When an AC is applied across two conductors whose spacing is large as compared to their diameters, there is no apparent change in the condition of atmospheric air around the conductors. If the applied voltage is increased and when it reaches certain voltage called as disruptive voltage.
The phenomenon of violet glow, hissing noise and production of ozone gas in an overhead transmission line is known as corona effect.
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.
This produces another ion and one or more free electrons, which is turn are accelerated until they collide with other neutral molecules, thus producing other ions. Thus, the process of ionization is cummulative. The result of this ionization is that either corona is formed or spark takes place between the conductors.
Formation of Corona effect.
Some ionization is always present in the air due to cosmic rays and ultra violet radiation and radioactivity. So under normal condition, the air around the conductor contains some ionised particles and neutral molecules.
When an electric field intensity is established between the conductors, these ions and free electrons experience force upon them. Due to this force, ion and one or more free electrons which in turn are oscillated until they collide with other neutral molecules, thus producing corona.
Important terms of Corona
1. Critical disruptive voltage
- The value of Vcd given by the below expression is called the critical disruptive voltage.
- its value is considerably reduced during bad atmospheric conditions, such as fog, sleet, rain and snow storms.
Vcd = g0 δ m0 r loge d/r
where, r = radius of conductor in cm
d = the spacing between conductors in cm
m0 = the wire surface condition factor
2. Visual critical voltage
- In case of parallel wires it is found that visual corona occurs at voltage higher than Vcd, which may be called as Visual critical voltage.
- The visual corona will occur when the break down value is attained at a distance of
r (1+0.3/√δr) log e kv
to neutral (rms). Author Peek states that, the roughness factor may be taken as unity for smooth conductors and 0.72 for stranded conductors and 0·72 to 0·82 for rough conductors.
3. Power Loss due to Corona
- Formation of corona is always accompanied by energy loss.
- which is dissipated in the form of light, heat, sound and chemical action. When disruptive voltage is exceeded.
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