Corona occurs when the potential gradient between two conductors exceeds a critical value, causing a faint violet glow. It is accompanied by a hissing sound, ozone production, and power loss. Corona results from ionization of air molecules caused by free electrons accelerated by the potential gradient, and is affected by factors like conductor size/spacing, line voltage, and atmosphere. Larger conductors and greater spacing reduce corona by lowering the surface potential gradient. Critical disruptive voltage is the minimum voltage for corona to occur, calculated based on conductor size and spacing. Visual critical voltage is the minimum for visible corona glow along the entire line.

1. Corona
When an alternating potential difference exceeds a certain value, called critical
disruptive voltage, is applied across two conductors, the conductors are surrounded by a
faint violet glow called corona.
The phenomenon of corona is accompanied by:
 a hissing sound
 production of ozone
 power loss and radio interference
The phenomenon of violet glow, hissing noise and production of ozone gas in an
overhead transmission line is known as corona.
If the conductors are polished and smooth, the corona glow will be uniform
throughout the length of the conductors,
If the conductors are the rough, points will appear brighter.
With d.c. voltage, there is difference in the appearance of the two wires. The
positive wire has uniform glow about it, while the negative conductor has spotty
glow.

2. Theory of corona formation:
1. Under normal conditions, the air around the conductors contains
some ionized particles (i.e., free electrons and +ve ions) and
neutral molecules.
2. When p.d. is applied between the conductors, potential gradient is
set up in the air which will have maximum value at the conductor
surfaces.
3. Free electrons acquire greater velocities.
4. When the potential gradient at the conductor surface reaches about
30 kV per cm (max. value), the velocity acquired by the free
electrons is sufficient to strike a neutral molecule with enough
force to dislodge one or more electrons from it.
5. The process of ionization is cumulative. The result of this
ionization is that either corona is formed or spark takes place
between the conductors.

3. Factors affecting Corona
1. Atmosphere: In the stormy weather, the number of ions is more than
normal and such corona occurs at much less voltage as compared with
fair weather.
2. Conductor Size: a stranded conductor has irregular surface ( i.e.
decreases the value of break down voltage) and hence gives rise to more
corona that a solid conductor.
3. Spacing between conductors:
4. Line voltage: The voltage of the line has a directly effects on the
probability of the corona formation
Disadvantages of Corona Effect
1- Corona is a accompanied by a loss of energy. This affects the
transmission efficiency of the line.
2- Ozona is produced by corona and may cause corrosion of the
conductor due to chemical action.
3- The current drawn by the line due to corona is non- sinusoidal and hence non- sinusoidal
voltage drop occurs in the line. This may cause inductive interference with neighboring
communication lines.
If the spacing between the conductor > their diameters
The electro- static stresses at the conductor surface will reduces
Thus avoiding corona formation.

4. Method of Reduction Corona Effect
1.By increasing conductor size
2.By increasing conductor spacing
By increasing conductor size, the voltage at which occur of corona is raised and hence
corona effects are considerably reduced.
This is one of the reasons that A.C.S.R. (Aluminum Conductor Steel Reinforced).
conductors which have a larger cross- sectional area are used in transmission lines.
Important Terms
The phenomenon of corona plays an important role in the design of an overhead
transmission line. Therefore, it is necessary to consider the following terms in the analysis
of corona effects
1-Critical disruptive voltage
It is the minimum phase – neutral voltage at which corona occurs.
Consider two conductors of radius ( r )cm and spaced (d )cm apart. If V is the phase-
neutral potential, then potential gradient at the conductor surface is given by :
𝐸 =
𝑉
𝑟𝑙𝑖𝑛
𝑑
𝐷
In order that Corona is formed, The value of ( E ) must be made equal to break down
strength of Air. (𝐸°) The break down strength of air at 76 cm mercury pressure and
temperature of 25 = 30 kv/cm (max) or (30/√2 =21.1 kv) as r.m.s value

5. 𝐸° =
𝑉°
𝑟𝑙𝑖𝑛
𝑑
𝐷
(
𝑘𝑣
𝑐𝑚
)
∴ 𝑉° = 21.1 𝑟𝑙𝑖𝑛
𝑑
𝐷
The value of 𝐸° is directly proportional with air density. Thus the break down strength of
air at a barometric pressure of b cm of mercury and temperature of t°C become δ𝐸°
where

6. 2. Visual critical Voltage
It is the minimum phase- neutral voltage at which corona glow
appears all along the line conductors. The phase- neutral voltage
of visual critical is
3. Power loss due to corona:-
Formation of corona is always accompanied by energy loss
which dissipated in the form of light, heat, sound, and chemical
action.
When disruptive voltage is exceeded, the power loss due to
corona is given by

7. Ex1) A 132kv 3Ø line with 1.956 cm diameter conductor is built so that
corona takes place if the line voltage exceeds 210 kv(r.m.s). If the value
of potential gradient at which ionization occurs can be taken as 30
kv/cm. Find spacing between the conductors.
Ex2) A 3-phase line has conductors 2 cm in diameter spaced equilaterally 1
m apart. If the dielectric strength of air is 30 kV (max) per cm, find the
disruptive critical voltage for the line. Take air density factor δ = 0·952 and
irregularity factor mo = 0·9.

8. Ex3) 3Ø, 220Kv, 50HZ transmission line consists of 1.5 cm radius
conductor spaced 2m a part in equilateral triangular formation. If
the temperature is 40°C and atmospheric pressure is 76 cm find the
corona loss per km of the line. Take 𝐦° = 0.85
So1)

9. Ex4) A certain 3-phase equilateral transmission line has a total corona loss of
53 kW at 106 kV and a loss of 98 kW at 110·9 kV. What is the
disruptive critical voltage? What is the corona loss at 113 kV?