Dissolved gas analysis (DGA) of natural ester insulating fluids with different chemical compositions

This study presents experimental results of the application of the dissolved gas-inoil analysis (DGA) technique to natural ester insulating fluids. The investigations covered two common types of transformer faults, discharge and overheating, and four different commercial insulating oils, one mineral and three natural esters: Biovolt A (from corn oil), Biovolt HW (from sunflower oil with high oleic acid) and Envirotemp FR3 (from soya oil). The natural esters insulating fluids (NEIFs) analyzed had different chemical compositions. Experiments were performed in the laboratory with the test oils at three different moisture levels (low, medium and high). The purpose was to see the effect of moisture content and oil composition on the quantities of fault gases dissolved in the different oil types. The overheating tests were performed by heating the oils in an oven at 150 °C. Discharges were generated using a flat plate-plate electrode arrangement immersed in oil. Five key dissolved hydrocarbon gases - hydrogen (H2), methane (CH4), ethane (C2H6), ethylene (C2H4) and acetylene (C2H2) - generated by the two fault types in NEIFs were analyzed and compared with mineral oil (MIO) fault gases. The classic Duval triangle 1, modified Duval triangle 3, Roger's ratio, Dornenburg and IEC 60599 methods were applied to predict the fault type. The findings confirmed that the direct application of classic DGA fault interpretation methods, developed for mineral oils, to non-mineral oils needs some modifications. IR and RMN spectroscopy analyses were performed to observe differences in triacylglycerol group compositions before and after heating tests.