Bok-Hee Lee

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.

A Monitoring Device of Leakage Currents Flowing through ZnO Surge Arresters

In this study, we developed a device for monitoring resistive leakage current and a technique for the diagnosis of the deterioration of zinc oxide (ZnO) surge arresters. The resistive leakage current that increases with time results in a thermally unstable state that may even lead to a serious problem. We developed the leakage current monitoring device for the ageing test and durability evaluation for ZnO blocks. The third-harmonic component of the leakage current of ZnO blocks may be used as an indicator of whether ZnO surge arresters are in a good state or in a bad state. Two methods for the diagnosis of the deterioration of surge arresters were assessed. Both the measurement instruments for the resistive leakage current with a compensation circuit and the third-harmonic current component were designed and fabricated. For higher precision and greater reliability of current detectors, the current transducer with a ferrite core, which has a very high relative permeability, was used to improve detection sensitivity. Laboratory tests for ZnO blocks were performed by measuring the resistive leakage current and third-harmonic current components, together with the fast Fourier transform (FFT) analysis. The third-harmonic current component was clearly separated from the leakage currents flowing through ZnO surge arresters for power frequency ac applied voltages. As a result, it was known that the proposed monitoring systems for both the resistive leakage current and the third-harmonic current would be useful for developing a device for predicting the defect and performance deterioration of ZnO surge arresters in power system applications.

Temperature dependence of magnetoresistance for tunnel junctions with high-power plasma-oxidized barriers: Effects of annealing

Magnetic tunnel junctions (MTJ) were fabricated with high oxygen-plasma power and the effects of annealing on the temperature dependence of tunneling magnetoresistance (TMR) were investigated experimentally. As grown, TMR increases, peaks around 160 K, and decreases with increasing temperature from 80 K to 300 K. When MTJs are annealed, Tmax, the temperature at which maximum TMR is obtained, decreases as annealing temperature increases to the optimal point. In order to explain the temperature dependence of TMR, the difference of conductance between parallel and antiparallel alignments of magnetizations as a function of temperature is also analyzed. The shifts of Tmax due to annealing are described phenomenologically with spin-dependent transfer rates of electrons through the barrier.

An Investigation on the Frequency Dependence of Soil Electrical Parameters

This paper presents the results of an investigation into the frequency-dependent electrical parameters for different types of soil as a function of moisture content. The frequency dependence of soil electrical parameters is very important in the design of grounding systems. In fact, the performance of grounding systems is greatly dependent upon various factors such as soil type, particle size, water content, temperature, frequency, and the like. The resistivity and relative permittivity for four different soils were measured and analyzed in the frequency range of 1kHz ? 1MHz. Soil resistivity declined as moisture content and frequency increased. In particular, the frequency dependence of soil resistivity was significant as the moisture content was low. In contrast, the relative permittivity of soil dramatically declined at the frequency of 10kHz or below as the moisture content increased, showing the opposite pattern in terms of variation patterns, compared to resistivity.

Analysis of Soil Ionization Behaviors under Impulse Currents

This paper presents the characteristics of soil ionization for different water contents, and the parameters associated with the dynamic properties of a simple model grounding system subject to lightning impulse currents. The laboratory experiments for this study were carried out based on factors affecting the soil resistivities. The soil resistivities are adjusted with water contents in the range from 2 to 8% by weight. A test cell with a spherical electrode buried in the middle of the hemispherical container was used. As a result, the electric field intensity Ec initiating ionization is decreased with the reduction of soil resistivities. Also, as the water content increased, the pre-ionization resistance R₁ and the post-ionization resistance R₂ became lower with increasing current amplitude. The time-lag to ionization t₁ and the time-lag to the second current peak t₂ at high applied voltages were significantly shorter than those of low applied voltages. It was found that the soil ionization behaviors are highly dependent on the water content and the applied voltage amplitude.

Characteristics of the radiation field waveforms produced by lightning return strokes

Some features of the radiation field waveforms produced by cloud-to-ground lightning return strokes were investigated. A transient analyzer was used together with electric and magnetic field measuring devices with frequency bandwidths of 200 Hz to 1.6 MHz and 270 Hz to 2.3 MHz, respectively. The initial peak of the first lightning return stroke fields is followed by several large subsidiary peaks, whose amplitudes are a fraction of that of the initial peak and decrease with time. The subsidiary peaks of the first lightning return stroke fields may be caused by the effect of branches of leader channels. The mean amplitude ratios of the subsidiary peaks to the first return stroke field peak were widely distributed over the ranges of 0.25–0.8, and the mean time interval between the subsidiary peaks ranged from 5 to 15 µs. Detailed statistical analysis showed that the time interval between the subsidiary peaks and the ratio of the subsidiary peaks to the first lightning return stroke field peak in the present work are less than those obtained in subtropical and tropical regions. The parameters characterizing the wave tail of lightning return stroke fields are closely related to the propagation velocity and length of return stroke throughout the entire leader channel, and the subsidiary large pulses in the wave tail of return stroke fields seem to be associated with the resonance in major branches of lightning return stroke channels.

Quaternary Alloy Films for Thin Film Resistors

In this study, we fabricated the quaternary alloy films of Ni–Cr–Al–Cu for thin film resistors using five different target compositions. Thin films were deposited by a DC/RF magnetron sputter and patterned for the measurement of temperature coefficient of resistance (TCR). The TCR of the Ni–Cr–Al–Cu thin films with composition contents and the effects of annealing in air atmosphere up to 400°C were examined. The TCR decreased as the O2 content decreased in film. We chose 5 mTorr as an optimum pressure. Transmission electron microscope (TEM) and X-ray diffraction (XRD) analysis of a thin film with a composition confirmed that the sharp increase in the TCR of films annealed at 400°C in air is due to the formation of the rhombohedral NiO phase in the film. The TCR decreased as the Cr content increased or the impurity (Al, Cu) content increased. Above 38 wt% of Cr, the TCR changed to a negative value. The result of XRD analysis revealed that the decrease in the TCR was due to the presence of microcrystalline microstructures in these thin films. The optimal composition was determined to be 51 wt%Ni–41 wt%Cr–4 wt%Al–4 wt%Cu (target 3) and the optimal annealing condition 250°C/3.5 h in air. We obtained thin film resistors of low TCR under ±10 ppm/°C.

Temperature dependence of tunneling magnetoresistance: Double-barrier versus single-barrier junctions

The temperature dependence of tunneling magnetoresistance (TMR) is studied for spin valve type double-barrier tunnel junctions. Normalized TMR values for double-barrier tunnel junctions (DBTJs) and single-barrier junctions (SBTJs) are plotted as functions of temperature and it is found that the DBTJ shows stronger temperature dependence of TMR than the SBTJ. The strong temperature dependence of TMR for the DBTJ is explained in terms of temperature dependence of the spin polarization of the middle magnetic layer and decrease of the spin coherence length with increasing temperature.

Energy Coordination of Cascaded Voltage Limiting Type Surge Protective Devices

For the purpose of designing and applying optimum surge protection, one of the essential points is to take into account the energy coordination between cascaded surge protective devices(SPDs) and it is important to obtain an acceptable sharing of the energy stress between two cascaded SPDs. In this paper, in case of two voltage-limiting SPDs connected in parallel, the amount of splitting impulse current and energy that flow through each SPDs is investigated as a function of the protective distance. As a result, the energetic coordination between cascaded SPDs is strongly dependent on the voltage protection level of SPDs and the protective distance. It was confirmed that the sharing of the energy between two cascaded SPDs and the limited voltage levels are appropriate when the voltage protection levels of both upstream and downstream SPDs are the same.