Gaseous electrical discharge characteristics in air and nitrogen at cryogenic temperature

In designing superconducting electrical power apparatus, the knowledge of cryogenic gas and liquid insulation characteristics is essential. The authors have studied the discharge characteristics of relatively long-gap configurations in air and nitrogen at a cryogenic temperature. A sphere-to-sphere electrode with a gap length of 20 to 150 mm is used for measurements in uniform electric field. The breakdown voltage characteristics basically obey Paschen's law at cryogenic temperature for 50 Hz, ac, dc and lightning impulse voltage applications. A rod-to-plane electrode with a gap length of 20 to 330 mm is used for measurements in non-uniform electric field. In air at cryogenic temperature and nitrogen gas at both room and cryogenic temperature, streamer-like corona discharge appears near the tip of the rod electrode before the breakdown, and the breakdown voltage increases linearly with gap length. In air at room temperature thin film-like corona discharge, however, appears near the tip of the rod electrode before breakdown, and the breakdown voltage becomes higher than the other case. In order to examine the variation of corona discharge characteristics, some additional experiments are conducted. As a result, it becomes clear that thin film-like glow corona discharge appears when electronegative gas is contained and sufficient electrons are supplied from the cathode.