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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.

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.