What is the Corona Effect in Overhead Transmission Lines?

Corona Effect in Overhead Transmission Line

The phenomenon of violet glow, hissing sound and production of ozone gas in an overhead transmission line is known as corona.

When a very high voltage is applied across two conductors of the overhead transmission line whose spacing is large as compared to their diameters. When this applied voltage exceeds a certain value (called critical disruptive voltage), then the conductors are surrounded by a faint violet glow that is known as corona effect.

In practice, the corona effect is accompanied by a hissing sound, production of ozone, power loss and radio interference. The higher is the transmission voltage, the larger and higher the luminous envelope becomes and the higher are the power loss and the radio interference. If the applied voltage attained the breakdown value, then a flashover will occur between the conductors due to breakdown of the insulation of air.

If the transmission conductors are polished and smooth, then the violet glow will be uniform throughout the length of the conductors, otherwise the rough points will appear brighter.

In case of DC transmission lines, there is a difference in the appearance of corona glow for the two conductors, i.e. the positive wire has uniform corona glow about it and the negative wire has spotty glow.

Factors Affecting Corona Loss

The physical state of the atmosphere and the conditions of the transmission line affects the phenomenon of corona. The factors which affect the corona loss in the overhead transmission line are given as follows −

  • Atmosphere – Since the corona loss is formed due to the ionization of the surrounding air of the conductors. Therefore, the physical state of the atmosphere affects the corona loss. In the stormy weather, the number of ions is more than the normal and hence the corona occurs at very low voltage as compared to the fair weather.

  • Line Voltage – The corona loss is greatly affected by the line voltage. If the line voltage is low, then there is no change in the condition of the air surrounding the conductors and hence no corona occurs. Although, if the line voltage has a very value such that there is the electrostatic stresses developed at the conductor surface making the surrounding air conducting, then corona effect takes place.

  • Spacing between Conductors – If the spacing between the conductors is made very large as compared to their diameters, then there may not be any corona discharge. It is because the large space between the conductors decreases the electrostatic stresses at the conductor which helps in avoiding the corona formation.

  • Size of the Conductor – The corona loss is also affected by the shape and size of the conductors. The rough and uneven surface will give rise more corona loss since the unevenness of the surface of the conductor reduces the value of breakdown voltage. Therefore, a stranded conductor has irregular surface and hence gives rise to more corona effect than a solid conductor.

Methods of Reducing Corona Effect

In practice, it has been observed that the corona effect occurs at a line voltage of 33 kV or above. Therefore, careful design of the overhead transmission lines should be made to avoid the corona loss at 33 kV or higher voltages, otherwise highly ionized air may cause flashover in the line insulators or between the line conductors, causing considerable damage to the equipment.

The methods used for reducing the corona effect / corona loss / corona discharge are given as follows −

  • By Increasing Conductor Size – The corona effect in the overhead transmission lines can be reduced by increasing the conductor size. By increasing the conductor size, the line voltage at which corona takes place is increased and hence the corona loss is greatly reduced. Therefore, this is one of the reasons that the aluminium-conductor-steel-reinforced (ACSR) conductors which have large area cross-section are used in overhead transmission lines.

  • By Increasing the Spacing between Conductors – The large spacing between the conductors reduces the electrostatic stresses developed at the conductors, thus avoiding the corona formation. Therefore, by increasing the spacing between the conductors, the line voltage at which corona occurs is raised and thus the corona effect can be reduced. However, the spacing between the conductors cannot be increased too much otherwise the cost of supporting structure becomes very high, which is uneconomical.