Superconducting fault current limiter with bulk materials

Superconducting materials offer the unique possibility to limit fault currents by their self-triggering quench to a resistive state above a given threshold current. Their use in power networks would improve the power quality, which is a real need today. YBCO or BSCO bulk materials are good candidates for current limitation. Their behaviours are rather different, in particular the electric field versus current characteristics. BSCO compounds are protected by their homogeneous quench due to the numerous defects (grain boundaries) distributed along the material. An assembling of 60 sintered Bi bars was tested under 1 kV. The current limitation is effective (a 5000 A short-circuit has been limited to 1080 A) but it occurs at 24 times the critical current. The increase of the critical current density of Bi compounds by their texturation reduces the current excursion above the critical value. In YBCO bulk elements, the defects are localized and make YBCO very sensitive to hot spots. Those are unavoidable and the operating conditions should be adapted in order to make them non-destructive. That is the self-protection concept. To fulfil it, we chose to operate near the critical temperature (above 90 K) to reduce the critical current density. The small difference with T, is very favourable for the quench. A fault current limiter based on 43 YBCO meanders working at 90.5 K in a pressurized liquid nitrogen bath is presented. The meanders are cut in 45 mm in diameter single domain pellets elaborated using the topseeding technique. Supplied under a maximum voltage of I kV, the current was limited at 740 A instead of the theoretical value of 6500 A without quench. (C) 2002 Elsevier Science B.V. All rights reserved.