US3086073A - High voltage liquid-free insulating bushing with improved voltage distribution - Google Patents

Description

Aprll lfi- 1963 N. F. ARONE EI'AL 3,036,073

HIGH VOLTAGE LIQUID-FREE INSULATING BUSHING WITH IMPROVED VOLTAGE. DISTRIBUTION Filed Dec. 20, 1961 m 30% J02 I //vvE/\/T0Rs: NICHOLAS FARONE, LAWRENCE L. MANKOFF,

BY 27m ATTORNEY.

United States Patent 3,686,073 HIGH VGLTAGE LIQUID-FREE INSULATING BUSHING WITH EMPROVED VOLTAGE DES- TRIBUTION Nicholas F. Arone, Upper Darby, and Lawrence L. Maniroiif, Eroornall, Pan, assignors to General Electric Company, a corporation of New York Filed Dec. 20, 1%1, Ser. No. M0344 8 Claims. (Cl. 174-442) This invention relates to a high voltage insulating bushing of the type that comprises an outer tubular shell of insulating material and a conductor within the shell radially spaced therefrom and extending longitudinally of the shell between its opposite ends.

It is customary in such bushings to fill the space between the shell and the conductor with an insulating liquid such as oil. .But the use of a liquid insulator has certain disadvantages, such as the need for sealing the space to prevent the loss of liquid through leakage and the need for providing for thermally-produced expansion of the liquid. If the liquid is omitted and only air is present in the space between the shell and the conductor, the dielectric strength, or basic impulse level, of the bushing is reduced .to an objectionably low level. This basic impulse level can be substantially increased by substituting a filling of solid insulation for the air filling. But this results in certain other problems. One of these is that the process for introducing the filling becomes unduly critical and must be carefully controlled to prevent voids and objectionable non-unifiormities in the solid filling. Another problem is that solid insulating materials suitable for such fillings have a high degree of yieldability and have high volumetric coeflicients of thermal expansion, and these properties tend to cause small quantities of the solid insulation to be forced out of the ends of the bushing under high temperature conditions.

One of the objects of our invention is to provide a high voltage bushing in which solid insulation is disposed between its outer insulating shell and its conductor, but the solid insulation is incorporated in such a manner that the above-described problems associated with solid fillings are minimized.

Another object is to provide a bushing of the type set forth in the preceding paragraph that is free of insulating liquid and that has a basic impulse level much higher than a corresponding air-filled bushing.

In carrying out our invention in one form, we provide an insulating bushing comprising a tubular shell of insulating material and a high voltage conductor centrally located within the shell and extending axially thereof. Closely encompassing the conductor, a tubular sleeve of insulating material is provided. This tubular sleeve is located within the tubular shell in radially-spaced relationship to the shell so as to leave an air gap between the shell and the sleeve. Grounded metallic structure is present about the outer periphery of the shell, and this grounded structure extends along a short portion of the total length of the shell. A first conductive shield in the form of a coating on the inner periphery of the shell is provided. This coating is electrically isolated from the grounded structure at the outer periphery of the shell. In addition, this coating extends along a portion of the length of the shell located axially of the shell in the region of the grounded metallic structure. A second conductive shield is provided on the insulating sleeve near its outer surface and this shield extends between points located axially outward of the respective ends of the first shield. The second shield is electrically isolated from the high voltage conductor by means of the insulating sleeve. The second conductive shield has its axially opposed ends covered 3,686,073 Patented Apr. 16, 1963 "ice by insulating material constituting a part of the sleeve so as to locate the high dielectric stress region adjacent these ends in the solid insulation of the sleeve. The two shields are electrically connected together by conductive bridging means that extends across the air gap.

For a better understanding of our invention, re erence may be had to the following specification taken in conjunction with the accompanying drawing, wherein:

FIG. 1 is a side elevational view partly in section of a high voltage bushing embodying one form of our invention.

FIG. 2 is an enlarged detail view of a component of the bushing of FIG. 1 shown prior to assembly in the bushing.

FIG. 3 is a cross-sectional view taken along the line 33 of FIG. 1.

FIG. 4 is an enlarged view of a portion of the bushing of FIG. 1 illustrating the approximate configuration oi the electric field.

Referring now to FIG. 1, the bushing shown therein comprises a tubular outer shell 11 of a suitable insulating material such as porcelain and a centrally-located conductor 12 extending between opposite ends of the shell. The conductor 12 is a rigid stud having clamping nuts 13 threaded on its opposite ends. Disposed between each of these nuts 13 and one end ofi the shell 11 is a metallic end cap 14 and a resilient washer 15. When the nuts 13 are tightened, the shell 11 is clamped between the end caps 14, and the rigid conductor 12 is thus firmly held in its central location shown. Radial shifting of the conductor 12 with respect to the shell 11 is prevented by providing on each of the end caps 14 a boss which fits within the bore of the shell 11 with only slight clearance.

For mounting the bushing on suitable electrical apparatus, an annular metallic flange 20 is provided about the exterior of the shell 11. This flange 20 is suitably bonded to the shell 11 and is preferably located centrally of the length of the shell. In most electrical apparatus this flange 20 will be at ground potential. The basic purpose of the bushing is to provide adequate insulation between the high voltage conductor 12 and parts at ground or at an intermediate potential located near the conductor 12. Since the grounded mounting flange 2G" is typically the grounded part closest to the high voltage conductor 12, the region of highest electrical stress in the bushing will be adjacent the mounting flange.

For withstanding a portion of the voltage that is present between the conductor 12 and the mounting flange 20, we provide within the shell 11 a sleeve 25 of solid insulating material closely surrounding the conductor 12. This insulating sleeve 25 is preferably cemented or otherwise bonded to the conductor 12 so that there are no voids between the sleeve and the conductor.

To allow for unequal thermal expansion of the sleeve 25 and the tubular shell 11, the external diameter of the sleeve 25 is made appreciably smaller than the internal diameter of the shell '11, so that an air gap 26 is present between the shell 1'1 and the sleeve 25. This air gap 26 permits the sleeve 25 and the shell 11 to expand or contract independently of each other without engaging each other and, thus, without significantly stressing the parts of the bushing. This is in distinct contrast to bushings that have solid insulation completely filling the space between the conductor and the outer shell.

In order for the bushing to withstand the high voltage for which it is intended, it is important that no significant amount of corona be present in the air gap 26. Such corona is objectionable not only because it produces excessive radio noise but also because it tends to ionize the air in the gap and thus produce electric stress conditions that could lead to a dielectric breakdown.

We prevent the formation of significant corona in the air gap 26 by providing means for eliminating dielectric stress in the air gap in the region of the bushing that is subject to the highest stress, i.e., the region of the mounting flange 2% This stress-relieving means comprises a conductive shield 39 in the form of a coating applied to the inner periphery of the porcelain shell 11, a conductive shield 32 on the sleeve 25 near its outer periphery, and conductive bridging means 34 extending across the air gap and electrically interconnecting these two shields 30 and 32. The presence of the conductive bridging means results in the two shields 30 and 32 being at the same potential. Accordingly, in the space between the shields 3i and 32, which space includes the air gap 2d, there is no significant amount of dielectric stress.

' The shields 3t? and 32 can be of any suitable conductive material, but we prefer to use a high resistance conductive paint comprising finely divided graphite suspended in a suitable resin. In a preferred form of the invention, the shield or coating 39 on the porcelain shell 11 extends about the entire inner periphery of the shell, and the shield 32 on the insulating sleeve 25 extends around the entire sleeve. The material of the insulating sleeve 25 is preferably paper coated and impregnated with a suitable epoxy resin. The paper is in the form of a sheet wound about the longitudinal axis of the sleeve to provide a laminated structure. During formation of the sleeve, the conductive shield 32 is sprayed as a coating about the periphery of the sleeve shortly before the full sleeve diameter is attained. When the application of the coating 32 has been completed, Winding of the impregnated paper is continued until the full sleeve diameter is attained. Thereafter, along a restricted portion of the sleeve length, insulating material is removed from the outer periphery of the sleeve to leave exposed a portion of the coating 32 centrally of the bushing length. The longitudinallyopposed ends of the coating 32, however, remain buried or imbedded in the insulating material of the sleeve. This latter feature contributes in an important manner to the high dielectric strength of the bushing, as will soon be explained in more detail.

Although we prefer to form the sleeve 25 of a paperepoxy resin material, other insulating materials may instead be utilized. For example, the sleeve can be formed of irradiated polyethylene tape or of varnished cambric tape. Both of these materials have lower dielectric constants than the paper-epoxy material but are not as easy to fabricate into a sleeve of the desired form. The lower dielectric constant of these materials, however, results in more of the dielectric stress being taken by the sleeve 25 and less by the porcelain shell 11 in the region adjacent the mounting flange 20. For reasons soon to be explained, this results in an even higher basic impulse level than is obtained with the paper-epoxy sleeve. The dielectric constant of the irradiated polyethylene is about 2.3 and that of the varnished cambric 3.5 to 4, as compared to 4.8 for the paper-epoxy material. For any of these materials, the ends of the shield 32 may be covered by tape or other suitable insulation instead of being covered in the particular manner described hereinabove. However, even where a tape covering is used, the covering is intimately bonded to the remainder of the insulating material of the sleeve and, thus, for the purposes of this application, constitutes a part of the sleeve. The tape covering is, in effect, integral with the remainder of the sleeve and completely covers the end of the shield 32 in the same manner as shown in FIG. 1.

Although the coating "30 on the inner periphery of the porcelain shell is preferably sprayed or painted on the shell, it is to be understood that other procedures could be used for applying this coating. For example, a layer of foil can be intimately bonded to the shell.

The bridging means 34 between the two shields 30 and 32 comprises a conductive leaf spring made, for example, of a copper alloy. In its unstressed form, the

spring 34 has a U-shaped configurationsuch as shown in FIG. 2. When the sleeve 25 is to be assembled Within the shell 11, the legs of the spring are wrapped aboutthe periphery of the sleeve in the region where the coat-' ing 32 is exposed so that the spring is generally of the shape illustrated in FIG. 3. The sleeve 25 is then slipped into the shell 11, and the spring 34 is held in its stressed condition of FIG. 3 by the inner periphery of the shell as the sleeve 25 is moved longitudinally of the shell to its final postiion. When the sleeve 25 is in its final position, the spring 34, in tending to expand to its original configuration, makes contact with the outer shell 30 at points 4t) and with the inner shell 32 at points 42.

During positioning of the sleeve 25 within the shell 11, the spring 34- is prevented from shifting longitudinally of the sleeve by reason of the fact that this spring 34 is captured between spaced-apant shoulders provided in the insulating material at opposed edges of the spring. The spring configuration is such that the relatively sharp ends 44 of the spring are spaced from the sleeve 25 and are located in the air gap 26 and thus do not bite into th material of the sleeve to impair its insulating properties. The rounded configuration of the spring 34 at its corners prevents the spring from biting into and thus impairing the shield 36 at contact points 49. Although the spring 34 is a preferred means for interconnecting our shields 3t) and 32, it should be understood that our invention in its broader aspects contemplates other forms of connecting means between the two shields. The connecting means used should, however, be of such a nature that it requires no auxiliary fastening means, such as pins or screws, which would impair the cylindrical character of the coatings.

In the disclosed bushing the inner shield 32 is substantially longer than the outer shield 30 and projects longitudinally beyond the opposite ends of the outer shield 3%. This relationship results in an electric field configuration in which there is no significant stress concentration adjacent the ends of the outer shield 3d. If the outer shield instead of the inner shield were made the longer of the two shields, then the equal potential lines would tend to concentrate in the region adjacent the ends of the outer shield. Since these regions are in the air gap 26, there Wou ld be a stress concentration in the air gap that could lead to objectionable corona. By making the inner shield project substantially beyond the ends of the outer shield, we, in effect, shield this idielectrically weak region at the ends of the outer shield 3t} and transfer the stresses associated with a shield end to the solid insulation of the sleeve 25. This solid insulation is much more capable than the air of withstanding this electric stress without harmful effect.

To illustrate the manner in which the regions at the ends of the outer shield 30 are relieved of electric stress, reference may be had to the electrostatic flux plot shown in FIG. 4. Here the equipotential lines are designated in terms of percentage of the total voltage between the high voltage conductor 12 and the grounded flange 20. It may be seen from this figure that the stress concentration that does result from the relatively sharp change in direction of the equipotential lines occurring adjacent a shield end is located in the solid insulation 25 adjacent the end of inner shield 32 rather than in the dielectrically Weak region adjacent the outer shield 30.

It will be apparent from PEG. 4 that in the region of the mounting flange Ztl, there is no electric stress in the air gap inasmuch as the shields 3t} and 32 on opposite sides of the air gap are equipotential surfaces between which there is no voltage. The voltage is distributed in this critical region entirely between the porcelain insulation of the shell 11 and the solid insulation of the sleeve 25. There is some voltage applied across the air gap in regions longitudinally-spaced from the central part of the bushing, but the stress is so low in these regions as to be non-objectionable. This low stress is indicated by the relatively great spacing between the equipotential lines in these regions of the air gap.

By utilizing the porcelain in the central region of the bushing to assume some of the voltage applied radially between the conductor 12 and the metallic flange 20, we can withstand this total voltage with a smaller diameter bushing than would otherwise be the case. Specifically, we can use a sleeve 25 of a considerably smaller diameter than if there was no insulation between the mounting flange and the shield 32 about the sleeve 25, which is a construction typical of some bushings.

When the sleeve 25 is made of a material, such as irradiated polyethylene, having a lower dielectric constant than the paper-epoxy material depicted, the sleeve 25 assumes a greater share of the electric stress. This results in the equipotential lines adjacent the mounting flange 29 being spaced at greater distance apart than shown in the drawing. The result is a reduced tendency for a dielectric failure to occur externally to the porcelain sleeve 11 in the region of the mounting flange 20 and, hence, a higher basic impulse level. The bushing is designed so that any breakdown that does occur takes place externally to the porcelain shell 11; so it will be apparent that an increase in this external dielectric strength results in an increase in the basic impulse level of the bushing.

To minimize the chances of a breakdown along the surface of the sleeve 25 as :a result of creep tracking due to moisture or other contaminants on the surface, We coat the surface with a track-resistant compound, preferably an acrylic base resin. This resin has a much higher resistance to creek-tracking than the remaining insulating material of the sleeve. The thickness of this coating is on the order of 20 mils. This high track-resistance makes it unnecessary to rigorously seal the interior of the bushing against the entry of moist air from the surrounding atmosphere.

Dielectric tests made on bushings constructed as disclosed in this application have shown that such bushings can consistently withstand impulse voltages of about 214 kv. and can consistently withstand 80 kv. of 60 cycle voltage applied for prolonged periods. Even with moisture frozen on the surface of sleeve 25, the bushing was able to withstand 80 kv. of 60 cycle voltage for many hours. In the tested bushings, the porcelain shell was about 26 inches in length and had an outside diameter of about 7 inches and an inside diameter of about 4% inches; and the sleeve 25 had an outside diameter of 3% inches. In comparison to the 214 kv. impulse strength of the disclosed bushing, a corresponding bushing without the sleeve 25 failed at impulse voltages slightly in excess of 150 kv.

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

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

l. A high voltage insulating bushing comprising a tubular shell of insulating material, a. centrally-located conductor adapted to be energized at a high voltage extending axially of said insulating shell, a tubular sleeve of insulating material closely encompassing said conductor and located within said tubular shell in radially-spaced relationship to said shell so as to leave an air gap between said shell and said sleeve, metallic structure about the outer periphery of said shell adapted to be connected to ground, said metallic structure extending along a short portion of the total length of said shell, a first conductive shield in the form of a coating on the inner periphery of said shell extending along a portion of the length of said shell located axially of the shell in the region of said metallic structure, said first shield being electrically isolated from said metallic structure about the outer periphery of said shell by means of said shell, a second conductive shield on said sleeve near the outer surface thereof extending between points located axially outward of the respective ends of said first conductive shield, said second shield being electrically isolated from said conductor by means of said sleeve of insulating material, said second conductive shield having its axially opposed ends covered by insulating material constituting a part of said sleeve so as to locate the high dielectric stress region adjacent said ends in the solid insulation of said sleeve, conductive bridging means extending across said air gap and electrically interconnecting said two conductive shields, the space between said sleeve and said shell being substantially free of insulating liquid, the voltage in the region of said metallic structure being distributed between said insulating shell and said insulating sleeve when said conductor is energized.

2. The bushing of claim 1 in which said second conductive shield has a portion of its outer periphery uncoated by insulation and exposed to said air gap, and in which said conductive bridging means comprises a conductive spring extending across said air gap and electrically interconnecting said two shields, said spring having one portion bearing against the exposed portion of said second conductive shield and another portion bearing against said first conductive shield, the resilience of said spring maintaining said spring portions in engagement with the respective shields against which said spring portions bear.

3. The bushing of claim 1 in which a circumferentially extending recess is provided in said sleeve to leave exposed a portion of the outer periphery of said second shield and to define a pair of longitudinally spaced-apart shoulders at opposite sides of said recess, a conductive leaf spring embracing said second shield and being captured between said shoulders, said leaf spring comprising at least one projecting portion having a rounded exterior surface with said rounded surface being urged into engagement with said first shield by the resilience of said spring and another portion urged into engagement with said second shield by said resilience whereby to electrically interconnect said two shields, the ends of said spring being spaced from said shields and located in said air gap.

4. A high voltage insulating bushing comprising a tubular shell of insulating material, a centrally located conductor adapted to be energized at a high voltage extending axially of said insulating shell, a tubular sleeve of insulating material closely encompassing said conductor and located within said tubular shell in radially spaced relationship to said shell so as to leave an air gap between said shell and said sleeve, metallic structure about the outer periphery of said shell adapted to be connected to ground, said metallic structure extending along a short portion of the total length of said shell, a first conductive shield in the form of a coating on the inner periphery of said shell extending along a portion of the length of said shell located axially of the shell in the region of said metallic structure, said first shield being electrically isolated from said metallic structure about said outer periphery by means of said insulating shell, a second conductive shield on said sleeve near the outer surface thereof extending axially of the sleeve in generally longitudinally aligned relationship relative to said first conductive shield, said second shield being electrically isolated from said conductor by means of said sleeve of insulating material, said second conductive shield having its axially opposed ends covered by insulating material constituting a portion of said sleeve so as to locate the high dielectric stress region adjacent said ends in the solid insulation of said sleeve, conductive bridging means extending across said air gap and electrically interconnecting said two conductive shields, the space between said sleeve and said shell being substantially free of insulating liquid, the voltage in the region of said metallic structure being distributed between said insulating shell and said insulating sleeve when said conductor is energized.

5. The bushing of claim 4 in which said second son ductive shield has a portion of its outer periphery uncoated by insulation and exposed to said air gap, and in which said conductive bridging means comprises a conductive spring extending across said air gap and electrically interconnecting said two shields, said spring having one portion bearing against the exposed portion of said second conductive shield and another portion hearing against said first conductive shield, the resilience of said spring maintaining said spring portions in engagement with the respective shields against which said spring portions bear.

6. The bushing of claim 4 in which a circumferentially extending recess is provided in said sleeve to leave exposed a portion of the outer periphery of said second shield and to define a pair of longitudinally spaced-apart shoulders at opposite sides of said recess, a conductive leaf spring embracing said second shield and being captured between said shoulders, said leaf spring comprising at least one projecting portion having a rounded exterior surface with said rounded surface being urged into engagement with said first shield by the resilience of said spring and another portion urged into engagement with said second shield by said resilience whereby to electrically interconnect said two shields, the ends of said spring being spaced from said shields and located in said air gap.

7. The bushing of claim 4 in combination with a trackresistant insulating coating on said sleeve made of a material having a higher resistance to creep-tracking than the remaining insulating material of said sleeve.

8. The bushing of claim 4 in which the material of said sleeve has a dielectric constant less than about 4.

References Cited in the file of this patent UNITED STATES PATENTS 2,157,815 Boyer May 9, 1939 2,423,596 Hollingsworth July 8, 1947 2,859,271 Johnson et a1 Nov. 4, 1958 3,055,968 Spiece Sept. 25, 1962 FOREIGN PATENTS 320,903 Switzerland May 31, 1957

Claims (1)

1. 4. A HIGH VOLTAGE INSULATING BUSHING COMPRISING A TUBULAR SHELL OF INSULATING MATERIAL, A CENTRALLY LOCATED CONDUCTOR ADAPTED TO BE ENERGIZED AT A HIGH VOLTAGE EXTENDING AXIALLY OF SAID INSULATING SHELL, A TUBULAR SLEEVE OF INSULATING MATERIAL CLOSELY ENCOMPASSING SAID CONDUCTOR AND LOCATED WITHIN SAID TUBULAR SHELL IN RADIALLY SPACED RELATIONSHIP TO SAID SHELL SO AS TO LEAVE AN AIR GAP BETWEEN SAID SHELL AND SAID SLEEVE, METALLIC STRUCTURE ABOUT THE OUTER PERIPHERY OF SAID SHELL ADAPTED TO BE CONNECTED TO GROUND, SAID METALLIC STRUCTURE EXTENDING ALONG A SHORT PORTION OF THE TOTAL LENGTH OF SAID SHELL, A FIRST CONDUCTIVE SHIELD IN THE FORM OF A COATING ON THE INNER PERIPHERY OF SAID SHELL EXTENDING ALONG A PORTION OF THE LENGTH OF SAID SHELL LOCATED AXIALLY OF THE SHELL IN THE REGION OF SAID METALLIC STRUCTURE, SAID FIRST SHIELD BEING ELECTRICALLY ISOLATED FROM SAID METALLIC STRUCTURE ABOUT SAID OUTER PERIPHERY BY MEANS OF SAID INSULATING SHELL, A SECOND CONDUCTIVE SHIELD ON SAID SLEEVE NEAR THE OUTER SURFACE THEREOF EXTENDING AXIALLY OF THE SLEEVE IN GENERALLY LONGITUDINALLY ALIGNED RELATIONSHIP RELATIVE TO SAID FIRST CONDUCTIVE SHIELD, SAID SECOND SHIELD BEING ELECTRICALLY ISOLATED FROM SAID CONDUCTOR BY MEANS OF SAID SLEEVE OF INSULATING MATERIAL, SAID SECOND CONDUCTIVE SHIELD HAVING ITS AXIALLY OPPOSED ENDS COVERED BY INSULATING MATERIAL CONSTITUTING A PORTION OF SAID SLEEVE SO AS TO LOCATE THE HIGH DIELECTRIC STRESS REGION ADJACENT SAID ENDS IN THE SOLID INSULATION OF SAID SLEEVE, CONDUCTIVE BRIDGING MEANS EXTENDING ACROSS SAID AIR GAP AND ELECTRICALLY INTERCONNECTING SAID TWO CONDUCTIVE SHIELDS, THE SPACE BETWEEN SAID SLEEVE AND SAID SHELL BEING SUBSTANTIALLY FREE OF INSULATING LIQUID, THE VOLTAGE IN THE REGION OF SAID METALLIC STRUCTURE BEING DISTRIBUTED BETWEEN SAID INSULATING SHELL AND SAID INSULATING SLEEVE WHEN SAID CONDUCTOR IS ENERGIZED.

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