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4th October 2011

«Ferranti effect»
Voltage drop:
a strange condition.

Can a voltage drop be a negative thing?
In other words, is it possible, of course without the aid of transformers that change the voltage conditions, for the supply voltage to be higher than the voltage generated at the beginning of the line?

This may seem strange, as one imagines that, as for power, the more you move away from the generation point towards the load, unfortunately (and today we all know the importance of energy efficiency) there is power dissipation, so much so that the yield of a line is always lower than 1.

Yet the answer is yes.

Let’s try to give an example, entirely theoretical but not impossible in real life, as we will see below. And lets’ make this analysis easier through a vectorial diagram, absolutely simple yet absolutely useful.

An example

So, let’s imagine a closed-energy transmission line on a fully capacitive load.
As you can see from the vectorial diagram (and as we all know from our knowledge of basic electrical engineering), the supply voltage Vu is 90 degrees in advance compared to the current Ic absorbed by the same load.
Now let’s get some help from a few “classical” electrical engineering concepts and from the vectorial diagram.

As it travels down the line, the current absorbed by the load causes two additional voltage drops, a resistive one, RLIc, in phase with the current, and resulting from the product between line resistance and the current travelling along said line, and one XLIc in delayed squaring, resulting from the product between the line reactance and the current that travels along the same line.

The vectorial sum of the three voltages Vu, RLIc e XLIC gives us voltage Vi at the beginning of the line. As you can see in the vectorial diagram, Vi is lower than Vu.

In other words, the supply voltage is higher than the voltage at the beginning of the line.

Of course, the diagram is for illustration purposes only, but the situation is not entirely impossible. This situation is (or better yet was) known as the “Ferranti effect” in the sacred texts of electrical engineering.

And it can occur on long transmission lines.

As we mentioned at the start, special conditions need to take place for this to happen, such a long transmission line, in which the capacitive effects developed due to the interaction between cables basically represent a load as close as possible to a purely capacitive load.

At this time, it is not necessary to specify how to protect ourselves from this type of condition (overvoltage), as in the traditional professional practice, especially if in low voltage, the situation described above does not take place (as we all know, loads are the inductive ohmic type, and overvoltages have quite different origins), but it is interesting to see how theory can help us solve situations that would otherwise be miraculous. But in a sector such as electrical engineering miracles are not contemplated.

And, as we all know, the relevant cause must be identified (because it does exist), as difficult to find as it may be.

Leran more
- InfoTECH: Technical standards




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