Variation of steady-state thermal characteristics of transformers with OFWF cooling in service

The paper presents the thermal model for top oil temperature calculation in transformers with forced oil and water circulation. The model is completely based on the heat transfer theory. The thermal parameters change in time, due to the sedimentation of substances from the water and the oil to the heat exchanger surfaces. Fouling factors at the water and oil sides can be determined based on measurements. The results of procedure application on the hydro power station block transformer 2 x 190 MVA/380 MVA are given in the paper. The model is aimed at estimation of transformer loading possibilities, but also at maintenance or reconstruction planning purposes.