Hybrid model of quench propagation in coated conductors for fault current limiters

International audienceWe developed a hybrid model of the quench propagation in coated conductors in the current limitation condition. This model combines the finite element method, to study the thermal behaviour of the coated conductors, and analytical calculation of the heat dissipation. We demonstrate that the evaluation of the heat dissipation can be conducted on a larger mesh than the FEM thermal problem. The results obtained with this model are in very good agreement with experiments, without the need of using free parameters for adjustment. Parametric studies are then conducted to evaluate the influence of both the substrate thickness and the layer interface thermal properties on the transition propagation behaviour. 3D simulations of a thin superconducting line placed on a wider substrate are also presented. Significant transverse heat propagation is observed in spite of the low thermal conductivity of the substrate, though this has little to no influence on the transition propagation along the line. These results are discussed in the context of FCL design

Comparison Between the Behavior of HTS Thin Film Grown on Sapphire and Coated Conductors for Fault Current Limiter Applications

The major drawback for the commercialization of fault current limiter (FCL) made of YBCO on sapphire is their expensive price. In the recent years, coated conductors (CC) have been extensively developed and, due to their lower prices, have been recently tested for current limitation application. One weakness of these CC is the very low electric fields they can sustain, typically below 1 V/cm as compared to 20-40 V/cm observed in YBCO films grown on sapphire. The limitation of this electric field in CC comes certainly from the very low propagation velocities of the dissipative state, a property which might be correlated with the poor thermal behavior of the architecture of these materials. Both the thermal conductivities of the Hastelloy substrate and of the conducting bilayer (superconducting DyBCO and Ag conducting layer) influence the thermal behavior of the CC and therefore have to be optimized to get the best performance. We have then investigated the thermal and electrical behavior and the propagation velocities in CC during constant current pulses above. The comparison with the results obtained on YBCO films grown on sapphire shows several differences. In CC, the flux flow resistivities are 2-3 orders of magnitude higher than in film grown on sapphire and quench propagation velocities are 2-3 orders of magnitude lower (of the order of cm/s). The propagation velocities in CC and in films on sapphire are analysed with a simple adiabatic model

Thermally Assisted Transition in Thin Film Based FCL: A Way to Speed Up the Normal Transition Across the Wafer

The adjunction of constrictions along the meander of a superconducting Fault Current Limiter (FCL) greatly improves its behavior thanks to a better distribution of the dissipative zones at the occurrence of a short circuit. This design works perfectly for symmetrical short circuit (i.e. short circuit at the maximum voltage). However for asymmetrical short circuits (at voltages close to 0), we are facing a problem due to the small number of the initially switched constrictions. To solve this problem, we test the possibility to speed up the transition into the normal state of the whole meander by heating it locally. This thermally assisted transition is realized by growing a gold layer on the backside of the substrate and by patterning it into a meander with its dissipative parts lying just underneath the constrictions of the FCL. This gold meander can be either connected in parallel with the superconducting meander or a capacitor bank can supply the current. In order to confirm the benefit of the thermally assisted transition we have carefully measured the behavior of the FCL during constant current and low voltage pulses as a function of the power injected into the gold line. We present results showing that the response of the FCL to the generated heat is very fast; typically less than 100 $mu{rm s}$. Furthermore the distribution of the dissipated power across the wafer, during asymmetrical AC short circuit, is clearly improved