In this paper, an analysis of experimental results on the behavior under impulse currents of various grounding electrodes: rod and horizontal electrodes, a ground grid, and tower footings, is presented. The parameters used for analyzing transient performance are reviewed, and the differences are highlighted based on test data. The analysis is extended to the following: 1) to quantify the effect of impulse shape; 2) to quantify the effect of current magnitude; 3) to compare low-frequency and impulse performances; 4) to compare impulse and high-frequency performances; and 5) to examine the effects of the test setup on measured results, e.g., in the case of field tests, the effect of current return leads, and proximity and extent of return electrodes. A generalized impulse index is introduced to help elucidate the differences between different parameters used for the analysis of transient test results on ground electrodes. It is found that the analysis of test data based on different parameters may lead to different assessments of impulse performance. The results also show that the impulse parameters used for the analysis of test data can be influenced by several factors such as electrode length, impulse current rise time, and experimental setup. In addition, variable-frequency test results are analyzed by introducing a “harmonic coefficient” which quantifies the deviations of the harmonic impedance from the low-frequency resistance over different frequency ranges covering the entire lightning frequency spectrum. Significant variations of the harmonic coefficient with frequency were observed, highlighting the importance of taking the frequency dependence of soil properties when modeling the impulse and high-frequency behavior of grounding systems.

The correct operation of surge arresters under lightning and switching transient conditions requires that grounding be effective at high frequencies as well as at power system frequency. Effective high frequency grounding also limits the electrocution risk to substation personnel due to transient ground potential rise. This article assesses the performance of high frequency electrodes at typical lightning impulse current magnitudes. The enhancement of such electrodes by addition of bonded horizontal arms in cross and star arrangement is also investigated. The effective impulse resistances for electrodes having lengths up to 4.8 m are calculated over a range of impulse current magnitudes at the same location. The results obtained indicate that, for low impulse current magnitudes, the preionization electrode's resistance (R $_{1}$ ) falls with increasing rod length, a behavior reflected in the measured dc resistance. The presence of horizontal enhancement was found to reduce R $_{1}$ in all cases. The occurrence of soil ionization in the immediate vicinity of the electrode resulted in a reduced postionization resistance (R $_{2}$ ) at higher currents, tending toward a common asymptotic value independent of both the length of the rod and the presence of electrode enhancements. The observed behavior is supported by numerical simulation of electric field and current density distributions, indicating that the high current performance of a grounding rod is heavily influenced by soil ionization and breakdown in high-field regions at the electrode extremities.

We report laboratory experiments in which lightning current impulses of magnitudes up to and above 150kA were applied to OPGW cables of length up to 18m. Unprecedentedly, fast SOP speeds exceeding 8Mrad/s were measured.

This paper presents tests carried out at an outdoor experimental facility, described in the companion Part 1 paper, to investigate the performance of practical grounding systems. Here, the results of low-voltage dc, ac, and impulse tests performed on rod and grid electrodes are described, and the measured quantities are compared with computed values obtained from numerical models. Measured ground resistance and impedance at low frequency (including power frequency) showed reasonable agreement with simple standard formulae and computational models, but revealed a significant falloff with current magnitude in the range often used for practical testing of high-voltage grounding systems. This may have implications for the specifications of grounding test equipment and extrapolation of measured ground resistance/impedance at low-voltage/current to values representative of realistic fault currents. A frequency dependence of ground impedance was also measured. Specifically, a fall in impedance over a frequency range up to 100 kHz is not generally accounted for in grounding models.

A technique was developed for the comparison of observed emission spectra from lightning current arcs generated through self-breakdown in air and the use of two types of initiation wire, aluminum bronze and nichrome, against previously published spectra of natural lightning events. A spectrograph system was used in which the wavelength of light emitted by the lightning arc was analyzed to derive elemental interactions. A lightning impulse of up to 100 kA was applied to a two hemispherical tungsten electrode configuration which allowed the effect of the lightning current and lightning arc length to be investigated. A natural lightning reference spectrum was reconstructed from literature, and generated lightning spectra were obtained from self-breakdown across a 14.0 mm air gap and triggered along initiation wires of length up to 72.4 mm. A comparison of the spectra showed that the generated lightning arc induced via self-breakdown produced a very similar spectrum to that of natural lightning, with the addition of only a few lines from the tungsten electrodes. A comparison of the results from the aluminum bronze initiation wire showed several more lines, whereas results from the nichrome initiation wire differed greatly across large parts of the spectrum. This work highlights the potential use for spectrographic techniques in the study of lightning interactions with surrounding media and materials, and in natural phenomena such as recently observed ball lightning.