Study On Transient Characteristics And Evaluation Technology Of Grounding Grid Of Hydropower Station

Due to the distribution of water resources,most of the hydropower stations are located in the mountains and valleys.The soil environment of hydropower station is more complex than that of the ordinary substation,and its grounding system is composed of a plurality of grounding grids in the station.For the grounding system,the grounding parameters such as the impulse grounding resistance and the transient ground potential rise are closely related to the soil environment,the topological structure and the electrical performance parameters of the conductor.According to the characteristics of hydropower station,the variation rules of the transient characteristics of grounding grid with the soil characteristic parameters are studied based on the horizontal two-layer soil model,the vertical two-layer soil model and the local heterogeneous soil model,respectively.According to the simulation results,under the horizontal two-layer soil,the impulse grounding resistance increases nonlinearly with the increase of upper soil resistivity and it is less affected by the lower soil resistivity.Under the vertical two-layer soil,the impulse grounding resistance increases nonlinearly with the increase of the soil resistivity of the side where lightning current injected,and the influence of the other side is smaller.Under the local heterogeneous soil,when the peripheral soil resistivity is certain,with the increase of heterogeneous soil resistivity,the impact grounding resistance increases.The larger cross section of heterogeneous soil,the greater impulse grounding resistance of the grounding grid,and the more significant impact on the impulse grounding resistance.Then,the lightning transient characteristics of grounding grid with vertical grounding electrodes and double-layer grounding grid are simulated and calculated.It is found that the impulse grounding resistance decreases obviously with the increase of the number of vertical grounding electrodes within a certain range of the lightning current injection point,and it is best way to reduce the impulse grounding resistance by adding the vertical grounding electrodes sparsely outside and densely inside.For the double-layer grounding grid,it is helpful to reduce the transient potential of grounding conductors on the upper grid when the lightning current injection point is close to the connecting conductor,but it will lead to the increase of the transient potential of grounding conductor on the lower grid.So it is necessary to consider the insulation performance of the primary and secondary equipments connected with the upper and lower grounding grids in the arrangement of the grounding wire.Finally,to study the secondary equipment fault of a 220kV switch yard,the model of grounding grid is established,and the transient potential distribution of the conductor in the fault zone of the grounding grid is simulated and the transient potential difference of the grounding conductor is analyzed,It is found that the increase of the conductor potential in the vicinity of the lightning rod is not a threat to the insulation of the primary equipment,but the transient potential difference of the conductors near the lightning current is very large which exceeds the insulation tolerance level of the secondary equipment.It is recommended at least 3 vertical grounding electrodes should be added to the grounding grid in the range of 1m of the grounding wire of the lightning rod or install the lightning rod in the position which the distance is more than 15m from the terminal box.