Ørsted and RWE have been fined £4.5 million ($5.9 million) each for their part in a major U.K. blackout last August.

A lightning strike on August 9 triggered a series of unlikely events that ultimately took offline the world's largest offshore wind farm, Ørsted's Hornsea One. At that time, 880 megawatts of the 1.2-gigawatt project had been approved for grid export.

RWE’s 740-megawatt Little Barford gas power plant also went offline along with hundreds of megawatts of distributed generation across the network as a series of protection measures did their job.

The local distribution network operator UKPN was fined £1.5 million for a “technical breach.” It connected customers back to the network without National Grid ESO, the system operator, asking it to.

The blackout lasted a maximum of 45 minutes and affected 1 million customers during a Friday evening rush hour. It also created a series of knock-on effects including huge disruption on the rail network, large chunks of which are electrified in Britain. 

A report looking into the event was delayed to avoid clashing with the U.K. general election last month. Finally released on Friday, the report makes a series of recommendations after assessing the causes of the blackout following the lightning strike.

Software update came too late at offshore wind farm

The trouble at Hornsea One was attributed to a single software update, according to the government's report. The update was made one day after the blackout and fixed the error that caused the plant's voltage control system to produce an unexpected result when faced with the fluctuation created by the lightning strike.

“The manufacturer of the offshore assets identified that this unexpected control system response could [have been] mitigated by implementing a software update that the manufacturer had already prepared,” states the report.

Frustratingly for Ørsted, it had scheduled the update for August 14, just five days after the blackout. The update was in fact made on August 10 in response to the events of the day before.

In a statement emailed to the media, Ørsted said it took any interruption to service “very seriously.”

“The power outage on the August 9 was caused by an extremely rare sequence of events, involving a number of parties, and the issue we experienced at Hornsea One was quickly resolved," the Danish energy company said. "However, in recognition of our role in the outage, we have offered to make a voluntary contribution to Ofgem’s redress fund. We have cooperated with Ofgem throughout their investigations and conducted a thorough internal review of the events in order to prevent a situation like this from happening again.”

The company declined to comment further on the status and findings of its internal review.

Looking for lessons from the blackout

For the regulator, Ofgem, the focus now turns to what lessons it can take from the experience. The bulk of these relate to assessing existing reserves held to protect the system from voltage fluctuations and improved communications between all stakeholders and the public. The report said this latter failing “compounded the severity” of the incident.

“Consumers and businesses rely on generators and network companies to provide a secure and stable power supply,” Jonathan Brearley, executive director of Ofgem, said in a statement. “August 9 showed how much disruption and distress is caused to consumers across the U.K. when this does not happen."

“Our investigation has raised important questions about National Grid’s Electricity System Operator, which is why our review will look at the structure and governance of the company," Brearley added.

“As the energy market changes, it is vitally important we future-proof the networks to ensure consumers continue to benefit from one of the most reliable electricity systems in the world."

National Grid ESO holds 1 gigawatt in reserve to address frequency changes, but the losses on August 9 were double this amount. The U.K. network operates at 50 Hz with a lower operational limit of 49.5 Hz. At one stage during the event, it fell all the way down to 48.8 Hz.

While the fact that part of the dropped generation was renewable is not specifically relevant, the decreasing volume of rotational inertia on the grid — as renewable sources replace coal and gas generation — is.

At the tail end of last year, National Grid ESO announced an inertia tender as it looks to reinforce grid stability and preempt the findings of the report in the process.

“The recommendations included in the reports, many of which we had already made as part of our own final technical report, include a review of the Security and Quality of Supply Standard. We’ve already begun this review and will update the SQSS industry panel by April 2020,” said National Grid ESO in a press statement.

A series of quarterly reports will now follow to ensure the actions recommended in Friday's report are followed up on.