On 8 January 2021 at 14:05 CET the synchronous area of Continental Europe was separated into two parts owing to outages of several transmission network elements in a very short time. ENTSO-E published its preliminary report on 8 January. Since then it has analysed a large portion of relevant data aiming to reconstruct the event in detail.
Its second update presents the key findings of detailed analyses, but these have a preliminary character subject to new facts that are expected to emerge in the still ongoing investigation.
The analysed sequence of events concludes that the initial event was the tripping of a 400 kV busbar in the Ernestinivo substation in Croatia by overcurrent protection at 14:04:25 CET. This resulted in a decoupling of the two busbars in the Ernestinovo substation, which in turn separated North-West and South-East electric power flows in this substation. North-west bound lines remained connected to one busbar, connecting Ernestinovo to Zerjavinec (Croatia) and Pecs (Hungary), while South-East bound lines, which remained connected to another busbar, connected Ernestinovo to Ugljevik (Bosnia-Herzegovina) and Sremska Mitrovica (Serbia).
The separation of flows in Ernestinovo led to the shifting of electric power flows to neighbouring lines which were subsequently overloaded. At 14:04:48, the line Subotica – Novi Sad (Serbia) tripped due to overcurrent protection. This was followed by the further tripping of lines due to distance protection, leading eventually to the system separation into two parts at 14:05:08.
The separation led to a deficit (approximately -6.3 GW) in the North-West Area, and a surplus of power (+6.3 GW) in the South-East Area, resulting in turn in a frequency decrease in the NW Area and a frequency increase in the SE Area.
At approximately 14:05 CET, the frequency in the North-West Area initially decreased to a value of 49.74 Hz within a period of around 15 seconds before quickly reaching a steady state value of approximately 49.84 Hz. At the same time, the frequency in the South-East Area initially increased up to 50.6 Hz before settling at a steady state frequency between 50.2 Hz and 50.3 Hz.
Owing to the low frequency in the NW area, contracted interruptible services in France and Italy (in total around 1.7 GW) were disconnected in order to reduce the frequency deviation, and 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 from the nominal value of 50 Hz was reduced to around 0.1 Hz in the NW area.
To reduce the high frequency in the SE area, automatic and manual countermeasures were activated, including the automatic disconnection of a 975 MW generator in Turkey at 14:04:57. This returned the frequency in the SE area to 50.2 Hz at 14:29 CET and it remained within control limits (49.8 and 50.2 Hz) until resynchronisation of the two separated areas at 15:07:31 CET.
Between 14:30 and 15:06 CET the frequency in the SE area fluctuated between 49.9 Hz and 50.2 Hz owing to the smaller size of the SE area, where several production units were also disconnected. During this period, the frequency in the NW area fluctuated far less and remained close to the nominal value, owing to the greater size of the area. This frequency behaviour is a subject of further detailed investigation.
Return to normal
The contracted interruptible services in Italy and in France were reconnected at 14:47 CET and 14:48 CET respectively prior to the resynchronisation of the North-West and South-East areas at 15:08 CET.
The investigation is ongoing. In line with the provisions of the relevant Commission Regulation, ENTSO-E will present the results of its investigation to the Electricity Co-ordination Group, and will subsequently publish a report once the analysis is completed.