The distribution of soil media in the compound layered soil structure is very complex, and it is difficult to describe and establish soil models at large scales. In the design and calculation of the ground electrode, a uniform layered soil model is generally used. Regardless of whether it is horizontal or vertical stratification, the calculation of the ground electrode is to solve the Green function of the soil model. The Green's function of the multi-layer soil structure can generally be obtained by the following Laplace equation satisfying certain boundary conditions: 20φ = (1) where φ is the potential. The cylindrical coordinate system and the boundary conditions of the soil interface can be used to obtain the Green's function form composed of infinite class numbers developed by Taylor's formula. Its physical meaning is to replace the boundary effect of layered soil with infinite multiple mirror images. The Prony method can also be applied, using a finite-term complex coefficient exponential series quasi and infinite order Taylor series, that is, the equivalent complex mirror image method. When studying the influence of the grounding electrode on the distribution of the ground potential, it is necessary to note that the ocean and land are medium of comparable scale, and the influence of the sea should be considered when establishing the soil model. Therefore, the soil model is a composite layered structure in a wide area, that is, the land and the ocean form a vertically layered soil structure, and the land itself can be regarded as a multi-layer horizontal layered structure. The solution of the Green function of this structural soil model is more complicated. But according to the physical meaning of the mirror image method, the analytical expression of the ground potential can be derived. The literature mentions an equivalent complex mirror image method to solve the Green's function in compound layered soil.
Compound layered soil structure Although the soil structure on land is complex and the soil resistivity varies unevenly, from the perspective of the entire AC-DC power grid crossing area, the ocean is much larger in area than the land, and the resistivity is much smaller than the soil. Therefore, the ocean is considered for grounding When the earth potential distribution caused by the pole-to-earth current is affected, the entire soil model is a composite layered structure as shown, that is, the land soil model is horizontally layered, and the land and ocean constitute a vertical layering. According to the physical meaning of the mirror image method, in the horizontal layer of land, the surface potential is caused by the mirror effect of infinite multiple power points; in considering the vertical layer of land and ocean, these power source mirror points can be regarded as the same soil The multiple power points in the layer are mirrored to the ocean layer according to the derivation of the vertical layered structure to meet the boundary conditions of the vertical layer. In this way, using the derivation in 1.2, the expression of the ground potential can be written.
The current distribution simulation calculation model under the compound soil model. When the DC single maximum loop is running, the potential difference at different stations causes part of the current to flow through the AC system through the grounding neutral point of the transformer. The AC system also changes the ground potential distribution, but it is extremely large. Part of the current still returns to the ground electrode through the ground, and the AC system has a limited impact on the ground potential. Therefore, this paper first solves the ground potential at any point within the range of the entire AC power grid based on the derivation of the above composite soil model; the AC network is a definite system, AC The DC path in the network is composed of the grounding resistance of the power plant or substation, the equivalent DC resistance of the transformer and the line resistance. According to the node voltage equation, the DC current injected into the AC system, that is, the DC current flowing through the neutral point of the transformer, can be obtained.
Calculation parameters Under the composite soil model, this paper simulates and calculates the DC current flowing through the grounding neutral point of each plant and station of the 500kV AC system in Guangdong Province when the DC single maximum loop is running. The interface is approximately determined according to the coastline, close to the Lingao and Daya Bay nuclear power plants. In the composite soil model, the horizontal soil resistivity of the two layers of land is 500 and 9000m, the thickness of the first layer of soil is 100m, and the resistivity of the vertical layer of seawater is 0.5m. The grounding electrode is selected according to the actual layout of the corresponding DC project. The parameters of Guiguang DC I Guangdong side are selected, that is, the grounding is very concentric double ring arrangement, the outer ring and inner ring diameter are 800 and 600m, and the buried depth is 4m. The equivalent grounding grid resistance of 500kV plant station is generally 0.4; 500kV neutral point grounding The DC equivalent resistance value of the transformer is taken as 0.10.2 according to the configuration; the line equivalent DC resistance is determined according to the actual wire type, length and number of loops.
Conclusion In this paper, the Green's function of horizontal and vertical stratification of soil is used to derive the analytical formula of the surface potential in the compound stratified soil structure based on its physical meaning through mirror image, so that the influence of the ocean can be quantitatively considered in the soil model. According to the simulation calculation results of the composite soil model, the surface potential near the ocean layer will drop to near zero potential under the influence of the ocean, resulting in more DC current flowing from the neutral point of the transformer near the ocean, affecting the normal operation of the transformer.
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