DC breakdown tests with pressurized liquid nitrogen

Superconducting fault current limiters can improve high voltage DC power transmission technique. The objective of the FASTGRID project is to support a liquid nitrogen cooled superconducting fault current limiter solution for DC grids. Liquid nitrogen is an inexpensive cooling medium for high temperature superconductor devices which can be applied for high voltage insulation purposes, too. The facility Fatelini 2 enables breakdown tests with liquid nitrogen up to DC voltages of 325 kV. The high voltage electrode has a shape of a bell. The ground plane allows heating of the liquid nitrogen in order to investigate the impact of a transition from the superconducting to the normal conductive state. Increase of dielectric strength by pressure increase was found in most cases. Degradation of the dielectric strength due to bubble generation occurred under saturated conditions. Polarity effect was condition-dependent. Lowest minimum slope in withstand voltage vs. gap length diagram was 3.5 kV/mm. Local withstand field maxima for gap lengths from 10 mm up to 96 mm were at least 6.1 kV/mm.