Abstract--The relationship between the size of water droplets on the surface of AC high voltage conductors, their vibration and corona onset is investigated. Measurements have been carried out on individual droplets in high voltage experiments, including high speed photography, and discharge inception measurements. These have been correlated with acoustic emission from larger arrays of droplets on overhead transmission line conductors. A numerical model based on the finite element method has also been developed to simulate the vibration of the droplet enabling evaluation of the electric field and the characteristics of the droplet vibration. It is shown that the size of water droplet is critical in its mechanical response to the field and that corona does not necessarily occur when the droplet is deformed into its most conical state as is the case under DC fields. For larger droplets the phase shift between field and droplet vibration results in the maximum field occurring when the droplet is in a 'flattened' profile rather than when it is 'pointed'. The phase relationship between the droplet vibration and the electric field thus controls the onset of corona. It is argued that conductors which facilitate the development of small, uniform, stable droplets on their surface are the cause of abnormally high levels of low-frequency audible noise sometimes observed by transmission utilities. Moreover the narrow resonant response of the droplet may lead to a difference in acoustic emissions experienced between power utilities with 50 Hz and 60 Hz system frequencies.

the surface potential gradient is a critical design parameter for planning overhead lines, as it determines the level of corona loss (CL), radio interference (RI), and audible noise (AN). The majority of existing models for surface gradient calculation are based on analytical methods which restrict their application in simulating complex surface geometry. This article proposes a novel method which utilizes both analytical and numerical procedures to predict the surface gradient. With this effective model, stranding shape, protrusions, proximity of tower, type of tower, bundle spacing and bundle arrangement can be taken into consideration when calculating surface potential gradients. A sensitivity study on the factors could affect surface gradient distribution is performed.

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