AC Breakdown Strength of Layered Tape Insulation Systems in Liquid Nitrogen Using Different Spacer Materials

Resistive type superconducting fault current limiters are in use at operating voltages up to 220 kV, and it is expected that they will also be necessary in the future in transmission grids up to 380 kV. To limit short-circuit currents at such high voltages, long high-temperature superconductor tapes are required typically wound into bifilar coils with low inductance. The windings are usually insulated from each other by a mixed insulating system, which consists of different sheets of insulation material like polyimide and a waved spacer ensuring sufficient cooling with liquid nitrogen. In this study the influence of the spacer material was investigated in detail. For this purpose, a method had to be developed to produce waved spacers from different materials on a laboratory scale, so that spacers with two dimensions, for a tape distance of 1.4 mm and 2.0 mm, could be manufactured from four different materials. The investigations reveal that the electric strength is only slightly dependent on the used spacer material, as a flashover of the spacer occurs. The increase of the tape distance from 1.4 mm to 2.0 mm leads to an increase of the breakdown voltage from roughly 31 kV to 38 kV and enlarging the flashover length at the spacer from 9 mm to 19 mm on each side results in a further increase to approximately 46 kV.