Classification of Overhead Transmission Line

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 3 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 to 80 km and the line voltage is low than that line is treated as short transmission line.

  • As the line length is small and the voltage is also small, the capacitive effects are small. Due to this effect , Capacitance is neglected.
  • Assume parameters to be lumped.

short_line

(ii). Medium transmission line : When the length of transmission line is lying between 80 to 200 km and the line voltage is moderately high, is treated is medium transmission line.

  • In the analysis of medium transmission line, Line to neutral capacitance is taken into account.
  • For easiness in the calculations, in place of distributed parameters, lumped parameters ( Parameters at only one or more point )  are used.

medium_line

(iii). Long transmission line : When the length of transmission line is more than 200 km and the line voltage is very high, the line is considered as long transmission line.

  • In long transmission line, Parameters are not lumped at one point, they are distributed uniformly ( Distributed parameters ) over whole length of line

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