Pattern analysis of discharge characteristics for hydrophobicity evaluation of polymer insulator

Hydrophobicity is a prominent characteristic of silicone rubber for polymer insulators. However, due to the environmental influence, the decrease even the loss of hydrophobicity will take place, which causes the essential concern on the electrical accidents induced by the ageing of polymer insulators. The accurate evaluation on the insulator hydrophobicity is increasingly required for the long-term performance of outdoor insulators in service. In this paper, based on dynamic dropping test (DDT), pattern characteristic of surface discharges generated by the dynamic behavior of water droplet was investigated to evaluate the hydrophobic properties of silicone rubber insulator at different ageing levels. The specimens were prepared by using the method of corona degradation and determined by HC levels from HC1 to HC6. Both the dynamic behavior of water droplet and the induced discharge phenomena were captured by using a high-speed camera fixed vertically to the sample surface. In order to quantify the surface discharges, methods of image processing and fractal dimension were employed to establish the relationship between the pattern characteristic of discharge light and the HC levels. The morphology of discharge region and the distribution of luminous brightness in relationship with the lapse time can provide an optical evaluation method for the insulator hydrophobicity. It is found that the number and duration of discharge light increase with the increase of HC level. The increasing tendency in brightness intensity and fractal dimension of discharge pattern can reveal the decrease of hydrophobicity. Therefore, the results obtained indicate that the pattern characteristic of discharge light in DDT method is sensitive to the hydrophobic properties, which can be applied as a non-contact method for hydrophobicity evaluation of polymer insulators.