One of the challenges that hasn’t been solved effectively in the study of direct current (DC) bias is how to establish the proper soil model to calculate the earth surface potential (ESP) accurately in large scales. In order to solve this problem, a soil model named as three-dimensional combined-layer soil model (TDCM) in which the layered soil might have any number of layers was introduced. Complex image method was applied for the derivation of the Green’s function of this model, and the general analytical solution was derived when the source point was in any soil layer. Based on the model, the effect of the parameters of vertical medium layer, including resistivity, thickness, depth, and location, on the ESP was investigated when a high-voltage direct-current (HVDC) system used earth as its current return path. Besides, the results were compared with that of horizontally multilayer soil model (HMSM) and two-dimensional heterogeneous structure soil model (HSSM). It was found that the different soil parameters have different effect on the ESP distribution, so the complexity of vertical soil layers should be considered in calculating the ESP. Moreover, the actual soil is able to be modeled by TDCM more accurately than HMSM and HSSM, and the ESP is able to be calculated more accurately based on the proposed model. The model and the results are useful for selection of substations near the HVDC ground electrode.
