Patent Description:
Most known lightning protection systems for wind turbine blades comprise one or more internally arranged down conductors and a number of lightning receptors arranged on the external surface of the blade.

It is a well-known problem of such systems, that lightning strikes do not only attach to the wind turbine blade in the intended positions, i.e. on the external attachment points, the so-called lightning receptors, but can also strike the internal conductive parts of the lightning protection system directly through the structure of the blade. Such incidents can cause severe structural damage to the wind turbine blade due to the large amounts of energy typically released in relation to lightning impacts.

Since the size of the wind turbines increases the size of the wind turbine blades increase as well. Hence the structural design of the wind turbine blades is being more and more important since it is always a desire to design the wind turbine blades as light as possible without jeopardizing the strength of wind turbine blade. Thus, it has been even more important to protect the wind turbine blade in the entire length for structural damages caused by lightning impacts.

Relevant prior art examples of lightning protection systems for wind turbine blades are found in for instance <CIT>, <CIT> and <CIT>.

It is an object of the present invention to wholly or partly overcome the above disadvantages and drawbacks of the prior art. More specifically, it is an object to provide an improved lightning protection system for wind turbine blades having structural element being a spar or beam made of fibre reinforced polymer (FRP).

The above objects, together with numerous other objects, advantages and features, which will become evident from the below description, are accomplished by a solution in accordance with the present invention by a lightning protection system for a wind turbine blade, the wind turbine blade comprises a root end and a tip end and a longitudinal axis, a pressure side and a suction side, the pressure side and the suction side are the outer faces of the wind turbine blade, and a structural element extending along the longitudinal axis, the structural element is a spar or beam made of fibre reinforced polymer (FRP), the lightning protection system comprising:.

the first sheet and the second sheet comprise a plurality of conductive connection points, the conductive connection points are arranged in the vicinity of the tip connection block and the root connection block and are electrically connected with the tip connector block and the root connection block, respectively.

Accordingly, the tip connection block serves as the interface between the single down conductor from the tip of the wind turbine blade, to the two sheets in each blade shell acting as down conductors from the tip of the FRP sparcaps. In addition, bonding or equipotentialization between pressure side and suction side down conductors is performed in one unit. At the same time, the second and third down conductors may act as a receptor base for strikes attaching to the tip of the sheets. The part is carefully designed to handle the full lightning current and avoid interception failures due to its insulating casting.

Furthermore, the structural element may be a spar or beam made of carbon fibre reinforced polymer (CFRP) being electrically conductive.

The predetermined distance may be <NUM> to <NUM> meters from the tip end of the wind turbine blade depending on the overall length of the wind turbine blade.

Also, the first sheet and the second sheet may be symmetrical arranged on opposite sides in relation to the structural element and being substantially equal in size.

Furthermore, the conductive connection points may be made of metal or other conductive materials, or a combination thereof.

Additionally, the metal may be tin, aluminium, copper, brass, silver, gold, or any alloys thereof.

Moreover, the conductive connection point may comprise a first layer and a second layer.

The first layer may be made of a first material and the second layer may be made of a second material. The first material is different from the second material.

Also, the conductive connection points may be directly or indirectly connected with the connection blocks.

In an embodiment, each conductive connection point may have a geometry exhibiting an outer and closed curvature with a minimum radius of curvature of between <NUM> to <NUM>, preferably between <NUM> to <NUM>.

Furthermore, the conductive connection points may have a semi major axis and semi minor axis.

The semi major axis and semi minor axis may be equal providing a circular outer periphery.

Moreover, the semi major axis and semi minor axis may be different providing an oval or elliptic outer periphery.

Additionally, the semi major axis may be oriented in a predetermined angle in relation to the longitudinal axis of the wind turbine blade.

The predetermined angle may be between <NUM> degrees and <NUM> degrees.

Also, the conductive connection points may be partly or fully circular or oval.

In addition, the connection points may have an asymmetrically shape.

In addition, the outer periphery of the conductive connection points may be defined by curved lines and straight lines.

Furthermore, the conductive connection points may be without any pointed corners.

Moreover, the conductive connection points may have a thickness, the thickness is larger than <NUM>, preferably larger than <NUM>.

Also, the thickness of the conductive connection point may extend in both directions with respect to a thickness of the sheets.

In addition, the conductive connection points may be mechanically connected with the sheet.

The conductive connection points may be adhered to the sheet by a conductive adhesive.

Furthermore, a receptor bolt may be screwed through the conductive connection point and into the connection block.

Moreover, a thread may be provided in the conductive connection point.

Also, a receptor bolt may be terminated in the conductive connection point.

Additionally, a side receptor may be connected to a conductive connection point.

A plurality of intermediate conductive connection points may be arranged opposite the structural element at a predetermined intermediate distance from the tip connection block.

Moreover, the intermediate conductive connection points may be connected with the structural element.

Furthermore, the predetermined intermediate distance may be less than <NUM>, preferably between <NUM> to <NUM>, more preferably between <NUM> to <NUM>, most preferably between <NUM> to <NUM>.

Also, the conductive connection point may be made by melting material so that the melted material in a liquid state is connected with the sheet thereby providing a mechanical and electric conductive connection between the conductive connection point and the sheet when the material hardens.

The melting of material may be performed by electrical induction heating or electrical resistive heating.

In addition, the conductive connection point may be made by spraying melted metal onto the sheet followed by soldering.

The conductive connection point may comprise at least two discs, the two discs being arranged on opposite sides of the sheet and subsequently mechanical fasten each disc to each other.

Furthermore, the conductive connection point may comprise at least two discs, the two discs being arranged on opposite sides of the sheet and subsequently being pressed together around the sheet by plastic deformation.

Also, the conductive connection point may comprise at least two discs, the two discs being arranged on opposite sides of the sheet and subsequently being spot welded together.

The conductive connection point may comprise at least two discs, the two discs being arranged on opposite sides of the sheet and subsequently being pulse-melted together.

Moreover, the conductive connection point comprises at least two discs, the two discs being arranged on opposite sides of the sheet and subsequently being glued together by a conductive adhesive.

In addition, the conductive connection point has an edge or outer periphery, a current density at the edge or outer periphery may not be greater than <NUM>,<NUM> A/mm.

Advantageously, the first sheet and the second sheet are expanded foils or meshes.

The conductive material of the sheet may be metal such as aluminium, copper, steel or associated alloys.

Also, the conductive material of the sheet may be non-metallic such as a composite or fibres.

The first expanded foil or first mesh may be arranged so as it at least fully cover a first side of the structural element facing the pressure side, and the second expanded foil or second mesh may be arranged so as it at least fully cover a second side of the structural element facing the suction side.

In addition, a length of the first expanded foil or first mesh and the second expanded foil or second mesh may be equal to or longer than a length of the structural element.

Furthermore, the first expanded foil or first mesh may have a first area and the second expanded foil or second mesh has a second area, the first area and the second area are substantially equal.

The expanded foil may be made as a monolithic whole.

Also, the mesh may be provided by weaving conductive threads.

The mesh may also be provided by non-woven conductive threads.

Furthermore, the tip connection block may electrically connect the single down conductor with the first expanded foil or first mesh and the second expanded foil or second mesh, respectively, via the conductive connection points.

Moreover, the tip connector block may comprises at least a first receptor base and at least a second receptor base, the first receptor base is configured to connect the conductive connection point of the first expanded foil to the tip connector block and the second receptor base is configured to connect the conductive connection point of the second expanded foil to the tip connector block.

Additionally, a receptor bolt may be screwed through the conductive connection point and into the receptor base.

Moreover, the tip connector block may comprise at least a first pair of receptor bases and at least a second pair of receptor bases, the first pair is configured to connect the conductive connection points of the first expanded foil to the tip connector block and the second pair is configured to connect the conductive connection points of the second expanded foil to the tip connector block.

Furthermore, an equal numbers of receptor bases may be arranged for connecting the first expanded foil or first mesh and the second expanded foil or second mesh to the tip connection block.

Also, the tip connection block may be configured to avoid interception failures by ensuring that a level of insulation is sufficient.

Moreover, the root connector block may be configured to electrically connect the first expanded foil and second expanded foil with a single root down conductor.

The root connector block may be Y-formed.

Also, a first intermediate connection block may be arranged between the first expanded foil and the root connection block, the first intermediate connection block electrically connects the first expanded foil with the root connection block, and a second intermediate connection block is arranged between the second expanded foil and the root connection block, the second intermediate connection block electrically connects the second expanded foil with the root connection block.

Each intermediate connection block may comprises at least a first receptor base and at least a second receptor base, the first receptor base is configured to connect the connection point of the first expanded foil to the intermediate connection block and the second receptor base is configured to connect the connection point of the second expanded foil to the intermediate connection block.

Also, each intermediate connection block may comprise at least a pair of receptor bases, the pair of receptor bases is configured to connect the conductive connection points of the expanded foils to the intermediate connector blocks.

In addition, a receptor bolt may be screwed through the conductive connection points and into the receptor base of the intermediate connection blocks.

Furthermore, a first cable may be arranged between the first intermediate connection block and the root connection block and a second cable is arranged between the second intermediate connection block and the root connection block. The first cable and the second cable function as a down conductor.

Moreover, the structural element may be conductive and manufactured by pultruded parts. The pultruded parts may be made in a dry fabric layup and a vacuum assisted resin infusion process, using pre-impregnated glasfiber sheets.

In addition, the structural elements may for instance be manufactured by.

The present invention also relates to a wind turbine blade comprising a root end and a tip end, and a longitudinal axis, a pressure side and a suction side, the pressure side and the suction side are the outer faces of the wind turbine blade, and a structural element extending along the longitudinal axis, the structural element is a spar or beam made of carbon fibre reinforced polymer (CFRP) being electrically conductive, and a lightning protection system as described above.

The present invention also relates to a wind turbine having one or more wind turbine blades with a lightning protection system as described above.

The present invention additionally relates to a method for providing a conductive connection point to a sheet such as an expanded foil or mesh in the lightning protection system, comprising.

The step of melting the material may be performed by electrical induction heating or electrical resistive heating.

The step of applying a melted connective material is performed by pouring.

The present invention also relates to a method for providing a conductive connection point to a sheet such as an expanded foil or mesh in the lightning protection system, comprising.

The present invention furthermore relates to a method for providing a conductive connection point to a sheet such as an expanded foil or mesh in the lightning protection system, comprising.

In addition, the discs may be fastened to each other by a mechanical connection.

Also, the discs may be fastened to each other by pressing the two discs together around the expanded foil or mesh by plastic deformation.

Furthermore, the discs may be fastened to each other by spot welding.

Moreover, the discs may be fastened to each other by pulse-melting.

Additionally, the discs may be fastened to each other by applying a conductive adhesive between the discs and maintaining the discs in position until the adhesive is cured.

The invention and its many advantages will be described in more detail below with reference to the accompanying schematic drawings, which for the purpose of illustration show some non-limiting embodiments and in which.

<FIG> shows a wind turbine <NUM> having a tower <NUM>, a nacelle <NUM> and three wind turbine blades <NUM>. Each wind turbine blade <NUM> has a root end <NUM> connected to a hub <NUM> and a tip end <NUM>. The wind turbine blade <NUM> has a longitudinal axis <NUM> extending from the root end <NUM> to the tip end <NUM>.

In <FIG> a cross-sectional view of wind turbine blade <NUM> is shown. The wind turbine blade <NUM> has a pressure side <NUM> and a suction side <NUM>, the pressure side <NUM> and the suction side <NUM> are the outer faces of the wind turbine blade <NUM>. The wind turbine blade <NUM> has also a leading edge <NUM> and a trailing edge <NUM>. The wind turbine blade <NUM> has a structural element <NUM> extending along the longitudinal axis, the structural element <NUM> is a spar or beam made of carbon fibre reinforced polymer (CFRP) being electrically conductive. The CFRP spar or beam enhances the strength of the wind turbine blade <NUM>. The present embodiment shows that the structural elements <NUM> are arranged in the shells of the wind turbine blade <NUM>. The CFRP elements may be connected via webs being made for instance as a sandwich structure of GFRP and core materials (PVC foam or balsa wood).

The present invention is especially directed to enhancing the lightning protection of a wind turbine blade <NUM> having a structural element being made of CFRP and thereby being electrically conductive.

For a numerous of years field inspections have consistently demonstrated how the blade tips are the most exposed parts of the wind turbines, see <FIG>, corroborating the numerical models describing lightning attachment process. The results have led to the development of a zoning concept for wind turbine blades, describing which parts of the blades are most exposed and to which lightning current amplitudes.

The numerical simulations were conducted for generic blades of <NUM>-<NUM> lengths, investigating the distribution of strikes attaching to the turbines at different current amplitudes. The results showed clearly that for downward initiated strikes the majority of all strikes attach to the blade tips, and that for lower amplitude strikes, the attachment can move inboard on the blade and attach to other and less exposed parts of the wind turbine (hub, nacelle, tower, etc.).

The zoning concept may be used to describe the possible strike amplitudes to different regions on the blade. In some interpretations of the LPL1 requirements in the IEC <NUM>-<NUM> standard all strikes with amplitudes between for instance 3kA and 200kA must be safely intercepted and conducted towards ground, whereas damages are tolerated for strikes outside these extremities. In practice, this would mean that since strikes may occur to the inboard sections of the blade (although with a very low probability), the blade must be able to withstand them so that unscheduled stop of operation is avoided. In practice the distribution of the amplitudes follows a normal distribution, meaning that the "little" currents are able to sneak inward, and the lightning protection system is designed to handle this.

Moreover, if the probabilities of having such small amplitude strikes to the blades are accounted for, by considering the probability density functions described in the lightning protection standard, one can come to the conclusion that protection according to strikes of such low amplitudes is unnecessary, because these only occur very rarely.

The design of lightning protection system should rather focus on the protection of the more exposed parts of the blade by ensuring the attachment to the intended air terminations at the tip region.

As seen in <FIG> and <FIG> over <NUM>% of damages are experienced on the first couple of meters from the tip end wherefore these zones of the blades are vital to protect. Even though the occurrence strikes are considerably lower <NUM> meters from the tip end the CFRP spar or beam is often arranged along the longitudinal axis from this point. Hence it is still very important to ensure proper lightning protection of the CFRP spar or beam.

The lightning protection system <NUM> according to the present invention is shown schematically in <FIG>. The lightning protection system <NUM> comprises a first down conductor <NUM> extending from the tip end <NUM> to a tip connection block <NUM> arranged at a predetermined distance from the tip end, the single down conductor <NUM> being electrically connected with the tip connection block <NUM>.

Furthermore, a second down conductor <NUM> extending from the tip connection block <NUM> between and along the structural element <NUM> and the pressure side towards a root connection block <NUM> arranged at the root end <NUM>.

In addition, a third down conductor (not shown in <FIG>) extending from the tip connection block between and along the structural element and the suction side towards the root connection block.

The second down conductor comprises a first sheet and the third down conductor comprises a second sheet, the first sheet and the second sheet are made of a conductive material for functioning as down conductors.

The first sheet and the second sheet comprise a plurality of conductive connection points <NUM>, the conductive connection points <NUM> are arranged in the vicinity of the tip connection block <NUM> and the root connection block <NUM> and are electrically connected with the tip connector block and the root connector block, respectively.

Hereby the tip connection block <NUM> serve as the interface between the single down conductor <NUM> from the tip of the wind turbine blade <NUM>, to the first sheet and the second sheet in each blade shell acting as down conductors from a tip part of the CFRP spar or beam. In addition, bonding or equipotentialization between pressure side down conductor, i.e. first sheet, and suction side down conductor, i.e. second sheet, is performed in one unit. At the same time, the second down conductor and third down conductor act as a receptor base for strikes attaching to the tip of the sheets. The part is carefully designed to handle the full lightning current and avoid interception failures due to its insulating casting.

In <FIG> an embodiment of a lightning protection system <NUM> according to the invention is shown. The lightning protection system <NUM> comprises a first down conductor <NUM> extending from the tip end <NUM> to a tip connection block <NUM> arranged at a predetermined distance from the tip end, the single down conductor <NUM> being electrically connected with the tip connection block <NUM>.

The second down conductor <NUM> extending from the tip connection block <NUM> between and along the structural element (not shown in <FIG>) and the pressure side towards a root connection block <NUM> arranged at the root end <NUM>. The third down conductor <NUM> extending from the tip connection block <NUM> between and along the structural element and the suction side towards the root connection block <NUM>.

The second down conductor <NUM> comprises a first sheet <NUM> and the third down conductor <NUM> comprises a second sheet <NUM>, the first sheet <NUM> and the second sheet <NUM> are made of a conductive material for functioning as down conductors.

As seen in <FIG> the first sheet <NUM> and the second sheet <NUM> comprise a plurality of conductive connection points <NUM>, the conductive connection points <NUM> are arranged in the vicinity of the tip connection block <NUM> and the root connection block <NUM> and are electrically connected with the tip connector block <NUM> and the root connector block <NUM>, respectively.

In the present embodiment two conductive connection points <NUM> are arranged for electrically connecting the first sheet <NUM> with the tip connection block <NUM>, and two conductive connection points <NUM> are arranged for electrically connecting the second sheet <NUM> with the tip connection block <NUM>. In other embodiments only one conductive connection point connects the first sheet to the tip connection block and one conductive connection point connects the second sheet to the tip connection block. In addition, a plurality of connection points may connect the first sheet to the tip connection block and a plurality of connection points may connect the second sheet to the tip connection block. Advantageously, the number of conductive connection points connecting the first sheet with the tip connection block is identical to the number of connection points connecting the second sheet to the tip connection block. The same applies in relation to the root connection block.

As indicated in <FIG>, the first sheet <NUM> and the second sheet <NUM> are symmetrical arranged on opposite sides in relation to the structural element and being substantially equal in size. Hereby is obtained that due to Faraday cage-like geometry the CFRP structural element is protected and the risk of high current density in the CFRP structural element and the risk of high voltage differences and internal flashover between parts of the CFRP structural elements are avoided.

The conductive connection points are preferably made of metal or other conductive materials, or a combination thereof. The metal may be tin, aluminum, copper, brass, silver, gold or any alloys thereof.

Also, the conductive connection point may comprise a first layer and a second layer. The first layer may be made of a first material and the second layer is made of a second material. The first material may be different from the second material.

In <FIG> an embodiment of a conductive connection point <NUM> is shown. Each connection point <NUM> may have a geometry exhibiting an outer and closed curvature <NUM> with a minimum radius of curvature of between <NUM> to <NUM>, preferably between <NUM> to <NUM>.

In addition, the connection point <NUM> has a semi major axis <NUM> and semi minor axis <NUM> as shown in <FIG>. When the semi major axis <NUM> and semi minor axis <NUM> are equal in length a circular outer periphery of the conductive connection point is provided as shown in <FIG>.

When the semi major axis <NUM> and semi minor axis <NUM> are different an oval or elliptic outer periphery of the conductive connection point <NUM> is provided as shown in <FIG>. In addition, the semi major axis <NUM> may be oriented in a predetermined angle a in relation to the longitudinal axis <NUM> of the wind turbine blade. The predetermined angle a may be between <NUM> degrees and <NUM> degrees.

Hence, the connection points <NUM> may be partly or fully circular or oval.

Also, the outer periphery <NUM> of the connection points <NUM> may be defined by curved lines and straight lines as shown in <FIG>.

It is presently preferred that the connection points <NUM> are without any pointed corners.

As seen in <FIG>, the connection points <NUM> may have a thickness t, the thickness t is larger than <NUM>, preferably larger than <NUM>. The thickness t of the connection point <NUM> may extend in both directions with respect to a thickness of the sheets.

The first sheet <NUM> and second sheet <NUM> are made of conductive material. The conductive material may be metal such as aluminum, copper, steel or associated alloys.

In other embodiments the conductive material is non-metallic such as a composite or fibers.

For minimizing the weight of the first sheet and the second sheet they may be provided as expanded foils or meshes.

The first expanded foil or first mesh may be arranged so as it at least fully cover a first side of the structural element facing the pressure side, and the second expanded foil or second mesh is arranged so as it at least fully cover a second side of the structural element facing the suction side.

Moreover, the first expanded foil or first mesh may have a first area and the second expanded foil or second mesh may have a second area, the first area and the second area are substantially equal.

Furthermore, the expanded foil may be made as a monolithic whole.

The mesh may be provided by weaving conductive threads.

In another embodiment, the mesh may be provided by non-woven conductive threads.

The conductive connection point <NUM> may be provided to a sheet, such as an expanded foil or mesh by.

The melted connective material may be applied by pouring it into a mould arranged in connection with the expanded foil or mesh. The mould defining the outer periphery of the conductive connection point. The conductive material may be a metal such as tin.

The conductive connection point <NUM> may also be provided to a sheet such as an expanded foil or mesh by.

The conductive connection point <NUM> may additionally be provided to a sheet such as an expanded foil or mesh by.

The discs may be fastened to each other by a mechanical connection.

In another embodiment, the discs may be fastened to each other by pressing the two discs together around the expanded foil or mesh by plastic deformation.

Furthermore, the discs may be fastened to each other by spot welding or soldering.

Also, the discs may be fastened to each other by pulse-melting.

In addition, the discs may be fastened to each other by applying a conductive adhesive between the discs and maintaining the discs in position until the adhesive is cured.

In <FIG> an embodiment of the tip connection block <NUM> connected to the first sheet <NUM> and the second sheet <NUM> are shown. The conductive connection points <NUM> have been arranged at predetermined positions on the first sheet <NUM> and second sheet, respectively, and the tip connection block <NUM> is arranged so that it is possible to electrically connect the conductive connection points <NUM> to the tip connection block <NUM>. In the present embodiment a connection bolt <NUM> has been screwed trough the conductive connection point <NUM> and into the tip connection block <NUM>. Hereby is obtained that the single down conductor <NUM> is electrically connected with the first sheet <NUM>, i.e. the second down conductor, and the second sheet <NUM>, i.e. the third down conductor so that lightning current from a lightning strike may be led from the single down conductor <NUM> through the tip connection block <NUM> and into the first sheet <NUM> and second sheet <NUM> via the conductive connection points <NUM> and thereby down towards the root end of the wind turbine blade.

The tip connector block <NUM> comprise at least a first block receptor base <NUM> and at least a second block receptor base <NUM>, the first block receptor base <NUM> is configured to connect the conductive connection point <NUM> of the first sheet <NUM> to the tip connector block <NUM> and the second block receptor base <NUM> is configured to connect the connection point <NUM> of the second sheet <NUM> to the tip connector block <NUM>. In the embodiment shown in <FIG> the tip connection block <NUM> has two first block receptor bases <NUM> and two second block receptor bases <NUM>.

As mentioned above a connection bolt <NUM> or a receptor bolt is screwed through the connection point <NUM> and into the block receptor base <NUM>, <NUM>. Hence, in this embodiment the connection points are directly connected with the connection block.

The tip connector block may comprise at least a first pair of block receptor bases and at least a second pair of block receptor bases, the first pair is configured to connect the connection points of the first expanded foil to the tip connector block and the second pair is configured to connect the connection points of the second expanded foil to the tip connector block.

An equal numbers of block receptor bases may be arranged for connecting the first sheet and the second sheet to the tip connection block.

In <FIG> another embodiment of a tip connection block <NUM> are shown. The tip connection block <NUM> is in the present embodiment designed with a first part <NUM> and a second part <NUM> connected via an intermediate part <NUM>. The first part <NUM> is larger in size than the second part <NUM> so that it may be arranged within the wind turbine blade having the first part <NUM> arranged closer to the leading edge than the second part <NUM>. Both the first part <NUM> and the second part <NUM> comprise on one side the first block receptor base <NUM> and on an opposite side the second block receptor base <NUM>.

In <FIG> a cross-sectional view of the tip connection block <NUM> of <FIG> is shown. The tip connection block <NUM> has a core <NUM> of conductive material extending from the first part <NUM> via the intermediate part <NUM> to the second part <NUM> so that all parts are electrically connected. The entire core <NUM> is covered by an isolation layer <NUM>. The single down conductor <NUM> is in this embodiment connected with the core of the second part <NUM>. In addition, it is important that the connection bolts are screwed trough the conductive connection points and into the conductive core of the tip connection block <NUM> for providing electrically connection. The conductive material of the core <NUM> may be metal such as aluminum or brass.

The design and shape of the tip connection block <NUM> may vary due to different wind turbine blade design and the positions within the blade it shall be positioned. However, the tip connection block is configured to avoid interception failures by ensuring that a level of insulation is sufficient. Furthermore, the tip connection block is configured to carry a lightning current sufficiently.

Before screwing the connection bolt or receptor bolt through the conductive connection point a thread is provided in the conductive connection point.

Also, a receptor bolt is terminated in the connection point and a side receptor may be connected to a connection point if it is necessary to provide the lightning protection system with side receptors opposite the first sheet and the second sheet.

Returning to <FIG>, the lightning protection system <NUM> also comprises tip unit <NUM> and a plurality of side receptors <NUM> arranged between the tip end <NUM> and the tip connection block <NUM>. The tip unit <NUM> and the side receptors <NUM> are connected via the single down conductor <NUM>. An embodiment of the tip unit will be described further below in connection with <FIG>.

The space between the blade shell parts at the tip end is rather limited compared to the space at the root end. Hence, the root connection block is often arranged a larger distance from the first sheet <NUM> and the second sheet <NUM>. In the lightning protection system <NUM> two intermediate connection blocks <NUM> are arranged for electrically connecting the conductive connection points of first sheet <NUM> and second sheet <NUM>, respectively. Each intermediate connection block <NUM> has a cable <NUM>, <NUM> electrically connecting the intermediate connection block with the root connection block <NUM>. The root connection block <NUM> is electrically connected with a single root down conductor <NUM> being electrically connected to earth through the wind turbine nacelle and tower (not shown).

In addition, the first sheet <NUM> and the second sheet <NUM> may have intermediate conductive connection points <NUM> arranged opposite the structural element. The intermediate conductive connection points closest to the tip end may be arranged at a predetermined distance from the tip connection block <NUM>. The predetermined distance is less than <NUM>, preferably between <NUM> to <NUM>, more preferably between <NUM> to <NUM>, most preferably between <NUM> to <NUM>.

Since the structural element of the blade is made of CFRP, directly coupled or induced lightning current from lightning strikes will be led through the structural element. The size of the lightning current in the CFRP beam or spar is minimized by arranging the first sheet <NUM> and the second sheet <NUM> outside and opposite the extension of the CFRP spar or beam so that the majority of lightning current will be conducted via the first sheet and the second sheet. The remaining current which still will be running in the CFRP beam or spar has to be controlled for equalizing potential differences in the material and avoiding unintended electrically flashovers between the CFRP beams or spars.

The intermediate connection points <NUM> are therefore electrically connected with the structural element when necessary so that a certain current is led from the first sheet and the second sheet over in the CFRP structural element, whereby the potential differences in the material is equalized and the CFRP structural element is not exerted for damaging currents and energies. Hence, the function of the CFRP structural element will be maintained by the lightning protection system <NUM> according to the present invention.

At the root end of the first sheet <NUM> and the second sheet <NUM> intermediate conductive connection points <NUM> are arranged in electrically connection with the structural element when needed for ensuring that the current running in the structural element is led out in the first sheet <NUM> and the second sheet <NUM> via the intermediate conductive connection points <NUM> and therefrom down to the root connection block <NUM>.

In <FIG> an embodiment of an intermediate connection block is shown. The intermediate connection block <NUM> function in the same manner as the tip connection block as described earlier.

However, as mentioned previously the first intermediate connection block is arranged between the first sheet <NUM> and the root connection block, the first intermediate connection block electrically connects the first sheet with the root connection block, and the second intermediate connection block is arranged between the second sheet and the root connection block, the second intermediate connection block electrically connects the second expanded metal foil with the root connection block.

Each intermediate connection block <NUM> comprises at least one receptor base <NUM> configured to connect the conductive connection point of the sheets to the intermediate connection. In the embodiment shown in <FIG>, the intermediate connection block <NUM> has two receptor bases <NUM>. In the same manner as with the tip connection block a connection bolt or receptor bolt is screwed through the conductive connection points and into the receptor base <NUM> of the intermediate connection blocks <NUM>. A first cable <NUM> is arranged between the first intermediate connection block <NUM> and the root connection block and a second cable are arranged between the second intermediate connection block and the root connection block.

In <FIG> another embodiment of an intermediate connection block is shown. This intermediate connection block is very similar to the one shown in <FIG>, however, the intermediate connection block <NUM> comprises an intermediate cable <NUM> electrically connected to the CFRP structural element as shown in <FIG>.

As seen in <FIG> a connection patch <NUM> is electrically connected to the CFRP structural element <NUM>. The connection patch <NUM> has a cable connector <NUM> which again is connected with the intermediate cable <NUM>, which in turn is connected with the intermediate connection block <NUM>. Hereby current running in the CFRP structural element <NUM> may be led to the intermediate connection block <NUM> in an expedient manner. The intermediate connection block <NUM> may in same manner as described earlier be connected to the second sheet <NUM> via conductive connection points <NUM>.

In <FIG> an embodiment of the root connection block <NUM> is shown. The root connection block function in the same manner as described above and the ligthning current from the first cable <NUM> and the second cable <NUM> is interfaced in the root connection block <NUM> and led into the single root down conductor <NUM>.

In <FIG> another embodiment of the root connection block <NUM> is shown. In the present embodiment the root connection block <NUM> is Y-formed, however it functions in the same manner as described in connection with <FIG>. By having a Y-formed root connection block <NUM> the weight of the connection block is minimized considerably.

As mentioned before the structural element is conductive and may be manufactured by pultruded parts. The pultruded parts are made in a dry fabric layup and a vacuum assisted resin infusion process, using pre-impregnated glasfiber sheets.

For instance, the structural elements may be manufactured by.

<FIG> shows a tip unit <NUM> arranged at the tip end of a wind turbine blade <NUM>. Preferably, the tip unit <NUM> is attached to the wind turbine blade <NUM> by means of a suitable adhesive, such as <NUM>-component epoxy adhesives, fast curing polyurethane adhesives, <NUM>-component polyurethane adhesives, <NUM>-component curing acrylate adhesives or other polymeric adhesives.

The tip unit <NUM>, is arranged with its longitudinal axis at least substantially parallel with the longitudinal axis of the wind turbine blade <NUM> in such a way that the external part <NUM> of the tip <NUM> forms the tip of the wind turbine blade <NUM> and the insulated cable <NUM> forms the outermost part of a single down conductor extending along the longitudinal axis of the wind turbine blade <NUM> in the direction of the tip connection block.

The tip unit <NUM> comprises four electrically conducting elements, namely an external <NUM> and an internal part <NUM> of the tip <NUM>, which is electrically and mechanically connected to a side receptor base <NUM> through an internal tip unit conductor. The internal tip unit conductor is connected to the side receptor base <NUM> by means of a connection element integrated therein. An insulated electric cable <NUM>, which forms the outermost part of the single down conductor of the lightning protection system, is connected to the side receptor base <NUM> by means of the same connection element. Thus, all the electrically conducting parts <NUM>, <NUM>, <NUM> of the tip unit <NUM> are electrically and mechanically connected to each other.

<FIG> shows the insulating material <NUM> is arranged around the components of the tip unit <NUM>. Thus, apart from the part of the insulated electric cable <NUM> forming the down conductor from the tip unit <NUM> and inwards towards the tip connection block and an end part of the internal part <NUM> of the tip <NUM>, all electrically conducting parts of the tip unit <NUM> are fully covered by an electrically insulating material <NUM>, such as polymer nanocomposites, thermoplastic materials, thermoset materials insulating foams or any combination thereof. The thickness, geometry and material properties of this insulating material <NUM> is dimensioned to withstand the environmental conditions (vibrations, temperatures, temperature cycles, humidity, etc.) and the electric fields during lightning exposure and normal operation of the wind turbine blade.

Thus, there are only two ways, in which a lightning strike can reach the internal parts of the tip unit <NUM> and, thereby the part of the singe down conductor extending through this part of the wind turbine blade <NUM>. One is through a tip receptor of the lightning protection system formed by an external part <NUM> of the tip <NUM>, which is connected mechanically and electrically to the internal part <NUM> of the tip <NUM> through the end part thereof, which is not covered by the electrically insulating material <NUM>. The other way is through a side receptor <NUM>, which is arranged on the outer surface of the shell or to be flush with the shell surface of the wind turbine blade <NUM> and is not a part of the tip unit <NUM> itself. The side receptor <NUM> is mechanically and electrically connected to the side receptor base <NUM> through penetration of the outer face of the wind turbine blade <NUM> and the electrically insulating material <NUM> covering the side receptor base <NUM>. The fact that lightning strikes can only reach the internal lightning protection system through the tip receptor and side receptor <NUM> arranged on the outer face of the wind turbine blade <NUM> means that no lightning strikes pass through the structural parts of this part of the wind turbine blade <NUM>. Thereby, the risk of damage or even destruction of the structural parts of the tip of the wind turbine blade <NUM> is eliminated or at least significantly reduced.

At the ends of the cylinder-shaped parts of the tip unit <NUM> around the side receptor base <NUM>, the insulating material <NUM> forms recesses in its surface for the placement of an adhesive material.

<FIG> are a side view and a cross-sectional view, respectively, of the tip unit <NUM> shown in <FIG>.

<FIG> illustrates schematically a side receptor <NUM>. The side receptor <NUM> is mounted within the outer face of the wind turbine blade, preferably aligned therewith, and mechanically and electrically connected with a side receptor base arranged within the wind turbine blade and covered by an insulation. In a not shown embodiment the side receptor <NUM> may be formed as a bolt, the connection to the side receptor base consists simply in a threaded connection.

<FIG> and <FIG> are a perspective view and a cross-sectional view, respectively, of a receptor assembly in the form of a side receptor <NUM> mounted within the outer face of a wind turbine blade. A receptor cylinder <NUM> constitutes the electrically conducting part of the side receptor <NUM>. Its upper circular end forms the external part of the side receptor <NUM> being substantially aligned with the surface of the wind turbine blade when the side receptor <NUM> is mounted therein. This is the part being impacted by lightning strikes.

The opposite end of the receptor cylinder <NUM> forms a contact surface <NUM> through which the lightning current passes from the side receptor <NUM> into the side receptor base <NUM> to which the side receptor <NUM> is connected. The receptor cylinder <NUM> is mechanically connected to the side receptor base <NUM> by means of a mounting bolt <NUM>, the head of which is concealed within the receptor cylinder <NUM> and the thread part of which protrudes through a centered hole in the contact surface <NUM> of the receptor cylinder <NUM>.

In the embodiment illustrated in these figures, the contact surface <NUM> is plane and perpendicular to the longitudinal axis of the receptor cylinder <NUM>. In other embodiments, the contact surface <NUM> or at least a part thereof can be slanted.

<FIG> shows how the insulation <NUM> covers the side receptor base <NUM> as well as the side receptor <NUM> connected thereto. This is very important for ensuring that the lightning strikes do, in fact, pass through the side receptor <NUM> rather than bypassing it by penetrating the shell of the wind turbine blade next to the side receptor <NUM> on its path to the side receptor base <NUM> and the down conductor inside the wind turbine blade.

A washer can be arranged between the head of the mounting bolt <NUM> and the internal surface of the receptor cylinder <NUM> for securing the mounting bolt <NUM>.

In the illustrated embodiment, the side receptor <NUM> comprises an optional blade surface protection <NUM> in the form of a circular sheet of a heat resistant material arranged around the receptor cylinder <NUM> for protecting the outer face of the wind turbine blade against being damaged from the excessive heat energy following lightning strikes impacting the side receptor <NUM>. Advantageously, this blade surface protection <NUM> is adhered to the surface of the wind turbine blade during the mounting of the side receptor <NUM> therein.

A sealant <NUM> ensures a tight connection between the side receptor <NUM> and the surrounding outer face of the wind turbine blade.

The open end of the receptor cylinder <NUM> is closed by a receptor plug <NUM>, which may either be made from a solid electrically conducting or insulating material or consist of a heat-resistant paste. This receptor plug <NUM> covers and protects the head of the mounting bolt <NUM> from being damaged from impacting lightning strikes. A screw cap <NUM> protects the slot of the mounting bolt <NUM>, for instance against entrance of paste, if the receptor plug <NUM> consists of such a paste.

Furthermore, this embodiment of the side receptor <NUM> comprises a bolt insulator <NUM> arranged around the head of the mounting bolt <NUM> for ensuring electrical insulation between the mounting bolt <NUM> and the receptor cylinder <NUM> so that the lightning current is forced to pass through the contact surface <NUM> rather than through the threads of the mounting bolt <NUM> on its path from the side receptor <NUM> to the side receptor base <NUM>.

In <FIG> another embodiment of a side receptor <NUM> is shown. The design of this side receptor <NUM> is substantially as shown in <FIG> and described above. In the embodiment shown in <FIG>, the receptor cylinder <NUM> is surrounded by isolation member <NUM>. The isolation member <NUM> is made of an isolation material and ensures electrical isolation in addition to the sealing <NUM>.

<FIG> is an enlarged cross-sectional view of the side receptor <NUM> of the two previous figures, whereas <FIG> is a cross-sectional view of a receptor assembly in the form of a side receptor <NUM> according to another embodiment.

One difference from the embodiment shown in <FIG> is that in the embodiment shown in <FIG>, the receptor cylinder <NUM> is provided with a receptor ruff extending outwards from the upper circular end of the receptor cylinder <NUM>. Such a receptor ruff is useful for ensuring a tight and weather-resistant connection between the side receptor <NUM> and the surrounding outer face of the wind turbine blade and provides additional material for the arc root erosion and, hence, the natural wear of the side receptor <NUM>.

Furthermore, the edge of the receptor cylinder <NUM> is beveled in such a way that at least a part of the contact surface <NUM> is slanted. This increases the area of the contact surface <NUM> and thereby improves the electrical connection to the side receptor base <NUM>. Furthermore, it ensures a better mechanical stability of the connection between the side receptor <NUM> and the side receptor base <NUM>.

<FIG> is a cross-sectional view of a receptor assembly in the form of a tip receptor assembly.

In this embodiment of the invention, a tip receptor <NUM> is mounted to a tip receptor base <NUM> by means of two threaded rods <NUM>. These threaded rods <NUM> are screwed into threaded holes within the tip receptor base <NUM> and an electrically conducting bushing <NUM> is arranged around each of the threaded rods <NUM>. The free ends of the threaded rods <NUM> are put into holes in the surface of the tip receptor <NUM> facing towards the tip receptor base <NUM>, and nuts are mounted and tightened around these ends of the threaded rods <NUM> within the tip receptor <NUM> through an opening in the side of the tip receptor <NUM>. In another not shown embodiment the tip receptor may be mounted by means of a single threaded rod or bolt. An alignment member may be present for aligning the tip receptor to the tip receptor base.

This means that the tip receptor <NUM> is in mechanical and electrical contact with one end of the conducting bushings <NUM> and the tip receptor base <NUM> is in mechanical and electrical contact with the other end of the conducting bushings <NUM>. The figure illustrates, how the tip receptor base <NUM> is covered by a layer of electrically insulating material <NUM>, through which two openings gives access for the threaded rods <NUM> and the surrounding bushings <NUM> to be in electrical and mechanical contact with the tip receptor base <NUM>.

The fact that the lightning current tends to pass along the surfaces of conductors rather than through the more central parts thereof means that the vast majority of the lightning current passes through the conducting bushings <NUM> and only a negligible part passes through the threaded rods <NUM>, which are therefore not damaged, when the receptor assembly is subject to a lightning impact. Thus, the threaded rods <NUM> and the nuts are kept intact and can be used normally in case of the need for replacement of the tip receptor assembly or parts thereof.

The opening, through which the nuts are mounted, can be closed with a receptor plug (not shown) consisting, for instance, of a heat resistant paste, such as silicone, or made from a solid material, such as a metal, a plastic material, rubber or fibreglass.

In <FIG> the connection patch <NUM> is shown. The connection patch <NUM> has a cable connector <NUM> being electrically connected with a cable <NUM>. The cable <NUM> electrically connects the cable connector <NUM> with a connection point connector <NUM>. The connection point connector <NUM> is connected with a conductive connection point <NUM> arranged in the first sheet <NUM>. Hereby is a flexible connection obtained.

In <FIG> are shown how the connection point connector <NUM> is connected with the conductive connection point <NUM>. A first connector disc <NUM> is arranged on one side of the conductive connection point <NUM> and a second connector disc <NUM> is arranged on the opposite side of the conductive connection point thereby encapsulating the conductive connection point <NUM>. The conductive connection point is electrically connected with the first sheet as described earlier. In the present embodiment the first connector disc and the second connector disk are connected by means of connector bolts <NUM> as seen in <FIG>.

In <FIG> the cable connector <NUM> of the connection patch <NUM> is shown. The cable <NUM> is connected to the cable connector via a bolt head <NUM>. This embodiment assists in ensuring that any manufacturing tolerances may be overcome by the cable <NUM>.

The present invention also relates to a wind turbine blade comprising a root end and a tip end, and a longitudinal axis, a pressure side and a suction side, the pressure side and the suction side are the outer faces of the wind turbine blade, and a structural element extending along the longitudinal axis, the structural element is a spar or beam made of glass fibre reinforced polymer (GFRP, and a lightning protection system as described above.

Claim 1:
A lightning protection system for a wind turbine blade, the wind turbine blade comprises a root end and a tip end and a longitudinal axis, a pressure side (<NUM>) and a suction side (<NUM>), the pressure side and the suction side are the outer faces of the wind turbine blade, and a structural element (<NUM>) extending along the longitudinal axis, the structural element is a spar or beam made of fibre reinforced polymer (FRP), the lightning protection system comprising:
- a first down conductor (<NUM>) extending from the tip end to a tip connection block (<NUM>) arranged at a predetermined distance from the tip end, the first down conductor being electrically connected with the tip connection block,
- a second down conductor (<NUM>) extending from the tip connection block between and along the structural element and the pressure side towards a root connection block (<NUM>) arranged at the root end,
- a third down conductor (<NUM>) extending from the tip connection block between and along the structural element and the suction side towards the root connection block,
- the second down conductor comprises a first expanded foil or a first mesh and the third down conductor comprises a second expanded foil or a second mesh, the first expanded foil or first mesh and the second expanded foil or second mesh are made of a conductive material, the first expanded foil or first mesh and the second expanded foil or second mesh comprise a plurality of conductive connection points (<NUM>), the conductive connection points are arranged in the vicinity of the tip connection block and the root connection block and are electrically connected with the tip connector block and the root connection block, respectively.