GB2356975A

GB2356975A - Vacuum switching device electrodes

Info

Publication number: GB2356975A
Application number: GB9928405A
Authority: GB
Inventors: Rama Shankar Parashar; Bernard Leo Ennis; Leslie Thomas Falkingham
Original assignee: Alstom SA
Current assignee: Alstom SA
Priority date: 1999-12-02
Filing date: 1999-12-02
Publication date: 2001-06-06
Anticipated expiration: 2019-12-02
Also published as: GB2356975B; GB9928405D0

Abstract

A vacuum switching device electrode has a substantially flat contact disc 13 adapted, in use, to contact another electrode wherein the disc 13 has formed therein one or more reinforcing means 28. The reinforcing means 28 are provided on a rear surface of the disc by a localised hardening process. The disc comprises a matrix of a first material containing therein a second material and the hardening process causes a smaller grain size of the matrix. A method of forming such an electrode is also disclosed and a vacuum switching device incorporating such an electrode.

Description

2356975 IMPROVEMENTS RELATING TO VACUUM SWITCHING DEVICE ELECTRODES AND

DEVICES INCORPORATING THEM This invention relates to improvements in vacuum switching device electrodes, in particular, but not exclusively, to electrodes used in vacuum interrupters, or vacuum switches, and also to vacuum switching devices incorporating such electrodes.

Vacuum switching devices adapted to switch large currents have been known for many years. An integral part of such switching devices are electrodes, which are engaged or disengaged to switch the current.

A portion of a typical electrode generally comprises a substantially flat disc. In the engaged position surfaces of two such discs contact one another. To minimise the current density it is desirable to maximise the contact area between the two discs and as such, the discs should be as flat as possible.

The disc of such a typical electrode is generally supported by a cylindrical coil forming means around its perimeter region. The coil forming means provides a current path to the disc, and current flowing in this path modifies the magnetic field surrounding the electrode. It is therefore desirable that as large a percentage as possible of the total current flow passes through the coil forming means. However, because the coil forming means only supports the disc in a perimeter region the centre region of the disc is unsupported which can lead to sagging of the disc with consequent reduction in contact area between discs in the engaged condition.

2 Supports can be placed in the vicinity of the centre region of the disc to reduce the sagging but such supports tend to provide current paths to the disc reducing the current flow through the coil forming means.

There are therefore apparently opposing requirements: the need to prevent sagging of the disc by providing support and the need to maximise current flow through the coil forming means.

According to a first aspect of the invention there is provided a vacuum switching device electrode having a substantially flat contact disc adapted, in use, to contact another electrode wherein the disc has formed therein one or more reinforcing means.

An advantage of such an electrode is that the disc is less likely to sag than an unsupported prior art disc. This can help to provide maximisation of contact area between two such electrodes in an engaged position. In addition, to helping to alleviate sagging of the disc such reinforcing means can help to avoid the need for a support for the disc which may make the electrode cheaper and simpler to manufacture.

The skilled person will appreciate that whilst the disc is generally circular in plan this need not be the case and other forms may be possible.

Perhaps the disc may be square, elliptical, etc.

The reinforcing means may comprise one or more elongate regions formed in the disc. Preferably, there are a number of elongate regions and in one embodiment three such regions are provided. However, it will be appreciated that more or less regions could be provided, perhaps 2, 4, 5, 6 etc. Such elongate regions provide a convenient way of stiffening and may be likened to supporting beams.

3 Conveniently, the elongate regions are provided in areas of the disc in which, in use, high stress occurs in the disc. The skilled person will appreciate that as such, sagging of the disc can be resisted most efficiently.

The disc has a front surface adapted, in use, to contact another electrode, and a rear surface and the reinforcing means is conveniently formed in association with the rear surface. Providing the reinforcing means here is advantageous because the environment around the rear surface is less harsh than that around the front surface. The front surface experiences high temperature plasmas when currents are interrupted which, in use, may be detrimental to the reinforcing means. Further, sagging may be more easily resisted by providing the reinforcing means in association with the side of the sagged disc which would be convex. Further providing reinforcing in the front face may cause the surface area over which the disc can contact another disc to be reduced.

Preferably, the disc is fabricated from a mixture of two or more conducting materials (generally metal) which do not readily form an alloy with one another. Most preferably the disc is fabricated from a mixture of copper (Cu) and chromium (CO. The advantages of such a disc are described in Patent No. GB 1 194 674. In one embodiment the disc contains 60% Cr and 40% Cu (percentages by weight). The skilled person will appreciate that other percentages are equally possible: perhaps about any of the following: 10%, 20%, 30%, 40%, 50%, 55%, 65%, 70%, 80%, 90% Cr (with the remainder being fabricated substantially from Cu).

4 The disc may be fabricated such that the majority of the disc comprises a matrix of one of the conducting materials with the other infiltrating the matrix. As described in GB 1 194 674 such a structure provides beneficial properties for the formation of a plasma between two such electrodes as they interrupt, in use, a current.

The reinforcing means may be provided by regions of the disc which have a material structure different from the majority of the disc. In particular, the reinforcing means may have a finer grain structure than the material in the majority of the disc (much less delineation between the conducting materials forming the disc). Therefore, the reinforcing means may have a greater hardness than the material in the majority of the disc.

Conveniently, the reinforcing means is provided by a material modification process which may include any of the following: electron beam, laser, plasma arcing, chemical vapour deposition. Perhaps the preferred modification process is electrical arcing. Any of the preceding modification processes may be followed by quenching.

Preferably the reinforcing means comprises a hardened region of the disc.

The skilled person will appreciate that it is beneficial to keep such a hardened area of material away from high temperatures which are likely to degrade the hardened properties. Therefore, it is beneficial to provide the reinforcing means in a rear surface of the disc where it will not experience the plasma created when a switching device breaks a current.

The reinforcing means will generally be provided in a surface region of the disc. It may protrude from the surface, or may be flush or may be slightly depressed from the surface. The skilled person should also appreciate that the reinforcing means could be provided within the disc.

One arrangement to achieve such a structure may be to treat the surface regions of two discs and then to bond the discs together. In such a bonded structure the position within the resulting disc may of course be varied by varying the relative thicknesses of the initial discs.

According to a second aspect of the invention there is provided a vacuum switching device having provided therein one or more electrodes according to the first aspect of the invention.

Preferably, the vacuum switching device has two such electrodes provided therein. Such an electrode may have an improved interrupting performance when compared to prior art switching devices.

According to a third aspect of the invention there is provided a method of producing a vacuum switching device electrode including a step of performing material modification processes to one or more regions of a surface region of a contact disc of the electrode.

As discussed hereinbefore, the discs of prior art electrodes have suffered from sagging and the provision of one or more regions which have undergone material modification processes may help to alleviate this problem.

The material modification process may increase the hardness of the 1 material of the disc. This may provide a structure having the necessary properties to increase the resistance to sagging from which prior art discs have suffered.

6 Generally electrode contact discs are provided from a mixture of two immiscible (in the solid state) conducting materials (generally metals). In such cases there exists a matrix of one material infiltrated with the other such structure having a relatively large grain size. The method may reduce the grain size. Indeed, the method may reduce the grain size such that the grains are substantially indiscernible.

The method may comprise heating the surface region of the disc. Indeed, in perhaps the preferred method the surface region is subjected to localised melting. Such a method provides the necessary material modification to help resist sagging.

Suitable material modification processes may include any of the following heating techniques: electron beam, laser, plasma arcing or chemical vapour deposition.

The heating of the surface may be followed by a cooling process. Indeed, localised melting may be followed by a re-solidification process. Suitable cooling/resolidification processes include: ambient air cooling, forced air circulation, forced inert gas circulation, inert liquid cooling (may be by demineralised water).

In perhaps the preferred method the heating may be provided by electrical arcing, wherein an electrical are is struck between an electrode and the disc. The current flowing in the arc may be around 120A. However, the skilled person will appreciate that other currents are possible and around any of the following currents may be suitable (or indeed in ranges between any of the following, or outside these values): 90A, 100A, 110A, 130A, 140A, 160A, 180A.

7 The method may include using additional material to aid material modification of the chemical and/or physical properties of the material.

However, in perhaps the preferred embodiment an arc is struck between an electrode and the disc without any additional material.

The method may increase the relative strength of the material to around twice that of the untreated material. (Possibly around 11/4, 11/2, 13/4, 21/4).

According to a fourth aspect of the invention there is provided a method of manufacturing a vacuum switching device comprising including an electrode fabricated according to the method of the third aspect.

The method may further include the step of heating the device to braze the components to one another. Preferably the modified material has a strength greater than the untreated material after the heating process has been performed. Conveniently, the modified material has a relative strength, compared to the untreated material, after the heating of around any of the following: 1.2, 1.25, 1.3, 1.4, 1.5.

There now follows by way of example only a detailed description of the invention with reference to the accompanying drawings of which:

Figure I shows a schematic longitudinal section through a typical vacuum switching device; Figure 2 shows a schematic longitudinal section through an electrode according to the present invention; Figure 3 shows a plan view of an electrode according to the present invention; 8 Figure 4 shows a photograph of a section through the material of a portion of the electrode; and Figure 5 is a graph showing the strength of material before and after modification.

Although vacuum interrupters are referred to hereinafter, the techniques could equally well be applied to vacuum switches. A typical arrangement for a vacuum interrupter is shown in Figure 1 wherein an evacuated envelope 11 comprising an insulating (generally ceramic) cylinder 1, with metallic end plates 2, 3 joined to opposite ends of the insulating cylinder 1. Within the cylinder 1 there is provided a stationary contact 7 and a moveable contact 9. Each of the contacts 7, 9 has an electrode at an end portion comprising a coil forming means 6, 8 and an engagement means 13, 14.

The contacts 7, 9 are mounted on rod members 4, 5 and the rod member 5 supporting the moveable contact 9 is sealed to the end plate 3 by a bellows 10. Shields 12 are arrange inside the walls of the insulating cylinder 1 to prevent the build up of conducting deposits along the walls of the cylinder 1 which would eventually cause a short circuit.

A vacuum interrupter is connected in a circuit which it is desired to protect. In normal operating conditions the movable contact 9 is positioned such that the engagement means 13, 14 are in contact. In such a condition current can flow through the interrupter. During a fault condition the movable contact 9 is moved away from the stationary contact 7 so as to break the current flow. However, as the skilled person will appreciate the current flow is initially maintained and causes an arc 9 between the separated contacts 7, 9 (current continues to flow through the interrupter).

The coil forming means 6, 8 of each electrode have current paths defined therein which cause current flowing through the interrupter to rotate substantially as if the current were flowing through a coil. This rotating motion sets up a magnetic field which can be designed to tend to break up the arc formed between the separated contacts 7, 9 and aid interruption of the current.

The contact means 13 comprises a substantially flat disc (which can be best seen in Figure 2). The contact means 13 is supported around a perimeter region by a coil forming means 22 which comprises a tubular portion 24 having an end face portion 26 extending underneath the contact means 13. Slits 20 are provided, spiralling around the tubular portion 24 and continuing into the end face portion 26, defining the current paths through the coil forming means 22.

The slits 20 lead to mechanical weakness in the coil forming means 22.

To prevent sagging of the contact means 13 a reinforcing means 28 is provided in the contact means 13. Figure 2 schematically shows the reinforcing means 28 as a layer along an underneath face region of the contact means 13. Whilst a straight reinforcing means as indicated by Figure 2 is possible in some embodiments, the curved shape shown by the shaded regions in Figure 3 may be preferred.

Although shown to be present in Figure 2, a stainless steel cylindrical shell 25 may or may not be provided. This shell provides mechanical support to the coil forming means 22 which is mechanically weak due to the slits 20. However, the shell 25 provides a current path to the disc 13 allowing some current to bypass the coil forming means 22, leading to a reduction in the interrupter's efficiency. The reinforcing means 28 may provide enough strength to allow the shell 25 to be omitted.

In any case, as will be appreciated from Figure 2 the shell 25 only supports the disc 13 at an outer region and the centre region of the disc 13 is unsupported leading to possible sagging. As discussed sagging of the disc 13 leads to a reduction in the contact area between discs 13 of neighbouring electrodes in an assembled interrupter, with an associated, undesirable, increase in current density. The reinforcing means 28 will help to prevent such sagging.

As can be seen in Figure 3, three reinforcing means are provided in the disc of the contact means.

Each of these reinforcing means 28 comprises a portion of a circumference of a circle formed into a lower face of the contact means 13 (i.e. the face that does not contact the contact means 14 of the other electrode). The reinforcing means 13 acts as a beam resisting bending of the disc of the contact means 13.

Each of the reinforcing means 28 comprises a region of the material of the disc in which the properties /structure of the material has been modified and this changed structure is best seen in Figure 4. The disc is provided from a 60% chromium, 40% copper mixture. As can be seen from region 30 of Figure 4 the copper 32 and chromium 34 do not mix and the copper 32 infiltrates a matrix of chromium 34. This structure provides desirable properties as is well known in the art of vacuum switching devices.

11 It can be seen that the mixture in region 32 of Figure 4 has a finer grain structure than the mixture of region 30. This results in a greater hardness and strength thus providing the desired mechanical reinforcement of the disc.

The altered material properties are achieved by localised melting followed by rapid quenching of the material mixture of the disc. As will be appreciated (and is shown in Figure 2) this process affects the material of the disc in a surface region and the effects do not pass through the disc.

In this particular embodiment the localised melting is achieved by striking an arc between an electrode and the surface of the disc. It will be appreciated that it does not matter how far into the disc the process extends from the surface.

Once all of the parts of the interrupter have been fabricated they must be assembled. This assembly involves brazing the separate components to one another and includes the step of heating the interrupter until such brazing is achieved. This heating process has an annealing effect on the reinforcing means 28 reducing their hardness. However, once the brazing process has finished the material within the reinforcing means 28 is still harderlstronger than the surrounding material of the disc.

The graph of Figure 5 shows the relative strength of a mixture of CulCr after various processes have been performed.

The pair of bars labelled 1 shows as a reference that an untreated sample of CulCr has a relative hardness of one (the unshaded bar) and that after the brazing process has occurred (i.e. post "seal off") the strength of the sample has been reduced. Thus, the heating process has an adverse affect on the strength of the disc.

12 The pair of bars labelled 2 shows that by performing a weld on a sample of Cu/Cr using filler material the strength of the sample before the brazing process (pre "seal off") is slightly increased. After seal off the strength of the material again falls but it is greater than the strength of untreated material after seal off.

The remaining three pairs of bars show the strengths of samples before and after seal off after an are has been struck between an electrode and the surface (no filler material having been used). The current flowing in the arc varies between each of the pairs of bars (pair 3; 90 amps: pair 4; 120A; pair 5; 180A). It can be seen that as the current is increased the strength of the material pre-seal off is increased but the reduction in strength post seal off is greater for the higher currents. Indeed, the strength achieved post seal off is about the same for 120 Amps and 180 Amps. The strength of the materials post seal off (for 120 and 180 Amps) is about 25% greater than the untreated material.

13

Claims

1 A vacuum switching device electrode having a substantially flat contact disc adapted, in use, to contact another electrode wherein the disc has formed therein one or more reinforcing means.

2. An electrode according to claim 1 wherein the reinforcing means comprises a region of hardened material of the disc.

3. An electrode according to claim 1 or 2 wherein the reinforcing means comprises one or more elongate regions formed in the disc.

4. An electrode according to claim 3 wherein there are three elongate regions.

is

5. An electrode according to any one of the preceding claims wherein the elongate regions are provided substantially in areas of the disc in which, in use, high levels of stress occur.

6. An electrode according to any one of the preceding claims wherein the disc has a front surface adapted, in use, to contact another electrode, and a rear surface and the reinforcing means is formed in a region of the rear surface.

7. An electrode according to any one of the preceding claims wherein the reinforcing means is provided by regions of the disc which have a material structure different from the majority of the disc.

8. An electrode according to claim 7 wherein the reinforcing means has a finer grain structure than the material in the majority of the disc.

14

9. A vacuum switching device having provided therein one or more electrodes according to any one of claims 1 to 8.

10. A device according to claim 9 which has two electrodes provided therein.

11. A method of producing a vacuum switching device electrode including a step of performing material modification processes to one or more regions of a surface region of a contact disc of the electrode.

12. A method according to claim 11 wherein the material modification process increases the hardness of the material of the disc.

13. A method according to claim 11 or 12 comprising providing the disc from a mixture of two immiscible conducting materials such that there exists a matrix of one material infiltrated with the other and the method comprises reducing the grain size in one or more regions of the material.

14. A method according to any one of claims 11 to 13 comprising heating the surface region of the disc.

15. A method according to claim 14 wherein the surface region is subjected to localised melting.

16. A method according to claim 14 or 15 wherein the heating is followed by quenching.

is

17. A method according to 'any one of claims 11 to 16 wherein the material modification processes in any one of the following heating techniques: electron beam, laser, plasma arcing or chemical vapour deposition.

18. A method according to any one of claims 11 to 16 wherein the material modification process includes the step of striking an electrical arc between an electrode and the disc.

19. A method according to claim 18 wherein the current flowing in the arc is around 120A.

20. A method according to any one of claims 11 to 19 which increases the relative strength of the material to around twice that of the untreated material on which the material modification process has not been performed.

21. A method of manufacturing a vacuum switching device comprising including an electrode fabricated according to any one of claims 11 to 20.

22. A vacuum switching device substantially as described herein with reference to and as illustrated in Figures 2 to 5 of the accompanying drawings.

23. A vacuum switching device substantially as described herein with reference to and as illustrated in Figures 2 to 5 of the accompanying drawings 16

24. A method of forming a vacuum switching device electrode substantially as described herein with reference to and as illustrated in Figures 2 to 5 of the accompanying drawings.

25. A method of forming a vacuum switching device substantially as described herein with reference to and as illustrated in Figures 2 to 5 of the accompanying drawings.