US3450922A - Triggerable vacuum gap having offset trigger - Google Patents

Description

June 17; 1969 c, G LL R 3,450,922

TRIGGERABLE VACUUM GAP HAVING OFFSET TRIGGER Filed Aug. 5, i966 g I /2 f J I :2 9 3 23 d- I I 23 J] W ,A 5 f l 2 LLine "2/ Voltage o s l L I5 22 I F 1 Trigger F 552756 I?) 20 /9 4 I v 6 Q 8 I I x Hg 2 /6$ l3" l5 Inven/ar:

' Char/es J. Gallagher His Attorney.

3,450,922 Patented June 17, 1969 U.S. Cl. 313-155 7 Claims ABSTRACT OF THE DISCLOSURE Triggerable vacuum gap devices having longitudinally fed, substantially plane-parallel arc-electrodes are triggered by a trigger assembly, integral with one arc-electrode and decentered from the longitudinal axis thereof.

The present invention relates to improved triggerable vacuum gap devices, and particularly those of the type wherein a triggering electrical breakdown occur across a ceramic surface between a pair of metallic members to cause breakdown of a primary gap.

Vacuum switches and vacuum gap devices generally have been of greater importance in the electric power distribution arts, particularly for switching and interrupting electric currents and as electric circuit protective devices. In certain applications it is desirable that such devices be adapted to be rendered conducting, though normally non-conducting, upon the occurrence of an external incident. In U.S. Patent No. 3,087,092, issued Apr. 23, 1963, to I. M. Latferty and entitled, Gas Generating Switching Tube, such a device is disclosed. In the specific disclosure of the device of the aforementioned patent, electrical breakdown between a pair of primary arc-electrodes is provided by, in one specific example, the evolution and ionization of an active ionizable gas from a gas storage member or members which constitute electrodes of a trigger gap.

Although the device of the aforementioned Lafferty patent and other similar devices utilizing such breakdown triggers have provided a great step forward in the art, and are devices of great value and utility, trigger electrodes and trigger assemblies involving the evolution of active ionizable gasses are subject to gas exhaustion with long use and are further subject to destructive attack by primary electric arcs and remain the most fragile part of such devices, in most instances.

Accordingly, it is an object of the present invention to provide improved vacuum gap devices having trigger electrode structures which are rugged, simple and effective for the establishment of an electric discharge path between a pair of primary arc-electrodes.

Another object of the present invention is to provide triggerable vacuum gap devices utilizing ceramic surface breakdown paths which are so positioned as to avoid destructive action of a high current electric are on the trigger assembly.

In accord with one feature of the present invention I provide a vacuum gap device including a pair of primary arc-electrodes defining therebet-wen a vacuum gap. These electrodes are located within an evacuable envelope which is evacuated to a pressure of 1O mm. of mercury or less and which includes at least a portion thereof which is an electric insulator, thus permitting electrical isolation between the primary arc-electrodes.

In further accord with the invention I provide, in conjunction wi-th at least one arc-electrode, a ceramic surface gap across which a trigger pulse may be applied to cause the establishment of a cathode spot upon the arcing surface of one primary arc-electrode. In accord with another feature of the present invention, the trigger assembly formed by the ceramic member is decentered from the center of the normal electrical conduction path through the electrode with which it is associated so that when a primary arc is stricken between the primary arc-electrodes the conduction path of the electric current flowing through the arc executes a loop which causes magnetic forces operating upon the arc to instantaneously remove the are from the area of the trigger assembly, thus protecting it from destructive effects of the arc footpoints. The novel features characteristics of the present invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood with reference to the appended drawing in which:

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

FIGURE 2 is an enlarged partial vertical cross-sectional view of one primary arc-electrode of the device of FIGURE 1 illustrating the trigger assembly in accord with the present invention; and

FIGURE 3 is a partial plan view of the arcing surface of one arc-electrode of the device of FIGURE 1.

In FIGURE 1 the illustrated triggerable vacuum breakdown device includes a hermetically sealed evacuable envelope 1 comprised of a bulbous cylindrical sidewall member 2 and a pair of apertured end plates 3 and 4 hermetically sealed thereto. Within envelope 1, there are located a pair of spaced primary arc- electrodes 5 and 6 defining therebetween a breakdown gap 7. Are electrodes 5 and 6 may conveniently by cylindrical disks disposed in plane-parallel array. Primary areelectrode 5 is mounted upon arc-electrode support member 8 which passes through a centrally located aperture in end wall member 3 and is insulatingly sealed therethrough by means of a bushing 9 having a re-entrant counterbore 10 therein for added self-shielding. Bushing 9 is hermetically sealed to end wall member 3 by means of an interior sealing ring 11 which makes suitable ceramic-to-metal seal with end Wall member 3, and is sealed to electrode support member 8 by means of fernico spinning 12 which makes suitable seals to bushing 9 and electrode support member 8. Arc-electrode 6 is disposed within envelope 1 upon electrode support member 13 which is hermetically sealed through a central aperture in end wall member 4 by means of flanged sealing member 14.

Trigger assembly 15 comprises a ceramic dielectric cylinder or collar 16 with a central, metallic, rod-like core 17 fitting tightly therein. Trigger assembly 15 is insulatingly and hermetically sealed through a decentered aperture in end wall member 4 and is sealed therethrough by means of sealing flange 18. Trigger assembly 15 passes through a longitudinal aperture 19 in arc-electrode 6 and ab'utts flushly with the arcing surface 20 thereof facing the arcing surface 21 of arc-electrode 5. Arcing surfaces 20 and 2.1 are adapted to sustain the footpoints of a high current electric arc. Thus the breakdown gap is between the inboard end of trigger electrode 17 and arcing surface 20 of primary arc-electrode 6 across an annular crosssection of the inboard end surface of ceramic cylinder 16, which surface is flush with the arcing surface 20 of arcelectrode 6. This structure is illustrated in vertical crosssection in FIGURE 2 and in plan view in FIGURE 3. As mentioned hereinbefore the trigger assembly 15 is decentered from the longitudinal axis of the device of FIG- URE 1.

Arc- electrodes 5 and 6 are constructed of a high purity oxygen-free metal which may be repeatedly arced with a high current electric are without evolution of gas or gasforming constituents principally oxygen and readily decomposable compounds thereof so as to sustain a vacuum of lO- mm. Hg for its duty life. Conveniently such a material may be zone-refined OFHC copper, as for example that formed by the process set forth in Hebb Patent No. 3,234,351, issued Feb. 8, 1966. Electrode support members 8 and 13 may conveniently comprise high purity OFHC copper which need not necessarily have been so treated but which must be, nominally free of oxygen and oxygen-forming constituents.

The materials of envelope wall 2 and end wall members 3 and 4 may be conveniently be stainless steel or nickel, or if direct ceramic-to-metal seals are to be formed, titanium may be used for end wall members 3 and 4. Insulating bushing 9 and trigger insulator cylinder 16 must be of high voltage dielectrics, as for example a high density alumina or a fosterite ceramic, capable of being impervious to gasses and of sustaining high electric fields. Such materials are well known to the art.

Trigger electrode 17 may conveniently be a tungsten, molybdenum or other refractory metal rod having a diameter of approximately 0.050" or thereabouts and is insulatingly and hermetically sealed to ceramic cylinder 16.

Devices in accord with the present invention are prepared by conventional techniques involving metal and ceramic seals which may involve processing in vacuum and outgassing at high temperature, since there is no need for special treatment for maintaining a special atmosphere of hydrogen-storing material within the device. Once the device is evacuated and all seals made, sealing the interior to a vacuum of mm. of mercury or better, the primary arc-electrodes are connected through their respective support members to an electrical load to be switched, protected, or otherwise controlled. This may involve connection in series or parallel circuit with a high voltage unidirectional or alternating current voltage depending upon the manner in which the device is to be used, i.e. whether as a circuit protector or lightning arrester or as a capacitor switching device or a circuit recloser, etc.

Typical circuit connections shown include a connection in parallel with a high voltage line and the connection of a grounded trigger pulse source 22 between arc-electrode 6 and trigger electrode 17.

In operation with a high voltage, of 100 kv. for example, across arc- electrodes 5 and 6, a high current arc may be initiated thereacross by the application of a triggering pulse of from 50 to several thousand volts applied for a pulse length of approximately 5 microseconds between trigger electrode 17 and arc-electrode 20 with the positive polarity of the pulse applied to trigger electrode 17. The application of such a pulse causes a triggering electrical breakdown between the trigger electrode 17 and the immediately adjacent portions of arcing surface 20 of arc-electrode 6 across the inboard surface of dielectric cylinder 16. This discharge is sufficient to cause the volatilization of the material of the arcing surface 20 of arcelectrode 16 and the establishment of a cathode spot thereupon. Immediately thereafter the positive footpoint of the trigger arc is transferred to the arcing surface 21 of arc-electrode 5. At this instance a high current path shown by arrows 23 exists between the connections to the electrical load. Since this pathof current flow, as indicated by arrows 23, includes a current loop to accomodate for the fact that the initial breakdown occurred at a point decentered from the center of the normal electrical conduction path between the primary arc-electrodes. As used herein the phrase normal electrical conducting path through the arc-electrodes is intended to connote that path of electrical conduction which would present the path of least resistance if the arc-electrodes were in actual abutting contact along the arcing surfaces. With a device as illustrated in FIGURE 1 having axial symmetry such path is the longitudinal axis running through electrode support members 8 and 13 and is the straight projection of the vertical ones of arrows 23 therein. In other config- .4 urations such path is generally the path of least resistance to electrical flow between the input terminals to the primary arc-electrodes.

The decentering of trigger assembly 15 from the longitudinal axis of electrodes 5 and 6 (and the center of the normal electrical conduction path therebetween) and the concomitant establishment of the curved conduction loop as indicated by arrows 23 constitutes the means by which devices constructed in accord with the present invention secure their advantage over prior art devices. Thus for example in the prior art as exemplified by Patent No. 3,207,947 to Goncz, a triggered spark gap having a similar dielectric surface trigger which is located at the longitudinal axis of the device and the longitudinal axis of the primary arc-electrodes is vulnerable to are erosion and deterioration. This is due to the fact that the primary arc, once stricken, will tend to dwell at the same spot and thus cause destructive eroding of the trigger to the extent that the metal-to-ceramic interfaces which make possible the operation of the trigger assembly, will deteriorate rapidly preventing such a device from having a long-duty life. Longer duty-life is achieved in the referenced patent by various expedients, as for example, the interposition of a protective grid to protect the trigger against the primary are, or a recessing of the metallic member so that the exposed surface of the ceramic is not on the arcing surface of the primary arc-electrode. Such expedients, however, give rise to many disadvantages, as for example, loss of efficiency due to the interposition of an additional element, destructive arc erosion of the interposed element, distortion of the electric field due to sharp points by virtue of a recessing and other such disadvantages.

In accord with the present invention, however, all such expedients are unnecessary since the decentering of the trigger assembly causes the main current conduction paths to describe a loop or arcuate path which results in a concentration of flux lines within the loop or arcuate path and an automatic blow-out of the arc in the direction away from the longitudinal axis as indicated by arrow 24. Such automatic blow-out occurs almost instantaneously with the establishment of the high current arc so that the arc does not dwell in the vicinity of the starter assembly, thus allowing the starter assembly to be operated for many arc establishments without visible sign of deterioration. This is particularly so since I have observed that breakdown of the trigger assembly of the devices of the present invention invariably occurs at a point remote from the center of arc-electrode 6 so that in blowing out, the arc footpoint does not traverse the trigger assembly.

One device constructed in accord with the present invention, as illustrated in FIGURE 1, utilized an alumina cylinder as element 16 having an outside diameter of 0.145 and an inside diameter of 0.60" and was filled with a 0.60 diameter tungsten rod providing an annular gap between the tungsten rod and zone-refined OFHC copper electrode 6, and specifically the arcing surface 20 thereof, having a radial dimension of 0.042". Although tungsten was utilized, molybdenum, rhodium, rhenium, or other refractory metals may readily serve as well. In order to cause the device to break down, positive pulses were applied to rod 17, typical pulses being of the order of 1000 volts of 5 microseconds duration. Upon each application it was noted by the monitoring equipment utilized that the gap across the ceramic surface always broke down to cause a trigger arc and the establishment of a cathode spot upon the surface 20 of arc electrode 6 during the first microsecond of the application of the trigger pulse. I am able to fire this device with one hundred percent reliability for hundreds of cycles. During such operation, no evidence was detected of conducting films being deposited upon the surface of the ceramic which would cause the degradation thereof. Neither was any erosion or other arcing deterioration of the trigger assembly observed, thus indicating clearly the validity of the mechanism by which the trigger electrodes of devices in accord with the present invention are protected by selfinduced blow-out.

Although zone-refined copper electrodes were utilized,- in accord with the present invention other high vapor pressure arc-sustaining electrodes may be utilized.

Although the device of the present invention is illustrated with two fixed electrodes defining a fixed electrical gap the invention is equally advantageous when incorporated in a device having a movable electrode and a variable gap. Thus for example, electrode 5 could be connected to a sylphon bellows for reciprocating movement to make and break contact with electrode 6.

Similarly, since devices constructed in accord with the present invention depend for operation upon the establishment of a cathode spot upon the electrode into which the starter electrode assembly is incorporated, it follows that this electrode must be the cathode of the vacuum gap device and must be biased with the negative polarity of the applied primary voltage. While this can always be arranged conveniently when unidirectional voltages are being switched or protected, or controlled, for use with alternating currents it is desirable that a similar trigger electrode assembly be incorporated with both electrodes since it is not possible to predict which electrode will be negative at any given instant when it is desired to cause the initiation of a high current are therebetween. According- 1y, as is set forth in the application of I. M. Lafferty, Ser. No. 510,562, filed Nov. 30, 1965, and now Patent No. 3,303,376, and assigned to the assignee of the present invention, it is advisable, in alternating current circuits, to incorporate such a trigger electrode assembly with each primary arc-electrode and to connect the two to a grounded pulse source so that a positive pulse is supplied simultaneously to both trigger electrodes. With such an arrangement, breakdown occurs only between that trigger electrode and the associated primary arc-electrode which is biased negative with respect thereto to cause the formation of a cathode spot of the arcing surface thereof.

While the invention has been set forth hereinbefore with respect to particular embodiments and specific illustrations thereof it is to be appreciated that many modifications and changes may be made by those skilled in the art. Accordingly I intend by the appended claims to cover all such modifications and changes as come within the true spirit of the present invention.

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

1. A triggerable vacuum gap device adapted to be changed from a non-conducting state to a conducting state by the application of a predetermined signal and comprising:

(a) an evacuable envelope evacuated to a pressure of 1O mm. of mercury or less (b) a pair of primary arc-electrodes disposed in electrical insulating relationship and spaced apart so as to define a primary gap supported within said envelope (bb) said electrodes each being in the form of disc-shaped members having an arcing surface adapted to sustain the footpoint of an electric arc and a normal electrical conduction path therethrough (bbb) said electrodes further being supported and electrically connected by axially centered longitudinal support members,

(0) a trigger electrode assembly associated with at least one of said arc-electrodes and including a trigger electrode and a concentric surrounding dielectric collar therefor extending through a portion of said arc-electrode with the inboard end thereof substantially flush with the arcing surface of said arc-electrode (cc) said trigger electrode assembly being decentered from the normal axially centered electrical conducting path through said electrode to cause the path of electrical current which exists in said device upon establishment of an are between said primary electrodes as a result of firing of said trigger assembly to describe an arcuate path so as to propel said are radially outward along said arcing surfaces whereby said trigger assembly is unharmed by the footpoint of said arc.

2. The device of claim 1 wherein the arc-electrodes exhibit axial symmetry and the normal electrical conducting path through said arc-electrodes is the longitudinal axis of the device.

3. The device of claim 1 wherein the arc-electrodes are substantially plane-parallel discs and the normal electrical conducting path is the axial center thereof normal to the arcing surfaces thereof.

4. The device of claim 1 wherein both arc-electrodes are fixed so as to define a fixed gap.

5. The device of claim 1 wherein one of said electrodes is fixed and one is reciprocable to define a variable gap.

6. The device of claim 1 wherein each of said arc-electrodes has a trigger electrode assembly associated therewith.

7. The device of claim 1 wherein said trigger electrode is a metal selected from the group consisting of tungsten, molybdenum, rhodium and rhenium.

References Cited UNITED STATES PATENTS 2,508,954 5/1950 Latour et al. 313- X 2,740,915 4/1956 Jennings 313-148 3,202,852 8/1965 Martinson 313-198 X 3,207,947 9/1965 Goncz 313-197X 3,230,410 1/1966 Hafkemeyer et al. 313-197 X 3,263,112 7/1966 Kolb et al. 313-198 3,267,320 8/1966 Melhart 313-214 X 3,303,376 2/1967 Latferty 313-148 JAMES W. LAWRENCE, Primary Examiner. P. C. DEMEO, Assistant Examiner.

US. Cl. X.R. 313-187, 198, 217

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