AN ANALYSIS OF SELECTED ATMOSPHERIC ICING EVENTS ON TEST CABLES
Jacques Druez, Universite du Quebec a Chicoutimi, Chicoutimi (Quebec), Canada, G7H 2B1
Pierre McCoMber, Ecole de Technologie Superieure, Montreal (Quebec), H2T 2C8
Jean Laflamme, Hydro-Quebec, Montreal (Quebec), Canada, H2L 4M8
ABSTRACT
In cold countries, the design of transmission lines and communication networks requires the knowledge of ice loads on conductors. Atmospheric icing is a stochastic phenomenon and therefore probabilistic design is more and more used for structure icing analysis. For strength and reliability assessments, a data base on atmospheric icing is needed to characterize the distributions of ice load and corresponding meteorological parameters. A test site where icing is frequent is used to obtain field data on atmospheric icing. This test site is located on the Mt. Valin, near Chicoutimi, Quebec, Canada. The experimental installation is mainly composed of various instrumented but non-energized test cables, meteorological instruments, a data acquisition system, and a video recorder. Several types of icing events can produce large ice accretions dangerous for land-based structures. They are rime due to in cloud icing, glaze caused by freezing rain, wet snow, and mixtures of these types of ice. These icing events have very different characteristics and must be distinguished, before statistical analysis, in a data base on atmospheric icing. This is done by comparison of data from a precipitation gauge, an icing rate meter and a temperature sensor. An analysis of selected icing periods recorded on the cables of two perpendicular test lines during the 1992-1993 winter season is presented. Only significant icing events have been considered. Between October 1992 and May 1993, twelve icing periods lasting from a few hours to three weeks have been observed. Curves representing the ice load variation with time, on four different conductor types, are presented together with the corresponding variations of air temperature, icing rate meter signal, precipitation gauge signal and wind velocity component normal to the main test line. Analysis of data shows a typical evolution of icing events with temperature during the winter season. The two first events, in October 1992, are accretions due to precipitation icing. The next three icing periods, from the end of October to the end of November 1992, are composed of successive accretions of rime and glaze. Then, during the coldest period o f winter, between the end of November 1992 and March 1993, the five icing periods observed are mainly rime accretions due to in-cloud icing with only very small and scarce precipitations. The next icing period, in April 1993, is composed of three rime accretions and an icing event caused by a mixture of in-cloud icing and precipitation icing. The four icing events composing this icing period are separated by partial or total shedding of t h e ice induced by melting. The last icing period, in May, is a mixture of in-cloud icing and precipitation icing, followed by shedding of ice induced by melting. A comparative analysis of the ice load on the four test cables is drawn from the data, and typical accretion and shedding parameters are calculated separately for icing events related to in-cloud icing and precipitation icing.
