Corona

When an alternating potential difference is applied across two conductors whose spacing is large as compared to their diameters, there is no apparent change in the condition of atmospheric air surrounding the wires if applied voltage is low. However, when the applied voltage exceeds a certain value, called critical disruptive voltage, the conductors are surrounded by a faint violet glow called CORONA.

The phenomenon of corona is accompanied by a hissing sound, production of ozone, power loss and radio interference. The higher the voltage is raised, the larger and higher the luminous envelope becomes, and greater are the sound, the power loss and radio noise. If the applied voltage is increased to breakdown value, a flash over will occur between the conductors due to breakdown of air insulation.

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

If the conductors are polished and smooth, the corona glow will be uniform throughout the length of the conductors, otherwise the rough points will appear brighter. With D.C. voltage, there is difference int he appearance of two wires. The positive wire has uniform glow about it, while the negative conductor has spotty glow.

Corona Formation

In a power system transmission lines are used to carry the power. These transmission lines are separated by certain spacing which is large in comparison to their diameters. In Extra High Voltage system (EHV system ) 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 virtual corona and followed by production of ozone gas which can be detected by the odor. Still further increase in the potential between the power conductors makes the insulating medium present between the power conductors to start conducting and reaches a voltage (Critical Breakdown Voltage) where the insulating air medium acts as conducting medium results in breakdown of the insulating medium and flash over is observed. All this above said phenomenon constitutes Corona discharge effect in electrical Transmission lines.