Estimation of the minimum shielding failure flashover current for first and subsequent lightning strokes to overhead transmission lines

• The minimum shielding failure flashover current of overhead transmission lines is estimated through ATP-EMTP simulations.
• This critical current depends on insulator flashover modelling and is lower for first than subsequent lightning strokes.
• A modification of the IEEE Std 1243-1997 simplified expression for the critical current of transmission lines is suggested.
• The estimated SFFOR of several overhead transmission lines is lower than that obtained according to the IEEE Std 1243-1997.

ATP-EMTP simulations are performed to estimate the minimum shielding failure current causing flashover in overhead transmission lines with operating voltage in the range of 66 kV up to 735 kV. This critical current, affecting shielding failure flashover rate, is of great importance for assessing the insulation coordination of overhead transmission lines and the connected substations. The minimum shielding failure current causing flashover of line insulation is highly dependent upon insulator string flashover modelling and, also, markedly higher than that calculated according to the relevant IEEE Std 1243-1997 simplified expression. A modification of the latter is suggested by using multiplication factors of 1.5 and 1.65 for first and subsequent lightning strokes, respectively, so as to account for the increased dielectric strength of line insulator strings under non-standard lightning overvoltage surges. Alternatively, the critical currents can be respectively estimated by using average negative breakdown gradients per unit length of insulator string of 680 kV/m and 750 kV/m. The shielding failure flashover rate of the overhead transmission lines, being greatly influenced by insulator string flashover modelling, is lower than that obtained based on the critical current according to IEEE Std 1243-1997.