Lightning flashovers on 77-kV systems: observed voltage bias effects and analysis

Twenty-nine backflashovers were recorded on a 77-kV double-circuit transmission line with single overhead groundwire. Twenty-six of these occurred on phases where power-frequency voltage bias was positive. EMTP analysis of the line shows that the multistory tower model, recommended by the Japanese Guideline of Insulation Design/Coordination against Lightning, suggests a stronger influence of phase position than was observed. Best simulation results were obtained with a simple tower model of constant surge impedance.