Study of DC bias of power transformer based on UMEC model

DC bias of power transformer can lead to distortion of excitation current, reactive power increasing and winding overheating, which will seriously affecting the safe operation of the power grid. To study DC bias is of great significance on the safe operation of transformer. Based on the Unified Magnetic Equivalent Circuit (UMEC) model of transformer, the excitation current and its harmonic when DC bias occurs had been analyzed by using PSCAD in this paper. The tolerance abilities of transformers with different structures under DC bias have been compared. With finite element software of Maxwell 2D, the model of transient electromagnetic field of single-phase transformer had been established to analyze the internal magnetic field variations in core and yoke under different DC currents. It can be seen that the leakage flux gradually increased with the saturation of the core, which lead to the increase of eddy-current loss. The highest temperature appeared in the junction of yoke and core. The influence of the transformer DC bias on the single-phase transformers is the most severe, and the least severe on 3-phase 3-limb transformers.