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NETWORK / 8 / FEBRUARY 2020 BLACKOUT FINDINGS What can we learn from August 9th? The summer of 2019 saw the disconnection of more than a million electricity customers in Britain's largest single electricity system disturbance in over ten years. Here, UKERC co-director, Professor Keith Bell, summarises the findings of the government and regulator investigations into the incident and unpacks potential lessons for the electricity sector and its readiness for further decarbonisation without compromising security of supply Loss of power On August 9th last year, the power sector hit the headlines in a most unwanted way: shortly before 5pm that Friday, over a million electricity custom - ers were suddenly discon- nected. They were reconnected again within 40 minutes but not without some important services being affected. These included trains in the south- east, particularly those that, as an investigation by the Office of Rail and Road (ORR) found, had disconnected themselves during the disturbance. Most of these could not then be restarted without a visit from a techni - cian with a laptop. As a result, the lines were blocked for many hours and rail services were disrupted into the next day. Not good at any time and certainly not on a Friday evening. The ORR investigation was one of three that was started in the a…ermath of the incident. The others were by the Energy Emergencies Executive Com - mittee (E3C), a body convened by the UK Government, and the electricity regulator, Ofgem. All three published their reports last month. But what did they find and what else can we learn from what happened? So, what happened? The initiating event was a short circuit on a 400kV overhead line caused by a lightning strike. Such faults typically happen tens of times each year on the transmission network and do not cause losses of supply. On this occasion, however, the voltage depression caused by the short circuit fault caused an incorrect response by wind turbines' control systems at Hornsea offshore wind farm leading to large oscillations of reactive power and the trig - gering of the turbines' own protection – causing almost all of the power being produced at Hornsea to be lost. This should not normally happen. The voltage depression apparently also caused a steam turbine at Little Barford combined-cycle gas turbine power station, owned by RWE, to trip. It is still not completely clear why this happened or why, to compound the problem, a gas turbine there also tripped a minute later. A further, known issue made the event worse. This concerned small scale generation – 'distrib - uted generation' (DG) – con- nected within the distribution networks. Two types of 'loss of mains' protection, intended to safely shut down a portion of the distribution network when it becomes isolated from the rest, are known to be triggered by certain disturbances on the transmission system. 'Vector shi…' protection is sensitive to short circuit faults and 'rate of change of frequency' (ROCOF) protection is activated by a large enough instantaneous loss of generation. Although the phenomena are known, what the ESO and the Distribution Network Operators (DNOs) have told Ofgem about how much DG was lost due to each of them on August 9th is based only on es - timates. This is largely because, many years a…er significant volumes of DG started to be connected, the DNOs still lack detailed monitoring of it. What were the consequences? The loss of DG due to 'vector shi…', the drop in power at Hornsea and the disconnec- tion of the steam unit at Little Barford meant there was not enough generation to meet