Question

Ferranti effect on long overhead lines is experienced when:
1. Power factor is unity
2. Line is lightly loaded
3. Line is heavily loaded
4. Line is fully loaded

Answer 1

Correct Answer - Option 2 : Line is lightly loaded

Ferranti effect:

• The effect in which the voltage at the receiving end of the transmission line is more than the sending voltage is known as the Ferranti effect. Such type of effect mainly occurs because of a light load or open circuit at the receiving end.
• Ferranti Effect occurs when the current drawn by the distributed capacitance of the line itself is greater than the current associated with the load at the receiving end of the line(during light or no load).
• This capacitor charging current leads to a voltage drop across the line inductor of the transmission system which is in phase with the sending end voltages. This voltage drop keeps on increasing additively as we move towards the load end of the line and subsequently, the receiving end voltage tends to get larger than the applied voltage leading to the phenomena called the Ferranti effect in the power system.
• Thus both the capacitance and inductor effect of the transmission line are equally responsible for this particular phenomenon to occur, and hence Ferranti effect is negligible in the case of a short transmission line as the capacitance of such a line is practically not considered.
• In general for a 300 Km line operating at a frequency of 50 Hz, the no-load receiving end voltage has been found to be 5% higher than the sending end voltage.

 

Note:

• Ferranti effect is not a problem for short transmission lines because there is no capacitance effect
• Ferranti effect means receiving end voltage is greater than sending end voltage
• This phenomenon occurs in medium and long transmission lines which are at lightly loaded or no-load condition. This is because of the capacitance effect.