Calculation of Excitation Current and Loss of 110kV Three-phase Transformer under DC Bias

Power transformers are the key equipment in the power grid. Direct current (DC) bias of transformer, which are often derived from solar storms, high voltage DC (HVDC) transmission and subway lines, poses a serious threat to the stable operation of the power system. To study the mechanism of the loss increase under DC bias, a magnetic-circuit coupling model was designed based on duality principle and applied to calculation of the excitation current of 110kV three-phase three-leg under different DC current injection. Subsequently, a three-dimensional (3D) simulation model of the 110kV transformer was built in which the losses of different parts under various DC currents were calculated. It is observed that the average value of the three-phase excitation current peaks increased approximately linearly with the increase of DC current. Different increasing trends appear in the loss of different components consisting of clamps, pull plates and oil tanks, which means that different temperature rises can appear in the above components probably causing local overheating. Based on the obtained results, it is necessary to distinguish the losses of different parts and further evaluate the limit of the transformer tolerance to DC bias current.