Correlation between the Optical Properties and the Degree of Polymerisation of Transformer Insulation Paper

Transformer's insulation system consists of oil-paper insulation, which is the most critical component that largely influences its service life. In service, the degradation of the transformer oil-paper insulation is inevitable, even under normal conditions. The degree of polymerization (DP) is a widely accepted and direct technique to assess the deterioration of the paper insulation. However, once a transformer is commissioned and energized, the direct assessment of the condition of the paper insulation is not possible. While insulating oil can be sampled for analyses when needed, this is hardly the case for the solid insulation system (paper). It is therefore, a usual practice to monitor the solid insulation system indirectly. Another incentive is that the life expectancy of a transformer is directly related to the aging condition of its paper insulation. Therefore, the direct degradation monitoring of solid insulation is an interesting approach. In this contribution, an optical aging marker is investigated to monitor the degradation of the paper insulation in oil-filled transformers. The reflectance of the aged papers has been monitored, and the relationship with DP is investigated at three different commercial wavelengths. It is noticed that a correlation between the reflectance spectrum and DP-values may be used to monitor the aging conditions of the paper.