Patent Publication Number: US-7905009-B2

Title: Method of forming a transformer coil

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional patent application of, and claims priority from, U.S. patent application Ser. No. 10/858,039, filed on Jun. 1, 2004, which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     Transformer coils used in high-voltage and other applications are formed by winding a conductor and casting and curing a thermosetting resin composition around the conductor windings to form a resin body covering the coil. The resin body provides dielectric properties to the transformer coil assembly, as well as holding the conductor windings in place. The resin also provides protection and more uniform thermal properties to the coil assembly. Without some form of support structure for the coil assembly, the resin may develop cracks during casting or during use when the assembly is subjected to external conditions, such as high temperature, high humidity, moisture penetration and the like, or due to internal factors, such as heat generation or stress due to high current flow, electrical fault conditions, and the like. 
     The resin body is subjected to thermal forces from coil temperatures well above ambient during operation due to I 2 R losses in the conductors, from eddy currents, from hysteresis losses in the core, and from stray flux impinging the axial ends of the windings. Further, the resin body may be subject to vibratory forces during operation. The resin body should satisfactorily restrain, resist, and withstand all of these forces over long term operation. 
     SUMMARY 
     A method of forming a transformer coil is disclosed that includes providing a fibrous layer that includes a fabric formed from a plurality of interconnected fibers and a plurality of spacers affixed to the fabric and protruding therefrom. A conductor layer is disposed over the fibrous layer such that a first side of the conductor layer contacts the spacers. A resin is applied so as to cover at least the fibrous layer and the conductor layer with the resin. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Objects and advantages will become apparent to those skilled in the art upon reading this description in conjunction with the accompanying drawings, in which like reference numerals have been used to designate like elements, and in which: 
         FIG. 1  is a perspective view of a transformer coil assembly. 
         FIG. 2  shows a support structure and spacers. 
         FIG. 3  shows an area of detail of the transformer coil assembly of  FIG. 1 . 
         FIG. 4A  shows a support structure, spacers, and a conductor. 
         FIG. 4B  illustrates a feature of a spacer pattern of  FIG. 4A . 
         FIGS. 5A-5D  show other possible arrangements of the spacers. 
         FIG. 6  is a flow chart illustrating a method of forming a transformer coil assembly. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a perspective view of a transformer coil assembly  100  according to an exemplary embodiment. The transformer coil assembly  100  includes a first layer  130  and a second layer  140 . Referring also to  FIG. 3 , which details an area of the transformer coil assembly  100  of  FIG. 1 , a first layer  130  of the transformer coil assembly  100  includes means for establishing a support structure  310 . 
     The means for establishing a support structure  310  can include multiple fibers interconnected to form a fabric. The fabric can include glass fibers and can include electrical grade glass. The fabric can include any of a variety of fibers that are known in this art to be suitable for transformer cast applications, such as polyphenylene sulfide (PPS), polyamides (nylon), polyvinyl chloride (PVC), flouropolymers (PTFE), and the like. 
     The first layer  130  of the transformer coil assembly  100  also includes spacer means  330 , affixed to the support structure means  310 . The spacer means  330  can include multiple spacers and is preferably formed of a less compressive material than fabric, such as resin or epoxy. The spacer means  330  are affixed to a surface of the support structure means  310 . Here, the term “affixed” means that the spacers can be secured adjacent to a surface of the support structure means  310 , by adhesives or other known means, or can be partially embedded in the support structure means  310 . The spacer means  330  protrude from the support structure means  310  by a distance, i.e., height,  335 . It should be appreciated that although the spacer means  330  are shown affixed to only one surface of the support structure means  310 , the spacer means  330  can also be attached to both opposing surfaces of the support structure means  310 . 
     The second layer  140  includes a conductor means  145  in contact with at least one of the spacers of the spacer means  330  on a second side  332  of each spacer that opposes the first side  331 . The conductor means  145  can be a single conductor that is wound continuously to form a single transformer coil winding, or can be multiple conductors, depending on the type of transformer coil assembly  100 . The conductor means  145  can include tabs  160  for accessing the conductor means  145  by other electrical components outside the transformer coil assembly  100 . 
     The transformer coil assembly  100  includes a dielectric means for covering the support structure means  310 , the spacer means  330 , and the conductor means  145 . The dielectric means can be a resin body  110  covering the layers of the transformer coil assembly  100 . Although the dielectric means will be described hereinafter as a resin body  110 , or simply resin  110 , one of skill in this art will recognize that a number of dielectric materials may be used that are suitable for use in a transformer cast. The thickness of the resin body should be uniform to provide dielectric properties that are uniform throughout the transformer coil assembly. Here, the term uniform means substantially the same throughout with some tolerance. A dielectric with favorable properties will resist breakdown under high voltages, does not itself draw appreciable power from the circuit, is physically stable, and has characteristics that do not vary much over a fairly wide temperature range. 
     The transformer coil assembly  100  can optionally include a third layer  150  having support structure means  315  and spacer means  335 . The third layer  150  can be made of the same materials as the first layer, although this is not a requirement. When the optional third layer  150  is employed, the dielectric means, such as a resin body  110 , can cover the first, second, and third layers  130 ,  140 ,  150 , providing an overall thickness  160 . 
     The means for establishing support structure  310  provides reinforcing support to the resin body  110  to prevent the development of cracks during casting or during use when the assembly is subjected to external conditions, such as high temperature, high humidity, moisture penetration and the like, or due to internal factors, such as high coil temperatures or vibratory forces during operation. 
     The spacer means  330  protrude from the support structure means  310  by a distance  335 . The protrusion of the spacer means  330  creates a space  320  between conductor means  145  and the support structure means  310 , where the resin  110  can more easily flow during the casting process. That is, without the spacers, the resin would have to “wick” into the support structure, which takes additional time and may produce uneven dispersion of the resin  110 . Uneven dispersion produces a resin body  110  that does not have uniform dielectric properties. The spacer means  330  provides a more even resin body  110  having more uniform dielectric properties than using, for example, a support structure  310  only. 
     Moreover, the height  335  of the spacer means  330  can be selected to provide a desired overall thickness  120  of the first layer  130  using less support structure means  310 , such as fabric. That is, to achieve the same thickness  120  of the first layer  130 , and therefore the same dielectric properties, without the spacer means  330 , many layers of fabric would typically be required. The layers of fabric would not only cause uneven dispersing of the resin  110 , as described above, but would be subject to compression by the conductor means  145  as the conductor means  145  is applied, e.g., wound, over the fabric layers. Compression is typically uneven and results in a non-uniform thickness of the first layer, causing non-uniform dielectric properties. The spacer means  330  therefore preferably is less compressive, i.e., is less subject to changes in volume when a force is applied, than the support structure means  310 . For example, epoxy spacers are less compressive than layers of electrical grade glass. 
       FIG. 2  shows a support structure  210  with spacers  230 . The support structure  210  includes a plurality of fibers  220  interconnected to form a fabric. Although a grid-like pattern is illustrated, any pattern can be used. Multiple spacers  230  are affixed to the fabric  210  and protruding from a surface of the fabric  210 . 
     The spacers  230  can be arranged in a plurality of rows  240 A,  240 B. The rows  240 A,  240 B can be segmented as shown.  FIG. 2  shows the spacers  230  arranged in one of many patterns that can be used.  FIGS. 5A-5D  show other possible patterns of the spacers that can be used. 
       FIG. 4A  shows a support structure, spacers, and a conductor. The spacers  230  are shown arranged in a plurality of rows  240 A,  240 B. A conductor  430  has a first end  410  and a second end  430  and is continuous such that segment ends  420 A and  420 B are connected, i.e., represent the same point, and so on. The spacers  230  are shown arranged in a pattern so that the conductor  430  contacts only the spacers  230 , and contacts a spacer  230  at least every two rows. This pattern provides support for the conductor  430  every two rows. 
       FIG. 4B  illustrates this feature of the spacer pattern of  FIG. 4A . The superimposition of row  240 A onto  240 B provides an unsegmented row of spacers. Here, the term “unsegmented” is meant to include both a contiguous row of adjacent spacers and a row of overlapping spacers. This feature helps define the pattern of  FIG. 4A . Likewise, as can be appreciated, in the pattern of  FIG. 5A , the superimposition of three rows onto each other provides an unsegmented row of spacers. In  FIG. 5B , the superimposition of four rows onto each other provides an unsegmented row of spacers. In  FIGS. 5A and 5B , the respective pattern provides support for the conductor  430  every three rows and every four rows. This can be expanded to any number of rows. 
     As can be appreciated from  FIG. 5C , the rows need not be segmented, although it is preferable as discussed below. Moreover, as can be appreciated from  FIG. 5D , the spacers can be of varying sizes and patterns, and need not be in rows. The spacer pattern can be purely random if desired. 
     It is, however, preferable to use segmented rows of spacers. The segmenting allows better flow of the resin around the spacers. In addition, longer spacers are more likely to conduct electricity from one area of the conductor to another, or create a voltage potential between spacers. 
       FIG. 6  is a flow chart illustrating a method of forming a transformer coil assembly. A method of forming a transformer coil assembly includes providing a first fabric layer having a plurality of fibers interconnected and a plurality of protruding spacers affixed to a surface of the fabric ( 600 ). A conductor layer is applied to the first fabric layer in contact with at least one of the plurality of protruding spacers ( 610 ). A resin is applied to cover at least the first fabric layer and the conductor layer ( 620 ). 
     It will be appreciated by those of ordinary skill in the art that the invention can be embodied in various specific forms without departing from its essential characteristics. The disclosed embodiments are considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims, rather than the foregoing description, and all changes that come within the meaning and range of equivalents thereof are intended to be embraced thereby. 
     It should be emphasized that the terms “comprises”, “comprising”, “includes” and “including” when used in this description and claims, are taken to specify the presence of stated features, steps, or components, but the use of these terms does not preclude the presence or addition of one or more other features, steps, components, or groups thereof.