Condition assessment of naturally aged composite insulators, aiming to better understand the ageing processes involved, is essential for maintaining high reliability in power networks. In this study field-ageing effects on insulating materials (EPDM and VMQ/HCR) of two 150 kV composite suspension insulators are evaluated. The insulators had been in service for 17 years on a coastal transmission network, exposed to seaborne salt spray, windborne soil deposits and intense UV solar radiation. Several diagnostic techniques, including visual inspection, wettability classification, FTIR spectroscopy, SEM and EDX analysis were employed to assess the condition of the insulators. Field-ageing effects were not assessed as critical to dictate insulator replacement. For both insulators morphological and material degradation due to field-ageing was restricted to the upper surface layer of the polymeric housing. The resistance of housing material against tracking and erosion remained unaffected by field-ageing. Surface deterioration of EPDM was more intense than that of VMQ/HCR, despite the fact that the EPDM insulator had been in operation at a site with a lower pollution severity. The EPDM housing was classified as hydrophilic; however, after removing pollutant deposits its hydrophobicity was improved. The hydrophobic properties of the VMQ/HCR housing were only slightly reduced. Field-ageing effects on insulator housing are discussed based on material structural changes caused by ageing stresses.

Soil resistivity is an important parameter for engineering applications. In this work the effects of soil characteristics and conditions on the DC soil resistivity are investigated experimentally using remolded soil samples. Several natural silica and artificial glass sands of uniform gradations were tested under variable water resistivity and content, using the two-electrode and the four-electrode soil box methods for dry and wet soil, respectively. The dependence of DC soil resistivity on soil characteristics and conditions, such as grain size, specific surface, pore water conductivity and degree of saturation, is assessed. Soil resistivity decreases with increasing water conductivity and content. It also decreases with decreasing grain size as a result of the shorter current conduction paths (lower tortuosity) and the higher amount of available electrolytes; the contribution of the latter was quantified by measuring the resistivity of several soil:water suspensions. The effects of soil characteristics and conditions on soil resistivity have been well accounted for by considering soil resistivity at saturation as reference. For degree of saturation higher than the percolation threshold soil resistivity is well approximated by Archie's law; parameter values applicable to sandy soil are proposed. A new expression has been derived on a physical basis to describe soil resistivity for degree of saturation values lower than the percolation threshold.