Modelling of the negative lightning impulse flashover of medium voltage overhead line insulators

• Negative lightning impulse flashover of typical MV insulators is investigated. • A leader progression model is developed describing accurately the flashover process. • The model is useful for evaluating the lightning performance of distribution lines. • The model can be applied in insulation coordination for power distribution networks. Medium voltage distribution lines are exposed to direct or nearby lightning strikes, causing, in the majority of cases, insulation flashover owing to its relatively low dielectric strength. Thus, achieving better understanding of the dielectric behavior of medium voltage line insulators under lightning overvoltages is essential for evaluating the lightning performance of distribution lines. This study presents an experimental investigation on the negative lightning impulse flashover of typical medium voltage line insulators. Salient discharge growth and flashover characteristics, obtained through simultaneous measurements of the applied voltage and discharge current, are assessed and discussed. The flashover process begins with streamer corona. This is followed by the inception and growth of a rudimentary leader, which is necessary for flashover to occur. Building on these experimental findings, the study models the flashover process of the insulators by implementing an appropriately modified CIGRE leader progression model in ATP-EMTP. A very good agreement between experimental results and simulations is achieved, regarding both discharge dynamics and flashover, using overvoltage dependent model parameters; the latter are expressed as functions of the steepness of the average potential gradient at leader inception. This approach provides valuable insights for improving insulation coordination for distribution networks, enhancing system reliability under lightning conditions.