In the 1880’s the development of transformers contributed essentially to the enforcement of alternating and three-phase current technology. Transformers enable the transmission of electrical energy using the voltage adapted to power and distance. A three-phase transformer is made of three sets of primary and secondary windings, each set wound around one leg of an iron core assembly. A three-phase transformers must have (3) coils or windings connected in the proper sequence in order to match the incoming power.
Three phase transformer core has three sets of windings. Those sets of primary and secondary windings will be connected in either ? (delta) or Y (wye) configurations to form a complete unit. The three primary windings (P1, P2, P3) will be connected at the factory to provide the proper sequence (or correct polarity) required and will be in a configuration known as Delta. The three secondary windings (S1, S2, S3) will also be connected at the factory to provide the proper sequence (or correct polarity) required.
Find more detail discussion on Three-Phase Transformers Circuits Diagram here:
Three-Phase Transformers Circuits Diagram – link (source: www.allaboutcircuits.com)
3 Phase Electrical Power Transformer – link (source: www.3phasepower.org)
Three-phase electric power – link (source: www.wikipedia.org)