Effect of humidity and air pressure on the discharge modes transition characteristics of negative DC corona

Corona discharges on high-voltage transmission lines cause the problems of energy waste and electromagnetic interferences. Previous studies on negative DC corona discharges are mainly on the effects of various environmental factors on the corona inception voltage, Trichel pulse frequency etc. However, few researches are focused on Trichel pulses-pulseless glow mode transition, and their transition mechanisms are still lacking. In this paper, a vacuum vessel is built up in which the humidity and pressure can be accurately controlled to simulate corona discharges under different climatic conditions. The pressure range is 50-101 kPa and the relative humidity (RH) is 10-90%. A needle-plane electrode with different radius of curvature is used to produce negative corona discharges. It is found that the transition voltage increases with the RH and air pressure. A higher humidity results in the formation of hydrated ions with lower ion mobility and the increase in the gas temperature around the needle electrode. The experimental results demonstrate that the effect of reduced ion mobility is greater than the gas temperature rise. Furthermore, an interim process, in which the Trichel pulses and the pulseless glow exist simultaneously, appears during the discharge modes transition for the needle electrode with a larger radius of curvature.