The dissolved gas analysis process is among the most widely used methods of diagnosing internal transformer faults. In Japan, judgment criteria have been established from the results of a questionnaire survey on DGA for 1,324 units of operating power transformers. The traditional dissolved gas analysis method of identifying abnormal modes perdorms less well at distinguishing between abnormalities in the winding, which could lead to a transformer shutdown, and the core. Abnormalities in the winding should require immediate intervention because they could develop into dielectric breakdown faults. A gas generation test has been conducted by the conventional method and relationship between equivalent overheating areas and C2H4/C2H6 ratios. And ldquohigh-accuracy dissolved gas analysis methodrdquo implements internal diagnostics through microanalyses on the ldquoppbrdquo order to possibly improve the accuracy with which to locate gas sources in transformers. As a result, improved accuracy to locate overheating in transformers is obtained.