Technical field

The present invention relates generally to a wind turbine blade and in particular to the lightning protection system of a wind turbine blade.

Background

Wind turbines are susceptible to lightning strikes, and the blades of wind turbines are particularly susceptible to lightning strikes. As a result, it is common for a wind turbine blade to include a lighting protection system to receive a lightning strike and safely discharge the lightning current to ground. Lightning current may flow in electrically conductive structural reinforcing materials and it is desirable to limit the amount of current flowing in such materials. of Invention

According to the present invention there is provided a wind turbine blade extending longitudinally in a spanwise direction between a root end and a tip end, and extending in a chordwise direction between a leading edge and a trailing edge, the blade comprising: a spar cap extending in the spanwise direction of the blade, the spar cap comprising electrically conductive fibres; the spar cap having an outer surface facing towards an exterior of the blade, an inner surface facing towards an interior of the blade, a first side surface facing towards the leading edge of the blade and a second side surface facing towards the trailing edge of the blade; wherein an electromagnetic shield is positioned on at least one surface of the spar cap, the electromagnetic shield extending in the spanwise direction.

The electromagnetic shield which extends along the spar cap protects the spar cap in the event of a lightning strike. In particular, in a blade where the spar cap is formed from a conductive material, the electromagnetic shield prevents high currents from flowing in the spar cap by acting as a Faraday cage around the spar cap.

The electromagnetic shield is not intended to protect the spar cap from direct lightning attachment. Instead, the electromagnetic shield provides shielding to the spar cap by preventing the formation of high electric fields in the spar cap and thus minimising high current flow in the spar cap. The electromagnetic shield may be positioned on and in electrically conductive contact with the at least one surface of the spar cap. In other words, the electromagnetic shield is in intimate contact with the surface of the spar cap. The electromagnetic shield may be in intimate contact with the surfaces of the spar cap so that current can pass from the spar cap to the electromagnetic shield. In the event of a lightning strike, lightning current may flow in the spar cap. The current in the spar cap will be diverted into the electromagnetic shield as this has a lower impendence than the spar cap, in particular due to the skin effect. This avoids high currents through the spar cap as the majority of the lightning current is carried by the electromagnetic shield which acts as a low impedance path. This in turn avoids heating of the spar cap. In particular, damage to composite parts by overheating is avoided

The electromagnetic shield may be separated from the at least one surface of the spar cap by an insulating layer, the insulating layer having a thickness of less than 1 mm, preferably less than 0.5mm, preferably less than 0.25mm. The electromagnetic shield acts as a Faraday cage to protect the spar cap. Where the electromagnetic shield is separated from the spar cap, this is by a very small distance. For example, there may be a single layer of glass fabric positioned between the spar cap and the electromagnetic shield. The lightning current will not flow through the spar cap, and there will not be a flashover between the electromagnetic shield and the spar cap due to the higher impedance of the spar cap.

Preferably the conductive material comprises carbon fibres. For example, the spar cap may comprise carbon fibre reinforced plastic (CFRP). The spar cap may include pultruded fibrous strips of material such as pultruded carbon fibre composite material or other carbon fibre reinforced plastic material.

Preferably, the electromagnetic shield is positioned on the upper surface and/or the inner surface of the spar cap. The upper surface and the inner surface may define major surfaces of the spar cap and in this way, lightning current can be transferred to and from the spar cap into the electromagnetic shield over a large surface area.

The electromagnetic shield may be positioned on the first side surface and/or the second side surface of the spar cap. Providing the electromagnetic shield in this way helps to ensure that lightning current can be transferred between the spar cap and the electromagnetic shield. The wind turbine blade may further comprise a down conductor to transfer lightning current from the tip of the blade to the root of the blade.

The down conductor may include a lightning protection layer in the form of a conductive sheet adjacent an outer surface of the blade, the lightning protection layer being separated from the spar by insulating material. Preferably the conductive sheet is in the form of a metal foil such as a metal mesh or an expanded metal foil.

The down conductor may be equipotentially bonded to the electromagnetic shield. By equipotentially connecting the down conductor and the electromagnetic shield ensures that there will be no voltage flashovers between the electromagnetic shield and the down conductor which could damage the blade.

The electromagnetic shield may comprise a metallic foil. The metallic foil may be in the form of a metal mesh or an expanded metal foil. The metal foil may have a thickness of less than 1 mm, optionally between 0.2 mm and 0.6 mm, and optionally between 0.25 mm and 0.5 mm or between 0.2mm and 0.3mm. The metallic foil of the electromagnetic shield may be the same material as the lightning protection layer in the form of a conductive sheet adjacent an outer surface of the blade. Suitable materials for the metallic foil may include aluminium, copper and stainless steel. If the conductive fibres are carbon, the metal may be tinned to prevent galvanic corrosion with the carbon. A metallic foil provides high conductivity so that it is a preferred current path for the lightning current.

The electromagnetic shield may comprise metallic wires incorporated into a glass fibre layer. This material may be easily handled so as to facilitate the manufacture and the assembly of the blade. The wires may be laid in a wavy pattern such that they do not suffer from fatigue damage during the use of the wind turbine blade.

The electromagnetic shield may be co-pultruded with the fibrous material. This provides a simple and effective way to attach the electromagnetic shield to the spar cap

Preferably, the electromagnetic shield has a higher conductivity than the spar cap. This makes the electromagnetic shield the preferred current path for lightning current.

The blade may be a single piece blade. In other words, the blade does not have a connection joint between the root end and the tip end. Brief of the drawings

The present invention will now be described by way of non-limiting examples only, with reference to the accompanying figures, in which:

Figure 1 shows a wind turbine.

Figure 2 shows a wind turbine blade.

Figure 3 shows a cross section of the wind turbine blade.

Figure 4 shows an enlarged cross section of the wind turbine blade.

Figure 5 shows a plan view of the wind turbine blade.

Figure 6 shows an electromagnetic shield around the spar cap in a perspective view.

Figure 7 shows an electromagnetic shield around the spar cap in a spanwise cross sectional view.

Figure 8 shows an electromagnetic shield around the spar cap in a chordwise cross sectional view.

Figure 9 shows a chordwise cross sectional view of the electromagnetic shield and a down conductor.

Detailed

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

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

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

The chord of a blade is the straight line distance from the leading edge to the trailing edge in a given cross section perpendicular to the blade spanwise direction. The term chordwise is used to refer to a direction from the leading edge to the trailing edge, or vice versa.

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

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

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

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

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

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

Figure 1 shows a wind turbine 10 including a tower 12 mounted on a foundation and a nacelle 14 disposed at the apex of the tower 12. A rotor 16 is operatively coupled to a generator (not shown) housed inside the nacelle 14. The rotor 16 includes a central hub 18 and a plurality of rotor blades 20, which project outwardly from the central hub 18. It will be noted that the wind turbine 10 is the common type of horizontal axis wind turbine (HAWT) such that the rotor 16 is mounted at the nacelle 12 to rotate about a substantially horizontal axis defined at the centre at the hub 18. While the example shown in Figure 1 has three blades, it will be realised by the skilled person that other numbers of blades are possible. When wind blows against the wind turbine 10, the blades 20 generate a lift force which causes the rotor 16 to rotate, which in turn causes the generator within the nacelle 14 to generate electrical energy.

Figure 2 illustrates one of the wind turbine blades 20 for use in such a wind turbine. Each of the blades 20 has a root end 21 proximal to the hub 18 and a tip end 22 distal from the hub 18. The blade 20 is arranged to extend away from the hub 18 in a spanwise direction S. A leading edge 23 and a trailing edge 24 extend between the root end 21 and tip end 22, and each of the blades 20 has a respective aerodynamic high pressure surface (i.e. the pressure surface) and an aerodynamic low pressure surface (i.e. the suction surface) extending between the leading and trailing edges of the blade.

Each blade has a cross section which is substantially circular near the root end 21 , because the blade near the root must have sufficient structural strength to support the blade outboard of that section and to transfer loads into the hub 18. The blade 20 transitions from a circular profile to an aerofoil profile moving from the root end 21 of the blade towards the tip end 22. The blade may have a "shoulder", which is the widest part of the blade where the blade has its maximum chord. The blade 20 has an aerofoil profile of progressively decreasing thickness towards the tip end 22.

As shown in Figure 3, which is a cross sectional view of the blade 20 taken along the line A-A, the wind turbine blade 20 may include an outer blade shell formed of an upper part 42 and a lower part 44, which together define a hollow interior space 34 with a shear web 40 extending internally between the upper and lower parts of the blade shell 42, 44. The blade shell parts may be two half-shells 42, 44 which are separately moulded before being joined together (at the leading edge 23 and the trailing edge 24) to form the blade 20. It will be appreciated that the blade shell 42, 44 need not be formed as two half-shells which are subsequently joined together but may be formed as a unitary shell structure, together with the shear web 40, in a "one shot" single shell process. The blade shell may include a laminate composite material such as glass fibre and/or carbon fibre for example.

Figure 4 shows a detail view of the region B, where the shear web 40 meets the blade shell 44. A spar cap 50 is incorporated into the outer shell 44, as shown in Figure 4, or may be attached to the outer shell 44. The spar cap 50 is an elongate reinforcing structure and may extend substantially along the full spanwise length of the blade 20 from the root end 21 to the tip end 22. The spar cap 50 includes conductive material, such as carbon fibres. For example, the spar cap may include pultruded fibrous strips of material such as pultruded carbon fibre composite material or other carbon fibre reinforced plastic material.

The spar cap 50 may include a stack of layers of the conductive material. The shear web 40 may be adhesively bonded to an inner surface of the spar cap 50. An outer surface of the spar cap 50 may sit adjacent a lightning conductor 46 in the outer surface of the blade shell 44. As shown in figure 4, the lightning conductor may be in the form a lightning protection layer 46 which may be separated from the outer surface of the spar cap 50 by one or more layers of insulating material 48, such as glass fibre reinforced plastic. One or more further layers of glass fibre reinforced plastic 47 may be provided over the outside of the lightning protection layer 46. The lightning protection layer 46 may be in the form of a conductive sheet, such as a metallic foil. The layers collectively form an outer skin of the blade shell 44. One or more further layers 45 of glass fibre reinforced plastic provide an inner skin of the blade shell 44 with a core material 49 between the outer skin 48 and the inner skin 45. The core material may be a light structural foam, though other core materials such as wood, particularly balsa wood may alternatively be used to provide a lightweight core material. It will be appreciated that a near identical connection may be made between the shear web 40 and the other side of the blade shell 42.

The blade materials are laid up in moulds, where they are then infused with resin to bond the blade materials together. As is well known in the art, the blade materials are covered with a sealed vacuum bag which is evacuated, and then resin is infused into the blade materials. The resin is then cured which may be at an elevated temperature. This is known as a vacuum assisted resin transfer moulding (VARTM) process.

Figure 5 schematically shows a plan view of the blade 20. The blade 20 includes one or more lightning receptors and one or more lightning down conductors which form part of a lightning protection system for the wind turbine. The lightning receptors attract the lightning strike and the down conductors conduct the energy of the lightning strike down the blade 20 via the nacelle 14 and tower 12 to ground. The lightning down conductor may take a variety of forms, such as the lightning protection layer 46 on the outer surface of the blade and/or a cable 51 , e.g. running through the interior of the hollow blade. The lightning receptors may include the lightning protection layer 46, and/or a solid metal tip 53.

The majority of the outer surface of the blade 20 may be covered with the lightning protection layer 46. The lightning protection layer 46 serves to shield conductive material in the blade from a lightning strike, and it may act as either a lightning receptor, a down conductor, or both. The lightning protection layer 46 may extend substantially the full length of the blade. Where the majority of the outer surface of the blade 20 is covered with the lightning protection layer 46, the cable 51 may connect to the lightning protection layer 46 adjacent the tip end of the blade and adjacent the root end of the blade, with no cable 51 along the majority of the length of the blade covered with the lightning protection layer 46. The lightning protection layer 46 may extend from root to tip in which case there may be no need for cable 51 .

Also shown in Figure 5 is the spar cap 50 which extends in the spanwise direction of the blade. It will be appreciated, that the blade will have another spar cap on the opposing side of the blade. Furthermore additional spar caps may be provided where needed to provide the required strength to the blade.

The spar cap has an outer surface 56 facing towards an exterior of the blade, an inner surface 57 facing towards an interior of the blade, a first side surface 58 facing towards the leading edge of the blade and a second side surface 59 facing towards the trailing edge of the blade.

In the event of a lightning strike at the tip of the blade lightning current will flow via the down conductors from the tip of the blade to the root of the blade. As the spar caps are formed of conductive material, lightning current may also be present in the spar caps.

Figures 6 ,7 and 8 show how the spar cap 50 has been protected so that the amount of lightning current that can flow directly through the spar cap is reduced. Figure 6 is a partial schematic perspective view of the spar cap. Figure 7 is a spanwise cross section through the spar cap looking towards the leading edge or the trailing edge. Figure 8 is a chordwise cross section through the spar cap looking towards the root of the blade or the tip of the blade.

An electromagnetic shield 60 covers the spar cap 50. As shown, the electromagnetic shield surrounds all surfaces of the spar cap 50. However, as will be explained below, the electromagnetic shield 60 may only cover one or more surfaces. In an example, the electromagnetic shield 60 is an electrically conductive metallic foil.

The electromagnetic shield 60 may be positioned on and in electrically conductive contact with the spar cap 50, i.e. the electromagnetic shield is in direct and intimate contact with the spar cap such that the electromagnetic shield is in conductive contact with the spar cap. In other words, there may be no insulating material between the electromagnetic shield and the spar cap. In the event of lightning striking the tip of the wind turbine blade, lightning current is transferred from near the tip of the blade to the root of the blade. The lightning current will flow through the blade’s lightning protection system (e.g. the lightning protection layer 46) but there may also be a proportion of lighting current in the spar cap 50. The spar cap 50 is formed from electrically conductive fibres (e.g. carbon fibres) and so it will conduct the lightning current.

Considering Figure 6, as lightning current flows through the spar cap 50 from the tip (at the top right of Figure 6) to the root (at the bottom left of Figure 6) the lightning current has two parallel paths. The first current path is through the spar cap 50. The second current path is though the electromagnetic shield 60 which may be in direct contact with the spar cap 50.

The second current path though the electromagnetic shield 60 is configured to be the preferred current path so that the majority of current takes this path, rather than the first current path through the spar cap. This is because (i) the electromagnetic shield 60 has a lower impedance than the spar cap 50; and (ii) as the electromagnetic shield 60 is positioned on the outer surface of the spar cap 50, the skin effect means that the current flows preferentially using the outermost volume of the conductor system, and therefore the current will be greater at the electromagnetic shield 60 than in the interior of the spar cap 50.

In effect, as the current flows from the tip to root direction, the current is diverted into the electromagnetic shield 60. This is advantageous as it avoids high current passing though the spar cap. Avoiding high currents in the spar cap means that there will be reduced heating of the material of the spar cap as lightning current passes through, and it will also prevent flashovers between the spar caps and other electrically components in the blade.

The electromagnetic shield 60 may be separated from the spar cap 50 by a thin insulating layer. In this case, the electromagnetic shield 60 will act as a Faraday cage to protect the underlying spar cap.

As shown in the cross sectional view in Figure 9, the electromagnetic shield 60 may also be electrically connected to the lightning protection system. In this example, the electromagnetic shield 60 is connected via a conductor 68 to the lightning protection layer 46. In this way the electromagnetic shield 60 is equipotentially bonded to the lightning down conductor preventing any flashovers which may cause damage to the blade. The conductors 68 may be discrete components positioned at spaced intervals along the length of the spar cap. In the example of Figure 8, the electromagnetic shield 60 is shown as covering all surfaces of the spar cap 50, namely the outer surface 56, the inner surface 57 and the first and second side surfaces 58, 59. However, the electromagnetic shield 60 may only cover a single surface of the spar cap. Preferably the electromagnetic shield 60 will cover all the surfaces of the spar cap as this will provide a greater level of protection to the spar cap, but a sufficient level of protection may be provided if three, two or just one surface is covered.

In a preferred example, only the outer surface 56 and the inner surface 57 of the spar cap 50 are covered with the electromagnetic shield. In this configuration, the electromatic shield parts on the outer surface and the inner surface operate at different frequency ranges of the lightning pulse to provide sufficient shielding to the spar cap. Conductive strips may be provided between the electromatic shield parts on the outer surface and the inner surface so that they are electrically connected so that current can be transferred between the different surfaces.

As mentioned, the spar cap may include pultruded fibrous strips of material such as pultruded carbon fibre composite material. The electromagnetic shield may be attached to the pultrusions as part of the pultrusion process, e.g. the carbon fibre is co-pultruded with the electromagnetic shield. This advantages provides an efficient way of connecting the electromagnetic shield to the pultruded strips.