Corona Discharge: What is the Corona Effect?

CORONA DISCHARGE

Hello friends, welcome to ELECTRICAL ENCYCLOPEDIA. In this article we will study about the Corona Discharge and its effect in transmission lines.

WHAT IS CORONA DISCHARGE?

It is an electrical discharge which occurs due to the ionization of fluid (mainly air) surrounding the electrically charged conductor.

In transmission lines, when a low Alternating Voltage is applied across two conductors whose spacing is large as compared to their diameters, then there is no apparent change in the condition of atmospheric air surrounding the wire.

But if Voltage exceeds a certain limit, called critical disruptive voltage , then conductors are surrounded by a faint violet glow. There is also production of ozone,hissing sound,power loss and radio interference.

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Phenomena of violet glow, hissing noise and production of ozone gas in an overhead transmission wire due to the ionization of air surrounding the conductor is called CORONA.

THEORY OF CORONA FORMATION 

We know that some ionic particles are always present in air due to cosmic rays, ultraviolet radiations, radioactivity etc. Therefore under normal conditions, air around conductor contains some ionized particles as well as neutral particles. When potential difference is applied between conductors, then under the potential gradient, existing ionized particles around the conductor acquire greater velocity. Greater the voltage, more will be the velocity of the ionized particles.

When this Voltage exceeds a certain value, called critical disruptive voltage  , the potential gradient reaches about 30 kV/cm and the velocity of ionized particles becomes sufficient to strike the neutral molecules with such force that it can dislodge one or more electrons from it. This produces another ion and this process adds on. Thus, the process of ionization is cummulative and it results either in corona or spark takes place between conductors.

FACTORS AFFECTING CORONA

Factors on which corona depends are:

1. Atmosphere – Atmospheric Conditions plays an important role in corona formation. Since corona is formed due to ionization of air surrounding the conductors, therefore it is affected by physical state of atmosphere. In stormy weather, number of ionized particles are more and corona can occur at much less voltage.

2. Conductor Size – Corona effect depends upon the shape and condition of the conductors. Rough and irregular surfaces will give rise to more corona. Thus stranded conductor has irregular surfaces and hence gives more rise to corona than solid conductor.

3. Spacing Between Conductor – When the spacing between the conductors is made very large, then the electro-static stresses at the conductor decreases and avoids the corona formation.

4. Line Voltage – It greatly affects the corona. If the line voltage is low, there is no change in the condition of air surrounding the conductors and hence no corona is formed.

CRITICAL DISRUPTIVE VOLTAGE

“It is the minimum phase-neutral voltage at which corona occurs”

It is given by;

Vc = m * g * r * t * log(d/r)

where,

 m = Irregularity factor of conductor (1 for polished smooth conductor)

 g = breakdown strength of air at 1 atm and 25 degree C (roughly, 30kV/cm)

 r = radius of conductor (in cm).

 d = distance between conductors (in cm).

 t = Air Density Factor.

POWER LOSS DUE TO CORONA

Power loss due to corona is given by:

P = 242.2 * {(f+25)/t} * {√(r/d)} * (V – Vc)² * 10⁻⁵ kW/km/phase

where,

f = supply frequency.

V = phase-neutral voltage.

Vc= critical disruptive voltage.

ADVANTAGES OF CORONA

1. Corona reduces the effect of transients produced by surges.

2. Due to corona formation, air surrounding conductor becomes conducting and hence virtual diameter increases which in turn reduces the electrostatic stress between conductors.

DISADVANTAGES OF CORONA

1. Corona causes loss of energy. This also affects the transmission efficiency.

2. Ozone gas produced due to corona may cause the corrosion of the conductor due to chemical action.

3. It causes interference with the communication lines and radio waves.

METHODS OF REDUCING CORONA

1. Increase Conductor Size – By increasing the size of conductor, voltage at which corona occurs is raised and corona effects are reduced. Therefore, ACSR conductors having larger cross sectional area are used in transmission lines.

2. Increase Conductor Spacing – By increasing the conductor spacing, voltage at which corona occurs is raised and corona effects are reduced. However, spacing cannot be increased too much otherwise the cost of supporting structure may increase.