GB2255233A - Arc extinguishing in switches. - Google Patents

Abstract

An electric switch includes arc extinction elements 41-47 spaced around and extending outwardly from a contact zone defined by at least two contacts 141-148 which are separable to provide a multiplicity of contact gaps around the contact zone. The spaced arc extinction elements 41-47 cooperate with the contact gaps to receive arcs arising at the contact gaps on the separation of the contacts, which arcs travel along the arc extinction elements 41-47 for subsequent extinction. Current supply conductors 51, 52 extend at least pan of the way around the arc extinction zone to provide an arc extinguishing magnetic field. The contacts may have interdigitated portions (Figs 5-7). <IMAGE>

Description

CONTROL OF ELECTRIC ARCS This invention relates to the control of an electrical arc as occurs for example in an electric switch when the switch is opened.

In many forms of electric switch there are contacts used to allow and stop the usual flow of current, when respectively closed or open. In addition if a fault leading to excessive current flow occurs the contacts are opened to stop the excessive current. For economy the contacts, and the whole switch, are made as small as possible so there must be a conflict between the contact size for usual currents and frequent use and that for fault currents and occasional use. If on opening the switch contacts in an attempt to interrupt a fault current an electrical arc develops between the contacts which is not rapidly extinguished not only will the fault current not be interrupted but it is very likely that the switch will be destroyed by the action of the arc. Frequently extra contact parts are provided to transfer an arc to an extinction area.Such extra parts are an economic and performance burden on normal use. Various techniques have therefore been developed to control such arcs and bring about the rapid extinction of the arc. An electrical arc is a plasma produced by a current flowing in a gaseous conductor such as ionised air. Such a current flow can interact with a magnetic field and be diverted to bring about extinction. The plasma can be cooled and the ionisation reduced by being directed into thermally and/or electrically conductive elements. A blast of air or other gas can also be used to "blow-out" the arc.

These techniques are useful and effective but improvements are always sought to improve the performance, economy and other aspects of the control of an electrical arc, reducing the time for which the arc persists and simplifying the switch.

It is an object of the present invention to provide more rapid but controlled arc extinction for electric switches.

According to the invention there is provided a control arrangement for an electrical arc including separable contacts closable for the flow of rated current and around the contacts an arc extinction array including elements diverging from an arc application area, the elements and contacts adjoining in said area, together with means operable at will to separate the contacts and said elements and by relative continued contact movement form therebetween arc sections in the arc application area of the arc extinction array whereby arc current expands the arc in sections away from each other and along the diverging elements for extinction.

The arc extinction array may include elements extending radially from a central area including said contacts. There may be supplementary arc extinction elements in the radially outward region of the array. The elements may be metal plates. The elements may be spaced and supported by insulating members. The insulating members may be surfaces grooved to receive the elements. The elements may carry or form one of the contacts.

The control arrangement may have at least one current flow conductor arranged at least part of the way around the arc application area and outwardly thereof. The control arrangement may have a circular cylindrical form with current flow conductors in a peripheral path at each circular face. The at least one current flow conductor may be arranged to have an electromagnetic field with the flow of arc current therein and to exert said field for action with current flow in said arc.

The contacts may be closable by respective contact to said elements and separable by respective movement away from said elements, the elements being ohmically conductive. The contacts may be separable by movement from each other past the elements.

The separable contacts may include a number of elements in contact until the contacts are separated and becoming individually isolated on separation of the contacts to encourage the formation of arc sections therebetween.

A method of controlling an electrical arc occurring between separating electrical contacts, at least one contact having a respective current flow conductor, including positioning adjacent said contacts conductive arc accepting elements diverging from adjacent said contacts, positioning said at least one conductor outwardly of said contacts and around said diverging elements, causing or permitting said arc to occur at said contactsas arc segments for entry between said diverging elements, and causing said segments to move along said elements by electromagnetic action with arc current flow in said at least one conductor for self-extinguishing extension by said divergence.

The method may include causing at least one of said conductors to provide a current flow around said diverging elements.

Embodiments of the invention will now be described with reference to the accompanying drawings in which, Figure 1 is a cross-sectional view partly in section of a switch of the circuit breaker type embodying the invention, Figure 2 is an "exploded" partly-sectional view of the switch in Figure 1, Figure 3 is a cross-sectional view of another switch of the circuit-breaker type embodying the invention and showing arc movement, and Figures 4, 5, 6 and 7 are views of contact arrangements useful in switches embodying the invention and showing arc position in some cases.

The basic form of the switches in the embodiments illustrated is a short cylinder formed of a ring 10 and two discs 21, 22 of plastics material such as PTFE (RTM). The diameter of the cylinder in one embodiment is some 75 to 100 millimetres with a height of some 20 to 30 millimetres. The ring and disc material are some five millimetres in thlckness. The ring has several vent-holes 11. On the axls A-A of the cylinder Is a spindle 30 of plastics material, again conveniently PTFE, rotatable in the cylinder on the axis. Between the discs is an arc extinction array of a number of spokewise directed spaced plates 41 to 47 of copper or other thermally and electrically conductive material some five millimetres thick.Conveniently the plates are regularly spaced, apart from one area where a wedge 31 of plastics, such as PTFE, fills at least partly a space formed by omitting one plate. The plates are conveniently supported and spaced by grooves such as 23 and 24 in the inward faces of discs 21 and 22. The inward ends of the plates are separate and spaced around the spindle to be close to the spindle. In the view in figure 1 plate 21 is removed for clarity.

On the outer surface of the spindle are two copper-tungsten contacts, To the switch electrical connection is made by one or more turns of respective copper coils 51, 52 (only one turn of one coil shown) supported in respective grooves in the outward surfaces of the plastics discs and extending around a large part of the periphery of the switch from a respective one of the plates, 47 in the case of coil 51 to a sultable terminal, not swown. The contacts may be moved by mechanical or electromagnetic means exerting either of a rotary or a linear action. The rotary action is in the plane of the discs and the linear action in a direction orthogonal to the discs. Connection to the switch contacts can be at the plates to which the coils are connected.

In the closed position the inner ends of the spokewise directed plates are bridged by the contacts so that current through the switch flows through one of the copper coils to a contact then past all the plates to the other contact and on through the other coil.

To open the switch, for example when a fault is detected, the contacts are moved in one of the ways set out above to slide behind the ends of the plates. In this way a series of arc discharges are immediately formed, with an arc discharge section between each two adjacent plates, see Figure 3 in which similar references are used. Each arc sections is inherently moved in the direction of divergence of the spokewise plates to be extinguished. While the arc exists the current continues to flow through one copper coil connection around the periphery, on to an "end" splitter plate and contact, through the arc to the other contact and "end" splitter plate and then through the other peripheral coil connection (not shown).

In one form the contacts on the surface of the spindle are conductive portions spaced and sized to bridge between adjacent plates so that a continuous conductive path is formed around the spindle. When the spindle is turned the spaces between the portions form gaps in the path, between the plates, and provide for the arc section discharges.

In another form, where the spindle moves axially, the spindle itself, or its surface forms a conductor which links all the inner ends of the plates in one position and can be displaced axially to open the current path and permit the arc section discharges to occur. Insulating fins may be provided on the spindle to fit between the splitter plates.

The extinguishing action is provided by self-induced magnetic forces generated as follows: a) the mutual interaction of the magnetic field caused by current flow through the arcs and the current flow itself; b) the magnetic field produced by current flow through the copper coil current supply leads and the current in the arc.

In this way the arc sections are formed immediately by the opening contacts in an area where the electromagnetic drive is strongest and are rapidly driven radially outwards by the magnetic forces to be lengthened by the divergence of the plates. The voltage drop in an arc section increases as the sections lengthen until there is insufficient voltage from any connected voltage source across the switch contacts to sustain the series of arcs, which are thus extinguished.

The opening of the contacts to produce a multiplicity of arc sections effectively produces a significant increase in the inductance in the current path (so long as continuity exists from one terminal of the opened switch to the other terminal through the arc arising on the opening of the switch). This increase in effective inductance is a significant feature in enhancing the performance of the switch and is distinct from switches used hitherto in which the inductance in the current path does not increase significantly on opening the switch.

Furthermore the close positioning of the actual current breaking contacts and arc extinction elements bring about direct application of the arc sections to the arc extinction elements and avoids the need for the transfer devices, such as runner electrodes, used hitherto to move the arc across the large space between the current breaking contacts and the arc extinction elements conventional in the art. This avoidance of the need for special transfer devices used only for the short time of the actual arc presence saves expensive contact material and permits a more compact arrangement. Also the arc reaches the arc extinction elements without the time delay needed for movement of the arc through the transfer devices. In this way the extinction of the arc begins more quickly.

Additionally the arc length is increased much more rapidly in a switch according the present invention than in the prior art.

The arc in the present invention can be substantially the whole of the periphery of the contact zone and is preferably as complete a circle as possible, subject to constructional needs.

To increase the effect of inductance on the breaking of the current by the switch of the present invention several turns can be provided in the coils 51 and 52.

The PTFE ring prevents the arc plasma moving out of the defined switch volume, avoiding the risk of "jump-over", and the vent holes permit gas to escape, avoiding overpressurisation.

Figures 4, 5, 6 and 7 show various contact arrangements to bring about the preferred manner of contact separation and position for any resultant arc in arc sections around the contact structure. These arc sections may together extend wholly or partly around the contact structure adjacent to the arc extinction array.

In Figure 4b conductive contacts are shown in the closed position for the flow of rated current. The contacts 141 and 143 are representative of movable contacts (shown solid) and six in number in this embodiment. The contacts 142 and 144 are representative of fixed contacts (shown clear) and seven in number. The movable contacts are supported by a movable central part 140 operable to move the contacts, in the rotary action mentioned above, at will between the closed position (Figure 4b) and the separated position shown in Figure 4a.

The movable contacts are not connected to each other. The movable contacts when positioned as shown in Figure 4b connect a fixed contact 142 connected to one "end" splitter plate 41 via individual fixed contacts, such as 142, for each splitter plate around to a fixed contact 148 on the other "end" splitter plate 47. The copper coil connections, exemplified by 51, convey current into and out of the switch, the current flowing through the metallically contacting fixed and movable contacts on the way.

To open the switch the movable contacts are moved to the position shown in Figure 4a where the fixed contacts are no longer metallically connected. By this action the current path is broken into sections and separate arcs are likely to be formed to bridge the gaps. The arrangement encourages these arcs to be formed where the arcs will tend to be urged into the arc extinction array for extinction as described above.

Considering now Figure 5 the conductive contact fingers such as 151, 152, 153, 154 are supported in two groups on respective insulating supports 150, 159 for interdigitated interaction to provide, as shown in Figure 5a, a conducting current path from end contact 151 to end contact 152 through the contacts. On moving the contact groups apart, as indicated by the arrow in Figure 5b, the current path is broken into sections and separate arcs are likely to occur all around the contact arrangement.

Figure 6 shows an arrangement broadly similar to that in Figure 5 except that the current supply directions at 161 and 162 are different. This arrangement will not be described further as it will be readily understood from the above descriptions.

Figure 7 shows elements of a contact arrangement operable by movement of two contact parts 171, 172 as shown by the arrow.

This arrangements encourages an azimuthal arc pattern.

Relating the contacts shown in Figures 4, 5, 6 to the arrangements in Figures 1 and 2 or 3 the end contacts are conveniently either side of the insulating wedge 31 or 131 and an arc position is conveniently encouraged at each opening in the arc extinction array by positioning a fixed operating contact by each arc extinction element. However there need not be an exact correlation, as will be apparent to those skilled in the art.

The appropriate arrangements for insulation and voltage ratings will also be apparent to those skilled in the art. The use of gas such as SF6 in the switch will also be apparent to those skilled in the art for appropriate applications. The switch is useful for most voltage ratings but the economic benefits are expected to be greater at higher voltage ratings such as those found in distribution and transmission (that is a kilovolt and upwards) rather than those in final circuits of a few hundred volts.

The presence of a tight closed loop of electromagnetic drive from the current in the connections to drive the arc outwards minimises contact wear and gives the most immediate transfer by direct application of the arc to to the splitter plates without the transfer delay hitherto present in circuit breakers. If required secondary splitter plates may be placed between the plates shown to increase the number of arc sections and thus the arc voltage drop as the arcs move outward.

Various constructions are of course possible while conforming to the various essential and optional requirements set out above. Thus the device may be of circular or helical form with appropriate motion of the movable contact to produce the arc sections between the diverging splitter plates.

Claims (13)

1. A control arrangement for an electrical arc including separable contacts closable for the flow of rated current and around the contacts an arc extinction array including elements extending from an arc application area, the elements and contacts adjoining in said area, together with means operable at will to separate the contacts and by relative continued contact movement form therebetween arc sections in the arc application area of the arc extinction array, whereby arc current expands the arc in sections away from each other and along the elements for extinction.

2. An arrangement according to Claim 1 in which the arc extinction array includes elements extending radially from a central area including said contacts.

3. An arrangement according to Claim 2 including supplementary arc extinction elements in the radially outward region of the array.

4. An arrangement according to Claim 1 having at least one current flow conductor arranged at least part of the way around the arc application area and outwardly thereof.

5. An arrangement according to Claim 1 having a circular cylindrical form with current flow conductors in a peripheral path at each circular face.

6. An arrangement according to Claim 4 in which said at least one current flow conductor is arranged to have an electromagnetic field with the flow of arc current therein and to exert said field for action with current flow in said arc.

7. An arrangement according to Claim 1 in which said separable contacts include a number of elements in contact until the contacts are separated and becoming individually isolated on separation of the contacts to encourage the formation of arc sections therebetween.

8. A method of controlling an electrical arc occurring between separating electrical contacts, at least one contact having a respective current flow conductor, including positioning around said contacts conductive arc accepting elements diverging from adjacent said contacts, positioning said at least one of said conductors outwardly of said contacts and around said diverging elements, causing or permitting said arc to occur at said contacts as arc segments for entry between said diverging elements, and causing said segments to move along said elements by electromagnetic action with arc current flow in said at least one conductor for self-extinguishing extension by said divergence.

9. An electrical switch, contactor circuit breaker or like device including an arc control arrangement according to Claim 1.

10. An electric power switch including a contact zone and an arc extinction zone characterised in that the arc extinction zone is around the contact zone, that the contact zone includes at least two contacts separable to provide a multiplicity of contact gaps around the contact zone and that the arc extinction zone includes arc extinction elements spaced around the contact zone and extending outwardly therefrom, the arrangement being such that said spaced arc extinction elements cooperate with the contact gaps to receive arcs arising at the contact gaps on the separation of the contacts to break a current flowing, in operation, through the closed contacts.

11. An electrical power switch according to Claim 11 in which a conductor of the switch extends at least part of the way around the arc extinction zone.

12. An electrical power switch substantially as herein described with reference to the accompanying drawings.

13. A method of controlling an electrical arc substantially as herein described with reference to the accompanying drawings.