Wet Snow Affecting Line Conductors & Ground Wire
Among the threats that impact overhead lines, perhaps the most critical is ‘wet snow’. Wet snowfalls are characterized by snowflakes with high liquid water content (LWC) that adhere easily to the external surface of conductors and ground wires. This phenomenon develops under air temperatures in the 0°C to 2°C range and an LWC greater than 30% of total snow mass. When precipitation rate exceeds 2 to 3 mm/h, an initial layer is deposited onto the external surface of the conductor. This constitutes the foundation over which a snow sleeve builds up. The phenomenon is often initially asymmetrical, since snow accumulates more on the side exposed to wind. This shifts the centre of gravity and generates progressive conductor rotation around its axis. The rotation process is the root cause of the cylindrical shape of sleeves, whose densities range from 150 to 500 kg/m3. When ambient temperature falls below 0°C, sleeves freeze and becomes highly compact (e.g. Fig. 5).
If snow accumulation generates mechanical loads exceeding the tensile strength of the conductor and causing mechanical strain, the combined effect under wind worsens the situation, often collapsing lattice towers and/or jeopardizing ground clearances. Moreover, abrupt detachment of a sleeve can result in violent rebound of conductors as well as ground wire, causing a permanent line trip. Although these types of events cannot be avoided, efforts can nevertheless be made to predict their occurrence in order to limit resulting contingencies and mitigate consequences. In Italy, for example, such an integrated monitoring and alert system, named WOLF (Wet snow Overload aLert and Forecasting), is already in place.
WOLF, developed over a GIS platform, provides a wet snow forecast covering the next 72 hours. It also calculates the associated potential mechanical load and estimates the anti-icing current (i.e. level of current in the conductors that would avoid formation of the sleeve due to heating caused by Joule losses), thus supporting operators in adopting active mitigation strategies. In this forecast system, shown in Fig. 7, each pixel in the screenshot depicts the mechanical load predicted by WOLF for each linear meter of conductor, with a resolution of mesh having 5 x 5 km sides. Specific reference is made to the typical two conductors used on Italian HV and MV networks, i.e. 31.5 mm and 4.5 mm diameter respectively, although other configurations can be implemented. By clicking each coloured dot, the user can open a summary chart that contains the main variables involved in sleeve formation, e.g. air temperature, wind speed, hourly equivalent precipitation, cumulative precipitation relevant to the current event, incremental ice load as well as the anti-icing current (AIC) curve.
In synergy with WOLF, an automatic test station named WILD (Wet Snow Ice Laboratory Detection) has been deployed in Vinadio in the Western Alps. Located at an altitude of 950 m, this facility allows validation of the forecast system, fine tuning parameters in the mathematical models of sleeve accretion, computation of AIC curves and also assessment of the relative effectiveness of different mitigation measures. The site includes 7 spans of about 14 m each to perform qualitative comparative tests of different types of conductors exposed to the same environmental conditions. Starting from the left, in Fig. 8, the first two conductors have been treated with black varnish – the first an ACSR-Z Ø19.04 mm conductor and the second a low-sag type TACIR Ø18.99 mm conductor. At the centre are two untreated conductors, respectively ACCC Ø24 mm and ACSR Ø22 mm. The remaining two conductors are ACSR-Z Ø19.04 mm with different hydrophobic coatings. The station is also equipped with two additional spans of ACSR conductors, with 3 m length, mounted on a system that forces them into a slow rotation, according to ISO 12494. These spans are equipped with load cells and ultrasonic sensors to measure mechanical load and thickness of the snow sleeve respectively. The rotation is able to represent conductor torsion and movement at the centre of the span in an actual overhead line. This allows capturing the total flow of snowfall over the conductor and is useful in validating the theoretical accretion model, which might otherwise assume a conservatively high growth rate.
The mathematical model for ice-accretion is based on the equation from the ISO standard for icing of structures and where all empirical parameters are adjusted based on a self-learning process founded on experimental observations from Vinadio. Fig. 9, for example, is an image captured by webcam at the end of a snowfall that occurred on Feb. 15-16, 2015. Snow formation can be seen on all spans, including on the hydrophobic-coated conductor, thus demonstrating the ineffectiveness of its formulation. Load measured on the ACSR conductor was 5 kg/m and thickness of the sleeve was 11 cm. Fig. 10 provides results of the simulation of that specific event, showing that the wet-snow load expected on that conductor was 4.2 kg/m, thereby confirming model accuracy.