US2484233A - High-voltage bushing - Google Patents

Description

Oct. 1-1, 1949.

H. J. LINGAL Al.

HIGH-VOLTAGE BUSHING Filed May 27. 1945 mw um wm Wm.

. mvENToRs Harry .Il/ngen and Tb omas E Wa.

WITNESSES:

Patented Oct. 11, 1949 S PATENT OFFICE HIGH-VGLTAGE BUSHING Harry i. Lingal, Wilkinsburg,

Pa., house Electric Corporation,

Watts, Forest Hills,

and Thomas R; assignors to Westing- East Pittsburgh,

Pa., a corporation of Pennsylvania `Application May 27, 1943, Serial No. 488,634

This invention relates to bushings for' the insulation of the leads to high-tension electrical apparatus, such as circuit breakers and transformers, and more particularly to a method of, and apparatus for, processing such bushings after assembly and before they are put into use.

Bushings of the type here involved usually comprise a central conductor having an insulated wrapping surrounding it and enclosed in a weather casing, usually of porcelain. The wrapping may comprise alternate layers of dielectric material and metal foil to constitute a condenser to grade the stress between the central conductor and the mounting `ange, which supports the porcelain, and which is to be mounted upon the grounded metal tank of the circuit breaker or transformer. The casing commonly is lled with an insulating gum or a fluid such as oil, and it has been difficult in the past tok properly impregnate the wrapped insulation without entrapping air or moisture which contribute to failure of the bushing.

In accordance with the present invention after the bushing 'has been assembled, the wrapping and the interior of the casing are dried and degassed after which heated degassed oil is introduced into the casing under pressure to completely fill the casing and completely impregnate the insulating wrapping.

It is an object oi the invention to produce e, method of properly processing a bushing to increase its insulating value and to insure that such value is maintained throughout the expected years of service of the bushing,

It is a further object of the invention to provide apparatus for properly processing such a bushing.

Other objects of the invention will be apparent from the following description y.and the accompanying drawing in which the single gure shows a bushing partly in section and the necessary apparatus for processing it.

Referring to the drawing, a bushing 2, which may be of the type disclosed in copending application Serial No. 479,891, iiled March 20, 1943 by H. J. Lingal, now Patent No. 2,372,098, issued March 20, 1945, comprises a hollow central conductor E having a threaded lower end and carries an insulating wrapping 8 secured thereto. The conductor and wrapping are enclosed in a casing formed by upper and lower porcelain shells It separated by a mounting flange structure I2 and maintained in position by an upper cap I4 and a lower plate I6 secured to the respective. ends of the conductor B.

2 Claims. (Cl. 174-31) The mounting iiange structure I2 includes a central flange to be secured about the opening in a transformer or circuit breaker tank, and upper and lower anges to abut the adjacent end faces of the upper and lower porcelain shells, with suitable gaskets, therebetween yto insure gas and fluidtight joints. Similarly,`suitable gaskets are provided between t'he porcelain shells and the cap I4 and lower plate I6. In' the interest of simplicity, such gaskets are not shown, but they may be of the type shown in said copending application.

The joint between' the upper face of cap I4 and the conductor E', as well as lthat between the lower end of the conductor and plate I6, are also ren.- dered gas and fluid-tight, so Vthat the assembled bushing comprises a central conductor and insulating wrapping enclosed in' a gas and fluid-tight casing.

After the bushing'is assembled, it is immersed in water while subjected to an internal air-pressure. If no leaks are apparent, as indicated by bubbles, it is then; baked to facilitate drying.

The dried bushing is then preferably mounted vertically on a treatment bench I8 with the lower threaded end' of thecentral conductor extending into a pedestal 9. It is contemplated that a plurality of bushings shall be undergoing treatment at the same time on said bench I8, although the apparatus for onli,r Qne bushing is shown, in the interest of' simplicity,

A steam lineZIlSupples steam to a manifold 2l common to all bushings to be treated, from which several branches extend upwardly through manually operatedsteam Valves, such as 22, to several bushing pedestals, such ,as l5; ,each in c0mmuncation with the borey of the hollow central conductor of the bushingmountedv thereon.

Steam at about 12.0 C. is admitted t0 saidr core under the controler` a valve 22 to heat the conductor and hence the insulating wrapping. The top of the conductor is closed by any suitable means,r such as theterminal cap I5, and the condensate formed inthe bQre of the Conductor runs down through valve 2,2 'and pipes 2| and 24 kinto a steam trap 26 of .anysutable well-known type. When a predetermined amount. of condensate accumulates insuchsteam trap, float valve 2S, of a usual construcvation,v operates] to permit the discharge of the condensate through drain pipe 3d. Until such amount of condensate accumulates, the trap, with its valve` 28' closed, maintains steam pressure in the bushing conductor.

Simultaneously withthis heating operation, or just following it if desired, the interior of the casing is subjected to avacuumtreatment as iollows.

A threaded outlet opening is provided in the cap i4 and a threaded outlet opening is provided through lower plate I6 to afford communication with the interior of the bushing.

A motor-driven vacuum pump is connected by a riser 42 and horizontally extending pipe 43 to an oil overflow tank 44, to which is connected a header-pipe or manifold 45, extending along the treatment bench. The upper opening of each bushing on the treatment bench is connected to the manifold 45 through individual connections such as the nipple 32, valve 34, union 35, pipe 48 (which preferably is a iiexible hose) oil ow sight glass 41, and valve 46.

The vacuum pump 40 also is connected through a vacuum manifold 49 to several individual branches each including a valve 50, flexible tube 5|, and union 52, valve 38 and nipple 35 which is screwed into the outlet opening in the lower plate I6 of a bushing casing.

By reason of the apparatus and connections thus far described, and with valves 34, 38 and 50 open, the vacuum pump evacuates the bushing casing and the insulating Wrapping. We have found that an absolute pressure of less than 5 mm. of mercury is desirable. The absolute pressure is conveniently read on a simple closed mercury manometer 4|, of Well-known construction. The vacuum treatment is adequate when the absolute pressure in the bushing casing will remain less than 5 mm. of mercury for a half hour or more after being shut off from the vacuum pump. Preferably, the vacuum treatment is applied simultaneously with the steam heating, so that any moisture present Will be vaporized more rapidly and evacuated from the wrapping and casing.

The removal of moisture may be accelerated by admitting dried air or other suitable gas, such as carbon dioxide or nitrogen, and pumping it out again, once or repeatedly, thus utilizing its capacity to absorb moisture as a scavenger to remove moisture from the bushing. The use of carbon dioxide in this manner serves an additional purpose, in that it accelerates the subsequent oil impregnation of the insulating wrap ping, as the solubility of CO2 in oil is much greater than that of air, and the small volume of gas remaining in the wrapping, because of the imperfect vacuum, must be absorbed by the oil as the oil impregnates the wrapping.

Adequate drying may conveniently be determined independent of the absolute pressure, by measuring the power-factor, or insulation loss, of the bushing, such as with the test set disclosed in United States Patent No. 2,130,865, issued September 20, 1938, to Frakes and Watts. A -cycle power-factor reading not exceeding .004 has been found to indicate suicient drying.

It is intended that, in the present case, the bushing casing shall be almost completely lled and the insulating wrapping 8 completely impregnated by an insulating uid, such as oil. To that end, a main oil supply 54 is connected through a motor driven pump 56 to supply oil to the lower part of the reservoir 51. Preferably, an electric heater 58 of a usual type is disposed in the reservoir to heat the oil to a desired temperature, to assist in the removal of water and gases from the oil, and to facilitate impregnation of the bushing by decreasing the oil viscosity.

Also in the reservoir 51 there is provided a motor driven atomizer pump 60, or an equivalent external pump, which draws oil from the lower part of the reservoir and atomizes or sprays it in the space above the oil. Any moisture and gas entrapped in the oil are thereby more readily released and may be pumped out through pipe 6|, including a valve 62, which is connected to the vacuum pump by riser 42. The valve 62 may be closed when it is desired to shut off the top of the reservoir from the vacuum system.

A n oil level regulator 63 of any usual type may be disposed in the reservoir 51 to control the liquid level therein. Such level controller would, of course, operate through suitable contacts to control the motor of the oil pump 58, in any well known manner which it is not considered necessary to illustrate in detail. Or, of course, the level may be regulated manually by valve 64 and oil pump 56 if a liquid level sight-glass 65 is provided, as indicated.

The oil is drawn from the reservoir 51 through pipe 66 which connects with motor driven pump 68, the output of which extends through pipe 69 to an oil manifold 10 and oil pressure gauge 1|; thence from the manifold 10, through several branches to the bushings being treated, such as are represented by valve 12 and pipe 14 to the connections which also serve for evacuation at the lower end of each bushing casing, namely; flexible tube 5| and union 52, which are connected to valve 38 and nipple 36 attached to the bushing.

Upon completion of the vacuum treatment, the vacuum manifold 49 is disconnected from the lower end of the bushing by closing valve 50; then valve 12 is opened (valves 61 and 38 being already open) whereupon heated and degassed oil from reservoir 51 will be forced under pressure into the lower part of the bushing to completely fill it, as indicated by the oil overflowing through the sight glass 41 at the top of the bushing, thence through valve 46 and manifold 45 to the overflow oil tank 44. 'Ihe valve 34 is then closed and a desired pressure is thereafter maintained on the oil by pump 68 to completely impregnate the insulating wrapping. A suitable pressure regulator 18 of any type commonly available is connected across the pipe lines 66 and 10, in a Well known manner to maintain said pressure constant and limit the value thereof.

With the system shown, vacuum is maintained at the upper end of the bushing, during the lling operation to minimize the possibility of absorption of air into the oil. After pressure has been maintained a sufficient length of time to insure complete impregnation, as indicated by measuring the capacitance of the bushing until no further increase occurs, which may be done with the set described in said Patent No. 2,130,865, the valves 22, 38 and 46 are closed and the unions 35 and 52 are removed to permit removal of the bushing from the treatment bench. The bushing may then be laid upon its side with the valves 34 and 38 on top, and the valves and connecting nipples may be removed without substantial loss of oil. The outlet opening in the lower plate I5 is then closed with a threaded plug and suitably sealed, and the outlet opening in the cap I4 is similarly closed and sealed.

As the bushing cools, the oil will contract, and when it reaches room temperature there will be a vacuum space in the cap I4, when the bushing is vertical and the liquid level will be indicated on the sight-glass 19 carried by cap |4. In order to take care of thermal expansion and contraction of the oil under usual service conditions, it is considered desirable to adjust the oil level to `about the level of the outlet opening in cap I4.

gas, such as nitrogen, may be admitted through sampling valve 80, under controlled pressure, to ll such space. It has been found that a pre sure of twenty pr" per rf; or adequate, and this drops, of course, when the cil is drained to bring it to the level of the outlet opening, as above described.

As a result of the treatment described, a confiplete oil lled bushing is obtained which is her metically sealed to insure against the entrance of any moisture or foreign material and the loss of oil, while the bushing is in service. Quite ob viously, variations may be made in the method disclosed and the apparatus described for accoInplishing it, and it is intended that the invention be limited only by the scope of the appended claims.

We claim as our invention:

1. In a high-tension electrical insulator of the bushing type in which a hollow conductor surrounded by an insulating wrapping is sealed in a tubular casing of dielectric material having a scalable outlet opening at each end thereof, the method of processing such bushing after assembly thereof including the steps of introducing steam into said hollow conductor to dry said sure until said wrapping is completely oi1 impregnated, and then sealing said openings.

2. In a high-tension electrical insulator of the bushing type in which a hollow conductor surrounded by an insulating wrapping is sealed in a tubular casing of dielectric material having a sealable outlet opening at each end thereof, the method of processing such bushing after assembly thereof including the steps of introducing steam into said hollow conductor to dry said wrapping, degassing said casing and wrapping by a vacuum applied to said outlet openings, shu ting off said vacuum and introducing oil under pressure through one of said openings to completely ll said casing, maintaining said pressure until said wrapping is completely oil impregnated, sealing said openings, and drawing off a quantity of oil to bring the level thereof, when the bushing is vertical, to a predetermined point, and admitting a relatively inert gas to fill the space above the oil in the bushing.

HARRY J. LINGAL. THOMAS R. WATTS.

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

UNITED STATES PATENTS Number Name Date 1,754,833 Mateer Apr. 15, 1930 1,760,583 Clark May 27, 1930 1,835,922 Zeiss Dec. 8, 1931 1,866,418 Neuhold July 5, 1932 1,941,913 Riley Jan. 2, 1934 2,053,163 Phillips Sept. 1, 1936 FOREIGN PATENTS Number Country Date 666,339 France May 18, 1929

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