What caused the UK’s power blackout and will it happen again?

On Friday evening, there was a widespread power blackout across many parts of the UK, with transport systems in particular being affected. Supplies were restored within an hour although the transport problems took far longer to resolve, with the knock-on effects of delayed trains and trains and passengers being stranded at different points on the network.

Around 500,000 customers in Wales, the south-west England and the Midlands were affected, along with 300,000 customers in south-east England, 110,000 in Yorkshire and north-east England and 26,000 in the north-west England, according to the distribution network operators in those areas.

Of course, questions are being asked about why and how this blackout happened, with some commentators such as Lord Adonis, the former Chairman of the National Infrastructure Committee, saying that two such occurrences in 11 years is a major failure on the part of National Grid, and utterly unacceptable.

So why did the outage happen, and is it a sign of poor management of the electricity system, a symptom of the changes taking place on the electricity network, or simple bad luck?

Causes of the blackout

The full causes of the blackout are not yet known. What is known is that at 16:57, the 727 MW CCGT at Little Barford in Cambridgeshire shut down, followed at 17:00 by units 2 and 3 of the windfarm at Hornsea, 120km off the Yorkshire coast, each with a nominal capacity of 406 MW. RWE said Little Barford shut down in line with normal practice when demand reached 23 GW.

The sudden removal of 1.5 GW of generation from the system caused a sudden drop in system frequency, to below 49 Hz, far outside the normal system tolerance of 50 Hz ±1%, meaning that automatic system protection protocols were triggered. These protocols involve rapid load-shedding in order to stabilise system frequency to avoid a cascading failure affecting the entire power grid. The process is automated.

National Grid told Bloomberg that the power failure amounted to an “unexpected and unusual event”:

“The system operated as planned by disconnecting an isolated portion of electricity demand,”
– National Grid

The UK’s large pumped storage facilities at Dinorwig and Festiniog immediately came online, followed by a rapid ramping up of other CCGTs. Interestingly, despite the day seeing higher wind generation, a coal unit ran for most of the day. Also, the bulk of the wind came from onshore sources.

Was this bad luck or bad planning?

Energy consultancy EnAppSys suggested that the system should have been able to cope with the loss of this amount of generation, and that there may have been other issues in play at the time of the trips. Indeed, from BM Reports it is clear that there were also problems at the huge Drax power station in Yorkshire thoughout the day.

It is also the case that there was an unusually large amount of wind generation on the system that day, with National Grid having tweeted earlier in the day that a record 67% of electricity demand was being met by wind. Having a high proportion of wind on the system presents two challenges to system stability:

• Wind is intermittent, so the output is constantly varying with time creating continuous changes in the supply and demand balance, and consequently in system frequency, that must be managed;
• Unlike thermal generators, wind power does not provide any inertia to the electricity grid, so when wind power displaces thermal generation, the amount of available inertia is significantly reduced. Inertia acts to dampen changes in system frequency, so not only does a higher proportion of wind on the system increase the levels of frequency change, it also reduces the natural mechanism by which changes in frequency are contained.

Some industry participants have argued that Friday’s events illustrate the need for more flexible generation and storage on the grid, to counteract these effects:

We thought we would be waiting for a cold winter for the impact of intermittent renewables to strike – however unwelcome, this is just another milestone in the energy transition from fossil fuels to renewables. It demonstrates with great clarity the urgent need for electricity storage at scale on the UK’s grid,”
– Mark Simon, chief executive at battery storage firm Eelpower

However, a single event does not constitute a trend, and wind has been contributing an increasing amount to the power grid for some time without incident. The last time the country saw a similar blackout was in May 2008, when wind penetration was much lower. In that instance, the now-closed Longannet coal-fired power station in Scotland went offline, followed 2 minutes later by the Sizewell B nuclear power station in Suffolk.

This resulted in a sudden loss of 1,510 MW of generation meaning the system experienced a sudden drop in frequency and emergency protection measures triggered load shedding. In addition, other power stations, including a significant volume of embedded generation, reacted to the drop in frequency by disconnecting themselves (also automatic protection measures). Some 500,000 people were affected.

It is rare for two large power stations to trip in such quick succession, so the question is whether the investment needed to create resilience in the face of such infrequent occurrences would justify the cost. Some people, such as Lord Adonis, believe these two events constitute a major failure by National Grid, and that the system operator had failed to learn the lessons of the previous event, but it is entirely reasonable to question whether the cost of maintaining supplies for a once-in-a-decade event would be justified.

It is also somewhat self-serving for providers of battery storage to point to this incident as illustrative of the need for more storage without clear evidence that Friday’s outages were directly the result of the impact of renewable generation on the grid, which so far does not appear to be the case.

In this latest large-scale blackout just under a million people were affected, but within 15 minutes DNOs were able to begin restoring demand, and all users were re-connected within 45 minutes. What made the incident more disruptive was the fact the transport infrastructure was impacted by the automatic load shedding, and took far longer to return to normal service. National Grid has said it will review its system protection measures, and should look in particular at which loads were disconnected and why to determine whether targeting other sources of demand would have resulted in lower disruption to the public.

Paying for a lower carbon elelctricity system

Over the next few weeks, more information about Friday’s blackout should emerge. In addition to understanding whether the protection systems operated appropriately, the reasons for the outages at Little Barford and Hornsea should become clearer, and in particular why the former suddenly went offline in response to lower system demand, and whether the latter tripped as a result of Little Barford going offline, whether there was a problem relating to high winds, or whether it was an un-related mechanical issue.

From the limited information available so far, it seems most likely that Friday’s events were simply bad luck, and sufficiently rare that the costs of preventing them would outweigh the benefits. However questions do need to be asked in relation to the impact on the transport infrastructure, and organisations across the country should review their own responses to the incident – for example why a back-up system at Ipswich hospital failed (fortunately not one affecting critical care).

One thing that is certain is that this major blackout is a useful reminder that maintaining security of supply is not a trivial problem at the best of times. De-carbonisation is changing the fundamental operation of the electricity system, and makes balancing supply and demand, and maintaining system frequency far more challenging.

Rising network costs are one of the drivers of higher electricity bills, but the fact that it has been more than a decade since the last major power outage indicates that these costs are in fact delivering a high level of reliability. It is the job of Ofgem, through the RIIO process, to ensure that these costs are reasonable and that this reliability is being delivered efficiently.