The increasing development in the field of wind energy plants is reflected in large units with still higher towers and longer blades resulting in the increased risk of being struck by a lightning. The plants are typically secured from lightning strokes, so that a possible lightning current is attracted and led to the ground in a controlled manner in order not to damage the sensitive elements in the wind energy plant. One of the most popular methods for securing lightning is by installing one or more so-called lightning receptors, which are conductive elements, and which are placed e.g. on the tip of the blade and connected to internal conducting cables in the blades. This is e.g. known from EP 0783629.
A similar method has been described in U.S. Pat. No. 6,457,943 according to which a wind turbine blade is constructed with long parts of carbon fibre material in the total length of the blade. The carbon fibres, which are conductive, are thus acting as a lightning receptor, and the lightning current is led through the material and down into internally placed conducting cables. This method thus relates to the construction of the entire blade, and it not only requires the use of carbon fibres in large parts of the blade, which is not always desirable, but also a specific thickness of the carbon fibre parts so that the material can lead the lightning current without being damaged.
The blades are usually the longest part of the wind energy plant, and they therefore have an increased risk of being struck by a lightning. Wind energy plants are placed in e.g. large numbers in the sea in the form of windmill farms, resulting in the fact that service and maintenance becomes rather expensive and complex due to weather conditions and difficult access conditions. Salt from the air deposits on e.g. the blades, thus making these conductive, which again increases the risk of these being struck by a lightning.
In WO 01/77527 it is suggested to glue or tape strips of copper strings on to the blades and connect the strings to the receptors. The strings are intended to conduct a lightning current to a receptor, from where it can be led to the ground via a cable. This implies that the strings have sufficient conductivity in order to be able to hold a lightning current which may exceed 50 kA. The strings must be exchangeable after strokes of lightning due to the damage caused by the strong heating resulting from the lightning current, which not only means monitoring of the wind energy plant and standstill during change, but also big expenses. There is furthermore the risk of the strings getting loose and consequently affecting the aerodynamic properties of the blade negatively. The strings must otherwise be made of a heavy material, resulting in undesired extra weight on the blade. One of the problems when designing long blades is to lower the weight, in that the weight in itself not only makes the blades even heavier, but also the hub, the nacelle and the tower.
Aircrafts are also exposed to lightning strokes and must therefore be secured from these. Aircrafts are equipped with radar installations, e.g. in connection with navigation, which are often placed in the nose of the aircraft in order to be able to look ahead. When radar equipment is placed in the nose of the aircraft, the nose is not made from aluminum like the remaining part of the aircraft. Rather, it is made of a certain plastic material, as the radar otherwise would be unable to see through the nose. The nose must also be secured from lightning strokes, but it must be done in a specific way in order not to disturb the radar. Since the 1960's, the idea of lightning protection by means of so-called lightning diverter strips, which may be of different shapes, has been known. One example is seen in U.S. Pat. No. 4,237,514, where a base, supplied with aluminum powder, is glued on to e.g. the nose of an aircraft in strips.
The aluminum powder does not constitute a continuous conductor, but rather disrupted or segmented, conductive particles. When these metallic and conductive particles are exposed to a strong electrical field as the result of a lightning stroke, short circuit among the particles takes place, and a conductive ionized channel is created in the air above the particles in which the current lightning can be led to e.g. some metal part of the aircraft. Instead of aluminum powder, U.S. Pat. No. 4,506,311 describes button- or diamond-shaped metal pieces, which are separately incorporated into a base shaped as a ribbon. Both ribbons and strips are intended for installation outside of the nose of the aircraft, where they are placed symmetrically radiating from the tip of the nose. Such a position provides a good protection, but at the same time it results in aerodynamic disturbance. Furthermore, these strips have the disadvantages of not lasting very long, as the segments easily get ripped off the base into which they are incorporated, either by the lightning or by simple wear. Therefore, these ribbons often need to be renewed when having been exposed to lightning. However, in many applications this is very costly and impractical. Diverter strips according to U.S. Pat. No. 4,506,311 are furthermore seen to have the problem that the lightning current can jump from underneath the segments to the next, which increases the risk of the current jumping into the structure, which should be protected by the diverter. Another disadvantage is that the strips are difficult to fasten onto the surfaces without unwillingly stretching the strips or ribbons a little, thereby changing the distances between the segments accordingly. This again changes the lift-off capacity of the strip as well as making small openings in the material where water can penetrate and start deterioration of the material.
USPTO Backgrounds
USPTO Backgrounds