A malfunction in Croatia’s substation Ernestinovo was the initial event that led to a split into two grid regions of the Continental Europe synchronous area on January 8, from 14:05 to 15:08 CET, according to the latest findings of ENTSO-E’s inquiry into the incident.
ENTSO-E has published the first update on the incident on January 15, and since then the association of the transmission system operators has analyzed a large portion of relevant data aiming to reconstruct the event in detail.
This second update presents the key findings of detailed analyses, which have a preliminary character subject to new facts that are expected to emerge in the still ongoing investigation, ENTSO-E said.
Tripping of a 400 kV busbar coupler in the Ernestinovo substation
According to the association, the analyzed sequence of events indicates that the initial event was the tripping of a 400 kV busbar coupler in the Ernestinovo substation by overcurrent protection at 14:04:25.
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 which remained connected to one busbar, connect Ernestinovo to Zerjavinec (Croatia) and Pecs (Hungary), while South-East bound lines, which remained connected to another busbar, linkErnestinovo to Ugljevik (Bosnia and Herzegovina) and Sremska Mitrovica (Serbia).
The separation of flows in the Ernestinovo substation led to the shifting of electric power flows to neighboring 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, according to ENTSO-E.
The route where the two parts of the Continental Europe Synchronous Area were separated.
Frequency in South-East area fluctuated more than in the North-West
According to ENTSO-E, the system separation resulted in a deficit of power (approx. -6.3 GW) in the North-West Area and a surplus of power (approx. +6.3 GW) in the South-East Area, resulting in turn in a frequency decrease in the North-West Area and a frequency increase in the South-East Area.
Between 14:30 CET and 15:06 CET the frequency in the South-East area fluctuated between 49.9 Hz and 50.2 Hz due to the rather small size of the South-East Area, where several production units were disconnected as well. During this period, the frequency in the North-West Area fluctuated far less and remained close to the nominal value, due to the rather large size of the North-West Area.
This frequency behaviour is a subject of further detailed investigation, ENTSO-E said.