Abstract:
A method and arrangement is provided for minimizing electrical field stress on circuit interrupters and their housings via the arrangement of the external sheds or skirts of the housing. In one arrangement, some of the sheds are grouped about each side of the open gap that is defined by the contacts of the circuit interrupter in an open position.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/165,415 filed on Nov. 13, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the field of electrical circuit interrupters for electrical power transmission and distribution, and more particularly to a method and arrangement for minimizing electrical field stress on circuit interrupters and their housings via the arrangement of the external sheds or skirts of the housing. 
     2. Description of the Related Art 
     Various circuit interrupters are known for the electrical power transmission and distribution field. For outdoor application, the insulating housings of these circuit interrupters include external sheds or skirts to satisfy desired power-frequency voltage withstand ratings in a variety of environmental conditions. As the diameter of such housings is decreased, high electric field stresses inside of the housing also appear on the exterior of the housing. At the desired BIL (basic insulation level) rating, the external electric field stresses must not exceed the breakdown level of air or other specified environment. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a principal object of the present invention to provide a method and arrangement for minimizing electrical field stress on circuit interrupters and their housings via the arrangement of the external sheds or skirts of the housing. 
     It is another object of the present invention to provide a grouping of external sheds on either side of the open gap defined by the open contacts of a circuit interrupter. 
     It is a further object of the present invention to provide a housing for a circuit interrupter that reduces the electrical field stress on the exterior of the housing via the grouping of sheds around the open contracts of the circuit interrupter. 
     These and other objects of the present invention are efficiently achieved by the provision of a method and arrangement for minimizing electrical field stress on circuit interrupters and their housings via the arrangement of the external sheds or skirts of the housing compared to a uniform spacing off the external sheds or skirts. In one arrangement, some of the sheds are grouped about each side of the open gap that is defined by the contacts of the circuit interrupter in an open position, i.e. increasing the leakage distance per unit length of the housing around the open gap compared to the middle of the open gap and other areas of the housing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the specification taken in conjunction with the accompanying drawing in which: 
     FIG. 1 is a cut-away view of a circuit interrupter with a housing having a pattern of external sheds in accordance with the prior art; 
     FIG. 2 is a cut-away view of a circuit interrupter with a housing having a pattern of external sheds in accordance with the present invention; and 
     FIGS. 3-5 are elevational views of alternate embodiments of the present invention illustrating circuit interrupters with respective housings and illustrating different patterns or grouping of external sheds. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 1, there is illustrated a circuit interrupter  10  including a housing  12  generally in accordance with the prior art. In specific implementations, the interior  15  of the housing  12  is filled with pressurized gas, e.g. SF 6 . The housing  12  includes external sheds or skirts  14 , e.g.  14   a, b, c  and  d  to satisfy power-frequency voltage withstand ratings in a variety of environmental conditions, e.g. to prevent external flashover of the circuit interrupter  10  due to the effects of precipitation and pollution. The diameter and spacing of the sheds  14  is varied in accordance with the desired ratings. For example, as known by those skilled in the art and as set forth in various standards etc., to achieve a particular power-frequency voltage withstand rating, a minimum creepage distance along the exterior of the housing  12  is required to achieve a particular rating, i.e. avoid flashover when a specified voltage is impressed across the interrupter  10 . This may be achieved by differing numbers of sheds of various diameters as long as the overall total creepage distance is achieved. Further, the sheds need not be evenly spaced. 
     The circuit interrupter  10  includes relatively movable contacts  16 ,  18  carried within the interior  15  of the housing  12 . The contacts  16 ,  18 , which are shown in an open position in FIG. 1 so as to define an open gap d 1 , are connected to respective terminals of the electrical power system. In such electrical power systems, the contacts  16 ,  18 , when open, as shown in FIG. 1, may be energized at significantly different system voltage potentials. The external electric field stresses must not exceed the breakdown level of air in order to pass BIL testing and ensure the avoidance of flashovers. As the diameter of the housing  12  is decreased to minimize size and material usage, high electrical field stresses inside the housing  12  begin to appear on the outside  13  of the housing  12 . 
     In accordance with important features of the present invention and referring now to FIG. 2, it has been found that these external electric field stresses may be reduced and thus a housing  52  of smaller diameter can be utilized for a circuit interrupter  50 . These features of the present invention are achieved by the provision of a predetermined grouping of the external sheds  54 , e.g.  54   a  to  54   h,  which may also be characterized as a non-uniform pattern. The predetermined pattern includes the spacing and/or grouping of the external sheds  54 . For example, in the illustrative embodiment of FIG. 2, the sheds  54  are grouped in the vicinity of and about the open gap of the contacts  16 ,  18 , as denoted by groups  56 ,  58 . Additionally, it has been found that it is preferable to have a minimum spacing d 2  between the grouped sheds,  56 ,  58  that is greater than or equal to the open gap d 1  defined between the contacts  16 ,  18  when in the open position. 
     For example, the arrangement of FIG. 1 results in relatively high and undesirable electrical field stresses on the exterior  13  of the housing  12  in the vicinity of the contacts  16 ,  18  while in FIG. 2 the electric field stresses are satisfactorily reduced. Specifically, for a BIL rating of 650,000 volts, while the stresses in the arrangement of FIG. 1 result in electrical field stress on the exterior  13  of the housing  12  that exceed 4000 volts/mm, this is reduced to approximately 3500 volts/mm for the interrupter  50  of FIG. 2, i.e. on the exterior  53  of the housing  52 . 
     In accordance with other important aspects of the present invention, it should be noted that the spacing of each shed  54  in each of the groups  56 ,  58  is substantially less than that which results from an even spacing of the number of sheds  14  along the housing  12  of the interrupter  10  of FIG. 1 that are required to meet the BIL rating. Accordingly, the present invention can be practiced by using the required number of sheds  54  to meet BIL ratings and spacing the sheds  54  in a non-uniform manner to achieve the groups  56 ,  58  and arranging the remaining number of sheds  54  at various positions along the exterior  53  of the housing  52 , e.g. uniformly or non-uniformly. Again, it should be noted that the grouping of the sheds  54  as illustrated by FIG. 2 is not required either for exterior environmental reasons or for internal reasons when the interrupter contacts  16 ,  18  are closed. Instead, the grouping of the sheds  54  is desirable to minimize stress on the exterior  53  of the housing  52  when the contacts  16 ,  18  are open. 
     While the spacing d 2  between the groups of sheds  56 ,  58  as shown in FIG. 2 may be varied without undesirable effects, e.g. in a range of 10-20% from that shown in FIG. 2, substantially greater or less spacing than that illustrated in FIG. 2 does begin to increase the electrical field stress and reduce the desirable features achieved by the grouped sheds. Additionally, it has been found that the geometry of the contacts  16 ,  18  also influences the desirable spacing of the groups  56 ,  58 . For example, for a contact such as  16  that includes a relatively abrupt increase in size at  17  benefits from a spacing d 2  that is larger and offset toward the contact  16  than if the contact  16  were of uniform diameter at the region  17 . The spacing between each shed  54  within the groups  56 ,  58  must also not be too large, e.g. less than a substantial portion of the open gap distance d 1  in order to avoid undesirable electrical field stress from occurring on the exterior of the housing  54 . 
     In accordance with other important aspects of the present invention and referring now additionally to FIGS. 3-5, the scope of the present invention is illustrated by different patterns, sizes, and/or grouping of sheds including sheds of diverse diameter on a particular housing. For example, the circuit interrupter  100  of FIG. 3 with housing  102  illustrates the general arrangement as in FIG. 2 with groupings  56 ,  58  about the contacts  16 ,  18  and a relatively uniform spacing of sheds  54  along the remaining portions  103  of the housing  102 , all of the sheds  54  being of the same diameter. This is useful where it is desirable to utilize only one size shed, e.g. for molding or manufacturing purposes. The interrupter  150  of FIG. 4 illustrates the use of larger diameter sheds  154  as compared to the sheds  54  of FIG. 3, with groupings  156 ,  158  about the open gap of the contacts  16 ,  18 . For this specific illustrative embodiment, it has been found useful to space the sheds  154  within the groupings  156 ,  158  so as to minimize electrical field stress on the exterior  153  of the housing  152  in the vicinity of the open gap. For example, the sheds  154  within each of the groupings  156 ,  158  include non-uniform spacing such that the spacing between the sheds  154   b  and  154   c  is greater than the spacing between the sheds  154   a  and  154   b  and the spacing between the sheds  154   c  and  154   d,  which are more closely spaced such as the smaller sheds  54  in FIG.  2 . The arrangement of FIG. 5 illustrates the use of both the smaller sheds  54  and the larger sheds  154 . Specifically, as before, the sheds are grouped about the open gap of the contacts  16 ,  18  with groupings  204 ,  206 . Each of the groupings  204 ,  206  utilizes a smaller shed  54  adjacent the contacts  16 ,  18  respectively and two larger sheds  154 . This arrangement has been found useful to minimize the electric field stress on the exterior  203  of the housing  202  in the vicinity of the open gap of the contacts  16 ,  18  of the interrupter  200 . It should be noted that the arrangement of FIG. 5 utilizes only 3 sheds per grouping to achieve the same results as the arrangement in FIGS. 3 and 4 that require 4 sheds per grouping. 
     While there have been illustrated and described various embodiments of the present invention, it will be apparent that various changes and modifications will occur to those skilled in the art. Accordingly, it is intended in the appended claims to cover all such changes and modifications that fall within the true spirit and scope of the present invention.