<![CDATA[Liquid nitrogen is the most common fluid for cooling superconducting power engineering devices. The dielectric strength of an insulator rod embedded in liquid nitrogen at a pressure of 0.3 MPa was investigated with lightning impulse voltage series of 20 impulses of ±473 kV for gap lengths up to 50 mm between a grounded plane and a high voltage electrode in the shape of a bell. The influence of boiling due to quenching of the superconductor was simulated by heating impulses with a duration of 10.1 s. Before triggering the heater impulse, the liquid nitrogen was in the subcooled state i.e., a pure liquid. Transient bubble generation due to the heater impulse was confirmed by video recording through an observation window of the cryostat. The voltage of 473 kV was kept by a gap length of 18 mm in case of impulses of positive polarity. A gap of 30 mm was necessary in case of negative polarity. Hence, a strong polarity effect was found. Calculated field values based on the experimental results do not exceed limits used for the high voltage design study for a support insulator of a superconducting fault current limiter.]]>
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