Abstract:
A dry type cast coil transformer ( 28 ) includes a hollow body ( 29 ), a dome structure ( 26 ) extending from the body, and undulation structure ( 30 ), defining at least a portion of an outer surface of the dome structure, constructed and arranged to increase an electrical track path in the dome structure.

Description:
FIELD 
     The invention relates to dry type transformers and, more particularly, to a dome area of the transformer that has features to increase the track path between taps. 
     BACKGROUND 
     A dry type transformer uses a complex system of air and solid insulation to prevent energized parts from contacting each other or ground. Many dry type cast coil transformers, such as disclosed in U.S. Pat. No. 6,445,269, are filled with epoxy in a horizontal orientation which makes a flat top surface called a ‘dome’. The dome area of a transformer houses the start and finish taps as well as voltage adjustment taps that have a large voltage gradient. This voltage gradient can cause solid insulations to electrically track due to material properties and distance. This dome area is where the customer makes connections to the transformer and where the voltage input/output of the transformer is adjusted to account for the incoming utility voltage. One of the main considerations is the track path from an energized part to another conductive part at a different potential. The flat top surface of the conventional dome area can lead to medium voltage tracking between energized parts when exposed to harsh environments such as off shore platforms, refineries, wind turbines, pulp and paper mills, etc. 
     Conventionally, increasing the track path requires the transformer coil to be cast with the voltage adjustment taps oriented downwardly or vertically to create bushings. Such a transformer coil has two common disadvantages. First, more epoxy is used than actually needed to fulfill the requirements of the coil. Secondly, the regions of the unnecessary epoxy are prone to the risk of cracks because of the large thickness of epoxy. 
     Thus, there is a need to provide a dome structure for a dry type cast coil transformer with undulation structure that allows a greater track path between taps, allows a casting process where the voltage taps face upwardly, and uses less epoxy than conventional dome areas. 
     SUMMARY 
     An object of the invention is to fulfill the need referred to above. In accordance with the principles of the present invention, this objective is obtained by a dry type cast coil transformer that includes a hollow body, a dome structure extending from the body, and undulation structure, defining at least a portion of an outer surface of the dome structure, constructed and arranged to increase an electrical track path in the dome structure. 
     In accordance with another aspect of the disclosed embodiment, a method of molding a dry type cast coil transformer having a dome structure is provided. The method provides a mold having a dome mold structure. The dome mold structure includes features for molding at least two tap connection bases from which a respective tap connection extends, and undulation forming structure adjacent to the bases for molding undulation structure. Windings are placed in the mold. The windings are coupled to the tap connections. The mold is oriented so that the tap connections are arranged upwardly. Epoxy is poured into the mold and permitted to cure. The mold is removed to obtain the cast coil transformer having the undulation structure adjacent to the tap connection bases. 
     Other objects, features and characteristics of the present invention, as well as the methods of operation and the functions of the related elements of the structure, the combination of parts and economics of manufacture will become more apparent upon consideration of the following detailed description and appended claims with reference to the accompanying drawings, all of which form a part of this specification. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood from the following detailed description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings, wherein like reference numerals refer to like parts, in which: 
         FIG. 1  is a top view of a mold for forming an outer surface of a dome structure of a dry type cast coil transformer, providing in accordance with an embodiment. 
         FIG. 2  is a view of the underside of the mold of  FIG. 1  showing undulation forming structure therein. 
         FIG. 3  shows the top surface of a dome structure of a dry type case coil transformer having undulation structure resulting from the mold of  FIG. 2 . 
         FIG. 4A  shows undulation structure having half-moon shape in accordance with another embodiment. 
         FIG. 4B  shows undulation structure having inverse half-moon shape in accordance with another embodiment. 
         FIG. 4C  shows undulation structure having saw-tooth shape in accordance with yet another embodiment. 
         FIG. 4D  is shows undulation structure having sine wave shape in accordance with another embodiment. 
         FIG. 4E  is shows undulation structure having cosine shape in accordance with still another embodiment. 
         FIG. 5  is a perspective view of a conventional mold for a dry type cast coil transformer, with the mold having an open top forming the dome structure. 
         FIG. 6  is a perspective view of mold for a dry type cast coil transformer in accordance with an embodiment, with the mold having additional structure on the top side for forming the dome structure. 
         FIG. 7A  is a schematic end view of a three-sided dome shape in accordance with an embodiment. 
         FIG. 7B  is a schematic end view of a five-sided dome shape in accordance with an embodiment. 
         FIG. 7C  is a schematic end view of a circle dome shape with an offset in accordance with an embodiment. 
         FIG. 7D  is a schematic end view of a three-sided dome shape with rounded edges in accordance with an embodiment. 
         FIG. 8A  is a schematic side view showing two end-tap molds and a spacer for an embodiment of a dry type cast coil transformer. 
         FIG. 8B  is a schematic side view showing two end-tap molds and three spacers for an embodiment of a dry type cast coil transformer. 
         FIG. 8C  is a schematic side view showing two-end tap molds, one center-tap mold and two spacers for an embodiment of a dry type cast coil transformer. 
         FIG. 8D  is a schematic side view showing two-end tap molds, one center tap mold and four spacers for an embodiment of a dry type cast coil transformer. 
         FIG. 9  is a top perspective view of a dome structure of a dry type cast coil transformer with tap connection bases resulting from the mold of  FIG. 6 . 
         FIG. 10  is a top perspective view of a dome structure of a dry type cast coil transformer with undulation structure adjacent to raised tap connection bases in accordance with and embodiment. 
         FIG. 11  is a perspective view of a mold for another shape of the dome structure of a dry type cast coil transformer. 
         FIG. 12  is a perspective view of a dry type cast coil transformer with a dome structure formed by the mold of  FIG. 11 . 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     With reference to  FIGS. 1 and 2 , a mold portion  10  is shown, for molding a dome structure of a dry type cast coil transformer  28  ( FIG. 3 ), in accordance with and embodiment. The mold portion  10  includes a base  12 , a pair of opposing side walls  14  and a pair of opposing end walls  16 . As shown in  FIG. 2  undulation forming structure, generally indicated at  18 , extends from the underside of the base  12 . The undulation forming structure  18  includes a plurality of alternating, continuously joined, peaks  20  and valleys  22 . In the embodiment, the peaks  20  define rounded fins and the valleys  22  are also rounded. 
     To form a cast coil transformer, a winding (not shown) with suitable is insulating material is placed in a mold (see, e.g., mold  68 ′ of  FIG. 6 ) that includes the mold portion  10 . Liquid epoxy is then poured into the mold and cured. With reference to  FIG. 3  an outer surface  24 , of a dome structure  26  of a dry type cast coil transformer  28 , is shown that results from using the mold portion  10  of  FIG. 2 . The body  29  of the coil transformer  28  is of conventional, hollow, generally cylindrically shaped configuration, with the dome structure extending from the body  29 . The winding is cast inside the body  29 . The outer surface  24  includes undulation structure, generally indicted at  30 , that includes a plurality of alternating, continuously joined, peaks  32  and valleys  34 . As shown in  FIG. 3 , each peak  32  is spaced from an adjacent peak  32  in a direction parallel to a longitudinal axis X of the body  29 . In the embodiment, the peaks  32  define rounded fins and the valleys  34  are also rounded. The undulation structure  30  can be separated by tap connection bases  36  that are directly adjacent thereto. The undulation structure  30  increases the effective track path and reduces the chances of dielectric failure. 
     Alternate contoured geometries for the undulation structure  30  can be used. For example,  FIG. 4A  shows the undulation structure  38  having half-moon shaped peaks  40  with alternating valleys  42 .  FIG. 4B  shows undulation structure  44  having half-moon shaped valleys with alternating peaks  48 .  FIG. 4C  shows undulation structure  50  of saw-tooth shape having alternating peaks  52  and valleys  54 .  FIG. 4D  is shows undulation structure  56  of sine wave shape having alternating peaks  58  and valleys  60 .  FIG. 4E  is shows undulation structure  62  of cosine wave shape having alternating peaks  64  and valleys  66 . Other shapes can be used with any amplitude and period. 
     The process of adding the undulation structure to the dome structure of the dry type cast coil transformer allows a greater track path to be established while using a horizontal casting method with the voltage taps facing upwardly. Currently, increasing the track path requires the transformer coil to be cast with the voltage adjustment taps down or horizontal to create bushings. The undulation structure also provides an improved cooling surface when transformer is in operation. 
     To minimize the volume of epoxy and thus reduce the risk of cracks in a cast coil transformer, the epoxy can be removed between the electrically connected sections and then, if desired, any of the undulation structures mentioned above can be applied to the dome structure  26 . 
       FIG. 5  shows a conventional mold  68  for molding a conventional cast coil transformer  70  having a dome structure  72  that includes the conductor leads (taps)  74 . The shape of the dome structure  72  results from the mold shape that is opened to the top side  76  (related to the casting and curing position) where the epoxy mixture is introduced into the mold  68 . 
     With reference to  FIG. 6 , instead of the mold structure which has an open top, in accordance with an embodiment, the mold  68 ′ includes additional dome mold structure, generally indicated at  78 , that limits the entire shape of the dome structure  26  on the top side  76 . The dome mold structure  78  ensures that the epoxy  80  can only fill out the necessary volume located around the tap connection bases  86  ( FIG. 9 ) for the taps  74 . More particularly, the dome mold structure  78  includes mold features  79  adjacent to the bases  86  that prevent epoxy from accumulating thereby reducing the amount of epoxy adjacent to the bases  86 . A special requirement is the possibility of adaption for the whole measurements spectrum of the coil outer diameter, the coils maximum height and the position of the taps but without the creation of a large variety of different dome mold parts. 
     To fulfill the requirement of the independence on the outer diameter of the coil, the dome mold structure  78  possesses a basic shape along the entire coil height (as in the conventional construction) but decreased to a minimum. Generally, the shape of the dome structure  26  (without considering the taps  74 ) should be part of a circle, similar to imitate the shape of the coil, and should minimize the epoxy volume. Some possible shapes of the dome structure  26  are shown in  FIGS. 7A-7D . For example,  FIG. 7A  shows the dome structure  26 ′ having a three-sided shape,  FIG. 7B  shows the dome structure  26 ″ having a five-sided shape,  FIG. 7C  shows the dome structure  26 ″′ having circle shape with an offset, and  FIG. 7D  shows the dome structure  26 ″′ having a three-sided shape with rounded edges. Other shapes are possible that reduce the volume of the dome structure. 
     The choice of the best shape of the dome structure  26  depends on the spectrum of the outer diameters and also the fabrication method may be a consideration. Furthermore, to fulfill the requirement of different heights, tap positions and their amount, the dome mold structure  78  needs to be parted in several sectors along the height. The amount of sectors depends on the amount of taps  74  and/or tap regions (if several taps are located very close it makes sense to combine their bases to one) and their positions (if the end taps are not very close to the face side of the coil a spacer between the end tap mold and the face sides is necessary). The general transformer configuration consists of two end taps and an area of several taps in the center of the coil. Several transformer configurations are shown in  FIGS. 8A-8D . For example,  FIG. 8A  shows two end-tap molds  82  and a spacer  84 ,  FIG. 8B  shows two end-tap molds  82  and three spacers  84 ,  FIG. 8C  shows two-end tap molds  82 , one center-tap mold  82 ′, and two spacers  84 , and  FIG. 8D  shows two-end tap molds  82 , one center tap mold  82 ′, and four spacers  84 . 
     The tap molds  82 ,  82 ′ are meant to be the same for every coil and shall be used many times. The spacers  84  just carry the shape of the dome structure  26  and may include the undulation forming structure  18  of  FIG. 2 . The spacers  84  can have different lengths depending on the position and amount of taps and the total length of the coil. The spacers  84  could be extruded aluminum profiles with shape of the dome structure  26  that allows a very easy and fast fabrication of the spacers. All fabricated parts can be stored and used again in later cases. To minimize a high variety of spacers  84 , standardized coil length and tap position could be defined. 
       FIG. 9  shows a cast coil transformer  28 ′ having a dome structure  26  (without undulation structure) that results from the mold  68 ′ of  FIG. 6 . In the embodiment, the dome structure  26  has a minimized volume along the whole coil height and has three tap connection bases  86  for the taps  74  which also have a minimum of volume. The tap connection bases  86  are raised with respect to an adjacent upper surface  88  of the dome structure, thus reducing the volume of the dome structure  26  due to the material omitted adjacent to the bases  86 . 
     In the embodiment of  FIG. 10 , the dome structure  26 ′ includes the undulation structure  30 ′ that is on a plane A that is below a plane B of the tap connection bases  86 ′,  86 ″ so that the tap connection bases are raised with respect to the undulation structure. As disclosed above, the undulation structure  30 ′ increases the effective track path and reduces the chances of dielectric failure. The volume of epoxy cast is also reduced due to the recessed undulation structure  30 ′. 
     The tap connection bases  86  can have different shapes as well. The configuration of the bases  86  basically depends on the best way to fabricate the bases. Some configuration of the bases can include a cone shape (especially for the end taps), a pyramid shape, rectangular, square, oval conic shape or other shapes.  FIG. 11  shows a transformer mold  68 ′ having a dome mold structure  78 ″ in accordance with another embodiment to produce end located, generally oval-shaped bases  86 ′ and a central, generally rectangular shaped base  86  of the dome structure  26 ″ of a cast coil transformer  28 ″ of  FIG. 12 . 
     The change of shape of the dome structure  26  down to a minimum volume and the addition epoxy tap connection bases  86  just surrounding the taps  74  reduces the volume and thus the cost of the coil transformer. Furthermore, the minimized thickness of the dome structure  26  reduces the risk of cracks which may occur after curing. 
     The foregoing preferred embodiments have been shown and described for the purposes of illustrating the structural and functional principles of the present invention, as well as illustrating the methods of employing the preferred embodiments and are subject to change without departing from such principles. Therefore, this invention includes all modifications encompassed within the spirit of the following claims.