Abstract:
A wrapped film sealing system includes a conductive stud, a film layer wrapped around at least a portion of the length of the conductive stud, and a bushing including a channel passing between two open ends. The conductive stud passes through the channel and a seal is formed between the conductive stud, the film layer, and the bushing.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to a wrapped film sealing system for electrical equipment.  
         BACKGROUND  
         [0002]    Many types of conventional electrical equipment contain a dielectric fluid for dissipating the heat that is generated by energized components of the equipment, and for insulating those components from the equipment enclosure and from other internal parts and devices. Examples of such equipment include transformers, capacitors, regulators, circuit breakers and reclosers. Transformers are used extensively in the transmission of electrical power, both at the generating end and at the load end of the power distribution system. A distribution transformer is one that receives electrical power at a first voltage and delivers it at a second, lower voltage.  
           [0003]    A distribution transformer consists generally of a core and conductors that are wound about the core so as to form at least two windings. The windings (also referred to as coils) are insulated from each other, and are wound on a common core of magnetically suitable material, such as iron or steel. The primary winding or coil receives energy from an alternating current (AC) source. The secondary winding receives energy by mutual inductance from the primary winding and delivers that energy to a load that is connected to the secondary winding. The core provides a circuit or path for the magnetic lines of force (magnetic flux) which are created by the alternating current flow in the primary winding and which induce the current flow in the secondary winding. The core and winding are typically retained in an enclosure for safety and to protect the core and coil assembly from damage caused by weather or vandalism.  
           [0004]    Transformers generate heat during operation through (1) electrical resistance in the conductors that constitute the windings, (2) alternating magnetic flux generating current flow in the core material as the flux passes through the core, and (3) hysteresis (i.e., the friction between the magnetic molecular particles in the core material as they reverse their orientation within the core steel, which occurs when the direction of the AC magnetic field reverses). The generated heat reduces transformer life by degrading the insulation of various internal components, which can lead to an internal fault or short circuit. To dissipate the heat, transformers may be filled with a dielectric coolant, which also functions to electrically insulate the transformer components from one another and from the enclosure.  
           [0005]    An electrical connection is formed from the inside of the transformer to the outside using an electrical bushing, such as an insulated component bushing well or tri-clamp bushing. The bushing must provide a seal through an internal stud or components and an external flange. The external flange is sealed by additional gasket components or welding to the flanges.  
         SUMMARY  
         [0006]    In one general aspect, a wrapped film sealing system includes a conductive stud, a film layer wrapped around at least a portion of the length of the conductive stud, and a bushing well including a channel passing between two open ends. The conductive stud passes through the channel and a seal is formed between the conductive stud, the film layer, and the bushing.  
           [0007]    Embodiments may include one or more of the following features. For example, the conductive stud may include a knurled portion and the film layer may be wrapped around the knurled portion. The knurled portion may include knurled surfaces interspersed with smooth surfaces. The conductive stud may include a smooth portion and the film layer may be wrapped around a portion of the smooth portion. The film layer may include an adhesive layer, such as a heat shrinkable plastic. The film layer also may include a thermoplastic.  
           [0008]    The bushing may include a thermoplastic, which may be a nylon. The bushing also may include a thermoset material. The bushing may be a bushing well or a tri-clamp bushing. The conductive stud in the channel in the bushing well may be a removable conductive stud.  
           [0009]    In another general aspect, a method of sealing a stud in a bushing includes providing a conductive stud and a film. The film is wrapped around a circumference of the stud along at least a portion of a length of the stud, and the wrapped stud is inserted into a molding machine into which a plastic is then injected. The plastic defines a bushing having a channel through which the stud and film extend. The plastic also bonds to the film such that the film forms a seal in the channel between the stud and bushing.  
           [0010]    Embodiments may include one or more of the following features. For example, the method may further include heating the film wrapped around the stud before inserting the stud into the molding machine, such that heating the wrapped film shrinks the wrapped film around the stud. The wrapped film may include an adhesive layer and a heat shrinkable plastic, such as a thermoplastic. The plastic also may include a thermoplastic, which may be nylon, or a thermoset material. The molding machine may be an injection molding machine or a transfer molding machine.  
           [0011]    Inserting the stud into the molding machine may include inserting the stud into a mold and placing the mold in the molding machine. The portion of the length of the stud may include a knurled section, with the film being wrapped around the knurled section. The knurled section may include knurled surfaces interspersed with smooth surfaces. The bushing may be a bushing well or a tri-clamp bushing.  
           [0012]    The wrapped film sealing system provides considerable advantages. For example, the system may be used to provide a seal between a conductive stud and a bushing to prevent leakage of dielectric fluid. The wrapped film layer can compensate for the difference in thermal expansion between the conductive stud and the plastic bushing, which improves the reliability of the seal.  
           [0013]    Conventionally, the seal is provided by spraying an adhesive on the conductive stud and then the bushing is injection molded around the stud. The adhesive may include a solvent that contains potentially environmentally harmful organic solvents that are released into the atmosphere during the spraying step. After the adhesive is applied to the stud, it is baked to cure the adhesive and bond the adhesive to the stud. The wrapped film sealing system advantageously avoids use of potentially harmful solvents, and also avoids the time and expense of baking, thereby resulting in a less complex and much cleaner process.  
           [0014]    Other features and advantages will be apparent from the following description, including the drawings, and from the claims. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a perspective view of an electrical transformer.  
         [0016]    [0016]FIG. 2 is a perspective view showing a core and coil assembly mounted within the transformer of FIG. 1 and connected to secondary terminals.  
         [0017]    FIGS.  3 - 6  are cross-sectional front views of a conductive stud in a tri-clamp bushing.  
         [0018]    [0018]FIGS. 7 and 8 are cross-sectional front views of a conductive stud in a bushing well.  
         [0019]    [0019]FIG. 9 is an enlarged cross-section front view showing a seal between the conductive stud and bushing of FIGS. 7 and 8.  
         [0020]    [0020]FIG. 10 is a flow chart of the steps used to form the seal between a conductive stud and bushing.  
         [0021]    FIGS.  11 - 14  are front and end views of a conductive stud wrapped with a film layer before and after application of heat to the conductive stud and film layer.  
         [0022]    [0022]FIG. 15 is a flow chart of the steps used to form the seal between a conductive stud and bushing when a separate heat treatment step is omitted.  
         [0023]    FIGS.  16 - 19  are front views of conductive studs having various configurations of knurled and smooth section to which a film layer is wrapped. 
     
    
     DESCRIPTION  
       [0024]    Referring to FIG. 1, a transformer  5  includes a core and coil assembly  10  (shown schematically in FIG. 1), an enclosure  15 , a high voltage bushing  20 , low voltage bushings  25 ,  26 ,  27 , and a ground lug  30 . The core and coil assembly  10  is positioned within enclosure  15  and includes a primary winding  35  and a secondary winding  40 . A dielectric fluid  45  fills enclosure  15  and surrounds the core and coil assembly  10 . Bushings  20  and  25 - 27  may be made of an insulative material, such as a polymer.  
         [0025]    Referring also to FIG. 2, a transformer primary lead  50  interconnects primary winding  35  with high voltage bushing  20 , which is sealingly mounted to enclosure  15  through an aperture  52  in the enclosure. Low voltage bushings  25 ,  26 ,  27  are constructed and sealingly attached to enclosure  15 . Bushings  25 ,  26 ,  27  include insulative bodies  55 - 57 , which extend through apertures  60 - 62  in the enclosure  15 . Bushings  25 ,  26 ,  27  further include conductive studs  65 - 67  and terminal end caps  70 - 72 . Secondary leads  75 - 77  connect the secondary winding  40  to conductive studs  65 - 67 .  
         [0026]    Referring to FIGS. 3 and 4, the low voltage bushings  25 ,  26 ,  27  can be implemented, for example, as tri-clamp bushings. FIGS. 3 and 4 illustrate one example of a tri-clamp bushing design. A tri-clamp bushing  80  includes a channel  83  and a mounting flange  85 . The tri-clamp bushing  80  is mounted through one of apertures  60 - 62  (FIG. 2), and forms a seal between mounting flange  85  and the edge of the aperture through which it is mounted. A conductive stud passes through channel  83  and forms a seal with the tri-clamp bushing. A conductive stud  87  differs from a conductive stud  88  in the configuration of the outside end. Stud  87  has a round end  90  whereas stud  88  has a flat end  92 . The outside end is connected to a wire that delivers high voltage electricity to the transformer  5 .  
         [0027]    The tri-clamp bushing  94  of FIGS. 5 and 6 differs from the tri-clamp bushing  85  of FIGS. 3 and 4 in the configuration of a channel  96  that has a reduced diameter to accommodate a narrow diameter stud. Like studs  87  and  88 , narrow diameter studs  97  and  98  differ in their outside ends. Stud  97  has a round outside end whereas stud  98  has a flat end. Conductive studs  87 ,  88 ,  97  and  98  are mounted in tri-clamp bushings  83  and  96 , respectively, such that a seal between the stud and tri-clamp bushing prevents the dielectric fluid from leaking out of the transformer enclosure  15  through the channel in the tri-clamp bushing.  
         [0028]    Referring to FIGS. 7 and 8, high voltage bushing  20  can be implemented, for example, as a bushing well. FIGS. 7 and 8 illustrate two different bushing well designs. FIG. 7 illustrates a bushing well  100  that includes a conductive stud  105  passing through a channel  110  in the bushing well. Bushing well  100  is mounted through aperture  52  (FIG. 1), and forms a seal between mounting flange  115  and the edge of the aperture through which it is mounted. The seal prevents dielectric fluid  45  from leaking out of the transformer enclosure  15 . Referring also to FIG. 8, another design of a bushing well  200  includes a conductive stud  205  passing through a channel  210  in the bushing well. Like bushing well  100 , bushing well  200  is mounted through aperture  52 , and forms a seal between a mounting flange  215  and the edge of the aperture through which the bushing well  200  is mounted to prevent dielectric fluid  45  from leaking out of the transformer enclosure  15 . Bushing well  200  differs from bushing well  100  in that the mounting flanges  115  and  215  differ, the bushing wells are designed to receive conductive studs of different shapes, and the stud  105  of the bushing well  100  is fixed whereas the stud  205  of the bushing well  200  is removable.  
         [0029]    Conductive studs  105  and  205  are mounted into bushing wells  100  and  200 , respectively, such that a seal between the stud and the bushing well prevents the dielectric fluid from leaking out of the transformer enclosure  15  through the channel in the bushing well. Referring to FIG. 8 for exemplary purposes, a seal  220  is formed between a knurled portion  225  of the conductive stud  205  and the channel  210 . Similar seals are formed in tri-clamp bushings  83  and  94  between the respective studs and channels.  
         [0030]    Referring also to FIG. 9, seal  220  includes a film layer  230  surrounding the knurled portion  225  and contacting the inner diameter of channel  210 . The film layer is bonded to the bushing well and may be bonded and/or tightly adhered to the conductive stud. The film compensates for the difference in thermal expansion between the stud and the bushing well to maintain the integrity of the seal during the different transformer environmental conditions that occur within the transformer during its use. Although FIG. 9 shows the film layer  230  surrounding only the knurled portion  225 , the film layer  230  can surround other portions of the conductive stud, and can be bonded or adhered to the channel.  
         [0031]    Referring to FIG. 10, film layer  230  is attached to the stud  205  and the seal  220  is formed in a multi-step fabrication process  300 . As illustrated in FIGS. 11 and 12, the film layer  230  is wrapped at least once around the entire diameter of the conductive stud  205  at knurled portion  225  (step  305 ). The film layer  230  may overlap itself and be wrapped more than once around the conductive stud  205 . Referring also to FIGS. 13 and 14, heat is optionally applied to the film layer  230  to cause it to shrink down around the stud  205  (step  310 ), which reduces the outer diameter of the film layer  230  and creates a seal between the tape and stud. Heat may be applied to shrink the film by using a heat gun or other heat device. Heating the film also may cause the film to bond to the conductive stud, which improves the seal between the tape and the stud.  
         [0032]    The conductive stud  205  then is inserted into an injection mold or transfer mold(step  315 ), which is placed into an injection or transfer molding machine. A plastic or thermoset material then is injected into the mold around the conductive stud  205  and film layer  230  to form the bushing well  200  (step  320 ). Injection molds, transfer molds and the processes of injection and transfer molding are well-known in the art. The molded plastic bonds to the film layer and, because the molded plastic heats the film layer, bonds the film layer to the stud. Consequently, the film layer creates the seal  230  between the stud and bushing well  200  to prevent dielectric fluid  45  from passing through channel  210 . After the plastic has cooled sufficiently, the bushing well  200  can be removed from the mold (step  325 ) and installed in the transformer enclosure  15 .  
         [0033]    The process  300  of FIG. 10 typically is applicable for using film layers in which neither side has an adhesive backing. By heating and shrinking the film around the conductive stud, the film is adhered to the stud so that it can be further processed without the concern that the tape may unwind and separate from the stud before the bushing well (or tri-clamp bushing) is formed around it. If, on the other hand, the film includes an adhesive backing on one or both sides, there is less concern that the tape will loosen and separate from the stud in the later processing steps. With such a tape, the heating step can be omitted, as illustrated in a process  400  of FIG. 15.  
         [0034]    In process  400 , the film layer is wrapped around the conductive stud (step  405 ) as described above with respect to step  305  except that the tape adheres to the stud. The conductive stud and tape then are inserted into the injection mold (step  410 ), which is inserted into the injection molding machine and a plastic material injected into the mold (step  415 ). As described above with respect to the process  300 , the film layer is heated by the injection molded plastic. Because the film layer has not been shrunk around the conductive stud in process  400 , the heat of the injection molded plastic causes the film layer to shrink around the conductive stud and potentially bond to the stud.  
         [0035]    Processes  300  and  400  can be modified in various manners. For example, although processes  300  and  400  are described and illustrated in terms of wrapping the film layer around the knurled portion of the conductive stud, the film layer may be wrapped around other portions of the conductive stud. The position of the film layer must be such that the injection molded plastic will contact and bond with the film layer. In general, it is easier to wrap the film around a smooth surface on the conductive stud but the film fills the crevices formed in a knurled surface, potentially providing a better bond between the film and stud.  
         [0036]    Although the conductive stud illustrated above included a knurled section only, various configurations are possible. Referring to FIGS.  16 - 19 , the surface to which the film is to be applied may have a number of configurations of smooth and knurled sections. For example, referring to FIG. 16, stud  505  has a surface  510  that is a combination of longitudinal knurled sections  515  and smooth sections  520 . As explained above, typically the film layer will be easier to apply to the smooth sections  520  but will fill the crevices in the knurled sections  515 . Referring to FIG. 17, a stud  530  has a surface  535  that is a combination of a circumferential smooth section  540  between a pair of circumferential knurled sections  545 . Referring to FIG. 18, in a related configuration, a stud  550  has a surface  555  with multiple circumferential smooth sections  560  separated by multiple circumferential knurled sections  565 . Finally, referring to FIG. 19, a stud  570  has a surface  575  with alternating helical smooth sections  580  and knurled sections  585 .  
         [0037]    With respect to the selection of materials, typically, the injection or transfer molded plastic will be a thermoplastic, such as Zytel HTN™, a high temperature polyphthalamide; Crastin™, a polybutylene terephthalate; or Rynite™, a polyethylene terephthalate. Each of these thermoplastic materials is sold by E.I. Du Pont de Nemours &amp; Co. of Wilmington, Del. The injection or transfer molded plastic also may be a thermoset plastic, such as E8353-706R or E8398, which are epoxidized novolac molding compounds sold by Rogers Corporation of Rogers, Conn.  
         [0038]    The film typically also will be a thermoplastic, such as the film sold under the trade name Surlyn™, which is marketed by E.I. Du Pont de Nemours &amp; Co. of Wilmington, Del. The film also may be a polytetrafluoroethylene film or tape, such as the PTFE tapes and films sold by 3M and E.I. Du Pont de Nemours &amp; Co. The tape may be formed with or without glass fibers and adhesive backings. The dimensions of the film, for example, may be one inch wide, five inches long, and have a thickness of approximately 2.0 mils. The film also may be an adhesive thermoplastic tape that adhesively bonds to the conductive stud. The conductive stud may be made from any electrically conductive material, such as copper or aluminum.  
         [0039]    Other embodiments are within the scope of the following claims.