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The ground potential differences due to the ground return current of a long-distance high-voltage direct-current (HVDC) system may make the grounded transformers in AC substations under DC bias. Complex image method is used to calculate the potential distribution in multilayer earth between ground electrodes of an HVDC system. The effects of the deep earth layers, including the crust, the mantle and the core, on the potential distribution along the earth's surface are analysed. Analysis results show that although the effects of the deep earth layers on the potential distribution near the ground electrode can be neglected, it should be taken into account for the ground potential rise over tens of kilometres away from the ground electrode, which is important for analysing the transformer's DC bias due to the ground return current. The method and the results are useful for analysing the influence of the ground return current from HVDC ground electrode on systems that use grounding technique.