Corona Effect and

Factors affecting

Corona Loss

Submitted By:

Mir Baaz Khan (FA14-REE-025)

Humayun Khan (SP15-REE-013)

Sajid ul Haq (FA15-REE-012)

Bakhtiar Khan (FA15-REE-007)

Corona Effect

1. Introduction

Corona loss is the other major type of power loss in transmission lines. Essentially, corona loss is caused by the

ionization of air molecules near the transmission line conductors. These coronas do not spark across lines, but

rather carry current (hence the loss) in the air along the wire. Corona discharge in transmission lines can lead to

hissing/cackling noises, a glow, and the smell of ozone (generated from the breakdown and recombination of

O2 molecules). The color and distribution of this glow depends on the phase of the AC signal at any given

moment in time. Positive coronas are smooth and blue in color, while negative coronas are red and spotty.

Corona loss only occurs when the line to line voltage exceeds the corona threshold. Unlike resistive loss where

amount of power lost is a fixed percentage of input, the percentage of power lost due to corona is a function of

the signal's voltage. Power losses due to corona discharge are also highly dependent on weather and temperature.

Electric power transmission practically deals in the bulk transfer of electrical energy, from generating stations

situated many kilometers away from the main consumption centers or the cities. For this reason, the long

distance transmission cables are of utmost necessity for effective power transfer, which in-evidently results in

huge losses across the system. Minimizing those has been a major challenge for power engineers of late and to

do that, one should have a clear understanding of the type and nature of losses. One of them being the corona

effect in power system, which has a predominant role in reducing the efficiency of EHV (extra

high voltage lines) which we are going to concentrate on, in this article.

2. HOW CORONA DISCHARGE EFFECT OCCURS:

In a power system, transmission lines are used to carry the power. These transmission lines are separated by

certain spacing which is large in comparison to their diameters. In Extra High Voltage system, when potential

difference is applied across the power conductors in transmission lines then, air medium present between the

phases of the power conductors acts as insulator medium however, the air surrounding the conductor subjects

to electro static stresses. When the potential increases still further then, the atoms present around the conductor

start to ionize. The ions produced in this process repel each other and are attracted towards the conductor at high

velocity which consequently, produces other ions by collision. The ionized air surrounding the conductor acts

as a virtual conductor and increases the effective diameter of the power conductor. Further increase in the

potential difference between the transmission lines results in the production of a faint luminous violet glow

along with hissing noise. This phenomenon is called virtual corona and is followed by the production of ozone

gas which can be detected by its odor. Still further increase in the potential difference between the power

conductors makes the insulating medium present between the power conductors to start conducting and reaches

a voltage (Critical Breakdown Voltage) where the insulating air medium acts as conducting medium. This

results in breakdown of the insulating medium and a flash over is observed. All this above said phenomenon

constitutes Corona Discharge Effect in electrical Transmission lines.

Corona Effect in Transmission Line

3. Factors Affecting Corona Effect in Power System:

3.1 Surface conditions

Corona also depends on the surface conditions. Rough and uneven surfaces will give rise to more corona loss

because unevenness of the surface decreases the value of breakdown voltage. The value of disruptive voltage is

less and corona effect is dominant. Stranded conductor has an uneven surface and hence attract more corona

effect than the smoothened conductor.

3.2 Line voltage

The line voltage directly affects corona and the corona loss. For lower line voltage corona loss may be absent.

But for voltages higher than critical disruptive voltage, corona starts. Higher the line voltage, higher is the

corona loss.

3.3 Atmospheric conditions

The atmospheric pleasure and temperature adversely control the presence of corona. The expression of corona

loss implies that it is a function of the air density correction factor value of δ. The lower the value of δ the higher

the loss. The pressure and temperature together decide the value of air density which affects critical disruptive

voltage and the corona loss. The lower the value of critical disruptive voltage the higher the corona loss. For

lower pressure and higher temperatures, the value of critical disruptive voltage will be small and corona effect

and loss is dominant. Hence in mountain areas the corona loss is high. Corona is affected by the physical state

of the atmosphere. In stormy weather conditions, dusty and rainy conditions, number of free electrons and ions

are more hence disruptive voltage is lower. This increases the corona loss considerably.

3.4 Size of conductor

Increase in the conductor size also increases the conductor surface field intensity. Also from Peek’s equation,

the corona loss is directly proportional to square root of radius conductor i.e. is α =√𝑟/𝑑the factor (V-Vd)2 is

proportional to the size of the conductor. So the loss is more if the size of the conductor is more. But for large

size conductors, Vd is more and hence the term (V – Vd) is less. Thus loss is less. The effect of Vd is more

dominating than the factor squared hence higher the size of the conductor, lower is the corona loss.

3.5 High Supply Frequency

From the Peek’s equation of corona loss, it can be observed that corona loss is affected by the supply frequency.

Higher the supply frequency, higher the corona loss. The d.c. corona loss is less compared to a.c. corona loss.

Due to corona effect in ac line, the third harmonic components are generated hence actual corona loss is higher.

3.6 Bundled conductors

For higher voltages a single conductor per phase produces large corona loss and consequently large radio

interference which affect communication lines. This can be overcome by using two or more than two conductors

per phase. This increases the geometric mean distance (GMD) of the conductors, which increases the disruptive

voltage and reduces corona loss.

3.7 Spacing between conductors

Spacing the conductors is an effective method of ameliorating the corona. If the spacing is made very large,

corona can be absent. Practically the spacing is selected so that corona is tolerable.

3.8 Profile of conductors

The shape of the conductors whether flat, oval, cylindrical etc. affects the corona loss. The cylindrical shaped

conductors have field uniformity which reduces corona loss when compared to any other shape.

3.9 Mean sea level

The height of the conductors from the sea level also affects the corona loss. At a very high level above the mean

sea level, the number of ion per c.c of air is quiet high. The smaller the clearance of the conductors from the sea

level, higher is the corona loss.

3.10 Load current

As the load current increases, the temperature of the conductors increases and does not allow snow and, dew to

deposit on the surface. Thereby minimizing the conductivity of the surrounding the conductor and limiting the

formation of corona. Sequel to the above factors, for the long transmission lines the corona loss per km at

various points is obtained. The net corona loss is taking average of all the values