Nordic System Disturbance

why this is interesting

No content here.

On April 26th the 400 kV voltage in Sweden dropped to 250 kV (0.62pu) for 7 seconds following a "manual error during maintenance of a substation".
System was N-1 secure and reserves available to cover a 1.4GW loss
Generation was 17GW. Load was 14.6 GW
The event started when someone tried to open a 220 kV disconnect remotely, but that didn't work because the interlock logic indicated the disconnect was in the intermediate position
The interlock logic said that it was in the intermediate position because the control cabinet had been demolished and no longer existed
So, the switching operators bypassed the interlocking locally and then opened the wrong disconnect
The current through the disconnect exceeded its rating, so it flashed over as a 2-phase fault and then evolved to a 3PH fault
Duplicate 220 kV busbar protection existed and both tripped, but one bay failed to open due to two separate failures of the protection design
The phase voltage in the area dropped to approx 0.6pu
Nearby Forsmark 1 & 2 nuclear gens both tripped from 2.13GW about 1 second later
The fault was finally isolated after 7 seconds by transformer undervoltage protection
After fault isolation, the voltage recovered and hit 1.17pu for several seconds before generators responded and down-regulated voltage again
A total of 25 cap and reactor banks tripped due to the low and high voltage conditions
The frequency fell to 49.3 Hz and recovered to 49.9 Hz about 12 minutes later