US2320796A - Method of making high tension electric cable - Google Patents

Description

H. H. RACE 2,320,796

METHOD OF MAKING HIGH TENSION ELECTRIC CABLE Filed Feb. 5, 1942 Fig.1.

June 1, 1943.

20 [1,17 Fig.5. /9

5 Inventor:

Hubert H. Race, by x/w ai His Attornev.

Patented June l 1% METHOD OF MAKING HIGH TENSION ELECTRIC CABLE Hubert H. Race, Schenectady, N. Y., asslgnor to General Electric Company, a

New York corporation of Application February 5, 1942, Serial No. 429,646

2 Claims.

My inventionrelates to high tension electric cable and particularly to an improved method of making the same.

It has long been known that high tension stranded cables insulated with fibrous material are subject to.ionization and corona eflects which manifest themselves at different places in the cable and specifically in the region of the outermost layer of the strands. This, I have ascertained, is due to the fact that the insulation which is wrapped around the conductor does not fill the outermost strand spaces but on the other hand, with the strand defines approximately V-shaped spirally disposed spaces extending lengthwise of the cable. Upon dissecting cables as ordinarily constructed, it will be found that in many cases the insulation has turned darker in the region where it contacts the peripheral surface of the strand. Such a darkening of the paper is evidence of ionization of the oil or compound contained in the V-shaped spaces which are the region of maximum electrical stress.

It has been proposed to prevent ionization and corona in the region of the V-shaped spaces above mentioned by various procedures and in my prior Patent 2,195,998, dated April 2, 1940, and assigned to the same assignee as the present invention, I disclosed and claimed a method of making high tension electric cable in which layers of insulation such as spirally wrapped absorbent paper are forced into the strand spaces while moist by means of dies. This paper is held in place by means of other layers of insulation, such as spirally wrapped layers of insulating paper, so that the strand spaces are completely filled. After the application of the outer layers of insulation and a sheath the paper is treated by drying and impregnation, if desired. While the method disclosed and claimed in my above mentioned patent oiiers numerous advantages, it has been found that the utilization of dies for forcing the moistened insulation into the strand spaces has a tendency to produce bare spots on the conductor surface if the conductor is irregular, that is, has high spots. The resulting thin spots or holes in the inner layer of fibrous insulating material impair the uniformity of the electrical properties and strengthoi' the cable. In accordance with the teachings of my present invention the strand spaces as well as the butt spaces between the inner-layers of insulation are filled while irregularities in the stranded cable are prevented froin damaging the inner layer of insulation.

the application of fibrous insulating material to a length of stranded cable, the subsequent moistening of the insulating material and finally the wrapping of insulation over the moistened material; Fig. 2 is a diagrammatic view in section or the usual construction of paper insulated cable; Fig. 3 is a cross sectional view of a cable manuiactured in accordance with my improved method; Fig. 4 is a greatly enlarged view showing a portion of the cable of Fig. 3, and Fig. 5 shows a length of sheathed cable with a vacuum pump for removing moisture and air therefrom.

Referring first to Fig. 2 which illustrates the usual form of cable, comprising stranded conductors in, a plurality of layers or wrapped insulating material H, such as paper, and a sheath i2. As illustrated in Fig. 2, the spaces between adjacent strands and the surrounding pap r are substantially V-shaped and are filled with gas or oil in the case of fiuid filled cable or with compound in the case of solid type cable. In Fig. 1 I have illustrated the manner in which these strand spaces are filled in accordance with my present invention so that the strand spaces are filled with a uniform mass of fibrous insulating material which is later preferably impregnated with oil or compound. As shown in Fig. 1, a stranded cable 13 is covered with a layer of fibrous insulating material and in the embodiment illustrated is wrapped with two or more layers H of absorbent or low density dielectric paper which is moistened by means of a suitable nozzle I 5 which discharges water, steam, or suitable vapor on the strands it to soften them. The paper It may also be creped before application. Instead of passing the moist paper through dies at a time when it i mechanically weak due to the moisture and subject it to tearing particularly by the action of high spots on the'cable and the cooperating faces of the dies. I apply outer layers of insulating material such as layers It of spirally wrapped insulating paper under suiilcient tension to force the moistened paper layers H into the strand spaces and also to force the outer layers of moistened paper into the butt spaces of the inner layers of moistened paper. By forcing the moist insulating material into the strand spaces by the tension or the outer layers of paper it is possible to completely fill the strand spaces without tearing the moist paper and in this way avoid holes in the first layers of paper. The structure resulting from my im- 7 proved process is illustrated in Fig. 3 and in a Referring now to the drawing. Fig. 1 illustrates 55 greatly enlarged view in Fig. 4. In Fig. 3 the conductors III of the cable I! are shown with the spaces between the outer layers thereof complete- 1y filled with the insulating paper layers H which have been deformed to provide strand space fillers [1. As many layers of insulating paper It are applied over the moistened insulating material as desired for the voltage rating of the cable. The wrapped conductors are then enclosed in the sheath II as shown in connection with the usual type of cable of Fig. 2. In the greatly enlarged view of Fig. 4 the manner in which the layers I4 are compressed to provide the strand space filler i1 is shown more clearly. While I have illustrated my invention as applied to cables having round strands, it is also applicable to cables of the compact strand type. In the latter case the strand spaces are somewhat smaller but the edges are sharper and regions of high potential gradient result in the adjacent dielectric.

In Fig. 5 is shown, by way of example, means employed for removing air and moisture from a sheathed cable. In this figure l8 indicates a sheathed cable having sealing caps I!) at its ends, one of which is connected to a vacuum pump 20 of any suitable construction. If it is desired to evacuate the cable before applying the sheath, it may be done in accordance with the usual method.

After removal of the moisture the cable is preferably impregnated with oil or compound so that the strand spaces are filled with a homogeneous mass of oil and fibrous insulation or compound and fibrous insulating material.

While I have described what I at present consider the preferred embodiment of my invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from my invention, and I, therefore, aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure y Letters Patent of the United States is:

1. The method of making a high potential stranded electric conductor comprising assembiing bare metal strands in concentric layers to form the conductor with small outwardly opening spaces between the strands of the outer layer, spirally wrapping a thin body of relatively dry fibrous material over the strands, thereafter adding sufficient moisture to the dry body to render it soft and pliable, wrapping additional insulation over said moistened body under suflicient tension to cause said thin body of fibrous material to substantially fill the strand spaces of the outer layer of strands, removing moisture from the body and the insulation, impregnating the fibrous body and the insulation, and enclosing the insulation in an impervious sheath.

7 2. The method of making stranded electric cable which comprises spirally applying over the bare strands of the conductor a body of fibrous paper, moistening the paper so applied to soften it. wrapping insulation over the moist paper under suflicient tension to force a portion of the paper into the strand spaces and reduce the thickness of the softened paper overlying the I

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