Evaluation of CFO for medium voltage insulators submitted to non-standard impulse shapes experimental results

This paper aims at presenting and discuss the critical flashover voltage - CFO results of medium voltage distribution insulators submitted to non-standard lightning impulse shapes. The insulators have been tested by applying the up-and-down method on dry and wet conditions, as well as positive and negative polarities for 13 non-standard shapes and the standard impulse shape 1.2 x 50 μs. CFO comparisons allow distinguishing the influence of front and tail times on the withstanding voltages. The results assured that the standard impulse shape is adequate to represent some induced lightning stresses during dielectric tests and lightning studies. Also, the CFO on positive polarity, dry condition, is in most of the cases, enough to determine the lowest CFO level for a particular shape. Considering insulation coordination, there is no loss of performance of distribution networks constructed with the tested insulators when stressed with fast front impulse shapes.