Balkans blackout on June 21 occurred after two overhead lines tripped – ENTSO-E

The European Network of Transmission System Operators for Electricity (ENTSO-E) said the cascading power failure of June 21 ensued amid outages of two overhead lines 13 minutes apart. Its expert group is investigating the incident that left Bosnia and Herzegovina, almost entire Albania and Montenegro and parts of Croatia without power supply for three hours.

Following the blackout incident in the southeastern part of the Continental Europe power system region, ENTSO-E, the European Union Agency for the Cooperation of Energy Regulators (ACER) and the regulators and transmission system operators (TSOs) in the region held the first meeting of their joint expert panel.

It will investigate in detail the root causes of the incident and make recommendations in a final report, the statement reads. ENTSO-E preliminary laid out the sequence of events and praised the coordinated efforts of the affected TSOs for restoring the power supply in three hours.

Affected area was separated from Continental Europe power system

On June 21 at 12:24, due to a major incident, Bosnia and Herzegovina, almost entire Albania and Montenegro and parts of Croatia were without electricity for three hours. Their transmission systems suffered a voltage collapse. The rest of Continental Europe was not significantly affected by the incident.

Rest of Continental Europe was not significantly affected

Just before the incident, the affected area was importing about 2 GW from neighboring countries and exporting 417 MW to Italy through the Monita high voltage direct current (HVDC) subsea cable between Montenegro and Italy.

Two unexpected tripping of overhead lines occurred at 12:09 and 12:22. Immediately after the second tripping, voltage started to drop in a wide area. It reached such low levels to cause high currents and further low voltages on many lines. The incident triggered protection devices to disconnect the 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, leading to a blackout.

First breakdown was inside Montenegro

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

At 12:22, the 400 kV line Zemblak-Kardia between Albania and Greece tripped due to a short circuit as well. Both events thus resulted in an incident with several system state violations in terms of line loading and voltage. The voltage in the South-Eastern part of Continental Europe decreased rapidly.

At 12:24 CET, several lines disconnected one after the other. Voltages continued to drop to zero, whereas on the other side of the separation line they returned to normal, ENTSO-E explained.

The 400 kV grid was restored in less than three hours. The power to the local load was restored shortly, the preliminary report adds.

Of note, the Balkans are experiencing an extreme heat wave of almost three weeks now. The electricity system is strained by a surge in demand and a severe drought, amid limited interconnection capacities and poor flexibility tools. Power prices have skyrocketed.