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Methods to protect superconducting transformers from premature quench due to local disturbances and excess current at accidental short-circuit were studied with regard to structure of the conductor and construction of the transformer. Average variations in temperature of the winding were calculated by a heat balance equation under the conditions both of constant current for the premature quench and of constant voltage for the quench due to the excess current. The superconducting transformer is protected from the quenches by suppressing the increase in the temperature of the winding from the initiation of quench to the interruption of the power supply. A permissive level of the maximum temperature restricted the volume fraction of copper in the conductor for the former type of quench and the length of the winding for the latter. The superconducting transformer protected from the above two types of quenches was designed and compared with conventional transformers concerning the ‘one-turn voltage’ of the winding. In the comparisons, the main characteristics of the superconducting transformer, efficiency and % impedance, were equivalent to the conventional one. In the range of capacity less than 300 MVA, the one-turn voltage can be larger than that of the conventional one for the maximum temperature of 200K. The maximum one-turn voltage acceptable for the protection of windings is not compatible with the % impedance for the capacity more than 300 MVA. The increase in the permissive temperature of the quenched winding may be neccessary for the superconducting transformer with high performances.

Quench protection of superconducting transformers.