Friday 9th August’s demand disconnection event or “blackout” was the result of a relatively rare event with two large power generation units disconnecting from the power system. This caused the frequency to drop and demand disconnections to occur across the country. But how did this happen and what are the learnings from it?
Unplanned outages or trips at power stations happen relatively often with 64 having occurred in the UK this year already. However, two large generators independently tripping within 15 seconds of each other is very rare with LCP calculating that the probability of this occurring is once every 25 years.
National Grid has several mechanisms available to manage these events which normally happen without consumers being aware. When a power station trips it lowers the frequency of the electricity system. We use a 50Hz system in the UK meaning that our thermal power stations spin at 3,000 RPM which is synchronised across the system. When a unit trips, National Grid has a licence condition to keep the frequency to 1% above/below 50Hz and it does so by using several ancillary services (backup systems) to rebalance the system.
Little Barford power station tripped at 16:52 when its declared maximum output dropped from 664MW to 0MW. This loss caused the system frequency to drop rapidly. Almost instantaneously, the Hornsea One wind farm (units 2 and 3) tripped, dropping its output from 756MW to 0MW, increasing the severity of the event.
At the time of the trips there was over 5GW of power available in the Balancing Market, so we can rule out an issue with a lack of capacity on the system or any problem with the level of capacity procured through the capacity market. But whether enough capacity was secured through the ancillary services market is another question.
One other factor that may have added to the problem and explain the secondary drop in frequency is small embedded generators also tripping because of the sudden initial drop-in frequency. These changes have been an ongoing issue for several years with Ofgem continuing its work to make embedded generation more resilient to changing frequency events.
National Grid are already reviewing the incident with an interim report being produced for Ofgem. Although there will be lessons to learn and improvements to be made to how the electricity system as well as other sectors function there are several conclusions we can make:
This wasn’t a case of insufficient generation capacity as there was significant capacity available through the Balancing Market;
The level of inertia was within operating limits when the trip occurred;
Ofgem’s changes to embedded RoCoF will help with future events but should be monitored;
The level of operational reserve may need to be increased considering this event;
National Grid can procure more frequency response and inertia, but this would come at a cost; and
Demand disconnections need to be reviewed.
Whatever the outcome of this review we should consider that any rushed decisions not supported by careful analysis of the data will quickly lead to increased costs for consumers and might even slow down the progress to decarbonising the generation system.
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