Patent Publication Number: US-2020300228-A1

Title: Lightning protection for a direct drive wind turbine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to European Application No. 19164682.7, having a filing date of Mar. 22, 2019, the entire contents of which are hereby incorporated by reference. 
     FIELD OF TECHNOLOGY 
     The following relates to a lightning protection arrangement for a direct drive wind turbine. 
     BACKGROUND 
     Wind turbines include lightning protection arrangements to guide the lightning strokes from the blades through the hub, the nacelle and the tower and down to earth. A direct drive wind turbine typically comprises:
         a hub,   a plurality of blades,   a nacelle on which the hub is rotatably mounted,   an electrical generator having an inner stator rigidly attached to the nacelle and an external rotor directly, i.e. without gearbox, rigidly attached to the hub,   a hollow fixed shaft for supporting the hub and the electrical generator, a main bearing being interposed between the hub and the free end of the hollow fixed shaft for allowing the rotation of the assembly including the hub and the electrical generator rotor,   a bedframe of the nacelle for supporting the hollow fixed shaft,   a tower on which the bedframe of the nacelle is mounted, in such a way that the nacelle can rotate about a yaw axis substantially perpendicular to the ground.       

     In the described above structure, a lightning protection arrangement is normally present, which includes a plurality of conductors, for example conducting cables, extending from the blades to the tower and the ground. The standard IEC 61400-24 defines a lightning protection zone (LPZ) concept. In the hub and the nacelle, three main LPZs (Lightning Protection Zones) are defined:
         the hub,   the space inside the hollow fixed shaft between the main bearing and the bedframe,   the annular space between the hollow fixed shaft and the rotor house of the electrical generator.       

     At the transitions between zones surge protection devices (SPDs) have to be installed for transferring the high current generated by a lightning stroke to the ground. The requirements of the SPDs depend on the classification of the lightning zones, as defined in the standard IEC 61400-24. 
     Embodiments of the invention is that of providing a lightning protection arrangement for a direct drive wind turbine allowing the reduction of the requirements to the SPDs, according standard IEC 61400-24, by changing the classification of the above defined three LPZs. This results in costs savings. 
     SUMMARY 
     An aspect relates to a lightning protection arrangement for a direct drive wind turbine is provided. The wind turbine includes:
         a hub   at least one blade fixed to the hub,   a nacelle on which the hub is rotatably mounted for rotating about a rotational axis, the nacelle comprising an electrical generator connected to the hub in order to receive rotational energy from the hub,   a tower on which the nacelle is mounted.       

     The lightning protection arrangement includes a plurality of protection conductors extending between the at least one blade and the tower, the plurality of protection conductors including at least a first protection conductor mounted on an external surface of the electrical generator. 
     Advantageously, by providing a protection conductor mounted on an external surface of the electrical generator the lightning protection zones inside the hollow fixed shaft and the annular space between the hollow fixed shaft and the rotor house can be classified as “zone 2” according to the standard IEC 61400-24. At the transitions between zones of this type no further protection through surge protection devices (SPDs) is required according to the standard IEC 61400-24. 
     In embodiments of the present invention, the electrical generator includes a rotor house rigidly connected to the hub for rotating about a rotational axis together with the hub the first protection conductor being mounted on an external surface of the rotor house. The external surface of the rotor house provides a convenient surface for the mounting of the protection conductor. 
     In other embodiments of the present invention, the plurality of protection conductors includes at least a second protection conductor mounted on an external surface of the hub and electrically connected to the first protection conductor. Advantageously, by providing a protection conductor mounted on an external surface of the hub, the hub lightning protection zone can be classified as “zone 2” according to the standard IEC 61400-24, thus obtaining the same advantage described above, i.e. that no further protection through surge protection devices (SPDs) is required according to the standard IEC 61400-24 at the transition between LPZs classified as “zone 2”. 
     In embodiments of the present invention, the lightning protection arrangement includes the following plurality of protection conductors arranged in series from at least one blade to the tower:
         a protection conductor mounted along the blade,   a protection conductor mounted on an external surface of the hub and electrically connected to the protection conductor mounted along the blade at the pitch bearing provided between the blade and the hub,   a protection conductor mounted on an external surface of the rotor house and electrically connected to the protection conductor mounted on the external surface of the hub at the main bearing of the wind turbine,   sliding contacts provided between the brake disk and the fixed shaft and electrically connected to the protection conductor mounted on the external surface of the rotor house,   a protection conductor mounted on a fixed shaft of the nacelle and electrically connected to the sliding contacts provided between the brake disk and the fixed shaft,   a protection conductor mounted on a bedframe of the nacelle and electrically connected to the protection conductor mounted on the fixed shaft,   a protection conductor mounted on the tower of the wind turbine and electrically connected to the protection conductor mounted on the bedframe.       

    
    
     
       BRIEF DESCRIPTION 
       Some of the embodiments will be described in detail, with references to the following Figures, wherein like designations denote like members, wherein: 
         FIG. 1  shows a schematic lateral view of a direct drive wind turbine including a lightning protection according to embodiments of the present invention; 
         FIG. 2  shows a more detailed lateral view of the direct drive wind turbine of  FIG. 1 ; and 
         FIG. 3  shows a schematic sectional view of the direct drive wind turbine of the  FIGS. 1 and 2 . 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 and 2  show a direct drive wind turbine  10  for generating electricity. The wind turbine  10  comprises a tower  11  which is mounted on the ground  9  at one bottom end. At the opposite top end of the tower  11  there is mounted a nacelle  14 . The nacelle  14  is usually mounted rotatable with regard to the tower  11  about a yaw axis Z substantially perpendicular to the ground  9 . The nacelle  12  includes an electrical generator  15 . Furthermore, the wind turbine  10  comprises a hub  13  which is rotatably attached to the nacelle  14  for rotating about a rotational axis Y, which is a longitudinal axis of the nacelle  14 . The hub  13  is hollow and space inside the hub  13  defines a first lightning protection zone according to the standard IEC 61400-24. When not differently specified, the terms axial or longitudinal, radial and circumferential in the following are made with reference to the longitudinal rotor axis Y. The electrical generator  15  is connected to the hub  13  in order to receive rotational energy from the hub  13 . 
     The wind turbine  1  further comprises at least one blade  17  (in the embodiment of  FIG. 1 , the wind rotor comprises three blades  17 , of which only two blades  17  are visible) mounted on the hub  13 . The blades  17  extend substantially radially with respect to the rotational axis Y. The hub  13  comprises three flanges for connecting a respective blade  17  to the hub  13 . A pitch bearing  16  is interposed between each blade flange and the respective blade  17 . A hydraulic pitch actuation circuit is associated to the pitch bearings  16  of the blades  17  for regulating the pitch angle of each blade, i.e. the angular position of each blade about a respective blade longitudinal axis X. The wind turbine  10  comprises a lightning protection arrangement  100 , as better detailed in the following. 
     With reference to  FIG. 3  the nacelle  14  includes a bedframe  22  having a flange for rotatably connecting the nacelle  14  to the tower  11  for allowing the rotation of the nacelle about the yaw axis Z. The nacelle further includes a hollow fixed shaft  20  extending coaxially with respect to the rotational axis Y. The hollow fixed shaft  20  coaxially extends along the rotational axis Y between two longitudinal ends, one of which is rigidly attached to the bedframe  22 . A main bearing  21  of the wind turbine  10  is mounted the other longitudinal end of the hollow fixed shaft  20 . The hub  13  is connected to the main bearing  21  for allowing the rotation of the hub  13  about the rotational axis Y. The hollow space  23  inside the hollow fixed shaft  20  defines a second lightning protection zone according to the standard IEC 61400-24. 
     The electrical generator  15  extending along the rotational axis Y between a drive end adjacent to the hub  13  and a non-drive end adjacent to the bedframe  22 . The electrical generator  15  comprises an outer rotor  4  and an inner stator  3 , separated by an air-gap  5 . The stator  3  is rigidly supported by the hollow fixed shaft  20 . During operation, the outer rotor  4  is caused to rotate about the rotational axis Y in order to transform mechanical energy into electrical energy. The rotor includes an external rotor house  40  coaxially extending along the rotational axis Y between a hub interface  42  and a brake disc  41 . The hub interface  42  is rigidly connected to the hub  13  and rotational connected to the main bearing  21  for allowing the rotation of the assembly including the hub  13  and the rotor house  40  about the rotational axis Y. The brake disk  41  locks the rotation of the rotor  4  with respect to the stator  3  when required by the operating conditions. The annular space  24  between the hollow fixed shaft  20  and the rotor house  40  of the electrical generator  15  defines a third lightning protection zone according to the standard IEC 61400-24. 
     The lightning protection arrangement  100  (schematically represented in  FIG. 3  by a dashed line) includes a plurality of protection conductors extending between at least one blade  17  and the tower  11  for transferring the electric current of a lightning stroke hitting the blade to the ground  9 . The plurality of protection conductors may include a plurality of conducting cable and a plurality of conducting brushes installed between components relatively rotating with respect to one another. The lightning protection arrangement  100  includes a first protection conductor  111  mounted on an external surface of the electrical generator  15 . In particular the first protection conductor  111  may be mounted on an external surface of the rotor house  40 . The first protection conductor  111  may include one or more conducting cables. The first protection conductor  111  longitudinally extends between the hub interface  42  and the brake disk  41 . The plurality of protection conductors includes a second protection conductor  112  mounted on an external surface of the hub  13  and electrically connected to the first protection conductor  111  at the main bearing  21 . The plurality of protection conductors include a third protection conductor  113  mounted on a flange of the bedframe  22  and electrically connected to the first protection conductor  111  by means of sliding contacts  117  provided between the brake disk  41  and the fixed shaft  20 . Alternatively, according to other embodiments of the invention, the third protection conductor  113  is mounted on the fixed shaft  20 , in particular on a flange of the fixed shaft  20 . The plurality of protection conductors include a fourth protection conductor  114  mounted on at least one blade  17  and electrically connected to the second protection conductor  112  at the respective pitch bearing  16  provided between the blade  17  and the hub  13 . The plurality of protection conductors include a fifth protection conductor  115  mounted on the bedframe  22  and electrically connected to the third protection conductor  113 . The plurality of protection conductors include a sixth protection conductor  116  mounted on the tower  11  and electrically connected to the fifth protection conductor  115  by means of sliding contacts  119 . The sixth protection conductor  116  is further connected to the ground  9 . 
     The lightning protection arrangement  100  provides an electrical connection between at least one of the blades  17  of the wind turbine  10  and the ground. According to the embodiment of  FIG. 3 , the lightning protection arrangement  100  includes the following plurality of protection conductors arranged in series:
         the fourth protection conductor  114  along the blade  17 ,   the second protection conductor  114  on an external surface of the hub  13 ,   the first protection conductor  111  on an external surface the rotor house  40 ,   the sliding contacts  117  between the brake disk  41  and a flange of the bedframe  22 ,   the third protection conductor  113  on a flange of the bedframe  22 ,   the fifth protection conductor  115  along the bedframe  22  up to the interface with the tower  11 ,   the sliding contacts  119  between the bedframe  22  and the tower  11 ,   the sixth protection conductor  116  along the tower  11  up to the ground  9 .       

     The presence of the protection conductors  111 ,  112  on external surfaces, i.e. exposed to the external environment, of the electrical generator  15  and the hub  13 , respectively permits to classify the three lightning protection zones above defined, i.e. inside the hub  13 , inside the fixed shaft  20  and in the annular space  24  between the hollow fixed shaft  20  and the rotor house  40 , as “zone 2” according to the standard IEC 61400-24. This allows reducing the requirements to the SPDs at the transitions between the three zones. 
     Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention. 
     For the sake of clarity, it is to be understood that the use of ‘a’ or ‘an’ throughout this application does not exclude a plurality, and ‘comprising’ does not exclude other steps or elements.