Contents

1. Abstract

2. Introduction

3. Mechanism of corona formation

4. Types of corona

4.1. Positive corona

4.2. Negative corona

5. Voltage parameters of corona

5.1. Disruptive critical voltage

5.2. Visual critical voltage

6. Factors affecting corona

7. Waveform of corona current

8. Disadvantages

9. Ways to reduce corona

10. Advantages and Applications

Abstract

Corona is a phenomenon associated with all energized transmission lines. Under certain conditions, the localized electric field near an energized conductor can be sufficiently concentrated to produce a tiny electric discharge that can ionize air close to the conductors (Electric Power Research Institute (EPRI), 1982). This partial discharge of electrical energy is called corona discharge, or corona.

Corona Discharge discharge results from electrical discharge and indicates ionization of oxygen

and the formation of ozone in the surrounding air. It can eat through a Polymer insulator until it

breaks! It produces extremely corrosive Nitric Acid when in a humid atmosphere!

Several factors, including conductor voltage, shape and diameter, and surface irregularities such as scratches, nicks, dust, or water drops can affect a conductor’s electrical surface gradient and its corona performance. Corona is the physical manifestation of energy loss, and can transform discharge energy into very small amounts of sound, radio noise, heat, and chemical reactions of the air components. Because power loss is uneconomical and noise is undesirable, corona on transmission lines has been studied by engineers since the early part of this century. Many excellent references exist on the subject of transmission line corona (e.g., EPRI, 1982). Consequently, corona is well understood by engineers and steps to minimize it are one of the major factors in transmission line design for extra high voltage transmission lines (345 to 765 kilovolts (kV)). Corona is usually not a design issue for power lines rated at 230 kV and lower. The conductor size selected for the project’s transmission line is of sufficient diameter to lower the localized electrical stress on the air at the conductor surface and would further reduce already low conductor surface gradients so that little or no corona activity would exist under most operating conditions.

To prevent the formation of corona, the working voltage under fair working conditions should be

kept atleast 10% less than the disruptive critical voltage. Corona formation may be reduced by

increasing the effective radius. Thus steel cored aluminium has the advantage over hard drawn

copper conductors on account of the large diameter, other conditions remaining the same. The

effective conductor diameter can also be increased by the use of bundled conductors.

The corona discharges emit radiations which may introduce noise signals in the communication

channels, radio and television receivers in the vicinity. This is called radio interference (RI). Radio

noise from overhead power lines is caused by corona on conductors and fittings ,surface discharges

on insulators and poor contacts in fitting and insulator strings.

The importance of radio interference problem in the present age can hardly be over-emphasized.

Great attention is paid to reduce the level of radio interference from EHV lines to tolerable limits.

Corona Discharge discharge results from electrical discharge and indicates ionization of oxygen

and the formation of ozone in the surrounding air.

Corona reduces the magnitude of high voltage steep fronted waves due to lightning or switching by

partially dissipating as a corona loss. Corona act as a safety valve for lightning surges, by causing a

short circuit.

In a positive corona the electron avalanche is initiated by an exogenous ionisation event in a region

of high potential gradient. The electrons resulting from the ionisation collision are attracted toward

the positive electrode, and the positive ions are repelled from it. The secondary electrons, required

to seed further avalanches, are generated at the boundary of ionisation region by photons of light

released during the ionisation process. When these photons strike neutral gas molecules they

liberate electrons (through to the photoelectric effect), which are then drawn to the positive

electrode. It is these electrons which seed and sustain further avalanches. While the electrons travel

to the positive electrode the positive ions drift away from it towards the earthed electrode.

A feature of negative coronas is that they can only be sustained in fluids which contain

electronegative molecules, such as O2, H20 and CO2. These gases have molecules which readily

scavenge free electrons. Without electronegative molecules to capture free electrons, small negative

ions cannot form, with the result that a simple path of electron flow of ionised gas will form

between the two electrodes and an arc will develop.

Introduction

The term corona has been derived from the glow surrounding the conductor when the operating

voltage is sufficiently high.

A corona discharge is an electrical discharge brought on by the ionization of a fluid surrounding a

conductor, which occurs when the potential gradient (the change in the strength of the electric field)

exceeds a certain value, but conditions are insufficient to cause complete electrical breakdown or

arcing.

If the electric field is uniform, a gradual increase in voltage across the gap produces a breakdown of

the gap in the form of a spark without any preliminary discharges. But if the field is non-uniform,

an increase in voltage will first cause a localized discharge in the gas to appear at the points with

the highest electric field intensity, namely at sharp points.

Corona discharge usually involves two asymmetric electrodes; one highly curved (such as the tip of

a needle, or a small diameter wire) and one of low curvature (such as a plate, or the ground). The

high curvature ensures a high potential gradient around one electrode, for the generation of plasma.

Coronas may be positive or negative. This is determined by the polarity of the voltage on the

highly-curved electrode. If the curved electrode is positive with respect to the flat electrode we say

we have a positive corona, if negative we say we have a negative corona. (See below for more

details.) The physics of positive and negative coronas are strikingly different. This asymmetry is a

result of the great difference in mass between electrons and positively charged ions, with only the

electron having the ability to undergo a significant degree of ionising inelastic collision at common

temperatures and pressures.

The phenomenon of corona is accompanied by the following processes:

1. A faint glow appears around the conductor s which is visible in the dark.

2. There is an acoustical noise.

3. There is a tendency in conductors to vibrate.

4. Ozone and oxides of nitrogen are produced.

5. There is a loss of power.

6. There is radio interference.

Mechanism of corona formation

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.

However 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, ultra violet radiations from the sun,

radioactivity of the soil etc. If the voltage impressed between the conductors is of alternating

nature, sustained charging current will flow due to the capacitance of the line. With the increase of

the voltage, there is a corresponding increase in the electric field intensity.

As long as the air is subjected to a uniform electric field intensity of peak value is less than 3×106

volt/m (3000 kV/m or 30kV/cm), the flow of current between the two conductor of the line is

negligibly small for practical purposes. But when the electric field intensity (voltage gradient)

reaches this critical value of 3×106 volt/m, the air in the immediate vicinity of conductors no more

remains a dielectric but it ionizes and becomes conducting this electric break down is accompanied

by a faint glow which appears around the conductor and is visible in dark.

If the voltage gradient is increased further , the size and the brightness of the luminous envelope

goes on increasing until finally a spark or arc is established between the conductor because of the

partial breakdown of the insulating property of air between them. The effect of corona is more

pronounced at the protruding points of the conductor due to local higher field intensity there.

Steps of corona discharge are as follows:

Corona discharge of both the positive and negative variety have certain mechanisms in common.

1. A neutral atom or molecule of the medium, in a region of strong electric field (such as the

high potential gradient near the curved electrode) is ionized by an exogenous environmental

event (for example, as the result of a photon interaction), to create a positive ion and a free

electron.

2. The electric field then operates on these charged particles, separating them, and preventing

their recombination, and also accelerating them, imparting each of them with kinetic

energy.

3. As a result of the energisation of the electrons (which have a much higher charge/mass

ratio and so are accelerated to a higher velocity), further electron/positive-ion pairs may be

created by collision with neutral atoms. These then undergo the same separating process

creating an electron avalanche. Both positive and negative coronas rely on electron

avalanches.

4. In processes which differ between positive and negative coronas, the energy of these

plasma processes is converted into further initial electron dissociations to seed further

avalanches.

5. An ion species created in this series of avalanches (which differs between positive and

negative coronas) is attracted to the uncurved electrode, completing the circuit, and

sustaining the current flow.

The formation mechanism of the surface corona on dielectric plates under negative impulse

voltages has been investigated with a high-speed gated image intensifier in atmospheric air. An

acrylic plate was inserted perpendicularly to the axis of a rod-plane electrode system, and as a

backside electrode, a translucent electric conductive film was used to observe the luminosity of the

surface corona. The size and shape of the negative surface corona were measured using acrylic

plates of 1, 2, 5 and 10 mm thick and rod electrode with diameters of 10, 20, 50 and 125 mm.

Experiments show that the negative surface corona occurred just after the impulse voltage

application and formed a ring-like luminescence. The diameter of the ring increased with the

thickness of the acrylic plate and the diameter of rod electrode. The formation mechanism of the

negative surface corona is studied using a numerical analysis of the electron avalanche developing

along the electric line of force around the rod electrode. As a result, it is found that the occurrence

positions of the surface corona agree with the theoretical computed points of the maximum linear

density of positive ion.

Types of corona

There are mainly 2 types of corona namely:

1. Positive Corona

2. Negative Corona

Positive corona

A positive corona is manifested as a uniform plasma across the length of a conductor. It can often

be seen glowing blue/white, though many of the emissions are in the ultraviolet. The uniformity of

the plasma owes itself to the homogeneous source of secondary avalanche electrons described in

the mechanism section, below. With the same geometry and voltages, it appears a little smaller than

the corresponding negative corona, owing to the lack of a non-ionising plasma region between the

inner and outer regions. There are many fewer free electrons in a positive corona, when compared

to a negative corona, except very close to the curved electrode: perhaps a thousandth of the electron

density, and a hundredth of the total number of electrons.

However, the electrons in a positive corona are concentrated close to the surface of the curved

conductor, in a region of high-potential gradient (and therefore the electrons have a high energy),

whereas in a negative corona many of the electrons are in the outer, lower-field areas. Therefore, if

electrons are to be used in an application which requires a high activation energy, positive coronas

may support a greater reaction constants than corresponding negative coronas; though the total

number of electrons may be lower, the number of a very high energy electrons may be higher.

Coronas are efficient producers of ozone in air. A positive corona generates much less ozone than

the corresponding negative corona, as the reactions which produce ozone are relatively low-energy.

Therefore, the greater number of electrons of a negative corona leads to an increased production.

Beyond the plasma, in the unipolar region, the flow is of low-energy positive ions toward the flat

electrode.

Mechanism

As with a negative corona, a positive corona is initiated by an exogenous ionisation event in a

region of high potential gradient. The electrons resulting from the ionisation are attracted toward

the curved electrode, and the positive ions repelled from it. By undergoing inelastic collisions

closer and closer to the curved electrode, further molecules are ionized in an electron avalanche.

In a positive corona, secondary electrons, for further avalanches, are generated predominantly in the

fluid itself, in the region outside the plasma or avalanche region. They are created by ionization

caused by the photons emitted from that plasma in the various de-excitation processes occurring

within the plasma after electron collisions, the thermal energy liberated in those collisions creating

photons which are radiated into the gas. The electrons resulting from the ionisation of a neutral gas

molecule are then electrically attracted back toward the curved electrode, attracted into the plasma,

and so begins the process of creating further avalanches inside the plasma.

As can be seen, the positive corona is divided into two regions, concentric around the sharp

electrode. The inner region contains ionising electrons, and positive ions, acting as a plasma, the

electrons avalanche in this region, creating many further ion/electron pairs. The outer region

consists almost entirely of the slowly migrating massive positive ions, moving toward the uncurved

electrode along with, close to the interface of this region, secondary electrons, liberated by photons

leaving the plasma, being re-accelerated into the plasma. The inner region is known as the plasma

region, the outer as the unipolar region.

Negative coronas

A negative corona is manifested in a non-uniform corona, varying according to the surface features

and irregularities of the curved conductor. It often appears as tufts of corona at sharp edges, the

number of tufts altering with the strength of the field. The form of negative coronas is a result of its

source of secondary avalanche electrons (see below). It appears a little larger than the

corresponding positive corona, as electrons are allowed to drift out of the ionising region,

and so the plasma continues some distance beyond it. The total number of electrons,

and electron density is much greater than in the corresponding positive corona. However, they are

of a predominantly lower energy, owing to being in a region of lower potential-gradient. Therefore,

whilst for many reactions the increased electron density will increase the reaction rate, the lower

energy of the electrons will mean that reactions which require a higher electron energy may take

place at a lower rate.

As with the positive corona, the electron avalanche in a negative corona is initiated by an

exogenous ionisation event in a region of high potential gradient. However, the electrons travel in

the opposite direction, away from the negative electrode, while the positive ions are drawn towards

it. Unlike the positive corona, electrons ionised from the neutral gas are of relatively little use in

sustaining a negative corona because the general movement of electrons is away from the negative

electrode. Instead, the dominant process for generating secondary electrons is the photoelectric

action of photons striking the surface of the negative electrode. Indeed, the energy required to

liberate the electrons from the electrode surface is considerably lower than that required to ionise

air at standard temperatures and pressures, making it a more liberal source of secondary electrons.

Consequently, the avalanche seed electrons are generated on the surface of the electrode and are

repelled from it. As these electrons leave the ionisation region they attach themselves to neutral gas

molecules to form small negative air ions which drift towards the earthed electrode.

Mechanism

Negative coronas are more complex than positive coronas in construction. As with positive

coronas, the establishing of a corona begins with an exogenous ionization event generating a

primary electron, followed by an electron avalanche.

Electrons ionized from the neutral gas are not useful in sustaining the negative corona process by

generating secondary electrons for further avalanches, as the general movement of electrons in a

negative corona is outward from the curved electrode. For negative corona, instead, the dominant

process generating secondary electrons is the photoelectric effect, from the surface of the electrode

itself. The work-function of the electrons (the energy required to liberate the electrons from the

surface) is considerably lower than the ionization energy of air at standard temperatures and

pressures, making it a more liberal source of secondary electrons under these conditions. Again, the

source of energy for the electron-liberation is a high-energy photon from an atom within the plasma

body relaxing after excitation from an earlier collision. The use of ionized neutral gas as a source of

ionization is further diminished in a negative corona by the high-concentration of positive ions

clustering around the curved electrode.

Under other conditions, the collision of the positive species with the curved electrode can also

cause electron liberation.

The difference, then, between positive and negative coronas, in the matter of the generation of

secondary electron avalanches, is that in a positive corona they are generated by the gas

surrounding the plasma region, the new secondary electrons travelling inward, whereas in a

negative corona they are generated by the curved electrode itself, the new secondary electrons

travelling outward.

A further feature of the structure of negative coronas is that as the electrons drift outwards, they

encounter neutral molecules and, with electronegative molecules (such as oxygen and water vapor),

combine to produce negative ions. These negative ions are then attracted to the positive uncurved

electrode, completing the 'circuit'.

A negative corona can be divided into three radial areas, around the sharp electrode. In the inner

area, high-energy electrons inelastically collide with neutral atoms and cause avalanches, whilst

outer electrons (usually of a lower energy) combine with neutral atoms to produce negative ions. In

the intermediate region, electrons combine to form negative ions, but typically have insufficient

energy to cause avalanche ionization, but remain part of a plasma owing to the different polarities

of the species present, and the ability to partake in characteristic plasma reactions. In the outer

region, only a flow of negative ions and, to a lesser and radially-decreasing extent, free electrons

toward the positive electrode takes place. The inner two regions are known as the corona plasma.

The inner region is an ionizing plasma, the middle a non-ionizing plasma. The outer region is

known as the unipolar region.

One feature common to both positive and negative coronas is the formation of an electron

avalanche. Such an avalanche occurs when a strong electric field acts on naturally occurring free

electrons in the air. The electric field accelerates these electrons so that they gain sufficient kinetic

energy to cause ionisation when they collide with neutral gas molecules in their path. Additional

electrons are liberated during these collisions, which after acceleration are also able to ionise. As

the process continues more and more electrons are liberated and an avalanche rapidly builds up. In

this way a small number of seed electrons can cause ionisation of an entire gas and turn it into a

plasma. In a positive corona the avalanche electrons are drawn towards the electrode, while the

resultant positive ions are repelled. In a negative corona the avalanche is in the opposite direction,

with the electrons repelled and the positive ions drawn to the electrode. In a positive corona the

electrons accelerate as the avalanche progresses, while in a negative corona they decelerate as they

travel away from the electrode.

In an avalanche the electron collisions excite the positive ions so that photons of short wavelength

light are emitted. It is this that gives a corona discharge its characteristic glow. These photons play

an important part in producing the new seed electrons which are required to sustain the corona.

Voltage parameters of corona

There are two voltage parameters associated with corona:

1. Disruptive critical voltage

2. Visual critical voltage

Disruptive critical voltage

The minimum voltage at which the breakdown of the insulating properties occurs and corona starts

is called the disruptive critical voltage. The potential gradient corresponding to this value of the

voltage is known as disruptive critical voltage gradient.

A transmission line should operate just below the disruptive critical voltage in fair weather so that

corona only takes place during adverse atmospheric conditions. Therefore, the calculated disruptive

critical voltage is an indicator of the corona performance of the line. However, a high value of the

disruptive critical voltage is not the only criterion of satisfactory corona performance. The

sensitivity of the conductor to foul weather should also be considered, and the fact that corona

increases more slowly on stranded conductors than on smooth conductors.

According to F W Peek, for air under conditions near see level at 25o C and at a atmospheric

pressure of 760 mm of mercury (Hg) without impurities, the maximum value of voltage gradient at

which ionization of air starts is about 3×106volt/m. That is maximum value of the voltage gradient

is disruptive critical voltage gradient. The voltage corresponding to this voltage gradient is the

disruptive critical voltage.

For a three phase line,

Eo=Go mo r ln Deq Volt/phase

where,

Eo=disruptive critical voltage in volts

Go=(3×106 )/2½ = disruptive critical voltage gradient in volts/m

mo=irregularity factor or surface factor

= 1 for smooth conductors

= 0.93 to 0.98 for rough and weathered conductors

= 0.80 to 0.87 for stranded conductors

r= radius of each conductor in meters

Deq= equivalent spacing between the conductors in m = (Dab Dbc Dca)1/3

air density factor = 0.392p/(273+t)

(p= pressure ; t= temperature in oC)

Visual critical voltage

When the voltage of the line is the disruptive critical value, there is no visible corona. This is due to

the fact that the charges ions in the air must be able to receive a finite energy before they can cause

further ionization by collision, which is necessary for the corona discharge.

Visual glow of corona occurs at a voltage higher than the disruptive critical voltage. The minimum

voltage at which the visual corona starts is termed as visual critical voltage.

According to F W Peek, the actual visual corona does not start at the disruptive critical value of

voltage. The maximum value of voltage gradient responsible for starting of corona is 3×106 volt/m.

but this value of voltage gradient at the surface of conductor will not ionize the air. The maximum

value of 3×106 volt/m will cause ionization when this value is reached at a distance of (r+0.0301

r1/2) from the conductor axis, where r is in meters. The reason being that some energy is required by

the charged ions to start corona. If the maximum voltage gradient at the surface of conductor be

3×106 volt/m, the value of maximum voltage gradient at any other point away from the centre

would be less than this and, thus there will be corona discharge at that point.

Under the standard conditions of temperature and pressure, the value of visual critical voltage for a

three phase line can be written as follows when the effects of irregularity of the surface of the

conductor and air density factor are considered.

Ev= (3×106)/2½ mv r (1+0.3/√δr) ln(Deq/r) rms volts/phase

Where,

Ev= rms value of visual critical voltage

mv= irregularity factor

= 1.00 for smooth conductors

= 0.70 to 0.75 for local corona when effect is first visible at some places along line

= 0.80 to 0.85 for decided or general corona along the whole length of conductor

Factors Affecting Corona

Due to the inherent complexity and predominance of weather conditions, the theoretical analysis of

corona has always lagged behind, and it studies are mainly based upon experimental evidences.

Large number of full scale experimental lines has been constructed. The main purpose of these lines

was to determine the factors affecting corona loss and radio interference so as to form a basis for

economical and satisfactory performance of the existing EHV/UHV lines and the economical

designs of the new ones.

Corona loss depends upon large number of factors, the important being broadly classified in the

following ways:

1. Conductor surface gradient

2. Condition of conductor surface

3. Atmospheric conditions

4. Air density factor

Conductor surface gradient

Corona loss on a conductor is a function of potential gradient at the conductor surface. The surface

gradient is given by

Gmax= En / r ln (D/r) V/m

which is applicable to single conductor only when the effect of stranding, surface condition, and air

density are not considered. The field strength will be different for bundled conductor. In such case,

the surface gradient becomes non uniform by the distortion produced by induction.

From the equations of surface gradients for single conductors and multiple conductor lines, it is

seen that radius and spacing of conductors affect the surface gradients and therefore, the voltage at

which corona starts. In addition to this surface gradient also depends on the configuration of the

line and the height of the conductor above the ground. For example, in case of a line and flat

spacing, the surface gradient at the middle phase conductor is higher than the outer. The corona will

therefore start first on middle conductor.

Corona loss also depends upon the frequency and waveform of the supply voltage.

Condition of Conductor surface

The voltage at which corona starts is also affected by the condition of conductor surface. If the

conductor is smooth, the electric field will be more uniform as compared to that when its surface is

rough. The roughness of conductor may be caused by stranding, die burrs, scratches or deposition

of dust or other air borne particles on its surface. Due to the concentration of field intensity at the

rough spots local corona discharges occur there. Therefore, corona starting voltage is lower in case

of conductors with rough surface than for those with smooth surface.

It has been seen that after an operation period of six months or so, the conductors surface becomes

smooth due to weathering effects. This has been explained by the fact that during corona discharge

ozone (o3) and oxides of nitrogen are produced. They are rather unstable to give atomic oxygen

easily, which is a very active oxidizing agent. With the action of atomic oxygen at the pronounced

points, they become blunted making the surface smooth. When the conductors get aged in this way,

the corona loss is brought from the initial high value to a comparatively smaller stable value.

Atmospheric conditions

Atmospheric conditions due to weather affect the corona loss to a great extent. The corona loss is

fairly large during foul weather. Foul weather reduces the uniformity of electric field and lowers the

corona formation voltage. The increase of corona loss due to rain fall is due to the fact that the rain

drops settle on the conductor surface to form protruding points, which become the sources of local

discharges because of field intensity concentration at those points.

When the rate of rain fall increases, it is observed that the corona loss for a given conductor at any

voltage increases. But the rate of increase of loss is lesser than that of rain fall. The loss becomes

practically constant when the rate of rain fall is very high.

Air density factor

Air density is also one of the main factors to affect the corona loss. The air density factor is

defined as the ratio of density of air at any given barometric pressure, and temperature to that at the

conditions 25oC (77oF) and 760 mm (29.9 inches) of mercury barometric pressure. The value of air

density is taken to be equal to unity at standard barometric pressure of 760 mm of mercury and

25oC .

The value of at a pressure p mm of mercury and toC can be found out from the well-known gas

equation, which states that the product of pressure and volume divided by the absolute temp of a

gas is constant.

(273+25)/760 × p/(273+t)

= 0.392 p/ (273+t)

Humidity also affects corona voltage. An increase of humidity decreases corona starting voltage.

With the increase of the altitude, the air density is reduced and corona starting voltage will be

lower. However, lower average temperatures of air at high altitude compensates this effect by

increasing the corona starting voltage.

Waveform of corona current

The current flowing in the corona discharge contains high frequency components. The current

carried by the corona is determined by integrating the current density over the surface of the

conductor.

The shunt current in a line is almost purely capacitive under normal conditions, and leads the

applied voltage by 90o, and there is no power loss in the line under no load conditions.

When the applied voltage is increased and corona is formed, the air is rendered conducting, and

power loss occurs.

The shunt current would no longer be leading the voltage by 90o. Thus the current wave form

consists of two components. The lossy component would be non-sinusoidal and would occur only

when the disruptive critical voltage is exceeded in either polarity.

The corona current can be analyzed and shown to possess a strong third harmonic component.

Disadvantages of Corona

Coronas can generate audible and radio-frequency noise, particularly near electric power

transmission lines. They also represent a power loss, and their action on atmospheric particulates,

along with associated ozone and NOx production, can also be disadvantageous to human health

where power lines run through built-up areas. Therefore, power transmission equipment is designed

to minimize the formation of corona discharge.

Corona discharge is generally undesirable in electric power transmission where it causes :

1. Power loss

2. Audible noise

3. Radio interference

4. Insulation damage

5. Ozone production

6. Static electricity discharge

7. Lightning

8. Long term exposure to these radiations may not be good to health

Power loss

The power dissipated in the system due to corona discharges is called corona loss. It is very

difficult to estimate corona loss accurately because of its extremely variable nature.

According to FW Peek, the corona loss for single phases and equilaterally spaced three phase

lines under fair weather condition is given by the formula

Where,

Pc = corona power loss

f = frequency of supply in Hz

= air density factor

E = rms phase voltage in kV

Eorms = disruptive critical voltage/ phase in kV(rms)

r = radius of conductor in m

d = equivalent spacing between comductor in m

Audible noise

Transmission lines can generate a small amount of sound energy during corona activity. This

audible noise from the line can barely be heard in fair weather conditions on higher voltage lines.

During wet weather conditions, water drops collect on the conductor and increase corona activity so

that a crackling or humming sound may be heard near the line. This noise is caused by small

electrical discharges from the water drops.

Audible noise values can be calculated for transmission lines experiencing corona activity. This

modeling indicates that, during wet weather conditions, audible noise levels of approximately 46.6

to 49.6 A-weighted decibels (dBA) would occur within the right-of-way for the proposed

transmission line loop.

Audible noise will decrease with distance away from the proposed transmission line loop.

Presently, the proposed transmission line loop is located within an open farmland area, where

residences, businesses, and other receptors are not present. Due to all of these factors, impacts from

corona noise would be less than significant.

Radio interference

Radio interference is the adverse effect introduced by corona on wireless broadcasting. The corona

discharges emit radiations which may introduce noise signals in the communication lines, radio and

television receivers. It is mainly due to the brush discharges on the surface irregularities of the

conductor during positive half cycle. This leads to corona loss at the voltage less than critical

voltage. The negative discharges are less troublesome for radio reception. Radio interference is

considered as a field measured in microvolts/m at any distance from the transmission line and is

significant only at voltage greater than 200kV. There is gradual increase in RI level till the voltage

is such that measurable corona loss takes place. Above this voltage, there is rapid increase in RI

level. The rate of increase is more for smooth and larger diameter conductor. The amplitude of RI

level varies inversely as the frequency at which the interference is measured. Thus the service in the

higher frequency band e.g. television, frequency modulated broadcasting, microwave relay, radar,

etc. are less affected. RI is one of the very important factors while designing a transmission line.

Insulation damage

Inside electrical components such as transformers, capacitors, electric motors and generators.

Corona progressively damages the insulation inside these devices, leading to premature equipment

failure.

The accumulation of the nitric acid and micro-arcing within it create carbon tracks across insulating

materials. Corona can also contribute to the chemical soup destruction of insulating cements on

insulators resulting in internal flash-overs. The corona is the only indication. Defects in insulating

materials that create an intense electrical field can over time result in corona that creates punctures,

carbon tracks and obvious discoloration of NCI insulators.

Electrical field intensity producing corona on contaminated areas, water droplets, icicles, corona

rings, ... This corona activity then contributes nitric acid to form a chemical soup to change the

bonding cements and to create carbon tracks, along with ozone and ultraviolet light to change the

properties of NCI insulator coverings. Other detrimental effects include water on the surface or sub-

surface freezing and expanding when thawing, as a liquid penetrating into a material and then a

sudden temperature change causes change of state to a gas and rapid expansion causing fracture or

rupture of the material.

Ozone production

Ozone (O3), or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an

allotrope of oxygen that is much less stable than the diatomic allotrope (O2). Ozone in the lower

atmosphere is an air pollutant with harmful effects on the respiratory systems of animals and will

burn sensitive plants; however, the ozone layer in the upper atmosphere is beneficial, preventing

potentially damaging electromagnetic radiation from reaching the Earth's surface. Ozone is present

in low concentrations throughout the Earth's atmosphere. It has many industrial and consumer

applications.

An electrical discharge (a spark) splits an oxygen molecule into two oxygen atoms. (Electrical

discharge is also referred to as corona discharge.) These unstable oxygen atoms combine with other

oxygen molecules. This combination forms ozone.

Ways to Reduce Corona

There are several disadvantages of corona. So it becomes necessary to reduce the corona formation

for efficient power transmission. Some of the ways to minimize corona are:

1. Use of Corona Rings

Installing corona rings at the end of transmission lines.A corona ring, also called anti-corona ring, is

a toroid of (typically) conductive material located in the vicinity of a terminal of a high voltage

device. It is electrically insulated. Stacks of more spaced rings are often used. The role of the

corona ring is to distribute the electric field gradient and lower its maximum values below the

corona threshold, preventing the corona discharge.

2. Inverter duty motors

Inverter duty motors are gaining popularity across the globe. Inverter duty motors are designed for

optimized performance when run with variable frequency controllers. A wide range of products are

available including energy efficient motors, RPM A-C motors offering superior constant

horsepower ranges, and V*S Master motors offering full torque from 0 – 60 Hz. Inverter Drive

Motors are wound with 200 C moisture resistant Corona resistant magnet wire which dramatically

extends the life of the motors compared to motors wound with non-corona resistant wire.

The Corona resistant magnet wire dissipates charge build up at the surface thereby minimizing the

damaging effects of corona formation. The Corona Clear coatings offered by The P.D. George

Company use semiconducting nanoparticles to achieve optimum charge dissipation while offering

excellent coatability and dispersion stability.

3. Greater spacing between the conductor

By increasing the spacing between the conductors, disruptive voltage of the conductor (i.e. voltage

at which corona starts) increases. But this method has also got its limitations as with the increase of

spacing between the conductors, the cost of tower increases and there is an increase in the voltage

drop in the line due to the increase in the inductive reactance.

4. Hollow conductors

The method of increasing the diameter of conductors has been found to be very effective in

reducing corona. The diameter of conductors can be increased by either using hollow conductors or

steel cored aluminium conductors (ACSR) or expanded ACSR conductors.

5. Bundled conductors

The present practice is to use bundled conductors for reducing corona. The use of bundled

conductor s has become so common for transmission of larger blocks of power over longer

distances at EHV and UHV that the bundled conductors may be considered as an integral part of

EHV and UHV transmission lines.

The bundle acts as far as the electric field is concerned, like a conductor of diameter much larger

than that of the component conductors. This reduces the voltage gradient. In other words a higher

voltage can be used for permissible level of RI. The GMD of a bundle is high and therefore the

inductive reactance of the line is low.

The bundled conductors have higher capacitance and therefore low surge impedence as compared

to single conductors of equivalent diameter. The lower value of inductive reactance helps in

reducing the cost of series capacitors which are used to increase the transient stability limit of very

long transmission lines.

Advantages and applications of corona

1. Surge arresters

One of the major advantage of corona is that it provides a relief valve for surges by acting as a short

circuit. So it is specially employed in surge arresters installed on transmission lines.

2. Chemical synthesis

Many chemical products can be synthetized by corona discharges, but ozone is so far the only

one of industrial importance. It is used for the treatment of water, preserving its natural taste

and avoiding the smell of chlorine, and for other applications utilizing its large oxidation power

for instance in textile and paper industries.

The streamer type coronas used for ozone generation generally use air or oxygen filled short

gaps where one of the electrodes is covered with an insulating layer in order to prevent

streamer-arc transition. The relevant features of these so-called silent discharges for ozone

production are

- High energy electrons, providing a high yield of atomic oxygen for the main reaction

O + 02 + M --03 + M where M is a molecule, often oxygen, carrying away the excess energy;

- Low gas temperature, necessary to avoid thermal dissociation of the ozone molecules already

produced;

- High working pressure, around atmospheric.

Research is still needed to improve both chemical and energy yields, still too low ( 2 % and 6 %

respectively in dried air). Progress might be obtained by different ways.

- There is no clear evidence that a unipolar conduction current does not flow in the silent

discharge considered, in parallel with the streamers.

The catalytic effect of selected surfaces in production of activated nitrogen in coronas might

also be used for chemical synthesis of other products, as ammonia.

3. Surface treatment

Coronas are widely used as chemical reactors for surface treatment. Their oxidation and

reduction effects, Dassivating or corroding, have already been mentioned in the section dealing

with the electrochemical behavior of the discharge. Depending on the material to be treated and

the desired surface properties, the optimal corona gas composition, discharge parameters and

operating procedure may be very different.

An industrially very important application is the surface treatment of polymers, in particular to

increase their wettability and their adhesivity to ease printing, painting, sealing, coating, etc.

This is achieved simply by corona discharges in ambient air. Discharge products transfer their

activation energy to the polymer by breaking chains and creating radicals. These will rapidly

react with the further impinging particles, with the environment, and even with gas coming

from the bulk material. Polar bonds and hydrogen bonds formed in this way will increase the

polymers surface energy. The bonds most frequently encountered are C - 0, C = 0, C - 0 - O, C

– OOH.

If gap length of an air or oxygen corona is reduced so that the drift region becomes very small,

the oxygen grafted on polymers rapidly diminish due to increased etching. However, the

induced surface energy (wettability, etc.) is not reduced, even if the mechanisms involved must

have changed.

The efficiency of a corona surface treatment also depends on polarity. Negative coronas

generally appear to be less efficient than positive coronas, both for oxidation and etching.

Contributing factors are that the energy delivered to the surface by charge neutralization is

lower for negative ions than for positive ones and that the streamers generated in positive

coronas submit the surface to concentrated bursts of high energy ions.

The use of coronas to improve the wettability and adhesivity of polymer and other surfaces is

well established. However, more recent work shows that coronas also may be used to reduce

these quantities, typically by surface polymerization induced by coronas in fluorinated gases of

pressures up to atmospheric. For instance, good results have been obtained on paper and textile

materials by corona treatment in a C2 H2 F2 atmosphere.

4. Sanitization

Finally, let us note that coronas have bactericidal effects. Exposure to air coronas will stop the

growth of bacteria cultures, as recently demonstrated on Escherichia Coli. That is why, it is

being used for the sanitization purposes.

5. Electrostatic applications

The fact that unipolar corona drift regions contain ions of one sign only, and no plasma, is the

obvious reason for their extended use as chargers in electrostatic apparatus, like precipitators,

paint guns, fertilizer projectors, separators, xerographic copiers, voltage generators, and even

lightning protectors. On the other hand, charges generated by other means, typically by

turboelectricity, may easily cause coronas and sparks of their own, causing a considerable

number of explosion accidents each year. A knowledge of coronas, their transition to sparks,

and their interaction with the chemical environment is essential to further progress in these

fields.

Few more applications are

Drag reduction over a flat surface

Removal of unwanted electric charges from the surface of aircraft in flight and thus

avoiding the detrimental effect of uncontrolled electrical discharge pulses on the

performance of avionic systems

Scrubbing particles from air in air-conditioning systems

Removal of unwanted volatile organics, such as chemical pesticides, solvents, or

chemical weapons agents, from the atmosphere

Production of photons for Kirlian photography to expose photographic film

EHD thrusters, Lifters, and other ionic wind devices

Surface treatment for tissue culture (polystyrene)

Ionization of a gaseous sample for subsequent analysis in a mass spectrometer or an ion

mobility spectrometer

Solid-state cooling components for computer chips

Photocopying

Air ionizers

Nitrogen laser

References

1. Electrical Power System by Ashfaq Husain

2. Electrical Power System by C.L. Wadhwa

3. en.wikipedia.org

4. High Voltage Breakdown and Testing (online syllabus) by Professor JR Lucas

5. www.scribd.com

6. www.nipponmagnetics.com

7. www.inderscience.com