Here present article about what is corona?? And factors affecting corona and advantage and disadvantage of corana.
➼ what is corona??
- It has been observed that,if the spacing between two conductrs is less than their diameters ,and if the applied potential between them gradually increased,a violet glow occurs round the conductor,along with a hissing sound,at a certain value of the potential.This phenomenon is known as Corona.
- The phenomenon of hissing noise,faint violet glow and production of ozone gas surrounding the overhead lines,due to ionization of air is called corona.
- If the conductors are uniform and smooth,the corona glow will be uniform throughout the length otherwise the rough points will appear brighter.
- In case of d.c.system,the glow due to corona will be different in the two conductors.
- The +Ve conductor has smooth and bright glow whereas the negative conductor has spotty glow.
Theory of corona formation:
Facts about corona:
(i) The spacing between the conductors should be very small as compared to their diameters.
(ii) The phenomenon occurs above a critical voltage, which has the maxi. value of 30 KV (or 21.0 KV rms) per cm. :
(iii) It is accompanied by a hissing sound along with violet glow discharges, Vibration of conductors, formation of ozone gas, and noise.
(iv) Its glow is brightest at rough or dirty surfaces of the conductor.
(v) It causes power loss and radio interference.
(vi) It introduces harmonic currents.
(vii)It causes ionisation of air between the conductors.
(viii)It causes much damage to lines/equipments, etc.
Factors affecting corona
The phenomenon of corona is affected by the physical state of the atmosphere as well as by the conditions of the line. The following are the factors upon which corona depends :
1. Atmosphere :
The corona effect depends upon the shape and condition 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 stranded conductor has irregular surface and hence give to more corona that a solid conductor Corona losses.
3. Spacing between conductor:
If the spacing between the conductor is made very large compared to their diameter,there may not be corona effect.It is because large distance between conductor reduces the electrostatic stress at the conductor surface thus avoiding corona formation.But the cost of installation is increased.
4.Line voltage:
The line voltage greatly affected corona.If it is low there is no change in condition of air surrounding the conductors and hence no corona is formed. However if the line voltage has such a value that electostatic stress developed at the conductor surface make the air around the conductor conducting, then corona formed.
5.Effect of Frequency:
Corona loss is directly proportional to system frequency.
6.Effect of Load Current:
The flow of the load current increases the temperature of the conductor.Thus,it prevents deposition of snow on the conductor's surface.This reduce corona loss.
- Phenomenon of corona is Ionization of air surrounding the power conductor. Due to cosmic rays, free electrons are normally present in free space.
- When the potential difference between the conductors to be large as compared to diameter of conductor, free electrons move with certain velocity depends on strength of field. Means greater the applied voltage, greater the potential gradient, more is velocity of free electrons.
- Due to large velocity of electrons, electrons have collision with the molecules of air, and dislodge the electrons from these molecules. This produces another ion and one or more free electrons. This process being commulative, will lead to an avalanche of electrons.
- Eventually there will be ionization of air surrounding the conductor.
- If spacing of conductor is less than 15 times diameter of conductor, flashover will take place between the conductor and either corona is formed or spark takes place between the conductors.
Facts about corona:
(i) The spacing between the conductors should be very small as compared to their diameters.
(ii) The phenomenon occurs above a critical voltage, which has the maxi. value of 30 KV (or 21.0 KV rms) per cm. :
(iii) It is accompanied by a hissing sound along with violet glow discharges, Vibration of conductors, formation of ozone gas, and noise.
(iv) Its glow is brightest at rough or dirty surfaces of the conductor.
(v) It causes power loss and radio interference.
(vi) It introduces harmonic currents.
(vii)It causes ionisation of air between the conductors.
(viii)It causes much damage to lines/equipments, etc.
Phenomenon of Corona :
- For an over head transmission system atomospheric air behaves practically like perfect insulator when potential difference between conductors is small. If we increase the voltage, there is a corresponding increase in the electric field intensity
- If air is subjected to a uniform electric field intensity of peak value less 30kV/cm, flow current between conductors is negligible. But when electric intensity increases and reaches this critical value of 30kV/cm air ionizes come conducting.
This breakdown accompanied by following phenomenon.
- Hissing Noise is produced.
- A faint violet glow appears which is visible in the dark.
- Production of Ozone gas.
- Radio interference occurs
- There is a loss of power.
If all these phenomenon occurs then we say corona produced in a transmission line.
Factors affecting coronaThe phenomenon of corona is affected by the physical state of the atmosphere as well as by the conditions of the line. The following are the factors upon which corona depends :
1. Atmosphere :
As corona is formed due to ionization of the air surrounding the conditions of the conductor, it is affected by physical state of atmosphere. In the stormy Weather the number of loss is more than normal and as such corona occurs at much less voltage as compared with fair weather.
2. Conductor size:The corona effect depends upon the shape and condition 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 stranded conductor has irregular surface and hence give to more corona that a solid conductor Corona losses.
3. Spacing between conductor:
If the spacing between the conductor is made very large compared to their diameter,there may not be corona effect.It is because large distance between conductor reduces the electrostatic stress at the conductor surface thus avoiding corona formation.But the cost of installation is increased.
4.Line voltage:
The line voltage greatly affected corona.If it is low there is no change in condition of air surrounding the conductors and hence no corona is formed. However if the line voltage has such a value that electostatic stress developed at the conductor surface make the air around the conductor conducting, then corona formed.
5.Effect of Frequency:
Corona loss is directly proportional to system frequency.
6.Effect of Load Current:
The flow of the load current increases the temperature of the conductor.Thus,it prevents deposition of snow on the conductor's surface.This reduce corona loss.
Advantages of Corona:
- Due to the formation of corona,the air surrounding the conductor becomes conducting and hence virtual diameter of the conductor is increased.The increased diameter decreases the electostatic stresses between the conductors.
- Corona reduces electro-static stresses between the conductors.This reduces the probability of flash over and improves the system performance.
- Effects of transients produced by lightning surges and other causes are reduced. Charge induced on the line by surges will be partially dissipated as a corona loss.In this way it acts as a safety valve.
Disadvantages of Corona:
The disadvantages are more than the advantages of corona:
- Corona causes power loss in the line and reduces efficiency.
- corona currents are non sinusoidal, which produce non- sinusoidal voltage drop. this may produce ratio interference in nearby communication lines.
- harmonics are produced in the line
- during the formation of corona, ozone gas is formed. ths gas reacts with the material of the conductors and causes corrosion.
Method of reducing corona
- the corona can be reduced by increasing diameters of the wires. this is the reasin that aluminium conductor steel reainforced(ACSR) of large cross section are used for transmission lines
- the spacing between the parallel conductors can be increased, but this has a limit as it will increase cost of supports, cross arms, etc.
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