Analysis of a passive superconducting fault current limiter

An analysis was carried out to predict the forced response of the equivalent circuit of an innovative superconducting fault current limiter (FCL). The FCL employs two superconducting coils with differing critical currents wound noninductively. The analysis shows that to reduce the voltage drop under normal operating conditions, the coupling coefficient should be kept fairly high (k>0.90). For a given coil configuration (/spl omega/L=constant), the limiting capability of the device increases with the resistance R of the trigger coil up to a certain value of R, then further increase in R changes very little the limiting capability of the FCL. However, further increase in R can reduce the heat generation rate in the device which will help alleviate the problem of relatively long recovery time. The fault current predicted from estimated values of R compares fairly well with results of an experiment reported in the literature. The discrepancy between the predicted and measured current is due mainly to the uncertainty in the estimated resistance R, because both the purity of Cu and the percentage of Cu in CuNi are not known. By varying the ratio of NbTi-Cu-CuNi in the matrix, the purity of Cu, and the percentage of Cu in CuNi various values of R, can be achieved, which should help to alleviate the problem of excessive Joule heating and recovery time.