Wet conductor surfaces and the onset of corona discharges

Corona discharges initiated from water droplets on the surface of overhead line conductors are considered. The size of droplets on conductors with a range of surface hydrophobicity is evaluated. A range of droplet sizes are then selected on which to perform experimental studies of their dynamic behaviour under electric fields. It is found that the droplet varies its shape at twice the power frequency through a semi-spherical, flattened (tortilla), and conical cycle. Further experiments performed with various sized droplets identified that the phase relationship between the vibration and applied voltage is dependent on the size of the droplet. High speed UV imaging is used to capture vibration and UV emission simultaneously. Corona discharge initiates from the tip of a 3 µL droplet when it is in a conical shape while it is initiated from the edge of a 5 µL droplet when its shape is flattened. It is concluded that corona inception is affected by the droplet size range which can form on the surface of a conductor. This is mainly dominated by the surface hydrophobicity of conductors, and will impact the acoustic emission from conductors in service. Significant differences may result between 50 Hz and 60 Hz power frequency systems.