US3223773A - High-tension cable insulated with synthetic tape and oil - Google Patents

Description

De 14, 1965 P. G. PRIAROGGIA HIGH-TENSION CABLE INSULATED WITH SYNTHETIC TAPE AND OIL Filed Feb. 27, 1962 INVENTOR. PAOLO Gazz/:INA PQ/AEOG G/A r wwmwmhmN United States Patent O HIGH-TENSION CABLE INSULATED WITH SYNTHETIC TAPE AND OIL Paolo Gazzana Priarogga, Milan, Italy, assignor to Pirelli Societa per Azioni, Milan, Italy, a corporation of Italy Filed Feb. 27, 1962, Ser. No. 175,991

Claims priority, application Italy, Mar. 31, 1961,

5,950/ 61 8 Claims. (Cl. 174-25) The present invention relates to a high-tension cable for direct or alternating current in which the insulation is partly constituted by synthetic plastic material.

In cables of this kind the insulation is generally constituted by helically Wound thin tapes of plastic material and by mineral oil filling the spaces existing between said tapes. As the synthetic material is not permeable to the oil, it is very difficult to carry out a good impregnation, so that air bubbles can easily be retained between the tapes.

As is known, the presence of air in the cable insulation is very harmful, as it can easily ,give rise to the known ionization phenomena which seriously impair the cable life and do not allow the retention of its ability to withstand high tensions. In order to solve the problem of a good oil circulation, the British Patent No. 854,797 provides for instance, in the preparation of the insulation, the use of tapes of synthetic plastic material previously roughened on at least one of their surfaces by means of a suitable treatment. This treatment consists in Obtaining on said surfaces parallel and straight grooves, disposed transversally with respect to the edges of the tape and extending for its whole width. These grooves are provided in great number and have a depth ranging between 1 micron and 10 microns. In this way it is possible to have a good mobility of the oil between thei tapes, as it easily penetrates between the facing surfaces, and the presence of air is eliminated. However, this expedient has some disadvantages which make its application unsuitable and which are in practice due to the method according to which the winding tapes are prepared.

The roughened tapes are in fact obtained by passing a sheet of synthetic material between two rollers, which may be heated, one of which is provided along its periphery with suitable projections which, pressing on the sheet, engrave the grooves, and the sheet is then cut into tapes. This particular method requires the adoption of a special machinery and affects in a considerable way the cost of the tapes. Furthermore, it is to be noted that the sur-face of the tape, owing to the presence of the grooves, has sharp corner-edges, two for each groove.

At the corner-edges, there are high voltage gradient concentrations, and this does not allow the employment of very high service gradients, if the formation of undesired concentrations are to be avoided.

The object of the present invention is to provide a high-tension electric cable for direct or alternating current in which the dielectric is constituted by insulating tapes of synthetic plastic material in con-tact with a film of impregnating oil, and in which the above indicated drawbacks are eliminated.

The cable of Ithe present invention is characterized in that thin separating and insulatin-g layers allowing a very good penetration of the impregnating oil are interposed between the insulating tapes. Said separating layers can be constituted by a winding of individual threads, of thin fabric tapes or by a continuous braid of threads of synthetic plastic material resistance to the impregnating oil of the cable.

Further details of the invention will more clearly appear from the following description and from the at- 3,223,773 Patented Dec. 14, 1965 ICC tached drawing, which shows examples of the invention and in which:

FIG. 1 is a perspective view, with certain parts removed, of a cable core constructed in accordance with the invention; and

FIG. 2 is a perspective View, with certain parts removed, of a further embodiment of cable core constructed in accordance with the invention.

In FIGURE l, the reference numeral 1 indicates the stranded conductor onto which there is wound a screening tape 2 of a known type, such as a conductive tape. Around the screening tape 2 there is a first winding 3 constituted by individual threads of synthetic material. This first winding 3 and the remaining windings 3 constitute thin separating insulating layers, composed of a plurality of individual threads, helically wound at a suitable pitch about the underlying tapes. About the first separating layer 3, covering the screening tape 2, there is then wound an impervious insulating tape 4, and the building up of the cable is carried on by alternating separating layers 3 and insulating tapes 4 until the desired thickness of the dielectric is obtained. The empty spaces existing between the threads constituting the separating layers and those existing between the edges of the tapes 4 are in this way easily filled by the insulating oil which is intended to impregnate the cable.

Instead of resorting to a winding of threads of synthetic plastic material to form the separating layers 3, each individual tape 2 or 4 can be covered with a woven fabric tape constituted by threads of synthetic material, or else each individual tape can be covered with a continuous braid, also of synthetic material, which can be applied with known methods.

FIG. 2 represents a cable core obtained by using a -fabric for the preparation of the separating layers. In said figure 1' indicates the stranded conductor, 2 the screening tape, 3' the separating layers composed of a thin fabric of synthetic material and 4 the imprevious insulating tapes. Also, in this case, a very good penetration of the impregnating oil between the various layers 4 constituting the solid dielectric is obtained.

The plastic material constituting the separating layers 3 or 3 is a synthetic plastic material and in a preferred form of the invention, said material has the same characteristics as those of the insulating tapes 4 and 4' of FIGS. 1 and 2.

Since, as is known, an oil film has a dielectric strength which is the higher the smaller is its thickness, the necessity arises of adopting very reduced diameters and thicknesses respectively for the threads forming the separating layers and for the fabric or braid, in order to minimize the thickness of the oil gap between the tapes 4 and to maintain therefore a high dielectric strength of the oil films existing in the cable insulation.

As a preferred value, a diameter of 0.025 mm. for the individual threads has been found suitable. Therefore, when a fabric is used in lieu of threads to form the separating layers 3', the maximum thickness of said fabric is 0.05 rnrn.

When the separating layers 3 are formed with tapes obtained from a fabric, it has been found convenient to use tapes having a width equal to that of the insulating tapes 4 as shown in FIG. 2 and to overlap them exactly, during the wrapping operation, on the underlying insulating tapes 4.

When the separating layers are formed with individual threads, a certain number of sufficiently close threads is wrapped on the tapes 4 at a suitable pitch in order to cover the underlying tape for its entire width.

The materials to be used for preparing the insulating tapes 4 or 4 and the separating layers 3 or 3 are necessarily those resistant to the action of the oil existing in the cable, and may, for example, be polypropylene, polytetrafluoroethylene, polyethylene terephthalate, polybutylene, polystyrene and polycarbonates. These materials, besides being resistant to the action of the oil at room temperature, are not affected by it even at the high service ternperatures of these cables, which can exceed 80 C. They have, moreover, a high dielectric strength.

Furthermore, polypropylene, polytetraffuoroethylene and the polycarbonates, besides having low dielectric losses, possess a reduced dielectric constant, slightly different from that of the oil and consequently they are particularly suitable for the alternating current cables.

The cables obtained as described above have the advantage of eliminating the inconveniences encountered in conventional cables of the same type. In fact, by means of a suitable selection of the fabric thickness or of the diameter of the individual threads, an oil layer of well defined thickness can be interposed between the insulating tapes 4 or 4' in order to comply with the two requirements of a high value of the dielectric strength and of a sufficient mobility of the oil, which requirements are in conflict with each other.

The individual threads and the threads forming the fabric or braid have surfaces of curvature considerably greater than those existing at the corner-edges of the grooves indicated in the above mentioned British Patent, and this contributes to proportionally reducing the concentrations of electric stresses, thus allowing the adoption of higher working voltages,

Obviously, the cables prepared in accordance with the present invention can be used either for alternating current or for direct current.

It is understood that the above described embodiments of the invention are given only by way of non-limiting examples and that the invention includes other forms of cables based on the foregoing principles and included within the scope of the appended claims.

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

1. A high-tension electric cable comprising a conductor surrounded by layers of fluid impermeable insulating tape of synthetic plastic material, fabric tape disposed intermediate each of said layers and the next layer and separating each of said layers from the others throughout their lengths, said fabric tape having the same width as the insulating tape and being wrapped around the underlying insulating tape in overlapping relation and with the edges of adjacent turns of the fabric tape abutting each other and said fabric tape being formed of individual filaments of synthetic plastic material which are thin relative to the thickness of said insulating tape, said filaments being disposed in a predetermined ordered arrangement throughout the length of said cable with at least portions of their periphery separated from each other and the adjacent insulating tape to provide fluid passageways therebetween and between the adjacent layers, each of which passageways extends from one end to the other end of said cable, and an insulating fluid filling said passageways, said synthetic plastic material being resistant to the action of said fluid.

2. A high-tension electric cable comprising a conductor surrounded by layers of fluid impermeable insulating tape of synthetic plastic material, fluid impermeable individual filaments of synthetic plastic material which are thin relative to the thickness of said tape disposed intermediate each of said layers and the next layer and separating each of said layers from the others throughout their lengths, said filaments being braided in the form of a continuous textile braid and being disposed in a predetermined ordered arrangement throughout the length of said cable with at least portions of their periphery separated from each other and the adjacent tape to ,provide fluid passageways therebetween and between the adjacent layers, each of which passageways extends from one end to the other end of said cable, and an insulating fluid filling said passageways, said synthetic plastic material being resistant to the action of said fluid.

3. A high-tension electric cable comprising a conductor surrounded by a plurality of layers of oil impermeable insulating tape of synthetic plastic material, each of said layers being formed by a single thickness of said tape, at least one oil impermeable, continuous filament of synthetic plastic material which is thin relative to the thickness of said tape disposed intermediate and in contact with each of said layers and the next layer and separating each of said layers from the others throughout their lengths, said filament being disposed in a predetermined ordered arrangement throughout the length of said cable with at least portions of its periphery separated from other portions thereof and from the adjacent tape to provide lengthwise and circumferential fluid passageways between the adjacent layers, each of which passageways extends from one end to the other end of said cable, and an insulating oil filling said passageways.

4. A high-tension cable as set forth in claim 3, wherein a single lsaid filament is intermediate said layers and is helically wound around the inner layer with the turns thereof abutting.

5. A high-tension cable as set forth in claim 3, wherein said filament has a diameter substantially no greater than 0.025 millimeter and the material thereof is selected from the group consisting of polypropylene, polytetrafluoroethylene, polyethylene terephthalate, polybutylene, polystyrene and polycarbonates.

6. A high-tension cable as set forth in claim 3, wherein a plurality of said filaments are interwoven to form a fabric disposed intermediate said layers.

7. A high-tension cable as set forth in claim 3, wherein a plurality of said filaments are braided together and are disposed intermediate said layers.

8. A high-tension electric cable comprising a conductor surrounded by a plurality of layers of fluid impermeable insulating tape of synthetic plastic material, each of said layers being formed by a single thickness of said tape, at least one fluid impermeable, continuous filament of synthetic plastic material which is thin relative to the thickness of said tape disposed intermediate and in contact with each of said layers and the next layer and separating each of said layers from the others throughout their lengths, said filament being disposed in a predetermined ordered arrangement throughout the length of said cable with at least portions of its periphery separated from other portions thereof and from the adjacent tape to provide lengthwise and circumferential fluid passageways between the adjacent layers, each of which passageways extends from one end to the other end of said cable, and an insulating fluid filling said passageways.

References Cited by the Examiner UNITED STATES PATENTS 451,586 5/1891 Williams 174-24 524,066 8/1894 Macfarland 174-120 1,574,076 2/1926 Fisher 156-48 e3 ,077,510 2/1963 Olds 174-25 3,077,514 2/1963 Kang 174-25 X FOREIGN PATENTS 147,915 9/ 1926 Switzerland.

OTHER REFERENCES Uhle: German printed application No. 1,020,075 (1 sht. dwg., 3 pp. spec), 11/1957.

JOHN F. BURNS, Primary Examiner.

JOHN P. WILDMAN, E. JAMES SAX, LARRAME A.

ASKlN, Examiners.

Claims (1)

1. 3. A HIGH-TENSION ELECTRIC CABLE COMPRISING A CONDUCTOR SURROUNDED BY A PLURALITY OF LAYERS OF OIL IMPERMEANBLE INSULATING TAPE OF SYNTHETIC PLASTIC MATERIAL, EACH OF SAID LAYERS BEING FORMED BY A SINGLE THICKNESS OF SAID TAPE, AT LEAST ONE OIL IMPERMEABLE, CONTINUOUS FILAMENT OF SYNTHETIC PLASTIC MATERIAL WHICH IS THIN RELATIVE TO THE THICKNESS OF SAID TAPE DISPOSED INTERMEDIATE AND IN CONTACT WITH EACH OF SAID LAYERS AND THE NEXT LAYER AND SEPARATING EACH OF SAID LAYERS FROM THE OTHERS THROUGHOUT THEIR LENGTHS, SAID FILAMENT BEING DISPOSED IN A PREDETERMINED ORDERED ARRANGEMENT THROUGHTOUT THE LENGTH OF SAID CABLE WITH AT LEAST PORTIONS OF ITS PERIPHERY SEPARATED FROM OTHER PORTIONS THEREOF AND FROM THE ADJACENT TAPE TO PROVIDE LENGTHWISE AND CIRCUMFERENTIAL FLUID PASSAGEWAYS BETWEEN THE ADJACENT LAYERS, EACH OF WHICH PASSAGEWAYS EXTENDS FROM ONE END TO THE OTHER END OF SAID CABLE, AND AN INSULATING OIL FILLING SAID PASSAGEWAYS.

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