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The features of lightning shielding of transmission lines have been calculated using the electrogeometric model (EGM) proposed by Armstrong and Whitehead. Discrepancies, however, have been found in the case of ultra-high voltage (UHV)-designed transmission lines. UHV-designed transmission lines often pass through high-altitude mountain areas, where foggy or rainy conditions are common when there is lightning. While the electrogeometric model determines the striking distance based on the lightning stroke current, environmental conditions, and how they might affect lightning discharge paths, they are not considered. It is possible, therefore, that foggy conditions may cause differences in the performance of transmission-line lightning shielding. This study used scale models of transmission lines for discharge tests with a 1-m air gap and analyzed differences in discharge paths and locations in dry and foggy conditions. The results showed that the polarity of the discharge had a significant influence, but differences in dry and foggy tests were negligible. Index Terms--Discharge characteristics, foggy conditions, lightning shielding, ultra-high voltage (UHV)-designed transmission lines.

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. Index Terms--Lightning, shielding, discharge characteristics, fog, long air gap, switching impulse.

The electrogeometric model (EGM) proposed by Armstrong and Whitehead has been used to calculate the lightning shielding features of transmission lines. While the striking distance equation in the EGM uses the 50% flashover voltage characteristics of negative switching impulses for rod-rod gaps, those characteristics for long air gaps have not been sufficiently examined. The present study has therefore focused on clarifying the 50% flashover characteristics of negative switching impulses for long air gaps. Consequently, saturating characteristics have been obtained for rod-plane gaps compared with almost linear characteristics for rod-rod and rod-conductor gaps. Furthermore, new knowledge was obtained indicating that, for air gaps of around 4 to 5 m, the 50% flashover voltage may possibly be lower for rod-plane gaps than for rodrod and rod-conductor gaps. Index Terms--Switching impulse, negative polarity, flashover characteristics, long air gap.

For rational insulation design of a UHV substation, full-scale switching impulse tests of phase-to-ground (shielding ring-to-tower) and phase-to-phase (shielding ring-to-shielding ring) insulating clearance have been carried out in dry and wet conditions. In the tests, shielding rings that will be installed on bushings at actual UHV substations were used. The sparkover voltages depend on not only gap configurations but also the waveform of applied switching impulses; in other words, they show what we refer to as U-characteristics. Critical sparkover voltages are higher than those of rod-rod gaps by 20%-25% in dry conditions and by 7%-10% in wet conditions. Index Terms--Air gaps, air insulation, flashover, surges, UHV substations.

A new approach to characterizing 'anomalous' flashover phenomena is presented and a method of calculating the probability of anomalous flashovers occurring is proposed. The calculated probabilities of anomalous flashovers are compared with experimental results, and the new method is used to estimate the safety clearance for UHV substations. Key Words: Anomalous flashover, Flashover probability, Parallel gap, Insulation coordination, Safety clearance

Experiments to guide electric discharges with a chain of air-breakdown plasmas (plasma channel) produced by a laser were carried out. Electric discharges were guided up to 4.5m with a high power CO2 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.

Experiments to guide electric discharges with a chain of apparently discrete air-breakdown plasmas (plasma channel) produced by a laser were carried out. The electric discharge was guided up to 2m with a high power CO2 laser focused by a 10-m focal length mirror. Voltage was applied at selected delay times tau following laser radiation. The relations between 50% flashover voltage of a gap filled with laser-produced plasmas and delay times tau, between the guided length and the peak of the applied voltage, were obtained experimentally. The effects of the polarity of an applied voltage and the position of the plasma channel on flashover voltage are described. Development characteristics of the guided discharge were also investigated.

A new shielding theory is proposed based on the assumption that the occurrence of lightning strokes on the Japan Sea coast in winter is due to the inception of upward leaders from tall structures. Ratios of the numbers of lightning strokes to high structures observed there in winter show reasonable agreement with values calculated by this theory. Shielding characteristics of a high structure in various conditions are predicted.