Definition of Corona Effect :
This phenomenon of ionization of surrounding air around the conductor due to which luminous glow with hissing noise is rise is called corona effect.
For overhead transmission system the atmospheric air, which is the dielectric medium, behaves practically like a perfect insulator when the potential difference between the conductor is small. If the voltage impressed between the conductor is of alternating nature, the charging current will flow due to the capacitance of the lines. This charging current increases the voltages of the lines and corresponding increase the electric field intensity of the lines.
When the value of electric field intensity is less than 30kV (disruptive voltage), the flow of current between two conductors of the lines is negligibly small. But when the electric field intensity reaches this critical value or disruptive voltage the airs between the conductors get ionizes and becomes conducting. If the voltage goes on increasing spark is established between the conductors until the complete breakdown of the insulating properties of the material.
Contents:
Air is not perfect insulator and even under normal condition, the air contains many free electrons and ions. When an electric field intensity is established between the conductors, these ions and free electrons experience force upon them. Due to this force, the ions and free electrons get accelerated and moved in the opposite direction.
The charged particles during their motion collide with one another and also with the very slow moving uncharged molecules. Thus, the number of charge particles goes on increasing rapidly. This increase the conduction of air between the conductors and a breakdown occurs. Which established the arc or discharge between the conductors.
Factors affecting corona:
Corona loss depends on a large number of factors, the most important being broadly classified in the following way:
Effect of supply voltage : If the supply voltage is high corona loss is higher in the lines. In low voltage transmission lines the corona is negligible, due to the insufficient electric field to maintain ionization.
The condition of conductor surface : If the conductor is smooth, the electric field will be more uniform as compared to the rough surface. The roughness of conductor is caused by the deposition of dirt, dust and by scratching, etc. Thus, rough line decreases the corona loss in the transmission lines.
Air Density Factor : The corona loss in inversely proportional to air density factor, i.e., corona loss, increase with the decrease in density of air. Transmission lines passing through a hilly area may have higher corona loss than that of similar transmission lines in the plains because in a hilly area the density of air is low.
Effect of system voltage : Electric field intensity in the space around the conductors depends on the potential difference between the conductors. If the potential difference is high, electric field intensity is also very high and hence corona is also high. Corona loss, increase with the increase in the voltage.
A spacing between conductors : If the distance between two conductors is much more as compared to the diameter of the conductor than the corona loss occurs in the conductor. If the distance between them is extended beyond certain limits then the dielectric medium between them get decreases and hence the corona loss also reduces.
Disadvantages of corona discharge:
Minimizing corona:
Corona decreases the efficiency of transmission lines. Therefore, it is necessary to minimize corona.The following factors may be considered to control corona:
Conductor diameter : For reducing corona loss this method of increasing conductor diameters is very effective. Diameters of conductors can be increased by using hollow conductors and by using steel-cored aluminum conductors(ACSR) conductors.
The voltage of the line : Voltage of transmission lines is fixed by economic considerations. To increase the disruptive voltage the spacing of the conductors is to be increased, but this method has some limitations.
Spacing between conductors : If the space between conductors increases, then the voltage drops between them also increases due to increase in inductive reactance.
Important points:
This phenomenon of ionization of surrounding air around the conductor due to which luminous glow with hissing noise is rise is called corona effect.
For overhead transmission system the atmospheric air, which is the dielectric medium, behaves practically like a perfect insulator when the potential difference between the conductor is small. If the voltage impressed between the conductor is of alternating nature, the charging current will flow due to the capacitance of the lines. This charging current increases the voltages of the lines and corresponding increase the electric field intensity of the lines.
When the value of electric field intensity is less than 30kV (disruptive voltage), the flow of current between two conductors of the lines is negligibly small. But when the electric field intensity reaches this critical value or disruptive voltage the airs between the conductors get ionizes and becomes conducting. If the voltage goes on increasing spark is established between the conductors until the complete breakdown of the insulating properties of the material.
- Corona effect
- Corona Formation
- Factors affecting corona
- Disadvantages of corona discharge
- Minimizing corona
- Important points
Air is not perfect insulator and even under normal condition, the air contains many free electrons and ions. When an electric field intensity is established between the conductors, these ions and free electrons experience force upon them. Due to this force, the ions and free electrons get accelerated and moved in the opposite direction.
The charged particles during their motion collide with one another and also with the very slow moving uncharged molecules. Thus, the number of charge particles goes on increasing rapidly. This increase the conduction of air between the conductors and a breakdown occurs. Which established the arc or discharge between the conductors.
Factors affecting corona:
Corona loss depends on a large number of factors, the most important being broadly classified in the following way:
Effect of supply voltage : If the supply voltage is high corona loss is higher in the lines. In low voltage transmission lines the corona is negligible, due to the insufficient electric field to maintain ionization.
The condition of conductor surface : If the conductor is smooth, the electric field will be more uniform as compared to the rough surface. The roughness of conductor is caused by the deposition of dirt, dust and by scratching, etc. Thus, rough line decreases the corona loss in the transmission lines.
Air Density Factor : The corona loss in inversely proportional to air density factor, i.e., corona loss, increase with the decrease in density of air. Transmission lines passing through a hilly area may have higher corona loss than that of similar transmission lines in the plains because in a hilly area the density of air is low.
Effect of system voltage : Electric field intensity in the space around the conductors depends on the potential difference between the conductors. If the potential difference is high, electric field intensity is also very high and hence corona is also high. Corona loss, increase with the increase in the voltage.
A spacing between conductors : If the distance between two conductors is much more as compared to the diameter of the conductor than the corona loss occurs in the conductor. If the distance between them is extended beyond certain limits then the dielectric medium between them get decreases and hence the corona loss also reduces.
Disadvantages of corona discharge:
- A faint glow appears around the conductor which is visible in the dark.
- There is an audio noise.
- There is the tendency in the conductor to vibrate.
- Corrosion due to ozone and oxides of nitride formation.
- There is the loss of power.
- There is radio and television interference
- Harmonic current flows resulting from corona formation.
Minimizing corona:
Corona decreases the efficiency of transmission lines. Therefore, it is necessary to minimize corona.The following factors may be considered to control corona:
Conductor diameter : For reducing corona loss this method of increasing conductor diameters is very effective. Diameters of conductors can be increased by using hollow conductors and by using steel-cored aluminum conductors(ACSR) conductors.
The voltage of the line : Voltage of transmission lines is fixed by economic considerations. To increase the disruptive voltage the spacing of the conductors is to be increased, but this method has some limitations.
Spacing between conductors : If the space between conductors increases, then the voltage drops between them also increases due to increase in inductive reactance.
Important points:
- Disruptive voltage is the minimum voltage at which the breakdown of air occurs and corona starts.
- Visual critical voltage is the minimum voltage at which visible corona begins.
Explain Corona Effect ?
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