Lightning protection system

Abstract

A pitch controlled wind turbine has a tower, a nacelle mounted on the tower, a hub mounted rotatably on the nacelle, and at least three wind turbine blades. Each wind turbine blade extends between a root end connected to the hub via a pitch mechanism, and a tip end. The wind turbine also has at least three blade connecting members, each blade connecting member extending from a connection point on one wind turbine blade towards a connection point on a neighbouring wind turbine blade, where the connection point on a given wind turbine blade is arranged at a distance from the root end and at a distance from the tip end of the wind turbine blade. The wind turbine also has at least three pre-tension members, each pre-tension member being connected to one of the blade connecting members and to the hub via a tensioning device, the tensioning device provides radial movement of a radially inward end of the pre-tension member with respect to an axis of rotation of the hub due to extension or retraction of the tensioning device, each pre-tension member thereby providing pre-tension in the blade connecting member to which it is connected. A lightning protection system protects each of the tensioning devices from lightning current and provides a lightning current path towards ground.

Description

[0001] LIGHTNING PROTECTION SYSTEM

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a pitch controlled wind turbine having a lightning protection system.

[0004] BACKGROUND OF THE INVENTION

[0005] Wind turbines are susceptible to lightning strikes. To protect the wind turbine, a lightning protection system couples various parts of the wind turbine, especially the wind turbine blades, to ground. Typically, the lightning protection system includes lightning receptors and conductors that are electrically connected from the tip of the blade, through the nacelle and tower, to ground.

[0006] WO 2022 / 128040 teaches a pitch controlled wind turbine comprising a tower, a nacelle mounted on the tower, a hub mounted rotatably on the nacelle, and at least three wind turbine blades connected to the hub via pitch mechanisms. The wind turbine further comprises at least three blade connecting members, each blade connecting member extending between a connection point on one wind turbine blade and a connection point on a neighbouring wind turbine blade. Pre-tension members coupled between each blade connecting member and the hub apply a tensile load in the blade connecting members which enables the wind turbine blades to mutually support each other, in the sense that loads on the wind turbine blades, in particular edgewise loads and flapwise (also known as flatwise) loads, are ‘shared’ among the wind turbine blades. This relieves loads on the blade structure inboard of the connection point, meaning that the blade structure inboard of the connection point can be designed for lower loads. The blade root diameter or reinforcement of the inboard blade portion can be reduced, achieving weight savings. This may also enable a smaller hub, smaller loads etc. achieving yet further mass savings for a given rotor diameter. Additionally or alternatively the rotor diameter may be increased without increasing the mass of the blades or hub. These load sharing components present new challenges in the design, manufacture and maintenance of wind turbines having a lightning strike protection system.

[0007] It is against this background that the invention has been devised.

[0008] SUMMARY OF THE INVENTION A first aspect of the invention provides a pitch controlled wind turbine comprising a tower, a nacelle mounted on the tower, a hub mounted rotatably on the nacelle, and at least three wind turbine blades, wherein each wind turbine blade extends between a root end connected to the hub via a pitch mechanism, and a tip end; the wind turbine further comprising at least three blade connecting members, each blade connecting member extending from a connection point on one wind turbine blade towards a connection point on a neighbouring wind turbine blade, where the connection point on a given wind turbine blade is arranged at a distance from the root end and at a distance from the tip end of the wind turbine blade; at least three pre-tension members, each pre-tension member being connected to one of the blade connecting members and to the hub via a tensioning device, the tensioning device provides radial movement of a radially inward end of the pre-tension member with respect to an axis of rotation of the hub due to extension or retraction of the tensioning device, each pre-tension member thereby providing pre-tension in the blade connecting member to which it is connected; and a lightning protection system for protecting each of the tensioning devices from lightning current and providing a lightning current path towards ground.

[0009] The lightning current path may extend from adjacent the radially inward end of each pre-tension member towards ground. The lightning current path may be electrically isolated from each tensioning device.

[0010] Each tensioning device may comprise an actuator having a first portion coupled to the hub and a second portion movable with respect to the first portion and coupled to the respective pre-tension member.

[0011] The lightning protection system may include a lightning receptor mounted to the second portion of the actuator adjacent the pre-tension member and electrically coupled to the lightning current path.

[0012] The lightning protection system may include a lightning receptor mounted to the first portion of the actuator nearest the second portion and electrically coupled to the lightning current path.

[0013] Preferably, the lightning receptor is an antenna. The lightning current path may include a flexible element to accommodate movement of the second portion of the actuator with respect to the first portion. The flexible element may allow for extension and retraction of the tensioning device whilst maintaining the lightning path protecting the tensioning devices from lightning current.

[0014] The actuator may be a linear actuator. The actuator may have a cylinder and a rod moveable into and out of the cylinder. The first portion of the actuator may include the cylinder, and the second portion of the actuator may include the rod.

[0015] The actuator may be a rotary actuator. The actuator may comprise a motor and a tension element, wherein a radially distal end of the tension element is connected to the radially inward end the pre-tension element, and a radially inward end of the tension element is wound around a drum connected to a rotatable shaft of the motor. The first portion of the actuator may include the motor, and the second portion of the actuator may include the tension element.

[0016] The lightning protection system may include at least one conductive cable providing at least a portion of the lightning current path.

[0017] A portion of the lightning current path may substantially follow the respective tensioning device.

[0018] A portion of the lightning current path may extend a shortest distance between the radially inward end of each pre-tension member and the hub.

[0019] The tensioning devices may be coupled to a projection extending forward of the hub in an upwind direction of the wind turbine.

[0020] A spinner coupled to the hub may cover a radially inner portion of each tensioning device.

[0021] A portion of the lightning current path may extend inside the spinner.

[0022] The lightning protection system may include a lightning receptor array extending over the tensioning devices and electrically connected to the lightning current path. The lightning receptor array may be provided additionally or instead of the lightning receptor mounted to the first and / or second portions of the actuator.

[0023] The lightning receptor array is shaped to provide lightning protection of all of the tensioning devices.

[0024] Preferably, the shape of the lightning receptor array is designed according to a rolling sphere method.

[0025] BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

[0027] Figure 1 shows a front view of a wind turbine according to a first example;

[0028] Figure 2 shows a side view of the wind turbine;

[0029] Figure 3 shows a blade of the wind turbine;

[0030] Figure 4 shows a detail front view of lightning protection system components adjacent the hub of the wind turbine;

[0031] Figures 5 and 5A show a partial side views of lightning protection system components adjacent the hub of the wind turbine;

[0032] Figures 6 and 7 show alternative lightning receptors for use with the lightning protection system;

[0033] Figure 8 shows an example of a flexible element of the lightning protection system;

[0034] Figures 9 and 10 show movement of the lightning protection system components;

[0035] Figure 11 shows a first variant of the lightning protection system;

[0036] Figure 12 shows a second variant of the lightning protection system;

[0037] Figures 13 to 15 show detail views of electrical connections in the lightning protection;

[0038] Figure 16 shows a third variant of the lightning protection system;

[0039] Figure 17 shows a fourth variant of the lightning protection system; and

[0040] Figure 18 shows a fifth variant of the lightning protection system.

[0041] DETAILED DESCRIPTION OF EMBODIMENT(S)

[0042] In this specification, terms such as leading edge, trailing edge, pressure surface, suction surface, thickness, and chord 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] The term outboard refers to a radial direction from the hub of the blade towards the tip end of the blade. The term inboard refers to a radial direction from the tip end towards the hub.

[0050] Figures 1 and 2 show a pitch controlled wind turbine 1 according to a first example. Figure 1 is a front view of the wind turbine 1 , and Figure 2 is a side view of the wind turbine 1. The wind turbine 1 comprises a tower 2 and a nacelle 3 mounted on the tower 2. A hub 4 is mounted rotatably on the nacelle 3, and carries three wind turbine blades 5 projecting outwardly from the nacelle 3. While the example shown in Figures 1 and 2 has three blades 5, it will be appreciated that other numbers of blades 5 are possible.

[0051] When wind blows against the wind turbine 1 , the wind turbine blades 5 generate a lift force which causes a generator (not shown) within the nacelle 3 to generate electrical energy.

[0052] 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 may be an onshore wind turbine such that the foundation is embedded in the ground, or the wind turbine 1 may be an offshore installation in which case the foundation would be provided by a suitable marine platform.

[0053] Three blade connecting members 6 interconnect neighbouring wind turbine blades 5 between connection points 7a, 7b on the wind turbine blades 5. The connecting members 6 are cables, e.g. metallic (e.g. steel) or polymer (for example comprising high density polyethylene - HDPE).

[0054] A pre-tension member 8 may extend between one of each of the blade connecting members 6 and a common point 17 arranged adjacent the hub 4, as best shown in Figures 4 and 5. The pre-tension members 8 are configured to provide pre-tension in the blade connecting members 6.

[0055] The pre-tensioned blade connecting members 6 cause the wind turbine blades 5 to mutually support each other, in the sense that loads on the wind turbine blades 5, in particular edgewise loads and flapwise loads, are ‘shared’ among the wind turbine blades 5.

[0056] The pre-tension members 8 are coupled to the hub 4 by respective tensioning devices 18. Each tensioning device 18 provides radial movement of a radially inward end 19 of the pre-tension member 8 with respect to an axis of rotation 20 of the hub due to extension or retraction of the tensioning device 18. Each pre-tension member 8 thereby provides pre-tension in the blade connecting member 6 to which it is connected. The common point 17 for connection to the pre-tension members 8 is forward of the positions where the wind turbine blades 5 are connected to the hub 4. This has the consequence that the pre-tension members 8 may also pull the blade connecting members 6 away from the hub 4 and away from the tower 2. This may also cause the wind turbine blades 5 to be pulled in this direction, thereby further reducing edgewise and flapwise loads at the root of the wind turbine blades 5 and securing tower clearance, similar to what is obtained when a coning angle is introduced. Due to the use of connecting members 6, this has been found to tend to lead to increased stiffness in the inner part of the blades 5.

[0057] The tensioning devices 18 are connected at a radial inner end thereof to the common point 17. The common point 17 may provide a pivotal connection to each of the tensioning devices 18. The common point 17 may be on a projection 21 extending forward of the hub 4 in an upwind direction of the wind turbine. The projection may be a framework or structure rigidly coupled to the hub 4. A spinner 9 may be coupled to the hub to cover a radially inner portion of each tensioning device 18. The spinner 9 may shroud the common point 17. The spinner 9 may be made of glass fibre reinforcement polymer and the spinner shell is not intended to provide lightning strike protection to components within the spinner.

[0058] As shown in Figure 3, the wind turbine blades 5 have a root end 11 proximal to the hub 4, adapted to be connected to the hub 4 via a pitch mechanism 50, and a tip end 12 distal from the hub 4. The blades 5 include a leading edge 13 and a trailing edge 14 that extend between the respective root end 11 and tip end 12. The blades 5 include a suction side 15 and a pressure side 16. A thickness dimension of the blade 5 extends between the suction side 15 and the pressure side 16.

[0059] Each blade 5 may have a cross section which has a substantially circular profile near the root end 11. The blade 5 may transition from a circular profile to an aerofoil profile moving from the root end 11 of the blade 5 outboard. The blade 5 may comprise a "shoulder" 28 outboard of the root end 11 , which is the widest part of the blade where the blade 5 has its maximum chord. The blade 5 may have an aerofoil profile of progressively decreasing thickness in an outboard portion of the blade. The progressively decreasing thickness may extend from the shoulder 28 to the tip end 12. The connecting points 7a, 7b may be between 10% and 60% of the length of the wind turbine blade 5 from the root end 11 to the tip end 12 in the radial direction but are preferably radially inboard of 50% of the length of the wind turbine blade 5 from the root end 11 to the tip end 12, and more preferably radially inboard of 45% of the length of the wind turbine blade 5 from the root end 11 to the tip end 12, e.g. around 35-40%.

[0060] The connecting points 7a, 7b may be arranged forward of the leading edge 13 and adjacent the pressure side 16. This provides additional clearance for the connecting members 6 as the wind turbine blades 5 rotate with the hub 4 about the nacelle 3. In particular, sufficient clearance is provided between the connecting members 6 and the blades 5 when the blades 5 are pitched between about -5 degrees and about +95 degrees.

[0061] Each of the blades 5 may be a split blade formed of a first blade portion 22 and an outboard blade portion 24 coupled together, such as shown in Figure 3. Each blade portion 22, 24 has a shell that defines a respective leading edge 30a, 30b, trailing edge 32a, 32b, suction side 34a, 34b, and pressure side 36a, 36b.

[0062] The inboard portion 22 and outboard portion 24 of each blade 5 may be connected at a connection joint indicated by connection line 40. The connection line 40 between the inboard and outboard blade portions 22, 24 may be a spanwise split, with the connection line 40 being chordwise. The inboard blade portion 22 extends from the blade root 11 to the connection line 40. The outboard blade portion 24 extends from the blade connection line 40 to the blade tip 12.

[0063] It will be appreciated that the blade 5 may have any number of blade portions 22, 24, with respective connection joints between them. Alternatively, the blades 5 may not be split blades and may instead extend continuously from the root end 11 to the tip end 12 without any connection joint.

[0064] It will be appreciated that the connection points 7a, 7b may be adjacent each other. Alternatively, the connection points 7a, 7b may be spaced from one another.

[0065] The connection points 7a, 7b may permit at least some freedom of movement of the connecting members 6 at its respective connection point 7a, 7b. For example, the connection points 7a, 7b may permit rotation of each blade connecting member 6 about the respective connection point 7a, 7b in two orthogonal rotational degrees of freedom. This allows each connecting member 6 to move independently of each other, thereby reducing constraints on the wind turbine 1 .

[0066] The wind turbine 1 includes a lightning protection system. The lightning protection system may include a down conductor, e.g. a down conductor cable and / or surface protection layer, in each blade 5. A surface protection layer may for instance be a metal mesh or foil surface protection layer incorporated into the blade shell at the outer surface of the blade and extending along at least a portion of the blade. The surface protection layer intercepts lightning strikes before reaching conductive components of the blade and is typically connected to the lightning strike protection system at numerous points so as to ensure a good electrical connection from the surface protection layer. The lightning strike protection system is arranged to ground via a suitable energy handling mechanism via the hub 4, the nacelle 3 and the tower 2. The lightning protection system may protect one or more of the hub structure, the main shaft or the main bearing of the wind turbine. A lightning path for carrying lightning current between the blades 5 and the nacelle 3 may be similar to that described in W02005 / 050008. In particular, the nacelle 3 may comprise stationary lightning protection parts, and the blades 5 (forming part of the rotor of the wind turbine) may comprise rotating lightning protection parts. The stationary and rotating lightning protection parts comprise contact surfaces connected by a lightning connector. The lightning connector comprises at least two contacts adapted for connecting the rotating and stationary lightning protection parts. The lightning connector further comprises at least one electric conductor establishing a dedicated connection between the at least two contacts and a force transferring part for the at least two contacts establishing a continuous connection between the at least two contacts and the contact surfaces of said stationary and rotating lightning protection parts. A part of the lightning current transfer unit (LCTLI) 10 for carrying lightning current between the blades 5 and the nacelle 3 is shown schematically in figure 5.

[0067] In addition to ‘sharing’ the loads among the wind turbine blades 5, the connecting members 6 and / or the pre-tension members 8 can also form part of the lightning protection system, as will be discussed below.

[0068] Whether or not the connecting members 6 and / or the pre-tension members 8 also form part of the lightning protection system, the tensioning devices 18 could be susceptible to lightning strikes. Therefore, the lightning protection system has been devised to protect each of the tensioning devices 18 from lightning current.

[0069] As best shown in figures 4, 5 and 5A, each of the tensioning devices 18 comprise an actuator 51 having a first portion 52 coupled to the hub 4 and a second portion 53 movable with respect to the first portion 52 and coupled to the respective pre-tension member 8. The tensioning device 18 extends and retracts by movement of the second portion 53 with respect to the first portion 52. Extension and retraction of the tensioning device 18 changes the tension in pre-tension member 8. The actuator 51 may be a linear actuator, e.g. having a cylinder and a rod moveable into and out of the cylinder. Alternatively, the actuator may be a rotary actuator, e.g. having a motor and a tension element, wherein a radially distal end of the tension element is connected to the radially inward end the pre-tension element, and a radially inward end of the tension element is wound around a drum connected to a rotatable shaft of the motor. The actuator may be hydraulic, electro-mechanical, pneumatic or a combination thereof, for example.

[0070] Where the actuator is a linear actuator, the first portion 52 of the actuator may include the cylinder and the second portion 53 of the actuator may include the rod, such as shown in Figure 5. Where the actuator 51 is a rotary actuator including the motor and the tension element, such as shown in Figure 5A, the first portion 52 of the actuator may include the drum 52a, shaft 52b and motor 52c, and the second portion 53 of the actuator may include the tension element, such as a belt 53a, which is wound upon the drum. The belt 53a may be coupled to the pre-tension member 8 by a thimble connection. The tensioning device 18 may further include electrical components such as power and control components. Protecting the tensioning devices 18 from lightning current aims to avoid damage to the electrical and electronic components.

[0071] The lightning protection system may provide a lightning current path which extends from adjacent the radially inward end 19 of each pre-tension member 8 towards ground. The lightning current path may be electrically isolated from each tensioning device 18. The lightning protection system may include a lightning receptor 54 mounted to the second portion 53 of the actuator 51 adjacent the pre-tension member 8. The lightning receptor 54 may be electrically coupled to the lightning current path. The lightning protection system may include at least one conductive cable 55 providing at least a portion of the lightning current path. For example, as shown in figures 5 and 5A, the lightning receptor 54 is mounted to the radially outboard end of the second portion 53 of the actuator 51 and is electrically connected to conductive cable 55 forming part of the lightning current path. The conductive cable 55 is electrically isolated from all parts of the tensioning device 18. Similarly, the lightning receptor 54 may be electrically isolated from all parts of the tensioning device 18.

[0072] Due to extension and retraction of the tensioning device 18, the length of the lightning current path, such as provided by conductive cable 55, may change. This is especially the case where the lightning receptor 54 is mounted to the second portion 53 of the actuator 51 . The lightning protection system may therefore comprise a flexible element 56, forming part of the current path, to accommodate movement of the second portion of the actuator with respect to the first portion. As shown in figures 5 and 5A, the flexible element 56 may be provided externally or internally on the spinner 9. The flexible element 56 may provide for a change in length of the conductive cable 55 between the mounting on the spinner 9 and a connection between the conductive cable 55 and the lightning receptor 54. The flexible element 56 may accommodate the changing length of the lightning current path for the full length of the stroke of the actuator 51 , e.g. several metres.

[0073] The flexible element 56 may be electrically connected via a second conductive cable 55 forming another part of the lightning current path to the LCTLI 10. The second conductive cable 55 may be at least partially routed inside the hub 4. Preferably, the lightning current path is electrically isolated from the hub 4. The hub 4 may be a cast metallic component. Alternatively, the metallic hub 4 may be used as a part of the lightning current path, acting as a Faraday cage to protect sensitive electrical and electronic components housed within the hub. The second conductive cable 55 may be electrically coupled to a known type of LCTLI between the blade 5 and the hub 4. Alternatively the second conductive cable 55 may be routed through an aperture in the spinner 9 or the hub 4.

[0074] The lightning receptor 54 may be a lightning antenna 57, a lightning rod, a lightning array 58 or similar. Examples of shaped lightning antenna 57 and lightning array 58 are shown in figures 5 and 6 and are compatible with the lightning protection system described above with reference to figures 1 to 5.

[0075] The lightning receptor 54 is positioned and arranged such that any incident lightning strike will attach to the lightning receptor 54 and lightning current will travel along the lightning path to ground and any sensitive components of the tensioning devices 18 will be protected from the lightning current.

[0076] An example of the flexible element 56 is shown in figure 8. The conductive cable 55 between the flexible element 56 and the actuator 51 may be a braided cable with conductive elements. The conductive cable 55 may be wound on a rotatable drum 59 and electronically connected via an electrical contact 60 mounted on a conductive spring 61 . Electrical connection to the second conductive cable 55 extending from the flexible element 56 towards the LCTL1 10 is made with the conductive spring 61 . As the drum 59 rotates the conductive cable 55 is spooled to pay in and out whilst maintaining electrical connection with the second conductive cable 55.

[0077] An alternative flexible element 56 may comprise a helical coil in a portion of the conductive cable 55. The path length of the lightning path remains unchanged as the helical coil is stretched or compressed but the flexible element 56 accommodates movement of the second portion of the actuator with respect to the first portion by changing pitch of the coils by elastic deformation of the helical coil.

[0078] Figures 9 and 10 show the dynamic movement of the lightning receptor 54 as the flexible element 56 accommodates the changing length of the conductive cable 55 as the tensioning device 18 moves between a fully retracted position (figure 9) and a fully extended position (figure 10) with consequential change in length of the pre-tension member 8 as the tension in the pre-tension member varies. It will be appreciated that the actuator 51 may be a linear actuator such as described above with reference to figure 5 or a rotary actuator such as described above with reference to figure 5A.

[0079] In figures 5, 5A, 9 and 10 the partial view shows in detail only one of the pre-tension members 8 and the tensioning device 18 coupled to the common point 17 but it be appreciated that the other pre-tension members 8 and tensioning devices 18 coupled to the same common point 17 (as shown in figure 4) operate identically and the equivalent parts of the lightning protection system are identically arranged for each of the tensioning devices 18.

[0080] A portion of the lightning current path may substantially follow the tensioning device 18. For example, the conductive cable 55 extending away from the tensioning device 18 may form a portion of the lightning current path which substantially follows directly downwind of the tensioning device 18, i.e. when viewing the wind turbine 1 in the downwind direction. In this way, interaction between the conductive cable 55 or other component forming the lightning path and the airflow may be suppressed, with consequential drag and noise suppression. This may also reduce aerodynamic forces on the conductive cable 55, leading to loads reduction on the cable and improved cable life or lower maintenance needs and potential light-weighting of the cable.

[0081] A portion of the lightning current path may extend a shortest distance between the radially inward end 19 of each pre-tension member 8 and the hub 4. For example, the conductive cable 55 extending away from the tensioning device 18 may form a portion of the lightning current path which substantially follows a shortest distance path towards the hub 4, i.e. when viewing the turbine 1 side on as such as shown in figure 5 or 5A. The cable 55 makes a diagonal path from the flexible element 56 to the actuator 51. In this way, the amount of flexibility required of the flexible element 56 may be minimised, reducing cost and weight, and a shorter cable will also result in a reduced voltage drop along the cable mitigating against flashovers from the cable to other conductive components.

[0082] The flexible element 56 may be located within the spinner 9 or within the hub 4. This may provide protection of the flexible element 56 from the ambient environment external to the wind turbine 1. If the flexible element 56 is located within the spinner or hub then the lightning path from the lightning receptor 54 extends from outside to inside the spinner or hub. For example, an aperture 62 may be provided in the outer wall of the spinner or hub. Figure 11 shows a first variant of the lightning protection system in which the only differences over figure 5, or 5A, is that the flexible element 56 is within the spinner 9 and the conductive cable 55 extends through aperture 62 and the second conductive cable 55 is routed through the spinner towards the LCTLI 10. It will be appreciated that all other variations of the wind turbine 1 and lightning protection system described with reference to figures 1 to 10 are applicable to the first variant of the lightning protection systems described above with reference to figure 11 . Where the flexible element 56 is mounted externally (to the spinner or hub) then aperture 62 can be avoided, which aids environmental protection of components within the spinner 9.

[0083] The lightning protection system may include a lightning receptor 54 mounted to the first portion 52 of the actuator 51 . The lightning receptor 54 may be mounted at the end of the first portion 52 nearest the second portion 53 and electrically coupled to the lightning current path. A benefit of this arrangement is that the first portion 52 of the actuator 51 substantially does not move with respect to the hub 4 as the actuator 51 is extended and retracted. Therefore, the lightning path may not need a flexible element 56 such as described above. Figure 12 shows a second variant of the lightning protection system in which the only differences over figures 5, 5A are the mounting of the lightning receptor 54 at the end of the first portion 52 nearest the second portion 53, the lightning path extending away from said end of the first portion 52, and the absence of the flexible element 56. It will be appreciated that all other variations of the wind turbine 1 and lightning protection systems described with reference to figures 1 to 11 are applicable to the second variant of the lightning protection system described above with reference to figure 12. The conductive cable 55 may be routed through an aperture 62 in the spinner or may connect to a known LCTLI between the blade 5 and the hub 4, similarly to the options described above.

[0084] It will be appreciated that a flexible element 56 similar to that described above may also be used with the second variant of the lightning protection system to avoid slack in the conductive cable 55 and to accommodate small angular movements of the actuator 51 about the common point 17. The amount of flexibility of the lightning path provided by the flexible element may however be significantly reduced due to the smaller movements of the position of the connection between the conductive cable 55 and the lightning receptor 54 relative to the hub 4 in this second variant.

[0085] The lightning receptor 54 may be sufficiently large or long so as to provide lightning strike protection for the whole of the actuator 51. This is important in the second variant of the lightning protection system as the second portion 53 of the actuator 51 when fully extended may be some radial distance well beyond the extremity of the first portion 52 of the actuator 51 to which the lightning receptor 54 is mounted. To that end, the lightning receptor 54 may be larger or longer in this second variant as compared with examples of the lightning protection system where the lighting receptor is mounted to the end of the second portion 53.

[0086] Figure 13 shows a detail view of the connection between the lightning receptor 54 and the end of the first portion 52 of the actuator 51 nearest the second portion 53. The lightning receptor 54 may be physically mounted to the radially outer end of the first portion 52 of the actuator 51. The conductive cable 55 is electrically connected to the lightning receptor 54 and is fixed to the radially outer end of the first portion 52 of the actuator 51 but electrically isolated from the first portion 52.

[0087] As mentioned previously, the pre-tension members 8 may also form part of the lightning protection system. In this case, the pre-tension members 8 may be configured to carry lightning current. It is however still undesirable that lightning current may interfere with the electrical or electronic components of the tensioning devices 18. In any of the examples of the lightning protection system described herein, the pre-tension members 8 may also form part of the lightning protection system (unless expressly excluded).

[0088] The actuator 51 may be configured such that the cylinder or first portion 52 of the actuator 51 is electrically isolated from the rod or second portion 53. To achieve this, an insulator, such as insulating sleeve 63 may be disposed between the first portion 52 and the second portion 53 of actuator 51 . A sliding electrical connector 64 may be in sliding electrical contact with the rod or second portion 53 of the actuator 51 . The sliding electrical connector 64 may be electrically connected to the lightning path and therefore to the conductive cable 55. As the second portion 53 of the actuator 51 moves with respect to the first portion 52 any lightning current in the pre-tension members 8 that may pass along a lightning current path into the rod or second portion 53 of the actuator will be routed via the sliding electrical connector 64 to the conductive cable 55 and will not pass to the first portion 52 of the actuator. Therefore, any sensitive electrical or electronic components of the tensioning device 18 will be protected from the lightning current even when the pre-tension members 8 provide a lightning current path. The sliding electrical connector 64 may comprises conductive brushes or similar.

[0089] Where the lightning protection system is arranged in the second variant such that the lightning receptor 54 is mounted to the first portion 52 of the actuator 51 , a conductive sleeve 65 may be arranged to extend from the first portion 52 and around the second portion 53 of the actuator 51 . The conductive sleeve may be electrically connected to the lightning path and to conductive cable 55 but isolated from the first portion 52. The conductive sleeve 65 is electrically isolated from the second portion 53 of the actuator 51 . The conductive sleeve 65 may extend radially beyond the outer extremity of the second portion 53 of the actuator 51 when the actuator 51 is fully extended. In this way, the lightning receptor 54 may be made smaller, and the conductive sleeve 65 acts as a Faraday cage to provide lightning strike protection to the second portion 53 of the actuator 51 . Figure 15 shows an example of an actuator with the conductive sleeve 65. A portion of the lightning current path may substantially follow the tensioning device 18. For example, the conductive cable 55 extending away from the tensioning device 18 may form a portion of the lightning current path which substantially follows directly the length of the tensioning device 18, i.e. when viewing the wind turbine 1 side on as shown in figure 16. This may avoid impacts on drag and noise of the conductive cable 55.

[0090] In a third variant of the lightning protection system, a pulley wheel 66 may be mounted near the base of the actuator 51 and the conductive cable 55 may follow behind the actuator 51 (downwind) and round the pully wheel 66 before passing generally parallel to the outer surface of the spinner 9 towards the flexible element 56. The pully wheel may be electrically isolated from the actuator 51. Figure 16 shows an example of this third variant of the lightning protection system which differs from figures 5 or 5A only in the presence of the pulley wheel 66 and the path of the conductive cable 55. In this way, interaction between the conductive cable 55 or other component forming the lightning path and the airflow may be suppressed, with consequential drag and noise suppression. This may also reduce aerodynamic forces on the conductive cable 55, leading to loads reduction on the cable and improved cable life or lower maintenance needs and potential light-weighting of the cable. The third variant of the lightning protection system may be combined with any of the examples of the lightning protection system described above with reference to figures 1 to 10.

[0091] In a fourth variant of the lightning protection system, a pulley wheel 66 may be mounted near the base of the actuator 51 and the conductive cable 55 may follow behind the actuator 51 (downwind) and round the pully wheel 66 before passing through the shell of spinner 9 and inside the spinner shell to the flexible element 56. The pully wheel may be electrically isolated from the actuator 51. Figure 17 shows an example of this fourth variant of the lightning protection system which differs from figure 11 only in the presence of the pulley wheel 66 and the path of the conductive cable 55. In this way, interaction between the conductive cable 55 or other component forming the lightning path and the airflow may be suppressed, with consequential drag and noise suppression. This may also reduce aerodynamic forces on the conductive cable 55, leading to loads reduction on the cable and improved cable life or lower maintenance needs and potential light-weighting of the cable. The fourth variant of the lightning protection system may be combined with any of the examples of the lightning protection system described above with reference to figure 11 .

[0092] The lightning protection system may include a lightning receptor array 67 extending over the tensioning devices 18 and electrically connected to the lightning current path. The lightning receptor array 67 may extend forward of the spinner 9. The lightning receptor array 67 may mounted to the spinner 9, or it may be included in the construction of the spinner. The lightning receptor array 67 may have a radius (when viewed in the downwind direction) greater than a radius of the radially extending tensioning devices 18 at full extension. The lightning receptor array may be generally disc-shaped, such as shown in figure 18. The lightning receptor array 67 may be arranged as series of concentric conductive rings, electrically connected together. The lightning receptor array 67 may be arranged in a plane generally upwind and parallel to the rotor plane of the blades 5. Alternatively, the lightning receptor array may be a set of discrete array elements each extending over an area larger than a respective one of the radially extending tensioning devices 18 at full extension when viewed looking downwind. Each element of the lightning receptor array may be a grid of conductors. The lightning receptor array formed as a set of discrete elements may be arranged in a plane generally upwind and parallel to the rotor plane of the blades 5. Alternatively, each element may be curved. The lightning receptor array 67 may be positioned upwind of the spinner (as shown in figure 18), or alternatively may be arranged on the spinner downwind of the tensioning devices 18 but upwind of the rotor plane.

[0093] The lightning receptor array 67 may be electrically connected to the lightning path, e.g. provided by conductive cable 55, and extending towards ground.

[0094] The lightning receptor array 67 may be used with the wind turbine 1 described previously. The lightning receptor array 67 may be used in place of any of the arrangements of lightning receptors 54 described above in the lightning protection system for the wind turbine 1 , or may be used in combination with any of those lightning protection systems having the lightning receptors 54. An advantage of the lightning receptor array 67 is that no flexible element 56 for accommodating the extension and retraction of the tensioning devices 18 is required. The shape of the lightning receptor array is designed according to a rolling sphere method. The rolling sphere method for lightning strike protection is an aid to identify the parts of the tensioning devices 18 that needs protection by lightning receptors. A sphere with a radius (defined by the class of lightning protection required), is theoretically rolled across the wind turbine 1. Wherever the surface of the sphere comes into contact with the tensioning device 18 is deemed to be unprotected, thus requiring protection with a lightning receptor. This method can therefore be used to determine the ideal shape of the lightning receptor array. Although the invention has been described above with reference to one or more preferred embodiments, 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 pitch controlled wind turbine comprising a tower, a nacelle mounted on the tower, a hub mounted rotatably on the nacelle, and at least three wind turbine blades, wherein each wind turbine blade extends between a root end connected to the hub via a pitch mechanism, and a tip end; the wind turbine further comprising at least three blade connecting members, each blade connecting member extending from a connection point on one wind turbine blade towards a connection point on a neighbouring wind turbine blade, where the connection point on a given wind turbine blade is arranged at a distance from the root end and at a distance from the tip end of the wind turbine blade; at least three pre-tension members, each pre-tension member being connected to one of the blade connecting members and to the hub via a tensioning device, the tensioning device provides radial movement of a radially inward end of the pre-tension member with respect to an axis of rotation of the hub due to extension or retraction of the tensioning device, each pre-tension member thereby providing pre-tension in the blade connecting member to which it is connected; and a lightning protection system for protecting each of the tensioning devices from lightning current and providing a lightning current path towards ground.

2. A pitch controlled wind turbine according to claim 1 , wherein the lightning current path extends from adjacent the radially inward end of each pre-tension member towards ground, the lightning current path is electrically isolated from each tensioning device.

3. A pitch controlled wind turbine according to any preceding claim, wherein each tensioning device comprises an actuator having a first portion coupled to the hub and a second portion movable with respect to the first portion and coupled to the respective pre-tension member.

4. A pitch controlled wind turbine according to claim 3, wherein the lightning protection system includes a lightning receptor mounted to the second portion of the actuator adjacent the pre-tension member and electrically coupled to the lightning current path, preferably wherein the lightning receptor is an antenna.

5. A pitch controlled wind turbine according to claim 3, wherein the lightning protection system includes a lightning receptor mounted to the first portion of the actuator nearest the second portion and electrically coupled to the lightning current path, preferably wherein the lightning receptor is an antenna.

6. A pitch controlled wind turbine according to any of claims 3 to 5, wherein the lightning current path includes a flexible element to accommodate movement of the second portion of the actuator with respect to the first portion.

7. A pitch controlled wind turbine according to any of claims 3 to 6, wherein the actuator is a linear actuator having a cylinder and a rod moveable into and out of the cylinder.

8. A pitch controlled wind turbine according to any of claims 3 to 6, wherein the actuator comprises a motor and a tension element, wherein a radially distal end of the tension element is connected to the radially inward end the pre-tension element, and a radially inward end of the tension element is wound around a drum connected to a rotatable shaft of the motor.

9. A pitch controlled wind turbine according to any preceding claim, wherein the lightning protection system includes at least one conductive cable providing at least a portion of the lightning current path.

10. A pitch controlled wind turbine according to any preceding claim, wherein a portion of the lightning current path substantially follows the respective tensioning device.11 . A pitch controlled wind turbine according to any preceding claim, wherein a portion of the lightning current path extends a shortest distance between the radially inward end of each pre-tension member and the hub.

12. A pitch controlled wind turbine according to any preceding claim, wherein the tensioning devices are coupled to a projection extending forward of the hub in an upwind direction of the wind turbine, and wherein a spinner coupled to the hub covers a radially inner portion of each tensioning device.

13. A pitch controlled wind turbine according to claim 12, wherein a portion of the lightning current path extends inside the spinner.

14. A pitch controlled wind turbine according to any preceding claim, wherein the lightning protection system includes a lightning receptor array extending over the tensioning devices and electrically connected to the lightning current path.

15. A pitch controlled wind turbine according to claim 14, wherein the lightning receptor array is shaped to provide lightning protection of all of the tensioning devices, preferably wherein the shape is designed according to a rolling sphere method.

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