Electrically conductive shield for a vacuum switching device

Abstract: An electrically conductive shield for a vacuum switching device, e.g. a vacuum interrupter, has a self-centering means for centering the shield within a ceramic insulator surrounding the shield. The self-centering means may comprise three outwardly facing domes (33) formed as "dimples" in the wall of the shield (19; 20). These domes make a clearance fit with the ceramic insulator (8) when cold, however when heated in order to effect brazing of the component parts of the interrupter, the domes (33) make first an interference fit with the ceramic while the shield "floats" relative to the ceramic, then are distorted by a further increase in temperature, the shield being then self-centred within the interrupter. The shield may consist of two shield members (19, 20), the first being as described above, the second having an outwardly turned lip (40) which is a clearance fit when cold, but causes distortion of an end-portion of the second shield member (20) at elevated temperature and acts to centre the second shield member (20).

Description: The invention concerns an electrically conductive shield for a vacuum switching device, in particular a vacuum interrupter, also a unit to be assembled by brazing and a method for assembling a unit by brazing. Vacuum interrupters are commonly used in electrical equipment for interrupting an AC supply in the event of a fault, e.g. a short-circuit on a power line. A typical vacuum interrupter is shown in very general terms in Figure 1. The interrupter comprises an insulator 10, normally a ceramic, housing two electrically conductive contacts 11, 12. Contacts 11, 12 are taken out of the interrupter unit by means of respective stems 13, 14, the stems terminating in end-portions 15, 16 for connection to further electrical equipment (not shown). The end-portions 15, 16 may have external or internal threads for effecting the connections. Also included in the interrupter is a bellows unit 17 and a shield 18. The bellows unit 17 allows axial movement of the stem 14 to make and break, selectively, electrical contact between the contacts 11 and 12, contact 11 and stem 13 being fixed relative to the insulator 10. The shield 18 is an electrically conductive component which serves two main purposes: to prevent an arc, which is drawn when the contacts are separated, from striking the insulator, and to impede the deposition of metal vapour, which is given off from the contacts when the arc is present, on the insulator. Document EP 0 409 047 discloses an assembly according to the preamble of claim 1. It is known to assemble a vacuum interrupter in a single brazing process, in which the various components are positioned as required in an external jig, with brazing discs between those items to be brazed, and the whole jigged arrangement is heated up to a temperature at which the brazing agent will flow and the temperature then reduced so that the brazing agent solidifies, the components of the interrupter being then set together to form a complete unit. A problem with this known procedure, however, is that internal items such as the shield are not normally alignable using the jig, so that during brazing, because the brazing agent is a liquid, these non-alignable components can move relative to the jig and assume non-central positions within the interrupter. Where this occurs with the shield components 18, the result can be unbalanced electrical stresses on the insulator 10, for example. In a first aspect of the invention, there is provided an assembly as defined in claim 1. The shield may be substantially circular cylindrical, said means for effecting self-centering comprising three outwardly facing domes formed from said shield as dimples therein, said domes being disposed at substantially equal distances around a circumference of said shield. The domes may be formed from a portion of the shield which lies substantially parallel to a longitudinal axis of the shield. The shield may comprise first and second shield members, said first and second shield members each comprising means for effecting self-centering of the respective shield member within the vacuum interrupter upon application of heat to said shield. Said shield members may be substantially circular cylindrical, a portion of at least said first shield. member being substantially parallel to a longitudinal axis of the shield, said means for effecting self-centering of said first shield member comprising three outwardly facing domes made from said portion of said first shield member as dimples therein, said domes being disposed at substantially equal distances around a circumference of said portion, and said means for effecting self-centering in said second shield member comprising an outwardly turned lip at one end of said second shield member. Said first and second shield members may have corresponding first ends to be positioned adjacent each other, said substantially parallel portion of said first shield member being disposed at the first end of said first shield member and said outwardly turned lip of said second shield member being disposed at the first end of said second shield member. The first shield member may comprise an attachment means for allowing attachment of said first shield member to an insulating wall of the vacuum interrupter. The attachment means may comprise a brazable, outwardly facing lip disposed at said first end of said first shield member. A maximum outside diameter of said first shield member, taking into account said domes, may be substantially the same as an outside diameter of the outwardly turned lip of said second shield member. The shield may be made from a soft, high-expansion material, which may be copper. In another aspect of the invention, there is provided a vacuum switching device to be assembled by brazing, as defined in claim 14. The means for effecting self-centering may comprise three outwardly facing domes formed from said inner element as dimples therein, said domes being disposed at substantially equal distances around a circumference of said inner element, said inner element having when cold, and taking account of said domes, a maximum outside diameter less than an inside diameter of said housing arrangement. The domes may be formed from a portion of said inner element which lies substantially parallel to a longitudinal axis of the inner element. Said housing arrangement may comprise two outer, substantially circular cylindrical, concentric elements, and said attachment means may comprise a brazable, outwardly facing lip for sandwiching between said outer elements. Said inner element may be composed of a softer material than said housing arrangement. Said inner element may be made from copper, and said housing arrangement may be made from a ceramic. In a brazed state of the unit, said lip of said inner element may be secured by brazing between said outer elements and said domes may touch one of said outer elements and be distorted at a point of contact with said one of said outer elements. In get another aspect of the invention, there is provided a method of assembling a vacuum switch device by brazing, as defined in claim 21. Said device may comprise a further circular cylindrical inner element having at one end thereof an outwardly facing lip, said lip of said further inner element having when cold a third, outside diameter less than said first, inside diameter, the method including after step (a) the step of positioning said further inner element on top of said inner element such that the lip of said further inner element touches the lip of said inner element, the method in step (b) allowing said third, outside diameter to increase until the lip of said further inner element touches one of said outer elements, and the method in step (c) allowing an end-portion of said further inner element adjacent said lip to distort in shape during said increase in temperature. Said device may be a vacuum switching device, said outer elements may be ceramic insulators and said inner elements may constitute a conductive shield. An embodiment of the invention will now be described, by way of example only, with reference to the drawings, of which: Figure 1 is a perspective and partially cutaway view of a typical vacuum interrupter; Figure 2 is a side view in partial cross-section of a vacuum interrupter incorporating a shield in accordance with a first aspect of the invention; Figure 3a is a perpective view of a shield according to the invention, namely a first shield member (a) and a second shield member (b). Figure 4 is a top view of the first shield member of the vacuum interrupter of Figure 2; Figure 5 is a side view in partial cross-section of a jigging arrangement for the assembling of the vacuum interrupter shown in Figure 2, and Figure 6 shows the effect of elevated temperature on the shield members within the vacuum interrupter of Figure 2. Referring now to Figure 2, a vacuum interrupter according to an aspect of the invention is illustrated comprising a pair of contacts 11, 12 on stems 13, 14 (these components being shown by dotted lines) with associated end-stubs 15, 16, two outer ceramic insulator elements 8, 9 making up an insulator housing arrangement 10, and an electrically conductive shield 18 consisting of first and second shield members 19, 20. (Like components are given the same reference designators throughout the drawings). Two end-rings 23 and 24 form anchoring points for their associated stems and end-stubs relative to the ceramic insulators 8, 9. The whole interrupter assembly is meant to be brazed together in one operation, and in order to maintain a central position of the shield members within the assembly, the shield members are equipped with self-centering means as shown in Figure 3. Figure 3(a) shows the preferred form of the first shield member 19, which is circular cylindrical like the ceramic insulator elements and includes portions 30, 31 substantially parallel to a longitudinal axis 26 of the assembly and non--parallel, i.e. tapering, portions 32, 33. The most notable feature of this shield member is the presence of small domes 33 (see also Figure 1) in the substantially parallel portion 30 of the shield. Domes 33 are made from the shield wall itself and form dimple-like structures viewed from inside the shield member 19 and roughly hemispherical projections when viewed from outside the shield member. The height of the domes from the wall of the shield portion 30 is such that an outside diameter 34 of the shield member is, when the assembly is cold and in an unbrazed state, slightly less than an inside diameter of the ceramic insulator elements 8, 9 and significantly less than an outside diameter 35 of a lip 36 formed in the end of the shield member 29 nearest the domes 33 (see Figure 4). The lip 36 is designed to be sandwiched between brazable end-faces of the insulator elements 8, 9, and to this end the outside diameter 35 is arranged to be approximately equal to the outside diameter of the insulator elements 8, 9 (see Figure 2). Figure 3(b) shows a view of the second shield member 20. This member has an outwardly turned lip 40 which is approximately of the same outside diameter as that of the first shield member 19, taking into account the domes 33, i.e. the outside diameter 34. The procedure for assembling the vacuum interrupter will now be described with reference to Figure 5. The interrupter is assembled with the aid of a jig 50, which comprises a base 51, three location rods 52 (only two are shown) fixed to the base 51 at equidistant points around its circumference and three location discs 53. The lower two discs 53 are maintained at a desired spacing relative to each other by way of spacers 54 (only one is shown). Assembly is commenced by the insertion of the end-stub 15 of the interrupter into the base 51. The fixed end-ring 23 (i.e. the end-ring associated with the fixed contact 11) is then placed on top of the end-stub 15 and the fixed conductor arrangement consisting of the contact 11 and the stem 13 is mounted on the end-ring 23. Next, the fixed-end ceramic 8 is positioned on top of the end-ring 23, being guided by the lowest of the three location discs 53, and the first shield member 19 is positioned on top of the ceramic 8 so that the lip 36 rests on the upper brazable surface of the ceramic 8. The domes 33 on the shield member 19 are dimensioned so that there is a clearance fit between that member and the ceramic element 8 when the member 19 is positioned in the jigged assembly. Next, the moving conductor arrangement consisting of the contact 12 and the stem 14 is placed on top of the contact 11, followed by positioning of the second shield member 20 on top of the first shield member 19. The upper ceramic element 9 is then placed on top of the lip 36 of the first shield member, being located by the middle locating ring 53. Again, there is a clearance fit between the second shield member and the upper ceramic element 9. A bellows unit (not shown) is then fitted, then the end-stub 16. The bellows unit is held in position by the upper locating ring 53. Finally, the end-ring 24 is mounted on the upper ceramic 9. It should be understood that, at all the junctions where brazing is to take place, a brazing ring is supplied in series with the components concerned. Once all the components are in position, a large weight 55 is placed on top of the end-ring 24 to squeeze the outer joints (e.g. the joint between the ceramic elements 8 and 9 and the lip 36 of the first shield member 19). The whole jigged assembly is then placed into an oven, which is evacuated down to 10 -6 mbar and then heated to a temperature of up to 800°C. As the temperature is raised, both the shield members 19, 20 and the insulator elements 8, 9 expand, but at different rates, the shield members, being copper, expanding at a greater rate than the ceramic elements 8, 9. At a particular temperature, the brazing agent melts, the shield members start to "float" with respect to the ceramic insulators and, as the temperature increases, the domes 33 of the first shield member and the lip 40 of the second shield member touch the inner walls of their respective ceramic elements to provide an interference fit between these parts, the shield members being then centralised within the assembly. A further increase in temperature causes both the domes 33 and the lip 40 to strain against the ceramic walls; however, because the shield is made from soft copper, the domes and the end-portion of the second shield member adjacent the lip both distort, so that no damage is caused to the ceramic. Figure 6 shows the distortion process of the first and second shield members, whereby the domes 33 are flattened against the ceramic 8, while the end-portion 41 of the second shield member 20 near the lip 40 distorts as shown. The assembly is then allowed to cool to a temperature at which the brazing agent sets and the assembly is made rigid. Upon further cooling, the metal parts of the assembly contract toward their normal dimensions, except those parts which are secured by brazing, e.g. the lip 36 of the first shield member 19 and the lip 40 of the second shield member 20. Although the domes on the first shield member have been described as being essentially hemispherical in shape, in practice any convenient shape may be employed which allows easy deformation on contact with the ceramic housing:

Claims: An assembly comprising an outer wall (8) of a vacuum switching device and an electrically conductive shield (19;20) within said outer wall, characterised in that said shield has centring means (33;40) projecting therefrom for effecting self-centring of said shield within said vacuum switching device upon application of heat to said assembly, said centring means (33;40) being deformable against said outer wall (8) during differential thermal expansion of said shield relative to said outer wall upon said application of heat, said centring means being dimensioned relative to said outer wall such that there is a clearance fit between said shield (19;20) and said outer wall (8) before said application of heat. An assembly as claimed in Claim 1, in which said shield is substantially cylindrical and said centring means for effecting self-centring comprises three outwardly facing domes (33) formed from said shield as dimples therein, said domes being disposed at substantially equal distances around a circumference of said shield. An assembly as claimed in Claim 2, in which said domes (33) are formed from a portion of the shield (30) which lies substantially parallel to a longitudinal axis of the shield. An assembly according to any preceding claim, in which the electrically conductive shield comprises first and second shield members (19,20), said first and second shield members each having centring means (33,40) thereon for effecting self-centring of the respective shield member within the vacuum switching device upon application of heat to said assembly. An assembly as claimed in Claim 4, in which said shield members are substantially cylindrical, said centring means for effecting self-centring of said first shield member (19) comprising three outwardly facing domes (33) formed from said first shield member as dimples therein, said domes being disposed at substantially equal distances around a circumference of said shield member, and said centring means for effecting self-centring in said second shield member (2) comprising an outwardly turned lip (40) at one end of said second shield member. An assembly as claimed in Claim 5, in which said domes (33) are formed from a portion of said first shield member (30) which lies substantially parallel to a longitudinal axis of the shield. An assembly as claimed in Claim 6, in which said first and second shield members (19,20) have corresponding first ends to be positioned adjacent each other, said substantially parallel portion (30) of said first shield member being disposed at the first end of said first shield member and said outwardly turned lip (40) of said second shield member being disposed at the first end of said second shield member. An assembly as claimed in Claim 7, in which said first shield member (19) comprises an attachment means (36) for allowing attachment of said first shield member to an insulating wall (10) of the vacuum interrupter. An assembly as claimed in Claim 8, in which said attachment means comprises a brazeable, outwardly facing lip (36) disposed at said first end of said first shield member (19). An assembly as claimed in Claim 8, in which a maximum outside diameter (34) of said first shield member (19), taking into account said domes (33), is substantially the same as an outside diameter of the outwardly turned lip (40) of said second shield member. An assembly as claimed in any one of the preceding claims, in which the shield is made from a soft, high-expansion material. An assembly as claimed in Claim 11, in which said material is copper. A vacuum switching device, comprising an assembly as claimed in any one of the preceding claims. A vacuum switching device to be assembled by brazing, comprising an outer, substantially cylindrical housing arrangement (10) and an inner, substantially cylindrical shield element (19) disposed within said housing arrangement, said inner element comprising an attachment means (36) for attaching said inner element to said housing arrangement by brazing, characterised in that said inner element (19) has a higher coefficient of thermal expansion than said housing arrangement, and in that centring means (33) project from the inner element for effecting self-centring of the inner element within said housing arrangement upon application of heat to said device, said centring means (33) being deformable against said housing arrangement (10) during differential thermal expansion of said shield relative to said outer housing arrangement upon said application of heat, said centring means being dimensioned relative to said housing arrangement such that there is a clearance fit between said shield element (19) and said housing arrangement before said application of heat. A vacuum switching device as claimed in Claim 14, in which said centring means for effecting self-centring comprises three outwardly facing domes (33) formed from said inner element as dimples therein, said domes being disposed at substantially equal distances around a circumference of said inner element, said inner element having when cold, and taking account of said domes, a maximum outside diameter less than an inside diameter of said housing arrangement. A vacuum switching device as claimed in Claim 15, in which said domes (33) are formed from a portion of said inner element (30) which lies substantially parallel to a longitudinal axis of the housing arrangement. A vacuum switching device as claimed in Claim 16, in which said housing arrangement comprises two outer, substantially cylindrical, concentric elements (8,9), and said attachment means comprises a brazable, outwardly facing lip (36) for sandwiching between said outer elements. A vacuum switching device as claimed in any one of Claims 14 to 17, in which said inner element is composed of a softer material than said housing arrangement. A vacuum switching device as claimed in Claim 18, in which said inner element is composed of copper and said housing arrangement is composed of a ceramic. A vacuum switching device as claimed in Claim 19, in which, in a brazed state of the unit, said lip (36) of said inner element is secured by brazing between said outer elements (8,9) and said domes (33) touch one of said outer elements (8) and are distorted at a point of contact with said one of said outer elements. A method of assembling the vacuum switching device of claim 14 by brazing, the device comprising two outer cylindrical elements (8,9) of a first, internal diameter, the outer elements having brazeable end faces, and an inner cylindrical shield element (19), said shield inner element having a higher coefficient of thermal expansion than, and being of a softer material than, said outer elements and comprising at one end a brazeable, outwardly facing lip (36) substantially perpendicular to a longitudinal axis of said inner shield element, said inner shield element comprising three outwardly facing domes (33) made from said inner shield element as dimples therein, said domes being disposed at substantially equal distances around a circumference of said inner shield element, said inner shield element having when cold, and taking account of said domes, a second, outside diameter (34) less than said first, internal diameter, the method comprising the steps of (a) assembling the device by sandwiching said lip (36) of said inner shield element (19) between adjacent end-faces of said outer elements together with a solid brazing agent; (b) in a vacuum, raising the temperature of the device and allowing said brazing agent to flow and said second, outside diameter (34) to increase until said domes (33) touch one of said outer elements (8), self-centring taking place by a floating of said inner shield element (19) relative to said outer elements (8,9); (c) increasing the temperature still further until said domes (33) distort in shape; (d) lowering the temperature so as to allow said brazing agent to solidify. A method as claimed in Claim 21, in which said vacuum switching device comprises a further cylindrical inner element (20) having at one end thereof an outwardly facing lip (40), said lip of said further inner element (20) having when cold a third, outside diameter less than said first, internal diameter, the method including after step (a) the step of positioning said further inner element (20) on top of said inner element (19) such that the lip (40) of said further inner element touches the lip (36) of said inner element (19), the method in step (b) allowing said third, outside diameter to increase until the lip (40) of said further inner element (20) touches one of said outer elements, and the method in step (c) allowing an end-portion of said further inner element (20) adjacent said lip (40) to distort in shape during said increase in temperature. A method as claimed in Claim 21 or Claim 22, in which said outer elements are ceramic insulators and said inner elements constitute a conductive shield.

Inventors: FALKINGHAM LESLIE THOMAS, RAND JOHN, FALKINGHAM,LESLIE THOMAS, RAND, JOHN
Applicants: ALSTOM UK LTD
Application year: 1996
Main classification: H01H33
IPC codes: H01H33