Over-voltage suppression in a fault current limiter by a ZnO varistor

A superconducting fault current limiter (SCFCL) of the transformer type with a ZnO varistor (metal oxide varistor) in parallel was investigated to determine the effects of the ZnO varistor as an over-voltage suppressor for the SCFCL. An SCFCL of the transformer type, which has an adjustable trigger current level, has been studied. A small model of this type of SCFCL was designed and built with NbTi superconductors. Since a fault current is reduced by its inductive component, a large over-voltage is observed at the beginning of the current limiting event. It is important to suppress the over-voltage to avoid any damages to the power system apparatus. Experimental results on the fault current limiting operation of the SCFCL with ZnO varistor in parallel are shown. It was confirmed that the surge voltage that appears at the terminal of the SCFCL can be successfully suppressed by ZnO varistor. Current limiting and recovery characteristics of the SCFCL with a ZnO varistor are investigated and discussed. The trigger current level of the SCFCL is not affected by the ZnO varistor. The recovery time is a little longer with the ZnO varistor than that without it. Energy dissipation in the ZnO varistor and the SCFCL is discussed.