Is instead of neutral wire can you ground it by putting the wire in to earth if yes then why is extra wire used for neutral by eleminating netrual wire cost of transmission can be reduced?

Answer 1

Transmission lines do not use a neutral conductor, so your question isn't actually relevant.

Neutral wires are only necessary where imbalance may be large, and under some conditions current may be flowing in the neutral. Under these conditions a low impedance path (the neutral wire) is more desireable than a high impedance path (the ground). In many overhead transmission lines, a fourth or fifth wire are placed above the current carrying conductors; these are shield wires (not neutral wires) and are used to minimize outages on the line due to lightning (the lightning will strike these instead of the phase wires, these will flash over to the structures and the energy will be shorted to ground without the line having to be removed from service). When you're dealing with things that cost ~$100,000 - 2M/mile, it may be worth the added expense of a shield wire to keep it in service as much as possible.