Superconducting transformers: key design aspects for power applications

Conventional power transformers are very simple and reliable electrical components and their efficiency, for large power rating, is well above 99 %. With such an excellent performance the improvement margin seems very limited. However, due to the large amount of power managed and the continuous service, also a small increase in the efficiency is desirable. As an example, consider that an efficiency increase of 0.5 % of a 25 MVA transformer may lead to savings in the order of 100 k€/year. The use of superconducting materials opens the way to efficiency improvements on power transformers, and also adds important advantages such as size and weight reduction, that are very attractive for urban substations and transport applications. Moreover superconductors eliminate the need for refrigeration oil, thus avoiding the risk of fire hazard and reducing the environmental impact, in accordance with recent EU guidelines. In this paper a design procedure for HTS power transformers is reported. This procedure, that includes an analytical method for the calculation of the AC losses, is used to design a 25 MVA – 154 kV / 20 kV transformer based on commercial BSCCO tapes, and the evaluated performance are compared with those of a conventional copper transformer. The optimal working temperature is evaluated, and allowable cooling technologies are discussed. Considerations on the use of future 2nd generation YBCO coated conductors are also reported.