US3914568A

US3914568A - High-voltage vacuum switch

Info

Publication number: US3914568A
Application number: US499740A
Authority: US
Inventors: Donald W Crouch; Donald R Kurtz; Joseph C Sofianek
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1974-08-22
Filing date: 1974-08-22
Publication date: 1975-10-21
Anticipated expiration: 1992-10-21
Also published as: BR7504950A; SE395792B; JPS5145770A; CH591156A5; ES438988A1; DE2536024A1; JPS5910006B2; SE7508494L; IT1041964B; CA1036203A; GB1520922A; FR2282712A1; AU8174375A

Abstract

The disclosed vacuum switch has features that give it exceptional high-voltage withstand ability even after its contacts have been roughened by the effects of arcing. Both contacts of the switch are arranged to move during opening and closing operations. When the switch is fully-open, the contacts are located in shielded positions behind the front faces of shielding electrodes which respectively surround the contacts. During closing, each of the two contacts is driven out of its fully-open shielded position into the gap between the two shielding electrodes, engaging the other contact when the contacts are approximately midway between the shielding electrodes.

Description

United States Patent Crouch et al.

[ HIGH-VOLTAGE VACUUM SWITCH [75] Inventors: Donald W. Crouch, Newtown Square; Donald R. Kurtz, West Chester; Joseph C.-Sofianelt,

[73 Assignee:

[22] Filed:

Broomall, all of Pa.

Philadelphia, Pa.

Aug. 22, 1974 21 Appl. No.: 499,740

[5 2] US. Cl. 200/144 B General Electric Company,

[ Oct. 21, 1975 240,805 8/1969 U.S.S.R. 200/144 B Primary Examiner-Robert S. Macon Attorney, Agent, or Firm-.1. Wesley Haubner; William Freedman [5 7 1 ABSTRACT The disclosed vacuum switch has features that give it exceptional high-voltage withstand ability even after its contacts have been roughened by the effects of arcing. Both contacts of the switch are arranged to move during opening and closing operations. When the switch is fully-open, the contacts are located in shielded positions behind the front faces of shielding electrodes which respectively surround the contacts. During closing, each of the two contacts is driven out of its fully-open shielded position into the gap between the two shielding electrodes, engaging the other contact when thecontacts are approximately midway between the shielding electrodes.

9 Claims, 3 Drawing Figures /I/I//// I 4 US. Patent Oct. 21, 1975 HIGH-VOLTAGE VACUUM SWITCH BACKGROUND This invention relates to a vacuum switch for high voltage applications and, more particularly, to a switch of this type which has exceptional high-voltage withstand ability even after its contacts have been roughened by the effects of arcing.

The following references are of interest with respect to this invention: US. Pat. Nos. 3,2l0,505-Porter; 3,2ll,866-Crouch; 3,261,954-Yonkers; 3,283,l- Frink; 3,555,223-Robinson; and German Federal Republic Pat. No. 1,248,775-Lukatskaya.

The switch of the present application comprises relatively-movable contacts that are driven together to effect switch-closing and are separated to effect switchopening. In the high voltage applications that we are concerned with, a pre-strike usually occurs during the final stage of a closing operation before the contacts engage each other. That is, as the contacts near each other during this final stage, the high voltage present therebetween causes a breakdown (or pre-strike) between the still-spaced contacts, which is followed by arcing until the contacts actually engage. This arcing tends to roughen the contacts, not only because of its erosive effect but also because it leads to welds between the contacts which must be broken on a subsequent opening operation. Such roughening ordinarily detracts from the ability of the switch to withstand high voltages when in its open position.

SUMMARY An object of my invention is to construct the switch in such a manner that when fully open it can withstand exceptionally high voltages despite contact-roughness produced by prior arcing and other related effects.

In carrying out the invention in one form, we provide two annular shielding electrodes respectively surrounding the rods on which the contacts are mounted, each shielding electrode being electrically connected to its associated rod. These shielding electrodes are mounted in fixed spaced-apart positions and have front faces facing each other. When the switch is fully open, its contacts are located behind the front faces of the shielding electrodes in regions of relatively lowintensity electric field. With the contacts thus shielded from high-intensity electric field by the shielding electrodes, there is a greatly reduced likelihood that a breakdown will be initiated between the contacts.

Initially the shielding electrodes have smooth front faces, and the breakdown voltage between these faces is high. But if these faces become roughened by arcing or the effects of arcing, the ability of the switch to withstand high voltages across the gap between these faces is seriously impaired.

One possible approach to protecting these electrode faces from impairment by arcing or arcing effects is to locate them in regions relatively remote from the arcing regions of the contacts. But this remoteness renders the electrodes less capable of providing effective shielding action for these contact regions.

Accordingly, another object of the invention is to construct the contacts and shielding electrodes in such a manner that the shielding electrodes can be located relatively close to the arcing regions of the contacts when the switch is fully open, thus providing effective shielding action, yet with relatively little exposure of the shielding electrodes to being roughened by arcing or the effects of arcing.

In accordance with one form of the invention, we provide for movement of both contacts during closing and opening operations. When the switch is fully open, the contacts are located in their above-described shielded positions behind the front faces of the aforesaid shielding electrodes. But during closing, each of the two contacts is driven out of its fully-open shielded position into the gap between the two shielding electrodes, engaging the other contact when the contacts are approximately midway between the two shielding electrodes. If a prestrike should occur when the two contacts near each other during this closing operation, it consistently occurs between the contacts, and the resulting arc is normally confined to the contacts and is excluded from the then relatively remote shielding electrodes.

BRIEF DESCRIPTION OF DRAWINGS 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 switch embodying one form of the invention. Solid lines are used to show the switch contacts in their fully open position and dotted lines to show the contacts in their fully closed position.

FIG. 2 is a schematic illustration showing the switch and a suitable operating linkage therefor.

FIG. 3 is an enlarged view of a portion of FIG. 1 showing a portion of the adjacent electric field.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT Referring now to FIG. 1, the vacuum switch shown therein comprises a highly-evacuated, sealed envelope 10. This envelope comprises a cylindrical casing 11 of insulating material and a pair of

metallic end caps

12 and 13 at opposite ends of the casing. Suitable seals 14 are provided between the end caps and the casing to provide vacuum-tight joints between these parts. The normal pressure within the envelope is lower than 10" torr.

In the central region of the shield, the internal insulating surfaces of the casing 11 are protected from the condensation of arc-generated metallic vapors thereon by means of a suitable metallic shield 15 suitably supported on casing 11 and preferably electrically isolated from both

end caps

12 and 13. This shield 15 acts in a known manner to'intercept arc-generated vapors before they can reach casing l 1. Located within the evacuated envelope l0'are two relatively

movable contacts

17 and 18 shown by solid lines in FIG. 1 in their fully open position. Contact 17 is suitably fixed to the inner end of a conductive contact rod 17a that extends freely through the lower end cap 12. Providing a seal between contact rod and end cap 12 is a flexible bellows 20 that is secured in sealed relationship at its respective opposite ends to the end cap 12 and the contact rod 17a. This bellows 20 allows contact rod 17a to move in a direction longitudinal of the rod without impairing the vacuum in the envelope. Contact 18 is fixed to the inner end of a corresponding contact rod 18 a that extends freely through the upper end cap 13. A flexible bellows 21 provides a vacuumtight seal about cont-act rod 18 and allows this contact rod to move longitudinally without impairing the vacuum in the envelope.

End cap 12 is maintained at the same potential as contact 17 by virtue of a flexible electrical connection 16 therebetween, and end cap 13 is maintained at the same potentialas contact 18 by virtue of a flexible electrical connection 19 therebetween.

Guiding the contact rod 17a for substantially straight line movement in a vertical direction is a fixed sleeve bearing 24 located outside the envelope and supported on end cap 12. A corresponding sleeve bearing 26 guides "the other contact rod 18a for substantially straight-line movement in a vertical-direction.

For electrostatically shielding the

contacts

17 and 18 when they are in their fully-open position of FIG. 1, we provide a pair of stationary, generally

annular shielding electrodes

30 and 32. Each of the shielding electrodes surrounds its associated contact and contact rod in radially spaced relationship thereto and is electrically connected to its associated contact so as to be at substantially the same-potential as the contact. For supporting the shielding electrode 30, a metallic tube35 is provided about contact rod 17a, and a cup shaped metallic shield 37 supports this tube 35 on the lower end cap 12. The cup shaped shield 37 has its lower end brazed to end cap 12 and its upper end brazed to the tube 35. The upper end of tube 35 is suitably joined to the lower end of shielding electrode 30.

Structure

35, 37 electrically connects shielding electrode 30 to the lower end cap 12 and hence to the contact rod 17a.

The other shielding electrode 32 is supported on and is electrically connected to the upper end cap 13 by metallic structure 35a, 37a corresponding to the

abovedescribed structure

35, 37.

Closing of the switch is effected by driving the two

contacts

17 and 18 toward each other from their solidline positions of FIG. 1 into their dotted-line positions of FIG. 1. Each of the contacts has a contact-making surface 40 on its inner end facing the other contact. When these contact-making surfaces 40 engage each other, the switch is fully closed and the contact-making surfaces are located in a reference plane 42. This reference plane 42 extends perpendicular to the longitudinal axes of the rods and is located substantially midway between the front faces 50 of the shielding

electrodes

30 and 32.

After a closing operation has taken place, as above described, opening of the switch is effected when desired by returning the two

contacts

17 and 18 from their dotted-line closed positions to their solid-line fully open positions.

A suitable mechanical linkage for effecting the abovedescribed equal and opposite motion of the two

contacts

17 and 18 with respect to reference plane 42 is illustrated in FIG. 2. This linkage comprises two portions 60 and 60a and a vertically movable operating rod 62 to which these two portions are pivotally connected. Portion 60 corresponds to a linkage 110 disclosed in US. Pat. No. 3,594,525-Miller assigned to the assignee of the present invention. As such, it comprises two

bellcranks

63 and 64, each mounted on a stationary pivot, and interconnected by a wipe mechanism 65. One arm of bellcrank 63 is pivotally connected to the contact rod 17a and the other is pivotally connected to one end of wipe mechanism 65. The other bellcrank 64 has one arm pivotally connected to the other end'of wipe mechanism 65 and its other arm pivotally connected to the operating rod 62. When the operating rod 62 is moved downwardly, the two

cranks

63 and 64 are moved in a clockwise direction about their pivots, thereby driving the contact rod 17a through an upward closing stroke. The wipe mechanism 65 has been shown in schematic form only since it can be of any suitable conventional type, for example, that shown in FIG. 5 of US. Pat. No. 3,025,173-Frank, assigned to the assignee of the present invention. One purpose of the wipe mechanism is to allow a certain amount of overtravel of the driving parts after the contacts have engaged at the end of the closing stroke.

The other linkage portion 60a is substantially the same as linkage portion 60 and therefore will not be described in detail. Corresponding components of the two linkages have been assigned corresponding reference numerals except for the suffix a attached to the parts of linkage 60a. It will be noted that the crank 63a is inverted relative to the crank 63 and thus produces downward motion of contact rod 18a when the operating rod is moved downwardly. Thus,

contact rods

17a and 18a move in opposite directions in response to unidirectional movement of operating rod 62. The parts of the linkage are so proportioned that the contact rods move through substantially equal strokes at substantially equal speeds.

In one circuit application of our switch, a high voltage is present between the

contacts

17, 18 when the switch is fully open and also during a closing operation, but no voltage is present across the contacts while the switch is in the process of opening. During a closing operation in this circuit application, the high voltage present between the contacts can produce a breakdown, or pre-strike, between the contacts as they near each other during the final stages of the closing operation. This breakdown is' followed by arcing between the contacts, whichcontinues until the contacts engage each other and thus short out the arc. This are tends to roughen the contacts not only because it erodes them but also because it leads to contact-welding when the arc-melted portions of the contacts engage. When the contacts are separated during a subsequent opening operation, the weld is broken, thus leading to additional surface roughness.

In most high voltage devices, such surface roughness has a-tendency to reduce the breakdown voltage between parts such as 17 and 18 when they are spaced apart and a high voltage is applied between them. We, however, are able to maintain a high breakdown voltage between

contacts

17 and 18 despite such surface roughness because, when our switch is fully open, the contacts are effectively shielded from the high voltage field by being located in regions where the field intensity is low. This is explained in more detail in the following paragraph.

When the switch is fully open as shown in FIG. 1, the equipotential lines of the electrostatic field in the region of the shielding electrode 30 have an approximate configuration such as depicted by the dotted lines 39a and 39b of FIG. 3. Line 39a represents approximately 95 percent of the potential present across the gap between the electrodes, and line 39b represents approximately percent. It will thus be apparent that the contact 17 by being withdrawn behind the front face 50 of the shielding electrode 30 is in a region where the electrostatic field has very low intensity. Similarly, the

equipotential lines of the field adjacent the other shielding electrode -32'have a correspondin'gconfiguration and the fully open c'ontact'l81 is-thus' also'in a region of correspondingly low field intensity. This low field intensity adjacent the contact-making faces 40 very'materially reduces the likelihood that-abreakdown will be initiated from the contacts, even-though the contacts may have been roughened by prior arcing andthe'effects fsuch arcing-gas described hereinabov'e 1 I l i In order to efficiently utilize the shielding electrodes 30 and- 32"to maintain a high breakdown voltage, it is important that their own surfaces in highly stressed regions be maintained relatively smooth, especially' the front surfaces 50." To help maintain this surface mote from the contacts. But such remoteness tends to lessen the effectiveness of the electrodes in performing their shielding function for the contacts. Forexample, if the diameter of shielding electrode 30 is increased to make it more remote from the contact 17; the equipotential lines such as 39a of the electrostatic field tend to dip moredeeplyinto the center region ofthe'shielding electrode '30and to place the region immediately adjacent contact-making surface 40 in a higher intensity field region. M I y We are able to utilize shielding electrodes which closely surround th'contactsl7, "18', and thus provide effective shielding, because we longitudinally displace both contacts from their shielded fully open positions 3 during closing "operation" and cause" contactengagement tooccur in the midregion of the gap between the shielding electrodes. In this respect, note that the contactmaking surfaces are on or closely adjacent the central reference plane 42 whenthey engage at the end "of' the closing operation. Usually, any arcing that results from a pre-strike occursbetw efen the sur faces 40 when these surfaces are near the central reference plane 42.

Since the illustrated switch is intended to be used in exceptionally high voltage applications, we make the gap between the

electrodes

30, 32 unusually long for a vacuum switch, e.g., about 2 inches, thus locating the central reference plane where the contacts engage about 1 inch from each of the shielding electrodes. This relatively large distance substantially reduces the chance that an arc resulting from a pre-strike when the closing contacts are nearing reference plane 42 will originate on or contact either shielding electrode. The fact that the pre-strike arcing location is longitudinally displaced from both shielding electrodes also plays an important role in keeping the pre-strike are off the electrodes and in reducing the chance that the electrodes will be splattered by arc-generated molten metal from the contacts. Most of this metal travels generally radially outward from the arcing region, and the longitudinal displacement of the shielding

electrodes

30, 32 from the arcing region results in their being out of the path of such splatter. This would not be the case, if the arcingregion was located partially within the confines of either of the annular shielding'electrodes.

In order to reduce the amount of metal splatter producedby a pre-striking arc and to assure that the gap length will be exceptionally short at the time of a prestrike, we make the contacts, in one embodiment of our invention, of beryllium or a primarily beryllium material, as is disclosed and claimed in U.S. Pat. No. 3,143,373-Horn, assigned to the assignee of the present invention. Beryllium has exceptionally good dielectric strength properties and exceptional resistance to being roughened by an are or the effects of arcing. The shielding electrodes are preferably made of a hard, high ductility" ferrous material such as one of those disclosed and claimed in U.S. Pat. No. 3,769,538-l-larris, assigned to the assignee of'the present invention. Such materials also have excellent dielectric strength properties in a vacuum. Stainless steel is also a suitable materialfor the shielding

electrodes

30,32. In general, we avbid the softer metals such as copper and aluminum for the

electrodes

30, 32 because of their inferior dielectric strength compared to the above-mentioned steels.

Although we have described our invention hereinabove as being of special utility in connection with a switch that is opened under no-voltage conditions and thus without forming an arc during opening, itisto be understood that the invention in itsbroa'der aspects is not solimited. It can also be usedto good advantage even in those applications where an arc is formed during opening because such arc is typically extinguished aftercontact movement through only a small portion of the full opening stroke length and well before the contact making surfaces 40 enter their shielded positions 'within' the

surrounding shielding electrodes

30, 32, Thus, the metal splatter resulting from such arcing can be expelled radially outward in the usual manner without encountering the shielding electrodes. Also, there is little likelihood of the opening arc transferring to the shielding electrodes since the arcing region between the contact surfaces 40 is longitudinally displaced from the shielding electrodes and also since there is normally little or no magnetic action on an arc onsurfaces 40 tending to produce arc transfer to the

electrodes

30, 32 from the

contacts

17, 18. n

An ancillary feature of our invention is that we utilize the cup-shaped metallic support 37 for the shielding electrode 30 as a means for reducing the electrical field intensity in the region of the seal 14. Since this cupshaped support 37 extends closely adjacent the inner insulating wall of the cylindrical casing 11 for a substantial distance past the seal 14 toward the central reference plane 42, the electric field in the region of the seal has a relatively low intensity as indicated by its equipotential lines of about percent and percent, the approximate configurations of which are shown at 70a and 70b, respectively. This shielding effect for the seal desirably counteracts the known tendency for electrical stress to concentrate at glass-to-metal interfaces such as present in the seal 14. Cup-shaped support 37a produces this same shielding effect for the seal 14 at the upper end of the cylindrical casing 11.

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 switch comprising:

a. a highly-evacuated envelope comprising spacedapart conductive ends and electrical insulating means located between said ends for insulating the ends from each other when the interrupter is open and the ends are at different potentials;

b. a pair of contact rods respectively extending through said ends and each being movable with respect to its associated end in a direction longitudinal of said rod,

c. sealing means providing a seal between each of said rods and its associated end and permitting longitudinal movement of said rod with respect to said associated end,

d. a pair of contacts respectively fixed to the ends of said rods that are located within said envelope and having contact-making surfaces facing each other,

e. said rods being longitudinally movable toward each other to drive said contacts into a closed position where said contact-making surfaces engage each other and being longitudinally movable away from each other to effect opening of said switch,

, f.va generally annular shielding electrode surrounding each of said rods and electrically connected to the I rod surrounded thereby,

g. and means for mounting said shielding electrodes in fixed'positions within said envelope where said shielding electrodes are located longitudinallyspaced from each other on opposite sides of the region where said contact-making surfaces engage when the switch is closed, h. each of said shielding electrodes having a front surface facing the other shielding electrode,

i. each of said rods being movable during a switchopening operation into a withdrawn open position wherein the contact-making surface of its associated contact is located behind said front surface of its associated shielding electrode.

2. The switch of claim 1 in which:

a. an arc is established between said contacts when they are separated during an opening operation, and

b. when the switch is in closed.position, said shielding electrodes are spaced from said contact-making regions by a sufficiently large distance that arcing during opening is normally confined to said contacts and excluded from said shielding electrodes. v

3. The switch of c laim,l in which pre-strikes typically occur during a closing operation, said contact-making surfaces both being located well ahead of the front surfaces of said shielding electrodes at the time of said prestrikes, thereby normally confining thearcs resulting from such pre-strikes to said-contacts and excluding said arcs from said shielding electrodes.

4. Switching equipment comprising the switch of claim l, means for applying voltages between said ends that normally cause pre-strikes to occur during a clos- I ing operation, and means for locating the contactmaking surfaces of both of said contacts well ahead of said front surfaces of said shielding electrodes at the time of saidpre-stikes, thereby normally confining the arcs resulting from such pre-strikes to said contacts and excluding said arcs from said shielding electrodes.

5. Switching equipment comprising the switch of claim 1, means for applying voltages .between said ends that normally cause pre-strikes to occur during a closing operation, and means for locating the contactmaking surfaces of both of said contacts in a region of the gap between said shielding electrodes that is longitudinally spaced from both of said shielding electrodes at the time of said pre-strikes, thereby normally confining the arcs resulting from such pre-strikes to said contacts and, excluding said arcs from said shielding electrodes.

'6. The switch ofclaim 1 in which said contacts are of a metal that'is principally beryllium. i

7. The switch of claim 6 in which said shielding electrodes are of a ferrous material. I

. 8. Thevacuum switch of claim 1 in which said region where said contact-making surfaces engage is located substantially midway between the front faces of said shielding electrodes.

9. The vacuum switch of claim 3 in which said region where ,said contact-making surfaces engage islocated substantially midway ,between the front faces of said shielding electrodes.

Claims

1. A high-voltage vacuum switch comprising: a. a highly-evacuated envelope comprising spaced-apart conductive ends and electrical insulating means located between said ends for insulating the ends from each other when the interrupter is open and the ends are at different potentials; b. a pair of contact rods respectively extending through said ends and each being movable with respect to its associated end in a direction longitudinal of said rod, c. sealing means providing a seal between each of said rods and its associated end and permitting longitudinal movement of said rod with respect to said associated end, d. a pair of contacts respectively fixed to the ends of said rods that are located within said envelope and having contactmaking surfaces facing each other, e. said rods being longitudinally movable toward each other to drive said contacts into a closed position where said contactmaking surfaces engage each other and being longitudinally movable away from each other to effect opening of said switch, f. a generally annular shielding electrode surrounding each of said rods and electrically connected to the rod surrounded thereby, g. and means for mounting said shielding electrodes in fixed positions within said envelope where said shielding electrodes are located longitudinally-spaced from each other on opposite sides of the region where said contact-making surfaces engage when the switch is closed, h. each of said shielding electrodes having a front surface facing the other shielding electrode, I. each of said rods being movable during a switchopening operation into a withdrawn open position wherein the contactmaking surface of its associated contact is located behind said front surface of its associated shielding electrode.

2. The switch of claim 1 in which: a. an arc is established between said contacts when they are separated during an opening operation, and b. when the switch is in closed position, said shielding electrodes are spaced from said contact-making regions by a sufficiently large distance that arcing during opening is normally confined to said contacts and excluded from said shielding electrodes.

3. The switch of claim 1 in which pre-strikes typically occur during a closing operation, said contact-making surfaces both being located well ahead of the front surfaces of said shielding electrodes at the time of said pre-strikes, thereby normally confining the arcs resulting from such pre-strikes to said contacts and excluding said arcs from said shielding electrodes.

4. Switching equipment comprising the switch of claim 1, means for applying voltages between said ends that normally cause pre-strikes to occur during a closing operation, and means for locating the contact-making surfaces of both of said contacts well ahead of said front surfaces of said shielding electrodes at the time of said pre-stikes, thereby normally confining the arcs resulting from such pre-strikes to said contacts and excluding said arcs from said shielding electrodes.

5. Switching equipment comprising the switch of claim 1, means for applying voltages between said ends that normally cause pre-strikes to occur during a closing operation, and means for locating the contact-making surfaces of both of said contacts in a region of the gap between said shielding electrodes that is longitudinally spaced from both of said shielding electrodes at the time of said pre-strikes, thereby normally confining the arcs resulting from such pre-strikes to said contacts and excluding said arcs from said shielding electrodes.

6. The switch of claim 1 in which said contacts are of a metal that is principally beryllium.

7. The switch of claim 6 in which said shielding electrodes are of a ferrous material.

8. The vacuum switch of claim 1 in which said region where said contact-making surfaces engage is located substantially midway between the front faces of said shielding electrodes.

9. The vacuum switch of claim 3 in which said region where said contact-making surfaces engage is located substantially midway between the front faces of said shielding electrodes.