At 4:52pm on Friday 9 August 2019, the UK suffered its first wide-scale blackout in more than a decade. More than 1.1 million consumers were plunged into the dark as rail lines screeched to a halt, traffic lights failed and even airports reported problems. Liam Stoker looks at the root causes, and how battery storage came to the rescue in an article which first appeared in PV Tech Power Vol.20.
16:52:33.490. Those nine consecutive digits won’t mean much outside of the UK’s energy sector, but they’re likely to be etched into folklore. It’s the precise timestamp for when, on 9 August 2019, a single lightning strike sparked a cascade of events that caused the UK’s first major blackout in more than a decade.
More than one million people experienced power outages and significant disruption, with not insignificant swathes of the country’s rail network taken out of action, albeit temporarily. The incident made national headlines for days after, as theory and rumour abounded.
A cyber attack? No, the UK’s transmission system operator National Grid quickly dismissed. Were renewables to blame? Earlier that day wind had provided more than half of the country’s power,
a feat which had the renewables lobby celebrating. That just hours later the lights had gone out was a fact not lost on a number of climate change sceptics.
But those theories were also dismissed by National Grid in the days after the event. While there was indeed marginally less inertia on the grid that day, courtesy of less synchronous generation, this was not something that ultimately contributed to the blackout.
The true cause, National Grid’s preliminary investigation, released on 19 August, was perhaps both simpler and more complicated at the same time.
Thunderbolts and lightning
National Grid’s timeline of events puts together a sequence of events that while individually manageable, together caused a drop in frequency sizeable enough to cause the blackout. Lightning struck a transmission circuit near Eaton Socon, a town near St Neots in Cambridgeshire.
That lightning strike, as tens of others that hit grid infrastructure that day, was said by National Grid to have been dealt with by its protections systems normally. It did, however, trigger a Loss of Mains protocol that took around 500MW of embedded generation – domestic solar panels, batteries and the like – off the system. That loss of generation would prove pivotal.
Seconds later, within a few miles of the lightning strike, a CCGT power station, named Little Barford and owned by European power giant RWE, came offline, taking around 700MW of generation with it. Within seconds of that occurring, Orsted’s Hornsea offshore wind farm also came offline, contributing to an event National Grid described as “rare and unusual”.
The reasons for two sizeable generators coming offline at once are the subject of continuing investigations. National Grid said it wanted to collaborate with both RWE and Orsted to better understand the respective power stations’ failure mechanisms, and both operators have remained fairly tight-lipped to date. What the system operator was almost immediately prepared to dismiss however was a previously held theory that the incidents were somehow connected to each other, or that a failure at one plant triggered a failure at the other.
Altogether, around 1,378MW of generation came off the UK’s transmission system within mere seconds, plunging grid frequency to an initial low of 49.1Hz, followed by a secondary dip which took frequency to 48.8Hz, far outside the safe operating limit. Attempts to use reserve capacity to restore the frequency failed, meaning that National Grid was forced to call on the UK’s regionalised distribution network operators to begin Low Frequency Demand Disconnection (LFDD); essentially switching customers’ power supply off.
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