Transmission efficiency

1.6 Classification of Overhead Transmission Lines

As mentioned earlier, 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 and neutral forms a shunt path throughout the length of the line. Therefore, the capacitance effects introduce complications in transmission line calculations. Depending upon manner in which capacitance is taken into account, the overhead transmission lines are classified based on its length and rated voltage line as follows;

1.6.1 Short transmission lines

A short transmission line is classified as a transmission line with:  A length less than 80km  Voltage level less than 20 kV  Capacitance effect is neglected.  Only resistance and inductance are taken in calculation.

Due to smaller length and lower voltage, the capacitance effects are small and hence can be neglected. The equivalent circuit of a short transmission line is shown in the Figure 1.7 below. In the equivalent circuit short transmission line is represented by the lumped parameters R and L. R is the resistance (per phase) while L is the inductance (per phase) of the entire transmission line. As said earlier the effect of shunt capacitanceand conductance is not considered in the equivalent circuit.

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