Modification of the CIGRE Formula for Calculating the Grounding Device Impulse Impedance Considering the Ionization of Two-Layer Soil
DOI:
https://doi.org/10.24160/0013-5380-2026-2-17-26Keywords:
lightning current, soil ionization, CIGRE formula, two-layer soil, grounding deviceAbstract
When lightning current spreads through a grounding device, it produces a strong electric field that causes soil ionization, which in turn causes a decrease in the grounding device impulse impedance. The CIGRE formula provides a simple analytical dependence of the grounding device impedance on the current, but it is applicable only in single-layer soil. A modified version of the CIGRE formula for two-layer soil is developed and investigated. The characteristics of ionization processes in two-layer soil are studied. The most significant decrease in the grounding device impedance is observed in soil with a high resistivity of the upper layer, in which the ionization processes are more intense in nature, and the ionization zone expansion brings the grounding device closer to the well-conducting lower layer. The critical ionization field strength in the single-layer model depends significantly on the current, whereas in the two-layer model it is almost constant, which was established in processing the experimental data and investigated theoretically. An equivalent single-layer soil is characterized not only by an equivalent resistivity, but also by an equivalent critical field strength, which can be significantly higher (if the upper layer conductivity is higher than the lower layer conductivity) or less than the standard values at high currents. For 110 and 220 kV double-circuit overhead power lines, a significant (tens of percent) decrease in the expected probability of the insulation back flashover has been obtained in the case of taking soil ionization into account.
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