CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to EP 15153989.7, having a filing date of Feb. 5, 2015, the entire contents of which are hereby incorporated by reference. 
       FIELD OF TECHNOLOGY 
       [0002]    The following describes a cooling arrangement for cooling the stator windings of a stator, a direct-drive wind turbine comprising such a cooling arrangement, and a method of cooling stator windings. 
       BACKGROUND 
       [0003]    In a wind turbine generator, some way of cooling the hot coils or windings of the armature (usually the stator) is necessary, since the electrical resistance of the windings is proportional to their temperature, and the power output of the generator decreases as the resistance increases. Furthermore, an air-gap between stator and rotor is generally very small, so that heat from the windings may also be transferred to the magnets. Excessively high temperatures in the magnets can cause these to deteriorate over time. Therefore, it is necessary to ensure that the winding temperature is kept at an acceptably low level. 
         [0004]    A winding or coil generally comprises a “loop” of a low-resistivity metal, with two halves extending along the length of the stator, and with each winding half arranged between parallel stator teeth. At either end of the stator, a winding undergoes a 180° turn. A wind turbine generator is generally realized as a three-phase machine, i.e. the windings are arranged in interleaved groups of three. Since each winding must undergo the 180° turn at each end of the stator, the ends of the windings for three phases must be interleaved, and the interleaved winding ends extend beyond the outer ends of the stator in a “winding overhang”. To ensure minimum losses and balanced phases, the winding overhang is generally realized in as compact a manner as possible. 
         [0005]    In one approach to cooling such a generator, a gaseous coolant such as air can be blown into the air-gap, where it absorbs heat from the windings. The warmed air can then be expelled from the generator. Alternatively, the air can pass from the air-gap through channels in the stator body and into the stator interior. The warmed air can be cooled using heat exchangers arranged in the stator interior cavity, or by means of any other appropriate cooling technique, and the cooled air can once again be blown into the air-gap. 
         [0006]    A problem with the known cooling arrangements is that the winding overhang is difficult to cool, since any cooling gas being blown or sucked into the air-gap will effectively bypass this region. The interleaved winding ends of the overhang extend to some distance beyond each end of the stator, and the  180 ° turns in the winding overhang effectively present a barrier or deterrent to any gaseous cooling medium. The gaseous coolant will always follow the easiest path on its way into the air-gap, and will therefore simply flow around the winding overhang. This leads to an insufficient cooling of the winding end regions. As a result, the temperature in one or more regions of the winding overhang can be significantly hotter than the temperature of the straight winding sections between the stator teeth. Since the conductivity of the winding ends decreases as their temperature increases, the heat-related losses of the generator increase as a result. Therefore, the power output of the generator is effectively limited by the hottest temperature in the winding overhang. 
       SUMMARY 
       [0007]    An aspect relates to an improved way of cooling the winding overhang region. 
         [0008]    According to embodiments of the invention, the cooling arrangement is realized to cool stator windings of a stator enclosed in a generator housing, and comprises a fan arrangement for directing a gaseous cooling medium into a cavity, which cavity is defined by a stator end face and the generator housing; an arrangement of bypass openings in a stator end face, wherein a bypass opening provides a path for an airflow into an interior of the stator; and a manifold arranged to extend over the bypass openings and at least partially over a winding overhang of the stator windings, wherein the manifold is realized to deflect a portion of the cooling airflow from the cavity into the stator interior. 
         [0009]    In the context of embodiments of the invention, the manifold may be understood to be a cover or shield that serves to encourage or compel an airflow to pass through the winding overhang and into the bypass opening(s). The manifold can be made of any suitable material, for example fibreglass or plastic, since these are light and can be formed with relatively little effort to give any desired shape. To act as a guide for the cooling airflow, the manifold is arranged as a “divider” or screen between a part of the cavity about the winding overhang region and the remainder of the cavity. The manifold can be secured to the stator end face in order to define a region or space that includes the bypass openings and the winding overhang region. This region or space is essentially closed off except for a gap or entrance close to the winding overhang to allow the cooling airflow to enter this space. An advantage of the cooling arrangement according to embodiments of the invention is that the combination of manifold and bypass openings acts to encourage or compel a portion of the cooling airflow through the problematic winding overhang region. Therefore, a cooling airflow can deliberately be guided through a winding overhang region. In this way, it is possible to maintain a favourably low temperature in the winding overhang, minimizing heat-related losses, so that the power output of the generator can be increased accordingly. 
         [0010]    According to embodiments of the invention, the direct-drive wind turbine comprises an outer rotor and an inner stator, wherein the outer rotor is arranged on a rotatable generator housing; and a cooling arrangement according to the invention, realized to cool stator windings of the stator. 
         [0011]    An advantage of the direct-drive wind turbine according to embodiments of the invention is that it can deliver a higher power output than a comparable wind turbine with a conventional cooling arrangement. Furthermore, this improvement in power output can be achieved with relatively low cost and effort. 
         [0012]    According to embodiments of the invention, the method of cooling stator windings of a stator enclosed in a generator housing comprises the steps of providing a number of bypass openings in a stator end face, wherein a bypass opening provides a path for an airflow into the stator interior; arranging a manifold to extend over the bypass openings and at least partially over a winding overhang of the stator windings; and directing a gaseous cooling medium into a cavity defined by a stator end face and the generator housing. 
         [0013]    An advantage of the method according to embodiments of the invention of cooling stator windings is that a portion of the cooling airflow can deliberately be compelled to pass through the relatively hot winding overhang, so that this hot region can be cooled in an effective manner. Furthermore, this improvement in the efficiency of the cooling arrangement can be achieved in a straightforward manner. 
         [0014]    According to embodiments of the invention, the method of retrofitting a wind turbine—comprising a stator enclosed in a generator housing and a gaseous cooling arrangement for cooling stator windings of the stator—comprises the steps of forming at least one bypass opening in a region of the stator end face corresponding to a hottest winding overhang region; and arranging a manifold to extend over the bypass openings and at least partially over a winding overhang of the stator windings such that a cooling airflow is directed from the cavity into the stator interior. 
         [0015]    An advantage of the method according to embodiments of the invention of retrofitting a wind turbine is that a significant improvement in the cooling efficiency can be achieved with relatively little effort and at a low cost. Any wind turbine that is already equipped with a suitable gaseous cooling arrangement can be adapted by forming the bypass openings in one or both stator end plates and by arranging a manifold over these. When the gaseous cooling arrangement is activated during operation of the generator, the increased cooling effect in a winding overhang region underneath a manifold results in an increase in efficiency of the generator. Such a wind turbine can therefore deliver an increased power output compared to a comparable wind turbine using a conventional cooling arrangement. 
         [0016]    In the following, it may be assumed that the generator is a wind turbine generator, particularly a direct-drive generator with an outer rotor that is directly caused to rotate by the action of the rotor blades. Furthermore, it may be assumed that the gas used by the cooling arrangement is air, although it shall be understood that any suitable gas could be used. The term “cooling airflow” as used in the following may therefore be understood as a cooling flow of air. The cooling airflow can absorb heat from a heat source and the warmed air can subsequently be expelled from the wind turbine. Equally, the warmed air can be “recycled”, i.e. it can be cooled and then re-used as a cooling airflow. 
         [0017]    In a direct-drive wind turbine, the stator windings are arranged on a cylindrical stator body, which can have a diameter of several metres. The interior of the stator is essentially hollow, and affords room for various components, particularly components of the cooling arrangement. The stator cavity can be closed off from the outside by an end plate at the drive end of the generator and an end plate at the non-drive end. 
         [0018]    Magnets, usually permanent magnets, are mounted on the interior surface of the outer rotor, and are separated from the stator windings by a narrow air-gap. As mentioned above, the generator construction makes it difficult to bring a cooling medium close to the heat source, i.e. the windings. In a preferred embodiment of the invention, the cooling arrangement comprises a plurality of axial cooling channels, wherein an axial cooling channel extends between adjacent windings arranged on the stator. The cooling airflow can be directed along these axial cooling channels, so that heat can be absorbed close to is source. 
         [0019]    In one embodiment, the cooling airflow can enter the airgap at one end of the stator, and warmed air can exit the airgap at the other end of the stator. However, such a realization might result in an uneven cooling of the windings. Therefore, in a further preferred embodiment of the invention, the cooling arrangement comprises a plurality of radial channels, wherein a radial channel extends from an axial cooling channel into the stator interior. In this realization, a cooling airflow can enter the axial cooling channels at both ends of the stator, and the warmed air can pass from the axial channels into the stator interior, where it can be collected and treated in a suitable manner. This embodiment ensures a more even cooling of the straight winding sections. 
         [0020]    The efficiency of the cooling arrangement will depend to some extent on the rate at which the cooling airflow passes over the hot windings. Therefore, in a preferred embodiment of the invention, the cooling arrangement comprises a suction apparatus realized to draw the cooling airflow through the stator windings and into the stator interior. The suction apparatus can comprise one or more fans arranged in the stator interior to draw or suck the warmed air from the radial cooling channels and the axial cooling channels. 
         [0021]    As mentioned above, the warmed air can be “recycled” and used again as a cooling airflow. To this end, in a preferred embodiment of the invention, the cooling arrangement comprise one or more heat exchangers arranged in the stator interior, wherein a heat exchanger is realized to cool a warmed airflow that is drawn into the stator interior. To re-use the cooled airflow, in a preferred embodiment of the invention the fan arrangement is also realized to direct the cooled airflow out of the stator interior into the DE cavity and/or the NDE cavity. The fan arrangement can comprise several fans arranged to direct the cooled air outward from the stator interior. To this end, a fan or blower unit can be mounted to a fan outlet opening formed in a stator end plate. 
         [0022]    In a further preferred embodiment of the invention, the cooling arrangement is realized to generate a pressure differential comprising a relative underpressure in the stator interior and a relative overpressure in a cavity. For example, this can be achieved by driving the blower units or fans to blow the cooling airflow into the cavity at the drive end and/or non-drive end in order to achieve the desired higher pressure. This can significantly improve the effectiveness of the bypass opening/manifold arrangement. Regardless of the way in which this pressure differential is obtained, the passage of air though the winding overhang and into the bypass openings will be facilitated as long as the pressure in the cavity is higher than the pressure in the stator interior. 
         [0023]    The interior cavity of the stator usually also accommodates a passage that is used to access the drive end at the front of the generator, for example to allow a technician to access the hub, the rotor blades, the blade pitch motors, etc. As indicated above, the cooling airflow originates in the stator interior cavity. However, the necessity of incorporating one or more such access passages in the stator cavity makes it difficult or impossible to direct the cooling airflow evenly into the hot winding regions. For this reason, conventional cooling arrangements are characterized by uneven temperature distribution in the windings and winding overhang regions. 
         [0024]    Therefore, in a preferred embodiment of the invention, the position of a bypass opening on an end face of the stator is determined on the basis of a temperature differential between a first winding overhang region and a second winding overhang region. For example, one or more bypass openings might be formed in a region of an end plate that is close to a service hatch. Similarly, one or more bypass openings might be formed in a region of an end plate that is removed from a fan outlet opening. The bypass openings and manifolds can therefore counteract the negative effects of having to restrict the placement of fan outlet openings on account of the service hatches in the stator end plates. 
         [0025]    In a preferred embodiment of the invention, the number of a bypass openings arranged in an end face is determined on the basis of a temperature differential between a winding overhang region and an axial winding region. For example, temperature can be measured at various relevant positions in an operational wind turbine—for example at various points in the winding overhang at the drive end and the non-drive end. These measurements can be analysed to determine the hottest winding overhang region(s) and to determine a temperature difference between these hottest regions and the coolest winding overhang regions. The temperature difference will indicate the additional effort required to cool the hottest winding overhang regions in order to reach a temperature close to that in the cooler winding overhang regions. 
         [0026]    Additionally, temperature may be measured at several points on the stator body in the straight winding sections. This information can be used to determine whether some of the existing cooling capacity can be spared for the additional cooling required for the hot winding overhang region(s), or whether the capacity of the cooling arrangement should be increased. For example, an existing heat exchanger unit and/or an existing blower unit could be replaced by a more powerful unit in order to increase the cooling capacity of the cooling arrangement. 
         [0027]    Depending on the generator construction, the stator interior may only accommodate a relatively low number of blower units and/or heat exchangers. In such a realization, the efficiency of the cooling arrangement can be significantly improved by a suitable arrangement of bypass openings and manifolds. In a preferred embodiment of the invention, therefore, the arrangement of bypass openings extends about the periphery of an entire end face. In this embodiment, the manifold can be realized in the form of a conical annulus or ring. 
     
    
     
       BRIEF DESCRIPTION 
         [0028]    Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein: 
           [0029]      FIG. 1  shows a direct-drive wind turbine with a first embodiment of a cooling arrangement; 
           [0030]      FIG. 2  shows an enlarged view of a portion of  FIG. 1 ; 
           [0031]      FIG. 3  shows another view of an embodiment of the cooling arrangement shown in  FIG. 1 ; 
           [0032]      FIG. 4  shows a second embodiment of a cooling arrangement  1 ; 
           [0033]      FIG. 5  shows a cooling arrangement of a prior art wind turbine. 
       
    
    
       [0034]    Objects in the diagrams are not necessarily drawn to scale. 
       DETAILED DESCRIPTION 
       [0035]      FIG. 1  is a very simplified diagram of an embodiment of a direct-drive wind turbine  2  according to embodiments of the invention. A nacelle is mounted on top of a tower. A hub with rotor blades is mounted to a direct-drive generator comprising an outer rotor  4  and an inner stator  3 . During operation, the outer rotor  4  is caused to rotate about an axis of rotation R. Windings  30  on the inner stator  3  are cooled by guiding a cooling airflow AFx into an air-gap  20  between rotor  4  and stator  3  and then drawing the airflow AFx through the stator windings  30  and into an interior cavity  31  of the stator  3 . This interior cavity  31  extends about a main shaft around the axis of rotation R. In this embodiment, the warmed air AF&#39; drawn into the interior cavity  31  is cooled by a number of heat exchangers  15  and then blown out of the interior cavity  31  by a number of fans  14  which direct a cooled airflow AF back into a cavity  200 R,  200 F at each end of the stator  3 . The diagram shows that a winding overhang  300  extends to some distance into the cavity  200 R between rotor housing  40  and brake-disc  41  at the non-drive end of the generator, and into the cavity  200 F between rotor housing  40  and a front face  32 F at the drive end. At the non-drive end, the cavity  200 R is bounded by the cylindrical rotor housing body  40  and a brake disc  41 , and is sealed off from the nacelle or canopy  44  by a suitable cover  45  or seal  45 . 
         [0036]    To ensure that the cooling airflow AF does not simply flow around the barrier-like winding overhang  300 , the cooling arrangement  1  according to embodiments of the invention comprises an arrangement of bypass openings  10  and manifolds  11 . In this exemplary embodiment, the bypass openings  10  are formed on the stator front face  32 F and stator rear face  32 R. As a cooling airflow AF is directed from the stator interior  31  into the cavity  200 R,  200 F and in the direction of the air-gap  20 , an underpressure inside the stator interior  31  acts to draw a portion AFy of the cooling airflow AF through the narrow spaces in the winding overhang  300 H. The manifold  11  is arranged to ensure that the warmed air then passes directly into the stator interior  31  through the bypass openings  10 . This portion AFy of the cooling airflow AF effectively “bypasses” the air-gap and enters the stator interior  31  by a shorter route. The underpressure in the stator interior cavity  31  can be relative, i.e. as long as the pressure inside the stator interior  31  is lower than the pressure in a cavity  200 R,  200 F, a portion of the cooling airflow AF will be encouraged to pass through the spaces in the winding overhang  300 , since the openings  10  offer a path into the stator interior  31 . The pressure differential can be achieved by driving a number of fans  14  to blow the cooled air AF into the cavity  200 R,  200 F. The relatively small space in the cavity  200 F,  200 R (reduced even further by the presence of the manifold  11 ) encourages such a pressure differential. 
         [0037]    Axial cooling channels  100  and radial channels  101  (shown in  FIG. 2 ) provide a path for a portion AFx of the cooling airflow AF alongside the windings  30  and into the stator interior  31 . The cooling airflow portion AFx can enter the air-gap  20  at either end, and can travel along the axial cooling channels  100  and then through the radial channels  101  into the stator interior  31 . Temperature measurements made for this type of generator can have been obtained at an earlier stage to determine the location of any winding overhang hotspot  300 H and the required number and positions of bypass openings  10 , and the dimensions and positions of the manifolds  11 . The temperature measurements can also have been analysed to determine how much of the cooling capacity can be diverted into cooling the winding overhang hotspots  300 H, and/or whether the cooling capacity of the cooling arrangement  1  should be increased. 
         [0038]      FIG. 2  shows an enlarged view of a portion of a winding arrangement  30  on the stator  3  of the wind turbine  2  of  FIG. 1 . The diagram shows the parallel arrangement of windings  30  and illustrates the complex interleaved shape of the winding overhang  300 . The diagram also shows how the presence of the manifold  11  in conjunction with the bypass openings  10  in the stator front face  32 F or rear face  32 R (the diagram applies to both drive end and non-drive end of the stator  3 ) acts to draw a portion AFy of the cooling airflow AF through narrow gaps in the winding overhang  300  and on through the bypass openings  10  and then into the stator interior  31 . 
         [0039]      FIG. 3  shows a view of the cooling arrangement of  FIG. 1  as seen from the non-drive end of the generator. The diagram shows two manifolds  11 , one on the left-hand side and one on the right-hand side of the rear face  32 R. The diagram also shows two service hatches  33  in the rear face  32 R. Since a fan cannot be arranged in those positions, temperature in the winding overhang regions  300 H in the neighbourhood of the service hatches  33  is higher than the temperature in the remaining winding overhang regions. These “hotspot” winding overhang regions  300 H behind the manifolds  11  are indicated by the broken lines. Bypass openings  10 , also indicated by broken lines, are arranged in the rear face  32 R. The combination of bypass openings  10  and manifolds  11  ensures that a cooling airflow AF (provided through the outlet openings of the fans  14 ) is encouraged to pass through the winding overhang hotspot  300 H. A similar arrangement can be used at the front end of the stator. The effectiveness of the hotspot cooling can be increased by ensuring that the pressure in the stator interior  31  is lower relative to the pressure in the rear-end cavity (or front-end cavity). 
         [0040]      FIG. 4  shows a second embodiment of a cooling arrangement  1  according to embodiments of the invention. Here, an annular manifold  11  extends about the entire winding overhang  300 , and bypass openings  10  are arranged essentially evenly about a stator face. Here, the rear face  32 R is shown. This embodiment can be beneficial for a stator design that results in a relatively uniform higher temperature over the entire winding overhang. This embodiment can be used at either end of the stator  3 . Alternatively, this embodiment can be used at one end of the stator  3 , and the embodiment described in  FIGS. 1-3  can be used at the other end, as appropriate. 
         [0041]    Of course, other versions of the manifold are possible. For example, a manifold can be realized to extend over any fraction of the circumference of the stator front end  32 F or rear end  32 R. Equally, the bypass openings  10  can be as small or as large as desired, depending on the desired rate of airflow through the winding overhang region. Any suitable number of bypass openings can be provided underneath a manifold, and can be arranged in any suitable pattern. 
         [0042]      FIG. 5  shows an enlarged view of a region in the non-drive end of a prior art wind turbine  5 . The diagram shows that the winding overhang  300  extends into a cavity  200 R between rotor housing  40  and brake-disc  41 , and into a cavity  200 F between rotor housing  40  and hub interface  42 . A cooling airflow AF is directed at the winding overhangs  300 , but bypasses these to a large extent since it will tend to take the easier path around the winding overhang  300  on its way to the air-gap  20 . The result is that the winding overhang  300  is not cooled sufficiently, and a significant temperature difference can develop between the winding overhang  300  and the more optimally cooled parallel winding arrangement between the stator teeth. This temperature difference shows that the winding overhang  300  is not cooled as well as the remainder of the windings. The hottest temperature of a conductor determines the current it will carry, so that the prior art cooling arrangements may be described as inefficient regarding this aspect. 
         [0043]    Although embodiments of the present invention have 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 embodiments of the invention. 
         [0044]    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.
A cooling arrangement realized to cool stator windings of a stator enclosed in a generator housing, the cooling arrangement includes a fan arrangement configured to direct a gaseous cooling medium) into a cavity, the cavity is defined by a stator end face and the generator housing; an arrangement of bypass openings in a stator end face, wherein a bypass opening provides a path into an interior of the stator; and a manifold arranged to extend over the bypass openings and at least partially over a winding overhang of the stator windings and configured to guide a cooling flow through the winding overhang into the stator interior is provided. A direct-drive wind turbine, a method of cooling stator windings of a stator, and a method of retrofitting a wind turbine is further provided.