Whenever an AC voltage is applied across two conductors whose spacing is large as compared to their diameter then there are no changes in the air surrounding the conductors if the applied voltage is low. However, when the applied voltage exceeds a certain value called Critical disruptive voltage then conductors are surrounded by a faint violet glow called the Corona effect.
The phenomenon of Corona is accompanied by a hissing sound, the production of ozone, power losses, and radio interference. The higher the voltage raised, the larger and higher be luminous envelope of violet glow, hissing sound, power losses, and radio interference. If the applied voltage is increased up to the breakdown value, then a flash-over will occur between the conductors due to the breakdown of air insulation.
The phenomenon of violet glow, hissing sound, and production of ozone gases in an overhead transmission line is known as Corona effect or Corona discharge.
Theory of Corona Formation
Under normal conditions, the air around the conductor contains some ionized particles (i,e. free electrons and +ve ions) and neutral molecules due to cosmic rays, ultraviolet radiation, and radioactivity. When potential difference is applied between conductors then a potential gradient is set up in the air which will have a maximum value at the surface of the conductors. Under the influence of a potential gradient, the existing free electrons acquire greater velocities. Since the greater the applied voltage, the greater be potential gradient, and hence due to this more will be the velocity of free electrons.
When the potential gradient at the conductor surface reaches about 30 kV/cm (max value) then the velocity acquired by free electrons is sufficient to strike neutral molecules with enough force to dislodge one more electron from it. This produces one more ion and free electrons, which in turn are accelerated until they collide with other neutral molecules thus producing another ion. Thus this process of ionization is commutative. As a result, either the Corona effect is observed or sparking takes place between the conductors.
Factors Affecting Corona Effect
The phenomenon of Corona is affected by the physical state of the atmosphere as well as the condition of the transmission line. The following are the main factors that affect the corona effect:
- Atmosphere:- As Corona is formed due to the ionization of air surrounding the conductor, it is affected by the physical state of the atmosphere. Since in the stormy weather, the number of ions is more than normal, and hence Corona discharge occurs at much less voltage as compared to fair weather.
- Conductor Size:- The Corona effect also depends on the shape and sizes of conductors. The rough and irregular shape of conductors gives rise to more Corona because the roughness of the surface decreases the value of breakdown voltage. Thus an irregular shape of the stranded conductor produces more Corona than a solid conductor.
- Spacing between Conductors:- If the spacing between conductors is made very large as compared to their diameter then there may not be any Corona discharge. It is because the larger distance between the conductors reduces its electrostatic stress at the conductor surface. Hence large space between conductors restricts Corona formation.
- Line Voltage:- The line voltage greatly affects the Corona effect. If it is low, then there is no change in conditions of air surrounding the conductor, and hence no Corona is formed. However if the line voltage has such a value that electrostatic stress developed at the conductor surface makes the air around the conductor is conducting, hence Corona discharge observed.
Advantages and Disadvantages of Corona Effect
Corona has many advantages and disadvantages. Let’s discuss it below.
Advantages of Corona Effect
- Due to the Corona formation, the air surrounding the conductor becomes conducting and hence the virtual diameter of conductors increases and hence it reduces the electrostatic stress between the conductors.
- Corona reduces the effect of transient produced by surges.
Disadvantages of Corona Effect
- Due to the Corona effect, power losses occur. Hence it affects the efficiency of the transmission line.
- Ozone produced by Corona may cause the corrosion of conductors due to chemical reactions.
- The current drawn by the line due to the Corona effect is non-sinusoidal and hence non-sinusoidal voltage drop occurs in the line. This may cause Inductive interference with the neighboring communication line.
Methods of Reducing Corona Effect
It has been observed that generally, Corona formed at a working voltage 33kV or above. Therefore a careful design of conductors should be made to avoid the Corona effect at the substation and bus bar rated for 33kV and higher voltage, otherwise, highly ionized air may cause flash-over in the insulator and between conductors. So the Corona effect can be reduced by following these methods:
- By increasing Conductor Size:- By increasing the size of conductors, the voltage at which the corona develops is raised, and hence Corona effect is considerably reduced. This is the main reason that ACSR conductors which have larger cross-section areas are used in the Transmission line.
- By increasing Conductor Spacing:- By increasing the spacing between conductors, the voltage at which the corona develops is raised and hence Corona effect can be eliminated. However, spacing between conductors can’t increase more than a specified limit because the cost of supporting structure (larger cross arms and tower support) may increase rapidly.
Important Terms Related to Corona Effect
The phenomenon of Corona plays a very important role in the design of transmission lines. Therefore it is preferable to discuss the following terms that are being used in the analysis of Corona effect or Corona discharge.
Critical Disruptive Voltage (Vc)
It is the minimum phase-neutral voltage at which corona occurs is known as critical disruptive voltage. It is denoted by Vc.
Visual Critical Voltage (Vv)
It is the minimum phase to neutral voltage at which Corona glow appears throughout the whole line conductor. It has been observed that in the case of parallel conductors, the Corona effect does not occur at a critical disruptive voltage (Vc) whereas it occurs at some higher voltage called visual critical voltage and denoted by Vv.
The phase-neutral effective value of visual critical voltage is defined by an empirical formula given below.
Power Losses due to Corona Effect
In the formation of corona, there are energy losses in the form of light, heat, and sound. So power loss due to the Corona effect is given below.
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