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 AC- and 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