Impulse-strength measurements on compressed-gas insulation for extra-high-voltage power cables

Impulse-flashover measurements on a number of solid insulating materials in compressed-gas atmospheres at pressures up to 500 lbf/in2 are described. From the results of tests made with uniform-field and concentric-cylinder electrode systems, it is concluded that flashover occurs when the breakdown stress of the gas itself is reached at any point. The influence upon gas breakdown of placing thin coverings of solid insulating material in contact with metallic electrodes has also received attention. With a variety of coverings, improvements in impulse strength of approximately 50% were obtained with nitrogen and to a lesser, although still significant, extent with sulphur hexafluoride. The general levels of breakdown stress obtained are considered in relation to possible designs of extra-high-voltage power cables employing compressed gas as the major insulation.