Abstract:
A method of making a wind turbine blade component incorporating a lightning protection system, the method comprising: providing a mould surface; arranging a forming element on the mould surface; providing an electrically conductive layer; reinforcing the electrically conductive layer in a predetermined region to create a reinforced zone; arranging the electrically conductive layer over the forming element so that the reinforced zone is superimposed on the forming element; arranging one or more structural components on the electrically conductive layer; consolidating the structural components under vacuum to form a blade shell having an integrated electrically conductive layer adjacent an outer surface of the shell; removing at least part of the forming element from the blade shell to define a recess in the outer surface of the shell so as to expose the reinforced zone of the electrically conductive layer; electrically connecting the electrically conductive layer at the reinforced zones to a respective electrical component located adjacent an inner surface of the blade shell. The invention also extends to a preformed component for use in fabricating a wind turbine blade.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to wind turbine blade structures and associated fabrication processes for improving the resilience of wind turbine blades to lightning strikes. The invention also extends to preformed components for use in such wind turbine blades, and methods for fabricating such preformed components. 
       BACKGROUND 
       [0002]    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. 
         [0003]    In general, lightning protection systems for wind turbine blades are known. In one example, an electrically conductive lightning receptor element is arranged on an outer surface of the blade to receive a lighting strike. Since the receptor element is electrically conductive, lightning is more likely to attach to the receptor element in preference to the relatively non-conductive material of the blade. The receptor element is connected to a cable or ‘down conductor’ that extends inside the blade to the root and from there connects via an armature arrangement to a charge transfer route in the hub, nacelle and tower 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. However, the discrete receptors are relatively complex to install during fabrication of the blade and, moreover, they leave a significant portion of blade area exposed to a risk of lightning strike. 
         [0004]    Such a receptor arrangement provides discrete conductive points to which lightning may attach. To increase the effectiveness of such a system, US2011/0182731 describes a wind turbine blade having a conductive layer that is laid over the outer surface of the blade shell so as to make contact with the 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 the blade shell has been fabricated. This requires an additional time-consuming manufacturing step thereby increasing assembly time and cost. 
         [0005]    It is against this context that the invention has been devised. 
       SUMMARY OF THE INVENTION 
       [0006]    In a first aspect, the invention provides a method of making a wind turbine blade component incorporating a lightning protection system, the method comprising: providing a mould surface; arranging a forming element on the mould surface; providing an electrically conductive layer, and reinforcing the electrically conductive layer in a predetermined region to create a reinforced zone. Then, the electrically conductive layer is arranged over the forming element so that the reinforced zone is superimposed on the forming element; and one or more structural components are arranged on the electrically conductive layer. The structural layers may include further composite-suitable layers such as GRP fabrics. The structural components are then consolidated under vacuum to form a blade shell having an integrated electrically conductive layer adjacent an outer surface of the shell, wherein the method includes removing at least part of the forming element from the blade shell to define a recess in the outer surface of the shell so as to expose the reinforced zone of the electrically conductive layer. Finally, the electrically conductive layer is electrically connected at the reinforced zones to a respective electrical component located adjacent an inner surface of the blade shell. 
         [0007]    The reinforcing of the conductive layer in a predetermined zone or region, or a plurality of such zones, optimises the electrical contact between the conductive layer and the electrical component. 
         [0008]    To simplify the assembly process, the conductive layer may be provided as a preformed component for arranging on the mould. In this case, the forming element could also be provided as part of the preformed component. 
         [0009]    The reinforced zone may be established by applying a conductive element, such as a metal plate or disc to one or both sides of the conductive layer. The conductive elements may be applied by a suitable technique such as being fused to the conductive layer by brazing, welding, or casting for example. 
         [0010]    To allow the reinforced zone to be accessed for electrical connection, the forming element may be provided with a removable element which, when removed, exposes the underlying reinforced zone. However, during the consolidation phase, the removable element beneficially protects the conductive layer from being infused with resin. 
         [0011]    The conductive layer may be any suitable thin sheet-like conductor such as metallic foil or a mesh. However, an expanded metal foil, of copper or aluminium for example, strikes a good balance between electrical conductivity, robustness, weight and cost. 
         [0012]    In order to connect the conductive layer to the electrical component, a connector element may be received through the conductive layer, for example by drilling a hole for its passage. The connector element may be provided with a flat face for mating with a correspondingly flat surface of the reinforced zone which results in a robust electrical contact. 
         [0013]    As has been discussed, the conductive layer may be formed as a preformed component for laying up in a suitable mould. Therefore, in a second aspect, the invention also resides in a preformed component for a lightning protection system of a wind turbine blade, wherein the preformed component comprises a conductive layer including reinforcing means arranged to reinforce the conductive layer in one or more discrete reinforced zones. 
         [0014]    The reinforcing means may include at least one reinforcing element applied to the conductive layer in a respective one or more locations so as to form one or more reinforced zones. The reinforcing element may be formed by applying a conductive material such as solder to the conductive layer to thicken the layer in localised areas or, alternatively, a reinforcing element such as a metallic plate or disc may be bonded or fused to the conductive layer. 
         [0015]    The preformed component may also include a forming element superimposed on the reinforced zone that helps to shape the preformed component as appropriate for receiving a receptor element. The forming element may include a removable plug that seals against the reinforced zone to prevent intrusion of resin. However, the plug can be removed to allow access to the underlying conductive layer. 
         [0016]    To assist technicians in locating the forming elements once the blade has been formed, the forming element may include locating means, preferably in the form of a magnet embedded in the plug, although other means are acceptable. 
         [0017]    Preferred and/or optional features of the first aspect of the invention may be combined with other aspects of the invention and vice versa. The invention in its various aspects is defined in the independent claims below and advantageous features are defined in the dependent claims below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    For a fuller understanding of the invention, some embodiments of the invention will now be described with reference to the following drawings, in which: 
           [0019]      FIG. 1  is a plan view of a wind turbine blade equipped with a lightning protection system; 
           [0020]      FIG. 2  is an enlarged view of a region of the turbine blade in  FIG. 1 , that illustrates a surface protection layer of the lightning protection system; 
           [0021]      FIG. 3  is a section through a leading edge region of the turbine blade in  FIG. 2  along the line C-C; 
           [0022]      FIG. 4  is a section through a trailing edge region of the turbine blade in  FIG. 2  along the line C-C; 
           [0023]      FIG. 5  is an enlarged perspective view of a surface protection layer in exploded format; 
           [0024]      FIG. 6  is an enlarged view of the region A indicated on  FIG. 3 ; 
           [0025]      FIGS. 7 a  to 7 e    illustrate a series of fabrication steps for a wind turbine blade shell incorporating a surface protection layer; and 
           [0026]      FIGS. 8 a  to 8 d    illustrate series of fabrication steps for a preformed component of a surface protection layer. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    With reference to  FIG. 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 half-shells 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. 
         [0028]    The blade 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 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  FIG. 1 , but which is indicated as reference numeral  16  in  FIGS. 3 and 4 , for example. 
         [0029]    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’. 
         [0030]    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. 
         [0031]    In this embodiment, the blade  2  is divided into three zones for the purposes of lightning protection—these are illustrated in  FIG. 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. 
         [0032]    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. 
         [0033]    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  20  in the form of a surface protection layer  20 . 
         [0034]    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 200 kA and total charge transfer in excess of 300 coulomb and, moreover, a high incident of strikes. To provide the required level of protection for the blade, zone C includes two further lightning protection sub-systems. 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. 
         [0035]    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 a 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 5 mm in thickness. It is currently envisaged that the conductive layer is less than 1 mm in thickness, preferably 0.3 mm. 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. 
         [0036]      FIG. 2  shows the surface protection layer  20  in more detail, although not to scale. Here it can be seen that 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 . 
         [0037]      FIG. 3  shows a leading edge connector arrangement  40  in more detail and  FIG. 4  shows a trailing edge connector arrangement  40  in more detail. 
         [0038]    With reference firstly to  FIG. 3 , the connector arrangement  40  includes a block-like connector component  42  that is shaped to fill 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 . 
         [0039]    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 will be polyurethane for its good dielectric properties and low cost, although other insulating materials are acceptable. 
         [0040]    In more detail, the insulating member  52  is generally C-shaped, and is defined by first and second arm portions  52   a,    52   b  that extend from each end of a yoke portion  52   c.  Each of the connector bases  48 ,  50  is encapsulated by a respective one of the arm portions  52   a,    52   b  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 drillability although other metals or alloys would be acceptable. 
         [0041]    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  44   a  and a shank  44   b.  Stainless steel is currently the preferred material for the bolt, although other conductive materials, particularly metals, are also acceptable. The shank  44   b  extends through the blade  2  and engages with the first connector base  48  by way of cooperating screw threads, and the head  44   a  is arranged to lie flush with the surrounding surface of the surface protection layer  20 . Note that the underside of the head  44   a  is substantially flat so as to establish a robust electrical connection with the surface protection layer  20 . An identical arrangement is provided to couple the surface protection layer  20  on the windward surface  16  to the second connector base  50 . 
         [0042]    A conductive link  56  is provided to electrically connect the first connector base  48  to the second connector base  50  and, in this embodiment, the conductive link  56  is a zinc coated copper braided wire. Although braided wire is not essential, it is useful from a manufacturing perspective since it is flexible and so can be suitably shaped to extend between the first and second connector bases  48 ,  50  prior to encapsulation by the insulating member  52 . 
         [0043]    Connection between the connector component  42  and the down conducting system  26  is 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 insulating member  52  around the components. In this way, the connector component  42  becomes installable as a unit together with the down conducting system  26 . 
         [0044]    The insulating member  52  is sandwiched between an interior of the leeward surface  14  and an interior of the windward surface  16 . Adhesive (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. It should be appreciated that  FIG. 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. 
         [0045]    Turning to  FIG. 4 , here is shown a section through one of the connector arrangements  40  on the trailing edge  12  of the blade  2 . In a similar manner to the connector arrangement  40  in  FIG. 3 , the connector arrangement  40  shown in  FIG. 4  also includes a block-like connector component  60  that connects to a first connector element  62  associated with the leeward surface  14  and a second connector element  64  associated with the windward surface  16 . 
         [0046]    However, here the connector component  60  comprises a single connector base  63  with which the connector elements  62 ,  64  engage, and an insulating member  66  that encapsulates the connector base  63 . The connector base  63  includes a corresponding recess  68  through which the trailing edge down conductor  30  is routed so as to connect the connector bases  63  into the down conducting system  26 . Therefore, the encapsulation of the connector base  63  also encapsulates the junction between the down conducting system  26  and the connector base  63 . 
         [0047]    Each connector element  62 ,  64  is in the form of a bolt having a head  62   a,    64   a  and a shank  62   b,    64   b.  The shanks  62   b,    64   b  extend into the blade  2  and engage into a threaded socket  70  in the connector base  63 . The heads  62   a,    64   a  lie against and are countersunk into the blade shell so that an upper face of the heads  62   a,    64   a  are flush with the surrounding surface of the blade  2 . 
         [0048]    Since the connector component  60  is installed in region of the blade  2  that has a relatively shallow depth, preferably the connector elements  62 ,  64  are joined to the connector base  63  so as to be offset from one another or ‘staggered’, as is shown in  FIG. 4 . This avoids the shanks  62   b,    64   b  of opposing connector elements  62 ,  64  contacting one another when installed. 
         [0049]    The insulating member  66  is sandwiched between an interior of the leeward surface  14  and an interior of the windward surface  16 . Adhesive (not shown) is located between the insulating member  66  and the interiors of leeward and windward surfaces  14 ,  16  to bond the insulating member to the interior of the blade. It should be appreciated that  FIG. 4  shows a cross section through the insulating member  66  and that the connector base  63  is fully encapsulated by the insulating member  66 . The insulating member  66  may have a width in the span-wise direction of around 15 cm. 
         [0050]    As has been mentioned above, the head of the connector elements  44 ,  46 ,  62 ,  64  defines an electrical coupling or interface between the surface protection layer  20  and the respective connector component  42 ,  60 . The surface protection layer  20  and, in particular, the electrical connection between it and the connector elements will now be described with reference to  FIGS. 5 and 6 . 
         [0051]    In overview, and as has been mentioned, the surface protection layer  20  incorporates a conductive layer currently envisaged to be expanded aluminium foil. In  FIG. 5 , 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 . 
         [0052]    Both the outer insulating layer  82  and the inner insulating layer 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 layer  82  is a relatively lightweight fabric, for example less than 200 gsm, so as not to inhibit the formation of leaders from the conductive layer  86  during lightning conditions. The thin outer layer also reduces the risk of surface damage during a strike. Conversely, since it is desirable to insulate in-board from the surface protection layer  20 , the weight of the inner insulating layer  84  is heavier, for example around 600 gsm, although these values should not be considered limiting. 
         [0053]    In order to promote a good electrical contact between the conductive layer  86  and the connector elements, the conductive layer includes reinforced zones, identified in  FIG. 5  generally as ‘ 90 ’. The reinforced zones  90  serve to strengthen the conductive layer  86  in localised regions by thickening the metal foil in some way. For example, the conductive layer  86  may undergo a soldering or casting process to solidify the expanded foil in localised regions. Alternatively, one or more conductive elements in the form of plates, discs or the like are bonded to the conductive layer  86  in the required zones. Bonding may be by way of brazing for example. 
         [0054]    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 . As will be described, the forming element shapes the conductive layer  86  during blade fabrication to provide a recess for receiving a respective connector element, as will now be described with reference to  FIG. 6  which is an enlarged view of region ‘A’ in  FIG. 3  and so shows the surface protection layer  20  in situ. The insulated forming element also prevents unwanted lightning strike attachment points adjacent to the connector bolts, thus preventing the surface protection layer from being damaged. 
         [0055]    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. 
         [0056]    The insulating member  52  is bonded to the structural blade components  96  by a layer of adhesive (not shown). 
         [0057]    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  92   a  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 . 
         [0058]    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  82 . Here, the reinforced zone  90  includes first and second metal discs  102  that are cast onto either side of the conductive layer  86 . 
         [0059]    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 wall  100  of the forming element  92 . The recess defined by the base  104  and the inner wall  100  has a depth that matches with the depth of the head  44   a  of the connector element  44 . Therefore, the underside of the head  44   a  abuts the reinforced zone  90  and the upper face of the head  44   a  lies substantially flush with the surrounding blade surface  14 . To optimise the electrical contact between the head  44  and the reinforced zones  90 , the faying surfaces thereof may be machined to an appropriately fine surface finish. This helps to avoid arcing during current transfer between the conductive layer  86  and the connector element  44 . 
         [0060]    The shank  44   b  of the connector element  44  extends through a bore  103  provided through the reinforced zone  90  and the structural components  96  so as to engage with the connector base  48  of the connector element  52  that is mounted adjacent the inside surface of the blade  2 . 
         [0061]    The surface protection layer  20  could be built up in a step-by-step process during fabrication of the blade  2  or, alternatively, the surface protection layer  20  could be provided as a preformed component that can be laid-up as a unit onto a blade mould. 
         [0062]    One embodiment of a method for assembling a blade incorporating the surface protection layer  20  will now be described with reference to  FIGS. 7 a    to  7   e.  As illustrated schematically in  FIG. 7   a,  a blade mould  109  having a surface  110  is provided as an initial step. Although not shown here, a blade mould is typically provided as two mould halves in which two half shells are formed. Once the half shells have been fabricated, the two mould halves are brought together and the half shells are joined thus completing the shell of the blade  2 . 
         [0063]    Turning to  FIG. 7   b,  the outer insulating layer  82  and the forming element  92  are provided on the mould surface. As in  FIG. 6 , the outer insulating layer  82  has an aperture  101  that lines up with the inner wall  100  of the forming element  92 . Here, the forming element  92  includes a plug member  112  that sits tightly inside the aperture  98  of the forming element  92 . The plug member  112  is preferably a polymeric part and serves two main purposes. Firstly, it defines the position of the recessed base  104 , as shown in  FIG. 6 , and so for this reason it has substantially the same shape and dimensions as the head  44   a  of the connector element  44 . Secondly, by plugging the aperture  98  it ensures that the conductive surface of the reinforced zone  90  of the conductive layer  86  remains free of resin during the resin infusion process. 
         [0064]    In  FIG. 7   c,  the conductive layer  86  is shown laid-up on the forming element  92 . It will be appreciated that the majority of the conductive layer  86  is substantially planar, although the (or each) forming element  92  shapes a localised region of the conductive layer  86 , and particularly the reinforced zone  90  over its outer profile. Here it can be seen that one of the conductive discs  102  of the reinforced zone sits onto the plug member  112  thereby defining the recessed base  104  of the inner wall  100 . Note that the conductive layer  86  has been provided as a preformed component with the conductive discs  102  pre-cast onto appropriate regions. 
         [0065]    To finish the surface protection layer  20 , as shown in  FIG. 7   d,  the inner insulating layer  84  is laid down on top of the conductive layer  86 . The inner insulating layer may be provided with a tackified surface so that it adheres to the conductive layer  86  prior the resin infusion process. Following this the half shell is laid-up by arranging the structural components  96  over the surface protection layer  20 , as is illustrated in  FIG. 7   e.  It should be noted that the structural components are shown here schematically and may include one or more glass fibre fabric layers, foam cores or spar caps. Once completed, the whole lay up is covered with an airtight bag that forms a sealed region over the components. The sealed region may then be evacuated using a vacuum pump, following which liquid resin is introduced into the sealed region so that it may flow through a series of channels to allow the resin to infuse completely through the lay up. 
         [0066]    Once the resin has infused through the lay up, the vacuum pump continues to operate during the subsequent curing process in which the mould may undergo controlled heating to cure the blade half shell effectively. 
         [0067]    During the resin infusion process, the component layers of the surface protection layer  20  are fixed in position and, in particular, the reinforced region  90  is set as the recessed base  104  of the aperture  98 . After curing, and removal of the part from the mould, the plug member  112  is removed, thereby exposing the underlying reinforced zone  90  of the conductive layer  86 . In order to locate the plug member  112  within the half shell, its position may be recorded so that it is readily locatable once the moulding is complete. Alternatively, the plug member  112  may incorporate an appropriate locator mechanism  120  which in this embodiment is a magnet  120  encased within the plug member  112 . Since the plug member  112  ensures that the reinforced region  90  of the conductive layer  86  remains substantially free from resin during the infusion process, once removed the connector element  44  may be inserted through a suitable bore drilled through the half shell so as to connect up to as associated connector arrangement  40 , as is shown in  FIG. 6 . 
         [0068]    Manufacturing benefits may be achieved by forming the first and second insulating layers  82 ,  84  and the conductive layer  86  as a preformed component, as will now be explained with reference to  FIGS. 8 a    to  8   d.  The same reference numerals will be used for consistency. 
         [0069]    In previous embodiments, as shown in  FIG. 5 , the conductive layer  86  has been illustrated as a single sheet. However, in  FIG. 8 a    the conductive layer  86  is shown as a pattern of rectilinear conductive layer portions, indicated generally as  202 , arranged to be overlapped with one another, as will be explained. 
         [0070]    The pattern of conductive layer portions  202  includes two main portions  202   a  and two side portions  202   b.  Each of the portions  202   a,    202   b  includes first and second folded end regions  204 . The folded end regions  204  serve to avoid any loose ends protruding from the expanded metal foil and increases current carrying capacity. Furthermore, the corners of the conductive layer portions may be formed with a radius to avoid sharp corners which may otherwise be vulnerable to lightning attachment. 
         [0071]    The two main portions  202   a  are provided with a reinforced region  90  at each of their folded ends, resulting in four reinforced regions  90  in total. The reinforced regions  90  may be formed by casting metallic discs onto the main portions  202  in the manner described above with reference to  FIGS. 7 a   - 7   e.    
         [0072]    With reference to  FIG. 8   b,  the pattern of conductive layer portions  202  are to be overlaid on a first insulating layer  82  which, as has been described above, is a glass fibre fabric. Since this layer  82  will form the outermost skin of the blade, preferably this fabric layer is relatively lightweight to avoid suppression of lightning leaders emanating from the conductive layer and to minimize surface damage during a strike  86 . The first insulating layer  82  is provided with four forming elements  92  located in positions so that they overlay the apertures  98  in the insulating layer  82 . As an alternative to this, the forming elements  92  may be applied directly to the reinforced regions  90  on the conductive layer  86  so that they are superimposed thereon. 
         [0073]    The pattern of conductive layer portions  202  are then overlaid on the first insulating layer  82  in an overlapping manner. Firstly, the two main portions  202   a  are overlapped slightly with one another along their long edges, in the order of a few centimetres, to define a central margin of overlap  208 . Then, the two side portions  202   b  are overlaid either partially or completely on the outer long edges of the main portions  202   a,  which provides two outer margins of overlap  210 . 
         [0074]    The assembled conductive layer  86  may then be stitched in place on the first insulating layer  82  to ensure that it is held securely in position. Alternatively, the first insulating layer  82  may be provided with a tackified surface so that the conductive layer portions  202  self-adhere to the first insulating layer  82 . The surface of the first insulating layer  82  may be tackified by the application of a suitable pressure sensitive adhesive. Tackified glass fibre fabrics are available commercially. 
         [0075]    By observing the completed conductive layer  86  as shown in  FIG. 8   c,  it can be seen that it is provided with a high capacity current channels  212  by virtue of the folded regions  204  and the margins of overlap  208 ,  210 . The channel  212  extends about the perimeter of the conductive layer  86 , and also longitudinally through the centre of the layer  86 . The channel  212  is beneficial in directing the energy from a lightning strike from the relatively thin regions of the conductive layer  86  along the channel  212  and to the reinforced regions  90  at which points the conductive layer  86  is coupled to the down conductive system  26  via the connector elements as discussed above. 
         [0076]    Finally, and with reference to  FIG. 8   d,  a second insulating layer  84  is applied on top of the conductive layer  86  so as to sandwich the conductive layer  86  between it and the first insulating layer  82 . The second insulating layer  84  may be stitched to the first insulating layer  82  and/or the conductive layer  86  in order to secure all of the layers together. Alternatively, the faying surface of the second insulating layer  84  may also be tackified with a suitable adhesive. 
         [0077]    It should be noted that in  FIGS. 8 a  to 8 d    the conductive layer  86  and the insulating layers  82 ,  84  are shown in simplified form to illustrate the concept for the purposes of this description and, as such, they are not necessarily representative of actual size or scale with respect to a wind turbine blade. 
         [0078]    The completed surface protection layer  20 , as shown in  FIG. 8   d,  therefore incorporates all components it needs to be laid up in a blade mould. This makes the assembly process simpler since the surface protection layer  20  can be applied to the mould as a single unit instead of needing to be laid-up in a step by step process in which each of the constituent layers needs to be aligned accurately. 
         [0079]    It will be appreciated from the above discussion that the surface protection layer  20  is equipped with measures to make it able to manage frequent high energy lightning strikes. For example, the reinforced regions  90  strengthen the conductive layer  86  in regions where it connects to the down conductor system  26 . Furthermore, the current channel  212  provides a means to manage the energy flow path through the conductive layer  86  which increases the robustness of the conductive layer. However, it will be appreciated that the preforming process as described above could also be applied to a more simple surface protection layer  20  including a single sheet of metallic conductor, expanded metal foil, for example, sandwiched between first and second glass fibre fabric layers. 
         [0080]    Some variants to the specifics embodiments shown in the drawings have been described above. Others will be apparent to the skilled person and some will now be explained by way of example. 
         [0081]    The specific embodiment of the surface protection layer  20  is shown as having four points of contact between it and the down conducting system  26  of the blade  2 . This is driven in part by the fact that the down conducting system  26  includes two down conductors  28 ,  30  that run the length of the blade near to the leading and trailing edges. However, although having four points of contact is beneficial, in terms of energy sharing during a lightning strike for example, it is not essential and the surface protection layer  20  may be provided with less than four points of contacts. One or two points of contact may be sufficient in the case where the blade is provided with a single down conductor. 
         [0082]    The forming element  92  has been described above as having a plug element  112  which incorporates a magnet which assists technicians in locating the forming element after the blade has been removed from its mould. It will be appreciated that this is one way to provide the forming element with locating means, and that other ways are possible. For example, a simple metallic inset could be located by a suitable hall sensor, for example. Other means include the use of RF transponders. Furthermore, it will be appreciated that the locating means could be incorporated into the outer part of the forming element instead of the plug member. 
         [0083]    In the above embodiments, it has been described that the blade is divided into three zones, A, B, and C, for the purposes of lightning protection. It will be appreciated that this is merely an example of how a blade may be configured for lightning protection and is not intended to be limited. For example, a blade may be configured so that zone A is omitted. In effect, therefore, the blade is protected along its entire length instead of leaving a zone relatively unprotected from lightning strikes.