As mentioned above, single-phase transformers may be connected to obtain three-phase power. These are found at many Reclamation facilities, at shops, offices, and warehouses. The same requirements must be observed as in section 2.18, “Parallel Operation of Singlephase Transformers for Additional Capacity,” with one additional requirement—in the manner connections are made between individual single-phase units. ANSI standard connections are illustrated below in the following figures. There are other angular displacements that will work but are seldom used. Do not attempt to connect single-phase units together in any combination that does keep the exact angular displacement on both primary and secondary; a dangerous short circuit could be the result. Additive and subtractive polarities can be mixed (see the following figures). These banks also may be paralleled for additional capacity if the rules are followed for three-phase paralleling
discussed below. When paralleling individual three-phase units or single-phase banks to operate three phase, angular displacements must be the same.
Figure 18 shows delta-delta connections. Figure 19 shows wye-wye connections, which are seldom used at Reclamation facilities, due to inherent third harmonic problems. Methods of dealing with the third harmonic problem by grounding are listed below. However, it is easier just to use another connection scheme (i.e., deltadelta, wye-delta, or delta-wye [figure 20]), to avoid this problem altogether. In addition, these schemes are much more familiar to Reclamation personnel.
Figure 18 – Delta-Delta Connections, Single-Phase Transformers for
Figure 19 – Wye-Wye Connections, Using Single-Phase Transformers
for Three-Phase Operation