National Grid had experienced three blackout “near-misses” in as many months before Friday’s major outage left almost a million homes in the dark and forced trains to a standstill around the UK.
The system operator, already under investigation by the energy watchdog, faces criticism from within the industry that it has not done enough to guard against the risk of blackouts.
National Grid blamed the “incredibly rare” nationwide power cut on a severe slump in the grid’s frequency – a measure of energy intensity – following the unexpected shutdown of two power generators.
It will face an investigation into its handling of the energy system after the first blackout in more than a decade following the shutdown of a gas-fired power plant in Bedfordshire and the Hornsea windfarm in the North Sea at about 5pm of Friday.
It said it would work with the regulator and energy companies to “understand the lessons learned” after two power plants shut down unexpectedly within minutes of each other, causing severe rush hour travel disruption across the country.
But industry sources claim National Grid has been aware of the growing potential for a wide-scale blackout “for years”, and has suffered a spate of near-misses in recent weeks.
The Guardian understands that in every month since May there has been a severe dip in the grid’s frequency from its normal range around 50Hz. Industry sources have confirmed that the grid’s frequency has fallen below 49.6Hz on three different occasions in recent months, the deepest falls seen on the UK grid since 2015. On Friday the blackout was triggered when the frequency slumped to 48.88Hz.
In June, the frequency of the grid plummeted to within a whisker of National Grid’s legal limit of 49.5Hz after all three units of EDF Energy’s West Burton gas-fired power plant in Nottinghamshire tripped offline without warning.
The unexpected outage triggered an emergency call for backup electricity supplies which stabilised the energy grid’s frequency before a blackout was triggered.
In addition, the grid’s frequency fell to 49.55Hz on 9 May, and 49.58Hz of 11 July.
A spokesman for National Grid said these events were “independent”. He added that there was “no trend or prediction of more frequency excursions”.
“Over the past four years frequency has regularly fluctuated between the agreed limits, as part of the normal day-to-day operation of the electricity system,” he added.
Steve Shine, chairman of Anesco, a battery company, said: “It would be easy for National Grid to write this incident off as a fluke event, but they have actually been aware of this potential issue for many years.”
The growing concern over National Grid’s grip on the energy system has emerged as the energy regulator, Ofgem, issued an ultimatum to the system operator calling for an interim report on the blackouts by the end of the week ahead of a full technical report by early September.
National Grid has managed to avoid wide scale blackouts by triggering last-minute contracts to help it stabilise the grid and avoid breaching the crucial 49.5Hz limit set by the regulator.
It contracts energy suppliers to ramp up their output from generators and batteries to make up for an outage, and offers contracts to companies such as factories and supermarkets, which can temporarily cut their energy demand to help stabilise the frequency of the grid.
But many of the companies tasked with supplying the “safety net services” – such as batteries and diesel farms, which are banks of small-scale generators – have warned that National Grid is not doing enough to safeguard the system against blackouts.
The UK’s booming renewable energy output 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, which generate more intensive energy.
National Grid said it had embraced the UK’s renewable industry by developing “frequency response” tools – such as quick-fire back-up supplies of extra electricity – which should make it technically possible to run the energy system without any fossil fuels by 2025.
Shine 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.
“It’s worrying that with just two generation sources dropping out of the supply mix, National Grid was still unable to deliver power to all areas, with no proper contingency plan in place,” he said.
“It’s exactly this kind of scenario the UK needs to be prepared for – these recent events demonstrate how important it is to have more, faster response services available, which can be called into action when the frequency drops,” he said.
Steven Meerman, the founder of battery firm Zenobe, called on National Grid to “update its old rules of thumb” to determine how many reserve services it kept on standby in the future.
“It may be the energy system is changing faster than expected,” he said.
John Pettigrew, National Grid’s chief executive, defended the grid’s response in a post on social media site LinkedIn entitled “there is never a good time for a power cut”.
He said: “Contrary to some erroneous media reports, I am not on holiday – indeed, I’ve been at my desk all weekend.
“As CEO of National Grid plc ultimately the buck stops with me.”