A delta-wye transformer is a type of three-phase electric power transformer design that employs delta-connected windings on its primary and wye/star connected windings on its secondary. Aneutral wire can be provided on wye output side. It can be a single three-phase transformer, or built from three independent single-phase units. An equivalent term is delta-star transformer.
Delta-wye transformers are common in commercial, industrial, and high-density residential locations, to supply three-phase distribution systems.
An example would be a distribution transformer with a delta primary, running on three 11kV phases with no neutral or earth required, and a star (or wye) secondary providing a 3-phase supply at 415 V, with the domestic voltage of 240 available between each phase and the earthed (grounded) neutral point.
The delta winding allows third-harmonic currents to circulate within the transformer, and prevents third-harmonic currents from flowing in the supply line.[2] Delta connected windings are not common for higher transmission voltages (138 kV and above) owing to the higher cost of insulation compared with a wye connection
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