DGA Comparison Between Ester and Mineral Oils

This paper reports on a comparative study between mineral and various ester oils based on Dissolved Gas Analysis (DGA). Electrical (breakdown of low energy) and thermal (stray gassing) faults were realized at laboratory scale, then gas formation as well as diagnosis methods were compared. Two extraction methods, described in IEC 60567 Ed.3 and independent from partition/solubility coefficients were used: multi-cycle vacuum degassing and stripping. Three different diagnosis methods were applied: IEC gas ratios, Duval's triangle and key gases. For electrical faults and especially for low energy discharges, the same main gases (Hydrogen and Acetylene) are created in relatively same ratios, thus confirming the efficiency of key gases method for ester oils. It is also shown that classic Duval's triangle (triangle 1) is well applicable for this type of fault confirming the results reported in literature. For thermal faults and especially for stray gassing issue (low thermal faults), Ethane (associated with Hydrogen) constitutes the key gas of natural esters. The new Duval's triangles can be applied: Triangle 4 for mineral oil and Triangle 6 for ester oils. For extraction methods and in both electrical and thermal faults cases, it can be noted that results are comparable for mineral oil whereas they are slightly underestimated with stripping method for ester oils. The higher viscosity of ester could interfere with the extraction.