Transmission line conductors constitute a capacitor between them. The conductors of the transmission line act as a parallel plate of the capacitor and the air is just like a dielectric medium between them. The capacitance of a line gives rise to the leading current between the conductors. It depends on the length of the conductor.
The capacitance of the line is proportional to the length of the transmission line. Their effect is negligible on the performance of short (having a length less than 80 km) and low voltage transmission line. In the case of high voltage and long lines, it is considered as one of the most important parameters.
Capacitance of two-wire line
The capacitance of the transmission line along with the conductances forms the shunt admittance. The conductance in the transmission line is because of the leakage over the surface of the conductor. Considered a line consisting of two conductors a and b each of radius r. The distance between the conductors being D shown in the diagram below:-
If the two conductors a and b are oppositely charged, and the potential difference between them is zero, then the potential of each conductor is given by 1/2 Vab.
Capacitance Cab is the combination of two equal capacitance a and b in series. Thus, capacitance to neutral is twice the capacitance between the conductors, i.e.,
Let a balanced system of voltage be applied to a symmetrical three-phase-line shown below