GB2512160A

GB2512160A - Improvements relating to vacuum switching devices

Info

Publication number: GB2512160A
Application number: GB201321132A
Authority: GB
Inventors: Leslie Thomas Falkingham
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 2012-12-14
Filing date: 2013-11-29
Publication date: 2014-09-24
Also published as: GB201321132D0; GB201222648D0; GB2508913A

Abstract

A vacuum switch (figure 1) comprising an evacuated envelope, an insulator and fixed and moving electrodes which engage and disengage mechanically to perform switching function and to generate a magnetic field predominantly in the direction of the contact axes includes at least one contact assembly with one or more arm conductors i, behind a contact disc h and a high conductivity backing plate m conductively bonded to the back face of the contact disc and electrically connected to the ends of the arms. The backing plate can be made of copper, be brazed to the contact disc, and have slots that are designed to reduce eddy currents. The backing plate may be in the shape of a disc, annulus or polygon and may vary in thickness. The backing plates and the rear of the contact plates may be formed with recesses to determine selected areas for bonding to take place. The contact assemblies may alternatively comprise cup-shaped conductors having sides with angled slots, the rims of which may be conductively bonded to the high conductivity backing plate m.

Description

Description:

This invention relates to improvements in vacuum switching devices, which may include br example vacuum interrupters or vacuum switches.

A typical idealised construction of a vacuum switching device is shown in figure 1.

These devices generally consist of an evacuated envelope which includes an insulating component (a) and end plates (b), together with a fixed electrode (c) and a moving electrode (ci). The electrodes can be engaged and disengaged mechanically to perform the switching function. Normally this movement is permitted without breaking the seas of the evacuated envelope by means of a bellows or diaphragm arrangement (e). Each electrode (c or d) consists of a conducting rod U) and a contact or contact assembly (g).

Vacuum switching devices that are dcsigned to switch arge currents have been produced for many years. A number of different designs of electrode assemblies have been proposed for use under high culTent conditions. These designs use sclf-generated magnetic fields to control electrical arcs that arise when the contacts are opened. The prior art falls into two categories, which work in different ways. In one category known as Radial Magnetic Field, (he contact geometry produces a magnetic field that is generally at right angles to the axis of the contacts. In the other category, which is known as an Axial Magnetic Field. the contact geometry produces a magnetic field that is substantially aligned with the axis of the contacts. With radial magnetic field geometries the contact faces are generally in the form of annular rings.

and the electric arc which forms during opening of the contacts is driven by the radial magnetic field to continuously move around the ring, so that excessive heating is not produced at any one point. Examples arc EP0525354 Al and US4553002 A. With axial magnetic field geometries the contact faces are most often in the form of discs, and the effect of the axial field is to diffuse the current arc fairly evenly over the contact surfaces, thereby preventing overheating of points on these surfaces.

Examples of such designs may be seen in EP 0349303. DE 3915519. DE 3610241, and GB 23811 1A. This invention concerns only axial magnetic field contact asscmhhes.

A number of different designs of axial field contacts and contact assemblies have been proposed to switch the current on or off under load and fault culTent conditions.

Examples of such designs may be seen in EP 0349303. DE 3915519. DE 3610241, and GB 238111A.

This invention relates to two particular types of prior art axial fidd contact geometry.

The first type is typified by US 39946179 (figure 2). In this geometry the contact assemblies each consist of a conducting rod (f), which is connected to a contact disc (h) by means ol a number ol arm conductors (i) along which the current must flow before aniving at the contact disc by way of pillars (j).

Figure 3 shows a cross section of this type of contact assembly. When the electrodes are first disengaged an arc of electric current forms in the vacuum space between their faces, which continues until it is extinguished by the switching effect of the device.

Due to aspects of the physics of the vacuum arc, the arc (Ii) is often spread unevenly over the contact surfaces and in particular on the cathode electrode there may be a concentration into a number of spots. Ahove a current of a few thousands of amps the spots can move towards cach other and the arc concentrates into a small area. This is called arc constriction. It is necessary to prevent arc constriction, which can inhibit successful interruption of the current due to local overheating of the contact surface.

In the prior art the currents through the circumferential parts of the arm conductors in the two contact assemblies circulate cooperatively to create a magnetic field generally in the direction of the axis of the contacts. This magnetic field acts to prevent the constriction of the arc and thereby allows the contacts to interrupt high levels of current successfully.

However due to the design of this type of contact the current enters the contact disc from sinai] areas at the ends of the pillars (j. figure 4). This produces concentrated areas of very high current U) leading from the pillars. In addition the relatively high electrical resistance of the special type of material commonly used for the contact disc then prevents the current from spreading evenly throughout the contact disc. This leads to an un-optimised arc distribution and thereby reduced current interruption capability for a particular diameter of contact disc.

According to the invention (figure 5). in contact assemblies as described in the prior art, a backing plate (iii) made of material with higher electrical conductivity, such as copper, is bonded to the rear surface of the contact disc, the bonding means having good electrical conductivity, for example brazing. In this invention the current path from the contact arms to the contact disc goes by way of the pillars to the hacking plate and then to the back face of the contact disc.

This invention allows the current to spread more evenly within the backing plate before passing into the contact disc, allowing the face of the contact disc to be more fully used for passing current into the vacuum. Also the magnetic field may be more uniform over the area of the contact disc. The result is that a higher current can he successfully interrupted for a given size of contact diameter, thereby making the vacuum switching device smaller and of lower cost.

In the prior art the magnetic field gives rise to eddy currents (n) in the contact disc, as shown in figure 6. One skilled in the art will know that this is detrimental to the generation of the axial magnetic field, reducing its effectiveness. Sometimes according to prior art, slots (o) are cut in the contact disc which reduce the eddy currents but this has the disadvantage that the slots have edges in an area where high electric fields occur. The edgcs concentrate these electric fields and reduce the voltage performance of the contacts. In a second aspect of the invention the backing plate may have slots so that eddy currents that might arise in it are reduced. Because the backing plate is in a position where the electric fields are not very great. the voltage perlormance is not affected. These slots therefore improve the arc contruil capability of the contact disc without detriment to its voltage performance. The slots may pass right across the backing plate if desired, dividing it into one or more pails. The slots need not be straight.

The backing plate may he in the form of a disc, hut can he other shapes, such a polygon, an annulus that does not cover the central part of the contact plate, or simply a ring connecting the tops of the pillars.

The thickness of the backing plate may be varied over its area for example to give a desired distribution of electrical conductivity. It can be designed to optimise the distribution of current entering the backing plate.

The backing plate need not be bonded to the whole of the rear surface of its contact plate. There may for example he advantage in conducting sonic of the current to the central portion of the contact disc or to an annular contact area at a distance between the centre and the edge of the contact disc, or to some other place. This can be achieved for example by forming the backing plate with recessed areas where brazing is not. required. Figure 7 illustrates this for the case where contact is required around the periphery and at a circular area in the centre. Alternatively or additionally the rear face of the contact disc may have recessed areas to achieve the same object.

In the second type of contact assembly to which this invention apples, shown in figure 8, an axial magnetic field is produced not with spiral arms but by bonding the contact plate, which is generally in the form of a disc, to the rim of a cup-shaped conductor whose sides have angled slots (p) which direct the current in a partly circumferential direction. In figure 8 a section of the contact disc is cut away to reveal the form of the cup-shaped conductor. A backing plate can also be applied with advantage to this configuration, as shown in figure 9.

The invention may he applied to the design ol vacuum switching devices over a wide range of contact diameters, the actual diameter chosen depending to a great extent on the intended maximum current to he interrupted, and the maximum load current intended to he carried.

Claims

Claims: I. A vacuum switching device consisting ci an evacuated envelope which indudes an insulating component. a lixed electrode, and a moving electrode which are designed to engage and disengage mechanically to perform the switching function and to generate a magnetic field which is predominantly in the direction of the contact axes and which includes contact assemblies with one or more arm conductors behind a contact disc and a high conductivity backing plate conductively bonded to part or all of the back faces of the contact discs and electrically connected to the ends of the arm conductors.
2. A vacuum switching device as described in claim 1 in which the high conductivity backing plates have slots designed to reduce eddy currents.
3. A vacuum switching device as in daim I or claim 2 in which the hacking plates arc in the shape of a disc.
4. A vacuum switching device as in claim 1 or claim 2 in which the backing plats are in the shape of rings that connects the pillars of the arm conductors to each other.
5. A vacuum switching device as in claim 1 or claim 2 in which the backing plates have the shape of a polygon.
6. A vacuum switching device as in claim 1 or claim 2 in which the backing plates arc in the shape of an annulus.
7. A vacuum switching device as in claim 1 or claim 2 in which the backing plates have a form intended to optimise the flow of current from the arm conductors into the contact disc.
8. A vacuum switching device as in daim I or claim 2 in which the thickness of the backing plates is varied.
9. A vacuum switching device as in claim 1 or claim 2 in which the backing plates are bonded to their contact discs only in selected areas.
10. A vacuum switching device as in daim I or claim 2 in which hacking plates are lornied with recessed areas to determine selected areas where bonding is to take place.
11. A vacuum switching device as in daim I or claim 2 in which the rear ol the contact plates is formed with recessed areas to determine selected areas where bonding is to take place.
12. A vacuum switching device consisting of an evacuated envelope which includes an insulating component, a fixed electrode and a moving electrode which are designed to engage and disengage mechanically to perform the switching function and to generate a magnetic field which is predominantly in the direction of the contact axes and which includes contact assemblies with cup shaped conductors having sides with angled slots behind contact discs and high conductivity backing plates conductively bonded to the rims of the cup shaped conductors and to selected areas on the hack laces of the contact discs.
13. A vacuum switching devicc as described in claim loin which thc high conductivity backing plates have slots designed to reduce eddy currents.
14. A vacuum switching device as in claim 10 or claim 11 in which the backing plates are in the shape of a disc.
15. A vacuum switching device as in claim 10 or claim 11 in which the backing plates are in the shape of m annulus.
16. A vacuum switching device as in claim 10 or claim 11 in which the thickness of the hacking plates is varied.
17. A vacuum switching device as in claim 10 or claim 11 which the backing plates are formed with recessed areas to determine the selected areas where bonding is to take place.

IS. A vacuum switching device as in daim 10 or claim II which the rear of the contact plates are foimed with recessed areas to determine selected areas where bonding is to take placc.