Reduction of Sheath Losses in Single-Conductor Cables

The use of single-conductor, lead-covered cable for high-voltage, three-phase transmission lines results in sheath losses ranging from 25 to 300 per cent of the conductor losses for cables installed in separate ducts, unless special methods for the reduction of the losses are used. Some of these methods, while practically eliminating sheath losses, cause a-c. sheath potentials which may be injurious. In this connection the authors have developed a new scheme of bonding and a new bonding device, which appear to have marked advantages. This article consists of a general discussion relative to the reduction of sheath losses with special reference to laboratory tests and field work done on 132 miles of single-conductor cable of the Commonwealth Edison Company. The economics of sheath losses and of the methods for their practical elimination are discussed. The theories of sheath losses and induced voltages are outlined and correlated, and new formulas and curves are developed. An analytical and graphical comparison of sheath bonding connections is presented. Investigations are reported on tests regarding the nature and extent of possible corrosion of sheaths caused by a-c. sheath voltages.