ENTSO-ETransmission system operators (TSOs) are responsible for operating, managing and developing the transmission system. At European level, transmission system investments are required to achieve the EU’s climate and energy policy goals. Besides ensuring security of supply and market integration, the integration of large and smaller scale renewable energy sources (RES) into the power system has been the most significant driver of grid investments, with up to 80% of grid investments related to the integration of RES. As a consequence, the power system has been undergoing transformative changes throughout which TSOs have to ensure system security.