US2263768A - Condenser bushing - Google Patents

Description

J. H. FRAKES CONDENSER BUSHING Filed Feb. 20, 1940 2 Sheets-Sheet 1 \vakk A QQ N 4 3 Q2 Q3 9:

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WlTNESSES: 697% Patented Nov. 25, 1941 CONDENSER BUSHING James H. Frakes, Edgewood, Pa., assignor to Westinghouse Electric 86 Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application February 20, 1940, Serial No. 319,869

2 Claims.

My invention relates to high voltage insulating bushings and it has particular relationship to oil filled bushings of the condenser type.

In a conventional type of bushing a conducting stud extends through a porcelain casing and a number of porcelain insulating tubes are arranged about the stud within the casing. Even though filled with oil such a bushing must be rather thick to provide the required insulating properties and, in addition, the voltage gradient necessarily varies considerably throughout. Because of the difllculty in forming porcelain accurately to a predetermined size, it is difilcult to obtain a uniform and accurate fit between the various insulating tubes.

In accordance with invention, I obtain a condenser effect by providing coatings of conducting material on portions of the inside and outside surfaces of the insulating tubes in a bushing as above described, and preferably the adjacent cOnducting coatings of adjoining tubes are electrically connected together to avoid high stresses on the material in the spaces therebetween. In addition I make the alternate insulating tubes of ceramic and fibrous materials which, because fibrous material may be readily machined, makes possible a more accurate fit. As a result of the condenser effect and a more satisfactory fit between the insulating tubes, it is possible to reduce the thickness of a bushing required to furnish insulation at a particular potential.

It is accordingly an object of my invention to provide a novel insulating bushing supplying increased insulating properties for each unit of space required thereby.

It is another object of my invention to provide an insulating bushing in which the insulating medium includes alternate fibrous and ceramic tubular members.

A further object of my invention is to provide a condenser type insulating bushing in which a conducting material is secured to the surfaces of tubular dielectric members.

Other objects and advantages of my invention will appear from the following description taken in conjunction with the accompanving drawings in which:

Figure 1 is a view in longitudinal section of an insulating bushing built in accordance with my invention;

Fig. 2 is a transverse sectional view of the bushing shown in Fig. 1;

Fig. 3 is a diagram showing the voltage gradient and distribution throughout the bushing shown in Figs. 1 and 2;

Fig. 4 is a diagram similar to Fig. 3 for a bushing having the same dimensions as that of Figs. 1 and 2, but in which no condenser effect is provided, and

Fig. 5 is a diagram similar to Figs. 3 and 4 of a non-condenser type bushing in which liquid filled spaces of considerable thickness exist between adjacent insulating tubes.

In Figs. 1 and 2 is shown an insulating bushing having an outer casing made up of an upper member I and a lower member 2, both of which may be made of insulating material such as porcelain. A conducting stud 3. which ma; be a copper tube or rod, extends axially through the outside casing and has a cap 4 sealed to it near its upper end. The cap 4 is disposed on the open upper end of the upper casing member I with a sealing gasket 5 therebetween. This cap is preferably provided with a plug 6 to facilitate filling the bushing with an insulating liquid. A nut I threaded 0n the lower end of the stud 3 secures the lower casing member in assembled relationship with a mounting fiange or supporting member 8 therebetween. Gaskets are provided at 9 and III to form liquid-tight seals between the two casing members and the flange 8 and between the lower casing member 2 and the stud 3, respectively.

Within the outer casing a fiber insulating sleeve or tube Ii fits around the stud 3. Porcelain insulating tubes I2 and I3 are concentrically arranged around the stud 3 with another fiber tube It disposed between them. While these insulating tubes are shown supported at their lower ends by the lower casing member 2, another suitable supporting means may be employed if desired. The outermost insulating tube I3 is preferably brazed to the supporting flange 8 and held in position in this manner.

A coating 15 of conducting material is disposed on the exterior of the fiber sleeve II for a portion of its length. Similar coats of conducting material I6 are secured to the inner and outer surfaces of the other insulating tubes l2, l3 and it. These conducting coats may be deposited in any 01' a number of different ways as, for example, by the well known metal spray process. The coating IS on the innermost insulating tube II is electrically connected by means of slugs of solder I1 to the conducting coating [6 on the inner surface of the porcelain insulating tube l2. In like manner the coating on the exterior surface of the tube I2 is electrically connected to the coating on the interior of the tube it, and the exterior coating on the tube It is connected to the interior coating on the tube I3. If preferred, other means such as metallic rings or springs disposed between the adjacent tubes may be used instead of the solder H to form the desired connections.

The length of the various conducting coverings l5 and I6 may be varied or graded as shown in the drawings to control the voltage gradient throughout the bushing. The coatings which are electrically connected to each other are preferably formed with substantially equal area since they are maintained at equal potentials.

It will be seen that the metallic coatings and the electrical connections result in a condenser arrangement throughout the bushing in which the insulating tubes form the dielectric between plates. The bushing is preferably filled with an insulating liquid, the primary purpose of which is to prevent creepage along the uncoated ends of the various insulating tubes. The insulating liquid will also aid in cooling the apparatus.

By comparing the graphs shown in Figs. 3 and 4, the advantages of the condenser arrangement in the bushing above described may be observed. Fig. 3 shows the voltage gradient and voltage distribution at a flashover value of 265 kilovolts for a bushing such as shown in Figs. 1 and 2. Fig. 4 shows th same quantities under the same conditions for a bushing of the same size in which no condenser effect is provided. The superiority of the described bushing is indicated by the fact that the maximum gradient to which its insulation is exposed amounts to 285 volts per mi1l,' while the bushing represented in Fig. 4 is exposed to a gradient of 380 volts per mill. In addition, the insulation of the first bushing is more effectively utilized since the gradient to ground amounts to 120 volts as compared to 94 in the second device. When the condenser effect is employed the same size bushing will furnish insulation for much higher voltages or a bushing of smaller dimensions may be utilized to provide the same protection.

Most insulating liquids have a lower specific inductive capacity than solid insulating materials. When an insulating structure is made up of solid and liquid materials in series the liquid will take the greater part of the stress because the voltage is distributed in inverse proportion to th specific inductive capacities of these materials. As a result of the relatively low dielectric strength of the liquid, it is preferred'not to use it directly as a dielectric medium between the conducting coatings which in practical effect are condenser plates. By means of the electrical connections between the adjacent conducting surfaces, such use is avoided. The effect which the omission of these connections would have on the bushing may be observed from Fig. 5 which shows the voltage gradient and voltage distribution throughout a bushing of slightly different dimensions from those previously described and in which an oil gap of considerable thickness exists in series with the other insulation. As shown at A and B on this figure, the oil in these gaps is stressed to a greater extent than the adjoining solid insulation.

In addition to the functional superiority above described, my invention offers advantages in its construction and assembly. Because of the readiness with which the fiber sleeves II and I4 between the porcelain sleeves may be machined, it is a relatively simple matter to secure a close and uniform fit. The ease with which the bushing may be assembled is believed to be evident from the above description.

While I have shown and described a particular embodiment of my invention, it will be evident that modifications may be made therein without departing from its spirit and scope as set forthin the following claims.

I claim as my invention:

1. An insulating bushing comprising a tubular casing of insulating material, an electrical conductor extending through said casing, and a plurality of tubular members of dielectric material disposed within one another about said conductor, the innermost of said tubular members being of fibrous material and disposed in snug engagement with said conductor while the other of said tubular members are alternately of ceramic and fibrous material.

2. An insulating bushing comprising a tubular casing of insulating material, an electrical conducting stud extending axially through said casing, a plurality of tubular members of dielectric material disposed concentrically about said conducting stud within one another, the innermost of said tubular members being of fibrous material and in engagement with said conducting stud while the other of said tubular members are alternately of ceramic and fibrous material, electrical conducting material on the outside surface of said innermost tubular member and on the inside and outside surfaces of said other tubular members, means electrically interconnecting the conducting material on the adjacent surfaces of adjacent tubular members, and an insulating fiuid disposed in said casing about said tubular members.

JAMES H. FRAKES.

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