Corona effect-Critical Disruptive Voltage & Factor Affecting

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 the 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 is observed in the high-voltage transmission line.

Corona produces 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.

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 is very large they dislodge electrons from air molecules and affect the number of electrons increases. This forms an avalanche of free electrons and result, the air around the conductor gets ionized.

When voltage is greater than critical disruptive voltage Corona occurs.

Critical Disruptive Voltage & Visual Critical Voltage

Critical Disruptive Voltage

Critical disruptive voltage is defined as the voltage at which complete disruption of the dielectric strength of air occurs. if the voltage increases above critical disruptive voltage corona occurs.

 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

g’=dielectric strength at any temperature and pressure

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 the 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

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 flashover 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, The corona effect or phenomenon can be identified by the Violet glow seen around the power conductor also called the corona ring. 

corona creates a hissing noise with ozone gas emission which can be identified by its smell.

Factors affecting to corona effect

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,

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 the 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 makes air around the conductor conducting then a corona is formed

Effect 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 conditions such as rain, snowfall, and storms, critical disruptive voltage decreases 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.

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 the 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

• Corona causes a loss of energy. This affects the transmission efficiency of the line.
• Ozone gas is produced by the corona effect and may cause corrosion of the conductor due to chemical action.
• corona effect has a disadvantage it can cause interference with neighboring communication lines.