As for impact on electrical insulation, service experience from AC systems indicates that rain is not a particularly severe environmental stressor for line insulators. In fact, higher impact severity has been reported for insulators at substations, especially when these have been designed with small spacing between sheds. Test regimens included in present standards are normally adequate to verify design of external insulation under rain conditions. Heavy rain tests might be recommended, however, in extreme conditions, e.g. rainfall rate of 10 mm/minute and water conductivity of 1000 μS/cm. Since reduction in performance due to heavy rain is most pronounced for insulators having small distance between sheds, mitigation measures include adopting insulators having shed spacing greater than 60 mm, or application of ‘water cut’ or booster sheds.
Environmental pollution and its impact on reliability of power system insulation is a known threat. Surface contamination due to salt deposits or other pollutants can, under certain circumstances, cause flashover of line and substation insulators, resulting in contingencies. A good example was the blackout that affected all of Sardinia on Sept. 21-22, 2001. An extended pollution event was the origin of a series of cascade contingencies that tripped most of the island’s transmission lines, the Sardinia-Italy HVDC cable link and more than 550 MW of local thermal generation. Local population, industry and the service sector were all left without power for more than 10 hours. Research to assess, monitor, prevent and correct such situations resulted in the latest version of AMICO (Artificially Moistened Insulator for Cleaning Organization) – a device able to measure site pollution severity, even in DC. The information and knowledge acquired relies on laboratory tests on insulators removed from service as well as on extensive mathematical modelling. The goal of this activity has been twofold: to update national pollution maps that are important for planning development of the power system as well as dictate performance and characteristics required of insulation at different locations; and, second, to forecast severe pollution events, thereby allowing preventive measures to be set up to mitigate impact on system reliability. An initial pollution map of Italy, elaborated during the 1990s, was based on geographical and local environmental conditions such as type and size of cities, industry, distance from the sea, network problems and weaknesses, local measurements of ESDD on insulators removed from overhead lines, etc. (Fig. 15).
But Italy has undergone numerous changes since that time. For example, evolution of the generation mix towards natural gas and renewables has impacted emissions from power plants while a prolonged economic downturn has reduced pollution from industry. On top of this, climate change has modified the entire panorama. Moreover, use of AMICO monitoring devices installed at critical locations has indicated the need to update this pollution map. Recent studies have demonstrated a strong correlation between dust concentration in the atmosphere and level of pollution deposited onto insulator surfaces. This has opened the way to a possible new approach in assessing and predicting pollution events, e.g. use of weather forecasting and air quality modelling. An entire suite of modelling tools are available: the WRF (Weather Research and Forecasting model) is a physical-mathematical high resolution meteorological model used for weather forecast with meshes of 5 km, while CAMx (Comprehensive Air Quality Model with eXtensions) is a Eulerian model of the chemical and transport processes that pollutants undergo within the lower atmosphere, also having a resolution of 5 km; the processors SMOKE (Sparse Matrix Operator Kernel Emissions), MEGAN (Model of Emissions of Gases and Aerosols from Nature) and SeaSalt are dedicated to human-related, biogenic and sea salt emissions respectively. Fig. 16 describes the general architecture of such a modelling system.
Pollution events on insulators are linked with a combination of two meteorological situations: an extended period of drought and low humidity responsible for accumulation of atmospheric pollutants over exposed surfaces of insulators (often associated with strong winds that lift material previously settled on the ground or generated by sea spray), followed by periods of high humidity linked to fog, light rain or night dew that moisten polluted surfaces and change their conductivity, potentially leading to flashovers. The physical elements that drive this process are humidity, wind, and the dynamics of precipitation and drought. An additional driver in the case of locations close to the seashore is spray or marine aerosol linked with a combination of waves and breeze from the sea that deposit salt over exposed insulator surfaces. Analysis of the applicability of the model approach was made step-by-step using a re-analysis process, i.e. through re-calculation by means of a model of actual situations observed in the field, specifically at locations close to AMICO sites and near data collection points. The end goal was to forecast build-up on insulator surfaces of a pollution layer characterized by a given ESDD and NSDD. This can only be achieved if three models are validated:
• dynamics of meteorological parameters that are drivers for development of a pollution event (i.e. temperature, humidity, wind speed and precipitation);
• mechanism of formation and transport of solid contaminants in air (e.g. PM10 and its inorganic components);
• mechanism of deposition of contaminants present in the air surrounding insulator surfaces.
The ability of the WRF Model to reconstruct temperature and ambient humidity at 2 m above ground and wind velocity at 10 m above ground was assessed. Results were satisfactory, especially for temperature, while discrepancies were more evident in humidity. Detailed analysis showed that meteorological simulations could be trusted for typical periods relevant for pollution events. The capability of the CAMx Model to estimate the drivers for salt deposit density and surface conductivity on insulators was subsequently verified considering solid particulates, with special reference to PM10 and its inorganic components. Field re-analysis was carried out given AMICO device installation sites as well as air quality monitoring stations belonging to the European Monitoring and Evaluation Programme network located in Montelibretti and Ispra. Comparison between measured and calculated daily concentrations of PM10 and its inorganic ions (i.e. SO4, NO3, NH4, NaCl) in the liquid phase was carried out over an entire year. Results showed an acceptable match, after adjusting for level of precipitation. Finally, ESDD could be estimated based on dry deposition rate for inorganic ions. Preliminary results, reported in Figs. 17 and 18 for standard and polymeric line insulators respectively, are encouraging, although maximum peaks in July and August are greatly underestimated.
The mechanism of deposition and possible dissolution of particulates across the complex surfaces of insulators is still not well understood and needs additional research. Although detailed evaluation and forecasting of pollution events is not yet possible with models, this analysis has nevertheless offered the ability of the modelling suite to represent, across all Italy, the driving force behind insulator pollution: i.e. concentration in air of particulates (PM10), with particular reference to that fraction from sea salt. This feature was used to update the Italian pollution map. The new map was constructed based on spatial estimation of PM10 and sea salt considered separately, each categorized over 8 classes of increasing severity and then re-aggregated, doubling the weight of sea salt concentration. Results were then re-grouped into five pollution severity classes, according to IEC 60815 (Fig. 19).
Compared to the previous map, the pollution class ‘exceptional’ – linked to peculiar situations close to heavily polluting plants – has disappeared. A new class rated ‘very light’ was introduced to characterize alpine areas where risk of flashover due to pollution can be considered negligible. The new map shows dramatic reduction in the level of contamination around the Pisa plain and urban Milan: this area, while heavily polluted by anthropogenic sources, is not affected by saline pollution. The new map also features reduction in severity along the Adriatic coast as well as in the north-eastern region of Puglia. Inland areas of Sardinia and Sicily have been upgraded from ‘light’ to ‘medium’ pollution level. Moreover, areas rated ‘heavy’ pollution in these two regions have been expanded, especially near the west coast. Finally, in Sicily, there was an increase in severity class along the coast of Trapani since it is frequently hit by Mistral winds carrying particles of sea salt. This new pollution map was extensively validated by comparison with measurements carried out by RSE and Italian TSO, Terna, based on more than 200 sampling sites located near high voltage infrastructure.