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 3.2.7.2 Wye-Delta. The Wye-Delta connection is shown in Figure 3-2. Wye-Delta
connections are generally used for large-ratio step-down transformers. The primary is a wye
connection due to the inherent mechanical and electrical advantages of wye circuits, while a delta
connected secondary provides an improved waveform to loads.
3.2.7.3 Wye-Wye. The Wye-Wye connection is shown in Figure 3-3. Wye-Wye
connections are seldom used since they are subject to disturbances from harmonic voltages and
currents. Unbalanced loads cannot be carried on the secondary unless the primary neutral is
provided. For those applications which require a wye-wye transformation, a third winding, delta
connected and designated as the tertiary is recommended to provide a low impedance path for
zero-sequence currents. The third winding can also be used to serve an auxiliary load if external
connection facilities are provided.
3.2.7.4 Delta-Delta. The Delta-Delta connection is shown in Figure 3-4. The Delta-Delta
connection is also seldom used. It is used on Naval ships to enhance damage control. A phase to
ground fault will be detected but does not trip breakers because there is minimal fault current.
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