The basics of power transformers in transmission and distribution grids

Three-phase transformer connections

Three-phase connections can be made either by using three single-phase transformers or by using a three-phase transformer. Advantages of the three-phase transformer are: it costs less, the weight is less, it requires less floor space and it has lower losses than three single-phase transformers.

The methods of connecting windings will be the same, whether using the one three-phase transformer or three separate single-phase transformers. The two general methods of connecting three-phase transformers are shown in figure 1.

Differences between wye and delta connection in that wye connection has two phases in series. And 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.

Wye–Wye Connections of Transformer

For high-voltage transmission systems, the use of the wye-connected transformer is more economical because the voltage across the phase of each winding is a factor of 1.73 less than the voltage between the lines.

If the neutral point is grounded, there is no need to insulate it for the line voltage.

Figure 2 shows a bank of three transformers connected in Y on both the primary and secondary sides. If the ratio of transformation of each transformer is K, then the same ratio will exist between the line voltages on the both sides.

This connection will give satisfactory service only if the three-phase load is balanced. When the load is unbalanced, the electrical neutral will shift from its exact center to a point that will make the line to neutral voltages unequal.

Advantages of the Y-Y connection:

The primary and secondary circuits are in phase; i.e., there are no phase angle displacements introduced by the Y-Y connection. This is an important advantage when transformers are used to interconnect systems of different voltages in a cascading manner.

If the neutral end of a Y-connected winding is grounded, then it is possible to use reduced levels of insulation at the neutral end of the winding. A winding, which is connected across the phases, requires full insulation throughout the winding.