WET-SNOW ACCRETION ON CONDUCTORS: THE ITALIAN APPROACH TO REDUCE RISKS ON EXISTING OHL.

SUMMARY The phenomenon of wet snow accretion affects a wide variety of man-made structures in many countries. Man-made structures at the top of the mountains are often exposed to rime icing; in other areas, wet snow or freezing rain likewise affect infrastructures at lower altitudes. Therefore, power lines like other buildings/structures are designed to withstand the loads and other adverse effects due to wet snow loads. Many attempts were made worldwide to establish understanding of wet snow accretion conditions in remote areas, especially in the mountains. Most countries have their own standards to take care of wet snow loads on their structures. Efforts are made to establish and improve standards and methodologies for handling the impact of wet snow accretion in the most economical and rational manner. Over the past 30-50 years, such knowledge was built up from field observations and measurements, laboratory studies and model development, however, despite this better understanding , actual weather conditions at a remote location are always a critical question for a potential overhead power line. The phenomenon of wet snow accretion on conductors of overhead lines is responsible in some parts of the Italian territory, because of its particular morphology, of important mechanical overloads, which may lead in some cases to power outages during the winter season. Wet snowflakes containing much free water easily accrete on electric wires; this often causes serious damage such as the breakage of power lines and crashing down of high-voltage power-transmission towers. In order to approach the problem Italian TSO (Terna) and RSE have developed two primary strategies: 1. Predictive system. Several years of research activity led to the setting-up of an alert system for wet snow formation on overhead lines. The purpose of the system, named WOLF (Wet-snow Overload aLert and Forecasting), is to provide the wet-snow load forecast on OHL during wet-snow events and, sufficiently in advance, to alert Terna to take appropriate preventive dispatching solutions. The system is based on the outputs of a Numerical Weather Prediction model (NWP), and the forecast is valid for the next 72 hours. The outputs of the NWP feed an ice accretion model for the evaluation of the snow sleeve dimension. All the outputs of the models are reported in a userfriendly WEB-GIS tool. In order to verify the models by the measurements, an innovative prototype of wet-snow station has been developed by RSE in a zone of Piedmont (northern Italy) particularly subjected to wet snow formations. Moreover a new laser system for measurements of ice sleeves accretion on actual OHL has been developed and tested in the same area. 2. Passive preventive method. In order to give a practical response preventing or limiting snow accretion on the OHL, the method of counterweight device is investigated. As known the use of counterweights increase the torsional stiffness of conductor spans limiting the typical rotation resulting from the snow loading. Report of experience from Terna experimental installation in Piedmont is given to demonstrate the efficiency of the counterweight method with experiences in situ.