US2786093A - High voltage electric cables - Google Patents

Description

m 1957 H. THEVENON HIGH VOLTAGE ELECTRIC CABLES Filed March 18, 1953 IHVENTOR.

H NR1 THE vEr/o/f HIGH VOLTAGE ELECTRIC CABLES Henri Thevenon, Lyon, France, assignor to Compagnie Generale dElectricite, Paris, France, a corporation of France Application March 18, 1953, Serial No. 343,043 Claims priority, application France November 18, 1952 1 Claim. (Cl. 174-25) My invention is relative to high voltage electric cables comprising a conductor, the periphery of which does not ofier a smooth or regular surface.

Said invention applies particularly to cables having a conductor formed of a plurality of wires cabled together, or to cables of the type known as segmental, in which the conductor is divided into two or more segments lightly insulated from one another and bound together by a metallic tape which is applied helically with a space between turns.

In such cables, the electric field in the vicinity of the conductor is not uniform.

To avoid this drawback, it is known to arrange on these conductors, a shield, formed by winding or wrapping on their periphery a number of layers of metal-coated or semi-conducting paper, which makes it possible to obtain a more regular conducting surface.

These tapes, however, are not wrapped in tight turns, so that there remains, between their turns, interstices, called joint gaps, which are filled with the fluid-oil or gas used for impregnating the cable insulation or keeping it under pressure.

The presence of these joint gaps in the inner layers of the shield olfers no drawback, since those inner layers are part of the conductor assembly. The last outer layer of the shield, however, constitutes the peripheral surface of the conducting assembly. The joint gaps in this layer, on which the cable insulation is placed directly, are thus in the Zone where the electric field is maximum.

Now it is known that the presence, in the insulation of a cable, of gaps filled with oil or gas has on the breakdown voltage of this cable an effect which is all the worse as they are in a zone having a more intense electric field.

It is also known that this unfavourable effect may be decreased by decreasing the height of the gaps, and it has been recommended, for this purpose, in U. S. Patent No. 2,532,152, to make the insulation for high tension cables, particularly in the vicinity of the conductor, out of very thin paper tapes.

It is thus particularly important, that the height of the gaps between the turns of the first inner layer of the insulating body of the cable be as small as possible.

Now the shields were made, heretofore, either out of metal coated paper or out of semi-conducting paper. The metal coated paper is formed of a paper strip on which a thin metal foil is glued, aluminum foil for instance; to be used as a support for this foil, it is necessary for the paper to be strong enough, and consequently thick enough. In addition, the semi-conducting paper is obtained by incorporating in the paper pulp acetylene or carbon black, but the effect of this addition is to decrease the mechanical strength of the paper, and this decrease in strength has to be compensated for by an increase in United States Patent thickness. These two types of paper, thus are not satisfactory for realizing a shield peripheral layer of small thickness.

An object of the present invention is a new form of embodiment of a cable of the above mentioned type, characterized in that the layer or layers of the shield being formed out of semi-conducting paper of the usual thickness, the first inner layer of the insulating body is formed out of a thin paper tape coated on one face with a thin semi-conducting layer, the face thus coated being applied against the semi-conducting paper in the outer layer of the shield.

Paper provided with a semi-conducting layer is known; it is easily obtained by coating the surface of the paper with a layer of paste with a base of acetylene black or the like, which does not decrease the mechanical strength of the paper in any manner.

When dealing with thin paper, the paper used may be, preferably, a flax based paper, the mechanical strength of which is higher than that of kraft paper.

The appended figures illustrate, by way of example, a form of embodiment of the invention, Figure 1 is a view in partial longitudinal section of a section of cable provided with a shield of the usual type, and Figure 2 is a View in partial longitudinal section, of a cable section according to the invention.

In these figures, l designates the conductor, formed of a number of cabled wires, and the surface of which, consequently, is irregular, 4 designates the tapes of semiconducting paper forming the screen arranged on this conductor, and 2 designates the tapes of insulating paper constituting the insulation of the cable. For the sake of clearness, the papers have been shown to a much larger scale than the conductor and their thicknesses have been strongly enlarged; a small number of paper layers only have been represented; finally, only a half section of the cable has been represented, the whole being symmetrical with respect to the axis xy.

The semi-conducting papers in the shield are separated by gaps 5, and the insulating papers 2 by gaps 3.

In the usual type of embodiment according to Figure l, the tapes of semi-conducting papers are, as explained above, much thicker than the thin insulating papers 2 and the gaps 5 are thus markedly higher than the gaps 3.

In the type of embodiment according to the invention, represented in Figure 2, the first inner layer of the insulating body is formed of paper strips 7, having substantially the same thickness as the paper 2 of the first other layers of said insulating body, and provided on one of its faces with a thin layer of conducting material 9, said first inner layer being arranged so as its coated face be in contact with the semi-conducting paper 4 of the shield. Thus the gaps 8 between the papers in the first inner layer of the insulating body are not substantially higher than the gaps 3 between the paper strips of the other layers of said insulation, and their breakdown strength is as high.

This breakdown strength could be further improved by using, for said first inner layer, a paper whose thickness, including the semi-conducting layer, would be smaller than or at most equal to that of the first other layers of the insulating papers 2.

I claim:

A high voltage cable comprising, in combination, a conductive core; a strip-shaped band of semi-conductive material wound upon said core to form at least a single helical wrapping on said conductor having non-overlap- 3 ping, spaced, adjacent edges; a strip-shaped band of thin insulating paper formed with a semi-conductive surface layer on one face thereof and wound upon said helical wrapping to form a single layer having non-overlapping, spaced, adjacent edges, said semi-conductive surface being in contact with said helical wrapping and covering the spaces between the turns of said helical wrapping; and a plurality of helical layers of thin insulating paper wound upon said single layer of thin insulating paper,

layers.

References Cited in the file of this patent UNITED STATES PATENTS Rist Mar. 14, 1939 Beaver et a1. July 23, 1940

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