Proposed heptagon graph for DGA interpretation of oil transformers

Abstract

Dissolved gas analysis (DGA) is one of the most acceptable methods used for detecting and evaluating gases dissolved in mineral oil and also in the solid insulation of internal incipient faults in the power and distribution transformers based on the oil samples. DGA has been proved the most accurate method for condition assessment of power and distribution transformers. The fault types depend upon type and concentrations of each gas dissolved in oil. IEC, Rogers, Doernenburg and Duval techniques, in spite of their importance in diagnosing certain types of faults, do not consider the concentrations of carbon monoxide (CO) and carbon dioxide (CO2) to evaluate cellulose insulation degradation. Some of these techniques do not take into account the impact of ethane (C2H6) and hydrogen (H2) for the evaluation of fault types. A new graphical method “Heptagon” based on seven gases produced in faulty transformer oil decomposition is presented. Based on about 452 test samples of transformer oil DGA, the Heptagon figure is drawn and the zones of different fault types are determined. The suggested technique is tested and compared with other techniques.