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.

Lightning usually contains multiple return strokes. Under multiple impulse currents, the main characteristics of soil media are not only the breakdown characteristics but also the recovery characteristics of the soil's electrical parameters. In this paper, the discharge recovery of soils under successive current pulses and the influential role of its various main factors are studied. The structural characteristics of soil successive-impulse discharge channels are analyzed from images taken during impulse discharges. Based on the experimental results, the modification method of soil resistivity under successive impulse currents is discussed. The results show that the peak value of the impulse current and the water content of the soil has a significant effect on the rate and duration of recovery. However, the effect of soil salt content on recovery is not significant. Under successive impulse currents, soil electrical characteristics need about tens to hundreds of milliseconds for a full recovery.

The on-site measurement of transient voltages is of great significance in analyzing the fault cause of power systems and optimizing the insulation coordination of power equipment. Conventional voltage transformers normally have a narrow bandwidth and are unable to accurately measure various transient voltages in power systems. In this paper, a wideband parallel resistive–capacitive voltage divider is developed, which can be used for online monitoring of transient voltages in a 220 kV power grid. The structures of the high-voltage and low-voltage arms were designed. The internal electric field distribution of the high-voltage arm was analyzed. The influence factors and improvement techniques of the upper frequency limit were studied. The parameters of the elements of the divider were determined. The voltage withstand performances and scale factors under lightning impulses and AC and DC voltages, the temperature stabilities of scale factors and the step response and bandwidth of the developed voltage divider were tested. The results show that the deviations of the scale factors under various voltage waveforms and different temperatures ranging from −20 to 40 °C are within 3%. The withstand voltage meets the relevant requirements specified in IEC60071-1-2011. The step response 10~90% rise time is approximately 29 ns, and the 3 dB bandwidth covers the range of DC to 10 MHz.

Partial discharge of soil occurs when a lightning current enters the ground, and the strength of partial discharge is closely related to the magnitude of its critical breakdown field strength. Therefore, how to accurately obtain the variation law of the typical soil critical breakdown field strength and residual resistivity is the key to realizing the safe operation of the grounding devices and cables in the ground. This paper first selects a variety of typical soils to study the influence of various factors on the morphology of the discharge channel, and then studies the calculation methods of the soil critical breakdown field strength and residual resistivity under the introduction of different discharge channel morphologies and structures, and further discusses the reason why typical soil media factors have a small impact on the critical breakdown field. The experimental results show that under the same conditions, the critical breakdown field strengths of different soils from small to large are sand soil, loam soil and Yellow cinnamon soil. The largest ratio of residual resistivity to initial resistivity of the three soils is sand soil.