Positive corona inception voltages and corona currents for air at various pressures and humidities

The variation of positive dc corona characteristics with air pressure and humidity using a point/plane electrode system was studied in a perspex chamber allowing the pressure and humidity to be varied. A method of calculation was developed to determine the corona current for humid air over a range of pressures and humidities in which the effective ionization coefficient was calculated as a linear interpolation of the effective ionization coefficients for dry air and water vapor. The calculation method involved the solution of partial differential equations using a particle-in-cell algorithm. This also allowed the determination of the corona inception voltages for a range of pressures and humidities as the voltage at which the corona current first appeared. The validity of these calculations was confirmed by a series of experimental determinations of the corona inception voltages and of the variation of the corona current with the applied voltage for the same range of pressures and humidities. It was found that the corona current increased when the air pressure or humidity decreased (at constant applied voltage) mainly because of the rise in the space charge density or the positive ion mobility, respectively. The coefficient C, in the relationship between the corona current, the applied dc voltage and the corona inception voltage, declined with the increase of humidity. The rate of decrease of C with humidity became larger as the pressure was reduced. An approximately inversely proportional relationship was found to exist between C and the relative air density at constant humidity. This variation of C was principally associated with the variation in the positive ion mobility.