Resumo:
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.