Most Shinkansen substations in Japan receive their electric power supply for traction from extra-high voltage systems. "Modified-Woodbridge connection" transformers have been used for this since 1972; however, these have a rather complicated structure. As such, studies were carried out to develop a new type of transformer, which had the same functions, but was simpler. The transformer has been coined the "roof-delta connection" transformer since it has both roof and delta windings. Test results from a prototype of this transformer revealed that it could be used in AC traction feeding systems. This paper describes the fundamental characteristics of the roof-delta connection transformer and results from tests.