Study of Turn-to-Turn Electrical Breakdown for Superconducting Fault Current Limiter Applications

Abstract

The rational insulation design of a resistive superconducting fault current limiter (r-SCFCL) requires data gathered from experimental setups representative of the final apparatus. Therefore, an experimental study was performed to characterize the electrical breakdown (BD) of liquid nitrogen (LN2) in the particular conditions of a quenching superconducting device. To reproduce the electrical stress occurring within a real r-SCFCL winding, BD voltages with a grounded tape electrode were investigated with different polarities and pressures, with and without transient heating on the tape electrode. Pictures obtained with a high-speed camera and breakdown delay measurements allow to further characterize breakdown mechanisms.

Breakdown Phenomena in Liquid Nitrogen Under Synchronized Transient Boiling and Impulse Voltage

“…With a similar setup in DC [14], VBD increased systematically versus pressure. Therefore, heating power and electrode shape are not the only relevant parameters.…”

“…In standard equilibrium conditions (temperature T = 77 K, pressure P = 0.1 MPa), the vaporization of LN2 by impulse heating induces a very large drop of breakdown voltage VBD DC under DC voltage: breakdown occurs through vapor bubbles, and VBD DC is reduced by a factor of nearly four as compared to the case with no heating [14]. Increasing the pressure up to P = 0.25 MPa allows recovering values measured in LN2 without heating [14].…”

“…In order to reproduce the transient heating of a superconducting tape during resistive limitation, a system using a resistive tape heated by Joule effect (impulse current applied through the tape) was designed [14,15]. In standard equilibrium conditions (temperature T = 77 K, pressure P = 0.1 MPa), the vaporization of LN2 by impulse heating induces a very large drop of breakdown voltage VBD DC under DC voltage: breakdown occurs through vapor bubbles, and VBD DC is reduced by a factor of nearly four as compared to the case with no heating [14]. Increasing the pressure up to P = 0.25 MPa allows recovering values measured in LN2 without heating [14].…”

“…The cryostat [16,17] and the tape-plane electrode setup, used in this experimental study, were presented in previous studies [7,14,15]. A schematic view of the electrode system is presented in Fig.…”

Some Results of the EU Project FASTGRID

“…Two corrugated fibre glass tapes with a straight fibre glass tape between the conductors provided the HV turn to turn isolation and an efficient cooling. DC breakdown and flashover experiments were successfully performed at KIT on the pancakes in addition to the numerous tests carried out by the SuperGrid Institute [15]. The mechanical strength was tested using a current pulse higher than the limited current.…”