접지계통에서 임피던스와 위험전압의 주파수 의존에 관한 연구

Abstract

This thesis deals with the frequency dependence of impedance and risk voltages to grounding systems by applying various current sources.

Ground impedance was calculated as the ratio of the potential rise to the applied current according to the fall-of-potential method, and the risk voltages were measured using the simulated-personnel method specified in IEEE Std. 81.2-1991.

The experiment was carried out in a triple-ground rod and a carbon ground electrode. Also, influence of ground lead on the ground impedance and risk voltages was analyzed in each grounding system with the length of 1[m] and 30[m] long. Applied current to the grounding system was sinusoidal waveform in ranges of 60[Hz]～3.5[MHz] and surge current with risetime of 50～500[㎱].

The experimental results showed that the ground impedance gradually decreases as the frequency of applied current increases in low frequency range by capacitance of the grounding system.

However, the ground impedance increases due to inductance of the grounding system over a specific frequency range, and the effect was remarkable in both the triple-ground rod and the carbon ground electrode with the 30[m]-lead.

The ground impedance by surge current was included in ranges of the ground impedance measured by the wideband sinusoidal current, and the risk voltages were strongly influenced by the ground impedance which shows frequency dependent characteristic.

Likewise, the ground impedance and the risk voltages show a capacitive or an inductive characteristic depending on the frequency of fault current flowing to grounding system.

Consequently, the performance of grounding system as the ground impedance and risk voltages should be evaluated by a sinusoidal waveform in ranges from a power frequency to a few MHz or a surge current with fast risetime.