AC Breakdown Strength of Parallel Cylindrical Conductor Arrangements in LN₂

In a first theoretical study, a compact 123 kV/50 Hz three-phase superconducting cable with a calculated smallest possible maximum peak field strength of 10.5 kV/mm was developed. The compact cable design uses three parallel-cylindrical high temperature superconductors. A special feature is that only the liquid nitrogen (LN2), surrounding the parallel conductors for cooling purposes, is used as electrical insulation system. Therefore, the aim of this study is to investigate the maximum electrical 50 Hz AC peak breakdown strength of parallel-cylindrical conductor arrangements in LN2. The highest electrical strength values for LN2 at 77.15 K and 1,013 mbar are obtained at the smallest spacing of 1 mm and smallest diameter of 8 mm with a peak breakdown field strength of 34.2 +/- 4.2 kV/mm. The influence of the volume effect is mainly evident when comparing the conductor diameters with the same small spacing. If the diameter increase, the decisively highly stressed insulating volume increases and the electrical strength decreases. The curves per diameter show that with increasing distance, i.e., decreasing utilization factor, the influence of the decisive stressed volume decreases and the electrical strength no longer decreases proportionally to the distance.