US1527250A - Cable for conveyance of electrical energy with graduated insulation - Google Patents

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Feb. 24. 1925- 1,527,250

L, EMANUELI CABLE FOR CONVEYANCE OF ELECTRICAL ENERGY WITH GRADUATED INSULATION FiledNo-f. 22. 1921 Patented Feb. 24, 1925.

UNITED STATES 1,527,250 PATENT OFFICE..

LUIGI EMANUELI, or MILAN, ITALY, AssIeNosv To SOCIETA ITALIANA PIRELLI, on

- MILAN, ITALY.

CABLE FOR CONVEIYANCIE OF ELECTRICAL ENERGY WITH GRADUATE!) INSULATION.

Application led November 22, 1921.

To all whom t may concern.'

Be it known that I, LUIGI EMANUELI, a subject of the King of Italy, residing at Milan, Italy, have invented certain new and useful Improvements in Cables for Conveyance of Electrical Energy with Graduated Insulation, of which' the following is a specification.

At present cables used for the conveyance of electrical energy are almost always insulated by meansot paper impregnated with suitable mixtures possessing high insulating power and great dielectric strength. A long series of investigations has demonstrated the existence of` a deiniterelation between the dielectric strength of impregnated paper and the mechanical permeability of paper to fluids.

The above mentioned permeability, the co-etticient of which may be indicated by A, is determined by means of a special apparatus, in which air may be lutilized as the working Huid. This apparatus may comprise an open-ended glass cylinder, ot known capacity, having its lower'l part immersed in a colored liquid. The air contained in the cylinder is exhausted from the top by means of an aspirator in such a manner that it must penetrate a small disk ot testing paper. The colored liquid then rises in the cylinder and fills the space V left by the air withdrawn. The time t necessary is calculated, and by means of a manometer the suction action I exerted on the paper disk by the aspirator is measured. The c0- eicient A whlch has been adopted to characterize the degree of permeability is determined by the formula PVS# A- im y in which S2 and .s respectively represent the surface and the thickness of the paper disk, while 'n is the coetiicient of viscosity of the fluid (air) employed in making the test. The dielectric strength of the paper can be determined by any of the well known methods.

The principal result of these experiments is, that with the same mixtru-re or oil for impregnating the paper the strength is found to be, up to a certain point, greatest in the papers least permeable to liquids. For example a type of paper which is very permeable A=50,000), is perforated after hav- Serial No. 517,099.

ing been impregnated with oil, at 15,000 volts per man; whilst, with a paper only very slightly permeable (A287 0,000), is similarly treated, a tension of 47,000 volts per m.m. may be attained before perforation. However no further augmentation of dielectric strength is obtained with papers still less permeable.

In order therefore to obtain a high resistance against perforation in cables for conveying electrical energy, at very high tension it would be necessary to use only paper possessing a very low degree of mechanical permeability to liquids or at least not beyond a certain limit (A=400.000). But this not only prolongs the time for the impregnating operation very considerably but it also results in a considerable increase in the net cost, and furthermore it is necessary to conduct this operation with the greatest care, in order that the paper shall not be exposed to a deterioration due to a long immersion in the mixture at a high temperature. Now since with high tension cables the distribution of the electric eld is such that the risk to which the insulation is exposed is greatest in the parts of the insulating cover which are in closest proximity to the conductor, the use of paper possessing a low degree of permeability may be confined to the layers which are closest to the conductor, whilst for the outer layers a paper of higher permeability may be used. By thse means it is possible to benefit by the 'result obtained during the course of the experiments mentioned above thus realizing an economical technological advance. y

The object of this invention is a new type of high tension cable insulated by means ot impregnated paper, in which, Vfrom the conductor outwards, papers are employed o t constantly increasing mechanical permeability to the oil or other liquid insulating material. Owing to this graduation, the insulation which is in immediate cont-act with the conductor possesses a. high dielectric strength, while this strength decreases towards the exterior layers; said exterior layers however, possessing great permeability. do not excessively prolong the duration of the impregnation of the cable. Y

The single figure of the accompanying drawing is a lsectional view of a cable constructed in accordance with the invention: l designating the conductor, 2 the innerl paper layers of high dielectric strength, 2? the outer paper layers of low dielectric l strength, and 3 the protective sheath or coating.

Having now particularly described and ascertained the nature of my said invention and in which Way the same is to vbe performed I declare that what I claim is:

l. An electric cable, comprising a conductor; layers of impregnated paper rolled upon the conductor, the mechanical permeability of said layers to the substance with which they are impregnated increasing from the innermost layers outwards; and a protective sheath encasing said layers.

2. An electric cable provided with a succession of layers of paper impregnated with In testimony whereof I aiiix my signature.

L. EMANUELI.

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