National Grid has suggested that higher levels of resilience in the UK’s electricity system may be appropriate, its report into the 9 August 2019 blackout has concluded.

Submitted to Ofgem last Friday (6 September 2019), National Grid ESO’s final technical report concludes that while protection mechanisms operated as expected to protect the grid from more significant faults, a review of the security standards could be necessary to determine whether greater quantities of reserve capacity are needed to prevent similar instances from happening again.

This should be done in a “structured” way to ensure proper balancing of risks and costs, the ESO said.

Meanwhile, it has also suggested that a review of the timescales for delivery of the Accelerated Loss of Mains Change Programme – which was responsible for taking ~500MW of embedded generation (domestic solar panels, battery storage etc) off the grid – could be necessary, especially as more significant quantities of such generation are connected.

Key conclusions and recommendations

The ESO’s analysis of the report has led it to come to various conclusions and make recommendations where it feels lessons can be learned, including;

New incident details

The full technical report also includes fresh details around the incident and its effects on the grid.

National Grid ESO maintains that the transmission circuit cleared the lightning strike as expected within 0.1 seconds, and the line returned to normal operation after circa 20 seconds. This was within its standards and thus, a fault on the line has been ruled out as a factor in subsequent impacts.

However, around 150MW of embedded generation is now revealed to have been disconnected due to vector shift protection immediately after the strike. Again, NG ESO stressed this was to be expected from a lightning strike.

Immediately after the lightning strike, Hornsea offshore wind farm reduced its contribution to the grid by 737MW, and a trip at Little Barford’s steam turbine reduced its load by 244MW in what National Grid is now referring to as an “extremely rare and unexpected event”.

The loss of 1,131MW of generation triggered the rapid fall in grid frequency, which in turn caused an additional 350MW of embedded generation to disconnect due to Rate of Change of Frequency (RoCoF) protection protocols.

Combined, this amounted to 1,481MW of generation coming offstream. While National Grid ESO worked to restore frequency using its backup power response fleet, amounting to roughly 1,00MW, a further trip at Little Barford’s gas turbine sent another 210MW off the system – this is now attributed to high pressure in the steam bypass system following a bypass valve to operate as expected – taking the total cumulative loss of load to 1,691MW.

The loss of that second load caused grid frequency to fall further, prompting National Grid to instruct distribution network operators to commence Low Frequency Demand Disconnection. While this enabled grid frequency to be recovered initially, a second subsequent gas turbine trip at Little Barford caused a further 187MW to deload. Total cumulative loss of generation therefore amounted to 1,878MW.

It has also now been revealed that some customers were without power for as long as 45 minutes, longer than originally anticipated.

Next steps

National Grid ESO has said that where possible, these lessons will be embedded within its business moving forward. It has also stressed that Ofgem, the Department for Business, Energy and Industrial Strategy (BEIS) and the wider industry need consult on its recommendations made surrounding industry standards and processes.

This should also incorporate any findings from subsequent investigations, including those conducted by Ofgem and the Energy Emergencies Executive Committee (E3C).

The industry will now look to those reports for additional detail, as well as any potential enforcement proceedings that could emerge.

While there is no specific deadline for Ofgem’s report, the E3C’s interim report to the Secretary of State is due by 18 September, followed by the final E3C report, as well an implementation plan for any lessons, is due by 6 November.

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