. Corona is a type of localised emission resulting from transient gaseous ionisation in an insulation system when the voltage stress i.e., voltage gradient exceeds a critical value,is also known a partial dischage. Corona can also occur within voids in insulators as well as at the conductor/insulator interference

A power system transmission lines are separated by certain spacing when potential difference is applied across the power conductors in transmission lines then air medium present between the phases of the power conductors acts as insulator medium however the air surrounding the conductor subjects to electro static stresses. When the potential increases still further then the atoms present around the conductor starts ionize

Then the ions produced in this process repel with each other and attracts towards the conductor at high velocity which intern produces other ions by collision. The ionized air surrounding the conductor acts as a virtual conductor and increases the effective diameter of the power conductor. Further increase in the potential difference in the transmission lines then a faint luminous glow of violet colour appears together along with hissing noise. This phenomenon is called corona

Corona effects in all types of transmission lines, but it becomes more noticeable at high voltages(345 k v and higher).under fair weather conditions the audible noise from corona is minor and rarely noticed. during wet and humid conditions, water drops collect on the conductors and increase corona activity. under these conditions, a crackling or humming sound may be heard in the immediate vicinity of the line:

Factors effecting corona : Corona Effect is considerably affected by the shape, size and surface conditions of the conductor. Corona Effect decreases with increases in the size of the conductor, this effect is less for the conductors having round conductors compared to flat conductors Line Voltage: Corona Discharge effect is not present when the applied line voltages are less. When the Voltage of the system increases corona Effect will be more. Atmosphere: Breakdown voltage directly proportional to the density of the atmosphere present in between the power conductors. In a stormy weather the ions present around the conductor is higher than normal weather condition. Spacing between the Conductors: Electro static stresses are reduced with increase in the spacing between the conductors. Corona Discharge Effect takes place at much higher voltage when the distance between the power conductors increases.

Corona effects: Light Ultra violet radiation Sound(hissing, or cracking by explosive gas explsions ) Ozone Nitric and various other acids Salts,and sometimes seen as white powder deposits Other chemicals depending on insulator material Mechanical erosion of surfaces by ion bombardment Heat Carbon deposits,thereby creating a path for severe arcing

Methods for reducing corona: By increasing the spacing between the conductors: Corona Discharge Effect can be reduced by increasing the clearance spacing between the phases of the transmission lines. However increase in the phases results in heavier metal supports. Cost and Space requirement increases. By increasing the diameter of the conductor: Diameter of the conductor can be increased to reduce the corona discharge effect. By using hollow conductors corona discharge effect can be improved.

By using Bundled Conductors: By using Bundled Conductors also corona effect can be reduced this is because bundled conductors will have much higher effective diameter compared to the normal conductors. By Using Corona Rings or Grading Rings: This now does not have much significance as Corona Rings or Grading RIngs are present on the surge arresters to equally distribute the potential along the Surge Arresters or Lightning Arresters which are present near the Substation and in the Transmission lines.

Advantages : Due to the corona formation,the air surrounding the conductor becomes conducting and hence the virtual diameter of the conductor is increased. The increased diameter decreases the electro static stresses between the conductors Corona reduces the effects of transients produced by surges

Disadvantages : Corona is accomplished by a loss of energy. This affects the transmission efficieincy of the line. Ozone is produced by corona and may cause corrosion of the conductor due to chemical action. The current drawn by the line due to corona is non-sinusoidal and hence non-sinusoidal voltage drop occurs in the line.This may cause inductive interference with neighbouring communication lines