Development of High Temperature Superconductors Coated Conductors for Resistive Fault Current Limiters with the Current Flow Diverter Architecture

Abstract

The primary benefit of a metallic stabilization/shunt in 2nd Generation (2G) High Temperature Superconductors (HTS) coated conductors (CC) is to prevent damage during quench by providing an alternative path for the current flow. On the downside, the increase in thermal capacity of a thick shunt makes the CCs prone to heat amassing due to hot-spots coming from variations in the critical current distribution of the HTS layer. This thermal issue makes the classical HTS tape design extremely vulnerable for operating in superconducting devices like high-field magnets and Superconducting fault Current Limiters SFCL. In SFCL, a prospective fault current level close to these tape’s average critical current (Ic) can lead to destructive hot-spots in a matter of milliseconds. The Current Flow Diverter (CFD) is a promising multilayer architecture concept that has proven to increase the conductor’s robustness against the inevitable hot-spot regime by inserting a high resistive layer partially covering the interface between the metallic silver shunt and the (Re)BCO film, thus alleviating shunt compromise. This relatively simple change creates a boost in the so-called Normal Zone Propagation Velocity (NZPV) and avoids the destructive heat amassing of hot-spots. However, since 2014 there has been a struggle in finding a practical manufacturing method compatible with the coating steps of reel-to-reel systems used by companies. In the framework of the H2020 project FASTGRID, this thesis reports on the technical journey of trying to achieve a feasible cost-effective method for implementing the CFD architecture in 12 mm wide 2G HTS tapes. Using chemical solution deposition (CSD) and chemical vapor deposition (CVD) methods, four materials, i.e epoxy, graphite, yttria and silver sulfide, led to four different manufacturing routes that helped identify the main practical constrains of the CFD fabrication and, for the first time, two viable CFD fabrication routes were found with yttria nanolayers and silver sulfidation reactions. Furthermore, for tapes with and without CFD, the Ic and NZPV, together with the maximum fault limitation conditions, electric field and fault time, were measured and compared using transport current in DC and AC respectively to confirm the advantages of the new proposed architectures.