Configurations for a Three-Phase Transformer

Three-phase transformers have six windings; three primary and three secondary. The six windings are connected by the manufacturer as either delta or wye. As previously stated, the primary windings and secondary windings may each be connected in a delta or wye configuration. They do not have to be connected in the same configuration in the same transformer. The actual connection configurations used depend upon the application.

In a three-phase transformer, there is a three-legged iron core. Each leg has one primary and one secondary winding. Figure 21 shows graphically what the delta and wye winding connections look like for a three-phase transformer.

The delta-connected winding in a three-phase transformer is a lot like three single-phase windings connected together.

A delta-connected secondary is illustrated here, with the primary not shown for the sake of simplicity. All of the illustrated voltages are secondary voltages available to the load.

There are equal voltages across each winding. Each of the voltages represents one phase of a three-phase system. The voltage is always the same between any two wires.

One of the phases, between X1 and X2 for example, can be used to supply single-phase loads. All three phases together supply three-phase loads. Notice in the illustration that X1, X2 and X3 are used to designate connection points. An "X" indicates a low voltage connection, and an "H" indicates a high voltage connection. This type of connection is referred to as a three-phase, three-wire connection.

The wye-connected winding is often called a "star connection." Wye connections provide two voltages because of the neutral connection. These are shown below. In other words, the wye has four leads, three phase leads and one neutral lead. This is also known as three-phase, four-wire connection.

Any line-to-neutral voltage (voltage across a phase) is always less than the line-to-line voltage.
If one voltage is known, the other can be calculated. The line-to-line voltage is 1.732 times the line-to-neutral voltage. You can prove this calculation for yourself using the voltages provided in the illustrations.