Classification of Overhead Transmission Line

A transmission line has four constants R, L, C and shunt conductance. But generally, three constants R, L and C are considered and they are uniform along the whole length of line. The fourth constant shunt conductance between conductors or between conductor and ground and accounts for the leakage current at the insulators. It is very small in case of overhead lines and may be assumed zero. The capacitance existing between conductors for 1φ line or 3φ line forms a shunt path throughout the length of line. Therefore capacitance effects introduce complication in transmission line calculation. Depending upon the manner in which capacitance is taken into account, the overhead transmission line are classified as,

1. Short transmission lines
2. Medium transmission lines
3. Long transmission lines

Short transmission lines

A short transmission line is one in which the line voltage is comparatively low (< 20kV) and the length of an overhead transmission line is upto about 50km. Due to smaller length and lower voltages the capacitance effects are small and hence can be neglected. Hence, whenever studying the performance of a short tranmssion line only resistance and inductance of the line are taken into consideration.

Medium transmission lines

The transmission line having length of an overhead transmission line in the range 50-150 km and the line voltage is moderately high (> 20 kV < 100kV) is considered as a medium transmission line. Since the line is having sufficient length and line voltage, the capacitance effects are taken into consideration. For the puropose of calcuklations, the distributed capacitance of the line is divided and lumped in the form of condensers shunted across the line at one or more points.

Long Transmission Line

When the length of an overhead line is more than 150 km and the line voltage is very high (>100 kV), it is considered as long transmission line. For the treatment of such line, the line constants are considered uniformly distributed over the whole length of the line and rigorous methods are employed for solution.