Caution: the corona is subjected to high voltage phenomenon and it can be very dangerous if one shall not be familiar to test or practical studies in high voltage atmosphere.
When a voltage higher than the critical voltage is applied between two parallel polished wires, the glow is quite even. After operation for a short time, reddish beads or tufts form along the wire, while around the surface of the wire there is a bluish white glow. If the conductors are examined through a stroboscope, so that one wire is always seen when at a given half of the wave, it is noticed that the reddish tufts or beads are formed when the conductor is negative and a smoother bluish white glow when the conductor is positive. The a.c. corona viewed through a stroboscope has the same appearance as direct current corona. As corona phenomenon is initiated a hissing noise is heard and ozone gas is formed which can be detected by its characteristic color.
When the voltage applied corresponds to the critical disruptive voltage, corona phenomenon starts but it is not visible because the charged ions in the air must receive some finite energy to cause further ionization by collisions. For a radial field, it must reach a gradient (visual corona gradient) gv
at the surface of the conductor to cause a gradient g0
, finite distance away from the surface of the conductor.
The distance between g0 and gv is called the energy distance. According to Peek, this distance is equal to (r + 0.301 √r ) for two parallel conductors and (r + 0.308√ r ) for coaxial conductors. From this it is clear that gv
is not constant as g0
is, and is a function of the size of the conductor. The electric field intensity for two parallel wires is given as
Investigation with point-plane gaps in air have shown that when point is positive, the corona current increases steadily with voltage. At sufficiently high voltage, current amplification increases rapidly with voltage up to a current of about 10–7
A, after which the current becomes pulsed with repetition frequency of about 1 kHz composed of small bursts. This form of corona is known as burst corona.The average current then increases steadily with applied voltage, leading to breakdown.