US3571773A

US3571773A - Center clamped bushing transformer

Info

Publication number: US3571773A
Application number: US873937A
Authority: US
Inventors: Donald J Johnston
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1969-11-04
Filing date: 1969-11-04
Publication date: 1971-03-23
Anticipated expiration: 1988-03-23

Abstract

A bushing current transformer is formed by mounting a pair of terminal bushings upon a tank with their axial conducting rods in laterally spaced side-by-side relation and extending into the tank, the proximate internal ends of the rods being bridged by a conductor to form a primary loop. A compression member between the tank wall and the bridging conductor applies a tensile stress to both bushing rods thereby to clamp each bushing assembly in position without the use of insulating sleeves internally of the tank.

Description

United States Patent Donald J. Johnston Pittslield, Mass.

Nov. 4, 1969 Mar. 23, 1971 General Electric Company Inventor Appl. No. Filed Patented Assignee CENTER CLAMPED BUSHING TRANSFORMER 5 Claims, 1 Drawing Fig.

U.S. Cl 336/92,

Int. Cl ..H0lf 27/02, H0lf27/28 Field of Search l74/(CT), 18, 31; 336/90, 92, 173, 174, 175; 200/150 (Cursory) [56] References Cited UNITED STATES PATENTS 1,899,658 2/1933 Ainsworth l74/(C.T)UX 2,849,694 8/1958 Prince 336/174 3,071,672 l/l963 Mitchell l74/(C.T)UX

Primary Examiner-Thomas J. Kozrna Att0rneysJ. Wesley Haubner, F rank L. Neuhauser, Oscar B.

Waddell and Joseph B. Forman .ABSTRACT: A bushing current transformer is formed by mounting a pair of terminal bushings upon a tank with their axial conducting rods in laterally spaced side-by-side relation and extending into the tank, the proximate internal ends of the rods being bridged by a conductor to form a primary loop. A compression member between the tank wall and the bridging conductor applies a tensile stress to both bushing rods thereby to clamp each bushing assembly in position without the use of insulating sleeves internally of the tank.

PAIENTEU m2 319m LEA tall CLAMPED BUSEHNG TRANSFORMER BACKGROUND AND OBJECTS OF THE INVENTION This invention relates to bushing transformers in generaland in particular to a high voltage bushing current transformer having a plurality of secondary coils.

Current transformers are commonly constructed utilizing a single insulating bushing housing the primary and secondary coils. in such transformers the primary conductor is ordinarily of eyebolt configuration and the secondary coils encircle and pass through the circular loop or eye of the primary conductor. One drawback of such a construction is that the number of secondary coils which can be effectively utilized is limited due to the limited available space within the single bushing and the eye of the primary conductor.

Accordingly, in the manufacture of current transformers comprising a plurality of secondaries the approach that has been taken is to mount a pair of bushings upon a tank in laterally spaced relation with one end of each bushing extending into the tank, and electrically bridging the internal ends of the bushing conductors thereby forming a single primary loop. With such construction numerous secondary coils can be disposed about the primary conductor due to the ample space provided within the tank.

Owing to the high voltages generally impressed on the primary of a current transformer, the bushings commonly utilized to form the primary loop are of oil filled type. Such bushings ordinarily comprise an axial central conductor or rod having threaded end portions, an insulating core surrounding the central conductor and at least two tubular porcelain shells in end-to-end relation enclosing the core and rod on opposite sides of an axially intermediate ground sleeve, an annular oil space being provided between the core and shells. An oil expansion chamber is commonly provided at one end of the bushing. A mounting flange is provided on the ground sleeve for attaching the bushing to a tank. Sealing gaskets are provided interposed between the porcelain shells and the ground sleeve.

In order to form a unitary insulating structure the porcelain shells, gaskets and mounting ring are held together by what is known in the art as center clamping'One manner of providing center clamping is through the use of two clamping members or nuts adjustably mounted upon opposite ends of the axial conducting rod thereby to hold the porcelain-shells, gaskets and ground sleeve in clamped relation therebetween. Upon the tightening of the clamping nuts the axial or central conductors are placed in tension and the porcelain shells, gaskets and ground sleeve are placed in compression.

Another manner of axially clamping the bushing parts in assembled relation is to utilize a porcelain shell having a large inside diameter. This shell enclosed not only the central conductor and core but also a number of longitudinal insulating tension rods disposed parallel to the axial conductor and attached to a clamping member at each end of the bushing, so that the porcelain shells and gaskets therebetween are placed in compression upon tensioning of the insulating rods.

Either of the aforementioned manners of applying the axial clamping force leaves something to be desired in applications where electrical considerations do not require an insulating outer sleeve at both sides of the bushing ground ring. In the former manner, duplicate porcelain shells are necessary for mechanical reasons, whereas in the later manner insulating tension rods are required. Further, both of these prior art arrangements apply the axial or center clamping force to the bushings individually so that each bushing is a complete selfsustaining unit.

it is a principal object of this invention to provide, in a bushing current transformer comprising a pair of bushings mounted upon and extending into a common tank of dielectric fluid, clamping means for the bushing assemblies operable in the absence of rigid internal insulating sleeves.

It is a further object of this invention to provide bushing type current transformers of reduced cost and simplified structure utilizing a common clamping means for all bushing parts.

SUMMARY OF THE INVENTION In carrying out the invention in one form a pair of oil-filled bushings are attached by their mounting rings to a wall portion of a tank containing a dielectric fluid, such as oil. The bushings are mounted transversely to the tank wall and in laterally spaced side-by-side relation so that one end of each axial bushing conductor or rod extends into the tank. An electrically conductive member inside the tank bridges the proximate end portions of the axial bushing conductors thereby to form a single loop primary. Disposed about the primary conductors within the tank are a plurality of secondary coils. A compression member or column of insulating material is compressed between the tank top and the bridging conductor whereby the force exerted by the compression member causes both axial bushing conductors to be jointly placed in tension. A clamping member is connected to the upper end of each axial bushing conductor with the porcelain shell and gaskets being embraced between the clamping member and the mounting ring so that the application of a tensile force on the axial bushing conductors places the porcelain shells and gaskets in compression.

BRIEF DESCRIPTION OF THE DRAWINGS This invention will be better understood and its various objects and advantages will be m ore fully appreciated from the following description taken in conjunction with the accompanying drawing in which:

FIG. 1 is a side elevational view, partly in section, ofa bushing current transformer embodying my invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in FIG. 1, an identical pair of high voltage terminal bushings 1 and 19 are attached by their mounting rings 2 to the top wall, or cover of a current transformer tank 3. The bushings l and 19 are each formed to provide an internal annular space for a dielectric fluid such as insulating oil, and are so mounted that these spaces communicate with the interior of the tank 3. The tank and bushings are filled to a level near the top of the bushings with a common body of oil 3a. The bushings are disposed in a laterally spaced-apart or side-byside relation to each other. While the elongate bushings 1 and 19 are shown in parallel spaced-apart relation and normal to the tank cover it is within the scope of this invention to dispose the bushings with their axes in any desired angularly displaced relation which is appreciably removed from a position of axial alignment. The reason for such a limitation will become apparent from a later discussion of the manner of providing central clamping.

Since the bushings are identical only one of the bushings will be described. Bushing l, for example, consists of a porcelain shell 4 surrounding an insulating core 5 which may be oil impregnated paper. The outer shell can be of a unitary construction, as shown, or can be made up of a number of stacked shells with gaskets therebetween. The porcelain shell 4 is stacked on top a metallic mounting ring 2. interposed between the porcelain shell and the mounting ring 2 is a sealing gasket 6. A transparent glass cylinder I1 is mounted as an upper extension of the insulating shell 4 and the bushing is closed at the top by a cap 10. Disposed inside the shell 4 and the core 5 and running the entire length of the bushingis an axial bushing conductor 7, each end of which is threaded. At the upper end of each bushing is a clamping member or stop nut 8 which is screwed to conductor rod 7 to apply clamping pressure to the cap llll in a manner to be more fully described hereinafter. As is well known to those skilled in the art, a spring assembly (not shown) may be provided within the cap 10 to maintain pressure on the clamped assembly.

Between the insulating core 5 and the insulating shell 4, 11, an annular space 9 is provided, and the space 9 is filled with the oil 38a to a level within the glass extension ill. The space 9 above the oil level provides an expansion chamber into which the oil may rise when it becomes heated during operation of the transformer. The glass shell 11 allows visual inspection of the oil level in the bushing assembly.

The lower end portions of the inner core and the axial bushing conductor 7 pass through an opening 12 and into the tank El. Surrounding this portion of the core 5 and axial bushing conductor 7 a number of transformer secondary coils 13 are provided. Although only two coils l3 are shown surrounding each core 5 and axial bushing conductor 7, it is of course contemplated that if desired a greater number of coils may be utilized. The coils l3 are supported by cup-shaped saddle 14 which is preferably formed of insulating material and suspended from the interior upper wall of tank 3. it should be noted that although the secondary coils 13 are shown disposed inside the tank, it is contemplated that if desired the coils 13 may surround the bushing conductors outside of the tank.

Electrically connecting the proximate lower ends 15 of the axial bushing conductors 7 inside the tank 3 is a bridging conductor lid which is attached to each rod 7 by a stop nut 17. The serial electrical path made up by one axial bushing conductor 7, the bridge member 16, and the other axial bushing conductor 7 forms a lJ-shaped primary circuit of the current transformer.

Within the tank 3 and between the bushings an insulating compression member or column 18 is interposed and compressed between the top of tank 3 and the bridge conductor in. The effect of such compression is to apply a downward force on the bridge member 16, which force by working against nuts l7 places the axial bushing conductors 7 in tension. With tension on the axial bushing conductors 7, a downward force will be imposed on the top clamping nut 8 and cap member ill thereby compressing the gasket glass shell ill, the porcelain shell 4 and the gasket 6 against the mounting ring l2.

Thus, according to my invention, the central clamping force is applied to the bushings jointly by the interaction of the stop nuts 17, the conducting bridge lit), the axial bushing conductors 7 and the top stop nuts As shown, the bushings l and l9 are disposed parallel to each other and normal to the conducting bridge 16. Although this is the most expeditious geometry nevertheless various other geometric arrangements are contemplated. it should be appreciated that since the compression member 18 applies a force to the bridge member l6 which in turn applies that force to the axial bushing conductors 7, the maximum force will be applied to the axial bushing conductors when the force applied to them by the bridge member has only one component and that component is parallel to the axes of the axial bushing conductors. This condition exists when the bushings are parallei to each other and normal to the bridge member. If, however, the bushings are disposed at an angle to each other and hence at an angle to the direction of the force exerted by the compression member 113, then only that component of the force which is parallel to the axes of the axial bushing conductors 7 is available to place them in tension. This component is necessarily less than the total force exerted by the compression member ill.

in practicing my invention, the bushings should be disposed relative to the direction of the force exerted by the compression member such that there will be a substantial component of that force parallel to the central conductors available to place the axial bushing conductors in tension. This condition will occur when the bushings are disposed in a laterally spaced-apart or side-by-side arrangement. By this arrangement ll mean to include all bushing arrangements from the preferred parallel disposition to an angular disposition wherein the bushings are appreciably out of mutual axial alignment.

As was heretofore noted when the bushing axes are parallel to each other and to the direction of the force exerted by the compression member l8 the tensile force on the axial bushing conductors will be at its maximum. As the angular disposition of the bushings increases toward a condition of axial align ment the tensile component of force on the central conductors decreases. For wide angular dispositions it may be expeditious to utilize a pliable flexible bridge member 16, such as a braided strap conductor or the like, in order to maximize the tensile force exerted on the central conductors.

In accordance with my invention, by eliminating the need for lower porcelain shells and insulating tension rods the manufacture of a bushing transformer is simplified thereby improving structural reliability while decreasing manufacturing costs.

Further, in my improved transformer structure, there is nothing to impede the free flow of oil between the bushings and the tank, thereby permitting the bushings to operate somewhat cooler than in prior art devices.

While a particular embodiment of the invention has been shown and described, it will be obvious to those skilled in the art that various changes and modification may be made without departing from the invention in its broader aspects. I therefore intend herein to cover all such changes and modifications as fall within the true scope and spirit of my invention.

1 claim:

1. A bushing current transformer comprising:

a. a tank having a wall portion;

b. at least two terminal bushings attached to said wall portion in a laterally spaced-apart relationship, each of said bushings comprising an insulating shell and an axial conductor extending longitudinally therethrough in spacedapart relation, one end portion of each said axial conductor being disposed inside said tank;

c. a common body of dielectric fluid filling said tank and at least partially filling said bushings;

d. a secondary coil disposed around one of said axial conductors in the region of said tank; and

e. means for imposing a central clamping force on each of said bushings comprising:

i. an electrically conductive bridging member connecting the proximate ends of said axial conductors within said tank;

ii. an insulating compression member interposed between said wall portion and said bridging member at a point intermediate said axial conductors; and

iii. stop means located at the upper ends of said central conductors; whereby a force exerted by said compression member acting through said bridging means to said axial conductors produces a substantial tensile component of force on said axial conductors and thereby produces a compression force on said insulating shells.

2. A bushing current transformer as specified in claim 1 wherein the bushings are disposed in a generally parallel relationship to each other.

3. A bushing current transformer as specified in claim ll wherein said bridging member is rigid.

4. A bushing current transformer as specified in claim l wherein said bridging member is flexible,

5. A bushing current transformer comprising:

a. a tank containing oil, said tank having a top wall portion;

b. a parallel pair of oil-filled bushings attached to said tank wall, in fluid communication with said tank, said bushings each comprising a mounting ring, an insulating shell, an insulating core and an axial conductor extending longitudinally thereof, a portion of said insulating core and a portion of said axial conductor extending into said tank;

0. a secondary coil disposed around one of said axial conductors in the region of said tank;

d. common means for imposing a joint central clamping force on each of said bushings comprising:

i. a rigid electrically conductive bridge member connecting the ends of said axial conductors extending into said tank;

ii. an insulating member compressed between said top portion of said tank and said bridge member at a point intermediate said axial conductors, and

means to said axial conductors produces a tensile force in said axial conductors and thereby causes the insulating shells to be placed in compression.

73 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 571, 773 Dated March 23, 197].

Inventor(s) Donald L. Johnston It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Front page, line 1 Change the inventor's name from "Donald J. Johnston" to Donald L. Jbhns" Signed and sealed this 13th day of July 1971 (SEAL) Attest:

EDWARD M.FLETGHER,JB. WILLIAM E. SCHUYLER, JR. Attesting Officer Commissioner of Patents

Claims

1. A bushing current transformer comprising: a. a tank having a wall portion; b. at least two terminal bushings attached to said wall portion in a laterally spaced-apart relationship, each of said bushings comprising an insulating shell and an axial conductor extending longitudinally therethrough in spaced-apart relation, one end portion of each said axial conductor being disposed inside said tank; c. a common body of dielectric fluid filling said tank and at least partially filling said bushings; d. a secondary coil disposed around one of said axial conductors in the region of said tank; and e. means for imposing a central clamping force on each of said bushings comprising: i. an electrically conductive bridging member connecting the proximate ends of said axial conductors within said tank; ii. an insulating compression member interposed between said wall portion and said bridging member at a point intermediate said axial conductors; and iii. stop means located at the upper ends of said central conductors; whereby a force exerted by said compression member acting through said bridging means to said axial conductors produces a substantial tensile component of force on said axial conductors and thereby produces a compression force on said insulating shells.

3. A bushing current transformer as specified in claim 1 wherein said bridging member is rigid.

4. A bushing current transformer as specified in claim 1 wherein said bridging member is flexible.

5. A bushing current transformer comprising: a. a tank containing oil, said tank having a top wall portion; b. a parallel pair of oil-filled bushings attached to said tank wall, in fluid communication with said tank, said bushings each comprising a mounting ring, an insulating shell, an insulating core and an axial conductor extending longitudinally thereof, a portion of said insulating core and a portion of said axial conductor extending into said tank; c. a secondary coil disposed around one of said axial conductors in the region of said tank; d. common means for imposing a joint central clamping force on each of said bushings comprising: i. a rigid electrically conductive bridge member connecting the ends of said axial conductors extending into said tank; ii. an insulating member compressed between said top portion of said tank and said bridge member at a point intermediate said axial conductors, and iii. clamping means located at the upper ends of said axial conductors; whereby a force exerted by said compressed insulating member acting through said bridge means to said axial conductors produces a tensile force in said axial conductors and thereby causes the insulating shells to be placed in compression.