OVERHEAD TRANSMISSION LINES
A transmission line has three constants R, L and C distributed uniformly along the whole length of the line. The resistance and inductance form the series impedance. The capacitance existing between conductors for 1 phase line or from a conductor to neutral for 3-phase line forms a shunt path throughout the length of the line. Therefore, capacitance effects introduce complications in transmission line calculations. Depending upon the manner in which capacitance is taken into account; The overhead transmission lines are classified as bellow.
SHORT TRANSMISSION LINES
When the length of an overhead transmission line is upto about 50 km and the line voltage is comparatively low (<20kV), it is usually considered as a short transmission line. Due to smaller length and lower voltage, the capacitance effects are small and hence can be neglected. Therefore, while studying the performance of a short transmission line, only resistance and inductance of the line are taken into account.
MEDIUM TRANSMISSION LINES
When the length of an overhead transmission line is about 50-150 km and the line voltage is moderately high (>20kV<100kV), it is considered as a medium transmission line. Due to sufficient length and voltage of the line, the capacitance effects are taken into account. For purpose of calculations, 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 LINES
When the length of an overhead transmission line is more than 150 km and the line voltage is very high (>100kV), it is considered as a long transmission line.For the treatment of such a line, the line constants are considered uniformly distributed over the whole length of the line and rigorous methods are employed for solution.