# Environmental Effects on HV Dielectric Materials and Related Sensing Technologies
## 2 Analyses and Discussions
These components are able to migrate and subsequently cover the aged surface layers to restore hydrophobicity. Studies on the flashover voltage (kV) of silicone rubber/ethylene propylene diene monomer blends of outdoor insulators in a desert environment [28] indicated that, for long-term use, the electrical performance of hydrophobic composite insulators such as the silicone rubber blend (%) should be increased. Studies in desert conditions also showed that thermoplastic insulators and their dielectric response under the tested thermoelectric-cum-ultraviolet irradiation performed better than the silicone rubber insulators [29]. The data provide information regarding the stability and performance of these composite insulators and allow an informed decision on the choice of the most suitable materials. The impact of birds' activity is also of concern, especially in areas with a large bird population. For example, the implications of excrement from birds perching on transmission towers on the 220-kV network installed by Southern California Edison Company led to reported flashovers [30]. Lightning strikes affect the performance of HV insulators both directly and indirectly. The direct effects are related to the direct interaction with insulators, while indirect effects are caused by transient HV pulses induced in other power network components such as power lines. Studies in this direction were carried out; for example, for 10 kV polymer insulators under different conditions for impulse, weather and insulator angle with respect to a cross-arm [31]. The results showed that the weather and insulator angle (with respect to the cross-arm) have a dominant influence on the values of breakdown voltage and leakage current for both negative and positive impulses. Industrial pollution commonly constitutes of gases and solid particles which are the products of combustion processes. When combustion products are transported by wind, suspended carbon particles (soot) may be adsorbed onto the HV insulator surface due to gravity and electrostatic forces [32; 33]. The layer of the soot particles on HV insulators become conductive due to the ionized atmosphere around the insulator and can cause a distortion of the electric field distribution [34; 35; 36]. This distortion enhances the electric field strength to a level which is larger than the air breakdown, leading to partial discharge. The typical pollutants that have a negative impact on HV insulator performance are summarised in Table 3.  
The severity of pollution can be quantified in terms of the equivalent salt deposition density (ESDD) in mg/cm\({}^{2}\). Table 4 presents the insulator leakage distance to avoid flashover according to the IEC 60815 standard [38]. The presence of environmental stresses such as heat, UV radiation, wind, rain, snow, salt, smoke, moss, algae, sand, dust, carbon particles, ash, avian excrements and gases results in material degradation, which then leads to a decrease in the mechanical and electrical properties of HV insulators. This ultimately facilitates the appearance of abnormal electrical stresses such as arching, corona, partial discharge, leakage current and flashovers. This process is presented in Figure 1.  
For outdoor electrical insulation in the power system, there are three different types of voltage stress [39]:  
1. Normal operation and temporary power-frequency over-voltages;
2. Switching surge voltages; \begin{table}
\begin{tabular}{c c c} \hline \hline
**ESDD (mg/cm\({}^{2}\))** & **Site Severity Based on Pollution** & **Leakage Distance (inch/kV\({}_{\text{L-G}}\))** \\ \hline \(<\)0.01 & Very light & 0.87 \\
0.01–0.04 & Light & 1.09 \\
0.04–0.15 & Medium & 1.37 \\
0.15–0.40 & Heavy & 1.70 \\ \(>\)0.40 & Very heavy & 2.11 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Pollution severity based on the equivalent salt deposition density (ESDD) [38]. Figure 1: The steps that lead to faults related to the electrical stresses of HV insulators.