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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The electrogeometric model has been used to calculate the transmission line lightning shielding characteristics. However, actual UHV design transmission line accidents and the lightning observation results indicate that there are differences between what actually happens and theoretical results. In this paper, long-gap discharge tests were conducted to better understand lightning shielding characteristics, mainly of UHV design transmission lines. As far as the test method is concerned, the arrangements of transmission line conductors were reduced, and simulated as ground electrodes, and a switching impulse voltage was applied to rod electrodes. As a result, discharges occurred most frequently to the conductors with the smallest gaps with the rod electrodes. On the other hand, it was seen that the electric field strength of the transmission lines and their position with respect to ground might have some influence. These results might also be valuable for long air gap dielectrics. [ABSTRACT FROM AUTHOR]