Patent Publication Number: US-9835141-B2

Title: Wind turbine blade and a lightning measurement system therein

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119(a) to Danish Patent Application No. PA 201170416, filed Jul. 28, 2011. This application also claims the benefit of U.S. Provisional Application No. 61/512,439, filed Jul. 28, 2011. Each of the applications is incorporated by reference herein in its entirety. 
    
    
     TECHNICAL FIELD 
     The invention generally relates to a blade for a wind turbine, and to a blade for a wind turbine comprising a lightning measurement system. 
     BACKGROUND 
     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. 
     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. 
     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. 
     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. It is desirable to understand the effects of a lightning strike on a wind turbine. 
     SUMMARY 
     One embodiment of the invention provides a wind turbine blade, comprising a proximal end, where the blade is attached to a rotor hub at a blade root portion; and a distal end, where the blade tapers to form a blade tip, a lightning protection system, comprising at least one lightning receptor exposed on a surface of the blade for receiving a lightning strike, and a lightning down conductor coupled to the lightning receptor for relaying lightning current from a lightning strike, the down conductor running internally within the blade and coupled to an electrical ground; and a lightning current measurement system, for deriving parameters of a lightning strike, comprising a lightning current sensor comprising a coil of metal substantially circumscribing the down conductor, the coil in proximity with the down conductor so as to detect magnetic field fluctuations, a lightning analytical system coupled to the lightning current sensor, for receiving an output reading from the lightning current sensor and to provide lightning current parameters, and a signal conversion unit coupled between the lightning current sensor and the lightning analytical system; wherein the signal conversion unit is physically located in the blade and is separated from the blade root by a spacing distance so as to reduce the occurrence of a flashover. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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. 
         FIG. 1  illustrates a wind turbine. 
         FIG. 2  illustrates a wind turbine blade according to an embodiment. 
         FIG. 3  illustrates a cross-sectional profile of the blade of  FIG. 2 . 
         FIG. 4  illustrates a section of a blade root portion of the blade of  FIG. 2 . 
         FIG. 4 a    illustrates a close-up view of a section of the blade root portion of  FIG. 4 . 
         FIG. 5  illustrates a schematic block diagram of a lightning measurement system according to an embodiment. 
         FIG. 6  illustrates an example of a conditioned current signal charted for the determination of peak lightning current. 
     
    
    
     DETAILED DESCRIPTION 
     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. 
     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). 
     One embodiment of the invention provides a wind turbine blade, comprising a proximal end, where the blade is attached to a rotor hub at a blade root portion; and a distal end, where the blade tapers to form a blade tip, a lightning protection system, comprising at least one lightning receptor exposed on a surface of the blade for receiving a lightning strike, and a lightning down conductor coupled to the lightning receptor for relaying lightning current from a lightning strike, the down conductor running internally within the blade and coupled to an electrical ground; and a lightning current measurement system, for deriving parameters of a lightning strike, comprising a lightning current sensor comprising a coil of metal substantially circumscribing the down conductor, the coil in proximity with the down conductor so as to detect magnetic field fluctuations, a lightning analytical system, for receiving an output reading from the lightning current sensor and to provide lightning current parameters, and a signal conversion unit coupled between the lightning current sensor and the lightning analytical system; wherein the signal conversion unit is physically located in the blade and is separated from the blade root by a spacing distance so as to reduce the occurrence of a flashover. 
     As wind turbine blades are prime locations for lightning strike attachments, the provision of such a system within the wind turbine blade allows the investigation of a lightning strike event at or close to the point of entry of lightning current into the wind turbine. This is especially useful in understanding how much lightning current the lightning protection system of the blade has to carry, as well as identifying which blade has been hit by lightning. Furthermore, the provision of the magnetic field lightning current sensor in proximity with the down conductor allows for an accurate measurement of the lightning current travelling within the down conductor and reduces any effect of loss due to attenuation. 
     By providing the signal conversion unit physically within the blade and separating from the blade root by a spacing distance provides electrical insulation between the signal conversion unit and metallic installations in the blade root. Electrical isolation of the signal conversion unit is provided as the lightning current sensor is electrically disconnected from the down conductor system. This allows for a portion of the lightning current measurement system, which is physically located in the blade, to be relatively protected from a direct lightning current transfer through conduction, or from a flashover. 
     In an embodiment, the entire lightning measurement system is physically located in the blade and is separated from the blade root by the same spacing distance so as to reduce the occurrence of a flashover. 
     In an embodiment, the output of the lightning measurement system is provided to a central controller in the nacelle of the wind turbine through fiber optic cabling. 
     In an embodiment, the lightning down conductor is coupled to a blade band external to the blade at a blade root portion, and the lightning current sensor is mounted on an internal surface of the blade, and circumscribing the lightning down conductor. 
     In another embodiment, the blade band is secured to the blade at a predetermined distance from the blade root, and the spacing distance is at least equivalent to the predetermined distance. 
     In an embodiment, the sensor is a large frequency bandwidth current sensor. 
     In another embodiment, the frequency bandwidth of the current sensor is from 0 to 10 MHz. 
     In an embodiment, the frequency bandwidth of the current sensor is from 0 to 10 MHz. 
     In another embodiment, the sensor measures a current range from ±100 A to ±10 kA. 
     In yet another embodiment, the sensor measures a current range from ±20 A to ±400 kA. 
     In an embodiment, the lightning current measurement system comprises two lightning current sensors. 
     In another embodiment, one lightning current sensor measures a current range from ±20 A to ±20 kA and the other lightning current sensor measures a current range from ±400 A to ±400 kA. 
     In an embodiment, the two lightning current sensors are identical. 
     In an embodiment, the lightning current sensor is a Rogowski-coil based current sensor. 
     In an embodiment, the lightning current sensor is provided with a sensor protection system for preventing dielectric breakdown. 
     In another embodiment, the sensor protection system comprises providing insulation about the coil of metal, the insulation having a dielectric breakdown voltage of at least 20 kV. 
     A wind turbine is further provided, comprising a wind turbine blade as described above. 
     Another aspect of the invention provides a method of measuring a lightning strike on a wind turbine, comprising: receiving a lightning strike on a lightning receptor on a wind turbine, directing the lightning strike onto a lightning down conductor coupled to electrical ground, obtaining, with a lightning current sensor, a measurement of magnetic field fluctuations due to the passage of electrical current from the lightning strike through the down conductor, determining an electrical current signal of the lightning strike from the magnetic field fluctuation measurement with a signal conversion unit, conditioning the electrical current signal for analysis with a current conditioning module, and extracting, from the conditioned electrical current signal, a parameter of the lightning strike which corresponds to a physical effect of the lightning strike on the wind turbine. 
     In an embodiment, a DC offset is reduced in the electrical current signal during the conditioning, with a DC offset module in the current conditioning module. 
     In another embodiment, the conditioning of the electrical current signal comprises identifying a lightning strike time period corresponding to the start and the end at which the lightning strike passes through the down conductor, and extracting the electrical current signal during the lightning strike time period. 
     In an embodiment, the method further comprises predetermining a noise floor of the electrical current signal prior to a lightning strike, determining the level at which the electrical current signal exceeds the predetermined noise floor as the start of the lightning strike, determining the level at which the electrical current signal drops below the predetermined noise floor as the end of the lightning strike. 
     In an embodiment, the electrical current signal is smoothened to provide a more accurate identification of the lightning strike time period. 
     In another embodiment, the electrical current signal is magnified to provide a more accurate identification of the lightning strike time period. 
     In an embodiment, at least one of the following lightning strike parameters are extracted from the conditioned electrical current signal: a peak current, a maximum current rise time, a specific energy of the lightning strike and a total charge of the lightning strike. 
     In another embodiment, all four of the lightning strike parameters are extracted from the conditioned electrical current signal. 
     In an embodiment, the method further comprises passing the conditioned electrical current signal through a low pass filter, prior to extracting any one of the specific energy and the total charge. 
     In an embodiment, the low pass filter has a cut-off frequency of about 250 kHz. 
     In an embodiment, the method further comprises converting the conditioned electrical current signal to positive values prior to extraction of the lightning strike parameters. 
     In an embodiment, an α-stroke peak current and a β-stroke peak current are identified from the peak current. 
     In an embodiment, the α-stroke peak current is identified as a current peak originating from a DC value, while the β-stroke peak current is identified as a current peak originating from zero. 
     In an embodiment, the method further comprises identifying a period of current measurement comprising the electrical current signal of the lightning strike for total charge measurement, establishing a noise floor in a noise floor check period during the period of current measurement, projecting a total noise floor in the period of current measurement, and estimating the total charge of the lightning strike by integrating over the period of current measurement and subtracting the total noise floor therefrom. 
     In an embodiment, there is provided an apparatus for measuring a lightning strike on a wind turbine, the apparatus configured to: obtain, with a lightning current sensor, a measurement of magnetic field fluctuations due to the passage of electrical current from a lightning strike on a wind turbine passed through a down conductor, determine an electrical current signal of the lightning strike from the magnetic field fluctuation measurement with a signal conversion unit, condition the electrical current signal for analysis with a current conditioning module, and extract, from the conditioned electrical current signal, a parameter of the lightning strike which corresponds to a physical effect of the lightning strike on the wind turbine. 
     In another embodiment, there is provided a computer readable medium having a computer program recorded thereon, the computer program comprising instructions which, when executed by a processor, causes the processor to perform a method of measuring a lightning strike on a wind turbine, comprising the steps of: receiving a lightning strike on a lightning receptor on a wind turbine, directing the lightning strike onto a lightning down conductor coupled to electrical ground, obtaining, with a lightning current sensor, a measurement of magnetic field fluctuations due to the passage of electrical current from the lightning strike through the down conductor, determining an electrical current signal of the lightning strike from the magnetic field fluctuation measurement with a signal conversion unit, conditioning the electrical current signal for analysis with a current conditioning module, and extracting, from the conditioned electrical current signal, a parameter of the lightning strike which corresponds to a physical effect of the lightning strike on the wind turbine. 
     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. 
       FIG. 1  illustrates an exemplary wind turbine  100  according to an embodiment. As illustrated in  FIG. 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. 
     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  FIG. 1 . In some embodiments of the invention, the tower  110  may be a lattice tower. Accordingly, the tower  110  may include welded steel profiles. 
     The rotor  130  may include a rotor hub (hereinafter referred to simply as the “hub”)  132  and at least one blade  140  (three such blades  140  are shown in  FIG. 1 ). The rotor hub  132  may be configured to couple the at least one blade  140  to a shaft (not shown). In one embodiment, the blades  140  may have an aerodynamic profile such that, at predefined wind speeds, the blades  140  experience lift, thereby causing the blades to radially rotate around the hub. The hub  140  further comprises mechanisms (not shown) for adjusting the pitch of the blade  140  to increase or reduce the amount of wind energy captured by the blade  140 . Pitching adjusts the angle at which the wind strikes the blade  140 . 
     The hub  132  typically rotates about a substantially horizontal axis along a drive shaft (not shown) 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. 
     Typically, the blade  140  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  140  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. 
       FIG. 2  illustrates as wind turbine blade  140  according to an embodiment. Blade  140  is a 50 m blade, but may be of any other length in other embodiments. A blade root portion  144  comprises a proximal end or blade root  142  of the blade  140 . The blade root portion  144  is typically a cylindrical section of the blade  140  which may taper to form a central spar  156  (see  FIG. 3 ) which acts as an internal support for the blade  140 . In such a case, two opposing blade shells, one forming a leeward surface of the blade, the other forming a windward surface of the blade, are thereafter joined together over the central spar to form the blade  140 . A blade band  160  is provided at the blade root portion  144 . 
     Blade  140  tapers towards the end the blade  140  at a distal end to form a blade tip  146 . Blade tip  146  comprises a tip receptor  148  which takes the shape and form of a typical blade tip and forms a part of the blade lightning protection system  150 . Tip receptor  148 , 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  140 , and which is able to receive and resist multiple lightning strikes, the tip receptor  148  is composed entirely of metal, and in the present embodiment, of copper. 
     The tip receptor  148  is coupled onto the blade  140  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 tip receptor  148  is also directly fastened, by crimping, to a down conductor (indicated as broken line  152 ) on the inside of the blade  140 . Other methods are possible. 
     Lightning protection system  150  also comprises a plurality of side lightning receptors  154  along the length of the blade  140 . The side lightning receptors  154  are provided along a central line of the blade  140 , and are located above the blade spar  156 . The side receptors  154  are exposed on both the leeward and windward surfaces of the blade and are coupled to the down conductor  152  inside the blade. Down conductor  152  comprises a core of copper wire, in the present embodiment around 50 mm 2  in cross-section. High voltage insulation is then provided about the copper core of down conductor  152 . In this embodiment, silicone rubber is provided, but any other electrical non-conductor or polymer-type insulation with a high dielectric breakdown voltage may also be used. 
     In the present embodiment, eight side receptors  154  are provided along the length of the blade, the average spacing between each receptor being about 4 m. In other embodiments, other arrangements of lightning receptors are possible, e.g. the lightning receptors are only provided for only a leeward or a windward surface, the lightning receptors are provided closer to the trailing edge, etc. 
       FIG. 3  illustrates a cross-sectional profile of the blade  140  at X-X in  FIG. 2 . Two side receptors  154  are shown as provided for the lightning protection system  150 , one on the lee-ward side  134  of the blade and another on the wind-ward side  136 . The side receptors  154  are located adjacent, and not contacting, spar  156 . The side receptors  154  are installed by means of screw formations through the shell of the blade  140  and are secured to a receptor base  158  on the interior of the blade  140 . The receptor base  158  is thereafter connected to the down conductor  152  running along the blade spar  156  with a braid of copper wires  153 . There may also be a block of low-density polyethylene foam holding the receptor base  158  in place. Any other lightning protection system arrangement may also be possible. 
     The down conductor  152  is a relatively thick bundle of copper wires running along the length of the blade spar  156  for the purpose of high voltage lightning strike current transmission. As indicated above, the down conductor  152  is connected to the side receptor  154  by a copper wire braid  153 . The connection from the down conductor  152  to the copper wire braid  153  is formed with a high quality weld or a pressed connection. The copper wire braid  153  may or may not comprise additional insulation, depending primarily on the distance from the blade tip. 
     The spar  156  is shown internal to blade  140  and acts as a support to the blade  140  and is substantially rectangular in shape. The spar  156  is composed of primarily of carbon fiber and epoxy for light-weight but resilient construction. The down conductor  152  is coupled to a tip receptor  148 , as mentioned, at one end and to a blade band  160  external to the blade  140  at the blade root portion  144 , at the other end. The blade band  160  thereafter provides a sliding surface for electrical contact with a lightning current transfer unit (not shown) between the blade band  160  and the nacelle  120  of the wind turbine  100 . A conventional lightning current transfer unit is described in U.S. Pat. No. 7,654,790, in which the proprietor is the present applicant. The lightning current transfer unit allows the electrical transmission of current from the blade  140  to the nacelle  120 , bypassing the hub  132 , and thereafter conducting the lightning current to electrical ground. In this way, lightning current is electrically isolated from the rotor hub  132  and rotor bearings supporting the rotor shaft. 
       FIG. 4  illustrates a section of the blade root portion according to the present embodiment. Blade band  160  is shown as coupled to a portion of the external circumference of the blade  140 . As indicated above, blade band  160  provides a sliding surface for electrical contact with a lightning current transfer unit, more specifically, the lightning current transfer unit comprises at least two sliding contacts, one for contacting the blade band  160 , and the other for contacting a separate band (not shown) on the nacelle  120 . As such, the blade band  160  should be of a length sufficient to ensure consistent contact between the lightning current transfer unit sliding contact and the blade band regardless of the pitch angle of the blade. In the present embodiment, the blade band  160  covers about 120° of the blade root portion  144 . Blade band  160  is secured onto the blade root portion  144  by means of nut and bolt fastening, but other methods may be possible. 
     Down conductor  152  is shown as extending proximally down to the blade root portion  144  and terminating into the wall of the blade  140 . As mentioned above, the blade root portion  144  may taper off distally to form a central spar  156  of the blade  140 . A hole is thus drilled through the spar  156  distal to the blade root portion  144  to allow the down conductor  152  to be transposed from the outside of the spar  156  to the inside. Opposing leeward surface and windward surface blade shells will provide a cover for the down conductor  152  until it is transposed internally into the blade. 
     The down conductor  152  is coupled to the blade band  160  through the wall of the blade  140  by a bushing  164  at the blade root portion  144 .  FIG. 4 a    illustrates a close-up view of a section of the blade root portion according to the present embodiment. A section of down conductor  152  is illustrated, terminating at the blade root end  142  and coupled to bushing  164  by a simple electrical socket connector. Other attachment means are possible. 
     Bushing  164  facilitates the electrical connection between the down conductor  152  and blade band  160 . In order to establish bushing  164 , a hole is first drilled in the wall of the blade  140  at the blade root portion  144  and bushing  164  is inserted into the cavity. Blade band  160  is then anchored on one end to the blade  140  by a secure connection to one end of the bushing  164 , and to the blade  140  itself by a bolt connection  166  on the other end of the blade band  160 . The down conductor  152  is as indicated coupled to the other end of bushing  164  by the electrical socket connector. There can also be other means of connecting the down conductor to the blade band. 
     Returning to  FIG. 4 , lightning current sensors  170 ,  172  are provided, circumscribing down conductor  152 . In the present embodiment, lightning current sensors  170 ,  172  are current sensors based on Rogowski coil current sensing technology. Such sensors utilize Faraday&#39;s law and output a low voltage output correlated to the rate of change of magnetic flux due to electrical current flow. Such sensors may also be known as magnetometers. Rogowski coil sensors are chosen as they allow for a wide bandwidth of frequency operation, from 0 Hz (Direct Current) to MHz levels. The current sensors  170 ,  172  are envisioned as flexible Rogowski coil comprising a metal wire loop. The wire loop may be configured as a single turn, a simple helix, a toroid, or other configuration used to form a sensor. 
     In the present embodiment, the wire loops of current sensors  170 ,  172  are shaped into a flexible coil of uniform cross section wound upon a non-ferrous core. The lead from one end of the coil is returned through the center of the coil to the other end, so that both terminals are at the same end of the coil. The voltage measured across the coil will be proportional to the rate of change of the magnetic field. Also present in the current sensors  170 ,  172  is the coaxial routing of the coil ends back to the beginning. This allows the current sensors  170 ,  172  to be temporarily separated to allow installation around the down conductor  152 . 
     The current sensors  170 ,  172  are coupled to the wall of the blade  140  by means of mounting arms  174  which are then attached to wall mount couplings  176 . Mounting the current sensors  170 ,  172  allow the sensors to maintain a relatively stable proximity about the down conductor  152  so as to optimize the conditions to detect magnetic field fluctuations for current measurement. Mount  178  is provided for the down conductor  152  to be spaced apart from the wall of the blade  140  so as to allow for the current sensors  170 ,  172  to be circumscribed about the down conductor  152 . 
     Further, a sensor protection system  180  is provided for each lightning current sensors  170 ,  172 . The protection system  180  provides a layer of insulation about the sensor coils and has a dielectric breakdown voltage of at least 20 kV. The sensor protection system  180  protects the sensors  170 ,  172  in the event of a lightning current surge during a lightning strike event on a lightning receptor  148 ,  154 . Particularly, the protection system  180  seeks to address the issue of dielectric breakdown due to the voltage rise caused by the lightning current. Further, as the coils within sensors  170 ,  172  comprise metal, being good electrical conductors, there may be occurrences of a sparkover between the down conductor  152  and the current sensors  170 ,  172  during a lightning strike event. Adequate insulation provided by the sensor protection system  180 , as well as the improved high voltage insulation on the down conductor  152 , seeks to inhibit such an occurrence. 
     Two current sensors  170 ,  172  are provided in the present embodiment, each one for a specified maximum current range—one for a range from ±20 A to ±20 kA the other for a range from ±400 A to ±400 kA. It is noted that present technology limits the optimal maximum current range of a Rogowski coil based current sensor to about 60 dB. However, should technology improve to allow a current sensor to operate with a bandwidth of about 80 dB, it may be envisioned that only one lightning current sensor may be provided in the lightning current measurement system, to cover the expected current range of a lightning strike. 
     The following equation provides the voltage output by the Rogowski coil based current sensors  170 ,  172 : 
                   V   =           -   AN     ⁢           ⁢     μ   0       l     ⁢       d   ⁢           ⁢   I       d   ⁢           ⁢   t                 (   1   )               
where A=πa 2  is the cross-sectional area of the current sensor coil, N is the number of turns in the sensor coil, and l=2πR is the length of the sensor coil.
 
               d   ⁢           ⁢   I       d   ⁢           ⁢   t           
is the rate of change of the current threading the current sensor coil, and is directly proportional to the rate of change of the magnetic field by the vacuum permeability factor (or magnetic constant) of μ 0 =4π×10 −7 . This formula assumes the turns are evenly spaced and that these turns are small relative to the radius of the coil itself. Such an arrangement also provides relative isolation from electromagnetic interference.
 
     As indicated above, the voltage generated that is induced in the coil is proportional to the rate of change of current in the straight conductor. To determine and record the reading as measured by the lightning current sensors  170 ,  172 , the output of the sensors  170 ,  172  are connected to a signal conversion unit  181 , which in the present embodiment is part of a lightning analytical system  182 . The lightning analytical system  182  is mounted on the wall of the blade  140  at the blade root portion  144  and is coupled to the wind turbine controller (not shown) in the nacelle  120  by means of fiber optic data cables running through the hub  132  and the drive shaft. In such a case, the lightning analytical system  182  is electrically isolated from the wind turbine controller. 
     The lightning analytical system is typically a post processing system and comprises at least a data management system, and a processor comprising means to execute instructions on a computer program recorded on a computer-readable medium. As such a system comprises metallic components, it is put at risk from a flashover from lightning stroke current carried in down conductor  152  or blade band  160 . Sufficient shielding is also provided for the lightning analytical system from magnetic field effects emanating from lightning current. 
     The blade band  160  is typically located a certain predetermined distance from the blade root  142 . This is so as to provide for sufficient electrical insulation between the lightning down conductor  152  and metal parts at the blade root junction at which the blade  140  is attached to the hub  132 , so as to reduce the occurrence of current flashover. 
     This predetermined distance is provided by the electrical insulation separation distance equation: 
                   s   =       k   i     ⁢       k   c       k   m       ⁢   l             (   2   )               
where:
     s is the calculated separation distance   k i  depends on the class of the lightning protection system—in the case of wind turbines—Class I
       (k i =0.08)   
       k c  depends on the lightning current flowing on the down-conductors (based on number of down conductors) (k c =1)   k m  depends on the electrical insulation material—in this case air (k m =1)   l is the length in meters, along the down conductor, from the point where the separation distance is to be considered (the blade band), to the nearest equipotential bonding point (in this case, where the current is to be grounded in the nacelle).   

     In a typical wind turbine blade, the predetermined separation distance works out to be about 50 cm; i.e. the blade band  160  is coupled to the blade  140  at a distance of 50 cm from the blade root  142 . 
     As to the lightning current measurement system of the present embodiment, the lightning current sensors  170 ,  172  provide nominal air separation from the down conductor  152 . As such, the lightning current measurement system circuit is considered to be electrically isolated from the lightning protection system of the wind turbine blade as well as the wind turbine controller. Consideration of lightning strike current utilizing the lightning measurement circuit as a conductive path in forming an electrical arc over to the blade root  142  is addressed by providing a spacing distance between the lightning analytical system  182  to reduce the occurrence of a flashover. This spacing distance is at least equivalent to the predetermined distance. In other words, the lightning analytical system  182  is mounted to the inside of blade  140  at the blade root portion  144  at least 50 cm, when measured perpendicularly, from the blade root  142 . 
     In another embodiment, the signal conversion unit  181  is an electrical-optical converter, with an output coupled to fiber optic cabling. In this case, the lightning analytical system  182  is no longer located in the blade  140 , but in the hub  132  or in the nacelle  120 , and is coupled to the electrical-optical converter through the fiber optic cables. 
     In the present embodiment, the lightning analytical system  182  comprises signal conversion unit  181  which is designed to determine an electrical current signal of the lightning strike from the magnetic field fluctuation measurement of the lightning current sensors. As shown in  FIG. 5 , the signal conversion unit  181  comprises a comparator  184  which receives the outputs of the lightning current sensors  170 ,  172  and provides a single overall measurement signal. Further, the signal conversion unit  181  comprises an electronic integrator unit  186  which receives the overall measurement signal and provides an output that is proportional to and representative of the current flowing through the down conductor  152 . The lightning analytical system  182  is further designed to provide four parameters of the lightning stroke measured by lightning current sensors  170 ,  172 , namely peak current, specific energy of the stroke, total charge and current rise time. 
     The above-named lightning stroke parameters are related to certain physical effects and design considerations of, for example, the lightning protection system of the wind turbine blade, in view of the information derived from the lightning stokes can then be addressed. Furthermore, the lightning stroke parameters can be used to provide an estimation of the effect of or damage caused by the lightning strike on the wind turbine blade. For example, the peak current of the lightning stroke, measured in kAmperes (kA), is directly related to mechanical forces. This in turn deals with design considerations such as the mechanical fixation of cables and connectors, the physical bonding of components, and the considerations of electrical energy in wind turbine components. 
     As for specific energy of the lightning stroke, measured in MJoules/Ω, the related physical effect is that of thermal heating. This corresponds to design considerations such as current conductor size/cross-section, the heating/melting of materials nearby the current conductor, and other aspects related to mechanical and thermal forces. 
     Total charge of the lightning stroke, measured in Coulombs, relates to effects such as surface erosion and melting of materials. As such, design considerations such as mechanical factors of blade lightning receptors, other designed lightning strike points and the lightning current transfer unit can be addressed during investigations. 
     Physical effects such as voltage rise are related to the current rise time of the lightning stroke, or 
                 d   ⁢           ⁢   i       d   ⁢           ⁢   t       .         
Such investigations provide information for the analysis of the coupling mechanisms and the shielding.
 
     In an aspect of the invention, there is provided a method of measuring a lightning stroke on a wind turbine blade, comprising at least determining an electrical current signal from a lightning current sensor output, conditioning the electrical current signal for analysis with a current conditioning module and extracting, from the conditioned electrical current signal, a parameter of the lightning strike which corresponds to a physical effect of the lightning strike on the wind turbine blade. Such a method may also provide an efficient way to process large amounts of lightning current effectively. 
       FIG. 5  illustrates a schematic block diagram of a lightning measurement system according to an embodiment. In  FIG. 5 , a block diagram of a lightning analytical system  182  is shown coupled to the outputs of lightning current sensors  170 ,  172 . The lightning analytical system  182  comprises, in general, a current conditioning module  188  and a lightning current parameter extraction module  200 . 
     Output V C1 * and V C2 *, from lightning current sensors  170  and  172  respectively are provided to comparator  184 , which combines the currents to output a single overall measurement V C *. V C * is then passed through an integrator unit  186  which generates a raw measured electrical current signal I raw *. 
     The electrical current signal I raw * is thereafter provided to the current conditioning module  188  which conditions the electrical current signal I raw * for analysis. Such a conditioning module allows for the manipulation of the raw current signal I raw * to allow lightning strike parameters to be extracted. 
     Current conditioning module  188  comprises a DC offset module  190  which receives electrical current signal I raw *. The DC offset module  190  functions to remove or reduce the effects of a DC offset on the analysis of the electrical current signal I raw *. As a definition, DC offset is an offsetting of signal from zero. DC offset is the mean value of a signal without an event; if the mean value is zero, then there is no dc offset. 
     A DC offset is problematic as it affects where the zero crossings of the electrical signal appear, which will affect the calculations of the lightning parameters. It is therefore desirable to remove or reduce the dc offset while processing the electrical current signal I raw *. This is achieved in the present embodiment by subtracting the mean value calculated from certain initial samples within the electrical current signal I raw *. The initial samples are taken in a pre-trigger period prior to the start of the lightning strike event. Trigger is defined as the start of the lightning strike event and triggered event is defined as the time period between the start and end of the lightning strike event. 
     In the present embodiment, to the triggered event accounts for 80% of data samples to be analyzed, and the pre-trigger period accounts for 20%. The size of the initial samples to be taken for DC offset calculations may be given to be the number of samples in the first 0.5% of the pre-trigger period. For example, if a sampling rate of 10 MHz is used in the lightning analytical system, a total time of 1000 ms is recorded for the pre-trigger and triggered even period, and pre-trigger event is 20% of the total recording time, the number of samples can be calculated as:
 
No. of samples=10×10 6 ×(0.5%×200 ms)=10000 samples
 
     The mean value is then taken from these samples and subtracted from the electrical current signal I raw * to produce I offset . 
     I offset  is then passed to a time interval module  192  for the identification of the start and end of the lightning strike event as the calculations of lightning parameters such as total charge, energy and rise time requires time identification of the lightning strike event. An embedded noise floor, such as white noise, usually causes difficulty in the clear definition of the start and end time for the lightning event. White noise is a random signal with a flat power spectral density, i.e., the signal contains equal power within a fixed bandwidth at any centre frequency and the average for the noise amplitude should be zero. 
     In the present embodiment, the time interval module  192  carries out a method of adjacent averaging to smoothen the current signal. Under adjacent averaging, the average value is obtained from a certain number of data points around each point in the current signal data and the point in the current signal data is replaced with the new average value. The new average value is further squared to magnify the current signal and in order to estimate the noise level. In the embodiment, any value of current signal which is lower than the predefined noise level will be considered as noise. The start time, Start_T, can be identified to be the value exceeding the predetermined noise level and the stop time, Stop_T, can be identified by the time when the current goes below the noise level. 
     In other embodiments, a separate noise reduction or signal smoothing module may be provided prior to the time interval module  192 . In yet other embodiments, other suitable smoothing methods, such as weighted moving average, or Savitz-Golay method, or FFT filter method, or any other applicable methods may be used. In another embodiment, in order to provide even greater noise reduction capabilities, the lightning current sensors  170 ,  172  are duplicated, i.e. two sets of identical sensors at about the same measurement location. Such noise reduction efforts attempt the cancellation of background noise and provide more accurate measurement results. Further, having two sets of sensors provides an additional advantage in that it will be possible to detect whether the current measured is due to lightning current through the down conductor or if the magnetic fluctuations are picked up due to a nearby lightning strike. The orientation of the magnetic field through the sensors are the same if the magnetic field-causing current is due to current in the down conductor. 
     Further, a filter  194  may be provided to allow the more accurate determination of certain parameters of the lightning stroke. Filter  194  is a low pass filter that passes low-frequency signals but attenuates signals with frequencies higher than a predetermined cut-off frequency. In particular, filter  194  is a low pass digital filter with a cut-off frequency of 250 kHz. 
     The output of the filter  194 , I off+LP-250 , is thereafter used for the calculations of the total charge and total specific energy lightning parameters. Such a filter  194  is provided for the calculations of these parameters to allow a focus on the information provided in the current signal at frequencies below about 250 kHz. 
     In the embodiment, two functional blocks  196  and  198  are provided for the signal I off+LP-250  to allow proper conditioning of the current signal in order to derive the respective lightning stroke parameters. Lightning currents consist of both positive and negative strokes, and it is preferable to work with only positive values (i.e., the mathematical integration of positive and negative currents would cause some discrepancies if for example the total charge of the lightning stroke is calculated). The current signal I off+LP-250  is first squared as defined by l 2  block  196  and then rooted by the √{square root over (I 2 )} block  198 . The results from √{square root over (I 2 )} block  198  can be used for calculating the total charge and the results from l 2  block  196  can be used to calculate the total specific energy. 
     Lightning current parameter extraction module  200  comprises several sub-modules for the extraction of lightning current parameters based on the measurement of lightning current by the lightning current sensors  170 ,  172 . In the present embodiment, parameter extraction module  200  comprises a peak current module  202 , a current rise time module  204 , total energy module  206  and charge module  208  are provided. In other embodiments, greater or fewer modules may be provided. 
     In peak current module  202 , a peak current of the lightning stroke, measured in kAmperes (kA), is determined from a conditioned electrical current signal provided by the current conditioning module  188 . The conditioned electrical current signal when charted out may also provide indications on the alpha(β)-pulses and beta(β)-pulses in the received lightning stroke. 
     In lightning analysis, for elevated objects, it is estimated that more than 90% of the flashes to the tower are upward initiated. An upward electrical leader bridges the gap between the grounded elevated object and a cloud and establishes an initial continuing current (ICC) with a duration of some hundreds of milliseconds and an amplitude of some tens to some thousands of amperes. In most cases, current pulses are superimposed on the slowly varying continuing current. These pulses are often referred to as ICC pulses or α-pulses. After the cessations of the ICC, one or more downward leader/upward return stroke sequences may occur—the associated current pulses are called β-pulses. Typically α-pulses are relatively small, less than 10 kA, while β-pulses have current peaks mostly in the range above 5 kA. 
       FIG. 6  illustrates an example of a conditioned current signal charted for the determination of peak lightning current. Essentially,  FIG. 6  illustrates on chart  240  the main criterions for defining the threshold for lightning stroke peak currents and classifying the type of lightning strokes. The noise floor of ±200 A, as described above, is taken into account in the defining of the threshold for the lightning stroke peak currents. For the sake of simplicity in processing a large amount of lightning current information, as well as in processing a large number of current peaks, a current peak may be identified by a threshold current rise of greater than 1 kA. The value may be different for other noise floor considerations. 
     In the case of a current spike  242 , which occurs near to another spike  244 , a lightning stroke current peak is identified when the current when measured from its trough to the peak is greater than 2 kA. This is to ensure that the current variations do not give current peaks which are not actually present. Spikes  242  and  244  can thus be identified as lightning stroke current peaks. To differentiate between α-pulse currents and β-pulse current peaks, α-pulse current peaks are defined as current peaks resting on a direct current (DC) value, such as spikes  242 ,  244 , while β-pulse current peaks are defined as current peaks residing on the zero line, such as spikes  246 ,  248 . The peak current module  202  thereafter consolidates the identified peak currents and corresponding information and generates them as output. 
     Current rise time module  204  determines the rise time of the conditioned current signal I offset . The rise time of the received lightning stroke will affect the coupling mechanisms in the wind turbine and may induce an electromagnetic field (EMF) which may affect the electronics in the wind turbine if cables and cabinets are not properly shielded. The faster the current rise time, the higher the EMF generated, and as such it is therefore useful to find out the rise time of the lightning flash for design considerations. The current rise may also cause a voltage drop on a wire due to the inductance of the wire. Higher current rises result in higher voltage drops. This voltage drop may also cause flashes to other structural portions of the wind turbine which are not part of the intended lightning dissipation current path to ground. 
     The rise times of the lightning stroke currents are calculated by differentiating the conditioned lightning current signal throughout the time interval Start_T and Stop_T as identified by time interval module  192 . The maximum rise time 
                       ⁢     d   ⁢           ⁢   i         d   ⁢           ⁢   t           
is then determined.
 
     The total energy module  206  receives the conditioned current I 2   off+LP-250k  from l 2  block  196  and provides the total specific energy of the received lightning stroke. The total specific energy is defined as the integration of the square of the lightning current of the limits set as the lightning stroke start/stop time:
 
Total Specific Energy=∫ Stop   _   T   Start     —T     I   2   dt  
 
     The charge module  208  receives the conditioned current I +   off+LP-250k  from √{square root over (I 2 )} block  198  and provides the total charge of the received lightning stroke. The total charge of the lightning stroke is defined as the integration of the lightning current over the limits set as the lightning stroke start/stop time:
 
Charge=∫ Stop   _   T   Start   _   T   Idt  
 
     Alternatively, the charge (C L ) can be calculated from the simple equation as follows: 
     
       
         
           
             
               C 
               L 
             
             = 
             
               
                 C 
                 AII 
               
               - 
               
                 C 
                 N 
               
             
           
         
       
       
         
           where 
         
       
       
         
           
             
               
                 C 
                 AII 
               
               = 
               
                 
                   ∫ 
                   Msmt_Stop 
                   Msmt_Start 
                 
                 ⁢ 
                 
                   I 
                   ⁢ 
                   
                       
                   
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                   d 
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                   t 
                 
               
             
             , 
             and 
           
         
       
       
         
           
             
               C 
               N 
             
             = 
             
               
                 [ 
                 
                   
                     ∫ 
                     
                       Noise_check 
                       ⁢ 
                       _Stop 
                     
                     
                       Noise_check 
                       ⁢ 
                       _Start 
                     
                   
                   ⁢ 
                   
                     I 
                     ⁢ 
                     
                         
                     
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                 ] 
               
               · 
               
                 
                   
                     t 
                     measurement_period 
                   
                   
                     t 
                     
                       Noise_check 
                       ⁢ 
                       _period 
                     
                   
                 
                 . 
               
             
           
         
       
     
     To determine the electrical charge of the lightning strike, a period of current measurement, defined by Msmt_Start and Msmt_Stop, and encompassing the lightning strike event, is used in the charge calculation. The measurement period is typically defined to be a period larger than the time period of the lightning event, and includes a noise floor check period, typically taken at the beginning of the measurement period, and prior to any current measurement rise. This noise floor is then assumed constant over the measurement period and subtracted from the charge calculation to provide a reasonable estimate of the electrical charge of the lightning event. 
     Outputs from the lightning analytical system  182  can thereafter be used as part of damage assessment for the blades and turbine, for input to a remaining life time estimation system or for charting lightning strikes. 
     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&#39;s general inventive concept.