Degradation of cellulosic insulation in power transformers: a SEC–MALLS study of artificially aged transformer papers
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The molecular weight distributions of artificially aged transformer papers were studied using SEC–MALLS with 0.5 % DMAc/LiCl as solvent, providing chain length distributions and averages. The slow dissolution and presence of non-cellulosic contaminants tend in some cases to introduce errors in the DPw estimates, and a modified data processing procedure was developed to correct for extraneous scattering. Data were compared to the intrinsic viscosities obtained in 0.5 M Cuen and the ‘viscosimetric DP’ calculated thereof according to ISO 5351. DPvisc was 2–3 times lower than the weight average DP (DPw), but closer to or slightly above the number average DP (DPn) obtained by SEC–MALLS in the case of pure cellulose. Ageing of transformer papers were in some cases associated with changes in the polydispersity (DPw/DPn). The apparent degradation rate (defined as δ(1/DP)/δt) gradually decreased with time, resulting in a tendency for a ‘level-off’ DP in the range 200–300 for DPvisc and DPn, and roughly 1,000 for DPw.
KeywordsCellulose Viscometry Size-exclusion chromatography Multi-angle laser light scattering Transformer paper Ageing
K.B. Liland, M.-H. G. Ese and L. Lundgaard (SINTEF Energy AS) are thanked for providing the aged samples. Cecilie M. Selsbak is acknowledged for performing viscometric measurements. We cordially thank Prof. A. Potthast, University of Natural Resources and Applied Life Sciences (BOKU), Vienna, Austria for valuable discussions and advice regarding SEC–MALLS of cellulosics. The work was supported by the Norwegian Research Council (Grant 173142/30), Hafslund Nett, Statnett SF, Statkraft Energi AS, Norsk Hydro, EDF, Nynäs AB, and ABB through the SINTEF Energy project “Thermal and electromagnetic performance of transformers”.
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