At seven minutes to five on the afternoon of 9 August 2019, a lightening strike caused the loss of 150MW of distributed power (i.e., a large number of small wind, diesel and solar generators) from the National Grid.  This sudden loss triggered the safety system on the giant Hornsea wind farm in the North Sea, taking another 799MW of power from the system.  Fortunately, regulations at the time required that the Grid operator keep 1GW of back up capacity for precisely this kind of emergency.  And so standby turbines at the Little Barford gas power station were started.  However, the system failed, taking 244MW of Little Barford’s power offline too.  This loss of power tripped a further 350MW of distributed generation bringing the total loss of power to 1,481MW.  Within the next minute, 900MW of National Grid’s 1GW of backup capacity was brought online, stabilising the frequency at 49.2Hz.  Seconds later, however, the gas turbine at Little Barford failed; bringing the loss of power to 1,691MW.  At this point, National Grid had consumed all of its 1GW backup capacity and had no resource to cope with further power losses or frequency fluctuations.  Then, half a minute later (16:53:50) the frequency fell to 48.8Hz; triggering the Low Frequency Demand Disconnection scheme and automatically disconnecting 1.1 million business and household consumers.  In response, and for yet to be discovered reasons, (16:53:58) a third turbine at Little Barford went offline bringing the total loss of power to 1,878MW – nearly double Ofgem’s stipulated backup capacity.

Network Rail was among the large industrial users in the Low Frequency Demand Disconnection scheme, so that at the peak of the Friday evening rush hour, a large part of the UK’s rail network was brought to a halt…

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