Power transformers are key elements for the safe and reliable delivery of electrical energy generated by renewable energy resources to consumers via transmission lines. Fault-tolerant current-limiting High Temperature Superconducting (FTCL HTS) transformers are type of superconducting transformers that tolerate fault for seconds and limit the fault current without the threat of burnout or delamination of tapes and deformation of windings. In this paper, the fault performance of a FTCL HTS transformer in a standard IEEE power system is investigated. The studied transformer is a 50 MVA 132 kV/13.8 kV transformer where both windings are made up of HTS tapes. The understudied power system consists of two microgrids with distributed generators. Part of the power in microgrids is supplied by the upstream grid which is connected to the microgrids through the HTS transformers. Two fault scenarios have been considered in this power system, in each one of these scenarios, a fault happens in one of the microgrids. Two considered fault scenarios have an approximate fault current of 18x to 23x of the rated current in the secondary windings. Results showed that insulated windings in FTCL HTS transformers could substantially reduce the peak temperature of the HTS windings, compared to bare windings. Afterwards, post-fault loading is imposed on the HTS windings, to observe their performance against the current increase after fault clearance. In this case, for the first scenario of the faults, the FTCL HTS transformer could tolerate 192% of post-fault overloading, while this number for the second fault scenario is 170%. Finally, the impact of post-fault loading on the full recovery time was discussed.

The homogeneity of the microstructure and composition are critical in determining the properties of rare earth-barium-cuprate, single grain bulk superconductors [(RE)BCO]. The magnitude of the trapped magnetic field achieveable in these technologically important materials, in particular, is influenced heavily by the size and distribution of (RE)₂BaCuO_x (RE-211) flux pinning inclusions in the bulk microstructure, whereas the size and distribution of silver agglomerates present within the bulk superconducting matrix correlate directly with improved mechanical properties. With careful engineering, these materials have significant potential for application in range of devices related to energy storage, medicine, electro-magnetic machinery and microelectronic technology. Fabrication of (RE)BCO bulk superconductors typically involves heating a powder compact above its peritectic decomposition temperature followed by slow cooling to facilitate the growth of a single grain. Each (RE)BCO composition has a different peritectic temperature and growth rate, which, therefore, necessitates different requirements in the heating profile. The fabrication temperature and growth rate, for example, may have an effect on the RE-211 and silver distribution, which may, in turn, affect the superconducting properties of the resulting single grain.

In this work we compare the distributions of silver and RE-211 in the single grain microstructures of YBCO-Ag, GdBCO-Ag, EuBCO-Ag and SmBCO-Ag bulk superconductors using optical microscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy. We observe that the distributions are very similar along both the a/b-axis and c-axis of these materials. This suggests that factors other than the maximum temperature used to achieve peritectic decomposition and the rate of single grain growth are particularly influential in determining the properties of the as-processed samples in the top seeded melt growth process. This observation demonstrates there is freedom to use (RE) materials interchangeably between different applications as required, for example, for functional or economic reasons.