Experimental setup showing Ferranti effect in Long Power Transmission lines

What is Ferranti Effect?

A long transmission line draws a substantial amount of charging current because of its distributed capacitance. If such a line is open ended or very lightly loaded, the operating voltage increases with the distance along the line and the receiving end voltage is greater than the sending end voltage. This effect is known as “Ferranti Effect”.

Why Ferranti Effect occurs?

Ferranti effect occurs when the charging current drawn by the long transmission line itself is greater than the current drawn by the load at the receiving end. Overvoltage occurs due to the voltage drop across the series inductance of the transmission line because of the flow of leading capacitive current or charging current. Overvoltage at the receiving end because of Ferranti effect can be controlled by using shunt reactor. The lagging reactive current drawn by the shunt reactor compensates for the charging current and hence the increase in voltage at the receiving end is controlled.

Typical values of inductance and capacitance parameters of a 400 kV transmission line are

L = 1.044 mH/km of line length and  C = 12 nF/km of line length. 

Demonstration of Ferranti Effect:

The effect of Ferranti effect can be well demonstrated in a lab using the tube light chokes and capacitors used for domestic fans and coolers connected in series and parallel respectively. The sending end can be provided a 230 V AC or some reduced voltage with the help of an auto transformer and the increase in voltage at the receiving end can be observed with the help of a voltmeter. Figure 1 shows the experimental board prepared to show the Ferranti Effect in the Lab.

Fig.1: Experimental board prepared to show the Ferranti Effect in the Lab.

The over-voltage caused by Ferranti effect can also be demonstrated with the help of MATLAB coding.