Effect of humidity on dielectric breakdown properties of air considering ion kinetics

This paper investigates the dielectric breakdown properties of humid air, taking into account ion kinetics. Initially, an overall kinetic scheme that includes all the main reactions likely to happen in humid air is established for the first time. Then, the calculation method of dielectric properties is improved, based on more comprehensive avalanche model considering both spatial growth and temporal processes. Better consistency between present results and experimental values confirms the validity of the improved method and kinetic schemes. Finally, the effective ionization Townsend coefficients a alpha(eff) in humid air at different humidity and gas pressures are calculated and analyzed considering ion kinetics. Reduced critical electric field strength (EIN)(cr) can be determined when a alpha(eff) = 0. The results show that (EIN)(cr) in humid air increases sharply in the beginning and then tends to be constant with the humidity rises. Besides, the mechanism of humidity's effect on dielectric breakdown properties is well explained: at relatively low EIN, alpha(eff) decreases with the increase of humidity mainly due to more frequent three-body conversions, while at higher EIN, the main reason is the increased number of attachment reactions.