Following the blackout incident in the south-eastern part of Continental Europe on 21 June, today ENTSO-E, ACER and regulators and transmission system operators (TSOs) in the region held the first meeting of their joint Expert Panel. The Expert Panel will investigate in detail the root causes of this incident and will make recommendations in a final report , which will also be made public .– With this update, ENTSO-E reports on the sequence of events of the incident which, thanks to the coordinated efforts of the affected TSOs, was concluded within a few hours with the power supply restoration.

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.

Figure 1 depicts the area affected by the voltage collapse and disruption of electricity supply:

Figure 1 – Geographic area affected by the incident of 21 June 2024 (in black).

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 (indicated in black in Figure 1) 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:

1. 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.
2. 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.
3. 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.
4. 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.

Figure 2 - Sequence of events at the beginning of the incident on 21 June 2024 

In the non-affected area (indicated in blue in figure 1), the voltage restored very quickly to 400 kV and the frequency remained within the normal interval (between 49.95 and 50.05 Hz).

Figure 3 – Voltages in the region during the event on 21 June 2024 

After the disruption of 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. 

All information published to date, including the present communication, is subject to the results of a detailed investigation on the incident. The investigation follows the legal framework under the Commission Regulation (EU) 2017/1485 of 2 August 2017 (System Operation Guideline), where National Regulatory Authorities and ACER are invited to join with the TSOs in the Expert Panel. 

In line with the provisions of the aforementioned Commission Regulation (EU) 2017/1485, ENTSO-E will present the results of the investigation to the Electricity Coordination Group and will subsequently publish them once the analysis is completed. 

Note: Any technical information is a subject of confirmation and can still change following the investigation

About ENTSO-E

ENTSO-E, the European Network of Transmission System Operators for Electricity, is the association for the cooperation of the European transmission system operators (TSOs). The 40 member TSOs, representing 36 countries, are responsible for the secure and coordinated operation of Europe’s electricity system, the largest interconnected electrical grid in the world. In addition to its core, historical role in technical cooperation, ENTSO-E is also the common voice of TSOs. More information on ENTSO-E can be found on our website www.entsoe.eu or contact media@entsoe.eu