Single Circuit Transmission Line refers to an arrangement of conductor over the Transmission Tower. In Single Circuit Transmission Line, three conductors corresponding to three distinct phases i.e. R, Y & B phase are run on the Transmission Tower.
A general thumb rule is that, if the number of conductor per bundle is 2 then it will be 220 kV line while if the number of conductor per bundle is 4 then it will be 400 kV line. This is general thumb rule and variation is expected.
Power supply using Double Circuit Transmission Line enables the transfer of more power over a particular distance. However, running two circuits in close proximity to each other will involve inductive coupling between the conductors. This needs to be taken into account when calculating the fault level and while designing the protection schemes. In case of Distance Protection of Double Circuit Line, a compensation for Mutual Coupling between the circuits shall be provided otherwise the Distance Relay may under-reach / overreach.
Let us suppose that we have a Power Generating Station and we want to supply three phase power to load centre A through two different lines. In this case it will be advantageous and economical to use Double Circuit Transmission Line as the cost of extra Towers will be saved if we used Single Circuit Transmission Line. The transmission is thus cheaper and requires less land and is considered ideal from an ecological and aesthetic point of view. Further, due to Double Circuit Transmission Line the reliability of power supply increases. Not only reliability rather maintenance opportunity also increases. If we want to take one circuit on maintenance then power supply through the other circuit will continue.