Jong-Hyuk Choi

Simulations of Frequency-dependent Impedance of Ground Rods Considering Multi-layered Soil Structures

Lightning has a broad frequency spectrum from DC to a few MHz. Consequently, the high frequency performance of grounding systems for protection against lightning should be evaluated, with the distributed parameter circuit model in a uniform soil being used to simulate grounding impedances. This paper proposes a simulation method which applies the distributed parameter circuit model for the frequency-dependent impedance of vertically driven ground rods by considering multi-layered soil structures where ground rods are buried. The Matlab program was used to calculate the frequencydependent ground impedances for two ground rods of different lengths. As a result, an increase of the length of ground rod is not always followed by a decrease of grounding impedance, at least at a high frequency. The results obtained using the newly proposed simulation method considering multi-layered soil structures are in good agreement with the measured results.

Underwater Discharge Phenomena in Inhomogeneous Electric Fields Caused by Impulse Voltages

The paper describes the electrical and optical properties of underwater discharges in highly inhomogeneous electric fields caused by 1.2/50㎲ impulse voltages as functions of the polarity and am- plitude of the applied voltage, and various water conductivities. The electric fields are formed by a point-to-plane electrode system. The formation of air bubbles is associated with a thermal process of the water located at the tip of the needle electrode, and streamer coronas can be initiated in the air bub- bles and propagated through the test gap with stepped leaders. The fastest streamer channel experiences the final jump across the test gap. The negative streamer channels not only have more branches but are also more widely spread out than the positive streamer channels. The propagation velocity of the posi- tive streamer is much faster than that of the negative one and, in fact, both these velocities are inde- pendent of the water conductivity; in addition the time-lag to breakdown is insensitive to water con- ductivity. The higher the water conductivity the larger the pre-breakdown energy, therefore, the ionic currents do not contribute to the initiation and propagation of the underwater discharges in the test conditions considered.

Frequency-dependent grounding impedance of the counterpoise based on the dispersed currents

When surges and electromagnetic pulses from lightning or power conversion devices are considered, it is desirable to evaluate grounding system performance as grounding impedance. In the case of large-sized grounding electrodes or long counterpoises, the grounding impedance is increased with increasing the frequency of injected current. The grounding impedance is increased by the inductance of grounding electrodes. This paper presents the measured results of frequency-dependent grounding impedance and impedance phase as a function of the length of counterpoises. In order to analyze the frequency-dependent grounding impedance of the counterpoises, the frequency-dependent current dissipation rates were measured and simulated by the distributed parameter circuit model reflecting the frequency-dependent relative resistivity and permittivity of soil. As a result, the ground current dissipation rate is proportional to the soil resistivity near the counterpoises in a low frequency. On the other hand, the ground current dissipation near the injection point is increased as the frequency of injected current increases. Since the high frequency ground current cannot reach the far end of long counterpoise, the grounding impedance of long counterpoise approaches that of the short one in the high frequency. The results obtained from this work could be applied in design of grounding systems.

An analysis of conventional grounding impedance based on the impulsive current distribution of a counterpoise

The performance of a grounding system should be evaluated in terms of the grounding impedance for a lightning surge containing high frequency components. Grounding impedance shows resistive, capacitive and inductive behaviors. Ground resistance is regarded as a particular grounding impedance measured in a low frequency range. When designing the grounding system, the grounding impedance and effective length should be considered. In this study, conventional grounding impedance is measured as functions of the front time of the impulse current and the length of the counterpoise used largely as the grounding electrode of the transmission tower. In order to find the relation between the conventional grounding impedance and current distribution, the magnitude of the dispersed impulse current at every 10 m interval along of the counterpoise, which is 50 m long, is measured and simulated according to the front time of the injected impulse current. As a result, the conventional grounding impedance of a long counterpoise is similar to that of a short counterpoise with a smaller front time. As the front time of the injected current becomes short, the current distribution is increased near the current injection point. In addition, the simulated results of multi-layer soil structure are very similar to the measured results.

Effects of Ac Mutual Coupling According to Location of Auxiliary Electrodes In Measuring the Ground Impedance of Vertically or Horizontally Buried Ground Electrode

In order to minimize ac mutual coupling, the auxiliary electrode are located at a right angle in measuring ground impedance. In case that the measurement space is limited, the alternative method is employed. At that time, it is necessary to investigate the measurement errors due to ac mutual coupling and earth mutual resistance in measuring the ground impedances. `This paper presents the measurement accuracy according to the location of the current and potential auxiliary electrodes in measuring ground impedance of vertically or horizontally buried ground electrode. The measurement errors due to ac mutual coupling were evaluated Consequently, the effect of ac mutual coupling on the measurement accuracy for horizontally buried ground electrode is greater than that for vertically buried ground electrode. Measurement errors due to ac mutual coupling is the largest when the current and potential auxiliary electrodes are located in parallel. The 61.8[%] rule is inappropriate in measuring ground measurement. Theoretically, in case that the angle between the current and potential auxiliary electrodes is 90, there is no ac mutual coupling. If it is not possible to route the current and potential auxiliary electrodes at a right angle with limitation of measurement space, the location of these electrodes with an obtuse angle is preferred to that with an acute angle in reducing the measurement errors due to ac mutual coupling.

Grounding impedance based on the current distribution for the horizontal ground electrode installed in two-layer soil structure

Large grounding systems such as ground grid and long horizontal ground electrodes are typically installed at non-uniform soil structure. In case of the ground resistance for these grounding systems, it is enough to reflect the average value of soil resistivity. However, the transient response of grounding system is much different with the ground resistance when the surge caused by lightning strikes is injected into the grounding system. The grounding system in lightning protection systems is designed and installed with considering the grounding impedance. Unlike the ground resistance, the grounding impedance is affected by the soil structure or locally different soil resistivity. This paper presents the grounding impedance and current rate dispersed into the soil as functions of the frequency of ground current and the fronted time of impulse current for the horizontal ground electrode of 40 m long when it is installed in two layer soil structure. As a result, the conventional and transient grounding impedance measured at the injection point with low soil resistivity shows low value even though the length of ground electrode is same since the fast fronted impulse current are much dispersed near the current injection point. As the effective length of the horizontal ground electrode is getting short with decreasing the fronted time of impulse current, the location of ground lead is important factor to decrease the grounding impedance. These results show the optimized technique for the location and condition of the ground electrode and the ground lead to decrease the grounding impedance.

Frequency-Dependent Resistivity and Relative Dielectric Constant of Soil on Water Content

In order to evaluate the performance of a grounding system against lightning or fault currents including high frequency components, the grounding impedance should be considered rather than the steady state ground resistance. To evaluate the ground impedance, the frequency dependence of resistivity and relative dielectric constant of the soil have to be analyzed. This paper deals with the frequency dependence of the resistivity and relative dielectric constant of three types of soil on water content. As a result, the resistivity of soil is getting lower with increasing of water content. It is nearly independent of the frequency in the range less than 1[MHz], and is decreased over the frequency range above 1[MHz]. On the other hand, the relative dielectric constant is rapidly decreased with the frequency in the range less than 1[MHz], but it is nearly independent on the frequency over the range of 1[MHz]. It was found from the experiments that the frequency-dependant resistivity and relative dielectric constant of soil should be considered when designing the grounding systems for protection from lightning or switching surges.

Frequency-Dependent Grounding Impedances of Counterpoises Associated with Soil Resistivity

This paper deals with the frequency-dependent grounding impedances of counterpoises relevant to the soil resistivity, the length of counterpoises and the feeding point of test current. The grounding impedances of counterpoises buried in one-layered and two-layered soils were measured and analyzed in the frequency range from 1[kHz] to 10[MHz]. As a result, the frequency-dependent grounding impedances strongly depend on the soil resistivity, and the grounding impedances within the frequency of several tens [kHz] are capacitive behavior in high soil resistivity. When injecting the ground current to the end of counterpoise buried in soil with high resistivity, the grounding impedances in high frequency are increased.

Potential Rise Characteristics of a Grounding Grid Subjected to Lightning Currents

This paper presents the results of an investigation into the factors that affect the impulse response of a grounding grid under the passage of lightning currents. We carried out simulations of potential rise of a 10m×10m square grounding grid comprising 4 meshes buried in the soil of about 140Ω․m resistivity. The potential rise profiles of the grounding grid stressed by lightning currents were computed by EMTP software. The dissipation current and the potential rise for the 0.25/100μs current wave were significantly concentrated around the current injection point. The impulse coefficient for a corner-positioned lightning strike point showed a doubling of the impulse coefficient compared to the corresponding central strike point of the grounding grid.

Frequency Response Characteristics of a Grounding Grid

The dissipation current distribution, potential rises and frequency-dependent impedances of the grounding grid were investigated. The computational simulations are made using the simplified EMTP-based model. Based on experimental and simulation results, the dissipation current at a frequency above ～100㎑ intensively converged near the current injection point of the grounding grid, and decreased far from the current injection point. When the test current was injected at the center of the grounding grid, the high frequency grounding impedances were lower than those measured at the corner or side of the grounding grid. The trend of simulation results for the grounding impedance and dissipation current distribution calculated using a non-uniform distributed parameter circuit model are similar to the data measured at the test site.