A study of surface discharges during the early ageing of silicone rubber insulation using partial discharge analysis

Polymer insulators, particularly those made of silicone rubber, are increasingly being used mainly because of their superior performance under wet and contaminated conditions compared to conventional porcelain and glass. Their hydrophobic behaviour, which prevents water filming on the insulator surface and consequently the leakage current (LC) development, is responsible for the superior performance. However, the changes in the surface properties under service conditions eventually lead to loss of hydrophobicity. The transition from a hydrophobic to a hydrophilic state is one of the main concerns with polymeric materials as they are susceptible to tracking and erosion due to dry-band arcing that takes place when LC develops.; To study the insulation surface conditions, LC measuring systems are generally used in laboratory and field; however, no correlation between the LC characteristics and the state of the surface has been established for silicone rubber. In addition, little useful information can be extracted from LC measurements at the early ageing stage when the surface is still hydrophobic, as no continuous wet path for the current has been established. On the other hand, it is known that the electric field distribution is significantly different when the surface is partially wet, and in some cases the resulting field may be high enough to initiate corona, which also plays a role in the degradation process. In this context, this research was focused on a new approach to study the ageing problem based on partial discharge (PD) analysis to complement the information from LC measurements. Surface discharges on silicone rubber insulation under varied experimental conditions, including uniform and non-uniform field, moist and salt-fog were investigated by means of PD measurements.; Tests were done by placing droplets on the surface using a micro-pipette, and by condensing water in a controlled humidity chamber. The field enhancement factor due to droplets placed on the insulation surface was experimentally and numerically determined. Values in the range of 3.8 to 6.3 were obtained depending on the size and number of droplets. The numerically computed field enhancement factors were between 3.9 and 5.5, which are in good agreement with those measured. The time resolved analysis of PD from water condensation indicated a threshold field, between 3.0kV/cm and 3.5kV/cm, above which the PD activity increased with time, bringing the insulation to a surface breakdown. The corresponding field enhancement factor obtained for the condensation experiments was between 5.8 and 6.8.; The effects of contamination and electric stress on the changes in the surface behaviour of practical moulded silicone rubber insulators were studied by means of PD measurements in a salt-fog chamber. Pattern recognition techniques were applied to investigate the transition from corona to dry-band arcing. A diagnostic scheme based on PD analysis was presented that allows to discriminate between the performance of different insulator designs under laboratory conditions.; In summary, PD analysis has been used to study the early ageing stage of silicone rubber insulation, and it has been demonstrated that PD measurement is a sensitive technique to estimate the hydrophobic conditions of the surface. The results have shown that the changes in the surface properties and the nature of the surface discharges during the early ageing period of silicone rubber insulators, when exposed to wet and contaminated conditions, are better detected with the use of PD analysis by observing the variations in the PD trends.