Takatoshi Shindo

Initial stage in lightning initiated from tall objects and in rocket‐triggered lightning

We examine the characteristics of the initial stage (IS) in object-initiated lightning derived from current measurements on the Gaisberg tower (100 m, Austria), the Peissenberg tower (160 m, Germany), and the Fukui chimney (200 m, Japan) and their counterparts in rocket-triggered lightning in Florida. All lightning events analyzed here effectively transported negative charge to ground. For rocket-triggered lightning the geometric mean (GM) values of the three overall characteristics of the initial stage, duration, charge transfer, and average current, are similar to their counterparts for the Gaisberg tower flashes and the Peissenberg tower flashes, while the Fukui chimney flashes are characterized by a shorter GM IS duration and a larger average current. The GM IS charge transfer for the Fukui chimney flashes is similar to that in the other three data sets. The GM values of the action integral differ considerably among the four data sets, with the Fukui action integral being the largest. The observed differences in the IS duration between the Fukui data set and all other data considered here are probably related to the differences in the lower current limits, while the differences in the action integral cannot be explained by the instrumental effects only. There appear to be two types of initial stage in upward lightning. The first type exhibits pulsations (ringing) during the initial portion of the IS, and the second type does not. The occurrence of these types of IS appears to depend on geographical location. The characteristics of pulses superimposed on the initial continuous current (ICC pulses) in object-initiated (Gaisberg, Peissenberg, and Fukui) lightning are similar within a factor of 2 but differ more significantly from their counterparts in rocket-triggered lightning. Specifically, the ICC pulses in object-initiated lightning exhibit larger peaks, shorter risetimes, and shorter half-peak widths than do the ICC pulses in rocket-triggered lightning.

A New Calculation Method of Breakdown Voltage-Time Characteristics of Long Air Gaps

A new calculation method of leader development which considers predischarge currents is proposed. Calculated leader delopment characteristics with this model are compared with photographs of a image converter camera and close agreement is shown. Further, breakdown voltage-time characteristics of rod-plane and rod-rod gaps with standard and non-standard lightning impulses of both positive and negative polarities are calculated and these values agree with experimental results. This new calculation method is applicable to long gaps of several meters.

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.

Model experiments of laser-triggered lightning

Experiments to guide electric discharges with a chain of apparently discrete air breakdown plasmas (plasma channel) produced by a laser are reported. The electric discharge was guided up to 2 m with a high-power CO/sub 2/ laser focused by a 10 m focal length mirror. Voltage was applied at selected delay times tau following laser radiation. The relationships between 50% flashover voltage of a gap filled with laser-produced plasmas and delay times, and between guided length and the peak of the applied voltage, were obtained experimentally. The effects of the polarity of the applied voltage and the position of the plasma channel on flashover voltage are described. The development of the guided discharge is discussed. >

Development of long gap discharges guided by a pulsed CO2 laser

The mechanism of the guidance effect of laser plasmas on electrical discharges has been studied. A 45 J CO2 laser pulse produces many spherical plasmas over a gap of up to 2 m, and the laser-guided discharge is formed along the plasma by the application of a lightning impulse voltage. Unusual properties of the laser guided discharge are obtained from streak photographs of the discharge development. The laser produced plasmas guide streamers and leaders from a negative electrode more effectively than from a positive electrode. The discharge development from the negative electrode plays an important role in the guidance effect.

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.

Discharge Path Model in Model Test of Lightning Strokes to Tall Mast

The discharge path model is proposed on the basis of the observations on the discharge path in the model test. The model gives satisfactory explanations of the striking probability to the model mast. The model yields the deterministic striking parameter which is independent of the wave shape of the applied impulses but depends only on its polarity for a given set of the mast height and gap spacing. The discharge path model is applied to the estimation of the frequency of the natural lightning strokes to the tall structures and to the estimation of their shielding effects.

Natural contamination test of insulators with DC voltage energization at inland areas

Results of tests for natural contamination of insulator strings in inland areas are discussed. Natural exposure tests with -280 kV DC voltage energization were carried out for 5 years. The following results were obtained: (1) The amount of contaminant on the bottom surface increased with the length of the exposure period, for the first year, at inland areas. Therefore, estimates of contamination severity should use data obtained for exposure periods of at least one year. (2) Insulators near the line and ground sides of a string collect more contaminant than the middle when DC voltage is applied. However, the effect of nonuniform contamination on DC withstand voltage is not as great as expected when ratios of the number of heavily contaminated insulators to that of insulator string are from 15% to 30%. (3) No significant effect of DC voltage energization on the ESDD of the middle part of insulator strings has been observed when the applied voltage is no higher than 280 kV DC. (4) A pollution severity map established for AC transmission lines can be used for the determination of design ESDD for HVDC transmission lines for inland areas without industrial contamination. (5) The maximum value of leakage current under natural conditions is less than 3 mA, and no flashover has been experienced during these 5 years for electrical stress from 90 to 120 kV/m. The application of these results to insulation design for HVDC transmission lines is discussed. >

Discharge characteristics of 5 m long air gap under foggy conditions with lightning shielding of transmission line

The electric geometry model suggested by Armstrong and Whitehead was used to calculate the performance of lightning-shielded transmission lines. However, since the ultra high voltage (UHV) designed transmission lines were brought into 500 kV operation, it has become evident that their actual performance is different from the calculated predictions in terms of their actual faults, lightning observation results, etc. It is thought that the UHV designed transmission lines are subjected to foggy or rainy conditions when lightning strikes, because they often pass through mountainous areas at higher altitudes. However, according to the electric geometry model, the striking distance is determined by the lightning-stroke current value only, and the model does not consider the environment of discharge path for the relevant lightning strokes. These considerations suggest that foggy conditions may affect transmission line lightning shielding performance. In this study, discharge tests were conducted using a scaled-down transmission line with a 5 m air gap. The discharge point to the simulated conductor was analyzed in both dry and foggy conditions. Tests were also conducted with a DC bias applied to the simulated power lines, to take the operating voltage of the lines into consideration. These experiments have shown that the effect of fog on the discharge ratio to the conductors is negligible.

Mechanisms of guiding ability of laser-produced plasmas on pulsed discharges

Bead-like plasmas produced by a CO 2 laser have guided and triggered electrical discharges in air gaps. The physical parameters (electron density, electron temperature, geometric structure) of the laser-produced plasmas have been measured using spectroscopy and laser interferometry. We compared the results of these measurements with the characteristics of laser-guided discharges, and found that the mechanisms of the guiding ability of the laser can be explained by the behaviour of the plasma as a conductor (metal ball model). We conducted model experiments of the laser-guided discharges using small metallic balls. The features of the discharge guided by metal balls were similar to those guided by laser-produced plasmas. We thus concluded that the behaviour of the plasma plays an important role in triggering and guiding the discharges.