A three-phase transformer bank can be easily created by using three single-phase transformers. The two sides of these three transformers can be either connected in a wye or a delta configuration, thus allowing four possible types of connections. These are:
• WYE WYE:
With the wye-wye (Y-Y) connection, the secondary side is in phase with the primary circuit, and the ratio of primary to secondary voltage is the same as the ratio of turns in each of the phases. A possible connection is shown in Figure 1. Power distribution circuits supplied from a wye-wye bank often create series disturbances in communication circuits (e.g., telephone interference) in their immediate vicinity. One of the advantages of this connection is that when a system is changed from a delta to a four-wire wye to increase system capacity, existing transformers can be used.
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Figure 1 Y-Y transformer with 0° phase shift between the
primary and the secondary sides.
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• WYE-DELTA:
In the Y-Δ connection, there is a 30° phase angle shift between the primary and secondary sides. The phase angle difference can be made either lagging or leading, depending on the external connections of the transformer bank.
The case with the primary side lagging is shown in Figure 2, and the case with the primary side leading is shown in Figure 3. The transformation ratio is times the ratio of turns in each of the phases.
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| Figure 2 Y-Δ transformer with the primary side lagging the secondary side by 30°. |
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| Figure 3 Y-Δ transformer with the primary side leading the secondary side by 30°. |
• DELTA-WYE:
With the 
Δ-Y connection, the neutral of the secondary wye can be grounded and single-phase loads connected across the phase and the neutral conductor.
Δ-Y connection, the neutral of the secondary wye can be grounded and single-phase loads connected across the phase and the neutral conductor.
Three-phase loads are connected across the phases. The phasor relationship between the primary and the secondary sides is shown in Figure 4. The transformation ratio is 1/√3 times the ratio of turns in each of the phases.
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| Figure 4 Δ-Y transformer with the primary side leading the secondary side by 30°. |
• DELTA-DELTA:
The Δ-Δ connection does not cause a phase shift between the primary and the secondary sides. The phasor relationship of this transformer is shown in Figure 5. The transformation ratio is equal to the ratio of the turns in each of the phases. There is no problem from third-harmonic over voltage or telephone interference because such disturbances get trapped in the delta and do not pass into the lines.
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| Figure 5 Δ-Δ transformer with 0° phase shift between the primary and the secondary sides. |
Although these four configurations are the most common ones used, other arrangements are possible, including:
• OPEN-DELTA:
An advantage of the Δ-Δ connection is that if one of the single- phase transformers becomes damaged or is removed for maintenance, the remaining two can be operated in a so-called open-delta connection. Because the currents in each of the two remaining transformers are the same as the line current, each transformer carries times the current it was carrying in the closed-delta connection. The open-delta bank continues to deliver three-phase currents and voltages in their correct phase relationship. To keep the transformers from being overloaded, however, it is necessary to reduce the line currents by approximately 1/√3 .
• SCOTT OR T-CONNECTION:
The Scott or T-connection is used when a two-phase (or a transformed three-phase) supply is needed from a three-phase system. In general, the T-connection is used for deriving a three-phase transformation, and the Scott connection is mainly used for obtaining a two-phase output. The two connections are similar in basic design. Either connection requires two specially wound single-phase transformers. The main transformer has a 50 percent tap on the primary winding, whereas the other transformer, called the teaser transformer, has an 86.6 percent tap. The main transformer is connected between two primary lines, whereas the teaser transformer is connected from the center tap of the main transformer to the third primary line. The secondary sides of the transformers provide two-phase service. A T-connection is shown in Figure 6.
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Figure 6 The T connection for a three-phase to two-phase
transformation.
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