Define Corona, List its causes and state how it can be avoided.
Define Corona: When AC Voltage given across two conductors separated by distance ‘d’ as shown figure is increased greater than breakdown voltage of air i.e. 30KV/cm, then air around the conductor gets ionized and ionized air is conducting under this condition corona will takes place (form) .
During corona following observations are noted:
* Luminous violet glow (typically a purple glow) occurs around the conductor.
* Hissing or cracking sound will produce.
* Ozone gas will produce. (smell the presence of ozone that was produced by the corona)
This phenomenon is known as “corona” effect.
The following causes:-
1. Magnitude of Voltage : If voltage across two conductors is greater than 30 KV/cm, i.e. breakdown voltage of air than corona formation starts. Corona will not start if voltage is below 30 KV/cm
2. Distance between two conductor: If spacing between two conductors is very large as compare to their diameter than there is no possibility of corona formation. Because value of voltage at which corona occurs increases.
3. Size of conductor: If size (Cross section) of conductor is more, than magnitude of voltage required to occur the corona increases.
4. Condition of conductor & Hardware: Rough and irregular surface of conductor and hardware will give more corona than solid, smooth body conductor & hardware.
5. Atmospheric Condition: As corona takes place due to ionization of air so it depends on condition of air so for dry air formation of corona occurs late than in wet air (damp atmosphere condition/ rainy season/thunderstorms/fog air becomes more conductivity)
6. Effect of supply Frequency: Corona loss varies directly as the supply frequency
7. Effect of density of air: Corona loss increases with the decrease in the density of air (The corona loss of transmission line passing through hilly area is higher than that of a similar line in plain due to reduced value of air density at high level /altitude)
Corona effect can be avoided for following way:
1. By increasing distance between two conductor i.e. by using longer cross arm.
2. By using larger size(diameter) of conductor e.g./ using ACSR, bundled conductor
3. By using smooth body conductor and hardware.