Definition: The ionization of air around the power conductor surface of the high voltage transmission line or the disruption of the dielectric strength of air near to conductor surface of the power line, producing hissing noise and causing a violet glow around the conductor called the corona effect or corona discharge.
Free electrons are present in the air surrounding the power conductor, some electrons are near the power conductor of the transmission line and some are very far from the conductor’s surface.
The electrons which are near to surface of the conductor move at a faster rate than the electrons away from the conductor’s surface.
Because electric field intensity is maximum near the conductor’s surface and gets weaker as moves away from the conductor’s surface.
The field intensity depends on the operating voltage of the transmission line.
When the operating voltage is more than 270Kv the field intensity will increase hence the mobility of electrons also increases.
The highly mobile electrons collide with other molecules present in the air and dislodge the electrons from collided molecules of air.
This process happens continuously and at a certain instant, an avalanche of electrons forms. arc will strike between the lines.
corona effect can be identified
- by hissing sound
- violet glow around a conductor
Factor affecting the Corona Effect
The phenomenon of the corona discharge is affected by the atmospheric condition around the conductor hence depends on
Atmosphere
Corona discharge occurs due to the ionization of air around the conductor and it is affected by the weather condition, in stormy weather, the numbers of ions present in the air are more than in normal atmospheric condition
Hence in this condition corona occurs at very low voltage as compared to fair weather condition.
Line Voltage
The supply voltage heavily affects the formation of the Corona.
if the low voltage is present there is no change in the condition of air around the conductor.
hence no visual critical voltage level is attained and no corona glow will appear around the line conductor.
However, if the voltage has such value that electrostatic stress developed at the conductor surface makes air surrounding the conductor conducting and corona glow start forming.
Conductor Size
The formation of the corona depends on the shape and condition of the conductor.
The rough surface will give rise to more corona loss Because the unevenness of the conductor surface decreases the breakdown voltage.
Spacing between conductor
If the spacing between the conductor makes large compared to their diameter, there may not be any corona effect.
This happens Because the larger distance between conductors applies less electrostatic stress at the conductor surface.
Advantages of the corona effect
Corona loss has some advantages
it reduces the magnitude of high voltage waves generated by lighting stroke or switching action by partially dissipating energy as corona loss.
Disadvantages of Corona effect
- Corona discharge causes a loss of energy hence the efficiency of the transmission line is affected.
- Ozone gas is produced during corona discharge and may cause corrosion of the conductor due to chemical action.
- corona can cause interference with neighboring communication lines.
