3-12
Figure 3-9.—Three-phase alternator or transformer connections.
A three-phase transformer may be made up of three, single-phase transformers connected in delta,
wye, or a combination of both. If both the primary and secondary are connected in wye, the transformer is
called a wye-wye. If both windings are connected in delta, the transformer is called a delta-delta.
Figure 3-10 shows single-phase transformers connected delta-delta for operation in a three-phase
system. You will note that the transformer windings are not angled to illustrate the typical delta (D) as has
been done with alternator windings. Physically, each transformer in the diagram stands alone. There is no
angular relationship between the windings of the individual transformers. However, if you follow the
connections, you will see that they form an electrical delta. The primary windings, for example, are
connected to each other to form a closed loop. Each of these junctions is fed with a phase voltage from a
three-phase alternator. The alternator may be connected either delta or wye depending on load and voltage
requirements, and the design of the system.
Figure 3-10.—Three single-phase transformers connected delta-delta.
