Frequency response performance ‘inadequate’ during blackout

The performance of frequency response providers during August’s blackout was “generally inadequate,” Ofgem has concluded in its final report on the incident.

According to the document, there was a 14 per cent shortfall from those required to respond within the first ten seconds and a 17 per cent shortfall from those required to respond within half a minute.

Mandatory providers and contracted providers of dynamic primary response performed “particularly poorly”, with both sets under-delivering by 25 per cent.

Ofgem said National Grid Electricity System Operator (ESO) has been “unable to demonstrate a robust process for monitoring and validating the performance of individual providers, including mandatory providers.”

“It is also unclear how such material under-delivery is accounted for in the ESO’s operational planning, how it is addressed on an ongoing basis to ensure delivery of these vital services, and furthermore, whether this represents value for money for consumers,” the regulator added.

“We do not believe that better response and reserve delivery would have been sufficient to prevent demand from being disconnected for this event.

“However, this is a significant finding given frequency response and reserve are vital balancing services that the ESO must continually procure to secure the system, and expenditure on these accounted for £132 million worth of balancing service charges in 2018/19.”

Furthermore, Ofgem said it had found examples where providers of reserve services were disconnected from the power grid due to the activation of the emergence load-shedding mechanism that cut power supplies to more than a million customers across the country.

“This is an area which needs addressing and we would like to see more industry engagement, particularly between the ESO, DNOs and generators on the impacts of distributed generation for restoring system stability,” said Ofgem.

“It is particularly important as the disconnection of balancing service providers can significantly undermine the recovery of the system frequency.”

The report also reiterated concerns over the Security and Quality of Supply Standards and how they are interpreted by the ESO when determining its procurement strategy for frequency response.

As required by the standards, the ESO had enough frequency response on hand to cover the largest single potential infeed loss on the transmission network. At the time, this was a group of three generators near Saltend with a combined capacity of 967MW.

Even when considering this loss alone, Ofgem said the volume of frequency response held by the ESO left a “narrow margin for error”. But the ESO also lacked sufficient reserves to cover the additional potential losses due to nuisance tripping by distributed generators, which was already known to be a risk during such incidents.

The immediate cause of the power cut was a lightning strike on a transmission line in Cambridgeshire. Within a second of the disturbance, around 150MW of distributed generation with vector shift loss-of-mains protection is thought to have tripped.

Along with the subsequent loss of output from the Hornsea One offshore windfarm and the Little Barford gas power station, this led to a rapid reduction in frequency, tripping a further 350-430MW of distributed generation with rate of change of frequency protection.

As the losses from Hornsea One and Little Barford eventually exceeded the 1GW of frequency response being held by the ESO, the power cut would likely have occurred anyway. However, Ofgem noted that a potential fault was simultaneously recorded on another transmission line, which could have led to the disconnection of the generators near Saltend.

The regulator said the tripping of distributed generators would likely have increased the total loss from this fault to as much as 1,600MW. The report said: “This loss would have exceeded the amount of back-up power the ESO was holding, causing the frequency to drop below standards, and could have resulted in a similar power outage to the one that occurred on 9 August.”

Ofgem said the ESO’s processes for estimating the impact of distributed generators “do not appear sufficiently robust given the marginal levels of system inertia and poor performance of frequency response providers on the day”. It said the issue is exacerbated by a lack of granular data on operational characteristics of distributed generators during network faults.

“We acknowledge that the ESO has had difficulty in obtaining accurate data on distributed generation,” it added.  “However, in our view the ESO could have been more proactive in raising the issue of distributed generation impacts on system security with the regulator and industry parties.”

Ofgem said it is the internal policy of the ESO to only cover for losses from distributed generation during periods of increase to the transmission system, for example, due to bad weather. The regulator said it would review both the Security and Quality of Supply Standards and their application by the ESO to see whether changes are required.

The ESO itself called for a review of the standards in its final technical report on the blackout.

Meanwhile, the owners of Little Barford and Hornsea One have each agreed to pay £4.5 million as redress for their roles in the power cut. UK Power Networks has also agreed to make a redress payment of £1.5 million for prematurely reconnecting demand following the activation of the Low Frequency Demand Disconnection scheme.