Quench Development in Bi-2212 Bulk Coils During Quenches

We investigated the quench development in resistive superconducting fault current limiters (SFCLs) based on Bi₂Sr₂CaCu₂O₈ (Bi-2212) bulk coils. In order to protect them from hot spot formation, the bulk coils were either attached with a metal shunt layer or connected with a shunt coil in parallel. The bulk coils also had a plastic support for enhanced mechanical strength and insulation. The SFCLs were subjected to simulated AC fault currents for quench measurements. They were immersed in liquid nitrogen during the experiment for effective cooling. The resistance of the Bi-2212 bulk coil stayed low for a few milliseconds, until it increased rapidly. The increase was more rapid in the SFCL with a shunt coil than with a shunt layer. Once the quench was completed, the resistance increased at slower rates. The average temperature of the bulk coil reached about 230 K at 3 cycles after quench start at application voltage of 2.4 V_rms/cm with a shunt coil, and at 0.4 V_rms/cm with a shunt layer. The resistance development in Bi-2212 bulk coils could be quantitatively explained with the concept of transfer of heat generated during quenches to the liquid nitrogen as well as within the Bi-2212 bulk. The fault period was divided into three regions: flux-flow region, region prior-to quench completion, and region after quench completion. Resistance was calculated separately for each region and compared with data. The results agreed well with data.