Due to the potential difference between the conductors we have to take into account the capacitance of transmission line. The capacitance causes the line to be charged similar to the plates of the capacitor when potential difference is applied to it. The capacitance between the conductors is the charge per unit of potential difference. Between the parallel conductors, the capacitance is constant which depends on size and spacing of conductors.
If the power transmission line is having the length of 80km, the effect of capacitance is assumed to be negligible and is not taken into consideration. But with increasing length of transmission line alongwith high voltage, the effect of capacitance becomes prominent and important in analysis.
When an alternating voltage is impressed on a transmission line, it causes charge on the conductors at any point to increase and decrease with instantaneous change in value of voltage between the conductors. Due to alternate charging and discharging of line, current starts flowing in the line. This current is known as charging current. This current flows even when the line is open circuited. The voltage drop along the line is affected. It also affects the efficiency and power factor of the line. Stability of the system is also affected because of this.