During a short hiking trip yesterday, we've heard loud buzzling sound as we move by electric trsnsmission cables. Quick Research Findings:
http://en.wikipedia.org/wiki/Corona_discharge
Problems caused by corona discharges
Coronas can generate audible and radio-frequency noise, particularly near electric power transmission lines. They also represent a power loss, and their action on atmospheric particulates, along with associated ozone and NOx production, can also be disadvantageous to human health where power lines run through built-up areas. Therefore, power transmission equipment is designed to minimise the formation of corona discharge. Corona discharge is generally undesirable in:
Electric power transmission, where it causes:
Power loss
Audible noise
Electromagnetic interference
Purple glow
Ozone production
Insulation damage
The strong electric field at the surface of a high voltage power line conductor ionizes the nearby air and causes a partial breakdown of its dielectric strength. The audible noise you hear is caused by the local sound-pressure level changes due to the individual corona discharges that in turn occur as part of electron avalances. During wet weather the noise level is much higher due to the presence of drops of water on the underside of the conductors. These weather drops break up and the departing portions cause discharges in the water-to-water gap.
Research by Hitachi Cable
http://www.hitachi-cable.co.jp/ICSFiles/afieldfile/2005/11/28/review02.pdf
CORONA NOISE
Actual state of corona noise
Corona noise occurs under rainfall and snowfall and comprises two sound components: one is irregular (random noise) sound, and the other is the pure sound (corona hum
noise) of“buzzing”.
The random sound has a wide frequency band because the impulsive sounds caused by corona discharge overlap randomly. The corona hum noise results from the excitation of ion groups, which was generated from corona discharge, caused by the electric field surrounding the conductors. The predominant frequency of the corona hum noise is double (100 Hz or 120 Hz) the commercial frequency.
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