BACKGROUND OF THE INVENTION  
       [0001]     This invention relates generally to electrical penetrations, and more particularly to corona-free bushings for medium to high voltage electrical equipment.  
         [0002]     Bushings are generally used for passing an electrical conductor through a vessel wall of, for example, a transformer or circuit breaker. The conductor is electrically isolated from the vessel wall by a non-conductive sleeve, such as a porcelain insulator. Concentration of charge along the conductor or at specific points on the conductor may cause a breakdown of the dielectric material between the conductor and other bushing components that are operating at a lesser potential. A partial breakdown of the dielectric material may cause partial discharge or corona at the point of concentration of charge. At least some known bushings use alternating foil/paper insulation in a mineral oil-filled porcelain to create a potential gradient between the conductor and the insulator that facilitates reducing corona. Other known bushings use a fluid dielectric medium, such as mineral oil or SF 6 , to facilitate reducing corona, and still other known bushings use a combination of a potential gradient and fluid dielectric medium to facilitate reducing corona. However, bushing utilizing these methods are costly, in terms of materials and labor, to fabricate and maintain.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0003]     In one aspect, a corona free bushing is provided. The bushing includes an insulator including an elongate body having a bore therethrough, and a conductor positioned at least partially within the bore. The conductor includes a load current carrying conductive core at least partially clad with at least one layer of a semiconducting insulation, and the semiconducting insulation facilitates grading a potential between a radial inner surface of the layer and a radially outer surface of the layer such that corona generated by the conductor is substantially zero.  
         [0004]     In another aspect, a method of fabricating a corona free electrical bushing is provided. The method includes providing an insulator that includes a body having a central bore therethrough, and inserting a corona free conductor at least partially into the central bore, the conductor includes at least one insulation layer including a semiconducting material, the semiconductor material facilitating grading a potential between a radial inner surface of the layer and a radially outer surface of the layer such that corona generated by the conductor is substantially zero.  
         [0005]     In yet another aspect, an electrical device is provided. The electrical device includes at least one of a vessel and a compartment wall, the wall having an opening therethrough, a corona free bushing inserted at least partially through the opening and fixed in a stationary position with respect to the wall, the bushing including an insulator including an elongate body having a bore therethrough, and a conductor positioned at least partially within the bore, the conductor including a load current carrying conductive core at least partially clad with at least one layer of a semiconducting insulation, the semiconducting insulation facilitates grading a potential between a radial inner surface of the layer and a radially outer surface of the layer such that corona generated by the conductor is substantially zero. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  is a cross sectional view of an exemplary electrical bushing assembly; and  
         [0007]      FIG. 2  is a cross sectional view of an exemplary portion of a conductor that may be used with bushing assembly shown in  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0008]      FIG. 1  is a cross sectional view of an exemplary electrical bushing assembly  100 . In the exemplary embodiment, bushing  100  is configured for mounting engagement with a transformer housing portion (not shown). Bushing assembly  100  includes an insulating member  102  that may be fabricated from porcelain, ceramic, polymeric resin, or other insulating material capable of performing the functions described herein. Insulating member  102  includes a plurality of integral annular fins or skirts  104  formed on at least a portion of a surface  106  of an exterior portion  107  of insulating member  102  to increase its insulating capacity. Insulating member includes an abutment or flange  108  configured to abut an annular seal or gasket  110  formed of rubber or synthetic elastomer material, such as Buna N. Gasket  110  may be engaged about an opening  111  in a transformer tank  112 , or vessel portion, such as a housing cover or lid. A lower or interior portion  114  of insulating member  102  may extend through opening  111  interiorly of transformer tank  112 . In the exemplary embodiment, a threaded clamping nut  116  is configured to engage a complementary threaded portion  118  of interior portion  114  to retain insulating member  102  in a substantially fixed position with respect to transformer tank  112 .  
         [0009]     Insulating member  102  also includes an interior end portion  120 , an exterior end portion  122 , and a central opening or bore  124  that extends through insulating member  102  along a longitudinal axis  126  between interior end portion  120  and exterior end portion  122 . A conductor  128  passes at least partially through bore  124  to facilitate transmitting electrical current between a first end  130  and a second end  132  of conductor  128 . In the exemplary embodiment, conductor  128  includes a conductive core  133  and at least one layer of an insulation  134  that may extend at least partially between first end  130  and second end  132 . A fastener  136  may engage conductive core  133  along a distal end portion  138 , where insulation layer  134  is stripped away, using a crimpable receptacle that is configured to receive distal end portion  138  and then be deformed into a gripping engagement with distal end portion  138 . Fastener  136  may also be configured to electrically couple to distal end portion  138  through other fastening means. Fastener  136  and distal end portion  138  may be covered with a second semiconducting insulation  139 , which may be, for example, a lapped covering or shrink tube composition. In the exemplary embodiment, fastener  136  includes an opposite threaded portion  140  configured to threadily engage a lead connector  142 , which may be configured to engage various electrical cable connector types (not shown). A flange  143  that extends radially outward from fastener  136  may engage an annular shoulder  144  of bore  128  to limit a range of travel of fastener along bore  128  from exterior end portion  122 .  
         [0010]     An end cap  146  may be used with various seals  148  to seal exterior end portion  122  from environmental intrusion into bore  124  and may also seal exterior end portion  122  from leaking a dielectric fluid (not shown), such as mineral oil, or SF 6  gas from bore  124  to ambient  149 . Similarly, an end cap and seal arrangement (not shown) may be incorporated into interior end portion  120  for leakage prevention and/or lateral stabilization of conductor  128  in bore  124 .  
         [0011]     Bushing assembly  100  may include one or more spacers  150  configured to maintain a gap  152  between conductor  128  and a surface  154  of bore  124 . Spacers  150  may surround conductor  128  circumferentially and be spaced axially along conductor  128  such that gap  152  is maintained. Spacers  150  may also be configured to extend axially along conductor  128  and be spaced about a circumference of conductor  128 . In an alternative embodiment, an outside diameter of conductor  128  is substantially equal to an inner diameter of bore  124  such that conductor  128  fills at least a portion of bore  124 .  
         [0012]      FIG. 2  is a cross sectional view of an exemplary portion of conductor  128  that may be used with bushing assembly  100  (shown in  FIG. 1 ). In the exemplary embodiment, conductive core  133  is fabricated using multiple individual conductive strands covered with an extruded insulation layer  134  comprising ethylene-propylene-rubber (EPR) in close contact with all exterior surfaces of conductive core  133 , and fills the outermost spaces between adjacent strands. In an alternative embodiment, conductive core  133  comprises a single conductor, such as a rod or a wire conductor. In other alternative embodiments, conductive core  133  may comprise aluminum or other metallic conductors. EPR has semiconducting properties such that a relatively constant potential gradient  200  is formed between a portion  202  at a first potential magnitude  204  and a portion  206  at a relatively lower potential magnitude  208 . Constant potential gradient  200  permits insulation layer  134  to substantially insulate conductive core  133  while suppressing corona and its onset for a predetermined range of voltages that may be applied to conductor  128 . In various embodiments of the present invention, other semiconducting materials may be used in place of EPR and the semiconducting material may be applied to conductive core  133  using molding, casting, or other processes. Insulation layer  134  may be at least partially covered by a second layer  210 , which may be, but not limited to an insulating layer, a semiconducting layer, and/or a protective sheath.  
         [0013]     While the present invention is described with reference to an electrical bushing, numerous other applications are contemplated. It is contemplated that the present invention may be applied to any electrical penetration, such as penetrations into electrical vaults, cable tunnels, bulkheads, and concrete containment buildings in nuclear power facilities.  
         [0014]     The above-described apparatus and methods of corona free electrical penetration through walls is cost-effective and highly reliable for electrical distribution and transmission. More specifically, the apparatus and methods described herein facilitate eliminating corona in conductor feed through applications. As a result, the apparatus and methods described herein facilitate reducing electrical distribution and transmission costs in a cost-effective and reliable manner.  
         [0015]     Exemplary embodiments of corona free electrical penetrations and methods are described above in detail. The systems are not limited to the specific embodiments described herein, but rather, components of each system may be utilized independently and separately from other components described herein. Each system component can also be used in combination with other system components.  
         [0016]     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.
Methods and apparatus for a corona free bushing are provided. The bushing includes an insulator including an elongate body having a bore therethrough, and a conductor positioned at least partially within the bore, the conductor including a load current carrying conductive core at least partially clad with at least one layer of a semiconducting insulation, the semiconducting insulation facilitates grading a potential between a radial inner surface of the layer and a radially outer surface of the layer such that corona generated by the conductor is substantially zero.