This study analyzes the impact of the European energy transition on system adequacy and cross-border electricity exchange under high shares of variable renewable energy sources. The focus is on Germany and its neighboring electricity markets under the assumption that all countries meet their 2030 National Energy and Climate Plan (NECP) expansion targets for wind and photovoltaic (PV) capacities. Particular attention is given to the simultaneity of extreme meteorological conditions, namely extended low-generation periods (Dark Doldrums) and high-generation surplus events (Bright Breezes).

A modified Sequential Peak Algorithm (SPA) with adaptive thresholds is applied to construct deficit-oriented residual load time series for Germany and interconnected European electricity systems. The method enables the identification of extreme scarcity and surplus events while explicitly accounting for transmission constraints, interconnector capacities, and synchronized renewable generation patterns across countries. The analysis further incorporates projected grid expansion, including additional High Voltage Direct Current (HVDC) links, as well as assumptions on storage and sector coupling development.

The results show a pronounced seasonal and spatial structure of system stress. Short-term imbalances, particularly during summer surplus periods, can be effectively mitigated through battery storage and flexible demand response. However, prolonged winter scarcity events remain a critical challenge even under significantly expanded renewable capacities. In addition, increasing wind and PV generation leads to growing curtailment, as projected electrolysis and other sector coupling capacities are insufficient to absorb sustained surplus energy.

A key finding is the high simultaneity of extreme weather-driven generation patterns across Germany and neighboring countries, which significantly reduces the effectiveness of geographic diversification within the European power system. Even with expanded cross-border transmission infrastructure, correlated meteorological conditions result in concurrent scarcity or surplus situations across multiple regions, limiting the balancing potential of electricity trade.

The study concludes that interconnectors alone are insufficient to ensure future system adequacy under high renewable penetration. While short-term variability can be managed through storage and demand-side flexibility, long-duration deficits require a combination of expanded transmission infrastructure, large-scale seasonal storage (e.g., hydrogen), and enhanced sector coupling. Overall, the findings highlight the importance of system flexibility and efficient utilization of renewable generation to maintain security of supply in a highly renewable European electricity system.