Negative corona inception voltages in rod-plane gaps at various air pressures and humidities

The variation of negative dc corona inception voltages with air pressure and humidity, using a point/plane electrode system, was studied using a perspex chamber in which the pressure and humidity could be varied. A method of calculation was developed to determine the corona inception voltages over a range of air pressures and humidities. The effective ionization coefficient was calculated as a linear interpolation of the effective ionization coefficients for dry air and water vapor. The validity of these calculations was confirmed by a series of experimental determinations of the corona inception voltages for the same range of pressures and humidities. A comparison of the negative and positive corona inception mechanisms was also made. Both the negative and positive corona inception voltages decreased with a decrease in the air pressure and an increase in the humidity. The greater values of the effective ionization intensity found in the critical zone for the negative corona accounted for their lower inception voltages. The mechanisms of the affect of the photo-absorption coefficient and Townsend's second coefficient on the negative and positive corona were also considered.