Test Results and Conclusions From a 1 MVA Superconducting Transformer Featuring 2G HTS Roebel Cable

We present results from commissioning testing of a 1 MVA three-phase transformer demonstration project. The three-phase transformer utilizes a 15 strand Roebel cable for the low voltage windings and 4 mm wide conductor for the high voltage windings. Both windings use ReBCO conductor and the iron core is external to the cryogenic region. The target operating temperature is 70 K. Key design features and operational performance measures of the advanced sub-cooled liquid nitrogen circulation system are described. The overall thermal budget is presented along with ac loss measurements for the windings and electrical test results of the complete transformer. There remain some unresolved issues with the stability of the cryogenic cooling system when operated sub-cooled, and with the high voltage withstand performance of the HV bushings. These issues are described and discussed. The utility of HTS Roebel cable as a convenient means of producing a compact, high current winding with low ac loss characteristics has been successfully demonstrated.