A 15-kV class intelligent universal transformer for utility applications

This paper discloses the development of 15-kV class intelligent universal transformer (IUT) to show system level design, circuit topology, and prototype test results. The complete system is split into two stages: (1) high-voltage ac to low-voltage dc and (2) low-voltage dc to low-voltage ac. With the adoption of silicon carbide (SiC) devices, the high-voltage front-end ac to low voltage dc conversion stage achieves 98.4% efficiency, and the complete power stage can be naturally cooled. With the adoption of auxiliary resonant soft-switching inverter, the second stage achieves 99.2% efficiency. Overall the system has been demonstrated at 97.5% efficiency without forced-air cooling. Extended 8-hour testing was conducted to ensure long-term operation reliability. Overall efficiency and voltage regulation were compared with that of a conventional transformer for justification of IUT adoption.