US1846917A - Bushing for high frequency high voltage current - Google Patents

Description

' Feb. 23, 1932. STEVENSON 1,846,917-

BUSHING FOR HIGH FREQUENCY HIGH VOLTAGE CURRENT Filed Sept. 1', 1925 Patented F eb. 23, 1932 UNITED STATES PATE NT OFFICE GEORGE STEVENSON, OF NEW YORK, N. Y., ASSIGNOR TO WESTERN ELECTRIC COM- PANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK BUSHING FOR HIGH FREQUENCY HIGH VOLTAGE CURRENT Application and September 1, 1925. Serial No. 53,774.

This invention relates to'elec'tric insulators of the bushing type and specifically to an insulator which isparticularly well adapted for use in high frequency and high voltage cur.-

a rentsystems such as radio broadcastingsystems and the like. Y

In ordinary high potential electric systems the failure of an insulator is generally due to flash-over, or to fracture of the insulating ma- 10 terial by the disruptive discharge of a high voltage surge, such as may be caused by lightning, or by some sudden change in the circuit conditions. The insulation of high frequency voltages, for example those occurring in a radio transmission system, is made more difficult by the presence of an additional cause of failure, namely the heat generated in the insulating material by the high frequency alternations of electric stress. It has been found 20 in practice that this is the most important cause of the failure of insulators at high frequencies, the heat generated being sufficient to soften the plastic dielectrics or to fracture the vitreous or ceramic dielectrics of insula- 25 tors of the common types.

' An object of the invention is to reduce to a minimum the effect of high frequency energy dissipation in the dielectric of an insulating bushing.

A further object is to prevent damage to the dielectric of an insulating bushing due to flash-over.

The invention accomplishes the objects broadly by the application of the following 5 rules: First, the insulating material and the surfaces of the metallic conductors should be in such geometrical form that the dissipation of energy in the form of heat is approx- 0 imately uniform at all point in the dielectric.

This implies that all points of the dielectric are subject to the same electric stress and requires that the metallic surfaces be carefully disposed so that a region of uniform potential gradient is produced between the conductors at different potentials. It also requires that the dielectric be placed in this region of uniform gradient in such manner that it does not by its presence distort the 50 electric field. In other words the surfaces of the dielectric must be parallel to the lines of electric force between the conductors.

Second, the material of the dielectric should be so disposed that the heat generated at internal points is readily conducted to the surface where it may be radiated to the surrounding air. The material should be thin and of large surface area in order that this may be best accomplished.

Third, to prevent injury to the dielectric by flash-over a safety gap should be provided so'that flash-over due to a sudden excess voltage maytake place in air as far away as possible from the insulating ma- .terial. i In accordance Wlthtllls invention a bushmg is provided comprising a central conducting rod having end discs of conducting material connected thereto, a torus of conductln material encircling said rod and be- .ing he (1 in spaced relation thereto by cylinin air for excess voltages.

Otherfeatures of the invention and advantages will appear from the following description and by the claim appended thereto, reference being had to the accompanying drawings in which:

. Fig. 1 is an assembly view of the bushing partially in section, and Fig. 2 an end view thereof also partially shown in section.

' The bushing made according to this in- 'vention generally consists of a lead-in rod 10 on the ends of which the metal discs 11' and Bare mounted. These discs are held in spaced relation with respect to a centrally disposed metal torus 13 by means of two cylindrical members :14 and15 which are made of a material having low dielectric hysteresis such as pyrex or high grade phenol The metal torus has a flange or fin portion 16 adapted for securing the bushing to the panel or casing from which the conductor is insulated.

Each disc 11 112 is provided a recessed'portion as shown at 17 which serves to hold the associated cylindrical insulators in t eir proper positons. Rings 21, of s t material such as ead be provided to ct as gaskets between the ends of insulating cylinders. and the discs. v 7

Similarly, the metal torus 13 is provided 5 with two oppositely disposed recesses V which the rings 19 and 20 of soft material are alsoinserted. The component parts of the bushing are clamped together by means of nuts threading on the ends of the lead-in rod 10. The threaded ends of conductor 10 serve also as means for connecting the external conductors 22 and 23 to the bushing.

The lines of force of the electric fieldare indicated by the dotted lines in Figure .1.

high voltage currents comprising a metal rod, metal discs mounted on the ends of said rod and each having a groove in its inner face, a ring of soft material in each groove, a flanged metal torus enclrcling sa1d rod and having 1n each iacea groove at the junction of the flange and tubular portion of the torus, a ring of soft material in each'groove, means for securing the flanged portion of the torus toa casing wall, a pair of straight in-- itegtra lcylinders of insulating material thin enough to maintain such material at a substantial-ly uniform temperature throughout its cross section when subjected to high frequency-high voltage currents, each cylinder engaging at one end a ring in a disc and at its other end a ring in saiditorsus, the curvature ef the torus being relatively ilargekand the discs having a substantial'overlap beyond the cylinders a TInWit-ness whereof, I hereunto subscribe my name this 24th day of August, A. D. 1925. GEORGE STEVENSON.

The principal elements determining their contour in the neighborhood of the iinsulatingcylinders are the end :discs and the central panel or casing. If these constituted a system of three "infinitely extended parallel plates,the two discs being at one potential and the panel at a different potentiai, the electric "field would be uniform at all points and the lines of force would be of uniform' 5 I density and parallel.

the discs and of the necessity of having a hole in the panel for thefl'ead-in conductor the field is distorted, but to large extent this distortion is overcome 'in the neighbor hood of the insulators by the relatively large :On account of the limited dimensionsof I curvature of the items '13" and by the ',-pro- 7 vision of a substantial overlap of the end discs 'as'indic'ated "at 18. v

' At the same time the 'cornbinat'ro'nni? the torus 1 3 and the rod IOpassing-through the a center forms a safety gap which has a very definite discharge voltage, particularly ifthe tween the torus and the red. When this is 7 established to provide a proper margin :of safety under operating conditions the other dimensions are readily -'letermined with :a

What is claimed is: r v -Anc=insulatingzbushing "for high 'suflicient degree of accuracy hythesimple rules given :above.

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