Impact of Unbalanced Loads on the AC Losses of Inductive HTS Fault Current Limiters

Superconducting fault current limiters (SFCLs) are recognized as one of the large-scale applications of high-temperature superconducting (HTS) materials with the highest potential to reach the commercial market. They can be integrated into multiple locations in electrical grids, contributing to the electrification of the energy system and, thus, to the Energy Transition. While limiting the levels of short-circuit currents, SFCLs allow, e.g., increasing the penetration of renewable generation while keeping the ratings of existing protections and postponing investments by utilities. Several topologies of SFCL have been developed, while the most technologically mature are the resistive and inductive ones. Among the latter, two different types have highlighted, namely the saturated cores and the transformer-type SFCL, which is focused on in this paper. This SFCL behaves as a short-circuited transformer, where its secondaries are made of HTS tape. Voltages are induced in the HTS short-circuited secondaries by the primaries (grid lines), and consequently, AC losses are generated. These should be estimated a priori to design the cryogenic system. When connected, e.g., in a busbar coupling or a transformer feeder in the distribution grid, SFCLs allow operating transformers in parallel, improving grid stability, among other advantages. Yet, the presence of single-phase loads in distribution grids leads to unbalanced currents, affecting the amount of AC losses. In this paper, the impact of unbalanced currents on the AC losses of a transformer-type SFCL, and its consequences in the design of the cryogenic system, are analyzed through extensive experimental measurements.