Calculation of DC current distribution in AC power system near HVDC grounding electrode in subregional layered soil

Abstract

The DC current from the grounding electrode of a HVDC system may result in great ground potential differences in a large area, and may flow into the widely distributed systems with dispersed grounding points such as AC power grid which may disturb the systems. Soil resistivity is the most important factor that affects the current distribution. The heterogeneous soil has a subregional layered structure on the whole. In this paper, a numerical approach coupling the boundary element method with complex image method is developed to calculate the current fields in the subregional layered soil. By using indirect boundary element method, the subregional layered soil is turned into a uniformly layered soil in which the electric field is calculated with the help of complex image method. The calculation scale is much smaller than that just using the boundary element method, and the complex image method can be extended to calculate the electric field in soil with complex structure. Then, an equivalent circuit network in the complex soil is extracted and is coupled with the equivalent circuit of AC power grid, with which the DC current distribution in an AC power grid around a HVDC grounding electrode is analyzed. The results show that even in the area where the ground electrode is located, the ground potential gradient in the subregional layered soil is different from the result in the uniformly layered soil, which leads to a difference in the current flowing through the neutral point of the substation. The closer the DC grounding electrode or the substation is to the interface of the subregional layered soil, the greater the effect on the ground potential around the DC grounding electrode and the DC current flowing through the substation.