US3792214A

US3792214A - Vacuum interrupter for high voltage application

Info

Publication number: US3792214A
Application number: US00221665A
Authority: US
Inventors: R Voshall
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1972-01-28
Filing date: 1972-01-28
Publication date: 1974-02-12
Anticipated expiration: 1991-02-12
Also published as: CA956683A

Abstract

An improved shielding arrangement in a vacuum-type circuit interrupter for increasing the basic insulation level (BIL) and preventing the accumulation of a metallic coating on the internal insulating surfaces which can cause failure of the interrupter.

Description

United States Patent Voshall Feb. 12, 1974 [54] VACUUM INTERRUPTER FOR HIGH 3,185,799 5/1965 Greenwood et a1 200/144 B V G APPLICATION 3,185,800 5/ 1965 Titus 200/144 B [75] Inventor: Roy E. Voshall, New Alexandria, FOREIGN PATENTS OR APPLICATIONS 261,513 1/1970 U.S.S.R. 200/144 B [73] Assignee: Westinghouse Electric Corporation,

Pmsburgh Primary Examiner-Robert S. Macon [22] Filed; Jan. 28, 1972 Attorney, Agent, or Firm-A. T. Stratton et al.

[21] Appl. No.: 221,665

[57] ABSTRACT [52] US. Cl 200/144 B An improved shielding arrangement in a vacuum type [51] Int. Cl. H01h 33/66 circuit interrupter for increasing the basic insulation [58] Field of Search 200/144 B level (BIL) and preventing the accumulation of a [56] References Cited tallic coating on the internal insulating surfaces which can cause failure of the interrupter. UNITED STATES PATENTS 2,892,912 6/1959 Greenwood et a1 ZOO/144 B 5 Claims, 1 Drawing Figure 46 4 47 46 43 494143 r 44 it e (32 f 46 1,7 6 ,,7-4 \t76 76 72 l 76 64 62 76 7O 72 a Q 76 7s 76 76 |4 min as .8 53 24 22 52 r I u a l VACUUM INTERRUPIER FOR HIGH VOLTAGE APPLICATION BACKGROUND OF THE INVENTION current flows therebetween. During operation, current interruption is initiated by separating the electrodes. When the electrodes separate an arc is formed across the annular gap. The arc vaporizes a portion of the metallic electrode material and these particles become ionized to help sustain the arc, through which current flows until a natural current zero is reached. After the current zero point has been reached, the recovery voltagetransient begins building up between the separated electrodes. If the dielectric strength of the gap is sufficiently strong to withstand'the recovery voltage transient, breakdown will not occur, the arc will not reignite and circuit interruption will be complete. If the internal insulating surfaces of the interrupter are not protected, the metallic vapor formed during arcing will condense on the internal surfaces and form a metallic coating. After a number of interruptions, the metallic coating will form a shorting path and cause the interrupter to fail. To protect the insulating surfaces of the vacuum interrupter it is customary to provide metal shielding, located between the insulating surfaces and the arc formed during interruption. This type of construction is exemplified by U.S. Pat. No. 3,185,800. Most of the metallic vapor then condenses on the shielding surfaces before reaching the insulating surfaces of the vacuum interrupter. The shielding thus collects any particles, and condenses the metal vapor, given off from the electrodes during the arcing, protecting the insulating envelope or housing of the interrupter.

During operation, it is desirable that, any arc generated during interruption should be confined to the electrodes and not come into contact with the metal shield. Due to the confined space in the vacuum interrupter, there is a possibility that, the arc will strike the shielding. If the arc contacts the shielding, it is desirable that the are which does occur be confined to the inside of the shielding surface facing away from the insulating envelope. On low pressure arcs formed between interrupter electrodes, designed in such a way that asymmetries will occur in the self magnetic field produced by the current, it has been observed that the cathode spots move in the retrograde direction, while the remainder of the plasma appears to to obey Amperes Law. It has been found that electrodes designed, according to Amperes Law, to drive the arc in a prescribed direction are not always effective. Because low pressure arcs in self-magnetic fields exhibit the phenomena that cathode spots travel in the retrograde direction, geometric shaping of the shielding ends, such as with curls, to control the movement of the are are not effective. If cathode spots travel to the outside of the shield, and these surfaces face the insulating surfaces of the interrupter, metal vapor sputtered from the cathode spots will be deposited on the insulating envelope and eventual interrupter failure will result.

In a commercially successful vacuum interrupter a high basic-impulse insulating level (BIL) is necessary. A non-uniform voltage distribution around the external insulating envelope adversely affects the withstand voltage of the interrupter and lowers the BIL. In the prior art of vacuum interrupters, the voltage distribution around the external envelope is non-uniform. One of the causes of this non-uniform electric field is the lack of symmetry in the shielding arrangement. This objection to the prior art structures is eliminated by the invention provided herein.

SUMMARY OF THE INVENTION A vacuum interrupter comprising an insulating envelope generally tubular in shape, two metal end plates mounted on opposite ends of the insulating envelope, a stationary contact assembly, a movable contact assembly, with the contact movable along the longitudinal axis of the insulating envelope into and out of engaement with the stationary contact, and a multiple electrically isolated shielding arrangement. The insulating envelope is evacuated to a pressure of 10 Torr or lower. The multiple shielding arrangement comprises three shields wich are electrically isolated or floating from each other, from the electrodes and from ground. The disclosed design of multiple floating shields is used to improve the voltage distribution across the interrupter, when electrodes are separated and a BIL voltage transient is applied to the vacuum switch. The spacing between the shields and the number of shields is determined from a field map of the internal and external voltages and voltage gradients when a 200 KV potential is placed across the tube with electrodes separated. Since the geometry of the shields, electrodes and other metallic parts of the unit are quite complex a computerized field map program which calculates and prints out the voltage gradient as well as the voltage is employed. The uniform voltage distribution across the outside of the interrupter envelope is attributed to the proper selection of intershield capacitance.

In the instant invention, when the vacuum switch is in the open position, the electrodes are separated and an annular arcing gap is present therebetween. Surrounding this annular gap is the main floating shield which is barrel shaped or more specifically, shaped like two frustums of right circular hollow cones joined base to base. The longitudinal axis of the barrel shaped shield and the longitudinal axis of the insulating envelope coincide. The ends of the barrel shaped shield extend inside hollow cylindrical portions of two end shields. The two end shields, which are identical in construction, are shaped like a frustum of a right circular hollow cone with a hollow cylindrical, or tubular, por-.

tion joined to the base of the cone. The longitudinal axis of the'end shields and the longitudinal axis of the tubular insulating envelope coincides. The three shields are electrically isolated from each other and from the electrodes. Note that the floating shield arrangement is symmetrical, about a plane through the annular arcing gap and perpendicular to the longitudinal axis of the insulating envelope, for a uniform voltage distribution.

A vacuum interrupter constructed in accordance with the present invention can have a greatly improved BIL. For example, under tests the BIL of an experimental 34 KV vacuum interrupter was raised from 140 KV to l86 KV, by constructing the shielding arrangement in accordance with the teachings of the present invention, without changing the physical dimensions of the insulating envleope.

Another advantage of the present invention is that by having the hollow cylindrical portion of the end shields overlap the end of the main center shield there can be no cathode spots tracking on the outer surfaces of the floating shields which face the insulating envelope. The absence of cathode spot tracking on these surfaces has been verified experimentally. If the cathode spot tracking were present, as would be possible if the floating end shields were inside the end of the center shield, metal vapors sputtered from the surfaces of the end shields could be directly deposited on the insulating envelope.

BRIEF DESCRIPTION OF THE DRAWINGS Other objectives of the invention will be apparent from the following detailed description taken in conjunction with the accompanying drawing in which:

FIG. 1 is a partial view, in side elevation, of a vacuum interrupter embodying the principle features of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1 there is shown a vacuum type circuit interrupter 10. The vacuum interrupter comprises a tubular insulating envelope or housing 12, a stationary end cap 14, a bellows end cap 16, a stationary contact 18 and a movable contact 20. The stationary end cap 14 and the bellows end cap 16 are located at opposite ends of the tubular insulating envelope or housing 12.

The insulating envelope or housing 12 comprises four tubular

insulating portions

29, 30, 31 and 32 made from a suitable insulating material, such as glass, ceramic, or the like. Attached to one end of the tubular

insulating portions

29, 30, 31 and 32, so as to form a vacuum tight seal 44, are flanged metal tubes 46 and attached to the other end, so as to form a vacuum tight seal 43, are flanged metal tubes 47. These flanged metal tubes 46 amd 47 have a coefficient of thermal expansion which is approximately equal to the coefficient of thermal expansion of the

insulating tubes

29, 30, '31 and 32. The

flanged metal tubes

46 and 47 are constructed from a suitable material, such as Kovar, which is a trademark of the Westinghouse Electric Corporation, consisting of 25-34 percent nickel, 4-17 percent cobalt, less than 1 percent manganese, and the balance iron. For more detailed information on Kovar consult US. Pat. Nos. 2,217,422; 1,872,354; and 1,942,261. The bellows end cap 16 and the stationary end cap 14 are joined to two of these flanged metal tubes 46, which are attached to the insulating tubes 29 and 32, by suitable means, such as welding or brazing, to form a vacuum tight seal 48.

Disposed within the tubular insulating envelope 12 is the stationary contact 18 and the movable contact 20. The stationary contact 18 is attached to a stationary contact rod 22 by suitable means, such as brazing. The movable contact is attached to a movable contact rod 24 by a suitable means, such as brazing. FIG. 1 shows the movable contact 20 in the open position. When the movable contact 20 is in the closed position, it is in contact with the stationary contact 18 and a continous current path extends from the movable contact rod 24, through the movable contact 20, through the stationary contact 18, and through the stationary contact rod 22. The movable contact 20 is movable along the longitudinal axis of the insulating housing 12 so that when the vacuum switch 10 is in the open circuit position there is an annular arcing gap 50 between the

contacts

18 and 20. During circuit interruption, as the movable contact 20 moves from the closed to the open position, an arc is formed between the

contacts

18 and 20 across the annular space 50. On an alternating current circuit, this are is normally extinguished at the first current zero of the alternating current wave. Very quickly, after the arc is extinguished, the high di electric strength of the vacuum is recovered and reignition is prevented thus completing interruption.

The stationary contact rod 22 is joined to the stationary end cap 14 by a suitable means, such as brazing or welding. The end 52 of the stationary contact rod 22, which extends through the stationary end cap 14, may be threaded to facilitate attachment of a terminal. A bellows 26 is attached to the movable contact rod 24 and to the bellows end cap 16 so as to permit movement of the contact rod 24 along its longitudinal axis without affecting the vacuum inside the envelope 12. A bushing 56 of a material having a low coefficient of friction such as polytetrafluoroethylene, which is sold under the trade mark Teflon," is disposed in the be]- lows end cap 16 so that the movable contact rod 24 passes therethrough. This bushing 56 facilitates opening and closing of the movable contacts 20. Attached to the movable contact rod 24 and surrounding most of the bellows 26 is a cup-shaped metal shield 28. The bellows shield 28 protects the bellows 26 from bombardment by particles expelled during arc interruption. Attached to the bellows end cap 16 are two threaded

studs

58 and 60 which are used for mounting the vacuum switch 10. The end 53 of the movable contact rod 24 is threaded for attachment of an operating mechanism (not shown) and an end terminal connection (not shown).

During circuit interruption the movable contact 20 separates from the stationary contact 18 and an are develops across the annular arcing gap 50. The are that is formed vaporizes some of the material from the

contacts

18 and 20 and the resulting metallic vapors are ejected radially outward from the arcing gap 50, in a straight line.

Three electrically isolated or floating shields 34, 36 and 38 and two tubular end shields 40 and 42, which are attached to the end caps 14 and 16, respectively, are are provided to collect the metallic vapors that are produced by the evaporation of material from the

contacts

18 and 20, which results from arching during circuit interruption. These

metallic shields

34, 36, 38, 40 and 42 in conjunction with the metallic end caps 14 and 16 and the bellows shield 28 intercepts substantially all of the particles expelled from the arcing contacts, and quickly condense any metallic vapor generated, thus protecting the insulating housing 12 from these metallic vapors. The speed with which the vapor is removed effects operation of the vacuum interrupter 10. If the metallic vapor is not quickly removed high voltage transients may cause the arc to reignite between the

contacts

18 and 20 resulting in failure of the unit 10.

The main barrel shaped center shield 34 is disposed within the insulating envelope 12, around the annular arcing gap 50, and is electrically isolated from other shielding

members

36, 38, 40, and 42 from

electrodes

18 and 20, and from ground. The main shield 34 is formed byelectron beam welding or vacuum brazing together two frustums of right circular hollow cones 62 and 64 at their base. For support of the barrel shaped shield 34 an annular metallic disc 66 is attached to the shield 34 by suitable means, at the point where the frusturns of the right circular cones 62 and 64 are joined base to base. The annular disc 66 is then attached to the two flanged metal tubes 46 which are joined to the tubular portions 30 and 31 of the insulating envelope or housing 12 so as to form a vacuum tight seal 68. The main barrel shaped shield 34 is thus disposed within the insulating envelope 12 so that the longitudinal axis of the barrel shaped shield 34 and the longitudinal axis of the insulating envelope 12 coincide. The barrel shaped shield 34 is formed from a suitable material, such as copper, nickel or stainless steel.

The first stationary end shield 36 and the second stationay end shield 38 which are identical in construction are disposed within the insulating envelope 12. These end shields 36 and 38 are electrically isolated from each other, from all other shielding, from the

electrodes

18 and 20 and from ground. Both shields 36 and 38 comprise a tubular portion 70 which is joined to the base of a portion 72, which is shaped like the frustum of a right circular hollow cone, by a suitable means, such as electron beam welding. An annular metal disc 74 is joined to shields 36 and 38 at the point where portions 70 and 72 are joined together. The annular metal disc 74, which is attached to the second end shield 38, is then attached to the two flanged metal tubes 47, which are joined to

tubular portions

29 and 30, so as to suspend the second electrically floating end shield 38 within the insulating envelope 12. The annular metal disc 74, which is attached to the first end shield 36, is then attached to the two flanged metal tubes 47, which are joined to tubular portions 31 and 32, so as to suspend the first stationary end shield 36 within the insulating envelope l2. Vacuum tight seals 49 are formed where the annular metal discs 74 are joined to the flanged metal tubes 47. The end shields 36 and 38 are disposed in the insulating envelope 12 so that the longitudinal axis of the end shields 36 and 38 coincide with the longitudinal axis of the tubular insulating envelope 12. The end shields 36 and 38 are constructed of a suitable material such as copper, nickel, or stainless steel.

The third end shield 40, which has a flanged portion, is attached to the stationary end cap 14. The third end cap shield 40 is at the same electrical potential as the stationary contact 18. The fourth end shield 42, which has a flanged portion, is attached to the bellows end cap 16. The fourth end cap shield 42 is at the same electrical potential as the movable contact 20. The end shields 40 and 42 are formed from a suitable material such as copper, nickel or stainless steel. The floating shields 34, 36 and 38 are symmetrical about a plane, through the annular arcing gap 50 and perpendicular to the longitudinal axis of the insulating envelope 12, for a uniform voltage distribution. To reduce high electrical stress concentration at the end of the

shields

34, 36 and 38, 40 and 42, the free shield ends are curled into a generally tubular shaped ring 76. A substantially ring shaped tube 76 is thus formed at the free ends of the

shields

34, 36, 38, 40 and 42. By thus shaping the ends of the shields, high electrical stress at the shield ends are prevented. Note that joining of the metallic shielding parts is accomplished by electron beam welding or vacuum brazing, these methods were selected because with them there is less probability of introducing gas into the shield.

The design of the floating shields 34, 36, 38 are such as to improve the voltage distribution across the interrupter 10 when the movable contact 20 is separated from the stationary contact 18 and a BIL voltage transient is applied to the interrupter 10. Electric field plots show that the external and internal voltage distribution of the disclosed interrupter 10 is more uniform than the voltage distribution of prior art interrupters. The uniform electric field distribution in the present invention, which is due to the proper selection of intershield capacitance, permits higher BIL levels for the same physical size interrutper 10.

The ends of the barrel shaped center shield 34 project inside the circular portions of the floating end shields 36 and 38. By having the circular portions 70 of the end shields 36 and 38 overlap the ends of the barrel shaped shields 34 no arc trails can proceed to the outside surfaces of the shields which are exposed to the insulating housing 12. This means that no metal vapor can be sputtered from these surfaces and directly deposited on the inside walls of the insulating envelope 12. If the end shields 36 and 38 were disposed within the end of the barrel shield 34 and the shields 34, 36 and 38 became involved in arcing then cathode spots could burn over the outer surface and metal vapor sputtered from the cathode spots on these surfaces could be directly deposited on the insulating envelope 12. The present invention prevents this possibility of direct deposit of metallic vapor or particles on the inside walls of the envelope 12.

The apparatus embodying the teachings of this invention has several advantages. For example, due to the multiple floating shields geometry taught in the invention, voltage gradients between the shields 34, 36, 38, I

40 and 42 are reduced both during arcing and for BIL requirements. Another advantage of this multiple floating shield arrangement is that the arc trails cannot proceed to the outside surfaces of the

shields

34, 36, 38, 40 and 42, which are exposed to the insulating envelope 12, therefore the possibility of depositing metal vapor on the inside walls of the insulating envelope 12 or causing other damage to the insulation is minimized. Another advantage of the disclosed multiple floating shield arrangement is that the voltage distribution across the interrupter 10 is uniform and a higher BIL can be attained.

Since numerous changes may be made in the above described apparatus and different embodiments of the invention may be made without departing from the spirit and scope thereof, it is intended that all the matter contained in the foregoing description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

I claim:

1. A vacuum type circuit interrupter comprising an evacuated tubular insulating housing;

a stationary electric contact rigidly positioned with respect to the insulating housing, said stationary contact being radially centered in said housing;

a movable electric contact, movable along the longitudinal axis of said insulating housing between a closed position in contact with said stationary contact and an open position displaced from said stationary contact to establish an arcing gap therebetween;

a central circular hollow metallic shield, disposed internally of said tubular housing, surrounding the arcing gap and said stationary contact and said movable contact, said central circular hollow metallic shield is spaced radially outward from the outer periphery of the arcing gap and is electrically isolated from said stationary contact and from said movable contact;

a first end circular metallic shield disposed within the end of said tubular insulating housing and surrounding one end of said central circular hollow shield, said first end shield being electrically isolated from said stationary contact from said movable contact and from said central shield;

a second end metallic shield disposed within the end of said tubular insulating housing opposite the end within which said first end metallic shield is disposd and surroundng one end of said central circular shield, said second end metallic shield being electrically isolated from said stationary contact from said movable contact from said central circular shield, and from said first end metallic shield; and including, a third end shield which is electrically connected to said stationary contact and surrounds the ends of said first end shield which faces the end of said insulating housing in which the first stationary end shield is disposed; a fourth end shield which is electrically connected to said movable contact and surrounding the end of said second end shield which faces the end of said insulating housing in which the second end shield is disposed.

2. The combination as claimed in claim 1 wherein said centralcircular shield is shaped like two frustums of right circular hollow cones joined base to base, said central circular shield being spaced a greater distance from the arcing gap than said movable contact is sepa rated from said stationary contact when in the open position so that an arc formed during circuit interruption does not normally intereact with said central circular shield, and said central shield being positioned so that its longitudinal axis coincides with the longitudinal axis of the insulating housing and the center of gravity of said central shield is located within the annular arcing gap formed between said stationary contact and said movable contact, when said movable contact is in the open position.

3. The combination as claimed in claim 1 wherein said first end circular metallic shield and said second end metallic shield are identical in shape and construction.

4. The combination as claimed in claim 3 wherein said first end circular metallic shield and said second end metallic shield comprise frustums of a right circular hollow metallic conical portion which have joined to their bases tubular portions, said tubular portion of said first end shield surrounds one end of said circular central shield and the smaller diameter opening in the frustum of the right circular conical portion of said first stationary end shield faces the end of said insulating housing nearest said stationary contact, said tubular portion of said second end metallic shield surrounds another end of said circular central shield and the smaller diameter opening in the frustum of the right circular conical portion faces the end of said insulating housing nearest said movable contact.

5. The combination as claimed in claim 1 wherein the longitudinal axis of said insulating housing coincides with the longitudinal axis of said central circular shield, the longitudinal axis of said first end shield coincides with the longitudinal axis of said insulating housing, the longitudinal axis of said second end shield coincides with the longitudinal axis of the insulating housing, and said central circular hollow shield and said first end shield and said second end shield all being symmetrical about a plane through the arcing gap and perpendicular to the longitudinal axis of the insulating housing whereby a uniform voltage distribution over the outer surface of said insulating housing is present when a potential is applied to the vacuum interrupter and the movable contact is in the open circuit position.

Claims

1. A vacuum type circuit interrupter comprising an evacuated tubular insulating housing; a stationary electric contact rigidly positioned with respect to the insulating housing, said stationary contact being radially centered in said housing; a movable electric contact, movable along the longitudinal axis of said insulating housing between a closed position in contact with said stationary contact and an open position displaced from said stationary contact to establish an arcing gap therebetween; a central circular hollow metallic shield, disposed internally of said tubular housing, surrounding the arcing gap and said stationary contact and said movable contact, said central circular hollow metallic shield is spaced radially outward from the outer periphery of the arcing gap and is electrically isolated from said stationary contact and from said movable contact; a first end circular metallic shield disposed within the end of said tubular insulating housing and surrounding one end of said central circular hollow shield, said first end shield being electrically isolated from said stationary contact from said movable contact and from said central shield; a second end metallic shield disposed within the end of said tubular insulating housing opposite the end within which said first end metallic shield is disposed and surroundng one end of said central circular shield, said second end metallic shield being electrically isolated from said stationary contact from said movable contact from said central circular shield, and from said first end metallic shield; and including, a third end shield which is electrically connected to said stationary contact and surrounds the ends of said first end shield which faces the end of said insulating housing in which the first stationary end shield is disposed; a fourth end shield which is electrically connected to said movable contact and surrounding the end of said second end shield which faces the end of said insulating housing in which the second end shield is disposed.

2. The combination as claimed in claim 1 wherein said central circular shield is shaped like two frustums of right circular hollow cones joined base to base, said central circular shield being spaced a greater distance from the arcing gap than said movable contact is separated from said stationary contact when in the open position so that an arc formed during circuit interruption does not normally intereact with said central circular shield, and said central shield being positioned so that its longitudinal axis coincides wiTh the longitudinal axis of the insulating housing and the center of gravity of said central shield is located within the annular arcing gap formed between said stationary contact and said movable contact, when said movable contact is in the open position.