AC Breakdown Strength of Concentric and Eccentric Cylindrical Conductor Arrangements in LN2

In order to make superconducting AC high-voltage three-phase power cables more compact and easier to manufacture, it may be possible to dispense with solid electrical insulation and use sole liquid nitrogen (LN2) instead. LN2 has very good electrical properties. However, it must be considered that the electrical stress from both the three-phase (unshielded conductor- conductor) and single-phase (unshielded conductor-grounded enclosure) voltage stress must be insulated in such an arrangement. Therefore, the electrical 50 Hz AC breakdown field strength of LN2 in concentric/eccentric cylindrical conductor arrangements is investigated. The analytical calculation of the highest peak breakdown field strength shows expected curves as a function of the homogeneity. With decreasing homogeneity of each electrode diameter D, resulting from the increasing distance d, the breakdown field strength decreases. Calculations of the peak field strength at 50% breakdown probability and its decisively stressed liquid volume shows that the results lie within the 95% prediction intervals of the empirical formalism for the volume effect of LN2. Although the average deviation of all results from the empirical formula is approx. 20%, which can be accepted as a first good approximation under the test conditions of 77 K and 1 bar. These results show that the empirical formalism is applicable for concentric but especially also for eccentric cylindrical conductor arrangements.