308CHAPTER 10. POLYPHASE AC CIRCUITS10.6Three-phase transformer circuitsSince three-phase is used so often for power distribution systems, it makes sense that we would needthree-phase transformers to be able to step voltages up or down. This is only partially true, asregular single-phase transformers can be ganged together to transform power between two three-phase systems in a variety of conﬁgurations, eliminating the requirement for a special three-phasetransformer. However, special three-phase transformers are built for those tasks, and are able toperform with less material requirement, less size, and less weight than their modular counterparts.A three-phase transformer is made of three sets of primary and secondary windings, each setwound around one leg of an iron core assembly. Essentially it looks like three single-phase trans-formers sharing a joined core as in Figure 317,10.40.Three-phase transformer coreFigure 10.40: Three phase transformer core has three sets of windings.Those sets of primary and secondary windings will be connected in either ∆ or Y conﬁgurationsto form a complete unit. The various combinations of ways that these windings can be connectedtogether in will be the focus of this section.Whether the winding sets share a common core assembly or each winding pair is a separatetransformer, the winding connection options are the same:• Primary - Secondary•Y-Y•Y-∆•∆-Y•∆-∆The reasons for choosing a Y or ∆ conﬁguration for transformer winding connections are thesame as for any other three-phase application: Y connections provide the opportunity for multiplevoltages, while ∆ connections enjoy a higher level of reliability (if one winding fails open, the othertwo can still maintain full line voltages to the load).Probably the most important aspect of connecting three sets of primary and secondary windingstogether to form a three-phase transformer bank is paying attention to proper winding phasing (thedots used to denote “polarity” of windings). Remember the proper phase relationships between thephase windings of ∆ and Y: (Figure 318,10.41)Getting this phasing correct when the windings aren’t shown in regular Y or ∆ conﬁgurationcan be tricky. Let me illustrate, starting with Figure 318,10.42.