Considerations about HTS superconducting transformers

A superconducting transformer brings savings in weight, loss and to a lesser degree, in volume. A transformer uses a magnetic circuit in order to decrease the no-load current. The magnetic circuit can operate either at the same temperature than the superconducting windings or at room temperature. In the first case the cryostat is simple and can be metallic but the iron losses must be multiplied by the refrigeration system coefficient of performance (about 30 W/W at 77 K). With a magnetic circuit at 300 K, there is no multiplying coefficient for the iron losses but the cryostat is more complicated and requires electrical isolating materials which are problematic in cryogenics. We have reviewed several magnetic materials and measured their iron losses at 77 K and compared the latter to the one obtained at 300 K. Scratched Fe-Si steel show reasonable iron losses and could be used in a cold iron HTS transformers. Amorphous materials offer good opportunities. Transformers with a rating of 30 MVA were designed. With PIT (powder in tube) conductors the AC losses are relatively high and a cold magnetic circuit can be considered. However with Y coated conductors a cold magnetic circuit penalises the transformer efficiency a lot.