An engineering approach to assessing static electrification phenomena in power transformers is presented. A theoretical model is developed to estimate streaming currents and space charge accumulation around the hydraulic oil flow loop in a three-phase transformer. The streaming current model is applied to a 345-kV, 240-MVA three-phase auto transformer with shell form winding design. The influence of space charge on the electric field and potential distributions in the upper plenum is modelled by a one-dimensional theory. Significant electric stress enhancement at coil edges on alternate half cycles of applied a-c stress is predicted for the 240-MVA transformer. Potential failure modes in power transformers due to static electrification are discussed.