WO2011131205A1

WO2011131205A1 - Lightning current transfer unit, wind turbine, method and use thereof

Info

Publication number: WO2011131205A1
Application number: PCT/DK2011/050127
Authority: WO
Inventors: Allan Laursen Molbech; Kim Bertelsen; Hans V. Erichsen
Original assignee: Vestas Wind Systems A/S
Priority date: 2010-04-19
Filing date: 2011-04-19
Publication date: 2011-10-27

Abstract

A lightning connection means for a wind turbine is disclosed, said wind turbine comprising stationary and rotating lightning protection means having contact surfaces connected by said lightning connection means. For a given wind turbine blade, said lightning connection means comprises a lightning transfer unit comprising a plurality of lightning current paths, said paths being arranged to divide a lightning stroke current from the wind turbine blade between them in a parallel impedance configuration. First ends of said paths of each lightning transfer unit are arranged to be connected electrically to at least one contact surface of said rotating lightning protection means, and second ends of said paths of each lightning transfer unit are arranged to be connected electrically to at least one contact surface of said stationary lightning protection means.

Description

LIGHTNING CURRENT TRANSFER UNIT, WIND TURBINE,

METHOD AND USE THEREOF

Background of the invention

The invention relates to lightning connection means for a wind turbine comprising stationary lightning protection means and at least one wind turbine blade comprising

rotating lightning protection means. The invention further relates to a wind turbine comprising said lightning connection means, a method and use hereof.

Description of the Related Art Typically, wind turbines are erected in areas with superior wind conditions such as rural areas comprising only a few tall obstacles e.g. trees or buildings. However, wind turbines are erected with increasingly higher towers and rotor blades in order to take the most efficient use of the wind and, with wind turbines being the tallest objects in an area, protection of the wind turbines against lightning strokes is thus a necessity.

A well-known lightning protecting system for a wind turbine involves a lightning receptor at the tip of every rotor blade in order to capture the lightning stroke. The lightning is discharged to a ground potential via a down conductor inside the blade and down conductors in the nacelle and tower. In order to transfer the lightning from the down conductor of the rotating blade to the down conductor of the stationary nacelle the lightning current is led through the rotor blade shaft to the nacelle. The nacelle comprises means, e.g. slip-rings which are connected to the shaft, and thus allows the current to be removed from the shaft and transferred to the down conductor of the nacelle. A problem with the above-mentioned system is the fact that the lightning current in its passage from the rotor blade to the nacelle flows via different components of the wind turbine such as the blade pitching means as well as the main bearings and the gear means. The significant energy in the lightning may bring about undesirable effects to the components e.g. by arc formations, and thus reduce the lifespan of the components. Further, the means connected to the shaft has proven to be rather inefficient in removing the lightning current from the shaft. This means that large parts of the lightning current find alternative and often undesired and destructive routes to the ground potential.

Lightning systems with different ways of transferring the lightning current from rotating to stationary lightning protection means of wind turbines have been suggested.

An example is disclosed in European patent application EP 1 154 537 in which the transfer of lightning current between the lightning protection system of a rotating blade and the nacelle is obtained by a sliding contact. The fixed part of the contact is connected to the nacelle and has spring engaged means in order to establish a sliding contact to a steel belt connected to the rotor blades. In an embodiment of the invention the steel belt and the lightning protection system for each rotor blade are connected to opposite contact plates. Hereby, it is possible to pitch a rotor blade and still maintain the lightning protection of the blade.

A problem with the lightning protection system of the EP patent application is the fact that the surfaces of the contacts quickly wear down and loose contact as they only comprise little opportunity of adapting to changes. The lightning current is hereafter at best transferred as electric arcs between the contact surfaces and thus deteriorating and eventually destroying the contacts.

A further problem is the complex and delicate mechanical construction of the lightning protection system involving a number of separate sections connected by cables. This makes the system less useable in connection with wind turbines due to the high risk of mechanical failures in the system.

A solution for this problem is disclosed in European patent application EP 1 692 395, in which a mechanically simple connection between the rotating and stationary parts of a lightning protection means, which is also reliable in service, is obtained.

This solution involves at least two contact means kept in a continuous connection with the rotating and stationary parts of the lightning protection system, respectively, by means of resilient force transferring means. At least one electric conductor establishes a dedicated connection between the contact means, thus providing a path for the lightning current to follow without passing and possibly damaging critical components of the wind turbine

However, one problem remains which is common to all known lightning protection systems for wind turbines and similar applications, namely the problem of large electric potential differences across the means connecting the rotating and the stationary parts of the lightning protection systems, which may cause at least parts of the lightning current to find alternative and often undesired and destructive routes through critical components of the wind turbine to the ground potential. This problem of large potential differences is especially typical in wind turbine constructions, in which there is a large distance between the rotating and the stationary parts of the lightning protection system, since the potential difference depends on the length of the path through the system and may, in severe cases, exceed 200 kV/m. Potential differences of such magnitudes require very large safety distances between the wind turbine hub, which is made from metal, and the lightning protection means on the blades in order to avoid flashovers and potential damage to critical components.

One of the objects of the invention is to establish a lightning protection system for wind turbines without the above-mentioned disadvantage. It is especially an object of the invention to create a system that is simple in construction as well as reliable in service.

Further, it is an object of the invention to establish a lightning protection system for wind turbines that is easy to integrate and install as a part of a wind turbine.

The invention

The invention relates to lightning connection means for a wind turbine, said wind turbine comprising stationary lightning protection means and at least one wind turbine blade comprising rotating lightning protection means, said stationary and rotating lightning protection means comprising contact surfaces connected by said lightning connection means, wherein, for a given wind turbine blade, said lightning connection means comprises a lightning transfer unit comprising a plurality of lightning current paths, said plurality of paths being arranged to divide a lightning stroke current from the wind turbine blade between them in a parallel impedance configuration, first ends of said plurality of paths of each lightning transfer unit being arranged to be connected electrically to at least one first contact surface of said rotating lightning protection means, and second ends of said plurality of paths of each lightning transfer unit being arranged to be connected electrically to at least one second contact surface of said stationary lightning protection means. By using more than one path, along which the current of a lightning stroke can find its way to ground potential, a larger total surface area and cross-sectional area of the current paths and, thus, a smaller effective impedance, resistive as well as inductive, is obtained.

Since current conductors normally have a very small resistance being made from materials having a very small resistivity, the resistive potential difference across the lightning connection means is usually of less importance.

The effective inductive impedance L of the conductors, however, is the main reason for the large potential differences seen across known lightning connection means as mentioned above. The inductive potential difference UL depends on L and the rate of change of the lightning current dl/dt according to the equation:

U_L = L * dVdt Therefore, by reducing the effective inductive impedance L, for instance by using a relatively flat and wide geometrical configuration of the current paths, also the inductive potential difference UL across the lightning connection means and, thus, the risk of lightning current finding alternative and undesired and destructive routes through critical components of the wind turbine to the ground potential is reduced.

The optimal number and dimensions of the current paths of a given lightning transfer unit as well as the optimal distance between them may vary depending on the dimensions and specifications of the wind turbine in which the lightning transfer unit is to be installed. Parameters to be taken into consideration include the cost price and weight of the lightning transfer unit, space and openings needed for enabling necessary maintenance and service work in the area around the lightning transfer unit and the distance between the lightning protection contact belt around the wind turbine blade and the metal parts of the wind turbine hub. In an embodiment of the invention, said plurality of paths is formed as metal bars.

Hereby is obtained a lightning connection means that is simple in construction, mechanically very stable, reliable in service and easy to integrate and install as a part of a wind turbine. Also, metal bars are very easy to produce in a flat and wide configuration reducing the inductive inductance of the current paths.

In an embodiment of the invention, said paths are positioned side by side at a certain mutual distance from each other, said distance being large enough to avoid inductive interference between the paths.

By keeping a certain mutual distance between neighbouring paths, it is achieved that inducted currents and, thus, potential differences in a given path caused by the electric fields around currents running in other paths can be avoided or at least reduced to an insignificant level.

In an embodiment of the invention, two or more lightning transfer units are coupled in parallel by connecting electrically said first ends of said two or more lightning transfer units to each other and/or by connecting electrically said second ends of said two or more lightning transfer units to each other.

Hereby it is achieved that the current of a lightning stroke from a given wind turbine blade is not only divided across the paths of the lightning transfer unit belonging to that blade, but also across the paths of the other lightning transfer unit(s) coupled in parallel therewith. Thus, the current running along each path is reduced and, therefore, also the rate of change of the current and the inductive potential difference is reduced according to the above equation. Also, in the transferral of the lightning current from the lightning connection means to the stationary lightning protection means, it is advantageous to divide the current between the contact means of more than one lightning transfer unit. In an embodiment of the invention, said lightning transfer unit further comprises at least one first contact means adapted for establishing direct electrical contact with said at least one first contact surface of said rotating lightning protection means. In an embodiment of the invention, said lightning transfer unit further comprises at least one second contact means adapted for establishing direct electrical contact with said at least one second contact surface of said stationary lightning protection means. Establishing the connections between the lightning connection means and the rotating and stationary lightning protection means, respectively, without the use of cables is advantageous in that it increases the reliability of the system.

In an embodiment of the invention, said lightning transfer unit further comprises at least one first electric conductor establishing a dedicated connection between said at least one first contact means and said first ends.

In an embodiment of the invention, said lightning transfer unit further comprises at least one second electric conductor establishing a dedicated connection between said at least one second contact means and said second ends.

Hereby, connections between the paths of the lightning transfer unit and the rotating and stationary lightning protection means, respectively, are achieved that are easy to integrate and install as a part of a wind turbine.

In an embodiment of the invention, said lightning transfer unit further comprises first force transferring means for said at least one first contact means establishing a continuous connection between said at least one first contact means and said at least one first contact surface.

In an embodiment of the invention, said lightning transfer unit further comprises second force transferring means for said at least one second contact means establishing a continuous connection between said at least one second contact means and said at least one second contact surface.

The force transferring means further ensure a continuously adapting of the contact means to changing positions of the contact surface. In an aspect of the invention, it relates to a wind turbine comprising stationary means such as a nacelle and a tower comprising stationary lightning protection means, rotating means such as a rotor including at least one wind turbine blade and shaft means, each of said at least one wind turbine blade comprising rotating lightning protection means, said stationary and rotating lightning protection means comprising contact surfaces, wherein said contact surfaces are connected by lightning connection means as described above.

In an embodiment of the invention, the at least one contact surface of said stationary lightning protection means is connected to a ground potential.

In an embodiment of the invention, the at least one contact surface of said stationary lightning protection means comprises a coherent circular metal plate ring at the front of the nacelle.

By using a coherent metal ring, it is ensured that all the wind turbine blades are always connected to the ground potential. The blades are connected to the ground potential regardless of the blades rotating or not. However, it shall be emphasized that the metal ring may be in one piece instead of being established by sections to a coherent ring. Further, the metal plate ring may be a section of a full circle in which the ground potential contact only is present in the section part.

In a further embodiment of the invention, the coherent circular metal plate ring is positioned on the front side of the drainage means of the nacelle.

By using the drainage means an advantageous position is ensured for the contact surface without interfering significantly with the existing parts of the wind turbine. As the drainage means usually is made in metal, the metal plate ring can easily be firmly mounted and the connection to the down conductor can easily be established.

In an embodiment of the invention, the contact surface of said rotating lightning protection means is connected to lightning receptor means through lightning down conductor means.

Hereby, an advantageous embodiment of the invention is achieved using well-proven rotating lightning protection means. In an embodiment of the invention, the at least one contact surface of said rotating lightning protection means comprises a metal belt positioned at least partly around a wind turbine blade above the pitching mechanism. Hereby, it is ensured that the wind turbine blade may pitch freely without the lightning connection means looses contact with the contact surface. The securing of the metal belt to the blade is enhanced with a complete surrounding of the blade. However, the steel belt does not necessarily need to surround the blade completely, since the maximum pitch angle of a wind turbine blade usually does not exceed approximately 110°.

In an aspect of the invention, it relates to a method of establishing a continuous contact between rotating and stationary lightning protection means of a wind turbine comprising at least one wind turbine blade, said method comprising for a given wind turbine blade the steps of: creating a lightning transfer unit comprising a plurality of lightning current paths, arranging said plurality of paths to divide a lightning stroke current from the wind turbine blade between them in a parallel impedance configuration, establishing at least one first contact between at least one first contact surface of said rotating lightning protection means and at least one first contact means, connecting said at least one first contact means electrically to first ends of said plurality of paths by establishing a first dedicated connection between said at least one first contact means and said first ends, establishing at least one second contact between at least one second contact surface of said stationary lightning protection means and at least one second contact means, and connecting said at least one second contact means electrically to second ends of said plurality of paths by establishing a second dedicated connection between said at least one second contact means and said second ends. In an embodiment of the invention, the step of creating said lightning transfer unit involves manufacturing said plurality of paths as metal bars.

In an embodiment of the invention, the step of arranging said plurality of paths involves positioning said plurality of paths side by side at a certain distance from each other, said distance being large enough to avoid inductive interference between the paths.

In an embodiment of the invention, the method further comprises the step of coupling in parallel two or more lightning transfer units by connecting said first ends of said two or more lightning transfer units to each other and/or by connecting said second ends of said two or more lightning transfer units to each other.

With the different embodiments of this method, lightning connection means with the above- mentioned corresponding advantages will be achieved.

In an aspect of the invention, it relates to the use of lightning connection means as described above in a wind turbine, such as a wind turbine as described above, in which the wind turbine blades are pitch controlled.

In an aspect of the invention, it relates to the use of a method for establishing a continuous contact between rotating and stationary lightning protection means of a wind turbine as described above, in which the wind turbine blades are pitch controlled.

Figures

A preferred embodiment of the invention will be described in the following with reference to the figures in which fig. 1 illustrates a large modern wind turbine, figs. 2a and 2b illustrate a wind turbine blade with a well-known lightning protection system, fig. 3 illustrates a contact surface of the stationary lightning protection means of the wind turbine, fig. 4 illustrates a contact surface of the rotating lightning protection means of the wind turbine, fig. 5a illustrates a preferred embodiment of a lightning transfer unit of a lightning connection means according to the invention, fig. 5b illustrates a preferred embodiment of a lightning connection means according to the invention, fig. 6 illustrates the connection between a lightning transfer unit according to the invention and the rotating lightning protection means of a wind turbine blade, fig. 7 illustrates the connection between a lightning transfer unit according to the invention and the stationary lightning protection means of a wind turbine, figs. 8a and 8b illustrate different parts of the lightning connection means according to the invention, fig. 9 illustrates a preferred embodiment of a contact means of the lightning connection means according to the invention, and fig. 10 illustrates schematically the functionality of a part of the lightning connection means. Detailed description

Fig. 1 illustrates a modern wind turbine 1 with a tower 2 and a wind turbine nacelle 3 positioned on top of the tower. The wind turbine rotor 5, comprising three wind turbine blades, is connected to the nacelle through the main shaft which extends out of the nacelle front.

As illustrated in the figure, wind beyond a certain level will activate the rotor due to the lift induced on the blades and allow it to rotate in a perpendicular direction to the wind. The rotation movement is converted to electric power, which is supplied to the utility grid.

Figs. 2a and 2b illustrate a normal wind turbine blade 5 with a well-known lightning protection system. Fig. 2a illustrates a front view of the wind turbine blade 5 seen and fig. 2b a close-up side view of the root portion of the blade 5.

Fig. 2a illustrates how the tip 6 of the wind turbine blade 5 with a metal receptor 8 which intercepts a lightning stroke 7. The receptor 8 is connected to the lightning down conductor 9 inside the wind turbine blade 5. The conductor 9 runs through the blade 5 in the longitudinal direction and ends at the wind turbine hub 4. The lightning receptor 8 and the lightning down conductor 9 of the wind turbine blade 5 are components in the rotating part of the lightning protection system in the wind turbine

Fig. 2b illustrates schematically how the lightning current is transferred from the lightning down conductor 9 through the pitching area 12 of the wind turbine blade 5 to the rest of the lightning protection system. From the down conductor 9, the lightning current path 10 is transferred to the main shaft 14 through the pitching mechanism 13 or any other mechanism between the blade 5 and the shaft 14. In the nacelle 3, sliding contacts are in contact with the shaft 10 in order to remove the current from the shaft 14 before entering the gearing means. The slip rings are connected to the stationary part of the lightning protection system in the wind turbine 1. The stationary part includes lightning down conductors of the nacelle 3 and the wind turbine tower 2 in which the conductors are mutually connected as well as connected to a ground potential 11. Fig. 3 illustrates schematically the contact areas 20b of contact means 19b (see Fig. 7) of the lightning connection means sliding on the contact surface 17 of a coherent ring 17a of the stationary lightning protection means at the front end of the nacelle 3 of the wind turbine 1. The figure illustrates the situation of a rotating three-bladed wind turbine rotor with lightning connection means including three contact means 19b. As the lightning connection means is mounted on the hub 4 and the main shaft 14, it will rotate with the main shaft 14 as the centre of rotation. Further, the contact means 19b are positioned at a distance from the centre corresponding to the diameter of the ring 17a. Thus, the contact means 19b will perform a circular rotation facing the coherent ring 17a while being continuously forced against the contact surface 17 of the coherent ring 17a (see Fig. 7).

In a preferred embodiment, the nacelle 3 is provided with water drainage means 16 (see Fig. 7) which is a circular and U-shaped water drain 16 positioned around the main shaft opening of the nacelle 3. The drain 16 helps draining water away from the main shaft 14 and the main shaft opening. The drainage means 16 makes a full circle around the main shaft opening in order also to protect against water tossed up from the rotating wind turbine blades. The sidewall of the water drainage means 16 facing the wind turbine blade comprises one or more metal plates. The metal plates create a coherent circular metal plate ring 17a on the drainage means as illustrated in the figure.

The coherent ring 17a establishes a contact surface 17 on the stationary part of the wind turbine 1 with the same circle centre and diameter as the water drainage means 16. Further, the ring has a width similar to the sidewall of the water drainage means 16, in which the width corresponds to the width of the contact surface 17.

The coherent ring is connected to the stationary part of the lightning protection system, e.g. the lightning down conductor (not shown) of the nacelle 3. The connection may be established as a direct connection between the metal plate ring 17 a and the down conductor, e.g. one or more conductors going on the outside or penetrating the water drainage means in order to connect to the ring. In a further embodiment, the drainage means is made in metal with a good electric conductivity and, thus, may be used as a connection between the ring 17a and the down conductor.

In a further embodiment the coherent ring 17a is the sidewall of the water drainage means 16 or is a ring of metal positioned directly on the front surface of the nacelle 3, e.g. in the situation in which a wind turbine does not include water drainage means 16. The ring of metal 17a may also be positioned on the nacelle 3 inside the perimeter of the water drainage means.

In an even further embodiment the coherent ring 17a is either positioned on the inside or outside surface of the water drainage means 16. The contact means 19b is consequently moved in order to contact the coherent ring 17a from above or below instead of horizontally.

It shall be emphasized that the water drainage means 16 may be replaced by another edge or a similar feature projecting from the front surface of the nacelle 3, e.g. a feature with the sole purpose of establishing contact between the stationary and rotating parts of the lightning protection system.

Fig. 4 illustrates schematically, how a wind turbine blade 5 comprises a metal belt 18a (see Fig. 6) surrounding at least partly the root of the blade 5. The belt 18a is connected to the lightning down conductor 9 inside the wind turbine blade 5, e.g. with one or more metal rods going through the blade 5. The belt 18a is preferably made in a metal with a good electric conductivity and may preferably surround the wind turbine blade 5 fully. However, less than a fully surrounding belt 18a, e.g. halfway around the blade, is also possible. Even further, the belt 18a is preferably mounted on the surface of the wind turbine blade 5 after the manufacturing of the blade 5, but may also be partly or fully integrated into the surface of the wind turbine blade 5. The integration may be performed after the manufacturing of the blade 5 by grinding a groove into the surface or at manufacturing by establishing the groove as a part of the manufacturing process.

The metal belt 18a establishes a contact surface 18 on the root of the wind turbine blade 5 above the pitching mechanism 13 of the wind turbine blade 5 and perpendicular to the longitudinal direction of the blade 5. Thus, the contact surface 18 will rotate with the pitching of the blade 5.

The figure illustrates how the contact surface 18 of a pitch-controlled wind turbine blade 5 connects in the contact area 20a with contact means 19a of a lightning connection means, which faces the blade 5. The contact means 19a is continuously forced against the contact surface 18 and slides on the surface 18 when the blade 5 is pitched to one or the other side. The lightning connection means will not rotate in relation to the matching blade 5 during rotation, as the lightning connection means are mounted on the flanges of the hub 4 together with the blades 5.

Fig. 5a illustrates a preferred embodiment of a lightning transfer unit 24 of a lightning connection means 15 according to the invention.

The shown embodiment comprises three lightning current paths 25 each constituted by a flat and relatively wide metal bar. A first end 31 of each of the metal bars 25 are connected by an electrical connection 33, and the second ends 32 of the metal bars 25 are connected by another electrical connection 34. Thus, the metal bars 25 are arranged in a parallel impedance configuration. The first ends 31 and/or the electrical connection 33 connecting them are electrically connected to the rotating lightning protection means of a blade 5 of the wind turbine 1 through a dedicated electric connection 30 (not shown on this figure, see Fig. 6) and contact means 19a (also not shown on this figure, see Fig. 6). Likewise, the second ends 32 and/or the electrical connection 34 connecting them are electrically connected to the stationary lightning protection means on the nacelle 3 of the wind turbine 1 through a dedicated electric connection 46 (not shown on this figure, see Fig. 7) and contact means 19b (also not shown on this figure, see Fig. 7).

Fig. 5b illustrates a preferred embodiment of a lightning connection means 15 according to the invention, comprising three lightning transfer units 24 as described like the one shown in fig. 5a.

Neighbouring lightning transfer units 24 are connected by electrical connections 35. Thus, the three lightning transfer units 24 together form one overall lightning connection means 15 connected electrically on one side 31, 33 to the rotating lightning protection means of all three blades 5 of the wind turbine 1 through three dedicated connections 30, one for each lightning transfer unit (not shown), in parallel. The lightning connection means 15 is also connected electrically on the other side 32, 34 to the stationary lightning protection means on the nacelle 3 of the wind turbine through another three dedicated connections 46, one for each lightning transfer unit (not shown) in parallel.

Fig. 6 illustrates the connection between a lightning transfer unit 24 according to the invention and the rotating lightning protection means of a wind turbine blade 5.

The base support part 22 for the force transferring means 26 is mounted on a section of the flange 21 of the wind turbine blade 5. The flange 21 is facing the corresponding flange of the wind turbine hub 4 and the two are connected to each other with a number of bolts 23. The bolts 23 of the section, which the base support part 22 covers, also go through and thus connect the base support part 22 to the flanges 21. As the wind turbine hub 4 is connected to the main shaft 14, the blade 5 and main shaft 14 are connected through the hub 4.

The force transferring means 26 expands from the mounting position at the rod of the wind turbine blade 5 up in the space between the wind turbine blade 5 and the front of the nacelle 3. By the force transferring means 26 being mounted to the wind turbine blade 5, the force transferring means 26 and, thus, also the contact means 19a will perform the same rotating movement as the blade 5, i.e. a circular movement with the shaft 14 as centre. Fig. 6 only illustrates the position of one contact means 19a and one blade 5. However, it shall be emphasized that under normal conditions, every wind turbine blade 5 of a wind turbine 1 will be equipped with its own contact means 19 a.

For example, a three-bladed wind turbine 1 will comprise three contact means 19a in order to protect all the blades 5. Each of the contact means 19a connects a part of the rotating lightning protection means with a lightning transfer unit 24 of the lightning connection means 15.

The figure also illustrates the different sections of the connection means including the base support part 22, a contact means 19a and the force transferring means 26 between the base support part 22 and the contact means 19a. The force transferring means 26 may be in the form of a flexible link and ensures that the contact means is actively forced against the contact surface 18 on the wind turbine blade 5. The contact means 19a and the contact surface 18 establish a contact area 20a. The contact area 20a ensures a constant contact to the rotating part of the lightning protection means. The contact means 19a is connected to the lightning connection means 15 by a dedicated electric connection 30.

Fig. 7 illustrates the connection between a lightning transfer unit according to the invention and the stationary lightning protection means of a wind turbine.

The figure illustrates the position of the contact means 19b in relation to the contact surface 17 on the sidewall 17a of the water drainage means 16. The figure also illustrates the different sections of the connection means including a mounting hook 47, a contact means 19b and the force transferring means 27 between the mounting hook 47 and the contact means 19b. The force transferring means 27 ensures that the contact means is actively forced against the contact surface 17 on the sidewall 17a. The contact means 19b and the contact surface 17 establish a contact area 20b. The contact area 20b ensures a constant contact to the stationary part of the lightning protection means. The contact means 19b is connected to the lightning connection means 15 at a second end of current path 32 of the lightning current path 25 by a dedicated electric connection 46.

Similarly, there are three contact means 19b in a three-bladed wind turbine 1 which preferably corresponds to three contact means 19a for each blade 5. Each of the contact means 19b, through a dedicated electric connection 46, connects a lightning transfer unit 24 of the lightning connection means 15 to the stationary part of the lightning protection means of the wind turbine 1, which for example is on the nacelle 3. The mounting hook 47 is used in this present embodiment for the mounting of the connection means as illustrated in Fig. 7 to the main shaft 14 (not shown). The mounting hook 47 is for example mounted directly to an external flange of the main shaft 14. The entire lightning connection means 15 is also mounted onto the main shaft 14 (also not shown). This may be achieved by the attachment of a flexible link similar to force transferring means 26, 27, onto the lightning current path 25 at one end and mounting the other end onto the main shaft. One flexible link may be attached to at least one of the lightning current paths 25 in each lightning transfer unit 24. This allows for a more securing mounting of the lightning connection means 15, with at least three flexible links mounted onto the main shaft. Mounting means for both the mounting of the connection means at the mounting hook 47 to the main shaft and mounting of the lightning connection means may be a simple nut and bolt arrangement or any other kind of securing arrangement. The main shaft, which is the base for mounting of both the lightning connection means 15 and the connection means to the stationary part of the lightning protection means of the wind turbine 1 through the mounting hook 47, is also the central axis of rotation around which the lightning connection means 15 and the connection means rotate. This would allow for the provision of a connection path from contact means 19a through dedicated electric connection 30 to a first end of current path 31 of a lightning current path 25. The connection path then preferably traverses the parallel impedances and exits the lightning connection means 15 at a second end of current path 32, which is connected to contact means 19b via a dedicated electric connection 46. There may be three such through connections in a three-bladed wind turbine 1.

The force transferring means 26, 27 in figs. 6 and 7 are preferably a flexible link and are made as boards in a composite material, e.g. reinforced glass fibre. The boards 26, 27 have a length, width and thickness which allow the boards 26, 27 to bend quite significantly without breaking when exposed to a force at one end. Further, the characteristics of the boards 26, 27 are such that the boards 26, 27 will respond with a significant counter-pressure at the end and return to the original positions when the forces are removed.

In a preferred embodiment, the boards 26, 27 are quadrangular in shape with a length between 200 and 500 millimetres, such as 300 millimetres, a width between 50 and 300 millimetres, such as 80 millimetres, and a thickness between 3 and 10 millimetres, such as 5 millimetres. The length normally depends on the size of the wind turbine 1. The width and thickness of the board 26, 27 are normally controlled by the length, e.g. a longer board 26, 27 also requires a thicker board 26, 27 in order to establish the correct and necessary flexibility. The contact means 19a, 19b are mounted on the side of the flexible links 26, 27, respectively, at the free end of the links 26, 27. The contact means are mounted pivotally on the boards 26, 27 by positioning the contact means in U shaped brackets 37, 45 with bolts 39 (not shown in these figures) in adjusting slots 38 (also not shown in these figures). By pivoting the contact means 19a, 19b, it is possible to alter the position of the contact area 20a, 20b in the up/down direction and, thus, adapt the contact means 19a, 19b to the contact surface 18 on the wind turbine blade 5 or the contact surface 17 on the sidewall 17a.

Each of the contact means 19a, 19b also comprises a retaining bolt 28, 29 (not shown in these figures) for the dedicated electric connections 30, 46, allowing the electric connection to be established and retained between the two contact means 19a, 19b and the lightning connection means 15. The electric connections 30, 46 are made in a flexible material with lengths corresponding to the distances between the two contact means 19a, 19b at their rest positions and the ends of current paths 31, 32 of the lightning connection means 15. If the contact means 19a, 19b are exposed to forces, the flexible links 26, 27 will bend resulting in more sagging dedicated electric connections 30.

The entire electrical connection between the blade(s) 5 and the nacelle 3, including at least the lightning connection means 15, the dedicated connection means 30, 46, and the contact means 19a, 19b, may also be referred to as a lightning current transfer unit of a wind turbine 1.

Figs. 8a and 8b illustrate different parts of the lightning connection means according to the invention.

Fig. 8a illustrates the contact means 19a facing the contact surface 18 of the wind turbine blade 5.

The contact means 19a comprises a connection to a flexible link 26 through a U shaped bracket 37. The bracket 37 and the flexible link 26 are bolted together with four bolts 36 penetrating the central part of the bracket 37 and the top of the link 26 before being terminated with nuts. The two opposite bracket sidewalls partly surround a frame unit 40 and are bolted to the unit with a bolt 39 through a hole in each bracket side. The bracket sidewalls further comprise substantially horizontal slots 38 below the holes. Further bolts are entered through the slots 38 and screwed into the frame unit 40. The bracket connection allows the contact means 19a and especially the contact area 20a to be moved pivotally within the limits of the slots 38. The frame unit is also the base for a sliding pad 41 of the contact area 20b and the means retaining the dedicated electric connection 30 including the retaining bolt 28 (as will be explained further in connection with fig. 9). The bracket 37 should be made in a durable material, e.g. glass fibre, as the flexible link 26 or in metal, e.g. steel. The material must be prepared for significant strains, such as high mechanical forces and temperatures.

Fig. 8b illustrates the contact means 19b facing the contact surface 17 of the coherent ring 17a of the stationary lightning protection means in the wind turbine 1. The construction of the contact means 19b preferably corresponds to the construction of the contact means 19a.

The figure further illustrates the integration of a rod 43 in the centre of a pad 44 in the contact area 20b.

The dedicated connection 30 establishes contact between the lightning transfer unit and the contact means 19a, 19b, especially, the contact areas 20a, 20b facing the contact surfaces 17, 18 of the stationary and rotating lightning protection means of the wind turbine 1. The dedicated connection 30 must be in a flexible material as the contact means 19a, 19b continuously adapt to the position of the contact surfaces 17, 18. The connections 30, 46 are preferably made as a flexible metal conductor, e.g. a woven copper or aluminium rope or cable, such as a tin coated copper cable, of diameters between 50 and 120 mm².

In a preferred embodiment of the invention, said force transferring means 26, 27 is at least one flexible link between said contact means 19a, 19b and a base support plate 22. By using a flexible link, which supplies a constant force to the contact means 19a, 19b, it is possible to continuously force the contact means 19a, 19b against the corresponding contact surface 17, 18. Furthermore, said force transferring means 26, 27 is bendable in one direction in relation to a stable position. By being bendable in one direction, it is ensured that each contact means 19a, 19b is continuously adapted to the position of the corresponding contact surface 17, 18. Further, the force transferring means not being bendable in other directions ensures that the contact means 19a, 19b does not slide of the contact surface 17, 18, e.g. sideways. The contact means 19a, 19b is moveable in at least one direction by the bending of said force transferring means 26, 27, said base support part 22 defining a fixed position for the other end of said force transferring means 26, 27.

In a preferred embodiment of the invention, at least a section of said force transferring means 26, 27 comprises a circular shape or establish a circular shape when exposed to a force, e.g. at the contact means 19a, 19b. Hereby, it is possible to use the material stress of the force transferring means 26, 27 in establishing the necessary force.

Said force transferring means 26, 27 is preferably made from a flexible material. Furthermore, said force transferring means 26, 27 is preferably in electrically insulating material, such as a composite material, e.g. glass fibre. The use of flexible material such as glass fibre material ensures a high flexibility together with a high durability. The use of an electrically insulating or non-conductive material ensures that the lightning current does not use the force transferring means 26, 27 as an alternative path toward a ground potential 11. In a preferred embodiment of the invention, said force transferring means 26, 27 comprises one or more boards with a length between 200 and 500 millimetres, such as 300 millimetres, a width between 50 and 300 millimetres, such as 80 millimetres and a thickness between 3 and 10 millimetres, such as 5 millimetres. The preferred movement of the force transferring means 26, 27 may be achieved with the board shape. Further, with the mentioned data a higher flexibility in one direction and less in other directions may be achieved.

Fig. 9 illustrates the contact means 19b of the lightning connection for contact with the stationary lightning protection means with a preferred embodiment of the contact area 20b. The contact means 19b comprises a frame unit 42 working as a common basis for the different features of the contact means 19b. The frame unit is connected to the flexible link (illustrated in fig. 8b) and to the front of the contact means 19b. The front includes the contact rod 43, which establishes a base for retaining the electric connection 46. The retaining is established by screwing a bolt 29 into a hole at the bottom end of the contact rod 43. The bolt 29 squeezes the electric connection 30 against the bottom of the contact rod 43.

The dedicated electric connection 46 is in direct contact with the contact area 20b through the rod extending through the frame unit 42 and pad 44. The surface of the rod 43 and the pad 44 jointly establish the contact area 20b, in which the contact area 20b preferably is flat with a smooth transition between the two. The rod 43 preferably makes the electric contact to the contact surface 17 of the stationary lightning protection system, and the pad 44 ensures a low friction slide on the contact surface 17. The area of the pad 44 will normally be significantly larger than the area of the rod 43 and thus ensures a common low friction slide on the surface 17.

The rod 43 is preferably made in an electric conductive material, such as copper, aluminium or a metal alloy based on one of the two, e.g. bronze. Further examples of suitable rod materials are metal graphites, in which the alloy has material contents of 15-95% copper, copper alloys or silver. Also, electro graphites are usable as rod materials.

The pad 44 is preferably made in a low friction material, such as nylon, acetal or Vesconite, e.g. glass filled nylon, in order to enhance the durability and the mechanical properties.

The contact surface 17 of the stationary lightning protection means is preferably made in stainless steel, such as a coherent ring 17a in sections of stainless steel.

Preferably, the construction of the contact means 19a of the lightning connection for contact with the rotating lightning protection means is the same as or at least very similar to the construction of the contact means 19b illustrated in fig. 9.

Fig. 10 illustrates schematically the continuous functionality of a part of the lightning connection means.

The contact means 19a is shown in a first situation (solid lines) forced against the contact surface 18 with little strain on the flexible link 26. This situation may be regarded as an example of the outer limit of a contact surface position. In the second situation (dotted lines), the contact surface 18 has moved somewhat forward, and the contact means 19a is thus forced back, and the flexible 26 link is bending due to the force applied on the contact means 19a and the fixed position of the base support part 22. The contact means 19a also performs a pivotally movement (in the adjustable bracket 37) as it is forced back. The contact means 19a may remain in good electric contact with the contact surface 18 by performing this adjustment. The adjustment is performed automatically as the applied forces to the contact means 19a from the contact surface 18 or the flexible link 26 will always press the contact means 19a toward the contact surface 18. Thus, the largest surface area 20a of the contact means 19a will be forced against the contact surface 18 and, preferably, the two surfaces 18, 20a are parallel when forced against each other.

The invention has been exemplified above with reference to specific examples. However, it should be understood that the invention is not limited to the particular examples described above but may, e.g., be used in connection with a wide variety of different wind turbine types in which the rotor blades 5 are more passively controlled types involving stall or active stall.

Further, it should be understood that especially the flexible link constituting the force transferring means for the contact means may be designed in a multitude of varieties within the scope of the invention as specified in the claims.

Even further, it shall be emphasized that the metal rings and belts of the stationary and rotating lightning protection means may be less than coherent, e.g. missing a section. The metal ring of the stationary lightning protection means may, for example, be without the lower third part. The lightning connection means will then be without connection to the ground potential 11 in the lower third part of a rotation.

The metal belt of the rotating lightning protection means may be diminished to half the normal size. The blade of the rotating lightning protection means may thus only be pitched a maximum of 90 degrees forward and backward in order to remain in contact with the lightning connection means.

Claims

1. Lightning connection means for a wind turbine, said wind turbine comprising

stationary lightning protection means, and

at least one wind turbine blade comprising

rotating lightning protection means, said stationary and rotating lightning protection means comprising

contact surfaces connected by said lightning connection means, characterised in that for a given wind turbine blade, said lightning connection means comprises

a lightning transfer unit comprising

a plurality of lightning current paths, said plurality of paths being arranged to divide a lightning stroke current from the wind turbine blade between the plurality of paths in a parallel impedance configuration, first ends of said plurality of paths of each lightning transfer unit being arranged to be connected electrically to at least one first contact surface of said rotating lightning protection means, and second ends of said plurality of paths of each lightning transfer unit being arranged to be connected electrically to at least one second contact surface of said stationary lightning protection means.

2. Lightning connection means according to claim 1, characterised in that said plurality of paths is formed as metal bars.

3. Lightning connection means according to claim 1 or 2, characterised in that said paths are positioned side by side at a certain mutual distance from each other, said distance being large enough to avoid inductive interference between the paths.

Lightning connection means according to any of the claims 1 to 3, characterised in that two or more lightning transfer units are coupled in parallel by connecting electrically said first ends of said two or more lightning transfer units to each other and/or by connecting electrically said second ends of said two or more lightning transfer units to each other.

Wind turbine comprising stationary means such as a nacelle and a tower comprising stationary lightning protection means,

rotating means such as a rotor including at least one wind turbine blade and shaft means, each of said at least one wind turbine blade comprising rotating lightning protection means,

said stationary and rotating lightning protection means comprising contact surfaces, characterised in that said contact surfaces are connected by lightning connection means according to any of the claims 1 -4.

Method of establishing a continuous contact between rotating and stationary lightning protection means of a wind turbine comprising at least one wind turbine blade, said method comprising for a given wind turbine blade the steps of: creating a lightning transfer unit comprising a plurality of lightning current paths,

arranging said plurality of paths to divide a lightning stroke current from the wind turbine blade between them in a parallel impedance configuration,

establishing at least one first contact between at least one first contact surface of said rotating lightning protection means and at least one first contact means,

connecting said at least one first contact means electrically to first ends of said plurality of paths by establishing a first dedicated connection between said at least one first contact means and said first ends, establishing at least one second contact between at least one second contact surface of said stationary lightning protection means and at least one second contact means, and

connecting said at least one second contact means electrically to second ends of said plurality of paths by establishing a second dedicated connection between said at least one second contact means and said second ends.

7. Method according to claim 6, wherein the step of creating said lightning transfer unit involves manufacturing said plurality of paths as metal bars.

8. Method according to claim 6 or claim 7, wherein the step of arranging said plurality of paths involves positioning said plurality of paths side by side at a certain distance from each other, said distance being large enough to avoid inductive interference between the paths.

9. Method according to any of the claims 6 to 8 further comprising the step of coupling in parallel two or more lightning transfer units by connecting said first ends of said two or more lightning transfer units to each other and/or by connecting said second ends of said two or more lightning transfer units to each other.

10. Use of lightning connection means according to any of the claims 1 to 4 in a wind turbine, such as a wind turbine according to claim 5, in which the wind turbine blades are pitch controlled.

11. Use of a method for establishing a continuous contact between rotating and stationary lightning protection means of a wind turbine according to any of the claims 6 to 9, in which the wind turbine blades are pitch controlled.