A delta-wye (Δ-Y) transformer is a transformer that converts three-phase electric power without a neutral wire into 3-phase power with a neutral wire. It can be a single three-phase transformer, or built from three independent single-phase units. The term Delta-Wye transformer is used in North America, and Delta-Star system in Europe.
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 400 V, with the domestic voltage of 230 available between each phase and an earthed neutral point.
The line-to-neutral voltage is the voltage between any phase and neutral. The line-to-line voltage is the voltage between any two phases, and is equal to the phase to neutral voltage x in balanced steady-state operation.
Phase-to-neutral voltage Phase-to-phase voltage (rounded) 120 208 127 220 220 380 230 400 240 415 250 433 277 480 347 600
The closed circuit on the delta side provides some benefits. Voltages on the secondaries have improved balance. Also it cancels third (or ninth et cetera) harmonics since these are not supported on the three-wire system.
Delta-wye transformers introduce a 30 degree phase shift. Thus they cannot be paralleled with wye-wye (or delta-delta) transformers; they can be paralleled with other delta-wye (or wye-delta with some attention) transformers.
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