US3376379A

US3376379A - High voltage bushing for horizontal mounting with sheds having rims protecting a substantial portion of the creep distance along the surface of the bushing from atmospheric fallout

Info

Publication number: US3376379A
Application number: US624634A
Authority: US
Inventors: Charles F Sonnenberg; James H Frakes
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1967-03-20
Filing date: 1967-03-20
Publication date: 1968-04-02
Anticipated expiration: 1985-04-02

Description

Apnl 2, 1968 c. F. SONNENBERG ETAL 3, 7

HIGH VOLTAGE BUSHING FOR HORIZONTAL MOUNTING WITH SHEDS HAVING RIMS PROTECTING A SUBSTANTIAL PORTION OF THE CREEP DISTANCE ALONG THE SURFACE OF THE BUSHING FROM ATMOSPHERIC FALLOUT Original Filed Aug. 24, 1966 INVENTORS ES Chorles F-Sonnenberga @lvwwg 3% -Jc|mes H. Frokes.

T RNEY United States Patent 3,376,379 HIGH VOLTAGE BUSHING FOR HORIZONTAL MOUNTING WITH SHEDS HAVING RIMS PRO- TEC'HNG A SUBSTANTIAL PORTION OF THE CREEP DISTANCE ALONG THE SURFACE OF THE BUSHING FROM ATMOSPHERIC FALLOUT Charles F. Sonnenberg and James H. Frakes, Pittsburgh, Pa., assignors to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Continuation of application Ser. No. 574,828, Aug. 24, 1966. This application Mar. 20, 1967, Ser. No. 624,634

3 Claims. (Cl. 174-211) ABSTRACT OF THE DISCLOSURE A bushing especially suitable for mounting in a horizontal position in a region of severe atmospheric contamination has alternate major and minor sheds at spaced intervals. Each major shed has a rim portion at the outer periphery thereof which extends in both directions and protects the underneath surfaces of the body of the bushing from vertical fallout; each minor shed has sloping surfaces and a diameter less than that of the major sheds while still extending a sufficient distance from the body portion of the bushing to substantially isolate scintillations formed by wet contaminant and restrict the migration of corona streamers and scintillations from one major shed to another, whereby heat produced by the scintillations is permitted time to dry up wet contaminant and extinguish the scintillations and prevent the formation of a conductive path between major sheds.

This application is a continuation of application Ser. No. 574,828 filed Aug. 24, 1966, which was a continuatiOn-in-part of application Ser. No. 343,369 filed Feb. 7 1964, now issued Patent No. 3,281,525.

In the past, trouble has been encountered with high voltage bushings mounted in a horizontal position, because over a period of time a deposit of contaminant material develops on the surface of a bushing. This contaminant or atmospheric fall out may be due to the presence of dust, smoke, pollen or other dry vegetable matter etc. in the atmosphere. When this deposit of contaminant is wet slightly, such as by fog or condensation, the resistance of the creepage path along the surface of the bushing may become so small that fiashover occurs. This fiashover may result from an ionized path, the formation of which is made relatively easy by the presence of impurities in the water or moisture on the surface.

Furthermore, one prior art bushing intended for horizontal mounting has shallow pockets or grooves between major sheds where collected contaminant becomes wet, and corona or are streamers are formed which make the surface between major sheds conductive.

Our invention provides a high voltage bushing composed of, for example, porcelain, suitable for mounting in a horizontal position, and in which a minimum of surface is exposed to direct vertical fallout; furthermore, the portions of the total surface between major sheds which are exposed to direct vertical fallout are isolated from one another by minor sheds so that electrical terminals at the ends of the bushing will not be short circuited by a column or path of ionized water. At the minor sheds the scintillations and associated heat soon dry up wet contaminant and scintillations are extinguished. Furthermore, a bushing constructed according to our invention will provide control of the path of an ionized stream of Water so that the water will fall through space rather than run across the complete creepage path between terminals.

Still further, our bushings are self-cleaning. This pro- 3,376,379 Patented Apr. 2, 1968 longs the period between necessary cleanings by servicemen, and permits more economical servicing.

In summary, in our invention shown and described herein we provide a bushing with a series of spaced maj r annular flanges or annular sheds of dog bone cross section, to provide a cylindrical rim portion, each of the sheds having an annular groove on the outside of the rim portion thereof which causes some of the deposited COntamination when wet to wash back to the bottom or root of this groove and around the periphery of the rim portion until it falls olT. The undersides of the rim portions have sharp circular edges which will throw off such a wash, particularly when the bushing has a high voltage applied thereacross, and prevent establishment of an ionized path on the protected surface underneath the rim. We provide that approximately 46.5% of the total creepage path is protected from direct vertical fallout by the overhanging rim portions of the major sheds. Between each pair of major sheds there is an annular minor shed of considerably smaller diameter than that of the major sheds, which prevents the formation of a conductive path on the bushing surface between the bases of the major sheds, by causing the wash off of the major rims to be carried around the periphery of the bushing and thrown off. The minor sheds isolate scintillations from the major sheds until heat dries up wet contaminant and the scintillations are extinguished. A bushing constructed according to our invention is characterized by the absence of contaminant trapping pockets on any side of the porcelain, and we can easily achieve a 2.83 creepage to strikedistance ratio between high voltage terminals.

Accordingly, a primary object of our invention is to provide a new and improved high voltage bushing Suitable for horizontal mounting.

Another object is to provide a new and improved high voltage bushing in which the possibility of electrical flashover due to atmospheric contamination and moisture condensation is reduced.

A further object is to provide a new and improved high voltage bushing suitable for mounting in other than a vertical position and having a large creepage to strikedistance ratio.

Still a further object is to provide a bushing having an increased surface flashover distance without necessitating lengthening the body portion of the bushing.

These and other objects of our invention will become more clearly apparent after a study of the following specification when taken in connection with the accompanying drawing, in which the single figure thereof is a crosssectional view through a portion of a bushing according to our invention.

Referring now to the drawing for a more detailed understanding of the invention, there is shown at 10 a high voltage bushing, which may be composed of porcelain, having a plurality of spaced major sheds generally desig nated 11 and a plurality of spaced minor sheds 12 between the major sheds, substantially equally spaced therefrom. The shell or tubular .body portion 13 is seen to have a substantial portion of the entire surface thereof formed with a major and minor sheds. It is seen that each of the major sheds 11 has an annular groove 15 on the outside thereof, that each of the major sheds has a rim portion 16 with sharp

circular edges

17 and 18 on the under sides thereof, and that the thickness of the disk or stem portion 21 is relatively small compared to the width of the cylindrical or rim portion 16, so that the under edges of the rim portions 16 overhang to a considerable distance the walls of the disc portions 21.

While the bushing of FIG. 1 is mounted in a horizontal position, when the bushing is wet some of the deposited contamination is caused to wash back to the root or bottom of the grooves 15 on the outsides of the major sheds 11 and around" the periphery of the shed until the contamination falls off. This prevents all the contamination on the outer surfaces of rims 16 from washing over the edge and down the side of sheds 11 and onto the root or bottom portions 19. Furthermore, the sharp

circular edges

17 and 18 on the underside of the shed rims 16 will throw off some of the wash from the outer surface of the rim, particularly when the high voltage is applied across the bushing, and prevent establishment of an ionized path on the underneath protected surface of the rim, the paths of the wash thrown off

edges

17 and 18 being shown at 25 and 26. The minor sheds 12 prevent shorting out of the surface of the tubular portion of the bushing between the bases of the major sheds by causing the wash off of the major sheds 11 to be carried around the periphery of the bushing and thrown off.

Typical dimensions and operating characteristics of a porcelain bushing manufactured according to the invention would show that the curvature of portion 22 corresponds to the curvature of the arc of a inch radius circle; the distance between the centerline of the first or nearest major shed and the end 23 may be 1 inch; the height of the rim 16 may be /8 of an inch; the groove 15 may be A of an inch deep; the curvature of the groove 15 may be that of the arc of a circle of inch radius; the distance between adjacent sides of adjacent major shed rims may be 1%; inches; the aforementioned sharp

circular edges

17 and 18 may be curved at a 7 inch radius; the underneath curve of the rim where the rim joins the vertical shed portion 21 may be curved at a & inch radius; the curve of the minor shed crests may be 4 inch radius; the distance between the centerline of a major shed and the centerline of an adjacent minor shed may be 1 inches; the diameter or the thickness of the stem or disk portion 21 may be inch; the minor sheds may be /1. inch in height; the width of the rims 16 of the major sheds 11 may be 1 inches; the overall height of the major sheds may be 2 inches; and the distance between centerlines of adjacent major sheds may be 2 /8 inches. This provides for a bushing with an overall length of 29 inches from end 23 to the opposite end of the bushing, not shown, having ten major sheds, nine minor sheds, a total creepage distance of 86% inches nominal with a distance of 82 inches minimum, and a protected creep distance of 40 inches nominal. The creep distance to strike distance ratio is 2.83, and the ratio of protected creep distance to total creep distance is 0.465. The creep to line ground kilovolts equals approximately 1.7 inches per kilovolt when, for example, six of the bushings are used in series on a 500 kilovolt, three phase line.

While a high voltage is applied across our bushing, drops of water just as they fall off the shed at, for example, the edge 18, will enter an electric field of very high strength. The field in question may include or consist of the corona field produced at the sharp circular edge 18, and, since corona results from the ionization of the air adjacent these edges, the corona field is of uniform polarity notwithstanding the fact that an alternating current potential may be applied across the bushing. The result is that a component of force is applied to the drop which tends to move it out of the vertical line of fall and to throw it over onto the slanting surface of a minor shed, as indicated by

lines

25 and 26 indicating the paths of material dropping off the major sheds, under one possible condition of operation of the bushing.

Furthermore, our bushing is characterized by the absence of any shallow grooves or depressions where a pocket of contaminant could collect, become wet and ionized, and emit corona or are streamers which in turn would make conductive an adjacent portion of the surface of the bushing, and perhaps make conductive one or more paths between major sheds.

Sharp as referring to

edges

17 and 18, means as small a radius of curvature as the material of the bushing will permit. For some porcelains, a radius of 7 might be the outer limit.

The term atmospheric fall out as employed herein includes smoke, dust and outer mineral matter, and vegetable matter.

Whereas the bushing has been described as having a shell or tubular portion, it will be understood that the bushing can be solid, if desired. The words body portion as employed in the claims appended hereto includes any tubular, hollow, or solid elongated portion of any suitable shape. Whereas round may be a convenient shape, wash off from, for example, a decagonal shaped body portion would also occur freely. 1

For a further discussion of inventive features of our bushing which provide advantages over any prior art horizontally mounted bushing, reference may be had to a work by Loeb, Electrical Coronas-Their Basic Physical Mechanisms, Univ. of Calif. Press 1965, and particularly to page thereof.

The annular grooves in the outer surface of the rim portions may be omitted if desired, where the self-cleaning feature provided by the grooves is not necessary.

The stated dimensions of the bushing are exemplary and not limiting. Dimensions may be varied to make a bushing fit available space, within limits which permit the advantages of the inventive features to be retained.

Whereas procelain has been described as a suitable material for constructing the bushing of our invention, other suitable materials such as cast resin may be employed, including suitable epoxy resins.

Whereas we have shown and described our invention with respect to an embodiment thereof which gives satisfactory results, it should be understood that the principles of the invention may be applied to other structures and modifications made without departing from the spirit and scope of the invention.

We claim as our invention:

1. As an article of manufacture, a high voltage bushing suitable for mounting in a horizontal position in a region of severe atmospheric contamination and fallout and in a region of substantial moisture condensation, the bushing being composed of insulating material and having an elongated substantially cylindrical body portion, a plurality of major sheds disposed at spaced intervals along the length of the body portion, each of the major sheds including a disk portion extending from the body portion and having around the periphery thereof a rim portion extending in both directions from the disk portion, the rim portion being spaced from the body portion and protecting the surface of the body portion underneath from vertical fallout while the bushing is mounted in a horizontal position, and a plurality of minor sheds, there being a minor shed disposed between each pair of adjacent major sheds, each minor shed having surfaces which slope from the crest of the minor shed to the body portion of the bushing, the minor sheds being of substantially smaller diameter than the inside diameter of the rim portions of the major sheds, the minor sheds having a suificient height in comparison to the height of the major sheds to provide substantially optimum creep distance per unit length while maintaining a sufficient spacing between the major sheds and minor sheds to minimize the possibility of fiashover between a major shed and a minor shed, each minor shed I extending a suflicient distance from the body portion of the bushing to substantially isolate scintillations formed by wet contaminant on the body portion on one side of the minor shed from scintillations formed by wet contaminant on the body portion on the other side of the minor shed, the distance between a major shed and an adjacent minor shed and the width of the rim portions of the major sheds being so dimensioned with respect to each other that moisture thrown off the edge of a rim portion may fall upon the sloping surface of a minor shed whence it is washed onto the adjoining bushing body portion and washed around to the underside of the bushing and discharged.

2. A bushing according to claim 1 additionally characterized in that each rim portion has an annular groove in the outer surface thereof, contaminating material on the outer surface of the rim portion being washed into the annular groove and washed around to the underside of the bushing and discharged.

3. As an article of manufacture, a high voltage bushing suitable for mounting in a horizontal position in a region of severe atmospheric contamination and fallout and in a region of substantial moisture condensation, the bushing being composed of insulating material and having an elongated substantially cylindrical body portion, a plurality of major sheds disposed at spaced intervals along the length of the body portion, each of the major sheds including a disk portion extending from the body portion and having around the periphery thereof a rim portion extending in both directions from the disk portion, the rim portion being spaced from the body portion and protecting the surface of the body portion underneath from vertical fallout while the bushing is mounted in a horizontal position, and a plurality of minor sheds, there being a minor shed disposed between each pair of adjacent major sheds, each minor shed having surfaces which slope from the crest of the minor shed to the body portion of the bushing, the minor sheds being of substantially smaller diameter than the inside diameter of the rim portions of the major sheds, each minor shed extending a sufficient distance from the body portion of the bushing to substantially isolate scintillations formed by wet contaminant on the body portion on one side of the minor shed from scintillations formed by wet contaminant on the body portion on the other side of the minor shed, the distance between a major shed and an adjacent minor shed and the width of the rim portions of the major sheds being so dimensioned with respect to each other that moisture thrown off the edge of a rim portion may fall upon the sloping surface of a minor shed, the annular edges of the underside of each rim portion of a major shed having a radius of curvature as small as permitted by the material forming the bushing, contaminating material washed over the sides of the rim portions while the bushing has a high voltage applied thereacross being thrown ofl? by the electric field adjacent said annular edges of the underside of each rim portion and thrown onto the sloping surfaces of the minor sheds and thence washed to the underside of the bushing and discharged.

References Cited FOREIGN PATENTS 871,851 1/ 1942 France. A.D. 4,450 1892 Great Britain. 214,170 4/ 1924 Great Britain.

LARAMIE E. ASKIN, Primary Examiner.