Long Transmission Lines

Long Transmission Lines

In the analysis of short and medium lines we were assuming that the capacitance associated with the line is lumped or concentrated at one or more point although in actual practice it is distributed along the length of line. If same assumption is made for the analysis of long transmission line then it is seen that serious errors occur in its performance parameters. Thus for better accuracy, the line constants in case of long transmission line are considered to be uniformly distributed throughout the length of line. Rigorous mathematical treatment is to be made for obtaining the solution in case of such lines.

The equivalent circuit of a 3 phase long transmission line is shown in the Fig. 2.10.1. For simplicity only one phase is shown. The entire length of the transmission line is divided into n sections with each section having the line constants 1/n times that for total line.

The resistance and the inductive reactance forms the series elements whereas the leakage susceptance (B) and leakage conductance (G) forms shunt elements. Because of the capacitance existing between line and neutral the leakage susceptance is present. The leakage taking place over the conductors or due to corona effect between the conductors forms leakage conductance. The leakage current through shunt admittance is maximum at the sending end and decreases continuously towards the receiving end.