Contribution to study of visible discharge initiation and development on the ice surface

The present study aims to further the knowledge of the fundamental processes of electrical discharges along ice surfaces. The understanding of these phenomena may help to develop numerical tools for prediction of flashover on ice-covered insulators and to improve the design of HV insulators for application to power networks in cold regions. Despite the numerous investigations on the electrical performance of industrial insulators under icing conditions, there still exist major gaps in comprehensive studies on the subject. Such fundamental studies on electrical insulation performance under atmospheric icing conditions consist one of the main objectives of the NSERC/Hydro Quebec/UQAC Chair on Atmospheric Icing of Power Network Equipment (CIGELE).;Sophisticated ultra high-speed photographic techniques were applied in order to observe and analyse the very fast processes of visible discharge initiation and development on the ice surface during the first nanoseconds. In addition, measurements of the 50% lightning impulse breakdown voltage and waveform measurements of voltage and current were realised. A physical model with two hemispherical capped electrodes, half submerged in ice, was used for the experiments, to which lighting impulse voltage was applied. The effects of various parameters were investigated: Environmental parameters (freezing water conductivity, ice temperature, ice surface uniformity, cooling rate, type of ice accumulation), electrical parameters (voltage polarity, voltage waveform, electric field during cooling period) and geometrical parameters (electrode distance, electrode curvature radius, electrode axis orientation).;The investigations show that streak photography is an adequate technique to study the very fast visible discharge processes along ice surfaces under various conditions. The study reveals that different regimes govern the ice surface discharge process in the early stages, depending on the ice temperature and the distance between the electrodes. Numerical data is provided on critical conditions of discharge initiation, time delay between start of pre-discharge activities and occurrence of breakdown, as well as on velocity of discharge development. In spite of the fact that the experimental conditions (simplified physical models under impulse voltage) did not perfectly portray the actual situation on power networks under icing conditions (complex-shaped insulators under ac or dc voltages), some good correspondences between previous field observations of power failures and the present laboratory investigations can be identified.