Electrical insulation characteristics in nitrogen gas and air at cryogenic temperatures

In designing superconducting power apparatus, gaseous insulation characteristics at cryogenic temperature are essential as well as liquid ones. The authors have studied the discharge characteristics of relatively longer-gap configurations in air and nitrogen gas at cryogenic temperature. A sphere-to-sphere electrode with the gap length of 20 to 150 mm is used for measurements in uniform electric field. The breakdown voltage characteristics obey Paschen´s law even at cryogenic temperature while quasi-uniform electric field is maintained in case of 50 Hz AC and DC voltage application, but doesn´t obey in the case of lightning impulse voltage application. A rod-to-plane electrode with the gap length of 20 to 330 mm is used for measurements in nonuniform 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 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 rod electrode before the 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 negative gas is contained and sufficient electrons are supplied from the cathode.