Experimental Study of the Discharge Area of Soil Breakdown Under Surge Current Using X-Ray Imaging Technology

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.

I. Introduction

As a good design of a grounding system can effectively protect power system equipment against abnormal events, thus, studying the impulse performance of grounding systems when excited by impulse currents is important. Research on the characteristics of electrical breakdown and discharge processes in soil has formed a fundamental component concerned with the impulse characteristics of grounding systems [1].