What is corona effect?factor affecting,Critical disruptive voltage,corona ring

What is Corona Effect or corona loss?

Corona effect (also known as Corona Discharge) is nothing but 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 occursCorona effect observed in the high voltage transmission line. Corona produced hissing and cracking noise with ozone gas emission, Corona loss cause radio interference with a neighboring communication line and also creates a violet glow around the power conductor. 

corona formation photo
corona formation
The free electrons move with a certain velocity depending upon field strength, These free electrons collide with the air molecules. if the speed of electrons very large they dislodge electrons from air molecules and effect numbers of electrons increases. This form avalanche of free electrons and result, air around the conductor get ionized.

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.

critical disruptive voltage formula
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 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
critical disruptive voltage at temperature
g'=dielectric strength at any temperature and pressure
g'=g₀𝛿
r: radius of the conductor
𝛿: air density factor
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= 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

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.

Formation of corona

If the ratio of the spacing between conductors to radius of the conductor is less than 15 then flash over will occurs between the conductor before the formation of the corona.

For the overhead line usually spacing between conductor to conductor, 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 Violet glow seen around power conductor also called corona ring. corona creates hissing noise with ozone gas emission which can be identified by its smell.

Factor affecting to corona loss:

There are many factors affect on corona effect some of the  Factors Affecting Corona Loss are discussed below 

i) Electrical factor

ii) Atmospheric Factor

iii)Shape and size of the conductor


Electrical factor:- Supply frequency:


 Power loss in corona  effect/ loss can be calculated using below formula,
power loss due to corona in transmission line
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 loss, we can see that corona loss is a function of frequency thus higher frequency of supply the higher corona losses. this shows that corona loss in DC transmission is less than compared to the AC line.



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 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 area corona loss is more than that of in the plain area.

Partials 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.


Shape and size of the conductor:

For higher voltage using one conductor per phase gives higher corona losses and radio interference with neighboring communication line 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.

Method of Minimizing Corona effect:-

The corona effects can be reduced by:-

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 which 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 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 loss has a disadvantage it can cause interference with neighboring communication lines.



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