Feasibility of a power transformer with superconducting windings

Although only 0.2% of the total power of a large generator transformer is accounted for by I2R losses, the capitalised value of this power dissipation is about half the price of an 800 MVA transformer. Superconducting windings with no power losses are therefore a potentially attractive substitute for existing copper windings. However, existing superconductors have a.c. losses which both reduce the additional capital sum that may be spent and increase the size and cost of the refrigerator. There is virtually no net saving when the cost additional materials is added to that of the refrigerator. Variations in the size and shap of the superconducting transformer, including an air-cored toroid, probably do not give any worthwhile advantage over a conventional transformer, although, with existing superconductors, the economically most competitive design is significantly larger than its conventional counterpart. It is probable that a straightforward niobium-on-copper winding would not be able to take a short-circuit fault and the immediately ready for normal use. Either a protective system using a switched-in separate copper winding or one using a type 2 superconductor might allow fault conditions to be withstood. No deep study has been made of either the electrical insulation or mechanical loads, both of which will present difficulties. The prospects for a viable superconducting transformer would be much brighter if a low-loss superconductor were developed having running a.c. losses not much greater than 0.1 W m ¿2 at Js = 110A mm¿1 and, under fault conditions, less than 500 Wm¿2 at 800 A mm¿1.