Present state of development of a gas-pressurised lapped-polythene cable for E.H.V. transmission

Polythene tape in association with sulphur hexafluoride under pressure is an attractive dielectric for underground transmission cables because it has low dielectric loss, low relative permittivity and high thermal conductivity. Unfortunately, polythene tape has a high coefficient of thermal expansion, and care is consequently needed in the design of the conductor electrostatic screen to achieve adequate electric strength. Because of the low thermal conductivity of sulphur hexafluoride between layers of tape, good contact needs to be obtained by controlling the interfacial pressure in the dielectric wall to achieve good heat transfer. It has been demonstrated by work on models and 650 mm2 275 kV cable that this type of cable is technically feasible