Determination of degree of polymerization of insulating paper using near infrared spectroscopy and multivariate calibration

A simple alternative method to determine the degree of polymerization (DP) in transformer insulating papers, using near infrared spectroscopy (NIR), was developed to reduce the costs and the analysis time of the conventional method. Seventy-five samples of Kraft, crepe and cardboard types of paper, in different stages of degradation (DP varying between 200 and 900 units) were collected from transformers over a period of 3 years. The sample set was analyzed according to the conventional method to compare with the proposed alternative spectroscopic method. The spectra were obtained by diffuse reflectance using the spectral range between 1260 and 2500 nm. Some pre-processing strategies of the spectral data were evaluated and the best results were obtained when the derivatives spectra were used (Savitzky–Golay algorithm employing a 21 point window). NIR spectroscopy combined with partial least squares regression method revealed a simple and fast technique to determine the DP of the papers. The prediction error of 83, obtained after external validation, is acceptable for evaluation of the remaining useful life of insulating paper.