High-tension single-conductor cable for polyphase systems

Three-conductor underground cable is limited to a line potential of about 25,000 or 30,000 volts. The transmission and distribution of large amounts of power at this voltage involves heavy investment in copper. The only factor that limits the voltage rating of single-conductor sheathed cable is over-all diameter. Line voltages of 44,000 and 55,000 volts are perfectly practical and mean a saving in copper over three-conductor cable of as much as 70 per cent. Induced sheath voltage or current, depending upon whether sheath is open- or short-circuited, is the only factor in the operation of single-conductor cable that needs serious consideration. In this paper the dielectric, inductive and general line characteristics of three-conductor and single-conductor cable are compared. The advantages and disadvantages of each type of cable are brought out in a way that will aid in deciding the merits of individual problems.