Delta-wye transformer

Not to be confused with Y-Δ transform.

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 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 400 V, with the domestic voltage of 230 available between each phase and an earthed neutral point.

Output voltages

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 $\sqrt{3}$ 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

Advantages

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 (since they would be in phase on all three wires). A delta connection on the secondary side provides the possibility of large circulating currents if the characteristics of the 3 windings are not perfectly balanced. A wye connection avoids this (even if there is no requirement to bring the neutral connection out).

Phasing

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.

See also

• Mains power systems
• Electric power distribution
• Motor soft starter
• high-leg delta

External links

• Three-phase transformer circuits