Soft rime

Abstract: The fundamental characteristics of ac contaminated flashover on iced insulators were studied for the three icing states, i.e., glaze, hard rime and soft rime, which are observed most frequently in service conditions. The results showed that thawed snow with a relatively low salt concentration can produce a salt deposit density (SDD) high enough for flashover on an iced insulator. For the same concentration of contamination liquid, average values of SDD for icing states are 5 to 9 times higher than those for icing-free conditions. The results also showed that the minimum flashover stress of the soft rime condition is 20% lower than that of icing-free conditions. For this reason, we deduced that the number density of Na atoms in the discharge space at flashover tends to become highest under the soft rime conditions.

Abstract: [1] A mesoscale atmospheric model, the Weather Research and Forecasting model (WRF), was used for a case study that reconstructs mid-spring episodes of rime formation at Mt. Zao, Japan. One particularly interesting and rare form of rime was observed. The formations were feathery, opaque aggregates of granular ice 15–30 cm long, called “shrimp tails” in Japanese. Based on an analysis of model-generated results, we find good quantitative agreement of modeled and observed wind and temperature time series at Jizosancho ropeway station. We identified two icing events (lasting for 36 and 41 h respectively, with surface air temperatures between −6.3° and −0.1°C, relatively constant westerly winds up to 26 m s−1, and maximum cloud liquid water contents (LWC) between 0.72 and 1.05 g m−3). We confirmed that high-resolution modeling (1.1 km grid spacing) was much more accurate than simulations with coarser grids (10 and 3.3 km). The LWC during the formation period of this rare type of icing was estimated for the first time using the WRF model at Mt. Zao, and it was found to be up to several times higher than values previously used in experimental studies. We found that the joint wind speed-air temperature distribution for this type of “tail” rime was more similar to that of a hard rime or glaze, than to a soft rime. We explain the formation of “shrimp tails” by wind impact angle and report previously made laboratory results on its effect on the droplet collision efficiency and the density of rime ice.

Abstract: The objective of the present study is to propose a practical laboratory method to simulate wet atmospheric ice accretion on H.V. insulators. The method developed is based on first accumulating a low density soft rime on energized insulators and then to heat the produced deposits. The liquid water fraction of the ice accretion is then determined using a centrifuge. This approach makes it possible to determine the performance of insulators under wet-snow-like conditions with relatively simple, low cost equipment.