Breakdown field strength of gaseous nitrogen at 3 bar under AC stress

With increasing possibilities for application of High-temperature superconductors, efficient design and understanding of the behavior of the insulation system becomes more important. An insulation, which often is already present in the HTS-system as coolant, is liquid nitrogen. In order to provide an efficient insulation design the dielectric strength and behavior of LN2 has to be known. Due to low latent heat and weak interactions of the nitrogen molecules, heat can easily lead to formation of vapor bubbles. It has already been established that vapor bubbles greatly influence the dielectric withstand capability. Therefore, if enough vapor is present in the discharge path, then breakdown may be governed by the dielectric strength of the gas. Thus, this paper continues to investigate the gas breakdown as a discharge process in liquid nitrogen insulation. AC breakdown measurements at 3 bar for uniform and nonuniform electric field setups with cryogenic and room temperature nitrogen gas are presented and compared to existing values for ambient pressure. Electrode gaps have been varied between 1 to 4 mm in 1 mm steps. Investigations are done at room temperature, for comparison to well-known discharge processes, and at a temperature close to the boiling point of liquid nitrogen for the real application. Due to the employed setup, the nitrogen gas at cold temperatures is a vapor model. To evaluate uniform and non-uniform fields, the measured breakdown voltages are converted into breakdown field strengths via electrostatic field simulation. With knowledge of the electric field strengths, simple calculations can be used to predict the break-down voltage of liquid nitrogen insulations with vapor content. The calculation requires material data such as primary ionization coefficient α in nitrogen vapor and discharge data such as the factor k. This factor k changes with the discharge process. It has to be distinguished between Townsend and Streamer mechanism and which is more probable to cause gas breakdown. Previous measurements, at ambient pressure, for cold nitrogen suggested a k factor associated to Townsend mechanism while at room temperature the k factor was in the range of the Streamer criterion. An evaluation of the k factor and comparison to k deduced for ambient pressure and 3 bar will be given and discussed.