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

Tripping out caused by lightning is an important reason of unplanned outage for transmission line and tower. Improving the lightning withstand level of transmission line and reducing the rate of lightning trip has a great significance on safe and stable operation of power system.The impulse characteristic of transmission line tower determines the effects of lightning protection of transmission line. The impulse characteristics directly depend on the flowing status of lightning current in soil. Tests have discovered that the soil electrical parameters have a characteristic of frequency dependence, namely, with the injected current frequency increases, the soil conductivity increases while the relative dielectric constant decreases. However, generally this law was ignored in the grounding impulse calculation: both the soil conductivitys and relative dielectric constant e keep power frequency constant.As the bandwidth of lightning usually ranges from several MHz even up to 10 MHz, the high frequency components are far more than power frequency component. Taking the power frequency parameters into grounding transient calculation will lead the calculated impulse grounding resistance to be overestimated more than 40% compared with actual value. Therefore, in the lightning protection and grounding modeling, it’s necessary to consider the soil frequency dependence to improve the calculation accuracy.In the frequency domain calculation, finite element method(FEM)as one kind of numerical methods, can effectively solve the electromagnetic field problems, is suitable for the study of grounding system frequency domain performance.In this paper,through combining the FEM with the Fourier transform, and introducing into the latest research achievement---Visacro-Alipio soil electrical parameters frequency dependence formula, a FEM model of grounding system that considering soil frequency dependence was established. Using the AC/DC module in Comsol software to model a 10-m level grounding electrode subjected to typical Heidler first and subsequent return lightning currents according to the test conditions in literature to validate the accuracy of the established model.To further research the effect of soil frequency dependence on the calculation of grounding grid:Firstly, conducting the simulation analysis of soil frequency dependence effect on a 3-m vertical grounding electrode which was respectively buried in 4 soils with the conductivity of 0.01 S/m, 0.002 S/m, 0.001 S/m, 0.0005 S/m. Analyzed the relationship between soil frequency dependence effect and initial soil resistivity, through comparing the calculated GPR of two cases including consideration of the soil frequency dependence and without consideration of the soil frequency dependence to summarize the application situation of soil frequency dependence, namely, for which initial soil resistivity, it’s need to consider soil frequency dependence in the transient calculation. Then, simulating a 2′3 style grounding grid(change the buried depth) to evaluate the effect of soil frequency dependence on the grounding grid transient calculation. Finally, taking the soil frequency dependence into account the grounding calculation to analyze the effect of soil frequency dependence on the grounding device effective lengtheL and the impact factor a, these work can provide an important theoretical reference for optimizing the design of grounding device of transmission tower.