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. We 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 described 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. It was shown that the maximum temperature rise of the conductor until interruption of the power line increases monotonically with the one-turn voltage, but has a minimum 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 that in the conventional transformers even if the copper ratio is optimized for the rated capacity and voltage.
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