Transformation of an ovehead AC transmission line into a DC transmission line : a study case


Intensive integration of renewable energy sources (RES) will lead to fundamental conversions of the existing transmission systems in order to adapt to the new context. Frequently, existing overhead AC transmission lines do not have a sufficient transmission capacity to meet the requirements involved by the connection of a huge number of new RES. Therefore, overhead lines with a higher voltage level and/or ampacity are required for their connection. The transformation of some overhead AC transmission lines, utilizing existing towers and Right-ofWay (ROW), into DC transmission lines can turn out to be a possible option to increase power transfer capacity. This paper describes the preliminary study of the transformation of an existing 220 kV AC transmission line into a bipolar DC line with a metallic return conductor. After some considerations regarding the selection of a line to be converted from AC to DC, the study is conducted considering a typical tower configuration and a typical span for the 220 kV transmission line from the Croatian transmission system operator (TSO) – HOPS’s network. The most critical of the existing overhead transmission line technical aspects and limitations are covered and analyzed to accommodate a DC bipolar configuration with a metallic return. Firstly, the DC line’s general design and the existing clearances are analyzed to determine the possible operating voltage level, taking into account the insulation coordination requirements and the influence of pollution. The mechanical strength of the towers is analyzed and the existing conductors are evaluated from a thermomechanical point of view. Electric and magnetic field including the corona phenomena of the newly upgraded DC line are calculated to check the fulfilment of the de-facto international reference thresholds. Finally, conclusions regarding the chosen voltage level and the practical feasibility of the upgrade are presented.

Domagoj Hart, A. Xemard, S. Poullain,
Z. Rubinic, T. Dolenc, K. Pavic, I. Uglesic, V. Milardic, G. Levacic, M. Mesic

Presented at 41st CIGRE International Symposium, Ljubljana