Discussion of Converting a Double-Circuit AC Overhead Line to an AC/DC Hybrid Line with Regard to Audible Noise

That the large-scale integration of new renewables requires a substantial increase in transmission capacity seems to be an undoubted fact today. As new rights-of-way for transmission lines are increasingly difficult to obtain, alternatives are discussed to increase transmission capacity within the existing rights-of-way. One possibility to do so is the conversion of existing AC lines to DC. In the past, already for the very same reason of limiting the land use, overhead transmission lines have been built as multi-circuit lines on the same structure in many countries. Especially in densely populated areas such as Central Europe, bundling of circuits is inevitable although it results in higher construction costs. In such situations, changing one AC circuit to DC will result in transmission lines with AC/DC hybrid towers. As, to the authors’ knowledge, practically no operational experience with such hybrid lines exists to date, such schemes have to be assessed rigorously before any realisation. Especially with the close proximity of ACand DC-circuits, many potential problems can be identified. Among them is the change in surface gradients, which may lead to different kinds of corona activity: The AC circuit induces an AC ripple in the surface gradients of the DC conductors, while the DC circuit induces a DC bias in the surface gradients of the AC conductors. According to the literature on corona and the resulting audible noise (AN) of such hybrid overhead transmission lines, calculation of the AN can be performed with the methods developed for pure DC and AC transmission lines. But disagreement remains on whether the DC-offset in surface gradients of the AC-conductors should be accounted for within these calculations or not. In this paper, these calculation methods are applied to a possible scheme of converting a 400 kV AC double circuit line to a hybrid line by replacing one AC by a ±500 kV DC circuit. After assessing the considerable increase of the thermal rating, different tower configurations are considered and the resulting electric surface gradients and AN are compared with those from the original 400 kV AC double circuit line. As to AN, the best configuration out of those considered is the horizontal arrangement of the AC-circuit on the lower cross-arms, while the DC-circuit is placed above, also in horizontal arrangement. As a further advantage, this geometry also provides the greatest shielding of the environment from ion currents originating from DC-corona. However, with this arrangement, the coupling between the two circuits is greatest, with largest DC-currents in the AC-system and vice versa.