GB2310760A - Vacuum switching device - Google Patents

Abstract

A vacuum switching device incorporates an electrically conductive shield 30 which extends beyond at least one end of a ceramic insulator housing 20. The result of this is reduced electrical stress particularly at the triple point 41 connecting the relevant end of the insulator and an end-ring 33, leading to a reduction in the length requirements for the switching device. Further, because the deposition of metal vapour onto the ceramic wall is virtually eliminated, the device does not have to be over-rated to take into account the degradation in its operating parameters that would occur during the lifetime of the device if the shield were of conventional design.

Description

VACUUM SWITCHING DEVICE The invention concerns a vacuum switching device, and in particular a vacuum interrupter or vacuum switch comprising an insulating housing and an internal shield.

Vacuum interrupters or vacuum switches (from this point on these devices will be referred to under the term "vacuum interrupters" only) are commonly used in electrical equipment for interrupting an AC supply in the event of a fault, e.g. a shortcircuit on a power line, as well as for performing switching duties at normal currents.

A typical vacuum interrupter is shown in very general terms in Figure 1, which is shown partially cut away for clarity. The interrupter comprises an insulator 10, which is normally a ceramic, housing two electrically conductive contacts 11, 12. Contacts 11, 12 are interfaced to external electrical equipment (not shown) by means of respective stems 13, 14, the stems normally terminating in screw-threaded portions 15, 16 for connection to the external equipment.

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, which is contained fully within the housing 10, 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 10.

In accordance with the invention, there is provided a vacuum switching device comprising an insulating housing, a contact means inside said housing for making or breaking electrical contact, and an electrically conductive shield disposed between said contact means and said housing, said shield forming a substantially continuous conductive surface which extends beyond said housing at at least one end of said housing.

The shield may extend beyond said housing by at least 2 mm.

An electrically conductive end-ring may be attached to said insulating housing at the end of the housing at which the shield protrudes, or at both ends if the shield protrudes at both ends. The end-ring is spaced apart from the 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 switching device; Figure 2 is a side view in partial cross-section of a preferred embodiment of a vacuum switching device in accordance with the invention, and Figure 3 is a diagram of a partial section of a typical vacuum switching device showing the effect of contact arcing on the device.

Referring now to Figure 2, a preferred embodiment of a vacuum switching device according to the invention is illustrated comprising a pair of contacts 21, 22 on respective stems 23, 24, two ceramic insulator elements 28, 29 making up an insulator housing 20, a bellows unit 27 anchored at one end to the ceramic housing via an end ringlguide arrangement 31 and at the other end to the stem 24 via a plate 32, and an electrically conductive shield arrangement 30. An end-ring 33 forms an anchoring point for the fLsed-end stem 23, while the end-ring/guide arrangement 31 at the other end of the switching device provides a guide for movement of the moving-end stem 24 towards or away from the fixed-end stem 23.

The shield arrangement 30 is secured by brazing to the insulator housing 20 at the junction 26 of the two insulator elements 28, 29 and, in the preferred embodiment illustrated, comprises cylindrical sections 35, 36 which are substantially parallel to the insulator arrangement 20 and one or more further sections 37, 38 which are tapered in form. The shield 30 may either be of one piece or comprise two pieces positioned one on top of the other in electrical contact with each other, or comprise any other suitable arrangement.

The upper part 39 of the shield arrangement 30 is fully inside the housing 20 below the bellows 27, whereas the lower part 40 of the shield arrangement protrudes beyond the lower ceramic 28, but without touching the end-ring 33. In the preferred embodiment the shield protrudes at least 2 mm beyond the housing.

Due to the protrusion of the shield, electrical stress is minimised at the so-called "triple point" 41. A triple point is an interface between a conductor, an insulator and a vacuum, and there are four triple points associated with the switching device, namely one at each end of the ceramic elements 28, 29, but it is the electrical behaviour at the triple point at the fixed-contact end 41 of the ceramic housing which is most affected by the shield design of the present invention.

The triple points are stressed electrically by the applied voltage if the conductor at that triple point is electrically connected, either directly or capacitively, to one of the fixed and moving conductors 21, 22. Breakdown across a solid insulator (the ceramic elements 28, 29) in a vacuum starts at the triple point. Once breakdown begins at that point, an avalanche effect occurs causing catastrophic breakdown across the whole insulator. The triple points between the middle part 26 of the shield and the ceramic elements 28, 29 are only moderately stressed and do not, therefore, present much of a problem. Similarly, the triple point 42 at the bellows end is in a very low stress region.

By contrast, the triple point 41 is subject to quite high stresses with a conventional shield design, and it is here that breakdown can begin.

Figure 3 shows the effect of metal deposition on the stress pattern of the ceramic insulator where a conventional shield design is employed. In Figure 3a, a potential difference exists between the conductor 33, which is the end-ring of the switching device connected to the fixed-contact stem 23, and the shield 30 at its central part 26.

It is assumed that the switching device has not yet been used and there has therefore been no arcing between the contacts. Due to the absence of any metal deposition on the ceramic element 28, the equipotential lines 43 shown in Figure 3a are fairly evenly spaced out along the ceramic element, such that there is only a moderate stress at the triple point 41. If now arcing is allowed to occur by operation of the contacts in actual use, a metallisation layer 44 will build up on the inside of the ceramic element 28 (see Figure 3b), such that bunching of the equipotential lines will occur at either side of the layer, but in particular at the triple point 41. In addition two new triple points are created, one at each end of the metallised layer.

The present invention avoids this problem by arranging for the shield to be extended beyond the end of the ceramic housing at the fixed-contact end of the switching device. This effectively precludes the formation of a metallisation layer on the inside face of the ceramic housing near that point, the result being the maintenance of a fairly even stress along the ceramic wall.

It will be noted that the moving-end ceramic element 29 (see Figure 2) is longer than the fixed-end element 28. This is to accommodate the bellows arrangement 27.

However, the invention includes within its scope a configuration in which the shield extends beyond the ceramic housing at both ends (this is assuming an appropriate disposition of the bellows 27). When this is the case, a considerable benefit occurs in that the reduction in dimensions of the switching device for a given voltage can be very considerable. As it is, the ceramic housing in the device shown in Figure 2 is already shorter than would be the case using a conventional shield design.

A further, related advantage of the present shield design is that the switching device does not need to be derated, as does a device using a conventional shield design.

Normally, a switching device, e.g. a vacuum interrupter, which is to be rated at a particular operating voltage capability has to be constructed to be able to withstand an impulse test of significantly higher voltage level; this results from degradation of the device during the time it is in service due to the above-described effects of metal deposition on the ceramic wall. In the design of Figure 2, however, no such degradation in performance takes place and the device therefore does not need to be "over-engineered".

Claims (4)

1. A vacuum switching device comprising an insulating housing, a contact means inside said housing for making or breaking electrical contact, and an electrically conductive shield disposed between said contact means and said housing, said shield forming a substantially continuous conductive surface which extends beyond said housing at at least one end of said housing.

2. A vacuum switching device as claimed in Claim 1, in which said shield extends beyond said housing by at least 2 mm.

3. A vacuum switching device as claimed in Claim 1 or Claim 2, comprising an electrically conductive end-ring attached to said insulating housing at said at least one end thereof, said end-ring being spaced apart from said shield.

4. A vacuum switching device substantially as shown in, or as hereinbefore described with reference to, Figure 2 of the drawings.