Comparison of the Formation of Aging Products in Mineral Oil and Decane Stressed by Thermal Transformer Stress

The generation of fault gases in the well-known conventional mineral oils due to different transformer fault types is well established and there are a number of dissolved-gas-analysis(DGA) interpretation algorithms for an early fault detection of the power transformer. However, new alternative insulation liquids show different gassing behaviors due to various transformer faults. Especially, a thermal hot spot fault is one of the most important transformer faults and the classification into the different temperature ranges is essential for a reliable fault detection and condition evaluation of the transformer. This study shows that not only the various types of insulation differ in their gas formation behavior as a result of thermal faults, but also within mineral oils large differences are observed in the generation of defect gases, their concentration and ratios. In addition to the conventional fault gases, also higher-value C
3
to C
5
hydrocarbons are formed that also offer characteristic fault gases. Four different mineral oils, including naphthenic and paraffinic based mineral oils, were investigated regarding their gassing behavior due to various Hot-Spot-temperature. Large differences were apparent when comparing the two mineral oils types. Paraffinic based mineral oils generate a higher amount and also further fault gases in comparison to the naphthenic based mineral oils. Thus, in addition pure n-decane were also stressed by the same thermal faults and the formed fault gases were compared with the various mineral oils. High amounts of the characteristic fault gases as in the paraffin-based mineral oils are also detected in pure n-decane, so that alcanes appear to be mainly involved as a reactant in the aging mechanisms. This result leads to the conclusion that oil-specific DGA interpretation schemes are required that also consider the main constituents of the mineral oil.