Modeling the saturation processes of a power-transformer core under simultaneous direct and alternating current passing through the winding

A mathematical model of a three-phase two-winding step-up power transformer with the winding circuit Δ/Yo with simultaneous ac and dc currents passed through the grounded winding has been developed. The proposed model is represented by a system of differential equations, which are nonlinear because of the nonlinearity of the inductive resistances of the magnetization curve. The calculation results for a TDTs-400000/242/20 shell–core transformer are given. The model takes into account the effect of dc on the saturation of the power-transformer core, which is especially important under asymmetrical modes that can occur during thunderstorms, seismic activities, or space-weather disturbances.