Why there is a delta connection and star connection in a transformer ?

for lighting loads it needs a Neutral which connected with ground hence secondary is always is star connected, to minimize unbalance current in three phases.

actually the primary concept why there is delta and wye connection of the transformer coz in the reality. It is emphasize in the theory of electrical that they are the two major connection of the three phase power system.

The delta connection is at the primary for step down transformer while the secondary has the star connection. The star connection enables the connection of the neutral and the earth conductors