Corona Effect in Overhead Transmission Line

Corona Effect in Overhead Transmission Line

The Corona is the term for the phenomenon of a violet glow, hissing sound and ozone gas production in an overhead gearbox line. when two overhead transmission line conductors with a wide spacing relative to their diameters are subjected to a very high voltage. The corona effect, also known as critical disruptive voltage, occurs when the applied voltage exceeds a specific threshold, encircling the conductors with a flimsy violet glow.

In reality, the corona effect also causes ozone to be produced, power loss, and radio interference. The luminous envelope grows larger and higher as the transmission voltage increases, and power loss and radio interference also increase. A flashover will happen between the conductors. If the applied voltage reaches the breakdown value because the air insulation will have failed.

The violet glow will be uniform along the length of the transmission conductors. If they are smooth and polished; 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 corona phenomenon is influenced by the physical characteristics of the atmosphere and the transmission line. The following are the factors that influence corona loss in an overhead transmission line:

• Atmosphere – Since the ionisation of the conductors’ surrounding air causes the corona loss to form. As a result, the corona loss is influenced by the atmosphere’s physical characteristics. When there are storms, there are more ions than usual, which causes the corona to occur at a much lower voltage than when it’s sunny.
• Line Voltage – The line voltage has a significant impact on the corona loss. Low line voltage prevents corona from forming because there is no change in the conductors’ surrounding atmosphere. However, the corona effect occurs when the line voltage is so high that electrostatic stresses form at the conductor surface and cause the surrounding air to conduct.
• Spacing between Conductors – There might not be a corona discharge. If the distance between the conductors is made very large relative to their diameters. The reason for this is that a wide gap between the conductors reduces electrostatic stresses at the conductor, assisting in preventing corona formation.
• Size of the Conductor – The size and shape of the conductors have an impact on the corona loss as well. More corona loss will result from a rough and uneven surface because an uneven surface on a conductor lowers the breakdown voltage. Because of its uneven surface, a stranded conductor exhibits more corona effect than a solid conductor.

Methods of Reducing Corona Effect

The corona effect is known to happen in real life at line voltages of 33 kV or higher. Since highly ionised air may cause flashover in the line insulators or between the line conductors, careful design of the overhead transmission lines is required to prevent corona loss at 33 kV or higher voltages. If this occurs, significant damage to the equipment will result.

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

• By Increasing Conductor Size – By enlarging the conductor, the corona effect in overhead transmission lines can be minimised. The line voltage at which corona occurs is raised by increasing the conductor size, and as a result, the corona loss is significantly decreased. This is one of the explanations for why overhead transmission lines use aluminum-conductor-steel-reinforced (ACSR) conductors with large area cross-sections.
• By Increasing the Spacing between Conductors – The formation of a corona is prevented by the large spacing between the conductors,. Which lowers the electrostatic stresses created at the conductors. As a result, the corona effect can be diminished by raising the line voltage at which corona occurs and decreasing the conductor spacing. However, if the distance between the conductors is increased too much,. The cost of the supporting structure will skyrocket, making the system uneconomical.