Abstract: The influence of magnetic bias current on the excitation characteristics of near-field transformers in

the DC grounding electrode from bipolar to unipolar earth mode was discussed. Based on the principle of field-circuit coupling，the DC magnetic bias co-simulation model of transformer was established and analyzed the action

mechanism of grounding electrode unipolar earth mode on AC power network and transformer excitation

characteristics. Through analysis of the simulation model，the results show that when the DC grounding pole from

bipolar to unipolar earth mode，magnetic bias current flowing through the AC power grid is pulsating DC current that varies with time，its instrusion into transformer winding causes the excitation current of transformer winding

fluctuation change，with the increase of pulse DC amplitude，between transformer excitation current peak and valley difference also gradually increased. When the operation mode of DC grounding electrode is adjusted，the simulation model can provide certain babis for the level monitoring of AC power network DC magnetic bias level and the evaluation of transformer excitation characteristics.

      Key words: grounding electrode；transformer；excitation characteristic；DC bias；pulsating DC