This paper discusses the state of the art in the application of self-healing silicone-based materials for outdoor high-voltage insulation. Both the dynamic behavior of the dimethyl side groups of silicone rubber and the diffusion of a bulk siloxane to maintain low surface energy are respectively reported as intrinsic mechanisms responsible for the self-healing of silicone rubber. Localization, temporality, mobility, and the type of synthesis are the aspects defining the efficiency of the self-healing ability of silicone rubber. In addition, the deterioration of the self-healing ability with filler loaded into silicone rubber insulation housing composites is discussed. Taking the self-healing property into consideration among the other properties of silicone rubber insulators, such as tracking and erosion resistance, can be a useful design practice at the material development stage. Hydrophobicity retention, recovery, and transfer measurements are discussed as useful indicators of the self-healing ability of silicone rubber. Nevertheless, there remains a need to standardize them as design tests at the material development stage. The paper is intended to shed the light on the hydrophobicity recovery, a key material design parameter in the development of silicone rubber outdoor insulating composites, similar to the tracking and erosion resistance.

High-voltage direct current (HVDC) transmission is known as green-energy transfer technology and has recently become an attractive alternative of high-voltage alternating current (HVAC) due to its high-power transmission capability and lower power loss. Use of composite insulators on direct current (DC) transmission lines experienced rapid growth in recent years due to their high hydrophobicity and better performance in contaminated environment than conventional ceramic insulators. During their service operation on DC lines, insulators are prone to more accumulation of contaminants due to unidirectional electric field. The contaminants under wet conditions allow leakage current to flow on the insulator surface. Being organic in nature, polymeric insulators have a tendency to age under the combined effects of electrical and environmental stresses. To fully understand the long-term aging performance of DC composite insulators, a detailed survey was considered necessary. Towards that end, this paper critically summarizes worldwide experience of aging performance of composite insulators in the field as well as in laboratory conditions.

Silicone rubber-based insulating polymers are extensively used in high voltage outdoor applications due to their high hydrophobicity and better withstand performance. However, the main problem with polymeric composites is their aging due to external environmental stresses (e.g. direct sunlight, high humidity and temperatures, ultraviolet radiations, etc.) as well as strength of the applied electrical field. In the present work, for examining weight gain, hydrophobicity, and hardness, five different types of HTV silicone rubber-based hybrid composites are prepared. All the samples are reinforced with micro-alumina (20 wt. %) and different concentrations of silica fillers. Material samples are immersed in distilled and boiling water for 24h and 2h, respectively. For analysis, hydrophobicity, weight gain and hardness tests are conducted. The results of the performed tests indicated that the sample filled with 6 wt. % of nano-silica has less percentage gain in weight due to water absorption compared to the other hybrid composites. The sample with the same composition showed high hydrophobicity as well.

This paper proposes ultra-high frequency detection of the eroding dry band arcing as a non-intrusive and online detection technique of erosion failure during the DC inclined plane test. Tests are performed on silicone rubber composites, while conducting leakage current, surface temperature and ultra-high frequency measurements using a horn antenna. No ultra-high frequency detection is evident during mild dry-band arcing, which does not lead to erosion. Whereas, the detection is evident during stable dry-band arcing, particularly when the intensity becomes remarkable. Thermogravimetry, differential thermal analysis and Fourier Transform spectroscopy are conducted to elucidate temperature measurements and visual observations. The eroding dry-band arcing is shown leading to depolymerization of silicone rubber at 400°C, thereby leading to combustion and erosion failure, and stimulating a radical-based crosslinking reaction at temperatures exceeding 500°C. The intensity of the ultra-high frequency detection is also shown to be sensitive to the severity of the eroding dry-band arcing as it increased with time during the test or with higher applied test voltages.

In this paper, the nitric acid degradation mechanisms of silicone rubber, filled with alumina tri-hydrate and silica, are investigated for outdoor insulation applications. Scanning electron microscopy and Fourier Transform infrared spectroscopy confirm the effect of nitric acid dissolving alumina tri-hydrate and causing chain scission in silicone rubber, thus leading to surface cracking. Nitric acid is also found to promote depolymerization and suppress radical-based crosslinking during the thermogravimetry and differential thermal analyses, which deteriorates the erosion performance in the inclined plane tracking and erosion test. The heat of fusion measured in the differential scanning calorimetry is shown to decrease with nitric acid immersion, indicating increased polymer chain mobility. Therefore, in a harsh acidic environment, chain scission of silicone rubber is not only reported to result in surface cracking but also erosion, due to the mobility of the chains promoting depolymerization during the dry-band arcing.