Abstract:
A high voltage bushing assembly includes an insulator shell adapted to enclose an electrical conductor. An annular flange is slidably received over the insulator shell, the annular sleeve formed with a radially outwardly directed flange at an upper end and a radially inwardly directed flange at a lower end, with a sleeve portion extending axially therebetween. The insulator shell has an outside diameter and the sleeve portion has an inside diameter sized to create an annular, radial gap therebetween filled with a high thermal endurance fiberglass-reinforced epoxy resin.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to large generator constructions, and specifically to a high voltage bushing utilized to pass an electrical conductor through a wall of a generator frame. 
     A high voltage bushing is used for passing an electrical conductor through a pressure vessel wall of, for example, a large generator, the conductor carrying electricity out of the generator to voltage and power transformers and then to an electrical grid or the like. It is important that such bushings prevent a cooling gas (e.g., hydrogen) inside the pressurized vessel (stator) from leaking out of the vessel through the bushing stator wall interface. In addition, the conductor must be electrically insulated from the pressurized vessel or stator wall. This is achieved by enclosing the conductor inside a porcelain or other insulating sleeve or shell. An annular, sleeve-like metallic bushing also referred to herein as a “bushing flange”) is telescoped over the exterior surface of the porcelain shell and is utilized to attach the porcelain sleeve to the pressure vessel wall. One such high voltage bushing flange is disclosed in commonly-owned U.S. Pat. No. 5,483,023. 
     Problems associated with such high voltage bushings include: 1) cracking of the porcelain sleeve due to mechanical stresses imparted by the thermally-mismatched bushing flange; 2) leaking of hydrogen gas from inside the generator stator through the bonding seals between the bushing flange and the porcelain shell; 3) micro crack formation of bonding materials induced by, for example, high density epoxies of virgin porosity, thermal-aging excess tensile stresses, thermal cycling, and/or vibrations experienced during operation. 
     There remains a need, therefore, for an improved high voltage bushing flange that alleviates excess tensile stresses on the porcelain shell for increased service longevity, and that more effectively blocks potential gas leakage pathways through the bonding seals utilized to provide a buffer between the metal bushing flange and the insulating shell such as, but not limited to, a porcelain shell. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with an exemplary but non-limiting embodiment, the present invention relates to a high voltage bushing flange assembly comprising an insulator shell adapted to enclose an electrical conductor; an annular bushing flange slidably received over the insulator shell, the annular bushing flange formed with a radially outwardly directed flange at an upper end and a radially inwardly directed flange at a lower end, with a sleeve portion extending axially therebetween; the insulator shell having an outside diameter and the sleeve portion having an inside diameter sized to create an annular, radial gap between the insulator shell and the sleeve portion, the radial gap filled with a high thermal endurance fiberglass-reinforced epoxy resin, supported axially by the radially inwardly directed flange. 
     In another aspect, the present invention relates to a high voltage bushing assembly comprising an insulator shell adapted to enclose an electrical conductor; an annular bushing flange slidably received over the insulator shell, the annular bushing flange formed with a radially outwardly directed flange at an upper end thereof and an axially-oriented sleeve portion, the insulator shell having a substantially uniform outside diameter and the sleeve portion having a substantially conically-shaped inside surface sized to create an annular, conically-shaped radial gap between the insulator shell and the axially-oriented sleeve portion, the conically-shaped radial gap filled with a high thermal endurance fiberglass-reinforced epoxy resin. 
     In still another aspect, the invention relates to a bushing assembly comprising a substantially cylindrical shell; an annular bushing flange slidably received over the substantially cylindrical shell, the annular bushing flange formed with a radially outwardly directed flange at one end and a radially inwardly directed flange at an opposite end, with a sleeve portion extending axially therebetween; the substantially cylindrical shell having an outside surface and the sleeve portion having an inside surface sized to create an annular radial gap therebetween, the radial gap filled with a high thermal endurance fiberglass-reinforced epoxy resin between the radially outwardly directed flange and the radially inwardly directed flange, bonding the bushing flange to the substantially cylindrical shell. 
     The invention will now be described in detail in connection with the drawings identified below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial, sectioned perspective view of a known high voltage bushing flange; 
         FIG. 2  is a partial section view of a high voltage bushing flange in accordance with a first exemplary but nonlimiting embodiment of the invention; and 
         FIG. 3  is a partial section view of a high voltage bushing flange in accordance with a second exemplary but nonlimiting embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference initially to  FIG. 1 , a known bushing flange  10  is shown enclosing a copper conductor  12  having a wrap  14  of asphalt or similar material between the conductor and a porcelain insulator sleeve or shell  16 . A metal bushing flange (or simply, “flange”)  10  is telescoped over the exterior of the porcelain shell  16  and is utilized to attach the porcelain shell  16  to the pressure vessel wall, indicated in phantom at  18 . The bushing flange  10  includes an axial portion  20  terminating at a tapered edge  22  at one end, and a radial flange portion  24  at an opposite end of the axial portion  20 . The radial flange portion  24  is provided with a plurality of axially oriented through holes  26  which enable the bushing  10  to be secured to the pressure vessel wall by means of bolts  28  or other suitable fasteners. 
     An annular support ferrule  30  is telescoped onto the shell  16  to a location where it abuts the radial portion  24  of the mounting flange  10 . The ferrule  30  serves as a seal, preventing escape of hydrogen from inside the pressurized vessel where the bushing flange  10  is joined to the pressurized vessel wall. A gasket  32  extends over the exposed side of the ferrule radial portion and is adapted to be compressed between the ferrule  30  and the pressurized vessel wall. 
     The bushing flange  10  is secured to the porcelain insulator shell  16  by means of an epoxy  34  located in a radial gap between the axial portion  20  of the bushing flange  10  and the porcelain insulator shell  16 . 
       FIG. 2  illustrates a high voltage bushing flange in accordance with an exemplary but nonlimiting embodiment of the invention. An annular, non-magnetic, metal (steel alloy, for example) bushing flange  36  is shown telescoped over a porcelain insulator sleeve or shell  38  that encloses a copper conductor  40 . The bushing flange  36  attaches the porcelain insulator shell to the wall of a generator stator frame  42 . The flange  36  is located radially outwardly of an enlarged diameter portion  44  of the insulator shell, commencing at a radial shoulder  46 , where the shell  38  projects through the stator frame wall. 
     The flange  36  includes an axial sleeve portion  48  and a radially outwardly directed flange portion  50  at one end thereof (the upper end as viewed in  FIG. 2 ), and a smaller radially inwardly directed flange  52  at the opposite end thereof. The radially outwardly directed flange portion  50  is formed with a plurality of circumferentially spaced bolt holes (one shown)  54  that facilitate attachment of the flange  36  (and hence the porcelain insulator shell  38 ) to the wall of the generator stator frame  42  in an otherwise conventional fashion. 
     In this embodiment, the porcelain insulator shell  38  is formed with at least one annular rib  56  in the enlarged diameter portion  44  at a location above and adjacent the radially inwardly directed flange  52 , with an axial gap sufficient to receive an o-ring  58  that is supported on the flange  52 . The axial portion  60  of the annular rib  56  leaves only a very narrow pathway or radial gap  62  for potential hydrogen gas leakage, thereby improving the effectiveness of the O-ring  58 . An epoxy bonding resin  64  fills both the narrow radial gap  62  and the relatively larger radial gap  66  (of about ½ inch in thickness) between the porcelain insulator shell  38  and the axial sleeve portion  48  of the flange  36 . It will be understood that one annular rib  56  would be sufficient, but it can be more than one, and may be spaced along the portion  44  of the shell  38 . 
     The epoxy resin  64  buffers the thermal expansion mismatch between the porcelain shell  38  and the metal flange  36  at high temperatures. Otherwise, the direct compression and tensile stresses of the brittle porcelain shell  38  by the sleeve portion  48  and radial portion  50  of the annular bushing flange  36  as a result of thermal mismatch could result in chip-off or micro-cracks formed in the porcelain sleeve. Note that the annular porcelain rib and the inwardly directed flange  52  also provide some axial support for, and thus reduce stress on, the epoxy bonding resin  64 . 
     The epoxy bonding resin  64  must be high in mechanical strength and toughness for supporting the weight of the porcelain shell  38  and for absorbing the thermal mismatch between the porcelain shell  38  and the metal flange  36 . In addition, it must have high thermal endurance capability and be void-free or void-less when cured. This is particularly important in the area of the narrow gap  62  where it is also a potential, high-pressure gas leakage pathway. Ordinary resins are subject to the formation of voids or bubbles caused by fast curing and skin effect, or by use of organic solvents or diluents that contain components that are readily trapped during the exothermal curing process. 
     In the exemplary embodiment, epoxy bonding resins such as ASTRO-6979 and ASTRO-6269 have proven suitable for bushing bonding application due to their lack of solvents which produce less porosity when cured. The utility of vacuum oven cure further reduces the bubble formation. The epoxy is reinforced with embedded fiberglass for increased bonding strength, increased mechanical strengths and a reduced coefficient of thermal expansion. As mentioned above, it is also important that the epoxy resin  64  be cured properly. The glass transition temperature should be between 90° C. and 120° C. so that flexibility and toughness are maintained. The thermal classification of said epoxy material should be Class 155 as per IEC 60216. This allows the epoxy bonding resins to have high thermal endurance capability to withstand the heat generated from the copper conductor (through the porcelain shell) as well as restive to heating due to Eddie currents induced on the flange. In one example the epoxy resin material may have the following properties: 
     
       
         
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                   
                 Temperature  
               
               
                 Epoxy Bonding Resin for Insulating Shell and Flange 
                 Value 
               
               
                   
               
             
             
               
                 Thermal Indices ° C. 
                 155-179 
               
               
                 Glass Transition Temperature of Bonding resin (° C. ) 
                  90-120 
               
               
                   
               
             
          
         
       
     
     In a second exemplary but nonlimiting embodiment as illustrated in  FIG. 3 , a high voltage flange bushing  67  is generally similar to the bushing flange  36  described above, but the inside diameter of the flange  67  varies along the length of the axial sleeve portion  68 . More specifically, the inside surface  70  is conical in shape, with the inside diameter decreasing substantially uniformly from the upper edge  72  of the flange portion  74  to the lower, radially inwardly-directed flange  76 . The resulting tapered gap  78  is filled with an epoxy bonding resin  80  that may be the same as the epoxy resin  64  described above. This arrangement further reduces tensile and shear stresses resulting from bushing body gravity, pressures, as well as the thermal mismatch between the porcelain shell  82  and the bushing flange  67 . While the annular rib  56  is omitted from  FIG. 3 , it will be understood that one or more such ribs  56  (and seal  58 ) may be included axially above the lower flange  76  of the bushing flange  67 . 
     In both embodiments, the bushing flange and high thermal endurance epoxy seal alleviates the excess mechanical stresses on the porcelain shell; reduces the potential for cracks in the porcelain shell by buffering the thermal mismatch between the porcelain shell and the bushing flange; and, as a result of reduced porosity in the epoxy resin, prevents gas leakage through the bonding regions. 
     The invention is widely applicable through a full range of hydrogen-cooled generators rated 24 KV and below. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.