3D Finite Element Simulations of strip lines of a YBCO/Au Fault Current Limiter

Geometrical aspects of the design of fault current limiters (FCL) have a great impact on their performance. Recently, the University of Geneva have presented optimized geometries obtained by splitting the FCL into many small dissipative lengths in order to distribute the power along the device. We have performed 3D finite element method (FEM) simulations for studying the behavior of strip lines of a YBCO/Au FCL in AC nominal use (sinusoidal current at industrial frequency) up to 3 Ic. Particular attention has been paid to the mesh, due to the very large involved aspect ratios. The numerical results show a concentration of the electric field in the sharp corners. This results in very large power dissipation, which has been experimentally confirmed by wafer cracks during over-Ic tests. A new geometry, taking into account the length of the connecting path and the corner optimization, has been proposed. Finally, simulations coupling electromagnetic and thermal equations show the behavior of the device when a default occurs on the electrical network. This work is supported by the Swiss National Science Foundation through the National Center of Competence in Research “Materials with Novel Electronic Properties – MaNEP

Quench propagation in coated conductors for fault current limiters

A fundamental understanding of the quench phenomenon is crucial in the future design and operation of high temperature superconductors based fault current limiters. The key parameter that quantifies the quenching process in superconductors is the normal zone propagation (NZP) velocity, which is defined as the speed at which the normal zone expands into the superconducting volume. In the present paper, we used numerical models developed in our group recently to investigate the quench propagation in coated conductors. With our models, we have shown that the NZP in these tapes depends strongly on the substrate properties

Quelques aspects des champs critiques dans les phases de Chevrel

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Computer Modeling of YBCO Fault Current Limiter Strips Lines in Over-Critical Regime With Temperature Dependent Parameters

We present the results of an advanced numerical model for fault current limiter (FCL) based on HTS thin films in which both thermal and electromagnetic aspects are taken into account. This model allows simulating the behavior of FCL in the over-critical current regime and we used it for studying strip lines of a YBCO/Au FCL on sapphire substrate. The electromagnetic and thermal equations have been implemented in finite-element method (FEM) software in order to obtain a model for investigating the comportment of the superconductor when the current exceeds $I_{c}$ . In particular, materials equations have been implemented in order to simulate the electrical behavior of superconducting devices with strong over-critical currents. We report results of simulations in voltage source mode where currents largely exceed $I_{c}$ . The global behavior of the FCL is compared with measurements, showing a good agreement. The use of FEM simulations offers the advantage to give access to local variables such as current density or temperature. Studies with this model can replace expensive experiments where very high current density might damage or destroy the FCL device

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

Interpretation of magnetization measurement data obtained on hot pressed Pb_1-xSn_xMo₆S₈ samples

Magnetization measurements have been carried out on samples prepared with varied Sn contents, using constant parallel applied field sweep rates in the range of 30 to 190 mT/s. The hysteresis loops observed up to about 6 T at 4.2 K are earmarked by characteristic flux jump phenomena. J/sub c/ critical current densities deduced from the loop widths can be interpreted, in agreement with the fine analyses of the loops, in terms of global critical screening currents, related to very good connectivity between the grains

Numerical studies of the quench propagation in coated conductors for Fault Current Limiters

A fundamental understanding of the quench phenomenon is crucial in the future design and operation of HTS based Fault Current Limiters (FCLs). The key parameter that quantifies the quenching process in superconductors is the normal zone propagation (NZP) velocity, which is defined as the speed with at which the normal zone expands into the superconducting volume. Recent experimental measurements in YBCO tapes have shown that the NZP velocity in these materials is extremely slow in comparison with theoretical predictions. In the present paper, we compared experimental results with FEM models developed in our group recently. With our models, we have shown that the NZP of YBCO tapes depends strongly on the substrate properties

Critical currents and pinning in powder metallurgically processed Chevrel phase bulk superconducting samples

Critical current density (Jc) has been measured in hot isostatically and uniaxially pressed bulk samples of PbMo6S8 (PMS) and SnMo6S8 (SMS) Chevrel phase superconductors (CPS). The obtained pinning force densities (Fp(B, T)) show a scsling in a wide range of temperature (T = 4.2 K−Tc) and field (B = 0–9 T), which is in good agreement with the predictions of the flux line shear (FLS) model. However, the magnitude of Fp is an order of magnitude smaller than the expected values. The PMS sample hot pressed at relatively high temperatures (1200°C) shows long tails in the scaled Fp(B, T) data. In contrast, the SMS samples sintered at similar temperatures show double peaks in Fp(B) for temperatures below 10.5 K. For both compounds the pseudo upper critical field () extracted from the Jc data, especially near Tc, shows deviation from the expected ideal Bc2(T) dependence. The results point out a correlation of Jc and B∗c2s with the morphology of the grain boundaries in terms of granularity and degradation of the superconducting properties