260 Electrical Power Systems Technology
formers is not needed until a later time. Two identical single-phase trans-
formers can be used to supply power to the load until, at a later time, the
third transformer is needed to meet increased load requirements.
Types of Three-phase Systems
T hree-phase power distribution systems, which supply industrial and
commercial buildings, are classified according to the number of phases and
Figure 10-3. Basic three-phase transformer connection methods: (A) The delta-
delta connection, (B) The delta-wye connection, (C) The wye-wye connection,
(D) The wye-delta connection, and (E) The open delta connection