A network outage last year that led to a widespread blackout in countries in south east Europe was initiated by two short circuit events in separate lines, both caused by vegetation too close to overhead lines, Entsoe-E said in a new report. The grouping of transmission network operators said the region’s record-breaking heat at the time of the event was a ‘contributing factor’.
The ‘significant incident’ on Friday 21 June 2024 led to major disruption in the power system, with substantial loss of load and generation in countries including Albania, Bosnia and Herzegovina, Montenegro and Croatia.
The sequence started in Montenegro when the Podgorica 2 – Ribarevine overhead line at 12:09 (Central Europe Summer Time) ,which tripped due to a short circuit caused by the shortening of the distance to the vegetation beneath the line. The Zemblak – Kardia line in Albania tripped shortly afterwards at 12:21 for a similar reason. This resulted in the tripping of 19 other transmission lines by 12.24 and a voltage collapse in Bosnia and Herzegovina, Montenegro, Albania and Croatia. The event caused a substantial loss of load and generation, with a total generation loss of 2,214 MW and significant load losses. Power was restored by 16.00.
Entsoe-E says “The analysis shows that the clear root cause of the trip was the vegetation not being trimmed in accordance with the operational limits (since the acceptable current is directly determined by the available clearance). The main reason behind this was the underestimation of the risk of a short circuit and the urgency of the situation.”
The report notes that the weather conditions were a ‘contributing factor’ in the event. It says the month was the joint-second warmest June on record for Europe. Air temperatures were “well above their 1991– 2020 average over southern Italy, southeast Europe and Turkey, reflecting the heatwaves that occurred in Cyprus, Greece and Turkey. Temperatures over 40 °C were recorded in many places” .
The ambient temperature is important because overhead lines can carry more power in cold conditions, as they are less likely to overheat. What is more, as the temperature increases in overhead lines they ‘sag’ more as the metallic components expand, meaning they are closer to the ground. This effect is more marked when wind speeds are very low as was the case on 21 June.
Entsoe-E also noted several other points around the weather conditions. The heat meant that there was increased power demand due to the increased use of air conditioning. Solar PV now surpasses gigawatt levels in some countries, but it is unequally distributed across the region and this increases power flows, including cross-border exchange. The group said “Comparing the power flows on several borders of the affected TSOs, one can note a change of historical patterns. … it is evident that on some borders, total physical energy exchange has been significantly increased or has even changed the historically dominant direction of flows.