Some characteristics and statistics of the electromagnetic fields produced by stepped-leaders just before cloud-to-ground lightning return strokes are described. A transient data acquisition system was used together with electric and magnetic field measuring devices with frequency bandwidths of 200 Hz to 1.6 MHz and 270 Hz to 2.3 MHz, respectively. In this recording mode, approximately 100 µs before the signal triggering, the transient data acquisition system could be recorded at a sampling time of 100 ns. The time intervals between peaks of the last leader pulse and return stroke field were widely distributed over the range of 5–70 µs, and the mean was approximately 16.2 µs with a standard deviation of 8.8 and 14.8 µs with a standard deviation of 5.2 for the positive and negative polarities, respectively. When the stepped-leader approaches closely to the ground, the time interval between leader steps tended to decrease with time. The mean peak amplitude ratio of the last leader pulse to the lightning return stroke field was 0.17±0.12 for the positive electric fields and 0.18±0.09 for the negative electric fields. On average, the full widths at half maximum of the stepped-leader pulses were 1.31±0.8 µs for the positive polarity and 1.71±1.12 µs for the negative polarity. All results presented here are somewhat different from those observed at other measuring sites, because lightning return stroke fields depend on geographical and meteorological conditions.
-When surges and electromagnetic pulses from lightning or power conversion devices are considered, it is desirable to evaluate grounding system performance as grounding impedance. In the case of large-sized grounding electrodes or long counterpoises, the grounding impedance is increased with increasing the frequency of injected current. The grounding impedance is increased by the inductance of grounding electrodes. This paper presents the measured results of frequency-dependent grounding impedance and impedance phase as a function of the length of counterpoises. In order to analyze the frequency-dependent grounding impedance of the counterpoises, the frequency-dependent current dissipation rates were measured and simulated by the distributed parameter circuit model reflecting the frequency-dependent relative resistivity and permittivity of soil. As a result, the ground current dissipation rate is proportional to the soil resistivity near the counterpoises in a low frequency. On the other hand, the ground current dissipation near the injection point is increased as the frequency of injected current increases. Since the high frequency ground current cannot reach the far end of long counterpoise, the grounding impedance of long counterpoise approaches that of the short one in the high frequency. The results obtained from this work could be applied in design of grounding systems.
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