Discharge Mechanisms in Liquid Nitrogen—Breakdown Field Strength of Gaseous Nitrogen

The application of high-temperature superconductors in medium- and high-voltage equipment requires an insulation system which prevents electric breakdown within the required working conditions. As dielectric insulation and coolant liquid nitrogen can be used, the design of the insulation requires knowledge of the dielectric behavior of liquid nitrogen. During normal operation, the insulation system usually does not generate vapor bubbles. But if fault or certain working conditions occur, vapor bubbles can occur. In this case, the dielectric behavior of gaseous nitrogen, close to the temperature of the liquid, becomes the discharge governing criterion. Furthermore, the discharge in liquid nitrogen has not been clearly explained. Therefore, this paper investigates the gas breakdown as a possible discharge process. To investigate if the discharge happens according to the gas breakdown, breakdown voltages of cryogenic and room temperature nitrogen gas are measured. This is done for uniform and nonuniform electric field at ambient pressure. Then the gas constantk, an integration of ionization coefficient over discharge path, is deduced from simulations and analytical formulas. These values ofkyield a possible criterion to decide if a gas breakdown happens according to Townsend or Streamer theory. This offers new insights into the breakdown of liquid nitrogen.