Off-switching Of Liquid-metal Arc Switching Device By Auxiliary Arc Liquid-metal Starvation

Abstract

An enclosed vessel containing one or more main anodes, one or more auxiliary anodes, a cathode, and means for maintaining a low pressure in the tube comprises a liquid-metal arc switching device. The cathode is fed with a metal which is liquid at convenient temperatures, so that limited quantities of the metal are present and available on the cathode for arcing. The interior of the vessel is maintained at a low background pressure so that during nonconduction, vacuum space insulation is provided between the anodes and the cathode. Arc initiation is accomplished by any convenient initiator, and the arc runs upon the small amount of liquid metal fed at an appropriate rate to the cathode. Arc extinction is accomplished by causing current to flow from the cathode to the auxiliary anodes at rates substantially higher than the main anode current. This causes consumption of the liquid metal at a higher rate than the liquid-metal feed rate, with consequent arc extinction by cathode starvation.

Description

BACKGROUND

This invention relates to the field of liquid-metal arc rectifiers and switches, such as mercury arc rectifiers.

Mercury arc rectifiers are well known in the art. They suffer from numerous problems, which problems primarily stem from the fact that a large mercury pool is present in the device, and this large mercury pool maintains, through evaporation, a fairly high background pressure within the rectifier vessel. In commercial devices, the pool temperature is kept as low as is practical and as is consistent with arc operation, in order to maintain this background pressure as low as is possible. However, despite this, at the pool temperature usually found in such devices, there is sufficient mercury vapor within the tube that de-ionization is slow upon voltage reversal. This in turn occasionally causes arcing back from the anode to the cathode, which is destructive to the device and highly objectionable regarding the circuit in which the device is used.

To overcome this limitation, state of the art high voltage mercury tubes are provided with grading electrodes. These grading electrodes lead to another limitation: the current which can pass to one anode through such a set of grading electrodes is limited to such extent that for higher currents a number of parallel anodes and sets of grading electrodes are required. These limitations then lead to the need for complex multianode tubes (with current-dividing transformers to divide the current uniformly between parallel anodes) and grading electrodes with attendant voltage dividers.

The voltage holdoff properties of the conventional mercury pool liquid cathode devices are determined by a tradeoff between the desired voltage holdoff, the peak current, the voltage drop across the arc, and voltage recovery rates. These conflicting requirements do not permit the device to be designed for high voltage holdoff and high current without the complex grading electrodes and the multiple anodes mentioned above.

Furthermore, prior mercury arc rectifiers and switching devices are not capable of forced current interruption against a voltage persisting in the forward direction between anode and cathode.

SUMMARY

In order to aid in the understanding of this invention, it can be stated in essentially summary form that is directed to a switching device including an enclosing tube or vessel in which are located one or more main anodes, one or more auxiliary anodes, a cathode, and means for maintaining a low pressure in the tube. The cathode is fed with a low arc-voltage metal which is liquid at convenient temperatures, so that a small quantity of this metal is available on the cathode to permit a current-carrying arc to run between the cathode and the main anode. When forced interruption of this current is desired, the auxiliary anodes are connected to draw an arc current from the cathode to the auxiliary anodes, in excess of the main current from the cathode to the main anode. This causes consumption of the low arc-voltage metal, which is fed at a rate suitable for the main anode current, with resultant starvation of the cathode and arc cessation.

Accordingly, it is an object of this invention to provide a liquid-metal arc switching device wherein only a small amount of metal available for arcing is positioned on the cathode so that during arcing the pressure in the tube is relatively low and upon cessation of arcing the pressure between the anode and the cathode is quickly reduced to high vacuum to provide high insulative value therebetween. It is a further object of this invention to provide a liquid-metal arc switching device wherein the liquid metal has a low arc voltage and is fed to a cathode structure having a higher arc voltage so that arcing is restricted to the low arc voltage material and arcing ceases when the low arc voltage material available in the cathode structure becomes exhausted. It is a further object of this invention to provide an arc switching device which has the capability of withstanding high voltages across a single anode-cathode gap because of the low pressure between the anode and cathode when the device is not arcing. It is a further object of this invention to provide a liquid-metal arc switching device which has a cathode which emits a plasma during arcing, and two or more anodes positioned to electrically couple to the plasma during conduction for efficient conduction of large currents to any of these anodes, and means to rapidly reduce the pressure in the tube between the anodes and the cathode upon cessation of arcing to provide a rapid rise of holdoff voltage versus time. It is a further object of this invention to use one or more of said anodes as auxiliary anodes and the others as main anodes, in a liquid-metal arc switching device which has liquid metal fed at a rate proportional to the current passing from the cathode to the main anodes so that upon initiation of an arc between the cathode and auxiliary anodes at arc currents in excess of that of the main anode, the liquid metal in the cathode is exhausted with consequent arc cessation so that current stops flowing to the main anodes of the switching device. It is a further object of this invention to connect the auxiliary anodes in a separate electric circuit from the main anodes in a liquid-metal arc switching device so that an auxiliary arc can be run from the cathode to the auxiliary anodes at currents in excess of the main anode current, and at voltages substantially lower than the standoff voltage of the switching device. Other objects and advantages of this invention will become apparent from a study of the following portion of this specification, the claims and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWING

The single FIGURE of the drawing is a longitudinal section through a liquid-metal arc switching device with six auxiliary anodes therein for off-switching by starvation of the liquid metal on the cathode, and with a schematic electric circuit shown in association therewith.

DESCRIPTION

Referring to the drawing, the liquid-metal arc switching device is generally indicated at 10. The general term "switching device" is used to generally describe a device which is capable of initiating and/or interrupting current flow in a circuit. This includes circuits wherein the current reaches a natural zero by means independent of and exterior of the switch tube, as well as devices in which the current zero is accomplished by the switch tube by an increase in voltage thereacross by arc cessation. The first case includes rectification of alternating current, and the second case includes DC circuit breakers and commutators for converting DC to AC. Thus, the device itself inherently interrupts current flow upon voltage reversal and can be operated to interrupt current without current or voltage reversal. For these reasons, the term switch device is used generically to define the scope of such devices.

The switch device 10 includes an envelope 12 which serves as a vessel or a tube within which the primary components of this switch device are located. Vacuum pump connection 14 is provided on the envelope 12 to either originally pump down the original noncondensable gases after manufacture, whereupon it is sealed, or can be continuously pumped to maintain the desired low background pressure of noncondensable gases.

Main anode 16 is positioned within the envelope and has an external connector which extends externally of the envelope for electrical connection. Anode 16 is shown as being in the shape of a flat circular disc so that a maximum area is exposed to the electrically conducting plasma. Anode 16 is preferably provided with heat exchanger 20. Heat exchanger 20 controls the temperature of anode 16 to prevent extreme temperature excursions. Anode 16 is preferably maintained at a temperature above the condensation temperature of the liquid metal, hereinafter described, so that condensation does not occur thereon. Such condensation leads to arc-back situations, and is thus undesirable. Heating of the anode is caused by its absorption of the kinetic energy of the plasma particles, by the recombination energy of electrons and ions, and by the I.sup.2 R drop of the current flow through the anode material. Thus, at high currents, cooling may be necessary through heat exchanger 20 to prevent the anode from reaching destructive temperatures. On the other hand, at low loads, heating by means of heat exchanger 20 may be necessary to maintain anode 16 above the liquid-metal condensation temperature. Heat exchanger 20 is conventionally externally connected and is controlled by any convenient temperature sensing means responding to the temperature of anode 16.

Envelope 12 is made at least partially of insulative material to provide electrical separation of the envelope walls between anode 16, condenser 50, and cathode 22. The envelope extends downwardly, as shown in the drawing, from anode 16 to below cathode 22. Offset 24 provides an internal offset adjacent which insulating disc 26 is positioned. Envelope 12 continues downward below offset 24 as a cylindrical tube, and curves inward and upward to form tubular reentrant section 28.

Cathode 22 is made of a metal which is a high arc voltage material, as compared with the liquid metal used in connection with it. The term arc voltage is defined as arcing voltage in Proceedings of the Institute of Electrical Engineers, Volume 110, No. 4, Apr. 1963, pages 796--797, Section 4.6. Furthermore, the metal of the cathode structure must be compatible with the liquid metal. Cathode 22 has pool-retaining walls 30 against which the liquid metal is located and against which the arc runs, at the juncture or junctures between walls 30 and the liquid metal. Above the walls, cathode 22 has a front face 32, which is substantially in the form of a planar disc. Below the front face 32, cathode 22 extends downwardly in a reduced diameter tubular neck 34 which is sealed to tubular reentrant section 28 of the envelope by means of an appropriate seal joint 36. In the embodiment shown, cathode 22 has a shoulder 36 positioned between the larger diameter front face and neck 34. Insulating disc 26 rests against this shoulder and is maintained in place by retainer 38. Other geometries of wall 30 than the one shown in the drawing can also be employed.

Heater 40 is located within the hollow interior of the cathode behind front face 32, and extends around the outer periphery, down inside of shoulder 36 and into neck 34 so that the entire exposed area of the cathode away from walls 30 can be controlled in temperature. The central portion of the cathode, below the recess formed by walls 30, extends downward to form neck 42, and heat exchanger 44 is positioned exteriorly of the neck. Both heater 40 and heat exchanger 44 have suitable connections extending downward within tubular reentrant section 28 to permit external connection and temperature control. If desired, temperature sensors can be mounted so that accurate temperature determinations of the respective portions of the cathode 22 can be made.

The liquid metal required on the surface of the cathode structure for arcing can be provided either by feeding of metal in the liquid state or by intermediate vaporization and recondensation.

The term liquid metal is used to define those metals which are liquid at or just somewhat above room temperature. While called liquid metal, the metal is not necessarily in the liquid state when fed to walls 30 which define pool-keeping surfaces. Mercury is a convenient liquid metal, because it is normally liquid at room temperatures. Furthermore, it has a suitably low arc voltage. Thus, the arc preferentially strikes upon the liquid metal when the external surfaces of cathode 22 are formed of a relatively high arc voltage material. Additional materials which are suitable to act as the liquid metal fed to cathode 22 are exemplified by cesium, lithium, and gallium. If necessary, liquid-metal feed line 46, and associated equipment to provide the liquid metal thereto can be heated to maintain the liquidity or vapor state of the liquid metal.

As indicated, cathode 22, as well as anode 16, are preferably formed of high arc voltage materials. When mercury is employed as the liquid metal, molybdenum serves as a suitable material for the anode and cathode. Liquid-metal feed line 46 extends from a source 47 of liquid metal of such nature as to provide the proper flow rate of liquid metal to cathode 22, as hereinafter described. Liquid-metal feed line 46 extends up through the reentrant section 28, and in the example of the drawings is connected centrally of cathode 22 to feed the liquid metal to the space defined between walls 30. If desired, a suitable flow restriction 48 can be positioned at the lower juncture of walls 30 with the opening therebelow connecting to feed line 46 to impede liquid-metal flow to the space defined between walls 30. When liquid metal is fed to the space in its liquid state, a porous flow restriction is desirable for it prevents an arc from extending down into the feed line when the pool between walls 30 is exhausted. However, when the liquid metal is fed in vapor form, a flow restriction in the form of a capillary passage is preferred, with the capillary somewhat larger than the passages through the porous mass employed in the example for liquid feeding.

Additionally, within envelope 12 condenser 50 is located. It serves emitted one means for continuously removing from the tube metal vapor emitted by the cathode. Condenser 50 is suitably externally connected to maintain the desired temperature upon the condenser surface, as described below. When condenser 50 is maintained at a temperature to condense the metal to the liquid state, it is collected by trough 52, and can be recirculated to the cathode. In the case of recirculation of the liquid metal, it can be drained out through line 54, and an appropriate isolator will be required in the recirculation line.

Auxiliary anodes 56 are also located within envelope 12. Since the plasma jet emerging from the cathode is substantially in the form of a solid cone having its apex on the cathode, the auxiliary anodes 56 are arranged as radial fins protruding into the plasma cone. Thus, during normal conduction of current from the cathode to the main anode 16, auxiliary anodes 56 do not substantially interfere with the operation. However, auxiliary anodes 56 provide sufficiently large surfaces within the plasma jet that they are in good electrical coupling therewith. Six auxiliary anodes 56 are illustrated.

The auxiliary anodes are connected through lines 58 to a source of electric current 60. In the example given by the FIGURE, source 60 may be a DC source, and all six auxiliary anodes may be connected in parallel, or source 60 may be a three or six phase AC source, and the auxiliary anodes may be connected to the different phases. source 60 in turn is connected through switch 62 to line 64 which is connected to cathode 22. Thus, continuity between cathode 22 and auxiliary anodes 56 is controlled by switch 62. Main anode 16 is connected by line 66 serially through load 68 and source of electric current 70. Source 70 is also connected to cathode 22 by means of line 64. Since the source of electric current 70 is driving a load, for example load 68, the voltage thereof may be quite high, and this voltage is the voltage switched by switching device 10. On the other hand, since the voltage against which source 60 operates is the voltage drop through the arc between cathode 22 and auxiliary anodes 56, source 60 can operate at fairly low voltages and at greater currents than the current at the main anode 16, and still be of considerably less power than that supplied by a source 70. For example, if the source 70 supplies voltage at 10.sup.5 volts, and at 1,000 amperes, the current supplied by a source 60 can be 10,000 amperes at the voltage drop across the arc, which is about 10 volts. If both currents flow for equal periods, the power spent in the auxiliary discharge to the auxiliary anodes 56 is 10.sup.-3 times the main circuit power passing through main anode 16.

When mercury is the liquid metal, the liquid-metal feed requirement for such an arc, drawn from a specific cathode may, for example, be one cubic centimeter of mercury per hour per 100 amperes of discharge current. Thus, in the example above, 10 cubic centimeters of mercury are required per hour to supply sufficient mercury to maintain a 1,000 amperes arc from the cathode to the main anode. When arcing to the auxiliary anode takes place at 10,000 amperes, the amount of mercury feed is only one-eleventh of that required to maintain arcing. If the mercury available on the cathode for arcing is 0.01 cubic centimeters, it is clear that the arc is extinguished in about 0.3 seconds.

The pressure within envelope 12 is pumped down through a vacuum pump connection 14, before the device is put into use. In some cases, when the contents of the envelope have a minimum of out-gassing, the envelope can be sealed by closure of connection 14. In other cases, it may be desirable to keep the connection with a vacuum pump so that noncondensables can be pumped down when operation of the device so indicates.

The arc is initiated by any convenient means. No specific arc initiator is disclosed in the drawings, but those well known in the art can be used. Examples of such are auxiliary electrode igniters, semiconductor igniters, and the like. Alternatively, a laser igniter directed onto the liquid-metal surface is suitable, as is an igniter which emits a puff of liquid-metal plasma into the space between the anode and the cathode to initiate arcing.

Presuming that a suitable voltage is applied across the anode to cathode space, and liquid metal is available on cathode walls 30, the ignition of the device will cause conduction. The amount of liquid metal available on walls 30 for evaporation is kept purposely low so that the electron to atom ratio is high. As a consequence, the pressure in the envelope will remain relatively low, even during conduction. A typical range for mercury as the liquid metal is from 10.sup..sup.-2 to 10.sup..sup.-6 Torr.

Heater 40 maintains the face of the cathode sufficiently hot that liquid-metal condensation cannot occur there. Similarly, the temperature of the pool-keeping walls 30 is controlled by heat exchanger 44. In cases where a liquid-metal pool is desired, heat exchanger 44 cools the pool-keeping walls to prevent excessive evaporation, especially in the case of high currents where the arc introduces a considerable amount of heat into the pool-keeping structure. In cases where the arc current is low, it may be necessary to heat the pool-keeping walls, especially in the case where the liquid metal is fed in vapor form to the walls 30. In such a case, the walls 30 are preferably slightly above the equilibrium condensation temperature of the liquid-metal vapor at that pressure. Thus, only transient condensation can occur. Transient condensation, in this sense, is a situation wherein slightly superheated metal vapor deposits upon the wall in a thin film over a portion of the area, with such condensation occurring for a short time. However, since the surfaces are above the equilibrium condensation temperature, no condensation of droplets or masses of metal occurs. Instead, liquid metal is continuously deposited and evaporated off, and occupies only a portion of the wall area at any one time.

As stated above, heat exchanger 40 keeps the remainder of the cathode above condensation temperature to prevent metal pools from forming thereon to prevent the arc from acting on any surface but the walls 30. In addition, the heating prevents metal condensation at the juncture between neck 34 and tubular reentrant section 28, for arcing at this juncture would be destructive. Additionally, insulating disc 26 prevents any appreciable quantity of the metal vapor from reaching that juncture in order to prevent the destructive arcing.

Condenser 50 is positioned to condense metal vapor in order to prevent a buildup of vapor pressure within envelope 12. In the case of mercury, in order to maintain the metal vapor pressure within the tube in the absence of arcing below 10.sup..sup.-5 Torr, when it is desired to retain the condensed mercury in the liquid state, the temperature of condenser 50 is maintained at about 240.degree. K. When a lower pressure is desired, a lower condenser temperature can be used, resulting in solidification of the condensed mercury. In the latter case, the condenser can be periodically warmed to permit liquid mercury to drain out of the bottom of trough 52 through drain 54.

In operation, switch 62 is open and the main circuit electron current is passing from cathode 22 to main anode 16 and thence through load 68 source 70. Liquid metal is supplied to the cathode at a suitable rate to maintain a sufficient quantity to feed the arc, and thus maintain continuity. However, when off-switching is desired, switch 62 is closed. Thereupon, the current supplied by source 60 also draws additional arc current. In the example given, this current is 10 times the current to the main anode 16. This auxiliary anode current consumes the liquid metal available on the cathode, and without increase in liquid-metal feed, the liquid metal on the cathode soon becomes exhausted and the arc terminates by starvation. In a switching device capable of switching the 10.sup.5 volts at 10.sup.3 amperes, as described above, and with an auxiliary anode current of 10.sup.4 amperes, the arc will be starved in approximately 300 milliseconds. With arc starvation, the source 70 builds up voltage across the switching device 10. With mercury as the liquid metal and molybdenum as the cathode structure material, a rate of voltage rise not exceeding 10 kilovolts per microsecond has been found permissible.

This invention having been described in its preferred embodiment, it is clear that it is susceptible to numerous modifications and embodiments within the ability of those skilled in the art and without the exercise of the inventive faculty. Accordingly, the scope of this invention is defined by the scope of the following claims.

Claims

What I claim is:

1. A switching device, said switching device comprising:

an enclosing envelope, a main anode and an auxiliary anode within said envelope, a cathode within said envelop;

electrical connection means on said main anode and said cathode for connection to an external serially connected load and a first source of electric power;

electric connection means on said cathode and said auxiliary anode, a second source of electric power positioned externally of said envelope and electrically connected to said cathode and said auxiliary anode, said second source of electric power being of such current capacity as to cause an arc current between said cathode and said auxiliary anode at least as great as the arc current between said cathode and said main anode;

said cathode comprising high arc voltage metal retaining walls, means for feeding lower arc voltage liquid metal to rest against a portion of said retaining walls at an adequate rate for supplying a current-conducting arc between said cathode and said main anode and not in excess of such a rate so that upon additional arcing between said cathode and said auxiliary anode caused by current flow from said second source of electric power, the lower arc voltage liquid metal available for arcing becomes exhausted to result in arc termination between said cathode and said main anode to stop current conduction therebetween even in the presence of an electric field therebetween.

2. The switching device of claim 1 wherein means are connected to said envelope for maintaining the pressure in the envelope sufficiently low to establish a vacuum of high insulative property after arc cessation.

3. The switching device of claim 2 wherein said means for maintaining the pressure in the envelope maintains the pressure in the envelope below 10.sup..sup.-2 Torr during arcing.

4. The switching device of claim 2 wherein said main anode is positioned to face said walls on said cathode so as to intercept the high velocity jet stream emitted from the arc spots at the junction between said lower arc voltage liquid metal and said high arc voltage metal retaining walls.

5. The switching device of claim 4 wherein said auxiliary anode is positioned to protrude into and partially intercept the arc jet stream issuing from said cathode to said main anode.

6. The switching device of claim 5 wherein there are a plurality of auxiliary anodes, each protruding into and partially intercepting the arc jet stream.

7. The switching device of claim 5 wherein the number of auxiliary anodes projecting into and partially intercepting the jet stream at least equals the number of phases of said second source of electric power.

8. The switching device of claim 1 wherein a source of electric energy and a load are serially connected between said cathode and said main anode, said source of electric energy being arranged to operate not in excess of a maximum current, and said second source of electric energy is connected to said cathode and said auxiliary anode, said second source of electric energy being arranged to operate at currents in excess of said maximum current of said first electric energy source.

9. A switching device, said switching device comprising:

an enclosing envelope, a main anode and an auxiliary anode within said envelope, a cathode within said envelope;

electrical connection means on said main anode and said cathode for connection to an external serially connected load and first source of electric power;

electric connection means on said cathode and said auxiliary anode, a second source of electric power connected to said cathode and said auxiliary anode, said second source of electric power having sufficient current capacity for drawing an arc between said cathode and said auxiliary anode having current in excess of an arc between said cathode and said main anode;

said cathode comprising high arc voltage metal retaining walls;

feed means for feeding lower arc voltage liquid metal to rest against a portion of said retaining walls at an adequate rate to supply sufficient liquid metal to supply an arc between said cathode and said main anode and insufficient to continually supply arcs between said cathode and both said main anode and said auxiliary anode so that, upon arcing between said cathode and said auxiliary anode, said liquid metal supply against a portion of said retaining walls is exhausted and the arc between said cathode and main anode is extinguished.

10. The switching device of claim 9 wherein the means are connected to said envelope for maintaining the pressure in the envelope sufficiently low to establish a vacuum of high insulative property after arc cessation.