Effect of oil replacement on dispersion staining colours of cellulose fibres in oil during the thermal ageing process

Ageing assessment of paper insulation is important to safe operation, condition based maintenance, and replacement of power transformers. This paper investigated the effect of oil replacement (OR) upon ageing assessment of paper insulation based on dispersion staining colours of cellulose fibres in oil. Accelerated thermal ageing experiments were conducted for oil-paper specimens for different durations and OR was conducted at different ageing stages. Degree of polymerization (DP) of paper and 2-furaldehyde (2-FAL) in oil were measured, and the dispersion staining colours of cellulose fibres were observed. DP of paper was predicted using traditional ageing marker 2-FAL and proposed ageing marker, i.e., primary colour ratio based Mahalanobis distance (PCR-MD) of (r, b) points. Results show that OR has little effect on DP of paper while it can results in substantial loss of carbon oxides and 2-FAL in oil. The OR has limited effect on Mahalanobis distance of (r, b) points. The prediction errors of DP with ageing marker PCR-MD is smaller than those with 2-FAL, which implies that the proposed ageing marker PCR-MD can be applied to ageing assessment of paper insulation that undergoes OR with accepted error level.