US3720867A

US3720867A - Fail safe vacuum type circuit interrupter and associated load current tap changer for electric induction apparatus

Info

Publication number: US3720867A
Application number: US00223631A
Authority: US
Inventors: W Rathbun
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1972-02-04
Filing date: 1972-02-04
Publication date: 1973-03-13
Anticipated expiration: 1990-03-13
Also published as: ES410993A1; FR2170229A1; BR7300836D0

Abstract

A load current tap changer for electric power transformers includes a vacuum type arcing-duty current interrupter immersed in dielectric fluid and biased to closed position by differential pressure. Overtravel limit switch means is actuated to shunt the vacuum arc gap upon excessive contact separation due to loss of vacuum and contact bias force. Limiting contact overtravel also disables the tap selector driving means.

Description

1111 3,720,867 1 1March 13, 1973 United States Patent Rathbun [54] FAIL SAFE VACUUM TYPE CIRCUIT 3,472,981 10/1969 McCarty et a1. ...............1..200 144 B 3,582,587 .200 144 B 3,626,123

6/1971 Barkan.............................

INTERRUPTER AND ASSOCIATED 12/1971 Pflanz et al. .............200/144 B LOAD CURRENT TAP CHANGER FOR ELECTRIC INDUCTION APPARATUS Primary Examiner-Gerald Goldberg Attorney-Francis X. Doyle et a1.

[75] Inventor: William H. Rathbun, Pittsfield,

Mass.

General Electric Company Feb. 4, 1972 Filed: A load current tap changer for electric power trans- [211 App! NOJ 223,631 formers includes a vacuum type arcing-duty current interrupter immersed in dielectric fluid and biased to closed position by differential pressure. Overtravel limit switch means is actuated to shunt the vacuum arc gap upon excessive contact separation due to loss of vacuum and contact bias force. Limiting contact overtravel also disables the tap selector driving means.

50 MB 39 2 3 1 R 8H1. 4 4 H 0 4 m p/ 9 0 RMO 5H 3 m" 4 n 3 n 2 n 3 "m m h C r 8 Hm 1 m l WM 1 2 I ll UhF 11] 2 8 555 [.11.

References Cited 12 Claims, 3 Drawing Figures 3,404,247 Glassanos .........................2o0/144 B PAIENTED MAR 1 3 I975 SHEET 1 BF 2 FAIL SAFE VACUUM TYPE CKlRCUllT HNTIERRUPTER AND ASSOCllATlElD ILQAD CURRENT TAP CHANGER FOR ELECTRIC HNDUCTllON APPARATUS My invention relates to protective means for vacuum type circuit interrupting devices and for load current tap changing apparatus including such devices, and

' more particularly to fail-safe means for preventing damage to such apparatus and devices in the event of significant loss of vacuum in an interrupting device immersed in a body of liquid. Published prior art presently known to applicant and concerning apparatus of this general type is illustrated in the following U.S. Pat. Nos.: 3,265,840 to Luehring; 3,404,247 to Glassanos; and 3,472,981 to McCarty et al.

Vacuum type electric current interrupting devices of high interrupting capacity are useful in electric switches and circuit breakers of all types, and are presently available in voltage and current ratings capable of interrupting power flow at voltages and frequencies commonly used in commercial and industrial distribution and transmission systems. in most applications the vacuum interrupting device is likely to be located in an ambient atmosphere of air, so that even a large leak in the evacuated envelope can do no more than fill the envelope with air or gas at ambient pressure. Under such conditions the device may fail to interrupt current, but the contained gas or air is not likely to build up sufficient pressure to fracture the envelope. in certain special applications it is desirable to immerse a vacuum interrupting device in a surrounding ambient liquid. In such locations it is possible that a leak of appreciable size and time duration will admit to the switch envelope a pool of liquid sufficient, if heated excessively by an arc, to develop a bursting pressure.

One special application where it is desirable to immerse a vacuum interrupter in a surrounding ambient liquid is in load tap changing apparatus for power transformers or other electric induction apparatus. Such transformers are usually immersed in a dielectric liquid such as oil or askeril, and the tap changing equipment is also liquid immersed. However, the tap changer ordinarily includes an arc current interrupting device which has been located in a separate liquid filled compartment to avoid contamination of the main body of transformer oil as a result of arching at the switch. To accommodate the mechanical interconnection of the arching switch with the transfer contacts and other switches in a tap changing apparatus, it has been customary to locate the entire tap changing apparatus in the separate liquid filled compartment. This requires all the tap leads from the transformer winding to be brought out from the main body of the transformer to the tap changing compartment. in high voltage apparatus this is an expensive procedure in both labor and material costs.

It would be desirable, if possible, to locate all the tap changing apparatus for high voltage power transformers, reactors and the like directly within the main enclosing tank and immersed in the main body of trans former oil or other dielectric liquid. This economical arrangement does become possible when the are interrupting switch is of the vacuum type. With the entire tap changing apparatus so located, the tap leads need not be brought out of the tank, and no separate tap changing compartment need be provided. Even if a separate compartment is otherwise desirable, a vacuum interrupting device is advantageous in a load tap changer in order to avoid contamination of the oil in that compartment and the consequent frequent maintenance. On the other hand a vacuum interrupting device so located in the dielectric liquid of an associated electrical apparatus should not be permitted to develop leaks sufficiently large to admit liquid to the switch envelope. Even in the absence of further damage, a fractured interrupter would so contaminate the dielectric liquid that expensive and time consuming maintenance work would be required.

Accordingly it is a principal object of my invention toprovide improved leak responsive protective means for vacuum type electric circuit interrupters and associated load current tap changing apparatus.

it is a more particular object of my invention to provide means for preventing long continuance of arcing in a vacuum interrupting device subjected to abnormal increase in internal pressure.

it is still a further object of my invention to provide a vacuum type electric circuit interrupter provided internally with means responsive to abnormal increase of internal pressure to short circuit the contacts and terminate arcing therebetween.

in carrying out my invention in one preferred embodiment, l utilize tap changing apparatus of the type including two movable tap selector switches connected in parallel circuit relation to a common line terminal and adapted to be moved in sequential step by step manner from one tap position to an adjacent tap position. The parallel branch circuit connections of the selector switches preferably include current limiting reactors and are adapted to be interrupted sequentially prior-to stepping movement of each associated tap selector switch. For such interruption l utilize a vacuum type circuit interrupter, either one in each branch circuit or a single interrupter connected to'be switched into the branch circuits sequentially during the interrupting interval of each circuit.

A vacuum interrupting device of the evacuated envelope type is usually a normally closed contact device by reason of the fact that the movable contact rod is sealed to the enclosing envelope by means of a flexible or deformable wall portion of the envelope (such as a metallic bellows), and external pressure on such wall portion biases the movable contact to engaging position. Thus actuating force applied to the movable contact rod to move it to disengaging position must act against the closing bias of the bellows or other flexible seal. If the actuating mechanism for the movable contact permits overtravel of the movable contact beyond its normal open circuit position abnormal overtravel will result from some predetermined diminution or reversal of differential pressure on opposite sides of the flexible wall seal.

In such a vacuum type load current tap changing apparatus l utilize overtravel, or abnormal opening movement, of the movable vacuum interrupter contact resulting from abnormally high internal pressure in the evacuated envelope to effect contacting engagement of a shunting device either within or outside the envelope, thereby to extinguish arcing between the contacts and prevent further build up of pressure. The abnormal pressure effective to actuate the shunting device is preferably less than a pressure which would rupture the switch envelope. In a preferred embodiment of my invention l utilize a metallic vapor shield within the vacuum interrupter envelope as the contact shunting device by so mounting the shield that when engaged by the movable contact in abnormal overtravel it completes a shunt circuit between the contacts. Such a shield may be normally connected electrically to one of the contacts or insulatingly mounted in electrostatically floating relationship.

In order to prevent further tap changing movement of the selector contacts with the vacuum interrupting device shunted and inoperable, and thus to eliminate arcing at the tap selector contacts, I also provide means actuated by abnormal overtravel of the vacuum switch contact to block the tap changing mechanism or disable the motor drive for the mechanism, or both.

My invention will be more fully understood and its several objects and advantages further appreciated'by referring now to the following detailed specifications taken in conjunction with the accompanying drawing wherein:

FIG. 1 is a schematic illustration of a transformer load tap changing apparatus including a vacuum type circuit interrupting device and embodying my invention; and

FIGS. 2 and 3 are fragmentary cross-sectional views of vacuum interrupting devices illustrating other embodiments of contact shunting devices which may be utilized in the apparatus of FIG. 1.

Referring now to the drawing, l have illustrated at FIG. 1 a high voltage autotransformer of the liquid immersed type including a tank or enclosure substantially filled with insulating oil or other dielectric fluid 11 and having positioned therein a winding 12. The transformer winding 12 is grounded at its low voltage end, as by connection to the tank 10,'and at its other end it is connected to a high voltage line terminal conductor 13 which passes out of the tank through an insulating bushing 14. The high voltage end of the autotransformer winding is provided with a plurality of selectable voltage taps each having a terminal contact 15. A selected one of the tap terminals is adapted to be connected through a load tap changing apparatus 16 to a secondary line terminal conductor 17 disposed in an insulating bushing 18. The load tap changing apparatus designated generally by the reference number 16 comprises a pair of movable tap selector contacts or fingers 20, 21, associated pairs of current limiting reactors 22, 23 and transfer switches 24, 25, a vacuum interrupting switch 26 and a motor driven cycle timer (designated generally as 28) for controlling the operation of the foregoing tap selector contacts and switches.

The tap selector circuit of the apparatus 16 is of a well known type. This circuit provides two parallel current paths, or branch circuits, between any selected tap terminal and the common secondary line terminal 17, each branch circuit includes one of the tap selector contacts 20, 21 in series with one of the reactors 22, 23 and one of the transfer switches 24, 25. Each transfer switch 24, 25 has a line contact connected to the terminal 17 and a load contact connected to one of the tap selector contacts 20, 21. The arcing duty vacuum interrupting device 26 is connected between the parallel branch circuits on the load sides of the transfer switches 24, 25 so that either transfer switch when opened is shunted by the vacuum interrupter and the other transfer switch.

The cycle timer 28 is selectably operable in either direction to effect sequential stepping movement of the tap selector fingers 20, 21 progressively in either direction along the group of transformer tap terminals 15. The cycle timer is driven selectively in either direction by a motor 30 through a reversing switch 31 from a suitable source of control voltage shown as a tertiary winding 32 on the transformer 12. When energized the timer starts from a normal rest position and operates for a single complete cycle to effect a single stepping movement of both tap selector contacts 20 and 21. The motor energizing circuit includes a normally open starting switch 33 which for the purpose of illustration may be considered to be manually operable. The cycle timer includes a motor holding and limit switch 35 connected in parallel circuit relation across the starting switch 33 and arranged to deenergize the motor at the end of a single cycle in either direction of operation.

The cycle timer 28 comprises a pair of earns 40, 41 arranged to open the transfer switches 24 and 25 sequentially in either direction of operation, and a cam 43 arranged to open the arcing contact switch 26 at two spaced apart intervals during each cycle. A pair of Geneva gears 44, 45 driven by the cycle timer is connected to actuate the tap selector contacts 21 and 20 in sequential relation at appropriate times during a cycle of operation in either direction.

A typical single cycle of operation of the tap changing apparatus described above takes place as follows: If the starting switch 33 is closed momentarily to energize the motor 30, the cycle timer begins to rotate in a selected direction (assume clockwise) as determined by the reversing switch 31. Immediately upon initiation of operation, the holding and limit switch 35 closes to maintain motor energization for a full cycle. During this cycle of operation the cam 40 first opens the transfer switch 24, thereby to substitute the contacts of the arcing contact switch 26 for the switch 24 in the branch circuit through the tap selector contact 20 and reactor 22. Following opening of the transfer switch 24, the contacts of the arcing vacuum switch 26 are opened by the cam 43, thereby to interrupt current through the tap selector contact 20. Thereafter the Geneva gear 45 moves the tap selector contact 20 to the next adjacent tap terminal 15 in the selected direction. Following this operation the cam 43 again closes the contacts of the arcing contact vacuum switch 26, and cam 40 closes transfer switch 24; thereafter the cam 41 opens the transfer switch 25. After opening of the switch 25 the cam 43 again opens the vacuum switch 26 thereby to interrupt current through the tap selector contact 21 and the reactor 23. While the tap contact 21 is deenergized, the Geneva gear 44 moves that tap contact to the next adjacent tap terminal 15 to which the tap contact 20 was previously moved. Thereafter the arcing contact vacuum switch 26 is again closed by the cam 43, and the transfer switch 25 is reclosed by the cam 41. This completes a single cycle of operation and the limit switch 35 thereupon deenergizes the motor 30. It will be evident to those skilled in the art that operation in the reverse direction is entirely similar.

Reference will now be had more particularly to the arcing contact vacuum interrupter 26 connected between the parallel tap selector branch circuitsv This circuit interrupter comprises an evacuated envelope 50, illustrated as a cylinder of insulating material such as glass having metallic end closure caps 50a and 50b and having mounted therein a pair of

separable contacts

51, 52. The

vacuum switch contacts

51, 52 are mounted, respectively, on the juxtaposed ends of axially aligned fixed and movable contact rods Sin and .5211. Contact rod 51a is fixed to the metal end cap 50a and the contact rod 52a is slidably mounted in a bearing 56 which is mounted upon the end cap 50b. The slidable contact rod 52a extends through the end cap 50b and is sealed to the end cap by means of a flexible metallic bellows 57. The bellows 57 is connected at one end to the end wallStlb of envelope 50 and at the other end to the contact rod 52a and thus constitutes a flexible portion of the envelope wall.

The bearing 56 does not provide a hermetic seal so that the interior of the bellows 57 is exposed to ambient pressure outside the switch envelope 50. In the example illustrated this is the pressure of the surrounding liquid body Ill and is ordinarily approximately atmospheric pressure. Since this external pressure is greater than the very low pressure within the highly evacuated envelope 50 it exerts upon the flexible bellows 57 a force tending to expand the bellows and move the switch contact 52 to its engaged or circuit closing position as shown in the drawing. Thus to separate the

contacts

51, 52 and to open the vacuum switch it is necessary to forceably pull the contact rod 52a outward (or downward as shown in the drawing) and to this end the opening cam 43 on the cycle timer 28 is illustrated as applying a downward force upon its cam follower. In the drawing I have illustrated by dotted lines adjacent the movable contact 52 the closed, or engaged, position (C) of contact 52, the normal open or disengaged position of the contact 52, and an overtravel or abnormally displaced open position (OT) of the contact 52. Abnormal displacement to position OT will be more fully described hereinafter.

Mounted coaxially within the vacuum interrupter envelope 50 l have shown a cylindrical metallic vapor shield 60 with the contacts 511, 52 and contact rods 51a, 52a in the central arcing region of the envelope. The shield 60 is mounted upon the insulating envelope t) and normally not electrically connected to either of the

contacts

51, 52 so that it is in electrostatically floating relation with respect to the contact potential. A shield mounting member 61 between the shield 60 and the envelope 50 is designed to be frangible or otherwise deformable under forces to be described hereinafter, but normally holds the shield 60 in fixed position out of engagement with the vacuum switch contacts and rods in both engaged and normal disengaged positions of the contacts.

In the embodiment of my invention herein described by way of illustration the electrically conductive me'tallic vapor shield 60 and the frangible, or otherwise deformable, shield mounting till is utilized to provide a conductive shunt connection across the arc gap and between the

contacts

51, 52 in the event the movable contact 52 is actuated to its abnormally displaced overtravel position OT. For this purpose the shield is provided at each end with in-turned end collars approaching but not engaging the aligned contact rods Sla, 52a and positioned for engagement with conductive interference detents or projections 51b, 52b, respectively, on the contact rods.

It will now be evident to those skilled in the art that in the normal cyclic operation of tap changing apparatus described above the

vacuum interrupter contacts

51 and 52 open and close recurrently without engagement of any contact parts with the enclosing vapor shield 60. In the event, however, that an abnormal increase in pressure occurs within the vacuum switch envelope 50, as for example because of a leak in one or more of the sealed joints constituting the envelope, the closing bias exerted by the bellows 57 will be partially or fully overcome depending upon the extent of internal pressure increase. Of course even without any leak, and in the absence of foreign material within the evacuated envelope, internal pressure increases in normal circuit opening operation as the result of arcing between the contacts and the presence of metal vapor. Such normal increase in pressure is not sufficient, to drive the movable contact 52 beyond the normal con tact opening position 0. At some predetermined high pressure, however, contact 52 will be driven to its overtravel position OT. Preferably the abnormally high pressure effective to produce such overtravelis less than that which would rupture the envelope 50.

In the event of leakage in the vacuum envelope 59, foreign material such as gas or liquid within the envelope will result in an increase in internal pressure which, without arcing, may or may not be sufficient to reverse the closing bias on the bellows 57. If, however, such increased internal pressure is of sufficient magnitude that further increase due to contact arcing does reverse the normal closing bias the movable contact rod 52a will be driven in the opening direction beyond its normal open position 0 and toward the abnormally displaced overtravel position OT. In moving to this position the contact rod projection 52b engages the lower end collar of the conductive shield 60, and if the opening pressure is sufficient the shield mounting 61 will be broken or otherwise deformed and the shield 60 moved downwardly into engagement with the contact rod projection 51b. Thus by excessive opening movement of the contact rod 52b the metallic shield 60 is brought into shunting relation with the arc gap and operates as a limit switch to bridge the separated

contacts

51, 52 and terminate arcing within the envelope 50. Bythis action the vacuum switch 26 is protected against further increase in internal pressure and against possible rupturing forces.

In the event of abnormal overpressure within the vacuum switch 26 and consequent failure of the switch to interrupt current it is desirable to protect the tap changing apparatus by disabling the cycle timer 28 and the stepping selector contacts 20, 21. For this purpose I have shown a blocking gear on the cycle timer shaft which is adapted to be engaged and held in fixed position by a detent 7ll actuated by the movable contact rod 52a. It will be understood that the detent 71 engages the blocking gear 70 only when the contact rod 52a is in its abnormally displaced overtravel position. On the drawing at FIG. 1 I have indicated by dotted lines the positions of detent 71 corresponding to the closed (C) open (0) and overtravel (OT) positions of the movable contact rod 52a.

. tively shunt said are gap.

As an alternate, or additional, means for disabling the cycle timer 28 l have shown also a clutch 75 between the driving motor 30 and the cycle timer shaft. By a schematic coupling connection 76 I have illustrated the clutch 75 as being actuated to open position only when the movable contact rod 52a is in its abnormal overtravel position.

At FIG. 2 I have illustrated another embodiment of overtravel shunting device operable by a vacuum interruption contact and operable in the apparatus shown at FIG. 1. In the vacuum interrupter shown at FIG. 2 the conductive vapor shield 60 is physically mounted upon and electrically connected at its upper end to the fixed contact rod 51a. An in-turned end collar at the lower end of the shield is positioned to be engaged by the movable contact 52 only in its overtravel position, as indicated on the drawing.

At FIG. 3 l have shown still another form of vacuum interrupter in which a shunting switch external to the evacuated envelope 50 comprises a fixed contact 80 connected electrically to the fixed vacuum contact 51 and a movable contact 81 mounted on the movable contact rod 520. As indicated the

limit switch contacts

80, 81 are positioned to engage only in the overtravel position OT of the contact rod 52a.

While I have shown and described a particular embodiment of my invention, 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; and I, therefore, intend herein to coverall such changesand 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:

l. A load current tap changer for electrical apparatus having a plurality of tap terminals and a line terminal comprising, a pair of movable tap selector contacts mounted for sequential stepping movement in either direction between adjacent tap terminals, a pair of parallel branch circuits each connecting one of said tap selector contacts to said line terminal, each said branch circuit including a transfer switch having separable line and load contacts, a vacuum type circuit interrupting device connected between the load contacts of said transfer switches and including separable arcing contacts within an evacuated envelope, said arcing contacts being biased into engagement by differential pressure within and without said envelope, means actuating said transfer switches alternately to open position thereby to transfer said arcing contacts into one said branch circuit, means actuating said arcing contacts to normal disengaged positions separated by an arc gap thereby to interrupt current through said one branch circuit prior to stepping movement of the associated tap selector contact, said actuating means and at least one said arcing contact being free to overtravel beyond its normal disengaged positionin response to abnormal increase of pressure within said envelope thereby to in-- crease the length of said arc gap, and limiting contact means engageable upon predetermined overtravel and abnormal separation of said arcing contacts to conduc- 2. A tap changer according to claim 1 wherein said limiting contact means is mounted within said envelope and actuated to engaged position in response to predetermined abnormal overtravel of at least one of said arcing contacts.

3. A tap changer according to claim 1 wherein said limiting contact means comprises a cylindrical metal shield mounted within said envelope and enclosing said arc gap in coaxial radial spaced relation and a contact means actuated by at least one of said arcing contacts to engage said shield in response to abnormal overtravel of said one arcing contact.

4. A tap changer according to claim 3 wherein one said arcing contact is fixed and the other movable, normally fixed deformable mounting means electrostatically isolated from said arcing contacts positions said shield normally beyond engagement with either of said arcing contacts and said movable arcing contact is positioned to engage said shield upon abnormal overtravel contact movement and move said shield into engagement with said fixed arcing contact.

5. A tap changer according to claim 3 wherein one said arcing contact is fixed and the other movable, said shield is mounted upon and electrically connected to the fixed arcing contact, and conductive means on the movable arcing contact engages said shield upon predetermined abnormal contact opening movement.

6. A tap changer according to claim 1 wherein said arcing contacts comprise a pair of axially aligned conductive rods one of which is fixed to said envelope and the other is movably mounted upon a flexible wall portion of said envelope, and said limiting contact means comprises a fixed contact and a movable contact actuated by said movable contact rod.

7. A tap changer according to claim 6 wherein said limiting contact means is positioned outside said evacuated envelope.

8. A tap changer according to claim 1 additionally including driving means for actuating said tap selector contacts and means operable upon abnormal separation of said arcing contacts to disable said driving means.

9. A tap changer according to claim 8 wherein said driving means includes alternately and intermittently rotatable gearing connected to actuate said tap selector contacts and said disabling means comprises blocking means engageable positively to lock said gearing against movement.

10. A tap changer according to claim 8 wherein said driving means includes a rotatable electric motor and said disabling means comprises a clutch for disconnecting said motor from said tap selector contacts.

1 l. The combination with a tap changer according to claim 1 of an electric induction apparatus including said tap terminals and said line terminal, a common housing for said induction apparatus and said vacuum interrupting device, and a body of dielectric fluid in said housing at least partially immersing said induction apparatus and said vacuum interrupting device.

12. In combination, an electric power transformer having a plurality of tap terminals and a line terminal, an enclosing housing for said transformer, a body of dielectric liquid in said housing substantially immersing said transformer, and the tap changer of claim 1 connected between said line terminal and said tap terminals with said vacuum interrupting device immersed in said dielectric liquid.

Claims

1. A load current tap changer for electrical apparatus having a plurality of tap terminals and a line terminal comprising, a pair of movable tap selector contacts mounted for sequential stepping movement in either direction between adjacent tap terminals, a pair of parallel branch circuits each connecting one of said tap selector contacts to said line terminal, each said branch circuit including a transfer switch having separable line and load contacts, a vacuum type circuit interrupting device connected between the load contacts of said transfer switches and including separable arcing contacts within an evacuated envelope, said arcing contacts being biased into engagement by differential pressure within and without said envelope, means actuating said transfer switches alternately to open position thereby to transfer said arcing contacts into one said branch circuit, means actuating said arcing contacts to normal disengaged positions separated by an arc gap thereby to interrupt current through said one branch circuit prior to stepping movement of the associated tap selector contact, said actuating means and at least one said arcing contact being free to overtravel beyond its normal disengaged position in response to abnormal increase of pressure within said envelope thereby to increase the length of said arc gap, and limiting contact means engageable upon predetermined overtravel and abnormal separation of said arcing contacts to conductively shunt said arc gap.

2. A tap changer according to claim 1 wherein said limiting contact means is mounted within said envelope and actuated to engaged position in response to predetermined abnormal overtravel of at least one of said arcing contacts.

11. The combination with a tap changer according to claim 1 of an electric induction apparatus including said tap terminals and said line terminal, a common housing for said induction apparatus and said vacuum interrupting device, and a body of dielectric fluid in said housing at least partially immersing said induction apparatus and said vacuum interrupting device.