Question

In a localized capacitance method for a Long transmission line what could be consider a drawback from it? Also when a long transmission line is open circuited on its receiving end, would there be any current at the sending end?

Answer 1

In a localised capacitance method for a long transmission line, the drawback could be explained by FERRANTI EFFECT. When a long line is operating under no load or light load condition, the receiving end voltage is greater than the sending end voltage. This is known as Ferranti-effect.

Assume no load condition,

  Reduces to…

when x= l and Ir = 0.

At l=0

As l increases, the incident component of sending end voltage increases exponentially and turns the vector anti-clockwise through an angle βl, whereas the reflected part of sending end voltage decreases by the same amount and is rotated clockwise through the same angle βl.The sum of these two components of sending end voltage gives a voltage which is smaller than .

A simple explanation of Ferranti-effect can be given by approximating the distributed parameters of the line by lumped impedance. The capacitive reactance of the line is quite large as compared to the inductive reactance, under no load or lightly loaded condition the line current is of leading p.f. The charging current produces drop in the reactance of the line which is in phase opposition to the receiving end voltage and hence the sending end voltage becomes smaller than the receiving end voltage.

Open circuit indicates with noload at the receiver end meaning I_r=0, and there is current at the sending end .

When I_r=0

.