Critical Disruptive Voltage & Visual Critical Voltage

Critical Disruptive Voltage is defined as the voltage level at which complete disruption of dielectric strength of air surrounding the conductor occurs. If the voltage further rise above critical disruptive voltage Corona starts occurring.

The air around the conductor has free electrons due to cosmic rays. As the potential between two conductors increases, the potential gradient near the conductor also starts increasing. When the voltage becomes greater than the critical disruptive voltage called visual critical voltage corona starts occurring.
Corona creates hissing and cracking noise with violet grow aground conductor and ozone gas emission.
Consider two parallel conductors having radius r cm and placed at d cm from each other.
If V is the phase to neutral voltage then the potential gradient near conductor surface is,
The value of g must be equal to the breakdown strength of air. At normal temperature and pressure breakdown strength of air is 30KV/cm.
If Vc is the phase to the neutral voltage required under these conditions then,
Where,
g0 is the breakdown strength of air
δ = Air density factor
r = Radius of the conductor
d = Spacing between the conductors.
Hence critical disruptive voltage
The above equation is at standard atmospheric condition and temperature.
However, if these standard conditions of the atmosphere change the air density also changes thus the value of g₀ is dependent upon the air density and directly proportional to the air density.
The breakdown strength of air at a barometric pressure of b cm of mercury and temperature t c becomes

g₀

Under standard condition  δ=1

In the above derivation, an assumption is made that the conductor is solid and the surface of the conductor is smooth and polished.
Correction must also be made for the surface condition of the power conductor
Critical disruptive voltage =Vc

Where Value of m0
m0= 1 for smooth and polished conductor
m0= 0.92 to 0.98 for rough conductor
m0= 0.87 for standard condcuctor

Visual critical voltage

Visual critical voltage is defined as the minimum phase to the neutral voltage at which corona glow occurs around the line conductors.

In the case of the two or more parallel conductors, the Corona glow does not start at the critical disruptive voltage Vc, it starts appearing at the voltage level higher than Vc called visual critical voltage.
The formula for the visual critical  voltage  is Vv
Where mv is
mv= for the polish condcutor
mv= 0.72 to 0.82 for rough conductor

Factor affecting the visual critical voltage

The phenomenon of the corona is affected by the atmospheric condition around the conductor hence  depends on

Atmosphere

Corona glow occurs due to the ionization of air around the conductor there for it is affected by the weather condition, in stormy weather, the numbers of ions present in the air are more than in normal atmospheric condition hence in this condition corona occurs at very low voltage as compared to fair weather condition.

Line Voltage

The supply voltage heavily affects the formation of the corona, if the low voltage is present there is no change in the condition of air around the conductor, hence no visual critical voltage level is attained and no corona glow will appear around the line conductor.
However, if the voltage has such value that electrostatic stress developed at the conductor surface makes air surrounding the conductor conducting and corona glow start forming.

Conductor Size

The formation of the corona depends on the shape and condition of the conductor. The rough surface will give rise to more corona loss Because the unevenness of the conductor surface decreases the breakdown voltage.

Spacing between conductor

If the spacing between the conductor makes large compared to their diameter, there may not be any corona effect. This happens Because the larger distance between conductors applies less electrostatic stress at the conductor surface.