What are the questions raised by the UK's recent blackout?

Britain’s most severe blackout in more than a decade caused rush hour travel disruption across the UK’s biggest train stations, railways, roads and airports last Friday evening, and left almost a million homes across the country in the dark.

The energy watchdog has asked National Grid, the energy system operator, to give an initial report into what went wrong by the end of this week, while the government has announced a separate inquiry. The network company could even face a fine.

Here are some key questions relating to Friday’s incident:

Temporary lighting is set up during the power cut affecting Clapham Junction train station. Photograph: Yui Mok/PA

Temporary lighting is set up during the power cut affecting Clapham Junction train station. Photograph: Yui Mok/PA

What caused the blackout?

National Grid blamed the “rare and unusual event” on a severe drop in the grid’s frequency – a measure of energy intensity flowing around the network – following a pair of major generator outages.

A key part of the operator’s job is to keep the frequency of the grid steady at around 50Hz; a deviation of more than 1% in either direction is enough to cause parts of the energy system to automatically shutdown as a safety precaution.

The blackout took place after two near-simultaneous power plant outages – at the Little Barford gas-fired power plant and the Hornsea offshore wind farm – caused the frequency to drop below the grid’s safety limits.

Little Barford, which is owned by German power group RWE, tripped shortly before 5pm on Friday evening. The outage was followed minutes later by the unexpected shutdown of the Hornsea offshore windfarm, which is owned by Danish wind power company Orsted.

RWE said its power plant shut down automatically due to a technical issue which is “not uncommon” at power plants. Orsted declined to comment on why its giant windfarm went offline while it investigates the issue alongside the energy regulator.

How was the problem fixed?

National Grid called on backup electricity providers to ramp up their output within seconds of the unplanned outages. Some of these backup suppliers include small-scale power plant owners, battery operators and even supermarket fridges.

Usually these backup contracts can stabilise the frequency of the electricity grid before the frequency drops too far, but on Friday the safety net was not enough to stop parts of the grid from automatically shutting down.

National Grid gave the all-clear for these networks to begin restarting their system 15 minutes after the outages. It said that by 5.40pm all the regional electricity networks had managed to restart their systems and restore power to their customers.

Could it happen again?

National Grid is working with the regulator, the generators and other stakeholders to “understand the lessons learned” from Friday’s blackout.

However, industry sources have warned that “near-misses” are on the rise and the system operator has been aware of the growing potential for a wide-scale blackout for years.

In the last 12 weeks the grid’s frequency has fallen dangerously low, below 49.6Hz, on three separate occasions. Prior to these near-misses the grid’s frequency had not fallen to this extent for at least the last four years.

National Grid had managed to avoid a wide-spread blackout until now by triggering a “low frequency event” which sends out a call to contracted energy companies for immediate back up, with only 10 seconds’ notice.

A spokesman for National Grid said the frequency shocks were independent events, and said there is “no trend or prediction of more frequency excursions”.

Why were rail lines hit particularly hard?

The power outage may have lasted less than an hour, but travel disruption across the country caused major delays for railways well into Friday night.

Nick King, a director for Network Rail, said the blackout caused a short-term loss of power to its signalling systems and power supply equipment across a wide area of the rail network. In some places its backup systems restarted but still needed an engineer to manually reset trains that had been brought to a halt by the blackout.

“Many of these trains were unable to restart on their own and had to be attended by an engineer and this caused significant disruption across parts of the network. We worked flat out with train operator colleagues and the British Transport Police to safely get passengers off impacted trains,” he said.

In some instances engineers needed to travel long distances to specific trains left stranded by the blackout. The blocked routes delayed train schedules across the network while the system was reset.

Does the blackout raise questions marks over renewables?

There is no question that the UK will need to increase its use of renewable energy sources to help reduce greenhouse gas emissions across the economy.

But there are questions over whether National Grid has done enough to adapt to the change renewable energy is bringing to the energy system.

High volumes of renewable energy can make it more difficult for National Grid to balance the frequency of the grid, which was originally built to accommodate fossil fuel power plants.

To prepare for the UK’s energy transition National Grid has developed “frequency response” tools – such as quick-fire backup supplies of extra electricity – which should make it technically possible to run the energy system without any fossil fuels by 2025.

Energy experts have suggested that National Grid may need to change its “rules of thumb” which govern how much electricity it keeps in reserve to manage the frequency of the grid as renewable energy plays a larger role.

Experts have been clear though: there is no suggestion that wind power is an unreliable source of electricity.

Does this strengthen the case for nationalising National Grid?

Some Labour MPs have pointed to the blackouts and subsequent rail chaos as further evidence that the privatisation of national utilities has failed.

The opposition party plans to renationalise energy networks and the railways amid criticism that private companies are providing poor service while hiking up consumer costs.

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According to energy industry sources the unprecedented political pressure on National Grid to keep costs down may have played a role in making the blackout more likely.

Some of the energy companies which provide backup services to help keep the lights on have warned that National Grid is being too tough on costs.

Steve Shine, chairman of Anesco, a battery company, said: “What is needed is a greater volume of faster response services, which can be called into action when the frequency drops. This would have prevented the need to turn the power off.”