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Lightning strokes to transmission lines have been estimated using the electro-geometric model proposed by Armstrong and Whitehead. The observed results of the lightning to large-scale transmission lines are reported here to validate the calculated results, in which ultra high voltage (UHV) designed transmission lines and 500 kV transmission lines were selected as the subjects of the observation. Lightning observations were carried out for direct strokes to phase conductor caused by shielding failures, as well as strokes to ground wires that have rarely been previously reported. The observed results showed that total number of direct lightning strokes to phase conductor were nearly identical to the calculated results based on the conventional method, while observed strokes to upper phase were larger and strokes to lower phase were smaller than those derived from calculations. It was also revealed that lightning strokes to ground wires of UHV designed transmission lines and 500 kV transmission lines were 5.1 times and 2.7 times larger, respectively, than calculated results based on the conventional method. [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]