# How does ferranti effect occur?

I search and read about Ferranti effect online, but none seems to be clear enough for me to understand how it actually happens. I must say, I do not have much prior knowledge about inductive/capacitive nature of the transmission lines - thus, it makes it harder to understand if the voltage drop is about these, or if it happens through the transmission line because of the no LOAD at the recieving end. I will appreciate if someone can explain "ferranti effect" giving enough, detailed explanations of the other factors as well (eg. current of the transmission line, etc.)

Here is my take on the ferranti effect.

Imagine a sea wall facing the ocean. Waves of height 1 meter are coming in toward shore and one of them strikes the wall. How high does the water splash vertically upward when the wave hits? If the wave splashes only one meter up (same height as the wave originally) then the wave is behaving as if it hadn't hit the wall at all, but we know it did, so the splash height has to be greater than the wave height.

The wall represents an impedance discontinuity to the waves hitting it, and in the process of being reflected off the wall, the peak height of the wave exceeds its original amplitude. Reflections that occur off of impedance discontinuities in electrical systems will exhibit similar behavior, especially if the discontinuity is not lossy.

• So, that means it just happens? Aug 13 '19 at 22:15
• it means this behavior is common in dynamical systems that exhibit resonance or that contain distributed compliance and inertance. Aug 14 '19 at 0:26

If you increased the frequency of the transmitted wave, eventually you would reach a frequency where the transmission line was resonating, with a node at one end and an antinode at the other.

If you plot the transfer function of a resonant system in the frequency domain, the response gradually increases as the frequency changes from 0 to the resonance. So even at frequencies a long way from resonance (e.g. 50 or 60 Hz for a line a few km long) there is some increase in amplitude compared with a non-resonant system.

The resonant frequency depends on the length of the line, and for a driving frequency a long way from resonance the shape of the transfer function explains the fact that the amplitude increase is proportional to the line length squared, and to the frequency squared.

• Sorry, but, how does it resonance actually occur within a no-load system? Aug 13 '19 at 23:07

Electrical impedance and acoustic impedance take the same form mathematically, so I personally find it super helpful to think of electricity in terms of acoustics.

Stand in an open field and shout. No impedance, no reflection.

Stand in a clothing closet and shout. Low impedance, low reflection. Quiet, but not as quiet as a field.

A living room is louder, and then a room with very hard surfaces, like a bathroom, have the highest impedances, largest reflections, and are thus the loudest places to be. Cathedrals are made of stone in part to "amplify" (not dissipate) voices.

Similarly, changes in electrical impedance will generate reflected waves. The larger the change in impedance, the larger the reflected wave. Infinite impedance, like an open circuit, generates the highest voltage reflected waves.

This is my point on Ferranti Effect.

The Ferranti effect is a temporary voltage pulse (a “glitch”) caused by discharge of the energy stored in a magnetic field around a wire, OR it is a continuous resonance in an AC transmission line that adds voltage (but not energy!) making the received voltage greater than the source voltage at low loading of the line. At the same time, the charging current also increases. Ferranti effect is mainly due to applied high voltage and rise in charging current.