WO1994014177A1

WO1994014177A1 - Arc extinguishing switch apparatus and method

Info

Publication number: WO1994014177A1
Application number: PCT/AU1993/000610
Authority: WO
Inventors: Gregory James Nunn; David Birtwhistle; Bruce Clifford Hicks
Original assignee: Nu-Lec Pty. Ltd.
Priority date: 1992-12-16
Filing date: 1993-11-30
Publication date: 1994-06-23
Also published as: CN1097898A

Abstract

There is provided switch apparatus (10) for interrupting an AC distribution grid and consisting of a single break switch assembly contained in a sealed enclosure (12) filled with sulphur hexafluoride gas (13). The switch assembly is interposed between a line conductor (14) and a load conductor (15) led into the enclosure (12) by gas tight insulator assemblies (16). The line conductor (14) is terminated by a fixed contact (21) having embedded therein a cylindrical rare earth metal magnet (23). The load conductor (15) is terminated by a moving contact assembly (24) including a contact mounting body (25) having pivotally mounted thereto an elongate main contact arm (26) having contact plates (27) adapted to electrically engage a copper-tungsten arcing fixed contact portion (33) of the fixed contact (21). Pivotally mounted between and electrically common with the contact plates (27) is an integral arcing contact assembly (37) comprising an arcing contact (40) having a cylindrical rare earth metal permanent magnet (41) embedded therein and an arcing contact arm (42).

Description

EXTINGUISHING SWITCH APPARATUS AND '"^"""HOD T, nvention relates to arc extinguishing switch apparatus ..nd methods of extinguishing electric arcs in medium to high voltage, high current switch gear and interrupters. This invention has particular but not exclusive application to extinguishing plasma forming arcs in medium to high voltage, high current AC distribution interrupter apparatus, and for illustrative purposes reference will be made to such application. However, it is to be understood that this invention could be used in other applications, such as medium and high voltage switch gear generelly.

The distribution of electr 1 power for domestic and industrial use is generally performed at medium and high voltages to reduce current and thereby reduce the cross section of conductors. The distribution grid of AC power is typically operated at voltages of tens of thousands of volts, typically in the order of 22,000 volts, and a particular grid sector may be config d to deliver in the order of a hundred to a t jsand a ares of current. at this voltage. Short term or it current loading on switch gear in such systems may exceed l-_,000 Amperes for several seconds. For any number of reasons including c: id maintenance or a fault condition it may be necessary to interrupt the circuit in a particular grid sector. Switch gear for providing this interruption must be capable of r.nterrupting large currents at medi^um to high voltages. In current usage is apparatus generally comprising an enclosure filled with a dielectric gas, typically air but preferably a gas of relatively high dielectric strength such as sulphur hexafluoride ( SFg ) , or an at least partially evacuated gas space. Mounted within the enclosure is generally a pair of contacts, one of which is fixed and the other moving. The moving contact generally comprises a main contact and a secondary contact, the object being to move the relatively large main contact out of circuit prior to rapidly breaking the circuit by moving the secondary, arcing contact, typically at speeds of several metres per second. A disadvantage of the apparatus described above has arisen from the generation of exceedingly hot plasma arcs in the breaking of the circuit. The free electrons and other charged species in the plasma provide a conduction path through the dielectric which is capable of carrying large currents. The root of the plasma arc itself causes rapid destruction of the contacts with the contact material feeding atoms and electrons into the plasma and thus providing more charge carriers.

Whilst the arc will generally self extinguish at large separation distances between the contacts, and whilst resistant materials such as tungsten-copper alloys may reduce erosion, the interruption will generally take several cycles of the alternating current to be effected, which results in an unacceptably high rates of wear on contacts together with slow interruption of the current. Such plain break apparatus are generally limited to use in interrupting currents of the order of a hundred amperes.

It has been proposed to utilize the current carrying characteristics 01 charge carriers in plasma arcs to cause more rapid extingυ. ng of the arcs in switches, lightning arresters and the li e by providing a magnetic field the lines of force of which are aligned with the arc and which cause swirling of the arc. Whilst the force exerted on a charged particle moving through a magnetic field is orthogonal to both the field and the direction of movement, and alignment of the field with the direction of motion would normally be expected to have no significant effect, magnetic coupling phenomena effects swirling of the arc.

In some apparatus the magnetic field is induced by the applied alternating current, the effect of which is to maintain the magnetic field substantially in phase with the current in the arc and thus provide a force of constant direction. In devices such as lightning arresters, permanent magnets about the contacts have been used to good effect. Also, use of magnets in high power circuit breakers exhibits iproved perfo^rmance over apparatus not using magnetic f_.≤ld means. The magne . i in such apparatus are located ourside the current path to improve the low current interrupting ability of the circuit breakers.

However in AC circuits of medium to high voltage and current, control of the arc using permanent magnets has proved to be less than effective in improving arc interruption performance of the switch gear. It is theorized that the poor performance of permanent magnets in such applications may result from the alternating nature of the current through the arc, thus reducing the swirling effect which contributes to extinguishing the arc. Additionally, the location of the magnets about the exterior of the fixed contact appears to reduce the field strength and consistency at the potential arc rooting region of the contact.

The present invention aims to substantially alleviate the above disadvantages and to provide apparatus which will be i reliable and efficient in use. Other objects and advantages of this invention will hereinafter become apparent. ^■ With the foregoing and other objects in view, this invention in one aspect resides broadly in switch apparatus of the type including: - a housing enclosing a dielectric environment; a pair of contacts within said housing movable relative to one another between a closed position in circuit and an open circuit position, wherein one or both of said contacts are provided with magnetic field means providing a magnetic field substantially aligned with a separation axis of said contacts.

Preferably one of the contacts comprises a fixed contact relative to the housing, the other of the contacts being movable relative to the housing such that the contacts may be drawn apart to break the circuit. Preferably, the contacts are drawn apart along at least a close approximation to a straight separation axis. However, it is also desirable from the point of view of mechanical simplicity to mount the moving contact on a radius arm about a radius, and in such cases the radius arm is preferably relatively long compared with the separation distance required between the contacts to extinguish the arc.

The magnetic field means may be selected from a permanent magnet or an inductive winding driven by alternating current to be switched or an external AC supply. The magnetic field means for interrupting high voltage/high current AC circuits is advantageously in the form of inductive windings driven by the AC to be switched, or by means of a permanent magnet. Typicall^y in apparatus for use in switching 36-24 kV at 400 to 650 Amperes continuous, prior art inductive or permanent magnetic fields of approaching 10 kilogauss at the magnet surface would be required. It has now been determined that the use of a relatively small powerful rare earth metal permanent magnet embedded in the contact and thereby protected from arc damage whilst being physically close to the contact face to maximize the magnetic field intensity achieves an improved result at lower field strengths. Accordingly, the magnetic field means preferably comprises a rare earth permanent magnet embedded in one or both of the contacts and protected from arc damage behind the contact face thereof. It has surprisingly been determined that extinguishing of an arc in medium to h_ h voltage, high current switch gear may be achieved by the first current-zero, by providing apparatus in accordance with the above, and it is further observed that this phenomenon is yet further enhanced by provision of a relatively small magnetic field associated with a contact, the increase in performance being out of proportion to the field strength and total flux of the magnetic field. It has further been determined that embedding a permanent magnet in the moving contact provides further advantages. The magnet may be small and so be adapted for inclusion in the relatively small secondary arcing contact of medium to high voltage, high current apparatus.

Preferably, the contacts are provided of a magnetically permeable material which is capable of withstanding erosive prolonged or multiple arc events in the operation of the switching apparatus. For example, the contacts may be formed of a refractory alloy having relatively high conductivity and is preferably selected from high temperature alloys of tungsten and copper or chromium and copper.

Preferably, one of the contacts comprises a contact assembly including an arcing contact and a main current contact. For example, the contact assembly may be operable to break the circuit by displacement of the main contact relative to the other contact to an insulating distance in the dielectric environment, followed by movement of the arcing contact away from the other contact to at least that distance. Conversely, such a contact assembly may be operable to make the circuit by bringing the arcing contact into engagement with the other contact, followed by engagement of the main contact with the other contact.

Whilst as described above, the permanent rare earth magnet may be embedded in the fixed contact as opposed to the moving contact assembly, the magnet may be embedded in the alternative in the arcing contact, where it has been surprisingly determined that small magnets of the kind which may fit within the relatively small arcing contact are efficacious in extinguishing the arc within the period to the first current zero point of high voltage AC circuits. Preferably, both of the fixed and arcing contacts are provided with rare earth magnet magnetic field means embedded therein, wherein the field axes of the respective magnets are substantially aligned with opposed poles adjacent when the circuit is closed. Preferably, the contacts are operable to make or break said circuit by automatic switching means under control of condition responsive control means. However, the moving contact assembly described above and comprising an arcing contact and a main current contact, may be moved by motive means such as one or more of manual switching means or automatic switching means under control of condition responsive control mr-ans, and is preferably provided with both.

Pre-.erably the switching means, whether automatic or manual, is adapted to break the circuit by first displacing the main contact away from the fixed contact to beyond the dielectric break .own distance of the medium at the applied voltage, followed by rapid movement of the arcing contact away from the fixed contact. To make the circuit, the arcing contact is preferably brought into engagement with the fixed contact rapidly, followed by substantially arc-free engagement of the main contact with the fixed contact. The main contact may comprise an contact adapted to pull straight away from the fixed contact or may pivot away from engagement with the fixed contact on a radius arm of length such that the main contact approximates a straight pull away from the fixed contact.

The main contact may be latched into contact with the fixed contact such that release of the latch manually or under the control of condition responsive control means results in the main contact falling away from the fixed contact or being driven away therefrom under bias. The arcing contact may be adapted to engage the fixed contact through a space or annulus provided through the main contact.

The arcing contact may be coupled to move dependently on the movement of the main contact such as by means of a spring loaded coupling operable to provide a bidirectional overcentering movement. Movement of the main contact away from the fixed contact may occur whilst maintaining the arcing contact against the fixed contact until a certain critical point whereupon the arcing contact separates rapidly under spring bias out of contact with the fixed contact. In closing the circuit, the movement of the main contact towards the fixed contact may load a spring to a critical point whereupon the arcing contact is accelerated into contact with the fixed contact prior to contact between the main and fixed contacts.

Alternatively, the main contact may comprise an assembly of the radius arm type hereinbefore described and having an arcing contact assembly pivoted thereto such that the arcing contact is biassed proud of the main moving contact portion such that the arcing contact is last to disengage and first to engage the fixed contact. In such embodiments the motive means may be configured to operate the apparatus via the main contact assembly.

When the apparatus is closed, the fixed and moviπ contacts are preferably held together under pressure, an . Load currents flow between the contacts. To interrupt current the arcing contact is pulled away from the fixed contact at a velocity of typically several metres per second.

In a further aspect, this invention resides broad1/ .in switch apparatus of the type including:- a housing enclosing a dielectric environment; a fixed contact within said housing, and a

ng contact assembly within said using .rid movable relative to said fixed contact between a closed position in circuit with said fixed contact and an open circuit position, wherein said fixed contact comprises a magnetically permeable contact body having a permanent magnet embedded therein of field substantially aligned with the separation axis of the contacts. Preferably, -he moving contact comprises a main contact and an arcing contact comprising a magnetically permeable arcing contact body having embedded therein a second permanent magnet of field substantially aligned with the field of the permanent magnet embedded in the fixed contact.

Preferably, the housing of apparatus in accordance with the present invention is a substantially sealed housing containing a dielectric environment selected from an at least partially evacuated gas space, air or sulphur hexafluoride gas. For example, the dielectric environment may comprise sulphur hexafluoride gas. Whilst dielectric performance of gases such as SF5 is enhanced at elevated pressures, the engineering of housings to contain gas under pressure may be avoided by utilizing the dielectric gas at atmospheric pressure.

Preferably, the housing is filled with SFg at atmospheric or moderately elevated pressures, this gas being preferred for its relatively high dielectric strength, heat capacity in terms of arc cooling and the capacity to scavenge stray electrons and thereby contribute to plasma decay.

Apparatus in accordance with the foregoing may include in typical distribution applications a copper tungsten fixed contact of about 32 mm diameter having embedded therein a cylindrical rare earth permanent magnet of 22 mm diameter and 25 mm length having a field strength of 4500 gauss at its circular faces, and being shielded behind a contact face thickness of 3mm. The use of a rare earth magnet of high magnetic field strength serves to minimize the magnet mass and volume in the contact, thereby providing maximum conductivity of heat and current in the contact for its size, reducing the temperature of operation of the contact.

Whilst optimum arc extinguishing requires significant field strength, it is envisaged that apparatus in accordance with the present invention will find application with field strengths at the contact face as low as 100 gauss.

Preferably, in apparatus operating at voltages and currents described hereinbefore, a copper-tungsten alloy arcing contact of 10 mm diameter may be provided with an embedded rare earth permanent magnet of about 6.35 mm diameter and 12.7 mm length, shielded behind a contact face thickness of from 6 mm. Typically such a magnet may have a field strength measured at the face of the magnet of about 2000 gauss although it is envisaged that the advantages provided by embedding the magnet in the contact may provide useful results in apparatus where the field strength at the contact face is as low as about 75 gauss. ^• ^' Whilst the moving contact generally terminates the other side of a distribution AC circuit to the fixed contact, in certain applications the moving contact may, for example, be switching a centre tap or the like. The moving contact is preferably connected by a tail or via its supporting structure to a transmission line external of the housing and is preferably insulated from the housing such that both sides of the circuit are above earth.

In order that this invention may be more easily understood and put into practical effect, reference will now be made to the accompanying drawings which illustrate a preferred embodiment of the invention, wherein: -

FIG. 1 is a side view in cross section of apparatus in accordance with the present invention;

FIG. 2 is a side view in vertical section of a moving contact portion for use in the apparatus of FIG. 1

FIG. 3 is a plan view of the moving contact portion of FIG. 2 , and FIG. 4 is an end view of the contact region of the apparatus of FIG.l.

In the figures there is illustrated switch apparatus 10 suitable for use as for example a pole mounted switch in an AC distribution grid. The switch consists of a single break switch assembly contained in a fully welded and sealed stainless steel enclosure 12. The enclosure 12 is filled with sulphur hexafluoride gas 13 at atmospheric pressure.

The switch assembly is interposed in an AC circuit comprising a line conductor 14 and a load conductor 15 each of which are led through the top of the enclosure 12 by gas tight insulator assemblies 16 each comprising an inner insulative spigot 17 and an outer polymeric insulated tail support 20. The line conductor 14 is terminated by a fixed contact 21 consisting of an outer copper chromium alloy body portion 22 having embedded therein a cylindrical rare earth metal magnet 23. The load conductor 15 is terminated by a moving contact assembly 24 including a contact mounting body 25 having pivotally mounted thereto an elongate main contact arm 26. The contact arm 26 comprises a pair of contact plates 27 spaced apart by a pivot bush portion 30 of the contact mounting body 25 and provided with a flared portion 31 at its outer contact end 32 adapted to electrically engage an copper tungsten arcing fixed contact portion 33 of the fixed contact 21. The contact end 32 is adapted to be expanded to engage the fixed contact body portion 22 by means of spreading of the contact plates 27 against the bias of spring 34 mounted on a pin 35 acting between a stop 36 and a contact plate 27.

Pivotally mounted between and electrically common with the contact plates 27 is an integral arcing contact assembly 37 comprising an arcing contact 40 having a cylindrical rare earth metal permanent magnet 41 embedded therein and an arcing contact arm 42. The arcing contact arm 42 provides an anchor for one end of a spring 43 adapted to urge the arcing contact 40 through the space between the contact end 32 of the contact plates 27 such that the arcing contact 40 is presented to the arcing contact portion 33 of the fixed contact 21.

Operating means for the moving contact assembly 24 is provided _^y means of an external controller (not shown) adapted to provide control pulses via a control cable 38 to an operating assembly 44. The operating assembly comprises a closing solenoid 45 adapted to close the moving contact assembly 24 via a mechanism plate 46 pivotally mounted to the enclosure 12 and insulative push ro 47, the upper end of which is pivotally mounted to the main contact arm 27. The solenoid 45 closes the moving contact assembly 24 against the bias of a spring 50, and the contact assembly is maintained in its closed position by a latch 51.

Upon operation of the control means by the operator or automatically in response to a selected fault condition the latch 51 may be released by delivery of a control pulse to a. trip armature 52 the response of which is to move a trip bar 53 to release the latch 51 and allow the mechanism plate 46 to be drawn down by gravity and the spring 50, resulting in withdrawal of the moving contact assembly 24 out of electrical connection with the fixed contact 21.

Control primary condition sensors are provided such as embedded voltage sensing screens ( not shown) within the gas tight insulator assemblies 16 and current transformers 54. Lightning arresters 55 may be provided for both sides of the circuit in pole mounted distribution apparatus or on the load side only for substations. An auxiliary voltage supply of may be provided to the controller or alternatively an internal voltage transformer 56 can be provided.

Apparatus in accordance with the abovedescribed embodiment is capable of interrupting electric load currents of several thousand amperes in 50 Hz circuits operating at tens of thousands of volts. An electric arc is formed between the electrodes which is extinguished at a 50 Hz current zero. The arc is generally extinguished at a contact separation of about 10 mm compared to several centimeters in the case of unmodified plain break apparatus. Whilst an unmodified plain break will interrupt currents of up to about one hundred amperes, in the present apparatus the current interrupting ability of the simple plain break arrangement is increased by more than one order of magnitude by embedding a cylindrical rare earth permanent magnet in the fixed contact. The magnet is arranged so that the axis of the magnetic field lies along the line between the centres of the fixed and moving electrodes, and the arc is exposed to high magnetic fields because the end of the magnet is within a few millimeters of the points where the arc is in contact with the electrode.

The effectiveness of the arrangement in interrupting current is increased in this embodiment by arranging the main current, in the closed position, is carried by a main moving contact that opens a few milliseconds before the arcing contact draws an arc between the fixed and moving contacts. The embeddir - of the permanent magnet in the arcing contact with the axi.. >^' both magnets co-incident and the fields in the same direction provides exemplary performance at especially high currents.

It appears that the field of the permanent magnets interacts with the arc to prevented rooting at one point on either contact hi :ause the magnetic field ir"³"ices continuous rotational ovement in the arc plasma. Thi ;nsiderably cools the arc plasma before the current zero at which interruption occurs an prevents the formation of i~-^'gh temperature plasma jets at the contacts. The cooler gas approaching the current zero results in an arc column at the zero which is more easily interrupted than the higher-temperature arc co±umn produced with an unmodified plain break-

After trie interruption at the current zero the temperature of the thin arc column decays in a few microseconds, the temperature of the hot gas remaining in the space between the contacts is affected by the temperature of the gas surrounding the arc before interruption. As the arc temperature with permanent magnets is considerably lower than that with an unmodified plain break the hot gas is cooler in the arc space than would be the case with an unmodified arrangement. Thus there is a considerably quicker build up of dielectric strength between the contacts of the present apparatus than in the prior art plain break apparatus. The rate of build up of dielectric strength between the contacts is also increased by removal of plasma jets which can produce high temperature, highly conducting clouds of plasma between the electrodes of the plain break device.

It will of course be realised that while the above has been given by way of illustrative example of this invention, all such and other modifications and variations thereto as would be apparent to persons skilled in the art are deemed to fall within the broad scope and ambit of this invention as is herein set forth.

Claims

1. Switch apparatus of the type including:- a housing enclosing a dielectric environment; a pair of contacts within said housing movable relative to one another between a closed position in circuit and an open circuit position, wherein one or both of said contacts are provided with magnetic field means providing a magnetic field substantially aligned with a separation axis of said contacts.

2. Switch apparatus according to Claim 1, wherein one said contact is fixed relative to said housing.

3. Switch apparatus according to Claim 2, wherein said magnetic field means is selected from a permanent magnet or an inductive winding driven by alternating current to be switched or an external AC supply.

4. Switch apparatus in accordance with Claim 3, wherein said magnetic field means comprises a rare earth permanent magnet embedded in one or both of the contacts and protected from arc damage behind the contact face thereof.

5. Switch apparatus in accordance with Claim 4, wherein said magnet embedding contact or contacts comprises a material selected from high temperature alloys of tungsten and copper or chromium and copper.

6. Switch apparatus in accordance with any one of the preceding Claims, wherein one of said contacts comprises a contact assembly including an arcing contact and a main current contact, said contact assembly being operable to break the circuit by displacement of the main contact relative to the other contact to an insulating distance in the dielectric environment, followed by movement of the arcing contact away from said other contact to at least said distance.

7. Switch apparatus in accordance with Claim 6, wherein said contact assembly is operable to make the circuit by bringing the arcing contact into engagement with the other contact, followed by engagement of the main contact with said other contact.

8. Switch apparatus according to any one of Claims 6 and 7, wherein said contact assembly comprises a moving contact movable relative to the other said contact which is fixed relative to said housing.

9. Switch apparatus according to Claim 8, wherein said fixed contact has embedded therein a rare earth magnet comprising said magnetic field means.

10. Switch apparatus according to Claim 8, wherein said arcing contact has embedded therein a rare earth magnet comprising said magnetic field means.

11. Switch apparatus according to Claim 8 or Claim 9, wherein both of said fixed and arcing contacts are provided with rare earth magnet magnetic field means embedded therein and wherein the field axes of the respective magnets are substantially aligned with opposed poles adjacent when the circuit is closed.

12. Switch apparatus according to any one of the preceding Claims, wherein said contacts are operable to make or break said circuit by automatic switching means under control of condition respt αsive control means.

13. Switch apparatus of the type including: - a housing enclosing a dielectric environment; a fixed contact within said housing, and a moving contact assembly within said housing and movable relative to said fixed contact between a closed position in circuit with said fixed contact and an open circuit position, wherein said fixed contact comprises a magnetically permeable contact body having a permanent magnet embedded therein of field substantially aligned with the separation axis of said contacts.

14. Switch apparatus according to Claim 13, wherein said moving contact comprises a main contact and an arcing contact comprising a magnetically permeable arcing contact body having embedded therein a second permanent magnet of field substantially aligned with the field of the permanent magnet embedded in the fixed contact.

15. Switch apparatus according any one of the preceding Claims, wherein said housing is a substantially sealed housing containing a dielectric environment selected from an at least partially evacuated gas space, air or sulphur hexafluoride gas.

16. Switch apparatus according to Claim 15, wherein said dielectric environment comprises sulphur hexafluoride gas at a pressure selected to be from substantially atmospheric pressure to moderately elevated pressures.