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Electrical power transmission is dominated by overhead line systems at present. This is mainly based on more than hundred years of experience of utilities in running overhead lines. Furthermore, overhead lines have proven their operational reliability and functional assurance. In the past, cables were used in distribution networks in urban areas for the most part with the exception of direct current submarine cables. New developments of high voltage XLPE cables make it possible to use this technology for EHV level applications in transmission networks. Within this paper, mixed network configurations, consisting of overhead lines and high voltage cables, are investigated. An exemplary EHV transmission line with a total length of about 100 km, which is quite typical for Central Europe, has been studied. Several different line combinations are discussed with varied rates between overhead line sections and cable sections length in practice. The length of the cable sections are ranging from several kilometers up to lengths of 100 km. In this paper the work focuses on the transient behavior of combined 400 kV overhead and cable lines during switching processes and lightning impacts. A number of calculations were carried out to get an overview of the transient stress in numerous network nodes along the transmission system. Numerical programs like ATP/EMTP have been used for these simulations. Peak values and wave shapes of the transient voltage stress have been evaluated, based on different systems and within possible combinations. In respect of the insulation coordination and transient stress at network nodes, the voltage-time trends are also analyzed. The combination of high voltage overhead and cable transmission systems, especially such with lengths of more than about 50 km, are making tightened and extended demands to the network design, to the operational management and of course to the network protection also. As an output of this investigations, the results might influence the strategy in running this new type of combined transmission systems.