Cables for 200,000 volt Pressures


    FOR several years, research has been made in the laboratories of Callender's Cable Company with the object of designing an underground cable which will withstand the high electric pressures necessary for the economic transmission of electric energy over considerable distances. The new impregnated cable has now undergone a continuous test of 5,000 hours at 200 kilovolts and the application of numerous heat cycles during this period. According to the Electrician of November 19, it has also passed through the official tests in Holland. This cable marks a very notable advance in the transmission of electric energy underground and will be most useful in the neighbourhood of towns and in populous districts. The dielectric is similar to that used in the 'solid' type of cable, but after the installation has been completed, dried nitrogen gas under pressure is admitted to the cable. The pressure used is 200 Ib. per square inch and the gas finds its way along the cable in the narrow space between the dielectric and the lead sheath. All the dielectric is subjected to this pressure and so any void spaces which form in it during the working of the cable must contain gas at this high pressure. The electric strength of the void space will therefore be much greater than if the gas were absent. Very long continuous cables can be made in this way ; no supplementary feed points are required. The strength required to withstand the high internal gas pressure is afforded by strengthening the lead sheath with copper tapes. The cable is treated just like the normal solid type cable during transport and installation. The point of entry of the gas is at the base of two sealing ends, and it is buried to a depth of about a yard. The cross-section of the conductor is about 0-65 sq. in. and the overall diameter is 3-46 in. The losses in the dielectric as compared with a solid cable have been reduced by 25 per cent, and there is no increase in the losses with rise of temperature.

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    Cables for 200,000 volt Pressures. Nature 140, 927–928 (1937).

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