US3469049A - High voltage vacuum device with improved means for inhibiting sparkover adjacent the edge of a tubular metal part - Google Patents

Description

p 23, 1969 o. w. CROUCH ETAL 3.46 9

HIGH VOLTAGE VACUUM DEVICE WITH IMPROVED MEANS FOR INHIBITING SPARKOVER ADJACENT THE EDGE OF A TUBULAR METAL PART Filed May 25, 1966 uvvmnom: DONALD fR/(uerz, DONALD VV. CROUCH,

B) A/Jawi'mmw A TTOR/VE) US. Cl. 200-144 2 Claims ABSTRACT OF DISCLOSURE Discloses a high voltage vacuum device comprising a tubular metal part having an. annular end portion located in a region of high dielectric stress. The end portion comprises a thin-walled annulus and an annular bead of arcmelted metal substantially aligned therewith, the bead being formed by locally melting the edge of the annulus with an electric arc to produce large grains and a smooth, rounded external surface.

This invention relates to a vacuum device for use in high voltage electrical applications and, more particularly, relates to means and a method for improving the ability of the device to withstand high dielectric stresses without sparking over.

In certain vacuum-type electrical devices, the edge of a tubular metal part is located in a region of high dielectric stress. It has been recognized that the dielectric stress adjacent this edge can be reduced by deforming this edge region into the general shape of a tore with a relatively large radius of curvature. But this deforming process is a relatively expensive one. Moreover, it requires a significant amount of otherwise unused metal, and the resulting tore consumes space that might be needed for other'purposes in the vacuum device.

An object of the present invention is to increase the dielectric strength of a vacuum device in the region adjacent the edge of such a tubular part by inexpensive means which requires little .or no excess metal and con sumes no significant amount of otherwise usable space.

Another object is to provide a new and improved method of treating the edge of such a tubular part to increase the vacuum devices capability of withstanding high voltages in the region adjacent said edge.

In carrying out our invention in one form, we treat this edge by establishing an electric are at the edge with one terminal of the arc attached to the edge. The attached are terminal is then moved along the entire edge to successively melt localized portions of the edge engaged by the arc. As the arc moves out of engagement with a given portion of the edge, the molten metal of the edge solidifies and forms a rounded bead having an extremely smooth external surface. The molten portions of the edge are maintained in an inert atmosphere until solidifica= tion, thus minimizing oxide formations which could interfere with subsequent attainment of the desired high dielectric strength.

For a better understanding of the invention, reference may be had to the following description taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a cross-sectional view through a vacuum type electric circuit interrupter embodying one form of the invention.

FIG. 2 illustrates a step in treating a part of the in-= terrupter of FIG. 1.

FIG. 3 is an enlarged cross-sectional view of a portion of FIG. 1..

3,469,049 Patented Sept. 23, 1969 Referring now to FIG. 1, there is shown a vacuumtype circuit interrupter that is designed for high voltage circuit applications. This vacuum-type circuit interrupter comprises a sealed envelope 10 that has its interior evacuated to a pressure of 10 mm. of mercury or lower. The envelope 10 comprises a tubular casing 11 of insulating material and a pair of metal end caps 12 and 13 sealing off the ends of the casing. The end caps are joined to the casing by suitable vacuum-tight glass-to-metal seals 14.

Located within the envelope 10 are a pair of separable contacts 17 and 18 of disc form. The upper contact 17 is at stationary contact suitably joined to a stationary con= tact rod 17a that extends in sealed relationship through the upper end cap 12. The lower contact 18 is a movable contact suitably joined to a movable operating rod 18a. The operating rod 18a extends freely through an opening 19 in the lower end cap, and a flexible metal bellows 20. is provided thereabout to permit movement of the rod 1821 without impairing the vacuum inside the envelope. The" bellows 18 is joined at its respective opposite ends to the end cap 13 and the rod 18a by means of suitable vacuum-tight joints.

The interrupter can be closed by driving the lower contact 18 upwardly into a dotted line position 21 where it engages the upper contact 17. When the interrupter is thus closed, current can flow through it via parts 17a, 17 18 and 18a. Contact-opening is elfected by returning the lower contact from its dotted line position of engagement to 'the illustrated solid line position.

If the interrupter is energized when the above opening operation takes place, an electric arc will be drawn between the contacts. This are is soon extinguished in a well-known manner by the effect of the vacuum; but while the arc is present, it vaporizes some of the contact metal, expelling it radially outward toward the insulating casing 11.

For intercepting this metal vapor, a tubular metal shield 30 is provided. This tubular metal shield 30 surrounds the contacts and is mounted on the casing 11 by suitable mounting means 32. In the preferred form of the inven= tion shown, the shield is electrically isolated from both contacts 17 and 18 and is at approximately a mid-potential with respect to the contacts lwhen the interrupter is opened. To aid in condensing the arc-liberated vapors, a pair of tubular end shields 34 and 35 are also provided; and these end shields are respectively connected to the end caps 12 and 13. These end shields are shown surrounding the respective oposite ends of the central shield 30.

When the interrupter is in its open position shown, the regions immediately adjacent the exposed edges 40, 41, 42 and 43 of the shields are regions of high dielectric stress. In these regions, the lines of force of the electrical field crowd closely together and produce conditions conducive to a sparkover.

The present invention is concerned with increasing the amount of voltage that the interrupter can withstand without a sparkover in the high stress regions. This we do by treating the edge region of each shield as follows. Before the interrupter is assembled, we establish an electric are at the edge of the shield with one terminal of the are attached to the edge. FIG. 2 shows such an are 49 established at the annular edge 41 of the central shield 30. One terminal of the are 49 is attached to the edge of the shield and the other to a refractory electrode 50, which is capable of withstanding the high temperature of the are without melting. As soon as the arc is established, the electrode 50 is moved rapidly along a circular path 52 that is in substantial alignment with the annular edge 41. The rate at which the electrode 50 is moved is made high enough to avoid any burning of the edge 41 3 but is kept low enough to melt the portion of the edge engaged by the arc. As soon as the arc moves out of a given region, the molten metal solidifies and forms a bead along the edge.

FIG. 3 is an enlarged cross-sectional view which shows the head at 55. As illustrated in FIG. 3, this bead 55 of arc-melted metal has a rounded, highly-smooth external surface. The bead is in substantial alignment with the thin-walled section of the shield immediately thereadjacent. The external surface of the bead smoothly merges with the cylindrical internal and external surfaces of the adjacent portion of the shield.

A factor that is believed to contribute to the roundness and smoothness of the bead surface is the thinness of the immediately adjacent shied wall. Because of this thinness, the arc can melt the edge of the shield wall across its entire thicknes at a given circumferential logation. The resulting pool of molten metal, aparently because of surface tension, tends to develop a peripheral configuration that is generally semicircular, when viewed in cross-section. This shape persists until solidification, after which it is, of course, permanently preserved, In a practical embodiment of the invention, a shield 30 made of nickel and having a wall thickness of about .05 inch is used.

Cross-sections have been taken through the bead and examined under a microscope to determine the grain structure of the metal in the bead. It has been found that the grains of the head are very large compared to the size of grains in the shield only a short distance away. For example, we have found that typically the grains in the bead have a diameter greater than about twenty times the diameter of the-grains located .15 inch and greater from the tip of the bead.

The above-described bead-forming process is performed in an inert atmosphere, preferably an argon atmosphere. This inert atmosphere can be maintained about the localized portionof the edge 41 exposed to the are at any given time, but preferably it is maintained around the entire annular edge. In other words, in the preferred form, the entire beading operation is performed in a chamber filled with argon. In any case, however, the inert atmosphere should be maintained long enough to allow the molten metal of the head to solidify before exposure to air or oxygen. The inert atmosphere serves to inhibit the formation of oxides and the possible burying of oxides in the metal of the bead After the shields are all treated in this manner, they are suitably assembled in the vacuum interrupter, after which the interupter is subjected to a conventional high temperature bakeout to remove contaminants from its surfaces. Such bakeout removes easily-decomposable oxides that may have formed on the bead surface after its formation. But such bakeouts cannot effectively remove oxides buried just beneath the surface of the metal, and it is therefore important that such oxide formation be minimized, as above-described, during the bead-forming process. It is highly desirable to avoid contaminants, such as oxides, at or near the surface of highly stressed parts in a vacuum, since these contaminants can cause dielectric breakdown at unduly low voltages.

By treating the edges of the shield in the above-de-= scribed manner, we have realized remarkably increased dielectric strength, For example, in vacuum interrupters built substantially as shown in FIG. 1, we have been able to increase the impulse-withstand level of the interrupter from about 90 kv. to about 150 kv. The compared interrupters were the same configuration except for the presence of the arc-melted beads in the improved interrupters. In the inferior interrupter, the edges 40-43 were rounded, but by a grinding process. This greater-than- 50 percent increase in dielectric strength is an unexpected new result of the bead-forming treatment.

Viewed in retrospect, the exceptional smoothness of the arc-melted head is believed to be a factor that contributes to the high dielectric strength. Another significant contributing factor is believed to be the larger grain sizes present in the bead, as was pointed out hereinabove. Grain boundaries may be regions of dielectric weakness, and the number of such. boundaries is drastically reduced because of the much larger grain sizes present in our arcmelted bead than elsewhere in the shield.

It is to be noted that our. arc-melted bead requires no significant additional shield metal and consumes no substantial amount of space beyond that otherwise needed in the interrupter. In this respect, note from FIG. 3 that the bead 55 at its thickest region, considered radially of the tubular shield 30, is only very slightly thicker than the adjacent wall of the shield. Moreover, the bead-forming process can be quickly and easily performed.

While we have shown and described a particular embodiment of our invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from our invention in its broader aspects; and we, therefore, intend in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of our invention.

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

.1. A high voltage vacuum device comprising:

(a) a highly evacuated envelope,

(b) a tubular metal shield located within said envelope and terminating in an end portion of annular form,

(c) said end portion comprising a thin-wall annulus and an annular bead of arc-melted metal substantially aligned with said thin-walled annulus at its extreme end, said bead being formed by locally meltting the edge of said annulus with an electric arc in an inert atmosphere and maintaining the molten portions' of said edge in said inert atmosphere until solidification,

(c') said head having a grain structure that is characterized by grains ofa large size in comparison to those present in said thin-Wall annulus at points spaced from said bead,

(d) said head having a smooth, rounded external surface that is located in a region of high dielectric stress during periods when said vacuum device is energized,

(e) said bead being so formed as to improve the ability of said high voltage vacuum device to withstand voltages applied between the edge of said thin-walled annulus and an adjacent part at a different potential spaced therefrom by evacuated space within said envelope.

2. The vacuum device of claim 1 in which said bead has a maximum thickness considered radially of said thin-wall annulus which is only slightly greater than the wall thickness of said annulus in immediately adjacent regions.

References Cited UNITED STATES PATENTS 1,273,015 7/1918 Thomas 174-140.2 2,892,912 6/1959 Greenwood et al.

2,050,416 8/1936 Blanchard 219-121 3,209,195 9/1965 Schade 313-317 X 3,307,246 3/1967 Gulliksen et a1 17468.5 X

ROBERT S. MACON, Primary Examiner US. Cl. X.R.

29155.5, 552.2; 72342; 174140; 200-166; 219l21;3l33l3

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