Thin-film washers as secondaries in superconducting inductive fault current limiters: the role of the cooling atmosphere

Abstract

Recently, we showed that the use of thin-film superconducting washers as secondaries in inductive fault current limiters may have considerable advantages when compared with the more conventional bulk rings or cylinders. Here we extend those previous works to determine if the cooling atmosphere plays a significant role on the recovery time, τR, of the inductive fault current limiter based on thin film superconductors. We have used helium as a cooling gas, from primary vacuum to atmospheric pressure. We have found that gas pressure has an appreciable influence on the recovery time of our inductive fault current limiter. For instance, our results show that τR under atmospheric pressure is almost thirty per cent shorter than when measured in primary vacuum. A numerical simulation realized by the finite element method shows that just after the fault the substrate is thermalized and, therefore, during the recovery convection is the only working heat exchange mechanism. To avoid this undesirable thermalization and to further reduce the recovery time the substrate should either be much more thicker or be in good contact with a thermal sink capable of removing the heat by conduction.