Oil-Forced Versus Oil-Directed Cooling of Power Transformers

In this study, an analysis of the oil-forced (OF) and oil-directed (OD) cooling of oil power transformers was performed. The starting point was a case study of an OF transformer with a declared rated power of 360 MVA having thermal problems in operation. The effects of possible improvements of the cooling were analyzed, where the limiting factor was the geometry of the transformer (winding, core, and tank); cross sections and cooling channels of the windings and the core were also resolved. All calculations, showing an influence of varying characteristics of the cooling (the pump, the cooler, modification from OF to OD construction), were realized with software based on a detailed thermal-hydraulic model. This method gives all relevant oil flows (including oil bypass between the winding and the tank) and all temperatures for a specified configuration and specified parameters of the cooling system. Based on the results, the limits of OF cooling in general are clearly described in this paper. Finally, the effect of applying oil guiding elements inside the windings is illustrated and discussed.