Power transformer temperature evaluation for overloading conditions

The calculation methodologies of transformersʹ hottest spot and top oil temperatures are evaluated in this paper. Thereafter, a comparison between the calculated and the values obtained during the assessment of a transformer factory test was realized. The referred tests were conducted using an ONAF type, 30/40 MVA, 138/13.8 kV transformer, in which sensors with fiber-optic cables, to record directly the internal parameters, were installed. In a first series of tests, transformer loadings corresponding to 100%, 70% and 116.5% of rated power, were used. The tests were completed by applying values of a daily load profile with several hourly power levels. Additionally, in two of the hourly periods, high temporary loadings one of 160% during 30 minutes, and another of 170% during 15 minutes, were applied.