Patent Publication Number: US-2022228563-A1

Title: Improvements relating to electrical power generators for wind turbines

Description:
TECHNICAL FIELD 
     The invention relates to an electrical power generator for a wind turbine, and more specifically to a system and method for installing a service conduit or ‘pitch tube’ and an end-shield in the electrical power generator. 
     BACKGROUND 
     As is well-known, wind turbines convert kinetic energy from the wind into electrical energy, using a large rotor with a number of rotor blades. A typical Horizontal Axis Wind Turbine (HAWT) comprises a tower, a nacelle on top of the tower, a rotating hub or ‘rotor’ mounted to the nacelle and a plurality of wind turbine rotor blades coupled to the hub. The nacelle houses many functional components of the wind turbine, including for example a generator, gearbox, drive train and rotor brake assembly, as well as convertor equipment for converting the mechanical energy at the rotor into electrical energy for provision to the grid. The gearbox steps up the rotational speed of the low speed main shaft and drives a gearbox output shaft. The gearbox output shaft in turn drives the generator, which converts the rotation of the gearbox output shaft into electricity. The electricity generated by the generator may then be converted as required before being supplied to an appropriate consumer, for example an electrical grid distribution system. 
     In some wind turbine designs, the hub houses several electrical and hydraulic sub-systems that are critical to the proper functioning of the wind turbine. For example, the hub may contain an electrically and/or hydraulically actuated blade pitch system, and a power control system for a blade anti-ice and de-ice system. In order to transport the necessary hydraulic and electrical services from the stationary frame of reference of the nacelle, to the rotational frame of reference of the hub, a service conduit or ‘pitch tube’ is typically provided. 
     Typically, in medium-speed wind turbines, such a pitch tube is usually located within the drive train of the wind turbine so as to extend through the generator and gearbox assemblies along their rotational axis and is configured to rotate in synchronisation with the low speed shaft. The pitch tube typically comprises a rotational transfer module which serves as a rotational interface between hydraulic and electrical supplies external to the pitch tube, and various hydraulic components and power cables within the pitch tube that extend along it towards the hub. In such medium-speed wind turbine systems, the pitch tube is normally supported by a bearing that is mounted within the generator. As is conventional in this technical field, the term “medium-speed” has been used to refer to wind turbines which operate at high power ranges (above 4-5 MW), typically have generator speeds in the range of around 100 to 600 rpm, and where, typically, only planetary gear stages are used and the generator and gearbox are co-axially positioned. 
     Since the pitch tube rotates with the low speed shaft, it is important that the pitch tube is aligned accurately with the rotational axis. Current approaches may involve mounting the pitch tube via a suitable bearing arrangement to a mounting bracket associated with the generator housing. However, such an approach has been found to present challenges in ensuring that the pitch tube is centred accurately on the rotational axis. 
     It is against this background that the invention has been devised. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention, there is provided a method of installing a pitch tube into an electrical power generator for a wind turbine. The method comprises: installing the pitch tube so that it is coaxial with a rotational axis of the generator; supporting a bearing arrangement associated with the pitch tube at an end of the generator using one or more primary supports, wherein each of the primary supports comprises a first end connected to the bearing arrangement and a second end connected to a component associated with a rotating reference frame of the generator; and supporting the bearing arrangement using one or more secondary supports, wherein each of the secondary supports comprises a first end connected to the bearing arrangement and a second end connected to a component associated with a stationary reference frame of the generator. 
     Advantageously, the method of the invention provides a more accurate assembly process for the pitch tube which ensures it is located at a precise central position with respect to the rotational axis of the generator. This is because an initial step involves supporting the pitch tube by the primary supports which are connected to a component associated with the rotating reference frame of the generator. This may be the rotor of the generator, for example. Since the rotor is datumed accurately at the rotational axis of the generator, the pitch tube will also be datumed accurately by being supported on the rotor by the primary support. Once accurately mounted, the pitch tube can then be connected to the generator housing using the secondary support arms, at which point the primary supports can be removed. 
     In some embodiments, the method may further comprise, after the bearing arrangement has been supported by the one or more secondary supports, disconnecting the one or more primary supports from the bearing arrangement and the component associated with the rotating reference frame of the generator. This advantageously enables the well-aligned pitch tube to be simply mounted only with respect to the stationary reference frame of the generator (e.g. to the generator housing) for the rest of the operational lifetime of the wind turbine. 
     In some embodiments, the method further comprises attaching a plurality of cover components to the one or more secondary supports to span an open area defined by an end opening of a generator housing. In some embodiments, attaching the plurality of cover components comprises fitting the cover components together to define a continuous cover over the end opening of the generator housing. 
     In some embodiments, the attaching of the plurality of cover components is carried out after disconnecting the one or more primary supports. 
     In some embodiments, the component associated with the rotating reference frame of the generator comprises a rotor hub component of the generator. Optionally, supporting the bearing arrangement using one or more primary supports comprises connecting the second end of each of the primary supports to the rotor hub component. 
     In some embodiments, the component associated with the stationary reference frame of the generator comprises a generator housing component. Optionally, supporting the bearing arrangement using one or more secondary supports comprises connecting the second end of each of the secondary supports to an end face of the generator housing component. 
     Optionally, each of the primary supports comprises an L-shaped bracket. 
     According to another aspect of the invention, there is provided a wind turbine comprising a wind turbine tower, a nacelle rotatably coupled to the tower, a rotating hub mounted to the nacelle, and a plurality of wind turbine blades coupled to the hub, wherein the nacelle comprises an electrical power generator having a pitch tube installed by the method substantially as described above. 
     According to another aspect of the invention, there is provided an electrical power generator for a wind turbine comprising: a generator housing; a generator rotor located within the generator housing and being accessible through an end opening of the generator housing, the rotor defining a rotational axis of the generator; a pitch tube extending through the rotor and being coaxial with the generator rotational axis, the pitch tube having an end that is supported by a bearing arrangement proximal with the generator housing end opening; a plurality of support members connected between the bearing arrangement and a component of the generator housing; and a plurality of cover components arranged on the plurality of support members so as to form a continuous cover for the end opening of the generator housing. 
     In some embodiments, each of the plurality of cover components extends between at least a pair of the plurality of support members. 
     In some embodiments, each of the support members is a planar member that when connected between the bearing arrangement and the generator housing component, is positioned to coincide with an end face of the generator housing. 
     Optionally, the electrical power generator comprises at least four support members and four cover components, wherein each of the cover components forms a quadrant of the continuous cover for the end opening of the generator housing. 
     According to another aspect of the invention, there is provided a wind turbine comprising a wind turbine tower, a nacelle rotatably coupled to the tower, a rotating hub mounted to the nacelle, and a plurality of wind turbine blades coupled to the hub, wherein the nacelle comprises the electrical power generator substantially as described above. 
     Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram of a wind turbine in which an electrical power generator according to an embodiment of the present invention may be implemented; 
         FIG. 2  is a schematic diagram of various functional power generating components located within the wind turbine nacelle of  FIG. 1 ; 
         FIG. 3  is a sectional view of a power generating assembly illustrating further details of the integrated gearbox and generator shown in  FIG. 2 ; 
         FIG. 4  is a perspective view of the wind turbine generator shown in  FIG. 2 , illustrating how a pitch tube may be mounted within the generator according to an embodiment of the present invention; 
         FIGS. 5 a  and 5 b    are perspective exploded views of the generator of  FIG. 4  illustrating an example implementation of a primary pitch tube installation step, according to an embodiment of the present invention, for mounting the pitch tube to a component associated with a rotating reference frame of the generator; 
         FIGS. 6 a  and 6 b    are perspective exploded views of the generator of  FIG. 4  illustrating another example implementation of the primary pitch tube installation step; 
         FIGS. 7 a  and 7 b    are perspective and perspective exploded views respectively of the generator of  FIG. 4  illustrating a secondary pitch tube installation step, according to an embodiment of the present invention, for mounting the pitch tube to a component associated with a stationary reference frame of the generator; 
         FIGS. 8 a  and 8 b    are front, and perspective exploded views respectively of the generator of  FIG. 4  illustrating the installation of an end-shield for the generator housing according to an embodiment of the present invention; 
         FIG. 9  is a perspective view of the generator, showing the end-shield installed in place following the method steps shown in  FIGS. 8 a  and 8 b   ; and 
         FIG. 10  is a flow diagram illustrating steps in the pitch tube and end-shield installation method shown in  FIGS. 5 a    to  8   b.    
     
    
    
     In the drawings, like features are denoted by like reference signs. 
     SPECIFIC DESCRIPTION 
     A specific embodiment of the invention will now be described in which numerous features will be discussed in detail in order to provide a thorough understanding of the inventive concept as defined in the claims. However, it will be apparent to the skilled person that the invention may be put in to effect without the specific details and that in some instances, well known methods, techniques and structures have not been described in detail in order not to obscure the invention unnecessarily. 
     In order to place the embodiments of the invention in a suitable context, reference will firstly be made to  FIG. 1 , which illustrates a typical Horizontal Axis Wind Turbine (HAWT) in which an electrical power generator according to an embodiment of the invention may be implemented. Although this particular image depicts an on-shore wind turbine, it will be understood that equivalent features will also be found on off-shore wind turbines. In addition, although such wind turbines are referred to as ‘horizontal axis’, it will be appreciated by the skilled person that for practical purposes, the axis is usually slightly inclined to prevent contact between the rotor blades and the wind turbine tower in the event of strong winds. 
     As previously mentioned, the wind turbine  1  comprises a tower  2 , a nacelle  4  rotatably coupled to the top of the tower  2  by a yaw system (not shown), a rotating hub or ‘rotor’  8  mounted to the nacelle  4  and a plurality of wind turbine rotor blades  10  coupled to the hub  8 . The nacelle  4  and rotor blades  10  are turned and directed into the wind direction by the yaw system. 
     With reference to  FIG. 2 , the nacelle  4  may include an electrical power generating assembly  20 , which includes a gearbox  22  and a generator  24 . A main shaft  26 , is supported by a main bearing housing  25  and is connected to, and driven by, the rotor  8  and provides input drive to the gearbox  22 . The gearbox  22  steps up the rotational speed of the low speed main shaft  26  via internal gears (not shown) and drives a gearbox output shaft (not shown). The gearbox output shaft in turn drives the generator  24 , which converts the rotation of the gearbox output shaft into electricity. The electricity generated by the generator  24  may then be converted by other components (not shown) as required before being supplied to an appropriate consumer, for example an electrical grid distribution system. So-called “direct drive” wind turbines that do not use gearboxes are also known. In a direct drive wind turbine, the generator is directly driven by a shaft connected to the rotor. 
     A so-called ‘pitch tube’  27  may be arranged to pass along the centre of the generator  24  and the gearbox  22  to provide hydraulic and/or electrical services to the hub. As indicated previously, the term ‘pitch tube’ is considered to be a conventional engineering term used to refer to the service conduit that extends through the generator  24  and gearbox  22 , and contains the appropriate hydraulic and/or electrical cables and other components within it that provide the required hydraulic and electrical services to the hub  8 . Thus, the pitch tube  27  is coaxial with the rotational axis of the wind turbine drive line, as depicted by the reference ‘R’. 
     The gearbox  22  and generator  24  may be coupled together in an integrated unit to form the electrical power generating assembly  20 . Such an integrated unit is shown in  FIG. 2 , but it should be noted that the gearbox and generator need not be integrated and could instead be separated axially. This close integration of the gearbox  22  and the generator  24  is a configuration typical of medium-speed wind turbines, as was previously mentioned. 
     With reference generally to the gearbox  22 , a gearbox housing  30  is generally cylindrical in form and is oriented such that its major rotational axis is horizontal, in the orientation of the drawings. The cylindrical configuration of the gearbox housing  30  is due to the specific type of gearbox that is used in the illustrated embodiment, which is an epicyclic gearbox. As the skilled person would know, an epicyclic gearbox comprises a series of planet gears that are arranged about a central sun gear, and which collectively are arranged within an encircling ring gear. The ratio of the number of teeth between the ring gear, the planet gear and the sun gears determines the gear ratio of the gearbox. For clarity, fine detail of the gearbox will not be described in further detail here as the gearbox is not the principal subject of the invention. Suffice to say that other gearbox configurations could also be used, although it is currently envisaged that an epicyclic gearbox provides an elegant solution fit for the confines of a wind turbine nacelle. 
     The focus of this discussion relates generally to the installation of the pitch tube  27  within the integrated power generating assembly  20 , with particular reference to the interface between the pitch tube  27  and the generator  24 , as well as installation and assembly of a closure or cover for an end opening of the generator  24 . For context and ease of reference, some details of the generator  24  will now be described with reference to  FIG. 3 , which shows the components of the generator  24  more clearly. 
     It should be noted here that for brevity and clarity some components of the generator are not shown or described so as not to detract from the focus of this discussion. For example, the drive shaft by which the generator  24  is coupled to the gearbox  22  is not shown. However, it should be noted that the output shaft of the gearbox  22  interfaces with a rotor  32  of the generator  24 . As such, the major axis of the gearbox output shaft defines the rotational axis of the generator  24 , which coincides with the horizontal path of the pitch tube  27  shown extending through the centre of the gearbox  22  and generator  24 . 
     The generator  24  in the illustrated embodiment is an IPM (interior permanent magnet) electric machine having an external stator  36  which surrounds the rotor  32 . By ‘external’, it is meant that the stator  36  is in a radially outer position relative to the rotor  32  with respect to the rotational axis R and surrounds it as compared to generator designs in which the rotor is external to the stator. In this way, the rotor  32  rotates in the cylindrical volume defined by the stator  36 . 
     The stator  36  includes stator windings  38 , a stator core  40 , and a stator frame (not shown in  FIG. 3 ) which surrounds and supports the stator windings  38  and stator core  40 . At this point, it should be noted that the invention is not restricted to the specific configuration of stator that is shown here. 
     The active components of the generator  24  are housed within a generator housing  42  which in this embodiment is cuboidal in form. The generator housing  42  comprises an end face  44  having an opening  46  via which the components of the generator  24  (in particular the rotor  32 ) may be accessed. However in operation, the end opening  46  of the generator housing is typically covered by a closure (not shown in  FIG. 3 ). 
       FIG. 4  shows further details of the generator  24  at an intermediate point of installation, as well as components of a mounting assembly  48  for the pitch tube  27 . This illustration is taken at a snapshot in time subsequent to the mounting of the pitch tube  27  within the generator  24  but prior to assembly and installation of the closure for the end opening  46  of the generator housing  42 . For clarity and ease of viewing of the components of the mounting assembly  48 , the pitch tube  27  is also not shown in the figure; its intended location is however indicated by an arrow for context. 
     The pitch tube mounting assembly  48  comprises a bearing arrangement  50  that is arranged concentrically around the pitch tube  27  and provides support to retain the pitch tube  27  in a desired location and orientation within the generator  24  relative to the rotor  32 . The bearing arrangement  50  is located proximate to the end opening  46  of the generator housing  42 , and is generally in line or aligned with the end face  44  of the generator housing  42 ; in other words the bearing arrangement  50  is generally ‘in plane’ or co-planar with the generator housing end face  44 . 
     The pitch tube mounting assembly  48  also comprises a plurality of supports  52 ,  54  for supporting the bearing arrangement  50 . Specifically, primary supports  52  are provided which connect the bearing arrangement  50  to a component associated with a rotating reference frame of the generator  24 ; secondary supports  54  are also provided which connect the bearing arrangement  50  to a component associated with a stationary reference frame of the generator  24 . 
     The term ‘rotating reference frame’ is used here and subsequently in the specification to refer to components of the generator  24  that, when the wind turbine is in operation, are undergoing rotational motion, for example components of the generator rotor  32  such as the rotor brake disk  56 . Similarly, the term ‘stationary reference frame’ is used here and subsequently in the specification to refer to components of the generator  24  that, during normal operation of the wind turbine, are substantially stationary relative to the rotor  32 . For example, this term could include not only portions of the generator housing  42 , but also portions of the stator  38  which surrounds the rotor  32  and which also does not carry out rotational motion during normal wind turbine operation. 
     In the illustrated embodiment, each of the primary supports has two ends: a first end  58  connected to the bearing arrangement  50 , and a second end  60  connected to a component of the rotor  32  (for example, the rotor brake disk  56 ). Similarly, each of the secondary supports has two ends: a first end  62  connected to the bearing arrangement  50 , and a second end  64  connected to a component of the generator housing  42  at the end face  44 . 
     The bearing arrangement  50  in the illustrated embodiment of  FIG. 4  takes the form of an annular central disk or plate  66  comprising a bearing seat  68  that is configured to receive and support the pitch tube  27  within it. Four primary supports  52  are provided which are arranged at substantially equal intervals around the circumference of the central plate  66  (i.e. the separation between adjacent primary supports is around 90 degrees and adjacent primary supports are arranged generally orthogonal to one another). Each primary support  52  takes the form of an L-shaped bracket that extends between the central plate  66  and the rotor brake disk  56 . 
     Four secondary supports  54  are also provided in the illustrated embodiment and these are also arranged at substantially equal angular intervals around the circumference of the central plate  66  (i.e. the separation between adjacent secondary supports is also around 90 degrees). Each secondary support  54  takes the form of an elongate, generally planar, rectangular support arm that extends radially outwards from the central plate  66  to interface with a portion of the end face  44  of the generator housing  42 . The primary and secondary supports  52 ,  54  are connected to the respective components of the generator  24  and bearing arrangement  50  (central plate  66  and generator housing  42  or rotor  32 ) by appropriate fasteners, for example bolts or screws. 
     It will be appreciated, however, that other embodiments are also possible within the scope of the present invention in which the form taken by the primary and/or secondary supports  52 ,  54 , and/or the number of primary and/or secondary supports  52 ,  54  may be varied, provided the appropriate support functionality for the pitch tube  27  is maintained. 
     Now that the components of the pitch tube mounting assembly  48  have been described, a method of installation of the pitch tube  27  according to an embodiment of the present invention will now be set out. In its most general sense, the installation method comprises two main steps—a first, primary installation step that is illustrated in  FIGS. 5 a  to 6 b   ; and a subsequent, secondary installation step that is illustrated in  FIGS. 7 a  and 7 b   . The primary installation step initially involves supporting the pitch tube  27  on a component associated with a rotating reference frame of the generator  24 , for example one of the components of the generator rotor  32  such as the rotor brake disk  56 . The secondary installation step then involves supporting the pitch tube  27  on a component associated with a stationary reference frame of the generator  24 , for example a portion of the generator housing  42  or a portion of the stator  38 . 
     Each of these steps themselves, as well as the order in which the steps are performed, is particularly advantageous. The primary installation step provides an important benefit of enabling accurate positioning and alignment of the pitch tube  27  along and relative to the rotational axis R of the generator  24 . Such accurate alignment is especially useful as it provides an accurate positioning datum for the pitch tube  27 , which in turn enables the desired clearances and orientations of the pitch tube  27 , relative to the other (rotating) components within the generator  24 , to be achieved. In addition, the gaps between the primary supports  52  after they have been installed facilitate accessibility of internal components of the generator rotor  32  for checks and maintenance services to be carried out. The secondary step in which the pitch tube  27  is connected to a stationary component of the generator  24  such as the generator housing  42  then ensures long-term strong and stable support to maintain the desired pitch tube alignment (accurately achieved via the primary installation step) during the operational lifetime of the wind turbine  1 . 
     After the secondary supports have been connected, a further step is then carried out. Specifically, once the secondary supports  54  have been secured in place and the pitch tube  27  is securely supported by the generator housing  42 , the primary supports  52  are removed or disconnected, such that only the secondary supports  54  are retained and provide long-term support for the pitch tube  27  and bearing arrangement  27  during the operation of the wind turbine. This configuration is shown in  FIG. 8   a.    
     Now considering the primary installation step (supporting the pitch tube  27  on a component associated with a rotating reference frame of the generator) in detail, it is noted that in practice, this step may be implemented in two different ways. 
     In a first implementation, as is illustrated in  FIGS. 5 a  and 5 b   , the pitch tube  27  is first positioned in its desired location within, and extending along the central rotational axis R of, the generator  24 . The central plate  66  of the bearing arrangement  50  is then located on and around a portion of the pitch tube  27 , proximate to the end opening  46  of the generator housing  42 , and generally in line with the generator housing end face  44 . A support bearing  69  of the pitch tube  27  is seated in the bearing seat  68  within the central plate  66 . Subsequently, the central plate  66  is secured to the generator  24  by attaching the primary support brackets  52  to connect the central plate  66  to the rotor  32 . Each of these primary support brackets  52  can initially be loosely attached in its desired position around the circumference of the central plate  66  (e.g. using fasteners  70  such as loose bolts and lock screws). This allows fine adjustment of the bearing arrangement  50  and the associated pitch tube  27  to be carried out as required thereby ensuring accurate positioning and alignment of the pitch tube  27  within the generator  24  and relative to the other (rotating) components of the generator  24 . The fasteners for the primary support brackets  52  are then tightened once the desired alignment has been achieved. 
       FIGS. 6 a  and 6 b    illustrate an alternative implementation of the primary installation step, in which the above-described steps can effectively be performed in reverse. As shown in  FIG. 6 a   , the central plate  66  of the bearing arrangement  50  is first located in its appropriate position within the generator  24 —proximate to the end opening  46  of the generator housing  42 , and generally in line, coplanar or otherwise aligned with the generator housing end face  44 . The primary support L-shaped brackets  52  are then connected between the central plate  66  and the appropriate component of the rotor  32  to secure the bearing arrangement  50  in place. Subsequently, as shown in  FIG. 6 a   , the pitch tube  27  is inserted through the annular central plate  66  and the support bearing  69  of the pitch tube  27  can be seated within the central plate  66 . 
     Regardless of which of the above implementations of the primary installation step is used, once the pitch tube  27  and bearing arrangement  50  are securely supported by the primary supports  52 , the secondary installation step of attaching the secondary supports  54  is then carried out. This step is illustrated in  FIG. 7 a   , where it can be seen that each of the secondary supports  54  is attached to the central plate  66  (via first end  62 ) and to the generator housing component (via second end  64 ) using fasteners  71 , such as bolts or screws. The end-product (initially shown in  FIG. 4 ) of the pitch tube installation method is shown in greater detail in  FIG. 7 b   ; this figure provides a close-up perspective view of the pitch tube  27 , the bearing arrangement  50 , and the primary and secondary supports  52 ,  54 , when viewed from inside the generator  24 . As can be seen from this figure, the first end  62  of each secondary support  54  extends a distance radially-inward towards the pitch tube  27  along a rear face  66   a  of the central plate  66  (i.e. the face of the central plate  66  that faces inwardly towards the generator&#39;s internal components). The secondary supports  54  thereby provide additional support to the central plate  66  and maintain the bearing arrangement  50  in its desired location along the rotational axis R of the generator  24 . It should be noted that each of the secondary supports is shown as a composite member in which the first end and second end of the support arms are separate planar sections that are able to slide relative to one another and be locked by the use of lock bolts. This allows fine adjustment of the support arms to connect between the generator housing and the pitch tube bearing. 
     Subsequently, appropriate steps can then be taken to close the end opening  46  of the generator housing  42 ; these steps are illustrated in  FIGS. 8 a    and  8   b.    
     Closure of the generator end opening  46  begins with the disconnection and removal of the primary supports  52 , the end-result of which is illustrated in  FIG. 8 a   . Subsequently, an end-shield  72  is installed across the end opening  46  of the generator  24  and is positioned generally in line with the end face  44  of the generator housing  42 . The end-shield  72  comprises a plurality of cover components  74  that are installed individually (as shown in  FIG. 8 b   ), and which are supported by and connected to the secondary support arms  54 , as well as to the generator housing  42 . The generator end-product following the end-shield installation is illustrated in  FIG. 9 , from which it can be seen that the end opening  46  of the generator  24  has been closed, and the internal components of the generator  24  (such as the rotor  32 ) are no longer readily accessible. It is in this configuration that normal operation of the wind turbine  1  will take place. 
     In the specific embodiment shown in  FIGS. 8 b    and  9 , the end-shield  72  comprises four cover components  74 , each one taking the form of a generally quadrant-shaped plate that is attached to at least one of the secondary support arms  54  and, when in situ, extends outwards to the generator housing  42  from the circumference of the bearing arrangement central plate  66 . This attachment of the cover components  74  is illustrated particularly clearly in  FIG. 8 b   , from which it can also be seen that the radially-inner edge of each cover component  74  is shaped as an arc that follows the curvature of the portion of the central plate  66  circumference with which the respective cover component  74  interfaces. In other words, the combination of the central plate  66  and the four cover component quadrants  74  spans and closes the generator end opening  46 , providing a substantially continuous cover or closure across the open end of the generator housing  42 . As can be seen from  FIG. 8 b   , the pitch tube  27  will extend outwards through the end-shield  72  after its installation, along the rotational axis R of the generator  24 , to interface with other components (not shown) of the wind turbine  1 . 
     The use of a plurality of cover components  74  supported on a plurality of secondary support arms  54  reduces the weight associated with each individual component of the end-shield  72 , especially relative to existing systems which comprise a single solid generator end-closure. Utilising a plurality of cover components  74  increases the ease of portability and installation of the end-shield, since each cover component  74  can be handled and installed separately, and is therefore less unwieldy than a single-piece end closure would be. Subsequent disassembly of (some or all of) the end-shield  72  after the generator  24  has been installed within the wind turbine is thereby also made easier. For example, some or all of the cover components  74  may be removed in order to permit access to internal components of the generator  24  during the operational lifetime of the wind turbine  1 . This facilitates carrying out of maintenance activities such as servicing the rotor brake disk, which may take place during the operational lifetime of the wind turbine, due to the gaps between the secondary supports  54  which remain in place to support the pitch tube  27  even after the cover components  74  have been removed. 
     The above-described method for installing the pitch tube  27  and the end-shield  72  is summarised in  FIG. 10 . This method  100  comprises primary and secondary pitch tube installation steps  105 ,  110  which are performed sequentially. As discussed previously, the primary installation step  105  involving accurate positioning of the pitch tube  27  can be broken down into two sub-steps: a first sub-step  105   a  of installing the pitch tube  27  within the generator  24 ; and a second sub-step  105   b  of connecting the supporting bearing arrangement  50  to a component of the generator rotor  32  via primary supports  52 . These order in which these two sub-steps are performed is interchangeable. The secondary installation step  110  then involves connecting the bearing arrangement  50  (and pitch tube  27 ) to a component of the generator housing  42  via secondary supports  54 . A third step  115  is then performed which involves disconnection and removal of the primary supports  52  whilst leaving the secondary supports  54  in place. The final step of the method is the end-shield  72  installation step  120 , in which the cover components  74  are attached to the secondary supports  54  and the generator housing end face  44 . 
     Many modifications may be made to the above examples without departing from the scope of the present invention as defined in the accompanying claims. 
     For example, although the illustrated embodiments show each cover component  74  being attached to only one of the secondary supports  54 , it will be appreciated that an alternative configuration would be possible in which each cover component  74  is attached to, and extends between, a pair of angularly adjacent secondary supports  54 . 
     In addition to the advantages described above in relation to the installation and assembly of the pitch tube into the generator housing, it should be noted that further advantages are apparent in relation to in-service maintenance and also decommissioning efforts, In such situations, it is often required to remove a blade from the hub and to do so the hub needs to be turned into a predetermined position to allow removal of the blade. Controlled rotation of the hub is carried out typically by a turner gear that couples to a point of the powertrain of the wind turbine. The point of coupling may be at the gearbox stage, but in the illustrated embodiment one option is to couple a turner gear device to the end of the generator housing so it engages with the generator and so is able to rotate the epicyclic gearbox and, thus, the main rotor shaft. In such a circumstance, it will be appreciated that it would be possible to remove the end shield cover components  74  and the secondary support arms  54 , and re-install the primary support brackets  52  in order to provide sufficient space to mount a turner gear to the housing. So, the replacement of the end-shield and secondary support arms with the primary support brackets will advantageously improve the ease with which such a component may be introduced into the system.