The Impact of Water Droplet Vibration on Corona Inception on Conductors Under 50 Hz AC Fields

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 the 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 the 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 and 60 Hz system frequencies.