Latest diagnostic methods of gas-in-oil analysis for oil-filled transformer in Japan

The working group in the Electric Technology Research Association (ETRA) in Japan has reviewed the actual status of conventional diagnosis by DGA in Japan and overseas. Various findings from the review are presented in this paper. (1) The required limits of detection and precision for gas-in-oil analysis are provided. (2) By classifying characteristic gases (C₂H₂, C₂H₄ ) which are present at the time of internal faults in a transformer and non-characteristic gases, three levels i.e., “Warning I”, “Warning II” and “Trouble” are set as judgment criteria. (3) Graphical representations of diagnostic chart determined for discharge by overheating, partial discharge and arc discharge by using the ratio of gases. (4) Other conventional, diagnostic methods by specific gases are also reviewed