US1702993A - Dielectric of condensers and the insulation of cables and other conductors - Google Patents

Description

Feb. 19, 1929. 1,702,993

S. G. BROWN DIELECTRIC OF CONPENSERS AND THE INSULATION OF CABLES AND OTHER CONDUCTORS Filed Jan. 27, 1928 ofgwd Patented Feb. 19, 1929.

UNITED STATES PATENT OFFICE.

SIDNEY GEORGE BROWN, OF NORTH ACTON, ENGLAND, ASSIGNOR 0F ONE-HALF TO THE TELEGRAPH CONDENSER COMPANY LIMITED, OF NORTH ACTON, ENGLAND, A

BRITISH COMPANY.

mnnnc'rnre or connnnsnns AND THE INSULATION or CABLES AND ornnn eonnnc'rons.

Application filed January 27, 1928, Serial No. 249,916, and in Great Britain January 19, 1927.

This invention relates to dielectric of c011- densers and the insulation of cables and other conductors carrying high tension current or subjected to large differences of potential. The chief object is to decrease power losses and to prevent the eventual breakdown of the dielectric by long continued action of high potential differences between the two sides of the insulating layer. It has been found that after use for a substantial period the insulating layer, commonly obtained by oil saturated material, such as paper or the like, deteriorates and is liable to break down completely. I have found that such deterioration is caused by the action of a voltage exceeding a predetermined limit and below such voltage the insulation maintains its efficiency, the actionbeing apparently due to an ionizing effect of the voltage upon the material of the insulator associated with a small amount of air trapped in the material, but the method adopted according to this invention is independent of any explanation of the action which is found to take place. The method employed isso to divide the dielectric into two or more layers (according to the voltage to which it is to be subjected) by an isolated conducting layer or layers that the voltage across any one section of dielectric is kept below the level at which thedestructive ionizing action on the insulation is found to be operative. With a dielectric such as is used in ordinary condensers employing oil or wax saturated paper the ionizing potential has been found to be in the neighbourhood of 250 to 800 volts, a potential differences substantially above the level causing more or less rapid deterioration of the insulating properties, with danger of a final complete breakdown. With such dielectric for the condenser or high tension cable the insulation should therefore be divided up by the interposed conducting layers so that from one layer to the next the potential should not to 300 volts.

There have been various proposals to provide the dielectrics of condensers for high potential use with interleaving of conducting sheets, but such interleaving has not been such as is directed to the limitation of the voltage across any dielectric division to a level below ionization and the present invention is based upon the discovery that there is exceed, say, 250

a definite limit below which the voltage must be kept, in order that the final breakdown of the ielectric in high voltage condensers or cables shall not take place owing to continuous ionizing action. In previous proposals interleaving of the dielectric has been suggested for the purposes of more uniformly loading the dielectric and to avoid a tendency towards brush or glow discharge. Such discharge occurs only at voltages far above that contemplated in the present invention, in which the fall of potential across a single dielectric section is below 300 volts.

The condenser according to this invention may be of plate form or rolled, in the latter case one ormore isolated conducting strips being rolled up with the insulating and the ordinary or active conducting strips constitnting the condenser. The isolated conductor may be made by coating a strip or sheet of paper or other insulating material with finely divided carbon, preferably in the form of vegetable black. Carbon in this form, owing to the fineness of its particles, forms an ex tremely thin continuously conducting film. One or both sides of one or more of the usual dielectric sheets or strips of a rolled or other condenser, or layers of insulation of a cable or other conductor, can be coated in this manner. The isolated conducting strips or layers may extendbeyond the ordinary conductors to give greater protection to the latter.

In order that the said invention may be clearly undersood and readily carried into effect the same will now be more fully de-.

scribed with reference drawings in which Figures 1, 2 and 3 illustrate in a diagrammatic form three types of condenser embodying this invention; and

igures 4 and 5 are cross sections of two forms of cable illustrating the use of. the invention in connection with the insulation of high voltage power transmission cables.

Referring to Figure 1, which illustrates a flat or platecondenser, A, A are the active condenser plates connected by bridges a. B, B are the dielectric plates and C, C are the isolated conductors, in the form of thin plates or flat sheets with conducting surfaces.

Figure 2 illustrates a rolled type of condenser with the interleaved strips shown parts 1y unrolled to facilitate illustration. A, A

to the accompanying are the usual or active conducting strips which may be of metal foil, separated by the insulating strips B of paper. C, are the isolated conducting strips rolled up with the other components of the condenser.

In Figure 3 the rolled type of condenser is also illustrated, the isolated conducting strip C being shown as of greater width than the usual active strip conductor A The- B with integleavcd isolated windings C in such number as to bring the voltage across any single layer of the paper or other insulation B below the ionizing potential.

In Figure '5 a three phase cable is illustrated, with three conductors D in a single sheath E and each surrounded by interleaved windings B C between the three covered conductors being insulation B of any suit able type, filling up the space within the sheath.

The isolated conducting layer B, B or B need not in most cases be one of high conductivity so long as it is suflicient to maintain the potential difference across the dielectric layer below the danger limit. The conducting layer, may for example, be in the form of thin metal foil or fine wire, or may even (2011- sist of a surface layer of graphite or amorphous carbon, preferably in the form of vegetable black, on one or both sides of the paper sheet or sheets formingthe basis of the insulator 0r dielectric, if the paper 'niethoglof formation is employed. layer is independent of any outside connec tion and its potential would depend entirely upon the potential difference between the plates of the condenser or the two conductors of the cable (core and sheath) 'at opposite sides of the dielectric. It has been found that the mere increase in thickness of the insulator or dielectric does not greatly increase the potential difference which can permanently be'employed, a high tension across any practicable thickness of dielectric finally causing breakdown by the ionizing action. The only method therefore to prevent such action taking place is to maintain the difference of potential across any layer of dielectric below the ionizing potential and this can be effected by a conducting layer which separates the dielectric although carrying no substantial current.

\Vhat I claim and desire to secure by Let-.

ters Patent of the United States is i- 1. An insulated conductor having layers of paper and a film of finely divided carbon interposed between the layers.

2. An insulated conductor having layers of paper and a film of vegetable black interposed between the layers.

SIDNEY GEORGE BROWN.

The conducting

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