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Monopole HVDC systems create dc current flow in the earth, which can cause the grounding location potential to rise relative to infinite spot and alters the working point of the transformer core. Because of the nonlinearity of the transformer core, the exciting current produces the amount of harmonics under dc bias. Past research has focused almost on simulation and testing dc bias problems of single-phase transformers. However, due to the complicated magnetic circuit structure in a three-phase five-limb transformer, there are some difficulties in studying its dc bias problem. In this paper, a new model and algorithm are proposed to find the nonlinear characteristics for the three-phase five-limb transformer under dc bias. Maxwell's equations are used to replace the magnetic circuit model of the transformer. By combining an electrical circuit with the nonlinear characteristic curve of the core, dc bias problems for a three-phase five-limb transformer are studied. Results show that the waveform of the no-load current and magnetic field intensity of the transformer are distorted under dc current inrush, and low-order harmonics increase with increasing dc current. Finally the interior and exterior nonlinear curve characteristics under dc bias are discussed.