Shozo Sekioka

Analytical Surveys of Transient and Frequency-Dependent Grounding Characteristics of a Wind Turbine Generator System on the Basis of Field Tests

To exploit high wind conditions, wind turbine generator systems are constructed in places with few tall structures; as a consequence, they are often struck by lightning. This results in breakdown and malfunction of electrical, communications, and control systems inside and adjacent to the wind turbine generator system because of ground potential rise. Impulse tests were conducted on an actual wind turbine generator system and analytical surveys based on field tests were carried out using electromagnetic field analysis. The ground potential rise of the system and that around its foundation was measured and analyzed. The grounding system employed in this study consisted of the foundation, grounding mesh, and foundation feet. The frequency characteristics were calculated using the Laplace transform to get voltage responses for all types of lightning current waveforms. Step and typical lightning current waveforms were used to calculate potential rise responses.

Current-dependent grounding resistance model based on energy balance of soil ionization

Soil ionization occurs around a grounding electrode when current density in the soil exceeds a critical value and reduces grounding resistance. This paper proposes a current-dependent grounding resistance model considering the soil ionization. The proposed model is derived on the basis of energy balance of the soil ionization. The resistivity of the ionization zone depends on energy stored in the zone. Analytical expressions of the model are proposed to estimate the zone resistivity. The model is verified by comparing it with experimental results.

Experimental study of current-dependent grounding resistance of rod electrode

This paper has experimentally investigated transient behavior of grounding electrode systems especially for a high impulse current up to 40 kA in particular reference with a mutual grounding resistance between the electrodes. The experimental results show that the mutual grounding resistance depends on the applied current similarly to the current dependence of the self-grounding resistance. The phenomenon can be explained by the increase of an effective rod radius and an effective length. The self-grounding resistance greatly depends on the rod dimension and on the soil resistivity for low currents as analytically well known. However, the parameters cause a less effect on the nonlinear characteristic for high currents exceeding 10 kA.

Analytical Formulas for Induced Surges on a Long Overhead Line Caused by Lightning With an Arbitrary Channel Inclination

This paper presents analytical formulas for lightning-induced surges (voltage and current) on an infinite length of overhead line, which are generated by a nearby lightning stroke with arbitrary striking angle. The formulas are derived using the Rusck model, which is one of the electromagnetic coupling models based on a transmission-line approximation of a return stroke. The return-stroke current is approximated by piecewise linearly varying characteristics. The formulas are simple and convenient for lightning-induced effect analysis. The analytical formulas are validated by the comparison with a finite-difference method. The paper proposes approximate formulas of peak value and time to peak value of the lightning-induced voltages for a vertical lightning channel.

An Experimental Validation of Lightning Performance in Distribution Lines

The Tokyo Electric Power Company had conducted field research of voltages and currents in actual distribution lines due to lightning strokes between 1997 and 2001. These results confirm that the distribution line faults are mainly caused by direct lightning strokes to the line. Thus, it is important to simulate lightning performance for direct strokes to establish a lightning protection design of distribution lines. In this paper, statistical data of the observed voltages and currents are analyzed, and the validation of the observation results for the direct lightning strokes using a lightning surge analysis model is conducted. The observation results and the simulation model are useful in clarifying lightning performance in distribution lines.

Simulation Model for Lightning Overvoltages in Residences Caused by Lightning Strike to the Ground

Lightning overvoltages coming from overhead lines, such as power distribution lines, telecommunication lines, and antennas to residences have been actively studied. However, the relation between a lightning strike to the ground and overvoltages in residences remains unclear. This paper proposes a simulation model to estimate lightning overvoltages caused by ground potential rise due to the lightning strike to the ground. The proposed model is derived on the basis of the Thevenin theorem, and can be realized in the Electromagnetic Transients Program.

An experimental study of sparkover between a rod and a photovoltaic panel

This paper describes experimental results of sparkover characteristics of a gap consisting of a photovoltaic panel and a rod which represents a final jump of a lightning stroke. Surface discharge occurs after a sparkover between the rod and the panel when a frame of the panel is grounded. Photovoltaic cells in sparkover phenomena act like a metallic plate. The experimental results suggest that lightning damages on a panel rarely occurs due to direct lightning hit to the photovoltaic panel by the surface discharge.

Lightning Overvoltages in Low-Voltage Circuit for Various Lightning Striking Points

Lightning overvoltages in low-voltage circuits in houses originating from a distribution line or an antenna have been actively studied. However, the mechanism by which damage is caused to home appliances due to a lightning strike to the ground near the house is still unclear. This paper discusses lightning overvoltages and currents in low-voltage circuits for various scenarios, such as a lightning strike to the ground, a direct strike to a distribution line, and a direct strike to an antenna. Simulations are carried out using the Electromagnetic Transients Program. From the simulation results, direct lightning strikes cause the most serious damage to home appliances, but nearby lightning strikes should also be considered when designing lightning protection schemes.

Return-Stroke Model Segmentation and its Application to Lightning-Induced Surges Calculation

This paper proposes an analytical formula for calculating lightning-induced voltages on an infinitely long overhead line generated by a lightning stroke departing from an arbitrary altitude. A calculation method of the lightning-induced voltages to consider various engineering return-stroke models that can account for the variation of both current amplitude and velocity in a lightning channel is proposed. The lightning stroke is divided into a number of segments, and the analytical formula of the lightning-induced voltage is applied to each segment. This paper discusses the influence of parameters of return-stroke models on lightning-induced voltages. The proposed method is applied to a case of lightning strike to a tall stack.

Approximate Formulas for Terminal Voltages on the Grounding Conductor

This paper proposes approximate formulas to estimate voltages on a horizontal grounding conductor. The formulas are derived in the time domain using Bewleys lattice diagram method based on transmission and reflection coefficients. The expression of the approximate formulas is simple, and the formulas are useful for engineers to carry out grounding system design for lightning protection. This paper concerns a voltage source with step waveform. A formula for an arbitrary voltage waveform can be calculated by using Duhamels integral. Surge characteristics of voltages and an effective length of a grounding conductor are discussed using the proposed formulas. It is easy to understand experimental and simulation results of a grounding system using the approximate formulas.