When we apply an alternating potential difference across two conductors whose spacing is large as compared to their diameters, then the atmospheric air surrounding the conductors is subjected to electrostatic stresses. At low p.d. between the conductors, there is no change in the condition of the air around the conductors.
However, if we increase the voltage gradually, a stage is reached when a hissing noise is heard, and there appears a glow of violet color around the conductor. This phenomenon is known as the corona effect in transmission lines. It is always accompanied by the production of ozone gas.
The glow is since the air around the conductor becomes conducting due to the electrostatic stresses. If p.d. is further increased, the intensity of sound and light will increase till flashover occurs between the conductors due to the breakdown of air insulation.
So the phenomenon of hissing noise, violet glow, and production of ozone gas in an overhead transmission line conductor is known as the corona effect.
If the conductors are uniform and smooth, the corona glow will be consistent throughout the length; otherwise, the rough points will appear brighter.
In the case of the DC system, the glow due to the corona effect in electrical transmission lines will be different in the two conductors. The positive conductor has a smooth and bright glow, whereas the negative conductor has a spotty glow.
Causes of Corona Effect in Transmission Line
If the spacing between the conductors is small, flashover may take place between the conductors without any hissing noise and glow. It is because the distance between the conductors being smaller; there is no time for the glow to occur. Following are the leading causes of corona effect in transmission lines:
Conductors shape: The corona effect in transmission lines depends upon the shape and conditions of the conductor. The rough and irregular surface will give rise to more corona. It is because the unevenness of the surface decreases the value of breakdown voltage. Similarly, a stranded conductor has an irregular surface and hence gives rise to more corona than a solid conductor having a smooth surface.
Atmosphere: Since corona is caused due to ionization of air surrounding the conductor, therefore, it is affected by the physical state of the atmosphere. In the stormy weather, the number of ions is more than normal, and corona occurs at much less voltage as compared with fair weather.
Spacing between conductors: If the spacing between the conductors is made very large as compared to their radii (r), there may not be any corona effect. It is because larger spacing reduces the electrostatic stresses at the conductor surface.
Line voltage: Line voltage significantly affects the corona effect. If it is low, there is no change in the condition of air surrounding the conductor and hence no corona. However, if the line voltage has such a value that electrostatic stresses developed at the conductor surface make the air around the conductor conducting, the corona effect appears.
Advantages of Corona Effect
- Due to the formation of the corona, the air surrounding the conductor becomes conducting. It increases the virtual diameter of the conductor. The increased diameter decreases the electrostatic stresses between the conductors.
- The corona effect reduces electrostatic stresses between the conductors. This reduces the probability of flashover and improves system performance.
- Corona reduces the effects of transients produced by lightning.
Disadvantages of Corona Effect
- The corona effect in electrical transmission lines is accompanied by a loss of energy, which is dissipated in the form of light, heat, sound, and chemical action.
- On the formation of the corona, ozone gas is produced, which chemically reacts with the conductor, causing corrosion.
- The current drawn by the line due to corona loss is non-sinusoidal, and hence non-sinusoidal voltage drop occurs in the line. This may interfere with the neighboring communication lines.
Methods of Reducing Corona Effect
It has been seen that intense corona effects are observed at a working voltage of 33 kV or higher. Therefore, careful designs should be made to avoid any corona on the substations or bus-bars rated at 33 kV or higher. Otherwise, highly ionized air may cause flashover in the insulators or between the phases, causing considerable damage to the equipment. The following methods can reduce the corona effect in transmission lines:
By increasing the conductor size: By increasing conductor size, the voltage at which corona occurs is raised, and hence corona effects are considerably reduced. It is due to this reason that ACSR conductors which have, a larger cross-sectional area, are used in transmission lines.
By increasing the spacing between the conductors: By increasing the spacing between conductors, the voltage at which corona occurs is raised, and hence corona effect can be eliminated. However, spacing cannot be increased too much; otherwise, the cost of supporting structure may increase too much.
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