Determination of interfacial tension of insulating oils by using image analysis and multi-way calibration

Power transformers are of great importance in the distribution of electrical energy. One of their most important parts is the insulating system, consisting of Kraft paper immersed in insulating oil. One of the most important parameters used for evaluating the degradation of this system is the oil interfacial tension. The aim of this study was to determine the interfacial tension in samples of insulating oils by using image analysis combined with a multi-way calibration method, N-PLS (multilinear PLS). Forty eight oil samples were obtained, whose values of interfacial tension were determined by a tensiometer, and divided into calibration (38) and validation (10) sets. Scanner images were obtained, converted to grey-scale, domain transformed and stacked in a three-way data array, before modelling. The best N-PLS model was obtained with mean centering and three latent variables and provided a RMSEP of 3 dyn cm− 1. This model provided individual prevision errors between − 14 and 16%, which were acceptable for electric energy companies. The proposed method was rapid and non-destructive, showing great advantages over the traditional ones, which are slow and produce chemical waste.