US1977325A - High voltage cable - Google Patents

Description

Oct. 16, 1934. w. PFANNKUCH HIGH VOLTAGE CABLE Filed Dec. 24, 1932 Inventor: Wilhelm FFom'nkuch, b9 m His Abborng.

Patented Oct. 16, 1934 UNITED STATES PATENT OFFICE HIGH VOLTAGE CABLE Application December 24, 1932, Serial No. 648,808

Germany January 7, 1932 3Clalma.

The unavoidable occlusions of air or gas occurring in the course of the hitherto usual methods of making the insulation of compound-impregnated or oil-impregnated high-voltage cables have, when they are permeated by the electric field, a detrimental influence upon the quality of the cable. This trapped air or gas causes phenomena, grouped under the term ionization, which are liable to produce local changes and therefore involve the destruction of the insulation. On the other hand, air and gas bubbles retained in the insulation in as uniform a distribution as possible are desirable from a mechanical point of view because they are easily compressible and allow the oil or the compound to expand when heated.

However the disadvantages of ionization, especially at high working voltages, are greater than the advantage of thermal adaptability, so that a number of proposals have been made to avoid gas occlusions in the dielectric and to give the oil the possibility of expanding with rising temperature. The construction of cable known as oil filled cable provides in principle a solution to this problem; the oil flows through channels, disposed in the cross-section of the cable, towards the ends of the latter and thence into suitable expansion vessels, whereby increases in the volume of oil caused by a rise in temperature are rendered harmless without any great increase in pressure. Oil for cables of this kind must of course be very mobile, but this is less satisfactory on electrical grounds. These channels moreover form a dead space in the cable and in certain circumstances considerably increase the cost of production.

In order to make possible the use of electrically valuable thick oils, it has been proposed to provide for expansion with the aid of media of dissimilar nature, for example, two oils of different viscosity, or oil and gas. Thus for example the insulation oil may be allowed to expand only radially while axial compensation is obtained by means of a thinner fluid oil separated by a flexible partition from the insulation oil. When using gas instead of thin-fluid oil, any damage done to the flexible partition involves the danger of a. considerable volume of gas penetrating into the dielectric. It has been attempted to remove this disadvantage by enclosing the gas in smallcapacity cells in the form of metal cases with diaphragm-like partitions. The manufacture of phragm metal cells of the size under consideration is attended by practical dimculties, especially because the space available in the cable is extremely limited.

The present invention has tor its object to avoid the objections above noted and to provide a cable of improved construction. In carrying out my invention, use is made of air or gas bubbles in the insulation but in such manner that their presence is no longer objectionable but on the contrary highly advantageous. To accomplish the new and improved result, the air or gas bubbles instead of being free to move about in a practically unrestrained manner within the sheath and by uniting form relatively large bodies are each enclosed in an envelope of elastic oil resisting material. As the impregnating fluid of the cable increases in volume due to heating, the gas containing envelopes are compressed thereby compensating for such increase.

In the accompanying drawing illustrative of my invention in a diagrammatic manner, Fig. l is a cross-section of a single conductor cable having a hollow core containing an elastic medium; Fig.

2 is a. cross-section of a single conductor cable having an elastic medium situated between the conductor insulation and the sheath, and Fig. 3 is a cross-section of a multi-conductor cable with an elastic medium in the filler spaces.

The separating layer or envelope for the gas bubbles is suitably made from a rubber-like, oilinsoluble and very elastic insulating agent com- 86 prising the products of polymerization of acrylic acid or a product of polymerization of its derivatives or again a product of polymerization of a homologue alone or in a mixture, if necessary, with an additional material such as paper, mica, etc. 90 Other synthetic and similar insulating agents may however be used. The subject-matter of the invention may be produced for example by introducing air or a chemically neutral gas into the interior of drops of solution of said agent with the aid of a nozzle in order to obtain small and balloon-like elements after the solvent has evaporated. These gas-filled elements can be deformed from their normal shape as desired and are disposed at suitable points of the cable strcture. I In order to make this step an easy stage of manuiacture by machinery, the said elements are suitably disposed in coherent arrangement and to this end they are for example spread over astrip which is impregnated with the said agent in solution and on which they are retained when the solution is dry. In this arrangement the gas cells or elements are fed into a machine and easily placed, for example, in a hollow conductor or in 110 a cross-section in accordance with its use as a.

the so-called filler spaces betweenthe cores of a multiple-conductor cable.

It is also possible to give these expansion means a structure generally similar to that o! spongy rubber. As an example, the material or agent may be 'honeycombed with bubbles by blowing gases therein or by mixing gas-evolving substances therewith. whereaiter the solvent is expelled. By a spraying process this material may be suitably formed into a continuous element and be given conductor filling or as a component led into the machine.

The balloon-like elements or the pores of said spongy material are preferably filled with a neutral gas so that it is possible for said material to be within the electric field, provided the latter is not sufllciently intense to give rise to corona discharges in the gas cells. It is therefore also possible to form a portion of the electric insulation with the layers of gas cells; to this end they may be disposed. in a single-conductor cable for example, between the wound layers of paper on the conductor and the lead covering. In this connection, the high dielectric constant of said agent or material has an advantageous efiect inasmuch as it lowers the stresses to which the gas-filled spaces or cells are subjected. On the other hand these spaces or cells may be entirely relieved from electric stresses even when they are in the electric field; to this end the enveloping material may be rendered slightly conductive by means of filling agents which have a certain degree of conductivity and which may be soot, graphite or metal colloids.

The volume of the hollow elements disposed in the cable per unit of length must be in accordance with the volume of oil contained in the cable and in accordance with the working temperature so that the outer covering of the cable will have a suflicient margin of safety to withstand the amount of pressure which is still exerted. The separating layers formed by said material are subjected to extremely small mechanical stresses because they have no overpressure to deal with. The expansive power of the material is so great that it easily answers the requirements involved by the very small alterations in the volume of the gas bubbles.

Should one or more gas-filled elements become leaky while in service, the volume of gas trans ferred to the oil is so small that it can be absorbed by the oil without making any appreciable alteration to its properties and without lowering the elasticity of the expansive layer as a whole.

In Fig. l, 4 indicates the core over which the wires 5 forming the conductor are stranded. Surrounding the conductor is a wrapping of insulaediating liquid which does not adversely affect the tion 6 such as paper which is impregnated with oil or compound. Surrounding the insulation is an impervious lead sheath '7. Inside of the core are small balloon-like elements .8 of the character hereinbetore described, each filled with gas and sealed one from the other, or the filling may embody other forms 0! my invention.

In Fig. 2, the gas-filled element or elements 8 is or are located between the insulation 6 and the sheath 7.

In Fig. 3 is shown a multi-conductor cable in which 5 indicates the conductors, 6 the insulation thereon and 7 the sheath. In the filler spaces 9 are located the elastic gas-filled elements 10 which extend longitudinally of the cable. The elements may 'entirely or only partly fill these spaces, the remainder of the space, it any, being filled with insulation 01' any suitable character. As previously indicated, the gas cells however formed are desirably very small and numerous so that the failure of one or evenen considerable number will have no serious efi'ect'."

In allot the foregoing constructions, the insulation is impregnated with oil or other inwalls of the gas-filled elements, and said liquid fills all of the spaces within the sheath not otherwise occupied'.

As previously stated, the gas cells are minute, the smaller and more numerous the better. On account of the nature of the cells, accurate illustration thereof is not feasible and hence the drawings are to be taken as illustrative and not as limitations of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. A cable comprising a conductor, insulation thereon, an impervious sheath, a filling of impregnating liquid, and an expansion means within the sheath comprising a polymerized product of the acrylic acid series containing small individual bubbles of gas.

2. A cable comprising a conductor, insulation wrapped thereon, an impervious sheath, a filling of impregnating liquid, and an additional filler for the spaces within the sheath not otherwise occupied comprising a polymerized product of the acrylic acid series containing minute gas bubbles and an agent which is a conductor of a low order.

3. A cable comprising a conductor, insulation thereon, an impervious sheath, a filling of impregnating liquid, and an expansion means within the sheath comprising a polymerized product of the acrylic acid series containing minute individual cells, and a chemically neutral gas filling for the cells.

WILHELM PFANNKUCH.

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