Contact angle measurement as a potential tool for electrical insulating paper condition assessment

forms such as Kraft paper, hard paper and wood. Cellulose is an organic compound, a polysaccharide consisting of a linear chain of linked D-glucose units. The average number of glucose monomers linked in a chain, so called degree of polymerization (DP) is determined by a viscometric method according to IEC 60450. The measurement is carried out on new insulating paper before use in the transformer to determine the initial state. The DP measurement is also carried out on paper from operating transformers with the aim to determine the condition of paper insulation and to assess the remaining transformer life. New insulating paper has a DP value ranging from 1100 to 1600. The lowest DP value below which the mechanical properties of the paper cannot withstand the stresses in the transformer is 200. DP is an indicator of mechanical properties of paper insulation and it is in direct correlation with its tensile strength. Both characteristics, DP and tensile strength, decrease during transformer exploitation and the rate of degradation depends on the type of paper, temperature, oxygen and water content in oil and other factors. This paper investigates the applicability of the contact angle method on insulating Kraft paper, as an additional parameter to assess the condition of the insulating paper. The contact angle method is used for the surface characterization of different materials and during the literature review no work on the application of this technique to characterize the electrical insulating paper was found. For determining the contact angle (θ) on Kraft paper three test liquids (water, diiodomethane and formamide) with different polarities were used. The contact angle method was applied on new Kraft paper and on Kraft paper which was subjected to laboratory accelerated ageing tests. The ageing tests were performed in new transformer oil at 120 °C and 130 °C. During the ageing, DP and contact angles of three liquids were measured and correlation curves between DP and contact angles were determined. The results show a significant increase of contact angle with paper ageing, measured with polar liquids (water and formamide), while the values of contact angles measured with diiodomethane as a nonpolar liquid remain approximately constant. The measured values of water and formamide contact angles suggest that the surface of the insulating paper during ageing becomes nonpolar and hydrophobic. It can be assumed that during ageing there is a reaction on the hydroxyl group (-OH) within the cellulose chains with the effect of reducing the number of -OH groups that are the most responsible groups for the hydrophilic surface of the material. The decrease of the number of -OH groups and an increase in the formation of new groups of nonpolar character, such as aldehyde and ketone groups, results in an increase of the hydrophobicity of the insulating paper. The method for measuring the contact angle is applicable to electrical insulating paper, and by measuring the contact angle of water and formamide the DP value of insulating paper can be estimated.