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Trends in snow deposition on overhead electric lines: an example of using synoptic data to investigate the relation black-out risk / climate change

pubblicazioni - Articolo

Trends in snow deposition on overhead electric lines: an example of using synoptic data to investigate the relation black-out risk / climate change

of using synoptic data to investigate the relation black-out risk / climate change Paolo Bonelli*, Matteo Lacavalla** Weather/Climate Risk Management for the Energy Sector S. Maria di Leuca (LE), 6-10 Ottobre 2008 PRESENTAZIONE POWER POINT * CESI RICERCA SPA ** COLLABORATORE A PROGETTO Icing on the overhead lines is a complex phenomenon in which many atmospheric variables, as precipitation, wind, air temperature, are involved to produce an extra load that may cause the break of the line and an electrical black-out. There are different classes of icing related to different climatic conditions: in Italy the most frequent is the accretion of ice due to wet-snow deposition. This phenomenon is triggered by the values of some variables during the snowfall event. CESI RICERCA is involved in studies about this topic as by means of specific measurements campaign as using synoptic weather data. One of the objectives is to characterize the optimum local meteorological conditions causing snow accretion on the lines and then to search this conditions along a time-series of synoptic data in order to find long term trends. Direct measurements of icing on overhead lines are very few in Italy and they are not linked to accurate meteorological data. Some reported events of broken electric lines over wide areas during snowfall situations are available for the last years and only in the present year CESI RICERCA has carried out accurate measurements during some snow events using different cable samples, meteo instruments and photographic reporting. This paper concerns the study of long time behaviour of snow deposition over Italy, that constitutes the basic knowledge for assessing risk of black-out or planning the construction of stronger lines. An investigation on a data-set of about 50 years of Italian synoptic data, provided by National Air Force Meteorological Service, was carried out. Snowfall events have been identified and described by means of “present weather” observations, air temperature, humidity, wind and precipitation amount. Only the events with duration of more than 6 hour have been selected. From them a subclass of events has been identified as likely wet-snow events, those causing snow accretion on the cables. The number of events per year, grouped in 5 year interval, has been counted from 1951 till 2005 and a trend coefficient has been computed together with a Mann-Kendal verification test. Due to the lack of data for many synoptic stations, only 5 long time series have been taken out, they are from the stations of Torino (16059), Milano (16080), Bologna (16140), Venezia (16105) and Passo della Cisa (16124). Four of these, show a positive trend of wet-snow event percentage on the total snow events. Total snow events don’t seem to decrease significantly. This result is in agreement with a general increasing of temperature, as highlighted by many studies accomplished on the topic of air temperature long term trends. From the electrical point of view, the results of the present climatic change in Northern Italy, seem to increase the risk of damage for snow accretion on the overhead lines and may be used in order to modified building criteria or to introduce weather forecast in the operational maintenance procedures. (*) Work financed by the Italian Ministry of Economic Development with the Research Fund for the Italian Electrical System under the Contract Agreement established with the Ministry Decree of march 23, 2006.

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