Modified Dissolved Gas Analysis with Additional Detection of Higher Hydrocarbons for Transformer Faults Diagnosis

Abstract

Dissolved gas analysis (DGA) is one of the well-established monitoring system for estimation of the transformer insulation condition. Commonly, the conventional fault gases C1 to C3 hydrocarbons as well as oxygen, nitrogen, hydrogen and carbon dioxide and monoxide are measured. The concentrations of the fault gases as well as the ratios among them are used in a number of DGA interpretations for fault diagnosis. However, most DGA interpretations are based on the well-known mineral oils. Currently, more and more alternative insulation liquids have been used in transformers. These alternative insulation liquids possess a different chemical structure compared to mineral oil, so that other aging reactions occur. For a more precise classification of the fault types, higher C3, C4 and C5 hydrocarbons could also be considered as new fault gases in addition to the conventional fault gases. The great advantage of higher hydrocarbons is their very high solubility in the insulation liquid, so that their volatilization is minimized. Other fault gases such as hydrogen and methane can volatilize easily into the liquid due to their low gas solubility. This complicates the DGA interpretation. In this study, three different types of insulation liquids were investigated for their gas formation behavior of higher-value hydrocarbons under different transformer faults as new suitable indicators for DGA interpretation. For this purpose, a modified DGA measurement system was developed and built, which allows the detection of higher-value hydrocarbons in addition to the conventional fault gases in different types of insulating liquids. Initial tests showed the general generation of higher C4 and C5 hydrocarbons as well as further C3 hydrocarbons depending on the transformer fault type. Two transformer fault types were reproduced in the laboratory, thermal and electrical faults. Large differences were apparent, when comparing the gas behavior of mineral oils to alternative insulation liquids. Furthermore, different fault gases were formed in the two fault types and correlations between temperature and energy for generation of the higher valence hydrocarbons were also obvious.