Analysis of aspects of lightning strokes to large-sized transmission lines

When designing large-sized transmission facilities, evaluating the risk of their lightning outage is very important, hence the authors have been conducting a study to determine a more precise method for calculating the lightning stroke rate and the lightning outage rate. In that study, it is essential to clarify the aspect of lightning strokes to large-sized transmission lines. In this context, the Tokyo Electric Power Company has observed lightning strokes to overhead ground wire or transmission tower (shielding success), for the first time as far as the authors are aware, and direct lightning strokes to a phase conductor (shielding failure). The present study analyzed and compared the ratios of shielding success or failure, and the (forward) flashover or back flashover outage rates based on the observation results of UHV-designed and 500 kV transmission lines. The number of lightning strokes to transmission facilities is 167.3 cases/100km/year for UHV-designed transmission lines and 130.8 cases/100km/year for 500 kV transmission lines, meaning the taller the facilities, the more lightning strokes occur. For these lightning strokes, the shielding failure rates were 1.80 and 0.98% for UHV-designed and 500 kV transmission lines, respectively, whereas the lightning outage rates due to direct lightning strokes (flashover) were almost equal at 0.37 and 0.39% for UHV-designed and 500 kV transmission lines, respectively. This is due to the balance between the shielding effect associated with the insulation clearance and the flashover voltage between arcing horns. Conversely, the lightning outage rate due to back flashover was mainly a factor of the distance between arcing horns and was 0.05% for UHV-designed transmission lines, which was very low compared with 0.59% for 500 kV transmission lines. Based on these results, since a comparatively high number of lightning outages were caused by direct lightning strokes on large-sized transmission lines, e.g. UHV class, the need for insulation design to tackle such cases was indicated.