When HVDC system operates in monopolar mode and uses the earth as a return path

partial entering ground current may flow into the power transformers

which neutral point grounded

and this will bring a series unfavorable consequences to system operation and transformer. Using electromagnetic coupling method

this paper establishes model of the single-phase transformer with considering the variation of load

and analyzes the effects of such DC current on a single-phase core-form power transformer

such as increment of current harmonics and DC flux in the iron core. The simplified transformer model

under unloaded condition

is analyzed based on analytic method to verify its validity. Furthermore

the influence of different transformer operation status has also been discussed. The simulation results indicate that lower operation point can be more advantageous against the DC bias effects.