For substation wiring(not for controls), stranded copper wire is used to reduced the vibrations caused by the currents passing thru at high voltages. Solid wires exhibit more vibrations compared to stranded ones. This is because, each strand in a stranded cable vibrates separately. However since...
Stranded copper wires have greater flexibility compared to solid ones. Solid wires also break easily compared to stranded ones. Have you tried cutting/breaking a small solid wire without a cutter and you just bend it (using your fingers) a lot of times until the bent area heats up then breaks...
