What is corona | Corona effect in power system - engineeringforu

Corona effect Definition:

The ionization of insulating material (air) surrounding the surface of the conductor of a transmission line or the disruption of the dielectric strength of air near the conductor of a transmission line.
The phenomenon of violet glow , hissing noise and production of ozone gas in an overhead transmission line is known as corona.

Corona effect in transmission line:

Most of the transmission systems are overhead systems on economical basis. There are certain free electrons which are available in the space in which some of the electrons are very near to the surface of the conductor and some of them are away from the surface conductor. The free electrons near the power conductor will move at a faster rate than that of electron away from the surface of the conductor, because the electric field intensity is maximum at the surface of the conductor. The electric field intensity will be proportional to operating voltage of the system.
Whenever the operating voltage is more than 270kv, the field intensity will also increases so that the mobility of the electron will increase. The high mobility electron will collide with the other air molecules, which will dislodge the electrons from those molecules. The process is continuous and at certain instant the electron avalanche will take place i.e an arc will struck between the two lines. The arc is an ionized body, which will be treated as line to line fault. The reliability of supply is reduced.
In order to eliminate the arc, the distance between the two lines are increased in a non-linear manner as the operating voltage increase, so that the field intensity will be restricted to be maximum at the surface of the conductor and almost zero between the two conductors.
However the two conductors which are employed for transmission line are stranded conductors having rough surface. Due to rough surface certain impurities are to be deposited. So a self sustained field emission will be initiated around the surface of the conductor at certain points. The voltage at which the self sustained discharge will be initiated is called the critical disruptive voltage.

The corona initiation can be identified as

  • Hissing noise.
  • Releasing of ozone gas.
  • Occurrence of beds and tufts.
The phenomenon of corona plays an important role in the design of an overhead transmission line. Therefore, it is profitable to consider the following terms much used in the analysis of corona effect.

Critical disruptive voltage: 

It is the minimum phase neutral voltage at which corona occurs.

Critical disruptive voltage formula for single phase 2-wire transmission line:
Consider two conductors of radii r cm and spaced d cm apart. If V is the phase neutral potential then the potential gradient at the conductor surface is given by

Vd = go r loge(d/r)
R is the radius of conductor in centimeters.
d is the distance of separation in meters.
go is the dielectric strength of air.
go value = 30 kv/cm at peak
               = 21.1 kv/cm rms at NTP
However the existing temperature and pressure at the surface of the conductor are different from NTP. So at any other temperature and pressure Critical disruptive voltage formula is given by
Vd = g' r loge (d/r) kv/rms
Where g' is the dielectric strength at any temperature and pressure = go δ
δ is the air density factor
δ = (3.92 x h )/ (273 + t)
h is the atmospheric pressure in cm of Hg
t is the temperature in Co
The surface of the conductor is irregular. So consider the surface irregularity factor (m)
Vd = 21.1 m δ r loge (d/r) kv/rms
m = 1 for smooth conductor.
m = 0.93 - 0.98 for rough or stranded conductor.
m = 0.83 - 0.87 for seven stranded cable.
m = 0.9 for more than seven stranded cable.

Visual critical disruptive voltage: 

It is the minimum phase neutral voltage at which corona glow appears all along the line conductors.
When the corona is initiated it will not be visible. In order to visualize the corona, further ionization needs to be done at the surface of the conductor.
The visual critical disruptive voltage is given by
Vv = 21.1 mv δ ( 1+ 0.3 / √ r δ) r loge (d/r)kv/rms.
mv = surface irregularity factor.
mv = 1 for smooth surface of the conductor.
mv = 0.93 - 0.98 for rough conductor or stranded conductor.
The visual corona will be observed as white bluish slow color.

Local corona:
Ionization of air at certain points along the power conductor when the conductor is deposited with ice. The 'm' value for local corona is 0.72.

Decided corona:
The visual corona at certain points when the conductor is deposited with ice. The 'm' value for decided corona will be 0.8.

Advantages of corona effect:

  1. Due to corona formation, the air surrounding the conductor becomes conducting and hence the virtual diameter of the conductor is increased. The increased diameter reduces the electro static stresses between the conductors.
  2. Corona will act as a safety valve against direct lightning stroke, by dissipating the peak magnitude of lighting stroke in the form of corona loss.
  3. Corona reduces the effects of transients produced by surges.

Disadvantages of corona effect:

  1. There is certain real power loss apart from ohmic loss. This affects the transmission efficiency of the line.
  2. Ozone is produced by corona and may cause corrosion of the conductor due to chemical action. 
  3. When the corona is initiated on the transmissions lines, the overall diameter of the conductor will increase so that the capacitance will increase. The net charging flux which will link with the neighboring communication line is not zero. So there will be communication interference. Apart from the charging flux, the zero sequence components during ground fault will also cause interference.