Optimal location and minimum number of superconducting fault current limiters for the protection of power grids

This paper presents a novel method to determine the optimal strategy for the allocation of multiple resistive superconducting fault current limiters (SFCLs) aiming to improve the overall protection of standard power grids. The presented approach allows for the straightforward determination of the optimal resistance of the SFCL, accounting for short circuit events occurring at different locations, by modelling the electro-thermal properties of the SFCL via a temperature dependent E-J power law. This material law, based on previous experimental evidence, allows for the introduction of flux pinning, flux creep, and flux flow properties of the superconducting material within a minimum level of complexity. Thereby, we have observed a distinctive kink pattern in the current limiting profiles of the SFCLs, from which no further reduction of the first peak of the fault current is achieved when a greater resistance is considered, allowing a univocal determination of the optimum SFCL resistance. This peculiarity is not observed when the model for the quench properties of the SFCL is simplified towards an exponential resistance, although the last can be used as an auxiliary process for addressing the first guess on the resistance value required for a specific strategy, as it demands less computing time. We have also determined that for many of the cases studied, i.e., for the combinations between one or more SFCLs installed at different locations, and those subjected to fault events located at different points in the network, the recovery time of the superconducting properties of at least one of the SFCLs can last for more than 5 min, constraining the feasibility of a large-scale deployment of this technology. However, by assuming that the practical operation of the SFCL is assisted by the automatic operation of a bypass switch when the SC material is fully quenched, we have determined that the optimal strategy for the overall protection of power grids of standard topology requires a maximum of three SFCLs, with recovery times of less than a few seconds. This information is of remarkable value for power system operators, as it can establish a maximum investment threshold which ultimately can facilitate making decisions regarding the deployment of SFCL technologies.This work was supported by the Engineering and Physical Sciences Research Council (EPSRC) project NMZF/064. X. Zhang acknowledges a grant from the China Scholarship Council (No. 201408060080)