Impact of soil structure adjacent to ground electrodes of UHVDC power transmission lines on DC bias of power transformers

During the period of the 12th Five-Year Plan of China, the construction of UHVDC power transmission projects will be widely carried out and a lot of new technical problems will appear in the construction. Firstly, a horizontal two-layer soil model was built and a formula to calculate surface potential of the proposed model was given. Taking six different types of soil for example, DC currents with magnitudes of 3000A, 4500A and 5000A were respectively led into the soil via the ground electrodes, and the surface potentials at the point with different distance to ground electrodes were calculated; then based on the surface potentials the DC bias currents of power transformer were calculated under different injected DC currents and the impacts of DC bias current of power transformer on the distortion rate of transformer magnetizing current were analyzed. Calculation results showed that the higher the soil resistivity, the more the DC current flowed into power transformer, thus the severer the caused DC bias would be. Taking two-layer soil for example, when the soil resistivity of the second layer of the soil increased from 1000 Ω·m to 3000 Ω·m, the distortion rate of transformer magnetizing current increased from 24.2% to 36.5%. With the increase of the distance from the ground electrode to the neutral of the transformer, the surface potential decreased; when the distance was further than 60km, the surface potential approached to zero. During the site selection of the substation, it is necessary to calculate the DC bias and the distortion rate of transformer magnetizing current according to soil structure.