Patent Description:
Wind turbine blades are the most exposed parts of the wind turbine with respect to lightning strikes. Therefore, it is critical that the lightning protection system of a wind turbine blade is properly designed to protect the blade from damage in the event of a lightning strike. This is conventionally done by having an internal down conductor in the blade, which is connected to a grounding system of the remainder of the wind turbine, thus providing a safe path to ground for the lightning current. Attachment points for the lightning - commonly known as lightning receptors - are positioned along the blade surface and are connected to the internal down conductor of the rotor blade. Typically, the internal down conductor contains metal blocks at the receptor positions - so-called receptor blocks - into which the respective lightning receptor are mounted.

In <CIT>, a wind turbine blade with a lightning protection system is described. The blade comprises a blade body made from a fiber-reinforced plastic and an aluminum blade point member joint to the blade body at the tip of the blade. The blade point member is connected to a down conductor to allow for grounding of lightning strikes.

In order for the lightning protection system to function properly, it is desired that the lightning protection system is designed in such a way that the lightning always attaches to the lightning receptors, and not to the internal down conductor. A well-known way of ensuring this is to provide sufficient insulation of the internal down conductor. This is especially critical in the outer <NUM>-<NUM> meters of the blade closest to its tip, which will experience the vast majority of lightning strikes. Sufficient insulation may be achieved by encapsulating the internal down conductor in thick insulation material, typically a polymer such as e.g. polyurethane.

Away from the tip section of the blade, the insulation may be provided by using a conventional insulated high voltage cable. The tip section of the blade is conventionally the narrowest part of the blade. This puts limitations to the size, and thereby the insulation strength, of the internal down conductor insulation required in the tip section of the lightning protection system. Furthermore, for blades casted using the integral blade process, the manufacturing method further puts restrictions on the size and geometry of the insulated lightning protection tip part due to the mandrels and vacuum bags required for the process. In addition to this, the insulation of the lightning protection tip part should only introduce a limited increase in total mass of the blade tip, which restricts the insulation volume and/or material.

In consideration of the aforesaid, it is an object of the present invention to provide an improved lightning protection system for a rotor blade of a wind turbine. The improved system shall particularly be able to reduce the risk of lightning strikes hitting the internal parts of the lightning protection system, such as the down conductor or receptor blocks, instead of the lightning receptors.

This object is achieved by the independent claims. Advantageous modifications and embodiments are disclosed in the dependent claims.

According to the invention, there is provided a rotor blade of a wind turbine, wherein the rotor blade comprises a trailing edge section with a trailing edge and a leading edge section with a leading edge, a root section with a root and a tip section with a tip. The rotor blade also comprises a shell with a suction side shell section and a pressure side shell section, wherein the outer surface of the shell defines the outer shape of the rotor blade and the inner surface of the shell defines a cavity of the rotor blade. Furthermore, the rotor blade comprises a lightning protection system with an internal down conductor extending from the root section of the rotor blade to the tip section of the rotor blade, wherein the internal down conductor is connectable at the root section to a grounding system of the remainder of the wind turbine and at the tip section to at least one tip lightning receptor which is positioned at the outer surface of the rotor blade. In addition, the rotor blade comprises an electrically insulating tip unit, wherein the tip unit is arranged in the tip section of the rotor blade in the cavity of the rotor blade and wherein the tip unit encapsulates at least a part of the internal down conductor in the tip section of the rotor blade. At least a part of the tip unit covers the entire area between a part of the suction side shell portion and a part of the pressure side shell portion, and the connection between the tip unit and the suction side shell portion as well as the connection between the tip unit and the pressure side shell portion is a cast interface, respectively.

A key aspect of the present invention is that at least a part of the tip unit covers the entire area between a part of the suction side shell portion and a part of the pressure side shell portion. In other words, the tip unit (or a part thereof) fills the entire cavity of the rotor blade in that section of the rotor blade where the tip unit is situated. As the tip unit advantageously only extends over a relatively small portion of the rotor blade (e.g. over less than three per cent of the length of the rotor blade), only relatively small parts of the suction side shell section and the pressure side shell section are directly connected with the tip unit.

An important feature of the rotor blade according to the invention is the cast interface between the tip unit and the shell of the rotor blade. A cast interface has the advantage of an improved fixation and an improved sealing joint of the tip unit to the shell of the rotor blade, compared to adhesive joints as disclosed e.g. in the patent applications <CIT> or <CIT>. This ensures that the risk of e.g. water ingress and consequently corrosion and explosive expansion of water during a lightning strike is minimized. A further advantage is that the electrical insulation properties of the tip unit in the interior of the rotor blade, i.e. in the cavity of the rotor blade, are improved.

The tip unit may exemplarily be a pre-casted component. By pre-casting the tip unit according to the desired blade geometry, and including that tip unit in the blade infusion process, per definition a perfect transition between the blade shell and the insulating tip unit is achieved due to the self-alignment of the parts' interfaces.

Another effect of the coverage of the entire area by at least a part of the tip unit is that the tip unit thus contributes to the stabilization of the rotor blade. The tip unit thus has a structural effect besides the fundamental function to insulate the internal down conductor by means of encapsulation.

In the context of this patent application, the tip section of the rotor blade is defined as that section which extends from the tip of the rotor blade inboard, i.e. towards the root of the rotor blade (the tip of the rotor blade can be seen as the "outermost" point of the rotor blade as seen from the hub, in case that the rotor blade is mounted to a hub of the rotor of the wind turbine). Exemplarily, the tip section of the rotor blade may have an extension of three meters.

The tip unit comprises a tip part which covers the entire area between the leading edge and the trailing edge of the rotor blade.

In other words, the tip unit advantageously comprises one part which covers the cavity of the rotor blade over the entire chordwise distance. As the shell of the rotor blade, which is typically made of a fiber-reinforced composite laminate, necessarily features a certain thickness, the distance from the leading edge to the trailing edge in the cavity, i.e. at the inner surface of the shell, is smaller than the distance from the leading edge to the trailing edge measured at the outer surface of the shell. This is particularly true for the trailing edge section, which is usually designed as a sharp and narrow region, and which could be filled with a so-called trailing edge core.

An advantage of filling the cavity at the entire distance from the leading edge to the trailing edge is that the stability and stiffening of the rotor blade is even further improved.

In the case that the rotor blade is manufactured according to the vacuum assisted resin transfer molding (VARTM) process, a further advantage of filling the cavity at the entire distance from the leading edge to the trailing edge is that manufacturing of the rotor blade, i.e. processing, is facilitated. This is due to the fact that otherwise vacuum bags would be required to fill out the relatively narrow space between the tip unit and the leading or trailing edge. By filling the entire cavity from the leading to the trailing edge, this is avoided.

The tip part is located at the outermost part of the cavity of the rotor blade.

Exemplarily, the tip part has a spanwise dimension of at least <NUM> per cent of the length of the rotor blade.

In terms of absolute numbers, the tip part advantageously has a spanwise dimension of at least ten centimeters, in particular of at least twenty centimeters.

In addition or instead of the tip part, the tip unit may comprise an extension part which covers less than the entire area between the leading edge and the trailing edge of the rotor blade.

Advantageously, the extension part is located at that region where the side lightning receptors of the lightning protection system of the rotor blade are located. As the side lightning receptors are oftentimes, as a consequence of the motion of the rotor blade, located at the trailing edge section of the rotor blade (motivated by the "hang-on zone" of the lightning), the extension part is preferably located in the trailing edge section of the rotor blade.

Assuming the case that the rotor blade comprises at least one side lightning receptor and that the side lightning receptor is located at the trailing edge section, it is not necessary that the tip unit extends at that section over the entire area from the leading edge to the trailing edge. With regard to savings in material, which is desirable for cost reduction and weight reduction reasons, it is thus advantageous that the tip unit extends only within a reduced area between the trailing edge and the leading edge, e.g. only in the trailing edge section.

Exemplarily, the extension part has a spanwise dimension, i.e. lengthwise extension of at least one per cent of the length of the rotor blade. Preferably, the spanwise extension of the extension part is in a range between one per cent and ten per cent of the total length of the rotor blade.

In another embodiment of the invention, the side lightning receptor is connected with the internal down conductor via a side receptor block. The side receptor block is then encapsulated by the extension part of the tip unit.

Likewise, in another embodiment of the invention, the tip lightning receptor is connected with the internal down conductor via a tip receptor block, which is encapsulated by the electrically insulating tip unit as well.

It is usual to connect lightning receptors via so-called receptor blocks with the internal down conductor. Preferably, not only the internal down conductor, but also the receptor block is encapsulated by the electrically insulating material of the tip part.

An attractive choice of a material for the tip unit is a polymer, for example polyurethane.

An important reason to choose a polymer, in particular polyurethane, is its favorable dielectric properties, particularly its high breakdown strength.

Another advantage is that a polymer, in particular polyurethane has a very low electrical conductivity (the electrical conductivity of polyurethane is negligible, such that it is regarded as an electrically insulating material), a low weight (which is important as it is desired to add as little weight as possible to the rotor blade) and is inexpensive.

In another embodiment of the invention, the internal down conductor comprises a high voltage insulation in the section which is adjacent to the tip unit of the rotor blade.

Additionally, in the inboard section of the rotor blade, the internal down conductor may comprise a low voltage insulation. Alternatively, in the inboard section of the rotor blade, the internal down conductor may as well have no specific insulation at all.

The risk of a lightning strike on a rotor blade is generally smaller for sections which are closer to the root of the rotor blade, compared to sections which are further away from the root. Therefore, in practice, it is oftentimes sufficient to only provide, in the inboard section of the rotor blade, a low voltage insulation (or even no specific insulation) for the internal down conductor. In this respect, the wording "low" voltage insulation compares to the "high" voltage insulation of the internal down conductor in mid-board section of the rotor blade and the maximum insulation provided by the provision of the entire encapsulation of the internal down conductor in the outermost section of the rotor blade.

Note that, apart from the reduced risk of lightning strikes in the inboard section of the rotor blade, the internal down conductor is also more protected in the larger cavity of the root section of the rotor blade, compared e.g. to the relatively narrow cavity in the outboard or even tip section of the rotor blade.

Finally, the invention is also related to a wind turbine for generating electricity comprising at least one rotor blade as described above.

Another aspect of the present invention relates to a method of manufacturing a rotor blade of a wind turbine according to one of the claims <NUM> to <NUM>. The method comprises the following method steps:.

A resin transfer molding process, in particular a vacuum assisted resin transfer molding (VARTM) process, typically uses a mold tool with a vacuum bag and the use of a vacuum to assist the resin flow. The VARTM process applied to wind turbine rotor blades is also referred to as the "Integral Blade" process. In contrast to this, a rotor blade which is manufactured according to the "butterfly concept" is made of two separately manufactured half shells which are - after casting - bonded together with an adhesive joint.

The present invention is particularly valuable for a rotor blade being manufactured by the VARTM process. In this manufacturing process, the fiber-reinforced laminate material of the blade shell is not yet stiff and rigid when the two half shells are brought together, as the resin has not yet been injected and, even if some parts of the fiber-reinforced laminate material are pre-impregnated by resin, the resin is in any case not yet cured. Given these facts, the tip unit, which has a solid structure and which is provided exemplarily as a pre-casted component, enables a self-alignment of the tip unit and the surrounding blade shell laminate. In other words, the geometry of the tip section of the rotor blade can be accurately and reliably determined by the tip unit which is placed upon the layout of the laminate blade shell material and to which the upper mould with the laid out laminate blade shell material of the upper shell clings to.

In a conventional rotor blade of a wind turbine, there might also exist pre-casted parts, e.g. the web, the spar caps or root reinforcement parts. These pre-casted parts are inserted into the fiber layout before injection of the resin and curing of the resin. In the tip section of the rotor blade, however, there are usually no pre-casted parts presents. Therefore, a tip unit as disclosed in the present invention could contribute in a favorable manner for stabilization and self-alignment of the rotor blade, especially in its tip section.

Embodiments of the invention are now described, by way of example only, with the help of the accompanying drawings, of which:.

Note that the drawings are in schematic form. Furthermore, similar or identical elements may be referenced by the same reference signs.

<FIG> shows a wind turbine <NUM> for generating electricity. The wind turbine <NUM> comprises a tower <NUM>. The tower <NUM> comprises one end by which the tower <NUM> is connected to the ground <NUM> and another, opposite end, by which the tower <NUM> is connected to a nacelle <NUM> of the wind turbine <NUM>. The nacelle <NUM> is rotatable mounted with regard to the tower <NUM>. This enables a yaw movement of the nacelle <NUM> with respect to the tower <NUM>. The nacelle <NUM> accommodates the generator of the wind turbine and further components of the wind turbine <NUM>. If the wind turbine is a geared wind turbine, the nacelle <NUM> also may comprise a gear box.

The wind turbine <NUM> furthermore comprises a rotor which is arranged rotatable with respect to the nacelle <NUM>. The rotor can thus be rotated about a rotor axis <NUM> which is located substantially horizontal with regards to the ground <NUM>. The rotor of the wind turbine is destined to capture the energy of the wind, transform this energy into a rotational movement and transmit the rotational movement to the generator of the wind turbine. The rotor of the wind turbine comprises a hub <NUM> and at least one, typically two or three, rotor blades <NUM>. The rotor blades <NUM> are mounted to the hub <NUM>. Each rotor blade <NUM> comprises a tip section <NUM> and a root section <NUM>. Each rotor blade <NUM> is mounted at its root section <NUM> to the hub <NUM>. In many current wind turbines, the rotor blades <NUM> are mounted rotatable to the hub <NUM>. This means that the rotor blade can be pitched about a so-called pitch axis <NUM> in order to optimize energy capture and loads acting on the rotor blade <NUM> of the wind turbine <NUM>.

The wind turbine <NUM> also comprises a lightning protection system. The lightning protection system comprises an internal down conductor <NUM>, extending from the root section <NUM> of the rotor blade <NUM> to the tip section <NUM> of the rotor blade <NUM>. The internal down conductor <NUM> is connected at the root section <NUM> of the rotor blade <NUM> to a grounding system <NUM> of the remainder of the wind turbine <NUM>. The grounding system <NUM> transmits the electrical current from the internal down conductor <NUM> to the ground <NUM>.

<FIG> illustrates a rotor blade <NUM> with a lightning protection system <NUM> according to the state of the art. The rotor blade <NUM> comprises a root section <NUM> with a root <NUM> and at its opposite end a tip section <NUM> with a tip <NUM>. The distance between the root <NUM> and the tip <NUM> is referred to as the blade length <NUM> or length <NUM> of the blade <NUM>. The outer shape of the rotor blade <NUM> is defined by the shell <NUM> of the rotor blade <NUM>. The shell <NUM> is typically made of a fiber reinforced composite material which is often times glued to another component such as Balsa wood. This results in a lightweight and stiff structure which can be described as a laminate. The rotor blade <NUM> comprises a leading edge section <NUM> with a leading edge <NUM> and a trailing edge section <NUM> with a trailing edge <NUM>. A chord or chordline is defined by the straight line between the leading edge <NUM> and the trailing edge <NUM> at each spanwise position ranging from the root <NUM> to the tip <NUM> of the rotor blade <NUM>. The leading edge section <NUM> is defined as that section of the rotor blade <NUM> which is adjacent until ten percent of the chord length as measured from the leading edge <NUM> of the rotor blade <NUM>. Likewise, the trailing edge section <NUM> is defined as the "last" ten percent in chordwise direction, i.e. the region which is encompassed by ninety percent chord length to one hundred percent chord length. The trailing edge <NUM> and the leading edge <NUM> divide the outer surface <NUM> of the rotor blade <NUM> into a suction side and a pressure side. With regard to the shell <NUM> of the rotor blade <NUM>, a suction side shell section <NUM> and a pressure side shell section <NUM> can be assigned to the rotor blade <NUM>. <FIG>, for example, shows a top view on the suction side shell section <NUM> of the rotor blade <NUM>.

The lightning protection system <NUM> comprises an internal down conductor <NUM> extending from the root section <NUM> to the tip section <NUM> of the rotor blade <NUM>. The internal down conductor <NUM> basically connects the lightning receptors such as the tip lightning receptor <NUM> and the side lightning receptors <NUM> to the grounding system <NUM> of the wind turbine (not shown in <FIG>). The connection between the internal down conductor <NUM> of the rotor blade <NUM> and the grounding system <NUM> is realized by the root terminal <NUM>. In the case of the exemplary rotor blade as illustrated in <FIG>, the rotor blade <NUM> comprises four side lightning receptors <NUM>, each of which is arranged in the trailing edge section <NUM> of the rotor blade <NUM> because the lightning will likely attach to this section due to the motion of the rotor blade. Another area where lightning strikes favorably or most commonly hit the rotor blade <NUM> is its tip section <NUM>. Thus, especially in the tip section <NUM> of the rotor blade <NUM> lightning receptors are advantageously provided.

The drawback of a conventional rotor blade <NUM> with a lightning protection system <NUM> such as shown in <FIG> is that especially in the tip section <NUM> of the rotor blade <NUM>, the internal down conductor <NUM> is relatively close to the shell <NUM> of the rotor blade <NUM>. This is due to the fact that the cavity <NUM>, which is defined by the inner surface <NUM> of the shell <NUM>, is narrow and small towards the tip section <NUM> of the rotor blade <NUM>. Thus, especially at the tip section <NUM> of the rotor blade <NUM>, the internal down conductor <NUM> is prone to lightning strikes. This is a serious problem because if a lightning strikes directly the internal down conductor <NUM>, the shell <NUM> of the rotor blade <NUM> can be damaged which may lead to costly repair works of the wind turbine.

<FIG> shows a rotor blade <NUM> with a lightning protection system <NUM> according to one embodiment of the invention. The rotor blade comprises a tip unit <NUM> which is arranged in the tip section <NUM> and further inboard of the tip section <NUM>. The tip unit <NUM> encapsulates the internal down conductor <NUM> at that section of the rotor blade where the tip unit <NUM> is provided. In other words, the internal down conductor <NUM> is best possibly insulated against lightning strikes. For this purpose, the tip unit <NUM> is made of an electrically insulating material.

As for the exemplary prior art rotor blade illustrated in <FIG>, the rotor blade <NUM> as illustrated in Figure also comprises exemplarily four side lightning receptors <NUM>, each of which is arranged in the trailing edge section <NUM> of the rotor blade <NUM> because the lightning will likely attach to this section due to the motion of the rotor blade.

Furthermore, in the example of <FIG>, the tip unit <NUM> comprises a tip part <NUM> and an extension part <NUM>. The tip part <NUM> fully extends from the leading edge <NUM> to the trailing edge <NUM> of the rotor blade <NUM>. Additionally, the tip part <NUM> covers the entire area between the suction side shell section <NUM> and the pressure side shell section <NUM>. This feature is not visible in <FIG> but will be clearer, for example in the context of <FIG>. The length of the tip unit <NUM> is composed by the length <NUM> of the tip part <NUM> and the length <NUM> of the extension part <NUM>. Note that the total length <NUM> of the tip unit <NUM> is still comparably small, for example less than ten percent of the entire length <NUM> of the rotor blade <NUM>.

As another measure to protect the internal down conductor <NUM> with regard to lightning strikes, in a mid-board and outboard part of the rotor blade the internal down conductor <NUM> is insulated by a high voltage insulation <NUM>. In the inboard part of the rotor blade, where the risk of lightning strikes is smaller, the internal down conductor <NUM> is only insulated by a low voltage insulation <NUM>. Note that, alternatively, the internal down conductor may as well have no specific insulation at all.

Note that the size of the high voltage insulation <NUM> and the low voltage insulation <NUM> is exaggerated in <FIG>, compared to the size of the tip unit <NUM> or the rotor blade <NUM> as a whole. More realistic sizes are depicted in the two concrete embodiments shown in <FIG>.

<FIG> show cross-sectional views of a lightning receptor <NUM> which is connected to a receptor block <NUM> and wherein the lightning receptor <NUM> and the receptor block <NUM> are embedded into a tip unit <NUM>. Also, the tip unit <NUM> covers the entire area between the inner surface <NUM> of the shell <NUM>. More precisely, however, the tip unit <NUM> does not cover the entire area between the blade shell laminates as the tip unit <NUM> is connected to the inner surface <NUM> via an adhesive <NUM>. Thus, an adhesive joint <NUM> is created between the tip unit <NUM> and the shell <NUM> of the rotor blade. Such a concept is particularly used in the context of the butterfly manufacturing method for rotor blades of wind turbines, wherein a pre-casted pressure side shell and a pre-casted suction side shell are bonded together. If in this case a tip unit <NUM> is inserted into the tip section of the rotor blade, this tip unit <NUM> needs in some way to be rigidly and fixedly attached to the shell <NUM>. Also, a perfect fit of the tip unit <NUM> into the cavity of the rotor blade is generally not possible and alignment is needed. Thus, a suboptimal connection between the tip unit <NUM> and the inner surface <NUM> of the blade shell <NUM> is achieved. This prior art situation is illustrated in <FIG>.

Note that in <FIG> a (generic) lightning receptor <NUM> and a (generic) receptor block <NUM> is depicted. The aim of <FIG> is to illustrate the concept of the invention. This concept can in principle be applied to any kind of concrete lightning receptors/ receptor blocks, such as e.g. a tip lightning receptor <NUM> being connected to a tip receptor block <NUM> or a side lightning receptor <NUM> being connected to a side receptor block <NUM>.

In contrast thereto, <FIG> shows a connection between the tip unit <NUM> and the blade shell laminate <NUM> according to the invention. As therein, the tip unit <NUM> is casted together with the blade shells <NUM>, such that these are self-aligned and a cast interface <NUM> is created between the tip unit <NUM> and the shell <NUM> of the rotor blade <NUM>. This cast interface <NUM> has, first, the advantage that water ingress between the tip unit <NUM> and the shell <NUM> is effectively suppressed and, secondly, the insulation in terms of electrical conductivity between the shell <NUM> and the tip unit <NUM> is improved. Thus, the joint or interface between the tip unit <NUM> and the shell <NUM> is significantly improved if a concept and set-up as illustrated in <FIG> is used.

In the following, two concrete embodiments of tip units <NUM> are shown. <FIG> shows a tip unit <NUM> which comprises a tip part <NUM> and an extension part <NUM>. The tip part <NUM> fills the cavity of the rotor blade in the very tip section and fills the entire space between the suction side shell section <NUM> and the pressure side shell section <NUM>. The tip part <NUM> also fills the entire space between the leading edge <NUM> and the trailing edge <NUM>. The open space between the tip part and the shell is filled by the laminate material of the blade shell itself. Also, a so-called trailing edge core <NUM> is commonly introduced into the sharp and narrow trailing edge section <NUM> of the rotor blade. In the rotor blade design according to embodiments of the present invention, the trailing edge core <NUM> may actually be replaced by the tip unit <NUM>. So, the trailing edge core <NUM> may only depart from the "root end" of the extension part <NUM> of the tip unit <NUM>, as seen from the tip <NUM> of the rotor blade <NUM>. The tip part <NUM> encapsulates the internal down conductor <NUM> which is not visible in that section of the rotor blade in <FIG>. Likewise, the tip part <NUM> also encapsulates the tip lightning receptor <NUM>. Furthermore, the extension part <NUM> of the tip unit <NUM> encapsulates the internal down conductor <NUM> in that section and also the side lightning receptor <NUM>.

<FIG> shows a cross-sectional view at the line A-A' as shown in <FIG>. In <FIG>, namely in the cross-sectional view, it can be clearly seen that the tip unit entirely fills the cavity (or "area") of the rotor blade in that section of the rotor blade. It encapsulates a part of the tip lightning receptors <NUM> (the respective upper part of each tip lightning receptor <NUM> is not encapsulated, since this is the designated attachment point for the lightning)), the tip receptor block <NUM> and the internal down conductor <NUM>, the latter being centrally located in the exemplary embodiment illustrated in <FIG>. Note that two tip lightning receptors <NUM> are provided, one at the suction side shell portion <NUM> and one at the pressure side shell portion <NUM>. As the tip part <NUM> fills the entire space, it adds to the structural stability and has the function of self alignment of the geometry and the outer shape of the two laminate materials during manufacturing of the rotor blade.

Finally, <FIG> shows a second concrete embodiment of the invention with a slightly different design of the tip unit <NUM>. Again, the tip unit <NUM> comprises a tip part <NUM> and an extension part <NUM>, wherein the extension part <NUM> is arranged further inboard relative to the tip part <NUM>. Again, the tip part <NUM> fully covers and fills the cavity <NUM> of the rotor blade in that section of the blade. In this case, the rotor blade comprises several tip lightning receptors <NUM> which are connected to the internal down conductor <NUM> via respective lightning receptor blocks (not visible in the top view of <FIG>). These lightning receptor blocks are all encapsulated and best possibly insulated by the tip part <NUM>. Likewise, the embodiment of the rotor blade as shown in <FIG> comprises a plurality of side lightning receptors <NUM>, which are also connected to the internal down conductor <NUM> via respective lightning receptor blocks (also not visible in the top view of <FIG>). These lightning receptor blocks are all encapsulated by the extension part <NUM> of the tip unit <NUM>. This ensures an optimum insulation against lightning strikes onto mainly the internal down conductor <NUM>.

Claim 1:
Rotor blade (<NUM>) of a wind turbine (<NUM>),
wherein the rotor blade (<NUM>) comprises
- a leading edge section (<NUM>) with a leading edge (<NUM>) and a trailing edge section (<NUM>) with a trailing edge (<NUM>),
- a root section (<NUM>) with a root (<NUM>) and a tip section (<NUM>) with a tip (<NUM>),
- a shell (<NUM>) with a suction side shell section (<NUM>) and a pressure side shell section (<NUM>), wherein the shell (<NUM>) extends to the tip (<NUM>) of the rotor blade (<NUM>) and wherein the outer surface (<NUM>) of the shell (<NUM>) defines the outer shape of the rotor blade (<NUM>) and the inner surface (<NUM>) of the shell (<NUM>) defines a cavity (<NUM>) of the rotor blade (<NUM>),
- a lightning protection system (<NUM>) with an internal down conductor (<NUM>) extending from the root section (<NUM>) of the rotor blade (<NUM>) to the tip section (<NUM>) of the rotor blade (<NUM>), wherein the internal down conductor (<NUM>) is connectable at the root section (<NUM>) to a grounding system (<NUM>) of the remainder of the wind turbine (<NUM>) and at the tip section (<NUM>) to at least one tip lightning receptor (<NUM>) which is positioned at the outer surface (<NUM>) of the rotor blade (<NUM>), and
- an electrically insulating tip unit (<NUM>), wherein the tip unit (<NUM>) is arranged in the tip section (<NUM>) of the rotor blade (<NUM>) in the cavity (<NUM>) of the rotor blade (<NUM>) and wherein the tip unit (<NUM>) encapsulates at least a part of the internal down conductor (<NUM>) in the tip section (<NUM>) of the rotor blade (<NUM>), wherein at least a part of the tip unit (<NUM>) covers the entire area between a part of the suction side shell portion (<NUM>) and a part of the pressure side shell portion (<NUM>), and wherein
the connection between the tip unit (<NUM>) and the suction side shell portion (<NUM>) as well as the connection between the tip unit (<NUM>) and the pressure side shell portion (<NUM>) is a cast interface, respectively, wherein the tip unit (<NUM>) comprises a tip part (<NUM>) which covers the entire area between the leading edge (<NUM>) and the trailing edge (<NUM>) of the rotor blade (<NUM>), wherein the tip part (<NUM>) is located at the outermost part of the cavity (<NUM>) of the rotor blade (<NUM>).