15 Min Dc breakdown tests with liquid nitrogen

The dielectric strength of liquid nitrogen is an important design issue for numerous cryogenic high voltage apparatus. In some cases the design must consider gas bubble occurrence caused by resistive heating, e.g. during activation of a resistive superconducting fault current limiter. The objective of the FASTGRID project is to support a liquid nitrogen cooled superconducting fault current limiter solution for DC grids. The high voltage test facility Fatelini 2 allows DC testing of liquid nitrogen up to voltages of 325 kV. A high voltage electrode with the shape similar to a bell and a ground plane electrode were used. A heater was installed within the ground plate in order to allow bubble generation which was verified via video observation. A one hour voltage stress was performed as an initial voltage step without activation of the heater. The test was continued with a duration of 15 min with the same voltage but including five 500 W heating impulses of 10 s duration, each. In case of no breakdown the test was followed by about 10% voltage increases for subsequent steps of 15 min DC operation including 5 heating impulses, each. The maximum examined gap length was 96 mm for negative polarity only. In most cases the breakdown occurred during bubble generation. An outliner with a very low disturbed voltage waveform did also occur. The breakdown voltage values obtained by these tests are lower than for test series which were performed without operation of the heater.