SHARDA UNIVERSITY

CORONA

EFFECT

CORONA ???

"Corona" is from the Latin word for crown.

It has been derived from the glow surrounding the conductors when the operating voltage is sufficiently high .

PHENOMENON OF CORONA

For an overhead transmission system the atmospheric air, which is the dielectric medium , behaves practically like a perfect insulator when the potential difference between the conductors is small. If the voltage impressed between the conductors of alternating nature , sustained charging current will flow due to the capacitance of the line. With the increase of the voltage, there corresponding increase in the electric field intensity.

Contd… As long as the air is subjected to a uniform

electric field intensity of peak value less than 3×106 V/m, the flow of current between the two conductors of the line is neglegibly small for practical purposes.

But when the electric field intensity (voltage gradient) reaches the critical value of

3×106 V/m, the air is immediate vicinity conductors no more remains a dielectric but is ionizes and becomes conducting

Contd…

This breakdown is accompanied by… A faint glow appears around the conductors

which is visible in dark. There is an acoustical noise. There is a tendency in the conductors to

vibrate . There is a loss of power. There is a radio interference.

Contd…

If the voltage gradient is increased further, the size and the brightness of the luminous envelope goes on increase until finally a spark is established between the conductors because of complete breakdown of the insulating properties of the air between them.

Theory of the Corona Formation The electrons and ions are always present to a small

extent in the atmospheric air due to the random action of the ionizing sources such as cosmic rays, ultraviolet radiation from the sun ,etc. when electric field of uniform intensity is established between the conductors, these ions and free electrons experience forces upon them proportional to the field intensity and the charge on them. Due to this force the ions and free electrons get accelerated and move in opposite directions. The charge particles during there motion collide with one another and also with the very slow moving uncharged molecules .

Contd…

The velocity acquired by the charge particle at collision depends on its mass , charge and the mean free path. The means free path depends upon the no. of molecules contained in a unit volume of the air and thus upon the size of molecule , the temperature and pressure of the air.

If atmosphere pressure of 760mm of mercury and normal temperature of 25*C when the electrical field intensity reaches a critical value of about 3000kV/ metre,the velocity attained by a charged ion or electron is sufficiently high to tear of one or more electron from the uncharged molecule which it makes collision , thereby leaving one or more free electrons and a new +ve ion behind . This process is called ionization.

Contd… These newly formed free electrons and ions in

turn are accelerated by a field and collide with other air molecules to ionize them further. As results of this the saturation is reached . The air become conducting and a complete breakdown occurs . At this stage a discharge or arc is established between the conductors.

FACTORS AFFECTING CORONA Corona loss depends upon a large no. of

factors , the important being broadly classified in the following way…

a) Conductor Surface Gradient:- From the equation Gmax = En

rlnD/r for single conductors and multiple conductors lines, it

is seen that radius and spacing of the conductors affect the surface gradients and therefore, the voltage at which corona starts. In addition to this, the surface gradient also depends on the configuration of the line and the height of the conductors above the ground.

V/m

b)Condition of Conductor Surface The roughness of conductor may be caused

by standing , scratches , deposition of dust or other air bone particles on its surface. Due to concentration of field intensity at rough spots local corona discharges occurs there .

This has been explained by the fact that during corona discharge ozone and oxides of nitrogen are produced.

c) Atmospheric Condition

When the rate of rainfall increases , it is observed that the corona loss for a given conductor at any voltages increases. But the rate of increase of loss is lesser than that of rainfall . The loss become practically constant when the rate of rain fall is very high.

d) Air Density Factor The air density factor δ is define as the ratio

of density of the air at any given baromatric pressure ..

So , The air density factor δ= 0.392p 273+tHumidity also effects corona voltage.

Advantages of Corona It reduces the magnitude of high voltage

steep fronted waves due to lighting or switching by partially dissipating as a corona loss . In this way it acts as a safety valve to some extent

Disadvantages of Corona 1. There is a definite loss of power even though it is not

much during fair weather condition.

2. When corona is present the effective capacitance of the conductors is increased because the effective dia of the conductor is increased. This effect increases the flow of charging current . Because of corona triple frequency currents flow through the ground in case of grounded system and they give rise to a voltage of triple frequency in an ungrounded system. These triple frequency currents and voltages interfere with the communication circuits due to electromagnetic and electrostatic induction effect.

Group Members

NAME ROLL NO.

Md. Shadab Khan 09o104058

Abhishek Kr. Roy 090104004

Ankit Osta 090104018