Comparison of gases generated in mineral oil and natural ester immersed transformer's models

Currently, the use of natural esters in transformers is increasing, from distribution transformers to power transformers. However, its application in power transformers is still limited by the low evaluation in service. Diagnostic tests of the condition of power transformers must be reviewed for whether they need modifications to their interpretation criteria, mainly in those tests carried out on the transformer's dielectric fluid. The dissolved gas analysis (DGA) interpretation criteria should be investigated since, although it has been shown that the types of fault gases are similar from each of the different fluids, for a given fault, the amounts and ratios of the gases may differ from fluid to fluid. Moreover, from the review of the reported research, it was observed that most of the results obtained are from laboratory scale test and also, they are somewhat dependant of the set-up used. This paper presents a case study of thermal faults model for different hot spot temperatures. The fault model was made using two kinds of oils: mineral and a natural ester, both without cellulose. A comparative study between mineral oil and natural ester oils based on DGA were done, identifying key gases for the different faults types and fluids. Both fluids were to be used to impregnate MV transformer windings by a manufacturer. Traditional mineral oil DGA interpretation techniques were tested on natural ester, such as Rogers ratio and Doernenburg ratio from IEEE Std C57.104:2008 and IEC ratio and Duval triangle from IEC 60599:2015. In addition, Duval triangle for non-mineral oils was also tested.