Oil-Filled Terminals for High-Voltage Cables

Underground cables for transmission of power at 33,000 ft. and above have only recently come into use in America, or received much attention here. In connection with such cables suitable terminals are necessary and present an important problem in high-voltage design. A marked tendency is noted toward the oil-filling of cable joints; terminal conditions make this procedure both logical and desirable. Dielectric strength must first be specified, and should exceed that of the cable; flashover should occur without puncture; lightning voltages should be guarded against in the design. Proper d-c. tests are still undetermined for various combinations of solid and liquid dielectrics, and a rigid practise can not yet be established with assurance. At present, high-voltage cable lines are intended for a-c. operation, and safety factors should be determined for that kind of service. High-voltage d-c. operation may come into practise later, and research in d-c. testing should be pushed. Standard ratings of terminals are proposed, corresponding to the accepted standard ratings for other high voltage apparatus. Consistency with other terminal insulation, such as apparatus bushings and line insulators, is desirable. Cable insulation may eventually experience similar standardization. The method of rating single-conductor and three-conductor cables should be harmonized, and both based on operating-line voltage. Four typical designs of high-voltage cable terminals are, described representing a carefully worked out and effective solution of the problem. These are (a) 37,000-volt three-conductor; (b) 50,000-volt single-conductor; (c) 73,000-volt, single-conductor; and (d) 110,000-volt single-conductor.