Yoshinori Aihara

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.

Influence of Humidity on Lightning and Switching Impulse Flashover Voltages

A long run flashover test was carried out from 1967 to 1969 to investigate the effect of atmospheric humidity on lightning and switching 50-percent flashover voltages of air gaps and suspension insulators. From the test results, it was found that the influence of humidity on flashover voltages of air gaps varied greatly from their flashover voltage-gap length characteristics.

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.

Development of a 500 kV transmission line arrester and its characteristics

A 275 kV gapped type arrester was developed and tested on an EHV transmission line which is liable to be extensively affected once a line fault occurs. The study was followed by the development of a 500 kV gapped type arrester, and its field test on an existing line was started in September 1990. The main characteristics of the 500 kV gapped type arrester are discussed, and evidence to corroborate its applicability to 500 kV transmission lines is presented. The series gap length was reduced by adopting an optimal electrode configuration and a reasonable concurrence probability of the arrester unit and a switching impulse, in order to design the practicable gapped type arrester for existing 500 kV transmission towers. It was demonstrated that the 500 kV gapped type arrester can be depended upon to cut off the follow current even when it is contaminated. Satisfactory insulation coordination with the existing insulator assemblies was demonstrated. >

Influence of Switching Impulse Wave Shape on Flashover Voltages of Air Gaps

Flashover tests were conducted by using three kinds of positive polarity switching impulses, exponential, damped oscillatory and linearly rising impulses. The influence of switching impulse wave shape on the flashover characteristics of air gaps was investigated and the basic data were obtained concerning the minimum 50-percent flashover oltage of air gaps and the time to crest of these switching impulses.

Polarity Effect on Breakdown Voltage in Organic Liquids

The polarity effect on breakdown strength is a well-known phenomenon using a nonuniform electrode system, especially point versus plate electrodes. When the point electrode is negative, particularly, free electrons move in a liquid to form negative ions due to the possibility of electron attachment to electrically neutral atoms or molecules. Negative ions form space charges, by which field stress around the point electrode relaxes. Breakdown voltages have been measured in four liquids having chemical structures differing markedly from each other. It has been found that the breakdown voltage with the point negative is higher than that with the point positive in toluene and n-octhane which have a methyl group in their molecular structure; but there is no such polarity effect in chlorobenzene and carbon tetrachloride which have chlorine atoms. It may be concluded that the polarity effect depends on molecular structure: Electrons attach to methyl groups to make negative ions which may form space charges.

Impulse Flashover Characteristics of Long Air Gaps and Atmospheric Correction

The flashover voltages of long air gaps such as rod-rod and rod-plane gaps vary with impulse waveform, polarity, gap spacing and atmospheric conditions.

Laser-guided discharges in long gaps

Experiments to guide electric discharges with a chain of air breakdown plasmas (plasma channel) produced by a laser were carried out for use in power system protection applications. Electric discharges were guided up to 4.5m with a high power CO/sub 2/ laser focused by a 10m focal length mirror. Unusual 50% flashover voltage characteristics were obtained and the mechanism on the laser-guided discharge is shown. Experiments of laser-guided discharge in fog or rain were also carried out to clarify the effects of precipitation on the guiding characteristics, and the results are analysed quantitatively. >

Model experiment of upward leaders-shielding effects of tall objects

Discharges originating from laboratory upward leaders are studied experimentally with a configuration consisting of two parallel vertical rods at ground and an elevated plane. The flashover probability of each rod, the effect of gap configuration on that probability, and the shielding effect of the taller rod on the smaller one are investigated. Experimental results are analyzed, taking into account the deviation of time to flashover of the gap. Application of the results to the case of natural lightning is also discussed. >