US1743268A - Single electric cable - Google Patents

Description

Jan. 14,1930. E. FERRO SINGLE ELECTRIC CABLE Filed Aug. 31, 1927 Patented Jan. 14, 1930 UNITED STATES PATENT OFFICE EBNESTOIERRO, OF TURIN, ITALY, ASSIGNOR T0 SOCIETA ANONIMA C. E. A. T. CON- ID'UTTORI ELETTRICI ED AFFINI, OF TURIN, ITALY, A COMPANY OF ITALY SINGLE ELECTRIC CABLE This invention provides improvements in single conductor electric cables intended to carry high-tension and very high-tension current, said cables being insulated with impregnated paper and enclosed in a leaden sheath.

It is known that for this type of cable, there is an optimum ratio between the diameter D, outside the sheathing paper wrapping, and the diameter d of the conductor properly socalled. According to the gradient'theory, this ratio is D: d=2.718.

In calculating the sectional area requisite for the conveyance of energy of very high tension, it is often found that the diameter of the conductor, corresponding to the sectional area found, is below D/cl 2.718, so that many manufacturers have endeavoured to increase the diameter, by various devices, with a view to obtaining the optimum diameter. These devices consist, generally, in disposing the individual wires forming the conductor round a fibrous or metallic support, or a support of some other material less expensive than the copper constituting the conductor.

It is also known that one of the most serious difiiculties encountered by manufacturers of cables for very high tensions is the determination of the length of time necessary for the complete drying of the paper and the complete impregnation of the same by the insulating mixtures.

To be able to recognize with certainty whether all'the humidity in the paper has been extracted is extremely important because excessive prolongation of the operation may injure the physical properties of the 'paper, whereas imperfect drying would adversely affect the Working efficiency of the cable.

The impregnation is usually effected after the determination of the necessary time, the viscosity of the mixture, the temperature and pressure employed, and the thickness and permeability of the dielectric being known.

However, owing to the large number of variable factors, it is impossible to limit the duration of the impregnation stage to the time strictly necessary, and it often happens that, although an ample time margin has been allowed over the calculated period for the operation, tests reveal places where the impregnation is defectivesometimes over yery extensive zones, and always in proxim- 1ty to the conductor, that is to say, where the efi'ort of the dielectric is strongest. These zones represent dangerous spots destined to (1) A wrapping of paper with a very high degree of porosity is provided round the copper cable, constituting the conductor of the requisite sectional area.

(2) A thin strip of copper, brass or other good conductive material is wound spirally on said conductor, so as to form a cylinder with a diameter at corresponding to the ratio D/d 2.718, in which D denotes the outside diameter of the ordinary insulating wrapper generally used for high tension cables and wound over said metallic spiral and special paper.

The conductor proper is therefore a cable of ordinary construction and suitably calculated sectional area. The thickness of the layer of highly porous paper should be such as to comply with said ratio, the metal spiral strip being connected to the inner conductor at the junctions of the lengths of cable and at the terminals of same.

The turns of said metal spiral strip are arranged to lie quite close together but without being in contact, in order to fulfil the dual purpose of maintaining the requisite flexibility of the cable and of nabling the impregnating mixture to penetrate completely as far as the inner conductor. Paper wrapping, which may consist of several layers is applied over the metallic spiral by the usual methods, until the thickness necessary for the tension for which the cable is designed is attained. Finally, the ordinary leaden sheathing is applied.

The functions of the layer of paper interposed between the conductor and the metallic spiral are as follows I (1) During the drying stage, the paper enables the progress of the operation. to be measured by electrical means and the extraction of moisture to be interrupted at the suitable moment, thereby achieving the dual purpose of not uselessly prolonging a very expensive operation and of not keeping the insulating material at a high temperature longer than necessary. I

The electrical measurements which may be carried out are insulation measurements, or better still, determining the losses of the dielectric by a bridge method with telephonic frequencies. V

The measurement affects the portion which is the most difficult to dry, namely the layer of paper interposed between the metallic Splarl and the'copper conductor. By following the drying process systematically, it is possible to determine the maximum resistance of the insulation, or the minimum dielectric losses, corresponding to perfect drying.

(2) In the impregnation stage it is possible to recognize when the impregnating mixture reaches the layer of porous paper interposed between the conductive spiral and the conductor proper, since a sudden change will be observed in the insulation resistance, owing to the high conductivity of the mixture at the impregnation temperature.

If, instead of the insulation resistance, the capacity between said two members is measured, a considerable increase in the capacity and the losses will be noted when the mixture reaches the layer of highly porous paper.

Since this latter is the last to be moistened by the mixture, the impregnation of the cable proceeding from the outside towards the inv terior, it is known with certainty that, when the porous paper layer between the metal spiral strip and the conductor is impregnated, the whole of the external insulating wrapper is completely saturated with the mixture.

In order to realize this condition, the mixture must be prevented from gaining entrance into the interstices of the copper cable at the ends of the'cable. As already mentioned the very reliable determination of the exact length of the impregnation stage represents a considerable advantage from both the technical and the economic point of view.

(3) When the cable isin use, the layer of highly porous paper interposed between the metal spiral strip and the conductor is not exposed to any electrical stress, because, as already mentioned, the metallic spiral strip is in electrical connection with the conductor proper at the junctions of the lengths of cable and at the extreme terminals thereof, so that said layer is situated between two surfaces of equal potential on the same electrical level.

In carrying out the invention, the edges of the metallic strip constituting the spiral are ably less than it would be on the surface tif I the conductor proper in the absence of the metallic screen represented by the sprial. Moreover, from research and experiments carried on in Italy and America, it appears that, when the surface of the conductive spiral is smooth and perfectly cylindrical, the

stress at the point of maximum gradient is considerably less than it would be if the conductor were artificially thickened, because the surface comprising individual wires wound round a supporting core gives rise to a dielectric stress from 15 to 20% greater than that due to a cylindrical conductor with a smooth surface.

In addition, the layer of highly porous paper forms a kind of reservoir for the insulating mixture, partly balancing the expansions undergone when the cable is in use and preventing any tendency of the expan- 510118 to affect the shape of the leaden sheath with the resulting formation (during the recooling of the cable) of empty spaces caused by the mixture shrinking to its original volume. By way of example, a cable according to the invention is illustrated on the accompanying drawing to which reference will now be made.

Fig. 1 represents a section at right angles to the axis of the cable, and Fig. 2 is an axial view, the Various layers composing the wrapping of the conductor being shown successively detached;

Fig. 3 represents a cross section, and Fig. 4 a partial axial section showing a method of electrical unction between the conductor and the metallic spiral for the junction terminals;

the diameter D resulting from the insulating layer 4, namely the ratio heretofore mentioned, D/d=2.718

A layer 2 of highly porous paper is interposed between the corductor 1 and the strip 3, and a leaden sheath 5 protects the outer insulating layer 4 against external agencies.

In Figs. 3 and 4, the adjacent conductor lengths 1: 1 of the cable are provided at the extremities with terminals 6 6 which are simply rings of conductive material threaded on the conductors, the two ends of which are placed in contact. The metal spiral 3 covers the terminals and the ends of the conductors are shown, and a soldering alloy 8 is poured in through the gap left between the edges of the strip 3, by suitable means, to form a rigid body between the terminals 6: 6 and an excellent contact between the conductors 1: 1 and the spiral 3.

1 Fig. 5 represents an end terminal comprising'a cylindrical sleeve 7 threaded on the completely bare end of the conductor, and flared at one end, namely the end that projects beyond the conductor. poured into this flared portion and also into the spiral gap of the strip 3 surrounding the sleeve 7, to replace the layer of porous paper 2. Said sleeve 7 is provided with a threaded extension for the'usual connections.

Obviously, the junction and end terminals may difler from those shown without departing from the scope of the invention.

What I claim is:

A single conductor electric cable for very high tension electric current comprising a metal core formed in sections, a highly porous paper wound on said core, a metal strip spi rally wound on the said porous paper so that the adjacent edges of the strip are in close proximity but out ofcontact, a ring of con-' ductive material threaded onthe terminal portion of each section of the core and electrically connecting the spiral strip and core, a layer of insulating material on said strip, the ratio between the outer diameter of the second layer and thediameter of said spiralbeing equal to 2.718 and a soldering alloy sur-' rounding the core sections between the conducting rings forming a contact between the spiral and core sections.

In testimony that I claim the foregoing a my invention, I have signed my name.

ERNESTO FERRO.

Soldering alloy 8 is

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