15: Three-Phase Transformer Connections

 Three-Phase Transformer Connections

Three-phase power is attainable with one three-phase transformer, which is constructed with three single-phase units enclosed in the same tank or three separate single-phase transformers. 

The methods of connecting windings are the same, whether using the one three-phase transformer or three separate single-phase transformers.

Wye and Delta Connections

The two common methods of connecting three-phase generators,

motors, and transformers are shown in figure . 

The method shown in at figure a is known as a delta connection, because the diagram bears a close resemblance to the Greek letter ), called delta.

The other method, figure b, is known as the star or wye connection.

The wye differs from the delta connection in that it has two phases in

series. 

The common point “O” of the three windings is called the neutral because equal voltages exist between this point and any of the three phases.

When windings are connected wye, the voltage between any two lines will be 1.732 times the phase voltage, and the line current will be the same as the phase current. 

When transformers are connected delta, the line current will be 1.732 times the phase current, and the voltage between any two will be the same as that of the phase voltage.