1992 Volume 27 Issue 6 Pages 473-484
The responses of superconducting transformers to lightning surge are discussed theoretically. For this purpose, a 300/3MVA, 275/√3kV superconducting transformer is conceptually designed, where the dielectric strength of supercritical He is supposed to be 35kV/mm. Equivalent circuits and basic equations which can describe the quench and evaluate the ac loss of superconducting windings are proposed. The responses of superconducting transfomers to the full wave with 1, 050kV in peak value of voltage and 1.2μs in front time are numerically simulated. In order to elucidate the design problems of superconducting transformers, the responses of four types of typical windings, i.e. ordinary circular-disk type, high-series-capacitance type, condenser-coupling-shield type and multi-layer type windings, are respectively examined numerically. It is shown that multi-layer type windings have excellent properties both for initial voltage distribution and current oscillation. Whereas high-series-capacitance type and condenser-coupling-shield type windings have been commonly used for conventional transformers in place of ordinary circular-disk type windings for improving the initial voltage distributions, ac losses due to current oscillation amount to 104 times as much as under rated operation and quenches are shown to be induced by temperature increment. The study on the insulation design of these windings by taking account of the degradation of dielectric strength of supercritical He due to temperature increment shows that dimensions of these windings should considerably be enlarged in comparison with ordinary circular-disk type windings. The present results indicate that the conventional analysis of only voltage response to lightning surge is insufficient for the insulation design of superconducting transformers.
