Abstract
Liquid nitrogen (LN2) is commonly used both as a coolant and electrical insulation in
high temperature superconductor (HTS) equipment for power applications. Hence it is
necessary to know the electrical breakdown characteristics of LN2 under a variety of
conditions which are likely to be encountered in practice. The ac breakdown and positive
and negative polarity breakdown results for lightning impulse (1.2 microsecond rise
time/50 microsecond fall time) are presented for LN2 using sphere to plane electrode
geometry for sphere diameters of 50.8 and 101.6 mm over a gap range of 1 to 15 mm.
Voltages up to 110 kVrms were studied for ac breakdown and up to 500-kV peak for
impulse. In this work both the ac and impulse breakdown voltages scale approximately
with distance over the limited gaps studied which is indicative of a quasi-uniform (near-
uniform) electric field between sphere and plane. These measurements were conducted in a
dewar which could be pressurized from 1 to 2 bar absolute which greatly reduces the
spontaneous formation of bubbles that can occur in open LN2 bath experiments and thus
potentially reduce the breakdown strength. Results from the pressurized system and near
atmospheric pressure similar to an open bath are compared.