High-voltage Electrical Cables

Abstract

High-voltage electrical cables having tapes of synthetic plastic material which form separating insulating layers. The tapes are formed with protuberances which facilitate the flow of insulating fluid through the cables.

Description

This invention relates to the insulation of high-voltage electrical cables and, more particularly, it concerns novel insulating arrangements for oil-filled electrical cables.

The present invention is especially useful in connection with electrical cables of the type shown and described in U.S. Pat. No. 3,223,773. As described in that patent, such electrical cables, which are suitable for both direct and alternating current, comprise a central electrical conductor and an insulation constituted by a plurality of tapings of synthetic plastic material helically wound about the conductor. An insulating oil such as mineral oil fills the spaces between the tapings.

In order that each of the hollow spaces throughout the tape insulation may be completely filled with oil, appropriate separating insulating layers are interposed between the layers of synthetic plastic tape. These separating insulating layers are each constituted by a helical winding of single filaments, or of a fabric or braid of filaments of synthetic plastic material, resistant to oil and yet able to facilitate the passage of oil along them.

In order to improve the insulating characteristics of these electrical cables and to ensure that all air spaces are filled with insulating fluid, a dry inert gas, for example, nitrogen or sulfur hexafluoride, has been substituted for the insulating oil.

The employment of separating insulating means, constituted by layers made of single filaments or of a fabric or braid of filaments as has been known in the past, may give rise to some disadvantages. For instance, when filaments are used, it is impossible to carry out a rapid-winding operation, without at the same time subjecting the filaments to excessive tension. Moreover, when fabric tapes or braided filaments are employed, they suffer considerable elongation during the winding operation due to their low modulus of longitudinal elasticity.

The present invention provides novel insulating means for separating the layers of synthetic plastic insulating tape in such manner as to facilitate the passage of insulating fluid between the layers, without, however, suffering from the above-described disadvantages.

According to the present invention, there is provided a high-voltage electric cable, suitable for direct or alternating current, in which a dielectric insulation, which is stratified and impregnated with insulating fluid, is constituted by layers of synthetic plastic insulating tapes with interposed separating insulating means. The separating insulating means are capable of providing good penetration of the insulating fluid throughout all of the hollow spaces existing between the layers. The separating insulating means, moreover, are constituted by tapes of synthetic plastic material having a substantially uniform thickness, and provided on at least one surface with a plurality of protuberances distributed uniformly thereabout. Additionally, recesses are provided on the opposite surface of the material in such a manner as to correspond to the protuberances.

Further details of the invention will appear from the following description and from the attached drawing which shows an illustrative example of the invention and in which:

FIG. 1 is a perspective view, with certain parts cut away, of an electric cable constructed in accordance with the present invention;

FIG. 2 is a perspective view showing a portion of a separating layer used in the cable of FIG. 1; and

FIG. 3 is a view similar to FIG. 2 showing a modified configuration of the separating layer.

In FIG. 1, there is shown a stranded electrical conductor 1 on to which is wound a screening tape 2 of a known type, such as a conductive tape. Around the screening tape 2 there is provided a first separating layer 3 constituted by a tape synthetic plastic material. About the first separating layer 3 there is wound an impervious insulating tape 4. The cable insulation is then built up by alternate separating layer 3 and insulating tapes 4 until the desired overall dielectric thickness is obtained.

As shown in FIG. 1, the separating layers 3 are each formed with a plurality of protuberances 5 which are distributed uniformly thereabout. Corresponding recesses, not shown, are provided on the opposite sides of the separating layers 3.

Turning now to FIG. 2, it will be seen that the protuberances 5 (which are shown exaggerated in size), and their corresponding recesses extend over substantially the whole of the surface of the separating layers 3. As previously stated, the separating layers 3 are constituted by tapes of synthetic plastic material, of substantially uniform thickness, wound about the cable in helical fashion. The thickness of the tape comprising each of the layers 3 is generally between 0.04 and 0.20 millimeters. The protuberances 5 have a height ranging between 0.01 and 0.20 millimeters.

While the protuberances 5 and their corresponding recesses may extend over the whole of the tape surface, nevertheless, portions of the surface are preferably left smooth between various protuberances. The spaces defined between protuberances and the intermediate smooth surface portions permit the passage of insulating fluid throughout the cable in which the separating layers 3 are used.

There are several advantages provided by the above-described separating layers 3 of synthetic plastic material when used in electrical cables. Firstly, the modulus of longitudinal elasticity is unaffected so that the tape can be wound under considerable tension without the occurrence of excessive elongation or rupture. Moreover, while the uniform thickness of the tape permits the attaining, after winding, of a high dielectric strength, nevertheless, the presence of the protuberances 5 on the tape surface reduces the modulus of elasticity to compression of the cable insulation in the radial direction. Consequently, radial pressures and the friction between the individual tapes is reduced. It is, therefore, possible to obtain an extremely flexible winding. It will be additionally noted that the protuberances 5 on the tape surface are round. Because of this rounded configuration, the protuberances 5 serve partially to reduce the concentration of electric stresses so that higher in-service voltages can be used.

The plurality of protuberances and associated recesses facilitates the passage of insulating fluid, permitting it to fill all of the hollow spaces existing between the layers of the insulating tapes of synthetic plastic material and thereby eliminating the presence of air or other gases of relatively low dielectric character.

The synthetic plastic materials used for the separating insulating layers 3 are selected from among those which have both good electrical and good mechanical characteristics, as for example, polypropylene, polytetrafluoroethylene, polyethyleneterephthalate, polybutylene, polystyrene, polycarbonates and polysulphones.

By appropriate selection of the impregnating fluid, and the synthetic plastic material for the tapes, the electrical characteristics of the fluid and tapes may be made consistent with each other, and they can be made to exhibit low dielectric losses even at the high operating temperatures of the cable (e.g., 80.degree. C.). In addition, the above-listed synthetic plastic materials have a high dielectric strength. Polypropylene, polytetrafluoroethylene and the polycarbonates, besides having low dielectric losses, have also a reduced dielectric constant which is slightly different from that of the conventional cable insulating oils. Thus, these tapes are particularly suitable for oil-filled cables used with alternating current.

The separating insulating layers 3 are made by passing a film of a synthetic plastic material, selected among the above-listed polymers, between two cylinders, at least one of which is provided with appropriate projections about its surface. The cylinders are heated, and as the film passes between them, the protuberances press into the film to form the desired configuration. Thereafter, the film is cut into tapes for winding about the cable.

FIG. 3 shows a modified insulating layer 3' . As shown, the layer 3' , which is also of synthetic plastic material, is formed with a plurality of protuberances 5 spaced apart from each other and having corresponding recesses on the opposite side of the tape. Interspersed among the protuberances 5 are depressions 5' which are provided with corresponding protuberances on the opposite side of the tape. The combination of the protuberances 5 and the depressions 5' serves to enhance the hollow space-forming characteristics of the tape and also serves to enhance the reduction in modulus of elasticity in the radial direction of the electrical cable with which the tape is used.

Although certain particular embodiments of the invention are herein disclosed for purposes of explanation, further modifications thereof, after study of this specification, will be apparent to those skilled in the art to which the invention pertains. Reference should accordingly be had to the appended claims in determining the scope of the invention.

Claims

What is claimed and desired to be secured by Letters Patent is:

1. A high-voltage electric cable, suitable for direct or alternating current, said cable comprising a central conductor surrounded by a dielectric, said dielectric being stratified, impregnated with an insulating fluid, and constituted by a plurality of layers of relatively smooth, fluid impervious insulating tapes of synthetic plastic material, with interposed separating insulating means which are physically different from said tapes and which permit the penetration and flow of said insulating fluid between the layers of insulating tapes, said separating insulating means being constituted by fluid impervious tapes of synthetic plastic material having practically a uniform thickness, and provided on at least one surface with a plurality of hollow, arcuate faced, fluid impervious protuberances spaced from each other both longitudinally and transversely of the tape and on the opposite surface with a plurality of concave recesses corresponding to said protuberances.

2. A high-voltage electrical cable as in claim 1 wherein said protuberances are distributed uniformly on the surface of the tape.

3. An electric cable as in claim 1 in which the tapes of synthetic plastic material constituting the separating insulating means have a thickness ranging from 0.04 to 0.20 millimeters.

4. An electric cable as in claim 1 in which said protuberances have a height ranging from 0.01 to 0.20 millimeters.

5. An electric cable as in claim 1 wherein the material constituting the separating insulating means is of the same material as the insulating tapes forming said layers.

6. An electric cable as in claim 1 wherein said separating insulating tapes have said protuberances and recesses on both surfaces thereof.