Studies of Breakdowns in Liquid Nitrogen at Different Pressures Between Rogowski Electrodes

The usage of superconducting machines in the power grid or other high energy application makes it necessary that the machine can withstand all electrical stresses which can occur during normal operation and at transient overload. To guarantee a sufficient insulation, it is essential to know the properties of the insulating material. For HTS applications liquid nitrogen is a possible cooling and insulation liquid. In this paper the influence of pressurized liquid nitrogen on the discharge voltage is observed. Therefore, a cryostat was used, that can be pressurized and the discharge voltages at 3 barabs and 5 barabs were investigated. The investigations were performed between Rogowski electrodes to guarantee a homogeneous electric field without discharges at the electrode edges. Experiments were done with gap distances up to 7 mm. The liquid nitrogen was stressed with lightning surge voltage of both polarities and AC ramp with a rise of 2000 V/s