Influence of polarity on breakdown voltage of gases in divergent electric field under lightning impulse voltages

This paper deals with the influence of the polarity of voltage of different gases (N₂, dry Air, SF₆ and CO₂) and mixtures (SF₆-N₂, SF₆-dry air and SF₆-CO₂) on breakdown voltage (U_BDV) in a sphere-plane and point-plane electrode arrangements under standard lightning impulse voltage stress. It's shown that in point-plane electrode geometry, the negative breakdown voltage UNBDV is always higher than the positive one (U_PBDV) for all investigated gases. While in a sphere-plane electrode system, this rule is not respected. Depending on the electrode gap, the type of gas and its pressure, U_NBDV can be either lower than U_PBDV (dry air), or first higher and then lower or yet first lower and then higher; the inversion appears at a given pressure that it's not the same according to the gas, gap and polarity. For gas mixtures, U_NBDV is always higher than U_PBDV whatever SF₆ content.