Cascading blackout across Albania, Bosnia and Herzegovina, Montenegro and Croatia after vegetation contact

In June 2024, shortly after midday, a disturbance on the Continental European interconnected transmission system caused a cascading blackout affecting Albania, Bosnia and Herzegovina, Montenegro and Croatia. Immediately before the event the affected area was importing in the order of 2,000 MW from neighbouring systems and exporting in the order of 400 MW over a sub-sea high voltage direct current link. An overhead line tripped on a short circuit about fifteen minutes before the collapse, and a second 400 kV cross-border overhead line tripped about two minutes before it. Together these produced an N-2 condition with violations of line loading and voltage limits, after which voltage in the south-eastern part of the synchronous area began to fall rapidly. Multiple lines then disconnected in sequence on undervoltage and overcurrent protection, the system separated, and voltage collapse led to loss of supply across Albania, Bosnia and Herzegovina, Montenegro and Croatia. The transmission system operators in these countries restored supply with assistance from neighbouring operators. The regional coordination body for European transmission system operators convened an expert panel, joined by the European regulatory agency and the relevant national regulatory authorities, which published an interim factual report in November 2024 and a final report in February 2025.

As stated in the expert panel's final report of February 2025: (1) the two initial short circuits were attributed to vegetation that had grown too close to the conductors at specific corridor sections; (2) the resulting outages affected several transmission system operators, including those in Albania, Bosnia and Herzegovina, Montenegro and Croatia, at the same time without the operators concerned having an overall view of the state of the regional grid, which the report identified as a gap in cross-border real time situational awareness; (3) the subsequent cascade reflected the reduced margin of the system under N-2 conditions together with limited local voltage support in the affected region. The report set out recommendations covering cross-border coordination, regional situational awareness tools and vegetation management practices.

1. Survey and clear vegetation on high voltage transmission corridors on a risk based cycle, giving priority to sections with high growth rates or difficult access, because a single clearance shortfall can be the initiating event for a wide area cascade when system margins are limited.
2. Provide control room operators with a shared, near real time view of the grid state across neighbouring transmission system operators, so that a disturbance spanning several control areas can be assessed against a common picture.
3. Calibrate N-1 and N-2 contingency assessments to the actual operating configuration at the time, including large import or export flows, since a heavily importing area is more sensitive to the loss of local circuits.
4. Verify that reactive power and dynamic voltage support resources are available locally within the area exposed to voltage stress, and review whether protection driven disconnection of circuits could further weaken voltage support.
5. Review undervoltage and overcurrent protection settings and any defence plan schemes against a regional cascade scenario, recognising that events of this type develop over seconds to minutes and may outpace manual operator intervention.
6. Publish factual post incident findings through a regional panel or equivalent mechanism so that operators elsewhere in the synchronous area can adjust their own settings, procedures and reserves.

entsoe.eu/news/2025/02/25/entso-e-publishes-the-final-report-on-th...