Study On Current Distribution Rule And Parameter Calculated Method Of Grounding System In Large Current And Complex Geological Conditions

'West-East Electricity Transmission Project'and'North-South Electricity Transmission Project'are main ways used to solve the distribution imbalance problem of power resource and load in China. Ultra-High Voltage (UHV) and Extra-High Voltage (EHV) transmission system have advantages such as high transmission capacity and long working distance. They are been built as trunks of Chinese Electricity Grid. Many UHV (EHV) substations and converter stations are located in Yunnan, Sichuan and Tibet border area. The geological conditions in these places are very complex. Soil resistivitys in some place are high than 2,000Ω.m. Short-circuit currents of UHV and EHV substations are usually large than ten thousand Ampere. Because of these problems, new researchs must be done in grounding engineering. The conductivity of soil, the current distribution characteristics of grounding system and the optimization methods are discussed in this thesis.Grounding system performance is affects by soil resistivity directly. Soil is a typical porous medium. Normally soil skeleton is composed by poor conductor and its resistivity is decided by pore water. A formula is given out to calculate resistivity of pore water. Research in this thesis presented an influence law of temperature to soil resistivity. At 0℃, pore water in soil is freeze and soil resistivity become very high. A diversification rule of soil resistivity is found out when DC current density increased by experiments. DC current makes the ion move out from soil, it make the concentration of the ions decline and soil resistivity increase. If DC field is large enough, water molecules will ionization and soil resistivity is drop down.Passive two-port network model is advanced to analyzing Double-grid grounding system in this thesis. Equivalent circuit of grounding system is obtained by the relationship of two port network input and output. A formula of double-grid system grounding resistance was got by this method. Relative error of result calculated by this formula to by simulation is lease than 0.1%. Parameters of grid and distance between two grids have effect on Double-grid grounding system. These influences have been analyzed by using passive two-port network model.Grounding grids of Extra High Voltage (EHV) and Ultra High Voltage (UHV) substations and convert stations are usually buried in two-layer soil. Grounding resistances of grids buried in uniform soil and two-layer soil are studied in this thesis and relationship functions between them are obtained. They do not relate to soil reisitivity but to the soil resistivity ratio of two-layer soil. Formulas to calculating grounding resistance of grid in horizonal two-layer soil or vertical two-layer soil are got by using these relationship functions and uniform soil grid resistance formulas. Results calculated by them are very close to simulation results. Their relative errors are less than 10%.Unequally spaced of grounding grid conductor can optimize the function of current distribution. Simulation models of square grounding grid are built in this thesis. Optimal location of each conductor is got by simulation. A relationship between conductor Number and its location is fined out and a new optimization formula is derived from it. This formula also works when the quantity of conductor is changed. The application of the formula to rectangular grid is studied in this paper. Optimization results are effect by the divided quantity of length and width. Best proportional of conductors and method to determine their quantity are obtained in this thesis.