A facility for DC breakdown voltage testing of liquid nitrogen up to 325 kV

Abstract:
Liquid nitrogen can be used for cooling as well as for electric insulation purposes of high temperature superconducting apparatus. In some cases the dielectric design must consider gas bubble occurrence caused by resistive heating, e.g. during activation of a resistive superconducting fault current limiter for a high voltage DC grid. The facility Fatelini 2 was equipped with a cryogenic DC bushing in order to allow DC testing of liquid nitrogen up to voltages of 325 kV in combination with gas bubble generation by a 500 W heater in the ground plane. Voltage tests with a ramping of 10 kV / s show strong but reversible conditioning effect, i.e. initial breakdowns are mostly lower than the later ones if the waiting interval for starting the next voltage ramp is only 3 min. After waiting at least over night the breakdown series begin again with low initial breakdown values. As second method tests with 1 h continuous voltage application followed by 10% voltage increases for subsequent steps of 15 min continuous voltage were performed with gap lengths up to 50 mm. Five heating impulses of 10 s duration were executed during each 15 min step. In most cases the breakdown occurred for these step tests during bubble generation but there was a low breakdown event even during 1 h conditioning without bubbles showing the need for using high safety factors for a reliable high voltage DC design.