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Stationary DC electric fields are determined by the resistivity distribution of the insulation materials. In the presence of interfaces between different insulation materials, charge accumulation at these boundary layers may have a significant influence on the electric field distribution and with it on the breakdown strength of the arrangement. In many previous works, surface charging of model spacers was investigated. However, the ion flow problem to properly account for the electric conduction in the gas has not yet been solved. The role of different ion sources and the behavior of the resulting ion currents in the gas phase on the charge accumulation process have to be investigated in more detail. In this paper, the transient charge accumulation process on a PTFE spacer surface with a focus on charges originating from the gas phase is investigated experimentally and by simulations. These charges are produced by natural radiation. The transition of the ohmic conduction regime in the gas to a saturated ion current with increasing voltage is analyzed. Furthermore, the importance of additional charge sources like partial discharges and field emission from the electrodes is highlighted. The experimental results indicate that additional charges are generated already at voltage levels significantly below the measured PD inception voltage. These charges originate from micro discharges on the electrodes and have to be taken into account when designing HVDC GIS applications. [ABSTRACT FROM AUTHOR]

Large-scale integration of new renewables requires substantial increases of transmission capacity on existing transmission corridors. To limit land use, overhead transmission lines have been widely built as multicircuit lines. On these, changing one ac circuit to dc to increase transmission capacity leads to lines with hybrid ac/dc towers. Since practically no operational experience with such hybrid lines exists to date, questions such as superimposed ac and dc fields intensified by corona-generated space charges arise. In this paper, a method to simulate these ion-flow fields using the discontinuous Galerkin method is for the first time applied to hybrid towers. Ways to considerably reduce the computational effort of the problem are discussed. The results indicate that hybrid lines are feasible in reference to space-charge-related problems. Shielding the ground is possible at the expense of increased coupling between the ac and dc circuits. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]