Study On The Variation Characteristics Of Soil Electrical Parameters And The Influence On Grounding Device’s Performance

An accurate numerical calculation and experiment research of grounding characteristics is the fundament of grounding design and lightning protection of power system. Nowadays, the change of soil electrical parameter is seldom analyzed in calculation of grounding characteristics, even constant value is adopted to simplify. It leads to the inaccurate grounding model and inaccurate conclusion. So it’s necessary to do research on the change characteristics of soil parameters, and use them in numerical calculation to analyze the imfluence on the grounding system.The soil dynamic ionization process under impulse current has great impacts on the impulse performance of the grounding system of the transmission lines. So it is necessary to reflect the dynamic physical process in the numerical calculation. The soil resistivity around the grounding conductor is an important parameter, and it changes with spatial distribution and time domain. So the change of soil resistivity is researched in this article by the experiments of the soil nonlinear ionization. The nonlinear relationship between the soil resistivity before soil breakdown and electric field in soil is built, the change rules of spark discharge, arc breakdown, soil resistivity in the ionization process on the impact of impulse current are analyzed, the finite element model(FEM) of the grounding devices containing the influence of the soil ionization is built. In order to solve the problem of the convergent difficulty, caused by the resistivity jumping on the medium interface(between the grounding conductor and soil, the nearby interface between the two layers of the soil), the Green formula and Divergence theory are used to reduce the order of the partial differential equation, meanwhile,the smoothing step function of continuous partial derivative is used in the transition. By the introduction of geometric coordinate transformation into the layered and blocking soil, the semi-infinite open region can be transferred into a bounded closed domain,it also can ensure the calculation accuracy and reduce computation. A criterion function is used to judge any unknown position before transformation in calculation region,which is used after the transformation. So the problem that the soil interface is mapped into complex ellipsoid domain is well solved, which is caused by the distortion after the introduction of geometric coordinate transformation. Also, the parameters in calculation can be determined accurately. In this article, the validity of this model to calculate the impulse properties can be verified by simulation and test results.The frequency dependence of soil parameter should be considered to evaluate grounding performance accurately. So the frequency dependence of soil electrical parameter with different water content is measured in the 50~106Hz range, the change rule of soil resistivity and relative permittivity with the current frequency variation is obtained. Also, both the function relationship between soil resistivity and the frequency of the injection current and the function relation between permittivity and the frequency of the injection current are concluded by mechanism analysis and numerical fitting. Furtherly, the finite element model of the grounding devices including the frequency dependence is built based on the differential form of Maxwell equations, and the influences of the frequency dependence of the soil resistivity and permittivity on the impulse performance of the grounding electrode are calculated and analyzed.While DC power transmission system is running, the current disperses into the surrounding soil through grounding electrode, then the temperature of soil will rise. The thermal parameter and soil resistivity have great influences on the temperature rise of the grounding electrode, so the temperature rise process of grounding electrode is analyzed systematically in this paper. The method of AC quadrupole is used in the soil temperature rise tests on three kinds of typical soil of the area where DC grounding electrodes widely distribute. In the test, the quantitative relationship among the soil water content, resistivity and temperature is deduced, and the relationship between the soil thermal parameter and temperature is inferred by the classic formula of the thermal parameter. Then, the finite model including the temperature rise characteristics of soil is built. In this model, the change of soil resistivity and thermal parameter with temperature are calculated, and the temperature rise of the DC grounding electrode is analyzed systematically. The results show that the model can reflect the temperature rise process of grounding electrode; there is a turning point in the temperature rise process of DC grounding electrode; after the point, the temperature of the grounding electrode rises sharply. So we proposed that the maximum temperature should be controlled in the smooth stage before the turning point of the temperature rise curve.The results in this paper can provide theoretical basis and data support for the evaluation of grounding performance and they are significant in academic research and engineering practice.