Snowfalls are unavoidable and projections of climate change indicate that the Italian region, for example, will potentially be subject to more severe such events in years to come. This presents a dilemma for network operators, i.e. either increasing mechanical strength of lines – including conductors, ground wires, poles and insulators – to withstand increased wet snowfall loads or using mitigation measures to limit impact of such events, i.e. adopting a resilience approach. Mitigation measures for wet snow are multi-faceted. In terms of impact for any user, the first measure can be achieved during line planning by taking into consideration the effects of climate change, i.e. taking into account evolution in thermal, electrical, mechanical and environmental stresses in light of the most critical events expected in that region. When the return period of heavy snowfall is shorter than the lifespan of a line, or shorter than the time considered as acceptable for any major supply interruptions in a risk analysis, several measures can be adopted:
• using underground cables;
• increasing transmission system meshing and back-supply capabilities (thereby avoiding supplying substations using lines in an ‘antenna configuration’);
• using physical means to avoid or limit accretion of snow sleeves, such as hydrophobic or ice-phobic surface treatments or coating conductors;
• applying anti-rotation devices (e.g. pendulum de-tuners, wind dampers, torsional damper de-tuners);
• installing interphase spacers, as used to reduce conductor galloping;
• increasing surface temperature of conductors to avoid sleeve build up by means of circulation of some minimum anti-icing current.
Mechanical dimensioning of overhead lines is currently carried out following requirements of the European standard EN 50341-1 “AC Overhead lines having rated voltage of 45 kV and above” first published in 2001 and integrated by National Normative Aspects (NNA) to take into account the specific situation in Italy. Among others, this standard prescribes that dimensioning of lines be carried out using a risk-based approach that should take into account actual local climatic conditions as well as ice and wet snow mechanical loads having a return period of 50 years. The dimensioning standard has been updated and amended several times since initial publication. Several editions of EN 50341 were published as well as of NNAs (the last being EN 50431 2 13:01 2017). It is intended that the requirements of the standard be used for new lines and for important overhauls to existing infrastructure, whereas most of the transmission system in Italy still complies with the previous CEI 11 4:1998. The risk-based approach requires understanding occurrence of events. This is why continuous update of the map of events is essential, especially for situations where local extreme weather conditions influence overall statistics. In the specific case of wet snowfall, the most recent map produced by RSE, shown in Fig. 11, considers the months from November to April, maximum duration of snow sleeve accretion of 48h, maximum possible load of 50 kg/m and temperature range of -2°C < Tavg < +2°C.
Among the most effective mitigation measures to limit or avoid a snow sleeve is anti icing current. Evaluation of AIC has been implemented in WOLF and validated at WILD. The thermal model proposed by Shurig and Frick is used to estimate the AIC necessary to maintain the conductor at a skin temperature of 1.5°C to keep it free from snow accretion. In light of the positive validation of the model and its continuous refinement using self-learning based on observations at Vinadio, AIC is systematically computed and reported for each pixel in the WOLF GIS considering all types of HV and MV overhead conductors and cables used in Italy.
In regard to impact on required mechanical characteristics of line insulators, wet snow can generate substantial increase in apparent weight seen by insulators on exposed lines. But is this a real threat for insulators? Dimensioning the mechanical ratings of insulators when equipping a line is generally founded on the most critical case, i.e. strain insulators. The tension that a strain insulator must accommodate varies continuously with conductor weight and temperature, wind load as well as snow and ice build-up and is influenced by specific stringing criteria. Mechanical dimensioning of insulators, e.g. required Mechanical Failing Load (MFL) is such that strain insulators must withstand all loads and adequate safety margins, even during worst cases conditions. Safety factors between maximum foreseeable load and minimum required MFL normally range between 2.5 to 5. Typical values range from 70 to 150 kN for 150 kV and from 120 to 360 kN for 380 kV lines. Straight and angle suspension insulators are normally selected with ratings similar to strain insulators, for uniformity along the line. The composition of forces on suspension insulators is such that the equivalent mechanical load they are subjected to is typically 4 to 5 times lower than for strain insulators. During a heavy wet-snowfall event the most critical portion of a line is represented by lattice towers with suspension insulators since these are dimensioned with the least stringent mechanical requirements. The safety factors adopted for suspension insulators are therefore much higher than for the towers themselves. Increased threat level due to wet snowfalls will therefore not directly affect insulators.
Mitigation measures, as discussed, must be taken for towers and conductors and different line and system management policies undertaken to avoid or limit line damage and its consequences. Snow sleeves can in some cases be limited or even avoided on conductors using anti-icing current to increase conductor temperature above some threshold value. In those situations where current flows cannot be directed, another possible mitigation measure consists of avoiding rotation of the conductor, thus preventing sleeve build-up. Pendulum de-tuners can be used for this purpose. Although applied mainly to avoid conductor galloping, these devices are tightly clamped onto conductors and increase their torsional behaviour. When such a measure is not applicable, interphase spacers can prove helpful given their duplicate function of clamping conductors and ensuring their correct mutual position. Dimensioning these devices must follow the same rules as for interphase insulation.
The impact of ice and snow on dielectric performance of line insulators has been investigated in past studies and related tests have been standardized by IEEE with special reference to icing conditions. Apart from known general factors such as type of voltage (AC or DC), insulator diameter as well as inclination and altitude, dielectric strength depends greatly on snow sleeve thickness and conductivity. Suspension insulators are the most critical here and dimensioning checks on real cases using a statistical approach have demonstrated that a 4 m arcing distance, as generally adopted on Italian 420 kV lines, is normally sufficient to maintain risk of insulation failure under snow conditions to as low as 0.01 per event when snow conductivity is less than 25 µS/cm (i.e. corresponding to clean air conditions encountered in most mountainous areas). However, arcing distances of 5 m may prove necessary in the event of snowfalls with higher conductivities, e.g. 100 µS/cm, or if snow is deposited on insulators whose surface has already been polluted by deposits of sea salt, as near seashores. Indeed, recent events and studies have suggested that climate change will multiply wet snowfalls at sea level altitude, potentially impairing reliability of line insulation.
In the case of Italy, violent, thermal atmospheric contrasts typically occur over the northern flats and coastlines. These situations generate intense localized storms characterized by extreme lightning activity, whirlwinds and even tornados and therefore can directly or indirectly impact power systems. Lightning induced overvoltages are normally kept under control by ground wires, insulator spark gaps and surge arresters. The effect of winds, heavy rain and hail can have a more serious impact, such as branches, signs and rooftops striking and damaging conductors and causing short circuits, or flooding substations. A whirlwind builds up within a storm super cell and can cause extreme localized damage.
Once again, a resilience approach can be envisaged with the goal of reducing the consequences of such contingencies. For example, a well-designed alert system can be useful in preventing injuries while precisely locating the event can reduce the time for repair crews to arrive. But forecasting such events is difficult since meteorological stations on the ground are not sufficient and only monitor wind or precipitation. On the other hand, ground-based C-band radar and the Meteosat satellite are able to forecast the intensity and position of a thunderstorm less than an hour in advance of the event. STAF (Storm Track Alert and Forecast), a ‘nowcasting’ system based on radar and Meteosat Second Generation data has been set up and selects only severe thunderstorms, tracks them and sends alert messages to users. STAF calculates the probability of damage from a thunderstorm based on a series of parameters derived from radar and satellite observations, with special reference to reflectivity of the cumulonimbus along the vertical axis. The method has been calibrated through re-analysis of events. For example, 359 such events have been analysed using information reported by local newspapers. An example of using STAF over the city of Milan is shown in Fig. 12. Re-analysis of the period from April to July 2009 identified 6 events as well as their development and direction over time. Analysing whirlwinds and tornados can be carried out using a high-resolution non-hydrostatic meteorological model, such as RAMS, complemented by another high-resolution model, WRF. Special attention is given to situations characterized by extreme baric gradients that are able to trigger wind gusts, whirlwinds and downbursts that threaten power lines. For this reason, the focus is on events occurring 35 m above ground level. Meteorological prediction models are not suitable for such analysis and ‘nowcasting’ systems based on radar and satellites are preferred.
Statistical analysis carried out on each pixel of the RAMS model, having spatial resolution of 0.065°, allows determining the Probability Density Function (PDF) of wind gusts over an entire territory and reporting, on a country-wide map (Fig. 13), the level of wind gusts having a probability as low as 0.1% of exceeding that threshold (i.e. with 99.9% percentile). A tornado, for example, occurred in Tuscany on March 5, 2015. Wind speeds of 210 km/h were recorded and more than 200,000 users were involved in an extended blackout that lasted several hours. Structural damage was estimated in the tens of millions Euros. EHV lines were damaged by the impact with detached metal roofs and trees falling across line corridors (Fig. 14).