Quench protection of superconducting transformers

Protection of superconducting transformers from the quench caused by excess fault current was studied theoretically with the aid of a simple equivalent circuit model which was composed of leakage inductance and resistances of the conductor. The authors distinguished the resistance in the flux-flow state from that in the normal state, and basic parameters of the transformer, such as sizes, leakage inductance, and conductor length, were determined as functions of one-turn voltage. The quench process after the short-circuit was numerically analyzed by this circuit equation with the aid of the heat-balance equation. The maximum temperature rise of the conductor until interruption of the power line was found to increase monotonically with the one-turn voltage, but the minimum is around several percent of the copper ratio in the conductor. To suppress the temperature rise, therefore, the one-turn voltage should be limited to smaller values than in conventional transformers, even if the copper ratio is optimized for the rated capacity and voltage.