The UK power cut that occurred on Friday 9th August at 4.54pm affected many across the country. At first, speculation that followed the events suggested the fault could have been due to wind generation reducing owing to reduced wind speed. In fact, the issue was the amount of generation that tripped, not the technologies involved.
Just before the teatime peak, the Little Barford gas generator tripped whilst exporting about 660MW. Two minutes later, the Hornsea offshore wind farm tripped, removing another 800MW or so from the supply side. The combined loss of around 1,440MW dropped the grid frequency to 48.8Hz, which was measured by one of our Energy Routers stationed at a customer site. This level of frequency is well below the statutory limits imposed on National Grid. The generation loss was above the level that National Grid plans for in a largest single loss, which is currently 1200MW from Sizewell B.
Early information suggests the outages were entirely coincidental and unrelated issues. At very low frequency levels, there is automatic load disconnection to protect the system and help to restore frequency to normal levels. The system was operating normally at a grid level within around 15 minutes. Had the frequency drop been allowed to continue past 47.7Hz for a sustained period, we may have seen more widespread blackouts, with greater impact.
Although it is rare for two trips to occur within a short period of time, this was not a unique event. A similar problem occurred on May 27th 2008, when half a million people were cut-off as a result of Sizewell B (nuclear, Suffolk) tripping two minutes after an outage at Longannet (coal, Fife). The system lost 1,510 MW of generation as a result of both of these stations tripping. There were several distributed generators that tripped in the Liverpool/Manchester area, which caused local issues due to their protection relays operating.
It is likely that blackouts of this nature will happen again. There is some speculation in the industry that lighter summer demand means that a large trip has a proportionally greater impact than a mid-winter event.