CORONA

THE LOSS WHICH IS REQUIRED IN A POWER SYSTEM

PRESENTED BY

N. AVINASH CHANDRA (01401A0206)

EEE ----- IV / IV

Avichandra06@yahoo.com

YASHODHA SUNKARA (01401A0259)

EEE ----- IV / IV

Yash_sunkara@yahoo.com

FOR

SRUJANA-2005.

DEPARTMENT OF ELECTRICAL AND ELECTRONICS.

DEVARAKONDA VITTAL RAO COLLEGE OFENGINEERING AND

TECHNOLOGY.

INDEX

TOPIC PAGE NO.

Corona 1

Corona loss 2

Factors affecting corona 3

Disadvantages 4

Advantages 4

Methods of reducing corona loss 4

Conclusion 5

Bibilography 6

ABSTRACT

The efficiency of any power system is estimated by the losses that occur in

its generating transmitting and distributing equipment. As efficiency is the main criteria,

measures are taken to reduce losses and improve the efficiency. However, there are losses

which are sometimes advantageous and which are not eliminated completely for these

advantages. One such loss is corona loss.

Corona is the phenomena of ionization of the air which surrounds the

power conductors. This phenomena when occurs, creates power loss in the power

system. Though there are many disadvantages in the occurrence of corona, due to

its advantages of protecting the power system, no attempts are made to eliminate it

completely. However, only few measures to reduce it to some optimum value are

employed.

CORONA :

Corona phenomenon is the ionization of air surrounding the power conductor.

It is caused by the electric field next to an object, exceeding breakdown value for air. It is

self-sustaining electric discharge in which the field intensified ionization is localized only

over a portion of the distance between the electrodes.

Free electrons are present in free space because of radio activity and cosmic

rays. As potential between the conductors is increased, the gradient around the surface of

the conductor increases.The free electrons will move with certain velocity, depending on

field strength.These electrons will collide with molecules of air,thereby releasing more

electrons.This process is cumulative and ultimately forms an electron avalanche.Since

corona is fundamentally a breakdown phenomenon, it follows Pachen’s law, the voltage

is a function of potential difference.

The corona starting voltage is typically 30 KV/cm. Dust or water particles

on surface of the object reduce the corona starting voltage, probably by providing local

areas of higher curvature and hence higher field stress.

CORONA LOSS:

The ions produced by electric field result in space charges which move

round the conductor. This energy is derived from supply system. In order to maintain

flow of energy, additional energy is required, it is necessary to supply additional ions

from supply referred to as corona loss. Peek made experiments to study the effect of

various parameters on the corona loss and gave the following empirical relation.

Corona loss, P=241 X 10-5 (f+25)/8 v---(r/d)(Vp-Vo) 2 KW/km/Ph , where

Vp = Operating voltage in kV,

Vo = Critical disruptive voltage.

This relation gives correct results only if :

1) ‘f ‘ lies between 25 to 120 HZ.

2) Radius is greater than 0.25 cm and

3) Vp/Vo is greater than 1.8

Also a small error in m o, the irregularity factor, will lead to wrong results when using

this formula.

FACTORS AFFECTING CORONA:

Many factors affect corona loss on overhead transmission lines, mainly

1) Electrical factors:

Depends on the field around the conductor.

2) Atmospheric factors:

a) Pressure and tempterature: Expression for critical disruptive voltage,

Vo=21.1mo&oln(d/r) KV

Where &o= air density correction factor, lower the value of &, higher is

the loss because loss is directly proportional to (V-Vo) 2 .

b) Dust, rain, snow and hail effect: The presence and occurance of these bad

atmospheric conditions reduce the critical disrupted voltage and hence

increase the corolla loss.

3) Factors connected with conductors:

a) Diameter of the conductor : Loss is directly proportional to square root of

(r/d). Loss is also proportional to (V-Vo) 2. But it is found in practice that

the effect of second proportionality is much more than the first. Therefore,

larger the size of conductor, lower the corona loss.

b) Number of conductors per phase: Corona loss is solved by using two or

more conductors called bundling of conductors.

c) Profile of conductor.

d) Surface conditions.

DISADVANTAGES:

1) Power loss is more during bad weather conditions.

2) When corona is present effective capacitance increases, there by increasing the

flow of charging current. This gives raise to a voltage of triple frequency in an

ungrounded system. These triple frequency currents interfere with the

communication circuits due to electro magnetic and electro static induction

effects.

ADVANTAGES:

It reduces the magnitude of high voltage steep fronted waves due to lightning

or switching by partially dissipating a corona loss. In this way, it acts as a safety valve

to some extent.

METHODS OF REDUCING CORONA LOSS:

1) Maintaining possibly low frequencies.

2) Distance of the conductors from the ground should be more.

3) Spacing between the lines should be regular.

4) Maintaining higher air density correction factor.

5) Avoiding hilly areas.

6) Large diameter conductors, hollow conductors, bundled conductors, cylindrical

conductors should be employed.

CONCLUSION:

Thus as it is seen, there are also losses which need not be totally eliminated in a

power system. As this phenomenon works as a safety valve which protects the equipment

from lightening effects, this effect is maintained to some optimum value using the

measures suiting the requirement.

BIBILOGRAPHY

1) Electrical power systems --------------------- C.L.Wadhwa

2) A text book on power system engineering --------------- M.L.Soni, P.V.Gupta,

U.S.Bhatnagar, A.Chakrabarti.

3) Electrical power transmission system engineering ------ Turan Gonen, John Wiley& Sons.