US2692297A - High-voltage bushing - Google Patents

Description

Oct. 19, 1954 H. M. owREN HIGH-VOLTAGE BUSHING Filed Jan. 2, 1952 TTORNEK Patented Oct. 19,` 1954 HIGH-VOLTAGE BUSHING Harvey M. wren,

the United States by sion Oakland, Calif., assignor to of America as represented the United States Atomic Energy Commis- Application January 2, 1952, Serial No. 264,447

1 Claim.

The present invention relates to an improved high voltage bushing for use in connection with the isolation of large alternating or direct current voltages encountered in connection with electrical equipment, such as transformers.

In the use ci very high voltages of the order of hundreds of kilovolts it is commonly necessary to employ bushings having a length of ten feet or more in order to prevent electrical breakdown, and elements of this size while quite practicable outdoors are often too large for indoor installations having even nominal space limitations. Additionally, bushings are conventionally designed and built for use with alternating current voltages or alternatively for direct current voltages and certain criteria governing bushing design makes a bushing designed for one use generally unsuited for another.

The present invention is adapted for use with alternating or direct current Voltage isolation and further may be made considerably smaller `than conventional bushings designed for comparable voltages. These advantages are obtained by the provision of a combination of insulations particularly adapted for alternating current voltage and for direct current voltage, by the elongation of the voltage creep path and by expansion of the areas having potential differences therebetween.

Accordingly, it is an object of the present invention to provide an improved high voltage bushing.

It is another object of the present invention to provide a high voltage bushing having expanded surfaces between which an impressed potential exists.

It is another object of the present invention to provide an improved high voltage bushing having means making substantially uniform the potential gradient across the insulation thereof.

It is a further object of the present invention to provide an improved high voltage bushing having in combination a coronashield and liquid and solid insulation.

It is a still further object or the present invention to provide an improved high voltage bushing having a corona shield immersed in insulating iiuid.

Numerous other advantages and possible obscription taken together with the attached drawings, wherein:

Figure 1 is a longitudinal sectional View of a preferred embodiment of the invention taken on a horizontal line through the center of Figure 2;

Fig. 2 is a sectional view taken at line 2 2 of Fig. 1; and

Fig. 3 is a schematic wiring diagram of a possible application of the invention.

Considering the invention in general, it will be seen to comprise a bushing it having central electrical conductor l l extending through an insulating envelope i 2 which engages same in sealed relation and contains an insulating iiuid i3. Mounting means lll about the exterior of envelope i2 extends interior thereto and is eX- panded along conductor ii as shown. Additional insulation is provided about conductor Il and numerous other elements are provided to complete an operable bushing assembly as set forth below.

Considering now the invention in more detail both as to elements and functions thereof, and referring to Figs. l and 2, there is shown an insulating envelope l2 which may include a plurality of stacked insulating cylinders It, il, and i3 which are formed of porcelain or the like and which are disposed concentrically about metallic tubular conductor l l. A pair of discs l@ and 20 are disposed about conductor I I with one adjacent each end thereof and engaging adjacent insulating cylinders It and i8 to denne an enclosed chamber which is filled with an insulating fluid i3, such as transformer oil. Bottom disc 2d is preferably formed of insulating material while top disc I9 may be made of metal or the like. A bottom retainer ring 22 is fastened about conductor il below insulating disc 20 to hold saine in position and a gasket disposed therebetween seals the connection about conductor Il. A top retainer ring 23 is disposed about conductor il near the top thereof and a hold-down nut 2i is threaded onto conductor li to force top ring 23 against disc lli and to seal the connection about conductor i i by forcing top ring against appropriate gasket means inserted in said connection. There may also be provided a terminal cup 26 secured atop insulating disc i9 by top' retainer ring 23 to contain appropriate terminal means.

With regard to the central portion of insulating envelope l2, it will be noted that the illustrated embodiment of the invention includes a third disc 2l about conductor il formed of insulating material and employed to join insulating cylinders i5 and il. The use of two insulating cylinders It and il and insulating disc 2l therebetween instead of a single insulating cylinder is advantageous when the total length of insulation is great, in that the cost and diilicultl7 of manufacture is decreased, and also is necessary when the insulating material is of laminar form in that it has been found that very high voltages tend to discharge along the laminations. Also, disposed Asomewhat centrally of the bushing assembly is mounting means I4 which is preferably discoidal and formed of metal or the like. Mounting means I4 is disposed about the eXterior of insulating envelope I2 and also extends interiorly thereof between insulating cylinders I1 and I8 in sealing relation thereto. Apertures may be provided through mounting disc I4 spaced about same exterior to insulating envelope for engagement with attachment means, such as bolts, by which the bushing assembly may .be mounted upon electrical equipment or thelike.

Internal to insulating envelope I2 there is disposed a corona shield 228 which is formed as a cylinder of somewhat lesser diameter than insulfating envelope I2 .and has the edges thereof rolled outwardly. The flaring or rolling of the ends or edges of shield 28 preferably includes a complete turn of the edge as may be accomplished by joining a bar or tube formed as an anchor ring 29 about each end of shield 28, as illustrated. Corona shield 28 is secured either directly or by intermediate electrically conducting support means to mounting means I4 with shield 28 preferably being electrically contacted about the entire circumference at a distance .from the ends thereof.

In addition to the above-noted elements there are provided liquid and solid dielectrics internal to insulating envelope I2 and about central electric conductor I I, The solid dielectric is formed as one or more tubes SI of insulating material, such as Lucite, disposed concentrically about conductor II and maintained in position by spacer rings 32 disposed between tubes 3l and between tubes 3l and conductor II, as shown. Insulating tubes SI extend along conductor II from insulating disc 2G to a point above mounting means ill that is substantially equal to the distance between insulating disc 20 and mounting means III, and tubes 3| are vdisposed between shield 28 and conductor II, preferably closer to conductor II. rEhe fluid dielectric I3 substantially fills the interior of insulating envelope I2 about conductor II and apertures in mounting means I4 and in spacer rings 32 provide communica-tion to all parts of the interior of insulating envelope I2 for lling same with fluid I 3.

With regard to relative dimensions of the elements of the bushing assembly, it is to be noted that the application for which the bushing is designed is at least in part determinative of certain of the relationships. In many applications the bottom portion of the bushing will be disposed in a medium havingr a different dielectric constant than the medium about the upper portion and the ratio of the dielectric constants may be used approximately as the ratio of the distance from the top of the bushing to mounting means III to the distance from the bottom of the bushing to mounting means I4. rIhus where the bushing is to be mounted partially within equipment including a transformer oil bath and insulating cylinder I8 is to be disposed in oil and cylinders I6 and I1 are to be disposed in air the vratio of the length of cylinder I8 to the combined lengths of cylinders I6 and I'I may be one-half, which is the ratio of Athe dielectric constants of air and transformer oil. Also the disposition of corona shield 28 is dependent at least in part upon the same criteria and in the above example corona shield 28 is connected to mounting means I4 with substantially two-thirds of the shield above mounting means I4 and onethird below. It will be appreciated that the above-noted dielectric ratio is important with relation to surface discharge on the insulators and it is presupposed that the insulators are of adequate size to withstand breakdown therethrough with the voltage to be employed.

With regard to the disposition of corona shield 28 and the fluid and solid insulation I3 and 3I, respectively, -it is to be noted that corona shield 28 in effect expands the area or surface of mounting means I4 internal to the insulating envelope I2. Ordinarily mounting means I4 is maintained at ground 'potential as by attachment to a transformer tank or the like, and whatever the connection, a potential exists between electrical conductor II and mounting means I4. This potential is maintained by insulating fluid I3 and by solid insulation 3|, and corona Shield 28 is preferably separated from central conductor II by a sufficient distance to dispose an adequate amount of insulating fluid I3 therebetween to hold the entire voltage for which the bushing is designed. In addition, solid insulation 3I between corona shield 28 and conductor II is preferably provided in sufficient thickness to withstand the entire voltage ratio of the bushing and this has been determined in the case of Lucite to be about one quarter of an inch per kilovolts. By the above provisions the bushing is capable of carrying alternating current voltages or direct current voltages and the possibility of the establishment of a voltage creep path directly between shield 2S and conductor I I by the alignment of impurities in the insulating fluid is prevented. Direct current voltages can only align the impurities in the insulating fluid from corona shield 28 to insulating tube 3I so that the voltage creep path must then extend along the insulating tube 3l to the end thereof and thus the creep path is made greater than the direct voltage path.

Equal potential lines extend between corona shield 23 and conductor II parallel thereto but at the ends of corona shield 28 they curve back toward mounting means I4 and if they are ltoo close together where they intersect the surfaces of insulating cylinders Il and I8 the potential gradient will be sufcient to cause local discharge which may then spread to produce electrical breakdown across the insulator surface. With sharp edges at the ends of corona shield 28 the equipotential lines will curve sharply back toward mounting means I4 and will become bunched together to produce regions of large potential gradient; however, the present invention obviates this difficulty by providing curved edges on corona shield 28 whereby the equipotential lines follow wide curves having an even greater separation than they do in the region between conductor II and mounting means I4. The potential gradients across all parts of insulating cylinders I1 and I8 are thus minimized and made more uniform to present discharge thereacross,

Considering now a typical application of rthe invention, reference is made to Fig. 3 wherein there is .shown in elevation a bushing constructed in accordance with the present invention and mounted upon the tank 4I of a high voltage transformer 42 shown schematically in section. Bolts or the like engage mounting rneans I4 and transformer tank 4| to hold bushing I0 in place and lower insulating cylinder IB is thus disposed within tank 4|. The transformer primary winding 43 is connected to an external power supply (not shown) and the secondary windings 44 are connected by leads 45 through the conducting tube of bushing I9 to the exterior of tank 4| where they may be attached to a terminal block or the like. Preferably tubular bushing conductor is sealed at the lower end thereof about secondary transformer leads 45 and the transformer windings 43 and 44 may be disposed in an insulating uid 46, such as transformer oil. There may be further provided within tank 4| a primary winding shield electrically connected to the primary winding and a secondary winding shield electrically connected to the central tubular conductor I I of bushing I0, In common practice transformer tank 4| is electrically grounded and thus mounting means I4 and connected corona shield 28 (Fig. 1) are grounded While the central conductor I I of bushing I is maintained at the high Voltage of secondary transformer winding 44. By the above-noted construction of bushing I0, electrical breakdown is prevented in bushing I0 whether direct current voltage pulses or alternating current voltages are passed by transformer 42.

While the present invention has been disclosed with respect to a single preferred embodiment, it will be apparent to those skilled in the art that numerous modifications and Variations are possible within the spirit and scope of the invention and thus it is not intended to limit the invention except by the terms of the following claim.

What is claimed is:

A high voltage bushing for use with electrical equipment disposed in a tank containing insulating fluid comp-rising, a sealed insulating cylinder, an insulating fluid Within said cylinder, discoidal mounting means extending through said cylinder at substantially one-third of the length thereof from one end of said cylinder for mounting said cylinder upon said tank with substantially one-third 0f said cylinder extending into said tank, an electrical conductor disposed along the axis of said insulating cylinder by connection to the cylinder ends and extending from both ends thereof for connection between electrical equipment and terminals exterior to the tank, a cylindrical corona shield secured to said mounting means within said insulating cylinder about said electrical conductor and extending along said electrical conductor in both directions from said mounting means, said corona shield being entirely surrounded by said insulating fluid with the exception of the connection thereof to said mounting means, and a plurality of insulating tubes disposed concentrically about said electrical conductor in radially spaced relationship within said corona shield and out of contact therewith, said tubes extending axially in both directions from said corona shield for preventing direct current electrical breakdown between `said corona shield and said conductor.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,160,702 Faccioli Nov. 16, 1915 1,526,023 Steinberger Feb. 10, 1925 1,945,917 Scarpa Feb. 6, 1934 1,996,423 Jansson Apr. 2, 1935 2,600,979 Dormont June 17, 1952 FOREIGN PATENTS Number Country Date 281,734 Great Britain July 26, 1928 309,936 Great Britain Apr. 18, 1929 334,093 Italy Jan. 21, 1936

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