As we discussed briefly at the beginning of Chap . 4, the shunt admittance of a transmission line consists of conductance and capacitive reactance. We have also mentioned that conductance is usually neglected because its contribution to shunt admittance is very small. For this reason this chapter has been given the title of capacitance rather than shunt admittance.
Capacitance of a transmission line is the result of the potential difference between the conductors; it causes them to be charged in the same manner as the plates of a capacitor when there is a potential difference between them. The capacitance between conductors is the charge per unit of potential difference. Capacitance between parallel conductors is a constant depending on the size and spacing of the conductors. For power lines less than about 80 km (50 mi) long, the effect of capacitance can be slight and is often neglected. For longer lines of higher voltage capacitance becomes increasingly important.
An alternating voltage impressed on a transmission line causes the charge on the conductors at any point to increase and decrease with the increase and decrease of the instantaneous value of the voltage between conductors at the point. The flow of charge is current, and the current caused by the alternate charging and discharging of a line due to an alternating voltage is called the charging current of the line. Since capacitance is a shunt between conductors, charging current flows in a transmission line even when it is open-circuited. It affects the voltage drop along the lines as well as efficiency and power factor of the line and the stability of the system of which the line is a part.