The phenomenon of ionization of surrounding air around the conductor, hissing noise and production of Ozone gas in an overhead transmission line is known as Corona Effect. Corona discharge is a process by which a current flows in a conductor with a high potential into a neutral fluid, usually air, by ionizing that fluid so as to create a region of plasma around the Conductor.
The ions generated pass charge to nearby areas of lower potential, or recombine to form neutral gas molecules. When the potential (electric field) is large enough (33 KV =disruptive potential gradient) the air ionizes HV, the ionized region continues to grow until it reaches another conductor at a lower potential, a low resistance conductive path between the two will be formed, resulting in an electric arc, then a faint luminous glow of violet color appears together along with hissing noise. This phenomenon is called corona.
Which factors affect Corona Discharge?
Conductor: 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.
There are many other factors that affect the corona effect in addition to atmosphere, conductor size, spacing between conductors, supply voltage, air density factor including many environmental Facts & Conditions.
Following environmental facts and conditions increase the corona effect:
- Low pressure conditions (Hilly terrains)
- High temperature areas
- Places where are heavy rain
- Snow areas
There are some of the advantages which are implied due to corona effect. Diameter of the conductor is increased that reduces the electro- static stresses between the conductors. Also corona effect reduces the effects of transients. Some industries use this effect to remove unwanted volatile organics from atmosphere.
Disadvantages of Corona Effect
The corona power loss reduces the efficiency of the transmission line. Corona effect produces Ozone which leads to corrosion of the conductor due to chemical action.. Voltage drops occur in the line. The radio and TV interference occurs on the line. The Corona effect can also damage the line insulation.
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 neighboring communication lines.
How to Reduce Corona Effect
Corona Effect can be reduced by increasing conductor spacing, increasing conductor size and using bundled conductors. ACSR conductors have a larger cross-sectional area are used to reduce the Corona effect in transmission lines. Bundled conductors produce less resistance and thus losses are reduced.
By using Corona Rings or Grading Rings: 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.