US3509405A - Coaxial vacuum gap devices including doubly reentrant electrode assemblies - Google Patents

Description

. J. A.- RICH 3,509,405 COAXIAL VACUUM GAP DEVICES INCLUDING DOUBLY April 28, 1970 REENTRANT ELECTRODE ASSEMBLIES 3 Sheets-Sheet 1 Filed July 1, 1968 I /g, A

i an Inverv 'or: Joseph A. Rich,

is Attorney.

Apnl 28, 1970 J. A. RICH 3,509,405

COAXIAL VACUUM GAP DEVICES INCLUDING DOUBLY REENTRANT ELECTRODE ASSEMBLIES Filed July 1, 1968 3 Sheets-Sheet 2 PULSE sou/ace [1? van 602*: Joseph A.F?/'ch,

is Atfior'n ey.

Aprll 28, 1970 J, cH 3,509,405

COAXIAL VACUUM GAP DEVICES INCLUDING DOUBLY REENTRANT ELECTRODE ASSEMBLIES Filed July 1, 1968 3 Sheets-Sheet 5 In ve n or: Joseph AMY/ch,

United States Patent 3,509,405 COAXIAL VACUUM GAP DEVICES INCLUDING DOUBLY REENTRANT ELECTRODE ASSEMBLIES Joseph A. Rich, Schenectady, N.Y., assigior to General Electric Company, a corporation of New York Filed July 1, 1968, Ser. No. 741,481 Int. Cl. H013 17/04, 17/30, 21/22 US. Cl. 313-155 8 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to high current vacuum arc devices, particularly those of the triggerable vacuum gap type, adapted to carry extremely high currents at high voltages.

The present invention is related to my copending application Ser. No. 639,693, filed May 19, 1967, and my concurrently-filed application Ser. Nos. 741,480 and 741,482 all of which are assigned to the present assignee, the disclosures of which are incorporated herein by reference thereto.

In the fabrication of vacuum arc devices adapted to carry, during arcing (currents in the tens of kiloampere ranges at voltage levels in the tens of kilovolt range, it has been conventional to utilize standard plane-parallel geometry arc-electrodes in such devices. More recently, I have discovered that a limiting factor in such devices is the current level at which destructive anode spots, with attendant melting of the anode electrodes, were formed. Most prior art devices were directed to structures in which formation of anode spots was accepted and improvement was sought by means other than elimination of anode spots, such as magnetic rotation of the arc, blowout of the arc from the arc gap into an arc-chute, or similar alternatives in seeking to utilize arcs at higher currents.

Such approaches were limited by the threshold current at which destructive anode spots occur. This formation of anode spots is generally due to the fact that current passing through a conventional vacuum arc device does so longitudinally, with the creation of a resultant azimuthal magnetic field. The combination of the action of the azimuthal magnetic field, which constricts arcing conduction paths longitudinally to the plane-parallel gap, and the radially applied expansive forces caused by the heating of the arc plasma, generally results in an anchoring of the arc footpoints at the periphery of a plane parallel type of electrode.

In accord with the invention disclosed and claimed in my aforementioned copending application, Ser. No. 639,- 693, I have provided an insight into the phenomena of arcing and have discovered that, by constructing the arcelectrode of the vacuum arc device so that the azimuthal component of magnetic fields formed due to the currents in the arc-electrodes is substantially eliminated or minimized, the constrictive forces upon the arc-conduction paths are removed or minimized and much greater current may be carried without the formation of destructive anode spots.

Although devices in accord with my aforementioned applications are of great value in raising the conduction current levels for vacuum are devices, I have found that, at exceeding high levels of current, the residual azimuthal component of magnetic field that is present within the arcing chamber does tend to propagate a disproportionate amount of the arcing currents in the inwardly depending end of the inner, reentrant cylindrical arc-electrode thereof, such that a large amount of current conduction occurs between the inwardly-dependent end of the inner reentrant cylindrical arc-electrode and the end of the outer coaxial arc-electrode. Under these circumstances, when the outer cylindrical arc-electrode is positive, or anode, a disproportionately high current density between the transverse member of the outer electrode and the folded end of the reentrant cylindrical member occurs, with a tendency for the formation of anode spots upon the end of the outer cylindrical arc-electrode assembly.

Accordingly, it is an object of the present invention to provide vacuum arc devices utilizing coaxial arc-electrode structures wherein the threshold current for formation of destructive anode spots is further increased.

Still another object of the present invention is to provide means for avoiding the formation of anode spots upon the outer conductor of a coaxial vacuum are device when said outer conductor is connected as anode.

Yet another object of the present invention is to provide new and improved coaxial vacuum are devices adapted to operate at exceedingly high voltages and conduct very high levels of current without the formation of anode spots.

Briefly stated, in accord with one embodiment of the present invention, I provide a coaxial vacuum arc device having a first, outer arc-electrode assembly including a closed cylindrical member and a second arc-electrode assembly including a folded reentrant cylindrical member partially inserted within the volume of said first arcelectrode assembly and defining therewith a hollow cylindrical primary arcing gap. Means are provided integral with said first arc-electrode assembly to provide a secondary arcing gap between the outer and inner electrode assemblies which means essentially comprises an annular cylindrical surface juxtaposed adjacent the cylindrical surface created by the inwardly-depending end of said second arc-electrode assembly. Provision of the secondary gap avoids the concentration of current on the end portions of the outer coaxial arc-electrode assembly.

The novel features characteristic of the present inven tion 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 following detailed description, taken in connection with the appended drawings in which:

FIGURE 1 is a vertical cross-sectional view of a vacuum arc device constructed in accord with the present invention and utilizing a reentrant cylindrical member as a part of the outer electrode assembly of the coaxial arcelectrode structure.

FIGURE 2 is a vertical cross-sectional view of a device similar to that of FIGURE 1, but utilizing a solid annular, inwardly-depending cylindrical member as an integral portion of the outer electrode assembly, and

FIGURE 3 is a vertical cross-sectional view of still another embodiment of the invention, wherein the structure of the outer cylindrical electrode is modified so as to provide the desired arcing surface without the provision of additional structural members extending within the outer arc-electrode cavity.

In FIGURE 1, a triggerable vacuum gap constructed in accord with the present invention is represented generally as 10. Trigger vacuum gap device 10 iucludes an outer arc-electrode assembly 11 and an inner arc-electrode assembly 12, generally concentric with one another and forming therebetween a primary arcing gap 40. An insulating member 13 is hermetically sealed therebetween, and forms therewith, an evacuable envelope which is evacuated to a pressure of millimeters of mercury or less for vacuum arc device operation. Outer electrode assembly 11 includes a cylindrical sidewall member 15, endwall member 16, and a reentrant, inwardly-depending cylindrical assembly 17. Assembly 17 includes an inner cylinder 18, physically and electrically affixed to endwall member 16 and an outer cylinder 19, larger in diameter and concentric with inner cylinder 18 and affixed thereto at the inwardly-depending end thereof by bridging member 20, which has a substantially planar inwardlydepending surface 21.

Inner electrode assembly 12 includes an endwall member 22 and an inwardly-depending reentrant concentric cylindrical assembly 23. Assembly 23 includes an inner cylinder 24 electrically and mechanically afiixed to endwall member 22 and an outer cylindrical member not in contact with endwall member 22, having a larger diameter and concentric with, inner cylinder 24 and affixed thereto at the inwardly-depending end by means of bridging member 26, which has a substantially fiat planar surface 27, which defines with the planar surface 21 of inwardly-depending assembly 17 a secondary arcing gap 38. The primary arcing gap 40 is comprised of the cylindrical space between the outer surface of cylinder 25 and the inner surface of outer cylinder 15. The distance therebetween is generally much smaller than the distance between substantially planar and parallel surfaces 21 and 27 constituting the secondary arcing gap and which are spaced at a distance therebetween which is approximately two to three times the radial dimension of the primary arcing gap.

Devices of the prior art, as represented by my aforementioned copending application, Ser. No. 639,693, are essentially similar to the device as illustrated in FIGURE 1 except that assembly 17 is not included. The inclusion of assembly 17 results in greatly improved current characteristics for such devices.

In the operation of the prior art devices, ideally, electrical conduction paths between the outer electrode assembly and the inner electrode assembly are all within the primary arcing gap 40. The azimuthal magnetic field within the primary arcing gap is minimized in such devices by the fact that currents flowing within cylinder 15 have no effect upon the magnetic field within the cavity created thereby. Additionally, the reentrant structure of folded cylindrical arc-electrode assembly 23 tends to result in a minimal field within the primary inter electrode gap 40. This is because the azimuthal component of magnetic field due to currents in one direction in outer cylindrical member 25 of inner cylindrical apparatus 23 tends to cancel the oppositely-disposed azimuthal component of the magnetic field due to the oppositely-directed electric current in cylinder 24. This is not true as a practical matter in all instances. Thus, for example, if one looks at the relative currents flowing in members 24 and 25 at region B, all of the current from the arcing current paths between the outer and inner electrode assemblies combine in inner cylinder 24 at B. On the other hand, all of the same currents are not present in the portion of outer cylinder member 25 at B. On the other hand, at region A, substantially all of the currents which exist in cylindrical member 24, likewise exist in cylindrical member 25. Accordingly, there is very little azimuthal magnetic field at region A within primary gap 40, but there is a substantial azimuthal magnetic field at region B within gap 40. The result is that the current conduction paths tend to be bunched about the upper half of the inner cylindrical member 23, adjacent the inwardly-depending end thereof, and as the current density thereat increases, this bunching tends to further increase.

At a given current level, in devices of the prior art which omit assembly 17, eventually the bunching of current conduction paths towards the bridging member 27 becomes so great that arcing occurs between surface 27 and endwall member 16. Although arcing therebetween does not deleteriously affect the surface of inwardly-depending reentrant cylindrical member 23, because the arcing therefrom is spread out over a large surface area, including the surface 27 of bridging member 26 and the outer surfaces of cylindrical members 24 and 25, the current density at the point of impingement of the current conduction paths at the arcing surface of endwall member 16 is not so limited. Thus, when endwall member 16 becomes anode, a distinct possibility exists that the constriction of current conduction paths thereat may result in the formation of anode spots at an undesirably low current value. If the gap between the inwardly-depending end of the folded electrode and the end of the outer electrode is increased, anode spots still tend to form, but at the center of the circular endwall.

In accord with the present invention, the formation of anode spots at the surface of endwall member 16 is avoided by providing an additional, inwardly-depending reentrant cylindrical arc-electrode member 17 which has an extended surface, including the inner surface of inner cylindrical member 18 and the outer surface of outer cylindrical member 19, as well as the surface 21 of bridging member 20. Accordingly, When at high currents, arc constriction due to the lack of compensation of the azimuthal component of the magnetic field within arcing space 40, causes an arcing over from the bridging member 26, the arcing is to inwardly-depending assembly 17, which is so constructed as to have a very large surface suitable for the residence of footpoints of arc conduction paths. This eliminates arc constriction and the formation of anode spots at relatively high currents.

In order to accomplish the foregoing, the diameter of inner cylindrical member 18 is essentially the same as that of inner cylindrical member 24, the diameter of outer cylindrical member 19 is substantially the same as that of outer cylindrical member 25. Bridging members 21 and 27 are substantially plane-parallel and have a sufiiciently large enough area to prevent constriction of an are at a pointed or near-pointed surface. The secondary gap 38 between surfaces 21 and 27 is made approximately two to three times that of the primary gap 40'.

In accord with the embodiment of the invention illustrated in FIGURE 1, are constriction upon the surface of inwardly-depending electrode member 17 is minimized by also providing this member in the form of a folded reentrant cylinder. While this is highly desirable and greatly preferred in the formation of the inner primary arc-electrode assembly 22, it is not an essential feature with respect to assembly 17. Since currents are conducted within the assembly 17 only at very high values of current, any azimuthal magnetic field created thereat does not have a. substantial effect uponvthe formation of destructive anode spots. Accordingly, the essence of the present invention may be achieved by the use of a solid member rather than a reentrant cylindrical member 17 in FIGURE 1.

Such a structure is illustrated in FIGURE 2, wherein like numerals are utilized to identify like parts. In FIG- URE 2, exterior coaxial assembly 15 encloses interior, inwardly-depending arc-electrode assembly 12 having an inwardly-depending structure 23 comprising inner cylinder 24 and outer cylinder 25 and defining the primary arcing gap 40. As in the device of FIGURE 1, insulator 13 is hermetically sealed between and to arc-electrode assembly 11, and arc-electrode assembly 12 to define an hermetically-sealed envelope which is evacuated to a pressure of 10- torr or less. As in the device of FIGURE 1, a trigger arc assembly 14 comprising a trigger cathode 28 and a trigger anode 29 is supported by an insulator 30 and fitted with a sealing plug 31, threaded into a flange portion 32 of endwall assembly 22, is effective to inject an electron-ion plasma into primary arc gap 40 upon the application of a suitable source of pulse voltage from a pulse source 41, for example.

Unlike the device of FIGURE 1, inwardly-depending cylindrical arc-electrode member 17 is comprised of a hollow cylindrical body having a solid sidewall. As is mentioned hereinbefore, the solid wall of the cylindrical body, although not avoiding arc constriction thereon, is sufficiently effective to provide a larger surface for the residence of footpoints of arc conduction paths upon the exterior concentric electrode assembly 11 when the latter is connected as anode at very high currents as is inwardlydepending assembly 17 in FIGURE 1. Although the azimuthal magnetic field due to current in member 17 is greater than in the device of FIGURE 1, the economy achieved by the provision of a solid-walled, inwardlydepending electrode member 17, greatly outweighs the light disadvantage of not having member 17 in the form of a reentrant cylindrical structure.

Still another embodiment of the invention, adapted to achieve substantially the major advantages of the increase in effective area of the arcing surfaces by the addition of the secondary arcing gap to the area of the primary arcing gap with the minimum modification from the original design, is illustrated in FIGURE 3 of the drawing, in which like numbers are utilized to denominate like parts to those of the embodiments of FIGURES 1 and 2.

In FIGURE 3, the inner of the coaxial primary arcelectrodes 12 is composed of an endwall member 22 and an inwardly-depending reentrant cylindrical structure 23 which, in turn, is comprised of an inner cylinder 24, an outer cylinder 25, and a bridging member 26 joining cylinders 24 and 25 at the inwardly-depending ends thereof and terminating in a substantially planar, annular arcing face 27. The outer of the electrode assemblies 11 comprises a first, larger, cylindrical portion 15, which is juxtaposed adjacent and which defines, with the outer surface of cylindrical member 25, the primary arcing gap. Above the depth of penetration of reentrant cylindrical structure 23 into the cavity of cylindrical member 15, the wall of exterior coaxial electrode assembly 11 is stepped to form a shoulder member 42 and a smaller concentric cylindrical member 45, having essentially the same inn r diameter as the inner diameter of inner cylindrical member 24 of the inwardly-depending reentrant cylindrical structure 23. The surface 43 of shoulder 42 facing arcing surface 27 also constitutes an arcing surface and defines therewith a secondary arcing gap 38 between the primary arc-electrode assemblies 11 and 12.

Although the area presented for the establishment of conduction current paths at the end cap portion of outer concentric electrode assembly 11 is lacking in the means for providing a reentrant path for current conduction within the apparatus which serves as the footpoint for current conduction at very high currents when the arc is constricted and forced to the end of inwardly-depending portion 23 of inner concentric electrode assembly 12, as is illustrated in FIGURE 1, and although this structure provides lesser area for the footpoints of are conduction paths than the apparatus of FIGURE 2, it is by far the simplest structure for achieving the general objective of increasing the area for the termination of arc conduction paths between the end of electrode assembly 23 and the end of outer electrode assembly 11, so as to avoid burning of end cap 16 by undue constriction of arc conduction paths. For this reason, in many instances, it is suitable that the relatively simple and much less difficult structure, from the fabrication point of view, as illustrated in FIGURE 3 be utilized, so long as the threshold for the formation of anode spots is not exceeded. In practice, it is found that, although not quite as effective in raising the threshold for anode spot formation as the embodiments of FIGURES l and 2, the embodiment of FIGURE 3 is, nevertheless, quite satisfactory and substantially easier and less expensive to manufacture than In all embodiments of the invention, the arc-electrode members which actually serve as footpoints for are coneither of the embodiments of FIGURES 1 or 2. duction paths during arcing are fabricated from high purity metals that are free of gases or gas-forming compounds to a degree at least no greater than one part per million of gas or gas-forming material contained therein. Such arc-electrode materials are also comprised of sufficiently-high vapor pressure material such that, upon the initiation of a current-conduction arc therebetween, sufficient metallic species is boiled therefrom to form the ionized electron-ion plasma which supports the are current during arcing.

Such arc-electrode materials may, for example, be copper, beryllium, copper-beryllium alloys, or any of the materials set forth in any of Patents 2,975,236, Lee et al; 2,975,225, Laiferty; 3,016,436, Lalferty', 3,140,373, Horn; and 3,246,979, Lafferty et al., for example, preferably these materials are prepared in high purity form, as for example, by multiple pass zone refining, or by a special zone refining zone technique, such as is set forth in Patent No. 3,234,351, Hebb. Insulating member 13 may be composed of Pyrex or Nonex glass or may be of a suitable ceramic, as for example, high density alumina. Any portion of the arc-electrode structures which is not intended and is not, in fact, exposed to the arc footpoint, as for example, base member 22 in each of the embodiments' of FIGURES 1, 2 and 3, may conveniently be composed of a highly-conductive, relatively-pure material, as for example, OFHC copper.

Trigger assembly 14 comprises a scored metal or hydride film formed upon an insulating ceramic disc 28, the outer portion of which serves as arc-cathode and is connected to inner concentric endwall assembly 12, and a trigger anode which is a hyride or metallic-coated ceramic post 29, having the upper end thereof coated and connected with a trigger anode lead wire which runs centrally therethrough and emerges as a trigger anode lead 33 and is connected to a pulse source 39, for exam le. Such trigger assemblies may conveniently be the triggers disclosed in Lafierty application Ser. No. 564,132, filed July 11, 1966, or, alternatively, may be that disclosed in Lafferty application Ser. No. 704,935, filed Feb. 12, 1968, both of which are assigned to the present assignee.

Devices constructed in accord with the present invention are suitable for providing a highly-stable and reliable means for establishing a high-current, high-voltage arc between the terminals thereof, which may be connected across a circuit or a circuit element as, for example, a capacitor bank to be protected by causing a short-circuit thereacross. Thus, for example, in a transmission line capacitor bank, should a fault current occur, the triggering of trigger anode 29 with a suitable positive pulse, either from a separate source or from the line, may cause the breakdown of the primary arc gap within a matter of microseconds to short-circuit a destructive voltage transient from the capacitor bank until a suitable circuit protective device as, for example, a vacuum switch may be mechanically closed within a matter of one or two cycles to establish a complete shortcircuit around the device or circuit to be protected.

Devices such as those illustrated in FIGURE 1 of the invention, having an inner diameter of approximately 4% inches and a longitudinal inner cavity of approximately 7 inches in length and having a primary arc gap annularly surrounding inner arc-electrode assembly having a radial dimension of approximately /2 inch with the secondary arcing gap, namely, the gap between surfaces 21 and 27 in FIGURE 1 of the drawing, having a dimension of approximately 1% inches has been suitable for and has repeatedly carried a 31,000 peak ampere, 60 cycle damped current wave. Such a device has been operated as high as 41,000 amperes, with only very slight melting of the inwardly-depending member 17 of outer concentric electrode assembly 11.

From the foregoing, it is apparent that I have devised new and improved structures for coaxial vacuum arc discharge devices wherein the primary arc is in the form of an annulus between two concentric cylindrical members, wherein the avoidance of the formation of destructive anode spots at the termination of the concentric structure is avoided by providing an inwardly-depending portion to the outer cylindrical arc-electrode assembly which substantially matches in shape and dimension the termination of the inner folded cylindrical, one of the concentric arc-electrode assemblies and forms therewith a secondary arcing gap. This improvement over prior art devices greatly increases the area available at the termination of the coaxial structure for the anchoring of the footpoints of current conduction paths to the outer one of a concentric pair of arc-electrode assemblies when it serves as the anode, greatly decreasing the current density thereat, and raising the threshold for the formation of destructive anode spots. Certain embodiments for the achievement of this end result have been disclosed, the

ideal of which is the most complicated, and the least complicated of which is, nevertheless, satisfactory to achieve a substantial proportion of the advantages proposed by the best embodiment thereof.

Although the invention has been set forth herein with respect to certain embodiments and examples thereof, many modifications and changes will readily occur to those skilled in the art. Accordingly, I intend, by the appended claims, to cover allsuch modifications and changes as fall within the true spirit and scope of this invention.

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

1. A coaxial vacuum arc device comprising:

(a) a first outer arc-electrode assembly including:

(a a cylindrical member,

(a an end wall member capping one end of said cylindrical member,

(a an annular, inwardly-depending member extending inward of said cylindrical member and having a substantially fiat annular inwardly-depending surface;

(b) a second reentrant cylindrical arc-electrode assem- .bly extending partially within said first arc-electrode assembly and including:

(b abase member,

(b an inner cylindrical member mechanically and electrically connected to said base member,

(b an outer cylindrical member concentric with and having a diameter greater than that of said inner cylindrical member, spaced apart from said cylindrical member of said first arc-electrode assembly to define a primary arcing gap therebetween, and not directly connected to said base member, and

(b a bridging member having a substantially annular configuration, joining the inwardly-depending ends of said first and second cylindrical members and having a substantially planar inwardly-depending surface, substantially parallel with the inwardly-depending surface of said inwardly-depending member of said first arc-electrode and defining therewith a secondary arcing p;

(c) insulating means hermetically sealed between said first and said second arc-electrode assemblies to form an evacua-ble envelope;

(d) means for injecting an electron-ion plasma into said primary arcing gap to cause the electrode breakdown thereof; and

(e) means for connecting said device in circuit with an electric circuit to be switched, controlled, or protected.

2. The device of claim 1 wherein the inwardly-depending member of said first outer electrode assembly is a folded reentrant cylindrical member.

3. The device of claim 1 wherein the inwardly-depending member of said first outer electrode assembly is an annular cylinder having a solid wall and a substantially flat inwardly depending surface.

4. The device of claim '1 wherein the inwardly-depending member of said first outer electrode is a reduced diameter portion of said cylindrical wall thereof connected to the outer cylinder thereof with a shoulder member which defines with the inwardly-depending end of said sec- 0nd arc-electrode assembly a secondary arcing gap.

5. The device of ,claim 1 wherein said secondary arcing gap is large in length as compared with the length of said primary arcing gap.

6. The device of claim 5 wherein said secondary arcing gap is approximately two to three times the length of said primary gap.

7. The device of claim 1 wherein the area of said secondary arcing gap is very small as compared with the area of said primary arcing gap.

8. The device of claim 1 wherein the arcing surfaces of each side of said secondary arcing gap are substantially equal.

References Cited UNITED STATES PATENTS 3,246,979 4/1966 Lafferty et al 200--144 X 3,320,478 5/1967 Harrison 3l3l55 X 3,328,632 6/1967 Robinson 313-233 X 3,331,981 7/1967 Lafferty 3l3-233 X 3,417,216 12/1968 Smith 200l44 3,450,922 6/1969 Gallagher 313- JAMES W. LAWRENCE, Primary Examiner C. R. CAMPBELL, Assistant Examiner Us. 01. X.R.

Images