface protection layer 20. However, in practice, the fabrication of a wind turbine biade is a labour-intensive process which requires that bores for the connector elements 44 are drilied through the biade sheil manually or using a suitable tool to ensure that the bores are formed as accurately as possible. However, even with the assistance of a suitable guidance tooi, there is the potential that the bores may be formed inaccurately so that they are not truly perpendicular to the surface of the blade 2. In such circumstances, the connector element 44 would be aligned incorrectly so that the head 44a may not sit accurately on the reinforced zone 90 of the conductive layer 86 thereby compromising the eiectrica! connection between those components. This could cause electrical arcing which has the potential to damage both the connector element and the conductive layer, thereby further reducing the effectiveness of the electrical connection to the point of failure.

Figure 5 is an enlarged view of region ‘A’ in Figure 3 showing one of the connector arrangements 40 in greater detail and illustrates an embodiment of the invention that provides a solution to the electrical connection issue discussed above.

Here, the surface protection layer 20 is shown as defining the leeward surface 14 of the blade 2 together with a set of structural blade components 96 with which the surface protection layer 20 is integrated during a resin infusion and curing process. The structural blade components 96 may include further fabric layers, foam core sections and the like, as would be known to a person skilled in turbine blade design, but which will not be described in detail here for the sake of brevity.

The forming element 92 is an outwardly-tapered annular disc that includes an inner aperture 98 defining an inner wall 100. The forming element is preferably a polymeric part, particularly polyurethane. The forming element 92 sits in-board of the outer insulating layer 82 such that the layer 82 extends over a flat outer face 92a of the forming element 92 and terminates at an aperture 101 aligned with the inner wall 100. Note, however, that the outer insulating layer 82 may instead terminate at the outer edge of the forming element 92.

The conductive layer 86 is in-board of the outer insulating layer 82 and is positioned such that a reinforced zone 90 thereof is in registration with, or ‘superimposed’ on, the aperture 98 of the forming element 92. Here, the reinforced zone 90 includes first and second metal discs 102 that are cast onto either side of the conductive layer 86 and so are integral parts of the surface protection layer 20,

The dished or domed shape of the forming element 92 raises the level of the reinforced zone 90 so that it defines a recessed base 104 adjacent the inner wail 100 of the forming element 92. The recess base 104 and the inner wail 100 thereby provide a countersink cavity 105 for the head 44a of the connector element 44.

When installed in the blade, the shank 44b of the connector element 44 locates through an opening 103 through the reinforced zone 90 and a drilling or bore 120 in the structural component 96. The bore 120 extends between an inner shell surface 121 and an outer shell surface 122, and also extends into receptor base 48 which, in this embodiment, may be threaded for receiving the connector element 44.

In this position, the underside surface of the head 44a of the connector element 44 is opposed to the surface protection layer 20 and serves as a contact face 124 to make electrical contact with it. In this embodiment, the contact face 124 makes indirect contact with the reinforced zone 90 via an intermediate adjustment element 130. The reinforced zone 90 can therefore be considered to be a contact region for the surface protection layer 20.

The adjustment element 130 is located between the surface protection layer 20 and the connector head 44a and provides a self-adjustment function which accommodates angular misalignment of the bore 120 in the blade shell. Together, the connector element 44 and the adjustment element 130 can be considered a self-adjusting connector assembly 129. Reference will now be made to Figures 7 to 11 which show the self-adjusting connector assembly 129 in more detail.

It should be noted at this point that Figures 7, 8 and 10 do not show a screw thread on the shank 44b of the connector element 44, although the presence of a screw thread is implied by the function of the connector element 44. Also, note that the head 44a of the connector element 44 include gripping features 127 that permit a driving tool to engage with those features and to drive the connector element 44 into engagement with the connector base 48.

Whereas Figure 7 shows the connector element 44 and the adjustment element 130 in assembled form, Figures 8 to 11 show the components individually. As can be seen, the adjustment element 130 is in the form of a shim or washer. In this embodiment, the adjustment element 130 is annular in form having a circular outer edge 131 and a central aperture 132 which is sized to fit onto the shank 44b of the connector element 44.

The adjustment element 130 has a concave upper surface 134 and an opposed or underside face 135 which is substantially flat. The concave upper surface 134 tapers radially inwardly from the outer edge 131 towards the central aperture 132. Therefore, the adjustment element 130 is relatively thick at the outer edge 131 compared to the thickness at the central aperture. The adjustment element 130 in this example comprises a concave washer.

The concave upper surface 134 serves as a seating surface for the head 44a of the connector element 44. To optimise the engagement between these parts, the contact face 124 of the head 44a is shaped to complement the concave profile of the upper surface 134 of the adjustment element 130, Thus, the contact face 124 is convex in this embodiment and has a radius of curvature about the same as the radius of curvature of the upper surface 134. In other words, the head 44a has a convex shape. In practice, the radius of curvature of the convex contact surface 124 will be slightly less than the radius of curvature of the seating surface 1,34 of the adjustment element 130.

The complementary curved surfaces of the contact face 124 of the connector element 44 and the seating surface 134 of the adjustment element 130 allows these components to slide relative to one another. This means that the connector element 44 can ‘seif adjust’ when being installed in its bore 120 so that the shank 44b remains in good angular alignment in the bore 120 whilst the head 44a maintains a robust electricai contact with the surface protection layer 20 via the adjustment element 130. To assist the sliding movement between the curved surfaces, those surfaces may be machined to an appropriately fine surface finish. Optionally, a suitable lubricant may be applied between the surfaces 124,134.

The material of the connector element 44 and the adjustment element 130 may be any suitable conducting materiai, although in practice a highly conductive, cost effective and strong material is preferred. Stainless steel is particularly suited for these parts, but this is not intended to be limiting. Although the connector element 44 and the adjustment element 130 may be different materials, it is currently envisaged that both parts be formed from the same material, which may avoid problems due to galvanic corrosion and differential thermal expansion.

It will be appreciated that the precise dimensions of the connector element 44 and the adjustment element 130 are specific to the application in which they are intended to be used. However, to provide an indication of suitable dimensions for these components, the diameter of the adjustment element 130 may be about 40mm and that the diameter of the central aperture of the adjustment element may be about 15mm, In terms of material thickness, the thickness of the adjustment element 130 at the outer edge may be about 3mm, whilst the thickness at the edge of the central aperture 132 may be about 1mm. Also, it is envisaged that an appropriate radius of curvature for the contact face 124 and the seating surface 134 is approximately 70 degrees. It should be understood that the aforementioned dimensions and radius of curvature are given by way of example only and are not meant to limit the scope of the invention, as defined by the claims. Furthermore, the dimensions and curvature are selectable depending on the angular misalignment that needs to be compensated for when installing the connector element 44 into the bore 120.

Figure 6 illustrates the principle of operation, although it should be noted that Figure 6 is a stylised view and is not intended to be a precise reproduction of Figure 5. Here, it will be appreciated that the bore 120 in Figure 8 is not perpendicular to the outer shell surface 122 but instead extends along an oblique axis which is marked as ‘B’. The oblique axis B can be compared with a perpendicular axis which is marked ‘A’.

Since the bore 120 is not perpendicular to the outer shell surface 122, this forces the shank 44b of the connector element 44 to be axially aligned with it. As a result, the head 44a of the connector element 44 has shifted angularly on the seating surface 134. However, the curved contact face 124 of the head 44a is still in good electrical contact with the adjustment element 130, despite the angular misalignment of the bore 120. As the flat face 135 of the adjustment element 130 sits squarely on the reinforced zone 90 of the surface protection layer 120, a robust electrical connection is maintained between the reinforced zone 90 and the connection element 44 that Is capable of carrying a high current load without adverse consequences. Given the dimensions and radius of curvature discussed above, the connector element 44 is able to move angularly with respect to the adjustment element 130 to compensate for up to about 3 degrees of misalignment of the bore 120. It should be appreciated, however, that the adjustment element 130 and the connector element 44 may be configured to have dimensions and radius of curvatures selected compensate for greater misalignment between the bore 120 and the perpendicular axis A. The correcting effect of the adjustment element 130 and the convex head 44a also allows for a wider tooling tolerance on achieving a perpendicular (to the exterior blade surface) bore at the connector element locations.

The process of driving the connector element 44 into engagement with the base with a predetermined force, for example by tightening in embodiment where the connector element 44 is a threaded bolt, encourages the adjustment element 130 to shift into the correct position. Thus, the process of establishing a good connection between the surface protection layer 20 and the connector element 44 is automatic and does not require monitoring by operatives.

Figure 12 illustrates a wind turbine blade 2 in accordance with an alternative embodiment in which the self-adjusting connector assembly 129 described above would be particularly useful. The wind turbine blade 2 of this embodiment is very similar to the blade 2 in Figure 1, so only the differences wili be described here. Also, common parts will be referred to using the same reference numerals. in this embodiment, the tip assembly 24, the receptor array 22 and the surface protection layer 20 are connected to a single down conductor 28. The down conductor 28 is coupled to the surface protection layer 20 at two connector arrangements 40. Notably, the down conductor 28 terminates at the connector arrangements 40 and does not extend up to the receptor array 22 and the tip assembly 24, as was the case with the blade 2 in Figure 1. Therefore, the surface protection layer 20 in effect acts as part of the down conductor since the electrical current generated from all lightning strikes on the tip assembly 24 and the receptor array 22, in addition to strikes on the surface protection layer 20, must pass through the two connector arrangements 40 in order to be channelled along the down conductor 28.

Since the surface protection layer 20 in this embodiment acts as part of the down conductor 28, the two connector arrangements 40 will necessarily handle current flow from many more lightning strikes. Therefore, the self-adjusting connector assembly 129 of the invention will be particularly suited to such a configuration as it ensures a robust electrical connection between the connector element 44 and the surface protection layer 20.

Claims (12)

1. A wind turbine blade comprising: a blade shell having a lightning protection layer; a down conducting component located within the blade; a connector element that extends through the blade shell and which is configured to make an electrical connection between the down conducting component and the lightning protection layer; wherein a contact face of the connector element opposes a contact region of the lightning protection layer and wherein an adjustment element is located between the contact face of the connector element and the contact region of the lightning protection layer.

2. The wind turbine blade, wherein the adjustment element is shaped to complement the contact face of the connector element.

3. The wind turbine blade of claim 1 or 2, wherein the adjustment element defines a concave seating surface for engaging with the contact face of the connector element.

4. The wind turbine blade of claim 3, wherein the contact face of the connector element is convex.

5. The wind turbine blade of claim 4, where the contact face of the connector element has a radius of curvature substantially the same as a radius of curvature of the concave seating surface of the adjustment element.

6. The wind turbine blade of claims 1 to 5, wherein the contact face of the connector element is provided by a head of the connector element.

7. The wind turbine blade of claims 1 to 6, wherein the connector element is a receptor that is exposed at a surface of the blade.

8. The wind turbine blade of claims 1 to 7, wherein the adjustment element is a washer that fits onto the connector element.

9. A method of assembling a wind turbine blade comprising a blade shell having a lightning protection layer, the method comprising: forming a bore through the blade shell between an exterior surface and an interior surface thereof; inserting a connector element into the bore, so that a contact face thereof opposes a contact region of the lightning protection layer; locating an adjustment element between the contact face of the connector element and the contact region of the lightning protection layer so that an electrical connection is formed between the lightning protection layer and the connector element.

10. The method of claim 10, wherein the adjustment element is fitted on the connector element before inserting the connecting element in the bore.

11. The method of claims 9 and 10, including engaging the connector element into a down conducting component provided in the wind turbine blade.

12. The method of claim 11, including driving the connector element against the down conducting component with a pre-determined force so that the adjustment element adjusts its position relative to the connector element and the contact region of the lightning protection layer.