UK blackout
London was left without power during peak commuting periods - Image credit: Teller Report

RWE, and 12 distribution network operators in England and Wales are to be investigated alongside the UK’s National Grid Transmission and Electricity System Operator (ESO) regarding the 9 August blackout.

National Grid’s much-anticipated interim report, which was delivered on Friday 16 August, cites a lightning strike as being the trigger in a chain of events which saw the Ørsted-managed Hornsea 1 offshore wind plant all-but go offline, followed milliseconds later by the tripping of a steam turbine at the gas-fired Little Barford power plant, run by RWE.

Ofgem will be investigating whether any party involved breached the conditions of their license, or other obligations. should contraventions have occurred, the regulator has the power to impose fines. The investigation is also being considered as an opportunity to review the grid’s operations.

The regulator’s investigations will consider:

  • The National Grid Electricity Supply Operator (NGESO’s) requirements to hold sufficient back-up power either through inertial response by generators in operation, or via frequency response.
  • How generators met their obligations with respect to the transmission fault.
  • Whether distribution network operators complied with their Low Frequency Demand Disconnection obligations.
  • The circumstances leading to the loss of power to critical infrastructure and if companies made the right decisions in terms of disconnections.

Here’s a breakdown of the events that triggered the blackout:

  • A lightning strike at approximately 4:50m UK-time on Friday 09 August causing a frequency fluctuation sufficient to trigger Hornsea 1’s protective safety systems.
  • The wind site’s output is triggered to deload from 799MW to 62MW.
  • Milliseconds after the deload event, a steam turbine at the gas-powered Little Barford site is tripped, taking 244MW off the system
  • Grid frequency response kicks in, taking a further 500MW of embedded generation offline as loss of mains protection systems are activated
  • Little Barford’s protection systems then automatically tripped a second steam turbine, causing frequency to drop below the 48.8Hz Low Frequency Demand Disconnetion threshold.
  • National Grid ESO’s system then ordered DNO’s to take 931MW of demand  offline
  • High steam pressure forced operators at Little Barford to shut down another turbine less than a minute later, and National Grid claim that additional power brought online would make up for this loss.
  • Frequency was restored to 50Hz within five minutes, and National Grid instructs DNO’s to start restoring demand, less than 15 minutes after the power failure.
  • DNOs were instructed to commence demand restoration in under 15 minutes – of the initial failure.
Source: National Grid ESO interim report

The ESO’s interim report lays out a timeline. Whilst the report shows Little Barford going offline first, then followed by Hornsea, data delays have to be taken into account. National Grid’s timeline suggests Hornsea started deloading milliseconds earlier.

Here's a link to the initial report.

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