To evaluate the degree of degradation of the transformer, average degree of polymerization (DP) of the insulating paper on winding is measured in general. This measuring process accompanies insulation risk because several tens centimeter of the insulating paper has to be cut off from the winding in transformer. In this paper, a new method is proposed to estimate the average DP of the insulating paper by measuring the refractive index of cellulose fibers on the paper. The relationship between wavelength characteristics of dispersion color on cellulose fibers and DP of the paper was revealed using Mahalanobis-Taguchi method. Only hundreds of cellulose fibers that come off from paper surface or paper edge are used in this method, it becomes possible to know DP without cutting of the insulating paper.
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