National Grid’s interim report into the blackout which hit Britain earlier this month provides a useful timeline of the faults and trips on the network, but leaves important questions about reliability unanswered.
The interim report blames the blackout on lightning strikes on the transmission system north of London, which caused a series of outages that left the grid with insufficient generation to meet demand.
The lightning strikes caused a very brief disconnection of a main transmission line and triggered a longer loss of around 500 megawatts (MW) of local distributed generation, which automatically disconnected to protect itself and the network.
But the lightning also caused the rapid disconnection of the Hornsea windfarm, cutting grid infeed by 737 MW, and the Little Barford combined cycle gas turbine plant, which cut infeed by a further 641 MW. The cumulative loss of 1,878 MW of infeed in less than 90 seconds overwhelmed the grid, which had only around 1,000 MW of fast-acting
Frequency response reserves delivered 1,000 MW of emergency power, as planned, but it was not enough to arrest the rapid decline in grid frequency well below its target of 49.5-50.5 Hertz (Hz).
When frequency declined to 48.8 Hz, automatic protection systems on the local distribution networks acted to reduce demand by 5 per cent by cutting supply to around 1.1 million customers.
Customers lost power for between 15 and 50 minutes, according to the report.
The report is careful to note that the grid control room was maintaining reserves in line with its obligations under the Security and Quality of Supply Standards (SQSS) set by government regulators.
The security standards require the grid to hold enough reserves to cover the loss of the largest single infeed to the network (n-1) while maintaining frequency and other power quality aspects within normal operating limits.
The security standards do not require the grid to cope with multiple simultaneous generation or transmission failures (n-2 or higher). The report also notes that frequency reserves behaved as expected and delivered 1,000 MW of extra power very quickly in response to the emergency.
But the extra power was not sufficient to cope with the multiple simultaneous failures of embedded generation, a large windfarm and a large gas-fired CCGT.
“These events resulted in an exceptional cumulative level of power loss greater than the level required to be secured by the security standards and as such a large frequency drop outside the normal range,” the report noted.
The key question is whether the reserve of 1,000 MW is adequate or should be increased to reduce the risk of load-shedding in future. The report treats the disconnection of the distributed generation, the windfarm and the CCGT as three separate failures.
“Two almost simultaneous unexpected power losses at Hornsea and Little Barford occurred independently of one another — but each associated with the lightning strike,” it said.
“As generation would not be expected to trip off or de-load in response to a lightning strike, this appears to be an extremely rare and unexpected event.” It went on to observe about the loss of local embedded generation: “The lightning strike also initiated protection on embedded generation in the area and added to the overall loss of power experienced.” “This is a situation planned for and managed by the electricity system operator and the loss was in line with ESO forecasts for such an event.” The grid’s study eliminates the possibility of a cascading failure in which the failure of one generator caused under-frequency and the failure of the others (the description of the failures as “independent”.) — Reuters