Please explain , why three phase power transmission is preferred over single phase transmission?

Answer 1

Power for 3 phase is 1.73VI (square root of 3 times voltage times current). This uses 3 wires. Power for single phase is just VI over 2 wires. For 1.5 times the wires you can transmit 1.73 times the power. Also, to transmit single phase would make conversion to 3 phase for 3 phase equipment difficult (getting single phase from 3 phase is trivial). And all generation is done as 3 phase.

For both generators and motors, the main advantage of 3 phase over single phase is constant power. The instantaneous power of any single phase device varies with the voltage and current (AC). The 60 W for your light bulb is average power. For 3 phase, the instantaneous power (sum of the 3 phases) is constant.