Statistical Description of the Accretion Phase of Atmospheric Icing Events

In cold climates, atmospheric icing is an important factor for the design and reliability of overhead transmission lines. For strength and reliability assessments, a data base is necessary to characterize the statistics of mechanical and meteorological conditions during accretion. Since the early 1980's, a natural icing test site located on Mt. Valin (Quebec), Canada, has been used to record such icing data. This paper presents results, with respect to the accretion phase, obtained from our analysis of this extensive database. The four types of ice defined were rime, glaze, wet snow and mixtures of these types. A total of one hundred and three icing events were selected on each of the four cables located at the test site. Forty-eight events produced rime, thirty-six produced glaze, six produced wet snow and thirteen produced mixtures of these types of ice. We then analyzed the distributions of the icing rate, the air temperature and the wind speed. By comparing the main test line and the perpendicular second test line (which is oriented along the prevailing wind direction) icing rates were found to be significantly lower in the latter case. This result indicates that for all types of ice accretion the cable orientation is an important factor on the accretion properties. The study of the icing rate distributions suggest that the normal probability law does not estimate high icing rates well. Other probability models that should be further investigated are the Weibull, gamma, beta and exponential distributions.