Withstand alternating voltage of liquid nitrogen in the presence of gas bubbles

Liquid nitrogen is often used in superconducting high voltage apparatus for cooling and for electrical insulation purposes. A temperature increase of the superconductor surface, e. g. to room temperature during a quench of a fault current limiter, may cause a decrease of the breakdown voltage within the apparatus by generation of nitrogen gas bubbles. A cryostat was equipped with a sphere to plane electrode arrangement with adjustable gap for the examination of the breakdown and withstand voltages of liquid nitrogen. Bubble generation of the heatable plane was verified by camera observation. Alternating voltage up to 230 kVrms was applied to the arrangement under the pressure condition of 0.1 MPa (absolute). Degradation of the withstand voltage up to 50% was found by using a 500 W heater in comparison to the case with no heating.