# Environmental Effects on HV Dielectric Materials and Related Sensing Technologies
## 2 Analyses and Discussions
\begin{table}
\begin{tabular}{l l} \hline \hline
**Contaminant** & **Source of Pollution** \\ \hline \multirow{3}{*}{Salt} & Coastal areas \\  & Salt industries \\  & Salt from snow melting \\  & Cement plants \\ Cement & Construction sites \\  & Fertilizer plants \\  & Fertilizers used in cultivated fields \\ Metallic & Mining process \\ Coal & Coal handling process \\  & Coal burning/brick-kil area \\ Defecation & Roosts of birds \\ Smog & Automobile emissions at highways and crossings \\  & Diesel engine emissions at railway yards \\  & Wild fire \\ Smoke & Industrial burning \\  & Agricultural burning \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The typical pollutants affecting the HV insulators and their sources [37]. Lighting surge voltages. During normal operation, the power-frequency voltage stress is low. Power-frequency overvoltage is caused by abnormal operation such as faults and resonances caused by weather conditions and structural aging and degradation associated with mechanical loads. The magnitude of overvoltage in high and extra-high systems is usually less than 2 V and lasts for less than 0.5 seconds. The switching surge voltages are produced by switching operations and their magnitude and duration are typically less than 3 V and hundreds of microseconds, respectively. A lightning surge voltage can be very high--larger than 1000 kV--however, it is very short in duration and is independent of the system operating voltage. For instance, along the sea shore in relatively clean environments where the contamination level of HV insulator surfaces are below a threshold (e.g., an equivalent salt deposit density (ESDD) < 0.01 mg/cm\({}^{2}\), the insulation level required is dictated mainly by the switching surge and lighting surge voltages. In places where insulation contamination is high--for example, ESDD > 0.01 mg/cm\({}^{2}\)--the length of the insulator is dictated by the power frequency operating voltage under specific weather conditions. In early years, these appeared to be satisfactory; however, as time passed and insulator surfaces, especially the under-surfaces of sheds that were not exposed to the cleaning action of the rain and snow, became contaminated, flashovers occurred in certain weather conditions and the aging of insulators was accelerated, leading to catastrophic insulator failure. A systematic study of leakage current on high-voltage polyamide insulators under different pollution conditions was reported [40]. The work showed that, in the case of clean/dry and clean/wet insulators, the leakage current increases linearly with the applied voltage. However, in the case of pollution with saline solution, there is a nonlinear relationship between the applied voltage and leakage current, evolving to the total flashover of the HV insulators, which occurs more rapidly than in the previous cases. The presence of saline solution also favors partial discharge, which precedes the flashover event, and causes traces of erosion and tracking on the surface of insulators. The flashover and aging of silicon insulators were also investigated under different pollution severity levels under dry band arcing [41]. The results show that the electric field stress occurs at the edges of the dry band and not at the electrode ends as in the case of uniform pollution layers. The study also suggested that the polymeric content in the silicone rubber surface decreases after flashover while the hydrophobic content increases. Hierarchical segmentation analysis was used to assess the effect of the atmospheric conditions on the risk of failure of HV insulators [42]. The significant variables of the study were the leakage current, ambient temperature, relative humidity, atmospheric pressure, solar radiation, level of precipitation and wind direction and speed. A flowchart of the process for the case of a substation is presented in Figure 2. The work also used the Kruskal-Wallis test [42] to determine the significant changes in the magnitude of pollution due to climate periods.