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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.

Resistive type superconducting fault current limiters (R-SFCLs) are currently developed up to an operating voltage of 220 kV and higher voltage levels seem to be necessary in the future. To limit the short-circuit current at such high voltages, long high-temperature superconductor (HTS) tapes are required, which are typically wound into bifilar coils with low inductance. The windings are insulated from each other by a mixed insulating system, which is stressed with AC voltage during the quench and with transient voltage in the case of lightning impulse overvoltages. To find an appropriate insulation concept, the dielectric strength of multi-layered insulation systems between the HTS tapes were investigated in liquid nitrogen. Therefore, a method to laminate tape conductors with polyimide sheets of different types was developed. Between these laminated tapes additional sheets of polyimide and a waved spacer layer were placed resulting in different thicknesses of the insulating system. The spacer ensures sufficient flow and cooling with liquid nitrogen. A test setup was developed to investigate the dielectric strength and the flashover strength of this mixed insulation system in dependence on the used layers. The results show that the dielectric strength of the whole systems depends mainly on the LN2 gap formed by the spacer and its dielectric strength. Further polyimide layers are not able to enhance the dielectric strength of the insulating system, which can be confirmed by electric field calculations. Flashovers occurred only at very small flashover distances.

Resistive type superconducting fault current limiters (R-SFCLs) are currently developed up to an operating voltage of 220 kV and higher voltage levels seems to be necessary in the future. To limit the short-circuit current at such high voltages, long high-temperature superconductor tapes are required, which are typically wound into bifilar coils with low inductance. The windings are insulated from each other by a mixed insulating system. During transient voltage stress, especially chopped lightning impulses, the voltage between adjacent windings can be increased due to propagation time delays and reflections of the transient voltage pulses, so-called travelling wave effects. With regard to the development of a 380 kV R-SFCL, simulations with a finite element method in the frequency domain were carried out and verified by measurements. For this purpose, small bifilar coil modules were used and exposed to fast rising pulses in the ns range to investigate the transient processes in detail. In a next step, the simulation results were approximately transferred to the design of the 380 kV R-SFCL exposed to a chopped lightning impulse. The results reveal, that transient impulses propagate on two ways through the bifilar coil. First, the pulses propagate in radial direction due to the capacitive coupling between the windings. Second parts of the pulses run in circumferential direction along the windings.