US2892911A - Vacuum-type circuit interrupter - Google Patents

Description

June 30, 1959 D. w. CROUCH 2,892,911

VACUUM-TYPE CIRCUIT INTERRUPTER Filed Dec. 24. 1956 Inventor: Donald W. Crouch United States Patent VACUUM-TYPE CIRCUIT INTERRUPTER Donald W. Crouch, Drexel Hill, Pa., assignor to General Electric Company, a corporation of New York Application December 24, 1956, Serial No. 630,247

12 Claims. (Cl. 200-144) This invention relates to a vacuum-type of circuit interrupter and, more particularly, to an improved shielding arrangement for protecting the insulation of the interrupter against the build-up of metallic coatings thereon.

In vacuum circuit interrupters, the metallic vapors which are produced by arcing tend to condense on the insulating surfaces of the interrupter, and, hence, to form metallic coatings which impair the insulating properties of such surfaces. The various shielding arrangements which heretofore have been proposed for protecting these surfaces against such metal deposition have not been entirely satisfactory. For example, some of these prior arrangements have been subject to the disadvantage that the shields are so shaped that a large portion of the metallic particles can by-pass the shields and still coat the insulating surfaces.

Another common deficiency of the prior art has been that a single internal breakdown from the shield could establish a power are capable of metallically coating the insulating surfaces to such an extent as to ruin the interrupter. The likelihood of such an occurrence becomes quite serious when considered in the light of the fact that the coating which accumulates on the surface of the shield sometimes consists of particles that are relatively loosely bonded to the surface, thus rendering the surface much more subject to the initiation of spark breakdown.

Accordingly, an object of my invention is to provide, for a vacuum interrupter, a vapor-condensing shield which effectively prevents metal-deposition on the insulation of the interrupter and which is so constructed that a single internal breakdown from the shield does not result in the establishment of a power are.

Another object is to construct the shielding in such a manner that any metallic coating resulting from areproduced metal vapors does not lessen the creepage distances present in the interrupter.

Still another object is to construct the interrupter and the shielding in such a manner that the interrupter has essentially the same breakdown strength irrespective of the polarity of the voltage applied to its terminals.

In carrying out my invention in one form, I surround the arcing gap between the electrodes of a vacuum circuit-inteirupter with a generally-tubular metallic shield which is electrically isolated from both of the electrodes and from ground. This construction provides a pair of series-related vacuum gaps respectively interposed between the two electrodes and the shield. The shield is arranged to intercept and condense essentially all metallic particles emitted from the arcing gap which, if not in tercepted by the shield, would form a conductive coating on those insulating surfaces which isolate the shield from the electrodes and the electrodes from each other. Since essentially all of said metallic particles are captured on the metallic shield and are prevented from coating said insulating surfaces, it will be apparent, especially as the description proceeds, that the percentage of the total potential between the spaced-apart electrodes that is applied between the shield and each electrode will be retained without being substantially changed by the condensation of said metallic vapors on the shield.

For a better understanding of my invention, reference may be had to the following specification taken in con junction with the accompanying drawing, wherein:

Fig. 1 shows a vacuum circuit-interrupter embodying my invention in one form.

Fig. 2 illustrates a modified form of vacuum interrupter.

Referring now to the interrupter of Fig. 1, there is shown a highly-evacuated envelope 10 comprising a casing 11 of insulating material, such as a suitable ceramic, and a pair of metallic end caps 12 and 13 closing off the ends of the casing. Suitable seals 14 are provided between the end caps and the casing to render the envelope 10 vacuum-tight.

Located within the envelope are a pair of separable electrodes, or rod contacts, 17 and 18 shown by solid lines in the closed-circuit position. The electrode 17 is a stationary electrode suitably united to the upper endcap 12, whereas the electrode 18 is a movable electrode suitably mounted for vertical movement and projecting through an opening in the lower end-cap 13. A flexible metallicbellows 20 interposed between the end-cap 13 and the movable electrode 18 provides a seal about the movable electrode and allows for vertical movement thereof Without impairing the vacuum inside the interrupter. As shown in the drawing, the bellows 20' is sealingly secured at its respective opposite ends to the electrode 18 and the end cap 13.

Coupled to the lower end of the movable electrode 18, I provide suitable actuating means (not shown) which is capable of driving the electrode rapidly downwardly from its solid line position of Fig. 1 to its dotted line position to open the interrupter and which is also capable of returning the electrode to the solid line position to close the interrupter.

When the electrode is driven downwardly to open the interrupter, a circuit-interrupting, or arcing, gap is established between the adjacent ends of the electrodes, and the resulting are, though quickly extinguished, vaporizes some of the metal of the electrodes. In order to prevent this metallic vapor from condensing on the internal insulating walls of the casing 11, I provide a metallic shield 25 which is constructed in accordance with the present invention.

This shield 25 is of a generally-tubular configuration and extends along the length of the insulating casing 11 for substantial distances on opposite sides of the gap between the electrodes. In accordance with my invention, the shield 25 is electrically isolated from both of the electrodes 17 and 18 and also from ground, or, in other words, it is at a floating potential with respect to the electrodes. For achieving this floating relationship or electrical isolation, I prefer to rely upon the insulating casing 11 as a supporting structure for the shield 25. To this end, the ceramic casing 11 is formed from two coaxiallydisposed tubes joined together by a ceramic-to-metal seal which comprises an annular metallic disc 27 sealingly interposed between the adjacent ends of the ceramic tubes. This disc 27 is suitably united, as by welding, to the tubular shield 25 and, thus, supports the shield 25 upon the insulating casing 11. At axially-opposite sides of the supporting disc 27, the entire floating shield 25 is spaced from the internal insulating surface of casing 11.

Cooperating with the floating shield 25, a pair of metallic end shields 30 and 31, respectively connected to the electrodes 17 and 18, are provided. These end shields cut invention. As described in greater detail in the Greenwood et al. application, the end shields 30 and 31 serve to transfer electrical stresses away from both the end seals 14 and the end-caps 12 and 13 and thereby to virtually eliminate these parts as possible sources of electrical breakdown. The end shields also provide condensation surfaces for metallic vapors produced by arcing and, thus, help to prevent the insulating casing 11 from becoming coated by the metallic particles contained in the vapors. Additional functions of the end shields are described in the aforesaid Greenwood et al. application.

It will be apparent from Fig. 1 that essentially all straight-line paths which extend from the general region of the arcing gap to the insulating casing 11 are intercepted either by the floating central shield 25 or by the end shields 30, 31. This relationship contributes in an important manner toward protecting the interior surfaces of the insulating casing 11 from the buildup of a metallic coating thereon. This follows from the fact that the metallic particles liberated from the electrode tips by arcing travel in generally straight-line paths once they leave the general region of the arcing gap. Arcing will produce slightly elevated pressures in the immediate region of the arcing gap, and in this region the metallic particles will travel in all directions. But once the particles leave this pressurized region, they travel outwardly therefrom in generally straight-line paths. Accordingly, the shielding 25, 30, 31 interposes in essentially all of these paths metallic surfaces upon which these particles condense before they can reach the casing 11. As explained in the aforesaid Greenwood et al. application, a small percentage of the metallic particles can bounce off the shields one or possibly several times before finally adhering, but only an insignificant number of such particles reach the casing 11 due to the tortuous and restricted nature of the gaps between the shields 25, 30 and 31. It will thus be apparent that the shield 25 is so located that it intercepts essentially all metallic particles emitted from the arcing gap which, if not intercepted by the shield, would form a conductive coating on the insulating surfaces which isolate the shield from the electrodes.

It has been observed that breakdowns may occur at random in an electrically-stressed gap even though the gap has sufficient dielectric strength to successfully withstand this same voltage for extended periods of time. A unique feature of a vacuum gap is that, in many such cases, the gap is capable of recovering its normal dielectric strength within a few microseconds after a breakdown is initiated thus quickly extinguishing the discharge.

The possibility that such a random breakdown will result in a destructive power arc being established from the shielding is minimized in my interrupter by relying upon the electrically-fioating relationship of my control shield 25. In this regard, the floating relationship of the central shield 25 results in the shield being separated from the two electrodes 17 and 18 by two series-related vacuum gaps. As explained hereinafter, each of these gaps has a dielectric strength which is normally capable of successfully withstanding the maximum voltage to which the interrupter may be subjected. Should one of these gaps spark-over in the above-described random fashion during or after an opening operation, then the dielectric strength of the other gap is available to prevent complete breakdown of the interrupter. It is extremely unlikely that the second gap would spark-over before extinction of the discharge at the first gap. Since a breakdown across a single gap does not establish a complete power circuit, no follow current results from such a breakdown, and, consequently, the resulting discharge can be quickly extinguished before any harm results to the interrupter. Follow current, on the other hand, generally requires a longer period for interruption and liberates vastly more metallic vapors. As a result, follow current flowing across one of the inter-shield gaps could quickly ruin the 4 interrupter by producing a rapid build-up of metallic particles on the interior of the insulating casing 11.

The importance of the above-described dual gap relationship is further emphasized by the fact that the metallic particles which accumulate on the shields are sometimes relatively loosely bonded to the surfaces thereof, and, as a result, breakdown from these surfaces can be initiated somewhat more readily than when the surfaces are free of such particles.

To minimize the possibility of concurrent breakdown of both gaps, I construct the shields 25, 30 and 31 in such a manner that the floating shield 25 is at approximately mid-potential with respect to the electrodes 17 and 18 when the interrupter is in its fully-open position. This relationship can be attained in a well-known conventional manner by suitably shaping the shields, by suitably selecting the size of the inter-shield gaps, and by suitably. selecting the location at whichthe mounting flange 27 is supported on the insulating casing 11. For example, in the disclosed interrupter this relationship is obtained by constructing the two gaps of the same size and configuration and by supporting the mounting flange 27 midway between the terminals of the interrupter. Obviously, these relationships would not be materially changed by the presence of a conductive coating on the internal surface of the shields 25, 30, 31. Accordingly, the production of such coatings, as a result of arcing, does not significantly vary the potential of the shield 25 relative to the electrodes. The mid-potential relationship of the shield is therefore retained without being substantially changed by the condensation of metallic vapors thereon. In other words, the percentage of the total potential between the spaced-apart electrodes that is applied between the shield and each electrode is not substantially changed by the condensation of metallic vapors on the shield. Preferably, the size and configuration of each of the intershield gaps are of such a nature that each of these gaps has an appreciably greater breakdown strength than the gap between the electrode tips when the electrodes 17, 18 are in fully-open position. This further minimizes the possibility of both inter-shield gaps breaking down concurrently. In this regard, if one of the gaps did spark-over, it is extremely unlikely, as explained hereinabove, that the other gap would spark-over before extinction of the discharge at the first gap.

Another important advantage resulting from the floating relationship of my shield 25 is that there is little tendency for the power are which is established between the electrodes 17, 18 to transfer to the shield 25 during the interrupting process. This is the case because the voltage available between the two separated electrodes is always greater than that available between either of the electrodes and the floating shield 25, which is at approximately mid-potential with respect to the electrodes. Precluding such arc-transfer is highly desirable because once an arc root attaches itself to the shielding, there is a serious possibility of the are shifting rapidly to a relatively vulnerable part of the interrupter.

To minimize the possibility of breakdown being initiated from the shielding, the shields preferably have smooth, well-polished surfaces which are free of insulating films.

It has been observed that arcing gaps in general have a lower breakdown strength when subjected to voltage of one polarity than when subjected to a voltage of an opposite polarity. In general, it appears that the more non-symmetrical is the electric field in the region of the gap the more pronounced is this polarity effect. In accordance with the present invention, my shielding 25, 30, 31 is shaped and located in such a manner that the con trolling electric field is generally symmetrical with respect to a reference plane which bisects the arcing gap between the fully-open electrodes and which extends perpendicular thereto. By providing this symmetrical relationship of the electric field about the interrupting gap,

I have been able to largely eliminate the above-described polarity efiect. As a result, when used for interrupting alternating-current circuits, my interrupter is not subject to the unduly prolonged arcing which could result from low breakdown strength during alternate half cycles.

Obtaining a symmetrical field for the interrupting gap is greatly facilitated by the fact that the shielding 25 which surrounds the gap is floating and, therefore, can itself be symmetrical with respect to the above-described reference plane. The fact that the floating shield is at a generally mid-potential also facilitates obtaining the desired symmetry. The fact that this mid-potential relationship is not changed by the deposition of metallic particles on the shield facilitates maintaining the desired symmetry of the field after repeated switch operations. In general, with collinear electrodes and a floating central shield, such as in my disclosed interrupter, the other conductive parts in the region of the arcing gap should be symmetrical with respect to the above reference plane in order to provide the desired symmetry of the electric field.

In prior vacuum interrupters, it has been common to support the condensing shield which envelops the arcing gap upon one of the electrodes. This renders it extremely difficult, if not virtually impossible, to obtain the desired symmetry of the electric field. In such arrangement, if the shield is of metal and is connected to one electrode, the electric field is inherently non-symmetrical, whereas if the shield is of insulating material, it quickly becomes metallically coated and thereafter acts in the same general way as a metallic shield.

Since my shielding arrangement prevents the deposition of metallic particles on any of the insulating surfaces of the interrupter, it will be apparent that the creepage distances along these surfaces remain constant despite the metallic vapors which are emitted from the arcing region.

Fig. 2 shows an alternative scheme for supporting the floating shield 25. In this arrangement the insulating casing 11 is formed as a single cylinder having an annular groove in its internal wall. Suitable tabs 27a welded to the outer periphery of the shield 25 project into the groove and properly position the shield. This construction allows the interrupter to be constructed with fewer seals than the interrupter of Fig. 1.

Although I have described my invention in connection with a vacuum switch, it is to be understood that certain aspects of the invention are also applicable to other types of circuit interrupters, such as lightning-arresters, wherein there is no relative movement between the electrodes.

Although the disclosed embodiment of my invention is provided with end-shields (30, 31), it will be apparent that the invention also may be utilized in those interrupters having no corresponding end-shields.

The particular interrupters shown and described are merely embodiments of my invention, and it will be obvious to those skilled in the art that various changes and modifications may be made without departing from my invention in its broader aspects. I, therefore, intend in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

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

1. In a vacuum circuit interrupter, a highly-evacuated vacuum-tight envelope comprising a generally-tubular casing of insulating material and a pair of end caps disposed at the ends of said casing, a pair of electrodes located within said insulating casing and disposed in spacedapart relationship during a circuit-interrupting operation to define an arcing gap therebetween, a generally-tubular metallic shield surrounding said arcing gap and extending along the length of said casing for substantial distances on opposite sides of said arcing gap, said shield having a metallic internal surface which acts to intercept and condense metallic vapors emitted from said arcing gap, said shield being electrically isolated from both of said electrodes and from ground, said internal surface being metallic even prior to the condensation of metallic vapors thereon and having a potential relative to one of the electrodes in open circuit position which is a predeter mined percentage of the potential between said electrodes in open circuit position, said predetermined percentage being retained without being substantially changed by the condensation of said metallic vapors on said shield even from a time prior to the condensation of said metallic vapors thereon, said shield being so located that it is interposed between said arcing gap and all of those insulating surfaces in the region of said shield which isolate the shield from said electrodes and between said elec trodes and the electrodes from each other, whereby to preclude a conductive path from being built-up between said shield and either of said electrodes.

2. The interrupter of claim 1 in which said shield is so constructed that it intersects essentially all straightline paths which extend from said arcing gap to said insulating casing, aside from those paths intersecting metallic structure electrically connected to said electrodes.

3. The interrupter of claim 1 in which said electrodes are generally collinear and which is further characterized by having in the open-position an electric field which, in the general region of the arcing gap, is generally symmetrical with respect to a central plane extending between said electrodes normal thereto and which retains its general symmetry despite the condensation of metallic vapors on said shield.

4. In a vacuum circuit interrupter, a highly-evacuated vacuum-tight envelope comprising a generally-tubular casing of insulating material and a pair of end caps disposed at the ends of said casing, a pair of electrodes located within said insulating casing and disposed in spaced-apart relationship during a circuit-interrupting op eration to define an arcing gap therebetween, a generallytubular metallic shield surrounding said arcing gap and extending along the length of said casing for substantial distances on opposite sides of said arcing gap for intercepting and cdndensing metallic vapors emitted from said arcing gap, said shield being electrically isolated from both of said electrodes and ground and having a generally mid-potential with respect to said spaced-apart electrodes, said mid-potential relationship being retained without being substantially changed by the condensation of said metallic vapors on said shield even from a time prior to the condensation of said metallic vapors thereon, said shield being so located that it is interposed between said arcing gap and all of those insulating surfaces in the region of said shield which isolate the shield from said electrodes and the electrodes from each other, whereby to preclude a conductive path from being built-up between said electrodes and between said shield and either of said electrodes.

5. In a vacuum circuit interrupter, a highly-evacuated vacuum-tight envelope comprising a generally-tubular casing of insulating material and a pair of end caps disposed at the ends of said casing, a pair of electrodes located within said insulating casing and disposed in spaced-apart relationship during a circuit-interrupting operation to define an arcing gap therebetween, a generally-tubular metallic shield surrounding said arcing gap and extending along the length of said casing for substantial distances on opposite sides of said arcing gap for intercepting and condensing metallic vapors emitted from said arcing gap, said shield being electrically isolated from both of said electrodes and from ground, said me tallic shield having a potential relative to one of said electrodes in open circuit position which is a predetermined percentage of the potential between said electrodes in open circuit position, said predetermined percentage being retained without being substantially changed by the condensation of said metallic vapors on said shield even from a time prior to the condensation of said metallic vapors thereon, means defining a pair of series-related vacuum gaps one of which is interposed between said shield and one of said electrodes and the other of which is interposed between said shield and the other of said electrodes, each of said vacuum gaps having a breakdown strength which is normally substantially above the breakdown strength of said arcing gap when the electrodes are in their fully-separated position, said shield being so located that it is interposed between said arcing gap and all of those insulating surfaces in the region of said shield which isolate the shield from said electrodes and said electrodes from each other, whereby to preclude a conductive path from being built-up between said electrodes and between said shield and either of said electrodes.

6. In a vacuum circuit interrupter, a highly-evacuated vacuum-tight envelope comprising a generally-tubular casing of insulating material and a pair of end caps disposed at the ends of said casing, "a pair of electrodes located within said insulating casing and disposed in spaced-apart relationship during a circuit-interrupting operation to define an arcing gap therebetween, a generallytubular metallic shield surrounding said arcing gap and extending along the length of said casing for substantial distances on opposite sides of said arcing gap for intercepting and condensing metallic vapors emitted from said arcing gap, said shield being electrically isolated from both of said electrodes and from ground, said metallic shield having a potential relative to one of said electrodes in open circuit position which is a predetermined percentage of the potential between said electrode in open cir cuit position, said predetermined percentage being retained without being substantially changed by the condensation of said metallic vapors on said shield even from a time prior to the condensation of said metallic vapors thereon, said shield being so located that it intercepts essentially all metallic particles emitted from said arcing gap which, if not intercepted by the shield, would form a conductive coating on those insulating surfaces isolating the shield from said electrodes and the electrodes from each other.

7. The interrupter of claim 6 in which said casing comprises a pair of insulating tubes disposed in substantially axially-aligned relationship, a metallic disc sealingly interposed between the adjacent ends of said tubes and projecting radially inward into said envelope, and means for supporting said shield on said disc.

8. The interrupter of claim 6 in which said casing has a recessed portion formed in its internal wall, and shieldsupporting structure projecting radially outward from said shield and seated in said recessed portion.

9. The interrupter of claim 6 in combination with support means wholly disposed radially outside of said generally tubular shield and supporting said shield on said casing in radially-spaced relationship to said casing.

10. In a vacuum circuit interrupter, a highly evacuated vacuum-tight envelope comprising a generally tubular casing of insulating material and a pair of end caps disposed at the ends of said casing, a pair of electrodes located within said insulating casing and disposed in spaced-apart relationship during a circuit-interrupting operation to define an arcing gap therebetween, a generally-tubular conductive shield surrounding said arcing gap and extending along the length of said casing for substantial distances on opposite sides of said arcing gap for intercepting and condensing metallic vapors emitted from said arcing gap, said conductive shield having a potential relative to one of said electrodes in open circuit position which is a predetermined percentage of the potential between said electrodes in open circuit position, said predetermined percentage being retained without being substantially changed by the condensation of said metallic vapors on said shield even from a time prior to the condensation of said metallic vapors thereon, said shield being electrically isolated from both of said electrodes and from ground, said interrupter having insulating surfaces which when coated with conductive material will form a conductive path which lowers the creepage distance between said electrodes and between said shield and said electrodes, said shield being so located that it is interposed between substantially all of said insulating surfaces and said arcing gap, whereby to substantially prevent said metallic vapors from building-up conductive paths along said insulating surfaces.

11. The interrupter of claim 10 in which said shield is at a generally mid-potential with respect to said spacedapart electrodes both when its internal surface is uncoated by metallic particles from said arcing gap and when its internal sunface is coated by said metal-lie particles.

12. The interrupter of claim 10 in which said electrodes are generally collinear and which is further charac terized by having in the open-circuit position an electric field which, in the general region of the arcing gap, is generally symmetrical with respect to a central plane extending between said electrodes normal thereto and which retains its general symmetry despite the condensation of metallic vapors on said shield.

References Cited in the tile of this patent UNITED STATES PATENTS 1,510,341 Proctor Sept. 30, 1924 1,819,154 Eschholz Aug. 18, 1931 1,836,725 Proctor Dec. 15, 1931 1,875,765 Scherbius Sept. 6, 1932 1,919,987 Prince July 25, 1933 2,794,101 Jennings May 28, 1957 and)...

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