GB2308498A

GB2308498A - Electrically conductive shield for a vacuum switching device

Info

Publication number: GB2308498A
Application number: GB9526232A
Authority: GB
Inventors: John Rand; Leslie Thomas Falkingham
Original assignee: GEC Alsthom Ltd
Current assignee: Alstom UK Ltd
Priority date: 1995-12-21
Filing date: 1995-12-21
Publication date: 1997-06-25
Anticipated expiration: 2015-12-21
Also published as: EP0780868A2; EP0780868B1; DE69631641D1; EP0780868A3; DE69631641T2; GB2308498B; GB9526232D0

Description

1 ELECTRICALLY CONDUCTIVE SHIELD FOR A VACUUM SWITCHING DEVICE 2308498 lhe

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. 'Me 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). 'Ite 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. lle 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.

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 2 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 electrically conductive shield for a vacuum interrupter unit, said shield comprising means for effecting self- centering of the shield within said vacuum interrupter unit upon application of heat to said shield.

The shield may be substantially circular cylindrical, said means for effecting selfcentering 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 selfcentering 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.

3 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 5 copper.

In a second aspect of the invention, there is provided a vacuum switching device, comprising an electrically conductive shield as described above.

In a third aspect of the invention, there is provided a unit to be assembled by brazing, comprising an outer, substantially circular cylindrical, housing arrangement and an inner, substantially circular cylindrical element disposed within said housing arrangement, said inner element having a higher coefficient of thermal expansion than said housing arrangement, said inner element comprising an attachment means for attaching said inner element to said housing arrangement by brazing and a means for effecting self-centering of the shield within said unit upon application of heat to said unit.

Said 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.

4 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 a fourth aspect of the invention, there is provided a method of assembling a 5 unit by brazing, the unit comprising two outer circular cylindrical elements of a first, internal diameter, the outer elements having brazable end faces, and an inner circular cylindrical element, said 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 brazable, outwardly facing lip substantially perpendicular to a longitudinal axis of said inner element, said inner element comprising three outwardly facing domes made 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 said domes, a second, outside diameter less than said first, internal diameter, the method comprising the steps of:

(a) assembling the unit by sandwiching said lip of said inner element between adjacent end-faces of said outer elements together with a solid brazing agent; (b) in a vacuum, raising the temperature of the unit and allowing said brazing agent to flow and said second, outside diameter to increase until said domes touch one of said outer elements, self-centering taking place by a floating of said inner element relative to said outer elements; (c) increasing the temperature still further until said domes distort in shape; (c) lowering the temperature so as to allow said braising agent to solidify.

Said unit 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 unit 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 6 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 Up 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. Ilis 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 5 1, 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 endstub 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 7 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 5 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).

lhe 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 8000C.

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 0 8 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 5 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.

While the invention has been mainly described in terms of a vacuwn interrupter, which is its main application, it may find application in any apparatus which is to be assembled by brazing and which includes a substantially circular cylindrical inner element to be centred within a substantially circular cylindrical outer element, the latter element having a lower coefficient of thermal expansion than the former element.

9

Claims

1. An electrically conductive shield for a vacuum interrupter unit, said shield comprising means for effecting self-centering of the shield within said vacuum interrupter unit upon application of heat to said shield.

2. An electrically conductive shield as claimed in Claim 1, in which the shield is substantially circular cylindrical and said means for effecting self-centering comprises 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.

3. An electrically conductive shield as claimed in Claim 2, in which said domes are formed from a portion of the shield which lies substantially parallel to a longitudinal axis of the shield

4. An electrically conductive shield for a vacuum interrupter unit, the shield comprising 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.

5. An electrically conductive shield as claimed in Claim 4, in which said shield members are substantially circular cylindrical, said means for effecting selfcentering of said first shield member comprising three outwardly facing domes 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 means for effecting self-centering in said second shield member comprising an outwardly turned lip at one end of said second shield member.

6. An electrically conductive shield as claimed in Claim 5, in which said domes are formed from a portion of said first shield member which lies substantially parallel to a longitudinal axis of the shield.

7. An electrically conductive shield as claimed in Claim 6, in which said first and second shield members 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 tumed lip of said second shield member being disposed at the first end of said second shield member.

8. An electrically conductive shield as claimed in Claim 7, in which said first shield member comprises an attachment means for allowing attachment of said first shield member to an insulating wall of the vacuum interrupter.

9. An electrically conductive shield as claimed in Claim 8, in which said attachment means comprises a brazable, outwardly facing lip disposed at said first end of said first shield member.

10. An electrically conductive shield as claimed in Claim 8, in which a maximum outside diameter of said first shield member, taking into account said domes, is substantially the same as an outside diameter of the outwardly tumed lip of said second shield member.

11. An electrically conductive shield as claimed in any one of the preceding claims, in which the shield is made from a soft, high-expansion material.

12. An electrically conductive shield as claimed in Claim 11, in which said material is copper.

13. A vacuum switching device, comprising an electrically conductive shield as claimed in any one of the preceding claims.

1

14. A unit to be assembled by brazing, comprising an outer, substantially circular cylindrical housing arrangement and an inner, substantially circular cylindrical element disposed within said housing arrangement, said inner element having a higher coefficient of thermal expansion than said housing arrangement, said inner element comprising an attachment means for attaching said inner element to said housing arrangement by brazing and a means for effecting self-centering of the inner element within said housing arrangement upon application of heat to said unit.

15. A unit as claimed in Claim 14, in which and said means for effecting selfcentering comprises 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.

16. A unit as claimed in Claim 15, in which said domes are formed from a portion of said inner element which lies substantially parallel to a longitudinal axis of the housing arrangement.

17. A unit as claimed in Claim 16, in which said housing arrangement comprises two outer, substantially circular cylindrical, concentric elements, and said attachment means comprises a brazable, outwardly facing lip for sandwiching between said outer elements.

18. A unit 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.

19. A unit as claimed in Claim 18, in which said inner element is composed of copper and said housing arrangement is composed of a ceramic.

12

20. A unit as claimed in Claim 19, in which, in a brazed state of the unit, said lip of said inner element is secured by brazing between said outer elements and said domes touch one of said outer elements and are distorted at a point of contact with said one of said outer elements.

21. A method of assembling a unit by brazing, the unit comprising two outer circular cylindrical elements of a first, internal diameter, the outer elements having brazable end faces, and an inner circular cylindrical element, said 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 brazable, outwardly facing lip substantially perpendicular to a longitudinal axis of said inner element, said inner element comprising three outwardly facing domes made 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 said domes, a second, outside diameter less than said first, internal diameter, the method comprising the steps of:

(a) assembling the unit by sandwiching said lip of said inner element between adjacent end-faces of said outer elements together with a solid brazing agent; (b) in a vacuum, raising the temperature of the unit and allowing said brazing agent to flow and said second, outside diameter to increase until said domes touch one of said outer elements, self-centering taking place by a floating of said inner element relative to said outer elements; increasing the temperature still further until said domes distort in shape; lowering the temperature so as to allow said braising agent to solidify.

(c) (d)

22. A method as claimed in Claim 21, in which said unit comprises 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, internal 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 13 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 5 temperature.

23. A method as claimed in Claim 21 or Claim 22, in which said unit is a vacuum switching device, said outer elements are ceramic insulators and said inner elements constitute a conductive shield.

24. An electrically conductive shield substantially as hereinbefore described 10 with reference to Figures 3, 4 and 6 of the drawings.

25. A unit to be assembled by brazing, substantially as hereinbefore described.

26. A method of assembling a unit by brazing, substantially as hereinbefore described.