WO2013007267A1

WO2013007267A1 - A wind turbine blade

Info

Publication number: WO2013007267A1
Application number: PCT/DK2012/050265
Authority: WO
Inventors: Hans V. Erichsen; Kent Bach LØNBÆK; Richard Baker
Original assignee: Vestas Wind Systems A/S
Priority date: 2011-07-14
Filing date: 2012-07-11
Publication date: 2013-01-17

Abstract

A wind turbine blade, comprising a proximal end, where the blade is to be attached to a hub at a blade root; and a distal end, where the blade tapers to form a blade tip, wherein the blade further comprises at least a first longitudinal blade zone comprising at least the blade tip and a second longitudinal blade zone comprising a longitudinal section of the blade, wherein the blade tip in the first blade zone comprises a metallic electrical conductor which is connected to a lightning down conductor, the second blade zone comprises a load-bearing spar in the blade for providing structural support for the blade and extends proximally from the second blade zone to the blade root, wherein the spar is covered with a medium in the second blade zone to reduce the occurrence of a direct lightning connection to the spar.

Description

A WIND TURBINE BLADE

FIELD OF THE INVENTION [001] The invention generally relates to a blade for a wind turbine.

BACKGROUND

[002] In recent years, there has been an increased focus on reducing emissions of greenhouse gases generated by burning fossil fuels. One solution for reducing greenhouse gas emissions is developing renewable sources of energy. Particularly, energy derived from the wind has proven to be an environmentally safe and reliable source of energy, which can reduce dependence on fossil fuels.

[003] Energy in wind can be captured by a wind turbine, which is a rotating machine that converts the kinetic energy of the wind into mechanical energy, and the mechanical energy subsequently into electrical power. Common horizontal-axis wind turbines include a tower, a nacelle located at the apex of the tower, and a rotor that is supported in the nacelle by means of a shaft. The shaft couples the rotor either directly or indirectly with a rotor assembly of a generator housed inside the nacelle. A plurality of wind turbines may be arranged together to form a wind park or wind power plant. [004] Lightning strikes are a major cause of concern for wind turbine sustainability. With wind turbines being located in remote areas for the best wind catchment, the turbines are a particularly attractive target for lightning strikes due to their height and material composition.

[005] Wind turbine blades typically encompass advanced lightning protection systems, some of which comprise features such as lightning receptors and a lightning down conductor for conducting lightning to ground to prevent lightning strikes from damaging the wind turbine blade. SUMMARY OF THE INVENTION

[006] One embodiment of the invention provides a wind turbine blade, comprising a proximal end, where the blade is to be attached to a hub at a blade root; and a distal end, where the blade tapers to form a blade tip, wherein the blade further comprises at least a first longitudinal blade zone comprising at least the blade tip and a second longitudinal blade zone comprising a longitudinal section of the blade, wherein the blade tip in the first blade zone comprises a metallic electrical conductor which is connected to a lightning down conductor, the second blade zone comprises a load-bearing spar in the blade for providing structural support for the blade and extends proximally from the second blade zone to the blade root, wherein the spar is covered with a medium in the second blade zone to reduce the occurrence of a direct lightning connection to the spar.

BRIEF DESCRIPTION OF THE DRAWINGS [007] Embodiments of the present invention are explained, by way of example, and with reference to the accompanying drawings. It is to be noted that the appended drawings illustrate only examples of embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. [008] Fig. 1 illustrates a wind turbine.

[009] Fig. 2 illustrates a wind turbine blade zoning concept according to an embodiment.

[010] Fig. 3 illustrates a portion of a wind turbine blade comprising a lightning protection system according to an embodiment.

[011] Fig 3a illustrates a cross- sectional profile of the blade of Fig. 3. [012] Fig 3b illustrates an internal view of a portion of the blade of Fig. 3 according to another embodiment.

[013] Fig. 3c illustrates a portion of a cross- sectional profile of Fig. 3b.

[014] Fig. 3d illustrates an internal view of a portion of the blade of Fig. 3 according to another embodiment.

[015] Fig. 4 illustrates a portion of a wind turbine blade according to another embodiment.

DETAILED DESCRIPTION

[016] In the following, reference is made to embodiments of the invention. However, it should be understood that the invention is not limited to specific described embodiments. Instead, any combination of the following features and elements, whether related to different embodiments or not, is contemplated to implement and practice the invention.

[017] Furthermore, in various embodiments, the invention provides numerous advantages over the prior art. However, although embodiments of the invention may achieve advantages over other possible solutions and/or over the prior art, whether or not a particular advantage is achieved by a given embodiment is not limiting of the invention. Thus, the following aspects, features, embodiments and advantages are merely illustrative and are not considered elements or limitations of the appended claims except where explicitly recited in a claim(s). Likewise, reference to "the invention" shall not be construed as a generalization of any inventive subject matter disclosed herein and shall not be considered to be an element or limitation of the appended claims except where explicitly recited in a claim(s).

[018] One embodiment of the invention provides a wind turbine blade, comprising a proximal end, where the blade is to be attached to a hub at a blade root; and a distal end, where the blade tapers to form a blade tip, wherein the blade further comprises at least a first longitudinal blade zone comprising at least the blade tip and a second longitudinal blade zone comprising a longitudinal section of the blade, wherein the blade tip in the first blade zone comprises a metallic electrical conductor which is connected to a lightning down conductor, the second blade zone comprises a load-bearing spar in the blade for providing structural support for the blade and extends proximally from the second blade zone to the blade root, wherein the spar is covered with a medium in the second blade zone to reduce the occurrence of a direct lightning connection to the spar.

[019] As earlier noted, lightning strike damage is a critical area of concern in wind turbine and wind turbine blade design and sustainability. The embodiment divides the blade into several longitudinal blade zones along the blade length in accordance to the impact of an expected lightning strike within the zone. By separating the blade into longitudinal blade zones and catering a particular lightning protection sub-system for each blade zone, a comprehensive lightning protection system can be provided for the blade, taking into concern the areas of higher risk of lightning strikes and providing more adept sub-systems for these areas. An overall cost savings can also be effected, by reducing or eliminating the lightning protection in areas which are deemed lower or negligible risk from lightning strikes.

[020] Having such configured lightning protection sub- systems provide a suitable match in accordance to the risk of lightning strikes in each blade zone. The embodiment provides a first lightning protection sub- system corresponding to a first longitudinal blade zone. Within the first lightning protection sub-system, a metallic electric conductor replaces the tip of the blade and provides an attractive target for downward initiated lightning strikes as well as allowing the easy formation of initial leaders for upward initiated lightning strikes. Further, the metal tip also improves the reliability and life-time of the blade tip and subsequently the blade; i.e. reduction in maintenance or repair of the wind turbine blade due to lightning strikes. [021] The second blade zone comprises a load-bearing spar extending proximally from the second blade zone to the blade root. Conversely, it may be noted that the spar originates at the blade root and ends off at the distal end of the second blade zone. There is no spar in the first blade zone. [022] The occurrence of a direct lightning connection to the spar is reduced by covering the spar with an effective medium. As will be later described, the spar is a substantially rectangular structure provided within the blade to provide support, the longer sides of the spar correlating to the windward and the leeward sides of the blade. These longer sides of the spar are also the most susceptible to a direct lightning connection, as the longer sides of the spar are also much closer to the blade shell, if not directly adjoined, at some areas. Covering the spar with a different medium in this case means that at least the longer sides of the spar are covered, in relation to the surrounding blade shell.

[023] In an embodiment, the medium is applied to a surface of the blade along the spar in the second blade zone.

[024] It is envisaged to protect the spar from lightning strikes by covering a longitudinal portion of the spar within the blade zone with a medium different in composition than the composition of the spar. Lightning strikes directly attaching to the spar leads to blade damage, and in most cases, a reduction in operability of the wind turbine blades and the wind turbine. Providing such a cover either provides a more attractive target for the lightning or reduces initial leader formation from the spar, thus reducing occurrence of lightning strikes attaching to the spar.

[025] In further embodiment, the medium has a dielectric breakdown voltage of at least 20kV, and is applied onto the internal surface of the blade, facing the spar. Providing the medium on the internal surface of the blade addresses the attractiveness of the spar by reducing or delaying any initial leader formation that propagates from the spar such that when lightning approaches the wind turbine, the leaders from the lightning will not connect to the leaders from the carbon fiber in the spar.

[026] In another embodiment, the medium is a metallic mesh, and is applied to the external surface of the blade. Such a covering mesh provides a more attractive option for the lightning to be attracted on to, the mesh being connected to a lightning down conductor for safe transmission of the lightning current to ground. In an embodiment, the mesh is made of copper as it provides a good balance of electrical conductivity and physical resilience. In another embodiment, the mesh is made of expanded copper foil thereby providing durability and ensuring electrical conductivity during and after a lightning strike connection.

[027] In an embodiment, the medium has a dielectric breakdown voltage of at least 20kV, and is applied to the surface of the spar. Applying the medium to the spar directly, allows the lightning protection sub-system to address the risk of lightning strike directly by covering the conductive carbon fiber spar. Having a dielectric breakdown voltage of over 20kV reduces or delays any initial leader formation that propagates from the carbon fiber of the spar.

[028] In an embodiment, the blade comprises a third longitudinal blade zone between the first and second blade zones, wherein the lightning down conductor runs longitudinally within the blade from the metallic electric conductor to the blade root, and is suspended within the third blade zone, and wherein the lightning down conductor is insulated with a material having a dielectric breakdown voltage of at least 20kV within the third blade zone. It should be noted that there is no spar within the third blade zone, as the spar only extends proximally from the second blade zone to the blade root. Without a spar in the blade zone, the lightning protection sub-system within the blade zone is the primary target for lightning strike attraction. Insulation of the down conductor allows the prevention or reduction of formation of electrical leaders and reduces any lightning attractiveness of the down conductor within the blade. [029] In an embodiment, the blade comprises a third longitudinal blade zone between the first and second blade zones, wherein the third longitudinal blade zone comprises a metallic conductor strip running along at least one of a leading edge and a trailing edge of the blade. In another embodiment, the lightning down conductor comprises the metallic conductor strip running along at least one of the leading edge and the trailing edge of the blade, and is coupled to electrical ground. Having such an edge-wise lightning down conductor allows the lightning down conductor to act as a channel for lightning current received through any blade zone lightning protection sub- system, as well as acting as a lightning receptor itself. An edge-wise strip is chosen, which further improves the durability of the blade as the edges of the blades are often susceptible to physical wear and tear.

[030] In an embodiment, the metallic conductor strip comprises an additional protective boundary layer over the metallic conductor strip. Having such a protective boundary area provides additional erosion protection for the metallic conductor strip.

[031] In another embodiment, the blade comprises a third longitudinal blade zone between the first and second blade zones, wherein the external surface of the blade in the third longitudinal blade zone is at least partially covered with a sheet of metallic conductor coupled to the lightning down conductor. As the third blade zone is towards the distal end of the blade, it is relatively susceptible to lightning strike attachments, and thus a relatively robust lightning protection sub-system is advantageous. A metallic conductor sheet on the surface of the blade provides a lightning protection sub- system that is attractive to lightning strokes and also able to withstand a high level of lightning stroke current.

[032] In a further embodiment, the blade comprises a third longitudinal blade zone between the first and second blade zones, the third longitudinal blade zone comprises a third blade zone lightning protection sub-system, and wherein the effectiveness of the third blade zone lightning protection sub-system is enhanced by the absence of the spar within the third longitudinal blade zone.

[033] In an embodiment, the blade comprises a plurality of lightning diverter strips on a surface of the blade and coupled to the lightning down conductor. In another embodiment, the blade comprises a plurality of discrete lightning receptors on the surface of the blade and coupled to the lightning down conductor. Such additions provide an improvement to the coverage of any lightning protection sub-system in the blade.

[034] In an embodiment, there is provided a wind turbine blade, comprising a proximal end, where the blade is to be attached to a hub at a blade root, and a distal end, where the blade tapers to form a blade tip; the blade further comprising a first longitudinal blade zone, wherein the first blade zone comprises the outermost lm of the blade, and further comprises a lightning protection sub- system in the first blade zone for intercepting a lightning strike impact of at least up to 200kA in current amplitude; a second longitudinal blade zone, wherein the second blade zone comprises from 5m inboard to 20m inboard of the blade, and further comprises a lightning protection sub-system in the second blade zone for intercepting a lightning strike impact of at least up to 50kA in current amplitude; a third longitudinal blade zone between the first and the second blade zones, wherein the third blade zone comprises from lm inboard to 5m inboard of the blade, and further comprises a lightning protection sub- system in the third blade zone for intercepting a lightning strike impact of at least up to lOOkA in current amplitude. [035] A wind turbine is further provided, comprising a wind turbine blade as described above.

[036] The following is a detailed description of embodiments of the invention depicted in the accompanying drawings. The embodiments are examples and are in such detail as to clearly communicate the invention. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.

[037] Figure 1 illustrates an exemplary wind turbine 100 according to an embodiment. As illustrated in Figure 1, the wind turbine 100 includes a tower 110, a nacelle 120, and a rotor 130. In one embodiment of the invention, the wind turbine 100 may be an onshore wind turbine. However, embodiments of the invention are not limited only to onshore wind turbines. In alternative embodiments, the wind turbine 100 may be an offshore wind turbine located over a water body such as, for example, a lake, an ocean, or the like. The tower 110 of such an offshore wind turbine is installed on either the sea floor or on platforms stabilized on or above the sea level.

[038] The tower 110 of wind turbine 100 may be configured to raise the nacelle 120 and the rotor 130 to a height where strong, less turbulent, and generally unobstructed flow of air may be received by the rotor 130. The height of the tower 110 may be any reasonable height, and should consider the length of wind turbine blades extending from the rotor 130. The tower 110 may be made from any type of material, for example, steel, concrete, or the like. In some embodiments the tower 110 may be made from a monolithic material. However, in alternative embodiments, the tower 110 may include a plurality of sections, for example, two or more tubular steel sections 111 and 112, as illustrated in Figure 1. In some embodiments of the invention, the tower 110 may be a lattice tower. Accordingly, the tower 110 may include welded steel profiles.

[039] The rotor 130 may include a rotor hub (hereinafter referred to simply as the "hub") 132 and at least one blade 134 (three such blades 134 are shown in Figure 1). The rotor hub 132 may be configured to couple the at least one blade 134 to a drive shaft (not shown). In one embodiment, the blades 134 may have an aerodynamic profile such that, at predefined wind speeds, the blades 134 experience lift, thereby causing the blades to radially rotate around the hub. The hub 132 further comprises mechanisms (not shown) for adjusting the pitch of the blade 134 to increase or reduce the amount of wind energy captured by the blade 134. Pitching adjusts the angle at which the wind strikes the blade 134.

[040] The hub 132 typically rotates about a substantially horizontal axis along the drive shaft extending from the hub 132 to the nacelle 120. The drive shaft is usually coupled to one or more components in the nacelle 120, which are configured to convert and the rotational energy of the shaft into electrical energy. [041] Typically, the blade 134 may vary from a length of 20 meters to 60 meters, and beyond. Such blades are precisely manufactured to ensure that the rotor remains balanced for optimum aerodynamic performance. The lightning protection system for use in the wind turbine blade is integrated into the manufacturing process, the end product being that the manufactured blade comprises a fully operable lightning protection system. Blade 134 is formed by a manufacturing process which includes pre-impregnation of composite fibers ("pre-preg"), which is well-known and will not be elaborated on. Other manufacturing methods may be used as well.

[042] Fig. 2 illustrates a wind turbine blade zoning concept according to an embodiment. Blade 134 is a 40m blade, but may be of any other length in other embodiments. Blade 134 is divided into four sections, Zone OAl at 140, Zone 0A2 at 142, Zone 0A3 at 144 and Zone OB at 146. Such zoning is a subdivision of traditional Zones OA and OB, and is based on the maximum peak current amplitudes expected from a lightning strike. The zoning concept only considers the longitudinal aspects of the blade and not to special geometries of trailing edges, tips, etc.

[043] Lightning protection Zone 0A1 140 has been designated as so, since the blade tip is exposed to the full lightning current of a lightning strike. This means that the blade tip should be designed to intercept lightning strikes of all current amplitudes, and should withstand the impact from the impulse current with the highest amplitude according to the IEC standards, i.e. 200kA.

[044] The second zone for the direct attachments is Zone 0A2 142, which covers the next 4 meters of the blade 134, from lm from the tip to 5m from the tip. In this area the peak amplitude current of the lightning strikes expected to attach here is less than for Zone 0A1 140. The lightning protection system installed in this area should intercept strikes to the area as well as to be able to conduct a stroke current of about lOOkA.

[045] The third section of the blade 134 exposed to lightning direct attachment is Zone 0A3 144, which is defined from 5m inboard (or from the tip) to 20m inboard. In this area the installed lightning protection system should intercept lightning strikes to the area and should also be able to cope with expected peak amplitude of stroke currents at a maximum of about 50kA.

[046] The last zone of the blade 134 is Zone 0B 146 from 20m inboard to the root end. In this zone 146, the blade is not expected to receive any direct lightning strike attachments, or even if so, a lightning strike at a low current amplitude which is acceptable for blade structural impact. The zone 146 will however still experience the full magnetic field and the full lightning current from a lightning strike in other zones, as current from a successful lightning interception in Zone 0A1 140, Zone 0A2 142 or Zone 0A3 144 is passed along an internal down conductor running longitudinally through the blade from tip to root. [047] Fig. 3 illustrates a portion of the wind turbine blade 134 comprising a lightning protection system 150 according to an embodiment. Lightning protection system 150 comprises multiple lightning protection sub- systems, 160, 170, 180, 190, relating to respective lightning protection zones as described in Fig. 2. The introduction of blade zoning for lightning protection allows the introduction of different and separate lightning protection sub-systems to address the different needs and risks of each blade zone.

[048] Fig 3a illustrates a cross-sectional profile of the blade 134 at X-X in Fig. 3. A spar 136 is shown internal to blade 134 and acts as a support to the blade 134. The spar 136 is substantially rectangular in shape. The spar 136 is composed primarily of carbon fiber and epoxy. Carbon fiber is by nature electrically conductive and is thus relatively attractive to a lightning strike, as it readily propagates electrical streamers which may evolve into leaders, and which may thereafter connect with oppositely charged leaders in the atmosphere to form a discharge path for a lightning strike. A lightning down conductor 138 runs through the longitudinal length of the blade 134 and is coupled to the spar 136 by means of a down conductor connector 152. The down conductor 138 is coupled to a tip receptor, as will be later described, at one end and to a blade band (not shown) at the blade root at the other end. The down conductor 138 is subsequently coupled to a lightning current transfer unit (not shown) between the blade band and the nacelle 120 of the wind turbine 100, which thereafter conducts the lightning current to electrical ground.

[049] Returning to Fig. 3, Zone 0A1 lightning protection sub-system 160 comprises a metal tip 162. The metal tip 162 takes the shape and form of a typical blade tip and is adapted to be a lightning receptor. Metal tip 162, being a good electrical conductor, provides for the easy formation and release of electrical leaders and is thus extremely attractive for lightning stroke formation and attraction. In order to provide an effective and durable segment to be incorporated into the blade 134, and which is able to receive and resist multiple lightning strikes, the metal tip 162 is composed entirely of metal, and in the present embodiment, of copper. [050] The metal tip 162 is coupled onto the blade 134 by a nut and bolt securing configuration, but any other means which allows the blade to be securely fasted on the blade may be possible. The metal tip 162 is also directly fastened, by crimping, to the down conductor 138 on the inside of the blade 134. Other methods are also possible. [051] Zone 0A2 142 covers from lm to 5m inboard from the blade tip. Within blade Zone 0A2 142, Zone 0A2 lightning protection sub-system 170 comprises at least partially covering the entire surface of the blade 134 in the blade zone 142 with a sheet of metallic conductor 172. It may be noted that lightning protection sub-system 170 is further enhanced by the absence of the spar 136 within the blade zone 142 as it makes the present metallic conductor 172 the most attractive target for lightning strike reception. A key consideration for the lightning protection of Zone 0A2 142 is thus the removal or absence of an internal carbon fiber spar within this zone. To this effect, the spar 136 is designed to terminate distally in Zone 0A3 144. As such, there is no longer an internal structural support for the tip-most 5m of the present embodiment of the wind turbine blade.

[052] The metallic conductor sheet 172 is formed over the surface of the blade 134 in the blade zone 142 and may be secured by adhesive and/or bolts. Further, the thickness of the sheet 172 is such that the aerodynamic profile of the blade is not affected by the inclusion of the sheet 172. In the present embodiment, the metallic conductor sheet 172 is formed by an aluminum sheet of about 2mm in thickness, but any other suitable conductive sheet or thickness may be alternatively used. The metallic conductor sheet 172 is also coupled to the lightning down conductor 138, in this case, through a direct connection to the metal tip 162 and then to the down conductor 138. The down conductor 138 is as such suspended within blade zone 142, and has no direct physical connection to any part of the blade 134 within blade zone 142. It should be noted that the down conductor 138 is fully insulated with a thermoplastic polymer with a high dielectric breakdown voltage of at least 20kV in the longitudinal entirety of the blade zone 142. This is to prevent or reduce electrical leaders originating from the down conductor 138 from connecting with lightning leaders. [053] Lightning protection sub-system 180 is provided for blade zone 0A3 144. The blade zone 144 marks the beginning of the internal spar 136 and starts from 5m inboard the blade tip.

[054] A conductive mesh 182 is provided over the surface of the blade 134 within the blade zone 144, and not only covers but fully encloses the portion of the spar 136 within the blade zone 144. Such a mesh provides adequate means for conducting a lightning strike connection on the surface of the blade 134 to a down conductor 138 which thereafter leads the stroke to ground. In the present embodiment, the mesh 182 is composed of an expanded copper foil, which is manufactured from a single piece of copper which has holes punched in it before being stretched, thereby providing a continuous mesh. Such a mesh ensures conductivity across the surface and also reduces problems during the manufacturing process. Alternatively, a mesh which is formed by criss-crossed wires may also be used.

[055] As the edges of the blade are identified as high-risk for lightning stroke connection, edge receptors 184 and 186 are provided for the trailing edge and leading edge respectively. Edge receptors 184, 186 are provided as metallic sheets in this embodiment, to provide structural durability as well as good electrical conductivity. In this embodiment, edge receptors 184, 186 are aluminum receptors. In other embodiments, copper or other metals maybe used instead. The edge receptors 184, 186 are placed over the mesh 182 and thus electrically connected. A definite electrical connection is also formed between the edge receptors 184, 186 and the conductor sheet 172 of lightning protection sub-system 170.

[056] To further supplement the mesh 182 and edge receptors 184, 186, lightning diverter strips 188 are provided as part of the lightning protection sub-system 180. In this embodiment, the diverter strips 188 are placed diagonally toward the tip of the blade, from the center line of the blade 134 to the edge receptors 184, 186. Other arrangements are possible. Lightning diverter strips 188 are highly-conductive segmented strips which provide an attractive connection for lightning strokes and also allow the lightning stroke to travel safely to the edge receptors 184, 186 in an ionized channel created in the air above the diverter strips 188. Such diverter strips 188 provide additional structural protection for the blade surface, as lightning stroke connections tend to cause damage to the blade surface, even if properly conducted away.

[057] As indicated above, lightning protection sub-system 170 and 180 are electrically connected and acts as the channel for conducting a lightning strike connection to ground. For example, a lightning strike connection in zone 0A3 144 on the surface of the blade 134 is attracted to a diverter strip 188 and conducted to the edge receptor 184 or 186 through an ionized air channel above the diverter strip 188. The lightning stroke current is then passed into lightning protection sub-system 170 and thereafter sub-system 160 and eventually into the down conductor 138. Such an electrical transfer for the lightning stroke current also applies for lightning strikes in zone 0B 146. Having a single channel for lightning stroke current conduction subjects the blade to fewer occurrences of arcing damage from the lightning stroke current trying to attach itself to other conductive materials in the blade 134 which may take place when there are multiple electrical connections to the internal down conductor 138.

[058] In blade zone 0B 146, lightning protection sub-system 190 is provided comprising edge conductors 192, 194, which are similar to and are extensions of edge conductors 184, 186. The zone 0B 146 begins from 20m inboard from the blade tip. In studies carried out by the applicants, it is found that the blade is not expected to receive any direct lightning strike attachments in the zone 0B 146, or even if so, receives a lightning strike at a low current amplitude which is acceptable for blade structural impact. The edge conductors 192, 194 are considered sufficient for lightning protection in this zone.

[059] To supplement the edge conductors, 192, 194, discrete lightning receptors (not shown), which are metallic conductors located within the blade shell and terminating at the surface of the blade may be provided as part of lightning protection sub-system 190. The discrete lightning receptors are connected to the down conductor 138 inside the blade 134. Lightning diverter strips, as earlier described, may also be used as a lightning protection supplement in the blade zone 0B 146. [060] Fig. 3b illustrates an internal view of a portion of the blade of Fig. 3 according to another embodiment. Lightning protection sub-system 180 for blade zone 0A3 144 comprises a glass fiber sheet layer 183 within the blade shell in zone 144 for lightning protection of the spar 136, instead of a conductive mesh. The glass fiber layer 183 may be formed in the blade shell, and on the internal surface of the blade, thereby covering the spar 136 within the zone 144.

[061] The glass fiber layer 183 is intended to be of a high quality, i.e. relatively defect- free in its formation. In manufacturing, the high quality glass fiber layer 183 should be without any air cavities. The glass fiber layer 183 is also of a thickness such that the dielectric breakdown voltage is at least 20 kV. This allows the glass fiber layer 183 to delay initial streamers that will propagate from the carbon fiber in the spar 136, when lightning is approaching the wind turbine 100, such that there is a reduced occurrence of leaders from the lightning connecting to the leaders from the spar 136.

[062] The width of the high quality glass layer 183, when looking from the leeward and windward sides of the blade, extends past the spar by about 10 cm on each side. Fig. 3c illustrates a portion of a cross-sectional profile of Fig. 3b. In other embodiments, the high quality glass layer 183 may also fully enclose the spar 136 by being formed as the innermost layer of the blade.

[063] Fig. 3d illustrates an internal view of a portion of the blade of Fig. 3 according to yet another embodiment. Lightning protection sub- system 180 for blade zone 0A3 144 comprises a heat shrinkable tubular sleeve 185 applied directly onto the surface of the spar 136, instead of a conductive mesh. The sleeve 185 could be made from any thermoplastic polymer material. This sleeve 185 delays the initial streamers that will propagate from the carbon fiber in the spar 136, such that there is a reduced occurrence of leaders from the lightning connecting to the leaders from the spar 136. Similarly, the sleeve 185 is of a thickness such that the dielectric breakdown voltage is at least 20kV and is manufactured and applied without any defects or air cavities.

[064] Fig. 4 illustrates a portion of a wind turbine blade according to another embodiment. Blade 200 is designed to be a length of 50m from tip to root, but may similarly be of a greater or shorter length. Blade 200 is also designed to be of a modular nature, due to its length and the various manufacturing issues in producing a blade of such length. A longitudinal spar 202 is provided as the central structural support, and modular pieces such as a leading edge module 204, a trailing edge module 206 and a blade apex module 208, are joined by means of composite adhesion processes, or other processes, onto the longitudinal spar 202. Other modules may be possible in forming the blade. As such, the spar 202 is actually exposed at, and comprises a portion of, the surface of the blade 200. As the spar 202 is composed partially of carbon fiber, which is an electrical conductor, lightning protection schemes have to be reconfigured in view of such a blade layout.

[065] Similarly, blade 200 has been demarcated into different zones for the purpose of lightning protection design. Lines A-A and B-B mark out zone 0A1 210, zone 0A2 220, and zone 0A3 230. An overall lightning protection system 240 is provided for the lightning protection of the blade 200, comprising several sub- systems. [066] Zone 0A1 210 is defined as the tip of the wind turbine blade 200 to line A-A, and as earlier set out, is expected to be the site of highest risk for lightning strike connections. Lightning protection sub-system 212 is provided for zone 210 and comprises a solid copper tip 214. Tip 214 is installed and secured by means of bolts onto the blade 200. Zone 0A1 210, and tip 214, is about lm in length, measured from the tip along the center line of the blade 200. It is noted that due to the modular nature of blade 200, an internal down conductor, running longitudinally down the entire blade, as described in Fig 3, is not feasible, especially in view of construction limitations.

[067] Zone 0A2 220 covers the next 4m inboard from the tip of the blade 200, from line A-A to line B-B. Lightning protection sub-system 222 is provided for zone 220. As before, the absence of spar 202 makes up part of the lightning protection consideration for the zone 220, and enhances the effectiveness of lightning protection sub-system 222. Lightning protection sub- system 222 comprises leading edge receptor 224 and trailing edge receptor 226. A polyurethane edge protective boundary 228 is also provided for structural durability along both edges of the blade and is laid over the edge receptors 224, 226. Any other material which provides protection may also be used.

[068] Leading edge receptor 224 and trailing edge receptor 226 also form part of an externally configured down conductor 242. Down conductor 242 is electrically connected to the solid copper tip 214 and runs longitudinally down the leading edge and the trailing edge of the wind turbine blade 200. It is electrically coupled to the blade band at the root of the blade (not shown) and electrical current is thereafter conducted to ground. In other embodiments, the edge down conductor may cease at the end of zone 0A3, and thereafter connect to at least one internal down conductor running down the side of the spar to the blade root.

[069] Zone 0A3 230 covers from line B-B, about 5m inboard, to 20m inboard. It may be noted that spar 202 starts distally in zone 230. As mentioned, spar 202 comprises carbon fiber which is attractive to a lightning stroke and is also exposed on the surface of the blade 200. Lightning protection sub-system 232 comprises covering the exposed spar 202 with a lightning protection conductive mesh 234. It is noted that the conductive mesh 234 is provided only to cover the portion of the spar 202 within the zone 230, and does not extend onto the rest of the blade 200, as the glass fiber composition of the modular blade surface has an acceptable lightning stroke connection risk. Mesh 234 comprises an expanded copper foil or any other form of conductive mesh. Mesh 234 is also electrically coupled to the leading edge receptor 224, which also acts as part of the down conductor 242. In the present embodiment, leading edge receptor 224 and trailing edge receptor 226 stretch longitudinally down the blade and encompass different lightning protection subsystems. Other embodiments may have the edge receptors 224, 226 cease at the end of zone 230 and linked to an internal down conductor. The lightning protection sub-systems for any further proximal zones are then supported by discrete receptors.

[070] Zone 0B is not shown in Fig. 8, but in the present embodiment, would comprise simply the down conductor 242, in the form of edge receptors 224, 226, coupled to the blade band at the root, as the lightning protection sub-system for the zone. [071] While the invention has been illustrated by a description of various embodiments and while these embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicant' s general inventive concept

Claims

WHAT IS CLAIMED IS:

1. A wind turbine blade, comprising:

a proximal end, where the blade is to be attached to a hub at a blade root; and a distal end, where the blade tapers to form a blade tip, wherein

the blade further comprises at least a first longitudinal blade zone comprising at least the blade tip and a second longitudinal blade zone comprising a longitudinal section of the blade,

wherein the blade tip in the first blade zone comprises a metallic electrical conductor which is connected to a lightning down conductor,

the second blade zone comprises a load-bearing spar in the blade for providing structural support for the blade and extends proximally from the second blade zone to the blade root, wherein the spar is covered with a medium in the second blade zone to reduce the occurrence of a direct lightning connection to the spar.

2. A wind turbine blade according to claim 1, wherein the medium is applied to a surface of the blade along the spar in the second blade zone.

3. A wind turbine blade according to claim 2, wherein the medium has a dielectric breakdown voltage of at least 20kV, and is applied onto the internal surface of the blade, facing the spar.

4. A wind turbine blade according to claim 2, wherein the medium is a metallic mesh, and is applied to the external surface of the blade.

5. A wind turbine blade according to claim 4, wherein the mesh comprises copper.

6. A wind turbine blade according to claim 5, wherein the mesh comprises expanded copper foil.

7. A wind turbine according to claim 1, wherein the medium has a dielectric breakdown voltage of at least 20kV, and is applied to the surface of the spar.

8. A wind turbine blade according to claim 1, further comprising a third longitudinal blade zone between the first and second blade zones,

wherein the lightning down conductor runs longitudinally within the blade from the metallic electric conductor to the blade root, and is suspended within the third blade zone, and

wherein the lightning down conductor is insulated with a material having a dielectric breakdown voltage of at least 20kV within the third blade zone.

9. A wind turbine blade according to claim 1, further comprising a third longitudinal blade zone between the first and second blade zones, wherein the third longitudinal blade zone comprises a lightning receptor which comprises a metallic conductor strip running along at least one of a leading edge and a trailing edge of the blade.

10. A wind turbine according to claim 9, wherein the lightning down conductor comprises the metallic conductor strip running along at least one of the leading edge and the trailing edge of the blade, and is coupled to electrical ground.

11. A wind turbine according to claims 9 or 10, wherein the metallic conductor strip comprises an additional protective boundary layer over the metallic conductor strip.

12. A wind turbine blade according to claim 1, further comprising a third longitudinal blade zone between the first and second blade zones, wherein the external surface of the blade in the third longitudinal blade zone is at least partially covered with a sheet of metallic conductor coupled to the lightning down conductor.

13. A wind turbine according to claim 1, further comprising a third longitudinal blade zone between the first and second blade zones, the third longitudinal blade zone comprising a third blade zone lightning protection sub- system, and wherein the effectiveness of the third blade zone lightning protection sub- system is enhanced by the absence of the spar within the third longitudinal blade zone.

14. A wind turbine blade according to claim 1, further comprising a plurality of lightning diverter strips on a surface of the blade and coupled to the lightning down conductor.

15. A wind turbine blade according to claim 1, further comprises a plurality of discrete lightning receptors on the surface of the blade and coupled to the lightning down conductor.

16. A wind turbine, comprising a wind turbine blade according to any of claims 1 to 14.

17. A wind turbine blade, comprising:

a proximal end, where the blade is to be attached to a hub at a blade root, and a distal end, where the blade tapers to form a blade tip;

the blade further comprising:

a first longitudinal blade zone, wherein the first blade zone comprises the outermost lm of the blade, and further comprises a lightning protection sub-system in the first blade zone for intercepting a lightning strike impact of at least up to 200kA in current amplitude;

a second longitudinal blade zone, wherein the second blade zone comprises from 5m inboard to 20m inboard of the blade, and further comprises a lightning protection sub- system in the second blade zone for intercepting a lightning strike impact of at least up to 50kA in current amplitude; and

a third longitudinal blade zone between the first and the second blade zones, wherein the third blade zone comprises from lm inboard to 5m inboard of the blade, and further comprises a lightning protection sub-system in the third blade zone for intercepting a lightning strike impact of at least up to lOOkA in current amplitude.