when a low luminous glow of violet colour will appear and a hissing sound will be heard. This phenomenon is known as Corona effect. Corona is also accompanied by the formation of ozone perhaps due to ionization of air in between the two wires.

1. Corona Effect
Physical Concept of Corona
Concept: On a transmissionlinetransmitting alternatingcurrent, if
potentialdifferenceapplied totwo wires, whose spacing islarge in
comparisonwith thediameter ofthe wire, is graduallyincreased, a point
will be reached when a low luminousglow of violet colour will appear
and a hissing sound will be heard. This phenomenon is known
as Corona effect. Corona is also accompaniedbythe formationof
ozone perhapsdue to ionizationof air in betweenthe two wires. The
ozone gasis indicatedbyitscharacteristic odour.
If the voltagebetween thewires is graduallyincreased still, further, A
type of hissing noise with violet glow will increase, intensityuntil
eventually, a spark takesover theplace.
If the twowires are perfectlysmooth and uniform, then the glow will
occur throughout thelength of the transmissionline. On theother hand,
the glow will be more pronounced at rough pointsand joints. However, if
the spacing betweenwiresis not largethen it is quitepossiblethat spark
over maytakeplace even before thereis any luminous glow.
Corona effect also occursif the transmissionofelectricalpower is by
direct current, theonly differencebeing that the positiveconductor has
uniform glow and the -ve conductor hasa patchyglow.
The corona is summarized asbelow:
(i) Violet light is noticed around theconductor.
(ii) It producesa hissing noise.
(iii) It created ozone which can be detected byits characteristic
odour.
(iv) The glow is maximum over rough and the dirtysurfaceof the
conductor.
(v) It is accompanied bya power loss – thewattmeter connected in
the electric circuitwillshow a reading.

2. Factors Affecting Corona Effect
The corona effect is affected by the givenfactors:
(i) Atmosphere: Corona iscaused by the rafale of molecules,
with subsequent dislodging of electrons, by theionized
particles. Corona will thus be affected by some physicalform of
the atmosphere. Thevoltage gradient for the breakdownof the
air is directlyproportionaltoitsdensity. In the heavy rainyand
stormyweather, the number of ions maybe more thannormal,
and as such the corona may occur at much fewer voltages
compared with thefair atmosphere.
(ii) Conductor: Thecorona effect is affected by the size as
diameter, shapeasstranded or smooth and surfaceconditions
(dirtyor clean) of the conductor. Whenthediameter of the
conductor hasincreased, thecorona effect decreases. A
stranded conductor enhancesmore corona that a solid
conductor. For theforlorn conductors, theshape of the x –
sectionis a series of arcsof the circleeach of smaller diameter
thanthe conductor. Thepotentialgradient for these types of
conductorswill, therefore, be greater thanfor the equivalent
smooth conductorsso the breakdownvoltagefor such
conductorswill be somewhat less than for a smooth conductor,
the ratiobeing between0.86 and unity. Theeffect of dirtiness
on thesurfaceis to increasethe aberranceand thereby
decreasing thebreakdownvoltagefurther, an averagevalue for

3. the ratiowhen both stranding and dirtinessareinvolved into
part lies about 0.86 and unity.
(iii) Spacing between Conductors: Withtheincreasein spacing
betweenthe conductors, theelectrostatic stressesarereduced
and therefore, the corona effect is reduced. If thegaping
betweenthe conductor islarger than to itsdiameter thenthere
may not be any corona effect in thetransmissionline.
(iv) Line Voltage: Line voltage mostly affectsthe corona. At the
low voltage, there is no corona effect in the transmissionline,
but when the line voltagestartsincreasing rapidlytosuch a
value that electrostaticstressesproduced on the surfaceof
conductor of the transmissionline, which createsthe
environmentalair nearbythe conducting conductor, causeof
that corona effect appears.
Advantages and Disadvantages of Corona
The following are the advantagesand disadvantagesofcorona:
Advantages
(i) Due to the emergenceof the corona effect, the sheath of air
surrounding theconductor becomesconductiveand thereis a
virtualincreasein conductor diameter and dueto this virtual
increasein the conductor thediameter ofthe maximum
potentialgradient or maximum electrostatic stressisreduced.
Thus, chancesof flashover arereduced and system performance
is improved.
(ii) Effects of transientsproduced bythunder and lightning and
other casesare decreased sincechargesinduced on the line by
lightning or other causeswill be partiallydissipatedasa corona
loss. In thisway, it works as a safetyvalve and sometimeslines
have been designed to have an operating voltagenear to the
censoriousvoltage in order to do awaywith the essentialfor,
and expenseof lightning arrestor gear. Anobjectiontothis
strategyisthat the criticalvoltageis not fixed for a given line,
but get vary considerablywith changesinthe atmosphere.
Disadvantages:

4. (i) There is a definiteloss of power, although thisis not important
except under abnormalweather conditions.
(ii) There is a non – sinusoidalvoltagesdrop due to non –
sinusoidalcorona current and these may causesome interface
with neighbouring communicationcircuitsdueto
electromagnetic and electrostaticinduction.
(iii) Owing to the distortionofthe wave form, the harmonics
predominatelythird harmonic, areintroduced intothe
transmissionline.
(iv) Owing to the formationof the corona, ozone gas is produced
which chemically reactswith theconductor and causecorrosion.
Methods of Reducing Corona Effect
It has been seen that high corona effects areobserved at a working
voltage of 33kV or higher. Consequently, the carefuldesignshould be
built to ignorethe corona effect on thesubstationsor bus – barsrated at
up to 33 kV or lighter otherwisehighly ionized air may causeflashing
over in theinsulatorsor between the phases, causing substantialdamage
to the equipment.
The corona effects canbe reduced by the given methods:
 1. By increasing conductorsize.
 2. By increasing spacing betweenconductors.
(i) By Increasing Conductor Size: By increasing the
conductor size, thepotentialdifferenceat which corona effect
occursand is raised and hence corona effect canbe reduced. It
is the causeof the reasonthat ACSR conductorshavea large
cross-sectionalarea and used in transmissionlines.
(ii) By Increasing the Spacing Between
Conductors: By increasing thespacing betweenthe
conductors, thepotentialdifferenceat which corona effect
occursis raised and hence corona effectscan be removed from
the transmissionlines. However, spacing cannot beincreased
too much otherwisethe cost of supporting structure, such as
bigger crossarmswill increasevery much.

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