Corona & its effects : 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 the 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 the radio noise. If the applied voltage is increased to breakdown value, a flash-over will occur between the conductors due to the breakdown of air insulation.
The phenomenon of violet glow, hissing noise and production of ozone gas in an overhead transmission line is known 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 in the appearance of the two wires. The positive wire has uniform glow about it, while the negative conductor has spotty glow.
Advantages:
(i) Due to corona formation, the air surrounding the conductor becomes conducting and hence
virtual diameter of the conductor is increased. The increased diameter reduces the electrostatic stresses between the conductors.
(ii) Corona reduces the effects of transients produced by surges.
Disadvantages
(i) Corona is accompanied by a loss of energy. This affects the transmission efficiency of the
line.
(ii) Ozone is produced by corona and may cause corrosion of the conductor due to chemical
action.
Methods of Reducing Corona Effect:
- i) By increasing conductor size. By increasing conductor size, the voltage at which corona occurs is raised and hence corona effects are considerably reduced. This is one of the reasons that ACSR conductors which have a larger cross-sectional area are used in transmission lines.
(ii) By increasing conductor spacing. By increasing the spacing between conductors, the voltage at which corona occurs is raised and hence corona effects can be eliminated. However,
spacing cannot be increased too much otherwise the cost of supporting structure (g., bigger cross arms and supports) may increase to a considerable extent.
Corresponding Author: Muhammad Nasir Abdullah, Hitec University Taxila