Overhead transmission line performance with respect to grounding impedance on alpine terrain

The present paper shows a study case of transient effects of direct and indirect lightning discharges on overhead transmission lines with respect to actual grounding impedances of the transmission line system. The transient measurement data was gathered from two 400 kV double-system overhead lines equipped with an overhead ground wire. Additionally installed RC dividers are used to perform transient voltage measurements with high accuracy in amplitude and frequency. The pairing of electric, time and geographic parameters of the used datasets provides the basis for an analysis of the performance of the overhead lines and the high-voltage system influenced by lightning discharges. For this case study, a novel approach is presented where the actual tower-footing grounding impedance of the two systems is related to the transient effects of direct and indirect lightning discharges on the lines as new parameter. The topic, whether it be exposed overhead transmission line systems in the alpine area or line sections with high grounding impedances that show a higher probability of power system faults, is analysed and interpreted in this case study. The results show no correlation between tower exposure (mountainous or flat terrain) and lightning discharge frequency. For the correlation of the tower footing grounding impedance with the recorded transient measurements, no accumulation of transient effects at towers with higher grounding impedance can be observed either. Regarding the analysed dataset, a high reliability of supply can be expected for systems with a similar setup and comparable lightning parameters. Grounding concepts of existing overhead lines can be validated by these results.