Three phase transformers are described by how they are configured including the primary (input) and secondary (output) coil windings and how the three phases are connected on either side. The two terms used to describe these phase connections are delta and wye. Separately, the terms delta and wye refer to either side (primary or secondary) of a three phase transformer. Combined, delta and wye describe the primary and secondary (in that order) configuration of a three phase transformer. (e.g. delta-delta, delta-wye, wye-delta, wye-wye)
Delta refers to the phase connection type that is schematically represented as a triangle. Delta connected phase windings are connected to each other in a “triangle” with respect to polarity where the positive of each phase winding is connected to the negative of the next.
Wye connected phase windings are schematically represented as a “Y” or star shape. Wye connected phase windings are connected to each other with either all of the positives or all of the negatives of the windings connected with each other. The common connection of the the phase windings (whether positives or negatives) are connected to a neutral.
A peculiar but functioning version of a three phase delta configured transformer allows two pairs (instead of three) of coils to be used in what is called an open delta or V configuration. This type of transformer configuration can transform three phase electrical power. The power that can be delivered is almost reduced by 1/2 of that which a three coil delta connection can deliver. (assuming equal rating of individual coil conductor and core material) An open delta transformer can be used in applications where the full punch of a closed delta configuration is not needed to run equipment but still the need for three phase power is present.