Shigeru Yokoyama

Thin wire representation in finite difference time domain surge simulation

Simulation of very fast surge phenomena in a three-dimensional structure requires a method based on Maxwells equations such as the finite difference time domain (FDTD) method or the method of moments (MoM), because circuit-equation-based methods cannot handle the phenomena. This paper presents a method of thin wire representation for the FDTD method that is suitable for the three-dimensional surge simulation. The thin wire representation is indispensable to simulate electromagnetic surges on wires or steel flames of which the radius is smaller than a discretized space step used in the FDTD simulation. Comparisons between calculated and laboratory-test results are presented to show the accuracy of the proposed thin wire representation, and the development of a general surge analysis program based on the FDTD method is also described in the present paper.

Calculation of Lightning-Induced Voltages on Overhead Multiconductor Systems

A computer program has been developed for numerically analyzing lightning- induced voltages on multiconductor lines. Thi. s program can be used to clarify the effects of lightning arresters and overhead ground wires on lightning-induced voltages, which has not before been possible, anid is potentiaLly of great uise in the lightning-resistant design of distribution lines. The results obtained with this numerical analysis program were compared to the results from an analog simulation using a 1:200 scale model, and found to be in good agreement, demonstrating the usefulness and validity of the program.

Experimental study of response of power distribution lines to direct lightning hits

In order to clarify the response of overhead distribution lines to direct lightning hits, experimental facilities were installed at the UHV Shiobara Testing Laboratory of CRIEPI. A lightning impulse current with a crest value of 18 kA and a waveform of (2/11) mu s was generated through the impulse current generator with a high internal resistance of 400 Omega . This impulse current is not easily affected by the conditions of a distribution line side. Protective effects of surge arresters and an overhead ground wire against direct strokes to the pole of a distribution line were clarified experimentally. Experimental results showed a better protective effect than earlier calculated results. Overvoltages between a line conductor and the earth are inclined to have a short wavetail. The 50% flashover voltage of an insulator against overvoltages with a short wavetail is higher than that against overvoltages with a standard wavetail. >

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.

Energy absorption of surge arresters on power distribution lines due to direct lightning strokes-effects of an overhead ground wire and installation position of surge arresters

The authors describe the characteristics of energy absorption of surge arresters on power distribution lines due to direct lightning strokes. Using the EMTP, the effect of an overhead ground wire and the influence of the installation position of a surge arrester on the energy absorption are investigated quantitatively. The analysis shows that: (1) an overhead ground wire is more effective in preventing damage to surge arresters than increasing the withstand capabilities of surge arresters by a factor of two; (2) the energy absorption without an overhead ground wire is about 3/spl sim/5 times larger than that with one; and (3) in the case of direct lightning strokes to an overhead ground wire, the energy absorption of surge arresters installed at the termination of a line is approximately 2.3 times larger at its maximum than that of arresters installed in the midst of a line.

Two types of lightning discharges to a high stack on the coast of the sea of Japan in winter

To clarify the characteristics of lightning on the coast of the Sea of Japan in the winter season, the current waveform and the progression of lightning discharge to a high stack were observed. Observation results from 1989 to 1994 show that: (1) there are two types of lightning discharges-one with large currents and strong luminosity in the lightning path, and the other with small currents of long duration and weak luminosity of lightning path; (2) the parameters of lightning currents of the two types of discharges are quite different; and (3) there is a quantitative correlation between the change of luminosity in a lightning path and the lightning current waveform.

Simultaneous Measurement of Lightning Induced Voltages with Associated Stroke Currents

Voltage surges induced in a distribution line situated at a distance of 200 m from the lightning flash position, and for which the associated lightning current waveforms have been determined, were measured.

Accurate modeling of core-type distribution transformers for electromagnetic transient studies

This paper proposes a model of core-type distribution transformers for electromagnetic transient studies. The model accurately reproduces not only the impedance characteristics seen from each terminal of a core-type distribution transformer but also the surge-transfer characteristics between the primary and secondary sides in a wide range of frequency. Because of this capability, the proposed model enables the accurate evaluation of overvoltages on distribution lines including consumer-side overvoltages. A 10 kVA transformer is modeled, and transient-simulation results agree well with laboratory-test ones.

Lightning occurrence data observed with lightning location systems in Japan: 1992-1995

Cloud-to-ground lightning occurrence for years 1992-1995 has been analyzed using the data obtained with nine different lightning location systems in Japan. A total of more than 2 million lightning strokes are observed for the four years and the number of annual lightning strokes is closely related to the weather conditions in summer. Lightning occurs all over Japan in summer but occurrence of lightning is concentrated in the coastal area of the Sea of Japan in winter. Data of thunderdays are compared with the data observed by the Japan Meteorological Agency. The relationship between number of lightning strokes and thunderdays is also obtained. It changes season by season and it is very difficult to express the relations with a single formula.

Voltage induced on a test distribution line by negative winter lightning strokes to a tall structure

Simultaneous measurements of lightning-induced voltages and lightning currents are presented and subject to analysis. The lightning current was measured at the top of a 200-m high stack by a current shunt. In the analysis, both the transmission-line model (TLM) and traveling current-source model (TCSM) of a return stroke are tested, taking account of reflection and refraction of the lightning current waves at the top and bottom of the stack. Analysis shows that the measured induced voltages on a test distribution line were influenced by finite ground conductivity. Agreement of measured induced-voltage waveforms with calculations demonstrates the validity of the adopted models of the stack and the lightning stroke. Both TLM and TCSM turn out to be useful in predicting induced voltage associated with a negative lightning stroke to a tall structure in winter.