Breakdown Characteristics of Liquid Nitrogen in Strongly Inhomogeneous Electric Fields

Abstract

Resistive superconducting fault current limiters (R-SCFCLs) based on second-generation superconducting tapes (2G tapes) are successfully used in the medium-voltage level. To develop R-SCFCLs for the high-voltage application exceeding 100 kV, it is necessary to know the dielectric properties of liquid nitrogen more in detail, particularly if sharp edges, occurring for example at 2G tapes or at connection conductors, cannot be avoided. Consequently, in this work, the breakdown characteristics of strongly inhomogeneous electrode systems in liquid nitrogen were investigated. An electrode configuration of a plane electrode and a needle electrode with a tip radius r of approximately 0.1 mm and sphere electrodes with radii r from 1 to 3 mm were used. The distance d between the electrodes was adjusted in the range of up to 15 mm. The electrode configuration was immersed in liquid nitrogen within an open isolated vessel, and the ac breakdown voltage was measured in dependence on the rounding radius r of the high-voltage electrode and the electrode distance d. For each combination of r and d, the mean breakdown voltage was calculated from 15 single measurements. The measuring results using sphere electrodes reveal that the mean breakdown voltage VBD approximately follows the empirical law V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">BD</sub> = c · d <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sup> with b = 0.46. For the needle electrode with a radius of roughly 0.1 mm, the mean breakdown voltages are remarkably smaller and increase with distance d only with an exponent b = 0.28.