Transient Thermal Condition of Natural Oil-cooled Disc-type Winding

The degradation of solid insulations in windings of a power transformer is most often due to thermal stresses. In this contribution, Computational Fluid Dynamics (CFD) is used to predict the hot-spot and top-oil temperatures by applying various boundary conditions during transient states following, which are validated with temperatures measured from an experimental setup. The experimental model, which consists of a disc-type winding with 20 discs and four turns, is divided into 3 passes and equipped with PT-100 temperature sensors. Heating cartridges are installed into each conductor of the winding model to supply the electrical heat losses in the winding turns. In order to consider, the variations of the thermal characteristics of oil, its properties are changed at each time step by increasing the temperature. This investigation takes into consideration the transient thermal behaviour of natural cooling method inside winding model at various starting oil temperatures 40°C and 80°C. Transient curves of top - oil and hot - spot temperatures are compared to identify the time constants. On evaluating the measurement data, the locations and the temperature time constants of hot - spots at different starting oil temperature are found to be the same. Moreover, it is observed that there is no flow reversal or blockage caused by separation eddies, which can be attributed to low oil flow rate through horizontal channels. Finally, it is noted that the average volume temperatures of conductors and top-oil temperature reach steady state thermal condition over time.