1. Introduction: The Key Role of the Transmission Network

The transmission network, bridging the generation and distribution of energy, plays a pivotal role in the power system. This system must be capable of long-term and continuous energy transfer [1], which, given the long distances covered, leads to networks that are built and operated in AC or DC at high voltage levels [2]. The increasing growth in energy consumption has led to significant changes in the types of consumption as well as power generation sources, most importantly increasing the penetration of renewables in recent decades [2–4]. This trend is set to continue: Europe aims to achieve a 30% improvement in energy efficiency and a 27% increase in the use of renewable energies by making a 40% reduction in greenhouse gas (GHG) emissions by 2030, with the ultimate goal of completely eliminating GHG from power generation by 2050 [4–6]. Renewables can be located anywhere on the grid, causing dramatic and direct changes in the need for transmission. Besides, the need for tighter market integration calls for the expansion of cross-border capacity. These two main drivers have led to an increased need for transmission expansion that should be undertaken in the next couple of decades.