Vacuum Insulation for Cryocables and Its Resistance to Discharges

A test length of a liquid-nitrogen-cooled vacuum-insulated coaxial cable for use in underground power transmission has been studied with respect to its high-voltage insulation properties. The critical element in the cable is the spacer between the conductors, since electrical discharges in the system are easily initiated across this spacer. Previous experiments with voltage sources capable of supplying low currents indicated insignificant discharge damage to the spacer as long as inorganic materials were used. The new results given in this paper show that insulator damage may still be small for discharges with peak current amplitudes up to 40 kA. The discharge current was supplied by an energy storage capacitor bank for a large number of tests with the same spacer. In addition to current measurements, photographic records of the discharges are presented, and an assessment of the damage to the insulator and conductors is given.