This paper carried out the comparative analysis on ground impedance of a carbon block and a copper rod. Two types of grounding electrode were compared; a carbon block (L: 1 m, ф : 245 mm)buried at depth of 0.8m and a three-linked copper rod (L : 1 m, : ф : 10 mm) of equilateral triangles with 1 m spacing. Ground impedance depending on applied current was evaluated by the application of a sine wave current with 60 Hz~3.5 MHz, fast-rise pulse with rising time of 200 ns, a standard lightning impulse of 8/20 ㎲ and a 600 Hz square wave. Ground impedance for both electrodes were almost the value below 100 KHz, and increased rapidly afterwards. The maximum ground impedance appeared 400 Ω at around 1.5 MHz. Ground impedance of the block was lower at the square wave and was higher at fast-rise pulse that of the copper rod. Also, impedance as ages showed no difference for the 8 months. From the results, it is likely that ground performance for both electrodes shows no difference against commercial frequency and lighting impulse current, while the copper rod shows better performance against fast-rise pulse with rise-time of a few hundred ns.

The basic performance of the ground system is evaluated as the ground resistance by applying low frequency current below 1 kHz. However, characteristics of the ground system should be analyzed by high frequency current up to 1 MHz since transient currents having a few hundred kHz component flow during a line-to-ground fault and/or a lightning strike. This paper deals with the design and fabrication of a wideband ground impedance meter (WGIM) which measures the impedance of ground systems in ranges from 65 Hz to 1.28 MHz. Also, a noise elimination algorithm using a digital bandpass filter is proposed. The maximum error of the WGIM is 4.91% in the measurement frequency range.