Hi Reza Rebk

In a uniform electric field, a gradual increase in voltage across a gap produces a breakdown of the gap in the form of a spark without any preliminary discharges. On the other hand, if the field is non-uniform, an increase in voltage will first cause a localised discharge in the gas to appear at points with the highest electric field intensity, namely at sharp points or where the electrodes are curved or on transmission line conductors. This form of discharge is called a corona discharge and can be observed as a bluish luminance.

When a gradually increasing voltage is applied across two conductors, initially nothing will be seen or heard. As the voltage is increased, the air surrounding the conductors get ionised, and at a certain voltage a hissing noise is heard caused by the formation of corona. This voltage is known as the disruptive critical voltage. A further increase in the voltage would cause a visible violet glow around the conductors. This voltage is the visual corona inception voltage.

The stress surrounding the conductor is a maximum at the conductor surface itself, and decreases rapidly as the distance from the conductor increases. Thus when the stress has been raised to critical value immediately surrounding the conductor, ionisation would commence only in this region and the air in this region would become conducting.

Under ordinary conditions, the breakdown strength of air can be taken as 30 kV/cm (E max = 30 kV/cm, so that RMS = 30/√2 = 21.2 kV/cm). Knowing that:

1. Corona will of course be affected by the physical state of the atmosphere,

2. When the surface of the conductor is irregular, it is more liable to corona,

3. When the surface of the conductor is irregular, it is more liable to corona.

The critical electric field can then be written as in the following equation:

Ec = 21.2*m1*m2[1 + 0.3/√(d*r)] kV/cm (Peek's xpression)

m1: factor depending of conductor surface,

m2: factor of weather conditions,

d : a correction factor ,

r : he conductor radius.

Good Luck,

Omar

Thanks Omar for explanation. Do you know any publication related on this question?

Would really appreciate if you could help with some examples.

Regards

There are different mathematical models for computing the electric field near discharge wire in corona discharge such as

The following explanation in the uploaded file can be useful in this case.

https://www.sciencedirect.com/science/article/abs/pii/S030438861630050X

Dear;

for more information please check the following documents :

Calculation of electric field in a coaxial cylinder (corona discharge configuration) http://ieeexplore.ieee.org/document/7936579/

Regards