Question

Why transmission voltages are high?

Answer 1

Transmission voltage is always higher because of it uses 

Answer 2

If current is high then I²R losses will be more .If I²R losses is more then heat generated is more. By heat or increasing temperature resistance will also be increase which will decrease transmission efficiency. 

Also we know P = V x I

where, P = Power,

           V = voltage,

           I  = current.

By changing the value of voltage and current we can get constant power. By increase the voltage we are decreasing current which is advantageous. That's why transmission voltages is high.