Whenever there are two conducting plates, separated by an insulating medium, a capacitance is formed. In a similar fashion, there exists a capacitance between two overhead transmission lines, and between a transmission line and the earth, which are separated by a distance with air acting as the insulating medium present in between them.
This capacitance is assumed to be distributed uniformly throughout the length of the line and hence constitutes the shunt parameter of the transmission line. Due to the effect of capacitance, there exists a charging current in the transmission line which is present even when the line is under no-load condition.
Factors Determining the Capacitance of Transmission Line :
The factors which govern the capacitance of a transmission line are,
Length of the Line :
The capacitance of a transmission line is directly proportional to the length of the line. If the length of the line is less than 100 km, the effect of capacitance will be negligible and hence can be neglected.
However, if the length of the line exceeds 100 km, the effect of capacitance becomes significant and can not be neglected. Hence, the effect of capacitance is neglected in short and considered in medium and long transmission lines.
Distance Between Conductors :
The capacitance of the transmission line is inversely proportional to the distance between the conductors. If the distance between the conductors is more, the effect of capacitance will be less and if the distance is less, the effect of capacitance will be more.
Presence of Earth :
As we know that there also exists a capacitance between conductor and earth, the presence of earth also affects the capacitance of the transmission line. The conductor-to-earth capacitance depends on the clearance of the conductor with respect to the ground.
If the clearance is more, the effect of the earth will be less and hence conductor to earth capacitance will be less. On the other hand, if the clearance is less, the effect of the earth will be more, and hence the conductor to earth capacitance will be more.
Electric Field Intensity :
The capacitance of a transmission line is directly proportional to the electric field intensity i.e., potential gradient. If the electric field intensity is more, the effect of capacitance will be more and if the electric field intensity is less, the effect of capacitance will also be less.
Effect of Earth on Transmission Line Capacitance :
The capacitance of a transmission line depends upon the ground effect. Generally, capacitance is formed when two charges are separated by some distance with a dielectric in between.
In the case of transmission lines, the phase conductor acts as one plate of the capacitor and the ground acts as another plate and the air, in between, acts as a dielectric. Thereby, causing the change in the calculation of capacitance of transmission lines.
Since the earth is an equipotential plane, it is assumed that there is an imaginary conductor, for every phase conductor, below the surface of the earth, such that whose depth is equal to the height of the actual phase conductor above the surface of the earth and it is known as the image of the phase conductor as shown in the figure below.
Now, the phase conductors R, Y, and B together with their respective image conductor R', Y', and B' constitute a capacitor whose capacitance adds up to the capacitance of transmission lines. This effect is very small hence, it is neglected in most cases.