Classification of Transmission Lines

In Transmission lines, the electrical parameters such as resistance, capacitance and inductance are distributed throughout the length of the conductors. The resistance and inductance form series impedance and the capacitance forms shunt impedances throughout the length of the line. This distributed capacitance introduce complications in transmission line calculations. Based on the manner in which the capacitance is taken into account the overhead transmission lines in power system are classified in to
three types namely; short, medium and long transmission lines

Short Transmission Line

Over head transmission lines of length about 50 km and below are classified as short transmission lines. In short transmission the line voltage is comparatively low (less than 20 kV). Due to smaller length and lower voltage, the capacitance effects are low and hence can be neglected. In the analysis of a short transmission line, only resistance and inductance of the line are taken into account.

Medium Transmission Line

Over head transmission lines of length in between 50 to 150 km are classified as medium transmission lines. In medium transmission lines the line voltage is moderately high  which is in the range of 20 kV to 100 kV. Due to sufficient length and voltage of the line, the capacitance effect should be taken into account but can be considered as a lumped parameter in the form of a shunt capacitor at one or more points.

Long Transmission Line

Overhead transmission lines of length more than 150 km are called as long transmission lines and line voltage is very high (greater than 100 kV). The line constants are uniformly distributed over the whole length of the line and cannot be taken as lumped parameter at one point while analysing. For analysis of long transmission lines rigorous methods are employed for solution.