봉상접지극의 과도임피던스 특성

Abstract

This thesis deals with the correlation of the transient impedance and its parameters with the stationary resistance of grounding systems to a square pulse current and a lightning impulse current.

In experiment, the grounding system consists of a single grounding rod(φ10[mm], 1[m]) and/or a triple-grounding rods of equilateral triangles with 5[m] spacing for operation.

To analyze the transient impedance characteristics of the grounding system, a pulse generator which can produce square wave of a 30[ns] rise-time and a 20[㎲] pulse duration is designed and fabricated.

Also, impulse current tests using the standard 8/20[㎲] wave specified in IEC 61000-4-5 were carried out on the grounding system to simulate the transient characteristics in an actual field condition such as a grounding system for power distribution lines.

The injected current in the grounding system and the developed potential were recorded, and the time variation of the transient impedance were calculated as the ratio of the potential rising to the injected current at each time.

The experimental results showed that the transient impedance reaches its maximum value very fast (around 250[ns]) and consecutively is returned to the value of the stationary impedance.

The transient impedance and the effective surge impedance Z3 which defines economic protection level were quite higher than the stationary resistance. The grounding impedance is decreased by the application of the triple-rods grounding system, and its effect is decreased as the frequency of the current is increased due to the inductance of the grounding leads.

It is therefore important to minimize the inductance of the grounding rods and leads to obtain the lowest grounding impedance and the lowest potential rising in a grounding system.