US2408990A - Bushing insulator - Google Patents

Description

t 8, 1946. J. E. M. MITCHELL 2 408,990

BUSHING INSULATOR Filed July 16, 1941 2 Sheets-Shee t 1 INVENTOR. (1.15M M/fm BUSHING INSULATOR Filed July 16, 1941 2 Sheets-Sheet 2 nh/u. 8 w 0 7 Tnfi a Patented Oct. 8, 1946 UNITED STATES PATENT OFFICE BUSHING INSULATOR John E. M. Mitchell, Parkersburg, W. Va., as-

signor to Porcelain Products, Inc., Findlay, Ohio, a, corporation of Delaware Application July 16, 1941, Serial No. 402,671

3 Claims. 1

The invention described in detail below relates primarily to insulators of the bushing type and more particularly to bushings made of porcelain or other suitable material and employing oil or other insulating media. Such bushings are designed for insulating the conductor which carries electric current through a grounded zone such as the cover of a transformer tank. Under such circumstances there is a concentrated Ilux or potential difference which may give rise to sparkover or even corona formation in quantities sufficient to cause disturbances for example, in terference with the reception of radio frequency currents. Numerous means have been suggested for the purpose of improving the voltage gradient as Well as the distribution of the flux density. Prominent among these is the idea of applying a conductor of relatively low resistance for an extended distance along the high tension conductor. When thi takes the form of a metallic sleeve around the conductor and within the bushing, it adds materially to the complication of the device and its cost. Furthermore, it in turn gives rise to stresses existing between this sleeve and the nearest insulating bafiie or other part of the bushing. Where it has been the practice tobond such a metallic sleeve to the porcelain of the bushing or its baffle, this can be accomplished only at the expense of the strength of the porcelain which in the treatment is materially weakened.

My invention has been developed for the primary purpose of obtaining a high electrical efficiency as well as attaining higher mechanical emciency and greater strength.

It is a further object of my invention to adequately eliminate the generation of voltage of the type causing radio interference.

A still further object of my invention is to obtain a more improved voltage gradient han is present in similar devices.

The method. used in this invention, in addition to greatly improving the voltage gradient and eliminating to a greater degree over stressed areas, actually increases the mechanical strength of the dielectric porcelain members.

Specifically my invention applies a special glaze upon insulating parts of the bushing. Thisglaze is selected for the special purpose of materially reducing the'electrical resistance and this taking the place of any grounding sleeve or other metallic conducting member. This glaze is prin cipally formed of silicates containing. conducting materials and which fires to an integral part of the porcelain body without the reduction of the conducting metallic oxides present, thereby cstablishing intimate connection with the porcelain body and at the same time producing improved voltage distribution without local differences of potential existing between the glaze and the adjacent porcelain. This glaze is moreover high in silica reaching its optimum at approximately so that the resultant compound has a coefficient of expansion conforming closely to that of the porcelain body and thus there is no tendency to crazing or shivering. Glazes of this type are said to fit the porcelain. It is to be understood that when the glaze is thus attached to the porcelain body there results an actual increase in the mechanical strength of the porcelain member. In fact the glaze becomes an integral part of the porcelain. p

In respect to the conducting character of the silicate glaze it is to be noted that the resistance betweenpoints one-half inch apart range from one-tenth up to fifty megohms which is an exceptionally low resistance compared with comparable electricalporcelain glazes.

The application of this conducting glaze may be made to any portion of the insulator where there is likelihood of over-stressing or abnormally high voltage gradient. In the example illustrated on the attached drawing I have shown it applied to several areas any one of which may in itself be the locality of sparkover or discharge.

The protection of any one of these areas is beneficial by itself. At the same time the protection of all of these areas is an added advantage and improvement in operating characteristics.

An important area to be protected by the conducting glaze from overstressing, as has already been suggested, is the surface of the bushing and specifically its internal baille, where it surrounds the high tension conductor. I have obtained improved voltage gradient as well as distribution of stresses by giving the inner surface of the baffle surrounding the conductor a uniform and complete coverage of the conducting silicate glaze.

Another area likely to be affected by abnormal voltage characteristics, is the outer surface of the insulating bushing extending downwardly from the mounting flange to below the surface of the oil surrounding the bushing. By applying a glaze of good conductivity over the outer surface of the bushing and extending this from below the surface of the liquid upwardly to the support in. g flange or other structure I avoid the possibility of a discharge between any part of the bushing and the adjacent areas.

This eliminates the necessity of a special outer grounding sleeve and also grading down the conductivity of the latter at it lower extremity as in Rorden 2,209,003. It also eliminates the necessity of providing special flux control structure at the lower end of the grounding sleeve as in Figure 3 of Rorden 2,188,417. In these prior structures, the lower ends of the grounding sleeves are maintained at ground potential, and the entire voltage drop between the high tension conductor passing through the insulator bushing and ground is concentrated within the oil space between the lower end of the ground sleeve and the lower terminal of the high tension conductor. The conducting glaze of the present invention provides for a different distribution of the voltage drop between the lower terminal of the high tension conductor and the supporting flange of the insulator, and the present invention is distinguished from these prior structures by the fact that instead of concentrating the entire voltage drop below the surface of the oil, the glazed outer surface of the insulator bushing provides for a substantial voltage drop along the outer surface of the bushing within the air space between the top of the oil and the supporting flange. The remainder of the voltage drop is taken up below the surface of the oil. This distribution of the potential drop reduces the amount of concentration of voltage at any one point and thereby reduces the tendency for the development of corona discharge.

These bushings contain an insulating oil up to within a short distance from the top.

An electrostatic charge may develop within the bushing and thus subject the space above the oil to an overstress. To reduce this condition I propose to provide an oil reservoir at the top of the bushing. This reservoir will be of conducting material, for example, a metal casting. It will have the same potential as the lead-in conductor and as high as any potential Within the bushing itself. Hence there will be no difference in potential to cause a discharge, Where the size of the bushing does not require a reservoir and the oil level does not come above the outer porcelain wall of the bushing then the latter may have a conducting glaze applied from a point below that oil level upwardly to the metal cap of the bushing and electrically bonded to this metal cap.

In like manner the surface inside of the bushing may be subject to overstress in the area opposite the fiange. To reduce this I may provide a conducting glaze on this surface extending above and below the flange and also on the opposite surface of the inner baifie.

To illustrate my invention I have shown it in its preferred form in the accompanying drawings in which Fig. 1 is a vertical half section of the improved bushing partly reduced in length;

Fig. 2 is an enlarged plan view of the reservoir with the cover removed and Fig. 3 is an enlarged vertical section of the cap on the line 3-3 of Fig. 2.

The bushing is mounted upon a flange I. This flange is of suitable form and shape to be attached to the top of a transformer tank A or the like. The flange has an upstanding collar 2 upon which a gasket 3 rests. The outer tubular bushing shell 4 rests upon the gasket 3 and is attached to the collar 2 by means of a body of cement 5 or the like.

Commencing with the enlargement of the bushing shell 4 which rests upon the gasket 3, and extending to a point below the oil level B of the transformer tank, the outer surface of the bushing 4 is given a glaze 6. This glaze is a silicate specifically one of the silicate compositions which are known to have definitely low resistance. This glaze is fired on the bushing d and becomes an integral part of this. It is characteristic of such fired glazes that the porcelain may thus be given an increased strength of as much as 40% over similar unglazed articles. This glaze is of a silicate type similar to those applied to the other portions of the bushing, with the exception that it possesses conductivity to the degree already mentioned.

Glazes of this type may be prepared by introducing titanium dioxide one or more other metallic oxides in a silicate glaze which would be a non-conducting glaze without the addition of these particular oxides. As examples, titanium dioxide with iron oxide or chromium oxide, or combinations of these oxides, will produce the required conductivity of the glaze. The resistivity of the glaze may be varied between wide limits hy the variation of the content of the titanium dioxide and the other metallic oxides introduced for the purpose of increasing conductivity. Dependent upon the proportions of the oxides, the resistance of the glaze over a one-half inch surface will vary from one-tenth megohm to fifty megoh'ms.

The bottom of the bushing shell 4 is closed by means of a rin I. This is separated from the bottom of the tubular bushing shell 4 by means of the oil-proof gasket 8 made of synthetic resin such as Corprene (from neoprene) or the like and the ring is cemented or otherwise attached to the outer wall of the shell as shown in Fig. 1.

Within the bushing the ring I is extended to form a flange support upon which a gasket 9 rests. A vertical tubular baffle ill rests upon the gasket 9 and extends to the top of the bushing. As the shell and baiiie are concentric, there is thus produced an annular oil chamber H.

The inner surface of the tubular baffle I!) has a conducting glaze it, This is coextensive with the inner surface of the baflle. It is also fired upon the baffle in the manner already indicated. The baflie provides a passageway for the high U311 sion conductor l3.

The metallic cap M is provided to rest upon the shell 4, to center the inner baiiie it, to support the high tension conductor I3 and finally to permit ready inspection of the oil with which the bushing is filled. This cap i4 is supported upon an oil-proof gasket i5 which in turn rests upon the upper edge of the shell 4. The extension l6 of cap [4 is cemented indicated at I! around the upper end of the shell 6.

The cap I4 is substantially cylindrical and provides a chamber l3 which is in communication with the space I i in the shell 4.

One portion of the side wall of the cap 14 is recessed as shown at H! to receive a vert cal gauge glass 20. This gauge glass rests upon the gasket 2| in the seat 22. Its upper end projects through the hole 23 at the top of the cap I4. A passageway 24 connects the lower end of the gauge glass to the chamber H3.

The cap [4 has a plurality of inwardly extending lugs 25 upon which an open spider 20 rests and is bolted by means of screws 21. This open spider 26 rests upon an oil-proof gasket 28 which is thus clamped firmly against the end of the baffle l0 and compresses the latter against the gasket 9 at the bottom of the bushing. In this way the chamber If is rendered substantially leak proof at the bottom and top of the inner baffle.

26 and the top of the cap Another oil-proof gasket 29 overlies the spider l4. A metallic cover 30 fits over the gasket 29 and has a rim 3| fitting within a stepped portion 32 of the side wall of the cap M. Bolts 33 pass through the cover 39 and seal the top of the cap M. A passageway 34 is provided to connect the upper part of the gauge glass 20 with the chamber l8 through gasket 29. A suitable vent may be provided in the cover 3% A spring contact 35 is screwed to the cover 30 by means of screw 36. Y The free end of the contact rests against the inner surface of the bafile H) where it is in contact with the conducting glaze E2. The high tension conductor [3 has a screw-threaded extension 31 extending through a center opening in the cover 39. A nut 38 on the extension 31 supports the conductor I3 on the cover.

A metallic cap nut 39 is also threaded on the extension 31 and by means of the oil-proof gasket 49 seals the passageway through the cover 30.

The usual terminal 4! is formed integrally with the cap nut 39.

This arrangement of a bushing insulator is a great simplification over existing types. In the first place by the use of a conducting glaze on the inner surface of the baffle, it is possible to eliminate entirely the conducting metal sleeve which has ordinarily been used for flux distribution at this point. Additionally the use of the conducting glaze gives added mechanical strength to the porcelain.

The top of the oil adjacent to the bushing is maintained at the same ground potential as the mounting flange by means of the conducting glaze 6. The glaze coatings 6 and I2 extend parallel for a considerable distance and thus distribute the flux in large degree.

The arrangement described in detail above, permits the support of a high tension conductor through a partition such as the cover of a transformer casing, a metallic deck or the like while reducing materially local electrical stresses. Thus the outer shell of insulating porcelain which would otherwise establish a zone of high electrical stress, is sheathed with a conducting glaze 6 in contact with the oil or other media surrounding it. This prevents objectionable electrostatic disturbances.

The same property is possessed by the conducting glaze 12 extending throughout the length of the inner bafile l3. Not only does each con ducting glaze surface perform a useful purpose by itself but also the extension of these surfaces in parallel serves to distribute the stresses between them and avoid electrostatic disturbances.

In some situations, for example where the conducting glaze is carried beneath the level of the oil, a corona glow may still occur if the glaze ends abruptly. Hence the glazed areas may be graded off in conductivity at their edges. This is acwith said housing at complished by providing a zone of intermediate conductivity.

- While I have illustrated by way of example my preferred form of an oil filled bushing insulator having integral stress-reducing facilities, the principle involved in the invention may be embodied in numerous other specific arrangements of details and materials and the invention is to be considered as limited only by the scope of the appended claims.

In the annexed claims the term high resistance path is intended to apply to a path of sumciently high resistance to produce an appreciable voltage drop along its length, as distinguished from a low resistance path provided by the grounding sleeve or metalized sleeve used in the prior art where there is substantially no voltage drop along the length of the sleeve.

What I claim is:

1. In electrical apparatus having a conducting housing containing insulating liquid and provided with an opening above the level of said liquid through which a high voltage conductor extends to a point below the level of said liquid, said conductor being supportedby an insulator bushing mounted in said opening and extending below the level of said liquid, the combination of means for establishing a substantial voltage drop along the outer surface of said bushing between said opening and the level of said liquid, said means comprising a high-resistance path formed solely of a conducting glaze forming the exterior surface of said bushing and having electrical contact said opening and extending to a point below the level of said liquid.

2. Electrical apparatus according to claim 1 wherein said conducting glaze has a resistance value not substantially less than one-tenth megohm and not substantially greater than fifty megohms as measured between points separated by a distance of one-half inch.

3. In electrical apparatus having a conducting housing containing insulating liquid and provided with an opening above the level of said liquid through which a high voltag conductor extends to a point below the level of said liquid, said conductor being supported by an insulator bushing mounted in said opening and extending below the level of said liquid, the combination of means for distributing a substantial portion of the voltage of said conductor along the outer surface of said bushing between said opening and the level of said liquid, said means comprising a conducting glaze forming the exterior surface of said bushing and having electrical contact with said housing at said opening and extending to a point below the level of said liquid, said glaze constituting a resistance path having a resistance value not less than one-tenth megohm as measured between points separated by a distance of one-half inch.

JOHN E. M. MITCHELL.

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