FAULT CURRENT LIMITER USING BULK OXIDES SUPERCONDUCTORS

We study the limitation possibilities of bulk Bi high T-c materials. F or this we test these materials with a.c. or d.c. currents above their critical currents. We study particularly the evolution of the voltage with time or with current. The material, the value of the current and the time duration play important parts. For sintered Bi samples the v oltage depends only on the current even for values much larger than th e critical current. With textured samples the V(I) curves shows an hys teretic behaviour due to a warming up. The textured materials are more interesting than sintered ones in terms of required volume for the cu rrent limitation. In both cases the superconductors are in a dissipati ve state but not in the normal state. This state is nevertheless reach ed if the dissipated energy inside the sample is sufficient. We have t ried to apply a magnetic field on the samples in order to trigger a mo re effective limitation. The voltage increases but with a limited effe ct for currents much higher (3-4 times) than the critical zero field c urrent. We think that the dissipative state is due mainly to the grain boundaries which become resistive above the critical current.