Hint:
Answer: (a) diameter of the conductor.
Explanation:
Corona: The phenomenon of violet glow, hissing noise, and production of ozone gas in an overhead transmission line is known as the corona.
Factors Affecting Corona:
Atmosphere:
- As corona is formed due to the ionization of air surrounding the conductors, therefore, it is affected by the physical state of the atmosphere.
- In stormy weather, the number of ions is more than normal, and as such corona occurs at much less voltage as compared with fair weather.
Conductor size:
- The corona effect depends upon the shape and conditions of the conductors.
- Corona loss varied along with the change in the diameter of the conductor.
- The rough and irregular surface will give rise to more corona because unevenness of the surface decreases the value of breakdown voltage.
- Thus a stranded conductor has an irregular surface and hence gives rise to more corona than a solid conductor.
Spacing between conductors:
- If the spacing between the conductors is made very large as compared to their diameters, there may not be any corona effect.
- It is because a larger distance between conductors reduces the electrostatic stresses at the conductor surface, thus avoiding corona formation.
Line voltage:
- The line voltage greatly affects corona. If it is low, there is no change in the condition of air surrounding the conductors and hence no corona is formed.
- However, if the line voltage has such a value that electrostatic stresses developed at the conductor surface make the air around the conductor conducting, then corona is formed.
Additional Information
Advantages of Corona:
- Due to corona formation, the air surrounding the conductor becomes conducting and hence virtual diameter of the conductor is increased.
- The increased diameter reduces the electrostatic stresses between the conductors.
- Corona reduces the effects of transients produced by surges.
Disadvantages:
- Corona is accompanied by a loss of energy. This affects the transmission efficiency of the line.
- Ozone is produced by corona and may cause corrosion of the conductor due to chemical action.
- 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.
Important Points
- Critical disruptive voltage: It is the minimum phase-neutral voltage at which corona occurs.
- Consider two conductors of radius r cm and spaced d cm apart.
- In order that corona is formed, the value of g must be made equal to the breakdown strength of air.
- The breakdown strength of air at 76 cm pressure and temperature of 25ºC is 30 kV/cm (max) or 21·2 kV/cm (r.m.s.).
- Visual critical voltage: It is the minimum phase-neutral voltage at which corona glow appears all along the line conductors.
Methods of Reducing Corona Effect:
It has been seen that intense corona effects are observed at a working voltage of 33 kV or above.
The corona effects can be reduced by the following methods:
- By increasing conductor size: By increasing conductor size, the voltage at which corona occurs is raised and hence corona effects are considerably reduced. This is one of the reasons that ACSR conductors that 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.