Corona discharge or corona effect cause air ionization around the electrical conductor at high voltage. The air ionization causes the electrical discharge.
The Electrical power transmission lines generally deal with high voltages as a huge amount of electrical energy transmits from generating stations to load centers. Usually, at this much high voltages, an effect called Corona Effect is introduced. We can describe it as Corona discharge as well due to its contribution to the loss of electrical energy.
More technically we can describe Corona Discharge or Corona Effect as the ionization of air surrounding the high voltage transmission lines causing the conductors to glow, producing a hissing noise.
A hissing or cracking audible noise usually accompanies corona discharge along with a visual violet glow, production of ozone gas around the conductor, power loss, and radio frequency interference(RFI). This phenomenon occurs when the electrostatic field produces a condition of the potential gradient across the transmission line conductors. The air surrounding the conductors gets ionized when the potential gradient at the conductor surface reaches the value of 30kV/cm at normal pressure and temperature.
Air acts as a dielectric medium. And we know that transmission lines are surrounded by air. The induced current between the conductor is not sufficient to ionize the air when the electric field intensity is less than 30kV/cm. However, when the potential gradient of air surrounding exceeds 30kV/cm, the charging current starts to flow through the air which indicates ionization of air. The ionized air acts as a virtual conductor, producing a hissing sound with a luminous violet glow.
Factors Affecting Corona Discharge
The following factors contribute to corona discharge and cause corona loss.
1. Supply Voltage
This plays one of the big factors in Corona Effect. As the electrical corona discharge mainly depends on the electric field intensity which is in turn produced by applied system voltage. If the applied supply voltage is high, corona discharge will cause excessive corona loss in transmission lines. And corona is negligible in the low voltage transmission lines due to the inadequate amount of electric field required for the breakdown of air.
2. Conductor Surface
The corona effect is hugely dependent on factors such as the shape, material, and conditions of the conductor. The value of breakdown voltage decreases whenever there is a rough and irregular surface. This decrease in breakdown voltage due to concentrated electric field at rough spots, give rise to more corona effect. Deposition of dirt, dust and scratching can be some reasons for the roughness of the conductor which increases the corona.
3. Air Density Factor
The corona loss is inversely proportional to the air density factor. Power loss is high due to corona in Transmission lines present in hilly areas because in hilly areas the air density is low.
4. Spacing between the Conductors
If the distance between two conductors is very large as compared to the diameter of the conductor, the corona effect may not happen. The electro-static stress reduces with a larger distance between conductors at their surface, thus avoiding corona formation.
Corona formation takes place due to the ionization of air surrounding the conductors; therefore, the atmosphere plays an important role. The number of ions is more in stormy weather than in normal weather. The number of ions increases with the decrease in the value of breakdown voltage. As a result of which corona occurs at much less voltage compared to the breakdown voltage value in fair weather.
Methods to reduce Corona Effect:
Corona discharge cause power loss in the form of light, sound, heat, and chemical reactions. Though these losses are individually small, however, over a period of time, the losses can add up to substantial power loss.
The following are the methods of reduction of the corona effect.
1. Increasing Conductor Size
It is possible to raise the voltage at which corona occurs by increasing the conductor size. Thereby decreasing the corona effect. This is one of the main reasons ACSR conductors having large cross-sectional areas are used in transmission lines.
2. Increasing Conductor Spacing
Increasing the spacing between the conductors raises the voltage at which corona occurs. Thus, the larger the conductor spacing, the lesser the corona. But it is limited due to the cost of supporting structure as bigger cross arms and supports to help in increasing the conductor spacing, and increase the cost of the transmission system.
3. Using Corona Ring
Wherever the conductor curvature is sharp, the intensity of the electric field is high at the point. Therefore, sharp points, edges, and corners are the areas where corona discharge mostly occurs first. Corona Rings are employed at the terminals of very high voltage equipment to mitigate electric fields.
Toroidal-shaped metallic rings are fixed at the end of bushings and insulator strings. The metallic ring distributes the charge across a wider area due to its smooth round shape which significantly mitigates the potential gradient at the surface of the conductor below critical disruptive voltage and thus prevents corona discharge.
4. Using bundled conductors
The bundled conductor increases the effective diameter of the conductor. This results in a reduction of the corona discharge.