Oil-filled terminals for high voltage cables

Abstract

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 safely 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. Flashover tests and time tests, corresponding to breakdown and endurance tests on equivalent cables, are reported to illustrate the ability of the terminals to withstand factory and field tests on the cables, and to show the ample factors of safety under operating conditions. Results of an experimental installation of the 110,000-volt terminals demonstrate the safety of the design, predicted from calculations and confirmed by laboratory tests. For temporary testing purposes these oil-filled terminals are most convenient and economical, and contribute to the uniformity and reliability of the results in cable testing, which are the factors of greatest importance.