DC-breakdown strength of liquid nitrogen under different voltage ramp conditions

The prospective use of superconductors with high critical temperature in power electricity needs information about the long term dielectric strength performance in large insulation systems. Liquid nitrogen is a promising cooling and basic insulating agent for fluid-transparent dielectric structures in power apparatus using superconductors with high critical temperature, However, evaluation of strength data from small-scaled multishot breakdown experiments requires extrapolation. The inherent statistical scattering of liquid nitrogen breakdown data then is a key phenomenon. Extrapolation which relies onto Weibull statistics has been claimed to be an adequate procedure, where breakdown experiments with different ramp speeds give background for Weibull parameter estimation. Thus, multishot breakdown tests with dc ramps ranging from almost 70 kV/s down to less than 0.05 kV/s have been carried out. Liquid nitrogen with technical quality was filtered and filled into a cryostat, which housed the concentric electrode arrangement; the electrodes consisting of polished stainless steel. Bubbles inside the 1.25 mm gap caused by heat transfer were eliminated by thermal double shielding. Breakdown data sets could be passably well fitted to a two-parameter Weibull function at a mean strength of roughly 15 kV/mm. It seems that liquid nitrogen exhibits almost no memory effect