Three phase transformers are constructed by winding three single phase transformers on a single core. These transformers are put into an enclosure that is then filled with dieletric oil.
Dieletric oil performs several functions for three phase transformers like providing electrical insulation between the windings and case due to its electrical nonconductivity, and it also helps prevent deterioration of the winding insulation by cooling and preventing moisture formation.
Three phase transformers use four possible transformer combinations: Delta to Delta, Delta to Wye, Wye to Delta and Wye to Wye. Delta transformer combinations are good short-distance distribution systems and are generally used for neighborhood and small commercial loads that are close to the supplying substation. Three phase transformers with Wye systems are good for longer distances.