When an alternating current is made to flow across multiple conductors of a transmission line where its spacing is large compared to their diameters, the air surrounding the conductors (composed of ions) is subjected to dielectric stress.
At low values of the supply voltage, nothing occurs as the stress is too small to ionize the air outside. But when the potential difference increases beyond a threshold value (known as the critical disruptive voltage), the field strength becomes strong enough for the air surrounding the conductors to begin to ionise – making it conductive. This critical disruptive voltage occurs at approximately 30 kV, depending on the conditions of the air, i.e. humidity, pollution etc.
The ionized air results in electric discharge around the conductors (due to the flow of these ions). This gives rise to a faint luminescent glow, along with the hissing sound accompanied by the liberation of ozone.
This phenomenon of electric discharge occurring in high voltage transmission lines is known as the corona effect. If the voltage across the lines continues to increase, the glow and hissing noise becomes more and more intense – resulting in a high power loss into the system.
Corona discharge always results in power loss. Energy is lost in the form of light, sound, heat, and chemical reactions. Although these losses are individually small, over time they can add up to significant power loss in high voltage networks.
One of the most common way to control this is to use corona rings. The electric field is stronger where there is a sharp conductor curvature. Because of this corona discharge occurs first at the sharp points, edges, and corners (As demonstrated in the video by the lecturer). Corona rings reduce the corona effect by ‘rounding out’ conductors (i.e. making them less sharp).
Video credit: Jefferson’s Lab YouTube Channel, click here to watch the full original video.