Abstract: The power transformer is one of the most important and critical assets involved in the grid and, at the same time, one of the most expensive. Several transformer condition parameters allow to assess the degradation of assets. These help in the decision-making process on the operation, repair, refurbishment, or replacement of transformers. Dissolved gas analysis (DGA) is one of the most commonly used methods to manage maintenance and establish the health index of power transformers, as well as to identify the type of fault. This paper examines and explores the studies related to graphical methods for the identification of faults in power transformers that had been developed over the last almost 50 years. The main types of faults and the sub-types presented in the analysed studies are compiled in this paper. The main differences between the methods in terms of their graphical representation, number of gases used, type of data used from the DGA results, and number of faults and sub-faults identifiable in each of them, are also presented. The application of the reviewed methods was carried out using two real DGA results.

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