Quench nucleation obtained by local reduction of Ic in coated conductors

Abstract

The normal zone propagation velocity in coated conductors is very small compare to YBCO on sapphire. For fault current limiter, it is more difficult to use them and obtain predictable behaviors under electrical faults. In the present work, we have measured the normal zone propagation on short coated conductors samples submitted to current pulses of variable length and amplitude (from 10-500 ms and 0.9-2.0 Ic respectively). The normal zone nucleation is obtained using a magnetic tip to reduce locally the critical current density of the tape. This reproducible technique allows to choose and modify the initial nucleation site position as well as to perform multiple experiments on the same sample without making any physical defect on the conductor. The normal zone propagation measurements as well as the characteristic shape of the resulting I-V curves obtained from various current pulse waveforms are presented here. From the experiments, it is shown that field-induced defects allow to increase the normal zone propagation velocity as well as to reduce the initial delay before quenches in coated conductors. In addition, sub-cooled measurements have shown that an increase of the heat diffusivity obtained by lowering the coolant temperature improve the quench propagation. © 2010 IOP Publishing Ltd.