Following the blackout incident in the south-eastern part of Continental Europe on 21 June, the the European Network of Transmission System Operators for Electricity (ENTSO-E), Agency for the Cooperation of Energy Regulators (ACER) and regulators and transmission system operators (TSOs) in the region held the first meeting of their joint Expert Panel.

On 21 June 2024 at 12:24 CET, due to a major incident in the Continental Europe power system region, a large part of the transmission systems of Albania, Montenegro, Bosnia and Herzegovina, as well as Croatia, suffered a voltage collapse followed by a total blackout in this area. The rest of the Continental Europe power system was not significantly affected by the incident.

Just before the incident, the affected area was importing about 2000 MW from neighbouring countries and exporting 417 MW to Italy through the HVDC sub-sea cable between Montenegro and Italy (Monita).

Within a short period of time (at 12:09 and 12:22 CET), two unexpected tripping of overhead lines occurred. Immediately after the second tripping at 12:22, the voltage started to drop in a wide area. These voltage drops continued until they reached such low levels to cause high currents and further low voltages on many lines in the region.

This triggered protection devices to disconnect these lines, resulting in the separation of the affected area from the Continental Europe power system. The affected area continued to experience a voltage decrease as well as a frequency drop. Finally, this led to a blackout in the affected area.

The investigations conducted so far, have yielded the following sequence of events:

At 12:09 CET, the 400 kV line Ribarevine – Podgorica2 in Montenegro tripped due to a short circuit. Following this outage, the loading of several other lines increased, without creating grid overloads, nor voltage or frequency problems.

At 12:22 CET, the 400 kV line Zemblak – Kardia between Albania and Greece tripped due to a short circuit as well. Both tripping events thus resulted in an (N-2) incident with several system state violations in terms of line loading and voltage. Following this second trip, the voltage in the South-Eastern part of the Continental Europe power system started to decrease rapidly.

At 12:24 CET, due to the continued drop in the region, several lines disconnected one after the other, due to undervoltage and overcurrent protections, causing a system separation.

The cascading power line tripping quickly led to a voltage collapse and a disruption of electricity supply in the affected area. Voltages continued to drop to zero, whereas on the other side of the separation line returned to normal.

After the disruption of the electricity supply, the affected TSOs worked together in a coordinated effort to restore the voltage on the 400 kV grid, which was restored in less than 3 hours. Subsequently, the power to local load was restored shortly thereafter.