Classification of overhead cable

  • A transmission line has three constants Resistance, inductance and Capacitor, 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 a 3-phase line forms a shunt path throughout the length of the line. 
  • Therefore, capacitance effects introduce complications in transmission line calculations.  
So we classified the transmission lines based on the capacitance of line


Depending upon the manner in which capacitance is taken into account, the overhead transmission lines are classified as :

(i) Short transmission lines. Due to smaller length and lower voltage, the capacitance effects are small and hence can be neglected, the length of an overhead transmission line is up to about 50 km and the line voltage is comparatively low it is usually considered as a short transmission line.
Therefore, while studying the performance of a short transmission line, only resistance and inductance of the line are taken into account.

(ii) Medium transmission lines. Due to sufficient length and voltage of the line, the capacitance effects are taken into account. For purposes of calculations, the distributed capacitance of the line is divided and lumped in the form of capacitor shunted across the line. The length of an overhead transmission line is about 50-150 km and the line voltage is moderatly high ( < 100 kV), it is considered as a medium transmission line.

(iii) Long transmission lines. In this case, length of an overhead transmission line is more than 150 km and line voltage is very high (> 100 kV), it is considered as a long transmission line.




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