2021 European Grid Split
What Happened
On a normal winter afternoon, a protection relay at the Ernestinovo substation in Croatia detected too much current flowing through a busbar coupler -- a switch connecting two sections of the substation. The relay tripped automatically, splitting the power flows through the substation.
Within 42.7 seconds, the resulting overloads cascaded across Croatia, Serbia, Hungary, and Romania. The entire Continental European grid -- serving 400 million people -- fractured into two islands. The northwest lost 6.3 GW of generation. Emergency reserves in France, Italy, and the Nordic countries kicked in within seconds.
Operators resynchronized the grid in 63 minutes. Consumer impact was limited (233 MW total), but the event proved that the same type of cascading split that hit Europe in 2006 could still happen 15 years later.
Timeline
Root Cause
A protection relay operated correctly -- and split a continent. The Ernestinovo busbar coupler was carrying heavy cross-border power flows near its rated capacity. When the overcurrent relay tripped, power redistributed to neighboring lines which also overloaded and tripped within seconds. Each protection relay did exactly what it was designed to do in isolation, but collectively they destabilized the entire system. Busbar couplers inside substations were not treated as critical elements in security assessments.