AC breakdown voltage of liquid nitrogen depending on gas bubbles and pressure

Liquid nitrogen is often used within superconducting high voltage apparatus as pressurized coolant and for electrical insulation purposes. A temperature increase of the super-conductor surface, e. g. up to about 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 for the examination of breakdown and withstand voltages of liquid nitrogen. The temperature of liquid nitrogen was kept at 78 K whereas the pressure values were adjusted to 0.1 MPa, 0.2 MPa and 0.3 MPa. Alternating voltage up to 230 kVrms was applied to the arrangement. Impact of bubble generation by heating impulses on the dielectric strength was not found for 0.3 MPa and it was found for 0.2 MPa only in case of low distances. This is different than the already known behavior at 0.1 MPa and boiling temperature.