15 min DC breakdown tests with pressurized liquid nitrogen

Liquid nitrogen is a widely applied cooling fluid for superconducting high voltage devices. The pressure dependence of the dielectric strength of liquid nitrogen is an important design issue in case of using liquid nitrogen for high voltage insulation purposes, too. The objective of the FASTGRID project is to support a liquid nitrogen cooled superconducting fault current limiter solution for DC grids. Occurrence of heat impulses, caused by transition from the superconducting to the normal conductive state, is an unavoidable feature of a superconducting fault current limiter and therefore the influence of such heat impulses were also investigated. The high voltage test facility Fatelini 2 allows DC testing of liquid nitrogen up to voltages of 325 kV and pressures up to 3 bar (absolute). A high voltage electrode with the shape similar to a bell and a ground plane electrode with an internal resistive heater were used. The pressurization causes subcooled conditions leading to a negligible sensitivity of the breakdown strength against heating which is different from the heat sensitive breakdown behaviour for the non-subcooled condition. The polarity effect changed after subcooling in case of testing the long gap setups. ... mehrCalculated maximum field values based on the experimental withstand voltages for gap lengths between 10 mm and 96 mm are above 6.0 kV / mm.