Propagation of creeping discharges in air depending on the electric field direction and insulator materials under lightning impulse voltage

This paper is devoted to the influence of the direction of the applied electric field (namely perpendicular and parallel to the insulator surface) and the type of insulator materials on the propagation of creeping discharges (pattern and stopping length) in presence of air under standard lightning impulse voltage. The investigated materials belong to two distinguished families among the mostly used in electrical industry: (i) thermoplastics (namely polyamide 6 and polyarylamide), and (ii) one cycloaliphatic filled epoxy resin. It is shown that the stopping discharges length on a solid/air interface covers generally 55% of the creeping distance for both polarities under perpendicular E-field right before the flashover. While with parallel E-field, the stopping length is included between 20 to 30% for positive polarity and less than 25% for negative polarity. Also, the application of a perpendicular E-field is more straitened than the tangential one due mainly to the dielectric constant of polymers. Numerical simulations show that the electric field is more reduced for perpendicular E-field inferring that discharges (streamers) lengthen and develop easily at lower voltage levels.