引外接地对降低接地网接地阻抗的作用分析

工程上常采用引外接地的措施,一般根据经验在离主接地网1~2km范围内另敷设一辅助接地网,并采用扁钢将此辅助接地网与主接地网相连,但国内现行接地设计方法及故障电流下地网等电位计算模型均不能计算引外接地电阻。为此采用接地网工频接地参数分析软件,计算了均匀土壤中引外接地网对降低主接地网接地阻抗的作用。结果表明,辅助接地网与主接地网间的距离超过一定数值后,辅助接地网的作用可忽略,且在土壤电阻率较高的地区,引外接地具有较好的降阻作用。

Grounding Impedance Analysis of Grounding Grids Connected With Buried Conductors

During a fault of an electric power system,the failure current drains quickly to the ground.The maximum ground potential rise can be controlled to ensure adequate safety for people and to protect the equipment.Therefore,an effective grounding grid is very important for the safe operation of the electric power system.Degreasing a grounding impedance of a grounding system in high resistivity soil is often a formidable task.In order to decrease the grounding impedance,it is usual to use a main grounding grid that is connected with another subsidiary grounding grid 1~2km near the main grounding grid.Both grounding grids are connected with buried conductors.The function of subsidiary grid for decreasing grounding impedance of main grounding grid is defined as the subsidiary grid effectiveness,which excludes the decreasing grounding impedance function of connecting conductors.The grounding impedance of grounding grids connected with buried conductors in the uniform soil is calculated based on numerical calculation of large substation grounding grids in industry frequency.The results show that the subsidiary grid effectiveness gets lower as the distance between grounding grids get longer and subsidiary grounding grid can be neglected when the distance between two grounding grids is beyond certain amount of distance.The subsidiary grid effectiveness is increased as the connecting conductors increase.The subsidiary grid effectiveness can be greatly increased if copper instead of iron is used as connecting conductor materials.The grounding impedance can be effectively reduced by subsidiary grids in high resistivity soil regions.A complex grounding system of a pumped-storage hydro generating station is designed.The calculation method presented in this paper gives scientific foundation for practical engineering.