Optimization of Corona onset and breakdown voltage of small air gaps stressed by DC and impulse voltages

The present paper aims to the investigation of the methods used to minimize or maximize the values of the Corona onset and Breakdown voltage in small rod-plate air gaps when stressed by dc or impulse voltages. The main factors which influence greatly the distribution of the electric field in the gap, and hence the above values are the geometry and the selection of grounding and charging of the electrodes, (ground effect), the gap length, the existence of barrier in the gap (barrier effect), and the Corona effects appearing prior to breakdown. Combining theoretical, simulation and experimental work, it is resulted that: a) The electrode chosen to be ground, strongly influences the distribution of the field and the Corona effects and hence the values of the Corona onset and breakdown voltage. When the rod is grounded and the plate is negatively charged the value of Corona onset voltage is higher, the corona effects are less intense and dc breakdown voltage, without Corona, is higher. When the plate is grounded and the rod is negatively charged the value of the Corona onset voltage is lower, the corona effects are more intense and hence the dc breakdown voltage is higher. b) A dielectric barrier, when placed in specific positions in the gap, decreases the Corona effects and raises the breakdown dc voltage. c) In air gaps stressed by impulse voltages the results are very different. The corona onset and the breakdown voltage are maximum when the rod is grounded and positive leading to more intense Corona effects.