How to Reduce the Ferranti Effect in a Transmission Line?

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Ferranti Effect in a transmission Line is reduced using voltage compensation which in turn is accomplished by Shunt Reactor. Shunt Reactor is just like a Transformer but it has only primary and no secondary. Shunt Reactor is connected to all the three phases i.e. R, Y and B phase. Figure below shows a Shunt Reactor. Isn’t it look like a Transformer? Yes..it does…

Ferranti Effect
 

For better understanding we will consider different aspects of Ferranti Effect.

What is Ferranti Effect?

A long transmission line can be considered to be composed of a considerably high amount of capacitance and inductor distributed across the entire length of the line. Ferranti Effect occurs when current drawn by the distributed capacitance of the line is greater than the current associated with the load at the receiving end of the line which occur during light or no load.

This capacitor charging current leads to 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 applied voltage leading to the phenomena called Ferranti Effect in power system.

Ferranti Effect

 

During no load or light load condition, the reactive power generated at a point on the Transmission Line will be more than the reactive power absorbed and therefore the voltage at that point will rise. Mind that during no load or light load condition the line capacitance will dominate to inductance.

The % Rise in voltage in an EHV Transmission Line depends on the square of its length of line.

% dV  is approximately = 0.55×(L/100)2

where L=line length in km and dV = Change in Voltage

If it’s a short line no control is needed as the voltage rise will not be significant during light load or no load condition i.e. Ferranti Effect will not be dominating.

For a medium length line of 250 km the rise in voltage will be about 9.4% and it is above tolerance limit of 5% for 400 kV and above.

Shut Reactor compensation at the receiving end might help to reduce the effect of Ferranti Effect. Shunt Reactor absorbs the excess reactive power generation during no load / light load condition and thus helps in stabilizing the voltage of Transmission Line. But for longer lines of about 400 km and above compensation has to be provided somewhere in the middle of the line in addition to the receiving end. For very long line compensation is needed after every 250 to 300 km.
 
For under-ground cable compensation may be needed at intervals of every 15 to 20 km. This is the reason why submarine EHV cables are technically not feasible.

Thank you!

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