Abstract:
A switchgear assembly includes a vacuum interrupter assembly having an internal switching contact. A conductive current exchange is in electrical contact with the switching contact, and the current exchange defines an internal chamber within the current exchange. A plug of non-conductive, compliant material has a first portion that extends into the internal chamber in contact with the current exchange. An insulative encapsulation surrounds the vacuum interrupter assembly, the current exchange, and the plug.

Description:
TECHNICAL FIELD  
         [0001]    This disclosure relates to the field of electrical switchgear, and more particularly to methods of increasing insulation levels in a vacuum interrupter encapsulation.  
         BACKGROUND  
         [0002]    High voltage vacuum current interrupters may be mounted or encapsulated at the upper end of an epoxy or porcelain structure or encapsulation that includes an internal chamber for supporting the interrupter and an operating rod.  
           [0003]    The structure must withstand the application of high voltage to the switchgear. In particular, the structure is designed to reduce “tracking,” which is the irreversible degradation of a surface of the structure due to the formation of carbonized or otherwise conductive paths. This may occur on any exposed surface of the structure, including the operating cavity, between the high potential to a frame below the encapsulation at ground potential, and may be due to either condensation or a build-up of surface contamination. The structure is also designed to prevent electrical arcing between the interrupter and the frame, and to prevent corona discharge caused by the ionization of air due to a high electric field gradient near a surface.  
         SUMMARY  
         [0004]    In one general aspect, a switchgear assembly includes a vacuum interrupter assembly having an internal switching contact. A conductive current exchange is in electrical contact with the switching contact, and the current exchange defines an internal chamber within the current exchange. A plug of non-conductive, compliant material has a first portion that extends into the internal chamber and is positioned against the current exchange. An insulative encapsulation surrounds the vacuum interrupter assembly, the current exchange, and the plug.  
           [0005]    Implementations may include one or more of the following features. For example, the plug may include a second portion that is positioned outside the internal chamber against the current exchange. The compliant material may include rubber. The switchgear assembly may include a shaft for moving the switching contact within the vacuum interrupter assembly A portion of the shaft may be located in the internal chamber, and the shaft may pass through a hole in the plug. At least a portion of the plug may be located between the shaft and the current exchange. The hole in the plug may have a cross-sectional area larger than the cross-sectional area of a portion of the shaft that passes through the hole such that the shaft does not contact the plug. The hole through the plug may be tapered from one side of the plug to another side of the plug.  
           [0006]    In another general aspect, insulatively encapsulating an electrical switchgear assembly includes surrounding with a mold a vacuum interrupter assembly having an internal switching contact, a current exchange in electrical contact with the switching contact and defining an internal chamber, and a plug of non-conductive, compliant material, having a first portion that extends into the internal chamber against the current exchange. An insulative encapsulation is formed around the vacuum interrupter assembly, the current exchange, and the plug, and the mold is removed.  
           [0007]    The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims. 
       
    
    
     DESCRIPTION OF DRAWINGS  
       [0008]    [0008]FIG. 1 is a cutaway side view of a vacuum interrupter encapsulation.  
         [0009]    [0009]FIG. 2 is a cross-sectional side view of an insulating plug for use with a vacuum interrupter encapsulation.  
         [0010]    [0010]FIG. 3 is a cross-sectional side view of an insulating plug positioned within a vacuum interrupter. 
     
    
       [0011]    Like reference symbols in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0012]    Referring to FIG. 1, an encapsulation  10  for an interrupter  12  includes an internal chamber  14 . An operating rod  16  passes through the internal chamber  14 . The operating rod  16  connects the interrupter  12  to an actuating mechanism (not shown) in the frame  18  upon which the encapsulation  10  is mounted.  
         [0013]    The interrupter  12  is connected at terminals  20  and  22  such that an electrical current passes from terminal  20  to terminal  22  through interrupter  12  when the interrupter is in a closed position. In doing so, the current passes through an electrically conductive current exchange  24 . In general, all electrically-conductive components, including terminals  20  and  22  and current exchange  24 , are maintained at a high voltage. Current exchange  24  is annular and has a generally cylindrical interior surface  25  that defines the internal chamber  14 . Operating rod  16  passes through an operating cavity  15  and connects to a movable piston within current exchange  24 .  
         [0014]    Encapsulation  10  may be cast from epoxy or any other suitable material capable of withstanding the mechanical, electrical, and thermal stresses that occur during use of interrupter  12 . For example, a cycloaliphatic, prefilled, hot-curing, two-part epoxy may be used to form encapsulation  10 .  
         [0015]    Referring also to FIG. 2, an annular, generally cylindrical plug  100  of compliant non-conductive material is adapted for fitting around operating shaft  16  and extending into operating cavity  15 . Plug  100  has a generally cylindrical hole  102  through which operating rod  16  passes without touching the inside surface  104  of the plug. Plug  100  has an outside surface  106  with a shape that is adapted for sealing against interior surface  25  of current exchange  24 , and a flange  108  that is shaped to seal against the bottom surface of current exchange  24 . The inside surface  104  of plug  100  may be slightly tapered, so that the diameter of the cylindrical hole  102  is slightly larger at the end closest to the flange  108  than at the end most distant from the flange  108 . Plug  100  is made of silicone rubber or another suitable compliant material.  
         [0016]    [0016]FIG. 3 shows plug  100  in a sealing position such that outside surface  106  of the plug seals against interior surface  25  of current exchange  24 , and flange  108  of the plug seals against the bottom surface of the current exchange. A layer of compliant material  26  (e.g., a stretched rubber sleeve) is placed over the outside surfaces of interrupter  12  and current exchange  24  before placing plug  100  in the sealing position and before encapsulating interrupter  12  in encapsulation  10 . The compliant material  26  extends from the outside surface of current exchange  24  around the bottom of the current exchange and along the interior surface  25  of the current exchange. Thus, compliant material  26  is positioned between the plug  100  and the interior surface  25  of current exchange  24  when the plug is positioned against the current exchange. Compliant material  26  helps to reduce mechanical stresses between interrupter  12  and encapsulation  10  that result from temperature changes and different coefficients of thermal expansion for interrupter  12  and encapsulation  10 .  
         [0017]    Compliant material  26  may be applied to interrupter  12  and current exchange  24  using a method such as is described in U.S. Pat. No. 5,917,167, which is incorporated by reference. Plug  100  may be placed in a sealing position within the bore of current exchange  24  by bonding or pressing the plug into position. A bonding agent may be applied to at least a portion of interior surface  25  of the current exchange and/or the compliant material  26  covering the interior surface. A bonding agent may also be applied to the external surface  106  of the plug  100 . The bonding agent may be a silane-based material, such as, for example, SILQUEST A-1100 (gamma amino propyl triethoxysilane). After the bonding agent has been applied to the interior surface  25  of current exchange  24  and/or the compliant material  26 , plug  100  is inserted into internal chamber  14  until flange  108  contacts the compliant material  26  covering the bottom surface of current exchange  24  and the outside surface  106  of the plug contacts the interior surface  25  of current exchange  24  or the compliant material  26  covering the interior surface. The bonding agent then bonds flange  108  of plug  100  to the compliant material covering the bottom surface of the current exchange  24  and bonds the outside surface  106  of the plug to interior surface  25  of current exchange or to the compliant material  26  covering the interior surface  25 .  
         [0018]    Plug  100  may also be placed in a sealing position by pressing the plug into position without a bonding agent. When a bonding agent is not used, the silicone rubber material of the plug&#39;s flange  108  and outside surface  106  may stick to the compliant material  26  and hold the plug in position.  
         [0019]    After plug  100  is sealed against current exchange  24 , the interrupter  12 , the current exchange  24 , and the plug  100  are encapsulated in encapsulation  10 . A mold is used to create the shape of encapsulation  10  around the interrupter  12 , the current exchange  24 , and the plug  100 . The mold core that forms the operating cavity  15  seals against the inner surface  104  of the plug  100  to prevent epoxy from entering internal chamber  14 . Positioning the plug  100  before encapsulation of the interrupter  12  and current exchange  24  eliminates the need for any complex hardware that previously was necessary to seal off internal chamber  14  during encapsulation. This hardware was troublesome in that it tended to leak, which caused the internal chamber  14  to fill with epoxy and prevented the interrupter  12  from actuating. The hardware also had to be removed after the encapsulation process, which required reaching through the operating cavity  15  with other fixturing to unthread and remove components of the hardware.  
         [0020]    Previous designs for current exchanges that used older methods of sealing had exposed metal surfaces, often with sharp corners, between the top of the operating cavity  15  and the internal chamber  14  in the current exchange. A high voltage potential on these metal surfaces with sharp corners could cause a high field gradient in air and could thereby lead to potential electric discharges. When plug  100  is sealed against the current exchange  24 , the bottom edges and surfaces of the conductive and high voltage current exchange are covered by the compliant, non-conductive material of the plug, thus containing these high field gradients in a solid material more capable of withstanding voltage stress. Also, the plug  100  lengthens the distance between exposed conductive portions of the current exchange  24  and the grounded base  18  of encapsulation  10 .  
         [0021]    The slight taper to the inner surface  104  of the plug  100  allows the mold for creating the encapsulation to seal easily against the plug  100  and then to be removed easily after the encapsulation  10  has been molded. The mold has a slight taper to mate against the inner surface  104  of the plug while the encapsulation  10  is being molded.  
         [0022]    After encapsulation, operating rod  16  is inserted through hole  102  of plug  100  and connected to interrupter  12 . The end of the operating rod  16  inserted through the hole  102  may be threaded or have a threaded insert for coupling the rod to a threaded protrusion or indentation of the interrupter  12  and enable actuation of the interrupter by the rod.  
         [0023]    A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made. Other implementations are within the scope of the following claims.