Assessment of shielding performance of 69/138 kV transmission lines exposed to lightning strikes

Lightning is the leading cause of power interruptions on overhead transmission lines. The insulators used on overhead 69 kV and 138 kV transmission lines are likely to flashover whenever there is a direct lightning strike on the phase conductors due to their relatively low insulation levels. However, significant improvement can be achieved with the appropriate design of transmission line tower structures. This paper presents an analysis of the shielding failure flashover rate (SFFOR) occurrence on both 69 kV and 138 kV transmission tower lines and tower structures. This analysis is based on an electro-geometrical model recommended by the IEEE Standards 1243:1997. The analysis accounts for the striking distance of the geometry of the transmission lines and tower structures, the critical impulse flashover voltage (CFO) of the insulators, the statistical distribution of lightning current parameters and the keraunic level of the location of the lines. The proposed assessment is supported by the recommendations of CIGRE TB 549 (2013) for lightning parameters, which define the statistical distributions of lightning current parameters. The results show that shielding angle lower than 30 degrees predicts substantially lower SFFOR. Also, insulators with higher CFO ratings contribute to lower SFFOR. The analysis presented, therefore, could be applied to the design of high voltage transmission line support structures.