Ageing phenomena of cellulose/oil insulation in natural ester and mineral oil

摘要

Degradation of electrical insulation due to ageing by combination of thermal, oxidative and hydrolytic processes is one of the main factors that affect power transformer service life. A better understanding of ageing phenomena is of great importance for transformer condition assessment. This enables reliable operation and affords application of adequate mitigation action to slow down the degradation process of insulating materials and extend transformer service life. This issue is all the more important when new insulating liquids as natural esters are being applied in growing number in transformer industry. For these reasons, this paper is focused on the ageing phenomena of cellulosic materials (paper and pressboard) impregnated with different insulating liquids as natural ester, inhibited and uninhibited mineral oils. A first part is dedicated to moisture equilibrium in impregnated paper and pressboard systems in order to give some guidelines on the evaluation of cellulose humidity through the oil. Moisture equilibrium isotherms were created for temperatures from 40 to 120°C and for water content in paper and pressboard from 0.5 to 5%. Differences in distribution of water within cellulose/oil insulation between natural ester and mineral oil were observed and discussed. Natural esters have higher water solubility and therefore, for the same water content in cellulose, at same temperature, water content in the oil was considerably higher in natural ester oil than in mineral oil. It was also found that water distribution between paper and pressboard in mineral and ester oils is quite different. Established isotherms constitute a useful tool for the condition assessment of green power transformers [1]. A second part is dedicated to thermal ageing at 140°C up to 120 days in order to evaluate ageing profile and long term stability of different oils and solids. Ageing experiments were performed using copper, paper, pressboard and insulating oils in proportions close to real design. Hermetic systems were used and oxygen content in the oil was adjusted to simulate conditions of sealed transformers (not free breathing). It was found that in conditions of particular ageing test, thermal degradation, i.e. oil pyrolysis was dominant degradation mechanism, while oxidation and hydrolysis were minor processes. This was confirmed by low to moderate rate of antioxidants depletion in both, natural ester and inhibited mineral oil. Stability and performance of natural ester were found to be very good, especially regarding the impact on solid insulation. Indeed, degradation of solid insulation was present at lowest rate, in comparison with mineral oils, on the basis of highest degree of polymerization values at the end of ageing.