Proposal for an RMS thermoelectric model for a resistive-type superconducting fault current limiter (SFCL)

In general, predictions of the limiting characteristics of a resistive superconducting fault current limiter (SFCL) include the electrical and thermal behaviour of the superconductor. Acknowledging that these two phenomena have very different time scales for limiters inserted in an electric power system, we propose in this paper a root mean squared (RMS) thermoelectric model for a resistive-type superconducting fault current limiter.Unlike other approaches, the thermoelectric model does not use the E(J,T) characteristic, which is an instantaneous feature of the superconductor. Instead, an RMS characteristic is directly estimated from experimental trials on the SFCL. These allow the establishment of a simple but precise lumped-parameter representation of the superconductor load, which is more adequate in predicting the performance of the limiter in an electric power system if a short-circuit fault occurs.We report experimental and simulated results based on the electrical testing of a small single-phase resistive-type SFCL using the thermoelectric superconductor model developed. The experimental limiter was made using liquid nitrogen-cooled Bi-2223 HTS tubes connected in series and located in the secondary of a transformer, which was inserted in the electric circuit to be protected.The results obtained have shown good agreement between the predicted results and the experimental measurements, indicating that it is possible to accurately simulate the effectiveness of the SFCL at limiting the electric current using the proposed RMS thermoelectric model for the superconductor load. Most importantly, the thermal model developed for the superconductor can be employed to establish the time period during which the superconductors could be inserted in the power circuit without reaching critical internal thermal stresses that could cause damage to them.