UK power cut 2019: How the system has changed since the last big blackout

Author Eleanor Green, Editor

The UK has suffered its most significant power cut in over a decade leading to widespread disruption on the transport network. In a rapidly evolving power system, what are the main challenges to ensure reliable power supplies?

Friday’s evening commute was more painful than normal for many in London, as a brief power cut led to the closure of some Underground lines and widespread problems on national railways as trains became stranded between stations.

It was the largest power cut since 27 May 2008, when a failure at the Longannet coal-fired power plant and then shortly after at the Sizewell B nuclear plant caused frequency to fall. To restore frequency, there was automatic load shedding — reducing demand by automatically shutting off parts of the grid.

Friday’s power cut appears to have been caused by something similar, with the failure of two generating units triggering a fall in frequency. The grid needs to keep frequency as close to 50HzT as possible and aim to remain within 1pc deviation in either direction. A fall or rise beyond this will automatically disconnect some demand — namely, regional distribution networks in some areas but can also lead to other power plants tripping and going off line.

But the big change since 2008 is the UK’s generation mix, and this is reflected in the types of generators that appear to have either triggered the frequency deviation, or potentially been the victims of the frequency deviation and to have been automatically disconnected from the grid.

National Grid transparency data show that the 727MW Little Barford gas-fired plant went off line in an unexpected outage first, followed two minutes later by the 1,218MW Hornsea 1 offshore wind farm

Wind and gas now dominate the country’s generation mix, unlike in 2008. Longannet has closed, and has been just part of the 8.9GW of coal-fired generation to have shut down in the past decade. Nuclear capacity has also fallen with the closure of the Wylfa plant, and there has been very slow progress on plans to build new nuclear plants.

In 2008, coal-fired generation provided 36pc of the UK’s power supply. On Friday, it averaged just 300MW, or 1.2pc of total power supply, while nuclear generation averaged 6,183MW, around 24pc of supply.

Wind generation supplied 37.48pc of demand on Friday, while gas took the second largest share at 26.9cpc. In 2008, wind and solar combined provided less than 1pc of total supply.

Inertia theory gains momentum

One challenge provided by high levels of renewable generation is ensuring that there is enough inertia in the system. Inertia was rarely an issue for the grid in previous years as conventional thermal generators automatically provide inertia.

Wind, solar and interconnectors provide no inertia to the grid. Low inertia means that frequency changes faster if there is an unplanned generation outage

At the time of the outage, wind and interconnectors were covering around 38pc of total UK power demand, or 10.5GW. Solar — which is predominantly connected to the low voltage distribution grid in the UK and so doesn’t appear in the transmission grid production mix — was generating around 3.7GW.

Proponents of small diesel-fired generators, modular nuclear reactors, large new nuclear reactors and all sorts of other thermal technology are likely to use this power cut as a reason to say that their favoured technology should be supported or purchased by anyone convinced that we are about to enter an age of regular blackouts.

But managing frequency deviations on the transmission grid at times of low inertia is already a priority for National Grid, which had a firm frequency response scheme last year in addition to a spinning reserve scheme that helps provide inertia. It has also been consulting on future frequency response schemes, which are likely to be largely delivered by relatively novel technology.

Batteries are the main technology that provides a fast frequency reserve, but other technologies that might be utilised include flywheels. Aggregators, who use software to bring together a wide range of small batteries, distribute generation and demand response into “virtual power plants” and are able to provide a range of rapid services to the grid by reducing demand at certain industrial sites or increasing demand by charging batteries.

National Grid will also need to review whether the automatic load shedding triggered by frequency deviations is set at the right level or may have been erring too much on the side of caution and triggering disconnections too soon.

Integrating high levels of wind generation is a continuing challenge for grid operators, but other countries manage to integrate much higher shares of wind generation into the supply mix — Portugal and Denmark have covered their entire demand with wind generation at times.

The most significant disruption caused by the power cut was on the rail transport network. Network Rail said that one particular type of train had a major systems failure when the power was cut, and it required an engineer to visit each individual train to restart it. Questions could be asked about whether the rail industry should be more resilient, with all trains capable of coping with the risk of disconnection.

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