The liquid-insulated power transformer is the most critical and expensive component of the power supply system. In order to improve monitoring of the state of health of these devices (transformers) installed on the power grid in the bank of the large metropolis of Abidjan, dissolved gas analysis (DGA) was used. Conventional interpretation techniques were developed to diagnose transformer oil. The database we used consists of those of the Compagnie Ivoirienne d'Electricité (CIE) in 2018. It includes 25 samples containing analysis of the five main dissolved gases (H2, CH4, C2H2, C2H4, C2H6). The five defect classes we considered, namely partial discharge (PD), D1 and D2 electrical defects, T1&T2 and T3 thermal defects, are taken from the IEC and IEEE. For interpretation, five methods were adopted, namely the IEC, Doenenburg, Rogers criteria using the ppm gas concentration ratio technique, the Duval triangle using percentage gas concentrations and the IEEE criterion using ppm gas concentrations. The highest success rate of 72% was obtained when using the Duval triangle criterion. But the greatest consistency with a rate of over 70% was observed using the IEC ratio criterion.

Electricity is missing or even unavailable in many localities in our country; while we have many power plants with large capacities (powers).This is partly due to energy losses in the networks. The transformers that are the essential components in the transmission and distribution of electrical energy have an aging population. The degradation of their paper/oil insulation system in the active part is the main cause. The dielectric losses in transformers due to this degradation (aging) are not taken into account in determining the efficiency of transformers. This paper shows that the dielectric losses in oil-impregnated paper-insulated transformers are becoming more and more important and are not to be neglected in the calculation of the efficiency of transformers due to their large number in electrical networks. With the help of DIRANA (Dielectric Analyzer), we made direct measurements of the dissipation factor and the capacity of the paper/oil insulation on real transformers in service in the electrical network in Côte d'Ivoire. In the following we calculated the dielectric power losses.