Experiment using variable reactor of rectifier type superconducting fault current limiter with a short-circuited trigger coil

A rectifier type superconducting fault current limiter using an additional short-circuited trigger coil has been studied. Such reactor consists of two well-coupled superconducting coils; the main coil and trigger coil. The main coil is connected to the DC link of the bridge circuit while the trigger winding is always short-circuited. As the trigger coil is short-circuited the reactor shows low inductance in normal operation. A change of load current due to load change or some other reasons increase the short-circuit current but inductance value remains unchanged as long as the trigger coil remains in superconducting state. During fault time, the fault current yields super to normal transition current in the secondary (trigger) winding and the trigger coil quenches. Thus higher impedance appears in the main coil that limits the fault current. The advantage of using an additional trigger coil with the main coil is that it reduces the impedance of the current limiter at normal operation and therefore reduces voltage drop across the limiter. To enhance the coupling factor of the windings iron core can be used. If the magnetic path is closed with the iron core even with some air gap it can be saturated during fault time because of the high DC current but would effectively reduce leakage reactance during normal operation. In this paper a small-scale experimental result of this type of current limiter has been presented.