Electrical Strength of Layered Insulation Systems in Liquid Nitrogen for Resistive Superconducting Fault Current Limiters

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.