Simulation of the space charge near coronating conductors of ac overhead transmission lines

In the case of ac high voltage overhead transmission lines, corona discharges may result in a broadband crackling and a humming noise at twice the mains frequency (2f). The computation of the emitted 2f-levels presupposes the knowledge of the apparent power and force passed by the ions to the gas in the surroundings of the conductors. In this paper a simulation determining these magnitudes used in a previous publication is presented. For that purpose the ideas of earlier publications by other authors to compute corona losses are followed. Their central idea is to define an onset field strength on the surface of the conductor which is not exceeded by injecting the right amount of charges in the vicinity of the conductor. This injection together with the recombination determines the amount of drifting ions. The novelty of the method applied in this work is the consideration of different onset field strengths over the conductor surface in dependence of the polar angle. Further 2f-force and 2f-power acting on the gas in the vicinity of the conductor are determined out of the resulting ion drift. The corona currents thus computed coincide fairly well with measured results.