Classification of Overhead Transmission Line

Classification of Overhead Transmission Line

AU :May-04, 07

The parameters R, L and C associated with any transmission line are distributed uniformly along the whole length of line. The series impedance is formed by the resistance and the inductance. In case of single phase circuits capacitance is present between conductors whereas in case of three phase circuits, conductor to neutral forms a shunt path throughout the length of line. Thus introducing capacitor makes the transmission line calculations more complex. There are different ways in which capacitance can be taken into account. Thus the overhead transmission line is classified into following types.

i) Short transmission line : If the transmission line length is about 50 km and the line voltage is low ie. about 20 kV or less than that then the line is treated as short transmission line.

As the line length is small and the voltage is also less, the capacitive effects are small. Due to this effect of capacitance is neglected. Thus only resistance and inductance is to be taken into account while analysing short transmission lines.

ii) Medium transmission line : When the length of transmission line is lying between 50 to 150 km and the line voltage is moderately high i.e. in between 20 kV and 100 kV, it is treated as medium transmission line. In the analysis of medium transmission line, the capacitance is taken into account as the line length is a appreciable. For easiness in the calculations, the distributed capacitance of the line is divided and is lumped across the line at one or more points. 

iii) Long transmission line : When the length of transmission line is more than 150 km and the line voltage is very high i.e. above 100 kV, the line is considered as long transmission line. In the analysis of such lines, the line constants are considered uniformly distributed over thee whole length of line. The rigorous methods are applied to obtain the solution.

In case of short and medium transmission lines, though we are taking the line parameters as lumped one instead of distributed throughout the length of line, sufficient accuracy is obtained.

Review Question

1. Give classification of transmission lines