Mechanism of the tonal emission from ac high voltage overhead transmission lines

Foul weather conditions can lead to corona discharges on high voltage overhead transmission lines which are perceivable as an audible broadband crackling and hissing noise. In such situations this noise is often accompanied by a humming noise at twice the mains frequency (2 f). While the crackling and hissing noise component was widely investigated in the past, the 2 f-component went largely uncommented. Only a scant amount of literature suggests mechanisms explaining the occurrence of this emission, but these suggestions are either vague or controversial. In this paper a mechanism analogous to that of ion loudspeakers is presented, which describes the sound levels of audible 2 f-emissions from high voltage model lines quantitatively in a satisfactory manner. While the broadband component of corona noise emissions has its immediate origin in the discharges themselves, the 2 f-emissions arise from the drift of ions left behind by the discharges. Due to collisions with gas molecules, these ions cause in the sum a rise of heat in and a force applying on the neutral gas, thus producing the 2 f-emissions. Depending on the situation, both can give rise to contributions of similar magnitude.