Manuel R. Osorio

Non-homogeneous quench of the superconducting secondary of an inductive fault current limiter: implications for current limitation

In this work, we have studied the performance of an inductive fault current limiter having biphase (Bi-2223 and Bi-2212) superconducting rings as the secondaries of the transformer. We have paid special attention to the role played by the inhomogeneous transition of the superconducting cylinder and rings. For that, we have measured the variations of the voltage and the temperature of several parts of these rings during current faults by using metallic contacts and thermocouples attached at different points along their perimeter. Our results show that the limiter response during a fault is governed by the overheating of a small part of the superconducting element. These measurements also show that the temperature and resistance evolution of this dissipative zone is in turn mainly determined by the progress of the resistance of the Bi-2212 minority phase. This behaviour reduces the current limitation in devices based on this kind of superconducting material because it provokes a smooth impedance increment during the faults.

Inductive Superconducting Fault Current Limiters With Y123 Thin-Film Washers Versus Bi2223 Bulk Rings as Secondaries

We present the comparison between the performance of an inductive superconducting fault current limiter operating with bulk Bi_1.8Pb_0.26Sr₂Ca₂Cu₃O _10+x rings or Y₁Ba₂Cu₃O_7-delta thin-film washers as secondaries. We have measured the impedance offered by the limiter under current faults and the recovery time once the faults are cleared. Our results show that the use of thin films can improve the impedance response of these devices and especially their recovery time, which can be about two orders of magnitude shorter than that of bulk samples

Thermal recovery of inductive superconducting fault current limiters with weak zones in the secondary

We study here the limitation performance and the thermal recovery of inductive superconducting fault current limiters (SFCL) with special emphasis on the case in which the superconducting element (i.e. the secondary) has weak zones. Our experimental results indicate that the conduction of heat between a hot weak zone and its cold surroundings plays an important role in its refrigeration. This suggests that it could be interesting to design superconducting elements based on an appropriate distribution of artificial weak zones. In this way, the heat would be removed more efficiently than in the case of homogeneous samples, where the heating would be uniform. By using a numerical simulation it is found that, with a proper distribution of artificially created weak zones, it could be feasible to simultaneously get a good current limitation (as big as with homogeneous secondaries) and a shorter recovery time. These results suggest an unexpected way to improve the performance of any SFCL.

Analytical approach to the thermal instability of superconducting films under high current densities

Using the Greens function of the 3D heat equation, we develop an analytical account of the thermal behaviour of superconducting films subjected to electrical currents larger than their critical current in the absence of an applied magnetic field. Our model assumes homogeneity of films and current density, and besides thermal coefficients employs parameters obtained by fitting to experimental electrical field - current density characteristics at constant bath temperature. We derive both a tractable dynamic equation for the real temperature of the film up to the supercritical current density J^\ast (the lowest current density inducing transition to the normal state), and a thermal stability criterion that allows prediction of J^\ast . For two typical YBCO films, J^\ast predictions agree with observations to within 5%. These findings strongly support the hypothesis that a current-induced thermal instability is generally the origin of the breakdown of superconductivity under high electrical current densities, at least at temperatures not too far from Tc.

Inductive fault current limiter based on multiple superconducting rings of small diameter

We present a fault current limiter prototype based on the use of a secondary comprised of an array of magnetic cores of small sections, each one of them with several superconducting rings. The main advantage of this configuration is that it is easier to make small diameter superconducting rings which, in addition, are more homogeneous and allow better refrigeration. We then present detailed measurements that show that, in addition to these advantages, this prototype offers the same limitation performances than when using a unique core and a superconducting ring with an equivalent area as the array of small section cores.

Normal-state resistivity versus critical current in YBa2Cu3O7−δ thin films at high current densities

Abstract We report measurements at zero applied magnetic field of the normal state resistivity against temperature, ρ ( T ), and of the characteristic curves ( E – J ) in YBa 2 Cu 3 O 7− δ thin films. The E – J curves are measured at different temperatures, between 77 K and the superconducting transition temperature, and up to current densities one order of magnitude larger than the critical current density, J c , with the samples submerged in a liquid nitrogen bath, controlled in temperature by its vapor pressure. These E – J curves allow us to observe the abrupt electric-field jump from the superconducting state to an ohmic highly dissipative state appearing at a current density J * ≫ J c . Our results confirm the absence of any scaling between ρ and J c for thin films having different structural quality. In contrast, we have found that J * scales with the slope of the E – J curves, ρ n (=E/J) , for J > J * , which is very close to the extrapolation of the normal-state resistivity to the range of temperatures where the E – J curves are measured.

Electric field versus current density curves in melt-textured samples of YBa2Cu3O7−δ under currents well above the critical current

Abstract In this work, we present measurements of the E – J curves of top seeded melt-textured YBa 2 Cu 3 O 7− δ . The samples, with a typical 0.5 mm 2 cross section, were cut from monoliths grown up to 2 cm in diameter ( ab -planes direction) and 0.7 cm in height ( c -axis direction). The E – J characteristics were measured up to current densities several times larger than the critical current density J C . To avoid spurious heating, current pulses up to 200 A and 1–3 ms long were applied through the samples immersed in liquid nitrogen. Measurements were carried out in the temperature range 80–93 K by pressuring the nitrogen bath. An abrupt electric-field jump is observed, in the E – J curves, for a current density J * several times higher than J C . This jump at J * is to some extent similar to the one observed in thin films.

Thermal recovery of an inductive fault current limiter under different refrigeration conditions

We report new experimental results on the thermal behaviour of an inductive fault current limiter prototype based on a superconducting Bi-2223 tube. Our studies mainly focus on the recovery after a fault of relatively long duration (of the order of seconds). Tests have been made with the device submerged in a liquid nitrogen bath, and also in He gas and vacuum environments. We analyse the advantages of the increase of the limitation in He gas and vacuum environments as a consequence of the heating in the superconductor but also the problem of the very slow thermal recovery of the limiter, in the range of tens of minutes for a dissipated power around 30 W.