LIGHTNING ARRESTER FOR WINDMILL BLADES

Abstract: The invention relates to a lightning arrester arrangement for mounting in a windmill blade (1). The lightning arrester arrangement consists of at least one lightning conductor (4a, 4b, 4', 4'') placed in the tip (1') of a windmill blade (1). The lightning conductor or the lightning conductors are connected to earth via an inner conductor (6). The novelty aspect of the invention is partly that the earthing of the lightning is effected only in or from the tip (1') of the blade, and partly that the further earthing towards the earth connection through the blade is effected via an inner conductor which wholly or partly extends between the blade tip and the mill hub (3). The invention also relates to a windmill blade with a lightning arrester arrangement of the type described above, and also to a lightning conductor for mounting in the tip (1') of a windmill blade (1).

Description: LIGHTNING ARRESTER FOR WINDMILL BLADES Background of the invention The invention relates to a lightning arrester arrangement for the blade of a windmill as disclosed in the preamble to claim 1. The invention also relates to a windmill blade with such a lightning arrester arrangement built into said blade, and a lightning conductor for building into the said lightning arrester arrangement. Finally, the invention re¬ lates to the use of said lightning arrester arrangement for mounting in an existing windmill blade. A number of known techniques are used in the normal pro¬ tection of constructions against lightning. The most used comprises an earthed cage around the construction it is desired to protect. In a more simplified form, the lightning arrester can be reduced to an earthed conductor which extends over the highest part of the construction, and said conductor can possibly be provided with one or more vertical projections. A traditional lightning arrester of this type is, however difficult to establish on, for example, a windmill, where the highest point is the blade, which from the point of view of security is desired to be protected against a stroke of lightning. The technique is complicated in com- parison with stationary constructions, in that the part of the construction which it is desired to protect is movable. This means that a possible stroke of lightning is not so well-defined or predictable, and it is herewith difficult to prevent or to lead away. Moreover, in connection with windmill blades struck by lightning, there is a great risk that the blade suffers damage. This necessitates time- consuming repairs, in that a possible replacement or repair of a windmill blade is complicated. In connection with composite materials, of which windmill blades are typically made, lightning conductor arrangements are known in the form of metal foil or wire laminated into or affixed by adhesive to the blade. Such lightning conductor arrangments are always based on conductors which are placed on or in the surface of the blade. Normally, it it is thus the intention to create a metallic net around the inner structure of the blade in order to create a combined longitudinal lightning arrester and conductor along the rear edge of the blade. However, it is common to all these arrangements that it is difficult to achieve the necessary cross-sectional area with hereby incurring undesired aerodynamic effects, for example in the form of noise and/or reduced efficiency. Furthermore, it can prove problematic to get a metallic material to main¬ tain the degree of tension in a blade of composite material, in that the degree of tension in a blade of composite material will normally be higher than the per¬ missible fatigue influence for a metallic material, and it will therefore require special arrangements to avoid fatigue cracks. Without the necessary coss-sectional area, it can be necessary to repair the lightning conductor arrangement after influence by lightning. Advantages of the invention As disclosed in claim 1, by letting the lightning arrester arrangment consist of one or more lightning conductors of an electrically-conducting material on or in the surface of the balde in the vicinity of the blade tip, where the lightning conductors are connected to an inner conductor which extends inside the blade, and connecting the lightning conductor or lightning conductors to an earth connection, an effective lightning arrester is achieved and herewith a good protection for the blade. It has thus proved to be sufficient to provide lightning protection for the area in the vicinity of the blade tip, in that lightning most often strikes the tip of the blade. By letting the inner conductor extend inside the blade, un- desired disadvantages with regard to strength, aerodynamics and accoustics are avoided. Moreover, the arrangement is ideal in connection with any subsequent mounting of a blade without lightning protection. As disclosed in claim 2, by connecting the lightning arrester arrangement's inner conductor to an earth connection via the windmill hub, the most simple conduction from the root of the blade is achieved. As disclosed in claim 3, by placing the inner conductor in or in the immediate vicinity of the neutral axis of the windmill blade, the degree of tension achieved at the inner conductor is lower than the fatigue limit for the material. An advantage from the point of view of strength can hereby be achieved by the placing of the inner conductor in a windmill blade. As disclosed in claim 4, by configuring the lightning in a streamlined manner, preferably with a torpedo-shape or guttiform, there is achieved a combination of the desired lightning arrester effect and a similarly-desired damping of acoustic noise stemming from the effects of turbulence at the tip of the blade. As disclosed in claim 5, by letting the inner conductor ex¬ tend in a hollow tip axle, when this may exist in a wind¬ mill blade, there is achieved an effective earthing through the inside of the blade, a reinforcement of the tip axle and a better securing of the rotatable tip for the windmill blade . As disclosed in claim 6, by letting the inner conductor wholly or partly consist of an electrically-conducting tip axle, the lightning arrester arrangement thus achieved can be used for known systems without any great reconstruction and adaptation of the technique. As disclosed in claim 7, by letting the inner conductor wholly or partly consist of a metal wire with or without a cladding of insulating material, a simple and inexpensive embodiment is achieved. As disclosed in claim 8, by configuring the lightning con- ductor or lightning conductors aerodynamically from an electrically-conducting material, and placing this in or in the vicinity of the tip of the windmill blade, the lightning conductor thus achieved is one which is ideal for use in a lightning arrester arrangement, in that this will be easy to mount afterwards, and moreover it will result in a damping of acoustic noise which may possibly arise at the tip of the blade due to the effects of turbulence. As disclosed in claim 9, by mounting the lightning arrester arrangment on a windmill blade, an inexpensive and effective protection for windmill blades against lightning is achieved, in that this can be mounted without any great side-effects in connection with the blade' s aerodynamic or acoustic characteristics or from the point of view of strength. Thus the blade of the windmill will be well pro¬ tected against damage as a result of being struck by lightning, and this will also afford protection for the system that it may be a part of, in that both the windmill housing and the electrical control will be better protected as a consequence, in that the lightning will be earthed in an expedient manner. It is thus particularly advantageous, in connection with windmill blades which have a rotatable tip and a metallic tip axle, that the lightning-arrester arrangement can be mounted afterwards, in that the blade tip can easily be dismantled for the building-in of the lightning conductor, after which this can be connected to the metallic part of the tip axle. The drawing In the foillowing section, example embodiments of the in¬ vention will be described in more detail with reference to the drawing, where fig. 1 shows a known windmill, fig. 2 shows an embodiment according to the invention, fig. 3 shows a cross-section of the embodiment in fig. 2 seen in the direction III-III, fig. 4 shows a further embodiment according to the invention, and fig. 5 similarly shows a further embodiment according to the invention. Description of the example embodiments In fig. 1 is seen an ordinary windmill, where the windmill blades 1 are suspended in a mill hub 3, which in turn is anchored on a tower 2. In fig. 2 is seen an embodiment according to the invention, where two lightning conductors 4a, 4b, which consist of an electrically-conducting material, are connected to an inner conductor 6 consisting of a steel wire. The inner conductor 6 is anchored to and electrically connected with the mill hub 3 (not shown). In the shown embodiment, there are mounted two lightning conductors 4a and 4b which project out from the windmill blade 1 and the blade tip 1 ' . In fig. 3 is seen a cross-section of the windmill blade 1 in fig. 2 in the section III-III, where the inner conductor 6 is seen to extend in the vicinity of the neutral axis of the blade. The two lightning conductors 4a, 4b placed at the tip 1' of the blade 1 will be sufficient to protect the whole of the windmill blade 1, in that experience has shown that lightning most often strikes at the tip 1 ' of the windmill blade 1. When the blade is struck, the lightning will thus flow from the lightning conductors 4a, 4b via the inner conductor 6 towards the mill hub 3 (not shown). The result of the inner conductor 6 being thus placed is that the blade will retain its original strength and dynamic characteristics, combined with an effective earthing of the lightning. In fig. 4 is seen a further embodiment according to the in- vention. A lightning conductor 4', which consists of an aluminium rod, extends to form a point towards the rear edge of the lightning conductor 4' . Via a bored hole 8 and a terminal 9, the lightning conductor 4' is connected to a blade tip 1 ' with an inner conductor 6. The inner con- ductor 6 extends towards the hub of the mill in a tip axle 10 which is configured from a composite material. The tip axle is connected at a terminal 14 to a guide 11 by means of a lock-washer 13 and a nut 12. The actual earthing of the lightning with the embodiment shown in fig. 4 will take place in the same way as with the system shown in fig. 2, in that the lightning will flow from the lightning conductor 4 ' via the inner conductor 6 in towards an earth connection at the root of the blade. The streamlined configuration of the lightning conductor 4' contributes towards giving the blade a further advantage, in that a blade configured with this streamlined conductor at the tip of the blade gives the blade an improved efficiency and a smaller emission of noise. The inner conductor 6 also serves to secure the rotatable tip 1' to the blade itself. In connection with the protection of a windmill blade 1 with a rotatable tip 1', the invention is particularly ad¬ vantageous in that the protection is implemented in the tip 1' of the blade, and in that the earthing takes place via an inner conductor. The inner conductor 6 earths a pos¬ sible stroke of lightning, regardless of whether or not the rotatable tip 1' is rotated. In fig. 5 is seen a further embodiment according to the in¬ vention. A lightning conductor 4' ' is disposed in the tip of a rotatable blade tip 1 ' , and is connected to a tip axle 15 which is made of stainless steel. The tip axle 15 is secured to a blade 1, in that the blade 1 has a pair of in- laminated brackets 16. An inner conductor 6, consisting of a stainless steel wire, is connected to the tip axle 15 by means of a clamp, and extends in towards the anchoring of the windmill blade 1. The shown embodiments are particularly advantageous in con¬ nection with the subsequent mounting of a lightning arrester on an existing windmill blade 1, in that the existing installations can easily be utilized or replaced. The actual lightning conductor 4' ' can thus easily be mounted, in that this must merely be mounted instead of a closing plug which sits in the same place in an existing blade.

Claims: C L A I M S 1. Lightning arrester arrangement for mounting in a wind¬ mill blade (1) of composite material, with or without ro- tatable tip ( 1 ' ) , c h a r a c t e r i z e d in that the lightning arrester arrangment consists of one or more lightning conductors (4a, 4b, 4', 4'' ) of an electrically- conducting material in or in the vicinity of the tip ( 1 ' ) of the blade and on or in the surface of the blade (1) , where the lightning conductor or conductors (4a, 4b, 4', 4' ' ) are connected with an inner conductor ( 6) which ex¬ tends inside the blade (1) and connects the lightning con¬ ductor or conductors (4a, 4b, 4', 4' ' ) to an earth con¬ nection. 2. Lightning arrester arrangement according to claim 1, c h a r a c t e r i z e d in that the inner conductor (6) of the lightning arrester arrangement is connected to an earth connection via the hub (3) of the windmill. 3. Lightning arrester arrangement according to claim 1 or 2, c h a r a c t e r i z e d in that the inner conductor ( 6 ) is disposed in or in the vicinity of the neutral axis of the windmill blade ( 1) . 4. Lightning arrester arrangement according to any of the claims 1 to 3, c h a r a c t e r i z e d in that the lightning conductor or conductors (4a, 4b, 4', 4' ' ) are streamlined, preferably with torpedo-shape or guttiform. 5. Lightning arrester arrangement according to any of the foregoing claims 1 to 4, c h a r a c t e r i z e d in that the inner conductor (6) extends inside a hollow tip axle. 6. Lightning arrester arrangement according to any of the foregoing claims 1 to 5, c h a r a c t e r i z e d in that the inner conductor ( 6 ) wholly or partly consists of an electrically-conducting tip axle (15). 7. Lightning arrester arrangement according to any of the foregoing claims 1 to 6, c h a r a c t e r i z e d in that the inner conductor wholly or partly consists of a metal wire with or without a cladding of insulation material. 8. Lightning conductor for mounting on a windmill blade, c h a r a c t e r i z e d in that this is aerodynamically configured of an electrically-conducting material and is disposed in or in the vicinity of the tip of the windmill blade. 9. Use of the lightning arrester arrangement according to claims 1 to 7, c h a r a c t e r i z e d in that the arrangement is used on a windmill blade (1) .

Inventors: STIESDAL HENRIK, STIESDAL, HENRIK
Applicants: BONUS ENERGY AS, STIESDAL HENRIK, BONUS ENERGY A/S, STIESDAL, HENRIK
Application year: 1995
Main classification: F03D11
IPC codes: F03D11, H02G13