A study of surface charges deposited on dielectrics by the breakdown of gases in short gaps

An investigation into the charges deposited on solid dielectrics due to surface discharge mechanism in short gap spaces containing air, oxygen and nitrogen gases is described. Experiments were carried out over a large range of electrical and geometrical parameters using a technique of powdering the insulation surface by a dielectric powder to reveal charge patterns produced. Particular attention has been paid to prespex, polythene and glass, these materials having been chosen as three of the most important dielectrics of present-day industrial practice. The effects of electric stresses, field configuration, electrode material and the time of voltage application using power-frequency alternating and direct-voltages at 293°K and 760 mm Hg has been studied. This paper describes a new technique which has been developed to study the mechanism of surface discharges in small gaps. From the analyses of the distribution of charges which develop the discharge sites it has been demonstrated that the discharge patterns were governed by the gap length, the electronegativity of the gas used, the magnitude of the applied voltage, the rise time of this voltage and the time taken to reduce it to zero. On raising the direct-voltage to the inception value direct discharges of certain polarity were formed. If the applied voltage is higher than the inception value discharges of direct polarity were noticed during the period of voltage rise, while discharges of different polarity were produced when reducing the voltage to zero and superimposed sites of alternative polarity were observed. When the voltage was shut-off immediately the reverse charges deposited were relatively small. This phenomenon was explained on the basis of the reversal of the local electric field. When power-frequency alternating voltages were applied, although discharge patterns of alternative polarities were produced at certain critical inception voltage, a discharge-free patterns were observed at voltages higher than the critical inception voltage. The dishcarge pattern was different in the positive half cycle than that in the negative half cycle. Experiments already made have given new informations on the fundamental processes associated with the electrical discharges and the process initiating breakdown.