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Korona effect
Corona is the effect caused by partial discharges on insulators of transmission lines and the air surrounding the conductor alone. High values of the electric field around the conductor lead to the ionization of the surrounding environment and at sufficiently high voltage leads to breakthroughs and electrical discharge in the form of pulsating current. Charge effects during these discharges are manifested as crackling sound - noise and since charges are within a variable electric field, currents are induced again emitting electromagnetic interference. Corona Effect depends on several factors of which most significant are voltage levels and atmospheric conditions. Corona usually occurs when humidity is high (typically above 80%).
Water facilitates the ionization and contributes to greater concentration of ions in the air, moisture itself increases conductance thus making electrical discharges more frequent.
Corona effect is most pronounced at 400 kV transmission line, less at 220 kV while at 110 kV and lower voltage level OHLs is almost negligible.
During the relatively low humidity (dry air) noise as a result of corona is 40 dB to 50 dB in close proximity (underneath) 400 kV transmission line. Under such conditions, sound associated witch corona often equalizes with the level of the sound of the environment in which it is located. Depending on the conditions, during wet weather conditions the noise level due to corona can be increased to values of 50 to 60 dB.
