Cross-Border Energy Infrastructure and Regional Energy Security: Empirical Evidence from the Poland–Baltic States Corridor

The Baltic states disconnected from the Soviet-era BRELL ring and synchronized with the Continental European grid in February 2025, completing a decade of new electricity and gas interconnections in the Poland–Lithuania–Latvia–Estonia corridor. This study examines how energy security evolved across the four markets as those interconnections were added, drawing on ENTSO-E cross-border flow data, Eurostat energy balances and ENTSOG gas transmission statistics for 2018–2025, and builds a composite Baltic Regional Electricity Security Index (BRESI) from three dimensions of electricity security: supply diversification, interconnection utilization and import dependency, with price convergence analyzed separately. The gains were uneven. Lithuania still imported 47% of the electricity it consumed in 2024, whereas Poland covered almost all of its own demand. Synchronization first sent prices sharply higher, with Lithuanian peaks of EUR 325/MWh against a January average of EUR 88/MWh, before the market settled. Across the corridor, electricity links ran at about 60 to 70 percent of capacity and gas links at 35 to 50; security improved where flows and market coupling were in place, while idle capacity added little. The index rose for all four countries between 2019 and 2024; the improvement holds under every aggregation and weighting variant tested, and monthly price spreads averaged EUR 19/MWh between Poland and Lithuania against under EUR 5/MWh inside the Baltic market, a pattern that persisted after synchronization. On this basis the study argues for more storage, earlier delivery of the Harmony Link, and shared balancing to cope with variable renewable output.