Acceptable direct current in three-phase power transformers: comparative analysis

This paper presents a comparative analysis of the temperature rise in three-phase core-form and shell-form power transformers in the presence of direct current. The temperature-rise assessment is compared on the basis of direct-current injection tests in single-phase core-form transformers, with a nominal voltage rating of 735 kV and nominal power ratings of 370 MVA and 550 MVA. The theoretical analysis of the saturation of three-phase transformers in the presence of direct current is based on the study of their magnetic circuit. The principle of duality was applied to obtain equivalent electric circuits simulated using the Electromagnetic Transients Program (EMTP) and validated using experimental results.