Large Grounding Grids Of Substations Power Frequency Parameter Calculation

Along with the development of electric power system capacity, the value of failure current flowing into ground has been increased greatly. Particularly, after the construction of the three gorge power plant, 34kA can be reached. In order to control ground potential rise(GPR), the area of the grid may be very large and maximum diagonal of the grid can be over a kilometer. Efficient simulation numerical calculation methods and software are developed in this paper based on electromagnetic field and basic electric theory to improve the level of grounding system design.Based on method of moment and field-circuit coupling theory, a numerical calculation software is developed. The resistance and self-inductance of conductors as well as mutual inductance between conductors are taken into account. It can be used to calculate grounding parameters in industry frequency. Based on the software, many practical engineering problems such as the grounding impedances in industry frequency, touch voltages , step voltages and potential differences within grounding grids are analyzed.In order to decrease the grounding impedance, it is often to use a main grounding grid that is intertied with another grid 1~2km near the main grounding grid. Both grounding grids are connected with buried conductors. The calculation method presented in this paper gives scientific foundation for practical engineering.Copper is used as grounding grid material in developed countries, but iron is used as grounding grid material in China. The permeability and resistivity of iron are higher than those of copper. It is a problem for us to have great potential difference within a grounding grid. Because of nonlinear permeability of iron, it is very difficult to calculate accurately the grounding parameters. A method based on disconnecting linearization is used to calculate parameters of iron grounding grids.