Occurrence probability of lightning failure rates at substations in consideration of lightning stroke current waveforms

To estimate lightning stroke overvoltages exactly, the occurrence probability of lightning stroke current waveforms must be accurately evaluated. This paper firstly formulated the occurrence probability distribution of lightning stroke current waveforms, taking into account the correlation between the current amplitude and the front duration. Next, lightning overvoltages were calculated, with the current amplitude and the front duration of lightning current as statistical parameters for Gas Insulated Switchgears (GIS's) and transformers in UHV substations. While overvoltages caused by back-flashovers at GIS's are affected by the front duration of lightning current, overvoltages at transformers are relatively less dependent on the front duration. Finally the failure rate was evaluated by considering not only the current amplitude but also the front duration. These values were smaller than those evaluated from the frequency of occurrence of current amplitude alone. Further, the proposed front duration of 1.7 μs was examined.