Spatial Characteristics of Electric Current and Field in Large Direct-Current Coronas

The distribution of corona currents arising at the plane of a rod/plane gap has been studied for the past 90 years by using small gaps ranging from 25 to 200 mm but it has not been known until recently whether this distribution changes markedly when the gap is increased to several metres, the sort of gap which is encountered in buildings where very high direct voltages are generated or distributed. The flow of corona creates a space-charge which, in turn, modifies the electric gradient across the gap; such gradients have not previously been measured in large gaps. The present paper reports the current density and field distributions over the plane of rod/plane gaps up to 3 m. The time taken for the space-charge to drift across the gap and the length of the trajectory have been deduced so that a general model of the current flow has been constructed with reasonable accuracy. The inter-relationship of the modification by space-charge of the original applied field and the current density has been established experimentally and theoretically. The phenomena have been studied for corona of both polarities