Patent GB-339990-A (GB)
Title: Improvements in or relating to electric cables
IPC: H01B3/48
Filed: 1929-09-18
Abstract: 339,990. Standard Telephones & Cables, Ltd., Scott, T. R., and Riley, T. N. Sept. 18, 1929. Drawings to Specification. Condensers.-A process for the impregnation of insulating materials such as fibrous insulation with a hydrocarbon oil in the manufacture of high tension cables or electric condensers consists in drying and exhausting the insulation and, before impregnation, replacing the residual air by the vapour of a substance which vapourizes under the temperature and low pressure conditions in the impregnating vessel and which is condensed to a liquid or a solid under the tern. perat,ure and pressure conditions of service. The impregnating vessel may be the lead sheath of a cable, and, preferably, the substance is such as. to condense during impregnation. In one method, a substance is introduced which wholly maintains the normal absolute pressure of the impregnating vessel by vapour pressure, the pressure remaining constant while the limit of vacuum is being reached by the pumping, and the action is stated to be a gradual replacement of residual air in the insulation by vapour. A condenser trap is fitted to the impregnating vessel to pass air and moisture but retain condensible vapour in the vessel. The impregnating medium is admitted to the vessel and the vapour is condensed by a gradual temperature decrease or by forcing in the impregnating medium at a pressure greater than the vapour pressure of the condensible substance at the temperature of impregnation. The insulation is impregnated at a temperature of 130‹-140‹ C. and an absolute pressure of 55 mm. of mercury or less. The pressure head of the incoming impregnating compound may be sufficient to condense the vapour without temperature change, the Specification stating that when a cable is impregnated at 5 mm. pressure and 108‹ C. with diphenyl as the vapourized substance, an oil head of three inches at the surface of the incoming compound is sufficient to condense the vapour. A mixture of vapourizable materials may be used such that the sum of the partial pressures at any temperature gives the desired pressure. The substance may be vapourized in a separate system and introduced as a vapour to the impregnating vessel or may be introduced as a solid or liquid and evaporated. The substance may be sprayed in at reduced temperature as a liquid into the fibrous insulation so that evaporation may occur in the interior of the insulation when the temperature is raised. The substance may be incorporated in a cable during manufacture and evaporated during or after the final drying operations. The substance may be applied to the cable core, and is preferably such as will not evaporate during the initial stages of drying. An example given consists of a copper core which after stranding, is. passed through a bath of a substance which does not evaporate below 100‹ C. at atmospheric pressure. The core is then insulated. The whole fibrous insulation may be impregnated with the vapourizable substance before, or after, application to the cable, and varnished with a retaining medium which may be of a resinous nature. In an example, methyl naphthalene is used as a vapourizable substance, the temperature being raised towards the end of the drying by passing an electric current through the core. Materials which may be impregnated are stated to be paper, textile materials, hydrocarbon oil with or without resins, tars, bitumens, resins and synthetic resins in solvents, cellulose acetate in solvents, sulphur, waxes such as paraffin, ceresin and beeswax, halogenated waxes such as chlorinated naphthalene, and petroleum jellies. Methyl naphthalene and diphenyl, when used as the vapourizable substance, are stated to reduce the dielectric losses of the oil in the finished cable. Diphenyl, methyl naphthalene, and aromatic hydrocarbons may be used as vapourizable materials in order to increase the resistance of the impregnating compound to deterioration. A vapourizable substance may be used which, on condensation, improves the absorption properties of the fibrous material, e.g. by wetting the surface of the fibres, or a substance may be used which forms a protective film on the fibres provided that the film is chemically inactive under electric discharge in the cable. For condensers, a vapourizable material of high specific inductive capacity such as glycol or ethylene glycol may be used. These substances are stated to raise the specific inductive capacity of the impregnating material. The vapourizale material may be an introfier for the impregnating material. It is stated that, for introfying sulphur into fibre boards or asbestos at 135‹ C., naphthalene, diphenyl, and triphenyl are vapourizable substances to be used.