Since three-phase is used so often for power distribution systems, it makes sense that we would need three-phase transformers to be able to step voltages up or down. This is only partially true, as regular single-phase transformers can be ganged together to transform power between two three-phase systems in a variety of configurationsWhether the winding sets share a common core assembly or each winding pair is a separate transformer the winding connection options are the same:
Primary - Secondary
- Y - Y
- Y - Δ
- Δ - Y
- Δ - Δ
, eliminating the requirement for a special three-phase.
The reasons for choosing a Y or Δ configuration for transformer winding connection are the same as for any other three-phase application: Y connection provide the opportunity for multiple voltages, while Δconnection enjoy a higher level of reliability (if one winding fails open, the other two can still maintain full line voltages to the load).
Probably the most important aspect of connecting three sets of primary and secondary windings together to form a three-phase transformer bank is paying attention to proper winding phasing (the dots used to denote “polarity” of windings). Remember the proper phase relationships between the phase windings of Δ and Y: (Figure below)