It has been known for many years that the grounding resistance of a concentrated electrode drops when it is subjected to a high current charge. This helps reduce the ground potential rise. The degree of the resistance depends on the magnitude of the ionization gradient of the soil E/sub o/. Based on both a theoretical analysis and a critical review of the large number of available measurements, this paper recommends that E/sub o/ be taken equal to 300 kV/m for typical soils. This is significantly less than the 1000 kV/m value used by some authors. Graphs are also given describing the behaviour of the rod electrodes which are used in many field installations. >

The soil ionization gradient associated with discharge of high currents into concentrated electrodes

Grounding is the fundamental measure of lightning protection for power systems, industries, and buildings, and the lightning impulse behavior of grounding electrodes plays a very important role. How to correctly consider the lightning impulse characteristics of grounding electrodes is the fundament in lightning protection design. This paper summarized the progress in lightning impulse characteristics of grounding electrodes with soil ionization, including the images of soil ionization, the residual resistivity of ionized soil, the critical breakdown electrical field, the impulse breakdown delay characteristics of soil, and the impulse breakdown mechanism of soil. Finally, the numerical simulation methods for lightning impulse characteristics of grounding electrodes are classified, and some key performances of grounding electrodes under lightning impulse were presented.

Progress in Lightning Impulse Characteristics of Grounding Electrodes With Soil Ionization

Fractal simulation of soil breakdown under lightning current

The predictions of a simple model of water vaporization in soil are determined and compared with previously published data for the time for the initiation of streamers which leads to soil breakdown under pulsed high-voltage excitation. This model is capable of predicting the relatively long initiation times which have been observed under certain experimental conditions and also provides an explanation for the strong, experimentally determined, dependence of the initiation time on electric field.

Initiation of Electrical Breakdown of Soil by Water Vaporization

Experiments have been carried out to study soil ionisation and the variation of the critical ionisation gradient against the soil resistivity. Compared with relevant experimental results of other researchers a satisfactory convergence has been proved. An accurate estimation of the critical ionisation gradient has been achieved, of the order of 200 kV/m, following the finding that the critical voltage gradient for wet soil decreases to approximately 35% of the corresponding value for dry soil.

Soil ionisation under lightning impulse voltages

The pulsed electrical breakdown characteristics of soil between a hemispherical and a planar electrode have been measured in meter-size geometries for three soil types with water contents from 1 to 7% by volume. The threshold electric field for breakdown is a function of the soil type and the water content, varying from about 1 to 2 MV/m locally near the hemispherical electrode. The delay time (tD) between application of the voltage and the occurrence of the breakdown is primarily a function of the applied voltage, which varies approximately as tD-3/8 for these samples. After breakdown, the impedance of the soil decreases with time until it reaches a minimum impedance of 0.2 to 5 ?/m for these meter-size samples, depending on the current that the pulser can drive through the arc. The discharge arc prefers to travel underground rather than on the surface of the soil unless there is some low-impedance object on the soil surface.

Ian Smith

Papers

Electrical Breakdown Characteristics of Soil