Analysis of service-aged 200 kV and 400 kV silicone rubber insulation in the Gulf region

This paper analyzes silicone rubber housing materials that have been aged in service in 200 kV and 400 kV transmission line systems in the Gulf region. Non-uniform discoloration with traces of white residue are more evident across the upper and lower sides of the 400 kV as compared to the 200 kV sheds located near both the energized and the ground ends of the insulators. Scanning electron microscopy (SEM) showed an increase in the roughness for all the surfaces of the materials analyzed as compared to the bulk. Surface oxidation was indicated by the increase in the Oxygen/Carbon (O/C) ratio determined using energy dispersive X-ray (EDX) and the existence of the silanol groups (Si-OH) as detected in the Raman spectroscopy. The effect of electric field causing surface discharges oxidizing the surface was more evident on the 400 kV as compared to the 200 kV materials, particularly on the lower side of the sheds located near the energized end of the insulators. The effect of photo-oxidation due to ultra violet radiation appears to be increasing as compared to the electric field enhancement on the upper side of the sheds, particularly for those located near the ground end of the insulators. Thermogravimetry analysis is found useful in indicating the relative amount of alumina tri-hydrate filler and the polymer in the silicone rubber housing composition. Both O/C ratio determined using EDX and the silanol groups detected using Raman spectroscopy are proposed as reliable aging indicators to assess the condition of the silicone rubber surfaces that have been aged in service under relatively light polluted conditions. Raman spectrometry has the additional advantage as portable Raman spectrometers can be used to conduct field measurements on site.