DC transmission system in monopole-grounded return operation mode may lead to DC magnetic bias of some transformers in AC power system. Based on the direct field-circuit coupled method

the DC magnetic bias calculation models of ±800 k V Xi-Zhe and ±800 k V Ling-Shao UHV transmission project were built. The modeling range and the ground grid parameters were determined

while the soil model was modified as well. The calculated values of DC biasing current were compared with the test results and the total relative average error of each testing station was no more than 5%

verifying the accuracy of the calculation model. The DC biasing current distribution regularities of the substations above 110 k V around the grounding electrode was calculated and analyzed further. The results indicate that the DC current of the substation decreases as the distance between the substation and the grounding electrode increases. When transformers are in parallel operation

the DC current of each transformer decreases greatly. The DC current flowing through the starting and terminal substations will increase with their distance. This research can provide reference to the calculation of DC current distribution in large AC grid and the management of DC magnetic bias.