Single-phase transformers can be connected to form three-phase transformer banks for stepping voltages up or down in three-phase systems. Four common configurations for connecting transformers in three-phase systems are delta–delta, wye–wye, wye–delta, and delta–wye.

The first three are shown in Fig. 3-9. The delta–wye is not shown because it is simply the reverse of the wye–delta connection.

Delta–delta connection
The delta–delta connection, shown in Fig. 3-9a, is widely used for moderate voltages. This connection has the advantage of remaining operational in what is known as the open delta or V connection if one transformer is damaged or taken out of service, leaving the remaining two functional.

If it is operated this way, the bank still delivers three-phase currents and voltages in their correct phase relationships. However, the capacity of the bank is reduced to 57.7 percent of the value obtained with all three transformers in service.

Wye–wye connection
In the wye–wye connection, shown in Fig. 3-9b, only 57.7 percent (or 1/1.73) of the line voltage is applied to each winding, but full line current flows in each transformer winding. The drawback to this connection is that power circuits supplied from a wye–wye bank generate serious electromagnetic interference, which could interrupt nearby communications circuits.

Because of this and other disadvantages, the wye–wye connection is seldom used. However, the wye–wye connection can be used to interconnect two delta systems and provide suitable neutrals for grounding both of them.

Delta–wye and wye–delta connections 
The delta–wye connection (not shown) is suitable for stepping up voltages because the voltage is increased by the transformer ratio multiplied by a factor of 1.73. Similarly, the wye–delta connection, shown in Fig. 3-9c, is used for stepping down voltages.

The high-voltage windings of most transformers operating at more than 100 kV are wye-connected. To match the polarities correctly in a wye connection, the H and X markings must be connected symmetrically.

In other words, if an H1 or X1 terminal is connected to the neutral, then all of the H1 or X1 terminals must be connected to the neutral and the remaining H2 or X2 terminals must be brought out as the line connections, as shown in Fig. 3-9b.

By contrast, in a delta connection, H1 must always be connected to H2 and X1 to X2, and the line connections must be made at these junctions, as shown in Fig. 3-9a.

When a large number of single-phase loads are to be served from a three-phase transformer bank, the wye connected low-voltage winding is recommended because the single-phase loads can be balanced evenly on all phases.

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