National Grid ESO, the UK electricity transmission system operator, in its previous report on the results of investigation of causes of a systemic crash in the UK energy grid on August 9, 2019, stated that the possible cause of the accident could have been a thunderbolt hitting one of the energy grid elements followed by frequency unbalance throughout the energy system.
According to the operator’s estimation, the pulse surge protection system (lightning protection) reacted in 0.1 second and after 20 seconds the power system returned to its normal mode of operation. In this case, about 500 MWt of distributed generation was disconnected from the distribution network, although the operator considers this situation normal in case of lightning strike.
Within minutes, however, there was a sharp reduction in generation at two large power plants – a gas-fired plant with the capacity of 740 MWt in Little Barford and an offshore Hornsea wind farm with the capacity of 1.2 GWt. As a result, the frequency in the network dropped sharply to 48.88 Hz (while 50.5 Hz – 49.5 Hz is the frequency regulatory range). The total amount of lost generation capacity constitutes 1,378 MWt, which exceeded the volume of first-line backup capacity of 1,000 MWt available in automatic mode, including the 472 MWt of accumulation capacity available to National Grid ESO for frequency control.
As a part of frequency recovery measures in accordance with the rules of operation of networks, National Grid implemented a secondary regulatory mechanism by means of controlled shutdown of a number of consumers to ensure the safety of operation of power system in accordance with the scheme, previously agreed with the operators of the distribution system. Shutdown of about 1 GWt of power facilities on the consumption side and attracting additional generation volumes allowed to restore the functioning of the energy system within the period from 20 to 40 minutes. Some problematic issues arose only for railway companies, as they were unable to start electric locomotives and were forced to send emergency crews of electrical engineers, which took from 4 to 6 hours.
The main findings of the National Grid ESO investigation are:
– lightning protection of the distribution system reacted in accordance with the procedures defined by the technical regulations;
– before and after the blackout there were repeated lightning strikes into the elements of the distribution system without significant impact on its further functioning;
– both power plants have drastically reduced generation due to the same lightning strike, although the technical parameters do not provide for such a reaction of the generation facilities to this natural phenomenon;
– the activation of the distribution network protection system in the lightning strike zone caused the distributed generation to be disconnected there, however, this situation is included in the forecast scenarios at the level of the system operator;
– the cumulative effect of loss of power in the network with a sharp drop in frequency, which is not stipulated by the current safety standards, was formed;
– the low-frequency shutdown system worked according to pre-set algorithms, and the renewal of energy supply to such facilities occurred in the shortest possible time;
– prolonged periods of downtime on individual railway branches were caused by technical difficulties with the launch of the locomotive fleet, not by power outages.
Investigation of technical reasons for shutdown of both power plants, including the causes and consequences of consumer shutdowns at the level of distribution system operators, as well as the procedure for interaction with state authorities, generating companies, critical infrastructure objects and media for the purpose of improving communication is still being conducted. National Grid ESO will prepare the final blackout report for the national regulator by September 6, 2019.
Energy Emergencies Executive Committee has been set up to independently evaluate the accident and prepare its own report and proposals to prevent and mitigate possible further energy system accidents.
According to the findings of the previous report, the blackout occurred in the result of cumulative effect of a number of circumstances caused by lightning hitting one of the elements of the energy network. It is likely that this system failure will cause the UK to review safety standards and increase the amount of reserve capacity required to balance the frequency. According to preliminary data, it could be an increase of such capacities up to 2.5 GWt due to additional tenders and increase of the tariff of the transmission system operator.
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