NEW DEVELOPMENTS IN TRANSFORMER COOLING CALCULATIONS

摘要

Thermal aspects affect transformer design. Therefore precise temperature calculations ensure good quality and long life expectancy of transformers.The temperature gradient between conductor and oil consists of a gradient inside the solid winding insulation and a gradient inside the boundary layer at the winding surface. The gradient inside the solid insulation depends on the thickness of the enamel, paper insulation and oil pockets between conductor and paper wrapping. The heat transfer at the winding surface is determined by the cooling conditions. Two general approaches are in use: the forced convective cooling (OD) and the natural convective cooling (ON).Another important thermal design aspect is the longitudinal temperature gradient inside ON-cooled windings. This value results from a balance of the natural convective oil driving forces accelerating the oil flow and hydraulic resistances in the oil circuit slowing down the oil flow. Further hydraulic resistances outside the winding are able to reduce the oil flow through ON-cooled windings additionally. Longitudinal temperature gradients measured in heat run tests with variable hydraulic resistances inside the winding were evaluated.Heat transfer coefficients at the winding surface were calculated using heat run test results of various ON- and OD-cooled winding types. Two different general approaches were made for ON- and OD-cooled windings. The characteristic of the heat transfer coefficients were calculated for each winding type.Further temperature rise experiments with an ON-cooled disc winding operated with variable heat flux densities were performed in order to investigate the influence of the heat flux density on the cooling efficiency of the boundary layer.