Monte Carlo evaluation of the impact of subsequent strokes on backflashover rate

The paper deals with the impact of subsequent strokes on the backflashover rate (BFOR) of HV overhead transmission lines (OHLs), assessed by means of an ATP-EMTP Monte Carlo procedure. The application to a typical 150 kV Italian OHL is discussed, simulating three different tower grounding system arrangements. Subsequent strokes parameters are added to the statistical simulation variables: peak current, front time, time-to-half value, lightning polarity, line insulation withstand, lightning location and phase angle of the power frequency voltage. The input data are fed to an ATP-EMTP circuit model of the OHL, including line insulation, lightning representation and tower grounding system, the latter simulated by a pi-circuit model able to simulate the effects due to propagation and soil ionization. Numerical results evidence a non-negligible BFOR increase for the simulated, spatially concentrated grounding systems due to subsequent strokes, especially for low grounding resistances.