Delayed Breakdown in Liquid Nitrogen for Sphere-Plane Geometry When Subjected to Lightning Impulse

Liquid nitrogen (LN) is often the primary insulating medium for high voltage, high temperature superconducting grid applications. A number of papers have been published on the AC and DC breakdown strengths but only a few have focused on lightning impulse. In general, when a 1.2 times 50 mus (rise/fall) impulse is applied to a sphere-plane gap in liquid nitrogen the breakdown does not always or typically occur at the peak voltage, but rather at times following the peak, ranging from 0 (no delay) up to 120 mus for the LN₂ pressures and gaps studied. Data will be presented on the delay times for different gaps, sphere diameters and at two pressures, 1 bar and 1.33 bar. Most of the data was obtained for negative impulse (on the sphere) and one data set was obtained for positive impulse. Delays were observed to increase with increasing gap spacing. However, for a given set of conditions (i.e. within a given data set) the breakdown delays decreased as the peak voltage increased. It is believed that liquid nitrogen bubble initiation and growth may account for the delay in breakdown.