Star Type Transformer

because cost of three conductor is lesser then the cost of four conductor for the transmission line. in short transmission line cost for delta system is lesser then the cost of star system

Bcoz for the same line voltage star connected transformer needs sqrt(3) times less turns . Hence its used for high voltage and delta for low voltage

Star/Delta windings have nothing to do with receiving or sending of power. On the contrary to your question you will find that all distribution transformers have secondary winding star connected in order to have 4 wires. In a star connection it is easy to detect a ground fault, whereas it is not in case of delta.

The delta winding in the transformer helps to circulate Zero sequence current generated due to ground fault on the star connected side. Hence, the unbalance created on one side of the transformer does not affect the other side. So, a circuit employing two transformers will definitely have a star & a delta to get benefit of both ground fault detection and circulation of ZSC.

bcoz at the transmission side especially for secondary distribution we need neutral wire for getting single phase supply and star type is only one which provide neutral. at receiving side to step up voltage we need delta type connection.

its not necessary to use only Delta type T/F at receiving end, we can also used star type but in this case at same voltage level two star neutral point create due to which Harmonic create for avoiding this we prefer opposite combination at sending end and receiving end.