Breakdown Characteristics Of Liquid Nitrogen In Non-Uniform Electric Fields

The AC breakdown (BD) voltages and partial discharge (PD) characteristics of liquid nitrogen (LN2) in non-uniform electric fields were investigated with the objective to improve insulating arrangements of superconducting devices cooled with LN2. The tested electrode configuration consisted of a plane earthed electrode with a diameter of 140 mm and tip electrodes with tip radii from 0.1 mm to 2.0 mm. The distance between plane and tip electrodes was varied from 10 mm up to 50 mm resulting in field utilization factors between roughly 0.006 and 0.233. The pressure of LN2 was changed from normal pressure up to an absolute pressure of 0.49 MPa at a constant temperature of 77 K. To investigate the ignition of the BD process more in detail PD inception and extinction voltages of the different arrangements were measured at normal pressure.The results reveal that the pressure has only a rather small influence on the BD strength of LN2 in non-uniform electrode arrangements. The pressure rise from 0.1 MPa to 0.49 MPa leads only to a maximum increase of the BD voltage of 40 %. With increasing electrode distance and decreasing tip radius the mean BD field strength decrease from values of roughly 8 kV/mm to values of 1.5 kV/mm, which are clearly lower than in uniform electric fields.The partial discharge measurements performed at normal pressure show that pre-discharges only occur in the case of field utilization factors smaller than 0.03 at inception voltages, which are roughly 50 % of the BD voltages. In all other cases the BDs occur without any pre-discharges.