WO1983002524A1

WO1983002524A1 - High current switching

Info

Publication number: WO1983002524A1
Application number: PCT/AU1983/000006
Authority: WO
Inventors: Of Australia Commonwealth; A. Gregory Clark; Yong Chia Thio
Original assignee: Commw Of Australia
Priority date: 1982-01-18
Filing date: 1983-01-17
Publication date: 1983-07-21
Also published as: JPS59500156A; GB2124435A; GB2124435B; AU1106483A; EP0098846A1; GB8324621D0; EP0098846A4

Abstract

A high current switch and a method suitable for switching stored energy in a large value capacitor C. The switch has electrodes 5 and 9 spaced apart a distance sufficient to withstand electrical arcing due to a voltage of said energy stored. Switching is effected by creating a plasma along the opposed faces of the electrodes 5 and 9. The plasma can be created in any suitable manner. In one embodiment the plasma is created by electrically exploding a conductor (11) placed between the electrodes 5 and 9.

Description

HIGH CURRENT SWITCHING TECHNICAL FIELD

This invention relates to high current switching and relates particularly but not exclusively to such for use in circuits where large value capacitors are charged so that the charge can be transferred into a load.

O PI BACKGROUND ART Hitherto currents in excess of 100 kilo - amperes with large transfer rates are switched by way of ignitrons, spark gaps or vacuum arc switches. Each of these prior art methods of switching has its own limitations.

In the case of ignitrons, the device is limited to a charge transfer of less than 2,000 coulombs. Further, the mercury pool has to be maintained level and thus, the switch cannot be used in a mobile environment. In respect of spark gaps and vacuum arc switches they have inherent problems due to the requirement of initiation of the plasma arc between the main electrodes to cause the charge to transfer. These switches are often used to short a capacitor in a circuit where the capacitor is charged and where that charge is then transferred to a load so that the load can receive a large D.C. current provided by that charge. In these circuits the capacitor is shorted across its terminals, by closure of a switch called a crow bar switch, at the instant when the first cycle of the current is nearly at a maximum. This crow-barring prevents oscillations of the voltage cycles from appearing at the capacitor terminals - a condition which if not prevented could destroy the capacitor because of the voltage reversal which occurs during the cycles. The effect of crow-barring the capacitor is to then provide a D.C. current into the load.

Spark gap and vacuum arc switches are inherently not suitable for use in crow-barring a capacitor to prevent voltage reversal of the capacitor as the gap in these switches must be large enough to stand off the high initial voltage across the capacitor and yet, they must be designed to ensure that electrical break-down occurs across the same gap at the instant when the current transfer is at a maximum At this particular instant the voltage across the terminals of the capacitor is almost zero as it is out of phase with the current by approximately 90 degrees. Therefore initiation of the plasma between the electrodes is exceptionally difficult.

DISCLOSURE OF THE INVENTION An object of the present invention is to provide a switch which can be used for switching high currents and which will be simple in construction. A switch in accordance with a preferred embodiment of the invention has particular application in crow barring a capacitor. Certain embodiments will have particular application for switching high currents at high voltages. Therefore according to one aspect of the present invention there is provided a high current switch comprising, two elongate switching electrodes spaced apart a distance sufficient to withstand an unwanted spark discharge between the electrodes consequent on a voltage which, in use, appears across the electrodes, and means for initiating a plasma along the length of the opposed faces of the electrodes, said length being substantially greater than the length of any plasma created by a spark discharge, whereby to enable the initiation of a current transfer through said switch. A circuit including the switch is also included within the scope of the invention.

In accordance with a further aspect of the present invention there is provided a method of switching a high current comprising, spacing apart two elongate switching electrodes said spacing being of a distance sufficient to withstand any unwanted spark discharge between the electrodes consequent on voltage which, in use, appears across the electrodes, and then introducing a plasma over a length of the opposed faces of the electrodes, said length being substantially greater than the length of any plasma created by a spark discharge, whereby to enable the initiation of a current transfer through said switch.

Most preferably the plasma is created over 'the length of the opposed faces of the electrodes by electrically exploding a conductor which is placed between the electrodes. Alternatively the plasma can be created by directing a laser beam between the electrodes in a direction extending longitudinally of the longitudinal extent of said elongate electrodes.

Another alternative is to accelerate a volume of plasma throughout the gap between the electrodes by using a plasma gun.

In the embodiment where the conductor is exploded, the explosion is achieved by way of electrically fusing the conductor.

BRIEF DESCRIPTION OF THE DRAWINGS _ ^'

In order that the invention can be more clearly ascertained preferred embodiments will now be described with reference to the accompanying drawings wherei :- Figure 1 is a perspective view of one embodiment;

Figure 2 is a perspective view of a further embodiment; Figure 3 is an end view of the embodiment shown in Figure 2;

Figure 4 is a diagramatic view of the embodiments of Figures 1 and 2 prior to creating the plasma; Figure 5 is a diagramatic view showing the creation of the plasma between the electrodes;

Figure 6 is a circuit diagram of one preferred circuit where the switch is used; and

Figure 7 is a alternative circuit diagram where the switch is used.

I DESCRIPTION OF PREFERRED EMBODIMENT Referring firstly to Figure 1 there is shown a bus-bar 3 terminating with a switching electrode 5. A similar bus-bar 7 is placed in the same plane as the bus bar 3 and terminates with a further switching electrode 9. The switching electrodes 5 and 9 have an elongate configuration. The outer surfaces of the switching electrodes 5 and 9 are shaped to smoothly blend with the surfaces of the respective bus bars 3 and 7- so as to minimize electrical field gradients. The elongate switching electrodes 5 and 9 extend parallel to one another and are spaced apart a distance, sufficient having regard to the potential between the electrodes 5 and 9 so that unintended electrical breakdown will not occur therebetween. A conductor 11 is placed between the switching electrodes 5 and 9. It is not necessary for the conductor 11 to be placed centrally in the gap, but it is most preferable that it be located close to the centre of the gap and directly between the opposed surfaces of the respective switching electrodes 5 and 9.

In the embodiment shown in Figure 1 there are insulating support members 13 at each side of the switching electrodes 5 and 9 which support the conductor. This conductor 11 is 0.05mm copper wire. The conductor 11 may alternatively comprise a sheet of metal foil or conductive film on a substrate. Electrical leads 15 terminate with each end of the conductor 11 and enable an electric current to be passed to the conductor 11 at the time when the switch is required to operate. In the embodiments shown in Figures 4 and 5 it can be seen that the conductor 11 is placed centrally between the switching conductors 5 and 9 and in line with the central planes of each of the respective bus-bars 3 and 7. - fa ¬

in this embodiment the position of the conductor 11 aligns with the position where the surfaces of the two switching electrodes 5 and 9 are closest to each other. Figure 3 shows the plasma that is created when the conductor -IX is exploded by fusing. It can be seen that a plasma^*is provided in the gap between the switching electrodes 5 and 9 and that the plasma extends along a length of the electrodes 5 and 9 for a substantial portion thereof. In the embodiment of Figure 1 it can be seen that the plasma will extend along the whole of the length of the respective switching electrodes 5 and 9. This should be contrasted with prior art spark gap switches where a conductor is brought into the gap between the electrodes and a high voltage applied to that conductor sufficient to cause a spark to discharge. The spark then provides a plasma but the nature of the spark is a point contact at one or more positions along the length of the switching electrodes 5 and 9. Thus, the current flows only through the plasma regions and accordingly, in this prior art embodiment it leads to pitting of the surfaces of the switching electrodes 5 and 9.

Figures 2 and 3 show an alternative embodiment. In this alternative embodiment like components to those shown in Figure 1 will be numbered with the same numeral. In this embodiment instead of the conductor 11 being supported directly between the contact electrodes 5 and 9 in air, the supporting means may comprise a suitable electrically insulating rod 17, e.g. a nylon rod. The rod 17 of nylon may have two holes 19 and 21 extending therethrough in a direction perpendicular to the longitudinal axis thereof. Thus, the conductor 11 can be placed along the outer surface of the rod 17 and each end arranged to pass respectively through the holes 19 and 21. Electrical leads 15 can then terminate with the - -

conductor 11 by suitable clamping terminals 23. The rod 17 of nylon,-¹ together with the conductor 11, are then simply placed on top of the switching electrodes 5 and 9 so that the xod 17 rests thereon. Thus, in this construction, the conductor 11 is suitably held between the switching electrodes 5 and 9. The insulating rod 17 has the effect of suppressing unwanted electrical break¬ down between the electrodes.

Referring now to Figure 6 there is shown. one particularly preferred circuit where the switch is used in a crow-bar situation. In this embodiment a large value capacitor C of 1600 micro-farads has bus-bars 25 extending therefrom The bus-bars 25 extend to a load which may include an inductance L and a resistance R. The respective bus-bars 3 and 7 extend from the bus-bars 25 to the switching electrodes 5 and 9. The conductor 11 is supported between the switching electrodes 5 and 9, as for example by either of the methods shown in Figures 1 or 2. In the embodiment shown in Figure 6 the conductor 11 is fused by a self-synchronized system utilizing two diodes D, and D_? connected to the respective bus-bars 25 which terminate with each side of the capacitor C. These diodes, in turn, connect with the respective ends of the conductor 11. A switch S. such as conventional spark gap switch isolates the main crow-barring switch composed of the switching electrodes 5 and 9 from the capacitor C. The capacitor C is charged by a conventional charging circuit not shown. It has a charge capacity of approximately 80 kilojoule.' In operation switch S -> is closed allowing the charge to transfer into the load; however, at a later instant after switch S, is closed, the voltage across the capacitor C terminals reverses. Thus, the diodes D, and D„ conduct allowing a current to flow through the conductor 11 to cause it to electrically explode and create a plasma in the gap between the ain

- RE

GMPI switching electrodes 5 and 9. At the instant when switch S-, is closed the charge transfers into the load and thus when the main crow-barring switch comprised of the switching electrodes 5 and 9 operates, the current which is retained within the inductance L in the load is caused to be applied to the resistance in the load as a D.C. current of slowly reducing magnitude. The diodes D -^ and D can be of any convenient type and in the preferred embodiment they are inexpensive types which conduct a high current for a very short period of time, sufficient only to allow the fusing conductor 11 to explode. Type VC 7OX by VARO have been found suitable.

In the embodiment of Figure 7 the construction is substantially identical to that shown in Figure 6 except that the conductor 11 is fused by a power source 30 external to the main capacitor C system. In this embodiment the conductor 11 is fused by a separate capacitor C2 which is charged by a high voltage power supply i.e. 1000 volts. Switch S₂ is provided so that it can be operated at a desired instant so that the charge retained within the capacitor C2 can be applied to the conductor 11.

It will be appreciated that in this embodiment the plasma can be created to occur at different times during the discharge cycle by exploding the conductor 11 at the required times. In the previous embodiment this could be controlled as for example, by using diodes D, and D_ with appropriate forward resistances.

The providing of the plasma over a substantial length of the switching electrodes 5 and 9 helps to ensure a homogeneous current density for the main discharge and thus reduces erosion with large charge transfers. By appropriate design, the plasma can be

OMPI ^" swept across large electrode faces at high velocities by the magnetic field produced by the trigger and/or main discharge currents.

It will be appreciated that the switch need not be pressurised or placed under vacuum or be in a fast gas flow environment as is required with prior art techniques. Furthermore the system proposed is such that the switch can be operated in any orientation. Thus, the switch described herein has particular application in D.C. welding applications where for example armour plate steel is to be welded. In these circumstances the necessary welding current can be stored in a capacitor and then the capacitor crow-barred at the appropriate time period as the current transfers into the load. The invention has application in many fields and the above is given by way of example only.

Modifications may be made to the invention as would be apparent to persons skilled in the electrical switching art and all.such modifications are deemed to be within the scope of the invention the nature of which is to be determined from the foregoing description.

. ,_"^ΛPO^"

Claims

1. A high current switch comprising, two elongate switching electrodes spaced apart a distance sufficient to withstand an unwanted spark discharge between the electrodes consequent on a voltage which, in use, appears across the electrodes, and means for initiating a plasma along the length of the opposed faces of the electrodes, said length being substantially greater than the length of any plasma created by a spark discharge, whereby to enable the initiation of a current transfer through said switch.

2. A high current switch as claimed in Claim 1 wherein said means for initiating a plasma comprises an electrical conductor between the electrodes extending in a direction along the longitudinal opposed faces thereof and means for supplying a current to said electrical conductor to cause it to electrically fuse and explode thereby creating said plasma.

3.- A high current switch as claimed in Claim 2 wherein said electrical conductor is carried on an electrically insulating member which rests on and spans between said electrodes.

4. A switch as claimed in Claim 2 or Claim 3 wherein said electrical conductor is connected with means which permit said electrical conductor to electrically fuse and explode and produce plasma at a particular time after said voltage appears across said electrodes.

5. A switch as claimed in Claim 2 or Claim 3 wherein said electrical conductor is connected in series with diode means which in turn are connectable in parallel with said electrodes whereby when said voltage is applied across said electrodes said diodes will conduct and permit said electrical conductor to explode and create said plasma.

6. A switch as claimed in Claim 4 wherein said means which permit said electrical conductor to electrically fuse and explode is an electrical means which is separate from said voltage.

7. A switch as claimed in Claim 5 wherein said diode means and said electrodes are connected in series with a large value capacitance which stores energy to be switched by said electrodes, and a further switch which when closed applies voltage from said capacitance across said electrodes and wherein a load is connectable in parallel therewith across said electrodes.

8. A switch as claimed in Claim 6 wherein said electrodes are connected in series with a large value capacitance which stores energy to be switched by said electrodes and a further switch which when closed applies voltage from said capacitance across said electrodes and wherein a load is connectable in parallel therewith across said electrodes.

9.. A method of switching a high current comprising, spacing apart two elongate switching electrodes said spacing being of a distance sufficient to withstand any unwanted spark discharge between the electrodes consequent on voltage which in use, appears across the electrodes, and then introducing a plasma over a length of the opposed faces of the electrodes, said length being substantially greater than the length of any plasma created by a spark discharge, whereby to enable the initiation of a current transfer through said switch.

10. A method as claimed in Claim 9 wherein said plasma is introduced by electrically fusing and exploding an electrical conductor between said electrodes.