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There are multiple discharges in one breakdown during a lightning incident. A lightning incident usually contains 3–5 return strokes. In this case, the lightning incident will not be well simulated by the first stroke. To date, a single impulse current waveform has been used to simulate the lightning impulse incident in research on the lightning impulse characteristics of soils, leading to some differences with observations of actual lightning discharges. In this paper, a time-delayed control technique and a controlled gaps-group isolation technique were used to produce the multiple high-impulse currents discharge to a soil sample, while nonintrusive X-ray imaging technology was used to obtain images of the discharge channel in the soil. The impulse characteristics of the soil under multiple impulse currents are studied. The differences between the discharge characteristics of the soil under multiple impulse currents and a single pulse are compared. It can be found that there has been a reduction in the effective soil resistivity under multiple impulse currents, and this reduction will be recovered over time. At the same time, a brief analysis of the discharge mechanism of the soil under multiple impulse currents is proposed, based on analysis of X-ray images of discharge channels. [ABSTRACT FROM AUTHOR]

Soil discharge occurs when it is subjected to high impulse current, and it is closely associated with the impulse performance of grounding systems. In this paper, a new method is put forward to visualize the inner impulse discharge region of simulate soil (fine sand) directly, using X-ray imaging technology. According to the three views of the discharge region, the boundary surface of the discharge region can be obtained based on image filtering and edge extraction by 3-D reconstruction. On this basis, the ionization parameters (i.e., the residual resistivity of soil discharge region \rhores and the critical breakdown electric-field strength E_c ) are calculated by a finite-element method. The results show that discharge region volume enlarges with water content increasing, and \rho_{0} . E_c$ increases from 100 to 150 kV/m, as soil water content decreases from 5% to 2%. [ABSTRACT FROM AUTHOR]

A penetrating X-ray imaging technology is applied to study the soil ionization under surge current, by which grayscale images are obtained. Based on soil breakdown mechanism, there are series of discharge channels generating in the soil. By comparing the discharge time and imaging time, the filamentary paths, which can be directly observed in these grayscale images, are determined to be discharge air channels. The principle and process of imaging are introduced in detail in this paper. According to the Beer–Lambert law, the relationship between grayscale value and equivalent air thickness of any points in the discharge channel area has been proposed. Image processing technology is used to dispose the grayscale images to make them more distinct for analysis without losing the accurate information of the original images by maintaining their grayscale value percentage. Analyses of experimental imaging results are also presented, and effects of factors, such as configuration of grounding electrode and amplitude of surge current, are considered on the distribution of discharge channels as well. In addition, soil residual resistivity of discharge area was calculated by a finite-element method, according to experiment parameters and 3-D diagram of discharge area. [ABSTRACT FROM AUTHOR]