What is Corona Effect in transmission lines?
Corona effect (also known as Corona Discharge) is nothing but the ionization of air around the power conductor. The air around the power conductor has free electrons due to radioactivity and cosmic rays. As potential between conductors increases potential gradient near to conductor surface increases. When the voltage is greater than the critical disruptive voltage Corona effect occurs. Corona loss observed in the high voltage transmission line. Corona produced hissing and cracking noise with ozone gas emission, Corona effect causes radio interference with a neighboring communication line and also creates a violet glow around the power conductor.
 |
| corona formation |
{tocify} $title={Table of Contents}
When voltage is greater than critical disruptive voltage Corona occurs.
Critical disruptive voltage :
Critical disruptive voltage is defined as the voltage at which complete disruption of dielectric strength of air occurs. if the voltage increases above critical disruptive voltage corona occur.
 |
| critical disruptive voltage formula |
𝛿: air density correction factor
r: radius of conductor
d: distance of conductor separation
g₀: dielectric strength
= 30kv/cm
However, the existing temperature and pressure at the surface of the conductor are different from NTP. so at any other temperature and pressure critical disruptive voltage equation become
 |
| critical disruptive voltage at temperature |
g'=g₀𝛿
r: radius of the conductor
𝛿: air density factor
h: Atmospheric pressure in cm of Hg
t: temperature in C degree
If the surface of the conductor is irregular then surface irregularity factor (m)will be introduced in the formula.
m= 1.0 for smooth conductor
m=0.93-0.98 for rough / stranded conductor
m=0.83-0.87 for seven stranded cable
m=0.9 for more than seven stranded cable
r: radius of the conductor
𝛿: air density factor
 |
| air density factor |
h: Atmospheric pressure in cm of Hg
t: temperature in C degree
If the surface of the conductor is irregular then surface irregularity factor (m)will be introduced in the formula.
m=0.93-0.98 for rough / stranded conductor
m=0.83-0.87 for seven stranded cable
m=0.9 for more than seven stranded cable
Visual Critical Voltage
The visual critical voltage is defined as the minimum phase to the neutral voltage which glows and appears all along the line conductor. It has been seen that in the case of parallel line conductors, the corona glow does not begin at the disruptive voltage but at a higher voltage called visual critical voltage.Theory of Corona formation
If the ratio of the spacing between conductors to the radius of the conductor is less than 15 then flash over will occur between the conductor before the formation of the corona.For the overhead line usually spacing between conductor to conductor, the radius is far more than 15 hence flashover is impossible. Hence corona is an electric discharge into the ionized air around the power conductor, corona effect or phenomenon can be identified by the Violet glow seen around the power conductor also called the corona ring. corona creates hissing noise with ozone gas emission which can be identified by its smell.
Factors affecting to corona effect:
There are many factors that affect on corona effect some of the Factors Affecting the Corona effect are discussed below- Electrical factor
- Atmospheric Factor
- Shape and size of the conductor
Electrical factor
Effect of Supply frequency
Power loss in corona effect/ loss can be calculated using the below formula,
 |
| corona loss formula/equation in transmission lines line |
Where,
f: supply frequency
𝛿: air density correction factor
r: radius of the conductor
d: distance of conductor separation
Vp: operation voltage
Vo: critical disruptive voltages
From the above equation of corona discharge, we can see that corona loss is a function of frequency thus higher the frequency of supply the higher the corona losses. this shows that corona loss in DC transmission is less than compared to the AC line.
Effect of Line voltage
The line voltage greatly affects corona. If it is low, there is no charge 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 stress developed at the conductor surface make air around the conductor conducting then corona is formedEffect of Atmospheric condition
From the equation of corona loss it's clear that corona loss is a function of air density correction factor “𝛿 ” which is directly in the denominator of the equation so conclude that the lower the value of "𝛿 "higher the losses and vice-versa.For the lower value of "𝛿 " pressure should be low and the temperature is high due to this reason in hilly areas corona loss is more than that of in the plain area.
Particles of dust on a conductor also increase corona losses because of a reduction in critical voltage. also in bad weather condition such as raining, snowfall, and storm, critical disruptive voltage decrease by increasing corona loss.
Effect of Shape and size of the conductor
For higher voltage using one conductor per phase gives higher corona discharge and radio interference with neighboring communication lines which always run parallel to the transmission line.This problem can be overcome by using a Bundled conductor instead of a single conductor, using more than two conductors per phase i.e bundled conductor increases critical disruptive voltage resulting reduction in corona losses.
Effect of Spacing between Conductors
If the spacing between the conductor is made the very layer as compared to their diameters, there may not be any corona effect. It is because layer distance between the conductors reduces the electrostatic stresses at the conductor surface, thus avoiding corona formation.Read Also...What is Skin Effect?
Method of Minimizing Corona effect
The corona loss can be reduced by:-Reduced Corona effect By increasing conductor size
By increasing conductor size, the voltage at which occur of the corona 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. However spacing cannot be increased too much otherwise the cost of supporting structure (i-e bigger cross- and supports) may increase to a considerable extent.Advantages of the corona effect
Corona loss has some advantages it reduces the magnitude of high voltage waves generated by lighting stroke or switching action by partially dissipating energy as corona loss.Disadvantages of Corona effect
1) Corona causes a loss of energy. This affects the transmission efficiency of the line.2) Ozone gas is produced by the corona effect and may cause corrosion of the conductor due to chemical action.
3) corona effect has a disadvantage it can cause interference with neighboring communication lines.
Tags:
Power System
