Corona Effect: Theory, Affecting Factors, Advantages
When an AC 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 around the conductors.
If the applied voltage is increased and when it reaches certain voltage called as disruptive voltage the conductors are surrounded by faint violet glow called corona.
Definition: Corona Effect
The phenomenon of violet glow, hissing noise and production of ozone gas in an overhead transmission line is called as corona.
Due to the corona a hissing sound may be produced, and it also produces ozone gas. Due to this power loss and radio interference will be occurred. If the applied voltage is increased this effect also will be increased.
Theory of corona effect:
- Some ionization is always present in the air due to cosmic rays and ultra violet radiation and radioactivity.
- So under normal condition, the air around the conductor contains some ionised particles and neutral molecules.
- When the potential difference is applied between the conductors a potential gradient is set up in the air which as maximum value at the conductor surfaces.
- Under the influence of potential gradient, existing free electrons acquire greater the potential gradient, and more is the velocity of free electrons.
- When potential gradient, at the conductor surface reaches about 30 kV/cm the velocity acquired by free electron is sufficient to strike neutral molecules with enough force to dislodge one or more electrons from it.
- This produces another ion and one or more free electrons which in turn are oscillated until they collide with other neutral molecules, thus producing corona.
Factors affecting corona effect:
- Conductor size
- Spacing between conductors
- Line voltage
Advantages of corona effect:
- The maximum electrostatic stress is reduced.
- The probability of flash over is reduced.
- System performance is improved.
- Virtual increase in conductor diameter.
- Effects of transients produced by lighting and other causes are reduced.
Disadvantages of corona effect:
- Loss of power.
- Third harmonics are produced.
- There is non sinusoidal voltage drop due to non sinusoidal corona current.
- Corrosion on the conductor due to chemical action takes place.
Important terms related to corona effect:
Critical disruptive voltage:
It is the minimum phase-neutral voltage at which corona occurs.
Visual critical voltage:
It is the minimum phase-neutral voltage at which corona glow appears all along the line conductors.
How to reduce corona effect?
By increasing conductor size:
- By increasing conductor size, the voltage at which corona occurs is raised and hence corona effects are considerably reduced.
- Because of this reason only, the ACSR conductors which have a larger cross-sectional area are used in transmission lines.
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.
- But the spacing can not be increased too much or else the cost of supporting structure may increase.
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