Development of a 3φ-66/6.9 kV-2 MVA REBCO superconducting transformer

We have designed and fabricated a 3φ-66 kV/6.9 kV-2 MVA transformer with RE₁Ba₂Cu₃O_7-δ(REBCO, RE:Rare Earth, Y, Gd etc.) superconducting tapes. It is a 1/10 model of a 3φ-66 kV/6.9 kV-20 MVA one for a distribution power grid. The superconducting windings were reduced only in current capacity by reducing the number of tapes in parallel conductors. In the primary side, a single REBCO tape with a width of 5 mm was cylindrically wound into 8 layers. In the secondary one, an 8-strand parallel conductor was wound similarly into 2 layers, where each strand was transposed 15 times per one layer. The REBCO tapes for the secondary winding were also scribed by laser into a 3-filament structure to reduce the ac loss. The windings for 3 phases were installed into a GFRP cryostat which had an elliptic-cylinder-shape and three cylindroid bore for an iron core at room-temperature. A Ne turbo-Brayton refrigerator with a cooling capacity of 2 kW at 65 K was developed and located close to the transformer. The windings were cooled with subcooled liquid nitrogen at 65 to 70 K, which was forced-flowed by a pump unit between the transformer and the refrigerator. The completed transformer was first tested in liquid nitrogen at 77 K according to the domestic regulation for conventional transformers. The load loss, i.e., ac loss of the windings, was 26.9 W for the rated operation. The dielectric strength was also verified by applying 350 kV impulse voltage and 140 kV ac voltage for 1 minute.