Lightning protection for wind-turbine blades

Abstract

A wind turbine rotor blade comprising: a plurality of carbon-containing structural spars, whereby each structural spar extends spanwise along the blade between a root-wards position and a tip-wards position, each structural spar having an outward facing surface; an electrically conductive lightning-protection structure, the electrically conductive lightning-protection structure being electrically connected to, or electrically connectable to, a ground connection at a root end of the wind turbine rotor blade; whereby the lightning-protection structure comprises a branch for each structural spar, each branch extending along and covering the outward facing surface of the corresponding structural spar; each branch electrically connected to the respective structural spar at least at a tip-wards location and a root-wards location.

Description

[0001] LIGHTNING PROTECTION FOR WIND-TURBINE BLADES

[0002] Field of the invention

[0003] The present invention relates to the field of lightning protection for wind turbine blades having carbon-containing structural spars.

[0004] Background to the invention

[0005] It is known to use carbon-containing materials, particularly carbon-fibre structures, to form structural spars extending spanwise along wind turbine blades. Carbon- containing structural spars can provide mechanical strength while enabling the blade to have a relatively low overall mass for a given length. However, carbon-containing structural spars can be conductive to some extent which increases the risk of structural damage to the blade in the event of a lightning strike, because they may act as the primary path for current flow to ground and so be damaged irreversibly.

[0006] It is known to provide a conductive (typically conductive metal) mesh down conductor arrangement in which a conductor, such as conductive metal mesh, extends along the length of a wind turbine blade (i.e. spanwise), from near the tip of the blade, to the root end of the blade where it is connected to ground. Metal mesh has the advantage of being highly conductive and yet relatively light. Typically, an array of lightning receptors is located near the tip end of the blade, and the lighting receptors are connected together and ultimately connected to the root end of the wind turbine blade by means of either an external conductive mesh or internal down conductor cable.

[0007] It is known to provide an equipotential bonding connection between the carbon fibre and the external conductive (e.g. conductive metal) mesh to avoid uncontrolled voltage flashover between the lighting protection system and the carbon fibre. Typically, equipotential bonding is located near the tip of the blade and also at the root end of the blade, and where required additionally at intermediate locations. This prevents uncontrolled electrical flashovers between the carbon fibre structure and the lightning protection system during the current flow associated with a lightning strike to the blade. However, the negative result of doing this is that the connection between the carbon fibre and lightning protection system results in a portion of the lighting threat current being transferred into the very carbon fibre that is being protected which can result in local damage to the carbon fibre structure at the connection locations. In order to ensure that no local damage occurs, both the design of the connections and the manufacturing process of the said connections are critical, and often results in very complex and difficult to control manufacturing procedures. Although the portion of lightning threat current flowing in the said connections is an order of magnitude lower than the threat current flowing on the surface mesh, the peak current and specific energy is still of a magnitude that can cause damage if the design is not robust or the manufacturing process not carefully controlled.

[0008] Accordingly, some embodiments of the invention aim to provide a practically realisable lightning protection surface mesh topology that by design optimises and reduces the peak current and specific energy at the equipotential bond connections, to thereby reduce the risk of damage, complexity, and / or cost of the equipotential bonds. A second aspect relates to the design of the equipotential bonds.

[0009] Summary of the invention

[0010] In accordance with a first aspect of the present invention, there is provided a wind turbine rotor blade comprising: a plurality of carbon-containing structural spars, whereby each structural spar extends spanwise along the blade between a root-wards position and a tip-wards position, each structural spar having an outward facing surface; an electrically conductive lightning-protection structure, the electrically conductive lightning-protection structure being electrically connected to, or electrically connectable to, a ground connection at a root end of the wind turbine rotor blade; whereby the lightning-protection structure comprises a branch for each structural spar, each branch extending along and covering the outward facing surface of the corresponding structural spar; each branch electrically connected to the respective structural spar at least at a tip-wards location (and typically also a root-wards location).

[0011] Advantageously, this branched arrangement restricts the peak current and specific energy flowing at the conductive connections between the carbon-containing structural spars and the electrically conductive lightning-protection structure. It forms a plurality of paths (one for each said structural spar and one for each said branch) for threat current and enables relatively high inductive and capacitive coupling between the branches of the lightning-protection structure and the respective structural spars, thus retaining current flow through the electrically conductive lightning-protection structure rather than the structural spars. It is relatively cost effective and light because it does not need to cover the whole blade surface.

[0012] It may be that each branch is electrically connected to the respective structural spar at the tip-wards end and at the root-wards end of the spar.

[0013] Typically, the carbon-containing structural spars comprise elemental carbon. Typically, the carbon-containing structural spars are carbon-fibre containing structural spars.

[0014] It may be that the outward facing surface of each structural spar defines the outer surface of the blade, at least in part. Nevertheless, outward of each structural spar is the electrically conductive lightning-protection structure and typically also a smooth surface layer.

[0015] Typically, the branches of the electrically conductive lightning-protection structure extend chord-wise beyond either side of the respective structural spar. Typically, the electrically conductive lightning-protection structure extends spanwise beyond the root-wards end and beyond the tip-wards end of each respective structural spar.

[0016] Thus, the branches of the electrically conductive lightning-protection system cover the respective structural spar both spanwise and chordwise.

[0017] Typically, the branches of the electrically conductive lightning-protection structure are electrically connected to each other in a root region of the blade.

[0018] Typically, the branches of the electrically conductive lightning-protection structure are electrically connected to each other at a tip region of the blade.

[0019] It may be that the branches of the electrically conductive lightning-protection structure are separate from each other along at least 50% of the length of the blade. It may be that the branches of the electrically conductive lightning-protection structure are separate from each other between the root region and the tip region. This gives improved capacitive and inductive coupling between the branches of the lightningprotection structure and the carbon-containing structure than if the electrically conductive lightning-protection structure covered the whole surface of the blade. This reduces the current passing through the carbon-containing structure as described further below.

[0020] Thus, typically the branches of the electrically conductive lightning-protection system define apertures in the electrically conductive lightning-protection system therebetween. Typically, the apertures in the electrically conductive lightningprotection system are between carbon-containing structural spars. Thus, there are regions (typically more than 50% or more then 75% or more than 90% of the length of the blade) where the electrically conductive lightning-protection system does not extend around the whole periphery of the blade. Instead, it has branches which cover the structural spars.

[0021] It may be that the electrically conductive lightning-protection structure comprises or is connected to an electrically conductive blade tip protector. The tip protector is located at a tip end of the blade. It may be that the tip protector is provided tip-wards of the tip-wards ends of the structural spars.

[0022] It may be that the inductance of a branch of the lightning-protection structure is less than 20% of (or less than 10% of, or less than 5% of, or less than 1% of) the inductance of the structural spar which it covers.

[0023] Thus, the electrical connections between a branch of the lightning-protection structure and a respective structural spar have a high impedance during the rise time of a threat current.

[0024] The blade may comprise one or more further conductive electrical connections between a branch of the lightning-protection structure and a respective structural spar, intermediate the said tip-wards location and root-wards location. Where required, for example for sufficiently long blades, this can prevent flashover between the respective structural spar and the overlying (covering) branch of the lightning-protection structure.

[0025] It may be that the blade comprises a first structural spar which supports a suction side of the blade, a second structural spar which supports a pressure side of the blade, and a third structural spar which supports the leading edge of the blade. Thus there may be a first branch of the electrically conductive lightning-protection structure covering the first structural spar, on the suction side of the blade, a second branch of the electrically conductive lightning-protection structure covering the second structural spar, on the pressure side of the blade, and a third branch of the electrically conductive lightning-protection structure covering the third structural spar, at the leading edge of the blade.

[0026] It may be that the electrically conductive lightning-protection structure is connected to an electrically conductive blade tip protector by two discrete connections, one of which is on the suction side of the blade, other of which is on the pressure side.

[0027] It may be that the electrically conductive lightning-protection structure is connected to an electrically conductive blade tip protector only by the said two discrete connections.

[0028] It may be that the third branch of the electrically conductive lightning-protection structure is electrically connected to the electrically conductive blade tip protector via electrical connections to the first and second branches of the electrically conductive lightning-protection structure at the tip-ward location (and typically also the root-wards location).

[0029] It may be that the electrically conductive lightning-protection structure comprises a root-wards common section which wraps around the circumference of the rotor blade and to which each branch is connected.

[0030] The branches may extend from the common section. The periphery of the common section may have a convex shape between each branch. This avoids sharp angles and corners which would negatively affect the electrical properties of the lightningprotection structure.

[0031] Typically, the blade comprises a suction side and a pressure side.

[0032] It may be that the electrically conductive lightning-protection structure, typically the root-wards common section, is connected to ground through two discrete connections, one of which is on the suction side of the blade, other of which is on the pressure side.

[0033] It may be that the electrically conductive lightning-protection structure, typically the root-wards common section, is connected to ground only through the two discrete connections.

[0034] The electrically conductive lightning-protection structure, typically the root-wards common section, may be connected to ground through a conductive connection to a tensioner.

[0035] The electrically conductive lightning-protection structure, typically the root-wards common section, may be connected to ground through a root plate of the blade.

[0036] Typically, there is a carbon-containing spar and branch of the electrically conductive lightning-protection structure at the leading edge of the blade and the branch of the electrically conductive lightning-protection structure at the leading edge of the blade is connected to branches of the electrically conductive lightning-protection structure on the suction and pressure sides of the blade at the root region of the blade and typically also at a tip region of the blade. It may be that the branches of the electrically conductive lightning-protection structure comprise conductive mesh, e.g. conductive metal mesh.

[0037] The electrically conductive lightning-protection structure may be formed of conductive mesh, e.g. conductive metal mesh.

[0038] It may be that each branch of the lightning-protection structure is formed as a layer over the respective structural spar.

[0039] It may be that each carbon-containing structural spar is formed as a laminate,

[0040] The wind turbine rotor blade may comprise a spacer layer between each respective structural spar and the branch of the lightning-protection structure which covers the structural spar.

[0041] The provision of a spacer layer enables the spacing between the structural spar and branch of the lightning-protection structure to be well-defined. The provision of a spacer layer enables a small distance to be maintained between the structural spar and the branch of the lightning-protection structure. Typically, the spacer has a predetermined thickness. Typically, the spacer has a predetermined dielectric strength. The spacer may comprise one layer of material within a laminate, for example one ply of glass fibre reinforced polymer (GFRP).

[0042] It may be that a structural spar, spacer layer, and branch of the lightning-protection structure are formed as a laminate.

[0043] This provides a reliable, cost-effective structure for manufacture. The spacer layer and lightning protection structure (e.g. conductive mesh) can be formed as additional layers in the lamination process which forms the structural spar.

[0044] It may be that an electrical connection is formed by a plurality of layers of conductive material (typically comprising one or more layers of conductive metal, for example copper) located between the outer surface of the structural spar and the branch of the lightning-protection structure covering the structural spar. The layers are typically in direct conductive contract. Thus, the structural spar, one or more layers forming a conductive connection, and a branch of the lightning-protection structure may be formed integrally by lamination. This enables formation of a reliable, clearly defined structure by a cost-effective manufacturing process.

[0045] The (equipotential) electrical connection is useful with blades with other types of lightning-protection structure and so, in a second aspect of the invention, there is provided a wind turbine rotor blade comprising one or more carbon-containing structural spars, (which may optionally extend spanwise along the blade), an electrically conductive lightning-protection structure covering (or underlying) the one or more one or more carbon-containing structural spars, and one or more electrical connections between a carbon-containing structural spar and a respective electrically conductive lightning-protection structure, the one or more electrical connections comprising a plurality of conductive layers. The plurality of conductive layers thereby form a layered structure, for example a laminate.

[0046] Typically, a conductive layer is in direct contact with the covering (or underlying) electrically conductive lightning-protection structure.

[0047] The conductive layers may comprise a plurality, or at least three, or at least four, conductive layers. The conductive layers may comprise one or more, or two or more conductive sheets (e.g. discs), for example conductive metal sheets (or discs), which may be made of copper. The one or more conductive sheets (e.g. discs) may be progressively smaller from the carbon-containing structural spar to the electrically conductive lightning-protection structure, for example they may be sheets or discs of monotonically (e.g. progressively) reducing diameter.

[0048] It may be that the conductive layers comprises at least one layer which comprises a carbon-containing material, for example a carbon-fibre layer, and at least one, or at least two, metal sheets.

[0049] Typically, there is a continuous electrical connection from the electrically conductive lightning-protection structure to the carbon material (e.g. carbon fibres) of the carbon- containing structural spar. This continuous electrical connection may pass through both conductive metal and carbon-containing material (e.g. carbon fibre layers). Thus, the plurality of conductive layers may comprise both conductive metal and carbon- containing material layers, for example at least two conductive metal and at least two carbon-containing material layers.

[0050] It may be that at least one conductive layer is a carbon-fibre layer having a nonconducting surface (e.g. a glass surface layer) with at least one conducting surface region (e.g. where carbon-fibres reach the surface) through which the continuous electrical connection is made. The surface of a carbon-fibre layer may comprise an abraded or cut portion forming a conducting surface region. For example, a carbon- fibre layer may be chamfered. This can be important because carbon-fibre structures, e.g. carbon-fibre pultrusions, typically have surface layers which are electrically insulating.

[0051] The conductive metal layer closest to the carbon-containing structural spar may be enclosed by a carbon-containing material, which may be formed as two layers, one on either side of the metal layer. This improves threat current diffusion from the carbon- containing structural spar into the metal layers and reduces edge sparking. Electrical connections of this type can be easily formed within wind turbine blades made by a typical blade layup process. The carbon-containing material may be a carbon-fibre material, for example a biaxial carbon-fibre, optionally with fibres running at 45° in either sense relative to the spanwise direction (and so crossing each other at 90°). This facilitates threat current being presented with a lower chord-wise resistance than would be the case with only unidirectional fibres.

[0052] The carbon-containing structural spar may comprise a plurality of layers, each layer comprising carbon fibres. A carbon-containing connector layer, for example a layer comprising carbon fibres, may conductively connect a plurality of structural spars to each other, for example, the carbon-containing connecter layer may wrap around the blade and be in conductive communication with a plurality of carbon-containing structural spars which extend spanwise. It may be that the carbon-containing structural spar, or spars, comprise cut-away regions, for example chamfers, to enable direct conductive communication with fibres with the carbon-containing structural spar, or spars. The carbon-containing connector layer may also comprise one or more cutaway regions, for example chamfers, such that carbon fibres within the carbon- containing connector layer may be in contact with carbon fibres in one or more carbon- containing structural spars. This facilitates a conductive electrical connection between one or more carbon-containing structural spars and the connector layer and / or other carbon-containing structural spars.

[0053] It is convenient from a manufacturing perspective to include layers of conductive material, as this is compatible with blade manufacturing processes, including lamination. This contrasts with conductive connections which use protrusions from structural spars or the lightning-protection structure, or other components such as screws or bolts, that are more difficult to include in a lamination procedure.

[0054] Typically, one or more layers of conductive material are formed within, or in a gap within, the spacer layer. Thus, the layers of conductive material can be readily included in a structure, including the spacer layer, which is to be laminated.

[0055] It may be that the lightning-protection structure forms part of the outer surface of the blade.

[0056] The blade may comprise an outer fixed layer which covers the lightning-protection structure and structural spars.

[0057] In this case, the outer fixed layer is typically a smooth outer fixed layer, for example a layer of glass. The outer fixed layer may be in direct contact with a layer comprising a branch of the lightning-protection structure.

[0058] The outer fixed layer is typically smooth, for example glass. It may be that a flexible textile layer overlays the outer fixed layer, over the structural spars.

[0059] It may be that the outer fixed layer is smooth, and a flexible textile layer overlays the outer fixed layer over the structural spars.

[0060] The use of a flexible textile layer as the outmost surface of the blade, at least over the structural spars, enables the mass of the blade to be restricted. A smooth outer layer, for example, a layer of glass over the lightning-protection structure, is compatible with a flexible textile layer.

[0061] The flexible textile layer is typically taut, but flowably mounted so that it may flow across at least a part of the outmost surface of the blade. The blade may comprise a tensioner which functions to keep the flexible textile layer under tension and also as part of the conductive connection between the lightningprotection structure and ground.

[0062] The invention also extends to a wind turbine comprising one or more said wind turbine rotor blades and one or more conductive electrical connections from the root end of the one or more wind turbine rotor blades to ground.

[0063] By the tip-wards end we refer to the end of the blade which is radially furthest from the axis of rotation of the wind turbine rotor and by the root-wards end we refer to the end of the blade which is radially closest from the axis of rotation of the wind turbine rotor. The tip-wards location is proximate the tip, and my for example be in the 20%, or 10% of the length of the blade closest to the tip and the root-wards location is proximate the root, for example, in the 20%, or 10% of the length of the blade closest to the root.

[0064] Features disclosed in respect of the first or second aspect of the invention are also optional features of the other aspect of the invention.

[0065] Description of the Drawings

[0066] An example embodiment of the present invention will now be illustrated with reference to the following Figures in which:

[0067] Figure 1 is a cross-section through a wind-turbine blade according to the invention;

[0068] Figure 2 is a perspective view of a wind-turbine blade according to the invention, and a connection to ground;

[0069] Figure 3 is a perspective view of the wind-turbine blade from a position adjacent the tip, facing towards the root;

[0070] Figure 4(a) is a perspective view of a laminate structure forming a carbon fibre spar, a mesh conductive layer overlying the spar and the outer layer of the shell; Figure 4(b) is a magnified view of a detail of Figure 4(a);

[0071] Figure 5(a) is a cross-section through the outer shell of the wind-turbine blade, through an equipotential connection between a carbon-fibre spare and mesh conductive layer; and Figure 5(b) is a plan view of an equipotential connection between a carbon-fibre spare and mesh conductive layer.

[0072] Detailed Description of an Example Embodiment

[0073] With reference to Figures 1 through 3, a wind turbine blade 1 comprises a shell 2 comprising an elongate hollow pole 4, formed of a composite of materials, which extend longitudinally (spanwise) and defines the leading edge 6 of the blade, and a plurality of generally triangular plates 8 extending from the pole towards the trailing edge 10 of the blade. The plates are longitudinally spaced from each other and retain a trailing edge support 12 which extends longitudinally and pivotably retains a trailing edge flap 14. A taut fabric 16 is attached to a longitudinally extending tensioner 18 on both the suction 20 and pressure 22 sides of the blade, which extends over the smooth outer surface 24 of the shell between the suction and pressure sides, covering the leading edge of the blade. Within the body of the composite shell there are provided three longitudinally (spanwise) extending carbon-fibre spars 26, 28, 30 respectively on the suction and pressure sides, and over the leading edge of the pole. This construction, with carbon-fibre spars, a hollow pole and spaced apart supports for a trailing edge support, with a fabric cover, provides a strong, lightweight and resilient blade. The fabric is not adhered to the surface of the shell and can flow around the pole when the trailing edge flap is adjusted.

[0074] With reference to Figures 1 through 5, the wind turbine blade has a lightning-protection structure comprising a mesh layer 32 of conductive metal which is split into three branches 34, 36 and 38, each branch covering a respective carbon-fibre spar 26, 28, 30. Thus, carbon-fibre spars on each of the suction side, pressure side and leading edge of the shell are covered with a metal mesh layer. The mesh layers extend beyond the edges of the carbon-fibre spars and run along the entire length of the spars. The carbon-fibre spars and mesh layers are formed within the composite structure of the pole, which may be predominantly made of GFRP (glass fibre reinforced polymer). At the root 40 of the blade, a root mesh region 42, extends around the shell, providing a continuous conducting loop. The branches 34, 36 and 38 each extend from the root mesh region and are thereby conductively connected to each other through the root mesh region. A tensioner 44 is provided to keep the fabric in tension. It is made of a conductive material (e.g. a metal such as aluminium) and so can provide part of a ground connection path between the mesh and ground, via a main root ground conductor 50 to a root plate 52 which is in turn connected to ground.

[0075] Towards the tip 60 of the blade, the mesh branches 34, 36, 38 recombine to a tipward mesh region 62 which again extends around the shell, providing a continuous conducting loop. The branches extend tipwards beyond the carbon-fibre spars and thus the spars are covered by mesh and the mesh branches are connected to each other at the rootward and tipward ends. The tipward mesh region 62 is conductively connected to a lightning protection tip array 64, which is a conductive structure known in the art, via a conductive down conductor connection 66. The spars do not extend to the tip of the blade and the tip may be formed of GFRP, for example.

[0076] Figures 4(a) and 4(b), show carbon-fibre spar 26, overlying mesh 34, and the outer layer of the shell the other spars and mesh correspond. The carbon-fibre spars are formed of elongate carbon-fibre layers 80. In this case, three parallel laminates 82 are provided for ease of manufacture and structural reasons, by this is optional. A spacer layer 84, e.g. of GFRP, of defined thickness is located between the carbon-fibre spar and mesh layer to ensure they are rigidly held relative to each other at a controlled and relatively small distance, to maximise inductive and capacitive coupling between the carbon-fibre and mesh. The mesh itself is covered with a smooth coating layer 86 which forms the outer surface of the pole and so the outer surface of the shell. The smooth coating layer avoids damage to the fabric by the mesh in use.

[0077] The carbon-fibre spars 26, 28, 30 are conductively connected to the overlying mesh branches 34, 36, 38 which cover them by a number of equipotential bonds 90. Tipward equipotential bonds 90 connect the spars to the overlying mesh branches near the tip ends of the spars, and rootward equipotential bonds 90 connect the spars to the overlying mesh branches near the root ends of the spars. Intermediate equipotential bonds 90 can also be provided to connect the spars to overlying metal branches at intermediate locations along the length of the spars.

[0078] The equipotential bonds are shown in cross section in Figure 5a and in plan view in Figure 5b. A carbon-fibre spar 26 is formed from a plurality of pultrusion layers, arranged in stacks and adjacent further stacks, and is covered (typically in a wrap around the pole covering all carbon-fibre spars) by a first carbon fibre BIAX layer 94. Although the absolute surface of individual pultrusion layers are typically non- conductive due to the surface resin layer, the pultrusion layers are also chamfered and these chamfered regions expose conductive carbon fibres. The first carbon fibre BIAX layer 94 also has an abraded surface to expose carbon fibres and so makes an electrical connection between the different stacks of pultrusions, thus connecting them all together in the blade through the thickness direction. This first carbon fibre BIAX layer 94 is applied over the entire chamfer area of the carbon-fibre spar at the tip end and root. A first conductive metal layer 92A, (in this example a disc of copper) is then located over the first carbon fibre BIAX layer, which has also been abraded on its outward surface to form a conductive electrical connection with the first conductive metal layer and is covered by a second carbon fibre BIAX layer 96 which contacts the first carbon fibre BIAX layer around the periphery of the disc and is again abraded on both sides to enable a continuous electrical connection. Outwards of the second carbon fibre BIAX layer are located three further conductive metal layers 92B, 92C, 92D (typically discs of copper) of progressively decreasing size from inside to outside (for example from about 160mm to 280mm diameter in the case of circular discs), the outmost of which 92D is in direct electrical contact with the mesh 34. The surrounding volume is filled with GFRP. The conductive metal layers and the first and second BIAX layers provide a direct continuous conductive connection from the carbon-fibre spar to the mesh (functioning as the lightning-protection system). This maintains the carbon- fibre spar and mesh at the same potential and avoids the potential of the carbon structure from floating. Furthermore, the two layers of carbon BIAX around the first conductive metal layer facilitate threat current diffusion from the carbon pultrusion (which forms the bulk of the carbon-containing spar) and reduce edge sparking at the copper to carbon interface. This structure provides a robust interface capable of handling threat current transfer while being relatively simple in structure. It is compatible with known blade layup processes and does not require any external parts to make the connection.

[0079] Thus, the mesh branches are conductively connected to each other at either end of the blade, and are connected to the carbon-fibre spars which they overlay at least at rootward and tipward regions of those spars. The mesh is held close to, but spaced apart, from the carbon-fibre spars, providing a controlled and relatively high capacitance and inductance between the mesh and spars. The mesh branches are separate to each other between the root mesh region and the tipwards mesh region. This further increases capacitive and inductive coupling between the mesh and spars in contrast to having mesh or another conductive material across the entire surface of the shell of the blade.

[0080] In use, when there is a lightning strike on the tip end of the blade, the current impulse splits amongst 6 possible current paths to ground, namely along any of the three carbon-fibre spars and any of the three mesh branches. The inductance of the paths along the mesh branches is an order of magnitude lower than the inductance of the paths through the carbon structure. Thus, current flows predominantly on the surface of the mesh. At the interfaces between the mesh and the carbon-fibre spars, the carbon appears as a high impedance connection during the rise time of the impulse. This results in the substantial majority of the current flowing through the mesh to ground rather than passing through the carbon. This avoids damage to the spars and reduces the impulse handling requirements of the equipotential connections. Thus, a reliable lightning protection system is provided with a relatively simple structure, minimising component complexity and installation time.

Claims

Claims1. A wind turbine rotor blade comprising: a plurality of carbon-containing structural spars, whereby each structural spar extends spanwise along the blade between a root-wards position and a tip-wards position, each structural spar having an outward facing surface; an electrically conductive lightning-protection structure, the electrically conductive lightning-protection structure being electrically connected to, or electrically connectable to, a ground connection at a root end of the wind turbine rotor blade; whereby the lightning-protection structure comprises a branch for each structural spar, each branch extending along and covering the outward facing surface of the corresponding structural spar; each branch electrically connected to the respective structural spar at least at a tip-wards location and a root-wards location.

2. A wind turbine rotor blade according to claim 1, wherein each branch of the electrically conductive lightning-protection structure extends chord-wise beyond either side of the respective structural spar.

3. A wind turbine rotor blade according to claim 1 or claim 2, wherein the electrically conductive lightning-protection structure extends spanwise beyond the root-wards end and beyond the tip-wards end of each respective structural spar.

4. A wind turbine rotor blade according to any one preceding claim, wherein the branches of the electrically conductive lightning-protection structure are electrically connected to each other at a tip region of the blade.

5. A wind turbine rotor blade according to claim 4, wherein the electrically conductive lightning-protection structure comprises or is connected to an electrically conductive blade tip protector.

6. A wind turbine rotor blade according to any one preceding claim, wherein the inductance of a branch of the lightning-protection structure is less than 20% of the inductance of the structural spar which it covers.

7. A wind turbine rotor blade according to any one preceding claim, comprising one or more further conductive electrical connections between a branch of the lightning-protection structure and a respective structural spar, intermediate the said tipwards location and root-wards location.

8. A wind turbine rotor blade according to any one preceding claim, wherein the electrically conductive lightning-protection structure comprises a root-wards common section which wraps around the circumference of the rotor blade and to which each branch is connected.

9. A wind turbine rotor blade according to claim 8, comprising a first structural spar which supports a suction side of the blade, a second structural spar which supports a pressure side of the blade, and a third structural spar which supports the leading edge of the blade.

10. A wind turbine rotor blade according to claim 9, wherein the electrically conductive lightning-protection structure is connected to ground through two discrete connections, one of which is on the suction side of the blade, other of which is on the pressure side.

11. A wind turbine rotor blade according to any one preceding claim, wherein the branches of the electrically conductive lightning-protection structure comprise conductive metal mesh.

12. A wind turbine rotor blade according to any one preceding claim, wherein each branch of the lightning-protection structure is formed as a layer over the respective structural spar.

13. A wind turbine rotor blade according to any one preceding claim, comprising a spacer layer between each respective structural spar and the branch of the lightningprotection structure which covers the structural spar.

14. A wind turbine rotor blade according to claim 13, wherein a structural spar, a spacer layer, and a branch of the lightning-protection structure are formed as a laminate.

15. A wind turbine rotor blade according to any one of claims 11 to 14, wherein an electrical connection is formed by one or more layers of conductive material located between the outer surface of the structural spar and the branch of the lightningprotection structure covering the structural spar.

16. A wind turbine rotor blade according to claim 15 when dependent on claim 13, wherein the one or more layers of conductive material are formed within, or in a gap within, the spacer layer.

17. A wind turbine rotor blade according to any one of claims 1 to 16, comprising an outer fixed layer which covers the lightning-protection structure and structural spars.

18. A wind turbine rotor blade according to claim 17, wherein the outer fixed layer is smooth, and a flexible textile layer overlays the outer fixed layer over the structural spars.

19. A wind turbine rotor blade comprising one or more carbon-containing structural spars, an electrically conductive lightning-protection structure covering the one or more one or more carbon-containing structural spars, and one or more electrical connections between a carbon-containing structural spar and a respective electrically conductive lightning-protection structure, the one or more electrical connections comprising a plurality of conductive layers.

20. A wind turbine rotor blade according to claim 19, wherein a conductive layer is in direct contact with the covering electrically conductive lightning-protection structure.

21. A wind turbine blade according to claim 19 or claim 20, where the conductive layers comprise two or more conductive metal sheets.

22. A wind turbine blade according to any one of claims 19 to 21 , wherein the metal sheets have monotonically (e.g. progressively) reducing diameter from the carbon- containing structural spar to the electrically conductive lightning-protection structure.

23. A wind turbine blade according to any one or claims 19 to 22, wherein the plurality of conductive layers comprises both conductive metal and carbon-containing material layers.

24. A wind turbine blade according to any one of claims 21 to 23, wherein the metal layer closest to the carbon-containing structural spar is enclosed by a carbon- containing material, formed as two layers, one on either side of the metal layer.

25. A wind turbine comprising one or more wind turbine rotor blades according to any one preceding claim and one or more conductive electrical connections from the root end of the one or more wind turbine rotor blades to ground.

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