What are the advantage of stranded conductor over solid conductors in a transmission line?

In a transmission line, voltage is stepped up to a very high value. This is so as to take advantage of lower line current and smaller conductor size. At these potentials1, however, like charges repel, so most of the current flow is along the skin of the conductor. Stranded conductors, even wound wire over insulating rope, is more efficient, and more cost effective, than solid conductors.

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1 Actually, this effect occurs at any voltage, but it is most notable at the very high voltages used in power line transmission systems.

Stranded conductors are used because of their flexibility compared with solid conductors. Normally, transmission lines consist of a steel-wire core (which provides strength) around which are wound stranded aluminium conductors.