US2298654A - Electric power cable - Google Patents

Description

Patented. Oct. 13, 1942 2,298,654 ELECTRIC POWER CABLE Thomas RobertsonScott, JohnlKrauss Webb, and

Stanley George Foord, London, England, assignors to International Standard.Electric Corporation, New York, N. Y.

No Drawing; Application February 14, 1941, Serial No. 378,956. In Great Britain July 25, 1939 12 Claims. (or. 154-220) This invention relates to the insulation of electrical apparatus, such as electric power cables and accessories.

In British Patent No. 429,611 there is described a method of forming a barrier joint in an electric cable by using a polymerisable material such as styrene. A further development ofthe process ing to which insulation is built up around a joint by winding tapes impregnated with polymerised material such as polystyrene and basting .the tapes at intervals with monomeric styrene in order to cause a certain amount of polymerised material to dissolve and the difierent layers of tape to adhere to one another.

This method has been adapted to the formation of accessories such as stress grading devices (see British Patent No. 506,277)- In United States application Serial No. 344,662, filed July 10, 1940, and assigned to the present assignee, the method has been extended to the use of other solvents for the. polymerised material than the monomer.

The present invention arises out of these preceding methods and it is to be understood that, althoughin the following description polymerised styrene is described as an example of a polymerised material. and. monomeric styrene as an example of a solvent therefor, other polymerised materials and'other solvents may be used.

The present invention has arisen from an attempt to improve the electrical characteristics oi and to seems more easily a barrier action in the insulation built up according to the above mentioned methods. In building up insulation according to those methods it has-been found that.

in order to secure absolute barrier action it has been necessary to apply heat so as to secure thickening of the insulation.

In order to obtain an insulation consisting solely of polystyrene and to eliminate the heating period, an attempt was made to use. the method described in British Patent No. 504,720 with tapes consisting of polystyrene only, but it was found that the surface of tapes already wound became so soft that no definite surface existed upon which a further layer of tape could be wound and it was impossible to make a satisfactory joint.

According to the present invention a method 0 forming a body of insulation around electrical apparatus comprises surrounding the apparatus with solid insulating material in a form shaped to afford interstices therein, filling the interstices with liquid insulating material which is a solvent for the solid insulating material in all proportions and taking precautions to delay the solvent action.

Thus, 'in applying the invention to building up a barrier joint or the like from tapes of polymerised styrene the monomeric styrene is diluted with. an inert solvent such as mineral oil or the is described in British Patent No. 504,720 accordpolystyrene film is coated with a material such as cellulose acetate which provides some resistance to the action of the solvent on the polymer during basting. This delaying action permits of tapes being wound on before appreciable softening of the layer already wound hastaken place. The invention mayalso be applied by packing the space between electrical apparatusg'and a surrounding sleeve with beads, rods, tubes or the like of polymerised styrene and filling up the remaining spaces with monomeric styrene, precautions being taken to delay the solvent action," of the monomeric styrene on the polymerised styrene. The delay action is necessary to permit the monomer to flood the interstices and sweep out residual gases before the solid beads, rods, tubes or the like collapse owingto the solvent action of the monomer.

The filling up of the spaces by monomeric styrene may be carried out by the application of a vacuum pump to one or more orifices in the surrounding casing whilst the monomer is introduced at other orifices.

The delay action may be secured by mixing a certain amount of an inert liquid such as mineral oil with the monomer, but preferably the beads, rods, tubes or the likeare coated with mineral oil. or with cellulose. acetate for this purpose.

In general, insulation built up by the above methods is almost homogeneous and has excellent barrier action andelectric insulating proper-. ties. It is however, somewhat deficient mechanically. To remedy this defect it is advisable to have a fibrous support for the polymerised material such as is afforded by a paper tape.

Thus instead of using tapes of polystyrene,

I tapes of fibrous material coated with polystyrene may be used. If the highest contact of. polystyrene is desired, a body of insulation may be built up by winding layers of paper tape impregnated with polystyrene and coated on both sides with a. film of polystyrene, the means mentioned above being used to delay the solvent action upon the polystyrene.

Inasmuch. as a body of insulation can be built up satisfactorily by the-method disclosed in British Patent No. 504,720, without using any delay a film of polystyrene is incorporated with the im-;

pregnated paper by lacquering it with polystyrene on one side, and the monomer is applied to the uncoated side during the basting, the delay action is aflorded by the paper and the solvent action ofthe monomer on the polymer is delayed sufiiciently to allow of a satisfactory joint being built up. Sufilcient adhesion between successive layers of tape to provide immediate barrier action is however, secured without heating.

Carrying this invention one step further, we find that satisfactory barrier action can be obtained in a very much cheaper manner than by the use of the method of British Patent No. 504,720 by using dry paper tapes coated on one side only with a film of polymerised material. Such tapes are economical to prepare and easy to apply. Before use the paper is vacuum dried. During application the paper is applied with the coated side towards the apparatus around which the insulation is being built up and just sufficient l monomer is basted on the paper to moisten it completely. The paper delays the solvent action of the monomer on the polystyrene coated on the other side of the paper, but the monomer acts upon .the polystyrene film applied upon it so that complete adhesion between layers is secured.

Fairly good impregnation of the paper fibres by a solution of polymer in monomer results and this solution is converted in the course of time into polymer. The impregnation isnot so good as" that secured by the vacuum impregnation used in .he preparation of pre-polymerised tapes according to British Patent No. 454,923. This fact is reflected in electrical tests of power factor and breakdown strength, but for many applications where the very highest quality insulation is not required, advantage may be taken of the cheaper form of dry paper coated with a film of polymerised material.

An intermediate case between the two last described is aiforded by partially impregnating dried paper' tapes by dipping in abath of polystyrene dissolved in solvent e. g. trichlorethylen'e, the bath being fluid enough to ensure penetration or wetting of the paper throughout its thickness. when the solvent is dried off a certain proportion of .olystyrene remains in the body of the paper. By repeating this process several times and scraping one side of the tape each time as it leaves the bath, an impregnated paper coated on one. side with a film of polystyrene is obtained. The quality of this paper is superior to the coated dry paper but inferior to that made by the vacuum impregnation process.

' .are intended to afford barrier action, the built up insulation should be caused to overlap the lead sheath or sheaths of the cable or cables as described in British Patent No. 508,036 in order to preventthe impregnant within the cable leaking between the outer layer of built up insulation and the protective sleeve inserted around the joint according to the usual practice.

Although polystyrene has been described by way of example as the polymerised material to be used in performing the invention and monomeric styrene as the solvent therefor, other polymers such as polyisobutylene, polyethylene or a mixture thereof or co-polymers formed by mixing monomers may be used with a suitable solvent or solvents. Again a natural polymerised material such as rubber or the like may be used.

What is claimed is:

'1. A method of forming a body of insulation around electrical apparatus which comprises surrounding the apparatus with solid insulating material comprising soluble solid material in a form shaped to afford interstices therein, covering said insulating material with a substance to delay solvent action on said solid material and filling the interstices with liquid insulating material which is a solvent for the soluble material in all proportions.

2. A method of forming a body of insulation in a confined space surrounding electrical apparatus which comprises filling the space with bodies of soluble solid insulating material shaped to afford interstices between them, covering said insulating material with a substance to delay solvent action on said solid insulating material and filling the interstices with liquid insiilating material which is a solvent for the soluble insulating material in all proportions.

- 3. A method as claimed in claim 2 in which the steps of covering said insulating material with .a substance to delay solvent action on said solid insulating material comprises coating the bodies of solid insulating material with an insert insulant before placing them in said confined space.

4. A method of building up a body of insulating material around electric apparatus which comprises winding a tape comprising soluble solid insulating material, covering said tape with a substance'to delay solvent action on said solid insulating material and basting the tape at intervals during the winding with a solvent for the soluble material.

5. A method as claimed in claim 3 in which the said soluble solid insulating material is polymerised material and in which said tape is a fibrous tape coated and impregnated with said soluble material.

6. A method as claimed in claim 4 in which.

the substance to delay solvent action on said solid insulating material comprises cellulose acetate applied to said solid insulating material before said tape is wound.

'7. A method of building up a body of insulating material around electric apparatus which comprises winding a tape comprising soluble solid insulating material and fibrous insulating material and basting the tape at intervals during the winding with a solvent for the solid material mixed with an inert insulant which delays the action of the solvent.

8. A method of building up a body of lnsulat ing material around electric apparatus which comprises winding a tape comprising soluble solid insulating material and fibrous insulating material, basting the tape at intervals during the winding first with a substance for delaying the action-of a solvent on the solid material'and then; with said solvent.

9. A method of building up a body of insulating material around electric apparatus which comprises winding a fibrous tape coated on one side with soluble solid insulating material around the apparatus with the coated side towards the apparatus and basting the uncoated side of the tape during the winding with a solvent for the solid material.

10. The method of forming a body of insulation around electrical apparatus which comprises applying a layer of polymerized styrene to one side of a paper tape, applying a layer of monomeric styrene to the other side of said tape, and winding said tape so trated upon said electrical apparatus in overlapping layers.

11. The method of forming a body of insulation 10 around electrical apparatus which comprises vacuum drying a paper tape, applying a layer of polymerized styrene to' one side of said tape, applying a layer of monomeric styrene to the other side of said tape, and winding said tape 50 treated upon said electrical apparatus in overlapping layers.

12. The method of claim 11 in which said paper tape is partially impregnated with polystyrene prior to the application of said layers.

THOMAS ROBERTSON SCOTT. JOHN KRAUSS WEBB. STANLEY GEORGE FOORD.