US3303376A - Triggered vacuum gap device employing gas evolving electrodes - Google Patents

Description

Feb. 7, 1967 J. M. LAFFERTY TRIGGERED VACUUM GAP DEVICE EMPLOYING GAS EVOLVING ELECTRODE S 70 L M/E VOLTAGE Filed Nov, 30, 1965 dd y as f

Inventor:

e M Ldffiert y,

a? is A ttor'n e y.

3,31%,3'3'6 Patented Feb. 7, 1367 nice 3,303,376 TRIGGERED VACUUM GAP DEVICE EMPLOYING GAS EVOLVING ELECTRODES) James M. Laiierty, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Filed Nov. 30, 1965, Ser. No. 510,562

Claims. (Cl. 313- -148) The present invention relates to improved vacuum gap devices and particularly those of the triggered. vacuum gap and triggered vacuum switch type which are adapted to operate on alternating or unidirectional voltages.

Vacuum switches and vacuum gaps have recently been brought to a stage of great commercial and technological importance. .The fundamental improvement which has greatly increased the versatility and the usefulness of such devices constitutes means for accurately and repetitively controlling, with great speed, the breakdown of such devices. In accord with my issued U.S. Patent No. 3,087,092 entitled Gas Generating Switching Tube issued April 23, 1963, I accomplish this by providing a trigger electrode assembly associated with the cathode electrode of the switching device for instantaneously and with great speed and accuracy establishing therein a high current arc between a pair of primary electrodes disposed therein so as to define a breakdown gap.

While the device set forth in my aforementioned patent is broadly adapted for many uses in the fields of equipment protection and high current switching, the specific embodiments disclosed therein are not well adapted for alternating current operation and often may require extra system components and circuits to provide for usage with alternating voltages because of the fact that either primary electrode may become the cathode electrode when so utilized.

According to it is an object of the present invention to provide improved vacuum gap devices having structure which uniquely adapts the same for use with alternating current voltages while, at the same time, not rendering the device as inoperable with unidirectional current voltages.

In accord with one feature of the present invention I provide a vacuum gap device including a pair of primary electrodes defining therebetween a vacuum gap. These electrodes are located within an evacuated envelope, which envelope is composed partially of metal and at least in part of the high voltage insulating substance so as to provide electrical isolation between the primary electrodes to prevent short-circuiting of the gap defined thereby. In one embodiment of the invention both electrodes may be fixed electrodes defining a fixed gap. In another embodiment one electrode may be attached to a suitable vacuum-tight bellows which permits relative motion between the two electrodes so as to allow the gap to be manually operable and or reclosable. In accord with this invention trigger electrode assemblies are located within central apertured portions of each electrode and are connected so as to cause the injection of a highly ionized gaseous plasma into the gap between the main or primary electrodes by causing a sprak discharge and the generation of a gaseous plasma between the electrode which is energized as cathode and the associated trigger electrode. a

The novel features characteristic of the present invention are set forth in the appended, claims. The invention itself, together with further objects and advantages thereof may be more readily understood by reference to the appended drawing in which:

FIGURE 1 is a vertical cross-sectional view of a fixed gap triggerable vacuum gap device constructed in accord with the present invention;

FIGURE 2 is a vertical cross-sectional view of a trigger electrode utilized in the device of FIGURE 1, and

FIGURE 3 is a vertical cross-sectional view of an alternative embodiment of the invention. '1

The vacuum gap device of FIGURE 1 includes an envelope represented generally as 1 containing therein is symmetrical along the longitudinal axis thereof, a first flanged metallic end piece 11 having a central aperture therein and a second flanged metallic apertured end plate. 12, also having a central aperture therein, said end pieces being disposed at opposite ends to define with cylindrical A pair of substantiallyv cylindrical centrally apertured insulated bushings 13 and 14 each ofW'hich has a large outside diameter portion,

body 9, a closed container.

and a smaller outside diameter portion connected by an annular shoulder serve as insulating members for the device and are assembled with the smaller outside diameter portion thereof extending inwardly with the annular shoulder resting against the planar exterior surface of aperture end plates 11 and 12 respectively. The central,

apertures within insulated bushings 13 and 14 are closed by the insertion therewithin of annular cylindrical electrode support members 15 and 16 respectively. The interior of cylindrical electrode support members 15 and 16 are hollow and sealed to vacuum by the insertion therein and sealing thereto of trigger electrode assemblies 5. and 6 respectively. Each of the cylindrical apertures in bushings 13 and 14 are slightly larger than the outside diameter of electrode support members 15 and 16 for added insulation properties and to provide an insulator surface not likely to be short-circuited by condensed metal vapor. The vacuum seal is made between these members by brazing thereto of annular seal members.

17 and 18 respectively, each of which has a larger diameter portion which fits tightly over the outside diameter of the respective insulator bushing and a smaller diameter portion which fits tightly over the exterior portion of the respectivel electrode support member.

The material comprising the devices illustrated in FIG-.

URE 1 may typically be as follows: Main envelope member 9 may be fabricated from a metal which will withstand high temperatures and which possesses sul'ficiently physical strength as to constitute the outside body of the.-

device, as for example, stainless steel. Flanged end plate members 11 and 12 and flanged seal members 17 and 18 are typically comprised of a material which makes good vacuum-tight seals to ceramic bushings 13 and 14 and to metallic members 9, 15, and 16 respectively and may conveniently be a Fernico or Kovar alloy generally utilized in electric discharge devices of this nature for this purpose. Bushings 13 and 14 are comprised of a high temperature gas impermeable'ceramic material as for example high density alumina (94% or higher A1 Electrode support members 15 and 16 are preferably comprised of a highly conductive, substantially gas free material as for example OFHC copper, premium grade. No special precautions need to be taken to render this material gas-free, other than vacuum firing prior to assembly. Primary electrodes 2 and 3 are composed of a high ly purified copper or high vapor pressure material as for example any of the materials set forth in Lee and Cobine Patent No. 2,975,256, issued March 14, 1961. This material is rendered substantially free of all gas and'gas forming compounds by some refining technique'as for example that set forth in copending application Serial No. 146,245, filed October 19, 1961, now Patent No. 3,234,- 351, of M. HgHebb, so as to reduce the concentration of gas and gasforrning impurities therein to a figure of less than 1 part in The trigger electrode assemblies 5 and 6 of FIGURE 1 of the drawing are illustrated in greater detail in vertical crosssection in FIGURE 2 of the drawing. In FIGURE 2, a typical figure represented generally as 20 comprises a hollow cylindrical metallized ceramic base member 22 which has a narrow circumferential groove 23 out therein, which may be tapered as shown, near the inward extremity thereof. Means for sealing the trigger electrode in a vacuum-tight seal to the interior of the electrode support member are provided in the form of an annular flanged member 24 having an extended shoulder and an aperture collar member 25 which rests thereupon and is sealed thereto. Both of these members are conveniently constructed of a metal suitable for making metal-to-seal ceramic seals as for example one of the general classes of metal alloys known as Fernico or Kovar. A hollow cylindrical shield member 26 having a first counterbore 27 therein and a second counterbore 28 of greater diameter also therein rests upon metal collar 25. The smallest inner diameter surface of cylindrical member 26 makes electrical contact with metallized ceramic cylindrical member 22. A first metallic sleeve member 29, which is beveled at one end thereof to fit the'lower bevel of the annular groove 23 in ceramic cylinder member 22, slides over member 22 and is aligned with the lower bevel of the annular groove therein and in electrical mechanical contact with the inner surface of smaller counterbore 27. A second cylindrical sleeve 30 which is likewise beveled at one of its ends to match upper bevel of the groove 23 in cylindrical member 22, slides over ceramic member 22 and is aligned with the upper bevel of groove 23 therein. A metallic cap and shield piece 31 having a re-entrant portion 32 and a central aperture 33 rests upon the upper end of cylindrical membert22 and within the interior end of second sleeve member 30. A trigger lead wire 34 enters the central aperture within'ceramic trigger support member 22 and is aflixed to the re-entrant end cap 31 at the uppermost portion thereof. Thislead may conveniently be of molybdenum or any'other low vapor material conventionally utilized providing lead wires in electric discharge devices. Members 24 and 25 are conveniently selected as a Fernico, or Kovar equivalent metal or alloy. The remaining metallic members 26, 29, 30, and 31 may conveniently be selected as a refractory metal, as for example, tungsten or molybdenum. Appropriate vacuum-tight seals are made to the mating surface members of all of the ceramic and metal parts by conventional or well known techniques. When assembled the trigger electrode assembly is aligned into the inwardly depending end of the appropriate electrode support member or 16 and rests in contact with a counterbore shoulder therein, at the point at which flanged member 24 is sealed with a vacuum tight seal thereto. Prior to final assembly the portions of metallic sleeve members 29 and 30 which are to be exposed between the upper portions of cylindrical shield member 26 and cylindrical end cap and shield member 31 are preferably coated with a material as for example a hydride of titanium, zirconium, hafnium, yttrium, erbium or other rare earth metals, which hydrides serve as a source of ionizable gas which may be emitted upon the initiation of a voltage pulse between sleeve members 29 and 30.

The device of FIGURE 1 may be fabricated in accord with standard ceramic and metal tube technology in which case thetrigger assemblies are independently fabricated and independently sealed to the respective electrode support members 15 and 16. The electrode support members, the apertured insulator bushings, and the primary electrodes are then assembled and sealed leaving only plug 35 unsealed. The device is then placed in an outgassing furnace and raised to a temperature of for example 850 C. and held at this temperature for one or two hours in order to cause outgassing and the removal of sorbed gasses from all of the constituent parts thereof. After sufficient outgassing and while the device is still at an elevated temperature, a suitable pressure of an active, ionizable gas as for example hydrogen, is introduced through plug 35 and the device is sealed. Since, in the case of titanium hydride coating upon metal sleeves 29 and 30, a large fraction of the hydrogen has been evolved therefrom, hydrogen is taken up from the furnace atmosphere to recharge the trigger electrode and to cause the establishment of a vacuum within the sealed envelope. In order to enhance this operation a reservoir for an active gas, as for example hydrogen, may be provided in the form of a suitably proportioned body'of a gettering material 37, as for example titanium, which is appropriately shielded, as for example, by bafiles from the direct lineof-sight with the region in-which the primary arc is active, so that the device may continue for many arcing operations without loss of hydrogen pressure. After the device has been baked out for an appropriate length of time an amount of hydrogen is metered into the device prior to vacuum sealing with plug 35 so that upon reabsorption of all of the hydrogen within the tube by the titanium hydride of the trigger or the appropriately chosen added hydrogen reservoir the device is returned to room temperature, the pressure therein is at a value of 10 millimeters of mercury or less. Alternatively the sealing operation may be carried out in a furnace charged with'an active gas, as for example a hydrogen furnace at one atmosphere pressure, in which case enough active, gas storing reservoir material, asfor example titanium, to absorb substantially all of the hydrogen vapor cooling and establish a pressure of 10- torr or less.

In FIGURE 1 of the drawing, electrical connections for operation of the triggered vacuum gap device are made schematically, indicating the connection of the primary electrodes, through the electrode support members to the line voltage or to the high which is to be switched or interrupted as the case may be. The potential of each of electrodes 2 and 3 will therefore bear a predetermined but varying relation to ground potential. Each of the trigger electrodes assemblies 5 and 6 are connected to a grounded pulse source 36. As will be described more fully hereinafter the gap is broken down by the establish ment of first a spark discharge across the trigger gap which causes a triggering arc to be struck between the triggering electrode and the cathode member. Obviously when alternating currents and voltages are associated with the gap device, the electrode which is the cathode for one operation, because of its being negative with respect to the other electrode, may be the anode upon the next operation. Accordingly, it is desirable and necessary in accord with the present invention that they trigger electrode assembly and pulse source be such that the primary gap is broken down by an appropriate starting arc irrespective of which electrode is cathode. It is for this reason that, in

accord with the present invention, I provide a trigger electrode assembly associated with each of the main electrodes. In this respect, it is possible to cause a breakdown irrespective of which electrode is cathode. This arrangement gives greater flexibility to the applicability of the triggered vacuum gap device in accord with the present invention and makes the external circuitry exceedingly simple, particularly as compared with the various polarity inversion circuits which might be required if only one trigger electrode were present. In further accord with the present invention a pulse source is provided which strikes a triggering are only with respect to the cathode, although it is feasible that the device could be operated with a pair of simultaneous trigger arcs. In accord with the illustrated embodiment a short electrical pulse which may vary in magnitude from 50 volts to 5000 volts depending upon the con-figuration of the device and the magnitude of currents to be interrupted is generated in pulse source 36 which may be any conventional pulse generating means, well known to the electronics arts. The pulse generated in pulse source 36 is a voltage which has a particular reference to ground potential since the pulse source is grounded. This pulse is applied to both trigger electrodes 5 and 6 through electrical conductors 38 and 39 which are connected to trigger electrode terminals 34 and 40. With the positive pulse applied to both trigger electrode assemblies and with the pulse source grounded, an electric arc will be stricken only between the trigger assembly which is associated with the cathode electrode since the cathode electrode will be the more negative of the two electrodes 2 and 3 and the potential difference between the cathode electrode and the trigger electrode associated therewith will be substantially greater than between the other primary electrode and the trigger electrode associated therewith.

The mechanism of breakdown of the primary gap of the vacuum gap device in accord with the present invention is substantially as follows: With the connection of primary electrodes 2 and 3 to the source of high voltage a high electric field is established within gap 4. When, because of a high voltage transient which could damage the equipment which may be protected by a device of this nature, or in accord with a preselected timing pulse for interrupting the main power, a preselected voltage pulse which is positive with respect to ground potential is generated from pulse source 36 and applied to electrode assemblies 5 and 6 respectively. As illustrated in FIG- URE 2, this voltage pulse is applied through conductor 34 and end cap 31 to one cylinder 30 bounding gap 23. The other side of the gap is connected through cylinder 29, counterbored member 26 and flange 25 to the primary electrode.

Assuming for the moment that primary electrode 2 is more negative with respect to ground than primary electrode 3, a spark breakdown will occur between the cylinders adjacent the gap 23 of electrode assembly 5. This is because a lower breakdown potential is sufiicient to cause breakdown at the interface between a metal and ceramic. The heat of this discharge immediately causes the evolution of the stored active gas, preferably hydrogen, from the active gas storing substance, which comprises the coating upon sleeves 29 and 30. As the hydrogen is evolved from the titanium hydride the hydrogen molecules are ionized and thus causing the arc discharge across the triggered gap to be intensified causing the further release and ionization of a large quantity of hydrogen gas. As the region between main electrodes 2 and 3 becomes increasingly filled with ion-electron plasma the main gap is broken down within a matter of mocroseconds or less.

Since the initial breakdown between primary electrodes 2 and 3 occurs in the immediate vicinity of the trigger electrodes at which point the path length between the two electrodes is long, and since the vacuum arc is essentially vaporstarved, the natural tendency of the arc is 6 to conserve vapor, causing a propulsion of the arc out into the small-gap length region between the two electrodes. This operation is highly effective to protect the trigger assembly from corrosive action of the main are once it has become established.

While a simple grounded pulse source has been described herein there are other suitable and functionally equivalent circuit connections which may be utilized, depending upon the type circuit in which the device is to be connected. One such modification contemplates a separate pulse source connected to the trigger electrode lead 34 and 40 respectively of each of the trigger electrode assemblies 5 and 6. These pulse sources would then be dependent for actuation upon a secondary pulse source which would feed a signal identical in magnitude and phase into each of the pulse sources so that the pulse sources would each generate a pulse at identically the same time.

Irrespective of the pulsing network that is utilized to trigger devices in accord with this invention, it is vastly superior to the highly complicated, expensive, and often fallible circuitry which must be utilized if only one trigger electrode is included because such a system must necessarily sense, not only the magnitude of a triggering pulse, but also the polarity of the respective electrodes at the time the pulse is applied to change the polarity thereof so as to cause breakdown of the trigger gap.

Although the invention has ben illustrated and described herein with particular reference to a specific embodiment utilizing a fixed gap triggera'ble device, the advantages of the present invention may be equally achieved when utilized in conjunction with a circuitry closer or circuit interrupter in which one primary electrode is fixed and the other primary electrode is operable to move from a circuit open position to a circuit closed position by mechanical actuation through a vacuum type bellows. Such an embodiment is illustrated in FIGURE 3 which is an alternative embodiment to the device of FIGURE 1 and wherein like parts are identified with the same reference numerals. In FIGURE 3 arc-electrode 2 is rendered reciprocably movable into, and out of, contact with arcelectrode 3 by means of bellows 41 which is hermetically sealed between bushing 13 and arc-electrode support member 15.

While the invention has been described in detail herein in accord with certain preferred embodiments thereof, many modifications and changes therein may be afiected by those skilled in the art. Accordingly it is the intention of the appended claims to cover all such modifications and changes as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. Triggerable vacuum gap apparatus comprising an envelope evacuated to a pressure of 10 mm. Hg or less; a pair of arc-electrodes disposed within said envelope in electrically insulated relationship and defining therebetwee-n a primary discharge gap; a trigger assembly associated with each of said arc-electrodes and containing means for storing an ionizable gas and spark gap means adjacent said first named means for releasing said ioniza'ble gas by the initiation of a spark discharge upon receipt of a suitable trigger pulse; means applying electrical potentials to each of said arc-electrodes, each of said potentials having a predetermined relationship to ground potential; and pulse source means associated with said trigger assemblies for applying a triggering pulse which is independent of the potentials applied to said arc-electrodes and positive with respect to ground potential to said trigger assemblies to cause only one of said trigger assemblies to be activated at any given time to cause the breakdown of said primary gap with the arc-electrode associate-d with the activated trigger assembly as cathode.

2. The apparatus of claim 1 wherein both arc-electrodes are fixed to define a fixed primary gap.

-3 The apparatus of claim 1 wherein at least one of said arc-electrodes is moveable to form a variable primary gap.

4. The apparatus of claim 1 wherein the same pulse source supplies the same electrical signal to both trigger assemblies 5. The apparatus of claim 1 wherein said trigger assemblies are located concentrically within longitudinal apertures with-in said primary arc-electrodes References Cited by the Examiner UNITED STATES PATENTS Marx et a1. 313,197 X Latour et a1. 3151ll X Jennings 313148 Lafferty 313197 X

Claims (1)

1. TRIGGERABLE VACUUM GAP APPARATUS COMPRISING AN ENVELOPE EVACUATED TO A PRESSURE OF 10**-5 MM. HG OR LESS; A PAIR OF ARC-ELECTRODES DISPOSED WITHIN SAID ENVELOPE IN ELECTRICALLY INSULATED RELATIONSHIP AND DEFINING THEREBETWEEN A PRIMARY DISCHARGE GAP; A TRIGGER ASSEMBLY ASSOCIATED WITH EACH OF SAID ARC-ELECTRODES AND CONTAINING MEANS FOR STORING AN IONIZABLE GAS AND SPARK GAP MEANS ADJACENT SAID FIRST NAMED MEANS FOR RELEASING SAID IONIZABLE GAS BY THE INITIATION OF A SPARK DISCHARGE UPON RECEIPT OF A SUITABLE TRIGGER PULSE; MEANS APPLYING ELECTRICAL POTENTIALS TO EACH OF SAID ARC-ELECTRODES, EACH OF SAID POTENTIALS HAVING A PREDETERMINED RELATIONSHIP TO GROUND POTENTIAL; AND PULSE SOURCE MEANS ASSOCIATED WITH SAID TRIGGER ASSEMBLIES FOR APPLYING A TRIGGERING PULSE WHICH IS INDEPENDENT OF THE POTENTIALS APPLIED TO SAID ARC-ELECTRODES AND POSITIVE WITH RESPECT TO GROUND POTENTIAL TO SAID TRIGGER ASSEMBLIES TO CAUSE ONLY ONE OF SAID TRIGGER ASSEMBLIES TO BE ACTIVATED AT ANY GIVEN TIME TO CAUSE THE BREAKDOWN OF SAID PRIMARY GAP WITH THE ARC-ELECTRODE ASSOCIATED WITH THE ACTIVATED TRIGGER ASSEMBLY AS CATHODE.

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