Thermal behaviour of high-temperature superconducting fault current limiters: Application to microlimiters

Abstract

a b s t r a c tFault current limiters are one of the most promising applications of high-temperature superconductors. Two important and interrelated aspects of these devices are their thermal behaviour and their refrigeration. Here we will present some results of our recent researches about this topic concerning the possibility of using superconducting thin-film microbridges as very efficient microlimiters intended to operate at very low powers as could be superconducting microelectronics applications (SQUID based electronics, infrared detectors, etc.).& 2010 Published by Elsevier Ltd.Superconducting fault current limiters (SFCL) based on high temperature superconductors (HTSC) of different types, inductive, resistive or hybrid (i.e. inductive-resistive), have being extendedly studied [1][2][3][4][5][6][7]. The refrigeration and thermal behaviour of the superconducting elements are aspects of crucial importance for the overall performance of these devices. Thin film samples present several advantages to be used in current limiting applications [4,8,9]. We have studied the use of this kind of samples in two up to now nearly unexplored configurations. One is a hybrid limiting device based on meandered HTSC thin-films refrigerated by using a thermoacoustic refrigerator [10], and the other a resistive FCL based on ''thermally small'' superconducting microbridges [9] intended to operate at very low powers (SQUID based electronics, infrared detectors, etc.). In this work we will summarize some of our recent results on this last issue.In Fig. 1 we show a typical electric field versus current density (EÀ J) curve, corresponding to one of the microbridges (denoted BS7) used in our experiments. This microbridge, whose length, width and thickness are, respectively, 385 mm, 28 mm and 300 nm, and with T c ¼ 88.6 K, has been grown on a sapphire substrate. The two characteristic current densities are indicated in this figure:The critical, J c , at which dissipation first appears, and the so-called supercritical, J * , at which the microbridge is triggered into highly dissipative states. Let us note here that we have chosen for our studies microbridges of these dimensions to guarantee a good thermal behaviour before, during and after the current fault, as we will see below. In addition, the widths of our microbridges are well above the threshold at which J * is sample-width dependent [11]. The strong increase of E for current densities around J * make this type of samples very useful for FCL. However, the current is effectively limited only for electric fields or, equivalently, applied voltages well above that at which the limiter is triggered from normal operation (sample in the superconducting state) to current fault mode. In addition, once J * is attained, a thermal runaway [12,13] can be provoked which causes the reduction of the circulating current well below the nominal value (i.e., the current in normal operation without a fault) or very important damage on the microbridge (that can be even burnt out).The superconducting ...