Size effect on breakdown strength in sub-cooled liquid nitrogen for superconducting power apparatus

Toward the practical and rational insulation design of high temperature superconducting power apparatus, the ac breakdown (BD) characteristics and mechanisms of liquid nitrogen (LN2) were systematically investigated and discussed for different pressures at P = 0.1-0.3 MPa and temperatures at T = 65-77 K under quasiuniform electric field with the field utilization factor ζ= 0.986-0.674. We obtained the BD strength (EB) in sub-cooled LN2 (SLN2) and analyzed them in terms of size effect, taking account of the stressed liquid volume (SLV) with the electric field distribution in SLN2. Together with the other researchers´ data on BD characteristics of LN2, we extended the evaluation by applying the concept of α% SLV (SLV with the electric field strength higher than the decisive electric field factor, α of the maximum electric field strength). As the result, we derived that EB with the 50 % probability decreased linearly with the increase in α% SLV on a universal line, EB = 78.4 x (α% SLV)-^1/8.15, which was irrespective of the pressure and temperature of LN2.