US2892912A - Vacuum type circuit interrupter - Google Patents

Description

J1me 0, 1 A. GREENWOOD ET AL 2,892,912

VACUUM TYPE CIRCUIT INTERRUPTER Filed Dec. 24. 1956 Z M a M 7 A .2 Q g a If w J a W Z Y a N Q N o Z 3 /l.// M. d A 4 l w |||w |nur| s w r E G t EH n PS 6a Km ,m E aT m United States Patent VACUUM TYPE CIRCUIT INTERRUPTER Allan Greenwood, Ardmore, Thomas H. Lee, Media, and

Harold N. Schneider, Springfield, Pa., assignors to General Electric Company, a corporation of New York Application December 24, 1956, Serial No. 630,248

Claims. (Cl. 200-144) This invention relates to a vacuum-type 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 and for effecting an optimum distribution of voltage stresses along the insulation of the interrupter.

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. For protecting these insulating surfaces against such metal-deposition, it has been found highly desirable to utilize an electrically-fioating shield which is isolated from the electrodes of the interrupter by means of a pair of series-related vacuum gaps. A shield of this type is disclosed and claimed in application S.N. 630,247, filed in the name of D. W. Crouch and assigned to the assignee of the present invention.

An object of the present invention is to form these series-related vacuum gaps in such a manner that the metallic particles liberated by arcing cannot readily pass therethrough to coat the adjacent insulation.

Another object is to form the gaps by relying upon a pair of end-shields which coact with the floating shield in such a manner as to transfer electric stresses from relatively vulnerable parts of the interrrupter to less vulnerable parts thereof.

Still another object is to assure that no arcing takes place from the usual conductive end-caps of the interrupter.

In carrying out our invention in one form, the arcing gap between the electrodes of a vacuum circuit interrupter is surrounded by a generally-tubular metallic shield which is electrically isolated from both electrodes. Coacting with this isolated shield are a pair of generallytubular end-shields which are respectively electricallyconnected to the electrodes. These end shields extend into proximity with the isolated shield so that a pair of series-related vacuum gaps are provided at opposite ends of the isolated shield. This entire structure is housed within an insulating casing, and the gaps are so disposed that they present a tortuous path for any metallic particles emitted from the arcing gap and travelling toward the insulating surfaces of the casing.

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

Fig. 1 is a vacuum-type circuit interrupter embodying our invention.

Fig. 2 illustrates certain electrical relationships which would be present in an interrupter similar to that of Fig. 1 but with certains parts omitted.

Fig. 3 is a modified form of vacuum-type interrupter embodying our invention.

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 01f 2,892,912 Patented June 30, 1959 the ends of the casing. Suitable seals 14 are provided between the end caps and the casing to render the envelope 10 vacuum-tight. Preferably, each of these seals 14 comprises a ceramic annulus 14a located at one end of the insulating casing 11 and an annular metallic disc 14b sealingly interposed between the casing and the annulus. A vacuum-tight joint is provided between the outer periphery of the metallic disc 14b and the adjacent end cap, preferably, by means of a suitable brazing or welding process.

Located within the envelope 10 are a pair of separable electrodes, or rod contacts, 17 and 18 shown in the open-circuit position. The electrode 17 is a stationary electrode suitably united to the upper end cap 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 metallic bellows 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, We provide suitable actuating means (not shown) which is capable of driving the electrode 18 upwardly from its position of Fig. 1 into circuit-closing engagement with the other electrode 17. The actuating means is also capable of driving the movable electrode 18 mp idly downwardly from the closed position to open the interrupter.

When the electrode is driven downwardly to open the interrupter, a circuit-interrupting 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, a metallic shield 25 is provided.

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. The shield 25 is electrically isolated from both the electrodes 17 and 18 and also from ground, or, in other words, it is at a floating potential with respect to the electrodes. This floating relationship or electrical isolation is achieved by relying upon the insulating casing 11 as a supporting structure for the shield 25. To this end, the ceramic casing 11 is formed from two coaxially-disposed ceramic 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.

In accordance with the present invention, we provide a pair of end shields 30 and 31 which are respectively electrically-connected to the electrodes 17 and 18. As shown in Fig. 1, these end shields are of cup-shaped, generally-tubular configuration and cooperate with the central floating shield 25 to provide a pair of seriesrelated vacuum gaps 32 and 34 at opposite ends of the central shield 25. Preferably, these ends shields are welded or otherwise secured to the electrodes 17 and 18.

Without these end shields, there is a tendency for electrical stresses to become concentrated in the region of the seals 14. Such stress concentration results not only from the relatively sharp inner peripheral edges 1 of the metallic discs 14b but also because of the tendency of an electric field to cencentrate near the boundary between conductive and insulating surfaces. Such stress concentrations tend to encourage breakdowns and are therefore most desirable.

In accordance with our invention, we materially lessen the possibility of such breakdowns by utilizing the end-shields 30 and 31 to transfer the electrical stresses away from the seals 14 and to distribute them over less vulnerable areas of the interrupter insulation. This may be more clearly understood by comparing the interrupter of Fig. l with a corresponding interrupter in which the end-shields are omitted, such an interrupter being shown in Fig. 2. Referring to Fig. 2, the dotted lines represent equipotential lines showing the general manner in which the voltage impressed across the interrupter terminals is distributed, for example, in the upper left hand region of the interrupter. It will be observed from Fig. 2 that the equipotential lines are distributed unevenly along the casing 11 and are rather severely concentrated in the region of the end seal 14. This, of course, results in high electrical stresses in the region of the end seals 14. With the end-shields present, however, as shown in Fig. l, the equipotential lines are distributed in a much more uniform manner along the insulating casing 11, and, particularly, without any material concentration in the region of the end seals. Thus, the end-shields 30 and 31 serve to reduce the voltage gradients in the region of the end-seals 14 and thereby greatly lessen the possibility of breakdowns from the seals.

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 build-up of a metallic coating thereon. This follows from the fact that the metallic particles liberated from the electrode tips by arcing tend to travel in generally straight-line paths once they leave the general region of the arcing gap. Our studies of this matter indicate that arcing will produce elevated pressures in the immediate region of the arcing gap, and in this particular 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. 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. 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 can reach the casing 11 due to the tortuous and restricted nature of the gaps 32 and 34. The fact that the tubular inner end of each end-shield is larger than and also surrounds the adjacent end of the central shield contributes to the desired tortousness of the paths extending through the gaps 32 and 34. Since substantially all the metallic particles are captured on the metallic shields and are prevented from coating the insulating surfaces, it will be apparent that the shields of Fig. 1 will retain the voltage distribution illustrated by the dotted lines of Pig. 1 without such distribution being substantially changed by the condensation of said particles on the shields.

The end-shields 30 and 31 also serve the very desirable function of shielding the end- caps 12, 13 and the bellows 20 from the effects of the electric field. This is most desirable because it is extremely diflicult to provide these latter parts with surfaces having the degree of smoothness and cleanliness which would otherwise be necessary to assure high dielectric strength. Surface irregularities or the presence O n l g films such as oxides, would cause these parts, if unshielded, to represent serious sources of breakdown. Since, in the disclosed interrupter, the voltage gradient in the region of these parts is very low due to the presence of the end-shields, as can be observed from Fig. 1, there is little likelihood of breakdown occurring from these parts. As can be further observed from Fig. l, the cup-shaped configuration of thes end-shields contributes in an important manner to their effectiveness in electrostatically shielding the end-caps.

The end shields 30 and 31, as well as the central shield 25, have smooth, highly-polished surfaces which are free from insulating films. They are preferably constructed of a material such as nickel, which can be subjected to relatively high degassing temperatures without melting. Because the shields are relatively thin, it is not diflicult to effect a thorough degassing thereof. "he fact that the shields are thoroughly degassed and have polished, film-free surfaces tends to lessen the possibility of breakdowns being initiated therefrom.

The end shields 30, 31 also minimize the possibility that an are established at the arcing gap will wander on to the end-caps. Though in nearly all cases arcing is confined to the region of the gap between the electrodes, it will occasionally happen that an arc terminus will escape from this region and wander about seeking to transfer the arc to another region where less energy is required for maintenance of the arc. With an end shield interposed as an obstruction which the arc would be required to circumvent if one terminus were on an end cap, it is extremely unlikely that the required arcing path would represent a region of least energy. Almost invariably, the energy required for such an arcing path would exceed that required if the terminus were located somewhere on the contacts in the volume generally enclosed by the shielding. Thus, with the endshields 30 and 31 interposed between the end caps and the arcing gap, there is little if any possibility that an arc will be transferred on to the end caps.

Thus, since, for all practical purposes, arcing from the end-caps is prevented, it is possible to more economically prepare the end caps for incorporation into the interrupter. For example, it becomes unnecessary to thoroughly degas the end-caps, as is required with the electrodes and shields. It also becomes unnecessary to provide highly-polished film-free surfaces for the endcaps.

As explained in the aforesaid Crouch application, the presence of the two series-related gaps 32 and 34 minimizes the possibility of a power are being established from the shielding. In this regard, it has been observed that breakdowns may occur at random in an electricallystressed gap even though the gap normally 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 power are being established from the shielding is minimized in the disclosed interrupter by the presence of the two series-related vacuum gaps 32 and 34. As explained hereinafter, each of these gaps has a dielectric strength which normally is capable of withstanding the maximum voltage to which the interrupter may be subjected. Should one of these gaps spark-over in the above-descrihed 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, thus precluding the formation of a power arc. 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 in the vacuum before any harm results to the interrupter.

Other advantages of utilizing a floating central shield, such as the ability of the floating shield to facilitate obtaining a symmetrical electric field for the arcing gap, are set forth in the aforesaid Crouch application, and reference may be had thereto for a more complete description of such advantages. The symmetrical electric field will, however, be discussed in greater detail hereinafter.

The possibility of concurrent breakdown of both gaps 32 and 34 is minimized in the disclosed interrupter by constructing the shields 25, 30 and 31 in such a mannet that the floating shield is at approximately a 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 which the 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, and 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. 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 intershield 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.

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. Because our central shield 25 is at a generally midpotential and because the internal structure of the switch is generally symmetrical with respect to a reference plane bisecting the arcing gap between the fully-open electrodes and extending perpendicular thereto, the controlling electric field is generally symmetrical with re spect to this reference plane. This symmetrical relationship of the electric field about the interrupting gap makes it possible to largely eliminate the above-described polarity effect, as is described in greater detail in the aforementioned Crouch application. As a result, when used for interrupting alternating-current circuits, the disclosed interrupter is not subject to the unduly prolonged arcing which could result from low breakdown strength during alternate half cycles.

' To prevent electric stresses from concentrating near the ends of the shields, these ends are rolled over, as shown at 36. The resulting improvements in stress distribution materially lessen the likelihood of breakdown occurring from these ends.

Fig. 3 shows a modified arrangement for supporting end-shields of the general type described hereinabove, with similar reference numerals being used to designate corresponding parts. Referring to Fig. 3, it will be noted that the end shield is formed as an integral part of the sealing disc 14b. This eliminates the need for welding 6 the end-shield to the electrode 17, as in Fig. 1. This construction also helps to eliminate the relatively sharp edge which is present in the seal 14 of Fig. 1. Preferably, the end-shield 30 of Fig. 3 extends into close proximity with the electrode 17, so as to provide an effective obstruction between the end-cap 12 and the arcing gap in the same general manner as the shield 30 of Fig. l. The shielding arrangement of Fig. 3 is especially suited for interrupters of relatively great length, and, for this reason, the interrupter of Fig. 3 is illustrated as being somewhat longer than that of Fig. 1.

While we have shown and described particular embodiments 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. In a vacuum circuit interrupter, a generally-tubular casing of insulating material, conductive end caps mounted at opposite ends of said casing, vacuum-tight seals located between said casing and said end caps, a pair of electrodes located within said casing and disposed in spaced-apart relationship during a circuit-interrupting operation to define an arcing gap therebetween, a first 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 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 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, a pair of generallytubular metallic end-shields extending from the region of said end caps into proximity with said first shield and cooperating with said first shield to provide a pair of series-related vacuum gaps at opposite ends of the first shield, said end-shields being respectively electricallyconnected to said electrodes and being shaped to lower the electrical stresses in the region of said seals in comparison to the stresses which would be present without said end-shields, said three shields being interposed between said arcing gap and essentially all of those insulating surfaces which isolate said electrodes from each other and from said first shield, said three shields coacting to maintain the voltage distribution along said tubular casing substantially unchanged by the condensation of said metallic vapors on said shields even from a time prior to the condensation of said vapors on said shields.

2. The interrupter of claim 1 in which each of said end-shields terminates in a open-end portion which is spaced radially-outward from an adjacent end of said first shield.

3. The interrupter of claim 1 in which said generallytubular end shields surround said first shield in the region of said inter-shield gaps.

4. The interrupter of claim 1 in which at least one of said seals comprises a radially-extending conductive disc which is integrally united with one of said end-shields.

5. In a vacuum circuit interrupter, a generally-tubular casing of insulating material, conductive end caps mounted at opposite ends of said casing, vacuum-tight seals located between said casing and said end caps, a pair of electrodes located within said casing and disposed in spaced-apart relationship during a circuit-interrupting operation to define an arcing gap therebetween, a first 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 vaports 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 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, a pair of metallic end-shields extending from the region of said end caps into proximity with said first shield and cooperating with said first shield to provide a pair of seriesrelated vacuum gaps at opposite ends of the first shield, the portion of each of said end-shields which is adjacent said first shield being located axially inward of said seals whereby to relieve said seals of electrical stress concentrations, said three shields being interposed between said arcing gap and essentially all of those insulating surfaces which isolate said electrodes from each other, said three shields coacting to maintain the voltage distribution along said tubular casing substantially unchanged by the condensation of said metallic vapors on said shields even from a time prior to the condensation of said vapors on said shields.

6. In a vacuum circuit interrupter, a generally-tubular casing of insulating material, conductive end caps mounted at opposite ends of said casing, vacuum-tight seals provided between said casing and said end caps, a pair of electrodes located within said casing and disposed in spaced-apart relationship during a circuit-interrupting operation to define an arcing gap therebetween, a first 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 first 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 said metallic vapors thereon and 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 opencircuit 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, a pair of conductive end-shields coopcrating with said first shield to provide a pair of seriesrelated vacuum gaps at opposite ends of said first shield, said end shields being respectively electrically-connected to said electrodes and being shaped to lower the electrical stresses in the region of said seals in comparison to the stresses which would be present without said end shields, said three shields being so located that they are interposed between said arcing gap and essentially all of the internal insulating surfaces of said insulating casing, said three shields coacting to maintain the voltage distribution along said tubular casing substantially unchanged by the condensation of said metallic vapors on said shields even from a time prior to the condensation of said vapors on said shields.

7. In a vacuum circuit interrupter, a highly evacuated vacuum-tight envelope comprises 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 first generallytubular metallic shield surrounding said arcing gap and extending along the length of said casing for substantial distances on opposite sides of said arcing gatp, said first shield having a metallic inner 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 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 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, a pair of metallic end-shields respectively electrically connected to said electrodes and cooperating with said first shield to define a pair of series-related vacuum gaps at opposite ends of the first shield, said three shields being so located that essentially all straight-line paths which extend from the region of said arcing gap to said insulating casing intersect at least one of said shields, and three shields coacting to maintain the voltage distribution along said tubular casing substantially unchanged by the condensation of said metallic vapors on said shields even from a time prior to the condensation of said vapors on said shields.

8. The interrupter of claim 7 in which said end-shields are of a generally-tubular configuration and extend from the region of said end caps into proximity with said first shield, said end-shields surrounding said first shield in the region of said inter-shield gaps.

9. The interrupter of claim 7 in which said end-shields are of a generally-tubular, cup-shaped configuration, each end-shield terminating in an open-end portion which is spaced radially-outward from an adjacent end of said first shield.

10. The interrupter of claim 1 in which said three shields coact despite vapor condensation of said shields to maintain a substantially uniform voltage distribution along the portion of said tubular casing extending from the general region of one of said seals to the general region of the other of said seals while said electrodes are 111 spaced-apart relationship.

References Cited in the file of this patent UNITED STATES PATENTS Jennings May 28, 1957

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