Transient Voltage Propagation in Bifilar Coils of a 380 kV Resistive Type SFCL

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.