Research On The Frequency Characteristics Of Grounding Device Considering Soil Frequency-dependence

The development of the electric power industry, leading the increasing system capacity and voltage level. In recent years, the EHV(extra-high voltage) transmission technology and UHV(ultra-high voltage) substations and other applications become more and more widely. Because of the EHV transmission lines span a wide range,when lightning transmission lines easily lead to a larger area of the fault. In addition,when the electrical switch of the UHV substation operation or other failure occurs, it will produce the higher frequency current. As an important part of the power system,the grounding device can discharge the fault current quickly when lightning or other failure occurs, it can provide protection for the safe and stable operation of power systems. Therefore, it is very important to analyze the frequency characteristics of grounding device when high frequency transient current injection.In the traditional grounding performance analysis, the setting of soil parameters is still in accordance with the parameters of power frequency. Actually the frequency range of the duration of impulse current is broad, during the process of calculating impact characteristics of grounding device, the high frequency current should be taken into consideration. First, this paper describes the mechanism of frequency-dependence of soil electrical parameters, summarized the test measurements about soil parameters and the frequency dependent formula at home and abroad, then select the Visacro-Alipio frequency dependent formula to build the numerical model in this paper. Then using COMSOL Multiphysics establishes the grounding device frequency characteristic finite element model involved soil frequency-dependence.Based on the full wave Maxwell equation considering the displacement current and induced current to describe the diffuser effectively. The open boundaries of earth are processed by introducing a spatial transformation which translates the semi-infinite space into the finite space.In order to solve skin effect problem on the high frequency condition, the model considers impedance boundary condition. Through the model in this paper calculate the grounding parameters of grounding grid and compare the results with the field test measurements of North China Electric PowerUniversity and CDEGS results to verify the accuracy of the model. Finally, using the model above-mentioned to analysis the grounding performance of the typical grounding device. When the frequency of the injection current is 50 Hz, 1MHz,2MHz and 4MHz, calculate the current density, potential distribution in the soil,and the potential distribution along the grounding conductor, the grounding resistance respectively with and without the soil frequency-dependence. The results showed that: when considering the frequency-dependence properties of soil, the ground potential and grounding resistance is reduced, the current density of the soil increased, and as the frequency increases, this phenomenon is more obvious. In addition, considering the soil frequency-dependent has greater impact in the high soil resistivity areas.