Under the no-load condition, the current in a transmission line is due to

• A. The capacitance of the line
• B. Corona effect
• C. Back Low from the earth
• D. Spinning Reserve

Answer: (A)

A long transmission line has a large capacitance. When a long line is operating under the no-load condition, the receiving-end voltage is greater than the sending end voltage. This is known as the Ferranti effect. This phenomenon can be explained by the following reasoning. It was first noticed by Ferranti on overhead lines supplying a lightly loaded network. The Ferranti effect is due to the charging current of the line. The value of current at the sending end at no-load and normal operating voltage applied at the sending end is called the charging current.
A simple explanation of the Ferranti effect can be given by approximating the distributed parameters of the line by lumped impedance as shown in Figure. Since usually, the capacitive reactance of the line is quite large as compared to the inductive reactance, under the no-load or lightly loaded condition, the line current is of leading pf. The phasor diagram is given below for this operating condition. The charging current produces the 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.
Note:-
The Ferranti Effect will be more pronounced the longer the line and the higher the voltage applied. The relative voltage rise is proportional to the square of the line length.
The Ferranti effect is much more pronounced in underground cables, even in short lengths, because of their high capacitance.