US2549426A - Electric transformer - Google Patents

Description

April 1951 F. M. CLARK ET AL ELECTRIC TRANSFORMER Filed Dec. 22, 1948 4 PiSgJ.

L/QU/O LEVEL Ihvehtors: Frank M. Clark Gaugliehmo Camilli Th eh Attorn ey.

Patented Apr. 17, 1951 ELECTRIC TRANSFORMER Frank Clark, Schenectady, N. 'Y., and-Guglielmo Camilli, Pittsfield, Mass assignors to General Electric Company, a corporation of New York A'pp'licationDecem'ber 22, 1948,, Serial 'No. 66,805

1 2 Claims. This invention relates to electric transformers and'more particularly to improvements .in the insulation of transformers.

Heretofore electric transformers have required solid insulation in the direct line between parts where the potential gradient is the highest. Usually this is in the space between different 'po tential windings because there the separation of the parts at the maximum potential difference is the smallest. Such solid insulation .is usually made oforganic material in the form of paper or cloth because of its flexibility and low costs. However, such material absorbs moisture which materially reduces its dielectric strength so that it is practically always necessary to subject it to an expensive and time-consuming drying treatment before the transformer can be used. The dielectric strength of such material is often further increased by submerging it in and impregnating it with a dielectric liquid, such as mineral oil. The action of the liquid is to fill minute voids in the solid insulation and thus eliminate gaseous ionization in such voids. Such ionization often causes eventual breakdown of the solid .insulation. It should be noted, however, that the solid insulation alone must be able to withstand the full voltage.

At first thought it might seem as though such liquid alone could be used and that the solid insulation in the space having the highest potential gradient could be dispensed with. .However, experience has shown that such an arrangement will not operate satisfactorily. The reason fo this is that there are always minute particles of foreign material such as dirt and fibre particles in the transformer tank and under the action of the electric field in the liquid these fibres and other particles are drawn toward the portion of the field where the stress concentration is the highest and they actually line up with each other thus producing a relatively low dielectric strength path and cause the electrical breakdown of the dielectric and failure of the apparatus.

We have discovered however that this lining up action of fibre and other particles in the liquid is a 'functionof the relative dielectric constant of the particles and of the liquid and that if the dielectric constant of the liquid is higher than the dielectric constant of the fibre and other particles they'will then be expelled from the regions of the electric field where the stress concentration is the highest and consequently these particles will not line up. The dielectric constant of most impurities in a transformer tank is about 3.7 and the dielectric constant of mineral oil is somewhat below this value. We have found however that certain askarels, such as chlorinated diphenyl and such as are described and claimed in Clark Patents 1,931,373, 1,931,455, 1,944,730, 1,964,684, 1,999,004, and 2,037,686, which are as-- signed to the present assignee, have a dielectric constant which is above 3.7 and that such liquid material can be used as the sole insulation between transformer parts at very substantial differences in the potential. However, such material has a lower impulse voltage strength than mineral oil (when the electrostatic field is not uniform) and we have, therefore, found it necessary to enclose the respective transformer windings in metal shields which are connected to their respective windings and which have smooth contours without any sharp points so as to control the distribution of electric stress and prevent it from being concentrated at high values which would be the case if the windings had sharp points.

An object of the invention is to provide a new and improved electric transformer.

Another object of the invention is to provide a transformer in which a liquid isthe sole insulation between the winding structures.

A further object of the invention is to eliminate the usual organic solid insulation in transformers and thus eliminate expensive and timeconsuming drying treatment of such solid insulation.

The invention will be better understood from the following description when taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

In the drawing, Fig. 1 is a sectional view of a transformer embodying the present invention and Fig. 2 is an enlarged view taken on line 2-2 of Fig. 1.

Referring now to the drawing and more particularly to Fig. 1, there is shown a high voltage current transformer comprising a metal tank I on the cover 2 of which is mounted a high voltage porcelain insulating bushing 3. At the top of the bushing is a smaller metal tank or compartment 4 in whose sides are mounted high voltage terminal bushings 5. The latter need only be insulated for the voltage difference between the ends of the primary winding, Whereas the main bushing 3 insulates the compartment 4 which is at substantially line potential from the tank I which is at substantially ground potential.

The primary or high voltage winding of the transformer consists of a conductor 6 which has its ends connected respectively to the inner ends of the bushings 5 and which extends down through the main high voltage bushing 3 and makes two turns around a magnetic core 1. This primary winding is enclosed in a toroidal metallic shield 8 which has a break in it at 9 so as to prevent its being a short-circuited turn around the core 1. As shown more clearly in Fig. 2, the neck of the shield 8 is oval in shape. This is for the purpose of insuring that the shield is mounted rigidly in place in the transformer so that it cannot turn or twist under the influence of electromagnetic forces. Thus, wedging strips 10 of insulating material are inserted between the flat sides of the neck of the shield 8 and a rigid insulating tube l I. In addition, studs [2 which are threaded into the tube I I have their ends in registering holes in the neck of the shield 8 so as to further insure that shield 8 is anchored rigidly in its proper angular position. The tube II in turn is mounted tightly in place by means of gasketed rings l3 fastened by means of bolts l5 respectively to the underside of the cover 2 and to the bottom of the top compartment 4.

A secondary winding l6 of the transformer is wound on the core I and is surrounded by a toroidal metal shield I! which is split at it so as to prevent it acting as a short-circuited turn on the core '1'. The low voltage secondary leads I9 are connected to an outlet box on the side of the tank l.

The shield 8 for the high voltage primary winding is electrically connected to this winding by means of a conductor 2i, and low voltage secondary wnding I6 is similarly electrically connected to the toroidal shield H.

A dielectric liquid such as chlorinated diphenyl having a dielectric constant of at least 4 at degrees C. and typically 5 or more fills the tank I and rises through the tube II to a level indicated in the upper compartment 4.

The liquid, of course, also fills the toroidal shields 8 and I! as it can enter them through the openings 9 and I8 respectively. However, the liquid within the shields is not under potential stress because each shield is electrically connected to the winding which it encloses so that there is no potential difference between them. Solid insulation is actually not essential between the turns of the windings, but in many cases it will be desirabl for insuring the physical spacing of the turns. However, such solid insulation, if it is used, will not be subjected to the principal voltage of the device because it will be shielded by the respective shields 8 and l! and such insulation will merely serve as turn insulation for the windings. If such insulation is used, it will not require any substantial drying treatment because of the small amount of it and the fact that it is not subjected to high voltage stress.

An important advantage of the present invention is that the sole insulation in the space between the interlinked windings is a liquid 22. The insulation is in effect self-healing in case it should be broken down as, for example, during an impulse voltage test. In an ordinary transformer if the transformer fails in impulse test, it is necessary to tear it down and rebuild it but with the present transformer the liquid will immediately reinsulate the device as soon as the lmpulse voltage has passed.

The shield l! for the secondary winding is rigidly anchored as by welds 23 so that the plane of the toroid I! is at right angles with the plane of the toroid 8.

Whil there has been shown and described a particular embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications can be made therein without departing from the invention and therefore it i aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A transformer having a high voltage winding and a low voltage Winding, a casing surrounding both windings, a separat metallic shell of smooth external surface surrounding each winding and electrically connected thereto, each shell having a discontinuity so as to prevent its being a short-circuited turn with respect to said transformer, said shells being inter-linked and physically separated from each other, and a liquid dielectric in said casing having a dielectric constant of at least 4 at 25 C., said liquid surrounding said shells and being the sole insulation in the space between their inter-linked surfaces.

2. A transformer having a high voltage winding and a low voltage winding, a casing surrounding both windings, an elongated hollow cylindrical bushing mounted on the top of said casing with the interior of the bushing communicating with the interior of said casing, a ring-shaped magnetic core inside said low voltage winding, a toroidally-shaped metallic shell surrounding said low voltage winding and being electrically connected thereto, said shell having a discontinuity so as to prevent its being a short-circuited turn on said core,'means for rigidly attaching said shell in an upright position to the bottom of said casing, a toroidally-shaped metallic shield surrounding said high voltage winding and electrically connected thereto, said shield having an elongated integral neck portion which extends through the interior of said bushing, said shield and said shell being interlinked and physically separated from each other, said shield having a discontinuity for preventing its being a shortcircuited turn with respect to said core, the elongated neck of said shield being val in crosssection, means attached to the top of said bushing for engaging said oval cross-section neck and rigidly clamping it in such a position that the planes of the toroidal shield and the toroidal shell are at right angles to each other, and a liquid dielectric in said casing having a dielectric constant of the order of 5 at 25 C., said liquid surrounding said shell and said shield and being the sole insulation in the space between their interlinked surfaces.

FRANK M. CLARK. GUGLIELMO CAMILLI.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Germany Sept. 24, 1924

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