Frequency-dependent grounding impedance and current distribution rates of the counterpoise

초록

When surges and electromagnetic pulses from lightning or power converting devices are considered, it is desirable to evaluate the grounding system performance by the use of grounding impedances. In the case of large sized grounding electrodes or very long counterpoises, the grounding impedances increase with increasing frequency. The grounding impedance is increased by the reactance in the high frequency range affected by the inductance of grounding electrode. This paper presents the measured results of frequency-dependant grounding impedance and impedance phase as a function of the length of a counterpoise. In order to analyze the characteristics of the grounding impedance of the counterpoise, frequency-dependant current distribution rates were measured. As a result, the current distribution rates are proportional to the soil resistivity around the counterpoise at low frequencies. On the other hand, the current distribution rates near the point of the injected current increase with increasing frequency. Since the high frequency ground current can not reach the end of the counterpoise, the grounding impedance of the long counterpoise is similar to the short one at high frequencies. Therefore, the effective length of the counterpoise is determined in the designing of the grounding systems.