Lightning current transfer system

Abstract

The present disclosure relates to a lightning current transfer system for a wind turbine. The lightning current transfer system comprises a contact device having a mounting part, an elastic non-conducting arm having a first end attached to the mounting part and a second end to which a contact part is attached. The arm has a length extending between the first end and the second end and a width which tapers from the first end towards the second end. It has been found that by tapering the width of the arm between the first and second end, the stress distribution across the arm can be improved.

Description

[0001] LIGHTNING CURRENT TRANSFER SYSTEM

[0002] FIELD

[0003] The present disclosure relates to a lightning current transfer system and to a wind turbine comprising said lightning current transfer system.

[0004] BACKGROUND

[0005] Modern wind turbines are generally erected in exposed and sparsely populated areas in which no (or very few other) tall objects (such as trees, buildings etc.) exist. This, in additional to their significant height, makes modern wind turbines susceptible to lightning strikes. Commercial wind turbines are therefore provided with lightning protection systems for discharging and grounding electrical currents in the event of a lightning strike.

[0006] Modern lighting protection systems typically comprise a plurality of lightning receptors and a series of down conductors for discharging lightning currents from the lighting receptors to a ground potential.

[0007] However, since modern wind turbines are made up of a plurality of different parts, modern lighting protection systems also comprise one or more lightning current transfer units for transferring lightning currents between the down conductors provided on adjacent parts of the wind turbine. The purpose of the lightning current transfer units is to transfer current between components which are rotating in relation to each other making direct cabling impossible, e.g. blade pitching in relation to a hub or hub rotation in relation to a nacelle

[0008] An example of a known wind turbine lightning current transfer unit is described in WO 2005 / 050008 A1.

[0009] However, as the size of modern wind turbines continues to increase, so too does the length (and hence the impedance) of the associated down conductors. This increase in impedance also increases the voltage drop between the down conductors provided on adjacent wind turbine parts, thereby making the connections between such parts more prone to electric arc discharge (or “flashover”).

[0010] Because of this, larger wind turbine blades require that a greater separation (or spanning) distance is provided between down conductors and the one or more metallic components provided at the interfaces between the adjacent wind turbine parts, meaning that the associated contact devices must also span greater distances. In addition, larger hubs and larger nacelles also contribute to an increased separation distance.

[0011] However, as the length of the contact devices increase, so too does the magnitude of bending stresses which are exerted on the arms of said devices during operation, which can reduce their operational life.

[0012] It is an aim of the present disclosure to provide a means for solving this problem.

[0013] SUMMARY

[0014] According to a first aspect, there is provided a lightning current transfer system for a wind turbine, the lightning current transfer system comprising: a first contact band mountable to a first wind turbine part adapted to rotate relative to a second wind turbine part; and a contact device mountable to the second wind turbine part, the contact device comprising: a first mounting part for mounting to the second wind turbine part; a first arm having a first end attached to the first mounting part and a second end opposite the first end, the first arm being elastic and nonconducting; and a first contact part attached to the second end of the first arm, wherein the contact device is configured such that the first contact part faces the first contact band and such that the first arm urges the first contact part against the first contact band for providing lightning current transfer between the first contact band and the first contact part, wherein the first arm has a length extending between the first end and the second end, and wherein the first arm tapers from the first end towards the second end.

[0015] Advantageously, it has been found that by tapering the first arm between the first and second end, the stress distribution across the arm can be improved thereby lowering the peak stresses experienced by the arm and, hence increasing its operational lifetime. This reduction in peak stress provided by the tapering design enables the arm to be used on a variety of wind turbines of different sizes, providing scalability which traditional arm designs (having a rectangular, non-tapered shape) cannot provide due to their higher peak stresses. The tapered arm may improve the stress distribution from operational bending loads and impact loads from lightning events. This may lead to lower operational fatigue loads extending the operational life of the arm while at the same time increasing the toughness of the arm for improved performance for shock loading from lightning events.

[0016] Optionally, the system may further comprise a second contact band mountable to a third wind turbine part, said third wind turbine part being adapted to rotate relative to the second wind turbine part, and wherein the contact device further optionally comprises: a second arm having a first end and a second end opposite the first end, the first end of the second arm is attached to the first mounting part or to a second mounting part for mounting to the second wind turbine part , and the second arm being elastic and non-conducting; and a second contact part attached to the second end of the second arm, said second contact part being electrically connected to the first contact part, wherein the contact device is further configured such that the second contact part faces the second contact band and such that the second arm urges the second contact part against the second contact band for providing lightning current transfer between the second contact band and the second contact part, wherein the second arm has a length extending between the first end and the second end, and wherein the second arm tapers from the first end towards the second end.

[0017] Optionally, the system may further comprise a second contact band mountable to the second or a third wind turbine part, said third wind turbine part being adapted to rotate relative to the first and / or second wind turbine part, wherein the contact device is a first contact device, and wherein the system further optionally comprises a second contact device mountable to one of the first, second or third wind turbine parts, the second contact device comprising: a second mounting part for mounting to another of the first, second or third wind turbine parts; a second arm having a first end attached to the second mounting part and a second end opposite the first end, the second arm being elastic and non-conducting; and a second contact part attached to the second end of the second arm, said second contact part being electrically connected to the first contact part; wherein the contact device is configured such that the second contact part faces the second contact band and such that the second arm urges the second contact part against the second contact band for providing lightning current transfer between the second contact band and the second contact part, wherein the second arm has a length extending between the first end and the second end, and wherein the second arm tapers from the first end towards the second end.

[0018] Preferably, the first and / or second arm may have a width which tapers from the first end towards the second end.

[0019] Advantageously, unlike other solutions such as increasing the thickness of the arm, the benefits described above can be achieved without substantially increasing the weight of the contact device. The tapered width arm design also shows lower tolerance sensitivity to contact pressure as compared with the traditional arm design.

[0020] Optionally, the first and / or second arm may have a substantially trapezoidal shape.

[0021] Optionally, the first and / or second arm may comprise a fibre-reinforced composite material. The arm construction may provide optimized fibre orientations to increase torsional strength and stiffness. The optimized fibre construction may prevent fibre splitting and may minimize torsion which subsequently improves the stress distribution leading to longer lifetime for impact loading from lightning events.

[0022] Optionally, the first and / or second arm may comprise a glass-fibre reinforced composite material. Optionally, the first and / or second arm may comprise a plurality of fibre layers, said layers being stacked one atop the other in the thickness direction.

[0023] Optionally, the plurality of fibre layers may include unidirectional fibre layers.

[0024] Optionally, the plurality of fibre layers may include multidirectional fibre layers.

[0025] Optionally, the first and / or second arm may comprise at least one fibre layer having a plurality of fibres orientated in a first direction relative to a longitudinal axis of the first and / or second arm, and at least one fibre layer having a plurality of fibres orientated in a second direction relative to a longitudinal axis of the first and / or second arm, said second direction being different to the first direction.

[0026] Optionally, the first and / or second arm may comprise at least one fibre layer having a plurality of fibres orientated at an angle of approximately 0 degrees relative to the longitudinal axis of the first and / or second arm.

[0027] Optionally, the first and / or second arm may comprise at least one fibre layer having a plurality of fibres orientated at an angle of approximately 45 degrees relative to the longitudinal axis of the first and / or second arm.

[0028] Optionally, the first and / or second arm may comprise at least one fibre layer having a plurality of fibres orientated at an angle of approximately 90 degrees relative to the longitudinal axis of the first and / or second arm.

[0029] Optionally, the first and / or second contact part may comprise a contact slider or roller or brush.

[0030] Optionally, the first and / or second arm may have a thickness which is significantly less than the width or length dimensions. By significantly less, in this context is meant that the thickness is less than 20% of the width at the second end of the respective arm. In an example, the thickness is less than 10% of the width at the second end of the respective arm.

[0031] Optionally, the first and / or second arm may have a substantially uniform thickness. The substantially uniform thickness may be simple and low cost to manufacture. Optionally, the first and / or second arm may have a tapering thickness. The tapering thickness, either in combination with the tapering width of the arm or with a uniform width arm, may allow for reduced peak stresses of the arm.

[0032] According to a second aspect, there is provided a wind turbine comprising: a first wind turbine part; a second wind turbine part, said first wind turbine part being adapted to rotate relative to the second wind turbine part; a first down conductor; and the lighting current transfer system according to any preceding claim, wherein the first contact band is mounted to the first wind turbine part and is electrically connected to the first down conductor; and wherein the first contact device is mounted to the second wind turbine part.

[0033] Optionally, the first wind turbine part is one of a wind turbine blade, a wind turbine nacelle or a wind turbine hub.

[0034] Optionally, the second wind turbine part is one of a wind turbine blade, a wind turbine nacelle or a wind turbine hub.

[0035] Optionally, the wind turbine may further comprise: a third wind turbine part, said third wind turbine part being adapted to rotate relative to the first and / or second wind turbine part, and a second down conductor; wherein the second contact band is mounted to one of the first, second third wind turbine parts and is electrically connected to the second down conductor, and wherein the second contact device is mounted to another of the first, second or third wind turbine parts.

[0036] Optionally, the third wind turbine part is one of a wind turbine blade, a wind turbine nacelle or a wind turbine hub.

[0037] BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Examples of the present disclosure will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of a wind turbine;

[0039] Figure 2 is a side-view of a lightning current transfer system according to one example of the present disclosure;

[0040] Figure 3 is a perspective view of a contact device of the lightning current transfer system illustrated in Figure 2;

[0041] Figure 4 is a plan view of an arm of the contact device illustrated in Figure 3;

[0042] Figure 5 is a graph representing the improved peak stress performance of the arm illustrated in Figure 4 when comparted to a conventional arm at 80N of nominal contact pressure;

[0043] Figure 6 is a graph representing the improved tolerance sensitivity of the arm illustrated in Figure 4 when comparted to a conventional arm;

[0044] Figure 7 is a table depicting an exemplary lay-up pattern for the fibre layers which make up the arm illustrated in Figure 4; and

[0045] Figure 8 is a side-view of a lightning current transfer system according to another example of the present disclosure.

[0046] DETAILED DESCRIPTION

[0047] 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.

[0048] The term “non-conducting” is defined herein as a material having an electrical resistivity greater than or equal to 1x108ohm-metres (Q.m).

[0049] The term “conducting” is defined herein as a material having an electrical conductivity greater than or equal to 1x105Siemens per meter (S / m).

[0050] The term “low friction” is defined herein as a material (or interface) having a coefficient of friction of 0.6 or less.

[0051] Figure 1 shows a wind turbine 1 according to an example. The wind turbine 1 includes a nacelle 2 supported on a tower 3 that is mounted on a foundation 4. It will be appreciated that the wind turbine 1 depicted may be any suitable type of wind turbine 1. The wind turbine 1 shown is an upwind wind turbine, although it will be appreciated the wind turbine 1 may be a downwind wind turbine. The wind turbine 1 depicted here is an onshore wind turbine such that the foundation 4 is embedded in the ground, but the wind turbine 1 could be an offshore installation in which case the foundation 4 may be provided by a suitable marine platform.

[0052] The nacelle 2 rotatably supports a rotor 5 comprising a hub 6 to which three blades 7 are attached. The blades 7 which make up the rotor 5 of the wind turbine 1 each comprise a tip end, which is located distal from the hub 6, and a root end, which is located proximal to the hub 6. While the example shown in Figure 1 has three blades 7, it will be appreciated that other numbers of blades 7 are possible.

[0053] It will be noted that the wind turbine depicted in Figure 1 is the common type of horizontal axis wind turbine (HAWT) such that the rotor 5 is mounted at the nacelle 2 to rotate about a substantially horizontal axis (B-B) defined at the centre at the hub 6 (see Figure 8). When wind blows against the wind turbine 1 , the wind turbine blades 7 generate a lift force which causes a generator (not shown) within the nacelle 2 to generate electrical energy.

[0054] The generating equipment is not shown in Figure 1 since it is not central to the examples of the present disclosure.

[0055] The wind turbine 1 has a plurality of lightning receptors 8 provided at the tip ends of each blade 7 for intercepting lightning strokes. The lightning receptors 8 form part of the wind turbine’s lightning protection system.

[0056] The lightning protection system also includes a first down conductor 9 which is mounted to a first wind turbine part (e.g., the blade 7), a second down conductor 10 which is mounted to a second wind turbine part (e.g., the nacelle 2) and a lightning current transfer system 100 for electrically connecting the first down conductor 9 to the second down conductor 10 so that lightning currents can be transferred between the two down conductors 9, 10.

[0057] Figure 2 shows an example of a lightning protection system for a wind turbine in greater detail.

[0058] The first down conductor 9 may be mounted to a blade 7 of the wind turbine 1 and electrically connected to one of the lighting receptors 8 (provided at the tip end of said blade 7) so that lightning currents can be transferred from the lightning receptor 8 to the first down conductor 9.

[0059] More particularly, the first down conductor 9 may be provided within the structure of the blade 7 and extends longitudinally along the blade’s length from the tip end, where it forms an electrical connection with the lighting receptor 8, to the root end where the first down conductor 9 terminates.

[0060] Meanwhile, the second down conductor 10 may be mounted to the nacelle 2 and is electrically connected to a ground potential 11 to enable lightning currents travelling along the second down conductor 10 to be safely discharged.

[0061] However, it will be appreciated that in other examples the first 9 and second 10 down conductors may be mounted to different parts of the wind turbine 1. For example, the first down conductor 9 may be mounted to the hub 6 or the nacelle 2 and the second down conductor 10 may be mounted to the hub 2 or to one of the blades 7.

[0062] The lighting current transfer system 100 includes a first contact band 102 which is mounted to the first wind turbine part (e.g., the blade 7) adapted to rotate relative to the second wind turbine part (e.g., the hub 6), and a contact device 110 mounted to the second wind turbine part (hub 6). The contact device has a first mounting part 120 mounted to the second wind turbine part (hub 6). The contact device 110 further includes a first arm 140 having a first end 142 attached to the first mounting part 120 and a second end 144 opposite the first end 142, the first arm being elastic and nonconducting. The contact device 110 also has a first contact part 130 attached to the second end 144 of the first arm 140.

[0063] The contact device 110 is configured such that the first contact part 130 faces the first contact band 102 and such that the first arm 140 urges the first contact part 130 against the first contact band 102 for providing lightning current transfer between the first contact band 102 and the first contact part 130.

[0064] The first arm 140 has a length extending between the first end 142 and the second end 142, a width which tapers from the first end towards the second end, and a thickness which is significantly less than the width or length dimensions. By significantly less, in this context it is meant that the thickness is less than 20% of the width at the second end. In an example, the thickness is less than 10% of the width at the second end. An example of the contact device 110 is shown in figure 3.

[0065] As is known, the blades 7 which make up the rotor 5 of the wind turbine 1 may be each rotatably mounted to the hub 6 via a pitching mechanism 12 which is configured to, upon actuation, enact pitchwise rotation of one or more of the blades 7 relative to the hub 6. In other words, the blades 7 of the wind turbine 1 are adapted to rotate relative to the hub 6 about their respective longitudinal axes to enable the pitch of the blades 7 to be adjusted during use.

[0066] As shown in Figure 2, the first contact band 102 has a substantially ring-shaped structure which extends around an external circumference of the blade 7 so that an electrical contact can be maintained between the first contact band 102 and the contact device 110 during pitchwise rotation of the blade 7 relative to the hub 6 (as will be described in greater detail below). The degree of pitch rotation of the blade 7 may be limited and accordingly the contact band 102 on the blade 7 may extend around only a portion of the circumference of the root of the blade 7.

[0067] As such, the first wind turbine part (e.g. the blade 7 in the example of Figure 2) is adapted to rotate relative to the second wind turbine part (e.g. the hub 6 in the example of Figure 2). It will be appreciated that in other examples, the first contact device 110 may be mounted to a different part of the wind turbine 1 such as to the nacelle 2 or to the blade 7, and the first contact band may then be mounted to another of the nacelle 2, the hub 6, or the blade 7.

[0068] The first contact band 102 is made of a conducting material (e.g., copper) and may be electrically connected to the first down conductor 9 such that lightning currents passing along the first down conductor 9 can be transferred onto the first contact band 102.

[0069] In the illustrated example, the first contact band 102 is mounted at the root end of the blade 7 and is spaced apart from the hub 6 in the longitudinal direction (i.e. in the spanwise direction of the blade) by a separation (or spanning) distance D which helps to reduce the likelihood of electric arc discharge (or “flashover”) between the first contact band 102 and the one or more metallic components located at the blade / hub interface. The first contact part 130 may be provided as a contact slider for sliding electrical contact with the first contact band 102. However, it will be appreciated that in other examples, the first contact part may be provided as a contact roller for rolling electrical contact with the first contact band 102 or as a contact brush or may have a different configuration for maintaining electrical contact with the first contact band 102.

[0070] The first contact part 130 may comprise a first contact surface 132 which may be pivotally mounted to the side of the first arm 140 facing the blade via a first bracket 134, such as shown in figure 3.

[0071] The first contact surface 132 may be made of a conducting, low-friction material (such as copper or stainless steel) which enables the first contact part 130 to slide along (and maintain an electrical contact with) the first contact band 102, e.g. during pitchwise rotation of the blade 7 relative to the hub 6.

[0072] The first arm 140 is configured to urge the first contact part 130 against the first contact band 102 to help ensure that an electrical contact is maintained between the first contact band 102 and the first contact part 130.

[0073] The first arm 140 comprises an elastic material such that, as the first arm 140 undergoes an elastic deformation, it applies a corresponding biasing force to the first contact part 130 thereby urging the first contact part 130 into electrical contact with the first contact band 102.

[0074] However, as well as applying a biasing force to the first contact part 130, as the first arm 140 is deflected during use, a corresponding bending stress is also applied onto the first arm 140.

[0075] The first arm 140 of the contact device 110 will now be described in greater detail, with reference to Figure 4. The first arm 140 has a first end 142 to which the mounting part 120 is attached, a second end 144, opposite to the first end 142, to which the contact device 130 is attached and a length (Y) extending longitudinally between the first end 142 and the second end 144 of the arm 140.

[0076] The first arm 140 also has a first edge 146 (which is the left-hand edge in the illustrated example), a second edge 148 (which is the right-hand edge in the illustrated example) opposite to the first edge 146 and a width (X) extending between the first edge 146 and the second edge 148 of the arm 140, transverse (or perpendicular) to the length (Y).

[0077] The width (W) of the first arm 140 tapers from the first end 142 towards the second end 144 such that a distance between the first 146 and second 148 edges at the first end 142 of the arm 140 is greater than a distance between the first 146 and second 148 edges at the second end 144 of the first arm 140.

[0078] Advantageously, it has been found that by tapering the first arm 140 between the first end and the second end, the stress distribution across the first arm 140 can be improved thereby resulting in lower peak stresses being experienced by the first arm 140 during use.

[0079] As shown in Figure 4, a plurality of fastener holes 143 may be provided at the first end 142 of the first arm 140 which are adapted to receive corresponding fasteners (not shown) so that the first end 142 of the first arm 140 can be bolted or otherwise fastened to the mounting part 120. Alternatively, a different form of connection (e.g., an adhesive connection) may be used for attaching the first end 142 of the first arm 140 to the mounting part 120.

[0080] The first arm 140 may also comprise a plurality of fastener holes 145 provided at the second end 144 of the first arm 140 which are adapted to receive corresponding fasteners (not shown) so that the second end 144 of the arm 140 can be bolted or otherwise fastened to the first contact part 130 (e.g. via the bracket 134). Alternatively, a different form of connection (e.g., an adhesive connection) may be used to attach the second end 144 of the first arm 140 to the first contact part 130.

[0081] As shown in Figure 5, the peak stress experienced by the first arm 140 having the taper when subjected to a bending stress of 80N was found to be in the region of 50 MPa, whereas traditional arm designs (having a rectangular, non-tapered shape) were found to experience peak stresses in the region of 400 MPa when subjected to the same loading. This reduction in peak stress provided by the tapering design of the first arm 140 enables the first arm 140 to be used on a variety of wind turbines of different sizes, providing scalability which traditional arm designs cannot provide due to their higher peak stresses. The lower operational stress may also correlate to increased operational lifetime with the tapered arm design. Furthermore, the aforementioned reduction of peak stresses can also be achieved without significantly increasing the weight of the contact device 110.

[0082] The tapered arm design also shows lower tolerance sensitivity to contact pressure as compared with the traditional arm design.

[0083] As shown in Figure 6, a contact pressure tolerance of + / - 15N can be attained within a deflection change of + / - 25mm for the traditional arm design, whereas the tapered arm design can achieve the same contact pressure tolerance of + / - 15N within a deflection tolerance range of + / - 35mm. This means that the tapered shape of the first arm 140 can accommodate larger deflections without increasing the contact pressure.

[0084] The first arm 140 is manufactured from a non-conducting material to help prevent the unwanted transfer of lightning currents from the first contact part 130 onto the part(s) of the wind turbine 1 to which the contact device 110 is mounted (e.g., the hub 6).

[0085] The first arm 140 may comprise a glass-fibre reinforced composite material having a plurality of fibre layers which are stacked one atop the other in a thickness direction.

[0086] The first arm 140 may be substantially planar and hence the first arm 140 may have a substantially uniform thickness dimension which is significantly less than its width (X) or length (Y) dimensions. The first arm 140 may have a thickness in the region of 4mm to 5mm (e.g. approximately 4.5mm), however it will be appreciated that in other examples, arms having different and / or non-uniform thicknesses may be envisaged.

[0087] It will be appreciated that in alternative examples, the first arm 140 may have a thickness which tapers from the first end 142 towards the second end 144 in addition to, or instead of, a width which tapers from the first end 142 towards the second end 144.

[0088] Advantageously, the provision of a first arm 140 having a constant thickness helps to improve the manufacturability of the arm. In other words, arms having a constant thickness are cheaper and easier to manufacture. However, the provision of a first arm 140 having a tapering thickness helps to further enhance the performance of the arm. Furthermore, it will also be appreciated that in other examples, the arm 140 may be manufactured from a different type of composite material such as aramid fibre reinforced composite or may be manufactured from another type of material such as an engineering polymer.

[0089] An exemplary lay-up pattern for the fibre layers which make up the first arm 140 is provided in Figure 7.

[0090] The plurality of fibre layers may be unidirectional fibre layers meaning that the fibres provided within each layer all extend in the same direction. However, it will be appreciated that in other examples, the first arm 140 may comprise one or more multidirectional fibre layers.

[0091] The plurality of fibre layers which make up the first arm 140 comprise at least one fibre layer having a plurality of fibres which are orientated in a first direction relative to a longitudinal axis (A-A) of the first arm 140, at least one fibre layer having a plurality of fibres which are orientated in a second direction relative to the longitudinal axis (A-A) of the first arm 140 and at least one fibre layer having a plurality of fibres which are orientated in a third direction relative to the longitudinal axis (A-A) of the first arm 140.

[0092] Advantageously, the provision of one or more fibre layers having fibres which are orientated in different directions helps to improve the resistance of the arm to multiple different loading types (e.g., bending, torsion, etc.).

[0093] The first arm 140 may comprise at least one fibre layer having a plurality of fibres orientated at an angle of approximately 0 degrees (i.e., parallel) relative to the longitudinal axis (A-A). Advantageously, the provision of one or more fibre layers having a plurality of fibres orientated parallel to the longitudinal axis (A-A) of the first arm 140 helps to improve the resistance of the first arm 140 to longitudinal loads.

[0094] The first arm 140 may also comprise at least one fibre layer having a plurality of fibres orientated at an angle of approximately 90 degrees (i.e., perpendicular) relative to the longitudinal axis (A-A). Advantageously, the provision of one or more fibre layers having a plurality of fibres orientated perpendicular to the longitudinal axis (A-A) of the first arm 140 helps to improve the bolt bearing strength of the first arm 140, e.g. at the fastener holes. The first arm 140 may also comprise at least one fibre layer having a plurality of fibres oriented at an angle of approximately + / - 45 degrees relative to the longitudinal axis (A-A). Advantageously, the provision of one or more fibre layers having a plurality of fibres orientated at an angle of + / - 45 degrees relative to the longitudinal axis (A-A) of the first arm 140 helps to improve the torsional performance of the first arm 140.

[0095] As shown in Figure 7, the first arm 140 may comprise two triaxial fabric layers having unidirectional (UD) fibre orientations of 0, +45 and -45 degrees respectively; and five biaxial fabric layers having unidirectional fibre orientations of 0 and 90 degrees respectively. The layup may be asymmetric about a mid-thickness plane of the first arm 140. The triaxial fabric layers may sandwich the biaxial fabric layers.

[0096] As such, in the example illustrated in Figure 7, the first arm 140 comprises 16 unidirectional (UD) fibre layers, 7 having a plurality of fibres orientated at an angle of approximately 0 degrees (i.e. , parallel) relative to the longitudinal axis (A-A), 5 having a plurality of fibres orientated at an angle of approximately 90 degrees (i.e., perpendicular) relative to the longitudinal axis (A-A) and 4 having a plurality of fibres orientated at an angle of approximately + / - 45 degrees relative to the longitudinal axis. The fibre orientations may therefore be balanced.

[0097] However, it will be appreciated that in other examples, the first arm may comprise a different number of fibre layers and / or may comprise fibre layers which are orientated in different directions (such as 30 degrees and 60 degrees).

[0098] Furthermore, whilst in the illustrated example the fibres which make up the plurality of fibre layers are orientated in three different directions (0, 45 and 90), in other examples fibre layer stacks having a different number of fibre directions such as 1 , 2, 4, 5 etc. UD fibre orientations may also be envisaged.

[0099] In the illustrated example, the plurality of fibre layers may be initially provided as dry fibre layers and once the dry fibre layers have been arranged in a stack, they may be impregnated with a resin matrix and cured to form the first arm 140.

[0100] In the illustrated example, the plurality of fibre layers may be impregnated and cured via vacuum-assisted resin transfer moulding (or VARTM) although it will be appreciated that in other examples, other methods (such as resin transfer moulding or pultrusion) may be used.

[0101] Furthermore, it will also be appreciated that in some examples, the plurality of fibre layers may be pre-impregnated with resin (i.e., the plurality of fibre layers may be prepreg fibre layers) and hence in some examples the step of impregnating the dry fibres with resin may be performed before the fibre layers have been stacked. After lay-up, the stack of pre-preg fibre layers may be cured using an autoclave or the like.

[0102] Returning to figures 1 and 2, it can be seen that the hub 6 of the rotor 5 is rotatably mounted about the nacelle 2, e.g. by a main shaft (not shown in figure 2). The main shaft is housed within the nacelle 2 such that the hub 6 (which is mounted to the main shaft) is able to rotate relative to the nacelle 2 about a horizontal axis which extends through the centre of the hub 6. As such, the nacelle 2 may be considered an example of a third wind turbine relative to which the second wind turbine part (e.g., the hub 6) can rotate.

[0103] The third wind turbine part (e.g. the nacelle 2) may have mounted thereto a second contact band 104. However, it will be appreciated that in other examples, the second contact band 104 may be mounted to other parts of the wind turbine 1 such as the hub 6 or one of the blades 7.

[0104] The second contact band 104 may be made of a conducting material (e.g., copper) similar to the first contact band 102. The second contact band 104 may be electrically connected to the second down conductor 10 such that lightning currents passing through the second contact band 104 can be transferred onto the second down conductor 10 and subsequently onto the ground potential 11 so that they can be safely discharged.

[0105] The second contact band 104 may be mounted to an exterior surface of the nacelle 2, adjacent to the hub 6, such as shown in figure 2. However, unlike the first contact band 102 which is circumferentially mounted about an exterior surface of the blade 7, the second contact band 104 is mounted to the nacelle 2 in a substantially vertical orientation. The contact device 110 may further comprise a second arm 160 substantially the same as the first arm 140. The second arm 160 has a first end 162 and a second end 164 opposite the first end. The first end 162 of the second arm 160 is attached to the first mounting part 120 (or to a separate second mounting part for mounting to the second wind turbine part (e.g. the hub 6) and it may be adjacent the first mounting part 120).

[0106] The second arm 160 is elastic and non-conducting. A second contact part 150 is attached to the second end 164 of the second arm 160, the second contact part 150 is electrically connected to the first contact part 130. The contact device 110 is further configured such that the second contact part 150 faces the second contact band 104 and such that the second arm 160 urges the second contact part 150 against the second contact band 104 for providing lightning current transfer between the second contact band 104 and the second contact part 150.

[0107] The second arm 160 has a length extending between the first end 162 and the second end 164, a width which tapers from the first end towards the second end, and a thickness which is significantly less than the width or length dimensions.

[0108] However, it shall also appreciated that whilst the illustrated example depicts a contact device 110 in which the second arm 160 has a width which tapers from the first end 162 towards the second end 164 and a substantially uniform thickness, it shall be appreciated that in alternative examples, the first arm may have a thickness which tapers from the first end 162 towards the second end 164 in addition to, or instead of, a width which tapers from the first end 162 towards the second end 164.

[0109] Advantageously, the provision of a second arm 160 having a constant thickness helps to improve the manufacturability of the arm. In other words, arms having a constant thickness are cheaper and easier to manufacture. However, the provision of a second arm 160 having a tapering thickness helps to further enhance the performance of the arm.

[0110] The second arm 160 ensures that an electrical contact can be maintained between the second contact band 104 and the contact device 110 as the hub 6 rotates relative to the nacelle 2 about a horizontal axis (B-B) defined through the centre of the hub 6. As shown in Figure 3, the contact device 110 includes a first mounting part 120 for mounting to a surface of the second part of the wind turbine (e.g., the hub 6) to which a pair of arms 140,160 are attached.

[0111] In the example illustrated in Figure 2, the first mounting part 120 is depicted as being mounted to a surface provided at a flange portion 6a of the hub 6.

[0112] The first mounting part 120 may include a support plate 122 which is mounted via a bolted connection to the flange portion 6a of the hub 6. The support plate 122 may also carry a pair of brackets 124, 126 to which the first 140 and second 160 arms are attached.

[0113] As shown in Figure 3, the pair of mounting brackets 124, 126 are each mounted to the support plate 122 in opposite orientations. The first 130 and second 150 contact parts are attached to oppositely facing sides of the first 140 and second arms 160 and hence the first 130 and second 150 contact parts face away from one another. As such, when the contact device 110 is installed between the first 102 and second 104 contact bands (as shown in Figure 2), the first contact part 130 is arranged to face the first contact band 102 and the second contact part 150 is arranged to face the second contact band 104 such that an electrical contact can be formed between the first contact part 130 and the first contact band 102 and between the second contact part 150 and the second contact band 104.

[0114] In the illustrated example, the first 130 and second 150 contact parts are provided as a pair of contact sliders. However, it will be appreciated that in other examples, the first and second contact parts may be provided as first and second contact rollers or brushes or may have an entirely different configuration.

[0115] As shown in Figure 3, in the illustrated example the first contact part 130 comprises a first contact surface 132 which may be pivotally mounted to the side of the first arm 140 facing the first part of the wind turbine (e.g. the blade 7) via a first bracket 134 and the second contact part 150 similarly comprises a second contact surface 152 mounted to the side of the second arm 160 facing the third part of the wind turbine (e.g. the nacelle 2) via a second bracket 154. The features and operation of the first arm 140 having the first contact part 130 are the same as the features and operation of the second arm 160 having the second contact part 150 acting on their respective first and second contact bands 102, 104 and will not be repeated.

[0116] The first contact part 130 is electrically connected to the second contact part 150 via an electrical connector 170 which is configured for transferring lightning currents across the contact device 110 from the first contact part 130 to the second contact part 150.

[0117] In the illustrated example, the electrical connector 170 is provided as an electrical cable which spans across the width of the contact device 110 between a rear surface of the first contact part 130 and a rear surface of the second contact part 150. However, it will be appreciated that in alternative examples, other types of electrical connector may be used to electrically connect the first 130 and second 150 contact parts.

[0118] Although only the first arm 140 is shown in Figure 4, it will be appreciated that the second arm 160 has a substantially identical configuration to that of the first arm 140 and so, for the sake of conciseness, the second arm 160 will not be separately described. However, like components of the second arm 160 (shown in Figure 3) have been denoted with corresponding reference numerals.

[0119] As the length of the grounding connection formed by the first and second down conductors continues to increase (due to modern wind turbines featuring longer blades and larger hubs I nacelles), so too does their respective impedance.

[0120] This increase in impedance also increases the voltage drop between the down conductors 9, 10 of the first and second wind turbine parts, thereby making the connections between such down conductors 9, 10 more prone to electric arc discharge (or “flashover”).

[0121] Therefore, the contact bands 102, 104 of modern lightning current transfer systems need to be spaced further and further way from the interfaces between adjacent wind turbine parts (such as the blade / hub interface and / or the hub / nacelle interface). In other words, at higher impedances, the separation (or spanning) distance D must be increased.

[0122] This in turn requires the lengths of the arms 140, 160 of the contact device(s) to be increased so that they can span across these extra distances which subsequently increases the bending stresses which are exerted on the arms 140, 160 during operation thereby adversely affecting their operational life.

[0123] A lighting transfer system 200 according to an alternative example will now be described with reference to Figure 8.

[0124] It will be appreciated that the example illustrated in Figure 8 has many features in common with the example illustrated in Figures 2 and 3 and hence only the differences will be described in detail below.

[0125] The lighting current transfer system 200 includes a first contact band 202 which is mounted to the first wind turbine part (e.g., the blade 7) and a second contact band 204 which is mounted to the third wind turbine part (e.g., the hub 6).

[0126] The first contact band 202 is made of a conducting material (e.g., copper) and may be electrically connected to the first down conductor 9 such that lightning currents passing along the first down conductor 9 can be transferred onto the first contact band 202.

[0127] As with the first contact band 202, the second contact band 204 is also made of a conducting material (e.g., copper) and may be electrically connected to a second down conductor 10 mounted to the second wind turbine part (e.g., the hub 6) such that lightning currents passing along the second down conductor 10 can be transferred onto the second contact band 204.

[0128] In the illustrated example, the first contact band 202 is mounted to the blade 7 and the second contact band 204 is mounted to the hub 6. More particularly, in the illustrated example the second contact band 204 is mounted to an exterior surface of the hub 6 adjacent to the nacelle 2.

[0129] However, it will be appreciated that in other examples, the first 202 and second 204 contact bands may be mounted to other parts of the wind turbine such as the hub 6 or the nacelle 2 in the case of the first contact band 202 and the nacelle 2 or one of the blades 7 in the case of the second contact band 204.

[0130] The lightning current transfer system 200 also includes a pair of contact devices 210, 310. The first contact device 210 includes a mounting part 220 for mounting to the first part of the wind turbine (e.g., the hub 6), a first contact part 230 for forming an electrical connection with the first contact band 202 (on the blade 7 or second part of the wind turbine) and a first arm 240 having a first end which is attached to the mounting part 220 and a second end which is attached to the contact part 230 for urging the contact part 230 against the first contact band 202 so as to form an electrical contact for transferring lightning currents between the first contact band 202 and the contact part 230.

[0131] It will be appreciated that the first arm 240 and the first contact part 230 illustrated in Figure 8 are of a substantially identical construction to those described above with reference to Figures 2 to 4 and so, for the sake of conciseness, will not be described again in detail.

[0132] In particular, in the example of Figure 8 the first arm 240 has a width which tapers from its first end towards its second end to help reduce the magnitude of bending stresses which are applied to the arm during use (as has been described in detail above).

[0133] However, it shall also appreciated that whilst the illustrated example depicts a first contact device 210 in which the first arm 240 has a width which tapers from the first end towards the second end and a substantially uniform thickness, it shall be appreciated that in alternative examples, the first arm 240 may have a thickness which tapers from the first end towards the second end in addition to, or instead of, a width which tapers from the first end towards the second end.

[0134] Advantageously, the provision of a first arm 240 having a constant thickness helps to improve the manufacturability of the arm. In other words, arms having a constant thickness are cheaper and easier to manufacture. However, the provision of a first arm 140 having a tapering thickness helps to further enhance the performance of the arm.

[0135] The second contact device 310 includes a second mounting part 320 for mounting to the third wind turbine part (e.g., the nacelle 2), a second contact part 330 for forming an electrical connection with the second contact band 204 and a second arm 340 having a first end which is attached to the second mounting part 320 and a second end which is attached to the second contact part 330 for urging the second contact part 330 against the second contact band 204 so as to form an electrical contact for transferring lightning currents between the second contact band 204 and the second contact part 330.

[0136] As such, in the example illustrated in Figure 8, each arm 240, 340 and associated contact part 230, 340 are provided on their own respective mounting parts spaced from each other, unlike in the figure 2 and 3 arrangement in which the mounting parts are either common or are separate but adjacent.

[0137] As shown in Figure 8, in the illustrated example, the second contact band 204 has a substantially ring-shaped structure which is mounted to the hub 6 such that the contact band 204 extends around the rotor shaft 14 connection on the hub 6, and the horizontal axis (B-B) about which the rotor shaft 14 and the hub 6 rotates. In other words, the second contact band 204 is mounted to the hub 6 such that the rotor shaft 14 and the horizontal axis (B-B) about which the rotor shaft 14 and the hub 6 rotates passes through a space defined at the centre of the second contact band 204.

[0138] This configuration enables an electrical contact can be maintained between the second contact band 204 and the second contact device 310 during horizontal rotation of the hub 6 relative to the nacelle 2.

[0139] Furthermore, as shown in Figure 8, the down conductor 10 which is mounted to the hub 6 has a first end which is electrically connected to the contact part 230 of the first contact device 210 and a second end which is electrically connected to the second contact ring 204 such that lightning currents are able to pass from the first contact part 230 to the second contact part 330 via the down conductor 10 and the second contact ring 204. In other words, in the example illustrated in Figure 8, the first 230 and second 330 contact parts are also electrically connected.

[0140] It will also be appreciated that the second arm 340 and the second contact part 330 illustrated in Figure 8 are of a substantially identical construction to those described above with reference to Figures 2 to 4 and so, for the sake of conciseness, will not be described again in detail.

[0141] In particular, the example of Figure 8 the second arm 340 has a width which tapers from its first end towards its second end to help reduce the magnitude of bending stresses which are applied to the arm during use (as has been described in detail above).

[0142] However, it shall also appreciated that whilst the illustrated example depicts a second contact device 310 in which the second arm 340 has a width which tapers from the first end towards the second end and a substantially uniform thickness, it shall be appreciated that in alternative examples, the second arm may have a thickness which tapers from the first end towards the second end in addition to, or instead of, a width which tapers from the first end towards the second end.

[0143] Advantageously, the provision of a second arm 340 having a constant thickness helps to improve the manufacturability of the arm. In other words, arms having a constant thickness are cheaper and easier to manufacture. However, the provision of a second arm 340 having a tapering thickness helps to further enhance the performance of the arm.

[0144] Although the present disclosure has been described above with reference to one or more preferred examples, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.

Claims

CLAIMS1. A lightning current transfer system for a wind turbine, the lightning current transfer system comprising: a first contact band mountable to a first wind turbine part adapted to rotate relative to a second wind turbine part; and a contact device mountable to the second wind turbine part, the contact device comprising: a first mounting part for mounting to the second wind turbine part; a first arm having a first end attached to the first mounting part and a second end opposite the first end, the first arm being elastic and nonconducting; and a first contact part attached to the second end of the first arm, wherein the contact device is configured such that the first contact part faces the first contact band and such that the first arm urges the first contact part against the first contact band for providing lightning current transfer between the first contact band and the first contact part, wherein the first arm has a length extending between the first end and the second end, and wherein the first arm tapers from the first end towards the second end.

2. The system according to claim 1 , wherein the system further comprises a second contact band mountable to a third wind turbine part, said third wind turbine part being adapted to rotate relative to the second wind turbine part, and wherein the contact device further comprises: a second arm having a first end and a second end opposite the first end, the first end of the second arm is attached to the first mounting part or to a second mounting part for mounting to the second wind turbine part, and the second arm being elastic and non-conducting; and a second contact part attached to the second end of the second arm, said second contact part being electrically connected to the first contact part, wherein the contact device is further configured such that the second contact part faces the second contact band and such that the second arm urges the second contact part against the second contact band for providing lightning current transfer between the second contact band and the second contact part,wherein the second arm has a length extending between the first end and the second end, and wherein the second arm tapers from the first end towards the second end.

3. The system according to claim 1 , wherein the system further comprises a second contact band mountable to the second or a third wind turbine part, said third wind turbine part being adapted to rotate relative to the first and / or second wind turbine part, wherein the contact device is a first contact device, and wherein the system further comprises a second contact device mountable to one of the first, second or third wind turbine parts, the second contact device comprising: a second mounting part for mounting to another of the first, second or third wind turbine parts; a second arm having a first end attached to the second mounting part and a second end opposite the first end, the second arm being elastic and non-conducting; and a second contact part attached to the second end of the second arm, said second contact part being electrically connected to the first contact part; wherein the contact device is configured such that the second contact part faces the second contact band and such that the second arm urges the second contact part against the second contact band for providing lightning current transfer between the second contact band and the second contact part, wherein the second arm has a length extending between the first end and the second end, and wherein the second arm tapers from the first end towards the second end.

4. The system according to any preceding claim, wherein the first and / or second arm has a width which tapers from the first end towards the second end.

5. The system according to any preceding claim wherein the first and / or second arm has a substantially trapezoidal shape.

6. The system according to any preceding claim, wherein the first and / or second arm comprises a fibre-reinforced composite material, and preferably a glass-fibre reinforced composite material.

7. The system according to claim 6, wherein the first and / or second arm comprises a plurality of fibre layers, said layers being stacked one atop the other in a thickness direction.

8. The system according to claim 7, wherein the plurality of fibre layers are unidirectional fibre layers.

9. The system according to claims 7 or 8, wherein the first and / or second arm comprises at least one fibre layer having a plurality of fibres orientated in a first direction relative to a longitudinal axis of the first and / or second arm, and at least one fibre layer having a plurality of fibres orientated in a second direction relative to a longitudinal axis of the first and / or second arm, said second direction being different to the first direction.

10. The system according to any of claims 7 to 9, wherein the first and / or second arm comprises at least one fibre layer having a plurality of fibres orientated at an angle of approximately 0 degrees relative to the longitudinal axis of the first and / or second arm.11 . The system according to any of claims 7 to 10, wherein the first and / or second arm comprises at least one fibre layer having a plurality of fibres orientated at an angle of approximately 45 degrees relative to the longitudinal axis of the first and / or second arm.

12. The system according to any of claims 7 to 11 , wherein the first and / or second arm comprises at least one fibre layer having a plurality of fibres orientated at an angle of approximately 90 degrees relative to the longitudinal axis of the first and / or second arm.

13. The system according to any preceding claim, wherein the first and / or second contact part comprises a contact slider or roller or brush.

14. The system according to any preceding claim, wherein the first and / or second arm has a thickness which is significantly less than the width or length dimensions15. The system according to any preceding claim, wherein the first and / or second arm has a substantially uniform thickness.

16. A wind turbine comprising a first wind turbine part; a second wind turbine part, said first wind turbine part being adapted to rotate relative to the second wind turbine part; a first down conductor; and the lighting current transfer system according to any preceding claim, wherein the first contact band is mounted to the first wind turbine part and is electrically connected to the first down conductor; and wherein the first contact device is mounted to the second wind turbine part.

17. The wind turbine according to claim 16, wherein the first wind turbine part is one of a wind turbine blade, a wind turbine nacelle or a wind turbine hub.

18. The wind turbine according to claim 16 or 17, wherein the second wind turbine part is one of a wind turbine blade, a wind turbine nacelle or a wind turbine hub.

19. The wind turbine according to any of claims 16 to 18, when dependent on claims 2 or 3, wherein the wind turbine further comprises: a third wind turbine part, said third wind turbine part being adapted to rotate relative to the first and / or second wind turbine part, and a second down conductor; wherein the second contact band is mounted to one of the first, second third wind turbine parts and is electrically connected to the second down conductor, and wherein the second contact device is mounted to another of the first, second or third wind turbine parts.

20. The wind turbine according to claim 19, wherein the third wind turbine part is one of a wind turbine blade, a wind turbine nacelle, or a wind turbine hub.

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