TECHNICAL FIELD

The present invention relates to a wind turbine blade , and more particularly to a wind turbine blade comprising a lightning protection system . The invention also relate s to a wind turbine compri sing one or more of said wind turbine blades , and to a method of manufacturing said wind turbine blades .

BACKGROUND

Wind turbines are highly susceptible to lightning stri kes , and in particular wind turbine blades , which extend to the highe st point of the turbine as during rotation of the turbine . If the electrical current of a lightning strike is not safely directed through the wind turbine and to ground, the electrical current may cause severe damage to component s of the wind turbine and cause failure of the turbine . For example , if the lightning current penetrates directly through the surface of a turbine blade , which i s typically made from composite non-conductive material unable to easily transfer electrical energy, the current may cause critical damage to the surface , causing catastrophic failure to the blade and requiring repair works . Further , wind turbine blades are typically hollow in order to reduce the weight of the blade . Should lightning arcs occur in the hollow interior of such a blade , this can cause damage to the interior structural supports and generate trapped gase s in the interior , li kely leading to failure of the blade . Repair works for wind turbines are costly, especially if the turbine is located in dif ficult to acces s areas such as off shore . Typical lightning protection systems comprise a plurality of conductive lightning receptors located along the length of the blade . The lightning receptors are electrically connected to a down conductor which safely direct s the lightning current towards the base of the blade and to ground, thereby mitigating the ris k of electrical current pas sing through another component and causing damage to the wind turbine . In such lightning systems , the lightning receptors are placed along the length of the blade on both the pre s sure side and suction side of the blade , which allows lightning to be safely directed to the down conductor regardles s of whether the lightning strikes the blade on the pres sure side or the suction side of the blade . Typically, the lightning receptors are connected to a conducting block extending between the pre s sure and suction side of the blade , which is in turn electrically connected to the down conductor .

In the regions of the blade further from the tip of the blade , the di stance between the pres sure side and suction side of the blade increa ses , meaning that the lightning receptors and conducting block must be longer in order to extend acros s the suction and pres sure sides of the blade . Thi s increases both the cost and complexity of manufacture of the turbine blade . The documents US 2016 / 258 423 Al , EP 4 019 772 Al , EP4 112 921 Al and GB 2 519 333 A disclose dif ferent blades with lightning protection systems .

There is therefore a need for a wind turbine blade having a lightning transmis s ion system which provides the same level of protection from lightning strikes whilst having a reduced cost and complexity of manufacture .

SUMMARY

The obj ect is solved by a blade for a wind turbine according to claim 1 , a wind turbine according to claim 12 and a method according to claim 13 . According to a first aspect of the invention there is provided a blade for a wind turbine comprising a lightning protection system and a non-conductive exterior surface , the lightning protection system compri sing : an internal down conductor for conducting lightning current towards a base portion of the blade ; one or more exposed lightning receptors , the one or more lightning receptors located on a suction side or a pres sure side of the blade at a spanwise pos ition of the blade , the one or more lightning receptors extending through the exterior surface of the blade and electrically connected to the internal down conductor ; wherein for one or more of the lightning receptors : for one or more points on the other of the suction side or pres sure side of the blade at the spanwise position corresponding to the lightning receptor , the total breakdown voltage along any path of lightning from any of the one or more points to the lightning receptor via the interior of the blade i s greater than the total breakdown voltage along an indirect path from the point to the lightning receptor around the exterior surface of the blade .

According to a second aspect of the invention there is provided a wind turbine comprising one or more blades according to the first a spect .

According to a third aspect of the invention there is provided a method of manufacturing a blade for a wind turbine , the method comprising : providing a blade having a non-conductive exterior surface ; providing a down conductor for conducting lightning current towards a base portion of the blade and locating the down conductor in an interior of the blade ; providing one or more lightning receptors and for each lightning receptor : extending the lightning receptor through the exterior surface of a suction side or a pres sure side of the blade at a spanwise position of the blade and exposing the lightning receptor externally to the blade ; and electrically connecting the lightning receptor to the internal down conductor ; wherein for one or more of the lightning receptors : for one or more points on the other of the suction side or pressure side of the blade at the spanwise position corresponding to the lightning receptor, the breakdown voltage for a path of lightning from any of the one or more points to the lightning receptor through the interior of the blade is greater than the breakdown voltage for an indirect path from the point to the lightning receptor around the exterior surface of the blade.

BRIEF DESCRIPTION OF THE DRAWINGS

To enable better understanding of the present disclosure, and to show how the same may be carried into effect, reference will now be made, by way of example only, to the accompanying schematic drawings, in which:

Fig. 1 shows a schematic view of a wind turbine for generating electricity according to one or more embodiments.

Fig. 2A shows a schematic view of a wind turbine blade known in the prior art.

Fig 2B shows a cross-sectional view of the wind turbine blade of Fig. 2A taken along line 2B-2B.

Fig. 3A shows a schematic view of a wind turbine blade according to one or more embodiments.

Fig. 3B shows a cross-sectional view of the wind turbine blade of Fig. 3A taken along line 3B-3B.

Fig. 3C shows possible paths of lightning current for the wind turbine shown in Fig. 3B.

Fig. 3D shows possible start and end points X, Y for leader paths . Fig. 3E shows a cross-sectional view of the wind turbine blade of Fig. 3A taken along line 3E-3E.

Fig. 4A shows a schematic view of a wind turbine blade according to one or more embodiments.

Fig. 4B shows a cross-sectional view of the wind turbine blade of Fig. 4A taken along line 4B-4B.

Fig. 4C shows possible paths of lightning current for the wind turbine shown in Fig. 4B.

DETAILED DESCRIPTION

As used herein, the term "lightning receptor" may be understood to mean any electrically conductive element configured to be exposed to the outside environment of a wind turbine blade to receive lightning current, and further configured to electrically connect to a down conductor of the wind turbine blade. The lightning receptors as disclosed herein may have an external surface which is flush with the exterior surface of the blade or may protrude from the exterior surface.

As used herein, the term "down conductor" may be understood to mean any electrically conductive cable which is configured to extend from a base portion of a wind turbine blade to a tip portion of the wind turbine blade, and to conduct lightning current from the tip portion to the base portion. The down conductor may be an electrically insulated metallic (e.g. copper) cable of a thickness which is sufficient to withstand the high level of lightning current without damaging the cable, for example through resistive heating. The down conductor may be electrically insulated with a polymer such as polyurethane. As used herein , the term " spanwise position" may be understood to mean a position of the blade along the longitudinal axis of a wind turbine blade , def ined by the line extending between the ( centre of ) the base of the blade and the tip of the blade .

As used herein , the term "airfoil" may be understood to mean the cros s -sectional shape of a wind turbine blade at a given spanwise position of the blade . The airfoil i s divided into a pre s sure side and a suction side .

As used herein , the term "pre s sure side" may be understood to mean the side of an airfoil which aerodynamically experiences a relatively higher pres sure than an opposite side of the airfoil .

As used here , the term " suction side" may be understood to mean the side of an airfoil which aerodynamically experiences a relatively lower pres sure than an opposite side of the airfoil .

Fig . 1 shows a schematic view of a wind turbine 10 for generating electricity according to one or more embodiments . The wind turbine 10 comprises a tower 11 . The tower 11 comprise s one end by which the tower 11 is connected to the ground 16 and another , oppos ite end, by which the tower 11 is connected to a nacelle 12 of the wind turbine 10 . The nacelle 12 is rotatably mounted relative to the tower 11 . This enables a yaw movement of the nacelle 12 with respect to the tower 11 . The nacelle 12 accommodates the generator of the wind turbine and further component s of the wind turbine 10 . If the wind turbine is a geared wind turbine , the nacelle 12 also may comprise a gear box . The wind turbine 10 furthermore comprise s a rotor which is rotatably arranged with respect to the nacelle 12 . The rotor can thus be rotated about a rotor axi s 14 which is located substantially horizontal with regards to the ground 16 . The rotor of the wind turbine 10 is configured to capture the energy of the wind by the aerodynamics of one or more blades 20, 200, and transform this energy into a rotational movement and transmit the rotational movement to the generator of the wind turbine 10. The rotor of the wind turbine comprises a hub 13 and at least one, typically two or three, rotor blades 20, 200. The wind turbine 10 comprises at least one blade 200 according to the invention but may additionally comprise one or more blades not according to the invention. The rotor blades 20, 200 are mounted to the hub 13. Each rotor blade 20, 200 comprises a tip section 22 and a base portion 21. Each rotor blade 20, 200 is mounted at its base portion 21 to the hub 13. One or more of the rotor blades 20, 200 may be rotatably mounted rotatable to the hub 13. This means that the rotor blade can be pitched about a so-called pitch axis 15, which may be considered the longitudinal axis of the blade 20, 200, in order to optimize energy capture and loads acting on the rotor blade 20, 200 of the wind turbine 10.

One or more blades 20, 200 of the wind turbine 10 also comprise a lightning protection system. The lightning protection system comprises an internal down conductor 31, extending from the base portion 21 of a given rotor blade 20, 200 to the tip section 22 of the given rotor blade 20, 200. The internal down conductor 31 is connected at the base portion 21 of the rotor blade 20, 200 to a grounding system 32 of the remainder of the wind turbine 10. The grounding system 32 transmits or conducts the electrical current from the internal down conductor 32 to the ground 16.

Fig. 2A shows a schematic view of a wind turbine blade 20 known in the prior art. The blade 20 comprises a base portion 21 and a tip section 22. The radially outmost point of the rotor blade 20 is the so-called tip 221 of the rotor blade 20. The rotor blade 20 furthermore comprises a trailing edge 23 and a leading edge 24. The leading edge 24 typically has a curved and round shape, while the trailing edge 23 may have a sharp or blunt edge. The straight line between the trailing edge 23 and the leading edge 24 defines a chord 27. The chord 27 divides the airfoil at a given spanwise position into a pressure side 25 and the suction side 26. One of the airfoils is exemplarily shown in Fig. 2A. It is to be understood that the rotor blade 20 comprises a plurality of airfoils - one next to the other - from the base portion 21 to the tip section 22. These gradually changing airfoils cause the gradual change of the shape of the rotor blade. The airfoil has a lift generating shape in most of the sections of the rotor blade.

The rotor blade 20 comprises a lightning protection system with one or more lightning receptors 30 and a down conductor 31 which may be also called a lightning conductor cable. When the blade 20 is mounted to the turbine 10 shown in Fig. 1, the down conductor 31 connects the lightning receptor (s) 30 with the ground 16 via the hub 13, the nacelle 12 and the tower 11. The rotor blade 20 usually comprises several lightning receptors 30 at different spanwise positions all electrically connected to the down conductor 31.

Fig. 2B shows a cross-sectional side view of the blade 20 according to the prior art taken along the line 2B-2B shown in Fig. 2A. At each spanwise position, a pair 300 of lightning receptors 30 are provided, a first lightning receptor 30 exposed on the pressure side 25 and a second lightning receptor 30 exposed on the suction side 26, so that lightning may be safely conducted to the down conductor 31 without passing through the non-conductive blade surface, regardless of whether lightning strikes the pressure side 25 or the suction side 26 of the blade at or near the spanwise position. Both lightning receptors 30 of each lightning receptor pair 300 are electrically connected to the down conductor, any may be connected via a conductive block 33 to which the lightning receptors are mounted. Especially at spanwise positions farther from the tip 221, in which the distance between the pressure side 25 and the suction side 26 is greater, this increases the complexity of manufacture, requires more materials, and also increases the weight of the blade as the metallic components of the lightning protection system must extend across the wider distance between the sides of the blade, which is undesirable as lighter blades are typically preferred for their aerodynamic qualities .

Fig. 3A shows a wind turbine blade 200 according to one or more embodiments. Like the blade 20 described with reference to Figs. 2A and 2B, the blade 200 comprises a base portion 21 and a tip section 22 and a tip 221 being the radially outmost point of the rotor blade 20. The rotor blade 20 furthermore comprises a trailing edge 23 and a leading edge 24. The leading edge 24 typically has a curved and round shape, while the trailing edge 23 may have a sharp or blunt edge. The straight line between the trailing edge 23 and the leading edge 24 defines a chord 27. The chord 27 divides the airfoil at a given spanwise position into a pressure side 25 and the suction side 26. One of the airfoils is exemplarily shown in Fig. 3A. It is to be understood that the rotor blade 20 comprises a plurality of airfoils - one next to the other - from the base portion 21 to the tip section 22. These gradually changing airfoils cause the gradual change of the shape of the rotor blade. The airfoil has a lift generating shape in most of the sections of the rotor blade. The blade 200 may comprise a shell having an exterior surface 201 (see Fig. 4B) and an interior surface 202 defining an interior cavity of the blade 200. The shell may be made of any suitable material, for example fibreglass, fibreglass- reinforced polyester or epoxy, carbon fibre, aramid or wood fibre, or laminates or composites thereof.

Also, as for blade 20 previously described, the rotor blade 200 comprises a lightning protection system with one or more exposed lightning receptors 400 and a down conductor 31 which may be also called a lightning conductor cable. When the blade 200 is mounted to the turbine 10 shown in Fig. 1, the down conductor 31 connects the lightning receptor (s) 400 with the ground 16 via the hub 13, the nacelle 12 and the tower 11. The rotor blade 200 may comprise several lightning receptors 400 at different spanwise positions all electrically connected to the down conductor 31 . The rotor blade may optionally also comprise one or more lightning receptor pairs 300 .

The blade 200 may compri se a tip region 2 extending from the distal tip 221 of the blade 200 to an intermediate spanwise pos ition 1 , and a proximal region 3 extending from the base 21 of the blade 200 to the intermediate spanwise position 1 .

For one or more of the exposed lightning receptors 400 , the lightning receptor 400 i s located on one of the suction side 26 and pres sure side 25 of the blade 200 . At the spanwise pos ition of the said lightning receptor 400 , the other side of the suction side 26 and the pre s sure side 25 may be free from lightning receptors . That is , at one or more spanwise pos itions of the blade 200 where a lightning receptor 400 i s located, the other side may not comprise a lightning receptor 400 , ( although in some embodiments the other side may comprise lightning receptors 400 at other spanwise pos itions ) .

The one or more lightning receptors 400 may all be provided in the proximal region 3 of the blade 200 . A plurality of said lightning receptors 400 may be provided in the proximal region 3 at different spanwise pos itions . When a plurality of lighting receptors 400 are provided at different spanwise positions .

In addition to the one or more lightning receptors 400 , there may additionally be provided one or more exposed lightning receptor pairs 300 . Each of the one or more receptor pairs 300 may be placed at a spanwi se position in the tip region 2 and may be placed at dif ferent spanwise positions . Each of the one or more lightning receptor pairs 300 may compri se a first lightning receptor 30 located on the suction side 26 of the blade 200 and a second lightning receptor 30 located on the pres sure s ide 25 of the blade 200 , the first and second lightning receptors 30 extending through the exterior surface 201 of the blade and electrically connected to the internal down conductor 31 , such as that configuration illustrated in Fig . 2B . As illustrated in Fig . 2B , one or both of the lightning receptors 30 may be electrically connected to an electrically conductive block 33 mounted to an interior surface of the blade 200 at the spanwise position of the lightning receptors 30 , wherein the electrically conductive block 33 is electrically connected to the down conductor 31 . In some embodiments , an electrical insulator covers one or both of the lightning receptors 30 in the interior of the blade 200 such that any path of lightning through the interior of the blade pa s ses through the electrical insulator . The insulator may be any suitable material such as a resin or polymer and may be precasted or cured .

Fig . 3B shows a cros s-sectional view of the blade 200 shown in Fig . 3A taken along line 3B-3B . The blade 200 comprises a non-conductive exterior surface 201 . The blade 200 may also comprise an interior surface 202 defining a hollow interior space of the blade 200 . The lightning receptor 400 extends through the exterior surface 201 of the blade 200 and i s electrically connected to the internal down conductor 31 . In some embodiments , and as illustrated in Fig . 3B , the lightning receptor 400 i s electrically connected ( for example mounted) to an electrically conductive block 402 mounted to the interior surface 202 at the spanwise position of the lightning receptor 400 , and the electrically conductive block 402 is in turn electrically connected to the down conductor 31 . In other embodiments , the lightning receptor 400 may be directly mounted and electrically connected to the down conductor 31 , for example by welding or similar . In some embodiments , an electrical insulator 404 may cover the lightning receptor 400 and or the electrically conductive block 402 in the interior of the blade such that any path of lightning through the interior of the blade pas se s through the electrical insulator 404 . The provi sion of such an insulator 404 increases the breakdown voltage of any lightning path which passes through the interior of the blade 200. The blade 200 may also comprise one or more structural supports 406 extending between the pressure side 25 and suction side 26, such as a web, at one or more spanwise positions of the blade 200, to provide structural support to the blade 200. The insulator 404 may be made of any electrically insulating material such as an insulating resin, polymer, or concrete. The insulator 404 may be precasted or cured. The structural supports 406 may be made of any suitable material providing the required structural strength to the blade.

It is noted that the insulator 404 should completely cover the electrical connections in order to prevent the existence of an air gap which would allow the electrical current to bypass the insulator 404 and pass directly to the lightning receptor 400 or conductive block 402.

Fig. 3C shows possible lightning paths for the blade 200 shown in Fig. 3B. In Fig. 3C, a lightning bolt strikes the blade at a location X opposite the lightning receptor 400. The path of the lightning current from point X towards ground depends on the formation of ionized paths or leaders which propagate between the lightning receptor 400 and point X. Fig. 3C shows possible lightning paths LI, L2 and L3 which the lightning current may take. Which path the lightning takes depends on the level of electrical insulation along each path (i.e. the inception voltage for a leader along each path) . The lightning will take the path with the lowest level of electrical insulation. Accordingly, if the insulation level for path L3 is higher than the insulation level for at least one of path LI or L2, then the lightning current will have a strong preference to travel to the lightning receptor 400 indirectly around the exterior of the blade 200 without passing through the surface of the blade 200 or the interior of the blade 200. In such circumstances, the lightning receptor 400 provides protection from lightning strikes on both sides of the blade, even though at the spanwise position a lightning receptor is provided on only one of the pres sure side 25 and the suction side 26 .

For a given path , the total insulation level can be calculated by summing the inception voltages for an ionized leader along each path . For example , for paths LI and L2 , which begin with the external part of the lightning receptor 400 in contact with the exterior surface 201 , the inception voltage is relatively low at about 10 kV/mm ( due to the three dif ferent permittivities of the receptor material , the exterior surface material , and the air ) . Once incepted, the leader propagates along the surface and requires about 0 . 5 kV/mm to propagate along the surface . The total required voltage can therefore be readily calculated . Path L3 pa s ses through the insulator 404 , the air internal to the blade and the shell material having the exterior surface 201 and the interior surface 202 . The inception voltage f rom the conductive block 402 would typically be about 30 kV/mm, with about 2 to 5 kV/mm required for the air gap through the interior of the blade 200 , and finally a further voltage to cros s the shell of the blade proximal to point X . In addition to the path L3 , the insulation level for a path along the interior surface 202 may also be calculated to determine if there is any path internal to the blade 200 having a lower insulation level than path LI or L2 . As path L3 is the most direct path through the air gap interior to the blade 200 , if the insulation level for the path L3 is higher than either path LI or L2 , it follows that the insulation level along any other path through the air gap interior to the blade is also higher than either path LI or L2 .

It is al so noted that for a given blade design including blade shape , dimensions and materials include the dielectric properties of the materials , the propagation of leaders can be simulated using simulators known in the art to determine whether the insulation level for all paths through the blade is higher than an indirect path along the exterior of the blade. The paths of lightning for a given design could also be measured empirically as known in the art.

It is noted that in the tip region 2, the distance between the suction side and the pressure side of the blade may be too narrow for the insulation level of path L3 to be greater than one of paths LI and L2. Accordingly, for some blade designs in the tip region 2 there are provided receptor pairs having a lightning receptor 30 on either side of the blade 200, so that the blade 200 is protected from lightning on both sides of the blade 200 in the tip region 2.

Preferably, in some embodiments and as illustrated in Fig. 3D, for all points on the other side of the blade 200 to the lightning receptor 400, such as points XI to X9, there exists a path (such as LI and L2) around the exterior of the blade 200 to the lightning receptor 400 having a lower insulation level than that of any path through the interior blade such as L3. In such embodiments, the lightning receptor 400 is configured to receive substantially all lightning strikes which strike the blade 200 at or near the spanwise position, regardless of whether the lightning strikes on the suction side 26 or pressure side 25 and therefore negates the need for a lightning receptor pair as in the prior art. It is noted that the internal paths such as L3 may be calculated for each extreme point Zl, Z2 on the internal side of the conducting block 402, if a higher accuracy is desired. Similarly, the extreme points Z3, Z4 may be used for the end points of the external paths.

Fig. 3E shows a cross-sectional view of the wind turbine blade 200 of Fig. 3A taken along line 3E-3E. As illustrated, the lightning receptors 400 may all be provided on the same side, namely all on the pressure side 25. In other embodiments, the lightning receptors 400 may all be provided on the suction side 26. Providing the lightning receptors 400 on a single side of the blade 200 simplifies the manufacturing process of the blade 200. In such embodiments, the down conductor 31 may extend along the blade 200 proximal to the s ide of the blade having the receptors 400 along its entire length , also simplifying manufacture of the blade 200 .

Therefore , according to the invention , there is provided a blade 200 for a wind turbine comprising a lightning protection system and a non-conductive exterior surface 201 , the lightning protection system comprising an internal down conductor 31 for conducting lightning current towards a base portion 21 of the blade 200 . The blade 200 al so comprises one or more exposed lightning receptors 400 which are located on a suction side 26 or a pres sure side 25 of the blade 200 at a spanwise position of the blade 200 , the one or more lightning receptors 400 extending through the exterior surface 201 of the blade 200 and electrically connected to the internal down conductor 31 . For one or more of the lightning receptors 400 , the following i s also true : for one or more points on the other of the suction side or pres sure s ide of the blade at the spanwise position corresponding to the lightning receptor , the total inception voltage for a leader along any path of lightning f rom any of the one or more points to the lightning receptor via the interior of the blade i s greater than the total inception voltage for a leader along an indirect path from the point to the lightning receptor around the exterior surface of the blade .

Fig . 4A shows a schematic view of a wind turbine blade 200 ' according to one or more embodiments . The blade 200 'may comprise any of the elements disclosed with reference to Figs . 3A to 3E . In addition , the blade 200 ' comprises two or more lightning receptors 400A, 400B which are located at the same spanwise position of the blade 200 ' and both located on the same side of the blade 200 ' ( i . e . , either the suction side 26 or the pre s sure side 25 as shown in Fig . 4B ) . Two or more lightning receptors may be place at the same spanwise position for one or a plurality of spanwise positions . That is , a first plurality of lightning receptors may be provided at a first same spanwise position , and a second plurality of lightning receptors may be provided at a second same spanwise position. The two or more lightning receptors 400A, 400B are electrically connected to the down conductor 31 as for the lightning receptors 400 described with reference to blade 200. Further, as for the lightning receptors 400 previously described, the two or more lightning receptors may be directly connected to the down conductor 31 or may be connected via one or more conductive blocks 402 such as the blocks 402 previously described.

It will be appreciated that the down conductor 31 may comprise a series of interconnected cables which electrically connect to all of the lightning receptors to transmit the lightning current from any of the lightning receptors to the base 21 of the blade 200 or 200' .

Fig. 4B shows a cross-sectional view of the wind turbine blade 200 ' of Fig. 4A taken along line 4B-4B. As shown in Fig. 4B, one of the two or more said lightning receptors (receptor 400A) may be provided more proximal to the leading edge 24 of the blade 200 ' at the spanwise position, and another other of the two or more said lightning receptors (receptor 400B) may be provided more proximal to the trailing edge 23 of the blade 200 ' .

The blade 200 ' may also be provided with a structural support 406. such as a web, extending between the pressure and suction sides of the blade 200', wherein one of the two or more said lightning receptors is provided on a first side of the structural support 406, and another of the two or more said lightning receptors is provided on a second side of the structural support 406 opposite to the first side.

In some embodiments, all of the lightning receptors 400A, 400B, 400 provided in the proximal region 3 are provided on a single side of the blade 200 ' . In such embodiments the down conductor 31 may extend along the entire length of the proximal region 3 proximal to a single side of the blade 200 ', such as illustrated in Fig. 3E.

Fig. 4C shows possible paths of lightning current for the wind turbine 200' shown in Fig. 4B. Whilst there is provided a new possible path L4 through the interior of the blade 200 ', this does not typically provide a substantially lower insulation level for the interior lightning paths. On the other hand, path L2 is substantially shortened meaning that the difference between insulation levels for the indirect exterior paths LI and L2 and those of the interior leader paths L3 and L4 may be increased, further strengthening the protection against lightning strikes on the opposite side of the blade 200 ' . As the lightning receptors 400A and 400B are provided on a single side of the blade 200', the manufacturing process remains simplified.

According to the invention, there is also provided a method of manufacturing a blade for a wind turbine. The method comprises providing a blade (for example blade 200 or

200 ') having a non-conductive exterior surface. The method further comprises providing a down conductor for conducting lightning current towards a base portion of the blade and locating the down conductor in an interior of the blade. The method also comprises providing one or more lightning receptors. For each lightning receptor, the method comprises: extending the lightning receptor through the exterior surface of a suction side or a pressure side of the blade at a spanwise position of the blade and exposing the lightning receptor externally to the blade; and electrically connecting the lightning receptor to the internal down conductor. For one or more lightning receptors, for one or more points on the other of the suction side or pressure side of the blade at the spanwise position corresponding to the said lightning receptor, the breakdown voltage for a path of lightning from any of the one or more points to the lightning receptor through the interior of the blade is greater than the breakdown voltage for an indirect path from the point to the lightning receptor around the exterior surface of the blade.

It is noted that the blade may be provided by producing a complete blade shell comprising pressure and suctions sides before the down conductors and lightning receptors are provided. For example, the blade may be provided using a process known in the art as a "one-shot" process of manufacture .

In some embodiments, the blade may be provided by separately forming a suction side of the blade and a pressure side of the blade, and subsequently joining the suction side and pressure side of to form the blade as known in the art. For example, the blade may be provided using a process known in the art as a "butterfly" process of manufacture. In such embodiments, the one or more lightning receptors may be extended through the exterior surface of either the suction side or the pressure side (i.e. , all on the same side) of the blade, and electrically connected to the down conductor on the same side, before the suction side and pressure side are joined. This provides a simple process of manufacture of the blades .

It will be appreciated that whilst in some embodiments one or more blades according to the invention are provided on the wind turbine described with reference to Fig. 1, in other embodiments one or more blades according to the invention may be provided on any suitable wind turbine which is configured to convert wind energy into another type of energy.

It will further be appreciated that the shapes of the blades shown in the figures are exemplary only, and the shape of the blade including the length, width, thickness, and airfoil at each spanwise position may be varied according to the desired aerodynamic properties of the blade without departing from the scope of the invention. Similarly, it will be appreciated that the various components of the blade and wind turbine may be made of any suitable material without departing from the scope of the invention.

All of the above are fully within the scope of the present disclosure, and are considered to form the basis for alternative embodiments in which one or more combinations of the above-described features are applied, without limitation to the specific combination disclosed above.

In light of this, there will be many alternatives which implement the teaching of the present disclosure. It is expected that one skilled in the art will be able to modify and adapt the above disclosure to suit its own circumstances and requirements within the scope of the present disclosure, while retaining some or all technical effects of the same, either disclosed or derivable from the above, in light of his common general knowledge in this art. All such equivalents, modifications or adaptations fall within the scope of the present disclosure.