Transition mechanism of negative DC corona modes in atmospheric air: from Trichel pulses to pulseless glow

Corona discharge in high voltage power transmission lines can generate audible noise and radio inference, causing environmental problems in daily life. There are three different discharge modes of negative corona depending on DC voltage amplitude, namely, Trichel pulses, stable glow and negative streamers. A 2D fluid model, incorporating three species (electrons, positive oxygen ions and negative oxygen ions), was successfully built to simulate the formation of repetitive Trichel pulses in air. However, this simplified model cannot correctly predict the transition to pulseless glow. The calculated transition voltage is higher than experimental results. In this work the gas dynamics and a detachment reaction are taken into account in the model. Gas heating due to the deposited discharge energy will influence the chemical reaction rates and reduce the gas density. Besides the mobility of charged particles, the ionization and attachment coefficients are determined by the reduced electric field. The calculation results of the gas density from the gas dynamics model are coupled back to the discharge model. The corona discharge in a needle-cylinder electrode configuration is simulated and the results show that the mode transitions at a lower voltage, which agrees well with the experimental data. The ionization rate is greatly enhanced around the needle because of the gas density reduction. Therefore, the reduced electric field will not easily be decreased by the generation and accumulation of negative ions. A higher detachment rate also causes the Trichel pulses to disappear. The simulation results suggest that gas heating is more important for the mode transition of negative coronas from Trichel pulses to pulseless glow than was previously expected.