The synchronous area of Continental Europe was split into two separated grid regions between 14:05h CET and 15:08h CET on 8 January 2021.
An area in the south east region of the interconnected grid was, during that period, separated from the rest of Continental Europe. A temporary frequency drop of approximately 250 mHz was registered. Co-ordinated actions and an immediate response taken by the Continental European Transmission System Operators ensured that the system stability was not affected in most European countries. An investigation is ongoing on the cause of this system split.
At about 14:05 CET, the frequency in the North-west Area of Continental Europe initially dropped to 49.74 Hz for about 15 seconds. Afterwards the frequency reached a steady state value of approximately 49.84 Hz. At the same time the frequency in the South-east area initially increased to a value of up to 50.6 Hz before settling at a steady state frequency between 50.2 and 50.3.
Due to the under-frequency in the North-west Area, contracted interruptible services in France and Italy, in total around 1.7 GW, were disconnected in order to reduce the frequency deviation. These services are large customers who are contracted by the respective TSOs to be disconnected if frequency drops under a certain threshold. In addition, 420 MW and 60 MW of supportive power were automatically activated from the Nordic and Great Britain synchronous areas respectively. These countermeasures ensured that by 14:09 CET the frequency deviation was limited to a deviation of around 0.1 Hz in the North-West Area from the nominal frequency of 50 Hz.
Due to the large overfrequency in the South-east Area, automatic and manual countermeasures were activated (ie the reduction of the feed-in of generation units) in order to stabilise the frequency. Thus, at 15:05 CET the frequency deviation in the South-East Area could be limited to +0.1 Hz from the nominal frequency of 50 Hz.
The automatic response and the co-ordinated actions taken by the TSOs in Continental Europe ensured that the situation was quickly restored to normal operations. Actions were then co-ordinated between the TSOs in order to reconnect the two areas as fast as possible. The contracted interruptible services in Italy could be reconnected at 14:47 CET and in France at 14:48 CET. At 15:08 CET, both areas were connected again to one synchronous area in Continental Europe.
A detailed investigation on the event has stated and is ongoing. The investigation follows the legal framework under the Commission Regulation (EU) 2017/1485 whereby National Regulatory Authorities and ACER are invited to join together with TSOs in the investigation panel.