Avaliação espectroscópica, cromatográfica e quimiométrica da degradação de óleos isolantes de transformador

Abstract

Concern about environmental issues, the need for installations employing safety dielectric fluids, and the search for new methodologies for fluid maintenance of electric power machines using environmentally friendly technology are increasingly pronounced. Although the characteristics and variations of the physicochemical and electrical properties of insulating oils are well established, less attention has been paid to the molecular level to understand the nature of the thermal and electrical aging processes of these fluids. Thus, the objective of this work was to develop a fast, simple, and environmentally safe method to evaluate and monitor the quality of vegetable insulating oil (IVO) through multivariate control charts and Fourier Transform Infrared (FTIR) spectroscopy, in addition to evaluating the evolution of chemical compounds formed during the degradation of IVO subjected to electrical and thermal stresses, comparing with insulating mineral oil (IMO). In order to simulate the internal environment of a transformer before and after energization, samples of IVO and IMO were kept in contact with paper and copper and subjected to accelerated electrical and thermal aging processes, in which the samples were subjected to disruptive discharges (from 10 at 1000 discharges) and heating to 130 °C for up to 1080 hours, respectively. Subsequently, the composition of the fluids before and after electrical and thermal treatments was studied through the techniques of gas chromatography coupled with mass spectrometry (GC-MS) and FTIR spectroscopy and the evaluation of acidity, viscosity, and density of the samples. The results obtained showed that the developed control chart was able to identify IVO samples at oxidation levels outside the acceptable quality standards for plant insulating fluids in transformers; and those new compounds were formed, due to the degradation process of mineral and vegetable oils, under the conditions studied. Partial discharges contribute to the degradation of oils, and this effect was more pronounced in the IVO and the factor that most influences the degradation of these fluids is heating. The insulating paper acts to delay the aging of mineral and vegetable oils, while copper acts as a catalyst for fluid oxidation. The degradation of the antioxidant 2,6-di tert-butyl-p-cresol (DBPC) from IMO is heating dependent, while the antioxidants tocopherol, stigmasterol, and sitosterol from IVO degraded even in systems that were not subjected to heating. The viscosity and acidity values of the oils increased, demonstrating the degradation under the conditions studied, and this effect was more pronounced in the IVO. In this way, the potential of FTIR spectroscopy, together with the strategy of control charts as a quick and simple tool for monitoring the quality of insulating fluids, allowing to guide the transformer maintenance plan, was demonstrated. The results contribute to studies carried out on insulating fluids and to companies involved in energy generation since with the results obtained, it will be possible to predict the aging time of oils, as well as make proposals to increase their useful life and to promote in a secures the replacement of mineral-type insulating fluid in transformers with vegetable oils, as they are more environmentally viable and have dielectric properties equivalent to IMOs.