Prospects for large high-temperature superconducting power transformers: conclusions from a design study

A design feasibility study has been conducted for a 240MVA high-temperature superconducting grid auto-transformer. Conclusions are relevant to superconducting power transformers in general. It is argued that economic benefits may be predicted, subject to assumptions about achievable conductor properties, costs of components and power system operating requirements. Liquid nitrogen cryogenics is relatively cheap and simple, and refrigeration power demand is reduced by a factor of the order of 20 compared to the low-temperature case. Attention is drawn to the importance of AC losses in the superconductor and the difficulty of keeping these sufficiently low. Various technical problem areas, and their likely influence on the overall design concept, are reviewed. Three particularly important influences are identified: insulating properties of liquid nitrogen coolant, required transformer performance in the through fault condition, and mechanical strength to withstand electromagnetic forces. Design proposals are detailed, and recommendations made for future development of high-temperature superconductors for power applications.