In over headed transmission line,when an alternating potential difference is applied across two conductors,atmospheric air around the conductor is subjected to the electrostatic stresses.when there is low voltage ,there is no change in the condition of atmospheric air around the conductors.But,when the voltage is gradually increased ,a time arise when a faint luminous glow of violet colors appears together with a hissing sound, by the production of ozone which is detected by its odor.The glow is mainly due to the atmospheric air around the conductors subjected to the electrostatic forces.In simple, when the voltage between conductors of an overhead line exceeded the disruptive critical voltage (minimum phase to neutral voltage at which corona occurs)value ,a hissing noise with a violet glow appears and all overall phenomenon is called corona.If we increase the potential difference the glow along with noise will increase in its intensity and air insulation breakdown takes place.(Note that the spacing between the conductor must be larger in comparison with their diameter to occur corona).If the spacing between the conductor is not very large as compared to the diameter then corona bridge the conductor and we may not be able to see the luminous glow.Mainly in EHV(extra high voltage ) and HVDC(High voltage ) line visible glow of corona is uniform.But in the case of HVDC the glow is brighter. corona loss increases in d.c system in bad weather like in a.c system.
picture source: WAZIPOINT ENGINEERING science and technology(corona seen on transmission line)
The effects of corona are as follows:
- Violet glow is mostly observed around the conductor
- Hissing noise
- Glow is maximum over rough and dirty surfaces of the conductor.
- power loss
- production of ozone.
There are various factors affecting the corona:
- Atmosphere:As the atmosphere plays the vital role ,corona is formed due to the ionization of air around the conductor.In stormy weather the number of ions is more than in normal weather.So corona occurs at less voltage as compared to normal weather.
- frequency: Corona loss is directly proportional to the system frequency,increases with increase in the system frequency.
- conductor:The shape,size and surface of the conductor affects the corona.When the diameter of the conductor is increased ,corona decreased.When the surface of conductor is dirty or irregular ,decrease in breakdown voltage.As a result results in increased in corona loss.
- Line voltage:At low voltage ,there is no corona effect,but when the line voltage is increased,the atmospheric air surrounding the conductor is subjected to the electrostatic stress and give rise to corona.
- Spacing Between conductors :When the spacing between the conductor is increased there is decreased in the electrostatic stresses and therefore the corona effect is reduced.If the spacing between the conductor is increased with compared to the diameter ,no corona effect is seen.
- Load current: When the load current increases,the surface of conductor gets heated and there will be no formation of dew or snow in the surface of the conductor.As a result there is decrease in the corona loss.
Advantages and disadvantages of corona
- since the formation of corona the air surrounding the conductor becomes conductive and there is increase in conductor diameter as a result lowers the electrostatic stress.Hence,the risk of flash-over is reduced.
- corona acts as the safety measure against the lightening.since the charge induced on line by lightning will be dissipated as a corona loss.Transient produces by the surge is reduced by the corona.
- There will be the corrosion of the conductor as the ozone gas produced during the corona reacts with the conductors.
- Loss of power.
- May cause interference with neighboring communication circuits.
- Third harmonics are produce.(corona forms when the voltage of a conductor passes the disruptive critical voltage and disappear when the voltage decrease by same value.In every half cycle on conductor develops a pulsation in the voltage wave having three times the generator frequency.In non earthed system triple frequency current flows through the ground .But in the case of earthed system triple frequency current flows through the capacitance of the conductor.)
- There is non-sinusoidal voltage drop due to non-sinusoidal corona current.
Dielectric strength of air
The value of potential gradient at which complete disruption of air occurs is called the dielectric strength of air.
The breakdown strength of air at a barometric pressure of b cm of mercury and temperature of t⁰C becomes αg0 where
and g0 is the breakdown strength of air at a barometric pressure of 760 mm,and temperature of 25⁰C.
Disruptive critical voltage
The disruptive critical voltage is defined as the minimum phase to neutral voltage at which corona occurs.The value of disruptive critical voltage is considerably reduced during bad atmospheric conditions,such as fog,sleet,rain and snowstorms.
Visual critical voltage
Visual critical voltage is defined as the minimum phase to neutral voltage at which glow appears all along the line conductors.Visual corona doesn’t occur when the electric filed intensity becomes equal to critical value rather it starts at higher value.
Corona Power Loss
when the surface voltage gradient of line conductor exceeds the critical breakdown stress,corona apperas and energy is dissipated in the form of light and heat.This is known as corona power loss.
corona loss in an EHV AC line can vary between a few kw/km in fair weather to hundreds of kw/km during thunderstrom or bad weather condition.
power loss due to corona (Pc)under fair weather condiotion is given by peek`s emperical formula.
Pc=243.5*(f+25/δ)*(√r/D)*(V-Vd)²*10^-5 Kw/phase /Km
V=phase voltage rms KV
Vd=disruptive critical voltage rms KV
r=radius of conductor in meter
D=GMD of conductor in meter
Peek`s formula is useful only when the corona losses are predominant and is less applicable when the losses are low.
peek`s formula is valid when
when (V/Vd)<1.8,we use peterson`s formula ,
F is the factor that depends upon V/Vd.
Factors affecting the corona Loss
- Effect of frequency: corona loss is directly proportional to system frequency.
- Effect of system voltage: As the electric filed increases with greater potential difference. Hence the corona loss increases with higher system voltage in a step way.
- Effect of conductivity of air:The conductivity of air increase if ions in air increases.During rain and thunderstorms ion content in the air enhances and thus corona loss become high during bad weather conditions.
- Effect of conductor diameter:The electric field intensity reduces if the conductor radius increases.Hence,with greater conductor diameter,electric field intensity reduces resulting in lower corona loss.
- Effect of load current:Flow of load current increase the temperature of the conductor .Thus,it prevents deposition of dew and snow on the conductor surface.This reduces corona loss.
- Effect of conductor surface: Roughness of the conductor surface result in a field distortion and give rise to high potential gradient causing higher corona loss.Standing of conductor increase corona loss.
- Effect of density of air:corona increase with increase in the density of air.In the hilly area the corona loss in higher than in plain area.
- .Effect of atmospheric condition:Rain drop and dust deposition increase the corona loss.Particularly local corona discharge.If the density of air decreases the corona loss increases.
Minimization of corona
Corona effects can be minimized in transmission line by using large diameter conductors which may be accomplished by using hollow conductors,ACSR(Aluminium conductor steel reinforced) or bundled conductors.Corona effect can also be reduced ,by increasing the spacing between the conductors.